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Hybrid plasma system for diamond-like carbon film deposition
Surface and Coatings Technology 131 Ž2000. 20᎐25
Hybrid plasma system for diamond-like carbon film deposition D. KorzecU , G. Fedosenko, A. Georg, J. Engemann Microstructure Research Center, Uni¨ ersity of Wuppertal, Obere Lichtenplatzer Straße 336, 42 287 Wuppertal, Germany
Abstract A novel hybrid plasma apparatus optimized for diamond-like carbon ŽDLC. film deposition is presented. A plasma was generated by use of a jet matrix plasma source ŽJeMPS. operated at 13.56 MHz and up to 1 kW. The 48 plasma jets were arranged as a hexagonal matrix within a 15-cm diameter circle. In the center of an argon plasma at pressure of 1 mbar the ion concentration is 4.82= 10 11 cmy3. At a distance of 6 cm from the plasma source a water-cooled substrate holder biased with 13.56 MHz power was positioned. A plasma-enhanced chemical vapor deposition ŽPECVD. process was used. Helium was used as a carrier gas excited in the jet matrix plasma source. Methane, used as a source of carbon, was introduced in the zone between the plasma source and the substrate holder. A fractal carrier and process gas distribution system allowed high film homogeneity. Typical gas flows were 500 and 100 sccm, respectively; typical process pressure was 1 mbar. A Root’s blower with a pumping speed of 250 m3 hy1 was used. A deposition rate of 78 nm miny1 at room temperature was achieved. The film thickness variation over a 5 inch wafer was less than 7%. As the dc bias of the substrate holder increased from 350 to 400 V the sp 2rsp 3 ratio increased from 0.79 to 0.88. The refractive index of almost 2.4 and a Vickers hardness of 3000 were evidence of high quality DLC films. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Rf discharge; Plasma source; Diamond-like carbon; Plasma processing; Remote plasma
1. Introduction The term diamond-like carbon ŽDLC. describes a very diversified group of materials with different properties implying a large number of applications. For example, DLC deposition is an established technology for improvement of biocompatibility w1x, tribology w2x, and optical applications, especially in the infrared range w3x. Due to a dielectric constant below 2.8 applications for ULSI-technology are also considered w4x. Comparable with the variety of the DLCs is the variety of techniques developed for deposition of these materials. One large group constitutes the physical techniques like ion beam deposition w5x, non-balanced magnetron sputtering w6x, pulsed vacuum arc deposition U
Corresponding author. Tel.: q49-202-59509; fax: q49-202595098. E-mail address: [email protected] ŽD. Korzec..
w7x, or high power pulsed laser evaporation and ion deposition w8,9x. Other approaches are physically activated chemical processes like excimer laser plasma activated deposition w10x, photon enhanced chemical vapor deposition ŽCVD. w11x, dc glow discharge decomposition w12x, or hot filament CVD w13x. Recently a big effort has been concentrated on plasma enhanced CVD. One advantage of this technique is the possibility of depositing different films like metal and DLC in the same process w14x. Examples of PECVD techniques are parallel plate PECVD w15x, ECR-PECVD w16x, rf bias ECR-plasma source PECVD w17x, or dc HCD activated CVD w18x. The presented technique is a novel type of PECVD process. The chemical reactions were initiated by a high-density plasma produced in a jet matrix plasma source ŽJeMPS. w19x. The efficient chemistry is combined with the physical influence of an independently controlled ion bombardment achieved by an rf biased
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 7 7 2 - 6
D. Korzec et al. r Surface and Coatings Technology 131 (2000) 20᎐25
substrate holder. Excellent films deposited with high rates are the result.
2. Experimental setup 2.1. Process chamber The central part of the DLC deposition apparatus is a cylindrical process chamber with diameter of 25 cm consisting of two parts: a base and a cover Žsee Fig. 1a.. On the top of the cover the JeMPS is flanged. The chamber can be opened for substrate handling by vertical shifting of the substrate holder mounted in the chamber base. All components of the chamber and substrate holder in contact with the plasma are made of aluminum. Both base and cover are equipped with a water-cooling ring. Due to the very high thermal conductivity of aluminum, a constant wall temperature of the entire chamber can be achieved. The non-movable
21
cover of the process chamber is equipped with DN 40 KF vacuum ports allowing plasma diagnostics, vacuum measurement and system venting. At the DN 63 ISO-K pumping port a 251 m3 hy1 Root’s blower with backing pump is connected. The pumping ring is used for an azimuthally homogeneous gas flow. An MKS system consisting of four flow controllers, a throttle valve and Baratron vacuum gauge for independent pressure and flow control was used. 2.2. Source design The 13.56 MHz JeMPS is based on the concept of an earlier multi-jet rf plasma source w20x. The low temperature plasma jets w21x are extracted through coaxially arranged orifices in the cathode and the grounded casing of the source. Separate hollow cathode chambers for each plasma jet are used to guarantee identical properties of all ignited plasma jets. The JeMPS is designed as a hexagonal matrix of 48 plasma jets. The distance between neighboring jets is 15.5 mm. The diameter of the anode orifice is 2 mm. Each cylindrical cavity in the hollow cathode has a diameter of 12 mm and a depth of 47.5 mm. Gas channels used for distribution of the carrier gas are integrated in the back wall of the cathode. Water cooling allows the time stable operation with 1 kW rf power. In Fig. 1b the generation principle of a single plasma jet is shown. Rf current flows between the cathode and anode surface via two coaxially arranged orifices of both electrodes. Due to the reduced cross section of the current flow in both electrode orifices the current density in the jet zone is very high. The second feature assuring a high plasma density in the jet zone is the hollow cathode discharge established in the cathode cavity. High-energy electrons oscillate in the hollow cathode because of the repelling potential of the plasma sheath. This pendulum motion of the electrons trapped in the hollow cathode discharge enhances ionization in the bulk plasma. This results in an increasing electron concentration in the hollow cathode plasma and consequently in even higher current flowing through the jet orifices. New in comparison with original JeMPS constructions are the anode hollows assuring the same effective anode surface area for each jet discharge w22x. 2.3. Gas flow
Fig. 1. Operation principle of the hybrid plasma processing unit. Ža. Cross section of the processing apparatus. Žb. Principle of single jet operation.
