Continuous Czochralski Growth of Silicon Single Crystals

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9414606 Prasad High yield, high performance microelectronic devices require large diameter silicon (Si) wafers with a high degree of crystallographic perfection, uniform and low axial and radial resistivity gradients, low oxygen impurity and highly uniform electrical and mechanical properties. Many of the inhomogenities in crystals grown by the Czochralski (CZ) method (which is used to manufacture almost all Si crystals for microelectronics applications) can be attributed to the non-steady nature of the growth kinetics due primarily to the continuous change in melt height from start to finish. As the CZ process is scaled up to grow large diameter crystals, the forces which produce flow instabilities and oscillations become much stronger. To overcome many of the shortcomings of the conventional CZ process, a polysilicon pellets-feed continuous Czochralski (CCZ) growth process will be investigated. By reducing the melt height and keeping it fixed, this novel process can suppress many kinds of unsteady kinetics and inhomogenities. A comprehensive program of modeling, simulation, design and experiments will be performed to develop a commercially viable CCZ process. To simulate three- dimensional (3D) transport processes in an irregular domain with free and/or moving boundaries and interfaces, a high resolution computer model based on multizone adaptive grid generation and curvilinear finite volume discretization will be developed. It will then be possible to examine accurately the effects of melt flow recirculation and oscillations, heat transfer from the melt and crystal, crystal/melt interface shape and its dynamics, impurity transport, and pellets melting in a range of parameters suitable for industrial processes. Non-invasive visualization of temperature (using liquid crystals) and flow fields, digital image processing and heat transfer experiments in an apparatus simulating the CZ system to obtain basic information on the physics of the process will be investigated. Computer reconstruction of 3D images from two-dimensional horizontal and vertical pictures will be attempted. The results from numerical computations and laboratory experiments will help in designing the CCZ growth experiments which will be conduced in a commercial puller at an industrial research facility to determine the optimal process conditions. This project builds on prior research demonstrating the feasibility of continuous Czochralski growth of silicon single crystals using small pellets instead of melting a large block of silicon. Anticipated high probability of success is further demonstrated by the industrial partner's commitment to allocate a researcher to this project. Successful completion of this project will break down the current technological barrier limiting the size of silicon single crystal that can be manufactured and lays the foundation for converting the current batch process to truly continuous process with the potential to increase the yield as well as quality. Successful commercialization of this technology will enable the U.S. to maintain its competitive edge in this important electronics market.