US3865072A

US3865072A - Apparatus for chemically depositing epitaxial layers on semiconductor substrates

Info

Publication number: US3865072A
Application number: US407569A
Authority: US
Inventors: Earl L Kirkman
Original assignee: HLS IND
Current assignee: HLS IND
Priority date: 1973-10-18
Filing date: 1973-10-18
Publication date: 1975-02-11
Anticipated expiration: 1992-02-11
Also published as: IT1011349B; FR2248610A1; GB1436051A; CH566076A5; DE2423303A1; FR2248610B1; NL7405317A; JPS50155172A

Abstract

Apparatus for chemically depositing an epitaxial layer on semiconductor substrates in which the substrates are carried in pockets formed on the exterior wall of a rotating, cylindrical opaque susceptor. Infrared radiant heaters are disposed centrally within the susceptor to project radiant heat onto the inner cylindrical wall of the susceptor. Spaced radially outward from the susceptor and coaxially therewith is a porous, undulated wall. The undulations in the porous wall travel in the circumferential direction. A plenum wall is disposed radially outward from and concentric with the porous undulated wall. The porous undulated wall has micro filtrative capability. Carrier and diluent gases enter the plenum chamber defined by the plenum wall and pass through the porous undulating wall to produce a thorough mixing of the carrier and diluent gases and to provide a highly uniform flow of the carrier and diluent gases. Reactant gases enter through the plenum wall and are discharged by a vertically aligned gas ejection orifices located at the maximum radially outward amplitude of the undulation of the porous reaction wall. As the carrier and diluent gases leave the porous, undulating wall, they encounter the reactant gases to provide a mixture thereof. The mixture of gases contact the exposed surfaces of the rotating substrates, which are heated to the desired reaction temperature through the susceptor, for producing an epitaxial layer on the exposed surfaces of the substrates.

Description

Elite tates atent [1 1 ltirltntan Feb.lll,1975

[ APPARATUS FOR CHEMHCALLY DEPOSITING EPITAXIAL LAYERS ON SEMICONDUCTOR SUBSTRATES [75] Inventor: Earl L. Kirkman, Felton, Calif.

[73] Assignee: HLS industries, Sunnyvale, Calif.

[22] Filed: Oct. 18, 1973 [21] Appl. No.: 407,569

[52] US. Cl. 118/4, 118/49 [51] int. Cl. C236 13/12 [58] Field of Search 118/4, 5, 50, 50.1, 4849.5,

OTHER PU BLICATIONS Western Electric C0. Technical Digest No. l 1, Apparatus for the Deposition of Silicon Nitride Whitner, R. A. (July 1968) pp. 5,6.

Primary Examiner--Morris Kaplan Attorney, Agent, or Firm-Jack M. Wiseman [57] ABSTRACT Apparatus for chemically depositing an epitaxial layer on semiconductor substrates in which the substrates are carried in pockets formed on the exterior wall of a rotating. cylindrical opaque susceptor. Infrared radiant heaters are disposed centrally within the susceptor to project radiant heat onto the inner cylindrical wall of the susceptor. Spaced radially outward from the susceptor and coaxially therewith is a porous, undulated wall. The undulations in the porous wall travel in the circumferential direction. A plenum wall is disposed radially outward from and concentric with the porous undulated wall. The porous undulated wall has micro filtrative capability. Carrier and diluent gases enter the plenum chamber defined by the plenum wall and pass through the porous undulating wall to produce a thorough mixing of the carrier and diluent gases and to provide a highly uniform flow of the carrier and diluent gases. Reactant gases enter through the plenum wall and are discharged by a vertically aligned gas ejection orifices located at the maximum radially outward amplitude of the undulation of the porous reaction wall; As the carrier and diluent gases leave the porous, undulating wall, they encounter the reactant gases to provide a mixture thereof. The mixture of gases contact the exposed surfaces of the rotating substrates, which are heated to the desired reaction temperature through the susceptor, for producing an epitaxial layer on the exposed surfaces of the substrates.

23 Claims, 4 Drawing Figures I NULL swncn DIFFERENTIAL PRESSURE SWITCH PATENTEU FEBI H975 SHEET 2 BF 3 APPARATUS FOR CHEMICALLY DEPOSITING EPITAXIAL LAYERS ON SEMICONDUCTOR SUBSTRATES BACKGROUND OF THE INVENTION The present invention relates in general to the fabrication of semiconductor devices and more particularly to apparatus and method employed in the vapor deposition of epitaxial layers on semiconductor substrates.