A remote kind of process is used, in which the energy needed for dissociation of the precursor gas Ži.e. CH 4 . is transferred to the process zone by use of the carrier gas Ži.e. He. excited in the primary high density plasma jet discharges. This scheme allows a selective chemistry and suppression of the plasma source contamination.
22
D. Korzec et al. r Surface and Coatings Technology 131 (2000) 20᎐25
Carrier gas is evenly distributed among the hollows of the cathode. Due to the large power coupling area, an efficient conversion of the rf energy into chemical energy of the carrier gas is possible. Especially important for the power transfer is the zone between the anode and cathode orifices, where the plasma jet is formed. The carbon-containing gas is introduced directly into the process chamber. For uniform film deposition an optimized gas distribution system is used, allowing very high homogeneity of the film deposition. Another feature for achieving a symmetric gas flow is the pumping ring. 2.4. Substrate holder Ion bombardment is needed for obtaining high quality DLC films at room temperature. An rf biased substrate holder is used for this purpose. Separating of the biasing and plasma production allows a balance between the chemical and physical components of the process. The rf bias allows an ion bombardment even for insulating surfaces. The biasing of the substrate holder is performed by use of 13.56 MHz rf with power up to 1 kW. To minimize the reflected power, separate impedance matching is used. Experiments with kHz biasing have also been performed, which should be better for ion bombardment than MHz w23x, but instabilities of the discharge and negative influencing of the plasma source operation, especially with insulating substrates, forced coating experiments to be performed with only 13.56 MHz biasing. The substrate holder is designed for specimens with a diameter up to 5 inches. Water cooling of the substrate is applied, because there is evidence for an increasing deposition rate with decreasing temperature w24x and because plastics are a potential substrate for DLC coating.
Fig. 2. Ion concentration vs. rf power for different pressures with an argon flow of 750 sccm through the plasma source and with a floating substrate holder, measured 30 mm from the source axis and 21 mm from the bottom edge of the source.
4.8= 10 11 cmy3 Žsee Fig. 3., which is quite competitive with inductively coupled rf discharges and overdense microwave discharges. Due to the complex geometry of the reactor, the radial distribution of the ion concentration cannot be fitted exactly by use of a simple analytical formula Žcosine or logarithmic., but the ion concentration change by an order of magnitude is evidence of a diffusion determined profile Žsee Fig. 3.. Because the bias rf power also contributes to the plasma generation, more homogeneous ion concentration distributions can be expected in the vicinity of the substrate holder surface w26x. The dc bias at the substrate holder is crucial for determination of the mean energy of ion bombardment. It was measured for helium as a function of the rf power for different pressures Žsee Fig. 4.. As expected, the dc bias increases with rf power according to the function:
3. Results and discussion 3.1. Plasma characterization The ion concentration in the JeMPS plasma was measured by use of a radially movable double Langmuir probe ŽDLP. w25x. For comparison purposes, an argon plasma has been characterized. The increase of ion concentration with rf power can be described by: n i s const = Prf␥
Ž1.
where ␥ is approximately 0.8 in the considered pressure range 0.6᎐1.4 mbar Žsee Fig. 2.. Ion concentration decreases with increasing pressure. The maximum ion concentration measured at 1 mbar for 800 W in a distance of 3 cm from the plasma source bottom is
Fig. 3. Ion concentration vs. radial position for different rf powers with an argon flow of 750 sccm through the plasma source at a pressure of 1 mbar, with floating substrate holder, measured 21 mm from the bottom edge of the source.
D. Korzec et al. r Surface and Coatings Technology 131 (2000) 20᎐25
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allowing a very efficient energy transfer from the excitation zone in the plasma jets to the plasma bulk, where excitation and dissociation of process gas molecules due to Penning collisions occurs. 4. Due to the very small cross sections for electronic collisions, the optimal pressure for volume chemistry is much higher than for argon, which allows much higher gas flows and hence higher process rates for a given pumping speed.
Fig. 4. The dc bias on the substrate holder vs. rf power for different process pressures measured for a helium flow of 500 sccm with the plasma source switched off.
Vdc s const = Prf
Ž2.
