Scaling the microwave plasma-assisted chemical vapor diamond deposition process to 150–200 mm substrates

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Diamond & Related Materials 17 (2008) 520 – 524 www.elsevier.com/locate/diamond

Scaling the microwave plasma-assisted chemical vapor diamond deposition process to 150–200 mm substrates D. King a , M.K. Yaran a , T. Schuelke a,⁎, T.A. Grotjohn a,b , D.K. Reinhard a,b , J. Asmussen a,b a

Fraunhofer USA Center for Coatings and Laser Applications, East Lansing, Michigan, United States b Michigan State University, Electrical & Computer Eng., East Lansing, Michigan, United States Available online 11 January 2008

Abstract The scale up of two microwave plasma assisted chemical vapor deposition processes from 75 mm to 200 mm substrates is investigated. A thermally floating 2.45 GHz reactor is scaled up by increasing its physical size by a factor of 2.7 and exciting the reactor with 915 MHz microwave energy. Two processes are investigated, 1) the deposition of ultananocrystalline diamond films (UNCD) and 2) the deposition of polycrystalline diamond films (PCD). Gas chemistries of argon/methane/hydrogen were used for UNCD deposition and hydrogen/methane was used for PCD deposition. Experimental pressures range from 40–110 Torr while microwave power input ranged from 1.9–7 kW resulting in steady state substrate temperatures from 630–950 °C. Uniform deposition was demonstrated over 150–200 mm substrates, i.e. thickness variations of 4% over 150 mm and 6% over 200 mm were achieved with deposition rates ranging from 30–460 nm/h. Low temperature deposition at 633 °C was achieved and thereby demonstrated the potential of integrating the process with temperature sensitive materials. A comparison of the power densities between the two reactors indicates that the large reactor operates at five to nine times lower discharge power densities than smaller reactors suggesting improved deposition efficiencies. © 2008 Elsevier B.V. All rights reserved. Keywords: Large area diamond synthesis; Microwave plasma assisted chemical vapor deposition; Ultranano crystalline diamond; Polycrystalline diamond

1. Introduction The successful commercialization of chemical vapor deposition (CVD) diamond synthesis requires the development of reactor technologies that combine economically viable linear growth rates with a uniform deposition of contamination free diamond over large areas. These reactor systems must be robust and capable of operating continuously in a multi-day production process environment. Currently various deposition techniques including hot filament and microwave plasma-assisted CVD are used to deposit diamond. Hot filament systems provide large deposition areas [1]. However, for some applications requiring the highest quality diamond, high rate deposition of diamond, and the deposition of diamond at low temperatures microwave plasmaassisted CVD (MPACVD) has inherent advantages. For these applications MPACVD has an important commercial potential ⁎ Corresponding author. E-mail address: [email protected] (T. Schuelke). 0925-9635/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.12.050

and, if the reactor and associated process scaling can be successfully achieved, MPACVD can make an important contribution to the industrialization of diamond CVD. Potential applications of large area deposition include diamond on silicon for thermal management in integrated circuits [1], surface acoustic wave device substrates [2], coatings for electrochemical electrodes [3], and large area diamond deposition on silicon for microelectro-mechanical systems (MEMS) applications [4]. This paper investigates the scaling of 2.45 GHz reactor technology to a larger size by employing excitation energy at a frequency of 915 MHz. The scaling is achieved by increasing the size of the various components in the 2.45 GHz reactor by a factor of approximately 2.7, i.e., increase the resonant cavity size, the fused silica dome size and substrate size and then exciting the system with 915 MHz energy to generate a discharge of an accordingly increased size. Scaling the substrate diameter by a factor of 2.7 suggests that a reactor design based on 915 MHz microwave frequency will achieve useful deposition areas of 150–250 mm in diameter, and perhaps even 300 mm.

