Donald R. Gilbert

Engineered Interfaces for Adherent Diamond Coatings on Large Thermal-Expansion Coefficient Mismatched Substrates

Adhesion of thin or thick films on substrates is a critical issue in systems where the thermal-expansion coefficients of the coating and bulk material are significantly different from each other. The large mismatch of the expansion coefficients results in the generation of very high stresses in the coating that may lead to delamination, cracking, or other deleterious effects. A method to increase the adherence of diamond coatings on tungsten-carbide and stainless steel substrates is reported based on a substrate-modification process that creates a three-dimensional thermally and compositionally graded interface. Scratch and indentation tests on diamond-coated steel and tungsten-carbide samples did not exhibit film fracture at the interface and concomitant catastrophic propagation of interfacial cracks.

Low‐pressure, low‐temperature, and remote‐plasma deposition of diamond thin films from water‐methanol mixtures

We have deposited diamond thin films remote from the active plasma region using an electron cyclotron resonance chemical vapor deposition technique. Diamond films were fabricated at temperatures in the range of 550–650 °C and gas pressures between 25 and 60 mTorr. The volume ratio of water to methanol was varied from 1:20 to 1:5 to optimize diamond film growth. High methanol content resulted in multiple nucleation in the growing diamond film, while higher water content led to complete etching of the film. A positive electrical bias was found to be essential for diamond thin film growth remote from the plasma region. The films were characterized by x‐ray diffraction, micro‐Raman, and scanning electron microscopy for phase identification, surface morphology, and bonding characteristics.

Growth of adherent diamond films on optically transparent sapphire substrates

The growth of continuous adherent diamond thin films on optically transparent substrates is important for the development of corrosion and erosion resistant infrared windows for many applications. Until now, the growth of adherent diamond films on optically transparent substrates like sapphire has been unsuccessful due to the large thermal mismatch between the film and the substrate and the absence of an interfacial carbide ‘‘glue’’ layer. By employing a low temperature (500–550 °C), low pressure (∼1 Torr) electron–cyclotron–resonance chemical‐vapor‐deposition process, and utilizing a dispersed–particulate diamond suspension for nucleation, adherent diamond thin films have been fabricated on sapphire substrates. Raman spectroscopy showed that the diamond peak was shifted approximately 6 cm−1 above its equilibrium position, suggesting the presence of very large compressive stresses (∼3.2 GPa) in the film.

DYNAMIC GROWTH EFFECTS DURING LOW-PRESSURE DEPOSITION OF DIAMOND FILMS

Diamond films were deposited in a modified electron–cyclotron-resonance plasma system operating at pressures between 1.0 and 2.0 Torr. This system provides the advantage of efficient plasma generation due to magnetic enhancement and high diffusion rates due to relatively low-pressure operation. Films were formed from preexisting seed layers providing high “nucleation” densities to promote rapid coalescence. Raman analysis of grown films showed a quality dependence on both deposition pressure and nucleation density. We speculate that the increased presence of amorphous carbon and larger film stresses is the result of grain-boundary impurity effects in the seeded films. Oxygen addition improved film quality by reducing nondiamond carbon incorporation.

Novel in situ production of smooth diamond films

We have developed a unique method to produce smooth diamond films using a modified microwave plasma process system. This method consists of sequential in situ deposition and planarization in an electron cyclotron resonance plasma system. Diamond films were deposited to a thickness of 3.0 μm in this system at a pressure of 1.000 Torr from gas mixtures of methanol and hydrogen. Deposition was followed by planarization using a two-grid ion beam extraction process with a pure oxygen plasma at 10 mTorr. The average roughness of the diamond films so produced was as low as 30 nm, which was a factor of two lower than that of the as-deposited diamond films.

High-pressure process to produce GaN crystals

High melt temperature and thermal decomposition prevent the use of standard bulk semiconductor crystal growth processes for the production of GaN. We have employed a hydrostatic pressure system to grow GaN crystals. An ultrahigh pressure, high temperature process was developed using a solid-phase nitrogen source to form GaN crystals in a Ga metal melt. Using a thermal gradient diffusion process, in which nitrogen dissolves in the high temperature region of the metal melt and diffuses to the lower temperature, lower solubility region, high quality crystals up to ∼1 mm in size were formed, as determined by scanning electron microscopy, x-ray diffraction, and micro-Raman analysis.

Surface composites: a novel method to fabricate adherent interfaces in thermal-mismatched systems

The utility of diamond as a wear-resistant coating has been severely limited by the inherent adhesion problems experienced in thermal expansion coefficient mismatched systems. Diamonds low thermal expansion coefficient relative to such materials as cemented carbides results in high residual stresses in the deposited film, which leads to poor adhesion characteristics. In this work, a novel process has been investigated for the formation of laser induced micro-rough surfaces on cemented carbide substrates. These surface structures have been used to produce diamond-coated cemented carbides (WC-6%Co) with compositionally graded interfaces, which are termed surface composites. Micro-Raman analysis of these structures showed significant modification of stress distribution within the deposited diamond film. Rockwell indentation testing of these structures showed concomitant improvement of film adhesion with increasing roughness of the interface.

A novel method to predict laser-induced, non-linear thermal effects in semiconductors

We have developed novel analytical methods to predict laser-induced, non-linear thermal effects in semiconductors. Equations based on energy balance considerations were used for the estimation of melt depths, surface temperatures and maximum solidification velocities of semiconductors exposed to nanosecond laser pulses. The effect of laser pulse duration and fluence on these parameters was displayed graphically. Analytical results were compared with detailed numerical solutions and experimental values where available. Good agreement between analytical and numerical calculations was observed, thus suggesting the universal application of this method to the estimation of non-linear thermal effects of laser-irradiated materials.

Crystal growth of gallium nitride and manganese nitride using an high pressure thermal gradient process

Abstract Standard, semiconductor-industry bulk crystal growth processes are virtually impossible for the production of GaN as this is prohibited by both the high melt temperature of GaN and thermal decomposition of the compound into Ga metal and diatomic nitrogen gas. In this study, a novel hydrostatic pressure system was employed to grow GaN crystals in a very high pressure ambient. The ultra-high pressure, high temperature process uses a solid-phase nitrogen source to form GaN crystals in a metal alloy melt. Using a thermal gradient diffusion process, in which nitrogen dissolves in the high temperature region of the metal melt and diffuses to the lower temperature, lower solubility region, high quality crystals up to ∼0.5 mm in size were formed, as determined by scanning electron microscopy, X-ray diffraction, and micro-Raman analysis.

Control of the Microstructure of Polycrystalline Diamond And Related Materials Via an Enhanced Cvd Process

High quality diamond films have potential applications in high speed, high temperature electronic devices and hard, wear resistant coatings for cutting tools. Diamond is an ideal material for substrates or thin films in integrated circuits because it has the combined properties of high thermal conductivity, high electrical resistivity, and low dielectric constant.1,5 In addition to passive roles as heat sinks, diamond is also being evaluated as a device material for microwave frequency and semiconductor applications. Diamond is desirable for electronics operating at microwave frequencies because of its low susceptibility to X-ray, ultraviolet, and gamma radiation damage.3,5 The interest in diamond as a semiconductor stems from its ability to operate at temperatures between 100°C and 500°C, beyond the range of most smaller band gap semiconductors.6