GaN Device Processing
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Mat. Res. Soc. Symp. Proc. Vol. 482 © 1998 Materials Research Society
implant activation efficiency, gate insulator, trench etching for capacitor formation and high temperature/high current stable ohmic contacts. p- Well Contact
Gate Source Contact Contact i
Drain Contact
10 pm GaN n--Layer Sapphire Substrate
Figure 1. Schematic of an ultra high breakdown voltage GaN power MOSFET. DRY ETCHING It is now well-established that high density plasma techniques such as Electron Cyclotron Resonance (ECR) and Inductively Coupled Plasma (ICP) produce much higher etch rates than conventional Reactive Ion Etching (RIE) or low pressure (0.3 mTorr) Reactive Beam Etching (RIBE).( 6 8 Figure 2 shows GaN etch rates in C12/Ar-based chemistries as a function of dc chuck bias (or grid acceleration voltage in the case of RIBE) for these four techniques. ECR and ICP produce faster rates at equivalent ion energies because of their higher ion flux and degree of dissociation. Figure 3 shows etch yields calculated from this data - the higher values for the ECR and ICP reactors indicates there is also an enhanced chemical component in these systems, in addition to the higher ion flux. This originates from the enhanced reaction C12 + e ->2 Cl, leading to higher efficiency of atomic neutral creation. 7000
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Figure 2. Dry etch rate of GaN in C12/Ar-based plasmas created in four different types of reactor, as a function of dc chuck bias.
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Figure 3. Etch yield of GaN in C12/Ar-based plasma created in four different types of reactor, as a function of dc chuck bias. Etching selectivity for one material over another occurs most often when there is formation of an involatile etch product upon exposure of one of the material to the plasma, i.e. presence of a strong chemical reaction. Due to the requirement for a substantial physical component in Ifi-nitride etching due to the strong bonds in these materials, selectivities are generally not that high. In electronic device structures there will be a need to selectively etch InN-based ohmic contact layers from underlying binary or ternary nitride active channel layers. Figure 4 shows that maximum selectivities of 3-10 for InN over the nitrides can be achieved in ICP C12/Ar plasmas as source power is increased. Shul et al.(9) performed a detailed study of etch selectivity of GaN with respect to AIN and InN in C12 and BC13 plasmas with addition of Ar or SF 6 and found maximum values typically of 2-4. Since the nitride etching is generally ion-driven, sidewall anisotropy is generally excellent as long as mask erosion is avoided. As an example, Figure 5 shows a feature etched into InN in an ICP 25C12/Ar, 2 mTorr 500W source power, 250W rf chuck power discharge. The SiNx mask is still in place. Note the excellent verticality of the sidewalls, although striations are present and these will lead to s
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