D. King et al. / Diamond & Related Materials 17 (2008) 520–524

In this paper we investigate the scaling of a 2.45 GHz reactor to 915 MHz excitation frequency and compare the performance for two separate process chemistries. The first, which uses argon/ hydrogen/methane gases, is the scaling of ultrananocrystalline diamond (UNCD) synthesis [9,10] and the second, which uses hydrogen/methane gases, is the scaling of polycrystalline diamond (PCD) synthesis. The focus of this study is to scale up the thermally floating configuration and to experimentally evaluate the scaled up UNCD and PCD processes in the 40– 110 Torr pressure range by measuring film uniformity, deposition rate, and film properties. The performance of the 915 MHz and 2.45 GHz reactors are compared. 2. Deposition system A cross-sectional view of the microwave plasma reactor system and vacuum system is displayed in Fig. 1. A 2.45 GHz plasma assisted CVD system with a 12 cm diameter discharge is our benchmark reactor and has been described in detail earlier [5–8]. This system works well for deposition across 50–100 mm diameter in a (1) thermally floating deposition configuration [6– 8] from a few Torr to about 100 Torr, and in a (2) water cooled configuration at pressures of 90 Torr to 200 Torr [5]. In this substrate holder geometry the size, shape, position and gas flows were designed to modify the shape of the typical ball plasma discharge to achieve a very flat and uniform film deposition profile. This reactor creates a hemispherical discharge located above and in close contact with the substrate and yields excellent deposition uniformities [5–8]. For example using this config-

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uration film uniformities over 75 mm-diameter silicon substrates of 75–95% were achieved for UNCD deposition [7] and 95% for PCD [8]. The scaled up system consists of a variable power, 30 kW maximum, 915 MHz microwave supply that is connected via coaxial and rectangular waveguides to a 46 cm inside diameter cylindrical microwave plasma cavity applicator. As shown in Fig. 1 the cavity applicator has a variable short and a variabledepth coaxial coupling probe, i.e. the cavity applicator is tunable. Microwave energy is coupled into the cavity applicator through the coaxial excitation probe and enters the applicator when the sliding short position is adjusted to excite the TM013 electromagnetic mode. With the application of microwave power a hemispherical discharge is created inside the 32.5 cm diameter cylindrical quartz dome that is located in the fixed base plate at the bottom of the applicator. A 24 cm diameter molybdenum substrate holder is placed on a stage located on the holder base plate. Process gases are introduced upward into the discharge region via a gas distribution ring located at the bottom flange of the quartz dome. The gases flow through the discharge and out the reactor via an annular ring of holes located on the outer rim of the substrate holder. The reactor base plate is watercooled and the quartz dome is nitrogen-cooled. The substrate temperature was measured by either a single-color pyrometer pointed at the top of the substrate surface (PCD) or by a twocolor pyrometer that viewed the backside of the center of the substrate (UNCD) through an approximately 8 mm diameter hole in the substrate holder as seen in Fig. 1. The experiments were carried out using a leak free vacuum system and with

Fig. 1. Schematic cross section of the 915 MHz reactor with thermally floating substrate holder.

