The Criteria Of Laser Irradiation Process & Effect On Irradiated Sample

Since it’s very important to understand the criteria of laser irradiation process and effect on irradiated sample some measurements and calculations were carried out to fully understand the influences of laser parameters and substrate temperature on the melt duration of molten diamond during excimer laser. First generated temperature on sample surface upon irradiation was simulated and it appeared to reach a high value of almost 4000 K at max fluence with time almost equal to the time of irradiation. In addition, self-reflectivity of diamond substrate was experimentally measured to compare results with simulation result and fin optimum parameters of irradiation to achieve best doping. Analysis of time-resolved reflectivity data during the laser heating of diamond was used to determine the onset of melting and the melt duration. Optical reflectance analysis has been utilized to decipher the details of excimer laser induced phase transformation mechanisms.

Laser induced surface temperature

Investigating surface change and induced temperature upon irradiation is of great importance. To accomplish this objective, a melt-mediated transformation scenario has been proposed, utilizing spatially selective melting and lateral temperature modulation, simulation was carried out using Quick Therm software to calculate the time dependence of temperature profiles in multilayered plane media. It is a program that automatically performs complicated heat transfer / stress analysis typified by heat processing. The time in the finite element method and excessive analysis is internally generated. It is to perform flexible and sophisticated analysis such as automatic processing of moving heat sources and heat radiation spaces.

First the sample orientations are desgned with dimentions similar to the exprimentally irradiated sample. After that the designed sample is devided into multiple meshes to enhance calculation and get more accurate results . The temperature distribution inside irradiated materials is related to multiple parameters which are listed in Table ‎3 1. Properiies of SCD sample was directly obtained from manufacturing compoany official site [ ].Air here is used as surrouinding medium for the irradiated sample since the expriment is performed at room temperature and open atmosphere. Body density Specific Heat Thermal Conductivity Temp. Diamond 3.515E6 / mm3 520 J Kg-1 K-1 1900 W/(m.K) 300 K air 6.5E-27 /mm3 20 5E-6 W/(mm.K) 300 K

Laser pulse is considered as the heat source for simulation, so input of laser beam parameters (watage ) is required. Wattage values were obtained from ArF generated pulse spectrum as shown. The pulse was captured by biplanar photodetector that consist of a circulaar, plane, lightsensitive surface called photocathode and a mesh type anode aligned in parallel to each other to make ot possible to generate high linear output current with sunnano-econd response time.

The fluence parameter is considered the major factor affecting the generated heat on sample. As we can see from Figure ‎3 9 the large change in temperature for two different fluences. the temperature increases with fluence, since the excess laser energy density beyond threshold fluence is used to heat the liquid ablated area, consequently raising the peak temperature With changing fluence of laser shot from 5 J/cm2 to 3 J/cm2 induced temperature was almost reduced by halfwhich in turns affects the time at which the surface remains ablated . The temperature response, melt propagation process are fundamentally different in the partial-melting and the complete-melting regimes. Since the absorbed laser energy in excess of the level needed for surface melting is consumed by the latent heat of phase change from solid to liquid .The maximum induced temperature in the partial-melting regime remains close to the melting point of diamond. The peak temperature rises in the complete-melting regime.

So from the Simulation result we can conclude that as the radiant laser energy increases, the diamond layer becomes completely molten and the melt duration is prolonged. In addition to confirm the ablation process of diamond film, direct reflectivity of the sample was measured at different fluences.

Singlecrystalline Diamond Self-Reflectivity

Since the optical properties depend on temperature and state of phase, the reflectivity and transmissivity are good probing indicators of the laser irradiation process. Analysis of time-resolved reflectivity data during the laser heating of diamond was used to determine the onset of melting and the melt duration. Optical reflectance analysis has been utilized to decipher the details of excimer laser induced phase transformation mechanisms.

Experimental setup

Diamond substrate was first cleaned with Acetone for 3 min in ultra-sonic followed by distillated water for 3 min as well. It Shows the schematic illustration of the specimen structure and experimental setup for measuring the melt duration of ablated diamond substrate. The film was irradiated by a ArF excimer laser operating at a wavelength of 193 nm with a short pulse of 20 ns in full width at half maximum (FWHM). The specimen was moved to a new position following each fluence change. Measurements were performed using a continuous wave laser operating at a wavelength of 600 nm. laser was focused on the center of the laser-irradiated spot. The reflected beam from the specimen was focused on a silicon photodiode. The photodetector is placed in an excellent position where the maximum signal is obtained. record of reflection spectra was captured using a fast digital storage oscilloscope (bandwidth=500 MHz, sample rate=2 GHz/s).

Fluence dependent self-reflectivity of 193 nm,20 ns pulses normally incident on diamond

With laser irradiation self-reflectivity begins to increase sharplywhich might be due to the initial dynamis of the melting process which occurs owning to high temperatures that can be developed during the laser pulse.As can b seen from the figure that both peak intensity and full width half maximum are greatly affected with fluence of the laser shot. At lower fluence (3 J/cm2) it appears that only partial ablation occures and the liquid still retains the higher density of diamond. With increasing Fluence up to 4 J/cm2, reflectivity greatly enhanced which indicate a higher ablation rate than lower fluence, in addition the ablation time also increased which in turn enhance and faclitate the doping process. Slight increase in reflectivity was obtained by increasing fluence up to 5 J/cm2 which indicates the the irradiated might have been fully ablated.

Conclusion

Laser beam was partially characterized by simulation and expriments to understand it’s behavior and effect on diamond substrate. Generated heat upon irradiation was found to increase with increasing fluence and melting depth also increased as well. Exprimentally measured reflectivity proved that melting duration is approximately same as laser pulse duration and reflectivity increase with increasing fluence which indicate if partial or full ablation occurred upon irradiation. Combining this result and result from simulation ,we believe that The rise in the ambient temperature of the substrate, the fluence of the laser beam, laser pulse duration and ambient pressure controls the total energy required to melt the film and increase the melting duration to facilitate doping, so performing the expriment in heated vaccumed chamber is expected to enhance the process greatly. In next chapter we will show effect of these parameter on diamond substrate in details.