Pulsed Laser Etching of GaN and AIN Films
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Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
Various dry etching techniques have been vigorously pursued in the interest of effective device processing [2].
These include reactive ion etching [3), plasma etching [4], chemically
assisted ion beam etching [5], and electron cyclotron resonance [ECR] etching [6]. To date, the highest etch rates of GaN are 4700 A/min in RIE [7], and 2850 A/min in ECR plasma etching at room temperature [8],
Typically, chemically assisted RIB tends to cause more damage to the
material than ECR because of higher ion energy. Here we report another dry processing technique, namely, pulsed laser etching that can be utilized for the controlled etching of heterostructures.
GaN and A1N
In the past, this technique has been successfully employed for dry etching of
metal-oxide films [9,10]. Recently, a similar approach has been demonstrated in the etching and patterning of GaN films using Nd:YAG laser operating at wavelength of 355 nm and the etch rates of 500-700A per pulse was obtained [II]. In this paper, we report pulsed laser etching of GaN and AlN films using a KrF excimer laser operating at 248 nm wavelength and pulse duration of 30 ns. The strong absorption of the deep UV laser causes forward directed ejection of the material yielding highly anisotropic etch profiles. We have studied the influence of laser energy density and the number of pulses on the etching characteristics of GaN and AIN. It is shown that
high etching
rates of about 0.1-1.5 gtm/sec can be obtained under optimized processing conditions. These etch rates are significantly higher than those achieved in the RIB, chemically assisted RIB, and plasma etching of GaN. EXPERIMENTAL The uniform portion of a KrF excimer laser beam was selected by a 15 mm x 7 mm thin metal aperture and subsequently focused on the samples by a lens of 25 cm focal length.
The GaN
film was mounted perpendicular to the laser beam behind the focal point and shielded by a copper plate resting against its surface. A rectangular array of circular holes (250 Itm in diameter) were drilled in the copper plate, so that only the surface exposed under the holes was etched.
This
arrangement produced several (8-10) identical etch patterns each time the film was laser etched. 2 A wide range of incident energy fluences (0.2 to 10 J/cm ) was explored for etching GaN and AIN
films. It was achieved by varying either the distance between sample surface and the lens or the output energy of the laser.
The samples of AIN and GaN films used in this experiments were
fabricated by chemical vapor deposition and pulsed laser deposition [12] techniques. The etching was carried out in air at room temperature. After the laser etching experiments, the samples were cleaned in dilute HC1 to remove a thin layer of Ga that was formed due to the thermal decomposition of GaN by the laser beam. The etch depth was measured using a mechanical stylus (Dektak IIA). The crystalline quality, morphology and optical properties of the laser processed GaN sur
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