Apparatus for and method of backside protection during substrate processing

Abstract: A suitable inert thermal gas such as argon is introduced onto the backside of wafers being processed in a CVD reactor during the deposition of tungsten or other metals and silicides, to avoid deposition of material on the backside of the wafers being processed. Each process station includes a gas dispersion head disposed over a platen. The platen has a circular depression for receiving a wafer, and an annular groove provided in the floor of the depression, near the wall thereof. Heated and pressurized backside gas is introduced into the groove so that the wafer is maintained in a position above the floor of the depression but still within it. In this manner, backside gas vents from beneath the edge of the wafer on the platen and prevents the process gases from contacting the wafer backside. The backside gas is also used for levitating the wafer in a transfer region above the platen, so that the wafer can be transported to or from the platen with a suitable wafer transfer mechanism. One suitable transfer mechanism is a multi-armed spindle, the arms being respective pairs of tines. Another suitable transfer mechanism is a removable structure in which the platens are provided with respective heaters, and the platen-heater assemblies are supported by respective tubes from a central support block. Gas and electrical power are furnished to the platen-heater assemblies from the central support block.

Description: This invention relates to substrate processing, and more particularly to the protection of the backsides of wafers during a semiconductor processing operation. 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 may also affect 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 US-A-4 817 558. A substrate support member having the form of a cylinder is provided with 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 aid to trap the reactive gas on the lateral face of the wafer, thereby preventing a film from being deposited on the wafer backside. The present invention provides an apparatus for processing a thin substrate in a process chamber of a cold wall chemical vapor deposition reactor, the apparatus comprising a platen mounted in the process chamber, the platen having a platen surface for underlying the substrate, a gas dispersion head for introducing a process gas into the process chamber above the platen, a gas channel integral with the platen and having an opening in the platen surface for dispensing a backside gas different from the process gas and during introduction of the process gas to the process chamber, and a substrate retainer arranged to retain the substrate over the platen surface whilst allowing levitation of the substrate, and so as not to seal the periphery of the substrate against the platen surface so that, in use of the apparatus, a peripheral space is defined by the platen surface and a backside periphery of the retained substrate, for providing unimpeded flow of the backside gas from the gas channel to the process chamber. The invention also provides a method of suppressing backside deposition on a thin substrate mounted on a platen in a process chamber of a cold wall reactor during a chemical vapor deposition operation, the method comprising the steps of introducing the substrate over the platen, introducing a process gas into the process chamber, uniformly introducing a backside gas different from the process gas from the platen into a region between the backside periphery of the substrate and the platen at a flow rate, retaining the substrate over the platen so as not to seal the periphery of the substrate against the platen surface, so that the backside gas is uniformly dispersed throughout a peripheral space defined by a surface of the platen and a backside periphery of the substrate, and so that, at the flow rate of the backside gas, a generally unimpeded outward flow of the backside gas is established from the peripheral space into the process chamber to exclude the process gas from contact with the backside periphery of the substrate, and the substrate is caused to levitate over the platen. The apparatus of the invention can also include means for transporting a substrate in the process chamber. A wafer transport mechanism having an arm can be included within the process chamber, the arm being selectively movable into a transfer region above the platen surface portion and into a space remote from the platen. The backside gas is arranged to levitate the substrate in the transfer region. It is known from WO-A-82/01482 to retain a flat workpiece on a frame by suction applied to the lower side of the workpiece during processing of the upper side. Processing here however is by a liquid. EP-A-260 150 is concerned with using a gas for improving heat transmission from the platen to the work piece. 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 platen illustrative of the platens shown in Figure 1; Figure 4 is a cross-sectional view of the platen of Figure 3 mounted on a pedestal base, taken along line F.4-F.4; and Figure 5 is a top plan view of a wafer transport and backside protection structure; and Figure 6 is a partial cross-sectionalview of the structure of Figure 5. 