Method and apparatus for depositing thin films on a surface

0001This application is a reissue of U.S. Pat. No. 7,537,662, which issued May 26, 2009, from U.S. patent application Ser. No. 10/428,207, filed Apr. 29, 2003.

FIELD OF THE INVENTION

0002The present invention relates generally to a semiconductor processing apparatus and more particularly, a semiconductor processing apparatus for depositing thin films on a substrate surface.

BACKGROUND OF THE INVENTION

0003Thin films may be grown on the surface of substrates by several different methods. These methods include vacuum evaporation deposition, molecular beam epitaxy (MBE), different variants of chemical vapor deposition (CVD) (including low-pressure and organometallic CVD and plasma-enhanced CVD), and atomic layer epitaxy (ALE), which has been more recently referred to as atomic layer deposition (ALD) for the deposition of a variety of materials.

0004In ALD, the sequential introduction of precursor species (e.g., a first precursor and a second precursor) to a substrate, which is located within a reaction chamber is generally employed. Typically, one of the initial steps of ALD is the adsorption of the first precursor on the active sites of the substrate. Conditions are such that no more than a monolayer forms so that the process is self-terminating or saturative. For example, the first precursor can include ligands that remain on the adsorbed species, which prevents further adsorption. Accordingly, deposition temperatures are maintained above the precursor condensation temperatures and below the precursor thermal decomposition temperatures. This initial step of adsorption is typically followed by a first removal (e.g., purging) stage, where the excess first precursor and possible reaction byproducts are removed from the reaction chamber. The second precursor is then introduced into the reaction chamber. The first and second precursor typically tend to react with each other. As such, the adsorbed monolayer of the first precursor reacts instantly with the introduced second precursor, thereby producing the desired thin film. This reaction terminates once the adsorbed first precursor has been consumed. The excess of second precursor and possible reaction byproducts are then removed, e.g., by a second purge stage. The cycle can be repeated to grow the film to a desired thickness. Cycles can also be more complex. For example, the cycles can include three or more reactant pulses separated by purge and/or evacuation steps.

0005Ideally, in ALD, the reactor chamber design should not play any role in the composition, uniformity or properties of the film grown on the substrate because the reaction is surface specific and self-saturating. However, only a few precursors exhibit such ideal or near ideal behavior. Factors that may hinder this idealized growth mode can include: time-dependent adsorption-desorption phenomena; blocking of the primary reaction through by-products of the primary reaction (e.g., as the by-products are moved in the direction of the flow, reduced growth rate down-stream and subsequent non-uniformity may result, such as when corrosive and less volatile halide products are produced as a byproduct of an ALD process alternating, e.g., TiCl₄+NH₃to produce TiN); total consumption (i.e., destruction) of the second precursor in the upstream-part of the reactor chamber (e.g., decomposition of the ozone in the hot zone); and uneven adsorption/desorption of the first precursor caused by uneven flow conditions in the reaction chamber.

0006These problems have been partially alleviated with the use of a showerhead-type apparatus used to disperse the gases into the reaction space, such as disclosed in U.S. Pat. No. 4,798,165. The showerhead-type apparatus, as found in U.S. Pat. No. 4,798,165, may be positioned above a substrate so that the reactants and purge gases flow through apertures that are located on the showerhead and the gas flow may be directed perpendicular to the substrate. However, in such a configuration, in the course of time the reacted gases may form a film in the apertures and the apertures may become blocked. Such blockage may result in uneven deposit of layers onto the substrate.

0007PCT publication No. WO 00/79019, published Dec. 28, 2000 discloses use of hollow tubes with apertures for ALD deposition. In addition to issues with respect to blockage of the apertures, the disclosed structure contemplates relative rotation of either the substrate or the tubes during deposition. Such a construction leads to the additional issue that, for most efficient saturation of the substrate with reactant, rotation must be calculated to be an integral value in each reactant pulse, limiting flexibility in recipe design and risking non-uniformity. Furthermore, the complexity of rotating elements leads to risks of reactant leakage between rotating parts, consequent particle generation and/or safety hazards.

0008Thus, there is a need for an improved apparatus and method for depositing thin layers that addresses at least some of the problems described above.

SUMMARY OF THE INVENTION

0009In accordance with one aspect of the invention, an apparatus is provided for depositing a thin film on a substrate. The apparatus includes a reaction chamber having a reaction space, a substrate holder, a gas outlet in fluid communication with the reaction space and a gas injector structure positioned with the reaction chamber fixed relative to the substrate during deposition. The gas injector structure includes a first gas injector in fluid communication with a first reactant gas source and a purge gas source, and a second gas injector in fluid communication with a second reactant gas source and a purge gas source. The first and second gas injectors include hollow tubes extending in the reaction space, with a plurality of gas flow apertures spaced along respective tube axes of elongation, where the apertures open to the reaction space.

0010In one embodiment, the hollow tubes with apertures therein are positioned on opposite sides of the substrate holder. The apertures can face each other or away from one another in different arrangements. A similar third injector can provide a third reactant from a third side of the substrate holder. In another embodiment, the hollow tubes of each injector include tubes branching from distribution tubes on opposite sides of the substrate holder, the branching tubes of the different injectors staggered with one another along an injection plane adjacent a major surface of the substrate. In either of these embodiments, the exhaust can flow through the plane of the injectors. In yet another embodiment, the injectors are hollow tubes extending parallel to one another but stacked upon one another on one side of the substrate.

0011In accordance with another aspect of the invention, an apparatus is provided for depositing thin films on a substrate. The apparatus includes a reaction chamber, a substrate support and a showerhead rake structure positioned adjacent the substrate support. The showerhead rake structure includes a first gas injector having a first rake in fluid communication with a first reactant source, including a plurality of first fingers extending from and being in fluid communication with a first gas distribution structure. The showerhead rake structure also includes a second gas injector having a second rake in fluid communication with a second reactant source, the second rake including a plurality of second fingers extending from and being in fluid communication with a second distribution structure. Each of the first and second fingers have apertures along a length thereof.

0012In accordance with another aspect of the invention, a method is provided for depositing a thin layer on a substrate within a reaction space defined by chamber walls. The reaction chamber has a gas feed and removal structure including a first gas injector, a second gas injector, and a gas outlet. The first and second gas injectors comprise elongated tubes disposed within the reaction space and having apertures spaced therealong in fixed relation to the substrate during deposition. The method comprises at least one cycle including the following steps in sequence: <ul id="ul0001" list-style="none"><li id="ul0001-0001" num="0000"><ul id="ul0002" list-style="none"><li id="ul0002-0001" num="0013">a first reactant step including injecting a first vapor phase reactant into the reaction space through the apertures in the first injector while injecting purge gas through the apertures in the second injector;</li><li id="ul0002-0002" num="0014">a first purge step including stopping the first reactant injection and injecting purge gas through the apertures in the first and second injectors into the reaction space;</li><li id="ul0002-0003" num="0015">a second reactant step including injecting a second vapor phase reactant into the reaction space through the apertures in the second injector while injecting purge gas through the apertures in the first injector; and</li><li id="ul0002-0004" num="0016">a second purge step including stopping the second reactant injection and injecting purge gas through the apertures in the first and second injectors into the reaction space.</li></ul></li></ul>

0017In one embodiment, gases are exhausted through the elongated tube in route from the substrate to the outlet. Both reactant and purge gas is allowed to diffuse across gaps between the apertures and thereby saturate the substrate in each step. Advantageously, the purge steps can include multiple sequential pulses causing pressure fluctuation, which expedite diffusion between gaps in the apertures, and especially diffusion into deep narrow trenches of the substrate. Similarly, the reactant steps can include multiple sequential pulses causing pressure fluctuations to aid in diffusing reactant across the substrate or into deep, narrow features on the substrate. In one particular arrangement, a booster purge pulse, immediately prior to a standard purge pulse, helps clear reactant from the prior pulse and creates a pressure gradient, while also clearing an inert gas diffusion barrier created by an inert gas valving arrangement.

0018In accordance with another aspect of the invention, a method is provided for atomic layer deposition. The method includes alternating reactant steps and intervening purge steps in a plurality of cycles, wherein at least some of the purge steps comprise multiple purge pulses. The multiple pulses creating pressure fluctuations within a reaction space.

0019Further aspects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

0020<figref idref="DRAWINGS">FIGS. 1-16</figref> are non-limiting illustrations (not to scale) of deposition apparatuses constructed in accordance with various preferred embodiments.

