Method of producing an electric connection using nanotubes and having air gaps

Technical Field of the Invention

p0001The invention relates to a method of making an electrical connection nanotube, between two conductive layers, respectively lower and upper, separated by a target layer, the method comprising successively:<ul><li>forming the target layer on the lower conductive layer,</li><li>training in the target layer of at least one hole having a bottom,</li><li>growth of the nanotubes in said orifice,</li><li>forming the upper conductive layer electrically connected to the lower conductive layer by the nanotubes,</li></ul>

State of the art

p0002Nowadays, electronic integrated circuits have a significant proportion of metallic elements. These can be used as interconnection elements, antennas, coils or inductors. With the reduction of distances between the metal layers, electrostatic or electromagnetic couplings between them are increasing, which has the effect of changing their electrical behavior. Generally, these electrostatic couplings are modeled by the appearance of at least a parasitic capacitance between the metal elements. Reducing the coupling through a decrease of the dielectric permittivity of the material between the metallic elements.

p0003For current generations of integrated circuits, the use of dielectric materials with low permittivity (of the order of 2.3 to 2.7) is compatible with the specified performance. These dielectric low-k advantageously replace silicon oxides. However, for technological generations to come (nodes below 32nm), performance requirements are much higher than the small improvement in permittivity currently measured on the deposited bulk materials. The use of air cavities ( "Air Gap" in English) in interconnection structures now seems inevitable. With such architectures, the air cavities present between the line conductors provide permittivities equal to 1.

p0004Currently, a possible route for the production of these cavities is the use of a deposit non-conformal dielectric layers on interline metallic material. This approach, described by<nplcit id="ncit0001" npl-type="s"><text>Gosset et al. ( "General review of issues and prospects for advanced copper interconnects using air gap as ultra low-K material", Proceeding of the IEEE 2003</text></nplcit>, International Interconnect Technology Conference (2003) 65), provides, under certain conditions of manufacture and with a surplus of complex and expensive technological steps, the desired cavities.

p0005Another alternative is to include a sacrificial material between the metallic material patterns, forming the intermetallic level. This material reacts after its integration through various physicochemical processes (eg by thermal annealing, UV treatment (<nplcit id="ncit0002" npl-type="s"><text>"Benefits and Trade-offs in Multi-Level Integration Air Gap" MRS Spring San Diego, April 2006</text></nplcit>) Or chemical attack (patent <patcit id="pcit0001" dnum="US7172980B"><text>US7172980</text></patcit>). In this way, it is possible to achieve in whole or in part, integration interconnection levels with the sacrificial material, and then forming the air gaps. However, this approach greatly limits the range of usable materials during integration. The sacrificial material, which must give way to the cavity, must indeed be compatible with semiconductor technology, be degradable and, like its withdrawal, the physico-chemical processes used should not damage the already integrated electronic devices ( CMOS transistors, capacity ...). Similarly, the materials adjacent to the sacrificial material must have a place in these physico-chemical processes in order not to alter, but some of these materials should enable the degradation and removal of the sacrificial material by being, for example, permeable multiple chemical processes.

p0006In addition, the carbon nanotubes are currently the subject of a major research effort as their monoatomic cylindrical structure gives them exceptional properties on the nanoscale. One promising application is to use the nanotubes in interconnects, particularly in the microelectronics industry, as described by<nplcit id="ncit0003" npl-type="s"><text> Nihei et al. ( "Electrical Properties of Carbon Nanotube Bundles Via Interconnects for Future" Japanese Journal of Applied Physics Vol. 44 No. 4A 2005 pp1626-1628</text></nplcit>). These interconnections are formed by two rows of conductive metal, currently copper, located one above the other, thereby forming two metal levels connected by conductive bridges called vias.

p0007To withstand the stresses imposed by reduced dimensions added to a complexity of integration parameters, it is envisaged to use carbon nanotubes as nanoscale metal son for interconnections. These have in fact very interesting intrinsic properties with respect to copper. Integration of carbon nanotubes in a microelectronic interconnection is disclosed in<patcit id="pcit0002" dnum="US20030179559A"><text>US-A-2003/0179559</text></patcit>Wherein the nanotubes connect two layers of conductive materials.