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 a central exhaust 6 in the bottom of the process chamber 2, which leads into a vacuum exhaust port 24. The system for moving wafers from station to station in the chamber 2 includes wafer transport mechanism 10 and a spindle rotation mechanism 26, the design of which are well known in the art. Wafer transport mechanism 10 is in the form of a multi-armed spindle having six arms 16, 17, 18, 19, 20 and 21. Each of the arms 16-21 is made of a pair of tines. Each tine may be perforated with numerous holes (not shown) or otherwise modified to minimize air flow interference. 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, and a tube 13c (hidden) for delivering a gas (a "backside" gas) through the platen 14c to the backside of a wafer being processed. Process station 4c is mounted on the heater 15, and tube 13c passes through the heater 15. Similarly, process station 4b includes gas dispersion head 12b, platen 14b, and tube 13b (hidden), and is mounted on the heater 15. Similarly, process station 4d includes gas dispersion head 12d, platen 14d, and tube 14d (hidden), and is mounted on the heater 15. A backside gas delivery system 30 includes a gas delivery tube 28 and various tubes 13a-13f (not shown) of the stations 4a-4e and 5 respectively, which are connected inside the heater 15 to the gas delivery tube 28. The tubes 13a-13f are heated within heater 15 by being mounted in proximity to heating elements (not shown) or in any other suitable manner, so that gas passing through them becomes heated. If desired, heated baffles, fine wire meshes, or metal sponges (not shown) are placed in the gas distribution system 30 to improve the transfer efficiency of heat to the gas. A platen 100 illustrative of platens 14a-14e of Figures 1 and 2 is shown in detail in Figures 3 and 4. A disk 110 made of a suitable material such as aluminum is provided with a recessed region 112, which functions both to receive and retain a semiconductor wafer to be processed. Other suitable materials include certain stainless steel alloys or nickel, which can be used in the reactive process environment without being degraded. The recessed region 112 has a sidewall 114 and a floor 116. The sidewall 114 is angled 5 degrees relative to the normal to the floor 116, the diameter of which is 10.31 cm. (4.06 inches) for accommodating a 10.16 cm. (4 inch) wafer. The floor 116 is provided with an annular groove 118, which has an outside diameter of 9.37 cm. (3.69 inches) a width of 0.48 cm. (0.19 inches) and a depth of 0.41 cm. (0.16 inches). The operation of recessed region 112 as a substrate retainer is discussed more fully below, but it will be appreciated that other types of substrate retainers may be used as well. For example, the upper surface of disk 110 may be made flat, and the wafer to be processed may be confined by pins rising from the upper surface of disk 110. Assume, for example, a prevailing gas flow during wafer processing from the bottom to the top of the illustrative platen 100 of Figure 3. In a two pin embodiment (not shown), two pins are provided in locations corresponding to, for example, two points just outside of the sidewall 114 of Figure 3, at about the angular position of radial bores 124e and 124g respectively. These pins resist the force imparted to the wafer being processed by the prevailing gas flow. Three or more pins may be provided if desired. If a clamping effect is desired, the pins may be slanted inward toward the center of disk 110, or may be hooked at their tops toward the center of the disk 110. Other suitable retainers include a ring which mounts over the periphery of its upper surface, or clamps which engage the outside edge of the wafer at three or more positions. The wafer to be processed may even be confined by jets of gas directed at the wafer backside, provided that the jets impart equal and evenly distributed forces to the wafer backside. The platen of Figures 3 and 4 is provided with a gas injector system that includes eight radial bores 120a-120h and respective orifices 124a-124h. The orifices 124a-124h are provided between the bores 120a-120h and groove 118. As shown, groove 118 is a continuous annular channel, but other forms are suitable as well. For example, the channel may be discontinuous (segmented), or configured to match the edge configuration of the wafers being processed, or configured in a particularly easily manufacturable shape. The cross-sectional channel shape is semicircular, but other shapes such as rectangular are suitable as well. The channel may be placed nearer the wafer edge or nearer the center, although a more even pressure distribution is believed to be established by placing the channel nearer the wafer edge. Plural channels may be provided if desired. The bores 120a-120h are equally spaced at 45 degree from one another and merge in the center of the disk 