Ion implantation apparatus with a cooled structure controlling the surface potential of a target surface

(54) Name: Device for ion bombardment equipment in parentheses indicates international identification code, INID code. Letters in clamps indicate international document code

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The present invention relates generally to soil bombarding or ion implantation equipment and more particularly to such equipment whose construction permits the control of the surface potential of the target, more particularly a target having a bombarded surface composed entirely or partially of an electrically insulating material.

Ion implantation is a technique of rapidly growing importance in the manufacture of integrated circuits, especially bipolar integrated circuits. Within this bipolar technique, there is an increasing demand for high dose ion implantation operations but relatively short time cycles, and ion implantation technology which is useful for introducing disruptors through openings with at least one side dimension not exceeding 0.025 mm. As the implantation dose depends on the combination of current and time, it follows that the technique - in order to achieve a high dose in a relatively short time - is moving in the direction of ion implantation rays with high current, greater than 0.5 mA. It has been found that when using such high current in ion implants of conductivity-determining interferences through openings in electrically insulating layers, which openings have dimensions of the order of 0.0025 mm - 0.025 mm as required for very dense, large-scale integrated circuits, a severe tendency for deterioration or destruction of parts of this electrically insulating layer and on exposed semiconductor areas;

7804844-4 which results in voltage short circuits which render the integrated circuit malfunctioning.

It is believed that such deterioration or destruction results from an electrical breakthrough of a voltage built up on the insulating layer as a result of the charge applied by the positive ions forming the primary ion trawl. This voltage build-up is particularly pronounced in rays of high current, which have a high density of positive ions. It is further believed that in such rays of high current, positive ions have such a high density that the liquid cloud of electrons naturally generated by the operation of the ion bomb apparatus e.g. in the form of secondary electron emission from material, which is hit by the ion beam and neutral background ionization of the ion beam, is not large enough to completely neutralize the charge generated by the positive ions on the target.

The concept of positive ion beam and the effect of the secondary electron cloud are addressed in detail in US patents 3-997,846, 4,011,449 and 4,013,891, and in the articles High Current Electron Scanning Method for Ion Beam Writing,

WC Ko, p. 1832-1835, IBM Technical Disclosure Bulletin, vol. 18, no. November 6, 1975 and Ion Beams With Application to Ion Implantation, RG Wilson and GR Brewer, (John Wiley & Sons, Hew York, 1973) on p. 132-143.

It is further found that when the openings through which the ions are to be implanted have small side dimensions of the order of 0.025 nm or less, secondary electrons normally generated by positive ions hitting the semiconductor substrate are minimized, which further contributes to the lack of available secondary electrons at the surface. to neutralize the positive ion accumulation and prevent charge buildup.

Although the stated problem occurs in connection with ion implantation through very small openings, similar problems can be expected to occur when the ion implantation is performed with rays of high current through thin regions in an electrically insulating layer over a semiconductor substrate instead of through openings in such an insulating layer. .

The prior art has proposed a solution to this charge buildup problem, which is based on direct irradiation of the surface of the electrically insulating material with electrons in sufficient quantity to generate a negative potential on the surface of the insulating material sufficient to remove any positive charge, generated by the ions in the beam.

It has been found that when such a direct radiation method is utilized, the desired effect is limited. First, the power source is usually a filament, a metallic part of some kind or a plasma. Such an electron source can be adversely affected by materials emanating from the target during ion bombardment, and in addition, these sources may emit materials that may contaminate the target. Since the electron source is usually a heated part, e.g. a filament, in addition

7804844-4 the heat from the source to exert an unwanted, heating effect at the target. If then the target is covered with an electrically insulating material, e.g. photoresist affected by heat, the filament can damage the target.

Finally, since ion beam dosimetry, ie measurement and control of the ion beam current, is considered to be important in ion implantation apparatus, there is a need in the art - and especially in high-intensity beams - of method and apparatus to control and minimize the positive surface potential of the target, which is compatible dosimetry equipment for measuring the jet stream.

Accordingly, an object of the present invention is to provide ion beam bombardment apparatus, where the build-up of positive surface charge at the target surface is minimized.

Another object of the invention is to provide ion beam bombardment apparatus, where the build-up of positive charge on a surface of insulating material present on a semiconductor substrate is minimized.

Yet another object of the invention is to provide ion beam bombardment apparatus, where the build-up of positive charge on a layer of electrically insulating material on the target is minimized. At the same time, the target pollution from the equipment is minimized to limit such build-up.

