PSG LAYERS FOR SURFACE MICROMACHINING

The effect of both the deposition parameters and subsequent thermal pr ocessing upon LPCVD phosphosilicate glass (PSG) is experimentally inve stigated with an emphasis on establishing the effect of phosphorus con tent and PSG processing on the essential characteristics of PSG, such as etch rate, shrinkage, etch rate reduction and distribution of phosp horus after annealing. The effect of PSG upon the stress in overlying micromachined polysilicon layers is also investigated. The entire proc essing was carried out at a maximum temperature of 850-degrees-C in or der to avoid a significant increase in the thermal budget and to maint ain fabrication compatibility with already integrated standard bipolar devices. The shrinkage of the PSG is shown to increase linearly with phosphorus content, while both the vertical-etch rate (as-deposited an d after annealing) and the under-etch rate demonstrated an exponential increase with phosphorus content. The large value of the etch rate re duction after anneal (which can be as high as 70%) and its inverse pro portionality with shrinkage (which is up to 8%) show that densificatio n cannot be the only cause of the large variations in etch rates after annealing. The chemical meta-stability of the deposited PSG is identi fied in this research as a major cause of etch rate reduction after an neal, rather than the densification. Stabilization is obtained within 20 min after a 850-degrees-C anneal. Increasing the phosphorus content of PSG from 2 to 6 at.% results in a reduction in sheet resistivity o f the subsequently deposited and doped polysilicon from 1.4 to 0.9 kOM EGA/square, both with and without anneal of the PSG. The phosphorus co ntent and the anneal of the PSG have negligible effect on strain in th e overlying polysilicon.