110. The radially remote ends of the bores 120-120h are closed by press fitted plugs 122a-122g. A tube 126 is press-fit into a hole passing from the bottom of the disk 110 into the chamber formed by the merging of the bores 120a-120h. Tube 126 has an outer diameter of 0.97 cm. (0.38 inches). The process stations 4a-4e and the load/unload station 5 are furnished with respective gas delivery tubes 13a-13f (hidden), which are similar to tube 126. Gas delivery tubes 13a-13f are routed through the heater 15 in any suitable manner. The diameter of the bores 120a-120h and the inner diameter of the tube 126 is 0.64 cm. (0.25 inches). The diameter of the orifices 124a-124h is 0.16 cm. (0.063 inches). Generally, the selection of the diameters of the bores 120 and the orifices 124 and the number of orifices 124 are a matter of design choice, consistent with the volume of gas intended to be supplied to the wafer backside. The design of wafer load/unload station 5 is essentially identical to the illustrative platen 100. The recess 112 need not be provided, however. 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 into an empty load/unload station 5. Suitable mechanisms for transporting the wafer from a carrier to the station 5 are well known, and include, for example, an multi-segmented arm mechanism terminating in a pair of tines on which the wafer to be transported rests. The tubes 13a-13f of the gas distribution system 30 are coupled to respective networks of bores within each of the stations 4a-4e and 5 which correspond to bores 120a-120h of the illustrative platen 100. A suitable inert thermal gas such as argon, helium, freon, C 2 F 6 , or CF 4 , or any suitable combination thereof, is introduced into the gas distribution system 30. A thermal gas is any gas having thermal conductivity and heat capacity sufficient to achieve good temperature uniformity across the wafer. 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. The flow of the introduced or "backside" gas is adjusted to levitate the wafers to be processed in the stations 4a-4e and 5. For example, a levitation flow of one standard liter per minute of argon is suitable. Once the wafers at the stations 4a-4e and 5 are levitated, wafer transport mechanism 10 rotates 30 degrees in a desired direction so that arms 16-21, which previously rested in positions between the stations 4a-4e and 5, assume new positions under each of the six wafers in the process chamber 2. As the tines of the wafer transport mechanism 10 are designed to minimize air flow interference, the floating balance of the six wafers is maintained. The backside gas pressure is reduced to a sufficiently low value or to zero for depositing the wafers on the respective tines of arms 16-21 of the wafer transport mechanism 10. Wafer transport mechanism 10 now rotates 60 degrees in a desired direction so that the wafers are transported to respective successive ones of the stations 4a-4e and 5. Once again, the flow of the backside gas is increased to the levitation value to lift the wafers up from the arms 16-21 of the wafer transport mechanism 10. Wafer transport mechanism 10 now rotates 30 degrees in a desired direction, so that the arms 16-21 once again rest in positions between the stations 4a-4e and 5. Backside gas pressure is reduced to a suitably low value or zero, so that the wafer at the load/lock station 5, which is fully processed, is deposited on the tines of the load/unload mechanism and removed into the load lock chamber 1. The flow of gas to load/unload station 5 is completely shut off using a suitable valve. The wafers to be processed now rest upon or are slightly levitated over respective platens 14a-14e, under respective gas dispersion heads 12a-12e. The flow of the backside gas is now coordinated with the flow of a process gas at the gas dispersion heads 12a-12e to levitate the wafers to be processed within the respective recessed areas of process stations 4a-4e corresponding to recess 112 of Figures 3 and 4. The flow of the backside gas is adjusted to compensate for the pressure of the process gas dispersed directly upon the wafer by the dispersion heads 12a-12e, which is somewhat in excess of the process pressure in chamber 2. The process pressure in chamber 2 typically is about 10 Torr, depending on the process in use. As the backside gas is introduced through groove 118 into the space between the wafer backside and the floor 116, a uniform pressure is maintained under the wafer and a positive backside flow is maintained from under the edge of the wafer into the process chamber 2. The wafers being processed are slightly levitated, due to the small pressure differential between the wafer backside and face. The backside gas is furnished under a pressure from 1 to 20 Torr and a flow rate of from about 0.3 to about 1.0 liter, depending on the process pressure in chamber 2 and the desired rate of venting from underneath the wafer's edge. It is estimated that the difference between the ambient pressure in the chamber 2 and the pressure under the wafer is from about 1 to about 5 Torr. The backside