GB2300424A - Diamond growth on ion implanted surfaces - Google Patents

Abstract

A method for producing a polycrystalline diamond film by plasma chemical vapour deposition on a substrate, such as a silicon wafer involves subjecting the substrate to ion implantation e.g. by bombarding the substrate surface with an ion beam to promote nucleation for subsequent diamond growth, exposing the resulting surface to a carbon-containing plasma to effect nucleation and depositing crystalline diamond on the nucleated surface from carbon-containing plasma. During nucleation the substrate is subjected to a negative bias.

Description

DIAMOND GROWTH ON ION IMPLANTED SURFACES This invention relates to the production of diamond films on substrates.

In our British Patent Application No. 2,270,326, we describe a method of preparing highly oriented diamond films on substrates, particularly semiconductor substrates, The resulting hetero-epitaxially grown diamond films are useful in the fabrication of semi-conductor electronic devices such as transistors, diodes and sensors.

In the process described in our above application, the substrate surface is preferably subjected to a pre-carburisation treatment. This process is carried out in the diamond deposition reactor and is difficult to control. It is an object of the present invention to improve the uniformity of the pre-nucleation of the substrate surface. It is a further object of the invention to reduce the cycle time for the fabrication of a diamond film by carrying out a necessary stage in the diamond deposition process outside of the reactor environment.

According to the present invention there is provided a method of producing a crystalline diamond film by plasma chemical vapour deposition on a substrate, which comprises subjecting the substrate to ion implantation, exposing the resulting surface to a carbon-containing plasma to effect nucleation of said surface for oriented diamond crystal growth, while subjecting the surface to electrical bias and depositing crystalline diamond on the nucleated surface from a carbon-containing plasma.

Currently, the preferred implantation ion is carbon in conjunction with a carbide-forming substrate. However, other crystalline compounds which promote the subsequent growth of an oriented diamond layer can be formed in the substrate layer using alternative implantation ions and the same or different substrates. For example, diamond growth is promoted on cubic boron nitride.

Crystalline boron nitride can be formed on a silicon substrate by implantation with boron and nitrogen. Other refractory materials can be used instead of silicon for the substrate, e.g. hafnium, tungsten, molybdenum, nickel, tantalum or titanium. Examples of other possible implantation ions are calcium, fluorine, chlorine, silicon and phosphorus.

The choice of a suitable implant ion, and the conditions under which implantation is carried out, is dependent on the type of substrate used. In the case of a single crystal silicon substrate, carbon ion implantation has been used to form a high quality, stoichiometric, silicon carbide film - see the paper by von Munch and Wiebach (Dia. Rel. Mater, 5, (1994), 500), and references cited therein. Implantation may be carried out at any temperature up to about 1 5000C.

It may also be carried out at or near room temperature, however, improved results are obtained at elevated temperatures, particularly at temperatures above 6000C, e.g. 700 to 12000C. Post-heating or annealing the carbon ion implanted surface may be desirable, depending on the substrate employed.

The objective of the ion implantation is to increase the concentration of diamond nucleation sites on which single crystal diamond films can be grown.

In the case of silicon, a large lattice mismatch exists between silicon and diamond. There is strong evidence in the literature to suggest that an intervening layer of silicon carbide is established to absorb this mismatch and in our British Patent Application No. PCT/GB94/00964, we show evidence of this layer. In order to minimize mismatch, the carbide is preferably in its cubic or beta form.

Similar considerations apply when other substrates are employed comprising an element capable of forming a refractory carbide. Examples of such elements include hafnium, tungsten, molybdenum, nickel, tantalum and titanium.

Candidate ions for implantation are dependent on the type of substrate employed.

Substrates which do not promote nucleation of diamond films per se may be modified either chemically andlor structurally by implantation such that a diamond layer is encouraged to nucleate. This may also involve simultaneous and/or sequential implantation of more than one ion species into a substrate; this aspect is considered to be within the scope of this invention.

It is also advantageous to anneal the implanted surface at elevated temperature. This is normally carried out either in vacuo or in a reduced pressure of a suitable gas. For silicon, this has been found to be about 1000 to 1 5000C in vacuo for a few seconds, e.g. 1 to 5 seconds, typically 12000C for 1 to 2 seconds.

This has the effect of converting any amorphous carbon in the surface to a carbide and healing' the topmost silicon or other substrate element which may have been damaged by the implantation.

Substrates are typically implanted at energies between 1 and 100 keV.

Preferably, the substrates are implanted at low acceleration energies, i.e. less than about 20 keV, and the ion beam is continuously swept over the substrate area in order to achieve a substantially homogeneously implanted area. The aim is to confine the bulk of the carbon implant to the immediate surface layer, e.g. within 150 to 1000 A of the surface.

Implant doses in the range 1014 to 1022 ions per cm-3 are typically employed, preferably 1018 to 1021 ions per cam~3. The aim is to provide sufficient species into the implantation region such that the surface is modified to promote diamond nucleation.