The invention

p0008The object of the invention is to provide a method to overcome the aforementioned drawbacks, in particular a less complex and less expensive manufacturing process.

p0009According to the invention, this object is achieved by a manufacturing method according to the appended claims and more particularly by the fact that the method comprises, between the nanotube growth and the formation of the conductive top layer,<ul><li>depositing a planar insulation layer, through which the nanotubes,</li><li>the selective degradation of the target layer, of degradable material, nanotubes, porous walls, forming passages between the target and the outer layer to a degradation agent and / or degradation by-products formed during said selective degradation.</li></ul>

BRIEF DESCRIPTION OF DRAWINGS

p0010Other advantages and features will become more apparent from the following description of specific embodiments of the invention given as non-limiting examples and represented in the accompanying drawings, wherein:<ul><li>The <figref idrefs="f0001 f0002">Figures 1 to 4</figref> show the successive steps of a first embodiment of the method according to the invention.</li><li>The <figref idrefs="f0002">Figures 5 and 6</figref> show the successive steps of a second process embodiment according to the invention. </li><li>The <figref idrefs="f0003">Figures 7 and 8</figref> represent various steps of a third embodiment of the method according to the invention.</li></ul>

Description of particular embodiments of the invention

p0011In a first embodiment, illustrated in <figref idrefs="f0001">figure 1</figref>, A target layer 3 is formed on a lower conductive layer 2, which is itself deposited on a substrate 1. In this particular embodiment, the target layer 3 is composed only of a degradable material 4. However, the target layer 3 may be formed of several degradable materials having different degrading agents and may comprise at least one non-degradable material. The degradable material 4 is, for example, silica (SiO₂), Which can be degraded by chemical attack, or a thermally degradable polymer, such as polymethylmethacrylate or alpha-terpinene. Target layer 3 is then patterned by any suitable technique, for example by photolithography and etching to form openings 5. Thus, the bottom of the opening 5 is formed by predetermined areas free of the lower conductive layer 2. the side walls of holes 5 are formed by the degradable material 4.

p0012The nanotubes 6 are then formed, from the lower conductive layer 2, inside the openings 5, by any known method, for example catalytically, by electric discharge at high temperature or by laser ablation. The nanotubes 6 then have one end in contact with the lower conductive layer 2.

p0013As shown in <figref idrefs="f0001">figure 1</figref>, A catalyst layer 7, enabling growth of nanotubes 6 is advantageously formed in the bottom of the holes 5. The catalyst 7 is for example Co, Ni, Fe, Al, Al₂O₃, Fe₂O₃, Mo. can also be achieved by a self-positioned deposit and is then, for example, CoWP, CoWP / B, NiMoP, NiMoB, their oxides or their alloys. The catalyst 7 can be deposited in the form of aggregates or may be composed of multilayers of different catalysts. In some cases, the catalyst 7 may be preceded by depositing a barrier layer and / or a bonding layer in the bottom of the holes 5.

p0014The nanotubes 6 are, for example, carbon nanotubes and are preferably of hollow cylindrical shape. The nanotubes 6 are not necessarily strictly vertical and can also be twisted. The majority of the nanotubes 6 is substantially perpendicular to the surface of the substrate 1 on which they are formed.

p0015In the case of 6 carbon nanotubes, the growth of the nanotubes may be performed by any suitable technique, for example by chemical vapor deposition CVD, chemical vapor deposition, plasma enhanced PECVD, electron cyclotron resonance ( "Electron Cyclotron Resonance" ) PECVD, chemical vapor deposition hot-filament chemical vapor deposition assisted laser ... Preferably, a technique for the growth of nanotubes 6 atoms from a catalyst and at a temperature below 900 ° C. is used. The gases used in the formation of carbon nanotubes can be CO, C₂H₂CH₄, Fe (C₅H₅)₂, Xylene, metallocenes, alcohols in the gaseous state and all carbonaceous gases, H₂, NH₃, H₂O, O₂ or a mixture of these gases. Carbon can also be provided with a graphite sole bombarded by a plasma.

p0016As illustrated in <figref idrefs="f0001">2</figref>, An insulating layer 8, substantially flat, is then deposited on the target layer 3 and the nanotubes 6. The deposition of the insulating layer 8 is advantageously non-design type. In this way, the layer 8 does not completely coat the patterns target layer 3 and the nanotubes 6 but forms a substantially horizontal uniform and continuous film, thereby forming air gaps by closing the openings 5. The deposition process layer 8 is conventional because it is commonly used to form insulating layers in interconnect structures. The insulating layer 8 may have a certain mechanical strength to enable the maintenance of nanotubes 6 or structure on which the layer 8 is deposited.

