Photothermally Assisted Dry Etching of GaN
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resonance (ECR) etching [3, 4], and chemically assisted ion beam etching (CAIBE) [5, 6] have been very popular for etching the HI-N materials. Although these techniques have high etch rates ranging between 200 to 3500 A/ min, they rely on bombardment of an electrically biased surface with ions that can produce damage, degrading the optical and electrical properties of devices, unless used at low ion energies [7, 8]. Wet etches are ineffective due to the III-N materials resistance to chemical attack, with low etch rates around 20 A/ min [9] for GaN. Conventional wet etches also have the drawback of isotropic etching, leading to undercutting of the mask; however, these etches can be effective for surface cleaning [10]. Photoassisted etching techniques using both wet and dry etchant species are being developed as alternatives to plasma etching III-N materials to potentially reduce the damage associated with ion beams. Photoassisted wet etching of undoped n-type GaN has also been shown effective in producing etch rates as high as 4000 A/ min using (1:3) 45% KOH and 400k/ min using (1:10) HCI; H20 coupled with a He-Cd (325 nm) laser [11]. The etch features were also reported to be smooth and distinct, with the etch mechanism suggested as photoelectrochemical in nature. Recently we have reported the photoassisted dry etching of GaN [12] using an ArF excimer laser (193 nm) and 100% HCI etchant gas at - 0.5 mTorr, producing distinct sidewalls and smooth etched features with an etch rate of - 80 A/ min. Results from so called dark etching, without laser interaction, have shown that no appreciable etching occurs. Also, no appreciable etching is observed with raising the sample temperature to - 400 'C under the same HCI conditions, again, without laser radiation. Both HC1 and UV radiation were necessary to produce etching of GaN. 157 Mat. Res. Soc. Symp. Proc. Vol. 423 0 1996 Materials Research Society
In this paper we will present the effect of surface temperature, HCI pressure, and laser fluence on both etch rate and etched surface morphology. EXPERIMENT The system used in this study has been previously described [12], and consists of a load locked ultra high vacuum chamber (base pressure 3x10 9 Torr) equipped with an internal shower head doser for delivering HC1, with the gas flux coincident with laser radiation and the GaN material. GaN samples grown in-house [13] epitaxially on (0001) sapphire and purchased from Cree Research, Inc. (undoped n-type carrier concentrations < lx l01l /cm 3) were used in this study. The experiments were performed with no thermal bias to the substrate at ambient temperatures near 23 'C, surface heating due to laser between - 200-300 'C, and HC1 (99.999% purity) pressures - 0.5 to 1 mTorr. An ArF (193 nm) excimer laser was used at 30 Hz and 26 kV to produce laser fluences which were attenuated using non-coated optic flats and all energy losses
were measured by a pyroelectric Joule meter. The laser power was measured at the entrance to the chamber, taking into account the loss due to the lase
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