Surface cleaning mechanisms utilizing VUV radiation in oxygen containing gaseous environments

Transcription

1 Surface cleaning mechanisms utilizing VUV radiation in oxygen containing gaseous environments Zoran Falkenstein Ushio America, Inc. Zoran Falkenstein

2 Surface cleaning mechanisms utilizing VUV radiation in oxygen containing gaseous environments Zoran Falkenstein * Ushio America, Inc. ABSTRACT This article presents results on surface cleaning with VUV radiation from dielectric barrier discharge-driven Xe 2 * excimer VUV light sources at 172 nm in oxygen-containing gases. The basic mechanism for the generation of excited rare gas and rare gas/halogen dimers in dielectric barrier discharges is described, which is utilized to generate powerful and efficient, incoherent excimer (V)UV light sources. After a brief discussion of the formation of atomic oxygen and ozone by irradiation of molecular oxygen with VUV light at 172 nm, the dominant chemical reaction scheme in the advanced oxidation of hydrocarbons on surfaces is outlined. Following to a comparison of the reaction rates of atomic oxygen and ozone with hydrocarbons, as well as a discussion of the mean free pathlength of atomic oxygen at or near atmospheric pressure, results on surface cleaning in air with VUV radiation at 172 nm will be presented. Keywords: surface cleaning, excimer, barrier discharge, atomic oxygen, ozone, advanced oxidation 1. INTRODUCTION An important consideration in the field of (semiconductor) surface modification and engineering is the cleanliness of the surface prior to surface treatment. Typically, chemical solvents are used in wet-chemical cleaning processes to remove (organic) contaminations on semiconductor surfaces. However, wet-chemical cleaning techniques require ultra pure chemicals, generate large quantities of organic waste and pose strong adverse health effects on exposed personal. Dry-chemical cleaning alternatives are needed. As a response, plasma physical and plasma chemical processes have emerged as effective methods for surface cleaning below the sub-monolayer level [1-6]. In all plasma-based surface processing techniques, activated species (e.g., ions, electrons, radicals and metastables from the working or background gas) are generated within a non-thermal plasma (usually at low gas pressure) [4,6]. These species are then utilized to induce plasma physical or plasma chemical reactions with hydrocarbons on the semiconductor surface. Unfortunately, surface charging that inevitably accompanies plasma treatment of surfaces can cause severe surface damage [7-9]. Additionally, low gas pressures require long pumping and sample handling times [10], and have not proven viable for semiconductor surface cleaning where small cycle and processing times are required to allow for large-volume production. A superior surface cleaning technique that has been established in the semiconductor and LCD display industry, is the advanced oxidation of surface hydrocarbons by the utilization of VUV radiation in oxygen-containing environments. This technique generates atomic oxygen and hydroxyl radicals - and not ions - by VUV irradiation of oxygen-containing gas mixtures (typically air) at or near atmospheric gas pressure. The various radicals are then used for the (photochemical) oxidation and mineralization of surface hydrocarbons. While low-pressure mercury discharge lamps at 185 and 254 nm are still being used in many cleaning systems, better-suited VUV radiation at 172 nm from Xe 2 * excimer light sources has established in many semiconductor and LCD display manufacturers. The utilization of Xe 2 * excimer VUV radiation at 172 nm in oxygen-containing gases is the subject of this article. * ZFalkenstein@Ushio.com; phone (949) ; fax (949) ; Ushio America, Inc., 14 Mason Drive, Irvine, CA, USA 92618

