Reactor susceptor temperature

Figure 3. Relationship of susceptor temperature to spin rate that is required to operate a particular reactor geometry in the one-dimensional regime.

A typical computation such as the ones described here used about 100 adaptively placed mesh points and required about 5 minutes on a Cray 1-S. Of course, larger reaction mechanisms take more time. Also, simpler transport models can be used to reduce computation time. Since the solution methods are iterative, the computer time for a certain simulation can be reduced by starting it from the solution of a related problem. For example, it may be efficient to determine the solution to a problem with a susceptor temperature of 900 K starting from a converged solution for a reactor with a susceptor temperature of 1000 K. In fact, it is typical to compute families of solutions by this type of continuation procedure. 

The vertical reactor simulations reported In this paper typically Involved 14,000 unknowns and took 25 CPU seconds per Newton Iteration on a Cray-2. The tracing of a complete family of solutions for one parameter (e.g. susceptor temperature) cost approximately 25 CPU minutes. The latter number underscores the advantage of using supercomputers to understand the structure of the solution space for physical problems which often Involve many parameters. 

Figure 7. Reduced Nusselt number for mass transfer to the substrate In a vertical reactor for varying Inlet flow rate and susceptor temperature.

The baseline reactor conditions in the following reactor analysis are susceptor temperature Ts = 1273 K, inlet temperature Tm = 333 K, reactor pressure p = 400 mTorr, gas velocity through the inlet manifold Vin = 100 cm/s, and the gap between inlet and susceptor L = 1 cm. Incoming gas-mixture mole fractions (e.g., from a gas-cylinder) are TEOS 0.25 and N2 (carrier gas) 0.75. You may use the files teos. gas and teos. surf for the gas-phase and surface reaction mechanisms. (Hint You may need the following initial guesses at the surface species site fractions SiG3(OH) 0.98, SiGsE 0.02, SiG(OH)2E 0.001. More details on the surface reaction mechanism and nomenclature are found in Ref. [69].)... 

Reactor operating parameters and conditions were pressure 140 Torr, susceptor temperature 1050°C, inlet manifold temperature 27°C, distance between susceptor and inlet 3.9 inches. The carrier gas was composed of H2 (mole fraction 0.846) and NH3 (mole fraction 0.154). The reactant gas trimethylgallium was also included in the flow, but can be neglected in the calculation below. 

Figure 11. Mass-transfer Nusselt number (Nu) for various susceptor temperatures and inlet flow rates for a vertical CVD reactor with a 900 K susceptor and 300 K reactor walls and operating at 10.1 kPa (Reproduced with permission from reference 24. Copyright 1987 Elsevier).

Heat transfer is an extremely important factor in CVD reactor operation, particularly for LPCVD reactors. These reactors are operated in a regime in which the deposition is primarily controlled by surface reaction processes. Because of the exponential dependence of reaction rates on temperature, even a few degrees of variation in surface temperature can produce unacceptable variations in deposition rates. On the other hand, with atmospheric CVD processes, which are often limited by mass transfer, small susceptor temperature variations have little effect on the growth rate because of the slow variation of the diffusion with temperature. Heat transfer is also a factor in controlling the gas-phase temperature to avoid homogeneous nucleation through premature reactions. At the high temperatures (700-1400 K) of most... 

The rotating-disk CVD reactor (Fig. 1) can be used to deposit thin films in the fabrication of microelectronic components. The susceptor on which the deposition occurs is heated (typically around lOOOK) and rotated (speeds around 1000 rpm). A boundary layer is formed as the gas is drawn in a swirling motion across the spinning, heated susceptor. In spite of its three-dimensional nature, a peculiar property of this flow is that, in the absence of buoyant forces and geometrical constraints, the species and temperature gradients normal to the disk are the same everywhere on the disk. Consequently, the deposition is highly uniform - an especially desirable property when the deposition is on a microelectronic substrate. 

Features common to all CVD reactors include source evaporators with an associated gas handling system to control input gases and gas-phase precursor concentrations, a reactor cell with a susceptor heated by either radio frequency or infrared radiation, and an exhaust system to remove waste products (which may include a vacuum pump for low-pressure operations). Substrate temperatures can vary from less than 200 °C to temperatures in excess of 1000 °C, depending on the nature of the material layer and precursor used. Schematic diagrams of some simple CVD reactors are shown in Figure 4. 

A silane-based CVD reactor suitable for performing high-temperatnre anneals in an Si- rich ambient was used for these experiments [86]. The samples were placed on a SiC-coated graphite susceptor and an RF induction coil used to heat the susceptor to temperatures on the order of 1,600-1,800°C. Silane and argon were the two process gases used, where Ar not only serves as a dilutant gas but also as a carrier gas to transport silane molecules to the crystal surface. All the implant annealing experiments were performed at atmospheric pressure. 

