FOR CHEMICAL VAPOR DEPOSITION PROVIDING

EXCELLENT CONTROL OF THE DEPOSITION

Background of the Invention The present invention relates most generally to chemical vapor deposition (CVD) reactor systems. The same principles may be applied equally aptly to organometallic vapor phase epitaxy technology.

CVD reactor systems may be used for the deposition of amorphous, homoepitaxial, heteroepitaxial and polycrystalline films of semiconductors, such as silicon or gallium arsenide, insulators and metals on the surface of semiconductor substrates. The underlying principle of this technology is that substrate wafers approximately 1-6 inches in diameter, possibly larger contained in a reactor vessel are heated to the desired reaction temperature and then contacted with a reactant gas. Film layers are deposited as a result of a thermal decomposition of reactant gas compounds as they impinge on the heated substrate.

Processes commonly employed in CVD operations include halide and organometallic reactions, examples of which are:

(1) SiCl4, + 2H2- -» Si + 4HC1 (2a) AsH ₃ -► l/4As ₄ + 3/2H ₂

(2b) Ga(CH ₃ ) ₃ + l/4As ₄ +3/2H ₂ * GaAs + 3CH ₄

It is also possible to dope.these epitaxial .film layers by introducing additional compounds into- the gas phase reactantε- When these compounds decompose, the selected dopant species are incorporated into the growing layer.

The primary goal in the design of CVD reactor systems is the production of high electrical quality

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films, i.e., films uniform in thickness and composition and free of contaminants. Such high quality films are necessary for subsequent fabrication of LSI and VLSI integrated circuits. Film quality demands are particularly stringent for superlattice and hererostructure devices.

Typical prior art reactor configurations are illustrated in Fig. 1. These include horizontal reactors, Fig. la, and vertical reactors, Fig. lb, lc, and Id.

Fig. la is a cross-sectional view of a horizontal CVD reactor 10. Reactant gases flow through a reaction vessel 11 as indicated by a flow arrow 12. An assembly of RF coils 14 heats the quartz reaction vessel 11 to prevent deposition of product materials on the reaction vessel 11. Reactant gases impinge upon a heated substrate 18 where they undergo thermal decomposition depositing a thin film of desired material. The substrate is supported at an angle to the gas flow and heated by a susceptor 19.

Fig. lb is a cross-sectional view of a vertical barrel-type reactor 20 such as that produced by the Spire Corporation, Bedford, MA. Here, reactant gases flow through a reaction vessel 22 in a downward fashion as indicated by a gas flow arrow 23. Reactant gases impinge upon heated substrates 24 and a film is deposited as a result of thermal decomposition of these gases. The substrates 24 are supported on a susceptor .26 and heated by various means. This configuration provides for rotation of the susceptor about an axis concentric with gas flow indicated by an arrow 28. This rotation provides for greater uniformity in deposited films.

Fig. lc is a cross-sectional view of another vertical CVD prior art reactor 30. In this configuration, reactant gases flow downward as indicated by an arrow 32. Contained within a quartz reaction vessel 34 is a susceptor 36 which supports and heats a substrate 37. An RF coil 38 heats the walls of the reaction vessel 34 to avoid deposition of film materials on the reaction vessel walls.

Fig. id is a cross-sectional view of another prior art vertical CVD reactor 40. In this configuration, gas flow is downward as indicated by an arrow 42. Within a containment vessel 44 is a susceptor 46 for support and heating of a substrate 47. Here film uniformity is enhanced by rotation of the susceptor 46 in the direction indicated by an arrow 48.

The key objectives of the geometrical arrangements shown in the prior art devices of Fig. 1 may be summarized as follows:

1. to confine the thermal decomposition of reactant gases to the substrate surface;

2. to provide a uniform spatial deposition rate; and

3. to allow the deposition of discrete multiple layers of the order of 100 Angstroms thickness with sharp, abrupt interfaces between layers of different composition by changing the composition of the input gas at rapid intervals.

