Characterization of particle contamination in process steps during plasma-enhanced chemical vapor deposition operation

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1 Aerosol Science 34 (2003) Characterization of particle contamination in process steps during plasma-enhanced chemical vapor deposition operation Heru Setyawan, Manabu Shimada, Yuji Imajo, Yutaka Hayashi, Kikuo Okuyama Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Kagamiyama, Higashi-Hiroshima , Japan Received 7 October 2002; accepted 8 March 2003 Abstract The occurrence time and the contribution level of particle contamination on the wafer in individual steps during plasma-enhanced chemical vapor deposition (PECVD) operation were investigated. A method was proposed to determine the occurrence time of particle contamination by making use of the capability of thermophoresis to shield the wafer from particle deposition. The level of particle contamination on the wafer was determined by a scanning electron microscopy (SEM) and the particle behavior in the reactor was observed using a laser light scattering (LLS) technique. The particles were continuously injected into the plasma reactor from the outside. Using this technique, the eect of particle size on the particle behavior can be studied with high certainty. It was found that the particle contamination occurred during the postplasma when the injected particles were trapped in the sheath region below the powered electrode. On the other hand, when the injected particles were not trapped due to a strong inertial eect of particle, the contamination occurred during plasma operation. There is a regime of operation condition in which the lowest level of contamination occurs. Most particles retained their negative charge in the postplasma as shown by their movement and deposition on the wafer in the presence of either a negative or positive dc eld. The charge on these particles was determined from particle motion with high certainty using the current experimental technique.? 2003 Elsevier Ltd. All rights reserved. 1. Introduction Particle contamination on wafers in plasma-assisted processes for semiconductor manufacturing is known to have harmful eects on device morphology, performance, reliability and yield. The particle contamination may occur under continuous plasma operation or at the end of the process when the plasma is turned o (Selwyn, Singh, & Bennett, 1989). During plasma operation, the particles Corresponding author. Tel.: ; fax: address: smd@hiroshima-u.ac.jp (M. Shimada) /03/$ - see front matter? 2003 Elsevier Ltd. All rights reserved. doi: /s (03)

2 924 H. Setyawan et al. / Aerosol Science 34 (2003) are negatively charged and are electrostatically trapped in localized regions at the plasma/sheath boundaries. Individual particles may grow so large that they can no longer be trapped. Barnes, Keller, Forster, O Neill, and Coultas (1992) showed that if the growing particles were trapped at the topwall surface, they would traverse the plasma to the bottom electrode where they would hover due to force balance between the electrostatic and gravitational forces. If they continued to grow, the gravitational force pulled the particles closer to the bottom wall sheath. There they reached a region where the electron density was low. At this point, most of the particle s negative charge was neutralized and they dropped to the wafer. At the end of the process step, the plasma is extinguished and the eect of electric eld on the particles decays very rapidly in the postplasma. As a result, the particles are released from the traps and may deposit on the wafer due to external forces, e.g., neutral gas drag, thermophoretic and gravity. The particle transport and behavior in the postplasma regime have been investigated by several researchers (Collins, Brown, O Hanlon, & Carlile, 1995, 1996; Yeon, Kim, & Whang, 1995; Garrity, Peterson, Garrett, & O Hanlon, 1995; Choi, Rader, & Geller, 1996). The role of various forces including thermophoretic, Coulomb, gravitational and neutral drag on the particle transport during postplasma was elucidated. Collins et al. (1996) observed that particles in the postplasma could have positive, negative and neutral charge that diers from the observation of Yeon et al. (1995) in which the particles had no charge. Garrity et al. (1995) and Choi et al. (1996) carried out numerical simulation of particle transport in the postplasma in order to evaluate particle deposition on the wafer during this period. However, the experimental data concerning with particle deposition on the wafer in the postplasma and the relationship with the plasma operating condition has never been reported. In addition, the contribution level of particle contamination on the wafer from individual steps of wafer processing in plasma-assisted operation, i.e. plasma operation and postplasma, has not been elucidated although it has been known that both steps can contribute to particle contamination. Knowledge about the contribution level of particle contamination on the wafer from individual steps during plasma processing would be very useful for the proper design of devices and of processes to control particle contamination. This work investigates the contribution level of particle contamination on the wafer placed on the grounded electrode from individual steps during wafer processing in plasma reactor. A method to determine the occurrence time of particle contamination on the wafer in individual steps of plasma operation is proposed by making use of the capability of thermophoresis to shield the wafer from particle deposition. In the previous work, we showed that typically there were two modes of particle trap found during plasma operation referred to as the lumping mode and the winding mode (Setyawan, Shimada, & Okuyama, 2002b). The particle trap modes were found to be dependent on plasma operating conditions and were supposed to inuence particle contamination on the wafer. In addition, experiments were carried out to investigate the role of the Coulomb force and the sign of the charge on particles in the postplasma. 2. Experimental works The schematic diagram of the experimental set up is shown in Fig. 1. The experiments were carried out in a parallel-plate type plasma-enhanced chemical vapor deposition (PECVD) reactor. The reactor used has been described in detail elsewhere (Setyawan, Shimada, Ohtsuka, & Okuyama,

