D. Cai

Numerical and experimental study of polysilicon deposition on silicon tubes

Abstract Computational and experimental methods were used to investigate the production of bulk polysilicon via a horizontal tubular CVD reactor. Experiments were conducted to study the effect of cooling gas on the system process while keeping the process gas flow rate at zero. A numerical model was also developed to simulate the process. Simulation results were compared to the experimental data to examine the effect of cooling gas on temperature distributions of the silicon tube and the inner quartz tube, as well as the effect of different process gas flow rates on heating power input and silicon tube temperature. Using the numerical simulation method, the investigation has also been conducted to reveal the correlation between the polysilicon production rate and the process gas flow rate, mass fraction of the silane gas in hydrogen and the temperature of the substrate silicon tube. This study has demonstrated the feasibility of high production rate for polysilicon in the new reactor.

Thermal Environment Evolution and Its Impact on Vapor Deposition in Large Diameter AlN Bulk Growth

In an AlN sublimation growth system, to obtain a large and thick single crystal, it is very important to maintain the thermal environment suitable for growth inside the crucible during a long period of time (>100 Hours). In this paper, an integrated model capable of describing inductive, radiative and conductive heat transfer will be used to simulate the transient behavior of thermal environment inside the crucible during a 40-hour experiment growth. The effect of graphite insulation degradation on the temperature distribution inside the crucible will be investigated. Simulation results will be compared with the experiments data to study the effect of the insulation thermal conductivity and geometry change, the degradation induced particle deposition, and the crystal size enlargement on the temperature distribution achieved in the crucible and the growth rate. The relationship of graphite insulation degradation with power input change of the induction heated system will be established. The evolution of temperature difference between the surfaces of source material and crystal, which is the driving force of the growth, will be presented.

Vapor Growth of III Nitrides

Good understanding of transport phenomena in vapor deposition systems is critical to fast and effective crystal growth system design. Transport phenomena are complicated and are related to operating conditions, such as temperature, velocity, pressure, and species concentration, and geometrical conditions, such as reactor geometry and source–substrate distance. Due to the limited in situ experimental monitoring, design and optimization of growth is mainly performed through semi-empirical and trial-and-error methods. Such an approach is only able to achieve improvement in the deposition sequence and cannot fulfill the increasingly stringent specifications required in industry. Numerical simulation has become a powerful alternative, as it is fast and easy to obtain critical information for the design and optimization of the growth system. The key challenge in vapor deposition modeling lies in developing an accurate simulation model of gas-phase and surface reactions, since very limited kinetic information is available in the literature. In this chapter, GaN thin-film growth by iodine vapor-phase epitaxy (IVPE) is used as an example to present important steps for system design and optimization by the numerical modeling approach. The advanced deposition model will be presented for multicomponent fluid flow, homogeneous gas-phase reaction inside the reactor, heterogeneous surface reaction on the substrate surface, heat transfer, and species transport. Thermodynamic and kinetic analysis will be presented for gas-phase and surface reactions, together with a proposal for the reaction mechanism based on experiments. The prediction of deposition rates is presented. Finally, the surface evolution of film growth from vapor is analyzed for the case in which surface diffusion determines crystal grain size and morphology. Key control parameters for film instability are identified for quality improvement.

Transport Phenomena in Growth and Annealing of Laser Crystals

Cloudiness, bubble core defects, anomalous absorption, low-angle grain boundaries, and cracking are the main problems in the growth of high quality and large diameter (>7cm) Yb:S-FAP crystals utilizing the Czochralski method. The generation mechanism of these defects is highly related to transport phenomena in the growth system. In this paper, firstly, inductive, conductive and radiative heat transport phenomena are examined in the entire growth system. Then, an integrated modeling and experimental study is presented to determine an efficient means to eliminate or reduce crystal cracking during cooling. A process model has been developed to simulate the crystal cooling process. The effect of temperature distribution on thermal stress in the crystal during cooling is predicted by a simple but effective computational algorithm. With the relationship between the power change and crystal surrounding temperature change, the process model is further used to optimize power ramp-down profile to avoid cracking of the crystal during cooling-down process.Copyright