3 2. EXCIMER FORMATION IN DIELECTRIC BARRIER DISCHARGES Originally only homonuclear dimmers, with a stable electronically excited state but a repulsive ground state, were called excimers (excited dimers) [11]. The term excimer has since been extended to mean any multi-atom molecule (exiplex) with a thermally unstable (repulsive or only very weakly binding) ground state. The best known include rare-gas excimers and rare-gas halogen excimers [11-16]. Rare-gas-halogen trimers, metal excimers, metal rare-gas excimers, metal-halogen excimers and rare-gas-oxygen excimers are also known [11]. To effectively form excimers, three criteria have to be met *. First, the bulk gas has to be provided with excitation energies of several ev (i.e., for rare-gas excimers the electronic excitation energy of the rare-gas; for rare-gas-halogen excimers the ionization energy of the rare-gas). Second, the gas pressure has to be close to atmospheric (100 to 1000 Torr) since the dominant excimer formation channel occurs through three-body reactions. Third, the gas temperature has to be cold since excimers are thermally unstable. An efficient electrical gas discharge that provides non-thermal gas discharge conditions at (or near) atmospheric pressure is the dielectric barrier discharge, also called silent discharge [17]. This type of discharge is obtained when at least one dielectric barrier (e.g., quartz, glass, ceramic) is placed between metallic electrodes that enclose a gas gap on the order a few mm. A schematic of the arrangement is depicted in Figure 1 for flat and double-coaxial tube arrangements. Figure 1 dielectric electrode Schematic of a double barrier arrangement used for ozonizers and excimer (V)UV sources When an electrical field is supplied across the atmospheric-pressure gas gap that is high enough to cause electrical gas breakdown, a gas discharge in the streamer mechanism is formed. Because of the presence of the dielectric barrier, the discharge is quickly terminated (depending on many discharge conditions, typical microdischarge durations are a few ns to 100 s of ns) [15,18,19], and the degeneration of the discharge into a thermal arc discharge is prevented. Instead of the formation of one (thermal) arc, a multitude of short-lived microdischarge filaments is formed which quickly fill the entire gas volume. Typical discharge conditions lead to electron energies of several ev at electron densities in the range of cm -3, while the gas temperature is ambient [20-22]. When discharging rare gases or mixtures of rare-gases and halogens at appropriate total and partial gas pressures, the dominant excimer formation channels can be simplified as follows [11]. Here, Rg stands for rare gas, X for halogen, and M for a third body, * denotes electronically excited states. e - + Rg e - + Rg* ( * ) e - + X 2 X + X - e - + Rg* e - + Rg** e - + X 2 e - + X + X e - + Rg 2 e - + Rg + e - + X + M X - + M Rg + Rg* + M Rg 2 * + M Rg + + X - + M RgX* + M e - + Rg 2 ** Rg 2 * + e - Rg* + X + M RgX* + M * Very similar requirements are given for the generation of ozone.

4 One unique feature of all excimers is that high-level electronically excited states, denoted Rg** and RgX**, cross repulsive states. This leads to the radiationless decomposition and subsequent re-formation of the excimer into its lowest excited electronic state. The excimer funnels to its lowest electronically excited state [11]. From here, the only way to de-excite is the emission of radiation into a repulsive or very-weakly binding ground state. Hence, excimer (V)UV light sources are very spectrally selective (V)UV light sources [12-16]. Figure 2 and Figure 3 show the potential diagram and emission spectra of two representatives for rare-gas and raregas-halogen excimers. Figure 2a and 2b Potential diagram and emission spectrum of the Xe 2 * excimer Figure 3a and 3b Potential diagram and emission spectrum of the XeBr* excimer A variety of gas mixtures can be employed in dielectric barrier discharge excimer (V)UV light sources to produce different, spectrally selective radiation in the VUV and UV spectral range. The most effective systems are Xe 2 * (emission centers at 172 nm), KrCl* (emission peaks at 222 nm), XeBr* (emission peaks at 282 nm) and XeCl* (emission peaks at 308 nm). These excimer UV sources are commercially available at power levels in the range of a few 10 s W to kw with an UV radiant efficiency of about 10% [12-16]. They are used for various cleaning applications in the semiconductor industry, photochemical purification and desinfection of gases [23] and water [24-26], and UV curing of resins and lacquers [27,28]. More recently, they have also been successfully applied in the synthesis of advanced materials [29,30]. In particular in surface processing, where thermally sensitive surfaces are to be irradiated, the non-thermal behavior of excimer (V)UV light sources is of great advantage in comparison to conventional high-pressure discharge lamps.

