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

Aerosol Science 34 (2003) 923–936www.elsevier.com/locate/jaerosci

Characterization of particle contamination in process stepsduring plasma-enhanced chemical vapor deposition operationHeru Setyawan, Manabu Shimada∗, Yuji Imajo, Yutaka Hayashi, Kikuo Okuyama

Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University,1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, 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 stepsduring plasma-enhanced chemical vapor deposition (PECVD) operation were investigated. A method wasproposed to determine the occurrence time of particle contamination by making use of the capability ofthermophoresis to shield the wafer from particle deposition. The level of particle contamination on the waferwas determined by a scanning electron microscopy (SEM) and the particle behavior in the reactor was observedusing a laser light scattering (LLS) technique. The particles were continuously injected into the plasma reactorfrom the outside. Using this technique, the e;ect of particle size on the particle behavior can be studied withhigh certainty. It was found that the particle contamination occurred during the postplasma when the injectedparticles were trapped in the sheath region below the powered electrode. On the other hand, when the injectedparticles were not trapped due to a strong inertial e;ect of particle, the contamination occurred during plasmaoperation. There is a regime of operation condition in which the lowest level of contamination occurs. Mostparticles retained their negative charge in the postplasma as shown by their movement and deposition on thewafer in the presence of either a negative or positive dc <eld. The charge on these particles was determinedfrom 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 isknown to have harmful e;ects on device morphology, performance, reliability and yield. The particlecontamination may occur under continuous plasma operation or at the end of the process when theplasma is turned o; (Selwyn, Singh, & Bennett, 1989). During plasma operation, the particles

∗ Corresponding author. Tel.: +81-824-24-7717; fax: +81-824-24-7850.E-mail address: [email protected] (M. Shimada).

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924 H. Setyawan et al. / Aerosol Science 34 (2003) 923–936

are negatively charged and are electrostatically trapped in localized regions at the plasma/sheathboundaries. 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 thetop wall surface, they would traverse the plasma to the bottom electrode where they would hoverdue 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 aregion where the electron density was low. At this point, most of the particle’s negative charge wasneutralized and they dropped to the wafer.

At the end of the process step, the plasma is extinguished and the e;ect of electric <eld onthe particles decays very rapidly in the postplasma. As a result, the particles are released from thetraps and may deposit on the wafer due to external forces, e.g., neutral gas drag, thermophoreticand gravity. The particle transport and behavior in the postplasma regime have been investigated byseveral 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 variousforces including thermophoretic, Coulomb, gravitational and neutral drag on the particle transportduring postplasma was elucidated. Collins et al. (1996) observed that particles in the postplasmacould have positive, negative and neutral charge that di;ers 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 outnumerical simulation of particle transport in the postplasma in order to evaluate particle deposition onthe wafer during this period. However, the experimental data concerning with particle deposition onthe wafer in the postplasma and the relationship with the plasma operating condition has never beenreported. In addition, the contribution level of particle contamination on the wafer from individualsteps of wafer processing in plasma-assisted operation, i.e. plasma operation and postplasma, has notbeen 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 stepsduring plasma processing would be very useful for the proper design of devices and of processes tocontrol particle contamination.

This work investigates the contribution level of particle contamination on the wafer placed onthe grounded electrode from individual steps during wafer processing in plasma reactor. A methodto determine the occurrence time of particle contamination on the wafer in individual steps ofplasma operation is proposed by making use of the capability of thermophoresis to shield the waferfrom particle deposition. In the previous work, we showed that typically there were two modes ofparticle 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 onplasma operating conditions and were supposed to inKuence particle contamination on the wafer. Inaddition, experiments were carried out to investigate the role of the Coulomb force and the sign ofthe charge on particles in the postplasma.

2. Experimental works

The schematic diagram of the experimental set up is shown in Fig. 1. The experiments werecarried 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,

H. Setyawan et al. / Aerosol Science 34 (2003) 923–936 925

Fig. 1. Schematic diagram of the experimental set up and the LLS system to observe the particle motion in the PECVDreactor.