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argon, hydrogen, and methane source gas purities of 99.999%. Silicon wafer substrates were seeded either by mechanical polishing with 0.1 μm diamond powder or by an ultrasonic seeding technique. Following seeding, an ultrasonic bath using acetone and a deionized water rinse was performed. As shown in Fig. 1 the silicon wafer substrates are placed on the molybdenum holder. At a set operating pressure the stage height and the sliding short position are adjusted to position the substrate surface in direct contact with the microwave discharge. The discharge then assumes an intense, flattened, hemispherical shape hovering over and in direct contact with the substrate. When operating in this configuration the substrate holder and the substrate are not actively cooled or heated and thus all the energy supplied to the substrate is supplied by the microwave discharge. The balance of heat flow from the discharge and the loss of heat due to conduction, radiation and convection determine the steady-state substrate temperature. In the thermally floating configuration the substrate temperature is a function of both the pressure and the absorbed microwave power; i.e. substrate temperature is not an independent experimental variable. 3. Experimental methods The process conditions used for UNCD were 300 sccm argon, 0.6–2% methane and 1–2.6% hydrogen. The pressures were set between 75–110 Torr and the absorbed power range was 1.9–7 kW. In the floating substrate holder configuration the resulting wafer temperatures in steady state conditions were 633–810 °C. These deposition conditions are comparable to earlier UNCD work reported in [7,9,10]. The PCD process conditions, based on [6], were 1200 sccm hydrogen and 1.7–7% methane. The reactor was operated at a pressure range from 40–80 Torr with absorbed powers of 3–8 kW. The steady state substrate temperatures ranged from 700–1000 °C. The higher input power levels were associated with higher-pressure operation and larger deposition areas. The diamond film thickness distributions across the coated wafers were measured by cross sectioning and scanning electron microscopy (SEM). For some UNCD films we utilized an optical technique evaluating the reflected light intensity versus wavelength using the constructive and destructive interference of the light [11]. For the analysis it was necessary to calculate the refractive index from the Sellmeier equation using coefficients for diamond [12]. To verify that other properties of the diamond are uniformly distributed we also measured the Young's modulus for a UNCD coated wafer as well as the surface roughness at five locations across the wafer. The modulus was measured using a surface acoustic wave technique [13] and the roughness was measured using an atomic force microscope. 4. Results and discussion PCD and UNCD films were deposited over 150–200 mm silicon wafers. In Fig. 2 we provide typical SEM micrographs that show in a) and b) the morphology of PCD films in top view

Fig. 2. SEM micrographs of grown diamond films.

and in c) a cross section of a UNCD film. In particular the PCD images display typical film morphology variations as the amount of methane increases as is described in earlier experiments [6,8]. The UNCD films with thicknesses of less than 2 μm are very smooth and as measured by atomic force microscopy display an average roughness of less than 20–30 nm. Deposition rates were calculated by dividing the average film thickness by the

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Table 1 Process parameters and measured average deposition rates and uniformities Process

Wafer size [mm]

Pressure [Torr] ([kPa])

Power absorbed [kW]

Temperature [°C]

Deposition rate [nm/h]

Thickness variation a [%]

UNCD UNCD PCD

150 200 150

110(14.7) 75–110(10–14.7) 40–70(5.3–9.3)

1.9–6.7 3.6–5.8 3–8

696–810 633–785 700–940

80–460 30–180 320–390

8–13 6–21 4–15

a

Thickness variations are calculated as percentage of the standard deviation from the average thickness across the wafers.

deposition time and are summarized in Table 1. The typical growth rates for the UNCD films were 30–460 nm/h and for PCD 320–390 nm/h. In all cases we did not observe bending of the wafer substrate indicating that the residual stresses in the deposited diamond films were minimal. The film uniformity, which is shown in Fig. 3, can vary from poor to very good dependent on the operating conditions, adjustments of the deposition system and the substrate size. General trends for obtaining uniform deposition are that increases in microwave power increase the discharge size and increases in pressure increases the growth rate while decreasing the plasma discharge size. Uniformity is also determined by the discharge shape. Adjustment of the input power, the sliding short height (cavity length) and the substrate holder height are also important for obtaining good uniformity. See for example [14] for a description of how the discharge shape varies as the input power is increased. To achieve optimal uniformity on a given wafer at a specific pressure, a series of experiments is performed such that the power, pressure, sliding short position and stage position are adjusted. Visual examination of the discharge is useful to judge substrate coverage and the measurement of substrate temperature uniformity over the substrate surface during the deposition process is useful for achieving uniformity, but ultimately deposition experiments with subsequent material analysis are necessary to optimize the process. In Fig. 3 the measured thickness distributions for three 150 mm and three 200 mm wafers are displayed and Table 1

