GaN、ZnO和SiC的湿法化学蚀刻

There are a number of challenges when processing compound semiconductors,  including the relatively high vapor pressures of the group V and VI elements compared to  the group III and II elements and the difficulty in forming highly reliable Ohmic and  rectifying contacts. It is necessary to develop highly selective, as well as non-selective,  etch processes for the different materials in the heterostructure systems available with  both GaN (ie. InGaN/GaN/AlGaN) and ZnO (namely ZnMgO/ZnO/ZnCdO). Much effort  is devoted to achieving lattice-matched compositions, to avoid the introduction of  threading dislocations which degrade the electrical transport and optical qualities of  devices subsequently fabricated. To some extent the InGaN/AlGaN system represents an  exception, since highly luminescent light emitting diodes (LEDs) and laser diodes have  been demonstrated.(2,3) For LEDs the resultant reliability is sufficient for commercial  applications, but the high dislocation density in heteroepitaxial material limits the lifetime  of laser diodes, where the much higher current density leads to metal migration that  shorts out the p-n junction. In material grown on quasi-GaN substrates, this mechanism  is absent (4), and the laser diodes have much longer lifetime.  

The etch rate may be limited by the diffusion of the active etchant species to the  semiconductor surface, or by the diffusion away of the soluble product.(5) In this case the  etching is termed diffusion-limited, and its characteristic include a square root dependence of etch depth on etch time, an activation energy ≤6 kCal.mol-1 and a strong  dependence of etch rate on solution agitation. This mode of etching is not desirable for  device fabrication, because of the difficulty in obtaining reproducible rates.

The other rate-limiting step may be the chemical reactions at the surface. In this  case, the etch depth depends linearly on time, the activation energy is ≥6 kCal.mol-1 and  the rate is independent of solution agitation. This is the preferred mode of etching for  device fabrication, since only temperature and solution composition need to be controlled.

Since wet etching tends to be isotropic in nature, the undercutting of the mask  makes it unsuitable for pattern transfer of small (<2 µm) features. There are a number of  other disadvantages relative to dry etching, including an increased safety hazard due to  potential exposure to chemicals and fumes, and bubble formation during the etching  which can lead to local non-etched regions.

Fig1

We have observed a strong effect of annealing on subsequent wet etch rate of  sputtered AlN films in KOH solutions, with over an order of magnitude decrease in rate  after annealing at 1100o C.(36) Similarly the etch rate for In0.2Al0.8N grown on Si was  approximately three times higher in KOH based solutions than for material grown on  GaAs, which is consistent with the superior crystalline quality of the latter. Etching of  InxAl1-xN was also examined as a function of In composition, with etch rate initially  increasing up to 36% In and then decreasing to zero for InN.(36)

Minsky, White and Hu(37) reported laser enhanced, room temperature wet etching  of GaN using dilute HCl/H2O or 45% KOH/H2O, with rates up to a few thousand  angstrom/min for HCl and a few thousand angstrom/min for KOH. The mechanism is  believed to be photoenhancement of oxidation and reduction reactions in what amounts to  an electrochemical cell. Etch rates were linearly dependent on incident HeCd laser power.