2002a; Setyawan et al., 2002b). BrieKy, it consists of two cylindrical plates 200 mm in diameterseparated by 35 mm. The upper plate is in showerhead con<guration where the gas Kows into thereactor and is coupled to a 13:6 MHz rf power supply. The lower plate is grounded and equippedwith an electric heater. Its temperature can be controlled by a controller. Additional circuitry allowsa dc bias voltage to be applied on the grounded electrode when the rf supply is turned o;. An inletgas tube <tted through one of the Kanges on the chamber sidewall is added to introduce nitrogengas at the end of each experimental run in order to increase the reactor pressure slowly. This inletscheme was designed in such a way so that particle deposition on the wafer during this step couldbe avoided.

The plasma reactor was seeded with monodisperse silica particles injected from the outside throughthe showerhead. The particles were dispersed into the gas in a vibrating tube prior to injecting intothe 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 us-ing a laser light scattering (LLS) technique. A laser beam derived from an Ar+ laser (Model2017, Spectra physics) was used to illuminate particles in the space between electrodes. The laserbeam was expanded using a beam expander so that a three-dimensional structure of particlemotion in the space between electrodes could be obtained. A high-resolution charge coupled de-vice (CCD) video camera (DCR-VX2000, Sony) was used to detect the light scattered by theparticles. Scanning electron microscopy (SEM) was used to observe the particles deposited on thewafer.

The basic experimental procedure is described in the following. The reactor walls and all lineswere cleaned up and washed before each experiment. This is to ensure that particles do not deposit


Fig. 2. Position of wafers on the grounded electrode in thedetermination of the contribution level and the occurrencetime of particle contamination.

Fig. 3. Transition condition of particle trap mode as afunction of plasma operating condition. L/W is the tran-sition from the lumping mode to the winding mode. W/Eis the transition from the winding mode to the escapingmode. 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 speci<edpressure. 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 nitrogengas through a bypass line by opening valve V2 and closing valve V1. After a steady condition wasachieved, the rf power was turned on to ignite the plasma and the particles were injected into thereactor by opening valve V1 and closing valve V2. The particle injection took place for about 3 minto 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 CCDcamera. The particle injection was stopped during the postplasma. Particles deposited on three siliconwafers of 5:0 mm × 5:0 mm in size placed on the grounded electrode were observed by SEM. Thewafer positions are shown in Fig. 2.

3. Results and discussion

Particle behavior below the powered electrode during plasma operation for various conditions hasbeen reported previously (Setyawan et al., 2002b). BrieKy, two modes of particle trap were observed.We referred them to as the lumping and the winding modes. The lumping mode was characterized


by the big particle clouds localized in the discrete regions between the showerhead holes, and theparticles entering the reactor do not overshoot the equilibrium position of the particle trap. Thismode occurs typically at conditions of relatively high rf power and low gas Kow rate. The windingmode was characterized by the small and thin particle clouds, and the particles entering the reactorovershoot the equilibrium position of the particle trap. This mode occurs typically at conditionsof relatively low rf power and high gas Kow rate. Upon increasing the gas Kow rate further, theelectric forces responsible for the particle trap was not able to trap the particles below the poweredelectrode 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 trapmode changes from one to another is shown in Fig. 3. The error bar shows the uncertainty in theobservation of the change of particle trap mode.

In the next section, the particle behavior in the postplasma will be presented. The behavior inthe postplasma can be divided into two regimes, namely early postplasma and late postplasma. Theearly postplasma refers to the period of a few milliseconds after turning o; the plasma and therest is referred to as the late postplasma. Then, the occurrence time of particle contamination andthe contribution level from individual steps on the wafer placed on the grounded electrode, andthe 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 ofthe clouds for the case of unbiased and positive bias will be discussed.