3 H. Setyawan et al. / Aerosol Science 34 (2003) Fig. 1. Schematic diagram of the experimental set up and the LLS system to observe the particle motion in the PECVD reactor. 2002a; Setyawan et al., 2002b). Briey, it consists of two cylindrical plates 200 mm in diameter separated by 35 mm. The upper plate is in showerhead conguration where the gas ows into the reactor and is coupled to a 13:6 MHz rf power supply. The lower plate is grounded and equipped with an electric heater. Its temperature can be controlled by a controller. Additional circuitry allows a dc bias voltage to be applied on the grounded electrode when the rf supply is turned o. An inlet gas tube tted through one of the anges on the chamber sidewall is added to introduce nitrogen gas at the end of each experimental run in order to increase the reactor pressure slowly. This inlet scheme was designed in such a way so that particle deposition on the wafer during this step could be avoided. The plasma reactor was seeded with monodisperse silica particles injected from the outside through the showerhead. The particles were dispersed into the gas in a vibrating tube prior to injecting into the reactor. The vibrating tube aerosol generator has been described previously (Setyawan et al., 2002b). The distribution and movement of the particles in the plasma reactor were observed using a laser light scattering (LLS) technique. A laser beam derived from an Ar + laser (Model 2017, Spectra physics) was used to illuminate particles in the space between electrodes. The laser beam was expanded using a beam expander so that a three-dimensional structure of particle motion in the space between electrodes could be obtained. A high-resolution charge coupled device (CCD) video camera (DCR-VX2000, Sony) was used to detect the light scattered by the particles. Scanning electron microscopy (SEM) was used to observe the particles deposited on the wafer. The basic experimental procedure is described in the following. The reactor walls and all lines were cleaned up and washed before each experiment. This is to ensure that particles do not deposit

4 926 H. Setyawan et al. / Aerosol Science 34 (2003) Fig. 2. Position of wafers on the grounded electrode in the determination of the contribution level and the occurrence time of particle contamination. Fig. 3. Transition condition of particle trap mode as a function of plasma operating condition. L/W is the transition from the lumping mode to the winding mode. W/E is the transition from the winding mode to the escaping mode. Error bars show the uncertainty in the observation. on the wafer during the initial step of operation, i.e., during evacuating the reactor to the specied pressure. The reactor was evacuated to a pressure of 4 Torr using a dry pump (A-30, Ebara Co. Ltd.). In this step, no particle was injected into the reactor. This was done by passing the nitrogen gas through a bypass line by opening valve V2 and closing valve V1. After a steady condition was achieved, the rf power was turned on to ignite the plasma and the particles were injected into the reactor by opening valve V1 and closing valve V2. The particle injection took place for about 3 min to allow stable particle clouds formed below the powered electrode (Setyawan et al., 2002b). Then, the rf power was turned o and the particle motion in the postplasma was recorded using a CCD camera. The particle injection was stopped during the postplasma. Particles deposited on three silicon wafers of 5:0 mm 5:0 mm in size placed on the grounded electrode were observed by SEM. The wafer positions are shown in Fig Results and discussion Particle behavior below the powered electrode during plasma operation for various conditions has been reported previously (Setyawan et al., 2002b). Briey, two modes of particle trap were observed. We referred them to as the lumping and the winding modes. The lumping mode was characterized