0021<figref idref="DRAWINGS">FIG. 1</figref> is a schematic top view of an ALD reactor constructed in accordance with one embodiment of the invention, showing a gas flow pattern during a first reactant pulse.

0022<figref idref="DRAWINGS">FIG. 2a</figref> is a schematic cross section view taken along lines <b>2</b>a-<b>2</b>a of <figref idref="DRAWINGS">FIG. 1</figref>.

0023<figref idref="DRAWINGS">FIGS. 2b-2d</figref> are schematic cross-sections showing alternative gas inlet configurations.

0024<figref idref="DRAWINGS">FIG. 3</figref> is a schematic cross-sectional side view of one of the reactant in-feed systems of <figref idref="DRAWINGS">FIG. 1</figref>, showing the gas flow pattern during a reactant pulsing step.

0025<figref idref="DRAWINGS">FIG. 4</figref> is a schematic top view of the ALD reactor as depicted in <figref idref="DRAWINGS">FIG. 1</figref>, showing the gas flow pattern during a purging step.

0026<figref idref="DRAWINGS">FIG. 5</figref> is a schematic cross-sectional side view of the reactant in-feed system of <figref idref="DRAWINGS">FIG. 3</figref>, showing the gas flow pattern during a purging step.

0027<figref idref="DRAWINGS">FIG. 6</figref> is the schematic top view of the ALD reactor as depicted in <figref idref="DRAWINGS">FIG. 1</figref>, showing a gas flow pattern during a second reactant pulse.

0028<figref idref="DRAWINGS">FIG. 7</figref> is a schematic cross-sectional side view of the reactant in-feed system of <figref idref="DRAWINGS">FIG. 3</figref>, showing the gas flow pattern during an inactive gas booster step.

0029<figref idref="DRAWINGS">FIG. 8</figref> is a schematic cross-sectional side view of an ALD reactor constructed in accordance with another embodiment of the invention.

0030<figref idref="DRAWINGS">FIG. 9</figref> is a schematic top view of an ALD reactor constructed according to another embodiment of the present invention.

0031<figref idref="DRAWINGS">FIG. 10</figref> is a schematic cross-sectional side view of an ALD reactor constructed according to another embodiment of the present invention.

0032<figref idref="DRAWINGS">FIG. 11</figref> is a schematic cross-sectional side view of an ALD reactor according to still another embodiment of the invention.

0033<figref idref="DRAWINGS">FIG. 12</figref> is a schematic cross-sectional view of the ALD reactor of <figref idref="DRAWINGS">FIG. 11</figref> with an alternate type of reaction chamber sealing system.

0034<figref idref="DRAWINGS">FIGS. 13a, 13b and 13c</figref> are schematic cross sectional views of gas flow geometry near the substrate in the ALD reactor of <figref idref="DRAWINGS">FIG. 11</figref>.

0035<figref idref="DRAWINGS">FIG. 14</figref> is a schematic cross-sectional view of an ALD reactor according to still another embodiment of the invention.

0036<figref idref="DRAWINGS">FIG. 15</figref> is a schematic top view of a dual spiral showerhead, constructed in accordance with another embodiment of the invention.

0037<figref idref="DRAWINGS">FIGS. 16a and 16b</figref> are schematic top down and end views, respectively, of shower tubes constructed in accordance with another embodiment of the invention, shown in relation to a substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0038While not separately illustrated, the skilled artisan will readily appreciate that the flow sequences described herein can be controlled by software programming or hardwiring arranged to open and close gas control valves in the desired sequence.

0039<figref idref="DRAWINGS">FIG. 1</figref> illustrates a schematic top view of an embodiment of an atomic layer deposition (ALD) reactor <b>10</b>. The ALD reactor <b>10</b> includes a reaction chamber <b>12</b>, which defines, in part, a reaction space <b>14</b>. A wafer or substrate <b>16</b> is disposed within the reaction chamber <b>12</b> and is supported by a platform or a single-substrate support. The substrate support of the illustrated embodiment also serves as a wafer handler <b>18</b> configured to move the wafer <b>16</b> in and out of the reaction chamber <b>12</b>. The wafer handler <b>18</b> can be configured to receive the wafer <b>16</b> in such a way that the wafer <b>16</b> touches the handler <b>18</b> itself. Alternatively, the wafer handler can operate on the Bernoulli principle, whereby jets of inactive gas produce a low pressure zone between the handler and wafer. In a Bernoulli configuration the wafer can be held on top or on the bottom of the handler. The handler <b>18</b> can be robot end effector, or more preferably, can be configured to exchange a wafer with a separate robot end effector when the handler is lowered relative to the reaction chamber <b>12</b>. While configured in the illustrated embodiments as a movable substrate support to facilitate loading and unloading substrates between depositions, the handler <b>18</b> preferably keeps the substrate <b>16</b> stationary relative to the gas injection structure (described below) during operation.

0040In the illustrated reactor <b>10</b>, a gas inlet or injection structure <b>20</b> is provided. The gas injection structure <b>20</b> supplies two precursors, A and B, and inactive purge gas to the reaction space <b>14</b> and is located in a plane above the wafer or substrate <b>16</b>. A skilled artisan will appreciate that the gas injection structure <b>20</b> may be positioned below the wafer or substrate <b>16</b>, or to the side of a vertically positioned wafer or substrate in other arrangements and that such position will be dependent on the position of the wafer or substrate. Preferably, the injection structure is positioned on an adjacent plane to the substrate's major surface (as best seen in the cross-sectional end view of <figref idref="DRAWINGS">FIG. 2a</figref>). As will be appreciated from the discussion of <figref idref="DRAWINGS">FIG. 2a</figref> below, this arrangement permits exhaust flow through or along the space between injectors. The gas injection structure <b>20</b> is preferably readily replaceable, but is arranged to remain fixed relative to reactor walls, and preferably also fixed relative to the substrate <b>16</b>, during deposition, facilitating rapid gas spreading by diffusion across the substrate during each reactant or purge pulse.

0041The gas injection structure <b>20</b> includes a first gas inlet or injector <b>22</b> and a second gas inlet or injector <b>24</b>. The first gas inlet <b>22</b> is in communication with a first precursor A supply source (not shown) and a purging gas supply source (not shown). The first gas inlet may be connected to the supply sources via a single tube or multiple tubes, where each connection can be a tolerance fitting, o-ring seal, an axial shaft seal, or any other method of connection known by those skilled in the art. The purging gas is preferably an inactive gas, and may be, by way of example, nitrogen or argon. The purging gas may be used to transport the first and/or second precursor from the supply source to the reaction chamber <b>12</b>. The purging gas may also be used to purge the reaction chamber and/or the gas inlets of excess reactant and reaction by-product gases. Note that, in <figref idref="DRAWINGS">FIG. 1</figref>, reactant gas is shown by black arrows in the first gas inlet <b>22</b>, while purge gas is shown by white arrows in the second gas inlet <b>24</b>.

0042As shown in <figref idref="DRAWINGS">FIG. 1</figref>, the first gas inlet or injector <b>22</b> includes a first outer tube <b>28</b>, a first inner tube <b>27</b> and a plurality of appropriately spaced apertures <b>30</b> located along a distributor section of the length of the first outer tube <b>28</b>. Generally, the length of the apertured distributor section in the first outer tube <b>28</b> is longer than the diameter of the wafer or substrate <b>16</b>. The structure of the first gas inlet <b>22</b> will be described in greater detail below.

0043The gas injection structure <b>20</b> is used to deposit a thin layer onto a substrate. In this embodiment, the first precursor A is injected into the reaction chamber <b>12</b>, it travels from the first precursor A supply source (not shown), through the first inner tube <b>27</b> (indicated by arrows <b>52</b>), then through the first outer tube <b>28</b> and out (indicated by arrows <b>54</b>) through the plurality of apertures <b>30</b> over the wafer or substrate <b>16</b> so that the first precursor A chemisorbs onto the wafer or substrate <b>16</b>. Preferably, a monolayer (single molecular layer) of the first precursor A molecules is chemisorbed onto the wafer or substrate <b>16</b>. During the first precursor A flow, a small amount of purge gas can simultaneously flow in a space <b>29</b> between the first inner tube <b>27</b> and the first outer tube <b>28</b> towards the reaction chamber <b>12</b>, allowing the upstream section of the outer tube <b>28</b> tubing to remain substantially free from first precursor A. The space <b>29</b> thus serves as a first purge channel while the inner tube <b>27</b> provides a first reactant channel, which is preferably less restrictive than the first purge channel. The flow rate of the purge gas during the first precursor A flow can be, for example, 5-20% of the flow rate of the purge gas during the following purge step.