The final implant annealing process schedule developed during this research is shown in Figure 4.19. A 6-slm UHP Ar flow is first established in the reactor. When the RF generator is turned on, the susceptor is heated to the annealing temperature (typically 1,600°C) using a controlled thermal ramp. To avoid the formation of Si droplets, silane is not introduced into the reactor until a substrate temperature of 1,490°C is reached. At that time the premixed silane in Ar gas is introduced into the Ar carrier flow at a flow rate of 20 seem. All flows are controlled using calibrated... 

To demonstrate the main features of the flow in horizontal CVD reactors, the deposition of silicon from silane is used as an example (87). The conditions are as follows an 8-cm-wide reactor with either adiabatic side walls or side walls cooled to the top wall temperature of 300 K, a 1323 K hot susceptor (bottom wall), a total pressure of 101 kPa, and an initial partial pressure of silane in H2 of 101 Pa. The growth rate of silicon is strongly influenced by mass transfer under these conditions. Figure 8 shows fluid-particle trajectories and spatially varied growth rates for three characteristic cases. 

In the first case (Figure 8a), the side walls are adiabatic, and the reactor height (2 cm) is low enough to make natural convection unimportant. The fluid-particle trajectories are not perturbed, except for the gas expansion at the beginning of the reactor that is caused by the thermal expansion of the cold gas upon approaching the hot susceptor. On the basis of the mean temperature, the effective Rayleigh number, Rat, is 596, which is less than the Rayleigh number of 1844 necessary for the existence of a two-dimensional, stable, steady-state solution with flow in the transverse direction that was computed for equivalent Boussinesq conditions (188). 

The treatment can be modified to include effects of the temperature development and tilting of the susceptor by using the temperature dependence of the diffiision coefficient and adjusting d and (191). In this manner, the experimental data can be correlated, but the model has limited capability for predicting behavior beyond the particular set of experiments used to fit the model. In fact, because of the low values of the Reynolds number (<50) in typical horizontal CVD reactors, film theory and simple... 

The research at MIT has been done in the cold-wall vertical tube reactor shown in Figure 14. The wafer is aligned almost parallel to the flow on a vertical silicon carbide-coated susceptor. The wafer is heated by optical radiation from high-intensity lamps to a temperature of 775°C. Silane was introduced... 

Figure 1 illustrates conventional CVD reactors. These reactors may be classified according to the wall temperature and the deposition pressure. The horizontal, pancake, and barrel reactors are usually cold-wall reactors where the wall temperatures are considerably cooler than the deposition surfaces. This is accomplished by heating the susceptor by external rf induction coils or quartz radiant heaters. The horizontal multiple-wafer-in-tube (or boat) reactor is a hot-wall reactor in which the wall temperature is the same as that of the deposition surface. Therefore, in this type of reactor, the deposition also occurs on the reactor walls which presents a potential problem since flakes from the wall deposit cause defects in the films grown on the wafers. This is avoided in the cold-wall reactors, but the large temperature gradients in those reactors may induce convection cells with associated problems in maintaining uniform film thickness and composition. 

Heat is lost from the surface by conduction through the susceptor and mount, by forced convection of gas over the substrate, and by radiation to the reactor walls, provided the temperature of the substrate is sufficiently high. Endothermic chemical reactions also result in heat loss from the film. The substrate temperature is monitored with a thermocouple or an optical pyrometer and controlled using a traditional proportional-integral-derivative (PID) controller and power source. 

The photograph in Figure 4-14 shows a Nippon Sanso horizontal reactor in operation at temperature. They use an inverted geometry with full rotation of the susceptor and offer units which handle up to three 100 mm or one 200 mm wafer(s) and offer cassette-to-cassette wafer susceptor transfer. Several other vendors, e.g., Thomas Swan or Aixtron, market horizontal tube systems. CVTis also a well-known horizontal tube system manufacturer and one of the few vendors specializing in HgCdTe systems. 

Killeen (1992) used a horizontal-flow metalorganic CVD reactor with trimethylgallium (TMGa) and H2 as the feed gases. When the graphite susceptor was left at room temperature, only absorption from TMGa was observed. When... 

The epitaxial growth was performed using low-pressure or atmospheric pressure MOCVD. The former consists of lamp-heated and the latter consists of rf-heated horizontal reactor with load lock chamber. The substrate was put on the SiC-coated carbon susceptor and the temperature was controlled by the thermocouple inserted into the susceptor. Source gases for Al, Ga, Zn, As and Se are trimethylallu-minum (TMA), trimethylgallium (TMG),... 

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