Effect of radiation on silicon and borosilicate glass

Abstract

A study was made that is logically divided into two parts, both involving radiation damage effects. The first is a study of the effects of neutron and gamma radiation on the dimensions of two borosilicate glasses, Pyrex® and Hova SD-2®. These two glasses are commonly used as substrates for silicon microelectromechanical (MEMS) devices, and radiation-induced compaction in a substra.te can have deleterious effects on device performance. Results are presented for density changes induced in both glasses by neutron irradiation. Pyrex was shown to compact at a rate of (in [delta]p[rho]/p[rho] per n/cm2̂) 8.14 x 10-̂20 (thermal) and 1.79 x 10-̂20 (fast). The corresponding results for Hoya SD-2 were 2.21 x 10-̂21 and 1.71 x 10-̂21, respectively. On a displacement per atom (dpa) basis, the compaction of the Pyrex was an order of magnitude greater than that of the Hoya SD-2. Our results are the first reported measurement of irridiation-induced densification in Hoya SD-2. The compaction of Pyrex agreed with a previous study. Our results for gamma irradiations were unexpected. Silicon MEMS strain gauges mounted on glass wafers were gamma-irradiated to hundreds of Mrad. Based on expectations from the literature, the Pyrex was supposed to compact to a level easily measurable by the MEMS strain gauges. Almost no substrate compaction registered in the strain gauges, however. It is hypothesized that the anodic bonding process (by which a silicon wafer was bonded to the glass before etching to create the MEMS strain gauges) was responsible for either 1) changing the bulk radiation response of the glass or 2) creating a layer near the bond interface which somehow prevented the MEMS strain gauges from registering the compaction that was occurring in the glass substrate. While not yet understood, this null result for apparent substrate compaction is of great importance to the problem of mechanically rad-hard MEMS, since it indicates that the response of an anodically bonded Si-glass system to radiation is not simply the sum of the effects on the unbonded materials. To investigate this further, glass samples were prepared in various stages of the anodic bonding process (which involves heating in the presence of an electric field), then irradiated with neutrons. No difference in bulk compaction was noted among the

(cont.) treated samples or the untreated glass, but this result may have been influenced by the high temperature at which the glass was irradiated; however, temperature alone could not have annealed away all the effects of treatment. We conclude that the unexpected results of the MEMS strain gauge experiment were caused by surface layer phenomena at the bonding interface, though we do not currently understand the exact mechanism for this. The second major topic of this study is the effect of neutron irradiation on the Young's modulus of silicon, the constancy of which is key to the operation of many MEMS devices. The elastic constants of defected and amorphous silicon simulation cells were calculated using EDIP. Simulation cells included some containing randomly generated defect distributions, as well as several that were completely amorphous and one containing a small amorphous region. An extensive and careful characterization of point defects was made ...