GB1601206A

GB1601206A - Enhancing epitaxy or preferred orientation

Info

Publication number: GB1601206A
Application number: GB976678A
Authority: GB
Original assignee: Massachusetts Institute of Technology
Current assignee: Massachusetts Institute of Technology
Priority date: 1978-03-13
Filing date: 1978-03-13
Publication date: 1981-10-28

Description

(54) ENHANCING EPITAXY OR PREFERRED ORIENTATION (71) We, MASSACHUSETTS INSTI TUTE OF TECHNOLOGY, of 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States of America, a Corporation organised under the laws of the State of Massachusetts, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 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 relatively large area thin films regularly oriented with a practical process.

Much of modern technology makes use of thin solid films on the surfaces of solid substrates. A number of methods have been used to deposit such thin films including thermal evaporation, DC sputtering, rf sputtering, ion beam deposition, chemical vapor deposition, plating, molecular beam deposition and deposition from the liquid phase.

The structure of thin films can be amorphous (that is the atoms of the film are not arranged in any crystalline order), polycrystalline (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 expitaxial (that is, the film is predominantly 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 composition from the substrate.

If the film is expitaxial, the former is called "homoepitaxy" and the latter "heteroepitaxy".

In general, techniques for obtaining high quality amorphous and polycrystalline films particularly metals) are well developed and well understood. However, techniques for obtaining high quality expitaxial films and films of preferred orientation are severly limited, and only a limited number of combinations of overlayer film and substrate have been achieved. In most cases, films exhibit a high concentration of crystalline defects [S. T. Picraux, G. T. Thomas, "Correlation 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 achieve epitaxy or preferred orientation, and differences in thermal expansion between film and substrate lead to high stresses and sometimes to cracking when samples are cooled to room temperature.Although there are many important technological opportunities for the application of preferred orientation and epitaxial films, particularly in electronic, acoustic, and optical devices, with a few notable exceptions, such films have not been consistently obtained with sufficient quality or in a sufficient number of combinations 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 composition 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 substrate surface favor the growth of the desired film orientation. 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.

This invention relates from the discovery that the phenomena of nucleation, growth, and changes in crystallographic orientation that occur during the early stages of film formation on solid surfaces can be influenced and controlled by means of surface relief structures and point defects. 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 [G. 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)] who 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.The invention consists in a method of enhancing expitaxy or preferred orientation which method includes the steps of intentionally forming at predetermined locations a plurality of artificial defects at the surface of a solid substrate, said artificial defects being artificial point defects and/or artificial surface relief structure, and thereafter depositing a film on said surface to form a substantially epitaxial or preferred orientation layer in said film having crystallographic orientation influenced by the geometric arrangement of adjacent ones of said artificial defects, the separation between adjacent ones of said artificial defects being sufficiently small so that both artificial defects in a pair of adjacent ones contribute to influencing said crystallographic orientation.

The creation of artificial surface defects, for example 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 contradiction to conventional methods of thin film growth. Conventional methods attempt to remove, to the fullest extent possible, any natural surface relief steps or point defects. This is usually done by polishing or etching the surface prior to film growth.

The invention involves the intentional establishment of of surface defects such as surface relief steps or point defects on a solid surface.

The geometric pattern of the surface relief structure or 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 nucleation and growth operable for that combination and deposition method.

The geometric pattern may conveniently by a simple grating or grid with repeating elements spaced by distances of the order of 1/2 micron or less, although in some cases a repeat distance of 1 micron is adequate. The depth of the surface relief structure can vary from less than one nanometer to of the order of one micrometer. Preferably, there are sets of steps and/or point defects with each set embraced by a plane generally perpendicular to the substrate surface and generally parallel to a plane embracing another set with the angle between intersecting planes being an integral multiple of 30 or rc/6 radians.

In order to make the invention clearly understood, reference will now be made to the accompanying drawings which are given by way of example and in which: Figure 1 is a fragmentary sectional view of a solid substrate coated with a thin film; Figure 2 is a combined block-sectional view illustrating the use of soft x-rays for forming a controlled relief pattern in a solid substrate according to the invention; Figure 3 is a sectional view of the relief pattern thus formed; Figure 4 is a sectional view illustrating the relief structure following etching; Figure 5 is a combined block-pictorial representation of a means for depositing a thin layer on the solid substrate with ion beam sputtering; and Figure 6 is a greatly enlarged perspective view of a solid substrate formed with regularly spaced steps according to the invention suitable for receiving an epitaxial or preferred orientation layer.