gas vents from beneath the wafer's edge into the process chamber 2. In the process chamber 2, the backside gas mixes with the process gas and is vented through the exhaust 6, thereby creating a radial gas flow from the circumference of the process chamber 2 toward its center. While this radial flow tends to entrain the wafers being processed at the wafer process stations 4a-4e, the recessed regions of the platens 14a-14e corresponding to the recessed region 112 of the illustrative platen 100 (Figures 3 and 4) function as substrate retainers. The presence of the backside gas between the wafer backside and the floor 116 and the outward flow from under the wafer edge and into the process chamber 2 sufficiently impedes process gas from reaching any portion of the wafer backside, thereby preventing backside deposition. The backside gas is furnished to the heater 15 through delivery tube 28. The heater 15 is heated by any suitable technique. The backside gas is heated both within the heater 15 and as it flows through the various bores of the stations 4a-4e corresponding to bores 120a-120h of the illustrative platen 100, and transfers heat to the wafers as it contacts them. Typically, the temperature of the backside gas is from between 350 and 450 degrees C. A variety of process gases may be selected. For example, in depositing a tungsten film at a deposition rate of 2000 A/min, for example, the product reactant WF 6 is used under the reactant conditions of H 2 at a deposition temperature of 400 degrees C and an operating pressure of 10 Torr. In the apparatus of Figures 1 and 2, the flow of process gas is on the order of 2.5 liters per minute. The actual pressure on the wafer being processed is somewhat greater than 10 Torr because the flow of gas from the gas dispersion head impinges directly on the surface of the wafer. Under these process conditions, a suitable backside gas is Argon. The volume of backside gas flowing from beneath the wafer under such conditions ranges from 500 cubic centimeters to 2 liters for each of the process stations 4a-4e. A structure that integrates the functions of wafer transport and wafer backside protection in a unit removable for maintenance and repair is shown in Figures 5 and 6. The removable integrated structure 200 includes platen-heater assemblies 204a-204f, which include individual platens 214a-214f similar to the illustrative platen 100 mounted on individual heaters 215a-215f. Gas delivery tubes 213a-213f (hidden), which are similar to tube 126, are routed through respective heaters 215a-215f and connect to tubes 202a-202f. Tubes 202a-202f function as gas delivery tubes and support for the platen-heater assemblies 204a-204f. Tubes 202a-202f are fitted into support block 206, which is suitably bored to receive them, and welded or otherwise suitably secured. Power is supplied to the heaters 215a-215f through paired conductors, which run through electrical conduits 208a-208f (hidden in Figure 5) and are interconnected in any suitable manner in female connector 210. The structure 200 is removably connected to the rotation mechanism 26 (Figure 2) as follows. Shaft 230 is driven by the rotation mechanism 26. Shaft 230 is fitted into mounting block 224, which is suitably bored to receive it, and welded or otherwise suitably secured to the mounting block 224. Shaft 230 is hollow, and the bore which receives shaft 230 is made through the mounting block 224 to a surface that is designed to mate in a gas-tight manner with the support block 206. Support block 206 is secured to the mounting block 224 by bolts or in any other suitable manner. Accordingly, backside gas introduced into the shaft 230 is conveyed to bores in the support block 206, which distribute the backside gas to the tubes 202a-202f. Female connector 210 is detachably connected to male connector 220, which receives power from a pair of conductors which run through a single electrical conduit 222. 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 process and backside gases discussed herein are illustrative, and other process and backside gases and gas mixtures 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: An apparatus for processing a thin substrate in a process chamber (2) of a cold wall chemical vapor deposition reactor, the apparatus comprising: a platen (14a - 14f;100;214a-214f) mounted in the process chamber, the platen having a platen surface (116) for underlying the substrate; a gas dispersion head (12) for introducing a process gas into the process chamber above the platen; characterised by a gas channel (118) integral with the platen and having an opening in the platen surface for dispensing a backside gas different from the process gas during introduction of the process gas to the process chamber; a substrate retainer arranged to retain the substrate over the platen surface whilst allowing levitation of the substrate, and so as not to seal the periphery of the substrate against the platen surface, so that, in use of the apparatus, a peripheral space is defined by the platen surface and a backside periphery of the