0044Once the chemisorption process has essentially self-terminated through consumption of available reactive surface sites, the purging gas flows from the purging gas supply source (not shown) first through the space <b>29</b> between the first outer tube <b>28</b> and the first inner tube <b>27</b>, then through the first outer tube <b>28</b> and out (indicated by arrows <b>54</b>) through the apertures <b>30</b>. The path by which the gases travel may be hermetically sealed. For purposes of the present disclosure, “hermetically sealed” means that all the gas inlet surfaces upstream of the reaction chamber are exposed to only one precursor. Thus, the first gas inlet and the second gas inlet are preferably physically isolated from each other.

0045Similarly, the second precursor B is supplied to the reaction chamber <b>12</b> through the second gas inlet or injector <b>24</b>, which is in communication with a second precursor B supply source (not shown) and a purging gas supply source (not shown). The second gas inlet may be connected to the supply source via a single tube or multiple tubes, where each connection can be a tolerance fitting, o-ring seal or axial shaft seal, or any other method of connection known by those skilled in the art. The second gas inlet <b>24</b> may be position in the same plane as the first gas inlet <b>22</b> (see <figref idref="DRAWINGS">FIG. 2a</figref>); in other arrangements, however, one skilled in the art may appreciate that the second gas inlet may be position above or below the level of the first gas inlet (see <figref idref="DRAWINGS">FIGS. 16a and 16b</figref>). In the illustrated embodiment, the distributer tube sections <b>3</b> (see <figref idref="DRAWINGS">FIG. 1</figref>) of the first gas inlet <b>22</b> and the second gas inlet <b>24</b> are positioned parallel to one another on opposite sides of the substrate <b>16</b>, leaving a wide space (as wide as the substrate) therebetween for controlling exhaust flow.

0046The second gas inlet or injector <b>24</b> depicted in <figref idref="DRAWINGS">FIG. 1</figref> is also shaped substantially tube-like and includes similar components as the first gas inlet <b>22</b>, including a second inner tube <b>74</b>, a second outer tube <b>34</b> and a plurality of apertures <b>36</b>. According to one embodiment, the apertures <b>36</b> are directed toward the wafer or substrate <b>16</b>. Thus, when the gases are injected into the reaction chamber <b>12</b>, the second precursor B travels from the second precursor B supply source (not shown), first through the second inner tube <b>74</b>, then through the distributor section of the second outer tube <b>34</b> and out through the plurality of apertures <b>36</b>. Typically, in ALD the second precursor B molecules or radicals will react with the chemisorbed first precursor A that is already present on the substrate surface. During the second precursor B flow, a small amount of purge gas can simultaneously flow in a space <b>72</b> between the second inner tube <b>74</b> and the second outer tube <b>34</b> towards the reaction chamber allowing the upstream section of outer tube <b>34</b> to remain substantially free from second precursor B. The space <b>72</b> thus serves as a second purge channel while the inner tube <b>74</b> serves as a second reactant channel, which is preferably less restrictive than the second purge channel. The flow rate of the purge gas during the second precursor B flow can be, for example, 5-20% of the flow rate of the purge gas during the following purge step.

0047Once the desired chemisorption reaction on the surface is self-terminated, the purging gas flows from the purging gas supply source (not shown) first through the space <b>72</b> between the second inner tube <b>74</b> and the second outer tube <b>34</b>. Then the purging gas flow is divided into two parts so that the first part flows through the distributor section of the second outer tube <b>34</b> and out through (indicated by arrows <b>82</b>) the apertures <b>36</b>. The excess second precursor B and possible reaction by-products are removed, e.g., purged from the reaction chamber <b>12</b>. The second part of the purging gas flow goes through (indicated by arrows <b>76</b>, <b>78</b>) the second inner tube <b>74</b>.

0048In the illustrated embodiment, excess first precursor A, excess second precursor B, reaction by-products and/or purging gas is removed from the reaction chamber <b>12</b> via a gas exhaust or outlet <b>56</b>. As may be appreciated by one skilled in the art, the gas outlet <b>38</b> may be a vacuum or a simple outlet that employs a pressure gradient between the chamber and an outer environment, causing the gas in the reaction chamber to escape into the outer environment. Here, the gas outlet <b>56</b> is in fluid communication with a vacuum pump <b>58</b> or some other device used for generating vacuum. The vacuum pump <b>58</b> has a pump exhaust <b>86</b> for letting out gases from the vacuum pump <b>58</b>. Additionally, in this embodiment, a flow pattern modifier <b>50</b> is shown at each of the exhaust end and the opposite end of the reaction space <b>14</b>. The shapes of optional flow pattern modifiers <b>50</b> are selected according to the purging requirements of the reaction chamber <b>12</b>, for example, circular or ovular or any other shape known by those skilled in the art. The flow pattern modifiers <b>50</b> prevents the formation of stagnant gas volumes inside the reaction chamber <b>12</b>. Computer programs, such as FLUIDS32 Fluid Flow Analyzer developed by Dr. Stanislaw Raczynski, can be used for simulating the gas flow patterns and determining the optimum shapes of the flow pattern modifiers <b>50</b>. Flow pattern modifiers can be independent parts that are attached into the reaction chamber <b>12</b> or they can be integral parts of the reaction space <b>12</b>.

0049This deposition cycle of first precursor A injection, first purge, second precursor B injection and second purge may be repeated as many times as necessary, depending on the desired thickness of the thin film to be deposited.

0050A schematic cross sectional view of the ALD reactor <b>10</b> depicted in <figref idref="DRAWINGS">FIG. 1</figref> is shown in <figref idref="DRAWINGS">FIG. 2a</figref> and provides further detail of the apertures <b>30</b> and <b>36</b>. A substrate <b>16</b> is supported by a heated platform or a wafer handler <b>18</b>. The first outer tube <b>28</b> and the second outer tube <b>34</b> are placed at opposite sides of the substrate <b>16</b>. Apertures <b>30</b> in the first outer tube <b>28</b> and apertures <b>36</b> in the second outer tube <b>34</b> are directed towards the substrate <b>16</b>. The apertures <b>30</b> are preferably about 10-30 mm apart from each other. The distance between the apertures <b>36</b> can be similar, e.g., about 10-30 mm. As illustrated, the gas injectors are on a plane above the substrate <b>16</b>, positioned such that exhaust can flow along or through the space between the injector tubes <b>28</b>, <b>34</b>. In the illustrated embodiment, at least part of the exhaust opening <b>56</b> is positioned above the injection plane defined by the apertures <b>30</b>, <b>36</b>.

0051Referring to <figref idref="DRAWINGS">FIGS. 2b-2d</figref>, in other arrangements, the apertures <b>30</b> and <b>36</b> can be directed towards the ceiling <b>202</b>, the floor <b>204</b> or the side walls <b>206</b> and <b>208</b> of the reaction chamber <b>12</b>. <figref idref="DRAWINGS">FIG. 2b</figref> shows a setup where the apertures <b>30</b> in the first outer tube <b>28</b> are directed away from the side wall <b>206</b> towards the substrate, similar to the arrangement of <figref idref="DRAWINGS">FIG. 2a</figref>. In the illustrated arrangement, the outer tube <b>28</b> is shown spaced away from the side wall <b>206</b>. It will be appreciated by the skilled artisan that, in other arrangements, the first outer tube can abut the side wall without spacing. Nevertheless, the first outer tube <b>28</b> preferably comprises an independent element that can be removed and replaced without effecting the integrity of the reaction chamber walls <b>202</b>, <b>204</b>, <b>206</b>. In other words, as with the other embodiments in the present application, the gas injectors are not integrated with the walls but are rather removable and readily replaceable elements extending within the reaction chamber.

0052<figref idref="DRAWINGS">FIG. 2c</figref> shows another setup where the apertures <b>30</b> in the first outer tube <b>28</b> are directed towards the side wall <b>206</b>. Gases exiting the apertures <b>30</b> will first hit the side wall <b>206</b> and then flow around the first outer tube <b>28</b>. After that the gases will flow towards the substrate. One benefit of this setup is that the space between the first outer tube <b>28</b> and the side wall <b>206</b> is purged efficiently during the purge time between reactant pulses.

