Apparatus for and method of protection during substrate processing

Abstract: A suitable inert gas such as argon or a mixture of inert and reactive gases such as argon and hydrogen is introduced onto the backside of wafers being processed in a CVD reactor during the deposition of tungsten or other metals, metal nitrides and silicides, to avoid deposition of material on the backside of the wafers being processed. Each process station (4b,4c,4d) includes a gas dispersion head (12b,c,d) disposed over a platen (14b,c,d). A vacuum chuck including a number of radial and circular vacuum grooves in the top surface of the platen is provided for holding the wafer in place. A platen heater is provided under the platen. Backside gas is heated in and about the bottom of the platen, and introduced through a circular groove in the peripheral region outside of the outermost vacuum groove of the vacuum chuck. Backside gas pressure is maintained in this peripheral region at a level greater than the CVD chamber pressure. In this manner, backside gas vents from beneath the edge of the wafer on the platen and prevents the process gas from contacting the wafer backside. If the process gas is also to be prevented from contacting the periphery of the wafer frontside, a shroud is urged against the platen to direct the backside gas into a cavity formed between the shroud and the platen. This cavity contains the wafer frontside periphery, so that backside gas venting from between the shroud and the wafer frontside periphery excludes the process gas.

Description: This invention relates to substrate processing, and more particularly to the protection of certain portions of wafers during semiconductor processing operations. Chemical vapor deposition ("CVD") is a gas reaction process commonly used in the semiconductor industry to form thin layers of material known as films over an integrated circuit substrate. The CVD process is based on the thermal, plasma, or thermal and plasma decomposition and reaction of selected gases. The most widely used CVD films are silicon dioxide, silicon nitride, and polysilicon, although a wide variety of CVD films suitable for insulators and dielectrics, semiconductors, conductors, superconductors, and magnetics are well known. Particulate contamination of CVD films must be avoided. A particularly troublesome source of particulates in the chemical vapor deposition of metals and other conductors such as tungsten, tungsten silicide, and titanium nitride, is the film that forms on the backside of the wafer under certain conditions. For example, if the wafer backside is unprotected or inadequately protected during deposition, a partial coating of the CVD material forms on the wafer backside. This partial coating tends to peel and flake easily for some types of materials, introducing particulates into the chamber during deposition and subsequent handling steps. Many approaches have been developed for addressing the problem of material deposition on the wafer backside. In one approach, the material is permitted to form on the backside, but then is removed immediately following the deposition step using an in-situ plasma etch. This approach entails additional process steps and requires additional equipment capabilities, and also affects the flatness of the wafer. In another approach, the wafer is clamped onto a substrate holder in an attempt to seal and isolate the backside region from the CVD gas. An adequate seal tends to be difficult to achieve in practice, and the mechanical motion between the clamp and the wafer itself causes particulates. Yet another approach is disclosed in United States Patent No. 4,817,558, issued April 4, 1989 to Itoh. A substrate support member having the form of a cylinder is provided with a flat bearing surface on which the wafer rests. Three pins protrude from the peripheral edge portion of the bearing surface. The sidewalls of the shield are insulated from the reactive gases by a cover, which is further provided with a lifted and bent region that surrounds the substrate at the level of the substrate. The lifted and bent region is said to trap the reactive gas on the lateral face of the wafer, thereby preventing a film from being deposited on the wafer backside. There is known from US-A-4 709 655 a chemical vapor deposition apparatus of a kind comprising a chuck or platen mounted to have a surface thereof within a process or deposition chamber, holding means arranged, in use, for holding the backside of a wafer or substrate to the surface of the chuck or platen, and a gas disperser at the surface of the chuck or platen. The present invention provides a chemical vapor deposition apparatus of this kind wherein the gas disperser is located in a peripheral region of the platen surface, and wherein the holding means comprises a vacuum chuck located in an interior region of the platen surface surrounded by the peripheral region, the peripheral region being shaped so as, in use, to establish in conjunction with a backside periphery of the substrate held by the vacuum chuck a thin peripherally continuous space communicating with the gas disperser and the process chamber. Undesirable deposition of materials on the wafer backside is diminished in the present invention, which in one embodiment includes a pedestal base, a platen mounted on the pedestal base and having a vacuum groove formed in an interior region of its upper surface and a gas groove formed in a peripheral region of its upper surface, a vacuum line integral with the vacuum groove, and a gas line integral with the gas groove. In another embodiment, the present invention includes a pedestal base and a platen mounted on the pedestal base and forming therewith an enclosed volume. A heating element is disposed within the enclosed volume, and an orifice is provided for introducing gas under pressure into the enclosed volume. A gas groove is formed in the peripheral region of the upper surface of the platen, and a gas line integral with the gas groove extends through the platen and into the enclosed volume. In a further embodiment of the foregoing advantageous for preventing undesirable deposition of materials on the wafer backside, edge, and frontside periphery, a shield is added that has a first surface region enclosed by a second surface region. The second