Investigation of diamond deposition uniformity and quality for freestanding film and substrate applications

Transcription

1 Available online at Diamond & Related Materials 17 (2008) Investigation of diamond deposition uniformity and quality for freestanding film and substrate applications S.S. Zuo a, M.K. Yaran b, T.A. Grotjohn a,b, D.K. Reinhard a,, J. Asmussen a,b a Michigan State University, 2120 Engineering Building, Electrical & Computer Engineering Department, East Lansing, MI 48824, United States b Fraunhofer USA, Center for Coatings and Laser Applications, B100 Engineering Research Complex, Michigan State University, East Lansing, Michigan , United States Received 24 April 2007; received in revised form 20 December 2007; accepted 31 December 2007 Available online 18 January 2008 Abstract In this paper, we report on microwave CVD deposition of high quality polycrystalline diamond and on related post-processing steps to produce smooth, flat and uniformly thick films or diamond substrates. The deposition reactor is a 2.45 GHz microwave cavity applicator with the plasma confined inside a 12 cm diameter fused silica bell jar. The deposition substrates utilized are up to 75 mm diameter silicon wafers. The substrate holder is actively cooled with a water-cooled substrate holder to achieve a substrate surface temperature of C. The pressure utilized is Torr and the microwave incident power is kw. Important parameters for the deposition of thick films with uniform quality and thickness include substrate temperature uniformity as well as plasma discharge size and shape. As deposited thickness uniformities of ±5% across 75 mm diameters are achieved with simultaneous growth rates of 1.9 μm/h. The addition of argon to the deposition gases improves film deposition uniformity without decreasing growth rate or film quality, over the range of parameters investigated. Post-processing includes laser cutting of the diamond to a desired shape, etching, lapping and polishing steps Elsevier B.V. All rights reserved. Keywords: Diamond film; Plasma CVD; Optical properties; Windows; Polishing 1. Introduction Corresponding author. address: reinhard@egr.msu.edu (D.K. Reinhard). High quality freestanding films and substrates of polycrystalline diamond have many applications including electromagnetic-wave windows, thermal heat sinks and stripping foils for high energy ion beams [1 4]. Fabrication of such layers require critical deposition and post-processing steps. Many variables can affect the CVD diamond quality, growth rate, and deposition uniformity. For microwave plasma-assisted CVD diamond deposition, these variables include the reactor pressure, substrate temperature, composition of the inlet gases, absorbed microwave power, nucleation method and substrate holder design. A key approach to increase growth rate while maintaining diamond quality is to increase reactor pressure. However, this presents challenges to achieving uniformity across substrates since plasma size tends to decrease with increasing pressure. In this paper we report on the synthesis of high quality diamond and the fabrication of uniformly thick, transparent diamond windows. Substrate temperature profiles are reported as a function of pressure and microwave power and correlated with film uniformity. The addition of argon gas to the conventional H 2 /CH 4 growth chemistry is shown to improve the uniformity of diamond deposition across up to 75 mm diameter substrates in a 2.45 GHz microwave plasma system. Lapping and polishing steps that reduce surface roughness to the nm range are also described. 2. Deposition method A general description of the microwave plasma-assisted CVD reactor used in this investigation has been previously reported [5]. A cross-sectional view is shown in Fig. 1. It is a /$ - see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.diamond

2 S.S. Zuo et al. / Diamond & Related Materials 17 (2008) Fig. 1. A cross-sectional view shows the four reactor variables that are adjusted to achieve temperature uniformity including the sliding short position, the antenna length into the cavity, the height of the substrate holder and the design of the substrate holder. LS, LP, and substrate height positions are adjusted to obtain a hemispherical shaped discharge that extends across the substrate with good contact between the plasma and substrate. resonant cavity operated in the TM 013 resonant mode that utilizes internal tuning to both shape the plasma over the substrate and to reduce reflected power. The internal microwave circuit tuning structure utilizes a sliding cavity top (sliding microwave short) and an adjustable microwave antenna length. Tuning is accomplished by varying the short height and the antenna length. In the version used in the present work, microwave power from a 6 kw microwave power supply is transmitted through a rectangular waveguide and delivered into the brass-wall microwave cavity plasma reactor by a coaxial structure. A dome shaped fused silica bell jar seals the processing region within the microwave cavity. The substrate and substrate holders are on a water-cooled stage near the bottom of the cavity and the height of the substrate is carefully adjusted relative to the bottom of the cavity. The baseplate to which the cavity is attached and the top sliding structure above the fused silica bell jar are also cooled by water. Flowing air cools both the bell jar external surface and the inside of the brass cavity. Uniform growth of diamond film requires that good temperature uniformity be maintained across the deposition substrate. Four reactor variables are adjusted to achieve temperature uniformity including the sliding short position, the antenna length into the cavity, the height