Method of Making Synthetic Diamond Film

Europaisches Patentamt European Patent Office © Publication number: 0 597 445 A2 Office europeen des brevets

EUROPEAN PATENT APPLICATION

@ Date of filing: 09.11.93

® Priority: 10.11.92 US 973994 32 Cornell Street Arlington, MA 02174(US) @ Date of publication of application: Inventor: Frey, Robert M. 18.05.94 Bulletin 94/20 77 Santa Isabel Nr.2-14 Laguna Vista, Texas 78578(US) © Designated Contracting States: DE FR GB 0 Representative: Diehl, Hermann, Dr. © Applicant: NORTON COMPANY Dipl.-Phys. et al 1 New Bond Street DIEHL, GLASER, HILTL & PARTNER Worcester, MA 01615-0008(US) Patentanwalte Fluggenstrasse 13 @ Inventor: Simpson, Matthew D-80639 Munchen (DE)

© The invention describes a method for making a free-standing synthetic diamond film of desired thickness, comprising the steps of: no SELECT TARGET providing a substrate; X THICKNESS OF DIAMOND selecting a target thickness of diamond to be TO BE PRODUCED produced, said target thickness being in the range 200 urn to 1000 urn; finishing a surface of the substrate to a rough- 1 20 finish surface of ness, RA that is a function of the target thickness, substrate to prescribed said roughness being determined from ra Roughness 0.38t/600 urn ^ Ra = 0.50 urn 200 urn < t ^ 600 urn 0.38 urn ^ Ra = 0.50 urn 600 urn < t < 1000 1 30, urn DEPOSIT INTERLAYER where t is the target thickness; said the inter- CM depositing an interlayer on substrate, layer having a thickness in the range 1 to 20 urn; depositing synthetic diamond on said interlayer, by I40. DEPOSIT SYNTHETIC DIAMOND chemical vapor deposition, to about the target thick- BY CVD TO ABOUT THE ness; and TARGET THICKNESS cooling said synthetic diamond to effect the release Oi thereof. 1 50 RELEASE DIAMOND FROM SUBSTRATE BY COOLING

