US 3661636 A
A method for forming uniform and smooth surfaces which can be used in any chemical vapor transport system and in general in any deposition system in which chemicals in a gas phase are reacted to cause deposition onto a substrate. A zone of operating conditions exists in which smooth uniform surfaces are obtained, while at the same time ridge growth and defect growth is minimized. By changing substrate temperature and the concentrations of input reactants, ridge growth and defect growth in deposited films are substantially reduced.
Claims available in
Description (OCR text may contain errors)
I United States Patent 51 3,661,636
Green, II et al. 1 May 9, 1972  PROCESS FOR FORMING UNIFORM References Cited AND SMOOTH SURFACES UNITED STATES PATENTS  In ento s: James M. Green, e York; Thomas 3,301,213 1/1967 Grochowski et al. ..1 17/106 AX O. Sedgwick, Crompond; Victor J. Silves- Mount KiSCO, all Of Primary Eraminer-Ralph S. Kendall  Assignee: International Business Machines Corpora- A'mr'le-YHamfin and Jancm and Jackson Stanland tlon, Armonk, NY. ABSTRACT 0') [u] Fllcd' Apr' 1970 A method for forming uniform and smooth surfaces which can I 21 I A l. No.1 30,789 be used in any chemical vapor transport system and in general in any deposition system in which chemicals in a gas phase are reacted to cause deposition onto a substrate. A zone of operat- 152] Cl l 17/20! 1 17/106 l ing conditions exists in which smooth uniform surfaces are ob- 5] I t Cl CBC 13/00 tained, while at the same time ridge growth and defect growth 2 R 201 is minimized. By changing substrate temperature and the conl o c centrations of input reactants, ridge growth and defect growth in deposited films are substantially reduced.
14 Claims, 4 Drawing Figures 1 s .E 10.0 SL/Hg E 0.06 5 0.04 E A 0.02 1 .0
I I'- g 0 L1 0: L9
l 1 1 l l SUBSTRATE TEMPERATURE (Cl Patented May 9, 1972 FIG. 1
Ge/Hg E V E5 126% FIG. 3
SU BSTRATE TEM PERATU RE BY 1600 T2 WWW Hm SN1WTQM MW WH N HM S w mm mmm IJTV 4 G F 2 f m M M m n w m W r n u 2 m m l A U v 0 0 0 1 0 izm I565 AGENT SUBSTRATE TEMPERATURE 1C) BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to chemical vapor deposition (CVD) processes, and more particularly to chemical vapor deposition of films having surfaces which are as smooth as those upon which deposition occurs, and which follow the topology of the underlying surface.
2. Description of the Prior Art In the preparation of semiconductor devices, including monolithic and integrated circuitry, CVD techniques are widely used. These processes enable the deposition of one single crystal layer upon another. In this manner, multilayer single crystal devices can be fabricated.
Although CVD processes are widely used in the fabrication of many semiconductor devices, it is a common problem that the use of such processes frequently leads to surfaces which are less uniform in thickness and less smooth than those upon which the deposition occurred. Because subsequent semiconductor fabrication steps such as photomasking and photolithography require smooth surfaces, prior epitaxial deposition techniques have not been entirely satisfactory.
The reaction conditions for CVD processes can be considered to involve two regions of growth. One region is such that the growth of the deposited layer is mass transport limited. This means that the growth rate of the depositing atoms depends upon only the amount of material reaching the substrate surface per unit time, by any mechanism whatsoever. in this region, growth is temperature insensitive. Another region of deposition is that where the growth is surface rate limited. In this region, the growth rate of the deposited layer does not depend upon the amount of material reaching the surface per unit time, but instead depends upon surface conditions. That is, there is some slow step in the mechanism by which depositing atoms adhere to or react on the substrate surface that determines the rate of growth of atoms on the substrate. There is some chemical mechanism by which atoms link onto the surface or are removed from the surface which determines the growth rate (or linking rate) of atoms to the surface. There is a critical temperature in the ,deposition process below which this slow step becomes predominant. The growth rate under surface rate limited conditions is quite temperature sensitive, in contrast with the growth rate under mass transport limited conditions.