summarizes the measured data. Overall, the thickness variation of the films over 150 mm is in the range of 4–15% for both UNCD and PCD films. The UNCD thickness varies in the range from 6–21% over 200 mm. The capability of tuning the 915 MHz reactor to achieve better uniformity can be discussed based on the UNCD plots C), D) and E) in Fig. 3. While plot C) shows a reasonable thickness variation of 13% over 150 mm applying a similar recipe to a 200 mm (plot D) gives a similar distribution over 150 mm in the center of the wafer, but the thickness decreases steeply at the edges yielding a higher thickness variation of 21%. However, the process conditions can be changed to achieve for example plot E) and plot F). Here the thickness variation across the 200 mm wafers can be reduced to 13% and 6% respectively. Films C) and D) have a dip in the thickness distribution in the substrate's center. This is caused by applying too much power (see [14] Fig. 4). If desired this dip in film thickness can be reduced by adjustment in power and substrate position and can further be smoothed by pulsing the discharge [14]. The UNCD experiments for plot E) and plot F) in Fig. 3 also demonstrate a uniform deposition over 200 mm at lower substrate temperatures. Here the steady state temperatures remained below 635 °C, which makes the process suitable for coating temperature sensitive substrates such as already processed microelectronics wafers or glass substrates. Thus the combination of larger area and low temperature deposition enables the integration of diamond material properties with applications such

Fig. 3. Thickness distributions of UNCD and PCD coated 150 mm and 200 mm wafers.

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as the fabrication of micro-electro mechanical systems (MEMS) or high power electronics. We also measured the Young's modulus and average surface roughness of the material across this 200 mm wafer to be E = (823 ± 50) GPa and Ra = (28 ± 2) nm. Both material property distributions demonstrate the very good uniformity of the deposition process. The scaling of the reactor allows a comparison of performance between the 2.45 GHz and the 915 GHz reactors. Of interest is the comparison of the discharge power density, the substrate temperature and deposition rate versus pressure. First we consider the UNCD process. In [7] the deposition was applied to a 7.5 cm diameter substrate at 2.45 GHz and as discussed here it is applied to a 20 cm diameter substrate at 915 MHz. In both cases the Ar/H2/CH4 discharge almost fills the entire volume between the substrate and the quartz dome for all operating pressures. Thus the 2.45 GHz discharge has an approximate volume of 450 cm3 and the 915 MHz discharge has a volume of 6000 cm3. The 2.45 GHz experimentally measured input power varies from approximately 1.2–2.4 kW (see road map in Fig. 2 of [7]) over the 60–120 Torr deposition pressure regime yielding a discharge power density of 2.7 W/ cm3 at the low operating pressures to 5.3 W/cm3 at the high pressures. The input power for the 915 MHz reactor varies from 2–7 kW over a 50–120 Torr pressure range yielding a discharge power density of 0.3 W/cm 3 at the low pressure to 1.1 W/cm 3 at the high pressures. Under similar experimental conditions the substrate temperatures for the 2.45 GHz reactor were observed to be approximately the same as observed for the 915 MHz reactor while the deposition rates in the 915 MHz reactor were 10% higher. A comparison of the PCD deposition process in the two reactors yields similar results. In this case the H2/CH4 discharge does not fill the entire volume of the quartz dome but takes on a hemispherical shape that covers an area slightly larger than the 7.5 cm and the 20 cm diameter substrates. Based on experimental observations we calculate a discharge volume of 160 cm3 for the 2.45 GHz reactor and 2200 cm3 for the 915 MHz reactor. Measured absorbed powers over a pressure range from 40–80 Torr are 2–2.7 kW for the 2.45 GHz reactor [8] and 3–8 kW for the 915 MHz reactor. The respective discharge power densities are 12.5–17 W/cm3 and 1.4–3.6 W/ cm3. Under comparable experimental conditions the substrate temperatures for the 2.45 GHz reactor are slightly (10%) higher while deposition rates are approximately the same. The experimentally observed discharge power density in the 915 MHz reactor is lower by approximately a factor of nine at low pressures (40–100 Torr) and approximately a factor of five at higher pressures (N 100 Torr) for both the UNCD and the PCD processes when compared to a 2.45 GHz reactor. Operating the 2.45 GHz and 915 MHz reactors in the 40–100 Torr pressure regime leads to similar floating substrate temperatures and thus gas temperatures, which then results in comparable deposition rates. This is consistent with the modeling work reported in [15].