p0017Depending on the deposition conditions of the insulating layer 8, the nanotubes 6 may then optionally present a free end, that is to say an uncoated end by the insulation layer 8. In the case where the majority of nanotubes 6 does not cross, from one side, the insulating layer 8, an additional step (not shown) is required to release the upper end of the nanotube 6. the majority of the nanotubes then passes through the insulation layer 8 and constitutes passages between the target layer 3 or the lower conductive layer 2 and the outside. This additional step is performed by any known means and may be, for example formed by a chemical-mechanical polishing, plasma etching or wet etching, which specifically removes the insulating layer 8.

p0018The insulating layer 8 is, preferably, sufficiently rigid (sufficiently dense and with an appropriate Young's modulus) so as not to deform or deform slightly, once the air gaps are formed in the target layer 3 . Advantageously, the support elements (not shown) are integrated into the structure to increase the mechanical strength. Similarly, the insulating layer 8 is chosen so as not to be attacked during the degradable material of the degradation process 4. The layer 8 is preferably impermeable to the etchant used to degrade the material 5 as well as byproducts of degradation. Advantageously, the insulation layer 8 is made of dielectric material and, even more advantageously, in a low dielectric constant dielectric material (of the order of 2 to 5). This insulating layer 8 has a thickness typically in the range 20-500nm.

p0019Advantageously, the insulating layer 8 is methylsilsesquioxane type and more particularly in BD1® material marketed by Applied Materials.

p0020The assembly thus formed is subjected to agents of degradation of the degradable material 4. This degradation can be performed by any known method, for example by a process heat, radiation (e.g. ultraviolet radiation) or by any appropriate undulatory phenomenon or etching by a liquid or gaseous form agent.

p0021As illustrated in <figref idrefs="f0001">3</figref>In the case of a chemical attack by an agent 9 in liquid form, the agent may, for example, be arranged as a solution on the insulating layer 8. The agent can then, through the nanotubes 6 through the insulation layer 8, then through walls porous nanotubes, reach into the holes 5 and degrade the sacrificial material 4. the agent 9 may also cross the insulation layer 8 along the external walls nanotubes 6 in the case where these are not perfectly joined with the encapsulant layer 8. nanotubes having walls porous to the chemical agents and by-products of reaction, thus constitute passages for the liquid or the gas between target layer 3 and the outside. The formation of nanotubes for use as switching to a gas or a liquid in the field of biology has been described in particular Article<nplcit id="ncit0004" npl-type="s"><text>"Enhanced flow in carbon nanotubes" Nature, 438 (2005), p.44</text></nplcit>. However, this article does not mention the fluid loss due to the porosity of the side walls and losses along the walls.

p0022The characteristics of the nanotubes 6, in particular their diameter, are defined, in the case of chemical etching, so as to allow their passage through an agent capable of degrading the degradable material 4. If using a thermal degradation process or wave of the degradable material 4, the nanotubes 6 are not used by the degradation agent. However, in all cases, the characteristics of the nanotubes 6 must be chosen so as to allow by-products of degradation of the degradable material 4 to be disposed outwardly through nanotubes (porous walls and central openings nanotubes).

p0023The passage forming constituted by the nanotubes 6 between the target layer 3 and the outside, in the insulating layer 8, and allows a medium 9, for example chemical, reaching and degrading the degradable material 4 and, after degradation of the material 4, to residues of degradation to be disposed through the insulating layer 8 to create or enlarge the cavities of air 10 adjacent the nanotubes passing through the target layer 3, as shown in <figref idrefs="f0001">3</figref>.

p0024Thus, the agent 9 of chemical degradation from outside enters the hollow nanotube 6, passes through the insulating layer 8 and then passes through the nanotube walls and / or runs along the outer walls of the nanotubes in order to reach the degradable material 4 of the target layer 3. Likewise, after selective degradation of the degradable material 4, the sub-degradation product formed following the same path in reverse.

p0025Degradation and removal of the material 4 is preferably total. As illustrated in<figref idrefs="f0001">3</figref>, The air gaps 10 thus formed in the target layer 3, then fully replace the degradable material 4, which thus constitutes a sacrificial material.

p00266 when the nanotubes are carbon nanotubes, the diffusion of the agent from degradation and / or by-products of degradation by means of the nanotubes s'effectue through the walls of the nanotubes, interface nanotube insulating sublayer and the hollow channel of the nanotube 6. the nanotube 6 may also be made of titanium oxide, gallium nitride or based on rare earth compounds.

