Reactive Ion Etching of Boron Nitride and Gallium Nitride Materials in C1 2 /Ar and BCl 3 /Cl 2 /Ar Chemistries
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unique properties. GaN-based bright light emitters in the violet, blue and green spectral regions have been fabricated. The BN/CBN materials also allow tunable band gaps from 0.4 eV (CBN) to 6.4 eV (BN) and thus can result in IR to deep UV optoelectronic devices. The very high thermal conductivity and stability of these materials, particularly BN/CBN, make them the ideal choice for high power and high temperature applications.' The use of BN thin films as insulating layers in wide band gap-based devices instead of silicon dioxide (SiO 2) or silicon nitride (Si3N4) stems from the fact that SiO 2 forms electrically conductive solid state alloys with metals used for contacts starting at 350 °C while Si3N4 has a high thermal mismatch with these materials and hence multiple breakdowns in the structure barrier can be caused by defects and cracks formed at high temperatures. The BN/CBN materials are also very good candidates for coating and tribological applications. However, these properties make these materials very difficult to process. As shown in Table I the bond energies of some of these materials are extremely high. Boron nitride has been etched in a CF4/0 2 plasma2 and in phosphoric acid with reported etch rates up to 150 A/min using the later process.3 Dry etching of GaN and its ternary alloys has resulted in acceptable etch rates, anisotropic features and surfaces. 4' 5 Wet etching is receiving lately an increased interest with etch rates reaching up to 1600 A/min.6 In this work, reactive ion etching (RIE) of BN films using C12/Ar chemistries is reported for the first time. Dry etching of GaN using BC13/CI 2/Ar and BCI3 chemistries is also reported. The effects of gas flow rates and rf power (d.c. bias) on the etch rate were investigated. 285 Mat. Res. Soc. Symp. Proc. Vol. 512 0 1998 Materials Research Society
Table I. Bond energies of some III-V nitride compounds. Material
Bond Energy (eV atom")
cBN
13.5
hBN
13.2
GaN
8.92
AIN
11.52
InN
7.72
InGaN
8.27
EXPERIMENT Boron nitride films were grown using ion beam and electron cyclotron resonance (ECR) assisted physical vapor deposition. Nitrogen species are delivered by a gridless end Hall ion source (Commonwealth MARK II) and an electron cyclotron resonance (CECR -ASTEX) plasma source. The former delivers to the substrate ions with energies between 40 and 60 eV and ion currents ranging from 110 mA to 150 mA. The ECR source allows the delivery of nitrogen species with energies varying between 4 and 20 eV. Boron was obtained by electron beam evaporation of pure boron (99.999 %). BN films are deposited on (001) silicon and (0001) sapphire substrates. The boron evaporation rates were fixed at - 0.2 A/s for the binary compound. The background pressure in the growth chamber was - 10-8 Torr which rose to 10during deposition. Unintentionally doped, n-type GaN layers were grown at 800 0C on (0001) sapphire substrates by gas source MBE technique. Ammonia was used as a source of nitrogen, and gallium was delivered from an effusion cell. Gallium and ammonia flu
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