Vapor Deposition Apparatus

Abstract

A vapor deposition reactor having a gaseous phase inlet and exit system including inlet means, located at one end of a reaction chamber, including a porous gas distribution baffle which forms a plenum and which uniformly delivers gaseous materials to substantially all of the horizontal cross-sectional area of the reaction chamber and further including an exit means, located at the other end of the chamber, comprising a porous pressure baffle for uniformly allowing the removal of gaseous materials from the chamber to prevent recirculation of reaction byproducts.

Description

This invention relates to vapor transport chemical vapor deposition processes, and more particularly, to an improved apparatus for carrying out these processes.

BACKGROUND

As is well known in the art, vapor transport chemical vapor deposition reactions have many applications in the area of semiconductor and other solid state device manufacturing. In essence, any heat induced gaseous reaction producing desirable byproducts might be utilized.

Many different processes and reactions have been used in what is generally termed vapor growth; for example, various pyrolytic and disproportionation reactions have been employed, in addition to the well-known epitaxial semiconductor deposition process, for the deposition of oxides, nitrides, and metals. One of the most frequently used of these already developed processes is one involving the hydrogen reduction of silicon tetrachloride at an elevated temperature.

One object of many chemical vapor deposition systems is to produce a uniform product. If uniform products cannot be produced, great expense is entailed either by redesigning device specifications to fit the variations of the product produced or by the rejection of a large percentage of the products.

The problems which have been faced previously in chemical vapor deposition processes include: contamination of the reaction chamber; lack of uniformity of deposit thickness on all substrates in a single reaction vessel, as well as individual substrates; the presence of spikes on the surface of the substrates; and long cycles involved in the batch operations currently employed.

PRIOR ART

The various different forms of apparatus disclosed in the prior art are of significance only to illustrate the unsolved problems heretofore involved in producing acceptable products. Early in the development of the vapor deposition art, when most manufacturers were merely experimenting with chemical vapor deposition, two common forms of apparatus known as "closed" or "open" tubes were used. The small capacity of the tubes and the time consumed preparing for the deposition process made economical manufacturing difficult. Subsequently, a reaction chamber known as a "barrel reactor" received wide application in the field. An example of such a reactor is disclosed in U.S. Pat. No. 3,424,629,to Ernst et al., issued Jan. 28, 1969,and assigned to the assignee of the instant invention.

The barrel reactor consists of a barrellike chamber which contains a cylindrical susceptor upon which a number of substrates may be mounted circumferentially. Gaseous reactants admitted at the bottom of the reactor by a halo-shaped inlet system are passed over the substrate surfaces, usually heated by an externally located RF coil. Exhaust gases are removed from the reactor through a port located in the top of the reactor.

However, the barrel reactor did not solve all of the problems found in the prior art. The primary advantage of the barrel reactor was to increase batch sizes over the open tube method. Deposits produced in barrel reactors still had many drawbacks. For example, dust particle counts as high as 100,000 particles per ft..sub.3 could easily cause contamination of substrates. Film thickness variations on the order of .+-. 0.5 microns on a single wafer are also common. Other problems such as the presence of epitaxial spikes are often found.

It is therefore an object of this invention to improve the quality of vapor deposition products and the operating characteristics of barrel reactors in general by decreasing the duty cycle required to perform each batch operation.

Another object is to reduce the contamination level in deposited films by reducing the dust count inside the reactor.

A further object of this invention is to provide more uniform chemical vapor deposition deposits.

A still further object of this invention is to substantially reduce or eliminate the number of spikes formed in deposited films.

SUMMARY OF THE INVENTION

Briefly considered, the reactor of the present invention is constructed to realize the aforementioned objects, goals and advantages and comprises a barrel reactor having a gaseous phase inlet system including a gas distribution baffle extending substantially across the entire horizontal cross-sectional area of the reaction chamber which provides uniform distribution of reactants entering the chamber and a planar velocity front substantially throughout the chamber. An exit pressure baffle also extending substantially across the entire horizontal cross-sectional area of the chamber is utilized to maintain a constant mass flow from the chamber and to prevent recirculation of the gaseous products leaving the chamber.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawing.

The FIGURE shows a partial sectional view of a preferred embodiment of the present invention.

DETAILED DESCRIPTION

The reaction that is illustratively employed to demonstrate the preferred embodiment is the reduction of silicon tetrachloride by hydrogen, described by the equation:

SiC1.sub.4 + 2H.sub.2 Heat Si + 4HCL.

In actuality the reaction is more complex and depends upon the reactant concentrations, temperature, pressure, and reactor geometry, all of which may result in various side reactions. Since the reaction is reversible, etching and mass transport processing may also occur.