FIG. I

Rank Xerox (UK) Business Services (3. 10/3.09/3.3.4) 1 EP 0 597 445 A2 2

This invention relates to a method of making released from the substrate after the desired thick- synthetic diamond film. ness of synthetic diamond film has been deposited, Diamond has a number of properties which such as by chemical vapor deposition. The dia- make it attractive for use in various applications. mond is deposited at a relatively high temperature Among these properties are extreme hardness and 5 and, as the diamond (as well as the titanium nitride excellent transmissivity of certain radiation. Dia- interlayer and substrate below) cools after comple- mond is also an extraordinary heat conductor, ther- tion of the diamond deposition, the diamond should mally stable, and an electrical insulator. However, be released from the substrate, preferably in one natural diamond is prohibitively expensive for ap- piece. However, problems have been found to oc- plications which require any substantial size and is io cur in the procedure. One of these problems is difficult to form into certain shapes. premature flaking off of the diamond and/or its In recent years, a number of techniques have underlayer during deposition or premature release been developed for synthesizing diamond and for of the diamond before deposition is complete. A depositing synthetic diamond on surfaces of var- further problem is cracking of the diamond upon its ious shapes to obtain a diamond film or coating. 75 release from the substrate. These techniques include so-called high-pressure It is therefore the object of the present inven- high-temperature ("HPHT") methods and chemical tion to a method for making a free standing syn- vapor deposition ("CVD") methods. The CVD thetic diamond film of desired thickness which methods include plasma deposition techniques avoids the above mentioned drawbacks of the prior wherein, for example, plasmas of a hydrocarbon 20 art. This object is solved by the method according and hydrogen are obtained using electrical arcing. to independent claim 1 . Further advantageous fea- The resultant plasma can be focused and acceler- tures, aspects and details of the invention are evi- ated toward a substrate using focusing and accel- dent from the dependent claims, the description, erating magnets. Reference can be made, for ex- the examples and the drawings. The claims are to ample, to U.S. Patent Application Serial No. 25 understood as a first non-limiting approach to de- 773,465, assigned to the same assignee as the fine the invention in general terms. present Application, for description of an example The invention specifically refers to generally of a type of plasma jet deposition that can be improve the fabrication of free-standing synthetic utilized to deposit synthetic diamond on a sub- diamond by chemical vapor deposition process. strate. 30 Applicant has discovered that for obtainment of Synthetic diamond film can be deposited as a relatively thick free standing diamond films having permanent coating on a substrate, such as on the a thickness in the range 200 to 1000 urn, the wear surface of a tool or as an environmentally roughness of the substrate surface, beneath the protective coating. Such films are generally consid- thin interlayer upon which the diamond is to be ered to be relatively thin films. Alternatively, a 35 deposited by CVD, should be closely controlled in synthetic diamond film that is generally considered order to maximize the efficacy of the diamond a thick film, can be deposited on a substrate and deposition and release process. In particular, the then removed, preferably intact as a single "free substrate surface roughness must not only be standing" piece, for use in applications such as smooth enough to permit release of the diamond heat sinks, optical windows, and in tools. However, 40 after deposition of a desired diamond thickness is the obtainment of such thick films, especially of complete, but also rough enough to prevent pre- relatively large area, has proven troublesome. In mature lift-off of the diamond or flaking-off of dia- addition to the difficulty of depositing quality syn- mond during the deposition process. thetic diamond of substantial thickness, there is the In accordance with an embodiment of the in- problem of removing the diamond intact from the 45 vention, there is set forth a method for making a substrate. The substrate material will generally free-standing synthetic diamond film of desired have a different coefficient of expansion than the thickness, comprising the following steps: providing diamond, as well as a different molecular and a substrate; selecting a target thickness of diamond chemical structure. The adherence and growth of to be produced, said target thickness being in the the diamond film, as well as its release, will de- 50 range 200 urn to 1000 urn; finishing a surface of pend, inter alia, on the materials used, surface the substrate to a roughness, RA, that is a function preparation, and deposition parameters. of the target thickness, said roughness being deter- Titanium nitride and other materials have been mined from used as a coating for a substrate, such as molyb- 0.38t/600 urn ^ Ra = 0.50 urn 200 urn < t denum, upon which synthetic diamond is to be 55 ^ 600 urn deposited. Titanium nitride adheres reasonably well 0.38 urn ^ Ra = 0.50 urn 600 urn < t < to molybdenum. Diamond can be deposited over a 1000 urn thin layer of the titanium nitride and then, ideally, where t is the target thickness; depositing an inter-

