Selective diamond seed deposition using electroplated copper

Selective diamond seed deposition using electroplated copper

Diamond and Related Materials, Ol (1992) 907 910 Elsevier Science Publishers B.V., Amsterdam 907 Letter Selective diamond seed deposition using elec...

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Diamond and Related Materials, Ol (1992) 907 910 Elsevier Science Publishers B.V., Amsterdam

907

Letter Selective diamond seed deposition using electroplated copper R. Ramesham, F. M. R o s e 1 a n d A. A l l e r m a n 2 Electrical Engineering Department, Alabama Microelectronics Science and Technology Center, Auburn University, Auburn, AL 36849-5201 ( USA ) (Received March 3, 1992)

Abstract A process for selective seeding of conducting substrates (molybdenum and copper) with submicron diamond particles has been studied. In this process, copper is electroplated through a photoresist mask, and the electroplating is performed in a stirred solution of CuSO 4 and HzSO 4 into which ~0.1 jam size diamond particles have been added. This results in a continuous layer of diamond particles embedded in the electroplated copper. After the removal of photoresist, this layer is used to seed further CVD diamond growth selectively. Morphology of both the as-plated copper/diamond matrix and the film that resulted after further CVD diamond deposition are reported.

The American Association for the Advancement of Science has recognized diamond as the Material of the Year 1990, based on progress in its applications. This material could certainly be used for innumerable applications owing to its remarkable physical and chemical properties. Diamond has the highest thermal conductivity, the highest molar density and the highest hardness, and the lowest compressibility of any other known materials; it has a high Young's modulus, a high electrical resistivity, high Johnson's and Keye's figures of merit, a wide bandgap, a wide optical transparency; it is chemically very inert, has a high breakdown strength, a high electron and hole mobility, etc. [-1, 2], and therefore, several potential applications may be anticipated in electronics, optoelectronics, optics, protective mechanical coatings, etc. These applications include devices which could operate at high temperatures (> 600 °C), at high frequency and high power; they include blue light emitting diodes, heat sinks, wear resistant coatings, X-ray lithographic masks, sensors, optical windows, cold cathodes, passivating and corrosion resistant coatings etc. However, progress in diamond technology is hindered to a large extent because the growth temperature in most of the techniques reported in the literature is higher than 800°C, because of the inability to grow single crystal diamond on non-diamond substrates, and the difficulties of incorporating n-type carriers, and finally, because of the necessity of scratching non-diamond 1Space Power Institute. 2Physics Department.

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substrates with a diamond paste scratch pretreatment for enhancing the nucleation density. Although it is well known that substrate abrasion enhances nucleation density by several orders of magnitude on many substrates [3 5], this is often an undesirable and infeasible approach. Diamond pretreatment is not always possible when one desires to deposit on non-planar substrates or large substrates, and on various substrates. Selective deposition is a critical requirement for many applications of synthetic diamond thin film, and many microelectronic applications require patterned diamond thin films [6]. There have been a few attempts to seed non-diamond substrates by diamond particles using the electrophoretic method of Valdes et al. [7] and spin coating of diamond seeds with a carrier [8, 9]. The latter method is not suitable for non-planar substrates since spin coating is not possible in this situation. More recently, Meilunas and Chang [10] have shown that the carbon clusters (buckminsterfullerenes) of C6o and C70 grown on silicon wafers acted as diamond nucleation sites only when the substrate was given a certain pretreatment prior to diamond growth, and this treatment involved biasing of silicon substrates negatively with respect to the plasma for several minutes. The enhancement of nucleation density by the C7o layer (thickness, 1000 A) is nearly ten orders of magnitude as a result of biasing treatment. Seeding of non-planar substrates is very useful, especially for protective diamond coating applications. In this paper, we provide a simple viable process for enhancing the diamond nucleation density by seeding