surface region is adapted to contact a continuous portion of the platen peripheral region, while the first surface region is adapted to define a cavity in conjunction with a portion of the platen peripheral region. The cavity is adapted to enclose the backside peripheral region, the edge, and a frontside peripheral region of a substrate mounted on said platen. A means for urging the shield against the platen is provided. In another embodiment of the present embodiment, a platen has a substrate retainer that is ineffective for sealing a periphery of the backside of a substrate against a peripheral surface region of the platen. A gas disperser is disposed in the platen peripheral surface region for introducing a backside gas over the platen peripheral surface region. A shield is provided that has an enclosing contact bottom portion for contacting an enclosing contact portion of the platen peripheral surface region. The shield contact bottom portion encloses a relieved bottom portion, the width of which corresponds to a substrate frontside periphery, and the depth of which corresponds to the substrate thickness. A positioner is provided for selectively lowering the shield into engagement with the platen along the respective contact portions. In another embodiment, the substrate is restrained and a process gas comprising a mixture of a reactant gas and a carrier gas is introduced at a preselected pressure into the process chamber. A backside gas comprising a mixture of a reactant gas and an inert gas is introduced to the periphery of the substrate backside, and a positive pressure differential is maintained between the pressure at the periphery of the substrate backside and the pressure in the process chamber. In a variation of this embodiment, the process gas comprises the product reactant WF₆, the reactant gas hydrogen, and the carrier gas argon or a mixture of inert gases, while the backside gas comprises the reactant gas hydrogen and the inert gas argon or a mixture of inert gases. In a further variation of this embodiment, the backside gas is directed through a cavity adapted to contain the periphery of the substrate backside, the edge of the substrate, and the periphery of the substrate frontside. A positive pressure differential also is maintained from within the cavity to the process chamber. The invention is further described below, by way of example, with reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: Figure 1 is a cut away view of a process chamber for a chemical vapor deposition system, as viewed from above; Figure 2 is a cut away view of the process chamber of Figure 1, as viewed from a side; Figure 3 is a top plan view of a pedestal base illustrative of the pedestal bases shown in Figure 2; Figure 4 is a cross-sectional view of the pedestal base of Figure 3; Figure 5 is a bottom plan view of the pedestal base of Figure 4; Figure 6 is a top plan view of a platen illustrative of the platens shown in Figure 2; Figures 7 and 8 are views through different cross-sections of the platen of Figure 6; Figure 9 is a bottom plan view of the platen of Figure 6; Figure 10 is a plan view of a heating element; Figure 11 is a cross section view of a portion of the platen of Figure 7 showing an additional "shroud" feature; Figure 12 is a top plan view of a shroud lift assembly; Figure 13 is a cut away plan view of a portion of the process chamber of Figure 1, as viewed from a side, showing additional and modified features, including the shroud lift assembly of Figure 12; and Figure 14 is a cross section view showing a relationship of the shroud lift assembly of Figure 12 with a wafer transport mechanism. An illustrative reaction chamber of a high pressure chemical vapor deposition ("CVD") apparatus is shown from a top view in Figure 1, and from a side view in Figure 2. The process chamber 2 communicates with a load lock chamber 1, from which wafers to be processed are introduced into the process chamber 2, and into which processed wafers are received from the process chamber 2. Within process chamber 2 are five wafer process stations 4a-4e and a wafer load/unload station 5. Chamber gases are exhausted through vent ports 6a-6f. The system for moving wafers from station to station in the chamber 2 includes pin lift platforms 8a-8c and wafer transport mechanism 10. Also shown in Figure 2 are a vacuum exhaust port 24, a spindle lift/rotation mechanism 26, and a pin lift mechanism 28, the design of which are well known in the art. Wafer process stations 4b, 4c and 4d are visible in more detail in the side view of Figure 2. Process station 4c, for example, includes a dispersion head 12c for introducing a process gas or gas mixture over a wafer to be processed; a platen 14c for supporting the wafer to be processed; a pedestal base 16c, which includes a heater for heating platen 14c and indirectly supplying heat to the wafer to be processed; and pin lift platform 8b, which is associated with pins 20c, 21c and 22c (hidden) for lifting and lowering the wafer to be processed in coordination with activation of the wafer transport mechanism 10. Similarly, process station 4b includes gas dispersion head 12b, platen 14b, pedestal base 16b, and pin lift platform 8a in association with pins 20b, 21b and 22b. Similarly, process station 4d includes gas dispersion head 12d, platen 14d, pedestal base 16d, and pin lift platform 8b in association with pins 20d, 21d and 22d (hidden). A pedestal base 100 illustrative of pedestal bases 16b, 16c and 16d is shown in detail in Figures 3, 4 and 5. In the top plan view of Figure 3 and the bottom plan view of Figure 5, the most prominent feature is the base plate 102. Base plate 102 is made of aluminum. Other suitable materials include certain metal or ceramic alloys, including stainless steel and nickel, that can be used in the reactive process environment without being degraded. Three holes 105a, 105b and 105c through base plate 102 and respective spacing sleeves 104a, 104b and 104c, which protrude from base plate 102, accommodate screws (not shown) used to connect illustrative platen assembly 200 (Figure 6) to the base pedestal 100. The outside diameter of the spacing sleeves 104a-104c is 0.79 cm. (0.31 inches), and the top of spacing sleeves 104a-104c is 1.04 cm. (0.41 inches) from the bottom surface of the base