Neutron Irradiation-Induced Dimensional Changes in MEMS Glass Substrates
- PDF / 70,291 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 9 Downloads / 199 Views
Neutron Irradiation-Induced Dimensional Changes in MEMS Glass Substrates Clark L. Allred1,2,3, Jeffrey T. Borenstein1, Linn W. Hobbs2 1. The Charles Stark Draper Laboratory, Inc., 555 Technology Square, Cambridge, MA 021393563 2. Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 021394307 3. Air Force Institute of Technology, 2950 P Street, Wright-Patterson AFB, OH 45433-7765 ABSTRACT A study is made of radiation-induced expansion/compaction in Pyrex and Hoya SD-2 glasses, which are used as substrates for MEMS devices. Glass samples were irradiated with a neutron fluence composed primarily of thermal neutrons, and a flotation technique was employed to measure the resulting density changes in the glass. Transport of Ions in Matter (TRIM) calculations were performed to relate fast (~1 MeV) neutron atomic displacement damage to that of boron thermal neutron capture events, and measured density changes in the glass samples were thus proportionally attributed to thermal and fast neutron fluences. The trend for strain with thermal neutron fluence (n/cm2) was found to be a linear compaction of -2.8×10-20 for Pyrex and -1.0×10-21 for Hoya SD-2. For fast neutron fluence, the trend for strain was also linear: -6.1×10-21 for Pyrex and -7.9×10-22 for Hoya SD-2. The measured radiation-induced compaction of Pyrex is found to agree with that of previous studies. To our knowledge, this work represents the first study of compaction in Hoya SD-2 with neutron fluence. Hoya SD-2 is of considerable importance to MEMS, owing to its close thermal expansivity match to silicon from 25-500 C. INTRODUCTION In order for a glass substrate-mounted MEMS device to behave predictably when used in a radiation environment, one must design it with an understanding of the effect of that environment on material properties such as density, coefficient of thermal expansion, and elasticity. This study treats density changes caused by neutron irradiation which induce differential strains between a MEMS device and its substrate. One mechanism for causing dimensional changes in glass is the collision of a fast neutron with atoms in the glass network structure. The atoms so struck can be forced from their network sites with substantial kinetic energy, in turn leading to other atomic displacements and subsequent network rearrangements. This sequence of atomic displacements is referred to as a collision cascade, and the atom directly displaced by an incident fast neutron is called the primary knock-on atom (PKA). Collision cascades can also be caused by nuclear reactions that yield energetic particles. The reaction of interest here is the absorption of a thermal neutron by 10B, an isotope 20% naturally abundant in boron and present in both Pyrex and Hoya SD-2. Upon absorbing a thermal neutron, the 10B nucleus splits into a lithium nucleus with a kinetic energy of ~0.9 MeV, and an alpha particle with kinetic energy of ~1.6 MeV.1 Both of these products can then initiate a collision cascade.
B5.17.1
The effects of thermal neutron irra
Data Loading...
