Buoyancy driven vortex flow and the associated thermal characteristics in mixed
receive increasing attention because the important role they play in the horizontal MOCVD processes for thin crystal film growth, as already mentioned above. It is well known that the buoyancy driven vortex flow can result in a non-uniform vapor deposition on silicon wafers and is harmful to the thin film properties [22]. At sufficiently high buoyancy-to-inertia ratios [23] the buoyancy driven vortex flow can even become unstable. Moreover, at high buoyancy the forced flow can be reversed resulting in a return flow zone in the channel. The return flow will result in a memory effect in the MOCVD processes. This memory effect is detrimental to the epitaxial layer and should be avoided. In order to eliminate or weaken the vortex and return flows, the conventional method which is widely used in the industry is to reduce the cross sectional area of the channel in the streamwise direction. Reduction of the channel cross section by the sidewall converging toward the channel axis can accelerate the forced flow so that the buoyancy-to-inertia ratio in the flow decreases in the streamwise direction. This was found to be very effective in suppressing the buoyancy induced temporal flow oscillation in MOCVD processes. But the details on how the vortex and return flows are affected by the sidewall converging remain largely unexplored. In a recent model experiment [10] we examined the buoyancy driven three-dimensional return flow pattern in a gas flow over a heated circular disk embedded in the bottom plate of a horizontal rectangular duct, simulating that in a horizontal longitudinal MOCVD reactor.
Here in the present study we move further to investigate how the buoyancy driven return flow is affected by the main flow acceleration due to the sidewall converging.
In the mixed convective flow through a bottom heated flat duct the return flow is known to result from the strong upward thermal buoyancy as the cold entering gas is suddenly heated in the entry heated section of the duct. The cold flow from the upstream is forced to lift up first and moves obliquely upwards. Then, the flow is blocked to move upstream by the strong retarding force of the upward buoyancy, forming a reverse flow zone. At high buoyancy the return flow can penetrate significantly into the upstream unheated section of the duct and becomes highly elongated. When the main flow is at a high Reynolds number, the return flow can extend substantially into the heated section of the duct and is also highly elongated.
The return flow encountered in the horizontal MOCVD reactors was first investigated by Eversteyn et al. [24] by visualizing the flow. They identified a particle-free zone above the susceptor and erroneously interpreted this zone as a stagnant layer of fluid. Instead, it is well known today as the return flow zone. Kamotani et al. [15] experimentally examined the thermal instability of a laminar flow in a horizontal flat duct heated from below and found that at a high buoyancy-to-inertia ratio with Gr/Re2 » 1 a reverse flow zone was induced near the upper plate. Flow visualization conducted by Giling [25] to investigate flow pattern and temperature
profile in horizontal MOCVD reactors proved that there was no stagnant layer above the susceptor. According to the experimental and numerical study of nitrogen and hydrogen gas flows in a bottom heated quartz reactor with a rectangular cross section, Visser et al.
[3] indicated that the returning flow was mainly dominated by two dimensionless parameters, the Grashof and Reynolds numbers, Gr and Re. They proposed a critical level γcrit for the mixed convection parameter Gr/Reκ and showed that no return flow occurred when Gr/Reκ＜γcrit. The exponent κ is equal to 1 at low Reynolds numbers (Re≦4) and goes to 2 at higher Reynolds numbers (Re≧8). A similar study from Fotiadis et al. [26] also indicated that either the heated susceptor was placed in the bottom or top wall a flow recirculation could be induced. Moreover, the recirculation was noted in the upper portion of the reactor.
A two-dimensional numerical simulation carried out by Ouazzani et al. [27] to predict the buoyancy driven flow recirculations in the entrance region of a horizontal MOCVD reactor manifested that the presence of the return flow could result in an increase of film growth rate at the leading edge of the substrate and a decrease in the downstream. Later, they [28] extended the analysis to include the three-dimensional effects and found that at a high Ra/Re ratio, a buoyancy-induced reverse flow existed in the transition region between the isothermal entrance and the reaction section. Einset et al. [29] moved further to quantify the onset of recirculation flows in the entrance region
of horizontal CVD reactors based on the relative magnitudes of the vertical and horizontal pressure gradients in the flow. The pressure effects are independent of whether the heated substrate faces up or down, which explains their experimental observation that the flow recirculations appear at the same position in either configuration. Besides, they noted that the recirculation rolls located near the top wall of the reactor and rotated counter-clockwisely for both top- and bottom-heated reactors.
Ingle and Mountziaris [30] again used a 2-D numerical simulation to investigate the onset of transverse buoyancy-driven circulations in a horizontal flat duct consisting of a cool upstream section, a middle section with a heated bottom wall, and a cool downstream section. Their results showed a transverse recirculation formed in the middle section near the top wall above the leading edge of the hot bottom wall, which rotated in a counter-clockwise direction. The other one formed near the bottom wall above the exit end of the hot bottom plate and rotated in a clockwise direction. At increasing inlet velocity, the downstream transverse recirculation was eliminated first and the upstream one shrank significantly. In addition, they proposed two criteria for the absence of the transverse recirculaton as (Gr/Re2)＜100 for 10-3＜Re≦4, and (Gr/Re2)
＜25 for 4≦Re＜100. A similar 2-D numerical simulation from Ingham et al. [31,32]
predicted that the transverse flow recirculation could extend to the upstream of the wall temperature discontinuity. The onset of the return flow for the heated lower wall was
shown to occur at Gr/Re2
≅
17 for Re=10. This critical value of Gr/Re2 slowly decreases at increasing Re. Besides, the recirculation zone is larger for a lower Re. Makhviladze and Martjushenko [33] conducted 2-D and 3-D numerical simulations to study the return flow in bottom wall heated horizontal CVD reactors. They showed the formation mechanism of the return flow and the return flow could be suppressed by cooling the side walls and/or by reducing the width of the reactors. The characteristics of three-dimensional flow, heat and mass transfer in a horizontal CVD reactor were numerically investigated by Park and Pak [34]. They concluded that for a large Gr/Re2 the return flow appeared at the leading edge of the susceptor, and it caused an increase in the growth rate of the film in this region and a decrease in the downstream of the susceptor. Recently, the three-dimensional return flow structure driven by a heated circular disk embedded in the bottom wall of a rectangular duct was examined by Tuh and Lin [10] through experimental flow visualization.A simple mean often used to suppress and stabilize the buoyancy driven vortex flow is to accelerate the forced flow, as demonstrated by Chen et al. [35]. Gau et al. [36]
revealed that in a convergent channel with the top plate inclination, the acceleration of the forced flow could delay the onset of thermal instability and effectively suppress the temperature fluctuation. Tseng et al. [37] experimentally showed that inclining the top plate of a rectangular duct uniformly heated from below could effectively and
completely wipe out the irregular temporal flow oscillation. But the induced vortex flow can only be weakened to some degree. Besides, more vortex rolls would be induced due to the increase in the aspect ratio of the duct in the mean flow direction.
The above literature review clearly indicates that how the buoyancy induced return flow is affected by the sidewall converging and duct inclination remains largely unexplored in horizontal MOCVD rectors. An experimental flow visualization is conducted here to explore the effects of the sidewall converging and duct inclination on the return flow in a horizontal rectangular duct with a heated circular disk embedded in the duct bottom, simulating that in a horizontal longitudinal reactor.