3.1. Particle behavior in the postplasma

The particles are attracted to the powered electrode once the rf power is turned o; as shown bythe video images in Fig. 4 and then halt before reaching the electrode surface. The time-resolvedimage shown in Fig. 4 is for the condition of rf power 100 W, particle size 0:6 �m and gas Kow rate50 sccm, and the particles are trapped in the lumping mode during plasma. A similar phenomenonwas 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 isturned o;, the negative self-bias voltage is retained for several milliseconds. Moreover, the electrontemperature rapidly decreases to gas temperature. Consequently, the electron current to the electrodeis zero because the repulsive barrier of the electrode that has residual negative self-bias voltage ismuch larger than the electron energy. Hence, only positive ion will Kow to the electrode surface.During their transport to the electrode surface, the ions collide with the particles and drag themtoward 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 insuMcient to neutralizeall of the particle charge as shown by the particle movement in the presence of a dc <eld duringthe postplasma period that will be discussed later.

After the e;ect of ion drag force on the particles disappears, the particles fall down under theinKuence of gas Kow and gravity. Fig. 5 shows typical time-resolved images of the particle movementin the late postplasma for the lumping and the winding modes. The plasma condition is the sameas before except the gas Kow rate is 200 sccm for the winding mode. For the lumping mode, wecan observe that the particle clouds fall down towards the wafer. When the gas Kow rate is low, thedominant Kow direction in the space between electrodes is downward. Thus, it is not surprising thatthe particles are carried towards the wafer. On the other hand, for the winding mode, the particles


Fig. 4. Particle behavior in the early postplasma showing the movement of particles attracted to the electrode for a rfpower of 100 W and a gas Kow rate of 50 sccm (lumping mode).

are swept away from the wafer and go to the exhaust port. When the gas Kow rate is high, thedominant Kow direction in the space between electrodes is radially outward except in the regionnear the centerline (Setyawan et al., 2002a).

In order to determine the charge on particles in the postplasma, either negative or positive dcbias voltage was applied on the grounded electrode after extinguishing the plasma. Fig. 6 shows


Fig. 5. Time-resolved image of particle motion in the late postplasma for a rf power of 100 W and gas Kow 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 rfpower 100 W and the gas Kow rate 50 sccm. We can observe that the particles are repelled awayfrom the grounded electrode for the case of negative bias and are attracted to the grounded electrodefor the case of positive bias. These suggest that most particles retain their negative charge in thepostplasma. In addition, SEM observations on the wafers placed on the grounded electrode showedthat more particles were deposited on the wafer for the case of positive bias compared to the caseof unbiased and negative bias as shown in Fig. 7. For the case of negative bias, particle depositioncould not be observed. These also suggest that the residual charges on particles in the postplasmaare mostly negative.


Fig. 6. Time-resolved image of particle motion in the late postplasma for a rf power of 100 W and a gas Kow rate of50 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 Kow 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.

3.2. Occurrence time of particle contamination

In order to determine the contribution level and the occurrence time of particle contamination inindividual steps during plasma processing, particles deposited on the wafers were observed usingSEM for the conditions of no heating and heating the grounded electrode to 300◦C during thepostplasma. All experimental data of particle deposition reported herein are for silica particles ofsize 0:6 �m. We have shown previously using an LLS technique that particles were repelled awayfrom the grounded electrode by heating the electrode to 300◦C (Setyawan et al., 2002a). Numerical


simulation by taking into account detailed con<guration of the reactor revealed that no particle wasdeposited on the wafer for this condition. Hence, we used this <nding as a basis of experiment todetermine the occurrence time of particle deposition. Before we used this method, we carried outexperiments 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 con<rm that particle deposition does not occurfor any conditions used in this study. In these experiments, after the temperature of the groundedelectrode has reached 300◦C, plasma was ignited, and then particles were injected. We con<rmedthat no particle was deposited on the wafers by heating the electrode to 300◦C for any experimentalconditions used in this study.