5 H. Setyawan et al. / Aerosol Science 34 (2003) by the big particle clouds localized in the discrete regions between the showerhead holes, and the particles entering the reactor do not overshoot the equilibrium position of the particle trap. This mode occurs typically at conditions of relatively high rf power and low gas ow rate. The winding mode was characterized by the small and thin particle clouds, and the particles entering the reactor overshoot the equilibrium position of the particle trap. This mode occurs typically at conditions of relatively low rf power and high gas ow rate. Upon increasing the gas ow rate further, the electric forces responsible for the particle trap was not able to trap the particles below the powered electrode due to the very high particle inertia. This mode has not been reported in the previous study. Hereafter, we will refer this mode to as the escaping mode. The condition in which the particle trap mode changes from one to another is shown in Fig. 3. The error bar shows the uncertainty in the observation of the change of particle trap mode. In the next section, the particle behavior in the postplasma will be presented. The behavior in the postplasma can be divided into two regimes, namely early postplasma and late postplasma. The early postplasma refers to the period of a few milliseconds after turning o the plasma and the rest is referred to as the late postplasma. Then, the occurrence time of particle contamination and the contribution level from individual steps on the wafer placed on the grounded electrode, and the relationships with the particle trap mode and the plasma operating condition will be presented. Finally, an estimation of residual particle charges in the postplasma based on terminal velocities of the clouds for the case of unbiased and positive bias will be discussed Particle behavior in the postplasma The particles are attracted to the powered electrode once the rf power is turned o as shown by the video images in Fig. 4 and then halt before reaching the electrode surface. The time-resolved image shown in Fig. 4 is for the condition of rf power 100 W, particle size 0:6 m and gas ow rate 50 sccm, and the particles are trapped in the lumping mode during plasma. A similar phenomenon was also observed for the winding mode. The reason of the behavior may be explained as follows. During plasma processing, the electrode surface has negative self-bias voltage. When the rf power is turned o, the negative self-bias voltage is retained for several milliseconds. Moreover, the electron temperature rapidly decreases to gas temperature. Consequently, the electron current to the electrode is zero because the repulsive barrier of the electrode that has residual negative self-bias voltage is much larger than the electron energy. Hence, only positive ion will ow to the electrode surface. During their transport to the electrode surface, the ions collide with the particles and drag them toward the electrode. The collisions also make the particles lose some of their negative charges. However, in this very short time, the ion collisions with the particles are insucient to neutralize all of the particle charge as shown by the particle movement in the presence of a dc eld during the postplasma period that will be discussed later. After the eect of ion drag force on the particles disappears, the particles fall down under the inuence of gas ow and gravity. Fig. 5 shows typical time-resolved images of the particle movement in the late postplasma for the lumping and the winding modes. The plasma condition is the same as before except the gas ow rate is 200 sccm for the winding mode. For the lumping mode, we can observe that the particle clouds fall down towards the wafer. When the gas ow rate is low, the dominant ow direction in the space between electrodes is downward. Thus, it is not surprising that the particles are carried towards the wafer. On the other hand, for the winding mode, the particles

6 928 H. Setyawan et al. / Aerosol Science 34 (2003) Fig. 4. Particle behavior in the early postplasma showing the movement of particles attracted to the electrode for a rf power of 100 W and a gas ow rate of 50 sccm (lumping mode). are swept away from the wafer and go to the exhaust port. When the gas ow rate is high, the dominant ow direction in the space between electrodes is radially outward except in the region near the centerline (Setyawan et al., 2002a). In order to determine the charge on particles in the postplasma, either negative or positive dc bias voltage was applied on the grounded electrode after extinguishing the plasma. Fig. 6 shows

7 H. Setyawan et al. / Aerosol Science 34 (2003) Fig. 5. Time-resolved image of particle motion in the late postplasma for a rf power of 100 W and gas ow rates of (a) 50 sccm (lumping mode), and (b) 200 sccm (winding mode). the time-resolved images of particle movement in the postplasma for the negative (a) and positive (b) dc bias voltage of 300 V. The particle trap is in the lumping mode with the condition of the rf power 100 W and the gas ow rate 50 sccm. We can observe that the particles are repelled away from the grounded electrode for the case of negative bias and are attracted to the grounded electrode for the case of positive bias. These suggest that most particles retain their negative charge in the postplasma. In addition, SEM observations on the wafers placed on the grounded electrode showed that more particles were deposited on the wafer for the case of positive bias compared to the case of unbiased and negative bias as shown in Fig. 7. For the case of negative bias, particle deposition could not be observed. These also suggest that the residual charges on particles in the postplasma are mostly negative.

