High throughput OMVPE apparatus

A cold wall reactor having inner and outer walls defining an annular reactor cell. A susceptor is rotatably mounted in the cell, and received wafers to be treated by gases flowing axially through the cell. The outer wall of the reactor is normally cooled, but is heated by a suitable furnace to provide a hot wall reactor when cleaning of the cell is required.

BACKGROUND OF THE INVENTION
 The present invention relates, in general, to apparatus for producing flow
 modulation epitaxy, and more particularly relates to a high throughput
 organometallic vapor phase epitaxy (OMVPE) apparatus for deposition of
 material on substrates. In a preferred embodiment, the invention is
 directed to a cold wall reactor which is convertible to a hot wall,
 reactor for epitaxial deposition of compound semiconductor materials.
 Reactors for use in chemical vapor deposition, for example for epitaxial
 processing of semiconductor materials, or wafers, are generally well
 known. Two types of reactor are available for epitaxial processing, one
 being referred to as a cold wall reactor and the other being referred to
 as a hot wall reactor. Both types are well known, and the particular
 reactor used depends upon the type of reaction to be performed. For
 example, silicon processing is normally done in a hot wall reactor device.
 In a chemical vapor deposition reactor, the chemicals used in the process
 have a tendency to decompose on the cell wall, as well as on the substrate
 as they flow through the cell. Layers of decomposed reactants build on the
 cell wall and eventually these layers begin to flake off, producing
 particulate contaminates in the cell which damage the wafer being
 processed. In addition, certain compounds produce a chemical memory
 effect; i.e., impurities accumulate on the cell wall, and then are
 released during a later run, contaminating that later run. To prevent such
 contamination, the cells must be periodically cleaned.
 SUMMARY OF THE INVENTION
 The present invention resolves the problems of prior reactor devices as
 discussed above. Accordingly, the invention provides, among other things,
 a vertical barrel, concentric cylinder design for a cold wall reactor cell
 which can be converted to a hot wall cell for cleaning the interior of the
 reactor.
 In accordance with the invention, the reactor includes inner and outer
 concentric cylinders which preferably are quartz tubes, which cooperate to
 define an annular reactor cell. A susceptor is mounted in the annular
 reactor cell, adjacent the exterior surface of the inner cylinder, and
 includes an outwardly sloping, or conical, outer surface which receives
 wafers to be treated. The susceptor is supported in the cell by a rotation
 fixture which includes a support cylinder, which may be another quartz
 tube, having an upper edge which engages the bottom of the susceptor and
 having a lower edge supported on a support bearing carried by a lower end
 cap for the reactor cell. The rotation fixture also includes a gear wheel
 mounted on the exterior of the support cylinder and driven by a
 corresponding drive gear mounted on the shaft of a drive motor.
 A lift fixture includes a top end cap supporting a lift cylinder, which
 preferably is a quartz tube surrounding the inner reactor cell cylinder.
 The lift cylinder has a lower shoulder which engages the susceptor and an
 upper shoulder which engages the end cap. When the lift fixture is moved
 upwardly, the susceptor is pulled through the top end of the outer reactor
 cylinder to provide access to wafers on the susceptor and to allow them to
 be inspected, adjusted and/or replaced. The lift cylinder is rotatable
 with respect to the top closure so the susceptor may be rotated when
 lifted for access to all the wafers on the susceptor.
 The outer reactor cell cylinder surrounds and encloses the inner reactor
 cylinder, the susceptor, and the upper lift cylinder, and is secured at
 its upper end to the top end cap and at its lower end to a bottom end cap.
 Both end caps preferably are stainless steel, with appropriate seals
 between the cylinder and the stainless steel end caps being provided. The
 inner reactor cell cylinder is closed at its top end, and extends
 downwardly through, and is sealed to, the lower end cap so that the
 interior of this tube is exposed to atmosphere while the annular region
 between the cylinders is sealed from ambient atmosphere. An induction
 heating coil, quartz lamps, or other suitable heat source extends into the
 inner reactor cylinder to heat the susceptor and thus the wafers which the
 susceptor supports. The sealed annular region between the inner and
 reactor cylinders functions as a closed reaction cell.