Existing deposition processes are operated in the mass transport limited region. There are many reasons for such operation. For instance, this is a temperature insensitive region so that highly complex and sensitive temperature controls are not required. Also, the effect of temperature variations across the entire wafer surface does'not influence growth rate across the surface. This means that smooth surfaces will generally be obtained if operation is in the mass transport limited region. In addition, it is generally the case that high growth rates occur in this region, thereby leading to more economical deposition processes.
With operation in the surface rate limited region, smooth surfaces do not generally result, since deposition rates vary with substrate orientation. Although substrates typically have flat and smooth surfaces with one principle crystal plane exposed, under surface rate limited growth conditions small faceted regions appear spontaneously on the wafer surface due to differing nucleation conditions and subsequent dif ferences in the growth rates of the facet planes exposed. Different amounts of semiconductor atoms will adhere on the different facets of the growth structures, thereby leading to significant surface irregularities.
Previously, reactor deposition conditions have not been changed in order to solve the aforementioned problem of maintenance of surface integrity. Although this problem was evident in almost every semiconductor epitaxial process, the previous attempts to solve the problem did not involve a precise analysis of the mechanism causing the surface irregularities. All previous attempts to rectify the problem did not procede from a correct understanding of the exact physical and chemical reasons for the problems, and consequently were very unsuccessful in facilitating device fabrications.
Included within the general criterion that the integrity of the initial surface be maintained throughout the material additive processes, i.e., the formation of the additional layers, are three basic problems. These are: (a) ridge formations around nonnucleating material; (b) enhanced growth of small defects on wafer surfaces; and (c) contour deposition in which the original surface topology is not maintained. In order to better understand these problems, each will now be considered in more detail. In addition, the prior art attempts to rectify these problems will be discussed.
The first problem area is ridge growth around non-nucleating materials on the substrate surface. Semiconductor processes require the use of many materials in addition to the semiconducting materials themselves. For instance, plastics, tefion compositions, ceramics, nitrides, oxides, carbon, and carbides are some of the more frequently used materials. However, some of these, and others (depending on the semiconductor deposition process and operating conditions), are nonnucleating. That is, semiconductor atoms being deposited will not adhere to these materials. Since growth of a new layer will not occur on a non-nucleating surface, the depositing atoms migrate to the surrounding semiconductor surface where they will deposit. This deposition increases the local deposit and leads to the build-up of a ridge around the non-nucleating area, thereby causing a surface irregularity.
A particular example involving ridge formation is that which occurs in epitaxial deposition when a pedestal-type structure is to be backfilled. Here, an oxide mask, which is non-nucleating, protects the semiconductor pedestal while a semiconductor material is deposited around the pedestal. What occurs is an increased growth at the edge of the oxide mask, and a ridge is thereby created. The ridge is a surface irregularity which interferes with further device fabrication steps.
Another example in which non-nucleating materials give rise to the formation of a ridge is that which occurs in selective masking. Here, deposition is to be through a patterned mask. However, enhanced growth of depositing atoms occurs at the boundary of the mask pattern. Again, this creates a surface irregularity which hinders further device fabrication.
The prior art was not successful in eliminating ridge growth around non-nucleating materials. In general, scientists were discouraged from using refilling or back-filling techniques, since it was almost inevitable that large ridges would develop. In an attempt to eliminate such ridges, the deposition reaction was reversed, as by adding l-lCl gas, in order to vapor-etch the ridge which would be formed during deposition of the epitaxial layer. However, this is an unsatisfactory solution, since it does not eliminate formation of the ridge but only attempts to remove it after formation. Another attempt to avoid the ridge formation problem involves the use of a subsequent polishing step in order to remove the ridge. As is evident from these examples, the prior art accepted the undesirable formation of the ridge and then tried to remove it.