With reference now to the drawing and more particularly Figures 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 Figure 1. An example of such a film would be polymethyl methacrylate. This film may then be exposed by x-ray lithography (U.S. Patent 3,743,842 (July 3, 1973) "X-ray Lithographic Apparatus and Process" H. I. Smith, D. L. Spears, E. Stern) as depicted in Figure 2. The mask 3 consists of a membrane 4 which is relatively transparent to x-rays, and an absorber 5, which is formed into a pattern of periodic or quasiperiodic elements, such as a grating or grid.Soft x-rays 6 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 relief 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 isobutyl ketone and 60% isopropyl alcohol, which removes those regions of the polymer directly exposed to the soft x-ray radiation. 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 Figure 3. Many other radiation sensitive polymers and developing methods could be substituted within the principles of the invention.

The pattern can be exposed in the radiation sensitive polymer film by a number of other methods including photolithography, electron beam lithography, and holographic methods. X-ray lithography is particularly well suited because of its capability of exposing patterns with linewidths of 1000A and less with sharp vertical sidewalls. It is estimated that a resolution of soA is possible with x-ray lithography, using the Carbon K x-ray at 44.7to wavelength. Polymer relief structures can also be created by in-site polymerization.

Following creation of the polymer relief 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 Figure 4. The method of etching depends on the chemical nature of the solid and the resistance of the polymer to various etching environments. For example, with PMMA as the polymer relief pattern relief structures with sharp vertical sidewalls can be etched into SiO2 substrates by a reactive ion etching process.

While steep vertical sidewalls are preferred, the principles of the invention are applicable to discontinuities formed by sloping sidewalls, including those formed by undercutting. Alternatively, ion beam etching, wet chemical etching 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 surface. To convert the relief structure to a high quality SiO2, the substrate can be baked in an oxygen oven at or nearly 1000"C.

An array of 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 over the sample surface creating lines of point defects. Alternatively, ion bombardment through a mask could be employed to create a pattern of defects. High energy photons could also be used.

Following the creation of the relief structure or array of defects 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 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 beam sputtering, chemical vapor deposition, molecular beam deposition, plating and deposition from the liquid phase. Ion beam sputtering as depicted in Figure 5 has been used, and the material deposited was germanium on SiO2 substrate. 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 with thin films 0-3 microns thick, the invention is also useful for growing larger crystals. The initial thin film may then function as a seed and larger crystals grown using conventional crystal growing techniques.

Referring to Figure 6, there is shown a greatly magnified perspective view of a substrate regularly stepped according to the invention ready for receiving an epitaxial surface layer. There are sets of steps each embraced in a plane parallel to the plane embracing another set of steps. Intersecting embracing planes intersect at an angle of 90" or z/2 radians, an integral multiple of 30 or n/6 radians. The planes might also intersect at an angle of 60 or z/3 radians, also an integral multiple of 30 or z/6 radians. It is also preferred that separation between adjacent parallel embracing planes be less than 1 micron, typically being 500 Angstroms as shown in Figure 6 or 1/20 micron. The step or point defect depth is preferably within the range of 1 atom to 1/2 micron.

WHAT WE CLAIM IS: 1. A method of enhancing epitaxy or preferred orientation which method includes the steps of intentionally forming at predetermined locations a plurality of artificial defects at the surface of a solid substrate, said artificial defects being artificial point defects and/or artificial surface relief structure, and thereafter depositing a film on said surface to form a substantially

