An experimental and numerical investigation of plasma effects during laser processing of diamond

Abstract

Experimental and numerical data have been acquired for a diamond substrate in atmospheric pressure air, 100 mTorr pressure air, and submerged in water, exposed to a Nd:YAG pulsed laser (10 ns temporal duration, 10 Hz repetition rate) at 1064 and 532 nm. An iteratively coupled two-part numerical model was developed to complement the experimental observations and provide a complete accounting of the laser energy. One part of the model uses as input the incident energy flux at the workpiece surface and outputs the workpiece temperature distribution, material removal rate, and topography. The second part uses as input the material removal rate to calculate the time-dependent plasma density and volume and outputs the energy flux at the workpiece surface by accounting for inverse Bremsstrahlung absorption and subsequent re-emission. Results in ambient air indicate the important role of the plasma formed by the ablated diamond material ejected at supersonic velocities. The ensuing plasma allows less than 55% of the total incident irradiation to reach the workpiece surface. In air, material removal efficiencies of 4-8% are observed—the remaining losses are due primarily to the energetic material expulsion and partially to conduction within the workpiece. When the ambient condition is changed to water, the material removal efficiency substantially increases to about 50%. This increase is attributed to a chemical reaction between water vapor and the workpiece. The spatially Gaussian laser profile imparts only enough energy to sublimate diamond within a 50 μm diameter region. However, out to about a 200-400 μm diameter, the diamond is heated sufficiently (over 1000 °C) to become reactive. Further, the energy is sufficient to vaporize a small amount of the water which then reacts with the workpiece to produce CO2 and H2 vapor. This chemical reaction rate is slower than the primary laser sublimation rate and is well demonstrated by scanning electron micrographs. Although this chemical etching leaves a relatively rough surface, it offers the promise of an improved process—one that is more efficient and produces no impurities—using carefully chosen ambient conditions.Experimental and numerical data have been acquired for a diamond substrate in atmospheric pressure air, 100 mTorr pressure air, and submerged in water, exposed to a Nd:YAG pulsed laser (10 ns temporal duration, 10 Hz repetition rate) at 1064 and 532 nm. An iteratively coupled two-part numerical model was developed to complement the experimental observations and provide a complete accounting of the laser energy. One part of the model uses as input the incident energy flux at the workpiece surface and outputs the workpiece temperature distribution, material removal rate, and topography. The second part uses as input the material removal rate to calculate the time-dependent plasma density and volume and outputs the energy flux at the workpiece surface by accounting for inverse Bremsstrahlung absorption and subsequent re-emission. Results in ambient air indicate the important role of the plasma formed by the ablated diamond material ejected at supersonic velocities. The ensuing plasma allows less than 55% of ...