R. Ramesham

Selective and low temperature synthesis of polycrystalline diamond

Polycrystalline diamond thin films have been deposited on single-crystal silicon substrates at low temperatures (not above 600 C) using a mixture of hydrogen and methane gases by high-pressure microwave plasma-assisted chemical vapor deposition. Low-temperature deposition has been achieved by cooling the substrate holder with nitrogen gas. For deposition at reduced substrate temperature, it has been found that nucleation of diamond will not occur unless the methane/hydrogen ratio is increased significantly from its value at higher substrate temperature. Selective deposition of polycrystalline diamond thin films has been achieved at 600 C. Decrease in the diamond particle size and growth rate and an increase in surface smoothness have been observed with decreasing substrate temperature during the growth of thin films. As-deposited films are identified by Raman spectroscopy, and the morphology is analyzed by scanning electron microscopy.

Fabrication of an electron multiplier utilizing diamond films

Abstract High secondary electron yields (σ=14–27) from polycrystalline diamond films on Mo substrates have been reported previously. We observed that continuous exposure to an electron beam degraded the secondary yield in vacuum as a function of fluence owing to a loss of surface hydrogen. However, the hydrogen partial pressure could be adjusted such that the high secondary yield remained stable during beam exposure. We have constructed a prototype electron multiplier using these diamond films for use in ultrahigh vacuum. A gain of 0.9 × 105 has been measured in a d.c. mode. Palladium, titanium and aluminum nitride have been studied for possible dynode substrate materials which might eliminate the need for hydrogen during operation. Total secondary yields as high as 50 were measured from diamond on palladium and σ was stable at 25 during heating at 700 K in vacuum. Raman spectroscopy and scanning electron microscopy showed that variations in σ from diamond films on Pd were due to differing concentrations of non-diamond carbon.

Polishing of polycrystalline diamond by hot nickel surface

A microwave plasma technique has been employed to deposit polycrystalline diamond film over a molybdenum substrate button using a gas mixture of hydrogen and methane at a substrate temperature of 851°C. A CVD diamond coated molybdenum substrate button was mounted with a load against hot nickel plate and rotated for 3.45 h in a hydrogen ambient. Hot tungsten filament was used as a heat source to maintain the temperature of the nickel block and CVD diamond coated molybdenum button at 848°C. This experiment has reproducibly shown the successful polishing of polycrystalline CVD diamond by hot nickel. A Tencor profilometer and scanning electron microscope have been used to evaluate the surface smoothness and morphology before and after polishing the polycrystalline diamond thin films.

Cyclic voltammetric, a.c. and d.c. polarization behavior of boron-doped CVD diamond

Abstract Boron-doped polycrystalline diamond films have been deposited over a molybdenum substrate by the microwave plasma CVD process using a methane and hydrogen gas mixture at a pressure of 35.7 Torr. Boron doping of diamond has been achieved in situ by using a solid boron source while growing diamond in the CVD process. The a.c. impedance of boron-doped diamond films in 0.5 M NaCl solution has been determined and compared with the results obtained with a molybdenum substrate. Capacitance, solution resistance, and polarization resistance (corrosion rate) have been determined using the experimental data plotted in Nyquist and Bode formats. D.C. polarization techniques such as linear and Tafel polarization have been used to evaluate the doped diamond coated molybdenum and molybdenum for corrosion resistance characteristics in terms of charge-transfer coefficients and corrosion rate. Cyclic voltammetry has been used to evaluate the molybdenum, platinum, and doped and undoped diamond coated molybdenum materials in 0.5 M NaCl solution. We have observed two time constants with a doped diamond electrode / solution interface. Solution resistance was found to be constant irrespective of the electrode in the same electrolyte solution.

Voltammetric studies at the polycrystalline diamond grown over a graphite electrode material

Abstract Boron-doped polycrystalline diamond thin films were grown over POCO graphite by a microwave plasma assisted chemical vapor deposition (CVD) using a gas mixture of methane and hydrogen. As-deposited films were analyzed by scanning electron microscopy (SEM) for their morphology. Cyclic voltammetry has been used to evaluate the background current response corrected for the uncompensated IR drop. Redox kinetics of ferri-ferrocyanide at the boron-doped diamond/graphite electrode has been investigated. The lowest observed peak separation was observed to be 222 mV at a scan rate of 5 mV/s. The heterogeneous electron transfer rate constant has been determined using the experimental data and a COOL algorithm. The rate constant was found to be ∼2×10 −4 ( α =0.5 or variable) cm/s, and subsequently, the reaction kinetics was considered to be sluggish at the diamond electrode/solution interface. The diamond films grown over graphite electrode material may have uses in electrosynthesis and electroanalysis since the doped diamond films are electrically conductive and chemically inert.