of the substrate holder and the design of the substrate holder. The adjustment of the sliding microwave short and the antenna length determine not just the resonant microwave mode and reflected microwave power as stated above, but they also affect the shape and position of the plasma discharge. In operation the sliding short, antenna position and substrate height position are adjusted to obtain a hemispherical shaped discharge that extends across the substrate with good contact between the plasma and substrate. The plasma is the source of heating for the substrate and the substrate temperature is determined by this heating and simultaneous cooling that occurs due to the water-cooled substrate holder that the substrate holder structure sits on. The electric field is ϕ symmetric for the utilized cavity mode resulting is a ϕ symmetric plasma discharge. The tuning process via the short position, antenna length and substrate height are adjusted so that the plasma discharge is as uniform radially as possible, but radial variations in plasma heat load to the substrate can not be totally tuned away. This results in radial non-uniform heating of a substrate by the plasma. To compensate for this a fourth reactor variable

3 302 S.S. Zuo et al. / Diamond & Related Materials 17 (2008) is addressed. Specifically, the substrate holder is designed to compensate for this radial gradient by removing more heat at the center of the substrate as compared to the edge. Silicon wafers with nominal diameters of 50 mm and 75 mm and thickness of 1 mm are used as the deposition substrates for this study. The silicon wafers are nucleation seeded by mechanically polishing using natural diamond powder of size 0.25 μm and then cleaned until they appear clean to the human eye. The reactor pressure utilized for this report was in the range Torr and the utilized output of the microwave power was in the range kw. Reflected power is on the order of 10 20% of the incident power, so the absorbed microwave power ranges from approximately kw. The hydrogen flow rate was fixed at 400 sccm and the methane flow was varied in the range of 4 8 sccm. The argon flow rate was either 200 sccm or zero. The substrate temperature was monitored by a one-color pyrometer with the emissivity set to 0.6. This emissivity value was based on the measured emissivity of the silicon and diamond substrate combination as well as light transmission loss of the fused silica bell jar [6]. 3. Reactor operation Fig. 2 shows substrate temperature as a function of pressure and microwave power in a plasma with gas flows of 400 sccm hydrogen and 4 sccm methane. In addition, the dotted-line parallelogram provides useful information concerning the approximate operation range of the reactor for coverage of a 75 mm diameter deposition substrate. For a given pressure, the minimum operational power, i.e. the left hand side of the parallelogram, is determined by the power required to generate a plasma of sufficient size to cover the substrate. The maximum power limitation, i.e. the right hand side of the parallelogram, is determined by the power at which the plasma size becomes large enough to approach the walls of the bell jar. Thus, at a pressure of 100 Torr for example, the minimum power (to cover the substrate) is 1.75 kw and the maximum power (to avoid bell jar contact) is 2.75 kw. One also observes that at 100 Torr the center temperature increases with power from 705 C at the minimum power to 845 C at the maximum power. The vertical bars associated with the data points in Fig. 2 represent the minimum/maximum temperature variation across a 75 mm Fig. 2. Substrate center temperature vs. pressure and absorbed microwave power for the deposition plasma without argon. The vertical bars represent the minimum/maximum variation of temperature across a 75 mm diameter wafer. Fig. 3. Substrate center temperature vs. pressure and absorbed microwave power for the deposition plasma with argon. The vertical bars represent the minimum/ maximum variation of temperature across a 75 mm diameter wafer. The same substrate holder was used for Figs. 2 and 3. diameter silicon substrate for a fixed substrate holder configuration. As pressure increases, the center temperature increases and the temperature variation across the substrate also increases. The operation map of Fig. 2 shows that although substrate temperature is influenced by pressure and power, it is most strongly influenced by pressure. It also shows that at a given microwave power the plasma size becomes smaller as pressure is increased. Lower pressures are desirable to provide uniform coverage of large substrates because the plasma size is larger for lower pressures. Conversely, higher pressures are desirable to achieve the substrate temperatures required for high deposition rate and also to increase deposition rate related constituents in the plasma. These two facts present opposing criteria in terms of selecting the plasma pressure. One may expect to solve the dilemma by applying sufficient microwave power to increase both plasma size and substrate temperature. However, such an approach had an undesirable effect on film uniformity in this study since it was observed that the temperature variation across the substrate increased with the combination of high power and high pressure. Therefore, the addition of a non-molecular gas to increase the plasma size was investigated. Fig. 3 shows substrate temperature as a function of pressure and microwave power with 200 sccm argon flow added to the 400 sccm hydrogen and 4 sccm methane flow. Now, at a pressure of 100 Torr, the minimum power (to cover the substrate) is 1.65 kw and the maximum power (to avoid bell jar contact) is 2.50 kw. At 100 Torr, the center temperature increases with power from 740 C to 882 C. The same substrate holder configuration was used for Figs. 2 and 3. The effect of argon is to increase the plasma size at a given pressure and power and also to increase the substrate temperature for a given pressure and power. Thus, less power per unit plasma volume, i.e. less plasma power-density, is needed with argon to achieve a given substrate temperature. One may postulate that less energy is lost to the cavity walls with the addition of argon, as the thermal conductivity of argon is substantially less than the thermal conductivity of hydrogen. Moreover, the temperature across the substrate is more uniform for a given plasma size and substrate temperature as documented in Table 1. For a given temperature at the substrate center, the addition of