Ion implantation is a standard technique for doping semi-conductor surfaces and is carried out in accordance with the invention using a 12C ion source, accelerated at ion energies from 1 keV to 100 keV; in the case of silicon, a 12C ion source is preferably used. Following ion implantation, the treated surface is introduced into a microwave plasma CVD apparatus, as described in our above mentioned British Patent Application No. 2,270,326. The substrate is then subjected to a negative bias while monitoring the completion of nucleation of the surface, e.g. using the pyrometer technique described in our above-mentioned British Patent Application. Deposition of a highlv oriented diamond film is then carried out on the nucleated surface as described in our British Patent Application.

The following Examples are given to illustrate the invention.

Example 1. HOD growth on carbon ion implanted silicon (100) substrate "Best Mode" conditions A 1" diameter silicon water oriented in the (100) direction was implanted with energetic carbon ions.

Implnntation Conditions Implantation was carried out using a 12C ion source. The ion dose was lx 101 8cm-3 at an energy of 40 keV. The substrate was held at 7000C during implantation. A beam current typically in the range 1-100 ,um.cm~9 was used and the ion beam was swept to obtain a homogeneous implantation area. Figure 1 is a computer simulation showing the depth of carbon ion implant at various implantation energies.

After implantation, the wafers were treated with a high temperature, rapid thermal-anneal process. The samples were annealed at 12000C for 1-2 seconds.

The implantation procedure does not cause physical damage to the substrate surface. This is illustrated by Figure 2 which is a scanning electron micrograph (SEM) of the implanted silicon substrate taken under the indicated conditions after the annealing step.

After implantation and annealing the samples were used for diamond deposition by bias enhanced nucleation. Bias conditions used are as given in our above British Patent Application with the following amendments: a bias endpoint was detectable using a Mikron infrared pyrometer with these modified wafers Optimum biasing time is between 1 and 60 minutes (especially I to 50 minutes).

This sample endpointed between 19 and 20 minutes. Biasing time and biasing conditions are sensitively dependent on the type of sample being nucleated.

Figure ; are SEM's of the resulting highly oriented diamond film. Figure 4 is a Raman spectrum of the highly oriented diamond (HOD) film showing a very strong peak at 1331 cm-1 which is characteristic of diamond. This spectrum is typical of a high quality HOD film having a thickness of less than 5 microns.

A wide range of growth conditions can be specified dependent on the type of implantation conditions selected. Optimum diamond deposition conditions used for this experiment are as given in our above British Patent Application. A continuous diamond layer of high quality, well faceted crystals with (100) texture and azimuthally aligned to one another, was deposited using this method.

Example 2. Textured (1ooh diamond growth on untreated carbon ion implanted silicon (100) substrate Pre treatment conditions No pre treatment was used after ion implantation and prior to loading into the growth chamber.

Imolantation conditions Implantation was carried out using a 12C ion source. Ion dose was 2.3x1012 cm-3 and at an energy of 5 keV. The substrate was kept at room temperature during implantation. A beam current typically in the range 1-100 pm cm-2 was used and the ion beam was swept to obtain a homogeneous implantation area.

After implantation the samples were loaded directly into the growth reactor for diamond deposition by bias enhanced nucleation. Bias conditions used are as given in our above British Patent Application. A bias endpoint was detected after 28 minutes. Diamond deposition was carried out using best mode conditions given in our above British Patent Application A continuous diamond layer of high quality, well faceted crystals with preferred (100) texture was deposited using this method. This is illustrated by Figure 5 which are SEM's of the diamond layer so obtained.

Example 3. Textured (100) diamond growth on silicon (100) substrate following carbon ion implantation and KOH wet chemical treatment Implantation conditions Implantation was carried out using a 12C ion source. The ion dose was 2.3x1012cm-3 and at an energy of 5 keV. The substrate was kept at room temperature during implantation. A beam current typically in the range 1-100pm cm-2 was used and the ion beam was swept to obtain a homogeneous implantation area.

Pre treatment and deposition conditions A wet chemical etching treatment was employed prior to loading into the growth chamber. This was used to expose the buried OSiC layer. The following etch conditions were used. The implanted wafers were immersed in a 7M solution of potassium hydroxide (KOH) which was held at a constant temperature of 85 C. The solution has a controlled etch rate of approximately 1-1.5 ,um/min.

Wafers were etched between 10 seconds and 10 minutes dependent on implant conditions chosen. After etching, the samples were loaded into the growth reactor for diamond deposition by bias enhanced nucleation. Bias conditions used are as given in the original biasing patent. A bias endpoint was detected after 28 minutes. Diamond deposition was carried out using best mode conditions given in our above mentioned British Patent Application. A continuous diamond layer of high quality, well faceted crystals with preferred (100) texture was deposited using this method. Figure 6 are SENDS of the resulting diamond film.

