Etch characteristics of KOH, TMAH and dual doped TMAH for bulk micromachining of silicon

Abstract

High precision bulk micromachining of silicon is a key process step to shape spatial structures for fabricating different type of microsensors and microactuators. A series of etching experiments have been carried out using KOH, TMAH and dual doped TMAH at different etchant concentrations and temperatures wherein silicon, silicon dioxide and aluminum etch rates together with <100> silicon surface morphology and <111>/<100> etch rate ratio have been investigated in each etchant. A comparative study of the etch rates and etched silicon surface roughness at different etching ambient is also presented.

From the experimental studies, it is found that etch rates vary with variation of etching ambient. The concentrations that maximize silicon etch rate is 3% for TMAH and 22 wt.% for KOH. Aluminum etch rate is high in KOH and undoped TMAH but negligible in dual doped TMAH. Silicon dioxide etch rate is higher in KOH than in TMAH and dual doped TMAH solutions. The <111>/<100> etch rate ratio is highest in TMAH compared to the other two etchants whereas smoothest etched silicon surface is achieved using dual doped TMAH. The study reveals that dual doped TMAH solution is a very attractive CMOS compatible silicon etchant for commercial MEMS fabrication which has superior characteristics compared to other silicon etchants.

Introduction

Bulk silicon micromachining is an essential process step for the fabrication of MEMS devices. Anisotropic wet chemical etching of silicon is frequently used for shaping quite intricate three-dimensional structures such as proof masses, cantilevers, diaphragms, trenches and nozzles on silicon substrate [1]. Presently, dry etching techniques (RIE and DRIE) are employed for high aspect ratio silicon micromachining but wet chemical etching still dominates over dry etching due to its low process cost, simple etch setup, higher etch rate, better surface smoothness, high degree of anisotropy and lower environmental pollution. The study of silicon, silicon dioxide, aluminum etch rates along with topology of the etched silicon surface for various silicon etchants at different temperatures is necessary to develop a CMOS compatible commercial process for fabricating micromechanical devices. Compatibility of MEMS fabrication process with commercial IC fabrication technology allows for integration of microtransducers with integrated circuits which provides on-chip signal conditioning, interface control and remote signal transmission. The commonly used silicon etchants are classified into three main groups: (i) Alkali metal hydroxides [3], [1], [2] (ii) Diamines based [4] and (iii) Quaternary ammonium hydroxides [5], [6]. KOH (potassium hydroxide) is a non-toxic, economical and commonly used alkali metal hydroxide silicon etchant which requires simple etch setup and provides high silicon etch rate, high degree of anisotropy, moderate Si/SiO₂ etch rate ratio and low etched surface roughness [1], [2]. But KOH damages exposed aluminum metal lines very quickly and is not CMOS compatible due to the presence of alkali metal ions in it. EDP (ethylenediamene pyrocatecol) is a diamine based silicon etchant which has moderate silicon etch rate, high Si/SiO₂ etch rate ratio, low degree of anisotropy and is partly CMOS compatible [4]. But EDP ages quickly, requires a complex etching apparatus and careful handling as it produces reaction gases which are health hazardous and so require special safety measures. TMAH (tetramethyl ammonium hydroxide) is the most preferred quaternary ammonium hydroxide based silicon etchant [5], [6]. TMAH is gaining popularity despite its high cost and complex etch setup because it is a non-toxic, CMOS compatible organic solution which has moderately high silicon etch rate and high selectivity to masking layers. Ordinary TMAH yields rough etched silicon surfaces but when TMAH is doped with suitable amounts of silicic acid and AP (ammonium peroxodisulphate, (NH₄)₂S₂O₈), the mixture provides complete aluminum passivation along with smooth etched surfaces [7], [8], [9].

In the present investigation, studies have been made on (i) silicon etch rate, (ii) <111>/<100> etch rate (ER) ratio and (iii) etched <100> silicon surface roughness using KOH, TMAH and dual doped TMAH solutions. The etching experiments were performed using different KOH (10, 22, 33 and 44 wt.%) and TMAH (3, 8, 12 and 20%) solution concentrations and at different bath temperatures (50 to 80 °C). The dual doped TMAH etching experiments were carried out using 2 and 5% TMAH solution. Our earlier study [9] reported that 2% TMAH doped with 30 gm/l silicic acid and 5 gm/l of AP whereas 5% TMAH doped with 38 gm/l silicic acid and 7 gm/l AP provides high silicon etch rate, smooth etched silicon surface and almost complete passivation of the exposed overlaying aluminum metal interconnection lines. This paper reports the experimental results and a comparison of the silicon etch rates, <111>/<100> ER ratio and etched silicon surface roughness for different Si-etchants, namely KOH, TMAH and dual doped TMAH solution.