5. Summary The experimental data displayed in Fig. 3 and summarized in Table 1 demonstrate the ability to scale up diamond deposition processes that have been established at 2.45 GHz to seven times larger deposition areas by reducing the excitation frequency to 915 MHz. The potential of a 915 MHz microwave plasma reactor to synthesize uniform, large area diamond films up to 200 mm in diameter has been demonstrated. This reactor system is capable of depositing both UNCD and polycrystalline diamond with rates and qualities that match or exceed the results obtained in a smaller 2.45 GHz deposition system. Considering that the same temperature conditions can be achieved in the larger reactor over larger substrate areas at lower discharge power densities means that the scaled up system is actually more efficient both in terms of power per unit plasma volume and power per unit deposition area. Additional experimental measurements that are preformed under precisely the same experimental conditions, i.e. gas chemistries and gas flow rates, etc., are required to determine any important differences in deposition rates between the reactors. In addition to the larger deposition areas we demonstrated uniform low temperature (b 650 °C) deposition of diamond materials. This is an important factor for the commercialization potential of this technology since it enables the integration of diamond materials for electronics, MEMS, and optical applications. References [1] J. Zimmer, K.V. Ravi, Diamond and Relat. Mater. 15 (2006) 229. [2] B. Bi, W.S. Huang, J. Asmussen, B. Golding, Diamond and Relat. Mater. 11 (2002) 667. [3] M. Hupert, A. Muck, R. Wang, J. Stotter, Z. Cvackova, S. Haymond, Y. Show, G.M. Swain, Diamond and Relat. Mater. 12 (2003) 1940. [4] O. Auciello, J. Birrell, J.A. Carlisle, J.E. Gerbi, X. Xiao, B. Peng, H.D. Espinosa, J. Phys., Condens. Matter 16 (2004) R539. [5] K.P. Kuo, J. Asmussen, Diamond and Relat. Mater. 6 (1997) 1097. [6] T.A. Grotjohn, J. Asmussen, in: J. Asmussen, D.K. Reinhard (Eds.), Diamond Films Handbook, Chapter 7, Marcel Dekker, New York, 2002, p. 211. [7] D.T. Tran, W.-S. Huang, J. Asmussen, T.A. Grotjohn, D.K. Reinhard, New Diam. Front. Carbon Technol. 16 (2006) 281. [8] J. Zhang PhD Dissertation, Michigan State University,1993; K.P. Kuo PhD Dissertation, Michigan State University, 1997. [9] O. Auciello, et al., J. Phys., Condens. Matter. 16 (2004) R539. [10] D. Zhou, D.M. Gruen, L.C. Qin, T.G. McCauley, A.R. Krauss, J. Appl. Phys. 84 (1998) 1981. [11] E. Hecht, Optics, 2nd Ed, Addison Wesley, 1990. [12] M. Thomas, W.J. Tropf, Johns Hopkins APL Tech. Dig. 14 (1993) 16. [13] D. Schneider, B. Schultrich, P. Burck, H.J. Scheibe, G. Joergensen, M. Lahres, J. Karner, Diamond and Relat. Mater. 7 (1998) 589. [14] A.M. Gorbachev, V.A. Koldanov, A.L. Vikharev, Diamond and Relat. Mater. 10 (2001) 342. [15] T. Grotjohn, R. Liske, K. Hassouni, J. Asmussen, Diamond and Relat. Mater. 14 (2005) 288.