Uhv-Tem Studies of Laser-Induced Damage in Silicon
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UHV-TEM STUDIES OF LASER-INDUCED DAMAGE IN SILICON T. S. SAVAGE, P. XU, R. Al, D. DUNN, and L. D. MARKS Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208 ABSTRACT The effects of pulsed laser irradiation on silicon (111) single crystal thin samples were studied in a ultra-high vacuum transmission electron microscope. Samples were found to cleave along (110) planes under the laser beam. The formation of dislocation networks was also observed. The cleaving did not seem to originate from previously observed defect areas, but from random places, and is believed to be caused by thermal shock from laser beam heating. Bulk defects in the specimens, such as stacking fault tetrahedra and dislocations, were not observed to be affected by the laser treatment. INTRODUCTION The use of lasers for surface annealing, heating and/or damage has become a routine practice in the study of materials. Lasers have been closely looked at as an annealing technique for silicon and other semiconductors, since they allow rapid local heating from a beam which can be focused and tuned to different wavelengthes for specific tasks. Many studies in pulse laser annealing and damage have been done on thin films deposited on a substrate or on bulk wafers [1-3]. In order to induce damage in these systems, very large laser powers were employed. Thermal stress cracking, micropits and thin film delamination were observed as the result of surface melting and quenching during the rapid heating and cooling cycles. The studies that have looked at laser annealing or damage with a TEM typically irradiate a bulk sample and then chemically thin the sample from the back side [4], thus direct comparison between before and after images were of limited value. In this study, the laser damage experiments were carried out on thin silicon samples in a UHV microscope and with in situ sample preparation. The laser power employed was smaller (one to two orders of magnitudes) than that used for bulk specimens. TEM images taken before and after the laser irradiation show the actual changes that had taken place in the specimen and possible relationships to the sample microstructure. EXPERIMENTAL A Candela SLL-250 pulsed dye laser, with a maximum power of 1 J/pulse over 350400 nsec was set up in conjuction with a Hitachi UHV-H9000 TEM. The laser beam was introduced into the side chamber attached to the microscope column through a series of mirrors, a focusing lens and a quartz window. The laser uses a Rhodamine 6G dye dissolved in a 50:50 mixture of methanol:water giving it a broad wavelength spectrum with a peak at about 590 nm. This arrangement allows TEM samples to be irradiated with laser light for annealing or damage and subsequently transferred under UHV pressure into the microscope for imaging. Mat. Res. Soc. Symp. Proc. Vol. 236. @1992 Materials Research Society
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Boron-doped silicon (111) specimens were prepared by ultrasonically cutting 3 mm disks from bulk wafers, mechnically thinning and polishing, followed by chemical
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