Large area, production MOCVD rotating disk reactor development and characteristics

Microelectronics Journal, 25 (1994) 757-765

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Large area, production MOCVD rotating disk reactor

development and characteristics G.S. Tompa, W.G. Breiland 1, A. Gurary, P.A. Zawadzki, G.H. Evans 2, P. Esherick 1, B. Kroll and R.A. Stall E M C O R E Research Laboratories, E M C O R E Corporation, Somerset, NJ 08873, USA. Tel: 11] 908-271-9090. Fax: 11] 908-271-9686 ISandia National Laboratories, Albuquerque, N M 87185, USA. Tel: 11] 505-844-7029. Fax: 11] 505-844-3211 ZSandia National Laboratories, Livermore, CA 94551, USA Tel: 11] 510-294-2795. Fax: 11] 510-294-1459

The compound semiconductor industry is poised for rapid market advances during the next several years and this is creating a need for development of economical and high yield production equipment. These development efforts increasingly rely on advanced modelling with experimental verification. We report here on the utilization of modeling to rapidly develop a large scale production Rotating Disk Reactor (RDR). The influence of the equipment design and deposition process parameters on flow and temperature uniformity and on process characteristics are analyzed in this paper. Three dimensional Navier-Stokes flow modelling has been used to study the effect of an asymmetric exhaust at the base of the reactor on the uniformity of the rotating disk boundary layer in a scaled Pd-)R. Results show that the asymmetric exhaust port does not disrupt the symmetry of

0026-2692/94/$7-00 ~ 1994 Elsevier Science Ltd

the flow above the rotating disk for typical operating conditions, and for hydrogen flow the convective heat transfer from the disk is quite uniform (2% variation) over most (80%) of the surface. Thermal modelling of the R D R , which includes heat transfer by radiation, convection and conductance, was used to improve the temperature uniformity. Use of the recently developed Rotating Wafer Thermal Mapping (RWTM) technique verified the temperature distribution across the wafer under operational conditions, with measured uniformities of 1.3°C and 2.5°C for 2' and 4" wafers, respectively, ln-situ thermocouples were used to control substrate heating. The substrate temperature uniformity was found to depend strongly upon the process temperature, process pressure, gas composition, gas flow, and wafer carrier rotation speed. This in turn affects the properties of the grown films. This new reactor has been used to produce multiple 4" GaAs/AIAs Bragg

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G.S. Tompa et al./Large area, production MOCVD rotating disk reactor development and characteristics

reflectors with <1.0% variation in peak reflectivity wavelength, 4" GaAs Si doped epilayers with <2.0% doping uniformity, and to simultaneously demonstrate nmltiple 2" InGaP films with better than -t-{).75nm photoluminescence wavelength uniformity.

1. Introduction

dvanced optoelectronic and high speed A compound semiconductor devices require epitaxial deposition processes. The evolution of MOCVD into a more versatile and more economical production technology than MBE has resulted in a drive to further scale this production technology. Rotating Disk Reactor (RDR) technology, as imaged in Fig. 1, is a leading design for large scale, high-throughput single and multi-wafer MOCVD systems. RDR's can also be conveniently configured to be compliant with the Modular Equipment Standards Committee (MESC) guidelines of SEMATECH. Modem MOCVD equipment has to demonstrate a well controlled and uniform reactant flow over the substrate and a uniform substrate temperature. In comparison with other reactor technologies, R D R technology has significant advantages for meeting both of these requirements on a large (and small) scale. RDRs have inherent advantages in hydrodynamic symmetry and flow dynamics that enable growth to be laterally uniform, abruptly switchable, and robust against variations in process parameters [1-8]. The R D R is ideal for producing films with uniform thickness and composition, atomically abrupt interfaces, and sharp dopant and alloy transitions. The capabilities we have had to develop include precise pressure/flow (carrier and reactants) control with high speed pressure balanced gas switching, minimal chemical memory materials of construction, sharp reactant concentration transitions, and uniform temperature in the deposition region. The sharp temperature gradients associated with the flow dynamics of the R_DR provide a uniform thermal history to the reactants and minimize parti-

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Fig. 1. Three dimensional Navier-Stokes flow and heat transfer simulation of hydrogen in a scaled R D R with an asymmetric exhaust and baffle plate, showing streamlines and gas temperature.

culate incorporation through the thermophoresis effect [9]. The vertical reactor geometry has a fundamental ability to compensate for depletion effects by controlling the reactant distribution radially within the carrier gas flow, which other reactor geometries lack. Modelling and experimental studies of the gas flow in the vertical high-speed R D R have been the subject of many investigations during the last decade. The wafer temperature distribution in functioning reactors has been experimentally investigated less frequently than the flow dynamics due to significant problems with experimental temperature measurements. Recent flow modelling advances allow the prediction of process conditions and equipment parameters necessary to provide optimum flows.