 Hot reaction gases are introduced into the reaction cell at its top end,
 one or more outlet pumping ports with included filter assemblies are
 located below the outer reactor cell cylinder, preferably in the lower end
 cap, for drawing the gases downwardly over the outer surface of the
 susceptor and the wafers mounted thereon for delivering unused reaction
 gases to an external vacuum source. Between the susceptor and the outlet
 port, and surrounding the rotation fixture, is an annular cooler which
 serves to cool the process gases prior to their exiting the cell. This
 condenses the majority of unused reactants into their solid phases for
 trapping by the filters in the outlet ports to prevent the exhaust gas
 plumbing and valves from being coated with film during reactor operation.
 Additional cooling is provided by a split clamshell cooling jacket which
 surrounds the reactor cell cylinder.
 The upper and lower end caps preferably are surrounded by conventional dry
 box enclosures which contain an inert gas and which thereby enable the
 upper and lower caps to be opened for access to the susceptor and access
 to and cleaning of the outlet port filters without contaminating the
 interior of the cell and without the risk of fire or smoke from pyrophoric
 deposits.
 The heat source used with the present invention preferably is a heating
 coil which is excited by a radio frequency (RF) generator, with the RF
 power being coupled to the graphite susceptor which forms an inductive
 load for the coil. The susceptor is thereby heated directly, while the
 surrounding outer reactor cylinder is heated indirectly, by radiation from
 the hot graphite, by conduction through the gas present in the cell, or
 through the supporting rotation fixture, rather than inductively. The
 reaction chamber is said to be a cold wall cell because of this method of
 heating. An alternative radiant heating method for cold wall operation is
 the use of an array of quartz lamps located inside the rotation fixture in
 place of the heating coil.
 To turn the cold wall cell into a hot wall cell for a "self cleaning"
 operation, the cooling jacket is removed and a split clamshell furnace is
 provided around the outside of the outer reactor cylinder. During cell
 cleaning, the wall of the outer cylinder is heated and a corrosive gas
 such as HCl or a corrosive plasma is injected into the reactor cell to
 etch deposits off the cell wall and the susceptor. Heating the wall also
 produces a heating mismatch which will cause deposits to crack and flake
 off the wall. Using this approach, the cell can be cleaned periodically so
 that the deposits do not build up and contaminate the cell with
 particulate matter or with previously used reactants, and this allows
 cleaning to be done without disassembly or exposure to the atmosphere,
 thereby preventing atmospheric contamination of the cell.
 The present invention is also directed to a reactor gas injection structure
 which is usable with a variety of reactor cells, but which is particularly
 adapted to use with the reactor of the present invention to allow the cell
 to operate in a variety of different modes. Injection ports are located at
 the top of the cell, adjacent the top end cap, and permit injection of
 reactant gases through selected ports located symmetrically around the
 exterior of the reaction cell. Selected gases or gas mixtures can be
 injected through single selected ports, or through several ports for
 dispersal around the entire cell. The injection ports preferably are
 located symmetrically around the cell, with four ports defining four
 growth zones, for example, and additional ports being provided to permit a
 uniform distribution of gases in the reactor. The localization of a
 reactant gas in each zone is provided by establishing a vertical laminar
 flow in the reaction cell and by ensuring that the lateral diffusion of
 vapor species is small compared to the cell dimension as the gas traverses
 the cell from top to bottom. This laminar flow is enhanced by placing
 corresponding filtered exit ports at the bottom of the cell. The exit
 ports may be vertically aligned with the injection ports and can include
 flow controllers if desired.
 In accordance with the present invention, the gas injection portion of the
 device includes a set of input mass flow controllers (MFCs) connected to
 supply to a distributor block a carrier gas and one or more desired
 reactant gases which are to be delivered to the reactor cell. An MFC is
 connected to a pressure transducer which adjusts the flow of carrier gas
 to maintain a constant pressure in the distributor block. The gas mixture
 exits the distributor block through one or more selected MFCs of a set of
 matched MFC devices which are connected between the distributor block and
 corresponding injection ports leading to the reaction cell. In this way,
 controlled quantities of the selected reaction gases enter each injection
 port.