The above and other objects of the invention are realized by apparatus for bombarding a target with a ray of ions, which apparatus has means for controlling the surface potential of the target, which means is a combination of an electron source adjacent to the beam to deliver electrons to the beam and means between the target and this electron source to block direct rectilinear radiation between the source and the target. Such radiation includes the electrons generated by the source as well as other particle radiation and photon radiation. In other words, the blocking means or screen prevents a rectilinear electron path from the electron source to the target. Consequently, there is no radiation that is directly projected onto or hits the target from the electron source. The screen also prevents materials that, in vapor form, leave the source from contaminating the target. During the function of the source - and especially when the source is a heated thread of e.g. tungsten, tantalum or thorium iridium - evaporates such materials from the source. The screen prevents target contamination. In addition, the screen prevents any positive ions, which can be sputtered away from the target by the ion beam, from being directly damaged or contaminating the source structure. In such a heat source as a wire, the screen also prevents the source from heating the target and thereby damage heat sensitive material, e.g. photoresist. The apparatus also includes an arrangement for keeping the screen at a lower temperature than the target. This is particularly favorable when the electron source is a heated source, e.g. a conventional electron emitting wire.

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According to another aspect of the invention, the ion homing apparatus of the target with a beam of ions contains one. means for measuring the ionic current current and controlling the surface potential of the target, which device comprises walls close and electrically isolated from the target and the surrounding beam, whereby the walls and target form a Faraday how, means for providing varying amounts of electrons within Faraday's how, means for measure the target current, means for combining and measuring the target and wall currents to provide said ion beam current measurement and means for varying the amounts of electrons obtained in and for controlling the target current and thus said target surface potential. Furthermore, this device contains means for holding the screen at a lower temperature than the target.

The key to the latter aspect of the invention lies in the ability to monitor or measure the net target current, which is an indicator of the surface potential of the bombed target. However, since the target current also represents a substantial portion of each ion beam current measurement, Faradays must be arranged so that the walls are electrically insulated from the target, i.e. that the total wall current is fed separately from the target current, after which the target current and the wall current can be combined to give the value of the ion beam current.

In utilizing the apparatus of the present invention, the target provided with layers of electrically insulating material and the like may, e.g. consists of semiconductor boards with applied insulating layers become the subject of ion implantation while minimizing positive charge buildup on the insulating surface, even if ion beams with high current - at least 0.5mA - are used. The positive charge enhancement is avoided by monitoring the target current. As long as the target current is kept at zero or at a negative level, preferably a slightly negative level, no build-up of positive potential on the target's insulating surface layer can take place. The template current can be regulated by changing the amount of electrons introduced into the apparatus. This can be done by conventional variations in the working conditions of the electron source.

The foregoing and other objects, features and advantages of the invention as defined in the following claims will become apparent from the following, more detailed description of preferred embodiments illustrated in the accompanying drawings.

Fig. 1 is a schematic representation of an ion implant and shows the functional connection between the total equipment and the arrangement according to the present invention to control the target surface potential and to measure the beam current. The apparatus of the present invention is shown in more detail in Fig. 1A, which is an enlargement of the portion within dashed lines, which is partially cut through in Fig. 1.

Fig. 2 is a partial cross-sectional illustration of an alternative embodiment of the current measuring and measuring potential control apparatus of the present invention.

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Fig. 3 is a partial front illustration of a portion of the apparatus for controlling the target surface potential, which portion includes a modification for cooling the electron screen. The viewer stands at the target position and looks along the beam axis.

Fig. 3B is a schematic partial cross-sectional illustration of the apparatus of Fig. 3A along line 3B-3B.

In Figs. 1, 1A and 2, an ion implant is shown to control the surface potential of a target surface. This apparatus contains means for controlling the surface potential of a target, which is subject to implantation by providing electrons to the beam, and furthermore, means are provided between the target and the electron source to block direct rectilinear radiation between the target and the source. The present invention is a modification of the one shown in FIG. 1, 1A and 2, the apparatus was shown to include means for ensuring that the radiation blocker or shield between the source of the electrons and the target is cooled so that it is kept at a temperature below that of the target. This cooler is described in more detail with reference to Figures 3A and 3B.

With reference to the drawings and closest to Fig. 1, the arrangement according to the invention is illustrated to measure the beam current and control the target surface potential within the dash lines 10, as far as a conventional ion implantation apparatus is concerned. It should be noted that the rest of the apparatus of Fig. 1 is outside the block. 10 is schematic in nature and represents conventional ion implantation apparatus, e.g. that disclosed in U.S. Patent 3,756,862. The apparatus of FIG. 1 contains a conventional ion source 12, which may be any suitable high density source. However, in the illustrated embodiment, a filament-electron-switching source is shown which is arranged to operate in oscillating electron-charging mode. An ion beam is withdrawn from the source by conventional means through the extraction electrode 16 through the opening 15. The electrode 16, also known as an acceleration electrode, is held at a negative potential by the deceleration supply. The source electrode 17 is maintained at a positive potential relative to the wire 12 through the anode supply. A deceleration electrode 18 is also provided, which is held at ground potential. It should be noted that the specified bias may be varied depending on the operation of the device in a manner well known in the art.