Although the method and apparatus of the present invention will be described in the specific context of the above reaction, it will be apparent to those skilled in the art that other vapor transport reactions may be similarly utilized. Thus, the apparatus of the present invention is not. Tied to a single reaction or process since the only limiting criteria for advantageous application of the instant apparatus is that a reducible vapor source, or a heat induced chemical reaction byproduct be available. For example, the following types of reaction are possible: disproportionation, decomposition, condensation and gas cracking. Additionally, since a number of such reactions are reversible, the removal of films, or material, from substrate surfaces, as well as the deposition of material, is possible. Thus, reference to vapor deposition processes in describing the instant invention may also be considered to include any heat induced chemical vapor deposition or etching process.

Referring to the FIGURE, there is shown a partial sectional view of a vapor deposition reactor, generally designated 10, comprising an opaque quartz cylinder 12, which is capped at both ends by hollow plates 14 and 16 made of stainless steel, through which cooling water may be circulated, by means not shown, defining a reaction chamber 18. The reaction chamber may, for example, be 9 inches in diameter and 18 inches high. Tie rods 20, with the aid of O-rings 22 and 24, enable chamber 18 to remain airtight during the vapor deposition process. The lower plate 16 and O-ring 24, are attached to a hydraulic cylinder (not shown) which opens and closes the reactor. Situated within the reaction chamber 18, and mounted on a fused-quartz rod 26 is a substrate holder, graphite susceptor 28, having mounted around its circumference, and substantially parallel to the chamber's longitudinal axis, as defined by rod 26, a plurality of substrates 30 upon which deposition is desired. The susceptor may be cut from commercially available high purity graphite and is substantially in the form of a hollow right cylinder having a wall thickness of about three-eighths inch. The substrate holder is preferably tapered about 3.degree. from bottom to top and is counterbored in order to provide recesses to support substrates. The susceptor may be mounted on rod 26 by a star plate.

Supported by the top plate 14 and extending across the horizontal cross section of the chamber, there is provided a gaseous phase inlet means including a gas distribution baffle 38 forming, with plate 14, a first plenum 36. Gaseous phase materials, reactant gases SiC1.sub.4 and H.sub.2, generally designated by arrow 32, are introduced through tube 34 into the plenum 36. Plenum 36 and gas distribution baffle 38 together provide a means for evenly delivering the reactant gases 32 over substantially all of the horizontal cross-sectional area at the top of the reaction chamber 18. Gas distribution baffle 38 may be constructed from a perforated plate or a sintered material having a gas resistance sufficient to develop substantially uniform back pressure to maintain even gas distribution over the entire surface area of the baffle, and thereby deliver a uniform mass flow into substantially the entire horizontal cross-sectional area of the chamber 18. Baffle 38 may be one-eighth inch thick sintered stainless steel filter plate having an average pore size of 10 microns. Additionally, gas distribution baffle 38 may be a perforated metal plate providing that the proper number and size holes may be provided to achieve the desired pressure drop and gas distribution in the reaction chamber 18.

Also shown is a heat shield 40 which may be mounted on the reaction chamber side of gas distribution baffle 38. The purpose of heat shield 40 is to reflect energy radiated from the heat shield 40 is to reflect energy radiated from the heated susceptor 28 which may prove harmful to baffle 38, depending upon the material of which the baffle is constructed. Heat shield 40 is merely a thin stainless steel, or molybdenum plate, about 0.040 inch thick, having a number of 0.081-inch diameter holes 41 drilled on 0.25-inch centers. The plate is constructed such that it will not substantially disturb the gas flow through the chamber but will effectively prevent baffle 38 from overheating and perhaps out-gassing or decomposing. It should be understood that the addition of heat shield 40 is merely optional as it is used, or not used, depending upon the temperature at the top of the reaction chamber and the material of which gas distribution baffle 38 is made.

The gaseous materials, after passing through heat shield 40, enter chamber 18 having a planar velocity front--i.e., the gas velocity at all points on a horizontal cross-sectional area of the chamber is constant. Because susceptor 28 is a thin walled cylinder and substrates 30 are substantially flush with the outer surface of the susceptor, little resistance is met by the gas as it passes over susceptor 28. Thus, a substantially planar velocity front is maintained throughout the length of chamber 18.

