C. Barratt

Inductively coupled plasma etching of GaN

Inductively coupled plasma (ICP) etch rates for GaN are reported as a function of plasma pressure, plasma chemistry, rf power, and ICP power. Using a Cl2/H2/Ar plasma chemistry, GaN etch rates as high as 6875 A/min are reported. The GaN surface morphology remains smooth over a wide range of plasma conditions as quantified using atomic force microscopy. Several etch conditions yield highly anisotropic profiles with smooth sidewalls. These results have direct application to the fabrication of group‐III nitride etched laser facets.

HIGH-DENSITY PLASMA ETCHING OF COMPOUND SEMICONDUCTORS

Inductively coupled plasma (ICP) etching of GaAs, GaP, and InP is reported as a function of plasma chemistry, chamber pressure, rf power, and source power. Etches were characterized in terms of rate and anisotropy using scanning electron microscopy, and root-mean-square surface roughness using atomic force microscopy. ICP etch rates were compared to electron cyclotron resonance etch rates for Cl2/Ar, Cl2/N2, BCl3/Ar, and BCl3/N2 plasmas under similar plasma conditions. High GaAs and GaP etch rates (exceeding 1500 nm/min) were obtained in Cl2-based plasmas due to the high concentration of reactive Cl neutrals and ions generated as compared to BCl3-based plasmas. InP etch rates were much slower and independent of plasma chemistry due to the low volatility of the InClx etch products. The surface morphology for all three materials was smooth over a wide range of etch conditions.

Smooth, low‐bias plasma etching of InP in microwave Cl2/CH4/H2 mixtures

Electron cyclotron resonance microwave (2.45 GHz) discharges of Cl2/CH4/H2 with low additional dc biases (−80 to −150 V) on the sample are shown to provide smooth, anisotropic dry etching of InP at ∼150 °C. Rates of 2500 A min−1 are obtained at a pressure of 0.5 mTorr and ∼80 V dc bias. SiO2 masks show no discernible erosion under these conditions, yielding a process that is extremely well suited for laser mesa fabrication. The CH4 addition promotes the anisotropy of the etching by a sidewall polymer mechanism, while the H2 addition significantly enhances the etch rate at low pressure.

Chlorine-Based Plasma Etching of GaN

The wide band gap group-III nitride materials continue to generate interest in the semiconductor community with the fabrication of green, blue, and ultraviolet light emitting diodes (LEDs), blue lasers, and high temperature transistors. Realization of more advanced devices requires pattern transfer processes which are well controlled, smooth, highly anisotropic and have etch rates exceeding 0.5 {micro}m/min. The utilization of high-density chlorine-based plasmas including electron cyclotron resonance (ECR) and inductively coupled plasma (ICP) systems has resulted in improved GaN etch quality over more conventional reactive ion etch (RIE) systems.

Inductively coupled plasma etching of III-V nitrides in CH{sub 4}/H{sub 2}/Ar and CH{sub 4}/H{sub 2}/N{sub 2} chemistries

Inductively coupled plasma (ICP) etching of GaN, AlN, InN, InGaN, and InAlN was investigated in CH{sub 4}/H{sub 2}/Ar and Ch{sub 4}/H{sub 2}/N{sub 2} plasmas as a function of dc bias, ICP power, and pressure. The etch rates were generally quite low, as is common for III-nitrides in CH{sub 4}-based chemistries. In CH{sub 4}/H{sub 2}/Ar plasmas, the etch rates increased with increasing dc bias. At low radio frequency power (150 W), the etch rates increased with increasing ICP power, while at 350 W radio frequency power, a peak was found between 500 and 750 W ICP power. The dc bias was found to increase with increasing pressure. The etch rates in the CH{sub 4}/H{sub 2}/N{sub 2} chemistry were significantly lower, with a peak at 500 W ICP power. The etched surfaces were smooth, while selectivities of etch were {le} 6 for InN over GaN, AlN, InGaN, and InAlN under all conditions.

Dry etch damage in inductively coupled plasma exposed GaAs/AlGaAs heterojunction bipolar transistors

The dc current gain and emitter and base sheet resistance of C-doped GaAs/AlGaAs heterojunction bipolar transistors (HBTs) have been used to measure damage introduced by exposure to Ar inductively coupled plasmas (ICP). As the ICP source power is increased at fixed rf chuck power, the damage-induced changes in device characteristics are reduced due to a reduction in ion energy. Beyond a particular ICP source power (∼1000 W for 50 W rf chuck power), the damage increases due to the increase in ion flux, even though the ion energy is low (<30 eV). These results are a clear demonstration of the advantage of high ion density plasmas for pattern transfer in damage-sensitive minority carrier devices such as HBTs.

Hydrogenation effects during high-density plasma processing of GaAs MESFETS

GaAs MESFETs may be exposed to -containing plasmas during various etch and deposition steps. We have found that both inductively coupled plasma (ICP) and electron cyclotron resonance (ECR) plasmas create severe reductions in MESFET mutual transconductance and reverse breakdown voltage through reductions in channel layer doping and surface stoichiometry changes. While changes in channel sheet resistance and diode ideality factor may be minimized by limiting the plasma exposure time, and are still reduced by up to a factor of two even for 30 s exposures. The results show that there are no conditions under which there are not substantial changes in device performance, and unless -free plasma chemistries are used, post-plasma annealing will always be necessary to restore the device characteristics.

Inductively Coupled Plasma Etch Damage in GaAs and InP Schottky Diodes

The effects of ion-induced damage in n- and p-type GaAs and p-type InP exposed to inductively coupled plasma (ICP) Ar discharges were measured by diode ideality factor and barrier height measurements. At fixed RF chuck powers, the electrical characteristics of the diodes generally improve with increasing ICP source power because the incident ion energy is decreased. In the particular case of p-GaAs, the higher ion flux at high-ICP source power leads to a degradation in barrier height. Exposure time and ion energy have a stronger influence than ion flux in all three materials. The results are compared to those obtained with electron cyclotron resonance Ar plasmas and show the same basic trends. The clear result is that at moderate ICP source powers, the suppression of self-induced dc bias on the powered electrode leads to a lower amount of ion damage to the semiconductors than for conventional reactive ion etch discharges.