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layer on the substrate, the interlayer having a thick- 0.38 urn ^ Ra = 0.50 urn 600 urn < t < ness in the range 1 to 20 urn; depositing synthetic 1000 urn diamond on said interlayer, by chemical vapor de- where t is the target thickness. position, to about the target thickness; and cooling A relatively thin interlayer, preferably in the the synthetic diamond to effect the release thereof. 5 range 1 to 20 urn, is then deposited on the finished In a disclosed embodiment hereof, the step of substrate surface (block 130), such as by physical providing a substrate comprises providing a molyb- vapor deposition ("PVD"). The interlayer, which denum substrate, and said step of depositing an may if desired comprise several sublayers, should interlayer comprises depositing a layer of titanium not bond strongly to diamond. A strong chemical nitride. In this embodiment, step of depositing an io bond will promote adhesion and ultimately prevent interlayer also preferably comprises depositing an removal of the diamond from the substrate in one interlayer having a thickness in the range 3 to 5 piece. The layer should be thick enough to prevent urn. chemical bonding of the diamond to the underlying As seen from the above indicated relationship substrate, and thin enough to maintain the neces- between target diamond thickness and surface 15 sary degree of roughness of the coated substrate roughness, for target thicknesses between 200 and surface to permit a degree of mechanical bonding 600 urn the minimum acceptable surface rough- that deters premature release. For titanium nitride, ness increases with increasing target diamond a preferred interlayer hereof, the layer will have a thickness. This results in reducing instances of the thickness in the range about 3 to 5 urn. Examples types of failure that were first described above. 20 of other interlayer materials that can be utilized Further features and advantages of the inven- herein are titanium carbide, hafnium nitride, zirco- tion will become more readily apparent from the nium nitride, aluminum nitride, and aluminum ox- following detailed description when taken in con- ide. Mixtures and compounds of these materials junction with the accompanying drawings. can also be utilized. Fig. 1 is an operational flow diagram of the 25 Synthetic diamond is then deposited, by steps of an embodiment of the method of the chemical vapor deposition, to about the target invention. thickness, as represented by the block 140. As Fig. 2 is a schematic diagram of a plasma jet used herein, deposition to about the target thick- deposition system which can be utilized for CVD ness means deposition to within plus or minus ten deposition of synthetic diamond for use in an em- 30 percent of the target thickness. The description bodiment of the invention. below, in conjunction with Fig. 2, illustrates a tech- Referring to Fig. 1, there is shown an oper- nique of CVD plasma jet deposition, but other ational flow diagram of the steps of a procedure for techniques of CVD synthetic diamond deposition obtaining free-standing synthetic diamond film of a can be employed. It can be noted that the invention desired thickness in accordance with an embodi- 35 is particularly applicable to techniques of CVD syn- ment of the invention. The block 110 represents thetic diamond deposition, such as plasma jet de- selection of the target thickness of diamond to be position, wherein the diamond is deposited at a obtained, the invention being directed to a target relatively high temperature and subject to substan- diamond thickness in the range 200 to 1000 urn. tial stresses during the deposition and removal The surface of the substrate to be used for dia- 40 processes. After the target thickness is reached, mond deposition is then finished to a prescribed the synthetic diamond layer can be released from roughness, (block 120). The substrate should have the substrate by cooling, as represented by the a coefficient of thermal expansion relatively close block 150. Release is largely due to mechanical (preferably within 10_5/°K) to that of diamond, and stresses upon cooling, and occurs between about should be a reasonably good thermal conductor. 45 800 and 400 °C. A jet of nitrogen gas can be The preferred substrates hereof are molybdenum, directed at the edge of the diamond to assist the tungsten, and graphite. Molybdenum (including its release. In general, when the diamond is released alloys such as TZM, which contains relatively small most of the interlayer will remain with the substrate, percentages of titanium and zirconium) is presently and any of the interlayer that is on the diamond considered particularly preferred. The surface of 50 can be removed chemically, such as by selective the substrate is finished to a roughness, RA [Ra etching. If the remaining substrate and interlayer being the universally recognized international pa- are in sufficiently good condition, they can be used rameter of roughness, which is the arithmetic mean again a number of times for diamond deposition. of the departure of the surface profile from the When necessary, the substrate surface can be mean line], as a function of the target diamond 55 refinished and recoated with an interlayer as pre- thickness, the roughness being determined from viously described. 0.38t/600 urn ^ Ra = 0.50 urn 200 urn < t It will be understood that, if desired, the steps ^ 600 urn of finishing the surface of the substrate (block 120)