4 S.S. Zuo et al. / Diamond & Related Materials 17 (2008) Table 1 Conditions for substrate temperatures of approximately 860 C and 930 C for a midsize plasma (see Figs. 2 and 3) for with and without argon flow T CENTER ( C) Argon flow Microwave power (kw) Pressure (Torr) Temperature variation ( C) With the addition of argon, a given substrate temperature can be realized with lower power and pressure than without argon. Also, for a given substrate temperature, the variation in temperature (maximum minus minimum) across a 75 mm diameter substrate is reduced by the addition of argon. The same substrate holder was used for all samples represented in this table. argon reduces the temperature variation across the substrate by approximately 50%. 4. Deposition results Coverage of 75 mm diameter substrates was achieved with and without the addition of argon. However, as may be anticipated from the temperature uniformity results of the previous section, improved deposition uniformity was observed with the presence of argon in the gas input. Three methods were used to determine the thickness of the diamond films. An average thickness was calculated based on the weight gain and the substrate area using a mass density for diamond of g/cm 3. Secondly, a thickness distribution across the substrate was obtained by using a scanning tip connected to a linear encoder with a vertical precision of 50 nm. The results from the linear encoder method are typically a few percent higher than the average thickness calculation because the tip diameter is larger than the diamond crystallites and thus rests on the tips of the crystals. However, both thickness measurements result in values that are consistent with SEM cross-sectional measurements, which was the third method used to assess film thickness. Fig. 4 shows the radial and circumferential diamond film uniformity for a film of 350 μm nominal thickness on a 75 mm diameter substrate for deposition with argon flow and a pressure of 100 Torr. Gas flows were 400 sccm H 2, 200 sccm Ar, and 8 sccm CH 4, and the growth rate was 1.9 μm/h. The variation in uniformity was ± 4.7% radially as measured from the substrate center to a distance of 31 mm and ±4.0% cirumferentially at a radial distance of 31 mm. These variations are based on maximum and minimum thickness values. Without argon flow, the variation in uniformity was twice that shown in Fig. 4, that is on the order of ±10.0%. This is consistent with the improvement in temperature uniformity afforded by the addition of argon. It may be noted that the deposition results of Fig. 4 were obtained with a substrate holder that was modified from the one used for the operation maps of Figs. 2 and 3 in order to improve temperature uniformity. The addition of argon had negligible effect on growth rate, for a given combination of methane flow and pressure. As shown in Table 2, for 120 Torr pressure and 8 sccm methane flow the growth rate was in the range of 1.6 to 1.9 μm/h and for 100 Torr pressure and 6 sccm methane flow, the growth rate was Fig. 4. Diamond film uniformity for a 75 mm diameter substrate showing: (A) radial distribution of thickness, d, and (B) circumferential distribution of thickness, d, at a radial distance of 31 mm from the center. Gas flows were 400 sccm H2, 200 sccm Ar,and6sccmCH4andthegrowthratewas1.9μm/h. in the range of 0.8 to 0.9 μm/h, regardless of the presence or absence of argon. Run-to-run variations exceeded or were comparable to variations with and without argon flow. One may have expected higher growth rates with argon flow because of higher substrate temperatures. This is apparently compensated for by a reduced concentration of chemically active species resulting from the addition of argon. White, translucent diamond films resulted both with argon and without argon in the input gas flow. Work by Gruen and others have shown that argon may be used to greatly decrease the grain size in diamond films, resulting in ultra-nano-crystalline-diamond (UNCD) [7]. However the argon gas flows used in this study were not large enough to produce this effect. The method of linear intercepts was applied to SEM views of 16 diamond films in order to assess the grain size [8]. Nine of these samples were deposited with argon in the gas flow and seven were deposited without argon, over a range of other deposition variables. The resulting grain sizes ranged from 15 μm to35μm. Thus the grain sizes are of a magnitude associated with polycrystalline films rather Table 2 Growth rates on 75 mm diameter silicon substrates with and without the flow of argon Sample Pressure (Torr) Methane flow Argon flow Growth rate (μm/h) A B C D E F G H At a given pressure and methane flow rate, the presence of argon does not have a significant effect on growth rate. The hydrogen flow rate is 400 sccm for all samples. Slight modifications of the substrate holder were used for deposition of samples represented in this table.