p0027Advantageously, if one wishes to obtain nanotubes 6, made by catalytically completely hollow, the catalyst particles used for their formation can be eliminated, for example by means of an acid solution (eg acid nitric HNO₃).

p0028The diameter of the nanotubes 6 is selected depending on the desired application and as a function of the degradation process and by-product thereof, typically between 1 nm and 300 nm. Indeed, most nanotube 6 is wider and the distribution of chemical species (agent 9) is easy. Similarly, the degradation by-products may be removed much faster if nanotube 6 is wide.

p0029If a very localized attack is desired, the use of nanotubes 6 purpose will be promoted, as well as suitable degradation processes (time relatively short etching or annealing). If, however, large cavities are searched around the nanotubes 6, while the nanotubes 6 are made wide, optionally with an agent 9 highly concentrated chemical degradation, or a time of slower attack will be used. The skilled person will adapt the etching time in the concentration of the etching solution.

p0030The density of nanotubes 6 is also chosen depending on the application. This density of nanotubes 6 is preferably of the order of 10¹⁰ nanotube / cm². With this order nanotube density, the distribution of the degradation agent is easy as the evacuation of degradation by-products. In addition, such a high density of nanotubes using the mechanical strength of the entire structure and facilitates the eventual formation of additional levels of interconnection.

p0031In the case where the degradable material 4 is constituted by silicon oxide, degradation can be performed by hydrofluoric acid. The material of the layer 8 is then chosen so as not to react with hydrofluoric acid, and the layer 8 is then, for example, silicon nitride or silicon carbide or SiOC.

p0032As shown in <figref idrefs="f0002">4</figref>, The degradable material 4 being at least partially removed, an upper conductive layer 11 is then deposited and patterned to connect the upper end of the nanotubes 6 opening of the insulating layer 8. Thus, an electrical connection between the conductive layer 2 and patterns of the upper conductive layer 11, advantageously of metallic material, is performed by means of nanotubes 6. in order vector, one or more levels of interconnection may be performed from the upper conductive layer 11. at this time, it is no longer possible to use the nanotubes 6 as a diffusion path of an agent 9 of degradation or by-product of degradation.

p0033In a second embodiment, shown in <figref idrefs="f0002">5</figref>, The lower conductive layer 2 is formed by a first interconnect level consisting of units (e.g., contact pads or lines) in my electrically terial conductor 12 and insulating material 13.

p0034As before, to integrate air cavities in this structure interconnecting metal, the target layer 3 can be constituted solely by a degradable material 4, for example of silicon oxide. The electrically conductive 12 of the lower conductive layer 2 is material, for example metal (e.g. copper), while the insulating material 13 is identical to the degradable material 4. Thus, during the action of the spoiling agent, the 4 and 13 materials will be at least partially removed (<figref idrefs="f0002">6</figref>).

p0035In a variant of this embodiment, the material 4 is thermally degradable, other materials can remain unchanged. The degradable material 4 is then, for example, polymer type (for example polymethylmethacrylate PMMA, blowing type of norbornadiene or alpha-terpinene ...). In this approach, the structure is then heated so as to achieve thermal degradation of the degradable material 4. As previously, the degradation by-products can then diffuse out of the target layer 3 by means of the nanotubes 6 and the walls the central holes serve as passages.

p0036In a third embodiment, shown in <figref idrefs="f0003">7</figref>, The lower conductive layer 2 comprises several superimposed levels of interconnection consist of units (e.g., contact pads or lines) of electrically conductive material 12, preferably metal, for example copper, in an insulating material matrix 13, preferably degradable. This integration can be performed by any known technique, for example by the damascene technique.

p0037In this embodiment, the substrate then comprises a plurality of interconnect levels comprising patterns of metallic material in a degradable matrix material. The openings 5 ​​in the target layer 3 have advantageously been located next memorandum material 12, so as to enable growth of the nanotubes 6 since the patterns material metal of the layer 2. As before, the removal of material degradable various interconnection levels is then achieved through the walls and central holes of the nanotubes and an air cavity structure is easily obtained (<figref idrefs="f0003">8</figref>).

p0038As the <figref idrefs="f0002">4</figref>When the air cavities are formed in target layer 3, the upper conductive layer 11 is deposited. The upper conductive layer 11 is then in electrical contact with the lower conductive layer 2 or the various levels of interconnection by means of nanotubes 6.