Inductively Coupled Plasma Etching of HgCdTe Using a CH^sub 4^-Based Mixture

Journal of Electronic Materials, Jun 2005 by Laffosse, E, Baylet, J, Chamonal, J P, Destefanis, G, Et al

We report results on a study on inductively coupled plasma (ICP) etching of HgCdTe using a CH^sub 4^-based mixture. Effects of key process parameters on etch rates were investigated and are discussed in this article in light of plasma parameter measurements, performed using a Langmuir probe. Process parameters of interest include ICP source power, substrate power, pressure, and CH^sub 4^ concentration. We show that the ICP etching technique allows us to obtain etch rates of about 200 nm/min, which is high enough to use this technique in a manufacturing process. We also observe that the ion bombardment has a strong influence on HgCdTe etch rate. Finally, we show that this etch rate is modified by the substitution of methane for hydrogen.

Key words: Inductively coupled plasma (ICP) etching, HgCdTe, CH^sub 4^-based plasma, Langmuir probe

INTRODUCTION

HgCdTe is a II-VI compound semiconductor, which is extensively used as an infrared detecting material for high-performance devices.1

Fabrication of dual-band infrared detectors developed at CEA-LIR requires a step of trench etching to separate the pixels.2 In order to increase the array fill factor and allow reduction of the pixel pitch, an anisotropic etching process is needed.3 Plasma etching offers an alternative to wet etching, because higher anisotropy and better uniformity, as well as a lower consumption of chemicals, can be achieved with this technique.

The first HgCdTe plasma etching studies reported were carried out in reactive ion etching (RIE) systems.4,5,6 Incident particle energy has to be as low as possible to minimize HgCdTe damage, and pressure must be kept low to enhance anisotropy. Unfortunately, it is difficult to obtain RIE processes combining low pressure and low ion energy.

High-density plasma sources have been developed to replace RIE systems in semiconductor device manufacturing. These sources offer higher plasma densities at much lower pressures than RIE systems, which often lead to higher etch rates. Moreover, these systems are usually combined with a substrate stage independently biased from the plasma generation, allowing etching materials with low and controllable ion energy.

Most HgCdTe high-density plasma etching studies reported so far were carried out with electron cyclotron resonance (ECR) sources.7-14 However, inductively coupled plasma (ICP) etching may be more suitable for production, mainly because of its better uniformity and its lower cost, due to its simple design,15-17 in contrast to ECR systems, which include complex components, such as microwave tuning networks and microwave guides. There are surprisingly few results in the literature concerning ICP etching of HgCdTe.18 In this paper, we report results on ICP etching of HgCdTe using a CH^sub 4^/H2-based mixture.

This CH^sub 4^/H^sub 2^-based chemistry has been extensively used to RIE and ECR etch II-VI compound semiconductors, including HgCdTe. Chlorine and fluorine-based chemistries are not suitable to etch HgCdTe because they do not produce volatile etch products with cadmium.19 It has been suggested that methyl radicals CH^sub 3^, coming from the dissociation of CH^sub 4^, form volatile metal organic species, while atomic hydrogen H, coming from CH^sub 4^ and H^sub 2^ dissociation, forms volatile hydride species with group VI elements.4,7 This chemical etching is enhanced under the bombardment of ions extracted from the plasma and accelerated through the sheath. The Ar is often used as an additive to this gas mixture, especially because it makes plasma ignition easier. It can also play an important role in the etching mechanism, by increasing the ion bombardment to the material.

The purpose of this study is to investigate the effect of key process parameters on etch rates and discuss these results in light of plasma parameters measurements, performed using a Langmuir probe. The process parameters of interest include ICP source power, substrate power, pressure, and CH^sub 4^ concentration. To the best of our knowledge, this article reports the first work, which combines studies of HgCdTe ICP etch rates and plasma parameters.

We have applied these results to deep trench etching, which is necessary for our dual-band infrared detector manufacturing. We present an example of scanning electron microscopy (SEM) micrograph of an ICP-etched trench.

EXPERIMENTAL DETAILS

The HgCdTe layers used in this work were grown by liquid-phase epitaxy on (III)-oriented CdZnTe substrates. After chemical cleaning, to remove surface contamination and oxide layers, wafers were patterned with photoresist stripes or trench features and cleaved into 8x8 mm2 samples. Samples were then mounted on a 4-in. silicon wafer.

Etching was performed in a commercially available ICP system. A radio-frequency (RF) generator supplied a source power at 13.56 MHz to the coil. The substrate holder was connected to a second 13.56 MHz generator so that the energy of the ions impinging the substrate could be controlled independently from the plasma source. Interferences between the two RF signals are avoided thanks to a phase shift controller. A mechanical ring clamp held the silicon wafer, the temperature of which was regulated by an external heat exchanger and a He backside cooling. The chamber was pumped with a turbomolecular pump to a base pressure typically less than 2.10^sup -7^ Torr. The working pressure could be adjusted by throttling the turbomolecular pump and using a capacitive gauge for pressure feedback. Gas flow rates were accurately controlled by mass flow controllers.