where is changing from 0.51 to 0.57 while the pressure increases from 0.1 to 2 mbar. The bias decreases with increasing pressure. Only a very small influence of the methane flow Žless than 5% for less than 50 sccm. on the bias is observed. Also the concurrent operation of the JeMPS has no significant influence Žless than 5%. on the dc bias of the substrate holder. 3.2. Deposition process For process and film characterization, the DLC films were deposited on n-doped silicon wafers with the crystallographic orientation ²111: and a resistivity of less than 1 ⍀ cm. Before plasma processing the wafers were cleaned in an ultrasonic bath, 5 min in acetone and 5 min in isopropyl alcohol and dried with nitrogen. An 8-min pretreatment in argon plasma with a plasma source rf power of 400 W and a bias rf power of 250 W at pressure of 0.5 mbar is routinely used. The focus of this work was on the influence of the dc bias on the deposition results. In a previous work with a linear jet matrix plasma source, satisfactory standard processing conditions had been found. An important feature of the process is the use of helium as a carrier gas. The following advantages in comparison with argon as a carrier gas can be achieved: 1. Due to small cross sections for elastic collisions at a given pressure, more helium ions arrive the substrate holder with an energy corresponding to the dc bias. 2. Due to the small mass, the energy transfer during the elastic collision between He and hydrogen is much better than between argon and hydrogen. A more efficient removal of hydrogen from the film structure occurs, resulting in hydrogen-free films. 3. Helium has long-lived high-energy metastable states
An optimal helium pressure, balancing the deposition rate and film quality, of 0.8᎐1.1 mbar has been found. Different process gases can be used as a source of carbon. It is known from other PECVD processes that molecules with a higher carbonrhydrogen ratio allow higher deposition rates. Deposition rates three times higher for ethane than for methane were reported for a hollow cathode discharge process w27x. But for comparison purposes and for better film quality, methane was used as the process gas in this study. As a result of balancing the deposition rate and chamber contamination by graphite, a methanerhelium ratio of approximately 1:5 is obtained. Both the film quality and the deposition rate improve with increasing rf power of the plasma source within the investigated power range. In Fig. 5 the influence of the bias power on the deposition rate is shown. The characteristic feature of this dependence is a distinct maximum for a bias power of 280 V that corresponds to a dc bias of 400 V. The following model is proposed for explaining this dependence: 1. The increasing energy of Heq ions results in a more efficient removal of hydrogen atoms connected to the carbon atoms. Consequently new dangling bonds for carbon crystal growth are created in molecular radicals sticking at the film surface and a quicker film growth occurs. 2. For higher bias levels the ion bombardment energy gets high enough to also remove the carbon atoms from the film structure. The removal process starts to dominate the growth for bias higher than 400 V. In this range the deposition rate decreases with increasing dc bias. The homogeneity of the film thickness is much better than the homogeneity of the ion concentration distribution. The distribution also has a quite different character. While the ion concentration distribution has a single maximum in the center of plasma Žsee Fig. 3., the film thickness has a maximum at the substrate edge. The conclusion is that chemical processes are more important for determination of the deposition rate than the ion bombardment. Both the process and the carrier gas are introduced homogeneously above
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D. Korzec et al. r Surface and Coatings Technology 131 (2000) 20᎐25
Fig. 5. Dependence of refractive index and deposition rate on bias power. Deposition conditions: source power, 400 W; pressure, 1 mbar; methane flow, 100 sccm; helium flow, 500 sccm; and deposition time 5 min.
the substrate holder surface. But despite this, some radial dependence of the deposition rate can be expected, because a radial gas flow from the center of the plasma to the circular pumping ring positioned at the periphery of the plasma Žsee Fig. 1a. occurs. Almost no gas refreshment just in the center of the substrate can be expected. This could be the reason for the minimum deposition rate at this location. Overall, a very good film-thickness homogeneity with a variation of less than 7% was measured Žsee Fig. 6.. 3.3. Film properties A standard method for characterization of the optical properties of DLC film is the refractive index w28x. This can be measured by use of ellipsometry. For this purpose a SOPRA ES4M ellipsometer has been used. The refraction index obtained in our study at a wavelength of 589 nm ranges from 2.2 to 2.4 Žsee Fig.
Fig. 7. Influence of the rf bias on the micro-raman spectra of the DLC film deposited on silicon. Deposition conditions: source power, 400 W; pressure, 1 mbar; methane flow, 100 sccm; helium flow, 500 sccm; and deposition time, 5 min
5. which is very close to the value of 2.417 reported for diamond, and assures a good film quality w29,30x. Such a high refractive index is confirmation of earlier results, showing that the refraction index is higher when He is used as the process gas instead of Ar or H 2 w23x. The refractive index dependence on the dc bias correlates with the deposition rate. Both increasing the content of hydrogen for dc bias below the optimum Ž400 V. and structural damage above the optimum result in a refractive index lower than the maximum. Raman spectroscopy is a fundamental technique used for characterization of carbon film properties w31x. With this technique the proportion of the signal related to sp, sp 2 and sp 3 bindings, and the dimensions of the domains can be determined. In Fig. 7 the micro-Raman spectra of DLC films deposited with different bias of the substrate holder are presented. All spectra show a shape characteristic for DLC films. A strong G-line Ž1560 cmy1 . with a broad D-line Ž1350 cmy1 . shoulder w32x can be recognized. When the dc bias of the substrate holder increased from 350 V to 400 V the sp 2rsp 3 ratio increased from 0.79 to 0.88.
4. Conclusions
Fig. 6. Typical radial film thickness distribution in two perpendicular directions over a 5 inch silicon wafer. Deposition conditions: source power, 400 W; bias power, 283 W; pressure, 1 mbar; methane flow, 100 sccm; helium flow, 500 sccm; and deposition time, 5 min
1. A novel hybrid plasma reactor for DLC film deposition has been presented. 2. For both substrate holder biasing and plasma excitation, an rf excitation of 13.56 MHz was used. 3. The application of helium as a carrier gas allows the deposition of low-hydrogen films. 4. Amorphous carbon films with a Vickers hardness of 3000 and a refractive index of 2.4 at 589 nm can be produced.
D. Korzec et al. r Surface and Coatings Technology 131 (2000) 20᎐25
5. A deposition rate of 78 nm miny1 was achieved with methane.
Acknowledgements The authors wish to thank K.-H. Weggasser for con¨ struction and G. Kaul for set-up of the system, KarlHeinz Schumann and F. Tammen for manufacture of the processing unit. This project was supported by Ministry of Science and Research and Ministry of Economics Technology and Transportation, North Rhine-Westphalia, Germany.