plate 102. Three other holes 107a, 107b and 107c through base plate 102 and respective index sleeves 106a, 106b and 106c are 0.48 cm. (0.19 inches) in diameter for accommodating wafer lifting pins such as, for example, pins 20c, 21c and 22c of Figure 2. Sleeves 106a-106c are 0.79 cm. (0.31 inches) in diameter and rise 1.9 cm. (0.75 inches from the upper surface of the base plate 102 to engage respective holes 224a-224c of platen 200 (Figures 8 and 9). Holes 107 are 0.48 cm. (0.19 inches) in diameter and eccentric with respect to their respective sleeves 106, although they may be concentric if desired. An annular groove 108 is provided in base plate 102 to accommodate the platen 200 and provide a seal between the platen 200 and the base pedestal 100. The top of groove 108 is 1.04 cm. (0.41 inches) from the bottom surface of the base plate 102. The pedestal 110 and pedestal mounting block 112 are visible in Figure 4. Pedestal 110 is made of tubular aluminum or other suitable material, and is welded at its outside diameter to the bottom of the base plate 102 at a circular opening provided therein. The annular mounting block 112 also is aluminum or other suitable material, and is provided with pin holes 114a-114f and index pins 116a and 116b for mounting on the floor of the process chamber in any suitable manner. The mounting block 112 is inserted into the pedestal 110 and welded thereto. An illustrative platen 200 is shown in Figures 6, 7 and 8. The major component of the platen 200 is a circular block 202 of aluminum or other suitable material such as stainless steel. As viewed from the top, the upper surface of the platen block 202 is provided with a vacuum line 204, which appears as a central orifice from which radial vacuum grooves 206a-206h extend. The vacuum line 204 is 0.63 cm (0.25 inches) in diameter, and extends through the platen block 202. The radial grooves 206a-206h are rectangular in cross section, although other shapes are suitable as well, and are 0.15 cm. (0.06 inches) deep and 0.15 cm. (0.06 inches) wide. The eight radial grooves 206a-206h are regularly disposed at forty-five degree angles from one another. Radial grooves 206a-206h intersect concentric annular vacuum grooves 208a and 208b, which like the radial grooves 206a-206h are rectangular in cross section and (naasura 0.15 cm. (0.06 inches) deep and 0.15 cm. (0.06 inches) wide. The outer radius of the inner annular groove 208a is 3.58 cm. (1.41 inches). and the outer radius of the outer annular groove 208b is 6.10 cm. 2.40 inches). An annular gas groove 210 is provided in the peripheral region of the top surface of platen block 202. The annular groove 210 is rectangular in cross section, measuring 0.23 cm. (0.09 inches) wide and 1.27 cm. (0.50 inches) deep. The outside radius of the gas groove 210 is 6.71 cm. (2.64 inches). The gas groove 210 is provided within a peripheral region 209 of the top surface of the platen block 202 outside of the outermost annular vacuum groove 208b. As shown in Figure 7, a portion 211 of the peripheral region 209 is depressed with respect to the top surface of the platen block 202. The depression shown in Figure 7 is 0.025 cm. (0.01 inches). Other configurations of the peripheral region 209 include having portion 211 level with or elevated above the top surface of the platen block 202, and providing a baffle in the form of an annular ridge or ledge just beyond the diameter of the wafer to be processed. The gas groove 210 intersects a network of radial gas lines 212a-212j for distributing gas to the backside of the wafer to be processed. Radial gas lines 212a-212j are shown in hidden lines in Figure 6, and selectively in cross section in Figure 7. Lines 212a-212j are radially distributed in the platen block 202 at 36 degree intervals. Each of the bores for the ten gas lines 212a-212j is 0.33 cm (0.13 inches) in diameter, begins in the vertical edge of the platen block 202 at a distance of 0.28 cm (0.11 inches) from the top surface of the platen block 202 to the bore centerline, is angled twenty degrees from the top surface of the platen block 202, and extends sufficiently into the platen block 202 to intersect a respective one of the ten vertical bores for gas lines 216a-216j, which extend from the bottom surface of the platen block 202 (Figure 9). The gas lines 212a-212j are plugged by respective plugs 218a-218j ( see. e,g., plugs 218d and 218i in Figure 7), which extend 1.9 cm. (0.75 inches) from the outside vertical edge of the platen block 202 to just short of the gas groove 210. The plugs 218a-218j are slip fitted and welded on the outside surface of the platen block 202. The diameter of the bores for gas lines 216a-216j is 0.33 cm. (0.13 inches). A second network of radial gas lines 214a-214c are bored in the platen block 202 for distributing backside gas to holes 107a-107c, which accommodate wafer lift pins such as 20c, 21c and 22c shown in Figure 2. Radial gas lines 214a-214c are shown in hidden lines in Figure 6, and selectively in cross section in Figure 8. Each of the bores for the three gas lines 214a-214c is 0.33 cm. (0.13 inches) in diameter, begins in the vertical edge of the platen block 202 a distance 1.93 cm. (0.76 inches) from the top surface of the platen block 202 to the bore centerline, is parallel to the top surface of the platen block 202, and extends sufficiently into the platen block 202 to intersect a respective one of the three vertical bores for gas lines 220a-220c, which extend from the bottom surface of the platen block 202 (Figure 9). The gas lines 214a-214c are plugged by respective plugs 222a-222c ( see. e.g., plug 222a in Figure 8), which extend 1.9 cm. (0.75 inches) from the outside vertical edge of the platen block 202 to just short of the respective gas lines 220a-220c. The plugs 222a-222c are slip fitted and welded on the outside of the platen block 202. The diameter of the bores for gas lines 220a-220c is 0.15 cm. (0.06 inches). Holes 107a-107c, which accommodate wafer lift pins such as 20c, 21c and 22c shown in Figure 2, are 0.48 cm (0.19 inches) in diameter and pass through platen block 202. Toward the bottom surface of the platen block 202, holes 107a-107c merge into, respectively, index holes 224a-224c, which are 0.79 cm. (0.31 inches) in diameter and provided to receive respective index