3.5 Wet Chemical Etching

Etching process, like metalisation, is an integral part of fabricating semiconductor devices on wafers with epitaxially grown active layers. These vertically stacked layers have to be defined by mesa etching in order to make contacts to underlying layers or simply to electrically isolate a working device to restrict undesirable leakage currents. In addition, certain metals and compounds can also be defined by etching after deposition. Chemical etching is one of the more convenient ways of achieving this. Table 3.2 lists a host of chemical solutions which were used in the course of this work for various device fabrications; a more comprehensive list of etching solutions is available in [110].

In its simplest mode, chemical etching involves the dissolution of the material without any change in the chemical nature of the dissolved species. Factors affecting etch rates include temperature, humidity, strength of the etching solution as well as the morphology and the cleanliness of the surface being etched. In addition, given the tendency of different crystallographic planes to etch at dissimilar rates, various orientations of single-crystal substrates may etch very differently in a given etchant [111]. An example is GaAs in ammonia solution (8NH3 : 3H2O2 : 400H2O) which is therefore referred to as an isotropic etchant in this case.

In this study, wherever possible all etching was carried out in a controlled environment with known temperature and humidity. The samples were cleaned thoroughly in organic solvents to remove surface contaminants in steps prior to etching.

One of the more difficult steps in the fabrication of Al0.3Ga0.7As/GaAs HBTs is reaching the thin base (usually 1000Å or less) and leaving enough material for a good ohmic contact; a schematic of the processing steps involved is given in section 3.11. Although selective etching techniques (involving KI + I2 + H2O) can be used to minimise the risk, it is inapplicable where the Al mole fraction is less than 40% [112]. Selective etching is a process which refers to difference in etch rates between different materials. Thus to etch these Al0.3Ga0.7As/GaAs HBTs the ammonia solution was used and the etching had to be carefully monitored. Estimated doping levels monitored from breakdown voltage of the surface layer were used as a guide. These aspects are further discussed in section 3.7.

Etching InGaP/GaAs or InP/InGaAs wafers is relatively simple as a selective etching solution is readily applicable. InGaP only etches in conc. HCl (dilute solution is ineffective) which does not attack GaAs while the ammonia solution does not etch InGaP. Similarly, the respective etchants for InP and InGaAs listed in Table 3.2 do not etch the other material. Although it is difficult to quantify the etch rates of InGaP in conc. HCl or InP in ortho-phosphorous acid (1HCl : 1H3PO4), it was found that approximately 1000Å of both these materials can be removed in 10 sec. in the respective etching solutions.

In the fabrication of GaAs based Schottky diodes, given the large thickness involved (several mm in total), the ammonia solution was used for etching to the highly doped substrate. Both breakdown voltage and Talystep was used for judging the depth of the etch. The HF (1HF : 1H2O2 : 10H2O) solution was also used in cases where etch depth was not very critical such as in the isolation of TLM mesas consisting of thin conductive layers over thick SI substrates.

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