0053<figref idref="DRAWINGS">FIG. 2d</figref> shows still another setup where the first outer tube is flattened, the flattened part preferably including the distributor section along the length of the first outer tube <b>28</b> where the apertures <b>30</b> are located. The flattened first outer tube <b>28</b> has two sets of apertures <b>30</b> and <b>210</b>. The first set of apertures <b>30</b> is directed “outwardly” towards the side wall <b>206</b> and the second set of apertures <b>210</b> is directed “inwardly” towards the substrate. One benefit of this setup is that stagnant gas pockets are eliminated from the reaction space and the gas volume of the reaction space can be purged very rapidly and efficiently.

0054The gas injectors are preferably made of materials that have very smooth surfaces so that the gas injectors can be purged rapidly. Examples of such preferred materials are glass (especially silica), electrochemically polished metal, silicon carbide, polymer, and ceramic- or glass-coated material. The gas injector structure is placed for example about 30-60 mm from the platform.

0055The tubes that form the gas injectors of the preferred embodiments can take other shapes in various arrangements. Thus, the outer and inner tubes can have, e.g., rectangular, polygonal or round cross sections. Preferably, however, the tubes that form the gas injectors are curved in a cross section taken along a plane perpendicular to the axis of the tube, as shown in <figref idref="DRAWINGS">FIGS. 2b-2d</figref>. A smooth curvature facilitates gas flow around the tube, particularly for embodiments in which gas is designed to exit an aperture on one side of the tube and either flow or have excess purged around the opposite side of the tube. In the illustrated embodiments, the injector tubes include a plurality of axially spaced apertures opening perpendicularly from the tube axis.

0056<figref idref="DRAWINGS">FIG. 3</figref> further illustrates a cross-sectional side view of a gas injection structure and shows the reactant in-feed system and gas flow pattern during a reactant pulsing step. When first precursor A gas is let into the reaction chamber, a purge gas valve <b>606</b>, a booster valve <b>608</b> and a source exhaust valve <b>614</b> are kept closed. The source control valve <b>618</b> of the first precursor A source <b>616</b> is opened. Vapor of the first precursor reactant A flows through the source control valve <b>618</b>, through (indicated by arrows <b>52</b>) the first inner tube <b>27</b>, through the first outer tube <b>28</b> and through the apertures <b>30</b> of the first outer tube <b>28</b> to the reaction space. In the case that the vapor pressure of the first precursor A is so low that the precursor vapor cannot come out of the first precursor A source <b>616</b>, a carrier gas line (not shown) can be connected to the source <b>616</b> so that pressure increase inside the source <b>616</b> forces reactant vapor out of the source <b>616</b> to the first inner tube <b>27</b>.

0057There are different ways of controlling the gas content of the flow space or purge channel <b>29</b> between the first inner tube <b>27</b> and the first outer tube <b>28</b>. In the embodiment illustrated in <figref idref="DRAWINGS">FIG. 3</figref>, the purge gas valve <b>606</b> is kept closed so that gases are stagnant in the first purge channel <b>29</b> during the pulse time of the first reactant A. Alternatively, a by-pass capillary (not shown) lets a small amount of inactive gas flow (e.g., 5-20% of the purge flow during a purge step) from the controlled inactive gas source <b>602</b> past the closed purge gas valve <b>606</b> to the purge channel <b>29</b>. One benefit of this embodiment is that the inactive gas keeps reactant A molecules away from the purge channel <b>29</b>. The flow rate of the inactive gas is set to such a low level that a gas diffusion barrier is not formed near the tip <b>304</b> of the first inner tube <b>27</b> and the reactant A can flow towards the apertures <b>30</b>. Those skilled in the art will appreciate that other ways to control the gas content in the flow space may also be employed.

0058<figref idref="DRAWINGS">FIG. 4</figref> shows a schematic top view of an ALD reactor as depicted in <figref idref="DRAWINGS">FIG. 1</figref> showing a gas flow pattern during a purging step. As illustrated, the first gas inlet <b>22</b>, including the first inner tube <b>27</b> and the first outer tube <b>28</b>, is purged with inactive gas. Inactive gas flows <b>302</b> through the flow space or purge channel <b>29</b> between the first inner tube <b>27</b> and the first outer tube <b>28</b> until the tip <b>304</b> of the first inner tube <b>27</b> is reached. At the tip <b>304</b> the inactive gas flow <b>302</b> is divided into two parts <b>308</b> and <b>306</b>. The various restrictions in flow encountered by the purge gas are arranged to ensure that the second part <b>306</b> of the inactive gas flows towards the inside of the first inner tube <b>27</b> and makes sure that any gaseous reactant residue inside the first inner tube <b>27</b> is pushed away from the tip <b>304</b> of the first inner tube <b>27</b>. The first part <b>308</b> of the inactive gas flow continues through the first outer tube <b>28</b>, exits from the apertures <b>30</b> and flows towards <b>310</b> the substrate <b>16</b>.

0059Similarly, the second gas inlet <b>24</b>, including the second inner tube <b>74</b> and the second outer tube <b>34</b>, is purged with inactive gas. Inactive gas flow <b>70</b> is directed through the space <b>72</b> between the second inner tube <b>74</b> and the second outer tube <b>34</b> until the tip <b>75</b> of the second inner tube <b>74</b> is reached. At the tip <b>75</b>, the inactive gas flow <b>70</b> is divided into two parts <b>80</b> and <b>76</b>. The second part <b>76</b> of the inactive gas flows towards the inside of the second inner tube <b>74</b> and makes sure that any gaseous reactant residue inside the second inner tube <b>74</b> is pushed further away from the tip <b>75</b> of the second inner tube <b>74</b>. The first part <b>80</b> of the inactive gas flow continues through the second outer tube <b>34</b>, exits from the apertures <b>36</b> and flows <b>82</b> towards the substrate <b>16</b>.

0060<figref idref="DRAWINGS">FIG. 5</figref> shows a reactant in-feed system that can be used with an embodiment of the invention. The schematic cross-sectional side view of the reactant in-feed system depicts a gas flow pattern during the purging step. The gas inlet or injector <b>22</b> comprises a first outer tube <b>28</b> having apertures <b>30</b>, a first inner tube <b>27</b> that is in fluid communication with a precursor source <b>616</b>, a controlled exhaust line <b>620</b> and a controlled inactive gas source <b>602</b>.

0061Inactive or “purge” gas is let from the inactive gas source <b>602</b> through a flow restrictor capillary <b>604</b> and a gas valve <b>606</b> to a flow space <b>29</b> between the first inner tube <b>27</b> and the first outer tube <b>28</b>. The inset shows a cross-section of the injector <b>22</b> along lines Y-Y and illustrates the position of the flow space <b>29</b> between the first inner tube <b>27</b> and the first outer tube <b>28</b>. A booster valve <b>608</b> is kept closed. The inactive gas flows <b>302</b> towards a gas diffusion barrier area that is located near the tip <b>304</b> of the first inner tube <b>28</b>. Near the tip <b>304</b> the inactive gas flow is divided into two parts. The first part of the divided inactive gas flow continues <b>308</b> along the first outer tube <b>28</b> and exits the first outer tube <b>28</b> through apertures <b>30</b> to the reaction chamber. The second part of the divided inactive gas flow turns to the inside of the first inner tube <b>27</b> and flows <b>306</b> towards the exhaust line <b>620</b>. The exhaust line <b>620</b> has a flow restriction capillary <b>612</b> for limiting the flow rate of gases towards the vacuum pump <b>58</b> and a valve <b>614</b> for closing the exhaust line <b>620</b> when needed. The exhaust valve <b>614</b> is kept open during the purge step.

0062<figref idref="DRAWINGS">FIG. 6</figref> is the schematic top view of an ALD reactor as depicted in <figref idref="DRAWINGS">FIG. 1</figref> showing a gas flow pattern during a second precursor B pulse. Inactive purging gas flows through the first gas inlet <b>22</b> that includes the first inner tube <b>27</b> and the first outer tube <b>28</b>, the same way as in <figref idref="DRAWINGS">FIG. 4</figref>. The second gas inlet <b>24</b>, including the second inner tube <b>74</b> and the second outer tube <b>34</b>, allows second precursor B to travel <b>402</b> from a second precursor B supply source (not shown) through the second inner tube <b>74</b>, through the second outer tube <b>34</b>, through the apertures <b>36</b> and into the reaction space <b>14</b>. The surface area exposed to the second precursor B pulse is restricted to a certain area <b>410</b> within the reaction chamber by the opposing flow <b>310</b> of the inactive purging gas that is coming out of the apertures <b>30</b> of the first outer tube <b>28</b>. The exposure restriction is beneficial for preventing thin film growth on the surface of the first outer tube <b>28</b>. The gas injection structure stays clean for a long time. In addition, the formation of particles is suppressed because thin film does not accumulate on surfaces near the gas injection structure.