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E M C O R E , Fluent and Sandia National Laboratories [4,10]. Evans and Greif compared the effects of finite-dimension boundary conditions with the infinite radius disk problem. Their calculations have been verified by Breiland and Evans, who showed that gas temperatures were in agreement with model predictions when the disk was operated under stable flow conditions.

They also demonstrate regions where instabilities exist, and help to optimize the thermalmechanical characteristics of the system. Functional issues are: reactant injector design and resulting flow distribution, injector temperature control, reactor chamber geometry, optimization of reactor thermal characteristics, susceptor and heater geometry, exhaust size and location, wall temperature, and susceptor rotation speed. All of these must be compatible with the physical process parameters: pressure, total flow, chemistries and deposition temperatures.

Recently, we have used three dimensional Navier-Stokes modelling to determine flow stability in a scaled R_DR, and to investigate the effects of various gas injection schemes and an asymmetric exhaust port and baffle plate on the flow patterns and convective heat transfer. The numerical model uses a control volume approach; conservation equations are solved for species mass and for the momentum and energy of the mixture. The model assumes that the gas flow is laminar and incompressible (low Reynolds and Mach numbers); variable fluid properties are allowed (non-negligible buoyant effects). These assumptions are reasonable for the operating regimes of scaled RDRs. A typical grid for a numerical simulation consists of 70 × 26 × 42 control volumes in the axial, radial, and circumferential directions.

In our present research we first used computational flow and thermal models t o investigate the influence of the process parameters such as temperature, pressure, flow, and rotation speed on the flow dynamics and wafer temperature uniformity. These data allowed us to identify optimum flow conditions, develop a multi-zone heater and rapidly optimize material growth in a large scale R.DR. 2. Experiment 2.1 Fluid dynamical modelling Extensive modelling of R D R s has been performed by several organizations including

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G.S. Tompa et al./Large area, production MOCVD rotating disk reactor development and characteristics

2.2 Thermal modelling and experimental set-up A commercially available, finite element software code was used to perform thermal-mechanical modelling o f the system. For thermal effect measurements, a test set-up, shown schematically in Fig. 2, was used to mimic the production reactor. Top and bottom flanges were water cooled, and the walls o f the reactor were cooled by fans. A three-zone heater, with independent temperature control, was used to optimize wafer temperature uniformity. A scanning pyrometer was used for wafer temperature measurements. We utilized bare Si wafers in all thermal experiments. A number o f temperature distribution measurements were performed on the carrier wafer pocket with and without a wafer. The M O C V D deposition process parameters were simulated in the following ranges: reactor pressure - from 0.05 to 750 Torr, susceptor rotation speed - from 30 to 1200 rotation per minute, hydrogen flow - from 1 to 301/m, wafer temperature - from 600 to 900°C This wide range o f parameters covers most conditions for compound semiconductor deposition. All experimental results reported in this paper were produced on the loadlocked E N T E R PRISE 300 m m diameter wafer carrier M O C V D system. This and generic R D R assemblies have been described in the past [7,8] and will only be briefly reviewed here. The growth reactor, as shown schematically in Figures 1 and 2, is o f stainless steel configuration with molybdenum support members in the hot zones. A fixed position multizone graphite filament is used to radiatively heat the rotating graphite wafer carrier assembly. TMI, TMGa, TMA1, PH3, and AsH3 were used as the precursors and hydrogen as the cartier gas for film deposition. The AlAs and GaAs were grown typically at 700°C, InGaP typically at 690°C and InGaA1P at 730°C. A GaAs buffer layer was deposited at 550°C. The rotation rate was varied from 400-800 rpm, the pressure from 15-45 Torr and the total hydrogen flow from 30-50 slm. A sophisticated real-time

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hierarchical process control system with spreadsheet process entry and reahime interactive process display are used to operate the system.

