BACKGROUND OF THE INVENTION

This invention relates to monocrystalline diamond films and, more particularly, a method for depositing monocrystalline diamond on substrates, including non-diamond substrates, by chemical vapor deposition.

Diamond has great potential for electronic, optical, mechanical, chemical, and nuclear applications due to its superior properties. However, natural diamond is rare and expensive, and most methods for producing synthetic diamond are not capable of easily and inexpensively producing large area monocrystalline diamond films. Accordingly, there remains a need for a method for producing large area monocrystalline diamond films for optical, electronic, and other devices.

Previous attempts to grow monocrystalline diamond by CVD have required the use of diamond substrates or specially prepared substrates. Beetz, Jr., U.S. Pat. No. 5,006,914 and U.S. Pat. No. 5,030,583 discloses the production of epitaxial single crystal diamond on a bulk single crystal diamond substrate. The substrate is prepared by having post surfaces formed therein by etching away of the substrate. A disadvantage of these and other prior monocrystalline diamond film production methods is that the required substrates are relatively expensive and are not readily available in large sizes which facilitate the production of large surface area thin films. Furthermore, such methods may require complex substrate surface preparation which adds to the expense and complication of the process.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method for producing monocrystalline diamond films; to provide such a method which does not require the use of monocrystalline diamond substrates; to provide a method for chemical vapor deposition of monocrystalline diamond films of relatively large area; to provide such a method which is applicable for use with a wide variety of simply prepared substrates; to provide such a method which may be carried out using conventional CVD equipment; and to provide such a method which is capable of producing high quality monocrystalline diamond film for optical and electrical applications.

Briefly, therefore, the invention is directed to a process for depositing a monocrystalline diamond film onto a continuous surface of a substrate by chemical vapor deposition from a carbon-containing gas in a chamber. The process comprises nucleating diamond crystallites substantially ordered in a plane of growth and of substantially only one orientation perpendicular to the plane of growth by applying a d.c. bias to the substrate surface which results in an applied current density having a magnitude in the range of about 0.01 A/cm ² to about 1 A/cm ² while flowing the carbon-containing gas into the chamber and energizing the gas. The process further comprises growing the diamond crystallites to coalescence as a monocrystalline diamond film having a continuous surface area of greater than about 1mm ² .

The invention is also directed to a process for depositing a monocrystalline diamond film onto a continuous surface of a substrate by chemical vapor deposition from a carbon-containing gas in a chamber. The process comprises nucleating diamond crystallites substantially ordered in a

plane of growth and of substantially only one orientation perpendicular to the plane of growth by applying a d.c. bias to the substrate surface which results in an applied current density having a magnitude in the range of about 0.01 A/cm ² to about 1 A/cm ² while flowing the carbon-containing gas into the chamber and energizing the gas. The process further comprises growing the diamond crystallites to coalescence as a monocrystalline diamond film having a continuous surface area of greater than about 1mm ² by maintaining the d.c. bias applied to the substrate surface which results in an applied current density having a magnitude in the range of about 0.01 A/cm ² to about 1 A/cm ² while continuing said flow of the carbon-containing gas into the chamber and energizing the gas. The invention is also directed to a process for depositing a monocrystalline diamond film onto a surface of a substrate by hot filament chemical vapor deposition from a carbon-containing gas in a chamber having a filament therein for energizing the gas. The process comprises nucleating diamond crystallites substantially ordered in a plane of growth and of substantially only one orientation perpendicular to the plane of growth on the substrate by applying an electrical d.c. bias to the substrate surface relative to the filament which results in an applied current density having a magnitude in the range of about 0.01 A/cm ² to about 1 A/cm ² as carbon-containing gas is flowed into the chamber and into contact with the filament while heating the filament to energize the gas. The process further comprises growing the diamond crystallites to coalescence as a monocrystalline diamond film having a continuous surface area of greater than about 1mm ² by maintaining the electrical d.c. bias voltage on the substrate surface relative to the filament which results in an applied current