0063<figref idref="DRAWINGS">FIG. 7</figref> is a schematic cross-sectional side view of the reactant in-feed system and gas flow pattern during a booster step. After each reactant pulse step, a booster purge step may be added to quickly push the reactant vapor from the gas inlet into the reaction chamber. The booster step is optional and can be eliminated from the pulsing sequence if it is not needed. In that case a six-step pulsing sequence of first reactant A pulse/booster purge A/purge A/second reactant B pulse/booster purge B/purge B is reduced into a normal four-step pulsing sequence of first reactant A pulse/purge A/second reactant B pulse/purge B.

0064When the booster purge step is executed, the source control valve <b>618</b> and the source exhaust valve <b>614</b> are closed. Then the purge gas valve <b>606</b> is opened to allow purge flow <b>302</b> through the purge channel <b>29</b> and booster valve <b>608</b> is opened to allow purge flow <b>802</b> through the reactant channel defined by the inner tube <b>27</b>. Vapor of the first reactant vapor A is pushed away from the first inner tube <b>27</b>, through the first outer tube <b>28</b> and through the apertures <b>30</b> into the reaction space. It can be understood that the booster step removes most of the first precursor A vapor from the gas inlet so that the following purge pulse only needs to transport the residual first precursor A vapor from the space between the tip <b>30</b> of the first inner tube <b>27</b> and the apertures <b>30</b> of the first outer tube <b>28</b> into the reaction chamber and further to the outlet of the reaction chamber. The booster purge (arrows <b>802</b> in <figref idref="DRAWINGS">FIG. 7</figref>) pushes quickly most of the remaining precursor molecules from the inner tube <b>27</b> (<figref idref="DRAWINGS">FIG. 7</figref>) and from the inert gas valving area (near point <b>304</b> in <figref idref="DRAWINGS">FIG. 7</figref>) to the reaction space. The booster purge pulse empties the gas injector from the precursor molecules more quickly than would the inert gas flow during inert gas valving. Inert gas valving is not operational during the booster purge pulse. Inert gas valving starts to operate after the booster purge and ensures that residual precursor molecules cannot escape from the inner tube towards the reaction space.

0065Use of the booster step is not limited to the injection structures disclosed herein. The booster step is particularly preferred for valving arrangements that include inert gas valving, in which inert gas walls or diffusion barriers are typically employed for switching flow within the hot zone of the ALD reactor, in place of physical valves. As is known in the art, inert gas valving can be employed by switching inactive gas flow from a carrier gas path, leading through a reactant source (e.g., bubbler) to the reaction chamber, to a by-pass path that rejoins the carrier gas path upstream of the reaction chamber but downstream of the reactant source. The inactive gas flow then splits to partially flow downstream into the reaction chamber and partially upstream a short distance toward the reactant source. The upstream flow is typically diverted through a back-suction line leading to a vacuum source. The section of line between the reaction chamber and the reactant source that is filled within inert gas is referred to as an inert gas diffusion barrier. The booster purge step advantageously clears the inert gas barrier of reactant prior to the second or main purge pulse. Tuomo Suntola has presented an informative theoretical background about inert gas valving. See T. Suntola, Handbook of Crystal Growth 3, Thin Films and Epitaxy, Part B: Growth Mechanisms and Dynamics, Chapter 14, Atomic Layer Epitaxy, edited by D. T. J. Hurle, Elsevier Science B.V., 1994, pp. 601-663, the disclosure of which is incorporated herein by reference. See especially pp. 624-626.

0066<figref idref="DRAWINGS">FIG. 8</figref> shows a schematic cross-sectional side view of another embodiment of the invention. The first gas inlet or injector <b>22</b> and the second gas inlet (not shown in this view) are arranged relative to the substrate <b>16</b> the same way as in <figref idref="DRAWINGS">FIGS. 1 and 4</figref>. The flow pattern modifier <b>50</b> opposite the exhaust <b>56</b> in <figref idref="DRAWINGS">FIGS. 1 and 4</figref> is replaced with a third-gas inlet <b>510</b> that has apertures <b>512</b>. The apertures <b>512</b> of the third gas inlet or injector can be aligned towards an end wall <b>514</b> of the reaction chamber <b>12</b>, as shown. One benefit of this type of alignment is that stagnant flow spaces that possibly exist between the third gas inlet <b>510</b> and the end wall <b>514</b> of the reaction chamber are eliminated. The third gas inlet <b>510</b> can be used for supplying a third precursor C into the reaction chamber <b>12</b> or it can be used for supplying inactive purge gas into the reaction chamber <b>12</b>. One benefit of the third gas inlet, regardless of whether a third reactant is employed, is that it enhances the purging of the reaction chamber <b>12</b>. During purge pulses, inactive gas coming out of the apertures <b>512</b> helps to push residual first precursor A or second precursor B out of the reaction chamber <b>12</b> into the exhaust line <b>56</b>.

0067During the deposition process the substrate <b>16</b> is resting on a heated susceptor plate or pedestal <b>502</b> that is sealed <b>508</b> against a base plate <b>520</b> of the reaction chamber <b>12</b>. The pedestal <b>502</b> can be raised or lowered <b>506</b> by a piston <b>504</b>.

0068<figref idref="DRAWINGS">FIG. 9</figref> is a schematic top view of an ALD reactor according to another embodiment of the invention. The gas injectors of the illustrated embodiment form a showerhead rake structure that includes a first showerhead rake <b>906</b> and a second showerhead rake <b>902</b>. The first gas injector includes a first showerhead rake <b>906</b> in fluid communication with a first gas inlet <b>922</b>, including a distribution structure. In the illustrated embodiment, the distribution structure comprises a distributor section similar to that of <figref idref="DRAWINGS">FIG. 1</figref>. Rather than opening directly into the reaction space <b>14</b>, however, the distributor section of the first gas inlet <b>922</b> opens into branching hollow tubes or fingers, as described below. The first gas inlet <b>922</b> allows a first precursor A and/or purge gas to travel from a first precursor A supply source (not shown) and/or a purge gas source (not shown) into the reaction space <b>14</b> through the first showerhead rake <b>906</b>.

0069While illustrated as branching from distributor tubes within the reaction space, it will be understood that, in other arrangements, the hollow tubes or fingers can branch or extend from other distribution structures. For example, the fingers may separately connect to openings of a gas distribution manifold that is integrated with the reaction chamber walls. Thus, the first gas inlet <b>922</b> may be connected to the supply sources via a single distribution tube, as shown, or via multiple tubes. Each connection can be a tolerance fitting, o-ring seal, axial shaft seal, or any other method of connection known by those skilled in the art.

0070The second gas injector includes a showerhead rake <b>902</b> in fluid communication with a second gas inlet <b>924</b>, also including a distributor section that opens into branching tubes or fingers (see below). The second gas inlet <b>924</b> allows a second precursor B and/or purge gas to travel from a second precursor B supply source (not shown) and/or purge gas supply source (not shown) into the reaction space <b>14</b> through the second showerhead rake <b>902</b>. The second gas inlet <b>924</b> is in fluid communication with a second precursor B gas source (not shown) and the purge gas supply source (not shown) and may be connected to the supply sources as described above with respect to the first gas inlet.

0071As depicted in the illustration, the first gas inlet <b>922</b> includes a first outer tube <b>920</b> that is in fluid communication with each of a first set of hollow fingers <b>960</b> such that when the first precursor A flows through the first outer tube <b>920</b>, it flows <b>944</b> to each finger <b>960</b> and is dispersed out each finger <b>960</b> by a plurality of apertures <b>908</b> located on the fingers <b>960</b>. The apertures can face away from the substrate <b>16</b> in some arrangements, but preferably face the substrate <b>16</b>. Similarly, the second gas inlet <b>924</b> includes a second outer tube <b>930</b> that is in fluid communication with a second set of hollow fingers <b>962</b> such that when the second precursor B flows through the second outer tube <b>930</b>, it flows <b>954</b> to each finger <b>962</b> and is dispersed out each finger by a plurality of apertures <b>904</b> located on the fingers <b>962</b>. The apertures <b>908</b>, <b>904</b> are positioned and configured to cause the gas that flows out of them to contact the wafer or substrate <b>16</b> below and rapidly diffuse across the gaps between apertures of the same rake <b>902</b>, <b>906</b>. The apertures <b>908</b>, <b>904</b> are spaced on each finger <b>960</b>, <b>962</b> so that the distance between the apertures along each finger <b>960</b>, <b>962</b> is preferably on the order of about 5-30 mm. The diameter of the apertures <b>908</b>, <b>904</b> is preferably in the range of about 1-5 mm. As shown in <figref idref="DRAWINGS">FIG. 9</figref>, the fingers <b>960</b> of the first showerhead rake <b>906</b> are interspersed with the fingers <b>962</b> of the second showerhead rake <b>902</b> so that the interspersed fingers positionally alternate or stagger between a first rake finger and a second rake finger across an injection plane adjacent the substrate <b>16</b>.

