Intrinsic Stress in a-Germanium Films Deposited by RF-Magnetron Sputtering
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INTRINSIC STRESS IN a-GERMANIUM FILMS DEPOSITED BY RF-MAGNETRON SPUTTERING
D. Fahnline, B. Yang, K.Vedam, R. Messier, and L. Pilione Materials Research Laboratory, The Pennsylvania State University, University Pari, PA 16802
ABSTRACT Thin a-Ge films deposited by rf-magnetron sputtering undergo a change from compressive to tensile stress as the argon gas pressure is increased. Spectroscopic ellipsometry, neutron activation analysis, and a dual laser beam reflection technique were used to determine the film's void fraction content, Ar/Ge atomic percent, and stress, respectively. The results of these three measurements indicate that as the film's Ar/Ge content increases, the void fraction decreases and the intrinsic stress becomes compressive. These findings are consistent with currently accepted bombardment mechanisms.
INTRODUCTION The intrinsic stress of thin films prepared by physical vapor deposition techniques is known to vary with preparation conditions. In particular, numerous reports have indicated that sputtering at low gas pressures results in compressively stressed metallic, semiconducting, and insulating films, while high gas pressures produce tensile films [1-6]. It is generally believed that changes in the arrival energies and numeric ratio of gas atoms reflected by the target and atoms sputtered from the target play a dominant role in the evolution of film structures accounting for these stresses. At low gas pressures the arrival energies of atoms of both groups are only slightly modified by collisions as they move from the target to the substrate. As a result energetic atoms from both groups bombard the uppermost layers of the film causing densification through atomic rearrangement as well as compressive film stress [7]. At high gas pressures collisions reduce such bombardment and increase the oblique angle of incidence of sputtered atoms arriving at the surface of the growing film causing atomic shadowing. This combination of effects results in films with a microstructure that is columnar and has voided growth boundaries. Such films are associated with in-plane tensile stresses resulting from attractive forces between columns [4]. The total void volume, distribution, and connectivity are also thought to affect a number of other film properties [2, 4, 6, 8, 9]. To study these relations experimentally we rf-magnetron sputtered a series of a-Ge films at various argon gas pressures and a fixed target to substrate distance. Spectroscopic ellipsometric measurements were conducted and subsequent data analysis yielded the void fraction of each film. Neutron activation analysis was performed to determine the argon content, and a laser beam technique was utilized to determine the intrinsic film stress.
Mt. Res. Soc. Symp. Proc. Vol. 130. c1989 Materials Research Society
356
DEPOSITION CONDITIONS Thin a-Ge films were rf-magnetron sputtered simultaneously onto paired c-Si and polycarbonate substrates at forward and reflected powers of 100 W and 3 W, respectively. The c-Si substrates were strips broken from
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