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R. Ramesham et al. / Selective diamond seeding using electroplated Cu

non-diamond substrates with diamond particles using electroplating of copper. As the plating of metal is feasible on practically any complex substrate shapes, this suggests it is possible to seed the plated metal with diamond particles during the process. The molybdenum substrates were thoroughly washed with deionised water, acetone and methanol, ultrasonically agitated in deionised water, and dried with a jet of nitrogen gas. Conventional photolithography was used to pattern thick photoresist (Shipley, Type S1650, 2000 rpm) with a desired negative mask of squares of various sizes on a molybdenum substrate. Finally, the sample was hardbaked at 115 °C for 1 h after patterning. To supplement the process description for the selective diamond seeding and selective CVD diamond growth, a schematic diagram of the process is shown in Fig. 1. Acid copper plating solution was used to plate copper as per the composition provided in ref. 11. The solution consisted of CuSO4 (210 g 1 -1), H2SO4 (55 ml 1-1), and water. The diamond particles (~ 0.1 ~tm average particle size; PSI Systems, Houston, TX) were added into the solution and the solution was then stirred vigorously with a magnetic pellet during electroplating of copper. Typical electroplating parameters were as follows: Working electrode (substrate), molybdenum or copper; counter electrode, platinum; current density, ,-~10mAcm-2; D.C. bias, 1.6-1.8 V; plating time, 5-10 min. A commercially available high-pressure microwave plasma-assisted CVD system was used in the experiments to grow polycrystalline diamond thin films. The typical deposition parameters were as follows: deposition pressure, 45.4 Torr; forward power, 1251 W; reflected power, 26 W; hydrogen flow rate, 500 sccm; methane flow rate, 3.6 sccm; substrate temperature, 925 °C. The schematic diagram of the microwave plasma system and the process

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details of diamond deposition are described elsewhere 1-12, 13]. Figure 2(a) shows a scanning electron micrograph of a selectively electroplated copper/diamond matrix using a square mask on a molybdenum substrate. Figures 2(b) and 2(c) show micrographs of the corner of a square and the typical morphology of the copper/diamond matrix respectively. Diamond particles may be seen clearly in Fig. 2(c). The size of the diamond particles is roughly ten times that of the particles added into the

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Fig. 2. Scanning electron micrographs of the selectivediamond seed particles using electroplated copper on a molybdenum substrate: (a) pattern of the square; (b) corner of the square; (c) typical morphology.

R. Ramesham et al. / Selective diamond seeding using electroplated Cu

solution. This may be due to copper plating around the diamond particles, or perhaps the added diamond particles are different from the size specified by the company from which they were purchased. It is an important observation to speculate that the diamond particles embedded in the copper plating could be used as seed crystals to nucleate and grow CVD diamond. These plated samples were subject to the Scotch tape test and to ultrasonic agitation for a few minutes, and we found that the adhesion strength was good since the copper/ diamond matrix remained intact. The Z-axis pull stud test used to study the adhesion strength of the matrix on different substrates with various pretreatments, using the procedure described in ref. 14. A nickel plating solution has also been used to plate Ni/diamond matrix over a molybdenum substrate. In other words, diamond particles may be added to virtually any plating solution to obtain a matrix of metal/diamond over a desired substrate. Figure 3 shows micrographs of the microwave plasmaassisted CVD diamond grown on a copper/diamond matrix. Figure 3(a) is an optical micrograph of the complete pattern of selectively deposited diamond, scanning electron micrographs are shown in Figs. 3(b) (a square) and 3(c) (stripes), and the typical morphology of

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Fig. 4. Scanning electron micrograph showing typical morphology of the polycrystalline diamond thin films grown on scratched silicon substrates [13].

diamond at the corner of a square is shown in 3(d). The morphology of the diamond is similar to the morphology of diamond grown on scratched silicon substrates, as shown in Fig. 4 [13]. Very thick and dense deposits may be seen in the desired areas and sparse deposits in the undesired areas. This could be due to the non-uniformity of the molybdenum substrates and/or the seed diamond particles being splattered from the matrix during CVD

Fig. 3. Selectively deposited diamond using selective diamond seed patterns on a molybdenum substrate: (a) optical micrograph of the complete pattern; scanning electron micrographs of (b) pattern of the square in (a), (cl pattern of the stripe's in (a), and (d) corner of the square and the typical morphology.