density having a magnitude in the range of about 0.01 A/cm ² to about 1 A/cm ² while continuing said flow of the carbon- containing gas into the chamber and into contact with the filament while heating the filament to energize the gas. The invention is further directed to a process for depositing a monocrystalline diamond film onto a surface of a substrate by hot filament chemical vapor deposition from a carbon-containing gas in a chamber having a filament therein for energizing the gas. The process comprises positioning the substrate surface at a distance from the filament of between about 1 mm and about 50 mm and nucleating diamond crystallites substantially ordered in a plane of growth and of substantially only one orientation perpendicular to the plane of growth on the substrate by applying an electrical d.c. bias voltage having a magnitude in the range of about positive 50 to about positive 150 volts and an electrical d.c. bias current having a magnitude in the range of about 0.06 A/cm ² to about 0.8 A/cm ² to the substrate surface relative to the filament to establish an electrical field between the filament and the substrate. The field lines of the established electrical field are generally perpendicular to the substrate surface, as carbon-containing gas is flowed into the chamber and into contact with the filament while the filament is heated to a temperature between about 1800°C and about 2400°C to energize the gas while the substrate surface is maintained at a temperature between about 800°C and about 1300°C, the carbon-containing gas having a ratio of hydrogen atoms to carbon atoms in the range of about 200:1 to about 10:1.. The process further comprises growing the diamond crystallites to coalescence as a monocrystalline diamond film having a continuous surface area of greater than about 1mm ² by maintaining the electrical field between the filament and the substrate, the field lines of which are

generally perpendicular to the substrate surface, while continuing said flow of the carbon-containing gas into the chamber and into contact with the filament while heating the filament to energize the gas. Finally, the invention is directed to an improvement in a hot filament chemical vapor deposition reactor for depositing diamond thin film onto a substrate from a carbon-containing gas, the reactor having a chamber having an inlet for the carbon-containing gas, means for evacuating the chamber, a filament for energizing the gas, and a support for directly holding the substrate or for holding a substrate holder in a position such that the substrate is exposed to the gas energized by the filament. The improvement comprises a dc bias power supply applying a positive electrical bias to the substrate relative to the filament, the power supply having a positive lead for electrical connection to the substrate and a negative lead for electrical connection to the filament.

Other objects and features of the invention will be in part apparent and in part disclosed hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic representation of an apparatus for use in connection with the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention involves the deposition of carbon to form a monocrystalline diamond film using one of several CVD techniques which, in a general sense, are well known in the art. These techniques include hot filament, microwave plasma, radio frequency plasma, arc discharge, electron assisted, direct current discharge, atmospheric pressure

plasma, combustion flame, for example, and others. Hot filament CVD (HFCVD) is the preferred technique.

Fig. 1 shows a schematic of a hot filament CVD (HFCVD) reactor 1 which can be used in connection with the method of the invention. This reactor includes a chamber 2, gas control system 3, DC bias power supply 4, filament 5, filament power control 6, exhaust system 8, and substrate temperature readout 9. Also shown in Fig. 1 are substrate holder 11, substrate 12, and adjustable substrate support 14. Further details of the reactor system shown are described hereinbelow. The general set up for HFCVD and similar reactors known in the prior art is disclosed in U.S. Patents 4,707,384, 4,740,263, 4,816,286, 4,981,818, 5,112,775, 5,147,687, 5,160,544, 5,169,676, and 5,186,973, the entire disclosures of which are expressly incorporated herein by reference.

In the preferred embodiment, filament 5 is a helical tungsten coil extending about 5 inches (about 12.5 cms.) between filament supports 15 and 16. The filament may, however, be constructed of rhenium or other suitable material. The preferred filament illustrated has a diameter of about 0.02 inch (about 0.5 cms.) . The coiled portion is about 2 inches (about 5 cms) in length and has an interior coil diameter of about 3/8 inch (about 1 cm. ) . It has been discovered that selection of a filament of these dimensions provides a mass and surface area which are capable of producing the electrical field and high energy conditions found to facilitate monocrystalline diamond film deposition under the conditions of the invention. The diameter of the filament itself is selected to be large enough to carry the relatively large currents which are flowed through the filament and to provide sufficient radiation heating of the substrate and of the gas phase. In general, a larger coil

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area for many applications. The dimensions of the substrate are determined by the surface area of the film which is required for the particular application.

The substrate 12 is chemically cleaned prior to deposition of the diamond film 17. In a preferred embodiment the substrate is sequentially cleaned by submersion in ultrasonic baths for about 3-5 minutes each of trichloroethanol, acetone, methanol, and de-ionized water, the substrate being rinsed with de-ionized water between each bath. The substrate is then submerged in hydrofluoric acid for about 3-5 minutes, in boiling hydrochloric acid for about 3-5 minutes, and in boiling nitric acid for about 3-5 minutes, followed by rinsing with de-ionized water and submersion in an ultrasonic deionized water bath for about 3-5 minutes.