Fabrication of Microchannels in Synthetic Polycrystalline Diamond Thin Films for Heat Sinking Applications

2,817,048 12/1957 Thuermel et al. .................. 317/234 3,142,595 7/1964 Wentorf, Jr....... 148/171 3,628, 106 12/1971 Frank et al. ... ... 357/55 3,678,995 7/1972 Collard .......... 165/85 3,840,451 10/1974 Golyanov . ... 204/92 3,872,496 3/1975 Potter ... ... 357/8 3,922,775 12/1975 Potter............ ... 29/589 3,925,078 12/1975 Kroger et al. . ... 96/36.2 3,973,320 8/1976 Greco ............ ... 29/578 3,974,514 8/1976 Kressel et al. ... ... 357/17 4,069,463 1/1978 McGroddy et al. ............... 33/94.5

Effect of annealing and hydrogen plasma treatment on the voltammetric and impedance behavior of the diamond electrode

Undoped polycrystalline diamond thin films have been grown on molybdenum substrates by microwave-assisted chemical vapor deposition using a hydrogen and methane gas mixture. Cyclic voltammetric behavior of diamond films has been studied in 0.5 M NaCl solution as a function of annealing in nitrogen gas at 425°C and hydrogen microwave plasma treatments at 800°C. Preliminary voltammetry studies have shown that the annealed diamond film has a high resistivity, and hydrogen microwave plasma-treated diamond has a low resistivity. These treatments have an effect on the extent of the gas evolution reaction either in the anodic or in the cathodic polarization of the diamond electrode. AC impedance and DC linear polarization techniques have provided the data on the increase and decrease in diamond film resistance upon annealing and hydrogen microwave plasma treatment, respectively, which further corroborates the voltammetry studies. These electrochemical measurements may qualitatively suggest that there may be a change in electron affinity characteristics of the diamond surface as a function of various pretreatments.

Characterization of polycrystalline diamond thin films grown on various substrates

Abstract Polycrystalline diamond thin films have been selectively grown on various substrates such as silicon, silicon nitride, silicon dioxide, alumina, molybdenum, and boron nitride. This has been achieved by selectively damaging the substrate by an ultrasonic agitation process using diamond particles (typical size approximately 90 μm) in methanol. Microwave plasma-assisted chemical vapor deposition is used to grow diamond thin films using a gas mixture of hydrogen and methane. The films were analyzed for morphology by scanning electron microscopy, chemical nature by Raman spectroscopy, and adhesion strength by z -axis pull stud testing. Films grown on boron nitride were characterized by X-ray diffraction.

Determination of flatband potential for boron doped diamond electrode in 0.5 M NaCl by AC impedance spectroscopy

Abstract Boron-doped polycrystalline diamond films have been grown on molybdenum substrates by microwave-assisted chemical vapor deposition using a gas mixture of hydrogen and methane. AC impedance of diamond electrode was carried out in a solution of 0.5 M NaCl solution under various DC polarization conditions. Mott–Schottky plots were generated to determine the flatband potential of the boron-doped semiconducting diamond electrode in 0.5 M NaCl solution. It was found to be 0.91±0.2 V vs. R.E. in the frequency range of 1–400 Hz. The flatband potential was found to be higher than 0.91 V when the frequency was higher than 400 Hz and it was found to be lower than 0.91 V when the frequency was lower than 0.1 Hz. The kinetics were faster when a negative bias was applied to the diamond electrode as it was evidenced in the impedance data due to a reduction in the charge-transfer resistance.

Characterization of Synthetic Diamond Thin Films

High‐pressure, microwave plasma‐assisted chemical vapor deposition is employed to deposit diamond thin films, using a gas mixture of methane and hydrogen on single‐crystal silicon substrates. The deposition rate is approximately 1 micron/h. As‐deposited diamond thin films on a silicon substrate and free‐standing diamond thin films are analyzed by scanning electron microscopy, Raman spectroscopy, and x‐ray diffraction. Thermal stability of free‐standing diamond thin films is studied in oxygen and argon ambient, separately, using thermogravimetric analysis. It is found that these films maintain integrity in oxygen up to 676°C.