0072As discussed above, a typical pulsing cycle for an ALD process includes four basic steps: first precursor A pulse, inactive gas purge (or other removal step), second precursor B pulse and inactive gas purge (or other removal step). While the first showerhead rake <b>906</b> is used for distributing the first precursor A vapor over the substrate <b>16</b>, the second showerhead rake <b>902</b> is filled with flowing a low level of inactive purge gas. Inactive purge gas <b>950</b> flows first through the space between the second inner tube <b>74</b> and the second outer tube <b>930</b>, then through the section of the second outer tube <b>930</b> that is in fluid communication with the second set of hollow fingers <b>962</b> so that the flow <b>954</b> is divided between the fingers <b>962</b> and then through the apertures <b>904</b> that are spaced along the fingers <b>962</b>. The inactive purge gas flow ensures that the first precursor A flow cannot enter the hollow fingers <b>962</b> through the apertures <b>904</b>. As shown, the second gas inlet <b>924</b> is configured such that some of the inactive gas flows <b>952</b> backward into the second inner tube <b>74</b>. However, the level of inactive gas flow through the second showerhead rake <b>902</b> is low enough to allow first precursor from the apertures of the first showerhead rake <b>906</b> to readily diffuse across the substrate <b>16</b>, filling in gaps between the apertures <b>904</b>. This arrangement is in contrast to conventional flow-through systems (whether laminar or showerhead), in which carrier gas tends to lead a large part of precursor straight to the exhaust, or opposite channel feeds that provide purge gas flow one side of the substrate while reactant flows from the other side. In either case, reactant is not permitted to diffuse evenly to all surfaces of the substrate.

0073Thus, during the first precursor A pulse the first precursor A gas flows <b>940</b> through the first showerhead rake <b>906</b> of the first injector and inactive purge gas flows through the second showerhead rake <b>902</b> of the second injector. During the inactive gas purge step, inactive purge gas preferably flows though both the first showerhead rake <b>906</b> and the second showerhead rake <b>902</b>, preferably at an increased flow rate relative to the trickle flows during precursor steps. During the second precursor B pulse the second precursor B vapor flows though the second showerhead rake <b>902</b> and inactive purge gas flows (at a lower flow rate than during purge steps) through the first showerhead rake <b>906</b>.

0074<figref idref="DRAWINGS">FIG. 10</figref> is a schematic cross-sectional view of the ALD reactor as depicted in <figref idref="DRAWINGS">FIG. 9</figref>. Only the cross section of the first showerhead rake <b>906</b> is shown. Excess first precursor A, second precursor B and/or purge gases may be removed from the reaction space via a vacuum, a simple outlet or through an exhaust that employs a reduction in pressure between the chamber and an outer environment, causing the gas in the reaction chamber to escape into the outer environment. According to one embodiment presented in <figref idref="DRAWINGS">FIG. 10</figref>, gases are expelled from the reaction chamber <b>12</b> through a flow restrictor channel <b>1010</b> that is annular or ring-shaped, a collector space which is also annular and is in fluid communication with the flow restrictor channel, and an exhaust duct <b>56</b> that is in fluid communication with the collector space <b>1012</b>. Those skilled in the art will appreciate that the flow restrictor channel and collector space can also be oval-shaped, square shaped or any other shape. The exhaust duct <b>56</b> guides the gases to a vacuum pump <b>58</b> that creates a pressure gradient to the volume between the reaction chamber <b>12</b> and the vacuum pump <b>58</b>. Gases flow from a higher pressure space to a lower pressure space along the pressure gradient.

0075<figref idref="DRAWINGS">FIG. 10</figref> also shows the substrate <b>16</b> supported upon a heated susceptor plate or pedestal <b>502</b>, which can be raised or lowered <b>1008</b> by a piston <b>1006</b> to open the reaction chamber <b>12</b> and allow a transfer mechanism (not shown) to load or unload the substrate <b>16</b>.

0076<figref idref="DRAWINGS">FIG. 11</figref> shows a schematic cross-sectional view of an ALD reactor according to still another embodiment of the invention. A first showerhead rake includes precursor A fingers <b>960</b> that are hollow tubes having multiple apertures <b>908</b>. A second showerhead rake consists of precursor B fingers <b>962</b> that are hollow tubes having multiple apertures <b>904</b>. The first and the second showerhead rakes define a showerhead rake or injection plane <b>1110</b> that is located above the substrate <b>16</b>, preferably about 5-50 mm from the substrate <b>16</b> surface. The distance between the first precursor A fingers <b>960</b> and the second precursor B fingers <b>962</b> is preferably about 1-50 mm, more preferably about 5-15 mm. The diameter of the finger tubes is preferably about 6-30 mm. The number of fingers depends on the dimensions of the substrate and the distance between the fingers. A gas flow space <b>1112</b> above the showerhead rake plane <b>1110</b> is sufficiently high to enable high gas flow conductivity. The height of the gas flow space <b>1112</b> is preferably about 20-100 mm, more preferably about 30-60 mm. Advantageously, exhaust flow <b>1116</b> can escape through spaces <b>1118</b> between adjacent fingers, through the injection plane <b>1110</b>.

0077As shown in <figref idref="DRAWINGS">FIG. 11</figref>, a substrate <b>16</b> is placed on a heated susceptor plate or pedestal <b>502</b>. The susceptor plate <b>502</b> is lifted <b>1008</b> upwards until a contact surface <b>1102</b> is created between the susceptor plate <b>502</b> and the base plate <b>1104</b> of the reaction chamber <b>12</b>. Inert gas flow is switched on to the first precursor A fingers <b>960</b> and the second precursor B fingers <b>962</b>. The temperature of the substrate is allowed to stabilize for a period of time. After the substrate temperature has stabilized the deposition of a thin film starts. The first step is to pulse the first precursor A to the reaction space. The first precursor A flow is switched on so that the first precursor A gas with or without inactive carrier gas is flowing out of the apertures <b>908</b> of the first precursor A fingers <b>960</b>. At the same time some inactive purge gas flows at a low or trickle rate out of the apertures <b>904</b> of the second precursor B fingers <b>962</b>. The purpose of the inactive purge gas flow is to prevent the apertures <b>904</b> of the second precursor B fingers <b>962</b> from any exposure to the first precursor A molecules, but the trickle rate of purge flow preferably does not interfere with the diffusion of precursor A across the substrate.

0078After the first reactant A is pulsed, the flow of the first precursor A gas is switched off and the reaction space is evacuated from the excess first reactant A and possible reaction byproducts. Inactive purge gas flows out of the apertures <b>908</b> and <b>904</b> at a higher flow rate than during precursor steps, while a vacuum pump <b>58</b> removes gases from the reaction chamber <b>12</b>.

0079When the pulsing sequence has proceeded to the second precursor B pulse, the second precursor B flow is switched on so that the second precursor B gas with or without inactive carrier gas is flowing out of the apertures <b>908</b> of the second precursor B fingers <b>962</b>, while inactive purge gas is flowing at a low rate out of the apertures <b>904</b> of the first precursor A fingers <b>960</b> and protecting the apertures <b>904</b> of the first precursor A fingers <b>960</b> against any exposure to the second precursor B molecules. The inner surfaces of the fingers <b>962</b> and <b>960</b> and related apertures <b>908</b> and <b>904</b> stay clean and remain free of particles, but the trickle rate of purge flow preferably does not interfere with the diffusion of precursor B across the substrate.

0080After the second reactant B is pulsed, the flow of the second precursor B gas is switched off and the reaction space is evacuated of the excess second reactant B and possible reaction byproducts. Inactive purge gas flows out of the apertures <b>904</b> and <b>908</b> at a higher rate than during the precursor steps while a vacuum pump <b>58</b> removes gases from the reaction chamber <b>12</b>.

