Elastic Properties of Silicate Glass and Spin-On Glass Thin Films
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ABSTRACT The elastic properties of dielectric films used in microelectronics, such as undoped, phosphorous doped and boron-phosphorous doped silicate glass, as well as spin-on glass, have been studied by means of the Brillouin light scattering technique. The phase velocity of both the surface Rayleigh wave and of the longitudinal wave in the film material have been measured and the two independent elastic constants c,, and Ca4 evaluated. This permitted us to derive the values of the Young's modulus and of the Poisson's ratio which are useful quantities for the modelling of the elastic properties of multilevel structures used in electronics. Moreover, the biaxial stress in the films has been measured by the substrate curvature method. This enabled us to study the evolution of the intrinsic stress with time, caused by water adsorption into the films. The evolution of the stress during thermal cycles has been also analyzed and the thermal expansion coefficient of the films estimated.
INTRODUCTION Dielectric materials such as undoped, phosphorous doped and boron-phosphorous doped silicate glass (USG, PSG and BPSG, respectively) as well as spin-on glass (SOG) are widely 2 used as protective, insulating and planarization layers in the technology of integrated circuits.'1 These films are usually deposited at high temperature in a multilayer stack on a thick substrate, so that large thermo-mechanical stresses can occur, affecting the reliability of the devices. Therefore, knowledge of the elastic properties and of the stress status of these films and of their dependence on the growth conditions is of current interest in view of an optimization of the electronic devices. Although a number of previous investigations have been devoted to measurement of the stress, the hardness and the macroscopic Young's modulus of silicate glass and SOG layers, 3- 5 no detailed study of their elastic stiffness constants has been performed to date. The amorphous dielectric films we are concerned with are elastically isotropic media, so that there are only two independent elastic constants, namely cll and c 44. We report here on the experimental determination of these two elastic constants in USG, PSG, BPSG and SOG films supported by silicon substrates, by means of the Brillouin light scattering technique. This technique, which relies upon detection of the small amount of light inelastically scattered by thermal phonons naturally present in the structure under investigation, has recently affirmed as a powerful non6 destructive tool for determining the elastic properties of thin films and layered structures. We have also determined the biaxial stress in the films and studied its evolution during thermal cycles, by use of the substrate curvature method, achieving an evaluation of the thermal expansion coefficient. 215
Mat. Res. Soc. Symp. Proc. Vol. 356 o 1995 Materials Research Society
EXPERIMENTAL The silicate glass films here analyzed have been deposited on 525 ýtm thick (100)-Si wafers, by either plasma enhanced chemical vapor deposition
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