Heretofore, apparatus has been employed in which substrates are carried by a rotatable graphite susceptor in a reaction chamber defined by a reaction wall. Such apparatus are disclosed in the patent to Briody U.S. Pat. No. 3,659,552; Ernst et al. U.S. Pat. No. 3,424,629; and I-Iugle U.S. Pat. No. 3,659,230. Generally the reaction wall is constructed of transparent quartz, such as in the patents to Marinace U.S. Pat No. 3,047,438; Gutsche et al. U.S. Pat No. 3,304,908; and Nick] U.S. Pat. No. 3,293,074. The patent to Hugle U.S. Pat. No. 3,645,230 disclosed a stainless steel reaction wall constructed to be water cooled. In the patent to Ernst et al. U.S. Pat. No. 3,424,629 an opaque quartz reaction wall is employed. Various heaters have been employed such as r.f. heating, induction heating, infrared heating and ultra-violet heating. Disclosures thereof can be found in the patents to Bhola U.S. Pat. No. 3,338,209; Briody U.S. Pat. No. 3,659,552; McNeilly et al. U.S. Pat. No. 3,623,712 and Grossman U.S. Pat. No. 3,200,018. The patent to I-Iugle U.S. Pat. No. 3,645,230 shows a cylindrical graphite susceptor with a centrally located heater.

Earlier apparatus for chemically depositing epitaxial layers on semiconductor substrates have the unsolved problem of particulate contamination caused by residual silicon dioxide particles. The silicon dioxide particles are a by-product of the reaction of SiH, and 0 In this regard, the free silica is suspended in the gas stream. A very high diluent and main gas flow is required to keep the particles in motion. Toward this end, the reaction on the semiconductor substrate surface is controlled to transfer the maximum amount of SiH, to the hot surfaces on the substrate surface before free silica forms.

An additional problem of earlier apparatus for chemically depositing epitaxial layers on semiconductor devices is one of particulate fall out. In this connection, the molecular percentage to surface area is critical. The particles, once deposited on the semiconductor substrate surfaces, form peaks in the epitaxial film deposited on the substrate surfaces. This conditions reduces the usable area of the substrate. As a consequence thereof, such steps in the fabrication of semiconductor devices, such as masking, may be troublesome because of scratching, product yield/loss and the like.

Apparatus heretofore employed for depositing epitaxial layers on semiconductor devices experienced dead flow"boundary areas at the walls thereof, at the corners, at the raised susceptor edges and the like. This condition upsets the gas flow dynamics and produces non-uniform deposition conditions on the semiconductor substrate.

Contamination and the pure mixing of gases cause "pin holes-"in the epitaxial film or layer.

SUMMARY OF THE INVENTION Apparatus for chemically depositing epitaxial layers on semiconductor substrates in which an undulated, porous wall is employed for the mixing of carrier and diluent gases.

By virtue of employing an undulated porous wall, carrier and diluent gases pass through the undulated porous wall in a thoroughly mixed state and provide a highly uniform flow. The porous wall improves molecular stirring and controls boundary layer static conditions. When the carrier and diluent gases are thoroughly mixed and flow in a highly uniform state, the boundary layer effect, which upsets flow dynamics and produces non-uniform deposition conditions, has been reduced.

Apparatus for depositing epitaxial layers on semiconductor substrates in which an undlulated, porous wall is employed for the mixing of carrier and diluent gases and in which the carrier and diluent gases pass from the undulated, porous wall to encounter reactant gases. The reactant gases are discharged from injection orifices located in the vicinity of the maximum radially outward amplitude of the undulations of the undulated, porous wall. In this manner, there is improved mixing characteristics between the carrier and diluent gases and the reactant gases and improved control over the gas flow dynamics.

A feature of the present invention is the additional control over the gas flow dynamics by providing finite control over the exhausting gas from the apparatus and the injection pressure in the exhaust area.

It is an object of the present invention to provide improved mixing of the gas stream, improved efficiency of thermal energy, and a reduction in the required volume of gas to achieve desired chemical epitaxial deposition on the semiconductor substrates.

It is a further object of the present invention to provide improved control over mass transfer conditions, increase in production capacity and a more economical operation.

