The Applications of Citric Acid/Hydrogen Peroxide Etching Solutions in the Processing of Pseudomorphic MODFETs

The etching characteristics of and in citric acid/hydrogen peroxide etching solution were studied. The etch rate and the selectivity were measured over a wide concentration range from room temperature down to 0°C. The results show that by changing the concentration of the solution we can vary the selectivity ( etch rate: etch rate) from over 80:1 to reverse selectivity of 1:1.4; the etch rate can also be varied from over 4000 Å/min to less than 150 Å/min. However, high selectivity process consistently shows high etch rate. We have developed mesa and gate recess etch processes for pseudomorphic modulation doped field effect transistor (MODFET) structures based on these results. Using a two‐step mesa etch process, we can produce a recessed sidewall profile of the channel layer and hence avoid the parasitic gate leakage path caused by the contact of Schottky gate to the exposed channel layer at the sidewall. A nonselective low etch rate process was chosen for gate recess etch since the selective etch shows high etch rate and causes excessive undercut in the cap layer. Using the citric acid/hydrogen peroxide etch process with low etch rate, very uniform threshold voltage (with a standard deviation of 25 mV across a 2 in. wafer) can be achieved. This is the best result reported using nonselective wet etch and compares very favorably to published results using dry etch.