sleeves 106a-106c of the base pedestal 100 (Figure 3). The center axes of holes 107a-107c are offset from, respectively, the center axes of holes 224a-224c, to accommodate the eccentricity between holes 107a-107c and sleeves 106a-106c. The platen 200 also accommodates vacuum line 204, which is a 0.63 cm (0.25 inch) bore drilled through the center of the platen block 202. Toward the bottom surface of the platen block 202, vacuum line 204 merges with a hole which receives an aluminum press-fit fixture 226, which is welded to the platen. The fixture 226 has a channel through it for receiving aluminum tube 228, which is welded to it. At its other end, tube 228 is welded to a suitable aluminum vacuum fitting 230. The bottom of platen block 202 is provided with a spiral groove 232, to accommodate a heating element such as element 300 shown in Figure 10. The groove 232 is variably pitched to provide a uniform distribution of heat in the platen 200, and is suitably sized to receive heating element 300. The heating element 300 is made of any suitable soft heat-conductive material, and is slip-or press-fitted into the platen block 202. The resistance of the fit and the radial force exerted by the spiral act to securely retain the heating element 300 in the groove 232. The base pedestal 100 is assembled to the platen 200 by aligning the index sleeves 106a-106c extending from the top of the base plate 102 with the index holes 224a-224c in the bottom of the platen block 202, and bringing together the pedestal base 102 and the platen 200 so that annular flange 234 extending from the bottom of block 202 is set within the annular groove 108 in the base plate 102. The annular flange 234 has an inside diameter of 17.55 cm. (6.91 inches), and is 0.46 cm. (0.18 inches) wide and 0.66 cm. (0.259 inches) in length. The groove 108 has an inside diameter of 17.30 cm. (6.81 inches) and an outside diameter of 17.81 cm. (7.01 inches); hence is 0.51 cm. (0.20 inches) wide. The tip of flange 234 and the bottom of groove 108 are serrated in a mating pattern for providing a gas seal. The base pedestal 100 and the platen 200 are secured to one another by the use of screws (not shown), which passed through holes 105a-105c from the bottom of the base plate 102 (Figure 5) and engage respective threaded holes 236a-236c in the bottom of the platen block 202. It will be appreciated that the base pedestal 100 and the platen 200 may be made by a variety of different techniques. For example, the base pedestal 100 and platen 200 may be machined from a single block of material, or the base pedestal 100 may be manufactured with a different shape, or the platen 200 may be assembled from a variety of tubes and plates. In the event that the base pedestal 100 and the platen 200 are separate components, they may be joined by any number of different techniques, including welding, clamping, and gluing. Although load/lock station 5 as shown is different from the process stations 4a-4e, it will be understood that load/lock station 5 could be nearly identical to the process stations 4a-4e, retaining the major components thereof, including the lift pin holes 107a-107c, but lacking the groove 210 and the various gas lines 212a-212j and 216a-216j of the backside gas system, and the various grooves 206a-206h and 208a-208b and gas line 204 of the vacuum chuck system. Alternatively, the load/lock station 5 could be identical to the process stations 4a-4e for providing additional processing capability. A variety of materials including tungsten, tungsten silicide, and titanium nitride are deposited on a wafer using the apparatus of Figures 1 and 2 as follows. The wafer to be processed is introduced into the process chamber 2 from the load lock chamber 1 at a reduced pressure such as 5332 Nm⁻² (40 Torr), and is received at an empty load/unload station 5 and lowered onto raised lift pins 20f, 21f and 22f. By coordinating the rotation of wafer transport mechanism 10 and the raising and lowering of the lift pins 20a-20f, 21a-21f, and 22a-22f, the wafers are transported to successive ones of the stations 4a-4e and 5. The wafer at the load/lock station 5, which is fully processed, is removed into the load lock chamber 1. When the pins 20a-20f, 21a-21f, and 22a-22f at the stations 4a-4e and 5 lower, the wafers to be processed are deposited on respective platens 14a-14e under respective gas dispersion heads 12a-12e. Once the wafers are deposited on the respective platens 14a-14e, a suitable vacuum such as, for example, a pressure 2666-5332 Nm⁻² (20-40 Torr) less than the process chamber pressure, is maintained in the vacuum clamps of the respective process stations 4a-4e. The term "vacuum" is used herein in a relative sense to mean a pressure less than another pressure, e.g. the pressure in the vacuum clamps at the respective process stations 4a-4e relative to a pressure in the process chamber 2. Each of the platens 14a-14e includes vacuum grooves such as radial grooves 206a-206h and annular grooves 208a-208b for holding a wafer in place on the surface of the platen block 202, thereby creating a vacuum clamp. The vacuum is exerted through vacuum line 204, tube 228, and fitting 230. Once the vacuum clamps at stations 4a-4e are activated, a gas is introduced at the backside of the wafers at the stations 4a-4e. The introduction of the backside gas is coordinated with the introduction of a process gas at the gas dispersion heads 12a-12e. To better appreciate the function of the backside gas, consider the illustrative platen 200 of Figures 6-9. The backside gas is introduced into the space between the wafer backside and the surface of the platen block 202 in the peripheral region 209 (in the embodiment of Figures 7 and 8, the region 211) through annular groove 210, even if the peripheral region 209 is even with the top surface of the platen block 202. The combined force exerted through the vacuum grooves 206a-206h and 208a-208b and the gas dispersed directly upon the wafer, the latter being somewhat in excess of the process pressure in chamber 2, is sufficient to overcome the opposite force exerted by the backside gas pressure developed under the wafer in the peripheral region 209. The volume of backside gas furnished to the groove 210 is determined based on the desired rate of venting from underneath the