w16x
w17x
w18x
w19x
w20x
References w1x L. Lu, M.W. Jons, Diamond-like carbon as biological compatible material for cell culture and medical application, Bio-Med. Mater. Eng. 3 Ž1993. 223᎐228. w2x C. Donnet, Recent progress on the tribology of doped diamond-like and carbon alloy coatings: a review, Surf. Coat. Technol. 100-101 Ž1998. 180᎐186. w3x A.H. Lettington, C. Smith, Optical properties and applications of diamond-like carbon coatings, Diamond Related Mater. 1 Ž1992. 805᎐809. w4x A. Grill, Diamond-like carbon: state of the art, Diamond Related Mater. 8 Ž1999. 428᎐434. w5x V. Palshin, E.I. Meletis, S. Ves, S. Logethetidis, Characterization of ion-beam-deposited diamond-like carbon films, Thin Solid Films 270 Ž1995. 165᎐172. w6x J. Deng, M. Braun, Residual stress and microhardness of DLC multilayer coatings, Diamond Related Mater. 5 Ž1996. 478᎐482. w7x B. Schultrich, H.-J. Scheibe, D. Drescher, H. Ziegele, Deposition of superhard amorphous carbon films by pulsed vacuum arc deposition, Surf. Coat. Technol. 98 Ž1998. 1097᎐1101. w8x S.S. Wagal, Deposition of diamond-like carbon film by laser ion source, Surf. Coat. Technol. 47 Ž1991. 257᎐262. w9x C.B. Collins, F. Davanloo, E.M. Juengerman, W.R. Osborn, D.R. Jander, Laser plasma source of amorphic diamond, Appl. Phys. Lett. 54 Ž1989. 216᎐218. w10x A.A. Gorbunov, V.I. Konov, Carbon films deposited from UV laser plasma, Surf. Coat. Technol. 47 Ž1991. 503᎐508. w11x A.J. White, W.I. Milne, Deposition of diamond-like carbon films by photon-enhanced chemical vapour deposition of methane using a windowless hydrogen lamp, Diamond Related Mater. 2 Ž1993. 255᎐258. w12x M. Malhotra, S. Kumar, Thermal gas effusion from diamondlike carbon films, Diamond Related Mater. 6 Ž1997. 1830᎐1835. w13x E. Buccioni, E. Braca, J.M. Kenny, M.L. Terranova, Processing-structure-adhesion relationship in CVD diamond films on titanium substrates, Diamond Related Mater. 8 Ž1999. 17᎐24. w14x H. Bolt, A. Buuron, V. Hemel, F. Koch, Gradient metal ᎏ a-C:H coatings deposited from dense plasma by a combined PVDrCVD process, Surf. Coat. Technol. 98 Ž1998. 1518᎐1523. w15x S.F. Durrant, E.C. Rangel, N.C. da Cruz, S.G.C. Castro, M.A.
w21x w22x
w23x
w24x
w25x
w26x
w27x
w28x
w29x
w30x w31x w32x
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Bica de Moraes, Amorphous hydrogenated carbon films produced by PECVD, Surf. Coat. Technol. 86-87 Ž1996. 443᎐448. V. Dusek, M. Vanecek, P. Siroky, V. Vorlicek, Influence of the electron cyclotron resonance microwave plasma on growth and properties of diamond-like carbon films deposited onto rf biased substrates, Diamond Related Mater. 2 Ž1993. 397᎐401. M. Zarrabian, N. Fourches-Coulon, G. Turban, M. Lancin, C. Marhic, Effect of negative bias voltage on a-C:H films deposited in electron cyclotron resonance plasma, Diamond Related Mater. 6 Ž1997. 542᎐546. A. Hellmich, T. Jung, A. Kielhorn, M. Rißland, CVD-processes by hollow cathode glow discharge, Surf. Coat. Technol. 98 Ž1998. 1541᎐1546. M. Mildner, D. Korzec, J. Engemann, 13.56 MHz hollow cathode jet matrix plasma source for large area surface coating, Surf. Coat. Technol. 112 Ž1999. 366᎐372. D. Korzec, J. Engemann, M. Mildner, K.-P. Ningel, O. Borgmeier, D. Theirich, Multijet hollow cathode discharge for remote polymer deposition, Surf. Coat. Technol. 93 Ž1997. 128᎐133. L. Bardos, Radio frequency hollow cathode for plasma processing technology, Surf. Coat. Technol. 86-87 Ž1996. 648᎐656. J. Engemann, D. Korzec, M. Mildner. Vorrichtung zur Erzeugung einer Vielzahl von Niedertemperatur-Plasmajets. European patent application EP-OS 0 881 865 and German patent application DE 197 22 624 Ž1997.. K. Donnelly, D.P. Dowling, T.P. O’Brien, A. O’Leary, T.C. Kelly, Comparison of diamond-like carbon films deposited from 40 kHz and 13.56 MHz r.f. plasmas, Diamond Related Mater. 5 Ž1996. 445᎐447. S.F. Yoon, H. Yang, A. Rusli, J. Ahn, Q. Zhang, T.L. Poo, Influence of substrate temperature and microwave power on the properties of a-C:H films prepared using the ECR-CVD method, Diamond Related Mater. 6 Ž1997. 1683᎐1688. A. Brockhaus, C. Borchardt, J. Engemann, Langmuir probe measurements in commercial plasma plants, Plasma Sources Sci. Technol. 3 Ž1994. 539. D. Korzec, J. Engemann, Large area lubricant removal by use of capacitively coupled rf and slot antenna microwave plasma, Surf. Coat. Technol. 89 Ž1997. 165᎐176. A. Hellmich, T. Jung, A. Kielhorn, M. Rißland, CVD-processes by hollow cathode glow discharge, Surf. Coat. Technol. 98 Ž1998. 1541᎐01546. D.P. Dowling, K. Donnelly, M. Monclus, M. McGuinness, The use of refractive index as a measure of diamond-like carbon film quality, Diamond Related Mater. 7 Ž1998. 432᎐434. Y. Lifshitz, G.D. Lempert, E. Grossman et al., Optical and photoemission studies of DLC films prepared with a systematic variation of the sp 3 :sp 2 composition, Diamond Related Mater. 6 Ž1997. 687᎐693. S. Vakula, V. Padalka, V. Strelnizki et al., Superharte Mater. N1 Ž1980. 18᎐22. M.A. Tamor, W.C. Vassel, Raman ‘fingerprinting’ of amorphous carbon films, J. Appl. Phys. 76 Ž1994. 3823᎐3830. S.F. Yoon, H. Yang, A. Rusli, J. Ahn, Q. Zhang, The effects of self-generated DC bias on the characteristics of diamond-like carbon films prepared using ECR-CVD, Diamond Related Mater. 7 Ž1998. 70᎐76.
