| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 90 === Wet etching has been widely used in mesa etch and gate recess process in GaAs device process. We prefer to use wet etching when low damage is required since dry etching always accompanies with damage.
In this thesis, two etchant systems were used for GaAs wet etching in heterojunction structure. H3PO4/H2O2/MX03 mixture was used to etch GaAs layer with thicker layer thickness due to its etch rate of about 200Å/sec. Citric acid (CA)/KCA/H2O2/H2O mixture was optimized to derive stable and low etching rate so that etch rate can be well controlled and lateral etching can be prevented. Besides, InGaP etchant was optimized to improve etch uniformity.
In H3PO4-based system, the etch process was optimized in horizontal and vertical directions respectively. The following experiment conducted for the whole wafer showed that the wet etching uniformity for GaAs wafers in the cassette can be improved to be 5.6%, with the setting of the optimum etching condition of 11000ml H2O2 mixed with 1000ml H3PO4 and 12 ml MX03 with 30psi N2 bubble around by 1.5oC.
In CA-based system, CA/KCA/H2O2/H2O=371g/120g/351ml/4983ml performed stable etch rate in an observation duration of 400 hours. Etchant composed of Buffer (CA and KCA in water): H2O2 = 5.6:2 in volume demonstrated GaAs/InGaP etch selectivity of 1277 with the average etch rate of 66.4 Å/sec and 0.052 Å/sec for GaAs and InGaP respectively. This is a suitable etchant for mesa etch and gate recess of GaAs/InGaP heterojunction device.
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