PSG layers for surface micromachining

Abstract

The effect of both the deposition parameters and subsequent thermal processing upon LPCVD phosphosilicate glass (PSG) is experimentally investigated with an emphasis on establishing the effect of phosphorus content and PSG processing on the essential characteristics of PSG, such as etch rate, shrinkage, etch rate reduction and distribution of phosphorus after annealing. The effect of PSG upon the stress in overlying micromachined polysilicon layers is also investigated. The entire processing was carried out at a maximum temperature of 850 °C in order to avoid a significant increase in the thermal budget and to maintain 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 and after annealing) and the under-etch rate demonstrated an exponential increase with phosphorus content. The large value of the etch rate reduction after anneal (which can be as high as 70%) and its inverse proportionality with shrinkage (which is up to 8%) show that densification cannot be the only cause of the large variations in etch rates after annealing. The chemical meta-stability of the deposited PSG is identified in this research as a major cause of etch rate reduction after anneal, rather than the densification. Stabilization is obtained within 20 min after a 850 °C anneal. Increasing the phosphorus content of PSG from 2 to 6 at.% results in a reduction in sheet resistivity of the subsequently deposited and doped polysilicon from 1.4 to 0.9 kΩ/□, both with and without anneal of the PSG. The phosphorus content and the anneal of the PSG have negligible effect on strain in the overlying polysilicon.