retained substrate, for providing generally unimpeded flow of the backside gas from the gas channel to the process chamber. An apparatus as claimed in claim 1 wherein the substrate retainer comprises a plurality of pins for projecting over the substrate from respective locations outside the platen surface (116), the pins being slanted inwardly of the platen. An apparatus as claimed in claim 1 wherein the substrate retainer comprises a plurality of pins for projecting over the substrate from respective locations outside the platen surface (116), the pins being hooked at their tops in a direction inwardly of the platen. An apparatus as claimed in claim 2 or 3 wherein the gas channel (118) is at a periphery of the platen surface (116) in proximity to the pins. An apparatus as claimed in claim 1, 2, 3 or 4 comprising a substrate transport mechanism (10) having an arm (16-21) selectively movable into a transfer region above the platen surface (116) and into a space remote from the platen, wherein the gas channel (118) is arranged, during a substrate transfer mode, to levitate a substrate in the transfer region upon the introduction of a gas into the gas channel (118). An apparatus as claimed in claim 5 wherein the arm (16-21) of the substrate transfer mechanism terminates in at least one tine. An apparatus as claimed in claim 6 wherein the arm of the substrate transfer mechanism terminates in two tines, the tines being narrow and spaced apart for supporting a substrate. An apparatus as claimed in claim 6 or 7 wherein the or each tine is perforated. An apparatus as claimed in any preceding claim wherein the gas channel comprises an annular groove (118) in the platen surface (116). An apparatus as claimed in any preceding claim comprising a gas line (120,124) integral with the gas channel (118) and extending through the platen (14a-14e;100;214a-214e). An apparatus as claimed in claim 9 comprising a gas line (120,124) integral with said annular groove gas channel (118), said gas line extending through the platen and comprising a network of radial bores (120a-120h) connected to the annular groove by orifices (124a-124h) at angularly spaced positions. An apparatus as claimed in claim 11 wherein the radial bores (120a-120h) originate from an edge of the platen and converge centrally of the platen, one of the radial bores (120h) being arranged to be connected to an external gas source and the or each other radial bore (120a-120g) being closed by a plug (122a-122g) at its outside opening. An apparatus as claimed in claim 10, 11 or 12 comprising a gas source (30) connected to the gas line (120,124). An apparatus as claimed in claim 13 wherein the gas source (30) connected to the gas line is a variable pressure gas source. A method of suppressing backside deposition on a thin substrate mounted on a platen (14a-14e;100;214a-214e) in a process chamber (2) of a cold wall reactor during a chemical vapor deposition operation, the method comprising the steps of: introducing the substrate over the platen; introducing a process gas into the process chamber; characterised by uniformly introducing a backside gas different from the process gas from the platen into a region between the backside periphery of the substrate and the platen at a flow rate; and retaining the substrate over the platen so as not to seal the periphery of the substrate against the platen surface, so that the backside gas is uniformly dispersed throughout a peripheral space defined by a surface of the platen and a backside periphery of the substrate, and so that, at the flow rate of the backside gas, a generally unimpeded outward flow of the backside gas is established from the peripheral space into the process chamber to exclude the process gas from contact with the backside periphery of the substrate, and the substrate is caused to levitate over the platen. A method as claimed in claim 15 wherein the retaining step comprises the steps of: disposing the substrate over the platen, the volume of backside gas exerting a force normal to the platen surface to raise the substrate above the platen surface; and resisting the normal force with a retaining effect to maintain the substrate at a fixed spacing over the platen surface. A method as claimed in claim 15 or 16 comprising the steps of transporting the substrate into a transfer region over the platen, and dispensing a volume of the backside gas over the platen for levitating the substrate above the transfer region. A method as claimed in claim 15, 16 or 17 wherein the backside gas is an inert gas having thermal conductivity and heat capacity sufficient to achieve good temperature uniformity across the substrate. A method as claimed in claim 18 wherein the backside gas comprises argon.

Inventors: BROADBENT ELIOT K, THOMAS MICHAEL E, VAN DE VEN EVERHARDUS P, BROADBENT, ELIOT K., THOMAS, MICHAEL E., VAN DE VEN, EVERHARDUS P.
Applicants: NAT SEMICONDUCTOR CORP, NOVELLUS SYSTEMS INC, NATIONAL SEMICONDUCTOR CORPORATION, NOVELLUS SYSTEMS, INC.
Application year: 1991
Main classification: H01L21
IPC codes: C30B25, H01L21, C23C16, C30B31