U.S. patent number 3,661,636 [Application Number 05/030,789] was granted by the patent office on 1972-05-09 for process for forming uniform and smooth surfaces.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to James M. Green, II, Thomas O. Sedgwick, Victor J. Silvestri.
United States Patent |
3,661,636 |
Green, II , et al. |
May 9, 1972 |
PROCESS FOR FORMING UNIFORM AND SMOOTH SURFACES
Abstract
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.
Inventors: |
Green, II; James M. (New York,
NY), Sedgwick; Thomas O. (Crompond, NY), Silvestri;
Victor J. (Mount Kisco, NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
21856046 |
Appl.
No.: |
05/030,789 |
Filed: |
April 22, 1970 |
Current U.S.
Class: |
117/89; 117/935;
117/936; 148/DIG.26; 148/DIG.50; 148/DIG.115; 148/DIG.49;
148/DIG.51; 148/DIG.145 |
Current CPC
Class: |
C30B
25/02 (20130101); Y10S 148/026 (20130101); Y10S
148/145 (20130101); Y10S 148/049 (20130101); Y10S
148/051 (20130101); Y10S 148/05 (20130101); Y10S
148/115 (20130101) |
Current International
Class: |
C30B
25/02 (20060101); C23c 013/00 () |
Field of
Search: |
;117/16A,17.2R,201 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3301213 |
January 1967 |
Grochowski et al. |
|
Primary Examiner: Kendall; Ralph S.
Claims
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 of growth;
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 non-structured 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 during said
deposition.
8. 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 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.
Description
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 differences 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
non-nucleating 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, teflon 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 HCl 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 may be 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.
If 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
The method of this invention provides smooth surfaces during
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
to those 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. Whereas the prior art has
always taught that epitaxial reactors should be operated in mass
transport limited regions in order 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.sub.4 and hydrogen is the carrier gas, the GeCl.sub.4 /H.sub.2
ratio (for simplicity indicated as Ge/H.sub.2) 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 non-faceted 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. 1 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
of a large dome-like defect.
FIG. 3 is a plot of growth rate versus substrate temperature for a
germanium epitaxial system.
FIG. 4 is a plot of growth rate versus substrate temperature for a
silicon epitaxial system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1, 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.sub.2). Surrounding pedestal 10 is another Ge layer
18, which could be an epitaxial layer also.
Because SiO.sub.2 is a non-nucleating material for depositing
germanium atoms, these atoms will not adhere to the SiO.sub.2 mask
12 and will move to the edge of the 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.sub.2. As the distance from the boundary 22 of the
SiO.sub.2 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 30 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:
1. 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 defect 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.sub.2
reduction of GeCl.sub.4 and deposition of Si by H.sub.2 reduction
of SiCl.sub.4, 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 GeCl.sub.4 --H.sub.2 System," which appeared in the J.
Electrochemical Society, Vol. 116, No. 1, Jan. 1969, 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. 113, 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.sub.2). The
dashed line L 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 [111] surfaces, while square facets develop on [100]
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.sub.2
between 3.6.times. 10.sup..sup.-4 and 9.1.times. 10.sup..sup.-3 and
at temperatures between approximately 570.degree. and 850 C.degree.
, respectively. Although data points have not been established over
this entire range of temperature and concentration ratios, theory
indicates that curves L1 and L2 will follow the paths indicated in
FIG. 3. For each concentration ratio Ge/H.sub.2, a temperature
range of about 60.degree. 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 GeCl.sub.4 --H.sub.2 reduction system of FIG. 3, the
temperature of the high purity GeCl.sub.4 is usually varied between
-40.degree. and 25.degree. C. If a hydrogen dilution line is
provided, hydrogen can be by-passed around the GeCl.sub.4 source.
In this way the input Ge/H.sub.2 ratio is varied by either changing
the vapor pressure at the GeCl.sub.4 source while keeping the total
flow constant, or by adding pure H.sub.2 gas through the dilution
line at a fixed GeCl.sub.4 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 R.F. 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.sub.2) is increased by varying the
concentration of the input reactants GeCl.sub.4 and H.sub.2. The
Ge/H.sub.2 ratio is increased until rough (faceted) surfaces are
noted. At this point, the concentration of the reactants is changed
slowly to reduce the Ge/H.sub.2 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.sub.2 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
effect 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.sub.2 O; 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.degree. 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 10:1 solution of H.sub.2 O:H.sub.2 O.sub.2 (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.sub.4. Because most deposition is done at a
SiCl.sub.4 /H.sub.2 concentration ratio (designated Si/H.sub.2) 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 FIG. 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.sub.2).
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.
In general, smooth surfaces of epitaxially deposited layers of Si
having no ridges or enhanced defects, can be obtained for
deposition at temperatures between approximately 1,110.degree. and
1,400.degree. C, and concentration ratios (Si/H.sub.2) 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 homo-layer
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.
* * * * *