Hybrid plasma system for diamond-like carbon film deposition D. KorzecU , G. Fedosenko, A. Georg, J. Engemann Microstructure Research Center, Uni¨ ersity of Wuppertal, Obere Lichtenplatzer Straße 336, 42 287 Wuppertal, Germany
Abstract A novel hybrid plasma apparatus optimized for diamond-like carbon ŽDLC. film deposition is presented. A plasma was generated by use of a jet matrix plasma source ŽJeMPS. operated at 13.56 MHz and up to 1 kW. The 48 plasma jets were arranged as a hexagonal matrix within a 15-cm diameter circle. In the center of an argon plasma at pressure of 1 mbar the ion concentration is 4.82= 10 11 cmy3. At a distance of 6 cm from the plasma source a water-cooled substrate holder biased with 13.56 MHz power was positioned. A plasma-enhanced chemical vapor deposition ŽPECVD. process was used. Helium was used as a carrier gas excited in the jet matrix plasma source. Methane, used as a source of carbon, was introduced in the zone between the plasma source and the substrate holder. A fractal carrier and process gas distribution system allowed high film homogeneity. Typical gas flows were 500 and 100 sccm, respectively; typical process pressure was 1 mbar. A Root’s blower with a pumping speed of 250 m3 hy1 was used. A deposition rate of 78 nm miny1 at room temperature was achieved. The film thickness variation over a 5 inch wafer was less than 7%. As the dc bias of the substrate holder increased from 350 to 400 V the sp 2rsp 3 ratio increased from 0.79 to 0.88. The refractive index of almost 2.4 and a Vickers hardness of 3000 were evidence of high quality DLC films. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Rf discharge; Plasma source; Diamond-like carbon; Plasma processing; Remote plasma
1. Introduction The term diamond-like carbon ŽDLC. describes a very diversified group of materials with different properties implying a large number of applications. For example, DLC deposition is an established technology for improvement of biocompatibility w1x, tribology w2x, and optical applications, especially in the infrared range w3x. Due to a dielectric constant below 2.8 applications for ULSI-technology are also considered w4x. Comparable with the variety of the DLCs is the variety of techniques developed for deposition of these materials. One large group constitutes the physical techniques like ion beam deposition w5x, non-balanced magnetron sputtering w6x, pulsed vacuum arc deposition U
Corresponding author. Tel.: q49-202-59509; fax: q49-202595098. E-mail address: [email protected] ŽD. Korzec..
w7x, or high power pulsed laser evaporation and ion deposition w8,9x. Other approaches are physically activated chemical processes like excimer laser plasma activated deposition w10x, photon enhanced chemical vapor deposition ŽCVD. w11x, dc glow discharge decomposition w12x, or hot filament CVD w13x. Recently a big effort has been concentrated on plasma enhanced CVD. One advantage of this technique is the possibility of depositing different films like metal and DLC in the same process w14x. Examples of PECVD techniques are parallel plate PECVD w15x, ECR-PECVD w16x, rf bias ECR-plasma source PECVD w17x, or dc HCD activated CVD w18x. The presented technique is a novel type of PECVD process. The chemical reactions were initiated by a high-density plasma produced in a jet matrix plasma source ŽJeMPS. w19x. The efficient chemistry is combined with the physical influence of an independently controlled ion bombardment achieved by an rf biased
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 7 7 2 - 6
D. Korzec et al. r Surface and Coatings Technology 131 (2000) 20᎐25
substrate holder. Excellent films deposited with high rates are the result.
2. Experimental setup 2.1. Process chamber The central part of the DLC deposition apparatus is a cylindrical process chamber with diameter of 25 cm consisting of two parts: a base and a cover Žsee Fig. 1a.. On the top of the cover the JeMPS is flanged. The chamber can be opened for substrate handling by vertical shifting of the substrate holder mounted in the chamber base. All components of the chamber and substrate holder in contact with the plasma are made of aluminum. Both base and cover are equipped with a water-cooling ring. Due to the very high thermal conductivity of aluminum, a constant wall temperature of the entire chamber can be achieved. The non-movable
21
cover of the process chamber is equipped with DN 40 KF vacuum ports allowing plasma diagnostics, vacuum measurement and system venting. At the DN 63 ISO-K pumping port a 251 m3 hy1 Root’s blower with backing pump is connected. The pumping ring is used for an azimuthally homogeneous gas flow. An MKS system consisting of four flow controllers, a throttle valve and Baratron vacuum gauge for independent pressure and flow control was used. 2.2. Source design The 13.56 MHz JeMPS is based on the concept of an earlier multi-jet rf plasma source w20x. The low temperature plasma jets w21x are extracted through coaxially arranged orifices in the cathode and the grounded casing of the source. Separate hollow cathode chambers for each plasma jet are used to guarantee identical properties of all ignited plasma jets. The JeMPS is designed as a hexagonal matrix of 48 plasma jets. The distance between neighboring jets is 15.5 mm. The diameter of the anode orifice is 2 mm. Each cylindrical cavity in the hollow cathode has a diameter of 12 mm and a depth of 47.5 mm. Gas channels used for distribution of the carrier gas are integrated in the back wall of the cathode. Water cooling allows the time stable operation with 1 kW rf power. In Fig. 1b the generation principle of a single plasma jet is shown. Rf current flows between the cathode and anode surface via two coaxially arranged orifices of both electrodes. Due to the reduced cross section of the current flow in both electrode orifices the current density in the jet zone is very high. The second feature assuring a high plasma density in the jet zone is the hollow cathode discharge established in the cathode cavity. High-energy electrons oscillate in the hollow cathode because of the repelling potential of the plasma sheath. This pendulum motion of the electrons trapped in the hollow cathode discharge enhances ionization in the bulk plasma. This results in an increasing electron concentration in the hollow cathode plasma and consequently in even higher current flowing through the jet orifices. New in comparison with original JeMPS constructions are the anode hollows assuring the same effective anode surface area for each jet discharge w22x. 2.3. Gas flow
Fig. 1. Operation principle of the hybrid plasma processing unit. Ža. Cross section of the processing apparatus. Žb. Principle of single jet operation.
