Laser etching of GaN

1. Laser etching of GaN Jonathan Winterstein Dr. Tim Sands, Advisor

2. Outline • Introduction • Experimental process • Results • Conclusions

3. Introduction • Gallium Nitride is a wide-band-gap semiconductor (Eg = 3.4 eV) with potential applications as: • Blue LEDs or Laser diodes. • And researchers have also suggested GaN could be used for transistors. • GaN resists chemical etching, laser etching is a possible alternative. • Laser etching should be less expensive and less damaging to specimens than current industry etching methods such as Reactive Ion Etching

4. Introduction: Technical aspects • Laser etching of GaN works by raising the temperature of the GaN above a decomposition temperature of around 900° C. • The GaN heated above this temperature decomposes into gallium metal and nitrogen gas. The gallium metal solidifies and remains on the sample surface.

5. Introduction: Technical aspects • Laser etching of GaN works by raising the temperature of the GaN above a decomposition temperature of around 900° C. • The GaN heated above this temperature decomposes into gallium metal and nitrogen gas. The gallium metal solidifies and remains on the GaN surface.

6. Introduction: Technical aspects • Laser etching of GaN works by raising the temperature of the GaN above a decomposition temperature of around 900° C. • The GaN heated above this temperature decomposes into gallium metal and nitrogen gas. The gallium metal solidifies and remains on the sample surface. • The gallium is reflective and could reduce the etch rate because some laser energy would not reach the GaN surface. • For this study all samples are processed using a KrF excimer laser producing 248 nm wavelength light for a 25 nanosecond pulse.

7. Outline • Introduction • Experimental process • Results • Conclusions

8. Experimental Process • Experiment 1: Ga removal in HCl bath as a function of time and temperature. • GaN samples on sapphire substrates are irradiated once with the laser at a fluence of 800 mJ/cm2 • Residual Ga is cleaned in HCl baths for times of 30 seconds and 5 minutes at room temperature (26-27° C), 40° C, 50° C and 60° C. • Optical microscopy is used to compare amount of Ga removed for each sample.

9. Experimental Process • Experiment 2: Laser etch rate as a function of time in HCl bath. • Samples receive 10 laser pulses at a fluence of 700 mJ/cm2 and are cleaned in HCl for times of 30, 60, 120, 150 and 300 s at 50° C between each laser pulse. • Amount of GaN removed determined by profilometry.

10. Experimental Process • Experiment 3: Etch rate as a function of laser fluence. • Samples receive 10 or 15 laser pulses at fluences ranging from 550 to 825 mJ/cm2. • Between laser pulses, Ga is removed by cleaning in HCl bath at 50° C for 3 minutes. • Amount of GaN removed is determined by profilometry.

11. Outline • Introduction • Experimental process • Results • Conclusions

12. Results Temperature: 50° C 30 seconds 5 minutes

13. Results Results for Experiment 2.

15. Results Results for Experiment 3.

19. Outline • Introduction • Experimental process • Results • Conclusions

20. Conclusions • Etching effectively stops near 550 mJ/cm² • A significant drop in etching rate occurs as the fluence decreases from 700 to 600 mJ/cm². • A saturation point appears to occur near 750-800 mJ/cm². Saturation occurring at this fluence follows the trend of the results published by Yang et al. • Thirty seconds in HCl bath is not sufficient to remove residual gallium. • Etching without cleaning between pulses significantly reduces etch rate. • Future work might include studying laser etching under vacuum conditions and GaN film transfer through laser liftoff.

21. Thank You. Any Questions?