Advantageously, the diamond film deposition in accordance with this method is made directly onto the mirror polished silicon or other substrate 12. No special surface enhancement or treatment is required other than cleaning as described herein or as is known in the art. In particular, there is no need to deposit an intermediate layer onto the substrate, to scratch it, etch it or otherwise chemically or mechanically modify it to promote the desired nucleation, growth, and coalescence. The substrate is placed in a thermally and electrically conductive substrate holder 11, for example, a 1.5 in. x 2.5 in. x 0.5 in. (about 3.8 cm. x 6.4 cm. x 1.3 cm.) graphite holder. More than one substrate may be placed in the holder at a time. This method is effective for growing monocrystalline diamond films having a surface area greater than 1 mm ² , even greater than 0.5 cm ² , and even greater than 1 cm ² . The monocrystalline diamond thin films produced by this method have special industrial utility in

view of their relatively large surface area. For example, such films are useful for semiconductor electronics.

In accordance with the method of this invention, a monocrystalline diamond film 17 is deposited onto substrate 12 by chemical vapor deposition under conditions of electrical d.c. bias and relatively high energy. Without being bound to a particular theory, it is believed that the applied d.c. bias and high energy conditions result in enhanced nucleation such that a higher density of diamond crystallites is formed than if no d.c. bias is applied or if deposition is carried out under lower energy conditions. During nucleation, the applied d.c. bias is thought to promote ordering of the nucleating crystallites such that they all grow in the same plane and such that they are highly ordered within the growth plane.

During continued growth of the crystallites, maintenance of the d.c. bias and high energy conditions facilitates eventual early coalescence of the ordered crystallites into monocrystalline diamond film. Without being bound to a particular theory, it is believed that bombardment of the substrate with electrons and negative ions as a result of the d.c. bias application and high energy conditions results in an increase in the surface energy of the substrate. This change is believed to result in a smaller critical size of the nuclei, which results in the formation of a much larger number of nuclei for a given substrate surface area. The d.c. bias between the substrate and the filament creates an electrical field therebetween, and the loops in the helical filament create a magnetic field. The Lorentz forces due to these electrical and magnetic fields help define a preferential direction of crystallite orientation. Because smaller nuclei are easier to re-orient than relatively larger nuclei, the smaller

critical size of the nuclei are more easily imparted with the preferential direction of crystallite orientation. Continued application of electrical bias such that the Lorentz forces continue to affect growth during nucleation, through subsequent growth until crystallite coalescence, is therefore believed to be important to the successful growth of monocrystalline diamond by this process. It is further believed that the d.c. bias may change the surface conditions of the growing film. The d.c. bias is maintained during growth of the crystallites until coalescence. After coalescence of the crystallites into a continuous monocrystalline diamond film, continued growth of the thickness of the film is homoepitaxial. As alternative embodiments of this invention, such continued growth may be carried out with or without application of the d.c bias voltage to the substrate.

The specific magnitude of bias current and bias potential which will produce single crystal diamond depends on a number of factors, including the particular substrate, the substrate and filament temperature, the carbon containing gas composition and flow rate, the deposition chamber pressure, the size and shape of the filament, and the distance between the filament or other power source and the substrate. The d.c. bias voltage is selected so as to result in a relatively high current density which has been found to facilitate the nucleation and growth of single crystal diamond as disclosed herein. In particular, the current density is between about 0.01 A/cm ² and about 1 A/cm ² , more preferably between about 0.06 A/cm ² and about 0.8 A/cm ² , still more preferably between about 0.06 and about 0.2 A/cm ² most preferably about 0.1 A/cm ² . To determine current density in this context, the relevant area

is the approximate surface area of the substrate contacted by a plasma ball which is formed by the energized reactant gases between the filament and the substrate.

In further regard to the high current densities required by this process, the preferred HFCVD arrangement disclosed herein is especially suited, in comparison to certain prior art systems, for obtaining such high current densities. Because the two electrodes in our most preferred system are the filament and the substrate, it is possible to have them spaced closely enough that high current densities, such as our most preferred 0.1 A/cm ² , can be obtained with relatively modest voltages.