wafer's edge and the effect of the backside gas on the wafer frontside deposition. The backside gas vents from beneath the wafer's edge, which is disposed over the peripheral region 209, into the process chamber 2. In the process chamber 2, the backside gas mixes with the process gas and is vented through the vent ports 6a-6f. The presence of the backside gas over the peripheral region 209 and the outward flow from the peripheral region 209 past the wafer edge and into the process chamber 2 sufficiently impedes the process gas from reaching any portion of the wafer backside, thereby preventing backside deposition. The backside gas also vents beneath the base plate 102 through the lifting pin holes 107a-107c, to prevent the process gas from reaching the area around the lifting pins and the wafer backside through the lift pin holes 107a-107c. The backside gas is introduced into the interior volume of the pedestal 110 through orifice 113 in the block 112, and into the interior volume under the platen block 202 and about the heating element 300 through the orifice 103 in the base plate 102. From the interior volume of the pedestal 110 and under the platen block 202, the backside gas enters gas lines 216a-216j and flows from there to groove 210 through respective gas lines 212a-212j. The backside gas also enters gas lines 220a-220c, from which it flows to lift pin holes 107a-107c through respective gas lines 214a-214c. As gas lines 220a-220c are smaller than gas lines 216a-216j, the flow through them is relatively restricted. The backside gas is heated both within the volume under the platen block 202 and as it flows through the various gas lines. A variety of process gases and backside gases may be selected. For example, in depositing a tungsten film at a deposition rate of 2 × 10⁻⁷ m/min (2000 Å/min), for example, the product reactant WF₆ is used under the reactant conditions of H₂ and Ar at a deposition temperature of 400 degrees C and an operating pressure of 5332 Nm⁻² (40 Torr). In the apparatus of Figures 1 and 2, the flow of process gas to each of the dispersion heads 12a-12e is on the order of two standard liters per minute. The actual pressure on the wafer being processed is someewhat greater than 5332 Nm⁻² (40 Torr) because the gas stream from the dispersion head impinges directly on the surface of the wafer. Under these process conditions, a suitable backside gas is Argon or a mixture of Argon and Hydrogen in proportion to the mixture of Argon and Hydrogen introduced through the gas dispersion heads 12a-12e into the process chamber 2. The various constituent gases are delivered to and mixed in a suitable manifold, as is well known in the art. The flow of backside gas provided to each of the process stations 4a-4e under such conditions ranges from about 500 standard cubic centimeters per minute to about 3 standard liters per minute. Uniformity of deposition near the edge of the wafers being processed is further improved by mixing a reactant gas with the selected inert gas or gases to obtain the backside gas. In the example of the preceding paragraph in which the product reactant WF₆ is used with the reactant gas H₂ and the carrier gas Ar or N₂ or a mixture of Ar and N₂, improved uniformity of edge deposition is obtained by mixing the reactant gas H₂ with Ar or N₂ or a mixture of Ar and N₂ to obtain the backside gas. The proper proportion of reactant gas to inert gas is determined empirically, as follows. Optimize the process gas mixture ( e.g. WF₆+H₂+Ar) for the desired results at the center of the wafer. Adjust the backside gas mixture so that the proportion of reactant gas to carrier gas ( e.g. H₂:Ar) is reproduced in the backside gas mixture ( e.g. H₂+Ar). vary the proportion of gases in the backside gas ( e.g. H₂:Ar) through several wafer test runs to determine which proportion yields the best uniformity, and select that proportion for production. From ten to twenty percent variation from the starting proportion to the final determined proportion is to be expected. Suitable inert gases for use in the backside gas mixture include argon, nitrogen, helium, freon, C₂F₆, or CF₄, or any suitable combination thereof. An inert gas is any gas that does not react adversely with the materials present in the process chamber 2 and in the gas distribution system, and that does not participate in the chemical reactions involved. Moreover, it is desirable that the thermal conductivity and heat capacity of the inert gas be sufficient to achieve good temperature uniformity across the wafers being processed. In some deposition operations, and particularly in the CVD deposition of metals and metal compounds such as tungsten, titanium nitride, and silicides, one may wish to exclude deposition of material not only from the wafer backside, but also from the edge and a peripheral portion of the wafer frontside. This result is achieved by the use of an annular "shroud" structure such as the structure 400 illustrated in Figure 11. A wafer such as wafer 402 is held in place on the platen block 202 by any suitable means, such as the vacuum clamp comprising radial grooves 206a-206h and annular grooves 208a-208b described above. The shroud 400, which is of any suitable material such as metal or ceramic (including, for example, alumina), is an annular structure suitably relieved at its inside basal region to create a suitable cavity for containing the edge and a peripheral portion of the frontside of the wafer on which deposition is to be excluded. For example, a suitable cavity is created by relieving the inside basal region, as shown in Figure 11. While the dimensions of the shroud 400 are not critical, they are selected in accordance with the dimensions of the wafer 402 and the flow capacity of the backside gas delivery system. For example, the dimensions listed in Table 1 for the various wafer sizes are suitable. Wafer Diameter Platen Diameter Shroud OD Relieved Region OD Shroud-Relieved Relieved Region ID 12.70 cm. 17.78 cm. 20.32 cm. 13.06 cm. 11.25 cm. (5 inches) (7 inches) (8 inches) (5.14 inches) (4.43 inches) 15.24 cm. 17.78 cm. 20.32 cm. 15.60 cm. 13.79 cm. (6 inches) (7 inches) (8 inches) (6.14 inches) (5.43 inches) 22.86 cm. 22.86 cm. 25.40 cm. 20.68 cm. 18.87 cm. (8 inches) (9 inches) (10 inches) (8.14 inches) (7.43 inches) Note that in Table 