Another problem mentioned above is that which relates to the enhanced growth of an existing small defect on a surface which is to serve as a substrate for epitaxial deposition. Such a defect may be caused by a number of mechanisms, such as vapor-liquid-solid growth in conjunction with an impurity particle. If the initial defect is small enough, and if the deposition of additive layers maintains the integrity of the substrate smoothness by not increasing the size of the defect, device fabrication is possible. However, it is generally the case that deposition of layers causes the defects to grow at a rate faster than the rest of the deposited layer, with the result that large spike-like protrusions are formed. These protrusions severely inhibit further processing. For instance, it is difficult to use a mask on such a surface, as the protrusions will damage the mask and will cause mask alignment problems.
Prior art techniques to avoid or eliminate the formation of defect protrusions involve two approaches. The first is a purely mechanical one in which the protrusions are intentionally broken, as maybe done by pressing and/or rotating a flat surface against the wafer. However, this approach has disadvantages in that there is a high risk of damage to the wafer when protrusions are mechanically removed. Also, mechanically removing the protrusions is an extra process step. The second technique to eliminate the problem of enhanced defect growth is a preventative one in which ultra-clean laboratory conditions are used to eliminate the source of the initial growth of small defects on the substrate. Here, there is an attempt to eliminate all impurities with the hope that the defect will not occur. However, 100 percent efficiency is never achieved and defects do occur. Subsequent deposition causes enhanced growth of these defects, and they have to be removed mechanically, as explained previously.
The third problem mentioned above is that which concerns contour epitaxy. By this it is meant that the layer to be deposited should conform to the topology (i.e., contour) of the substrate surface. lf the substrate surface is uneven (for instance, if it contains grooves) it is desirable that the deposited layer follow the contour of the substrate and have a uniform thickness across the total substrate area. However, it is frequently the case that the deposited epitaxial layer is of varying thickness, so that surface irregularities develop. As with the other problems considered here, the prior art has not been able to adequately provide contoured epitaxial surfaces which follow the topology of the underlying substrate.
Accordingly, it is a primary object of this invention to provide a process of device fabrication in which the integrity of the initial surface'is maintained.
It is another object of this invention to provide an epitaxial deposition process which will retain the smoothness of the initial surface.
Still another object of this invention is to provide a deposition process which will eliminate both ridge growth and enhanced defect growth while maintaining smooth semiconductor surfaces.
A further object of this invention is to provide an epitaxial deposition process suitable for maintaining the topology of an initial surface during contour epitaxy deposition.
SUMMARY OF THE INVENTION ing epitaxial deposition. It eliminates problems due to ridge growth around non-nucleating materials and due to enhanced growth of defects existing on substrate surfaces, while at the same time providing smooth (non-faceted) surfaces.
The method can be applied to any chemical vapor transport system, to glow discharge reaction systems (in which gaseous reactants are ionized by electrical energy to form deposits on the substrate),'and to plasma anodization systems (in which one of the reactants is located within the film to be anodized and the others are in the gas phase). In general, the method applies to these processes and to any process in which chemicals must be brought to a substrate in a gas phase, for reaction in the vicinity of the substrate. The materials to be deposited can be metals or semiconductors, including single elements,
such as germanium (Ge) or silicon (Si), or compounds, such as gallium arsenide (GaAs). In the latter case, the input chemicals will probably include several gaseous species, which may contain either gallium or arsenic, in a manner well known tothose of skill in this art. The substrates can be the same material as the film deposits, or different materials, and the deposits may be epitaxial.
' Essentially, the method defines a zone of epitaxial deposition between surface rate limited conditions and mass transport limited conditions. Epitaxial deposition within this zone provides smooth (non-faceted) surfaces and defectless growth. Here, growing deposits have no defects and ridges are not formed around non-nucleating surfaces. Generally, this means that all surface irregularities are below 0.1 micron.
to provide more economical growth, smooth (non-structured) surfaces, and less transient temperature control requirements, the present invention teaches that there is a zone of operating conditions which will provide both defect-free growth and non-faceted surfaces even though the process may be operated in the surface rate limited region, or near that region.