 In addition to the spaced growth zone injector ports, additional
 intermediate injectors may be provided for use in cases where a diffused
 flow of reactant gas is desired. Such injectors may be spaced to yield a
 nearly uniform concentration of gases around the cell.
 In the preferred form of the invention, the susceptor is rotatable to carry
 each wafer in turn through the growth zones, exposing the wafers to doses
 of the selected constituents in the desired sequence to produce flow
 modulation epitaxy of the wafers.
 The annular flow cell of the present invention improves the fluid dynamics
 of the gas flowing into and through the cell. The annular cell has a
 longitudinal, or axial, vertical flow which prevents recirculation within
 the cell by causing the gas to flow downwardly from the inlet ports across
 the surfaces of the wafers, to the outlet ports. It will be understood
 that the flow direction could be reversed, to cause the gas to move
 upwardly across the wafers. Further, a variety of substrates may be
 treated in the reactor cell of the invention, but the specific substrates
 and specific reaction processes are not a part of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS
 Turning now to a more detailed description of the present invention, there
 is illustrated at 10 in FIG. 1 a vertical barrel concentric cylinder,
 organometallic vapor phase epitaxy (OMVPE) apparatus which includes an
 annular reaction cell for high throughput epitaxial deposition of compound
 semiconductor materials and for other reaction processes. The OMVPE
 apparatus 10 is of the cold wall type, and includes a generally
 cylindrical outer wall 12 and a coaxial, generally cylindrical inner wall
 14. The outer and inner walls are spaced apart to provide an annular
 flow-through reaction cell, or chamber, 16.
 The outer cylindrical wall 12 may be formed of a cylindrical tube which may
 be approximately 10 inches in diameter in one embodiment of the invention.
 The tube is closed at its top end by a stainless steel, water cooled top
 end cap 18 and at its lower end by a stainless steel, water cooled bottom
 end cap 20. The tube 12 is preferably quartz, although other materials may
 be used for this outer wall, depending on process parameters such as
 deposition temperature and the purity of the material being used. A
 quartz-to-metal seal 22 is formed at the junction of an upper edge 23 of
 the outer wall 12 and a lower surface 24 of the end cap 18. The seal 22
 may include first and second O-rings 25 and 26 of Viton, silicon rubber,
 or a suitable commercially available elastomer material such as Kalrez.
 Ring 25 is located between edge 23 and surface 24, while ring 26 is
 secured against the outer surface of tube 12 by a clamp which includes an
 annular shoulder 27 formed on the bottom surface 24 of cap 18 and an
 annular movable clamping jaw 28 which is secured to cap 18 by a series of
 spaced bolts 29. The shoulder 27 includes an outwardly sloped outer
 surface 30 which is parallel to an outwardly sloped inner surface 31 of
 clamping jaw 28, with O-ring 26 being captured between the shoulder 27 and
 the surface 31 of jaw 28. By tightening the bolts 29, the O-ring is
 squeezed against the outer surface of cylinder 12 to hermetically seal the
 junction of cylinder 12 with cap 18.
 In similar manner, bottom edge 32 of wall 12 is sealed to a top edge 33 of
 an upstanding stainless steel cylindrical flange 34 which forms a part of
 the bottom end cap 20, as by means of a quartz-to-metal seal 35. This
 quartz-to-metal seal preferably is of the same construction as the seal
 22, including an O-ring 36 between the end of cylinder 12 and the top of
 flange 34, and an O-ring 37 captured between a shoulder 38 on flange 34
 and a movable clamping jaw 39 secured by bolts 40 to an outwardly
 extending tip 41 on flange 34.
 The inner concentric wall 14 preferably is a quartz tube having a diameter
 of about 8 inches, for example, which is closed at its upper end by a
 dome-shaped integral end wall 42. The open lower end 43 of the tube 14
 protrudes downwardly through the end cap 20 so that the interior 44 of the
 tube is exposed to the ambient atmosphere. In the preferred form of the
 invention, the top end 42 of tube 14 engages a downwardly-extending
 filler/bearing support portion 45 integral with end cap 18. The end cap
 and filler portion are water cooled, and are preferably of stainless
 steel. The filler portion 45 is generally cylindrical, and of
 approximately the same diameter as the inner cylinder 14 to fill the upper
 end of the annular reactor cell so as to reduce gas flow turbulence.