Modeling of Multi-Species Transfer During Aluminum Nitride Vapor Growth

AlN has attracted much attention in the past few years as a highly promising material for electronic and opto-electronic device applications. A halide vapor phase epitaxy (HVPE) system has been designed to grow high quality aluminum nitride layers at the growth rate up to 60 μm/h with the deposition temperature of 1000–1100°C and the pressure ranging of 5.5–760 Torr [1]. A 3-D numerical model that is capable of describing multi-component fluid flow, surface chemistry, conjugate heat transfer, and species transport has been developed to help in design and optimization of the epitaxy growth system. The effects of reactor pressure on heat transfer and reactive mixing process are studied. The effects of carrier gas (N2 +H2 ) and reacting gas (AlCl3 +NH3 ) flow rates on species mixing process and deposition uniformity have also been investigated. To achieve a uniform reactive species distribution above the substrate under a high carrier and reacting gases flow rate, a baffle is added in between the adduct boat and the substrate. Different baffle sizes, shapes and locations are tested to examine the optional conditions for the best uniformity.© 2004 ASME

Modeling of a Gallium Nitride Epitaxy Growth System

An iodine vapor phase epitaxy (IVPE) system has been designed and built at North Carolina State University to grow high quality thick gallium nitride layer at the growth rate up to 80 μm/h with the deposition temperature of 1010 °C and the pressure of 200 Torr. In order to optimize the growth process, a numerical model, which is capable of describing multi-component fluid flow, gas/surface chemistry, conjugate heat transfer, radiation heat transfer and multi-species transport, has been developed to help in design and optimization of the IVPE reactor. The gallium source weight reduce rate is converted into flow rate of gallium vapor and has been simulated as an inlet boundary condition of the tubular reactor. By matching predicted and experimental deposition rates, the heterogeneous reaction boundary condition is determined and applied to the substrate. Comprehensive two-dimensional computational simulations have been performed to study the temperature distribution, species mixing process and GaN deposition rate distribution on the substrate under different geometrical configurations and operating conditions; and the operating parameters have been optimized.Copyright

Simulation of Heat Transfer in a Vertical Tubular CVD Reactor

Polysilicon growth has increased due to its broad applications and market demand. The traditional method for polysilicon growth is based on the Siemens process. To improve the throughput, a new system with either large growth surface or other mechanism for high deposition rate is necessary. A novel design, using a vertical tubular CVD reactor has been recently proposed, in which an enlarged surface reaction area exits. This study is to investigate the optimal conditions for growth through numerical simulation of heat and mass transfer in the proposed vertical tubular CVD reactor. A complex computational model is developed that is capable of describing multi-component fluid flow, gas/surface chemistry, conjugate heat transfer, thermal radiation, and species transport. Different from the classical Siemens system, the bulk poly-silicon in a vertical tube growth has a complicated geometry. To accurately predict the various parameters covering broad range of scales, a multi-block grid generation system is used. Numerical computation has been conducted under different operating conditions, and in particular the effect of cooling gas flow direction and flow rate on the temperature distribution of the system and the polysilicon deposition rate has been investigated. Numerical results show that cooling from the top of the system is preferred.Copyright

CFD-Aided Design for Polysilicon Production System

This paper presents the CFD-aided design for polysilicon production system that utilizes an innovative technique of silicon tube-based CVD process. Virtual experiment has been conducted, which involves the development of a complex computational model capable of describing multi-component fluid flow, gas/surface chemistry, conjugate heat transfer, thermal radiation, and species transport. Theoretical analysis has been conducted and a desirable velocity regime for silane and hydrogen mixture has been found. The simulations of the flow field, temperature and species transport have been performed for various reactor geometries, operating conditions (e.g., flow rates of primary silane and secondary hydrogen gases), and heating power design. The deposition rate of polysilicon has been derived analytically as well as computationally. The effects of various conditions on deposition rate have been investigated, and optimal geometry and operating conditions have been obtained for the targeted deposition rate.Copyright