A remote kind of process is used, in which the energy needed for dissociation of the precursor gas Ži.e. CH 4 . is transferred to the process zone by use of the carrier gas Ži.e. He. excited in the primary high density plasma jet discharges. This scheme allows a selective chemistry and suppression of the plasma source contamination.
22
D. Korzec et al. r Surface and Coatings Technology 131 (2000) 20᎐25
Carrier gas is evenly distributed among the hollows of the cathode. Due to the large power coupling area, an efficient conversion of the rf energy into chemical energy of the carrier gas is possible. Especially important for the power transfer is the zone between the anode and cathode orifices, where the plasma jet is formed. The carbon-containing gas is introduced directly into the process chamber. For uniform film deposition an optimized gas distribution system is used, allowing very high homogeneity of the film deposition. Another feature for achieving a symmetric gas flow is the pumping ring. 2.4. Substrate holder Ion bombardment is needed for obtaining high quality DLC films at room temperature. An rf biased substrate holder is used for this purpose. Separating of the biasing and plasma production allows a balance between the chemical and physical components of the process. The rf bias allows an ion bombardment even for insulating surfaces. The biasing of the substrate holder is performed by use of 13.56 MHz rf with power up to 1 kW. To minimize the reflected power, separate impedance matching is used. Experiments with kHz biasing have also been performed, which should be better for ion bombardment than MHz w23x, but instabilities of the discharge and negative influencing of the plasma source operation, especially with insulating substrates, forced coating experiments to be performed with only 13.56 MHz biasing. The substrate holder is designed for specimens with a diameter up to 5 inches. Water cooling of the substrate is applied, because there is evidence for an increasing deposition rate with decreasing temperature w24x and because plastics are a potential substrate for DLC coating.
Fig. 2. Ion concentration vs. rf power for different pressures with an argon flow of 750 sccm through the plasma source and with a floating substrate holder, measured 30 mm from the source axis and 21 mm from the bottom edge of the source.
4.8= 10 11 cmy3 Žsee Fig. 3., which is quite competitive with inductively coupled rf discharges and overdense microwave discharges. Due to the complex geometry of the reactor, the radial distribution of the ion concentration cannot be fitted exactly by use of a simple analytical formula Žcosine or logarithmic., but the ion concentration change by an order of magnitude is evidence of a diffusion determined profile Žsee Fig. 3.. Because the bias rf power also contributes to the plasma generation, more homogeneous ion concentration distributions can be expected in the vicinity of the substrate holder surface w26x. The dc bias at the substrate holder is crucial for determination of the mean energy of ion bombardment. It was measured for helium as a function of the rf power for different pressures Žsee Fig. 4.. As expected, the dc bias increases with rf power according to the function:
3. Results and discussion 3.1. Plasma characterization The ion concentration in the JeMPS plasma was measured by use of a radially movable double Langmuir probe ŽDLP. w25x. For comparison purposes, an argon plasma has been characterized. The increase of ion concentration with rf power can be described by: n i s const = Prf␥
Ž1.
where ␥ is approximately 0.8 in the considered pressure range 0.6᎐1.4 mbar Žsee Fig. 2.. Ion concentration decreases with increasing pressure. The maximum ion concentration measured at 1 mbar for 800 W in a distance of 3 cm from the plasma source bottom is
Fig. 3. Ion concentration vs. radial position for different rf powers with an argon flow of 750 sccm through the plasma source at a pressure of 1 mbar, with floating substrate holder, measured 21 mm from the bottom edge of the source.
D. Korzec et al. r Surface and Coatings Technology 131 (2000) 20᎐25
23
allowing a very efficient energy transfer from the excitation zone in the plasma jets to the plasma bulk, where excitation and dissociation of process gas molecules due to Penning collisions occurs. 4. Due to the very small cross sections for electronic collisions, the optimal pressure for volume chemistry is much higher than for argon, which allows much higher gas flows and hence higher process rates for a given pumping speed.
Fig. 4. The dc bias on the substrate holder vs. rf power for different process pressures measured for a helium flow of 500 sccm with the plasma source switched off.
Vdc s const = Prf
Ž2.