5 304 S.S. Zuo et al. / Diamond & Related Materials 17 (2008) than UNCD films. When deposited films were compared in pairs, with the difference between input variables being the presence or absence of argon in the gas input flow, in some cases the grain size decreased slightly with the addition of argon and in other cases it increased slightly. In all cases, the change was not statistically significant. Thus, for the concentration of argon used in this study the grain size was not significantly dependent on the presence of argon. 5. Post-processing Once the diamond is deposited on the silicon substrate a number of post-processing steps are performed to fabricate smooth, flat and uniformly thick films or substrates. These processing steps include laser cutting of the samples, lapping and polishing of the growth side of the diamond, removal of the silicon substrate, and plasma etching to remove a thin layer on the nucleation side of the diamond film. Laser cutting is performed with a pulsed Nd-YAG laser operating with the third harmonic and removal of the silicon substrate is accomplished with wet etching using a combination of hydrofluoric and nitric acid. Plasma etching of the nucleation surface is performed Fig. 5. (Upper) Free-standing diamond substrate, 75 μm thick, (Lower) lapped and polished diamond sample, 35 μm thick, mounted for an ion beam electron stripping application after silicon substrate removal and laser cutting. The window opening is 1 cm 1 cm. Fig. 6. The plot shows optical transmission for three samples for which the silicon substrate has been removed and the growth surface has been lapped and polished. Data for the samples labeled from left to right as I, J, and K are in Table 3. using an electron cyclotron-resonant plasma using O 2, Ar, and SF 6 as previously reported [9]. Lapping and polishing is performed with a Logitech LP 50 system using procedures recommended by the system provider [10]. The initial lapping step utilizes a 50 μm diamond slurry and the second lapping step utilizes a μm diamond slurry. In both cases, the sample is placed against a rotating metal plate. After lapping, the samples are polished. For the polishing step, the sample is placed against a rotating felt surface wetted with solutions of the type provided by the industry for chemical mechanical polishing of silicon and other semiconductors. Although the precise compositions of such commercial solutions are proprietary, their generic description is that of an alkaline slurry of colloidal silica. The surface Ra roughness values are reduced from hundreds of nm for as-grown films, to several tens of nm after lapping, to a few nm after polishing. Fig. 5 shows a 50 mm diameter diamond substrate, 75 μm thick, after silicon removal but prior to any lapping or polishing. Also shown is a laser cut, lapped and polished sample, 35 μm thick, mounted in a fixture for an ion beam electron stripper application. Fig. 6 shows the optical transmission of three lapped and polished samples for which information is provided in Table 3. Optical transmission is observed until the onset of band gap absorption, which occurs in the ultraviolet at approximately 225 nm and the transmission approaches the ideal value of approximately 70% in the infrared. Between these Table 3 Deposition conditions and results for the three samples of Fig. 6 Sample CH 4 flow Ar flow R a value (nm) Thickness (μm) Implied α (cm 1 ) I J K All three samples were deposited at a pressure of 140 Torr and with a hydrogen flow rate of 400 sccm. The wafer diameter for these three samples was 50 mm. The R a value refers to the surface roughness of the lapped and polished growth surface. The average surface roughness on the nucleation side of films is 12 nm with a standard deviation of 6 nm. The implied α refers to optical absorption coefficient at a wavelength of 250 nm.