1, "OD" means outside diameter, and "ID" means inside diameter. For many wafers, a relief of 0.119 cm. (0.047 inches) is suitable. If precise alignment accuracy is desired, the shroud 400 may be provided with any suitable type and arrangement of alignment means such as, for example, hole-pin pairs (not shown) or slot-wedge pairs (not shown) provided within the contact region of the shroud 400 with the peripheral region 211 of the platen block 200. Another such alignment means, which is illustrated in Figure 11, involves the use of three or more flanges such as 404, which are spaced along the outside edge of the shroud 400 and extend down in such a manner as to engage the outside edge of the platen block 202 and compel alignment of the shroud 400 with the platen block 202, hence with the wafer 402. The flange members 404 may be tapered as shown to engage progressively the outside edge of the platen block 202. The process stations 4a-4e within process chamber 2 are either shrouded or unshrouded as desired. A shroud lift assembly 420 suitable for shrouding or unshrouding the process stations 4a-4e is shown in Figure 12. A circular shroud lift plate 422 is provided with six semi-circular cutout regions 430-435 respectively associated with the process stations 4a-4e and the load/unload station 5. Cutout regions 431-435, which are associated with the process stations 4a-4e, are of a diameter just greater than the diameter of the platen block but just less than the outside diameter of the shroud 400, and are designed to engage respectively five shrouds 441-445 similar to the shroud 400. Typically, all of the process stations 4a-4e are either shrouded or unshrouded, although the process stations 4a-4e may be variously shrouded and unshrouded as desired merely by mounting or omitting respective ones of the corresponding shrouds 441-445. The shrouds 441-445 are engaged by the shroud lift plate 422 in any suitable manner. For example, in Figure 12, the shrouds 441-445 are respectively aligned with cutout regions 431-435 and are merely contacted and lifted by the upper surface of the shroud lift plate 422 as the shroud lift plate 422 rises above the platen block 202. Note that the shrouds 441-445 are accessible for cleaning and changing merely by removing the top of the process chamber 2. Note also that no shroud is mounted to the shroud lift plate 422 at cutout region 430, which is associated with the load/unload station 5. The shroud lift assembly 420 is mounted within process chamber 2 as shown in Figure 13. A suitable mechanism is provided for coupling the shroud lift assembly 420 in vertical movement with the wafer transport mechanism 10, while allowing the shroud lift mechanism to be rotationally static relative to the process stations 4a-4e during rotation of the wafer transport mechanism 10. For example, in the approach shown in Figure 14, a shaft 450 is rigidly coupled to the shroud lift plate 422 at the mounting point 424 by any suitable means, such as, for example, by bolting or welding. The shaft 450 extends into the spindle lift/rotation mechanism 26 (portions omitted for clarity) through a hollow shaft 454 that supports the wafer transport mechanism 10, and a seal such as, for example, O-ring 452 is provided to prevent particulates generated by the mechanical operations inside the spindle lift/rotation mechanism 26 from contaminating the process chamber 2. Inside the spindle lift/rotation mechanism 26, bearings 456 and 458 couple the shafts 450 and 454 for vertical motion, and decouple the shafts 450 and 454 for rotational motion. The shaft 450 includes a rigid extension 460 that engages a vertical slot 462 in a block 464. The block 464 is stationary relative to the process chamber 2. The engagement of the extension 460 with the slot 462 in block 464 prevents the shroud lift assembly 420 from rotating relative to the wafer transport mechanism 10 while allowing vertical movement. In an alternative approach, a bearing (not shown) is rigidly mounted on the shroud lift plate 422 at point 424. The bearing rotationally engages a race (not shown) coaxially and rigidly mounted on the top of the wafer transport mechanism 10. The bearing rotationally decouples the shroud lift assembly 420 from the wafer transport mechanism 10. A rigid extension (not shown) of the shroud lift plate 422 engages a vertical slot provided at a convenient location on the wall of the process chamber 2, which prevents the shroud lift assembly 420 from rotating while allowing vertical movement. The operation of the apparatus of Figures 12 and 13 is the same as the operation of the apparatus of Figures 1 and 2, except as augmented by the function of the shroud lift assembly 420. The wafer to be processed is introduced into the process chamber 2 from the load lock chamber 1, and is received at an empty load/unload station 5 and lowered onto raised lift pins 20f, 21f and 22f. By coordinating the rotation of wafer transport mechanism 10 and the raising and lowering of the lift pins 20a-20f, 21a-21f, and 22a-22f, the wafers are transported to successive ones of the stations 4a-4e and 5. As the wafer transport mechanism 10 rises toward a level suitable for engaging wafers at the stations 4a-4e and 5, the shroud lift plate 422 also rises, thereby lifting shrouds 441-445 to clear the space above the process stations 4a-4e for transport of the wafers. As the wafer transport mechanism 10 lowers from the level suitable for engaging wafers at the stations 4a-4e and 5, the shroud lift assembly 420 also lowers. Note that the motion of lift pins 20a-20f and 22a-22f follows the upward motion movement of transport mechanism 10 and shroud lift assembly 420, and proceeds the downward movement of transport mechanism 10 and shroud lift assembly 420. When the shroud lift assembly 420 is lowered, the shrouds 441-445 are deposited on the top of the platens at the respective process stations 4a-4e, thereby shrouding the wafers. Various measures may be taken to retain the shrouds 441-445 in place. For example, Figure 11 shows an approach in which the shrouds 441-445 are made to be of a suitable weight so that gravity acts to resist the displacement force applied by the backside gas. Once the vacuum clamps at stations 4a-4e are activated, a gas is introduced at the backside of the wafers at the