where is changing from 0.51 to 0.57 while the pressure increases from 0.1 to 2 mbar. The bias decreases with increasing pressure. Only a very small influence of the methane flow Žless than 5% for less than 50 sccm. on the bias is observed. Also the concurrent operation of the JeMPS has no significant influence Žless than 5%. on the dc bias of the substrate holder. 3.2. Deposition process For process and film characterization, the DLC films were deposited on n-doped silicon wafers with the crystallographic orientation ²111: and a resistivity of less than 1 ⍀ cm. Before plasma processing the wafers were cleaned in an ultrasonic bath, 5 min in acetone and 5 min in isopropyl alcohol and dried with nitrogen. An 8-min pretreatment in argon plasma with a plasma source rf power of 400 W and a bias rf power of 250 W at pressure of 0.5 mbar is routinely used. The focus of this work was on the influence of the dc bias on the deposition results. In a previous work with a linear jet matrix plasma source, satisfactory standard processing conditions had been found. An important feature of the process is the use of helium as a carrier gas. The following advantages in comparison with argon as a carrier gas can be achieved: 1. Due to small cross sections for elastic collisions at a given pressure, more helium ions arrive the substrate holder with an energy corresponding to the dc bias. 2. Due to the small mass, the energy transfer during the elastic collision between He and hydrogen is much better than between argon and hydrogen. A more efficient removal of hydrogen from the film structure occurs, resulting in hydrogen-free films. 3. Helium has long-lived high-energy metastable states
An optimal helium pressure, balancing the deposition rate and film quality, of 0.8᎐1.1 mbar has been found. Different process gases can be used as a source of carbon. It is known from other PECVD processes that molecules with a higher carbonrhydrogen ratio allow higher deposition rates. Deposition rates three times higher for ethane than for methane were reported for a hollow cathode discharge process w27x. But for comparison purposes and for better film quality, methane was used as the process gas in this study. As a result of balancing the deposition rate and chamber contamination by graphite, a methanerhelium ratio of approximately 1:5 is obtained. Both the film quality and the deposition rate improve with increasing rf power of the plasma source within the investigated power range. In Fig. 5 the influence of the bias power on the deposition rate is shown. The characteristic feature of this dependence is a distinct maximum for a bias power of 280 V that corresponds to a dc bias of 400 V. The following model is proposed for explaining this dependence: 1. The increasing energy of Heq ions results in a more efficient removal of hydrogen atoms connected to the carbon atoms. Consequently new dangling bonds for carbon crystal growth are created in molecular radicals sticking at the film surface and a quicker film growth occurs. 2. For higher bias levels the ion bombardment energy gets high enough to also remove the carbon atoms from the film structure. The removal process starts to dominate the growth for bias higher than 400 V. In this range the deposition rate decreases with increasing dc bias. The homogeneity of the film thickness is much better than the homogeneity of the ion concentration distribution. The distribution also has a quite different character. While the ion concentration distribution has a single maximum in the center of plasma Žsee Fig. 3., the film thickness has a maximum at the substrate edge. The conclusion is that chemical processes are more important for determination of the deposition rate than the ion bombardment. Both the process and the carrier gas are introduced homogeneously above
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Fig. 5. Dependence of refractive index and deposition rate on bias power. Deposition conditions: source power, 400 W; pressure, 1 mbar; methane flow, 100 sccm; helium flow, 500 sccm; and deposition time 5 min.
the substrate holder surface. But despite this, some radial dependence of the deposition rate can be expected, because a radial gas flow from the center of the plasma to the circular pumping ring positioned at the periphery of the plasma Žsee Fig. 1a. occurs. Almost no gas refreshment just in the center of the substrate can be expected. This could be the reason for the minimum deposition rate at this location. Overall, a very good film-thickness homogeneity with a variation of less than 7% was measured Žsee Fig. 6.. 3.3. Film properties A standard method for characterization of the optical properties of DLC film is the refractive index w28x. This can be measured by use of ellipsometry. For this purpose a SOPRA ES4M ellipsometer has been used. The refraction index obtained in our study at a wavelength of 589 nm ranges from 2.2 to 2.4 Žsee Fig.
Fig. 7. Influence of the rf bias on the micro-raman spectra of the DLC film deposited on silicon. Deposition conditions: source power, 400 W; pressure, 1 mbar; methane flow, 100 sccm; helium flow, 500 sccm; and deposition time, 5 min
5. which is very close to the value of 2.417 reported for diamond, and assures a good film quality w29,30x. Such a high refractive index is confirmation of earlier results, showing that the refraction index is higher when He is used as the process gas instead of Ar or H 2 w23x. The refractive index dependence on the dc bias correlates with the deposition rate. Both increasing the content of hydrogen for dc bias below the optimum Ž400 V. and structural damage above the optimum result in a refractive index lower than the maximum. Raman spectroscopy is a fundamental technique used for characterization of carbon film properties w31x. With this technique the proportion of the signal related to sp, sp 2 and sp 3 bindings, and the dimensions of the domains can be determined. In Fig. 7 the micro-Raman spectra of DLC films deposited with different bias of the substrate holder are presented. All spectra show a shape characteristic for DLC films. A strong G-line Ž1560 cmy1 . with a broad D-line Ž1350 cmy1 . shoulder w32x can be recognized. When the dc bias of the substrate holder increased from 350 V to 400 V the sp 2rsp 3 ratio increased from 0.79 to 0.88.
4. Conclusions
Fig. 6. Typical radial film thickness distribution in two perpendicular directions over a 5 inch silicon wafer. Deposition conditions: source power, 400 W; bias power, 283 W; pressure, 1 mbar; methane flow, 100 sccm; helium flow, 500 sccm; and deposition time, 5 min
1. A novel hybrid plasma reactor for DLC film deposition has been presented. 2. For both substrate holder biasing and plasma excitation, an rf excitation of 13.56 MHz was used. 3. The application of helium as a carrier gas allows the deposition of low-hydrogen films. 4. Amorphous carbon films with a Vickers hardness of 3000 and a refractive index of 2.4 at 589 nm can be produced.
D. Korzec et al. r Surface and Coatings Technology 131 (2000) 20᎐25
5. A deposition rate of 78 nm miny1 was achieved with methane.
Acknowledgements The authors wish to thank K.-H. Weggasser for con¨ struction and G. Kaul for set-up of the system, KarlHeinz Schumann and F. Tammen for manufacture of the processing unit. This project was supported by Ministry of Science and Research and Ministry of Economics Technology and Transportation, North Rhine-Westphalia, Germany.