A beam emitted from the ion source by the described electrode arrangement is passed along a beam path, generally indicated at 19. to an assay magnet 20 of conventional embodiment. The beam is further defined in a conventional manner by apertured plates 21 and 22, which are located on either side of the analyzing magnet. A conventional beam defining aperture 24 is included as shown. Next, the beam is again defined by the opening 26 of the plate 25 and hits the target 23. i

With particular reference to the device of the present invention, which allows control of the surface potential of the target while practicing precise dosimetry, i.e. measuring the ion beam current, reference is now made to the portion within the dash line block 10, which is enlarged in Fig. 1A. The device is a modified Faradays

7804844-4 cage of the type disclosed in U.S. Patent 4,011,449 for measuring the jet stream. The target 23, in combination with the closer walls 27 and the distant walls 28, forms a Faraday cage which surrounds the ion beam 29. The target 23 comprises a semiconductor disk holder 30 which carries a plurality of discs 31. a standard scavenger, e.g. that described in U.S. Patent 3,778,621, to ensure uniform distribution of the ion beam 29 across the surfaces of all discs 31 mounted on the target support 30. Alternatively, of course, the Faraday cage device of the present invention may operate with a stationary target. 23. Faraday's cage including the target is enclosed in a suitable conventional chamber (not shown) for maintaining a strong vacuum in the ion implantation apparatus.

The closer walls or side walls 27 must be electrically insulated from the target 23. In the present embodiment, these are shown separated from the target.

The side walls 27 are biased at a negative potential than the potential imposed on the target 23. In the embodiment shown, the target 23 is biased to ground via the conductor 32, and the walls 27 are more negatively biased relative to the supply V. Ink sources 33 and 33 'are conventional electron sources designed to introduce varying amounts of electrons 34 into the ion beam 29 to provide a sufficient amount of electrons in the ion beam region to neutralize any charge buildup that may result in an undesirable positive potential at the surface of the disc 31 subject to implantation. The problem of such undesirable charge structure has been described above, which problem is particularly pronounced when ion beams of high current - at least 0.5mA - are used to bombard or implant semiconductor discs with very small openings or no openings at all through insulating layers. Electron sources 33 and 33 'may be conventional electron sources, e.g. a filament, which emits electrons. Alternatively, the source may be conventional plasma bridges, electron guns with or without magnetic fields or field emission electrodes. The electron source wire 35 is supplied by a conventional power supply unit, not shown, which can be varied to increase or decrease a current through the wire 35 and thereby increase or decrease the electrons 34 delivered to the path of the ion beam 29. The wire is preferably biased by the bias to a negative level relative to the sidewalls 27 · It is important that the sources of electrons 33 and 33 'be placed in recesses in the sidewalls 27 in such a way that no rectilinear or linear path connects any part of the wire to any part of the wafer. . The wall portions of the walls 27 have the task of blocking such a path.

The far wall 28 is separated from the side wall 27 by a layer of electrically insulating material 37 · The voltage source V has the task of biasing the wall D so that it becomes negative than the side walls 27 and the wire 35 · With the bias arrangement shown, the electrons 34 introduced in beam

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as well as the secondary electron cloud accompanying the ion beam, in Faraday's cage, which is formed by the distant walls 28, the sidewalls 27 and the target 23, and is removed from the walls in the direction of the target. When using equipment operating at an energy level of the order of 50keV with ions such as arsenic and beam currents of the order of 0.5mA or more, best results are obtained with the target at ground potential, a bias on the sidewalls 27 of approx. -50V, a total bias on the wires 35 of approx. -6θ to -100V and a total bias on the far walls 28 of -200V. These values ​​can be considered as typical function parameters. The beam current value is determined by combining the current from all elements, i.e., the current from the target 23, from the side walls 27 and from the distant wall 28 at the ammeter 38, which gives a reading of the beam current in a manner similar to that described in U.S. Patent 4,011,449. At the same time, the target current alone can be monitored by means of the ammeter 39, which provides a reading of the target current, which allows adjustment of the electrons 34 introduced in the beam 29 from the wire 35 · As pointed out above, it is to prevent the build-up of a positive potential on a insulating layer formed on the surface of a target disk 31, desirable that the target current is either at zero or has a certain negative value.