stations 4a-4e. The introduction of the backside gas is coordinated with the introduction of a process gas at the gas dispersion heads 12a-12e. The volume of backside gas furnished to the groove 210 is determined based on the desired rate of venting of the backside gas from between the shroud and the wafer frontside. Note that the use of the shroud may in some instances permit a reduction in the volume of backside gas furnished to the groove 210. As shown by the flow arrows in Figure 11, the backside gas introduced into groove 210 flows first under the wafer 402, then past the edge of the wafer 402, then over a periphery of the front side of the wafer 402, and then into the ambient of the process chamber 2. In the process chamber 2, the backside gas mixes with the process gas and is vented through the vent ports 426a-426f and 6a-6f. The presence of the backside gas over the peripheral region 211 and the outward flow from the peripheral region 211 past the edge of wafer 402 and into the process chamber 2 via the top periphery of the wafer 402 sufficiently impedes the process gas from reaching the wafer backside, edge and top periphery, thereby preventing deposition on these surfaces. While our invention has been described with respect to the embodiments and variations set forth above, these embodiments and variations are illustrative and our invention is not to be considered limited in scope to these embodiments and variations. For example, the various shapes and dimensions and the various flow rates and pressures set forth herein are illustrative, and other shapes, dimensions, flow rates, and pressures may also be effective for the intended purpose. Moreover, the deposition process discussed herein is illustrative, and other processes may also be effective for the intended purpose. Accordingly, other embodiments and variations not described herein are to be considered within the scope of our invention as defined by the following claims.

Claims: A chemical vapor deposition apparatus comprising a platen (14;200) mounted to have a surface thereof within a process chamber (2), holding means (206,208) arranged, in use, for holding the backside of a substrate to the surface of the platen, and a gas disperser (210) at the surface of the platen, characterised in that: the gas disperser (210) is located in a peripheral region (209) of the platen surface and, in that the holding means comprises a vacuum chuck (206,208) located in an interior region of the platen surface surrounded by the peripheral region (209) the peripheral region being shaped so as, in use, to establish in conjunction with a backside periphery of the substrate held 'by the vacuum chuck a thin peripherally continuous space communicating with the gas disperser and the process chamber (2). An apparatus as claimed in claim 1 wherein at least a portion (211) of the peripheral region (209) of the platen surface is depressed relative to the interior region of the surface. An apparatus as claimed in claim 2 wherein the depressed portion (211) of the peripheral region (209) is depressed about 0.0254 cm. (0.01 inches) relative to the interior region. An apparatus as claimed in claim 1, 2, 3 wherein the gas disperser (210) is provided with an integral gas line (212) for providing gas thereto and the vacuum chuck is provided with an integral vacuum line (204) for providing vacuum thereto. An apparatus as claimed in claim 1, 2, 3 or 4 wherein the gas disperser comprises an annular groove (210). An apparatus as claimed in claim 4 or 5 wherein the gas line (212) extends through the platen (14;200). An apparatus as claimed in claim 5 or 6 wherein the gas line comprises a plurality of bores (216,212) originating at a bottom surface of the platen and intersecting the groove (210) at respective annular positions. An apparatus as claimed in any one of claims 4 to 7 comprising a gas source connected to the gas line. An apparatus as claimed in claim 8 wherein in use the gas source comprises a source of inert gas, or of an inert gas mixture. An apparatus as claimed in claim 8 wherein in use the gas source comprises a source of mixed inert and reactant gases. An apparatus as claimed in claim 8 wherein in use the gas source comprises a source of any one or more of argon, nitrogen, helium, freon C₂F₆, and CF₄. An apparatus as claimed in any preceding claim wherein the vacuum chuck comprises at least one vacuum groove (206,208) in the interior surface region of the platen (14;200). An apparatus as claimed in claim 12 wherein the at least one vacuum groove comprises at least one radial groove (206) intersecting at least one annular groove (208). An apparatus as claimed in any one of claims 4 to 13 wherein the vacuum line (204) extends through the platen (14;200). An apparatus as claimed in claim 13 wherein the vacuum line comprises a vertical bore (204) through the platen (14;200), the or each annular groove (208) being centred on the intersection of the vertical bore with the platen surface. An apparatus as claimed in claim 15 comprising a tube (228) connected at one end to the vertical bore (204) at the intersection thereof with a bottom surface of the platen (14;200), the tube having a vacuum fixture (230) connected at the opposite end for connection to a vacuum system. An apparatus as claimed in any one of claims 4 to 16 wherein the platen (14;200) is mounted on a pedestal base (16;100) mounted in the process chamber (2) with a lower surface of the platen and an interior portion of the pedestal base forming an enclosed volume for receiving a pressurized gas, and wherein the gas line (212) extends through the platen to the enclosed volume. An apparatus as claimed in claim 17 wherein the pedestal base (16;100) comprises a substantially hollow cylindrical pedestal (110) of which the interior forms part of the enclosed volume, an end of the cylindrical pedestal having an orifice (113) for introducing gas under pressure into the enclosed volume. An apparatus as claimed in claim 17 or 18 comprising a heating element (300) disposed within the enclosed volume. An apparatus as claimed in claim 19 wherein the heating element (300) is mounted on the lower platen surface within the enclosed volume and the pedestal base (16;100) comprises a base plate covering the heating element. An apparatus as claimed in any