Efforts to confine thermal decomposition of reactant gases to the substrate and thus minimize the problem of premature reactive gas losses by reducing peripheral heating of the inlet manifold include a perforated radiation shield. It consists of a pair of plates coated with a heat reflecting film which is

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interposed between the inlet gas manifold and the substrate to protect incoming reactant gases from thermal radiation emanating from the susceptor and substrates, as disclosed in U.S. Patent No. 3,196,822. This radiation shield also allows reaction product gases to pass out of the reactor. Further, this patent teaches that such a radiation shield helps to prevent forced convection in the vicinity of the substrate. Susceptor rotation, as depicted in Fig. lb and Id, is another commonly used mechanism for attaining film spatial uniformity. Such rotating susceptor designs require leak proof rotating seals to minimize system contamination and thus significantly complicate reactor design. For the most part, these existing processes have not been fully successful in production of spatially uniform, high quality wafers. A common characteristic of these processes is that they fail to address adequately the problem of balancing forced convection (i.e., flow due to the inlet gas stream) against flows produced by buoyancy. For example, Fig. 2 shows a schematic illustration of the so-called Spire reactor (Spire Corporation, Bedford, MA), together with the recirculating flow loops which may exist in such a system. The recirculating loops 21 marked with the double arrows 25 are due to natural convection, which, if not properly controlled, e.g., by shaping the reactor vessel and carefully adjusting the gas flow .rate, could lead to marked -non-uniformities in the deposition rates. The..other geometric arrangements that are currently employed in practice suffer from similar drawbacks.

A number of mechanisms for improving delivery and distribution of inlet reactant gases exist in the

literature. One approach focusses on the geometric configuration of the reactant gas supply inlets. U.S. Patent No. 4,033,286 seeks to achieve film thickness uniformity by injection of reactive gases through multiple jets positioned at uniformly distributed locations. Another arrangement consists of stationary wafer holders positioned radially about a perforated inlet gas tube (U.S. Patent No. 4,421,786). Wanlass (U.S. Patent No. 4,649,859) has designed a reaction chamber to achieve a helical gas flow pattern.

Susceptor rotation has been utilized to produce orderly laminar flow over the substrate surface (U.S. Patent No. 4,772,356) which insures that each portion of active gas impinges only once on a substrate yielding a uniform deposit.

MacDonald (U.S. Patent No. 3,916,822) utilized a perforated radiation shield between the reactant gas inlet manifold and the substrate to reduce convection and protect inlet gases from heat emanating from the susceptor. A quartz insert has been introduced into the inlet gas stream (Landgren et al., J. Crystal Growth, 77 (1986) 67-72) to spread the gas flow uniformly across the susceptor. The CVD reactor specified by U.K. Patent Application GB 2181460 achieves stagnation point flow by distributing reactant gas through a multiplicity of apertures and also utilizes susceptor rotation to enhance further film uniformity.

However, Berkman in U. ^* S. Patent No. 4,430,149 asserts that a system employing perforated or porous members is susceptible to clogging and subsequent breakdowns. Further, he observes that when such porous members are fabricated from quartz, they tend to be delicate and easily broken. Metal porous members, moreover, are likely to react with chemical source gas

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resulting in production of contaminants which can compromise the purity of substrates.

Wang et al. (J. Crystal Growth 77 (1986) 136-143) state that an optimal reactor design must provide for (1) epilayer uniformity by insuring that the thickness of the gas boundary layer near the substrate is uniform so that deposition occurs at the same rate for all points on the substrate; (2) interface abruptness by establishment of a flow field in the reactor which is free of laminar vortices, thus minimizing gas residence times. These workers state that a uniform boundary layer can be achieved by utilizing the stagnation flow that occurs when fluid impinges normally on a flat surface or when the inlet gases are introduced through a porous plug. The porous plug which they describe in their study consists either of a 3.8 or 7.6 cm. diameter metal screen. These requirements are confirmed by their gas visualization studies. Summary of the Invention

A reactor, according to the invention, includes a porous distributor interposed between the inlet gas stream and a substrate. The distance between this porous distributor and the substrate is small compared to a dimension of the substrate. This distance is optimized to reduce premature thermal decomposition of the gaseous reactant precursor. A distributor to substrate distance of between 5-30 mm may be optimal in many circumstances, e.g., when the substrate and the distributor have diameters in the 150 mm range. ^' A uniform thickness plug insures a substantially uniform gas flow rate toward the substrate. In an important aspect of the invention, the porous distributor has a variable thickness for regulating the carrier gas flow

velocity profile at the substrate. In yet another aspect of the invention, the porous plug is cooled by embedded cooling coils.

With gas entrainment minimized and recirculation eliminated by the short distance between the porous distributor and substrate, it becomes possible to obtain the very abrupt changes in composition required for the deposition of multilayer structures such as those used in quantum well and superlattice structure based devices for electronic and opto-electronic applications.

