Vapour phase HF and XeF2 etching methods with improved selectivity for MEMS manufacturing
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Fast and high selectivity vapour phase etching techniques for silicon dioxide and silicon materials are necessary for cost-effective and high precision manufacturing of future MEMS devices. The thesis describes the development and characterisation of vapour phase HF etching methods for silicon dioxide films and XeF2 based etching processes for silicon. In studies of vapour phase HF etching of silicon dioxide films the effects of catalyst choice, water and alcohol, are investigated in terms of etching rate and selectivity against silicon nitride films. The former is commonly used as the sacrificial material to release the fabricated microstructures from a substrate while the latter is used as the structure or passivation material. The dependence of etching rates and selectivity on gas flow, pressure and temperature were studied. It has been found that the best etching rate and selectivity for a water based catalyst can be achieved at low temperature and high pressure conditions while for the alcohol based catalyst high temperature is preferred. Selectivities of 26:1 and 12:1 have been obtained for the water and alcohol based catalysts respectively. In order to improve the selectivity, the effects of deposition and post processing conditions for silicon nitride on the resultant etching behaviour have been investigated. The silicon nitride films were made to contain more silicon (silicon-rich) than in the standard nitride films by increasing the gas ratio between silane and ammonia in the deposition process. The effect of annealing was also studied. It was found that the etching rate by HF vapour is reduced by a factor of 3.6 from 9.5 nm/min to 2.6 nm/min. After annealing at 500°C the etching rate can be reduced further to 1 nm/min. The total reduction of etching rate is by approximately an order of magnitude indicating that the selectivity can be improved by a factor of about 10. XeF2 based vapour phase etching of silicon has also been studied since silicon is also used as a sacrificial material in MEMS manufacturing. The undercut etching behaviour was investigated for both single crystal silicon and polysilicon films produced by the PECVD method. An improvement in undercut etching rate by almost a factor of 2 has been achieved for polysilicon, 18 µm/min over the value of 10 µm/min as in the previous work. A new etching process by adding hydrogen in the reaction gas was studied showing significant improvement in selectivity from 500:1 to 1000:1 between the single crystal silicon and the PECVD silicon dioxide films, and it is from 19:1 to 10000:1 between single crystal silicon and the PECVD nitride films.