3 5 EP 0 597 445 A2 6 and/or of depositing the interlayer (block 130) can prepared on 7.62 cm (3 inch) diameter round be performed beforehand, to obtain a supply of mesas on the 15.24 cm (6 inch) discs. The sub- substrates and/or coated substrates from which to strate surfaces were lapped with a slurry of dia- choose after the target thickness is selected. This mond or boron carbide grit to a roughness, RA, sequence of steps is equivalent to performing the 5 ranging from about 0.33 urn to about 0.51 urn. finishing and coating of the surface after the target After coating by PVD with a titanium nitride inter- thickness is selected. layer of thickness in the range 3 to 5 urn, equip- Referring to Fig. 2, there is shown a diagram of ment of the general type shown in Fig. 2 was used a plasma jet deposition system 200 of a type which to deposit synthetic diamond in diameters of about can be utilized in practicing an embodiment of the io 7.6 to 10.2 cm (about 3 to 4 inches), and at invention. The system 200 is contained within a thicknesses in the approximate range 200 to 1000 vacuum housing 211 and includes an arc-forming urn. The temperatures at which the diamond re- section 215 which comprises a cylindrical anode leased (if no premature lift-off) were in the range 291, a rod-like cathode 292, and an injector 295 about 800 to 400 °C. Samples having intended mounted adjacent the cathode so as to permit is thicknesses of above about 600 urn, where the RA injected fluid to pass over the cathode 292. In the roughness was less than 0.38, had a higher in- illustrated system the input fluid may be a mixture cidence of premature lift-off, and samples where of hydrogen and methane. The anode 291 and the RA roughness was greater than 0.50 had a cathode 292 are energized by a source of electric higher incidence of cracking upon release. For thic- potential (not shown), for example a DC potential. 20 knesses less than about 600 urn, the minimum Cylindrical magnets, designated by reference nu- roughness needed to prevent premature release meral 217, are utilized to control the plasma gen- was observed to vary approximately linearly with erated at the arc forming section. The magnets thickness as 0.38t/600 urn. Further samples were maintain the plasma within a narrow column until made in the same manner, but with the substrate the plasma reaches the deposition region 60. Cool- 25 first polished to RA < 0.1 urn. Deposited diamond ing coils 234, in which liquid nitrogen can be cir- spalled off before its thickness reached 75 urn. culated, are located within the magnets and sur- round the focused plasma. Claims In operation, a mixture of hydrogen and methane is fed to the injector 295, and a plasma is 30 1. A method for making a free-standing synthetic obtained in front of the arc forming section and diamond film of desired thickness, comprising accelerated and focused toward the deposition re- the steps of: gion. The temperature and pressure at the plasma providing a substrate; formation region are typically in the approximate selecting a target thickness of diamond to be ranges 1500-15,000 ° C and 133-931 mbar (100-700 35 produced, said target thickness being in the torr), respectively, and in the deposition region are range 200 urn to 1000 urn; in the approximate ranges 800-11 00 °C and 0.13 - finishing a surface of the substrate to a rough- 266 mbar (0.1-200 torr), respectively. As is known ness, RA,that is a function of the target thick- in the art, synthetic polycrystalline diamond can be ness, said roughness being determined from formed from the described plasma, as the carbon 40 0.38t/600 urn ^ Ra = 0.50 urn 200 pm in the methane is selectively deposited as dia- < t ^ 600 urn mond, and the graphite which forms is dissipated 0.38 urn ^ Ra = 0.50 urn 600 urn < t < by combination with the hydrogen facilitating gas. 1000 urn The bottom portion 105A of the chamber has a where t is the target thickness; base 106 on which can be mounted the substrate 45 depositing an interlayer on said substrate, the 10 with the titanium nitride layer 30 on which the interlayer having a thickness in the range 1 to synthetic diamond is to be deposited. The base 20 urn; can include a temperature controller. depositing synthetic diamond on said interlayer, by chemical vapor deposition, to about EXAMPLES 50 the target thickness; and cooling said synthetic diamond to effect the A number of samples (about forty) of synthetic release thereof. diamond film, with thicknesses in the approximate range 200 to 1000 urn, were deposited using CVD 2. The method as defined by claim 1, wherein plasma jet deposition equipment of the type de- 55 said step of providing a substrate comprises scribed in conjunction with Fig. 2. The substrates providing a substrate of a material selected used were molybdenum discs of about 15.2 cm from the group consisting of molybdenum, (about 6 inch) diameter. Some of the samples were tungsten, and graphite.

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3. The method as defined by claim 1 or 2, wherein said step of depositing an interlayer comprises depositing a material selected from the group consisting of titanium nitride, titanium carbide, hafnium nitride, zirconium 5 nitride, aluminum nitride, and aluminum oxide.

4. The method as defined by one of the preced- ing claims, wherein said step of depositing an interlayer comprises depositing an interlayer 10 having a thickness in the range 3 to 5 urn.

5. The method as defined by one of the preced- ing claims, wherein said step of depositing synthetic diamond on said interlayer comprises is depositing synthetic diamond by plasma jet CVD.

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SELECT TARGET X THICKNESS OF DIAMOND TO BE PRODUCED

finish surface of 120. substrate to prescribed

ra Roughness

130 DEPOSIT INTERLAYER

140 DEPOSIT SYNTHETIC DIAMOND BY CVD TO ABOUT THE TARGET THICKNESS

150^ RELEASE DIAMOND FROM " SUBSTRATE BY COOLING

FIG. I

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106

FIG. 2