Most noteworthy is that the reaction vessel of the invention provides for the introduction of reactant gas through a porous plug distributor positioned close to the substrate compared to a dimension of the substrate which insures spatially uniform gas flow toward the substrate with the uniform thickness plug, configuration. Since this distance between the porous plug and susceptor is relatively small, the active volume containing reactant gases is also small. Thus, the rapid switching of gases necessary to produce abrupt interfaces is readily accomplished. This porous plug provides the largest resistance to gas flow of any element in the reactor and the- variation in its thickness can be used to impose any desired approach velocity of the carrier gas stream, which contains the gaseous reactants. Gas flow uniformity near the substrate surface may be further ^• enhanced by • introduction of slow susceptor rotation. .Provision is -' made for the cooling of this porous plug distributor either through the flow of incoming reactant gases or by installation of cooling coils within the porous plug.

The reactor of the invention may be used in the performance of atmospheric pressure, low pressure and

plasma assisted CVD processes.

Brief Description of the Drawing In the drawing:

Fig. la is a cross-sectional view of a prior art horizontal CVD reactor.

Fig. lb is a cross-sectional view of a prior art vertical barrel type CVD reactor, such as that produced by the Spire Corporation, Bedford, MA.

Fig. lc is a cross-sectional view of another vertical CVD reactor design.

Fig. Id is a cross-sectional view of a vertical CVD reactor.

Fig. 2 is a schematic illustration of a section of a prior art Spire (Spire Corporation, Bedford, MA) type reactor with recirculating flow loops.

Fig. 3a is a cross-sectional view of a CVD reactor according to the invention incorporating a porous plug diffuser of uniform thickness.

Fig. 3b is a cross-sectional view of a CVD reactor according to the invention incorporating a porous plug diffuser where the thickness variation is concave.

Fig. 3c is a cross-sectional view of a CVD reactor according to the invention incorporating a porous plug diffuser where the thickness variation is convex.

Fig. 3d is a cross-sectional view of a CVD reactor according to the invention where two or more reactant gases are combined in a mixing zone before . passing through the porous plug.

Fig. 3e is a cross-sectional view of a CVD reactor according to the invention incorporating a porous plug diffuser in a plasma assisted CVD process. Fig. 4 is a detail view of the diffuser

assembly showing an alternate embodiment of the porous plug, allowing for external cooling (e.g., by embedded cooling coils) .

Fig. 5 is a detail view of the diffuser assembly showing an alternate embodiment of the porous plug where the outer surface of the porous distributor is coated with a high emissivity coating.

Fig. 6 is a graph showing computed streamline patterns in one side of a reactor of the invention. Fig. 7 is a graph depicting computed temperature profiles in one side of an apparatus constructed according to the invention.

Fig. 8a is a graph of the computed deposition rate as a function of radial distance from the center of a wafer substrate.

Fig. 8b is a superposition of the deposition rate profile of Fig. 8a on a wafer substrate. Description of Preferred Embodiment The present invention is designed to produce films having superior uniformity -with respect to both thickness and composition than the prior art reactors previously described.

With reference to Figs. 3a, 3b and 3c a cross-section of a CVD reactor 50 according to the present invention is shown incorporating a porous plug distributor 54. In the figures the small distance between distributor and substrate has been exaggerated for clarity. ^' This plug may be Of uniform thickness ^' (Fig. 3a) or ^* may have concave (Fig\ 3b) ^' or convex (Fig. 3c) shape. A suitable material for the porous distributor plug 54 is a porous fritted glass or porous refractory ceramic such as alumina, silica or zirconia. In operation, a reactant gas mixture enters the CVD reactor 50 at an inlet manifold 55. A direction of the

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gas flow is given by an arrow 56 which is upward in these Figs. 3a-3e and in the preferred mode of operation. Upward gas flow is preferable so as to minimize buoyancy effects. It may also be downward in an alternative mode of operation. Reactant gases then pass through the porous plug distributor 54 which serves to insure that a substantially spatially ^" uniform gas flow rate is directed toward a substrate 57 for the uniform thickness plug (Fig. 3a). The concave (Fig. 3b) and convex (Fig. 3c) plugs are used to impose a desired, spatially varying gas velocity profile at the substrate surface. The substrate 57 is supported on a susceptor 58 which is stationary and serves to maintain the substrate 57 at an elevated temperature such that pyrolysis of the reactant inlet gases may occur to result in deposition of a film of desired composition.