5 3. RADICAL FORMATION AND ADVANCED OXIDATION OF (NON-HALOGENATED) HYDROCARBONS Advanced oxidation, or cold combustion of hydrocarbons on surfaces of delicate substrates such as semiconductors is accomplished by radical (chain) reactions of mainly atomic singlet oxygen O( 1 D), atomic triplet oxygen O( 3 P), electronically excited molecular oxygen O 2 ( 1 ) and O 2 ( 1 Σ), hydroxyl radicals OH, and ozone. All these species are generated by the excitation or dissociation of molecular oxygen, hence we will discuss the photodissociation of oxygen. The potential diagram, as well as the spectral absorption cross section of molecular oxygen is given in Figures 4a through 4c. Figures 4a, 4b, 4c Potential diagram and absorption cross section of oxygen (units in atm -1 cm -1 ; base e; from [31]) It can be seen that effective dissociation of molecular oxygen can be achieved in the Schumann-Runge band and Schumann-Runge continuum [31], resulting in atomic oxygen in singlet and triplet state. In oxygen-rich gases at high gas pressure (i.e., atmospheric), the dominant subsequent reaction of atomic oxygen will be chemical quenching with molecular oxygen, resulting in the formation of ozone. O( 1 D) or O( 3 P) + O 2 + M O 3 + M The rate of this reaction at room temperature depends on the nature of the third body (k = cm 6 s -1 for M=O 2, k = cm 6 s -1 for M=N 2, k = cm 6 s -1 for M=Ar) [32]. It is important to note that the dominant reaction of atomic oxygen with nitrogen is not the formation of oxides of nitrogen (NO, NO 2, N 2 O, etc.), but rather physical quenching that merely leads to the vibrational excitation of nitrogen. The physical quenching of atomic oxygen, for example, allows the generation of considerable amounts of ozone in air (such as in ozonizers) [33]. With increasing ozone concentration caused by VUV irradiation of the oxygen-rich gas, the dissociation of ozone is gaining importance. The spectral absorption cross section of ozone is given in Figures 5a and 5b [31]. Figures 5a and 5b Spectral absorption cross section of ozone (units in atm -1 cm -1 ; base e; from [31])

6 The dissociation product of ozone in the Hartley band and VUV spectral region, as shown in Figure 5, is again molecular oxygen and atomic singlet oxygen [31]. Hence, the irradiation of oxygen-containing gases leads to the formation of a steady-state level of ozone, corresponding to a dynamic equilibrium of ozone formation and dissociation. To discuss surface cleaning processes, heterogeneous reactions (on or close to the sample surface) have to be considered which occur in the removal of surface hydrocarbons (contaminations). Here, the chemical reaction pathway can be extremely complex, particularly when halogenated hydrocarbons are being oxidized. Still, for nonhalogenated hydrocarbons, the dominant reaction pathway can be simplified as (a) initial attack by a radical (mainly hydrogen abstraction reactions) or direct photolysis, (b) oxidation of the hydrocarbon radical, and (c) fragmentation of the very unstable carbonyl [34]. This chain reaction results in the abstraction of a carbon dioxide and water molecule from the hydrocarbon. As the remaining hydrocarbon fragment is repeatedly attacked, oxidized and fragmented in several reaction cycles, the hydrocarbon will eventually be completely mineralized into carbon dioxide and water [34]. Initial reaction: H-C C-H + OH H-C C + H 2 O (k OH ) H-C C-H + O( 1 D) H-C C + OH (k O ) H-C C-H + O( 3 P) H-C C + OH (k O ) H-C C-H + O 3 H-C C-O + HO 2 (k O3 ) H-C C-H + O 2 ( 1 ) H-C C + HO 2 (k O2 ) H-C C-H + hν H-C C + H Second reaction: H-C C + O 2 + M H-C C-O=O + M H-C C-O=O + M H-C C + CO 2 +M Subsequent reactions: H-C C + O 2 + M CO 2 + H 2 O The crucial reaction step in these chain reaction(s) is the initial radical attack, which is initiated mainly by the photolysis of molecular oxygen (to produce atomic oxygen, ozone, and, in presence of water vapor, hydroxyl radicals). Once the hydrocarbon is converted into a radical, the subsequent reaction(s) with molecular oxygen are endothermic. In that sense, it is essential to understand that the reaction rates k OH and k O for hydroxyl radicals and atomic oxygen are about 100 to 10,000 times higher for almost all (non-halogenated) hydrocarbons than the reaction rates k O3 and k O2 for ozone and molecular singlet oxygen [32,35-38]. As a result, the removal rate (or cleaning rate) of hydrocarbons on surfaces is determined by the amount of atomic oxygen reaching a surface, and not ozone. This result is further complicated by the fact that molecular oxygen is not only a source of fast reacting atomic oxygen, but also a sink of atomic oxygen through the formation of relatively slow-reacting ozone. As a result, it was found in the advanced oxidation of hydrocarbons (toluene and trichloroethylene) at various oxygen concentrations that the removal efficiency initially strongly increases with increasing oxygen content in a balance gas of Ar or N 2 (0 to 0.1% O 2 in Ar), peaks (0.2 to 0.3% O 2 in Ar), and then quickly decreases with further increased oxygen concentrations (1 to 20% O 2 in Ar) [39]. This result indicates that while with increasing oxygen concentration the formation of atomic oxygen increases monotonically, the hydrocarbon is competing with molecular oxygen for the atomic oxygen. At small oxygen concentrations, nearly all the atomic oxygen is claimed by the hydrocarbon while at high oxygen concentrations (even though more atomic oxygen is concentrated than in the oxygen-starved gas) most of the atomic oxygen is claimed by molecular oxygen to produce (inefficient) ozone. In the concentration range of 0.2 to 0.3% O 2 in Ar, the fate of atomic oxygen is optimized for utilization by the hydrocarbon. Following the results of this work [39], it is expected to see a similar behavior for the cleaning rate with VUV/O 2 as a function of the amount of oxygen present in the processing gas (air). The main difference between heterogeneous (surface reactions) and homogeneous (gas phase reactions) removal rates (cleaning rates), is given by the small

