Simulation of Microstructure and Surface Profiles of Thin Films for VLSI Metallization
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MRS BULLETIN/NOVEMBER 1995
In addition to these concerns with the film surface profile, another factor is becoming increasingly significant. Both sputtering and CVD produce thin films with characteristic microstructures. This microstructure consists of columns or grains separated by grain boundaries and voids. A number of film properties are determined by this microstructure, including resistivity, etching characteristics, diffusion properties, surface roughness, and electromigration resistance. The microstructure of the film is, in part, determined by the orientation of the film to the depositing flux and is therefore a function of the local topography. Traditionally, the development and optimization of interconnect processes have been performed by expensive and time-consuming experimentation. However, increasingly stringent requirements on VLSI metallization reliability, smaller feature sizes, and the use of complicated structures have all made necessary the development of physically based simulation packages. These packages are needed to predict surface profile evolution and film microstructure for metal films deposited over VLSI topography by sputtering and CVD. A number of simulators that predict only the film profile have been developed, including simulation and modeling of profiles for lithography and etching (SAMPLE), standard profile emulator for etching and deposition for integrated circuit engineering (SPEEDIE), and EVOLVE.2'3'4 However, the state of film microstructural model-
ing is less mature. Monte Carlo atomic deposition models5 and molecular dynamics programs'1'7 have been developed to study the detailed atomic structure of thin films. These programs are not wellsuited to studies of VLSI metallization structures due the atomic scale lengths used. Even a 0.1-^tm via would contain of the order of 1010 atoms, which is beyond current computing capabilities. Currently, the most widely reported thin-film growth simulator that produces microstructural information is simulation by ballistic deposition (SIMBAD).9-11 This article will discuss the models and techniques needed to simulate both the profile evolution and microstructure formation of sputtered thin films. The SIMBAD simulation package and the recently reported grain oriented film microstructure simulator (GROFILMS) model12 will be used to illustrate the importance of these effects and to evaluate the degree of success in predicting experimental results. Although this article is primarily concerned with the simulation of microstructure in sputtered films, the microstructure of CVD films is due to the same fundamental phenomena and can be simulated by SIMBAD and GROFILMS. To correctly predict the evolution of the surface profile of a sputtered thin film over topography, two basic effects need to be considered. A primary effect is the shadowing of the incident flux. If the film is deposited over topography (such as a via or trench), certain regions of the film will receive less flux than others, resulting in a nonuniform film thickness (see Figure
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