6 S.S. Zuo et al. / Diamond & Related Materials 17 (2008) two parts of the spectrum, there is a gradual fall-off of transmission with decreasing wavelength. Such a decrease may be explained at least in part by surface roughness considerations. Surface roughness causes loss in transmission, even if all scattered light is collected. This is because of phase cancellation of light rays leaving or entering an uneven surface. For a Gaussian distribution of surface roughness with a standard deviation of R a, the light transmission into diamond from air is reduced by a factor of S R where n D is the wavelength dependent refractive index of diamond and λ is the free-space wavelength of " S R ¼ exp 2pR # aðn D 1Þ 2 k light [11]. The refractive index may be calculated using Sellmeir's equation using reported parameters for diamond [12].For the case of the polished sample I in Fig. 6,theR a value of polished surface (6.5 nm) yields S R =0.992 for 650 nm red light and for 250 nm ultraviolet light. The R a value for the nucleation side (12 nm) results in an S R value of for 650 nm red light and for 250 nm UV light. Accounting for reflection from the front and back surfaces, the expected transmission may be calculated as " # T ¼ ð1 RÞ2 1 R 4 S RP S RN ð2þ where the term in the square brackets is the average power transmission considering the series of front and back surface reflections reflection [13], S RP and S RN refer to the scattering term for the polished and nucleation surfaces, respectively and R is calculated from the refractive index of diamond, n D, and the Fresnel coefficient as R ¼ n D 1 2 ð3þ n D þ 1 Applying Eq. (2) to sample I results in predicted transmissions of 68% at 650 nm and 49% at 250 nm, close to the measured values. The thicker samples J and K, however, show substantially less UV transmission than would be predicted by Eq. (2). This implies non-zero absorption in the films in the UV portion of the spectrum. As a result, the transmission is reduced by a factor of exp( αd) where α is the absorption coefficient and d is the film thickness. An implied absorption coefficient can be determined from the difference in measured transmission values and calculated values from Eq. (2). The resulting absorption coefficient at 250 nm wavelength ranges from 80 cm 1 to 143 cm 1 for the three samples, as shown in Table 3. However, the significant ð1þ standard deviation of R a values causes an uncertainty in implied α values. For example, the standard deviation of surface roughness on the nucleation side is 6 nm, which corresponds to approximately ±50 cm 1 in the calculated absorption coefficient. This prohibits drawing comparisons between the samples. A more detailed assessment of optical quality differences, if any, caused by the addition of argon is underway. 6. Summary Thick and uniform polycrystalline diamond films are synthesized on up to 75 mm diameter substrates by using a 2.45 GHz microwave plasma-assisted CVD reactor. Non-uniform diamond film growth in the radial direction in relatively high pressure plasmas can be compensated by using a substrate cooler designed to preferentially remove heat from the central portion and by adding argon to the deposition gases. The addition of argon at a given pressure increases the plasma size, decreases the input power required to cover the wafer, and therefore decreases the plasma density. Also, substrate temperature increases slightly. Thus the addition of argon to the input gas flow is useful to achieve deposition uniformity. Processing of the diamond layers after diamond growth is described. Acknowledgements Mr. Michael Becker is thanked for providing laser cutting of samples. Lambda Technologies is thanked for their support of this work. References [1] E.V. Koposova, S.E. Myasnikova, V.V. Parshin, S.N. Vlasov, Diamond and Related Materials 11 (2002) [2] A.P. Malshe, W.D. Brown, in: J. Asmussen, D.K. Reinhard (Eds.), Diamond Films Handbook, Marcell Dekker Inc., New York, 2002, Chapter 10. [3] R.W. Shaw, A.D. Herr, C.S. Feigerle, R.I. Cutler, C.J. Liaw, Y.Y. Lee, Proceedings of the Particle Accelerator Conference, Portland, OR, May, 2003, IEEE, New York, 2003, p [4] D.K. Reinhard, T.A. Grotjohn, M. Becker, M.K. Yaran, T. Schuelke, J. Asmussen, Journal of Vacuum Science and Technology B 22 (2004) [5] K.P. Kuo, J. Asmussen, Diamond and Related Materials, 6 (1997) [6] Ircon, Inc N. Nathez Avenue, Niles, IL 60714, USA. [7] D.M. Gruen, Annual Review of Materials. Science, 29 (1999) 211. [8] M.I. Mendelson, Journal of American Ceramic Society 52 (1969) 443. [9] R.N. Chakraborty, D.K. Reinhard, P.D. Goldman, Extended Abstracts, Vol. 95 1, The Electrochemical Society, 1995, p [10] Logitech Ltd, Erskine Ferry Road, Old Kilpatrick, G60 5EU, Scotland. [11] I. Filinski, Physica Status Solidi (b) 49 (1972) 557. [12] M. Thomas, W.J. Tropf, Johns Hopkins APL Technical Digest 14 (1993) 16. [13] M. Born, E. Wolf, Principles of Optics, 7th ed., Cambridge University Press, Cambridge, 1999.