Silicon surface micromachining is a new technology that uses many of the common microfabrication techniques found in silicon integrated circuit (IC) production. The key processing step that is the sacrificial layer etch for releasing microstructures which is not analogous to silicon IC wet etching. Selective, isotropic hydrofluoric acid etching is used to remove a temporary support layer of silicon dioxide that is sandwiched between a patterned polycrystalline silicon structural layer and the silicon substrate. The goal of this work is to predict and optimize sacrificial layer etch times.
Experimental test structures were developed and fabricated in-house with optically visible markings for the observation of HF etching of SiO2 sacrificial layers. Increasing the HF concentration, increasing the phosphorus content in the glass, and adding HCI to constant HF molarity solutions increased process rates. Diffusion limitations were observed in all cases for long etch times. These effects have been modeled using a steady-state diffusion/chemical reaction model. Non-first order chemical reaction kinetic expressions for the HF/SiO2 system have been determined by independent etching experiments conducted on unpatterned silicon dioxide-on-silicon chips. The model uses a Newton-Raphson technique for linearizing the kinetics and a finite difference routine for stepping in time. The steady-state diffusion relationship requires values for the diffusivity of hydrofluoric acid in water that are estimated from independent Rayleigh interferometric restricted diffusion measurements.
Silicon surface micromachining structure release poses two additional problems. Cantilevered microstructures have been observed to stick onto the underlying substrate during the release process. A surface tension/contamination mechanism for this phenomenon has been developed, and surface tension experiments were performed. In addition, hydrofluoric acid has been observed, through microscopy (SEM and TEM), to degrade polysilicon mechanically. Phosphorus from underlying phosphosilicate glass films greatly enhances this process whereas annealing slows it. Potentially, however, these "permeable" polysilicon films may replace patterned etch openings for large-area microstructures
May 30, 1993
Monk, D. J. (1993). Controlled Structure Release for Silicon Surface Micromachining. United States: University of California, Berkeley.