3. Results The three dimensional fluid dynamic modelling studies have shown that there is a negligible effect o f the asymmetric exhaust on the symmetry o f the flow and heat transfer above the rotating disk (Fig. 1) for typical operating conditions o f the scaled R.DR. For the flow o f hydrogen, the streamlines shown in Fig. 1 reveal that the fluid flows toward the rotating disk uniformly without recirculation. The thermal boundary layer above the rotating disk is uniform over most o f the radius o f the disk (Fig. 1). Uniformity o f the boundary layer is shown quantitatively in Fig. 3 where the convective heat flux varies by only 2% over 80% o f the radius o f the disk. A finite element analysis modelling o f the static heat flow characteristics o f an optimized system is shown in Fig. 4. For boundary conditions we used the heat generation rate, a preset temperature o f 90°C for the water cooled top and base Radial Heat Flux Distribution

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Microelectronics Journal, Vol. 25

reactor plates, and convection for the reactor wall. Such thermal models allowed us to rapidly optimize experimental heater designs. Under a vacuum (without hydrogen flow) a uniform power distribution results in a higher temperature at the outside edge of the wafer [7]. We ascribe this type of temperature distribution t o conductive heat loss through the spindle, while heat losses to the reactor wall (made from stainless steel with low emissivity) take place only by radiation and are less significant. An increase in temperature at the rotation centre and a decrease in temperature at the edge occurs under gas flow; this is attributed to thermal conductance through the gas between the heated susceptor/wafer carrier and the cooled reactor wall. At some pressure level the temperature of points located close to rotation centre exceed the temperature points located far from the centre . The wafer pocket temperature uniformity as a function of the reactor pressure with a uniform power distribution is shown in Fig. 5.1. The thermal conductivity o f the gases depends upon

Temperature Distribution Inside Vertical MOCVD Rotating Disk Reactor

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the gas pressure at low gas density (low pressure range), but is less affected at higher gas pressure [11,12]. Increasing the process temperature requires additional power to compensate for the heat sink to the reactor wall; a single zone heater could not easily provide the necessary additional power to this region. The temperature distribution measured across the wafer pocket also depends significantly on wafer carrier rotation speed and total gas flow, see Fig. 5.2 and Fig. 5.3 respectively. We also observed a significant temperature uniformity dependence upon total gas flow at fixed pressure and rotation speed. The observed thermal and flow dependencies can only be explained on the basis of combined flow and thermal dynamic analysis 171. We chose a three-zone heater where the middle heater heats the bulk of the wafer carrier area, and the inner and outer heaters compensate for heat flow through the shaft and to the cool reactor wall, respectively. Figure 6 shows wafer temperature uniformity for process temperatures o f 650, 730, and 800°C. The three-zone heater produces a temperature uniformity which is +2.5°C for the 4" diameter wafer and -t-1.3°C for 2" diameter wafers over the temperature range. The uniform temperature over such a wide temperature range is unachievable from singlezone heater system. The three-zone heater requires ~1.5 × more power than a single-zone heater for the same average temperature, this being attributed to the additional energy necessary to compensate for significant heat flow through the spindle and to the cool reactor wall. The relative power density (power per surface unit) of the inner, middle, and outer heaters is 2.8/1.0/7.7 respectively. Average temperature deviation for all twelve 2" wafers located on the wafer carrier was in the range of + I ° C . The details of this experimental set-up are reviewed elsewhere [7]. Bragg reflectors have achieved widespread acceptance for visually evaluating overall deposition uniformity to ,-1.2%. As previously repor-

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ted, E M C O R E has demonstrated the growth of multiple 2", 3" and 4" wafers with GaAs/A1As Bragg reflectors showing better than 1% uniformity at several colours [8], as confirmed by SEM measurements. We have previously used TEM studies to demonstrate that R D R systems can also produce monolayer abrupt interfaces [12].