7 mean free pathlength of atomic oxygen in atmospheric pressure gases (12 µm in O 2 at 760 Torr) [6]. To effectively attack hydrocarbons on the surface of a sample, a high atomic oxygen concentration has to be generated within the mean free pathlength of atomic oxygen from the sample surface. Powerful (V)UV light sources that emit in the Schumann-Runge band and continuum are low-pressure mercury lamps (185 and 254 nm) and Xe 2 * excimer VUV light sources (172 nm), respectively. For proper choice, we believe it is very important to note that the extinction coefficients of molecular oxygen and ozone are very different at 172 nm, or 185 and 254 nm, respectively. While the extinction coefficient of molecular oxygen and ozone at 172 nm is practically the same, the extinction coefficient of molecular oxygen at 185 nm (which leads to the formation of atomic oxygen from molecular oxygen) is by far smaller than the extinction coefficient of ozone at 185 and 254 nm (which leads to the formation of atomic oxygen from ozone). As a result, according to the law of Lambert-Beer [31], the penetration depth of Xe 2 * excimer VUV radiation at 172 nm, or 185/254 nm in oxygen-containing gases is very different, as shown in Figure 6a for dry air (N 2 /O 2 ratio of 80/20) and Figure 6b for ozone-containing, dry air (N 2 /O 2 /O 3 ratio of 79/20/1). 100 Intensity [%] nm in N2/O2 (80/20) 185/254 nm in N2/O2 (80/20) penetration depth [cm] Figure 6a Propagation of (V)UV at 172 nm and 185/254 nm in dry air 100 Intensity [%] nm in N2/O2/O3 (79/20/1) 185/254 nm in N2/O2/O3 (79/20/1) Figure 6b penetration depth [cm] Propagation of (V)UV at 172 nm and 185/254 nm in dry ozone/air mixtures While VUV at 185 nm travels substantially farther in atmospheric-pressure air (to produce atomic oxygen throughout the irradiated gas volume), VUV at 172 nm is absorbed in a few mm (to produce ozone in a relatively thin gas volume), as shown in Figure 6a. Assuming the radiant power densities of the two sources to be equal, the concentration of atomic oxygen and ozone (not the total amount of atomic oxygen and ozone) is going to be higher for VUV at 172 nm. Hence, the cleaning rate and efficiency using the two (V)UV light sources at equal radiant power density should be higher at 172 nm than at 185/254 nm. At the same time, since the absorption of VUV at 172 nm by ozone is the same as for molecular oxygen, re-utilization of ozone is going to be very effective. This is fundamentally different for 185/254 nm, because of the very strong absorption coefficient of ozone at 254 nm.