It is a further object of the present invention to minimize the formation of free silica particles by inhibiting the reaction of the reactant gases and oxygen from mixing until approximate contact with the heated surface of the substrate and then create the maximum molecular mixing of the reactant gases and oxygen to occur at the approximate time of contact with the heated surface of the substrate.

DESCRIPTION OF THE DRAWINGS FIG. 1 is of an elevation view of the apparatus embodying the present invention for chemically depositing an epitaxial layer on semiconductor substrates with portions of the apparatus broken away to illustrate the internal structure thereof.

FIG. 2 is a plan view of the apparatus shown in FIG. 1 with portions thereof broken away to illustrate the internal structure thereof.

FIG. 3 is a fragmentary plan view of the undulated. porous wall and plenum wall employed in the apparatus illustrated in FIGS. 1 and 2 shown with the vertical aligned orifices for discharging reactant gases.

FIG. 4 is a diagrammatic axial section view of the heater assembly undulated, porous wall and plenum wall, shown in FIGS. 1 and 2 illustrated with the vertically aligned orifices for discharging reactant gases and with parts removed for clarity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Illustrated in FIGS. 1 and 2 is the apparatus of the present invention for chemically depositing an epitaxial layer on semiconductor substrates. The apparatus 10 comprises a suitable heater assembly 15. In the preferred embodiment, the heater assembly is an infrared radiant heater employing tungsten tubular quartz radiant heat lamps mounted with the filaments parallel to the vertical axis of the apparatus. A heater assembly of this type is manufactured by Research Inc. of Minneapolis, MINN., as model numbers SA 4926 and SA 4927. In the apparatus 10, the radiant heater assembly 15 is mounted in a fixed position and is disposed as a core along the vertical axis of the apparatus.

A heater assembly of the foregoing type includes a hot air exhaust port 16, cooling air input connector 17, a cooling water inlet connector 18, a cooling water outlet connector 19 and a terminal 20 for a source of power. Methods of cooling the apparatus 10 by water and air are well known in the art. For example, water may be circulated through watertight tubular rings 22 to cool the apparatus 10 at locations where the most heat is generated. A clear quartz tube 21 surrounds the heat lamps. The cover and base plate of the heater assembly are of stainless steel. As is well-known in the art, the quartz tube 21 prevents free silica from entering the heater assembly 15 and provides an enclosure to isolate the heater cooling air and water from the reactant gas stream mixture. The quartz tube 21 is transparent to the infrared radiation eminating from the infrared lamps of the heater assembly 15. Periodically the heater quartz tube 21 is removed and cleaned by chemical etching before being replaced. The tungsten quartz iodide lamps of the heater assembly 15 radiate at about 0.8 to l.2 micron wavelength. Heater assemblies of this type are well-known in the art. Another example thereof is an infrared radiant heater produced by Hugo N. Cahnman Associates, Inc. of Kew Gardens, N.Y., which employs resistance wire windings refractory material support forms contained within a translucent quartz tubular outer wall.

Surrounding the heater assembly 15 and disposed concentrically therewith is a rotatable, cylindrically shaped, opaque susceptor 30, which in the preferred embodiment, is made of graphite. As shown in FIGS. 1 and 2, the susceptor 30 is formed of axially disposed mounting plates 31 of graphite or other suitable opaque material. Formed in the mounting plates 31 are recesses or pockets 32 in which are disposed, respectively, the semiconductor substrates. The back of the plates 31, which form the susceptor 30, are relatively smooth and, of course, are made of opaque graphite material so as to be desirable heat absorbers. The substrate support plates 31 are spaced apart by slots 33. In a manner to be described hereinafter, the carrier and diluent gases flow from the plenum side (the side facing the outer wall of the susceptor 30) and advance between the vertical substrate support plates 31 by way of the slots 33 into a negative pressure area and downward into an exhaust duct 40. The negative pressure area, in a manner to be described hereinafter, is present on the heater side (the side facing the inner wall of the susceptor 30) of the vertical substrate support plates 31. There is a negative gas pressure on the inner side of the susceptor 30. The vertical substrate support plates 31 are individually removable by means of pins 34 and fatsteners 35 at the ends thereof. Clamp ring section 36 compresses the support plates 31 against the bottom bearing ring, 37 and the top cover 38. To replace a support plate 31, the clamp ring section 36, and fastener 35 are removed so as to provide a path for the removal of the support plate 31. The support is replaced and the fastener 35 and the clamp ring section 36 is once again secured. A fastener 39 holds the clamp ring 36 against the bearing ring 37.

