Inductively coupled plasma etching of III–V semiconductors in Cl2-based chemistries

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Abstract

Inductively coupled plasma (ICP) etching of all of the main III–V compound semiconductors is reported as a function of source and chuck power, pressure and gas additive (Ar, N2, H2) in Cl2 plasmas. Etch rates between 2000 and 5000 Å min−1 were obtained for all materials at moderate source power (750 W) and low d.c. self-bias (≤ −150 V). Surface morphologies were smooth over a broad range of conditions for Ga-based materials, while for In-based semiconductors there was a much narrower window of appropriate ion-to-neutral ratios to obtain smooth etched surfaces on In-based semiconductors.

Introduction

As in the Si electronics arena, there are two basic trends in compound semiconductor technology, namely a push to larger wafer sizes (maximum diameter is now 6″) and smaller critical device dimensions1, 2, 3. For these reasons, increasing demand is being placed on the anisotropy and uniformity of plasma etch processes for pattern transfer. High density plasma tools are now relatively common in fabrication lines for devices such as heterojunction bipolar transistors (HBTs) and heterojunction field effect transistors4, 5, 6, 7, 8, 9, 10, 11. The dominant high density plasma source employed has been electron cyclotron resonance (ECR) to this point, but this technology has significant drawbacks, especially in its electromagnet configuration. Firstly, there is a divergent ion flux emerging from the resonant zone, due to the magnetic field geometry and even with lower magnet collimation, etch uniformity is suspect for large diameter wafers. Secondly, the cooling requirements for the magnets are substantial (kW of power need to be dissipated), and cost of ownership escalates rapidly as the source size increases.

To overcome these problems, much of the equipment business has turned to radiofrequency sources which have the additional advantage of more mature automatic tuning capabilities relative to microwave sources12, 13, 14, 15. We have previously found that inductively coupled plasma (ICP) processing of devices such as HBTs and metal-semiconductor field effect transistors can produce essentially damage-free pattern transfer provided the ion flux-energy product is maintained below a particular threshold4, 16, 17, 18. In these tools the chuck self-bias is a strong function of both radio frequency (RF) chuck power and the high density source power because of the influence of the latter on ion density.

In this paper we report the results of a parametic study of the influence of chuck and ICP source power, pressure and plasma composition on the etch rate and surface quality of the most common Ga-based (GaAs, AlGaAs, GaSb, GaP) and In-based (InP, InGaAs, InAs, InGaAsP, InSb and InAlAs) III–V semiconductors in Cl2 plasmas. Controllable etch rates in the range 1000–6000 Å min−1 with smooth, stoichiometric surfaces can be achieved for all of these materials under optimized conditions.

Section snippets

Experimental

Wafers of undoped GaAs, GaSb, GaP, InSb, InAs and Fe-doped InP were taken from Czochralski crystals, while epitaxial layers of Al0.3Ga0.7As (∼1 μm thick) grown on GaAs substrates or In0.53Ga0.47As, Al0.48In0.52As or InGaAsP (λ=1.3 μm) grown on InP substrates by either metal organic chemical vapor deposition (MOCVD)[19]or metal organic molecular beam epitaxy (MOMBE)[20]were employed for these experiments. The samples were lithographically patterned with AZ5209E photoresist in a resolution test

Results and Discussion

Fig. 1 shows the dependence of etch rate [Fig. 1(a)] and etch yield [Fig. 1(b)] of the Ga-based materials in 10Cl2/5Ar plasmas as a function of ICP source power at fixed pressure (2 mTorr) and d.c. self bias (−80 V). The same basic trends are found for all four materials, with the etch rates increasing quickly beyond 500 W for GaAs, GaSb and GaP or 900 W for AlGaAs due to the increase in ion flux producing more efficient desorption. The AlGaAs etch rates are likely lower because of the native oxide

Summary and conclusions

ICP tools appear to be well-suited to dry etch processing of all the common III–V semiconductors, using a single plasma chemistry, that is, Cl2/Ar. This is not possible in conventional reactive ion etch systems, where Cl2 chemistries are employed for Ga-based materials, and CH4/H2 for In-based materials. Etch rates of 2000–5000 Å min−1 are obtained for all of the common III–V materials at source powers of 750 W (half of the maximum power available) and low rf chuck powers (corresponding to d.c.

Acknowledgements

The work at UF is partially supported by a DOD MURI monitored by AFOSR (H.C. DeLong), contract No. F49620-96-1-0026. Sandia National Laboratories is a multi-program laboratory operated by Martin Marietta for the US Department of Energy under Contract No. DE-AC04-94AL85000.

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