Note: Descriptions are shown in the official language in which they were submitted.
AA MIT P06
CH/rc BACKGROUND OF THE INVE~NTION
12/28/76
This invention relates in general to improving the
crystallographic quality of solid films grown on the surfaces
of solid substrates, and more particularly to enhancing epitaxy
or preferred orientation to provide rel~tlvely large area thin
~ilms regularly oriented by means of a practical process.
Much of modern technology makes use of thin solid films
on the surfaces of solid substrates. A number of me~hods have
been used to deposit such thin films including ~hermal evapora-
tion, DC sputtering, rf sputtering, ion beam deposition, chemical va~or deposi-
tion, plating, molecular-~e~m deposition and depositicn from the liquid phase.
The structure of thin films can be amorphous (that is,
the atoms of the film are not arranged in àny crystalline order),
polycry~talline (that is, the film is composed of many small
regions, in each of which the atoms are arranged in a regular
crystalline order, but the small regions have no mutual alignment
of their crystallographic axes), preferred orientation (that is,
the film is composed of many small regions, in each of which the
atoms are arranged in a regular crystalline order, and one or
more of the crystalline axes of the majority of said small
regions are parallel), or epitaxial tthat is, the film is pre-
dominantly of a single crystallographic orientation). A thin
film can be the same material (that is, the same element or
compound~ as the substrate, or it can differ in chemical composi-
tion from the substrate. If the film is epitaxial, the former is
called "homoepitaxy" and the latter "heteroepitaxy''.
In general, techniques for obtaining high quality
amorphous and polycrystalline films (particularly metals) axe
well developed and well understood. However, techniques for
obtaining high quality epitaxial films and films of preferred
orientation are severely limited, and only a limited number of
combinations of overlayex film and substrate have been achieved.
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AA MIT P0~
CH/rc In most cases, films exhibit a high concentration of crystalline
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defects [S. T. Picraux, G. T. Thomas, '1Correlation of ion
channeling and electron microscopy results in the evaluation of
heteroepitaxial silicon" J. Appl. Phys. vol 44, pp 594-602 (1973)].
In some cases, high temperatures are required to ac'hieve epitax~
or preferred orientation, and differences in thernal expansion
between film and substrate lead to high stresses and sometimes
to cracking when samples are cooled to room temperature. Although
there are many importan~ technological opportunities for the
application of preferred orientation and epitaxial films,
particularly in electronic, acoustic, and optical devices, with
a ~ew notable exceptions, such films have not been consistently
obtained with su~'ficient quality or in a su~icient number of
com~inations and orientations to meet the requirements.
Present or conventional methods for obtaining preferred
orientation and epitaxial film growth are based on choosing a
combination of deposition parameters (such as substrate composi-
tion and orientation, deposition method, deposition rate,
temperature and pressure) such that the nucleation and growth
processes which take place at a microscopic level on the sub-
strate surface favor the growth of the desired fi]m or:ientation.
The fundamental difficulty with this approach is that :it is not
always possible to control or reproduce all the factors which
affect film nucleation and growth. Moreover, this approach
limits the number of epitaxial combinations and orientations.
i
It is an important object of this invention to overcome
the shortcomings of conventional methods for producing epitaxial ;'
- and preferred orientation films and directly influence in a
controllable manner the nucleation, growth and orientation of
films grown on solid surfaces.
It is another object of -~his invention to control the
crystallographic orientation of thin films grown on solid
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AA MIT P06
CH/rc
12/28/76 surfaces in accordance with the preceding object.
It is a :Eurther obiect of this invention to achieve one
or more of the preceding Gbjects while reducing the density and
magnitude of defects in crystalline thin films grown on solid
surfaces.
- It is a further object of this invention to achieve
one or more of the preceding objects while obtaining epitaxial
or preferred orientation films at moderate temperatures and there-
by avoid stresses induced by differences in thermal expansion
between film and substrate.
This invention results from the discovery that the
phenomena of nuclcation, growth, a~d changes in crystallographic
orientation that occur during the early stages of film formation
on solid surfaces can be influenced and controlled by means of arti~
ficial surface relief structures and point defécts. It is well known
that naturally occurring defects such as steps or point defects
on crystal surfaces can act as nucleation sites for deposited
material. Some indication of the effects of arrAys of point
defects on the nucleation and growth of epitaxial films can be
found in the work of Distler et al LG. I. Distler, "Epitaxy as
a Matrix Replicating Process", Thin Solid Films, vol.32, pp. 157-
162 (1976); G. I. Distler, V. P. Vlasor, V. M. Kaneosky,
"Orientational and Long Range Effects in Epitaxy", Thin Solid
Films, vol. 33, pp. 287-300 (1976)] whb observed that naturally
occurring point defects on solid surfaces act as nucleation sites.
Distler et al further suggest that the point defects on a surface
naturally occur in some form of matrix or lattice and that the
orientational effects in epitaxy and crystallization in general
are due to the existence of the lattice of point defects.