In the operation of a process according to this invention, if it is desired to operate at a given temperature, the ratio of the concentration of the input reactants containing the materials to be deposited to the concentration of the carrier gas is first chosen to place the deposition in the mass transport limited region of operation. This will provide non-structured surfaces, although defects (including ridges) may be present. The concentrations of these input reactants and the carrier gas are then changed so that the ratio of these concentrations is increased. This change in ratio continues until structured (faceted) surfaces are obtained. As an example, if germanium is to be deposited from input reactant GeCl and hydrogen is the carrier gas, the GeCh/H ratio (for simplicity indicated as GelH is changed until the film being deposited exhibits oriented growth facets characteristic of the orientations of the substrate. Then, the concentration ratio is slowly changed in a reverse direction until surface facets disappear and the surface is tolerable for further device processing. At this temperature and these input concentrations, defectless growth and nonfaceted surfaces will be obtained. The final operating point may be in the surface rate limited region of growth, but sufficiently smooth surfaces will be provided without ridge growth and without enhanced defect growth.
Of course, if it is desired, the initial operating point (temperature and concentration ratio) can be such as to place deposition in the surface rate limited region. At this initial point, the deposited material will have facets thereon, but will contain no defects or ridges. Maintaining the temperature constant, the concentration ratio is then changed in a direction which will yield deposits having non'faceted surfaces. However, defects and ridges will begin to develop when the concentration ratio is changed in this direction. The change in ratio is stopped when the defects and ridges begin to occur. At this point, the concentration ratio is changed in a reverse direction until the defects and ridges disappear. A final operating'point of temperature and concentration ratio will be obtained at which the deposited surface is non-faceted, and does not obtain ridges or defects greater than 0.1 micron.
It is also possible to operate at a fixed concentration ratio, while varying the substrate temperature. Essentially, the same processes as those outlined in the preceding two paragraphs are followed. It is only necessary to determine whether the initial' operating conditions will place the deposition in the surface rate limited region or in the mass transport region. For instance, if the concentration ratio is fixed and the first temperature is chosen at which deposition occurs in the mass transport limited region, the deposited surfaces will be non-faceted, but will contain defects and ridges. The temperature is then changed in a first direction in order to move the deposition toward the surface rate limited region. As the temperature is changed, the deposited surfaces will retain their smoothness and the defect growth and ridge growth will diminish. This change in temperature is continued until the surfaces exhibit facets. However, the defect and ridge growth will have disappeared at this point. Then, the temperature is slowly changed in a reverse direction until the facets disappear. It will be found that, at this final temperature and concentration ratio, the surfaces of the deposited material will be free from facets, defects, and ridges.
In a manner analogous to that stated above, the initial operating conditions can be such as to place the initial deposition in the surface rate limited region. This will mean that faceted surfaces having no defects or ridges will be obtained. By changing the temperature in a first direction and holding the concentrations fixed, the surfaces of the deposited material will become more smooth, but defects and ridges will start to develop. When the defects and ridges occur, the substrate temperature is changed in a reverse direction until the deposited surfaces exhibit no defects or ridges. It will be found that such surfaces are free of facets, defects, and ridges at this final temperature and fixed concentration ratio.
The method of this invention depends upon the discovery that a region of operation surrounding the transition between mass transport limited growth and surface rate limited growth exists in which deposited surfaces are smooth, and defects and ridges are eliminated. This region of desired operation can extend far into the surface rate limited region of growth and, with good substrate cleaning processes, may be unlimited in the direction of surface rate limited growth. However, it is recognized that for some device fabrication, ultra-smooth surfaces are not required. In this case, as in all others, it must be decided what type of surfaces will be suitable for further device processing. It may be that ridge growth and defect growth are more hazardous in processing than the presence of surface facets. If this is the case, operation far into the surface RATE limited region may be desirable. In other situations, the presence of surface facets may be the primary consideration. In the latter case, it may be desirable to operate closer to or further into the mass transport limited region. The teaching of this invention allows deposition over a wider range than was heretofore possible, since the operator can balance the deposition to produce surfaces ideally suited for his own device fabrication.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic diagram illustrating the concept of ridge growth around a non-nucleating material.
FIG. 2 is a schematic diagram illustrating enhanced defect growth ofa large dome-like defect.