8 930 H. Setyawan et al. / Aerosol Science 34 (2003) Fig. 6. Time-resolved image of particle motion in the late postplasma for a rf power of 100 W and a gas ow rate of 50 sccm (lumping mode) with an applied dc bias of (a) 300 V, and (b) +300 V. Fig. 7. SEM photographs of particles deposited on the wafer for a rf power of 100 W and a gas ow rate of 50 sccm (lumping mode) for the cases of (a) negative dc bias of 300 V, (b) unbiased, and (c) positive bias of 300 V Occurrence time of particle contamination In order to determine the contribution level and the occurrence time of particle contamination in individual steps during plasma processing, particles deposited on the wafers were observed using SEM for the conditions of no heating and heating the grounded electrode to 300 C during the postplasma. All experimental data of particle deposition reported herein are for silica particles of size 0:6 m. We have shown previously using an LLS technique that particles were repelled away from the grounded electrode by heating the electrode to 300 C(Setyawan et al., 2002a). Numerical

9 H. Setyawan et al. / Aerosol Science 34 (2003) simulation by taking into account detailed conguration of the reactor revealed that no particle was deposited on the wafer for this condition. Hence, we used this nding as a basis of experiment to determine the occurrence time of particle deposition. Before we used this method, we carried out experiments at the condition of heating the grounded electrode to 300 C in the entire process, i.e. during plasma operation and in the postplasma, to conrm that particle deposition does not occur for any conditions used in this study. In these experiments, after the temperature of the grounded electrode has reached 300 C, plasma was ignited, and then particles were injected. We conrmed that no particle was deposited on the wafers by heating the electrode to 300 C for any experimental conditions used in this study. The temperature gradient created in the gas by heating the grounded electrode to 300 C is about 50 K=cm. In order for the particles to be repelled away from the wafer, the thermophoretic velocity should exceed the terminal settling velocity. The thermophoretic velocity is expressed as v th = K T T; (1) where T is the temperature gradient, is the kinematic viscosity of the gas, and K is the thermophoretic coecient. The thermophoretic coecient can be expressed as (Talbot, Cheng, Schefer, & Willis, 1980) ( +2:20Kn)[1+Kn{1:2+0:41 exp( 0:88=Kn)}] K =2:294 : (2) (1+3:438Kn)(1+2 +4:40Kn) Here, is the thermal conductivity ratio of gas to particle, and Kn is Knudsen number. The terminal settling velocity is expressed as v t = d2 p p gc c 18 ; (3) where d p is particle diameter, p is particle density, g is the gravitational constant, C c is Cunningham correction factor, and is viscosity of gas. The Cunningham correction factor can be expressed as C c =1+Kn[1:257+0:4 exp( 1:1=Kn)]: (4) For 0:6-m silica particles used in the deposition experiment and using = 0:023, the thermophoretic velocity is estimated to be 1:7 cm=s. Assuming p =2:3 g=cm 3 yields v t =0:035 cm=s. This value is 50 times smaller than the thermophoretic velocity. Hence, we can expect that the thermophoresis due to the temperature gradient created by heating the grounded electrode to 300 C can shield the wafer from particle deposition. Therefore, particles deposited on the wafer for the case of heating the grounded electrode to 300 C can be considered to occur only during plasma operation because the thermophoresis shield the wafer from particle deposition in the postplasma. For the case of no heating, the particle deposition can occur during both plasma operation and postplasma. Hence, we deduce that the occurrence time of particle contamination can be determined by this method. The particle deposition varies with the radial position of the wafer. As expected, a higher particle deposition is observed at the wafer placed in the center. Fig. 8(a) (c) shows the SEM photographs of silica particles deposited on the wafer for the lumping, winding, and escaping modes, respectively. The average value of the number of particles deposited on the wafers correspond to Fig. 8 is presented in Fig. 9. The operating conditions for the lumping and the winding modes are the same as before. For the escaping mode, the gas ow rate is 350 sccm using the same level of rf power. The lowest