 The filler 45 carries a bearing groove 46 to support and to facilitate
 rotation of a cylindrical lift tube 48, which surrounds the upper part of
 inner cylinder 14. The lift tube extends approximately one-half the
 distance down the axial length of the wall of inner cylinder 14 and
 terminates at a lower end 49, which incorporates an outwardly extending
 flange 50. Surrounding the lift tube 48 and its flange 50 and extending
 downwardly therefrom is an annular susceptor 52 for receiving wafers 53 or
 other materials to be treated in reactor cell 16. The susceptor preferably
 is formed of graphite, although other materials such as refractory metals
 are feasible in some applications, and includes a cylindrical interior
 wall 54 which is coaxial with the lift tube 48 and spaced outwardly
 therefrom to permit rotation of the susceptor with respect to the tube 48.
 Materials to be treated are mounted on a downwardly and outwardly tapered
 outer wall 56 of the susceptor which is spaced inwardly from the outer
 cylinder 12 and thus lies in the path of gases flowing through the
 reaction chamber 16. The interior surface 54 of susceptor 52 includes an
 inwardly extending shoulder 58 having a lower annular surface which
 engages the flange 50 of tube 48 when the lift tube 48 is moved upwardly,
 so that the susceptor will be lifted.
 The top end 59 of the lift tube 48 is secured to an annular lift bearing
 60, or to multiple bearing segments spaced around end 59, as by bolts 61
 extending through apertures in the quartz lift tube. The lift bearing
 extends inwardly into the bearing groove 46 so that when the top end cap
 18 is removed from the reactor cell, the bearing groove 46 will engage
 lift bearing 60 to raise the lift tube 48. This will cause the flange 50
 to engage shoulder 58 so that the susceptor 52 will be raised by the
 upward movement of the lift tube. Accordingly, tube 48 can be used to lift
 the susceptor 52 out of the reactor chamber, with the bearing surfaces
 allowing the susceptor to be rotated for easy access to all the wafers on
 surface 56.
 The susceptor 52 has a groove 63 formed in its bottom surface 64. This
 allows the susceptor to be positioned in the reaction chamber by lowering
 the lift tube so that groove 63 rests on a corresponding ridge 65 formed
 on the top edge of a support cylinder 66. The cylinder 66 preferably is a
 quartz tube which rests on a support bearing 68 secured to the top surface
 70 of the end cap 20. The support bearing 68 positions tube 66 so it is
 coaxial with inner tube 14 and allows tube 66 to rotate with respect to
 end cap 20. The ridge 65 on the top of support tube 66 centers the
 susceptor with respect to the axis of tube 14 and lifts the susceptor off
 the lift tube flange 50 as the top end cap 18 is lowered into place to
 close the top of the reaction cell 16. This frees the susceptor to rotate
 with support tube 66.
 A gas cooler such as a stainless steel can 76 surrounds the tube 66 and has
 an outer wall 78 lying in the path of gases flowing through the reaction
 chamber 16. The interior of can 76 may contain water or LN.sub.2 for
 cooling purposes, for example.
 Located below the cooling can 76, and spaced around the upstanding flange
 34 are a plurality of exhaust filters and pump assemblies generally
 indicated at 80 and including a plurality of exit ports illustrated at 81
 to 88. Preferably four or eight exit ports are equally spaced around the
 circumference of flange 34 and are connected by way of outlet conduits
 such as those illustrated at 90 and 92 to a manifold 94 which is, in turn,
 connected through a valve 96 to a vacuum pump 98. Each of the exit ports
 includes a filter (not shown) preferably carried on a support screen and
 constructed from non-reactive material such as Teflon, for example.
 A ring gear 100 is secured to the exterior of quartz support tube 66 for
 rotation therewith. The ring gear is driven by a drive gear 102 mounted on
 a drive shaft 104 which passes through the bottom end cap 20 by way of a
 feedthrough 106 to a drive motor 108. The feedthrough may be an ultra high
 vacuum bellows or a ferrofluidic drive, allowing the drive motor 108 to
 rotate the tube 66. The ratio between gears 100 and 102 will depend upon
 the speed of the drive motor, and on the size and the desired speed of
 rotation of the support tube.