FIG. 3 is a plot of growth rate versus substrate temperature for a germanium epitaxial system.
F IG. 4 is a plot of growth rate versus substrate temperature for a silicon epitaxial system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. I, the concept of ridge growth is illustrated. A substrate 14 of a semiconductor (such as germanium) has an epitaxial layer 16 of Ge thereon. Epitaxial layer 16 is formed into a pedestal structure 10, on which is located a masking layer 12 of silicon dioxide (SiO Surrounding pedestal is another Ge layer 18, which could be an epitaxial layer also.
Because SiO is a non-nucleating material for depositing germanium atoms, these atoms will not adhere to the SiO mask 12 and will move to the edge ofthe mask where they will be deposited. The increased concentration at the boundary of the non-nucleating material causes a ridge 20 to be formed at the edge of the SiO As the distance from the boundary 22 of the Si0 mask increases, the deposition becomes more planar. The ridges can be quite high (greater than 1 micron) and their presence will seriously interfere with further device processing.
In FIG. 2, there is illustrated a large dome defect which is many times the height of the epitaxial layer 32. Such defects are relatively common on epitaxially deposited germanium and silicon layers. These defect protrusions greatly hinder subsequent device fabrication procedures-especially that of masking. These dome-shaped defects may extend to a height of some 20 times the epitaxial layer thickness and are characterized by geodesic faceting.
The exact origin of these defects is difficult to determine but experimentation indicates that they probably nucleate at the site of some surface contamination. As with the ridge growth problem, if the defect can be kept below approximately 0.1 micron high, further device processing will not be seriously hindered. Generally however, both the defects and the ridges are quite large and will interfere with further processing.
If a small, spike-like crystallite 34 is assumed to initially protrude from the wafer surface 36, the growth rate of the spike is increased over that of the surrounding, flat surface, probably due to the following reasons:
I. The rate controlling step of the chemical reaction is generally considered to be that of diffusion of the reactants through the gas phase to the reaction site (i.e., the rate of epitaxial growth is diffusion limited). Since the apex of the spike is accessible to reactants diffusing from all directions, whereas other regions of the wafer surface effectively receive material only from the volume directly above these regions, more reactant species are attracted to the spikes per unit time than to the surrounding wafer surface.
2. A concentration gradient of the reactant species extends normally to the substrate. Consequently, the apex of the spike is in a region of higher reactant concentration than either the base of the spike or the balance of the wafer surface. Therefore, the chemical reaction for epitaxial deposition proceeds more rapidly at the apex, and large dome-like defects result.
These mechanisms assume that the concentration of the important reactant increases significantly at the site of the defect. Such a spike could nucleate on a particle of non-metallic contamination on the wafer. The depositing semiconductor would subsequently enclose the particle to form a protruding mound. However, other origins may be more likely. For instance, the core of material near the tip of the cone of the defeet was found to have an etch rate different than the balance of the defect. This core may be the result of the alloying of a submicroscopic impurity with the semiconductor, accompanied by a VLS-like (vapor-liquid-solid) mechanism. This would lead to a miniature spike. The small amount of impurity would be deleted so that the VLS growth stops. By that time, a sufficiently large spike would develop for the above-described dome growth mechanism to start.
In both ridge growth and enhanced defect growth, it is important to eliminate these problems in order to obtain good surfaces. A good surface is defined as that which enables further processing without any compensation (i.e., cleansing of the surface, etc). Generally speaking, a ridge or a defect having a height as small as 0.5 micron is harmful to further device processing. Of course, the harmful effects of the defects depend on the size of the components and the resolution of these components. For some devices, larger defects may be tolerable.
As examples of chemical vapor transport systems using this invention, FIGS. 3 and 4 illustrate deposition of Ge by H reduction of GeCl and deposition of Si by I-I reduction of SiCl,,, respectively. Although FIG. 3 relates to a germanium deposition system and FIG. 4 relates to a silicon deposition system, it is to be understood that such graphs can be generated for any deposition system. For instance, the method of this invention is applicable to the deposition of gallium arsenide on germanium, and any other deposition systems.
