Feature Size and Density Effects in Wet Selective Etching of GaAs/AlAs p-HEMT Structures with Organic Acid - Peroxide So
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Feature Size and Density Effects in Wet Selective Etching of GaAs/AlAs p-HEMT Structures with Organic Acid - Peroxide Solutions. Vinay S. Kulkarni1, Kanti Prasad1, William Quinn2, Frank Spooner2 and Changmo Sung3 1 Electrical and Computer Engineering Department, 3 Chemical Engineering Department, University of Massachusetts Lowell Lowell, MA, USA. 2 Skyworks Solutions, Inc., Woburn, MA, USA. ABSTRACT Pseudomorphic HEMT (p-HEMT) devices are used in a number of wireless communication applications including power amplifiers in the 1-50 GHz range, low noise amplifiers and switches. Selective wet etching is often used to form the gate regions of these devices to avoid plasma damage associated with dry etching. We have investigated the wet etching of small (8µm to 0.5µm) features with organic acid - hydrogen peroxide solutions. Two acid solutions were used as a selective etchant for GaAs using AlAs etch stop layers in a p-HEMT structure grown by MBE. The etched features were characterized by AFM, SEM, and TEM techniques. The etch depth uniformity and reproducibility were found to depend on a number of factors including feature size, feature density, etching chemistry, agitation and surface tension. When features with a range of size and density were placed in close proximity in a layout we found that the etch rate of the different features was a function of density, size and most importantly the etch chemistry. One etchant solution exhibited a 12% difference in etch rate from the smallest feature to the largest, while another solution exhibited uniform etching of all features regardless of size or density. Both solutions produced specular etched surfaces in GaAs and AlGaAs. However, the AlAs etch stop showed a non-uniform surface morphology after etching. The surface morphology of the AlAs etch stop is one factor that limits the over etch which can be designed into the process. The most important factors to be considered in designing a selective etch process will be presented. INTRODUCTION Gate and Channel recessing are the most important process steps in the fabrication of GaAs devices because they determine the device performance characteristics such as threshold voltage (Vth) and source to drain current (Idss). In wet etching, there are two different methods of front side etching of GaAs, one is the conventional etch and measure technique and the other is selective etching technique. The selective etching process has various advantages over the conventional etch process; it can produce an uniform etch depth, residue free etched surface after etching and also has excellent reproducibility. In GaAs devices, AlAs or AlGaAs is often used as an etch stop layer. Various selective etchants, including organic acid – peroxide mixtures are for etching III-V semiconductors [2]. Succinic acid and citric acid were the organic acids that selectively etch GaAs/AlAs or GaAs/AlGaAs in this study. There are a number of features (e.g. Channel and Gate recess) of different sizes (area) on the same wafer that are being etched at t
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