The distribution of diamond grains in the diamond layer can be modified by introducing a grid above or adjacent to the substrate surface in the CVD chamber during the diamond growth phase. This technique is described in our above-mentioned British Patent Application filed concurrently herewith (Patent Agent's ref. No. DCW/Case A)

Claims (15)

CLAIMS:

1. A method of producing a polycrystalline diamond film by plasma chemical vapour deposition on a substrate which comprises subjecting the substrate to ion implantation, exposing the resulting surface to a carboncontaining plasma to effect nucleation of said surface for oriented diamond crystal growth while subjecting the surface to electrical bias and depositing crystalline diamond on the nucleated surface from a carbon-containing plasma.

2. A method as claimed in claim 1 wherein the substrate is modified so as to be able to promote diamond film nucleation.

3. A method as claimed in claim 1 or 2 wherein the implantation is effected using as the implant ion carbon, boron, nitrogen, calcium, fluorine, chlorine, silicon or phosphorus.

4. A method as claimed in any one of the preceding claims, wherein the substrate comprises an element capable of forming a refractory carbide.

5. A method as claimed in claim 4 wherein said element is silicon, hafnium, tungsten, molybdenum, nickel, tantalum or titanium.

6. A method as claimed in any one of the preceding claims wherein more than one ion species is implanted into the substrate surface, either in a sequential or simultaneous manner.

7. A method as claimed in any one of the preceding claims, wherein the ion implanted surface is annealed prior to the nucleation step at a temperature and time sufficient to repair any damage to the substrate layer caused by said ion implantation.

8. A method as claimed in claim 7, wherein the substrate is annealed by heating to a temperature of from about 500 to 25000C in vacuum for 1 to 30 seconds.

9. A method as claimed in claim 7 wherein the substrate is annealed by heating to a temperature of from about 500 to 25000C in a gaseous environment for a time up to 100 minutes.

10. A method as claimed in claim 9 wherein the gaseous environment comprises hydrogen, nitrogen, oxygen, argon, neon, helium, fluorine, chlorine, bromine, carbon monoxide, carbon dioxide, ammonia, phosphine and/or diborane.

11. A method as claimed in any one of the preceding claims wherein the substrate is at an elevated temperature during the ion implantation.

12. A method as claimed in claim 10 wherein the substrate is at a temperature between about 500 to 1500 C.

13. A method as claimed in any one of the preceding claims wherein the ion beam is swept over a substrate surface in order to establish a substantially homogeneously implanted area.

14. A method as claimed in any one of the preceding claims wherein the ion implantation is carried out at an acceleration energy between about 1 and 100 keV.

15. A method as claimed in any one of the preceding claims wherein the substrate is subjected to a negative bias during the nucleation step.

15. A method as claimed in any one of the preceding claims wherein the substrate is subjected to a negative bias during the nucleation step.

Amendments to the claims have been filed as follows

1. A method of producing a polycrystalline diamond film by plasma chemical vapour deposition on a substrate which comprises subjecting the substrate to ion implantation, exposing the resulting surface to a carbon-containing plasma to effect nucleation of said surface for oriented diamond crystal growth while subjecting the surface to electrical bias and depositing crystalline diamond on the nucleated surface from a carbon-containing plasma.

2. A method as claimed in claim 1 wherein the substrate is modified so as to be able to promote diamond film nucleation.

3. A method as claimed in claim 1 or 2 wherein the implantation is effected using as the implant ion carbon, boron, nitrogen, calcium, fluorine, chlorine, silicon or phosphorus.

4. A method as claimed in any one of the preceding claims, wherein the substrate comprises an element capable of forming a refractory carbide.

5. A method as claimed in claim 4 wherein said element is silicon, hafhium, tungsten, molybdenum, nickel, tantalum or titanium.

6. A method as claimed in any one of the preceding claims wherein more than one ion species is implanted into the substrate surface, either in a sequential or simultaneous manner.

7. A method as claimed in any one of the preceding claims, wherein the ion implanted surface is annealed prior to the nucleation step at a temperature and time sufficient to repair any damage to the substrate layer caused by said ion implantation.

8. A method as claimed in claim 7, wherein the substrate is annealed by heating to a temperature of from about 500 to 25000C in vacuum for 1 to 30 seconds.

9. A method as claimed in claim 7 wherein the substrate is annealed by heating to a temperature of from about 500 to 25000C in a gaseous environment for a time up to 100 minutes.

10. A method as claimed in claim 9 wherein the gaseous environment comprises hydrogen, nitrogen, oxygen, argon. neon, helium, fluorine, chlorine, bromine carbon monoxide, carbon dioxide, ammonia, phosphine and'or diborane.

11. A method as claimed in any one of the preceding claims wherein the substrate is at an elevated temperature during the ion implantation.

12. A method as claimed in claim 10 wherein the substrate is at a temperature between about 500 to 1500 C.

12 A method as claimed in any one of the preceding claims wherein the ion beam is swept over a substrate surface in order to establish a substantially homogeneously implanted area.

14. A method as claimed in any one of the preceding claims wherein the ion implantation is carried out at an acceleration energy between about 1 and 100 keV.