In the preferred embodiment, the distance between the variable thickness porous distributor 54 and the substrate 57 is small compared to a dimension of the wafer substrate 57. In particular, it is preferred that the distance between distributor and substrate be less than the diameter of the substrate wafer and preferably significantly less than the diameter of the substrate wafer, e.g. less than one half of the wafer diameter. Further, the dimensions of the porous distributor 54, susceptor 58 and a containment vessel 59 should be comparable. The distance between the porous plug distributor 54 and the substrate 57 /susceptor 58 assembly is also selected to minimize the temperature of the porous distributor 54 where, more specifically, this distance is in the range of 10 mm for a suceptor having a diameter in the range of 160-180 mm.

The porous plug 54 may have a convex (Fig. 3c) or concave (Fig. 3b) shape depending upon the desired -

gas velocity profile.

The porous plug diffuser 54 can be used to restore controlled, spatially uniform gas flow after mixing of constituent reactant gases in deposition processes involving two or more reactant gases. Plural reactant gases may be mixed before passing through the distributor 54 by a stirring apparatus 51 shown schematically in Fig. 3d.

The porous plug diffuser 54 can also be used to condition gas flow in a reactor where a plasma 51 is created between an upper electrode 52 and a lower electrode 53 in a plasma assisted CVD process as shown in Fig. 3e.

In operation, the temperature of the porous plug distributor is minimized in order to reduce premature heating and pyrolysis of the inlet reactant gases. This cooling may be accomplished simply by the flow of the reactant gases over the porous plug distributor. Cooling may also be accomplished as depicted in Fig. 4. Here an array of cooling coils 60 are embedded in the variable thickness porous plug distributor 54. A gas, such as helium (not shown) is circulated through the coils by standard circulation apparatus, also not shown. During operation of the reactor 50 it is desirable to reduce the rate of radiative heat transfer between the heated substrate 57 and the porous plug distributor 54. An outer surface 61 (Fig; 5) of the variable thickness porous distributor ^" -54 ^' is treated with a coating 62 of an inert, noble metal such as gold, having high e issivity.

Table 1 shows a typical set of operating conditions for the production of gallium arsenide thin films.

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TABLE 1

A Typical Set of Operating Conditions

Inlet velocity 50 mm/s

Diameter of inlet 140 mm Diameter of susceptor 140 mm

Diameter of reactor 154 mm

Distance between the inlet and 20mm susceptor surface

Reactor pressure 500 Torr Mass fraction of tri-methyl gallium .02 in gas stream

Mass fraction of arsene in gas stream .16

Carrier gas Hydrogen

Temperature of top susceptor surface 923 K Temperature of reactor walls and susceptor 350 K side wall

Temperature of inlet gas stream 350 K

Total mass flow of inlet gas stream 3 x 10" ⁵ kg/s We emphasize that this gallium arsenide deposition is cited as an example and that the technique possesses broad applicability for the formation of metallic, semiconductor or other thin films. However, the preferred application of the technique is likely to be for the production of very high quality wafers for semiconductor or optical detector applications.

The advantages of this invention become clear when we examine Fig. 6 and Fig. 7. A representative computed streamline pattern 70. shown in Fig. 6 demonstrates the improved gas flow which can be achieved with this design. Gas recirculation is absent. A representative steep temperature profile 80 shown in Fig. 7 demonstrates the improved temperature control possible with the apparatus of this invention. Most importantly, one can obtain extremely uniform spatial deposition rates (up to 0.06 m from the center of a wafer substrate for this particular computer simulation) with this reactor configuration as evidenced in the plot shown in Fig. 8. The actual numerical value of the

deposition rates computed here is within the range normally encountered in many vapor phase processing applications in the semiconductor industry. It is possible that higher deposition rates may also be produced in this equipment, while maintaining wafer quality.

The gains realized by incorporating the porous plug distributor into the inlet gas port of a CVD reactor apparatus are demonstrated in computer simulations of gas flow, temperature profiles and deposition rates depicted in Figs. 6-8. Examination of these figures shows how the invention succeeds in fulfilling some of the key objectives of CVD reactor design outlined above. It is evident that use of the present invention permits confinement of thermal decomposition to the substrate surface, as demonstrated in the computed temperature profiles of Fig.. 7, while elimination of gas recirculation and entrain ent, as reflected in the computed streamline patterns of Fig. 6 is accomplished.

According to the method of the invention, a uniform film is grown on a substrate by chemical vapor deposition where gas flow may be directed upward or downward with respect to the substrate position. It is particularly significant that this technique can be useful for the growth of superlattices where sharp interfaces between layers are required. This technique will allow for production of such multilayers by rapidly changing input gas composition in a continuous growth process. ^~ '

What is claimed is:

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