0081<figref idref="DRAWINGS">FIG. 12</figref> shows a schematic cross-sectional view of an ALD reactor as depicted in <figref idref="DRAWINGS">FIG. 11</figref> with another type of reaction chamber sealing system. The susceptor plate <b>1202</b> is lifted <b>1008</b> upwards until contact is made against the reaction chamber <b>12</b> so that a contact surface <b>1206</b> seals the reaction chamber space from the surrounding gas space. The sealing length <b>1208</b> in the contact surface <b>1206</b> is preferably about 20-80 mm, more preferably about 40-60 mm.

0082<figref idref="DRAWINGS">FIGS. 13a, 13b and 13c</figref> show schematic cross sectional views of gas flow geometry near a substrate <b>16</b> in an ALD reactor with dynamic pressure control of the reaction space. As discussed in detail below, the purge gas flow may be modulated by dynamic pressure control. A first showerhead rake includes precursor A fingers <b>960</b> that are hollow tubes having multiple apertures <b>908</b>. A second showerhead rake includes precursor B fingers <b>962</b> that are hollow tubes having multiple apertures <b>904</b>. The first and the second showerhead rake define an injection plane <b>1110</b> that is located over the substrate <b>16</b> preferably about 5-50 mm from the substrate <b>16</b> surface. The distance between the first precursor A fingers <b>960</b> and the second precursor B fingers <b>962</b> is preferably about 1-50 mm, more preferably about 5-15 mm. The number of fingers depends on the dimensions of the substrate and the distance between the fingers.

0083<figref idref="DRAWINGS">FIG. 13a</figref> shows the deposition sequence in a precursor A pulse step. The flow rate of inactive purge gas through the apertures <b>904</b> of the second precursor B fingers <b>962</b> is low enough that the inactive gas flow <b>1308</b> does not shield the substrate surface below the second precursor B fingers <b>962</b> against the exposure to the precursor A molecules. On the other hand, the flow rate of inactive purge gas <b>1308</b> through the apertures <b>904</b> of the second precursor B fingers <b>962</b> is high enough to keep the first precursor A molecules from entering the apertures <b>904</b> of the second precursor B fingers. The flow rate ratio of the inactive gas is for example about 1:10, so that during precursor A pulse the flow rate of the inactive purge gas <b>1308</b> is 1 unit and during the following purging step 10 units. The first precursor A gas flows <b>1304</b> out of the apertures <b>908</b> of the first precursor A fingers <b>960</b> first towards the substrate <b>16</b> and then the gas spreads laterally over the substrate <b>16</b> surface. Gradually the flow direction of the first precursor A gases changes away from the substrate <b>16</b> surface and the gases flow through the showerhead rake or injection plane <b>1110</b> in the spaces between alternated fingers towards the exhaust <b>56</b> and the vacuum pump <b>58</b>.

0084<figref idref="DRAWINGS">FIG. 13b</figref> shows the deposition sequence in a purging step. Inactive gas is flowing through both the apertures <b>908</b> of the first precursor. A fingers <b>960</b> and apertures <b>904</b> of the second precursor fingers <b>962</b>. When the inactive purge gas has exited <b>1322</b>, <b>1324</b> the apertures <b>908</b> and <b>904</b>, respectively, the gas molecules first flow towards the substrate <b>16</b>. Then the flow direction of gases gradually changes by about 180 degrees and the gases flow through the showerhead rake or injection plane <b>1110</b> in the spaces <b>1118</b> between the rake fingers <b>960</b> and <b>962</b>. After passing through the showerhead rake plane <b>1110</b> the flow direction <b>1116</b> gradually changes towards the exhaust <b>56</b> and the gases flow to the vacuum pump <b>58</b>.

0085<figref idref="DRAWINGS">FIG. 13c</figref> shows the deposition sequence in a precursor B pulse step. The flow rate of inactive purge gas through the apertures <b>908</b> of the first precursor A fingers <b>960</b> is low enough so that the inactive gas flow <b>1342</b> does not shield the substrate surface below the first precursor A fingers <b>960</b> against the exposure to the precursor B molecules. On the other hand, the flow rate of inactive purge gas <b>1346</b> through the apertures <b>908</b> of the first precursor A fingers <b>960</b> is high enough to keep the second precursor B molecules away from the inside and the apertures <b>908</b> of the first precursor A fingers <b>960</b>. For example, the flow rate of the inactive purge gas is lowered from 10 units that was used during purge step in <figref idref="DRAWINGS">FIGS. 13b</figref> to 1 unit during the precursor B pulse step in <figref idref="DRAWINGS">FIG. 13c</figref>. The second precursor B gas flows <b>1346</b> out of the apertures <b>904</b> of the second precursor B fingers <b>962</b> first towards the substrate <b>16</b> and then the gas spreads laterally over the substrate <b>16</b> surface. Gradually the flow direction of the second precursor B gases changes away from the substrate <b>16</b> surface and the gases flow through the showerhead rake or injection plane <b>1110</b> towards the exhaust <b>56</b> and vacuum pump <b>58</b>.

0086After the precursor B pulse step the deposition system proceeds to a purging step as shown in <figref idref="DRAWINGS">FIG. 13b</figref>. The pulsing sequence of these four deposition steps are repeated as many times as is needed for growing a thin film of desired thickness. Each pulsing sequence typically adds about 0.2-1 Å to the film thickness, depending on the precursors and deposition process.

0087As mentioned above, the purge gas flow may be modulated by dynamic pressure control of the reaction space. To do so, the reaction space pressure is first kept at a low level, for example, at a pressure range of approximately 0.1-1 mbar. Low reaction space pressure causes faster distribution of the precursor molecules, particularly when the precursor molecules are provided from a higher pressure source, because the diffusion rate of molecules increases.

0088The precursor doses may be divided into multiple short pulses, which can improve the distribution of the precursor molecules into the reaction chamber. Just before switching on the precursor pulse the pressure of the reaction space is approximately at 0.1-1 mbar. The first short precursor pulse increases the reaction space pressure temporarily to a higher level, for example, at a range of approximately 3-10 mbar. The short precursor pulse lasts for approximately 0.04-0.10 seconds. Then the precursor pulse is switched off for about 0.04-0.50 seconds. Gases flow to the gas outlet and the pressure of the reaction space decreases again to the low level.

0089The switch-on and switch-off stages are repeated at least two times. As a result, the pressure of the reaction space fluctuates rapidly between the low level and higher level pressure. The resulting pressure gradient in the reaction space during the switch-on stage pushes the precursor molecules efficiently to all areas of the reaction space, while the resulting pressure gradient in the reaction space during the switch-off stage pulls gaseous reaction by-products away from the surfaces of the reaction space to the gas outlet. If a conventional, relatively long pulse (e.g., 1 second) is released to the reaction chamber, the pressure is allowed to equalize, such that dynamic spreading effect is lost and the main part of the gas flow tends to head directly to the gas outlet. When several short pulses (e.g., 3 times 0.3 seconds) are released, a much more even distribution is achieved in a similar time period.

0090Controlling the evacuation speed with variable evacuation capacity is optional, because it is more convenient to carry out the dynamic pressure control with inactive gas valves <b>606</b>, <b>608</b> and source valve <b>618</b> at the gas inlet or with a mass flow controller connected between the inactive gas source <b>602</b> and the inactive gas valves <b>606</b>, <b>608</b> (see <figref idref="DRAWINGS">FIG. 3, 5 or 7</figref>). Another possibility is to replace the inactive gas valves <b>606</b>, <b>608</b> with a sensitive mass flow controller. MicroFlo™, available from Pneucleus Technologies, LLC, 169 Depot Road, Hollis, N.H., USA, is provided as an example of a mass flow controller that is sensitive enough to control sub—0.10 second pulses. The MicroFlo™ controller has a response time of about 0.04 s (within 1% of the flow set point).

0091The purge gas flow may also be divided into multiple short pulses that can last for approximately 0.04-0.50 seconds each, preferably between about 2 to 6 pulses per purge step, more preferably between about 2 to 4 pulses per purge step. During the multiple short pulses, pressure in the reaction space fluctuates between the low level and the high level. Switching on the purge flow increases the pressure of the reaction space to the high level, while switching off the purge flow decreases the pressure of the reaction space to low level. The flow rate of the switch-off purge flow may be lowered, for example, to 10% of the switch-on purge flow. In that case the flow rate changes rapidly by one order of magnitude between high flow rate level (e.g., 200 std. cm³/min or sccm) and low flow rate level (e.g., 20 sccm). Pressure gradients inside the reaction space push inactive gas molecules towards the surfaces during the switch-on stage and pull the gas molecules away from the surfaces during the switch-off stage.