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References w1x L. Lu, M.W. Jons, Diamond-like carbon as biological compatible material for cell culture and medical application, Bio-Med. Mater. Eng. 3 Ž1993. 223᎐228. w2x C. Donnet, Recent progress on the tribology of doped diamond-like and carbon alloy coatings: a review, Surf. Coat. Technol. 100-101 Ž1998. 180᎐186. w3x A.H. Lettington, C. Smith, Optical properties and applications of diamond-like carbon coatings, Diamond Related Mater. 1 Ž1992. 805᎐809. w4x A. Grill, Diamond-like carbon: state of the art, Diamond Related Mater. 8 Ž1999. 428᎐434. w5x V. Palshin, E.I. Meletis, S. Ves, S. Logethetidis, Characterization of ion-beam-deposited diamond-like carbon films, Thin Solid Films 270 Ž1995. 165᎐172. w6x J. Deng, M. Braun, Residual stress and microhardness of DLC multilayer coatings, Diamond Related Mater. 5 Ž1996. 478᎐482. w7x B. Schultrich, H.-J. Scheibe, D. Drescher, H. Ziegele, Deposition of superhard amorphous carbon films by pulsed vacuum arc deposition, Surf. Coat. Technol. 98 Ž1998. 1097᎐1101. w8x S.S. Wagal, Deposition of diamond-like carbon film by laser ion source, Surf. Coat. Technol. 47 Ž1991. 257᎐262. w9x C.B. Collins, F. Davanloo, E.M. Juengerman, W.R. Osborn, D.R. Jander, Laser plasma source of amorphic diamond, Appl. Phys. Lett. 54 Ž1989. 216᎐218. w10x A.A. Gorbunov, V.I. Konov, Carbon films deposited from UV laser plasma, Surf. Coat. Technol. 47 Ž1991. 503᎐508. w11x A.J. White, W.I. Milne, Deposition of diamond-like carbon films by photon-enhanced chemical vapour deposition of methane using a windowless hydrogen lamp, Diamond Related Mater. 2 Ž1993. 255᎐258. w12x M. Malhotra, S. Kumar, Thermal gas effusion from diamondlike carbon films, Diamond Related Mater. 6 Ž1997. 1830᎐1835. w13x E. Buccioni, E. Braca, J.M. Kenny, M.L. Terranova, Processing-structure-adhesion relationship in CVD diamond films on titanium substrates, Diamond Related Mater. 8 Ž1999. 17᎐24. w14x H. Bolt, A. Buuron, V. Hemel, F. Koch, Gradient metal ᎏ a-C:H coatings deposited from dense plasma by a combined PVDrCVD process, Surf. Coat. Technol. 98 Ž1998. 1518᎐1523. w15x S.F. Durrant, E.C. Rangel, N.C. da Cruz, S.G.C. Castro, M.A.
w21x w22x
w23x
w24x
w25x
w26x
w27x
w28x
w29x
w30x w31x w32x
25
Bica de Moraes, Amorphous hydrogenated carbon films produced by PECVD, Surf. Coat. Technol. 86-87 Ž1996. 443᎐448. V. Dusek, M. Vanecek, P. Siroky, V. Vorlicek, Influence of the electron cyclotron resonance microwave plasma on growth and properties of diamond-like carbon films deposited onto rf biased substrates, Diamond Related Mater. 2 Ž1993. 397᎐401. M. Zarrabian, N. Fourches-Coulon, G. Turban, M. Lancin, C. Marhic, Effect of negative bias voltage on a-C:H films deposited in electron cyclotron resonance plasma, Diamond Related Mater. 6 Ž1997. 542᎐546. A. Hellmich, T. Jung, A. Kielhorn, M. Rißland, CVD-processes by hollow cathode glow discharge, Surf. Coat. Technol. 98 Ž1998. 1541᎐1546. M. Mildner, D. Korzec, J. Engemann, 13.56 MHz hollow cathode jet matrix plasma source for large area surface coating, Surf. Coat. Technol. 112 Ž1999. 366᎐372. D. Korzec, J. Engemann, M. Mildner, K.-P. Ningel, O. Borgmeier, D. Theirich, Multijet hollow cathode discharge for remote polymer deposition, Surf. Coat. Technol. 93 Ž1997. 128᎐133. L. Bardos, Radio frequency hollow cathode for plasma processing technology, Surf. Coat. Technol. 86-87 Ž1996. 648᎐656. J. Engemann, D. Korzec, M. Mildner. Vorrichtung zur Erzeugung einer Vielzahl von Niedertemperatur-Plasmajets. European patent application EP-OS 0 881 865 and German patent application DE 197 22 624 Ž1997.. K. Donnelly, D.P. Dowling, T.P. O’Brien, A. O’Leary, T.C. Kelly, Comparison of diamond-like carbon films deposited from 40 kHz and 13.56 MHz r.f. plasmas, Diamond Related Mater. 5 Ž1996. 445᎐447. S.F. Yoon, H. Yang, A. Rusli, J. Ahn, Q. Zhang, T.L. Poo, Influence of substrate temperature and microwave power on the properties of a-C:H films prepared using the ECR-CVD method, Diamond Related Mater. 6 Ž1997. 1683᎐1688. A. Brockhaus, C. Borchardt, J. Engemann, Langmuir probe measurements in commercial plasma plants, Plasma Sources Sci. Technol. 3 Ž1994. 539. D. Korzec, J. Engemann, Large area lubricant removal by use of capacitively coupled rf and slot antenna microwave plasma, Surf. Coat. Technol. 89 Ž1997. 165᎐176. A. Hellmich, T. Jung, A. Kielhorn, M. Rißland, CVD-processes by hollow cathode glow discharge, Surf. Coat. Technol. 98 Ž1998. 1541᎐01546. D.P. Dowling, K. Donnelly, M. Monclus, M. McGuinness, The use of refractive index as a measure of diamond-like carbon film quality, Diamond Related Mater. 7 Ž1998. 432᎐434. Y. Lifshitz, G.D. Lempert, E. Grossman et al., Optical and photoemission studies of DLC films prepared with a systematic variation of the sp 3 :sp 2 composition, Diamond Related Mater. 6 Ž1997. 687᎐693. S. Vakula, V. Padalka, V. Strelnizki et al., Superharte Mater. N1 Ž1980. 18᎐22. M.A. Tamor, W.C. Vassel, Raman ‘fingerprinting’ of amorphous carbon films, J. Appl. Phys. 76 Ž1994. 3823᎐3830. S.F. Yoon, H. Yang, A. Rusli, J. Ahn, Q. Zhang, The effects of self-generated DC bias on the characteristics of diamond-like carbon films prepared using ECR-CVD, Diamond Related Mater. 7 Ž1998. 70᎐76.