| AVS 61st International Symposium & Exhibition | |
| Plasma Science and Technology | Thursday Sessions |
| Session PS2+TF-ThM |
| Session: | Atomic Layer Etching (ALE) and Low-Damage Processing |
| Presenter: | Daisuke Ogawa, Chubu University, Japan |
| Authors: | D. Ogawa, Chubu University, Japan Y. Nakano, Chubu University, Japan K. Nakamura, Chubu University, Japan |
| Correspondent: | Click to Email |
We have developed the technique to reduce the damage that is induced by argon plasma to the surface of a gallium nitride (GaN) film. Our technique especially reduces the damage relating to the band gap more than 2.5 eV. Our in-situ monitoring showed that a GaN film cooled with liquid nitrogen (LN2) has a different pattern of the damage induced by the plasma.
In order to make the in-situ monitoring of the GaN surface, we made real-time measurements with photoluminescence (PL) spectrum emitted from the GaN film. Here, the GaN film was excited with a light illumination generated from a xenon lamp passing through a 313 nm band pass filter. This configuration allows us to monitor the volume-averaged material condition from the surface to ~75 nm depth. M. Chen previously found that the ratio of the blue luminescence (BL) band over the near-band edge (NBE) band is effective to make the in-situ monitoring of the damage induced by plasma.[1] The ratio basically gets larger as the film gets more damages. However, our result showed that the ratio stayed almost constant only when the film was cooled with LN2. This means that the damage induced by the argon plasma was likely avoided by using LN2 cooling.
To find the effect of the cooling with LN2, we made X-ray photoelectron spectroscopy (XPS) measurements for three samples (pristine, LN2, and no LN2) after the plasma exposures. We sputtered the film with argon ion beam for 3 minutes in vacuum every cycle of the XPS measurements to obta in the depth profile. The XPS spectrum from the GaN film that was exposed in argon plasma with LN2 cooling was matched well with the spectrum from the pristine GaN film after the first sputtering. On the other hand, the XPS spectrum from the GaN film that was exposed in argon plasma without LN2 cooling showed a chemical shift at the gallium line and a decrease at a nitrogen line. All three spectra matched well after the second sputtering. This profile indicates that the plasma-damaged layer thicker than ~30 nm was formed in the case of no LN2 cooling. (Assumed the sputtering rate at 5 nm/min.)
Our in-situ temperature monitoring on the GaN surface that was exposed in the argon plasma showed that the temperature stayed below 150 ° C with LN2 while the temperature exceeded over 300 ° C without LN2. This is the indication that we should be able to reduce the damage by controlling the GaN film temperature during the plasma processing.
In this presentation, we will show the evolution of the PL spectrum, connecting with the surface temperature, XPS results supporting with more details.
[1] M. Chen et al., App. Phys. Let. 101, 071105 (2012)