With respect to bias potential, in the preferred embodiment where the deposition is accomplished by HFCVD, the bias potential is between about 10 V and about 1000 V, either positive or negative. In one preferred embodiment which has been found to result in monocrystalline diamond growth under the preferred HFCVD conditions disclosed herein, the d.c. bias applied to the substrate 12 relative to the filament 5 is preferably between about positive 10 V and about positive 1000 V, more preferably between about positive 50 V and about positive 150 V, still more preferably between about positive 80 V and about positive 100V, most preferably about positive 100 V. For example, under the conditions disclosed in Example 1, 100 V positive bias was applied. The bias is applied from the positive pole of d.c. bias power supply 4 via d.c. bias positive lead 20 to substrate holder 11 to substrate 12. The negative pole of d.c. bias power supply 4 is connected via d.c. bias negative lead 21 and conductive filament power support 15 to the filament 5.

Upon application of potential between two poles, the positive pole being the substrate 12 and the negative

pole being the filament 5, an electrical field between the substrate and the filament is established which is applied to a continuous surface of the substrate 12 on which the diamond film is grown. In the preferred embodiment where HFCVD is used and a positive bias is applied between to the substrate relative to the filament, the selected applied d.c. bias and distance between the poles results in the formation of a glowing plasma ball visible between the substrate and the filament. The magnitude of the electrical field, which is primarily dependent upon the quantity of voltage applied and the distance between the poles, is difficult to determine as it is not uniform. The distance between the substrate and the filament is significantly closer than in prior art HFCVD systems and is maintained as close as possible without risking melting of the substrate, preferably between about 1mm and about 50 mm, more preferably between about 1 mm and about 10 mm, and most preferably between about 2mm and about 4 mm. This distance is adjustable prior to beginning the deposition process by manipulation of adjustable substrate support 14.

In the preferred embodiment where the deposition is accomplished by HFCVD, the bias current which has been found to result in monocrystalline diamond growth under the preferred HFCVD conditions disclosed herein is between about 0.1 A (A = amperes) and 2.3 A, more preferably between about 0.32 A and 0.39 A.

The filament temperature, where HFCVD and a tungsten filament are used, is preferably maintained between about 1800°C and about 2400°C, more preferably between about 1900° and 2300°C, and most preferably about 2200°C. If desired, a higher filament temperature may be used, for example, where a rhenium or other more heat resistant filament material is used. The relatively high filament

temperature employed in this invention has been discovered to favor monocrystalline diamond nucleation and growth. The filament temperature is measured by two-wavelength, optical pyrometer 25. Power is supplied to the filament via filament power lines 22 and 23 and filament power supports 15 and 16 for resistively heating the filament.

The substrate temperature on the surface on which the diamond is grown is preferably maintained between about 800°C and about 1300°C, more preferably between about 900°C and 1200°C, most preferably between about 950°C and about

1150°C. Such relatively high substrate surface temperatures have been discovered to facilitate nucleation and growth of monocrystalline diamond film. Substrate temperatures significantly above about 1300°C are avoided because such temperatures favor the deposition of graphite over diamond. The growth surface temperature is typically not measured directly, but the underneath surface of the substrate is measured using a thermocouple positioned where the substrate is supported on a graphite support. As described in Example 1, for instance, this temperature was maintained at about 815°C. The substrate temperature is measured using a Chrome1-Alumel thermocouple 26 and substrate temperature readout 9. The substrate is independently heated if the heat supplied by the filament or other power source is insufficient to maintain the substrate at the selected temperature. The substrate is independently cooled if the heat supplied by the power source is such that the substrate would be overheated without such cooling.

The pressure in the deposition chamber is preferably maintained between about 10 torr and about 1 atm, more preferably between about 20 torr and 40 torr, and most preferably between about 30 and 40 torr. Mechanical vacuum pump 30 continuously withdraws gas from the chamber via

vacuum exhaust tube 31. During diamond growth, main manual valve 32 is closed and metering valve 34 is operated to control the chamber pressure. Constant or near constant pressure in the chamber is maintained by matching the gas flow into the chamber with the conductance through metering valve 34. A preselected pressure can be set on pressure readout/controller 33. When the system pressure read by pressure manometer 35 (Baratron™, MKS Instruments, Inc., Andover, Mass.) is greater than the preset value, controller 33 will direct closure of the valve 56, which is operated by nitrogen source 57, via signal line 55 to interrupt the supply of reactant gases and thereby reduce the chamber pressure. Once the chamber pressure is reduced to a point below the preset value, the flow of reactant gases is resumed.