8 4. RESULTS AND DISCUSSION Figures of merit for the cleanliness of surfaces are the optical transmission for optically transparent samples, or the contact angle for optically opaque materials. Figure 7 illustrates the contact angle of a droplet of (purified and standardized) water on a polyimide film before and after cleaning with VUV/O 2. Before Contact angle 72º After Contact angle 6º Test condition: Radiation time 80 sec Sample PI film Figures 7a and 7b Contact angle measurements on uncleaned and VUV/O2 cleaned polyimide samples It can be seen that the initial contact angle (which represents some surface contamination of the polyimide film by various organics, as qualified and quantified by GC/MS), can be greatly reduced by irradiation of the processing gas with VUV at 172 nm. The result of a parametric study, where initial contact angle of a water droplet on a Si wafer was measured as function of the irradiation time, is shown in Figure 8. In these experiments, the radiant power density on the window plane of a modified Xe 2 * excimer VUV source (USHIO, UER20H) was measured at 6.7 mwcm -2, and the distance of the Si wafer to the VUV source window was 5 mm. The total gas pressure was controlled between and 1 atmosphere (oxygen to nitrogen ratio of 20/80). Si wafer cleaning results with VUV/O 2 contact angle [ ] p=1 p=0.1 p=0.01 p= irradiation time [s] Figure 8 Irradiation time versus total gas pressure as function of the irradiation distance for VUV/O 2 cleaning of a Si wafer

9 The results show that the contact angle (i.e., the amount of hydrocarbons on the Si wafer surface) decreases for each irradiation condition with increasing irradiation time. The results further indicate an exponential functionality, which would confirm the dominant cleaning mechanism to be of first chemical order [40]. More importantly, it can also be seen from Figure 8 that the contact angle can be more effectively decreased at an air pressure of 0.1 and 0.01 atmospheres, as opposed to working at an air pressure of 1 or atmospheres, respectively. To better see the dependency of the contact angle (i.e., surface cleanliness) as a function of the air gas pressure, the results of another parametric study, where the cleaning time needed to reduce the initial contact angle on a Si wafer from 48 to 6 is plotted against the total air pressure (oxygen to nitrogen ratio of 20/80), is given in Figure 9. Like for the results given in Figure 8, the radiant power density on the window plane of a modified Xe 2 * excimer VUV source (USHIO, UER20H) was 6.7 mwcm -2. The distance of the Si wafer to the VUV source window was 1 and 5 mm, respectively. The atmosphere between the Si wafer and the VUV source plane was again dry air, controlled at a total gas pressure of to 1 atmosphere. Si wafer cleaning with VUV/O irradiation time [s] d=1 mm d=5 mm pressure [atmospheres] Figure 9 Irradiation time versus total air pressure as function of the irradiation distance for VUV/O 2 cleaning of a Si wafer The results of these experiments show that the irradiation time at a given distance of the Si wafer surface to the VUV source window, is a strong function of the total gas pressure (air). With VUV irradiation of the silicon wafer and the gas (air) at atmospheric gas pressure, a processing time of 70 seconds (VUV source window Si wafer distance of 5 mm) or 20 seconds (VUV source window Si wafer distance of 1 mm) are needed to reduce the surface contact angle from 48 to 6, respectively. When reducing the total air gas pressure to 7.6 or 76 Torr (or 0.01 or 0.1 atmospheres), the irradiation time can be reduced to 5 to 10 seconds to achieve the same cleaning result, corresponding to an increased cleaning efficiency and cleaning rate. Further reduction of the total gas pressure to 760 mtorr (or atmospheres) leads to increased cleaning times of 50 (VUV source window Si wafer distance of 5 mm) or 65 seconds (VUV source window Si wafer distance of 1 mm). This behavior can be explained by considering that the amount of atomic oxygen, the quenching of atomic oxygen by molecular oxygen to produce ozone, as well as the mean free pathlength of atomic oxygen in air all depend on the total gas pressure. While with decreasing total gas pressure the absolute amount of atomic oxygen is decreased, ozone formation is reduced, and the mean free pathlength is increased (λ=σ n -1, where λ is the mean free pathlength, σ the cross section of atomic oxygen, and n the particle density). When decreasing the total gas pressure, starting from atmospheric pressure gas, the amount of atomic oxygen reaching the Si wafer surface is initially increased. This is due to both reduced ozone formation, and an increased mean free pathlength of atomic oxygen in the gas. As a result, the cleaning rate (and efficiency) is increased. When further reducing the air pressure, the gas is starved for oxygen, and even though the mean free pathlength is high, less atomic oxygen is reaching the Si wafer surface.