The deposition systems to be used for the practice of this invention are conventionally well known. Also, the equations determining these depositions are well known and are described in the literature. For instance, reference is made to an article by V. J. Silvestri, entitled Growth Rate and Surface Morphology Studies in the GeCh-I-I System, which appeared in the J. Electrochemical Society, Vol. 1 16, No. 1, Jan. I969, p. 81. In this article, the author describes a germanium epitaxial deposition system and a method for operation with this system. Similar systems are known for silicon epitaxy, as can be seen by referring to E. G. Bylander, J. Electrochem. Soc. 109, 1171 (1962); and A. Reisman, M. Berkenblit, J. Electrochem, Soc. 1 13, 146 (1966).
Referring in more detail to FIG. 3, the growth rate of deposited germanium is plotted as a function of substrate temperature for various input reactant concentration ratios (Ge/H The dashed line I which intersects each growth curve represents the crossover line between the surface rate limited region (A) and the mass transport limited region (B). This dashed curve generally demarks conditions for obtaining smooth surfaces or structured surfaces. In the mass transport limited region, smooth (non-faceted) surfaces are-obtained while in the surface rate limited region structured (faceted) surfaces are obtained and these facets are dependent on substrate crystallographic orientation. That is, in the surface rate limited region of operation, there is a growth rate dependence on the variations of substrate orientation and on substrate temperature, while in the mass transport limited region there is no growth rate dependence on surface orientation and temperature. Film deposits are considered structured" when they exhibit growth figures (facets) characteristic of the particular substrate orientations. For instance, triangular facets develop on  surfaces, while square facets develop on  surfaces. These facets are easily seen by the observer during deposition. "Smooth" surfaces exhibit essentially a mirror finish and are free of these growth figures.
The dash-dot lines L1 and L2 on each side of curve L define a zone of deposition in which smooth surfaces can be obtained, while at the same time maintaining ridgeless growth with no enhanced defect growth. Whereas the prior art used film deposition only in region B, in order to obtain non-structured surfaces, (even though ridge growth and enhanced defect growth would occur), this invention teaches that elimination of defect growth and ridge growth can occur while smooth surfaces are obtained if deposition is in accordance with selected process steps. That is, contrary to the teaching of the prior art which taught away from the present invention in order to reduce ridge growth, a detailed study of the growth mechanism of ridges and defects has surprisingly indicated that operation toward and into the surface rate limited region of deposition is the way to eliminate such spurious growth. With a germanium system, good surfaces together with ridgeless growth and no enhanced defect growth occur for deposition having Ge/H between 3.6Xl and 9.l and at temperatures between approximately 570 and 850 C respectively, Although data points have not been established over this entire range of temperature and concentration ratios, theory indicates that curves Ll and L2 will follow the paths indicated in FIG, 3. For each concentration ratio Ge/H a temperature range of about 60 C exists between L1 and L2.
To determine the range of operating conditions which will give optimum results, one need only adjust 'his deposition system consistent with the concept that operation into the surface rate limited region will not destroy the smoothness of the surface, nor lead to drastically reduced economy of deposit. As a measure of the surface roughness required for further device processing, generally such roughness cannot exceed 0.1 micron, although this varies, as indicated previously.
In FIG. 3, operation within the zone defined by curves L1 and L2 will keep both ridge growth and enhanced defect growth to less than 0.1 micron. This compares favorably with the best previously reported results for the reduction of ridge growth, in which ridges of approximately 2 microns were produced. Usually, ridges are upwards of 3 microns high.
In the GeCh-H reduction system of FIG. 3, the temperature of the high purity GeCl, is usually varied between 40 and 25 C. If a hydrogen dilution line is provided, hydrogen can be by-passed around the GeCl, source. In this way the input Ge/H ratio is varied by either changing the vapor pressure at the GeCl, source while keeping the total flow constant, or by adding pure H gas through the dilution line at a fixed GeCl, source temperature. The substrate temperature is obtained by R.F. induction to a germanium pedestal which acts as the susceptor. Control of the substrate temperature is obtained by means of a temperature sensing device, in combination with the RF. induction system.