We find that the thickness uniformity can be easily adjusted by controlling the reactant distribution into the reactor. In addition to deposition of GaAs/A1As in our new large system, we have also examined the deposition of InGaP on multiple 2" wafers. With

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a limited number of runs, we have been able to demonstrate uniform, morphologically smooth and highly crystalline films. Figure 7 shows an evenly spaced 9 point photoluminescence (PL) map along a cross pattern (the flat is perpendicular to a susceptor diagonal). The results show a

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wafer exhibiting a PL peak wavelength uniformity of <+0.75 nm as measured using 0.8 m W of 514 nm Argon ion laser excitation. This PL uniformity implies a high degree of compositional uniformity control and of the order/ disorder phenomenon. Figure 8 is a sheet resis-

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Summary

In conclusion, modelling and experimental techniques proved extremely useful to rapidly scale and demonstrate a large area, production

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scale R D R system; a system which efficiently produces uniform films over large areas in both GaAs/AIAs and InGaA1P materials. The process parameters have scaled in accordance with theory and experience. Using computer simulation, we have developed and tested a three-zone heater system. With total flows of ~50 slm and a growth pressure in the ,-15-45 Torr range, we have produced GaAs/AIGaAs films with <+0.9% uniformity with no edge exclusion on 4 × 4" wafers and InGaP films with a high degree of thickness and compositional uniformity. We have demonstrated that the wafer temperature uniformity in a vertical M O C V D rotating disk reactor depends on process parameters such as reactor pressure, deposition temperature, reactant flow, and wafer carrier rotation speed; and that a multi-zone heater is needed to control wafer temperature uniformity over a wide range of deposition parameters. The inner heater compensates for heat flow through the shaft, the middle heater heats the wafer carrier, and the outer heater compensate for heat flow through the gas to the reactor wall. Through our modelling and experimental efforts, we have scaled our 7" diameter disk reactors to a larger geometry, high throughput production system, which can sequentially process 12" diameter wafer carriers that can hold up to 17 x 2", 4 x 4", or 1 x 8" substrates. By forming a generic MESC compatible platform for compound semiconductor production, we have developed the only commercially available tool for these strategic materials compatible with "Si-world" cluster tool production technology. 5. A c k n o w l e d g e m e n t s

Fig. 8. Sheet resistivitymappingof Si doped GaAs on one of four 1(X)mnlwafers, showing <2% uniformity.

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The authors would like to thank P. Broskie and S. Durgett for their assistance in this work. This work was funded in part by EMCOtLE-Sandia National Laboratory C R A D A agreement number SC93-01186 and Air Force Large Area Contract No. F19628-93-B-0087.

Microelectronics Journal, Vol. 25

References 11] G. Evans and R. Greif. Numerical Heat Transfer, 12 (1987) 243-252. 121 W.G. Breiland and G.H. Evans, J. Electrochem. Soc. 138 (1991) 1806-1816. [3] G. Evans and R.. Greif, A numerical model of" the flow and heat transfer in a rotating disk chemical vapour deposition reactor, J. Heat Transfer, 109, (1987) 928-935. [4] I).l. Fotiadis, A.M. Kremer, D.R. McKenna and K.F. Jensen.J. Crys. Growth, 85 (1987) 154. [5] C.R. Biher, C.A. Wang, and S. Motakef. J. Cry. Growth, 123 (1992) 545. [6] H.E. Rebcnne and R.. Arora, FDI Technical Report: Design of a CVD Reactor using F1DAP, FDI, Evanston, IL 1992 [7] G.S. Tompa , M.A. McKee, C. Beckham, P.A. Zawadzki, J.M. Colabclla, P.l). Reinert, K. Capuder,

R.A. Stall and P.E. Norris.J. Crys. Growth, 93 (1988) 22O. [8] G.S. Tompa, P.A. Zawadzki, K. Moy, M. McKee, A. Thompson, A. Gurary, E. Wolak, P. Esherick, W.G. Breiland, G.H. Evans, N. Bulitka, J. Hennessey and C.J.L. Moore, ICMOVPE-VII, J. of Crys. Grou,th, 1994, in press. [9] R..W. Davis, E.F. Moore and M.R. Zachariah,J. of Crys. Gn~wth, 132 (1993)513-522. [10] D.I. Fotiadis, S. Kieda and K.F. Jensen, J. Crys.

Growth, 102 (1990) 441-470. [11] A.I. Gurary, G.S. Tompa, A.G. Thompson, R.A. Stall, P. Zawadzki and N.E. Schumaker, J. Crys. Growth, 1994, in press. [12] G.S. Tompa, P.A. Zawadzki, K. Moy, M. McKee, A. Thompson, R.A. Stall, A. Gurary, N.E. Schumaker, P. Esherick, W.G. Breiland and G.H. Evans, Proc. 1994 U.S. Conf. GaAs Mam!f Tedmol., in press.

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