 In the illustrated embodiment, the inner quartz tube 14, which extends
 downwardly through the bottom end cap 20, is hermetically sealed by a
 circumferential seal 110, including an O-ring 112, to the end cap. The
 seal 110 arrangement is similar to the seal 22 described above. The quartz
 tube 14 provides a continuous inner wall for the annular reactor cell 16,
 and surrounds the interior cavity 44 which forms a central, or axial inner
 chamber opening downwardly through the open bottom 43 to the ambient
 atmosphere. In this case, the ambient atmosphere is provided by an inert
 gas contained in a dry box 124 which surrounds the lower portion of the
 reactor cell, and particularly the lower end cap portion below seal 36, to
 permit the lower end of the reaction chamber 16 to be opened by lowering
 end cap 20. This provides access to the filters in the exhaust filter
 assembly 80 without contaminating the reaction chamber.
 An RF coil 130 is mounted in the inner chamber portion 44, and is located
 near the inner wall 54 of the susceptor 52. The coil preferably is made
 from copper tubing and is connected by way of leads 132 and 134 extending
 through the quartz tube 14 to an RF generator 136 which may be located
 outside the dry box 124. The coil 130 inductively heats the graphite
 susceptor, with its location ensuring that only the susceptor is directly
 heated. The surrounding quartz tubes, and in particular the tubes 12, 14,
 48 and 66 are not directly heated, but are indirectly heated by radiation
 from the hot susceptor material, by conduction through gas present in the
 reaction cell 16, or by conduction to the supporting quartz tube 66. The
 reaction chamber 16 is said to be a cold wall cell because of this method
 of heating.
 In a prototype of the apparatus 10, the RF generator was operated at 85
 kHz, and up to 11 kW was delivered to the coil. It was found that about 7
 kW was sufficient to heat the susceptor to temperatures exceeding
 1000.degree. C. An alternative heating method for cold wall operation is
 available through the use of an array of quartz lamps placed inside the
 susceptor in the location of the coil 130. If desired, the heating coil
 could be placed outside the outer tube 12, but that is not the preferred
 arrangement.
 Surrounding the outer quartz tube 12 is a water jacket 140 which provides a
 flow of cooling fluid to the outer wall 12 during operation of the
 reactor. This jacket preferably is of a split clamshell design so that it
 is removable to convert the reactor cell to a hot wall device for
 cleaning, as will be described.
 A suitable controller 142 may be provided to operate the drive motor 108
 and to regulate the RF generator 136 in response to the temperature of the
 susceptor, as detected by a thermocouple probe 144 connected to the
 controller by way of a line 146.
 Reactant gases are supplied to the reactor chamber 16 from a gas delivery
 system 150 illustrated in greater detail in FIG. 2, to which reference is
 now made. The illustrated gas delivery system allows the reaction cell to
 operate in a variety of different modes. For example, reactant gases can
 be injected with selected vapor species locally confined to one or more of
 four separate injection ports 152, 154, 156 and 158, and labeled as zones
 1-4. Each port may provide an injection pattern covering a controlled arc,
 so that the gases from adjacent ports overlap by a few degrees during
 operation. Alternatively, gases can be injected as a single gas mixture
 dispersed around the entire cell through four or more injection points
 such as 160, 162, 164 and 166, or a combination of all of the ports can be
 used. As illustrated, the latter injection points are located
 symmetrically between the four zone injection points, with all of the
 injection points being equally spaced around the circumference of the
 reaction chamber 16.
 FIG. 2 is a top view of the apparatus of FIG. 1, with the top end cap 18
 sectioned and the filler 45 removed. As illustrated, the injection ports
 extend through a downwardly extending annular flange portion 168 of end
 cap 18 to inject reactant gases into the reaction chamber 16. The
 injectors 152, 154, 156 and 158 may be connected through corresponding
 inlet conduits 152', 154', 156' and 158', respectively, to corresponding
 sources of carrier or reactant gases each of which may carry a selected
 vapor species for epitaxial deposition on wafers 53, which may be compound
 semiconductor materials, for example.