" .
SU~M`ARY QF THE INVENTION
According to the lnvention intentionally create at pre-
determined locations on a solid surface an array of artificial
surface relief steps or artificial point defects and thereby
control in a predetermined way the process of film formation
and growth.
rrhe creation of a regular array of surface relief
steps or point defects on a solid surface in order to enhance
the crystallographic quality of thin solid films grown on said
surface is in direct contradic-tion to conventional methods of
thin film growth. Conventional methods attemp-t to remove, to
the fullest extent possible, any natural surface relief steps
or point defects. This is usually done by polishing or etch-
ing the surface prior to film growth.
I~le invention includes a process of preparing an
array of artiEicial defects such as surface relief steps or
point defects at predetermined locations on a solid surface,
and a process for depositing material onto the solid surface
in such a way that the crystallographic orientation of the
deposited material is controlled by the array of surface
relief steps or point defects, more particularly, by the geo-
metric arran~ement of ad~acent defects~ :
The geometric pattern of the surface relief struc-
ture or array of point defects that will be effective for a
given combination of overlayer film and substrate and a given
deposition method depends on the exact mechanisms of nuclea- .
tion and growth operable for that combination and deposition
method~ r~he geometric pattern will in general ~e a simple
grating or grid with repeating elements spaced by distances
of the order of 1/~ ~m or less, although in some cases, a re-
peat distance of 1 ~m is adequater The depth of -the surface
: relief struc-ture can vary from less than one nanometer to of
the order of one micrometer~ ~referably, there are sets of
steps and/or point defects with each set embraced by a plane
generally perpendicular to the substra-te surface ancL generally
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parallel to a plane embracing another set with the angle
between intersecting planes being preferably an integral
multiple of 30 or ~/6 radians.
According to a further broad aspect of the present
invention, there is provided a method of enhancing epitaxy
and preferred orientation, which method includes the steps
of intentionally forming at predetermined locations a
plurality of artificial defects at the surface of an amorphous
solid substrate. The artificial defec-ts are selected from
the group consisting of (1) artificial point defects and
(2) artificial surface relief structure. Thereafter, a film
is deposited on the surface to form a substantially epitaxial
or preferred orientation layer in the film hav:ing crystallo-
graphic orientation controlled by the geometric arrangement
of adjacent artificial defects.
According to a further broad aspect of the present
invention there is provided a device comprising a solid
substrate having a surface with artificial defects selected
from the group consisting of (1) artificial point defects and
(2) artificiaL relief structure. A film is provided on the
surface including a substantially preferred orientation layer
having crystallographic orientation influenced by adjacent
ones of the artificial defects. The separation between
adjacent ones of the artificial defects is sufficiently small
so that both artificial defects in a pair of adjacent ones
contribute to influencing the crystallographic orientation~
~ umerous other features, objects and advantages
of the invention will become apparent from
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AA MIT ~6
CH/rc
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the following specification when read in connection with the
accompanying drawing in which:
BRIEF DESCRIPTION OF T-IIE DR~WING
FIG. 1 is a fragmentary sectional view of a solid
coated with a thin film;
FIG. 2 is a com~ined block-sect:ional view illustrating
the use of soft x-rays for forming a cont:rolled relief pattern
in a solid substrate according ta the invention;
FIG. 3 is a sectional view of the relief pa~tern thus
formed;
FIG. ~ is a sectional view illustrating the relief
, s-tructure following etching;
; FIG. 5 is a combined block-pictorial representation
of ~ means or depositing a thin layer on the solid substrate
with ion beam sputtering; and
FIG. 6 is a greatly enlarged perspective view of a
solid substrate formed with regularly spaced steps according to
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the invention suitable ~or receiving an épitaxial or preferred orientatlon layer.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS -
With reference now to the drawing and ~ore particu-
larly FIGS. 1-4, there is illustrated a method for creating a
relief structure on the surface of a solid according to the
invention. The solid 1 is coated with a radiation sensitive
polymer film 2 (commonly called a "resist") as seen in the
fragmentary sectional view of FIG. 1. An example of such a film
would be polymethyl methacryl.ate. This film may then be exposed
by x-ray lithography (Patent 3,743,842 (July 3, 1973) "X-ray
Lithographic Apparatus and Process" H. I. Smith, D. L. Spears,
E. Stern) as depicted in FIG. 2. The mask 3 consists of a
membrane 4 which i9 relatively transparent to x-rays7 and an
absorber 5, which is formed into a pattern of periodic or quasi-
periodic elements, such as a grating or grid. Soft: x-rays 6
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AA MIT Pu~
CH/rc
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from source 7 pass through the mask 3, thereby casting a shadow
of the absorber pattern 5 on the radiation sensitive polymer 2.