After passing over the susceptor 28, reactant gases leave the bottom of the chamber 18 through the following means. Mounted in the bottom of the reaction chamber, and extending across substantially the entire horizontal cross-sectional area of the chamber is a gaseous phase exit pressure baffle 42 which like gas distribution baffle 38 may be made of a sintered or porous material. Although it is desirable to provide the same pressure drop across exit pressure baffle 42 as that provided by gas distribution baffle 38, it is preferable that the porosity of baffle 42 be greater than baffle 38. The reason for this is twofold; first, because various deposits may tend to form in the pores of exit pressure baffle 42 thereby gradually increasing its resistance to gas flow, and second, because it is important only to maintain the planar velocity front until the gaseous materials have passed the susceptor, it is only necessary to maintain a pressure drop across exit pressure baffle 42. Therefore, the gas resistance presented by baffle 42 is substantially less than that of gas distribution baffle 38, on the order of one-twentieth. This may be achieved by utilizing a perforated plate, or sintered material, that is more porous, or less dense, than utilized at the inlet of the chamber. Pressure baffle 42 is mounted on bottom plate 16 and maintained by rings 44 and 46. Exhaust gases are passed from a second plenum 48 through exhaust tubes 50 and delivered to the atmosphere or a reclamation process.

In order to raise and maintain the temperature of the substrate surfaces to the proper reaction temperature, a heating means is required. An RF source is preferred, Although a resistance heater may also be used. An RF generator, not shown, is used to inductively heat susceptor 28 to the required temperatures. The generator is coupled to a water-cooled helical coil 52 which is permanently positioned outside quartz cylinder 12. It will be noted that this arrangement of a cylindrical load coupled to the helical RF coil 52 provides excellent heating efficiency Since all points on the circumference of the susceptor 28 are the same distance away from the RF coil, temperature uniformity can be readily established in the horizontal direction (within a row of substrates 30 on susceptor 28). To achieve temperature uniformity in a vertical direction, the coil spacings are adjusted. The susceptor 28 is rotated at a rate of approximately 3 r.p.m. by motor 54 to maintain temperature uniformity of .+-. 5.degree. C. at a temperature of 1130.degree. C. (which is the preferred temperature selected for the aforedescribed reaction) over the entire circumferential surface area of susceptor 28. For a more complete description of the structural details of the graphite susceptor 28, reference is made to the aforementioned patent.

EXAMPLE

In order to show an example of the operation and results achieved by the instant invention, the following description of the reactor's operation and performance is provided.

The basic operating procedure of the instant invention is best described in terms of the specific example of its operation as already referred to above--i.e., the hydrogen reduction of silicon tetrachloride.

Substrates 30, highly polished semiconductor wafers, are loaded onto graphite susceptor 28 while bottom plate 16 is in the previously referred to lowered position. During this time, it may be desirable to allow a nominal flow of inert gas, such as argon, to enter inlet tube 34 in order to maintain chamber 18 in a relatively clean condition. Bottom plate 16 is then raised in order to close reactor 10 for the deposition process. Chamber 18 is then purged for about one minute by an inert gas, preferably argon, which is maintained at any desirable flow rate which will insure adequate purging. Due to the high cost of argon, it is acceptable to utilize a purge rate of about one-tenth that of the reactant gases to be referred to later. The rate used in the preferred embodiment should be sufficient to produce a streaming velocity in excess of the diffusion velocity for impurities in the purging gas.

As the purge gas passes into plenum 36, it is uniformly distributed across the surface of gas distribution baffle 38 providing a back pressure of about 2-4 p.s.i. and a substantially planar velocity front within chamber 18. Due to the relatively wide chamber cross section, as compared with its length, the formation of a boundary layer, caused by frictional contact between gas flowing through the reaction chamber and the internal surfaces of quartz cylinder 12, is for all practical purposes, insignificant.

Because the purged gas enters chamber 18 with a planar velocity front and because of the low gas resistance presented by susceptor 28, reaction chamber 18 is effectively purged in a very short period of time.

After the chamber has been purged, the inert purge gas is replaced by hydrogen at a flow rate of about 150 liters per minute for a period of about 2 minutes to displace the purge gas and establish a total hydrogen ambient for the vapor deposition reaction. This rate will produce a streaming velocity in excess of the diffusion velocity for impurities in the hydrogen gas. The RF heating coils are energized while additional hydrogen is passed through the chamber for about 7 minutes while the substrates are raised to the reaction temperature of 1130 .+-. 5.degree. C., Just prior to admission the reactant gas flow is vented to purge the feed system and establish equilibrium flow before it is injected into the reactor. For a 7-inch long susceptor the SiC1.sub.4 is admitted to the reaction chamber in a hydrogen carrier at a rate of about 150 liters per minute in a SiC1.sub.4 /H.sub.2 ratio of about 0.01 for 6-14 minutes depending upon the film thickness desired. In general, higher streaming velocities are required for longer susceptors.

The gas distribution baffle 38 and exit gas pressure baffle 42 act on the reactant gases in the same manner as described above with reference to the purge gas. Additionally, due to the fact that the reactant gas concentration is substantially equal throughout the cross-sectional area of the chamber, because of the relatively short length of susceptor 28, uniquely uniform deposition of reaction byproducts are obtained.

After the desired deposition time has elapsed hydrogen alone is passed through the chamber for about 15 seconds to remove all traces of SiC1.sub.4 .