For imparting rotation to the cylindrical susceptor 30 above the common axis of the heater assembly 15 and the susceptor 30, a drive shaft 41 is rotated by a suitable drive motor and gear train arrangement, not shown. Fixed to the drive shaft 41 for rotation therewith is an adapter drive plate 42 and fixed to the adapter drive plate 42 for rotation therewith is the rotating cover plate 38. The rotating cover plate 38 is supported by a suitable plate 44 and is journaled for rotation with respect thereto by suitable seal ring bearings 45. As shown in FIG. 1, the cylindrical susceptor 30 is attached by suitable means, such as fasteners 35, to the rotating cover plate 38 for rotation therewith. Suitable seals 46 are provided between the plate 44 and the plenum assembly. The seal ring bearings are of glass filled teflon or graphite or mechanical felt to seal off the reaction chamber to avoid cross contamination. Also, seals 46 are mechanical felt or silicon rubber to seal off the reaction chamber to avoid cross contamination. The rings 22 protect the

seals

45 and 46 by conduction from excessive heating.

The lower end of the susceptor 30 is supported for rotation by means of the bearing ring assembly 37 and a stationary bottom plate 51, which is supported by a bracket 50 suspended from a plate 52. Disposed between

plates

52 and 37 are bearing rings 53, which are cooled by the water cooling ring 54 in the base of the plate 51. On plate 52 are bearing strips 54' and radial seals 55 for supporting the plenum chambers in opening and closing and for sealing the reactor from atmosphere. Similarly, the bottom plate of the heater assembly 15 is fixed to the frame 44.

According to the present invention, undulated, porous wall is provided. The undulated, porous reaction wall 60is made of suitable material, such as a sintered, stainless steel sheet and is spaced radially outward from the cylindrical susceptor 30 and has its axis coincident with the axis of the cylindrical susceptor 30 and the heater assembly 15. The porosity of the wall 60 is in the range 2 microns to I00 microns dependent on the desired flow characteristics and velocity of gas across the wall 60. The undulated, porous wall 60, has a micron filtration capability. Also, however, it is selected for optimum pressure flow and velocity. The undulations of wall 60 travel circumferentially around the porous wall 60. Carrier and diluent gases, such as nitrogen and oxygen gas mixture, are introduced into a plenum chamber 61 at uniform pressure and advances through the undulated, porous wall 60. The carrier and diluent gases leaving the porous reaction wall 60 is thoroughly mixed through the selection of the micron filtration capability thereof and because of the random nature of the gas passage through the undulated, porous wall 60. Additionally, the flow of carrier gas through the undulated, porous wall 60 provides a highly uniform flow of the carrier and diluent gas because of the controlled back pressure in the plenum chamber 61. Thus, the boundary layer effect is virtually non-existent. The undulated, porous wall 60 is sealed at the top and bottom thereof and also at the end plates. This can be accomplished by welding, by the use of refractory cement and the like.

Reactant gases are discharged by vertically aligned orifices 63 in the vicinity of the maximum radially outward amplitude 62 of the undulated porous wall 60. A porous tube, not shown, may be employed as well. The location of the orifices 63 is preferably to create the maximum molecular mixing of the reactant gases and the oxygen in the carrier stream. Typical reactant gases are silane, phosphene and diborane. There is an axial row of vertically aligned orifices 63 in the vicinity of each axial row of maximum radial outward amplitudes 62. Each vertical row of openings 63 is formed in a reactant gas manifold tube member 64, which is disposed in the axial direction. The openings 63 of the vertical reactant gas manifold tube members 64 face only the undulated, porous wall 60. After the carrier and diluent gases advance beyond the undulated, porous wall 60, they encounter the reactant gases discharged by the orifices 63 in the vicinity of the maximum amplitudes 62 of the undulated, porous wall 60. This action provides superior mixing characteristics and control over gas flow dynamics. Reactant gas manifolds 65 communicate with the tube members 64 to conduct the reactant gases and diluents thereto.

The substrates of the susceptor 30 are located so that the exposed surfaces thereof are spaced relative to the porous wall 60 (FIG. 3) to attain uniform deposition of the epitaxial layer without pin holes or particulate contamination.