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. grid. Soft x-rays 6 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 relief 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 isobutyl ketone and 60% isopropyl alcohol, which removes those regions of the polymer directly exposed to the soft x-ray radiation. 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 Figure 3. Many other radiation sensitive polymers and developing methods could be substituted within the principles of the invention. The pattern can be exposed in the radiation sensitive polymer film by a number of other methods including photolithography, electron beam lithography, and holographic methods. X-ray lithography is particularly well suited because of its capability of exposing patterns with linewidths of 1000A and less with sharp vertical sidewalls. It is estimated that a resolution of soA is possible with x-ray lithography, using the Carbon K x-ray at 44.7to wavelength. Polymer relief structures can also be created by in-site polymerization. Following creation of the polymer relief 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 Figure 4. The method of etching depends on the chemical nature of the solid and the resistance of the polymer to various etching environments. For example, with PMMA as the polymer relief pattern relief structures with sharp vertical sidewalls can be etched into SiO2 substrates by a reactive ion etching process. While steep vertical sidewalls are preferred, the principles of the invention are applicable to discontinuities formed by sloping sidewalls, including those formed by undercutting. Alternatively, ion beam etching, wet chemical etching 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 surface. To convert the relief structure to a high quality SiO2, the substrate can be baked in an oxygen oven at or nearly 1000"C. An array of 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 over the sample surface creating lines of point defects. Alternatively, ion bombardment through a mask could be employed to create a pattern of defects. High energy photons could also be used. Following the creation of the relief structure or array of defects 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 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 beam sputtering, chemical vapor deposition, molecular beam deposition, plating and deposition from the liquid phase. Ion beam sputtering as depicted in Figure 5 has been used, and the material deposited was germanium on SiO2 substrate. 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 with thin films 0-3 microns thick, the invention is also useful for growing larger crystals. The initial thin film may then function as a seed and larger crystals grown using conventional crystal growing techniques. Referring to Figure 6, there is shown a greatly magnified perspective view of a substrate regularly stepped according to the invention ready for receiving an epitaxial surface layer. There are sets of steps each embraced in a plane parallel to the plane embracing another set of steps. Intersecting embracing planes intersect at an angle of 90" or z/2 radians, an integral multiple of 30 or n/6 radians. The planes might also intersect at an angle of 60 or z/3 radians, also an integral multiple of 30 or z/6 radians. It is also preferred that separation between adjacent parallel embracing planes be less than 1 micron, typically being 500 Angstroms as shown in Figure 6 or 1/20 micron.The step or point defect depth is preferably within the range of 1 atom to 1/2 micron. WHAT WE CLAIM IS:

1. A method of enhancing epitaxy or preferred orientation which method includes the steps of intentionally forming at predetermined locations a plurality of artificial defects at the surface of a solid substrate, said artificial defects being artificial point defects and/or artificial surface relief structure, and thereafter depositing a film on said surface to form a substantially

epitaxial or preferred orientation layer in said film having crystallographic orientation influenced by the geometric arrangement of adjacent ones of said artificial defects, the separation between adjacent ones of said artificial defects being sufficiently small so that both artificial defects in a pair of adjacent ones contribute to influencing said crystallographic orientation.

2. A method as claimed in claim 1, including the step of forming the artificial defects in rows.

3. A method as claimed in claim 2, including the step of forming said artificial defects in intersecting rows.

4. A method as claimed in claim 3, wherein the rows intersect at an angle that is an integral multiple of s/6 radians.

5. A method as claimed in claims 2, 3 or 4, wherein the separation between the rows or between parallel rows is substantially equal.

6. A method as claimed in any one of claims 1 to 5, including the step of forming artificial point defects among said artificial defects.

7. A method as claimed in any one of claims 1 to 6, including forming artificial steps among said artificial defects.

8. A method as claimed in claim 7, wherein said artificial steps are substantially equidistantly spaced.

9. A method as claimed in claim 7, including forming further artificial steps that are substantially orthogonal to the firstmentioned artificial steps.

10. A method as claimed in claim 7, 8 or 9, wherein said steps have substantially vertical walls.

11. A method as claimed in claim 7, 8 or 9, wherein said steps have sloping walls.

12. A method as claimed in any one of claims 1 to 11, wherein the artificial defects are formed with the dimension of each defect in a direction perpendicular to the substrate surface being within the range of 1 atom to 1/2 micron.

13. A method as claimed in any one claims 1 to 12 wherein the substrate is an amorphous solid substrate.

14. A method as claimed in claim 13, wherein said substrate is silicon dioxide and said film comprises at least one germanium.

15. A method as claimed in any one of claims 1 to 14 wherein said artificial surface relief structure comprises structure having a preselected shape bounded by substantially planar facets.

16. A device having a substrate with an epitaxial or preferred orientation layer on the substrate surface, made in accordance with the method of any one of claims 1 to 15.

17. A device as claimed in claim 16, comprising a solid substrate having a surface with intentionally formed defects at predetermined locations, said artificial defects being artificial point defects and/or artificial surface relief structure, the separation between adjacent defects being less than 1 micron, said layer comprismg said film having crystallographic orientation determined by the geometric arrangements of adjacent artificial defects.

18. A device as claimed in claim 17, wherein said layer is of different material than said substrate.

19. A device as claimed in claim 18, wherein said substrate comprises silicon dioxide.

20. A device as claimed in any one of claims 16 to 19, wherein said layer is semiconductive.

21. A method for enhancing epitaxy or preferred orientation substantially as hereinbefore described with reference to the accompanying drawings.

22. A device having a substrate with an epitaxial or preferred orientation layer thereon, substantially as hereindescribed with reference to and as shown in the accompanying drawings.