The RF power is then turned off and the substrates are cooled in hydrogen for about 6 minutes. Finally, the hydrogen is purged from the chamber with argon which further cools the substrates for about 2 minutes. The reactor is opened and the coated substrates are removed from the susceptor.

A summary of the improvements achieved by the novel inlet and exit system of the subject invention, as compared with the prior art barrel reactor is given below.

The dust particle count of particles(greater than one-half micron) in the chamber, as determined by a commercial dust counter, is about 100 particles/ft..sup.3. This is a 1,000 to 1 reduction from the count of 100,000 particles/ft..sup.3 as found in a reactor not equipped with the novel inlet and exit system.

The uniformity of deposit, as determined by the infrared interference technique, for a 6-micron film, previously .+-. 0.5 microns on a single substrate, was reduced to .+-. 0.1 microns for all substrates within the chamber.

The presence of spikes found on substrates was reduced from about 80 to 100 per substrate to about one or two per substrate.

The duty cycle for the entire deposition operation was reduced by a factor of from two to three times that required by prior art reactors due primarily to the efficient purging action of the planar gas velocity front.

In summary, what has been disclosed is a novel structure for a vapor deposition reactor which significantly improves the quality of vapor deposition products. A structure which enables shortened duty cycles due to more efficient purging of the reaction chamber both before and after deposition. Such a reactor includes a gas distribution baffle which delivers gases to the reaction chamber uniformly throughout its horizontal cross section and a pressure baffle which assures uniform removal of the gases from each portion of the horizontal cross-sectional area of the chamber at a uniform rate.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. Apparatus for performing vapor transport processes on substrate surfaces utilizing gaseous phase materials, comprising:

a reaction chamber having a longitudinal axis;

a substrate holder for maintaining substrate surfaces substantially parallel to said longitudinal axis;

heating means for uniformly heating substrate surfaces to the reaction temperature;

gaseous phase inlet means located at one end of said chamber comprising: a gas distribution baffle, forming a plenum, said baffle extending substantially throughout the cross-sectional area of said chamber, said baffle capable of providing sufficient gas resistance to develop substantially uniform back pressure across said baffle, and said baffle also capable of delivering substantially uniform mass flow into said chamber with a planar velocity front, said velocity front moving parallel to said chamber axis and said substrate surfaces; and

a gaseous phase exit pressure baffle located at the other end of said chamber and extending over substantially all of the cross-sectional area of said chamber, said exit baffle capable of providing substantially uniform gas resistance for maintaining a uniform mass flow rate per unit area across said other end of said chamber to prevent recirculation of the gaseous phase materials and reaction byproducts in said chamber.

2. Apparatus as defined in claim 1 wherein said gas distribution baffle and said exit pressure baffle are perforated steel plates.

3. Apparatus as defined in claim 1 wherein said gas distribution baffle is a sintered steel plate and said exit pressure baffle is a perforated steel plate.

4. Apparatus as described in claim 1 wherein the gas resistance of said exit pressure baffle is about one-twentieth that of said gas distribution baffle.

5. Apparatus as defined in claim 1 wherein said substrate holder is a graphite susceptor.

6. Apparatus as defined in claim 1 wherein said substrate holder is substantially in the form of a hollow right circular cylinder.

7. Apparatus as defined in claim 6 wherein said heating means is an RF coil for inductively coupling energy to said substrate holder.

8. Apparatus as described in claim 7 wherein said substrate holder is rotated in said chamber to insure uniform heating of said substrate surfaces.

9. Apparatus as claimed in claim 1 wherein there is provided a heat shield mounted between said gaseous phase inlet means and said substrate holder to protect said gas distribution baffle from energy radiated by said substrate holder.

10. In a vapor transport reactor including a reaction chamber having a longitudinal axis, a substrate holder for holding substrates having reaction surfaces, heating means for heating means for heating substrates to a reaction temperature, gaseous phase inlet means and gaseous phase exit means, the improvement comprising:

gaseous phase inlet means located at one end of the reaction chamber comprising: a gas distribution baffle forming a plenum, said baffle extending substantially throughout the cross-sectional area of the chamber, said baffle capable of providing sufficient gas resistance to gaseous phase material to develop substantially uniform back pressure across said baffle, and said baffle also capable of delivering substantially uniform mass flow into the chamber, said flow having a substantially planar velocity front moving parallel to the longitudinal axis of the chamber and the substrate surfaces; and

gaseous phase exit means comprising: an exit pressure baffle located at the other end of the chamber and extending over substantially all of the cross-sectional area of the chamber, said exit pressure baffle capable of providing substantially uniform gas resistance to gaseous phase material in the chamber to maintain a uniform mass flow rate per unit area across the chamber to prevent recirculation of gaseous phase materials and reaction byproducts.