Surrounding the porous, undulated wall 60 and spaced radially outward therefrom is a plenum wall 70 that is rigid and sealed at all sides. The plenum wall 70 defines the plenum chamber 61. In the exemplary em bodiment, the plenum wall 70 is made of aluminum. The carrier and diluent gases are fed into the plenum chamber 61 for passage through the undulated, porous wall 60 by way of input connectors 71 and openings 74. The connectors 71 are placed at the top and at the bottom of the plenum wall 70. The top connectors 71 are spaced 180 apart and the bottom connectors 71 are spaced 180 apart. The reactant gases are conducted into the plenum chamber 61 through input connectors 73. There are three upper input connectors 73 spaced 120 apart and three lower input connectors 73 spaced 120 apart. The input connectors 73 are connected to the reactant gas manifold 65 for communication therewith.

The carrier and diluent gases advance through the undulated, porous wall 60 where it encounters the reactant gases ejected from the orifices 63. This mixture of gases forms a gas stream which contacts the exposed surfaces of the substrates carried by the rotating susceptor 30. The gas stream flows in a linear fashion. The rotating susceptor 30 is heated to a desired temperature. As a result thereof, an epitaxial layer is formed on the exposed surfaces of the substrates. The exposed surfaces of the substrates are substantially perpendicular to the flow of the gas stream mixture of carrier and diluent gases and reactant gases. The exposed surfaces of the substrates are in close proximity to the orifices 63, such as in the neighborhood of A; to

Exhaust fumes from the apparatus are conducted into an exhaust chamber 80 through slots 33 formed in the susceptor 30. The exhaust fumes in the chamber are drawn through the exhaust duct 40. There is a negative pressure in the exhaust chamber 80. The gas stream of the mixture of carrier and diluent gases and reactant gases advance through slots 33 past the susceptor 30 into the negative pressure area on the heater side thereof and are drawn downward into the duct 40. Suitable damper means 85, which is adjustable by a differential pressure control, controls the flow of reactor fumes from the exhaust duct 40. The damper may be motor driven in a well-known manner. The control over the flow of exhaust gas from the exhaust duct 40 and the negative pressure in the exhaust chamber 80 provides further control over the gas flow dynamics.

For controlling the flow of reactant gases to the susceptor 30, gas operated double acting cylinders are provided. There are four of such cylinders 90. A piston rod therein adjustably opens and closes the plenum assembly.

While the preferred embodiment shows a rotating susceptor on a vertical axis, it is apparent that the present invention is equally applicable to a susceptor moving along a horizontal path or in a horizontal plane.

I claim:

1. Apparatus for depositing an epitaxial layer on substrates by the chemical vapor deposition of reactant gases and carrier and diluent gases on the exposed surface of the substrates, comprising:

a. an outer wall defining a chamber;

b. a susceptor disposed within said chamber, said susceptor being arranged to support substrates;

0. a porous wall in said chamber disposed in the path of travel of the carrier and diluent gases for mixing the carrier and diluent gases advancing toward said susceptor, said porous wall defining one wall of a reaction chamber and confronting said susceptor;

(1. means external to said reaction chamber for conducting carrier and diluent gases toward said porous wall;

e. means for supplying reactant gases to said reaction chamber;

f. conduits in said reaction chamber communicating with said reactantgas supply means and having orifices disposed adjacent the gas-exiting surface of said porous wall for discharging reactant gases to mix with the mixed carrier and diluent gases, said mixed carrier and diluent gases and said reactant gases mix after the mixed carrier and diluent gases advance from the gas-exiting surface of said porous wall to form a mixture of gases contacting the exposed surfaces of the substrates; and

g. a heater for heating said susceptor for effecting an epitaxial deposition on the exposed surfaces of the substrates in response to said mixture of gases contacting the exposed surfaces of the substrates.

2. Apparatus as claimed in claim 1 wherein said porous wall is undulated.

3. Apparatus as claimed in claim 2 wherein said orifices are located in the vicinity of maximum radially outward amplitudes of said porous undulated wall for discharging reactant gases to mix with the carrier and diluent gases, said orifices eject said reactant gases in a direction perpendicular to the stream of flow of said carrier and diluent gases from said porous wall.

4. Apparatus as claimed in claim 3 wherein said porous wall has a cylindrical configuration with the undulations thereof disposed circumferential therearound.

