Selective Wet and Dry Etching of NiO over β-Ga2O3

Abstract

In this study, a reaction-limited chemical wet etching process of NiO by an HNO3-based solution, with a complete selectivity over β-Ga2O3 was investigated. The threshold ion energies for inductively coupled plasmas (ICP) dry etching of NiO in BCl3 discharges, its selectivity to β-Ga2O3, and the cleaning effect for chlorine-based residues remaining on the surface after plasma exposure was also examined. One of the major drawbacks of β-Ga2O3 device design is the lack of p-type dopants. An alternative approach is to use a p-type oxide, NiO, to form heterojunctions with n-type Ga2O3. Therefore, a selective wet and dry etching process development is in demand for the optimized pattern transfer process. A solution of 1 HNO3: 4 H2O exhibited measurable etch rates for NiO above 40°C and wet etching activation energy of 172.9 kJ.mol-1 (41.3 kCal.mol-1, 1.8 eV/atom), which is firmly in the reaction-limited regime. The selectivity over β-Ga2O3 was infinite for temperatures up to 55 °C. The strong negative enthalpy for producing the etch product Ga(OH)4 suggests that HNO3-based wet etching of NiO occurs via the formation and dissolution of hydroxides. The BCl3/Ar ICP etching produced maximum etch rates up to 300 Å.min-1 for NiO and 800 Å.min-1 for β-Ga2O3 under moderate plasma power conditions. The selectivity for NiO: Ga2O3 was < 1 under all conditions. The ion energy threshold for the initiation of etching NiO was between 35-60 eV, depending on the plasma power conditions. And its etching mechanism was ion-driven, as determined by the linear dependence between etch rate and the square root of ion energy incident on the surface. By sharp contrast, the etching of Ga2O3 had a stronger chemical component without a well-defined ion energy threshold. After being exposed to BCl3 plasmas, both NiO and Ga2O3 surfaces show chlorine residues, which can both be removed by the standard 1NH4OH: 10H2O or 1HCl: 10H2O solution used for native oxide removal. In addition, according to the location of the Cl 2p3/2 peak in the XPS analysis on the surface, the chlorine particles were ionically bonded. Figure 1