In the arrangement of Fig. 1A, the far wall 28, which is biased at the most negative level in Faraday's cage, has the task of ensuring that a minimum of the electrons leave the cage from its open back. At a modified C.

In the alternative embodiment of this arrangement of FIG. 3, the far wall 28 can be eliminated and a magnetic field 40 at right angles to the ion beam is created by a pair of magnets 4L and 42. This field essentially prevents backward movement of the electrons belonging to the ion beam 26 by functioning. in a conventional manner such as an electron barrier.

In connection with ion beams, which implant certain dopants, e.g. Arsenic, which evaporates easily below the working temperature, poses a problem as a result of the precipitation of evaporated arsenic on the target. In normal operation of ion implantation equipment, any arsenic evaporated during operations would be precipitated on the Faradaysbur walls near the target. In the present arrangement, where the electrons are delivered to the ion beam from a heated source, e.g. however, the wire 35 operating at temperatures in the range 1500-2700 ° C, the walls 27, and especially the screen portions 36, become quite hot. Because the walls and the screen have a higher temperature than the target, any arsenic vapor tends to be deposited on the target plate surface. This interferes with the treatment and especially the arsenic doping level, which is measured from the arsenic implant. This happens because the evaporated arsenic is not in the ionic state (it is essentially neutral), and consequently is not measured with the dosimetry apparatus during the implantation step. However, since it is deposited on the disc surface, it is driven into the disc during later high heat treatment steps for the disc. Consequently, arsenic amounts, which are not taken into account by the ion implantation dosimetry, eventually end up in the disc and will potentially

7804844-4 interfere with the desired implant doses and dopant concentration levels in the disc.

Furthermore, arsenic, which may have been plated on the walls of the arrangement during an earlier implantation cycle, may evaporate from the walls during a subsequent implantation cycle and adversely affect the dosimetry in the subsequent cycle.

As previously mentioned, cooled screens and wall portions can be utilized in apparatus which is particularly useful for bombardment or implantation with materials such as arsenic, which are easily evaporated under operating conditions. Of such cooled arrangements, of which the arrangement of Fig. 1A is an example, the device according to the invention is a modification of Fig. 3B, which is a cross-sectional illustration along the line 3B-3B of Fig. 3A. FIG. 3A is a front illustration of the implantation apparatus starting from the target, which faces the beam along the beam axis. Since a large portion of the apparatus of Figures 3A and 3B is substantially similar to that shown in Figures 1 and 1A, for the sake of simplicity, the elements of Figures 3A and 3B which are equivalent to the elements of Figures 1 and 1A, to be denoted by the same reference numbers as those preceded by the number 1. So, e.g. the sidewall 27 of Fig. 1A is equivalent to the sidewall 127 of Figs. 3A and 3B. In love with an element of FIG.

3A and 3B, which is equivalent to an element of Figures 1 and 1A, thus no further description will be made, but it is assumed that it operates in the same manner as in the original arrangement. In Figs. 3A and 3B, the disks are implanted with the ion beam 129 · The disks are mounted on the target support 130 within the painting arrangement 123. The side walls 127 have been modified to accommodate cooling pipes 150 which are connected to the input pipe 151, through which liquid enters the cooling system, and the pipe 152, through which liquid leaves the cooling system. A coolant, e.g. compressed air or fluorocarbon, can pass through the tubes to cool the walls 127, and especially the screen portions 136, so that these walls are maintained at a temperature below the target independent of the temperature of the wires 135 introducing electrons 134 into the ion beam 129. The refrigerants must be electrically insulating in nature, so that they do not affect the dosimetry, ie the ion beam measurement operation of the apparatus.

Similarly, the outer parts of the cooling system should be electrically insulated from the walls of Faraday's cage. As shown in Fig. 3A, the conductors 153 consist of electrically insulating material and have the task of insulating the pipes 151 and 152 from the walls 127 ·

In Fig. 3A, a section has been removed to show the location of one of the wires 135 within the electron source portion 133 relative to the beam 139 · In all respects, the elements of Figures 3A and 3B operate in substantially the same manner as the corresponding elements of Figures 1 and 1A. . Furthermore, the cooled beam current measuring and surface potential controlling apparatus of Figures 3A and 3B is used in combination with conventional ion implantation apparatus, the apparatus of which the remainder is shown schematically in Figure 1.

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With the described cooling device, when the wire is heated to temperatures in the range 1500 ~ 2700 ° C, the walls 136 will be maintained at less than 100 ° C during the ion beam operation, while the target, mainly heated by the ion beam, reaches a higher temperature, approx. 150 ° C.