To use the teaching of this invention, it is only necessary to consider the best point of operation of the epitaxial reactor. For instance, if it is desired to operate at a particular temperature, the concentration ratio (Ge/H is increased by varying the concentration of the input reactants GeCl, and H The Ge/l-I ratio is increased until rough (faceted) surfaces are noted. At this point, the concentration of the reactants is changed slowly to reduce the Ge/l-I, ratio until surface roughness disappears or is tolerable for the subsequent required device processing. In general, it will be noted that the smoothness of the deposited layer is essentially that which would occur if deposition were well within the mass transport limited region. That is, faceting can be eliminated without producing ridges or large defects.
If it is desired to operate the epitaxial reactor with a given concentration of input reactant species (fixed Ge/H, ratio), then the the substrate temperature should be reduced to move the deposition toward the surface rate limited region. The temperature is lowered until rough (faceted) surfaces are noted, at which time the temperature is slowly increased until the surface roughness disappears, or becomes tolerable. As before, it will be found that the surface roughness of the deposited layer is essentially the same as that-obtained when operation is in the mass transport limited region, even though the final operating point may be to the left of dashed line L. Also, no ridges or large defects will result.
While the initial operating points placed operation in the mass transport limited region, the process can start in the surface rate limited region, as previously explained in the summary. For instance, if temperature is held constant, the ratio is changed until defects appear, at which time it is slowly reversed until the defects disappear. This will determine the final operative point. Also, both temperature and concentration ratio can be changed simultaneously (or separately) to carry out the process. This may efi'ect more rapid determination of the final operating point.
It should be recognized that, if the substrate surface is poor, it may be very difficult for the investigator to discern when deposition in the surface rated limited region has occurred, since it will be difficult to detect the onset of rough surfaces. However, careful deposition runs made over a range of temperatures and concentrations will establish curves similar to those of FIGS. 3 and 4. In this way, it will be possible to determine a dashed line L for demarcation between a temperature sensitive growth region and a temperature insensitive growth region.
Generally, it is the practice to clean the substrate surface before epitaxial deposition. This aids immensely in being able to determine the crossover point from mass transport operation to surface rate limited operation. A standard cleaning procedure in the case of germanium is given on page 82 of the above referenced article by Silvestri. This cleaning procedure consists of a 2-minute etch in a 3:1 solution of H 0; NaOCl (5%). After rinsing in distilled water, the substrates are dried in a high pressure nitrogen stream. They are then heated in a reactor at temperatures greater than 700 C for 15 minutes. In the case of germanium, it may be possible to operate even further into the surface rate limited region if further cleansing is done before deposition. In this case, the standard cleaning procedure mentioned above would be used, followed by an etch for approximately 15 seconds in a 10:] solution of H O:I -I O( 30%). The substrates are then flushed in distilled water for 5-6 minutes. Of course, it will be realized by those of skill in the art that other cleansing techniques are used for silicon, gallium arsenide, etc.
FIG. 4 shows curves for the deposition of silicon by hydrogen reduction of SiCl Because most deposition is done at a SiCI /H concentration ratio (designated Si/H of 0.02, data for additional growth curves has not been extensively developed. Therefore, further growth curves are shown as solid lines where data has been well established and as dotted lines where the data is not well established. However, the dotted line extensions are believed to be quite accurate, based on experience with these systems. The dashed line L separating surface limited region A and mass transport limited region B is similar to that of FIG. 3. Again, a zone of operation (defined by L1 and L2) exists wherein it is possible to obtain smooth deposits of silicon, while eliminating ridge growth and enhanced defect growth. As with FIG. 3, this zone also extends over the large deposition range of silicon by chemical vapor transport processes.
The basic procedures in determining a desired operating point are the same as in the germanium deposition system of H6. 3. That is, it is first decided whether operation is to be at a given temperature or at a given input concentration ratio Si/H Concentration of input reactants or temperature, respectively, is then varied in order to bring the deposition into the surface rate limited region. This will produce rough (faceted) surfaces and the direction of variation of concentration ratio and temperature is reversed and slowly changed in order to note the onset of smooth surface deposition. The optimum operating point is that point which provides a smooth surface while eliminating ridge growth and enhanced defect growth.