5. Apparatus as claimed in claim 4 wherein said susceptor has a cylindrical configuration with the axis thereof coincident with the axis of said porous wall.

6. Apparatus as claimed in claim 5 and comprising means for supporting said susceptor for rotation and means connected to said susceptor for rotating the same.

7. Apparatus as claimed in claim 1 and comprising means for supporting said susceptor for movement and means for moving said susceptor relative to said porous wall.

8. Apparatus as claimed in claim 5 wherein said heater is disposed along the axis of said susceptor.

9. Apparatus as claimed in claim 8 wherein said heater comprises infrared lamps.

10. Apparatus as claimed in claim 8 wherein said susceptor is formed with pockets at its outer cylindrical wall for the seating of substrates therein and wherein said heater heats the inner cylindrical wall of said susceptor.

11. Apparatus as claimed in claim 10 wherein said heater comprises infrared lamps and includes a transparent quartz tube surrounding the infrared lamps.

12. Apparatus as claimed in claim 1 and comprising a differential pressure operated damper for controlling reactor back pressure in said chamber, a motor for operating said damper and differential pressure sensing means for controlling the operation of said motor.

13. Apparatus as claimed in claim 1 wherein said susceptor is formed with openings to define a path for the flow of exhaust fumes to produce a uni-directional flow of gases in said chamber to control the extraction of particulate suspended in the gas stream.

14. Apparatus as claimed in claim 1 wherein said porous wall has a cylindrical configuration and said susceptor has a cylindrical configuration with the axis thereof coincident with the axis of said porous wall.

15. Apparatus as claimed in claim 14 and comprising means for supporting said susceptor for rotation and means connected to said susceptor for rotating the same.

16. Apparatus as claimed in claim 1 and comprising means for supporting said susceptor for movement and means for moving said susceptor relative to said porous wall.

17. Apparatus as claimed in claim 14 wherein said heater is disposed along the axis of said susceptor.

18. Apparatus as claimed in claim 17 wherein said heater comprises infrared lamps.

19. Apparatus as claimed in claim 17 wherein said susceptor is formed with pockets at its outer cylindrical wall for the seating of substrates therein and wherein said heater heats the inner cylindrical wall of said susceptor.

20. Apparatus as claimed in claim 19 wherein said heater comprises infrared lamps and includes a transparent quartz tube surrounding the infrared lamps.

21. Apparatus as claimed in claim 1 wherein said orifices eject said reactant gases in a direction perpendicular to the stream of flow of said carrier and diluent gases from said porous wall.

22. Apparatus as claimed in claim 14 wherein said conduits are disposed in a direction parallel to the axis of said porous wall and wherein said orifices eject said reactant gases in a direction perpendicular to the stream of flow of said carrier and diluent gases from said porous wall.

23. Apparatus as claimed in claim 1 wherein said porous wall is formed with a micron porosity for controlling the boundary layer at the gas-exiting surface of said

Claims

1. Apparatus for depositing an epitaxial layer on substrates by the chemical vapor deposition of reactant gases and carrier and diluent gases on the exposed surface of the substrates, comprising: a. an outer wall defining a chamber; b. a susceptor disposed within said chamber, said susceptor being arranged to support substrates; c. a porous wall in said chamber disposed in the path of travel of the carrier and diluent gases for mixing the carrier and diluent gases advancing toward said susceptor, said porous wall defining one wall of a reaction chamber and confronting said susceptor; d. means external to said reaction chamber for conducting carrier and diluent gases toward said porous wall; e. means for supplying reactant gases to said reaction chamber; f. conduits in said reaction chamber communicating with said reactant gas supply means and having orifices disposed adjacent the gas-exiting surface of said porous wall for discharging reactant gases to mix with the mixed carrier and diluent gases, said mixed carrier and diluent gases and said reactant gases mix after the mixed carrier and diluent gases advance from the gas-exiting surface of said porous wall to form a mixture of gases contacting the exposed surfaces of the substrates; and g. a heater for heating said susceptor for effecting an epitaxial deposition on the exposed surfaces of the substrates in response to said mixture of gases contacting the exposed surfaces of the substrates.

2. Apparatus as claimed in claim 1 wherein said porous wall is undulated.

23. Apparatus as claimed in claim 1 wherein said porous wall is formed with a micron porosity for controlling the boundary layer at the gas-exiting surface of said porous wall.