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R. Ramesham et al. / Selective diamond seeding using electroplated Cu

diamond growth at the substrate temperature of 925 °C. Diamond growth time was reduced by approximately half to obtain a continuous layer of diamond thin film on the seeded samples when compared with the scratch pretreatment used on molybdenum substrates. In our experiment the chamber wall was coated with copper (melting point of Cu, 1083 °C) from the matrix/ diamond during diamond growth. This may be improved if the diamond is grown at lower substrate temperatures. Preliminary experiments show that the diamond films have poor adhesion strength when they are grown at a substrate temperature of 800 °C. Contamination of the films with copper may be avoided if the plated material has a higher melting point. If one wants to grow diamond on copper substrates for any particular application e.g. for microwave tubes or as a heat sink, this method of seeding diamond particles in the copper plating over a copper substrate could definitely be useful; other combinations such as Ni+diamond/nickel substrate or Cr + diamond/chromium substrate would also be useful. Although the seeding of diamond particles in nickel plating over molybdenum was possible, it resulted in poor CVD diamond growth. Polycrystalline diamond grown on metals is of wide interest for heat transfer from hybrids and for printed circuit boards. In summary, a simple technique for seeding diamond particles over conducting substrates is proposed, and was successfully demonstrated to grow CVD diamond where the seeded diamond particles are nucleation sites for diamond growth. This technique may be useful for seeding diamond particles over three-dimensional objects for CVD diamond growth for some protective mechanical applications. It is easy to seed the substrates in few minutes, no physical damage is required, the process results in selective seeding using a photoresist mask, and subsequently results in selective diamond growth. Analysis of the as-plated copper/diamond matrix and the films resulting after further CVD diamond growth by various techniques such as secondary ion mass spectrometry, Raman spectroscopy, Auger spectroscopy etc., contamination of CVD diamond films by the metal in the matrix and interfacial characterization will

be reported elsewhere. Additional utility of this technique may result from the application of electroless plating of copper onto dielectric materials.

Acknowledgments This work was supported by the Strategic Defense Initiative Organization's Office of Innovative Science and Technology (SDIO/TNI) through Contract Number N60921-91-C-0078 with the Naval Surface Warfare Center, and in part by the Alabama Microelectronics Science and Technology Center.

References 1 J. E. Field, The Properties of Diamond, Academic Press, London, 1979. 2 J. H. Edgar, J. Mater. Res., 7 (1992) 237. 3 K. Mitsuda, Y. Kojima, T. Yoshida and K. Akashi, J. Mater. Sci.. 22 (1987) 1557. 4 Y. San, J. Zeng-sun, L. Xian-yi and Z. Guang-tian, Chin. Sci. Bull., 36 (1991) 1438. 5 R. Ramesham and C. Ellis, J. Mater. Res., 7 (1992) 1189. 6 R. Ramesham, Review of Selective Growth and Microstructure Fabrication Processes of Diamond Thin Films, in T. D. Moustakas, J. I. Pankove and Y. Hamakawa (eds.), Wide Band-Gap Semiconductors, Vol. 242, Materials Research Society, Pittsburgh, PA, 1992, in the press. 7 J. L. Valdes, J. W. Mitchel, J. A. Mucha, L. Seibles and H. Huggins, J. Eleetrochem. Soc., 138 (1991) 635. 8 M. Geis, H. I. Smith, A. Argoitia, J. Angus, G. H. M. Ma, J. T. Glass, J. Butler, C. J. Robinson and R. Pryor, Appl. Phys. Lett., 58 (1991) 2485. 9 A. Masood, M. Aslam, M. A. Tamor and T. J. Potter, J. Electrochem. Soc., 138 (1991) L67. 10 R. J. Meilunas and R. P. H. Chang, Appl. Phys. Lett., 59 (1991) 3461. 11 Metal Finishing Guidebook and Directory Metals and Plastics Publications, Inc., Hackensack, NJ, 1981, p. 225. 12 J. L. Davidson, C. Ellis and R. Ramesham, J. Electron. Mater., 18 (1989) 711. 13 R. Ramesham, T. Roppel, C. Ellis, D. A. Jaworske and W. Baugh, J. Mater. Res., 6 (1991) 1278. 14 R. Ramesham, T. Roppel, R. W. Johnson and J. M. Chang, Thin Solid Films, 207 (1992).