After exposure, a reliPf pattern is created in the
radiation sensitive polymer by a development step. In the case
of polymethyl methacrylate, development may be accomplished, for
example in a solution of 40% methyl isolutyl ketane and 60%
isopropyl alcohol, which removes those regions of the polymer
directly exposed to the soft x-ray radiat:ion. Those regions of
the polymer 2 which were protected from the full intensity of the
x-ray radiation by the obstruction of absorber pattern 5 remain
undissolved and hence stand in relief, as depicted in FIG. 3.
Many other radiation sensitive polymers and developing methods
could be substituted within the principlesof the invention.
The pattern can be exposed in the radiation sensitive
polymer film by a number of other methods including photolithog-
raphy, electron beam lithography, and holographic methods. X-ray
lithography is particularly well suited because of its capability
of exposing patterns with linewidths of lOOOA and less with sharp
vertical sidewalls. It is estimated that a resolution of 50A is
possible wi~h x-r~y lithography, using the Carbon K x-ray at 44.7A wavelength.
Polymer relief structures can also be created by in-site polymerization.
Following creation of the polymer rellef structure 8,
the solid substrate 1 is etched and the polymer is removed,
thereby leaving a relief structure 9 on the surface of the solid
as seen in FIG. 4. The method of etching depends on the chemical
nature of the solid and the resistance of the poly~er to various
etching environments. For example, with PMMA as the polymer
relief pattern, reIief structures with sharp vPrtical sidewalls
can be etched into SiO2 substrates by a reactive ion etching
process.
While steep ve~tical sidewalls are preferred, the
principles of the invention are applicable to discont:inuities
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formed by sloping sidewalls, including those formed by under-
cutting. Alternatively, ion beam etching, wet chemical etch-
ing or gaseous plasma etching could be used for etching.
Another approach to creating a relief structure on sio2 is to
deposit sio2 or SiO, or a mixture of the two, over the polymer
relief structure, and then dissolve the polymer in a suitable
organic solvent. This leaves a relief structure of the sio2,
sio or mixture of the two on the sur-face~ To convert the ~-
relief structure to a high quality sio2, the substrate can be
baked in an oxygen oven at or near 1000C.
An array of artificial point defects can be created
on the substrate surface by exposing it to radiation in some
pattern. For example, a high energy finely focused electron
beam can be scanned in an appropriate pattern ovex the sample
surface creating lines of point defects. Alternatively, ion
bombardment through a mask could be employed to create a pat-
tern of defects. High energy photons could also be used.
Following the creation of the artificial relief
structure or array of de~ects on the solid surface, material
is deposited on top of it to form a thin film. The relief
structure or array of defects has the effect of controlling
the nucleation and/or growth of the film, thereby resulting
in a film with a determined crystallographic orientation and
low defect density. Many methods can be used to deposit thin
film material on the solid with surface relief structures or
array of point defects. These include evaporation, rf
sputtering, DC sputtering, ion bearn sputtering, chemical
vapor deposition, molecular beam deposition, plating and
deposition from the liquid phase. Ion beam sputtering as
depicted in ~IG. S has been used, and the material deposited
was germanium on sio2 substrate, a substance of amorphous
material. An ion source 10 emits an ion beam 11 which impinges
, ~
on target 12 of the material to be deposited. The material
13 sputtered from the target 12 deposits on the substrate 1
with surface relief structure 9.
While the invention is useful for making devices
w.ith thin films 0-3 microns thick, the invention is also use-
ful for growing larger crystals. The initial thin film may
then function as a seed and larger crystals grown using con-
ventional crystal growing techniques~
Referring to FIG. 6, there is shown a greatly magni-
fied perspective view of a substrate regularly stepped accord-
ing to the invention ready for receiving an epitaxial or pre-
ferred orientation surface layer. There are sets of steps
each embraced by a plane parallel to the plane embracing
another set of steps. Intersecting embracing planes intersect
at an angle of 90 or ~/2 radians, an integral multiple of 30
or ~/6 radians~ The planes might also intersect at an angle
of 60 or ~/3 radians, also an integral multiple of 30 or
~/6 radians. It is also preferred that separation between
adjacent parallel embracing planes be less than 1 micronr
typically being 500 Angstroms as shown in FIG. 6 or 1/20
micron. The artificial step or point defect depth is prefer- ~:
ably within the range of 1 atom to 1/2 micron. . `
There has been described novel structure and tech- :
niques for providing epitaxial and preferred orientation films
of relatively large area with an economically practical repeat-
able process, It is evident that those skilled in the art may
now make numerous uses and modifications of and departures from
the specific structure and techniques disclosed herein without
departing from the inventive concepts. Consequently, the in- :
vention is to be construed as embracing each and every novel
feature and novel combination of features present in or possessed
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by the apparatus and techniques herein disclosed and limited
solely by the spirit and scope of -the appended claims.
This is a division of Canadian patent application
serial number 298,810 filed March 13, 1978.
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