10 932 H. Setyawan et al. / Aerosol Science 34 (2003) Fig. 8. SEM photographs of particles deposited on the wafer for the cases of (a) lumping mode (100 W, 50 sccm), (b) winding mode (100 W, 200 sccm), and (c) escaping mode (100 W, 350 sccm). level of particle contamination is obtained for the winding mode and the highest level is for the escaping mode. The particle contamination is found to occur during the postplasma for the lumping and the winding modes whereas it occurs during the plasma operation for the escaping mode. For the lumping and the winding modes, the balance of electrostatic and ion drag forces acting on the particles are able to trap the particles in the region below the powered electrode. Some particles leak from the trap through the edge of the powered electrode. The trapped particles fall down to the wafer and contaminate it when the plasma is turned o. Because the particle concentration in trapfor the lumping mode is higher than that in the winding mode (Setyawan et al., 2002b) and the dominant gas ow direction in the lumping mode is downward, it is not surprising that the particle deposition in the lumping mode is higher than that in the winding mode. For the case of the escaping mode, the particle deposition on the wafer does not occur in the postplasma because there is no particle trapped just before turning o the rf power. As shown earlier, the particle movement in the postplasma is inuenced by the presence of a dc eld. Fig. 10 presents the number of particles deposited on the wafer for dierent dc bias voltage applied on the grounded electrode during the postplasma for the lumping mode. The rf power is 100 W and the gas ow rate is 50 sccm. When the negative bias of 300 V was applied on the grounded electrode, particle deposition on the wafers cannot be observed. On the other hand, a positive bias of 300 V increases the number of particles deposited on the wafers over unbiased voltage by approximately three times. These suggest that the dominant charge sign on the particles in the postplasma is clearly negative Estimation of residual particle charges in the postplasma In order to estimate the charge on particles in the postplasma, the cloud displacement as a function of time was plotted for the unbiased and positive dc bias cases. Particle cloud displacement as a function of time is shown in Fig. 11. We can observe that the cloud displacement is approximately linear both for the unbiased and positive bias with the slopes representing terminal velocities of the particle clouds. The terminal velocity obtained from the plot takes the values of about 13.2 and 15:3 mm=s for the unbiased and positive bias, respectively. Then, a force balance on the particles

11 H. Setyawan et al. / Aerosol Science 34 (2003) Fig. 9. Number of particles deposited on the wafer placed on the grounded electrode as a function of particle trap mode. The contribution level of particle contamination from individual steps, i.e. during plasma operation and during the postplasma, is displayed. The gas ow rate is 50, 200, and 350 sccm for the lumping, the winding and the escaping modes, respectively with a constant rf power of 100 W. Fig. 10. The inuence of dc bias voltage on the number of particles deposited on the grounded electrode for a rf power of 100 W and a gas ow rate of 50 sccm (lumping mode). Fig. 11. Particle cloud displacement after extinguishing the plasma for the cases of unbiased and positive bias of 300 V. The gas ow rate is 50 sccm and the rf power is 100 W (lumping mode). for the unbiased and positive bias was derived to estimate the charge from the terminal velocities of the particle clouds. Without an applied dc bias, the force balance for particles can be expressed as dv m p dt = F g + F D ; (5)

