Characterisation of Wet Chemical Etching of Algaas Layers Using Dynamic Optical Reflectivity
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CHARACTERISATION OF WET CHEMICAL ETCHING OF AlGaAs LAYERS USING DYNAMIC OPTICAL REFLECTIVITY J. V. ARMSTRONG, T. FARRELL AND G. TURNER Department of Materials Science and Engineering, The University of Liverpool, Liverpool L69 3BX, England.
ABSTRACT Wet chemical etching of AlGaAs layers using a dilute mixture of ammonium hydroxide and hydrogen peroxide is monitored in real-time using dynamic optical reflectivity (DOR). The AIGaAs layers were grown as a multilayer reflector stack by chemical beam epitaxy (CBE). The reflectivity of a normal incidence 3 mW, 670 nm semiconductor diode laser is monitored during the etching process and analysis of the DOR trace gives the etch rate for the different etchant concentrations used.
INTRODUCTION Optical interferometric techniques have been applied for some time to monitor in-situ the end-point detection while etching Si layers [1]. Use of diffraction of patterned photoresist structures has allowed analysis of etch undercutting [2]. Recently, non-normal incidence reflectometry has been used to study temperature variations during the reactive ion etching of InP/InGaAsP hetrostructures [3]. Here we report on the use of dynamic optical reflectivity (DOR) to monitor the wet chemical etching of a AlGaAs layers grown by chemical beam epitaxy (CBE). The DOR technique, which monitors the optical interference due to multiple reflections from the interfaces of multilayer structures, has proved to be a valuable tool in real-time growth monitoring of 111-V hetrostructures [4,5]. The technique yields similar information to that provided by ellipsometry but does not require polarised light and is used at normal incidence. The AlGaAs layers were grown as a high reflector, mirror stack [6] designed to maximize the change in the reflectivity oscillations during the etching process.
EXPERIMENTAL A multilayer reflector stack was grown on a 2" (100) GaAs wafer in a VG-V80H CBE chamber. The source gases used were triethylgallium, trimethylaminealane and cracked arsine. The growth temperature and pressure were 550 'C and 5x 10-5 mbar respectively. After growing a GaAs buffer layer the reflector stack was grown from alternating layers of high (Al 0 .4GaO. 6As) Mat. Res. Soc. Symp. Proc. Vol. 282. 01993 Materials Research Society
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and low (AlAs) refractive index layers, with each layer designed to have an optical thickness of a quarter wavelength for 670 nm light. The growth was stopped after 21 layers giving a total thickness of =lIgm. Due to substrate rotation during growth, the wafer appeared uniformly red across its 2" diameter. Small samples, typically 5x2 mm 2, were cleaved from the centre of the wafer and were mounted by epoxy on the flat bottom of a shallow glass container. The etching experiment is shown schematically in figure 1. About 20 cm 3 of etchant was poured into the glass container, covering the sample by at least 5 mm. The etchant was a dilute mixture of hydrogen peroxide (H 2 0 2 , 30%) and ammonium hydroxide (NH 4 OH, 30% NH 3 ) [7] with the mixture ratio determined by vo
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