preceding claim wherein the peripheral region (209) comprises an inner and an outer peripheral region, and wherein a shield (400) has a first surface region enclosing a second surface region and being shaped to contact the outer peripheral region, the second surface region being shaped to establish a cavity in conjunction with the inner peripheral region when the first surface region is in contact with the outer peripheral region, the cavity being arranged to contain the backside peripheral region, the edge, and a frontside peripheral region of the substrate, and the peripherally continuous space being in continuity with the process chamber through the cavity. An apparatus as claimed in claim 21 wherein the shield (400) is an annular member, wherein the second surface region corresponds to an annularly relieved inside bottom portion thereof, and wherein the first surface region corresponds to the bottom of the annular member. An apparatus as claimed in claim 22 wherein the annularly relieved portion is an annular step. An apparatus as claimed in claim 21, 22 or 23 wherein the inner peripheral region is depressed relative to the interior region. An apparatus as claimed in claim 21, 22, 23 or 24 comprising means for aligning the shield (400) with the platen (14;200). An apparatus as claimed in claim 25 wherein the alignment means comprises at least two pin-hole pairs associated with the first shield surface region and with the platen peripheral region. An apparatus as claimed in claim 25 wherein the alignment means comprises a wedge-slot pair associated with the shield and with the platen outside peripheral region. An apparatus as claimed in claim 25 wherein the alignment means comprises at least three flanges (404) associated with the shield (400) and extending beneath the outside edge thereof, the flanges being arranged to engage progressively an outside edge of the platen (14;200). A method of holding the backside of a substrate to a surface of a platen mounted to have the surface in a process chamber of a chemical vapor deposition apparatus, the surface of the platen having a gas disperser, the method being characterised by: applying a vacuum to an interior region of the backside of the substrate via the platen to restrain the substrate thereon, the platen having a peripheral region shaped to establish in conjunction with a backside periphery of the substrate a thin peripherally continuous space communicating with the gas disperser and the process chamber; introducing a backside gas into the thin peripherally continuous space; introducing a process gas into the process chamber; and maintaining a positive pressure differential between pressure at the backside periphery of the substrate and pressure in the process chamber. A method as claimed in claim 29 wherein the backside gas introducing step is further characterized by introducing the backside gas into a gas disperser located in the peripheral region of the platen through a gas line integral therewith. A method as claimed in claim 29 or 30 wherein the vacuum applying step is further characterized by applying the vacuum to a vacuum chuck located in an interior region of the platen through a vacuum line integral with the vacuum chuck. A method according to any one of claims 29 to 31 wherein the backside of a substrate disposed in a process chamber during processing is protected, the method being characterized by: the process gas comprising a mixture of a first reactant gas and a carrier gas at a preselected pressure into the process chamber; and the backside gas comprising a mixture of a second reactant gas and an inert gas. A method as claimed in claim 32 comprising the steps of: directing the backside gas through a cavity adapted to contain the periphery of the substrate backside, the edge of said substrate, and the periphery of the substrate frontside; and maintaining a positive pressure differential from within the cavity to the process chamber. A method as claimed in any one of claims 29-33, wherein the process gas comprises a mixture of a product reactant, the first reactant gas, and the carrier gas optimized for a desired result at the centre of said substrate, wherein the first and second reactant gases are the same type of gas, and wherein the backside gas introducing step comprises the steps of: initially setting the proportion of the second reactant gas to the inert gas in the backside gas to be substantially equal to the proportion of the first reactant gas to the carrier gas in the process gas; introducing varying proportions of the second reactant gas to the inert gas in the backside gas while monitoring deposition uniformly across a plurality of test wafers; and selecting one of the varying proportions of the second reactant gas to the inert gas in the backside gas that yields good deposition uniformity across a test wafer. A method as claimed in claim 34, wherein the carrier gas and the inert gas are the same type of gas. A method as claimed in claim 35, wherein the carrier gas and the inert gas are argon. A method as claimed in claim 35, wherein the carrier gas and the inert gas are a mixture of argon and nitrogen. A method as claimed in any one of claims 29 to 33, wherein: the process gas comprises a product reactant of WF₆, reactant gas of hydrogen, and a carrier gas of argon; and the backside gas comprises a reactant gas of hydrogen and an inert gas of argon. A method as claimed in any one of claims 29 to 33, wherein: the process gas comprises a product reactant of WF₆, a reactant gas of hydrogen, and a carrier gas of argon and nitrogen; and the backside gas comprises a reactant gas of hydrogen and an inert gas of argon and nitrogen. A method as claimed in any one of claim s29 to 33, wherein said inert gas is also a thermal gas. A method as claimed in any one of claims 29 to 33, wherein the inert gas is selected from the group comprising argon, nitrogen, helium, freon, C₂F₆, or CF₄, and any combination thereof.

Inventors: BENZING JEFFREY C, BROADBENT ELIOT K, BURKHART CHRISTOPHER W, CHIN BARRY L, EVERHARDUS VAN DE VEN P, BENZING, JEFFREY C., BROADBENT, ELIOT K., BURKHART, CHRISTOPHER W., CHIN, BARRY L., VAN DEN VEN, EVERHARDUS P.
Applicants: NOVELLUS SYSTEMS INC, NOVELLUS SYSTEMS, INC.
Application year: 1991
Main classification: C23C16
IPC codes: H01L21, C23C16