Characterization of Dry Etching Processes of III-V Semiconductors in Silicon Tetrachloride Plasmas

[摘要] This thesis is concerned with the development of Reactive Ion Etching (RIE) processes for GaAs/AlGaAs material system in Silicon Tetrachloride (SiCl4) and mixed wSilicon Tetrachloride and Silicon Tetrafluoride (SiCl4/SiF4) plasmas. Etch processes which etch GaAs and not AlGaAs (selective) and ones etch both (non selective) have been developed. Particular attention has been paid to achieving a damage free etch. The chemistry and mechanisms behind the etching process in these plasmas have been thoroughly studied using Optical Emission Spectroscopy (OES) as a tool for plasma diagnosis. Optical Emission Spectroscopic analysis of the SiCl4 plasma showed that the chemistry and the etching mechanism are strongly dependent on the applied rf power. Two chemical regimes and hence two mechanisms of etching were identified including: "the low power regime" in the rf power range of 5-20 W and "the high power regime" in the range of 25-150 W. In the low power regime, SiC l4 molecules breakdown gradually by multiple electron impact excitations into SiCl2 and SiCl radicals, Si and Cl atoms and molecular ions of CI2+. In the high power regime, the breakdown of SiCl4 molecules is mostly through one electron impact excitation into SiCl2 and SiCl radicals. Cl and Si atoms, and atomic Cl ion Cl+. This gives rise to two etching mechanisms, the first is in the low power regime where the etching depends on the concentration of mainly Cl2+ ions and chlorosilicon radicals. The second is in the high power regime where the etching depends on the concentration of Cl+ ions and chlorosilicon radicals. A very low damage, anisotropic selective or nonselective RIE has been developed in SiCl4 plasma for etching GaAs/AlGaAs which stops on an extremely thin AlGaAs layer (1.13 nm thick). Using a very low rf power of 10-15 W and a low dc bias < 60 V, this process can be selective or nonselective over the AlGaAs depending on the SiCl4 flow rate and pressure; selectivities of the order of 10000:1 are readily obtained whilst maintaining excellent verticality. Nanostmctures 50-60 nm wide in GaAs have successfully been etched to a depth of 1.5 mum under very low damage conditions. Selective RIE for gate recessing for MESFET fabrication has been successfully implemented, where T- shaped gates with vertical gate profiles with little gate off-set have been obtained. The mechanism of the selectivity depends on the formation of some form of AlxOy or AlxNy from the residual O2, air or water in the chamber. Both the surface and sidewall damage were measured and the results were confirmed by the evaluation of the performance of Metal Semiconductor Field Effect Transistors (MESFET) whose gate recess etching was performed using this process. The results of sidewall damage made on quantum wires showed that this process is virtually damage free in the first two minutes of etching. Results from Raman scattering and MESFET characteristics confirmed that no significant degradation of carrier mobility or free carrier concentration in the channel occurs even after 3600% of overetching. A "damage free" and selective RIE process for etching GaAs/AlGaAs has been developed in a mixture of SiCl4/SiP4 plasma using very low rf power and high pressure. The selectivity results showed that very high selectivities of the order of > 5000:1 can be achieved by the addition of large amounts (70-85 %) of Sip4 to SiCU at high pressures and reduced dc biases. On the other hand the results of the OES on SiCl4/SiF4 plasma showed that no free atomic F exists in the plasma and that the dominant species are the silicon chlorides and silicon fluorides. Dry etch damage assessment carried out by Raman scattering on heavily doped GaAs showed that "damage free" etching is possible. The above results were confirmed by Hall measurements on pseudomorphic High Electron Mobility Transistor (HEMT) structures which showed that no detectable change in the free carrier concentration of the channel nor in the electron mobility can be observed even after 5000% of overetching. This SRIE process was applied to T-gate recessing and the results showed a controllable gate off-set can be readily obtained.

[发布机构] University:University of Glasgow

[关键词] Materials science, Condensed matter physics