0092Thus, each precursor and purge pulse may consist of multiple switch-on and switch-off stages. Local pressure gradients enhance the exchange of gases in the reaction space and enhance the exchange of molecules between the substrate surface and the gas phase of the reaction space. It has been found that multiple pulses of the same gas per step, whether purge step or reactant step, is particularly advantageous when depositing on wafers with high aspect ratio features, such as deep, narrow trenches or vias in semiconductor substrates. Thus, the process of multiple same-gas pulses in a row, and the consequent pressure fluctuations, are particularly advantageous for deposition inside vias and trenches of greater than 20:1 aspect ratio, and more particularly greater than 40:1 aspect ratio. The pressure fluctuations enable saturation of the surfaces within such vias and trenches in less overall time than a single prolonged pulse. Thus, overall cycle time is reduced.

0093<figref idref="DRAWINGS">FIG. 14</figref> shows yet another embodiment of the invention. A wafer or substrate <b>16</b> is positioned within an upper part of a reaction chamber <b>12</b> so that the deposition surface faces down towards the reaction space. As shown in this embodiment, a wafer holder <b>1402</b> secures the wafer or substrate <b>16</b> above gas injector tubes or fingers <b>960</b>, <b>962</b>. A heating plate <b>1406</b> may be positioned over the substrate <b>16</b>. According to another embodiment the heating plate <b>1406</b> is replaced with a radiative heater (not shown) that is located above the substrate <b>16</b> and the substrate is heated with thermal radiation or visible light from the backside, preferably through a transparent window. When gases are dispersed from the apertures <b>904</b>, <b>908</b> of the gas inlet tubes, the gases first flow up towards the wafer or substrate <b>16</b>, then the flow direction <b>1420</b> of the gases gradually changes so that the flow direction finally points towards a flow restrictor grid <b>1408</b>. After flowing through the flow restrictor grid <b>1408</b>, the gases will flow <b>1422</b> towards an exhaust duct <b>1424</b> that guides the gases to a vacuum pump <b>58</b>. According to the embodiments, the evacuation of gases from the vacuum chamber <b>12</b> is carried out with a vacuum pump, a simple venturi outlet or any other means that result in a reduction in pressure from the chamber to an outer environment causing the gas in the reaction chamber to escape to the outer environment. It will be appreciated that the flow restriction grid <b>1408</b> can also be employed between the gas inlets and exhaust of previously described embodiments.

0094<figref idref="DRAWINGS">FIG. 15</figref> shows a gas injection structure in accordance with another embodiment, that includes a dual spiral showerhead that consists of a spiral tube <b>1502</b> for a precursor A and a spiral tube <b>1504</b> for a precursor B. Spirals are made, for example, of hollow silica or metal tubes. Apertures <b>1506</b> are arranged on the spirals so that the distance between the apertures is preferably about 5-50 mm. The dual spiral showerhead can be placed next to a wafer in a similar position as showerhead rakes were placed in <figref idref="DRAWINGS">FIG. 9</figref>. Apertures <b>1506</b> are pointing towards a substrate or a wafer or away from a substrate or a wafer. Precursor pulses can be alternated, with intervening purge steps, as described above with respect to previous embodiments. Advantageously, spaces between the spiral tubes <b>1502</b> and <b>1504</b> enable exhaust flow through the plane of the injectors for embodiments in which the dual spiral showerhead intervenes between the plane of the substrate or wafer and the plane of the exhaust. Such an arrangement, as noted elsewhere herein, facilitates purge pulses sweeping all gas away from the substrate by rapid diffusion across the substrate. before being lifted away from the substrate (rather than flowing across the substrate in a defined path in route to the exhaust).

0095<figref idref="DRAWINGS">FIG. 16a</figref> shows a schematic top view of a gas injection structure with a shower tube arrangement in relation to a wafer. L-shaped shower tubes are placed in front of a wafer <b>16</b>. Reactant vapor or inactive gas flows first along a hollow feed section <b>1602</b> of the shower tube then along the lateral distributor section <b>1604</b> in the shower tube and finally out of the apertures <b>1606</b> to the reaction chamber where the wafer <b>16</b> is located. Also shown in <figref idref="DRAWINGS">FIG. 16a</figref> is a hollow feed section <b>1612</b> of a second shower tube, as will be appreciated from <figref idref="DRAWINGS">FIG. 16b</figref> and corresponding description below.

0096<figref idref="DRAWINGS">FIG. 16b</figref> shows a schematic side view of the shower tube arrangement of <figref idref="DRAWINGS">FIG. 16a</figref> wherein two shower tubes are placed one above the other. Reactant A vapor flows out of the first apertures <b>1606</b> in the first lateral distributor section <b>1604</b> during the pulse time of reactant A. When the reactant. A pulse is switched off, inactive gas flows out of the first apertures <b>1606</b> of the first lateral distributor section <b>1604</b>. Reactant B vapor flows out of second apertures <b>1616</b> of a second lateral distributor section <b>1614</b> during the pulse time of reactant B. When the reactant B pulse is switched off, inactive gas flows out of the second apertures <b>1616</b> of the second lateral distributor section <b>1614</b>. During a purge time inactive gas flows out of all the apertures <b>1606</b>, <b>1616</b> into the reaction chamber where the substrate <b>16</b> is located. Dynamic pressure control of the reaction chamber can be applied during the deposition process so that gases coming out of one shower tube do not noticeably shade any substrate area from the gases that are coming out of the other shower tube. An optional booster step with inactive gas after each precursor pulse enhances the removal of residual precursor vapor from the shower tubes and the reaction chamber.

0097The gas injectors of the preferred embodiments, including the single distributor tube (per reactant) and showerhead rake structures, are simple and cheap to manufacture. They may be configured so that they may be suitable as consumable items for after sales marketing. For instance, as mentioned above, the gas injectors described herein can be attached to gas channel openings for example with tolerance fitting, o-ring seal, axial shaft seal, or by any other means known by those skilled in the art. By providing hollow tubes with apertures therein, where the tubes are independent of and connectable to the walls, the gas injectors are replaceable and they are preferably discarded when reactant buildup results in less than optimal operation. Additionally, because the gas injectors are replaceable parts protruding into the reaction chamber, the customer can select an injector structure according to specific needs. For example, the size, the number and the location of the apertures can be optimized so that the whole wafer will be exposed uniformly to the reactant gas. Advantageously, the entire chamber does not require disassembly in order to replace the gas injectors; rather, the chamber can simply be opened and the gas injector part (e.g., gas inlets of <figref idref="DRAWINGS">FIG. 1</figref> or the showerhead rakes of <figref idref="DRAWINGS">FIG. 9</figref>) can be readily replaced with minimal reactor downtime and minimal retuning after replacement.

0098Furthermore, the illustrated gas injectors are particularly conducive to efficient purging without a flow-through arrangement. For example, the gas injectors described herein readily distribute gas across the surface of the wafer, and are not constrained to a particular flow path, in contrast to laminar flow reactor designs. Unlike conventional showerhead arrangements, however, excess reactant, by-product and purge gases are allowed to flow through the plane of the gas injectors (see, e.g., <figref idref="DRAWINGS">FIG. 13b</figref>), thereby enabling placement of the spaced injector tubes between the planes of substrate and the exhaust outlet. One advantage of such an arrangement is that reactants are allowed to spread out over the substrate surface during reactant steps or pulses, without creating pockets of dead zones where reactant gas can avoid the purging process.

0099The skilled artisan will readily appreciate from the disclosure herein that the gas injectors of the preferred embodiments allow gas spreading across the substrate by diffusion and/or pressure fluctuation after exiting the apertures. Similarly, purging is made more efficient by allowing purge gas to sweep away reactant gases from the substrate, aided in certain embodiments by multiple sequential purge pulses and the pressure gradients thereby created. These functions are facilitated by keeping the substrate stationary relative to the injectors during deposition in the illustrated embodiments, allowing gas spreading by diffusion and/or pressure fluctuation even in rather short pulse intervals required by commercial implementation of ALD. Accordingly, the substrate does not rotate during operation of the illustrated embodiments.

0100While the invention has been described with reference to certain preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to any particular embodiment disclosed for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.