 In similar manner, the injection ports 160, 162, 164 and 166 are connected
 by way of corresponding conduits 160', 162', 164' and 166' to
 corresponding sources of reactant or carrier gases, which in this case are
 illustrated as including input mass flow controllers (MFCs) 170, 172, 174,
 and 176. The MFCs are connected to a common distributor block 180, which
 may be a high purity stainless steel vessel and which may contain a
 carrier gas and desired reactant gases which are to be distributed around
 the reaction cell 16. In one embodiment of the invention, the carrier gas
 was H.sub.2, while the reactant gases were HCl for cell cleaning, and were
 hydrides containing group V components for epitaxial deposition on a
 compound semiconductor. For example, ASH.sub.3 can be deposited for
 gallium arsenide semiconductors, NH.sub.3 can be deposited for GaN
 semiconductors, and InP can be used with a PH.sub.3 gas.
 The carrier gas is supplied to the distributor block through MFC 182 which
 is slaved to a pressure sensor 184 connected to the distributor block. The
 pressure transducer produces a carrier gas flow adjustment to maintain a
 constant pressure in the distributor block. Reactant gases are supplied to
 the distributor block through MFCs 186 and 188 and are mixed with the
 carrier gas in the block 180. The gas mixture exits the distributor block
 through MFCs 170, 172, 174, 176 and are injected into the reaction chamber
 16. Equal quantities of the distributed gases enter each injection port
 between zones 1 to 4 and are diffused around the reaction cell 16,
 yielding a nearly uniform concentration of distributed gases around the
 cell. The number of injection points can be increased or decreased, as
 desired, to obtain the desired uniformity or nonuniformity of vapor
 concentration.
 The gas injection ports located at the top of the reaction chamber, and in
 particular the four zone injection ports, preferably are vertically
 aligned with corresponding outlet ports at the bottom of the chamber to
 ensure a vertical laminar flow of gases from the top to the bottom of the
 chamber, thus avoiding uncontrollable swirling of gases about the vertical
 axis of the cell. This vertical laminar flow ensures that the vapor
 species injected at zones 1 to 4 will have only a small lateral diffusion
 compared to the cell dimension as the gas traverses the cell from top to
 bottom. In one embodiment of the reactor operating at a pressure of 25
 torr with carrier gas flows of roughly 25 SLPM, this condition was easily
 met. It was found that by the time the vapor species reached the
 susceptor, they had spread roughly 120.degree. around the cell, providing
 adjacent growth zones which overlapped only slightly.
 To ensure an equalized laminar flow in the reactor chamber 16, the outlet
 ports 81-88 are connected to manifold 94 through corresponding MFCs, five
 of which are illustrated in FIG. 3 at 81' and 85' through 88'. These
 controllers provide equal exit pumping at each port, and facilitate
 control of gas flow in the reactor cell.
 Access to wafers 53 is obtained by removing the top end cap 18 and lifting
 the quartz lift tube 48. The flange 50 at the bottom of tube 48 engages
 the shoulder 58 in the susceptor 52, enabling the tube 48 to pull the
 susceptor upwardly and out of the top of the reaction chamber. A dry box
 enclosure 200 surrounds the top end of the reactor to provide a controlled
 ambient atmosphere to prevent contamination of the reactor during wafer
 loading and unloading.
 In operation, the support tube 66 is rotated to move the susceptor 52 and
 thus the wafers 53 sequentially through the four growth zones provided by
 the vapor species injected at ports 152, 154, 156 and 158 and flowing
 vertically down through the reaction chamber. The surfaces of the
 substrate wafers are exposed to the selected constituent vapors in
 sequence, to provide flow modulation epitaxy on the wafers.
 Flowing water or liquid nitrogen is supplied to the cooling can 76 to cool
 the process gases as they leave the reaction chamber. This condenses
 unused reactants into their solid phases for subsequent trapping by the
 filters in the outlet ports 81-88 to prevent the exhaust gas plumbing and
 valves from being coated with solid films during reactor operation. As
 noted above, the filters can be maintained by lowering the bottom end cap
 20 into the dry box 124.