The supply of carbon for the diamond film formation is solid graphite, a carbon-containing liquid such as acetone, methanol, or ethanol, or a carbon-containing gas such as methane, ethane, propane, ethylene, acetylene, cyclohexane, or benzene. Where a liquid carbon source is used, it must be volatilized and transported into the reaction chamber as a gas. The preferred carbon source is methane, which is preferably transported into the deposition chamber as a gaseous mixture of 1% methane and 99% hydrogen. As shown in Fig. 1, the gas is supplied to the chamber via pure copper gas inlet tube 18 to which is supplied gas via reactant gas supply line 50. Flow control system 3 includes mass flow controller (MFC) 52 to control the supply of gas from supply tanks 58 and 59, which, in the preferred embodiment contain methane (99.99% pure) and hydrogen (99.999% pure) .

Under the conditions described herein we have achieved nucleation and growth of diamond crystallites,

substantially all of which grow in a single plane, for example, (111) , substantially all of which are highly ordered within the plane of growth and have only one orientation perpendicular to the plane of growth. According to this invention, crystallites nucleate and grow, substantially all of which are not randomly oriented in the plane of growth, that is, which are highly ordered in the plane of growth. The continued growth of these crystallites as disclosed causes coalescence of the highly ordered crystallites into a continuous monocrystalline diamond film. The nucleation and growth of diamond crystallites, substantially all of which are highly ordered in the plane of growth and substantially all of which are oriented perpendicular to the plane of growth, and coalescence of the monocrystalline film, is achieved by deposition under the electric bias and high energy conditions disclosed herein.

In one particular embodiment of the invention, the bias applied to the substrate is positive relative to the filament and in the bias voltage ranges described above. This positive bias results in an electrical field which points outward from the substrate and which has field lines which are generally perpendicular to the substrate. To establish an electrical field having these characteristics, the filament must be carefully configured and positioned. Without being bound to a particular theory, in this particular embodiment of the invention, the success of the process is believed to be sensitive to the configuration of the electric field, as influenced by the physical characteristics of the filament, as a slight variation of the filament configuration or position which produces the perpendicular field lines can upset the substantially uniformly perpendicular nature of the field lines.

The following example further illustrates the invention:

Example 1 A mirror-polished silicon wafer (3 in. diameter) obtained from Si-Tech, Inc. having (100) crystal orientation was chemically cleaned using the procedure described hereinabove. A helical tungsten coil was selected which extended about 5 inches between filament supports in an HFCVD reactor. The filament had a diameter of about 0.02 inch, a coiled portion about 2 inches in length and an interior coil diameter of about 3/8 inch. The filament was carburized by heating to about 2300°C and flowing a gas containing 10% methane and 90% hydrogen into the deposition chamber for several hours. The substrate was cut into several pieces and mounted on a graphite holder (1.5x2.5x0.5 in ³ ) . The holder was positioned such that the silicon substrate was 2-4 mm from the tungsten coil filament of a hot filament CVD chamber and a positive d.c. bias was applied to the substrate relative to the filament. A deposition chamber pressure of 30-40 torr was established and 1 seem of methane and 99 seem of hydrogen (measured by MKS flowmeters, MKS Instrument, Inc., Andover, Mass.) were caused to flow into the chamber. The filament temperature was maintained at about 2200°C as measured by a Micron digital two wavelength optical pyrometer. The temperature of the substrate was such that measurements taken on its underside were 815°C +/- 20°C as measured by a Chromel- Alumel thermocouple. A positive bias potential of 100 V was applied to the substrate relative to the filament with a current density of about 0.1 A/m ² . The bias current ranged from 0.32 to 0.39 amp. As the bias voltage was applied, a bright plasma ball formed between the filament and the substrate. The chemical vapor deposition and application of

bias was continued through a thin film nucleation and growth period of 30 minutes.

The thin film deposition was terminated by turning off the biasing power supply source and the methane source. The metering valve was fully opened to gradually bring the pressure in the chamber to about 10 torr. The hydrogen gas source and power supply to the filament were terminated. The main valve was opened to pump the chamber down to about 10 microns, i.e., about 0.01 torr. Both the metering and main valves were closed to maintain the established vacuum in the chamber. After about two hours, or when the chamber was cooled to about room temperature, the vacuum in the chamber was released and the substrate and thin film were removed. The deposited film was determined by x-ray diffraction to be monocrystalline.

As various changes could be made in the above embodiments without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.