As with FIG. 3, initial operation can be in the surface rate limited region. Also, both temperature and concentration ratio can be varied simultaneously or separately to effect the process.
1n general, smooth surfaces of epitaxially deposited layers of Si having no ridges or enhanced defects, can be obtained for deposition at temperatures between approximately 1,1 and 1,400 C, and concentration ratios (Si/H between approximately 0.02 and 0.06, which covers essentially all of the operating range for deposition of silicon.
While this invention has been explained in terms of a homolayer deposition, it is to be understood that it is equally 'applicable to heterolayer deposition. Further, it is to be understood that, with improved substrate cleaning procedures, it will be possible to operate even more fully into the surface rate limited region, while still obtaining smooth surfaces and ridgeless growth. This invention is based on a detailed study outlining the mechanisms for ridge growth and enhanced defect growth and establishes a deposition process in which reactors are used in a way contrary to that taught by the prior art. Whereas previous operation was at specified temperatures and concentration ratios, it is now possible to deposit better films over a wider range of temperature and concentration ratio.
What is claimed is:
1. In a process for epitaxially depositing material onto a substrate, wherein gaseous species containing said material and a carrier gas are reacted to deposit said material on said substrate, said process being characterized by a surface rate limited region in which deposition rate is dependent on substrate temperature and a mass transport limited region in which deposition rate is temperature insensitive, comprising:
reacting said gaseous species and said carrier gas at first concentrations and first substrate temperature such that said deposition occurs in said mass transport limited region said material exhibiting unstructured growth and defects when deposited in this region ofgrowth;
changing said substrate temperature and said concentrations in directions which cause said deposition to move into said surface rate limited region, said changes continuing until said deposited material becomes structured; reversing the direction of change of said substrate temperature and said concentrations and changing said substrate temperature and concentrations until final values are reached at which said deposited material exhibits nonstructured surfaces and no defects;
continuing said epitaxial deposition at said final values until said deposited material has a desired final thickness.
2. The process of claim 1, wherein said depositing material is germanium.
3. The process of claim 1, wherein said deposited material is silicon.
4. The process of claim 1, wherein said substrate is different from the material to be deposited.
5. The process of claim 1, wherein said material to be deposited is a semiconductor.
6. The process of claim 1, wherein only said concentrations are changed, said temperature being held constant during said deposition.
7. The process of claim 1, wherein only said substrate temperature is changed, said concentrations being held constant durin said deposition.
8. 1% process for epitaxially depositing material onto a substrate, wherein gaseous species containing said material and a carrier gas are reacted to deposit said material onto said substrate, said process being characterized by a surface rate limited region in which deposition rate is dependent on substrate temperature and a mass transport limited region in which deposition rate is temperature insensitive, comprising:
reacting said gaseous species and said carrier gas at first concentrations and first substrate temperature such that said deposition occurs in said surface rate limited region, said deposited material exhibiting faceted growth structure;
changing said substrate temperature and said concentrations in directions which cause said deposition to move into said mass transport limited region, said changes continuing until said deposited material exhibits defect growth thereon and said faceted growth ceases;
reversing the direction of change of said substrate temperature and said concentrations and changing said substrate temperature and concentrations until final values are reached at which said deposited material does not exhibit said defect growth and said growth facets;
continuing said epitaxial deposition at said final values until said deposited material has a desired final thickness.
9. The process of claim 8, wherein only said concentrations are changed, said temperature being held constant during said deposition.
10. The process of claim 8, wherein only said substrate temperature is changed, said concentrations being held constant during said deposition.
11. The process of claim 8, wherein said depositing material is germanium.
12. The process of claim 8, wherein said depositing material is silicon.
13. The process of claim 8, wherein said material to be deposited is a semiconductor.
14. The process of claim 8, wherein said substrate is different from the material to be deposited.