 Some condensation can occur on the quartz outer wall 12 as well as on other
 parts of the chamber during a reaction process, and it therefore becomes
 necessary to clean the chamber periodically. This is accomplished, in
 accordance with the present invention, by converting the cold-wall
 reaction chamber into a hot wall chamber for a self-cleaning operation.
 During the cell cleaning step, a corrosive gas, such as HCL, is injected
 into the reaction cell and serves to etch deposits off the chamber walls
 and the susceptor. This preferably is done periodically so that the
 deposits do not build up and do not contaminate the cell with particulate
 matter or with previously used reactants.
 Conversion of the reactor to a hot wall chamber device is accomplished by
 removing the cooling jacket 140 from the chamber and replacing it with a
 split clamshell furnace, diagrammatically illustrated at 210 in FIG. 2.
 This furnace may include a pair of arcuate sections 210 and 214 hinged at
 one end at 216 and abutting at their opposite ends 218 and 220. The
 arcuate segments curve around the outer wall 12 to enclose the reactor
 chamber and to heat it to a suitable temperature. Although the clam shell
 furnace 210 is illustrated for convenience as being spaced away from the
 outer wall 12, it will be understood that in fact the furnace interior
 wall may engage or be closely spaced from the outer wall 12 for maximum
 heat transfer. This clamshell furnace arrangement allows the reactor to be
 cleaned without disassembly and consequent contamination.
 In summary, then, hot reactive gases enter the annular reaction chamber 16
 at the top of the chamber and flow downwardly across substrates or wafers
 to be treated. These wafers are located on the downwardly and outwardly
 sloping outer wall of a graphite susceptor which is heated by an internal
 coil which, in turn, heats the wafers which are to be treated by the
 reactant gases. These gases flow vertically through the chamber and are
 removed through outlet filter ports by suitable external vacuum pumps. As
 the gases flow downwardly and out of the chamber, they are cooled by a
 cooling can to force the chemicals carried by the gases to precipitate
 before they reach the pump outlet port. This allows the filter assembly at
 the outlet ports to remove the particulates before they reach the vacuum
 pump and valve arrangement. The outlet ports are located in a dry box
 which is filled with an inert gas which not only prevents contamination of
 the reaction chamber when the filters are cleaned, but prevents fires in
 those cases when the compounds used are pyrophoric.
 The cell geometry provides an improved gas distribution system, providing
 an extremely simple construction which has not previously been possible
 because of difficulties in obtaining seals for the reaction chamber. The
 herein-disclosed structure is capable of sustaining ultra high vacuum
 pressures approaching 10.sup.-8 torr with an appropriate pumping system.
 The cell is not operated under ultra-high vacuum (UHV) conditions, but
 leaks are diagnosed under such conditions. The annular shape of the flow
 cell improves the fluid dynamics of the gas flowing into and through the
 chamber, with the sloped surfaces of the wafers preventing gas rebound and
 eliminating undesirable recirculation of the gases. The downward flow of
 the gases from inlets in the top cap reduces the amount of dead air in the
 chamber and the cylindrical inner wall 14 together with the distributed
 gas injectors provides an improved operation.
 The illustrated vertical chamber structure is preferred; however, it will
 be understood that a horizontal chamber device can be constructed
 utilizing the features illustrated herein. Additionally, the vertical flow
 chamber can be inverted, so that the reactant gases are introduced at the
 bottom of the reaction chamber, flow upwardly across the wafers on the
 substrate, and exit at the top of the chamber. This flow direction has
 some advantages, since the flow is in the same direction as the convection
 forces, but it requires the susceptor to be turned over. This, in turn,
 requires that the wafers be secured to the susceptor surface, and this is
 a major problem.
 Although the reaction chamber is described as being formed by concentric
 quartz tubes, it is apparent that other materials can be used. For
 example, pyrolytic Boron Nitride would be an excellent material, and
 stainless steel could be used for low temperature processes. The susceptor
 is described as being graphite, but it could be any electrically
 conductive material, when RF inductive heating is used. Other thermally
 conductive materials such as Aluminum Nitride can be used with other
 heating sources. Various other materials will be apparent to those of
 skill in the art.
 Although the invention has been described in terms of a preferred
 embodiment, it will be understood that numerous variations and
 modifications may be made without departing from the true spirit and scope
 thereof, as set forth in the following claims.