10 5. CONCLUSION The photochemical cleaning of surfaces with VUV irradiation at 172 nm in oxygen-containing gases (i.e. air) has established itself in the LCD and semiconductor processing industry, and is emerging in other surface cleaning applications to replace wet-chemical cleaning steps. For these applications, uniform and large-area (V)UV sources are needed that lead to effective formation of atomic oxygen, which has proven to be the key species in the cleaning mechanism of (non-halogenated) hydrocarbons on such surfaces. It has been rationalized that the absorption of molecular oxygen (to produce atomic oxygen) and ozone (to re-utilize oxygen to produce atomic oxygen) at 172 nm is better suited than 185/254 nm for producing high atomic oxygen concentrations at distances within the mean free pathlength away from the sample surface. Results on Si surface cleaning with VUV irradiation of dry air at 172 nm show that the total gas pressure and distance of the sample to VUV source window distance have a major impact on cleaning speed and efficiency. ACKNOWLEDGEMENTS The author would like to acknowledge the help from our colleagues at Ushio Inc. and the useful discussions that lead to this publication. REFERENCES 1. M. Zeuner, J. Meichsner, and H.-U. Poll, Plasma Sources Science & Technol. 4, pp. 406, E. J. H. Collart, J. A. G. Baggerman, and R. J. Visser, J. Appl. Phys. 78, pp. 47, J. Robertson, Pure & Appl. Chem. 66, pp. 1789, J. L. Vossen and W. Kern, Thin Film Processes II, Academic Press, D. B. Graves, IEEE Trans. Plasma Sci. 22, pp. 31, M. A. Liebermann and A. J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, John Wiley and Sons, NY, S. Ma and J. P. McVittie, J. Vac. Sci. Technol. B 14, pp. 566, S. Ma, J. P. McVittie, and K. C. Saraswat, IEEE El. Dev. Lett. 16, pp. 534, S. Fang and J. P. McVittie, Trans. El. Dev. 41, pp. 1034, D. J. Rej and R. B. Alexander, J. Vac. Sci. Technol. B 12, pp. 2380, Ch. K. Rhodes, Excimer Lasers, Topics in Appl. Phys. 30, Springer Verlag, B. Gellert, Contr. Plasma Phys. 31, B. Gellert and U. Kogelschatz, Appl. Phys. B 52, pp. 14, U. Kogelschatz, Appl. Surf. Sci. 54, pp. 410, K. Stockwald and M. Neiger, Contr. Plasma Phys. 35, pp. 15, Z. Falkenstein and J.J. Coogan, J. Phys. D: Appl. Phys. 30, pp. 2704, E. Warburg, Ann. d. Phys. 13, pp. 464, Z. Falkenstein and J.J. Coogan, J. Phys. D: Appl. Phys. 30, pp. 817, Z. Falkenstein, J. Appl. Phys. 81, pp. 5975, M. D. Koretsky and J. A. Reimer, J. Appl. Phys. 72, pp. 5081, T. J. Sommerer and M. J. Kushner, J. Appl. Phys. 71, pp. 1654, E. J. H. Collart, J. A. G. Baggerman, and R. J. Visser, J. Appl. Phys. 78, pp. 47, H. Scheytt, H. Esrom, L. Prager, R. Mehnert, and C. von Sonntag, in Proceedings of the NATO Advanced Research Workshop on Non-Thermal Plasma Techniques for Pollution Control, Springer, Berlin, pp. 91, J. R. Bolton, and S. R. Carter, in Aquatic and Surface Photochemistry, Lewis Publishers, Boca Raton, FL., pp. 467, O. Legrini, E. Oliveros, and A. M. Braun, Chem. Rev. 93, pp. 671, F. J. Beltran, G. Ovejero, and B. Acedo, Wat. Res. 27, pp. 1013, H. Boettcher, J. Bendig, M. A. Fox, G. Hopf, and H.-J. Timpe, in Technical Applications of Photochemistry, Interdruck Leipzig, R. S. Nohr, J. G. MacDonald, Radiat. Phys. Chem. 46, J.-Y. Zhang, H. Esrom, and I. W. Boyd, Appl. Surf. Sci. 96, 1996.

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