12 934 H. Setyawan et al. / Aerosol Science 34 (2003) where v is particle velocity, F g is the gravitational force, and F D is the drag force. With a dc bias applied, the force balance on the particles becomes m p dv dt = F g + F D F E ; (6) where F E is the Coulomb force on the charged particles due to the applied dc eld. Because of the very short relaxation time of particle, the particle clouds can be assumed to reach their terminal velocity very rapidly. Combining Eqs. (5) and (6) at the terminal velocity yields F D (v t) F D (v t )=F E ; (7) where F D (v t) and F D (v t ) are the drag forces on the particles at terminal velocity for the cases of the positive bias and unbiased, respectively. The drag force can be assumed to follow the Stokes drag law and is expressed as F D = 3d p(u v) C c ; (8) where u is the gas velocity. The Coulomb force is given by F E = pee; (9) where p is number of elementary unit of charge, e is elementary unit of charge, and E is the intensity of electric eld. Solving for the number of charges per particle by assuming that the gas velocity is equal for the case of unbiased and positive bias yields p = 3d p C c ee (v v ): (10) The electric eld intensity due to a dc bias voltage applied on the grounded electrode in the postplasma can be assumed to be linear. When the rf power is turned o, electrons disappear by diusion to the walls and attachment to gas molecules in a very short time. The characteristic time is less than 1 ms (Childs & Gallagher, 2000). Then, the electron density and temperature are too low for any signicant electrostatic elds to exist in the postplasma. Therefore, the electric eld exist in the postplasma is solely due to the applied dc bias voltage. It takes the value of about 85 V=cm for a dc bias voltage of 300 V applied on the grounded electrode. Using Eq. (10), the number of charges per particle is about 11.4 electron charges. This is in the range of particle charges predicted by Collins et al. (1996) that made an estimate for the residual charge in the postplasma to be electron charges for particles ranging in size from 200 to 600 nm. In their experiments, Collins et al. (1996) used particles that were generated in the system by sputtering. Because the particles generated by this method are not monodisperse, but have a distribution on particle size, the predicted charge number has uncertainty due to the uncertainty in the particle size. This diers from our technique in which the particles are injected from the outside so that it enables us to use a well-dened particle

13 H. Setyawan et al. / Aerosol Science 34 (2003) size. Therefore, the uncertainty in particle charge prediction due to the uncertainty in particle size can be surely eliminated. 4. Conclusions It has been demonstrated that the proposed method has been used successfully to determine the occurrence time and the contribution level of particle contamination in individual steps during operation of a PECVD reactor. The proposed method is based on the capability of thermophoresis due to a temperature gradient to shield the wafer from particle deposition. It is found that the particle contamination occurs during the postplasma for the lumping and the winding modes whereas it occurs during the plasma operation for the escaping mode. The highest level of particle contamination occurs in the escaping mode and the lowest level is in the winding mode. Observation of the particle movement in the presence of a dc eld using LLS technique and particle deposition on the wafer using SEM gives evidence that most particles in the postplasma retained their negative charge. The injection of particles into the plasma from the outside has been proven very useful in studying the eect of particle size on particle behavior and transport in a PECVD reactor, both during plasma operation and in the postplasma period. We have shown that the uncertainty in the prediction of residual particle charge in the postplasma due to the uncertainty in particle size could be reduced using this experimental technique. Acknowledgements This work has been supported in part by Innovation Plaza Hiroshima of the Japan Science and Technology Corporation (JST). The nancial support provided by the QUE project, Department of Chemical Engineering, Sepuluh Nopember Institute of Technology (ITS), Indonesia for H. Setyawan is gratefully acknowledged. References Barnes, M. S., Keller, J. H., Forster, J. C., O Neill, J. A., & Coultas, D. K. (1992). Transport of dust particles in glow-discharge plasmas. Physics Review Letters, 68, Childs, M. A., & Gallagher, A. (2000). Plasma charge-density ratios in a dusty plasma. Journal of Applied Physics, 87, Choi, S. J., Rader, D. J., & Geller, A. S. (1996). Massively parallel simulations of Brownian dynamics particle transport in low pressure parallel-plate reactors. Journal of Vacuum Science and Technology A, 14, Collins, S. M., Brown, D. A., O Hanlon, J. F., & Carlile, R. N. (1995). Postplasma particle dynamics in a gaseous electronics conference reference cell. Journal of Vacuum Science and Technology A, 13, Collins, S. M., Brown, D. A., O Hanlon, J. F., & Carlile, R. N. (1996). Particle trapping, transport, and charge in capacitively and inductively coupled argon plasmas in a gaseous conference reference cell. Journal of Vacuum Science and Technology A, 14, Garrity, M. P., Peterson, T. W., Garrett, L. M., & O Hanlon, J. F. (1995). Fluid simulations of particle contamination in postplasma processes. Journal of Vacuum Science and Technology A, 13, Selwyn, G. S., Singh, J., & Bennett, R. S. (1989). In situ laser diagnostic studies of plasma-generated particulate contamination. Journal of Vacuum Science and Technology A, 77,

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