The temperature gradient created in the gas by heating the grounded electrode to 300◦C is about50 K=cm. In order for the particles to be repelled away from the wafer, the thermophoretic velocityshould exceed the terminal settling velocity. The thermophoretic velocity is expressed as

vth =−K �T∇T; (1)

where ∇T is the temperature gradient, � is the kinematic viscosity of the gas, and K is the ther-mophoretic coeMcient. The thermophoretic coeMcient can be expressed as (Talbot, Cheng, Schefer,& Willis, 1980)

K = 2:294(�+ 2:20Kn) [1 + Kn{1:2 + 0:41 exp(−0:88=Kn)}]

(1 + 3:438Kn) (1 + 2�+ 4:40Kn): (2)

Here, � is the thermal conductivity ratio of gas to particle, and Kn is Knudsen number. The terminalsettling velocity is expressed as

vt =d2p�pgCc

18�; (3)

where dp is particle diameter, �p is particle density, g is the gravitational constant, Cc is Cunninghamcorrection factor, and � is viscosity of gas. The Cunningham correction factor can be expressed as

Cc = 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 thermo-phoretic velocity is estimated to be 1:7 cm=s. Assuming �p = 2:3 g=cm3 yields vt = 0:035 cm=s.This value is 50 times smaller than the thermophoretic velocity. Hence, we can expect that thethermophoresis due to the temperature gradient created by heating the grounded electrode to 300◦Ccan shield the wafer from particle deposition. Therefore, particles deposited on the wafer for the caseof heating the grounded electrode to 300◦C can be considered to occur only during plasma operationbecause the thermophoresis shield the wafer from particle deposition in the postplasma. For the caseof 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 particledeposition is observed at the wafer placed in the center. Fig. 8(a)–(c) shows the SEM photographsof 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 presentedin Fig. 9. The operating conditions for the lumping and the winding modes are the same as before.For the escaping mode, the gas Kow rate is 350 sccm using the same level of rf power. The lowest


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 theescaping mode. The particle contamination is found to occur during the postplasma for the lumpingand the winding modes whereas it occurs during the plasma operation for the escaping mode. Forthe lumping and the winding modes, the balance of electrostatic and ion drag forces acting on theparticles are able to trap the particles in the region below the powered electrode. Some particlesleak from the trap through the edge of the powered electrode. The trapped particles fall down tothe wafer and contaminate it when the plasma is turned o;. Because the particle concentration intrap for the lumping mode is higher than that in the winding mode (Setyawan et al., 2002b) andthe dominant gas Kow direction in the lumping mode is downward, it is not surprising that theparticle deposition in the lumping mode is higher than that in the winding mode. For the case ofthe escaping mode, the particle deposition on the wafer does not occur in the postplasma becausethere is no particle trapped just before turning o; the rf power.

As shown earlier, the particle movement in the postplasma is inKuenced by the presence of a dc<eld. Fig. 10 presents the number of particles deposited on the wafer for di;erent dc bias voltageapplied on the grounded electrode during the postplasma for the lumping mode. The rf power is100 W and the gas Kow rate is 50 sccm. When the negative bias of 300 V was applied on thegrounded electrode, particle deposition on the wafers cannot be observed. On the other hand, apositive bias of 300 V increases the number of particles deposited on the wafers over unbiasedvoltage by approximately three times. These suggest that the dominant charge sign on the particlesin the postplasma is clearly negative.

3.3. Estimation of residual particle charges in the postplasma

In order to estimate the charge on particles in the postplasma, the cloud displacement as a functionof time was plotted for the unbiased and positive dc bias cases. Particle cloud displacement as afunction of time is shown in Fig. 11. We can observe that the cloud displacement is approximatelylinear both for the unbiased and positive bias with the slopes representing terminal velocities of theparticle clouds. The terminal velocity obtained from the plot takes the values of about 13.2 and15:3 mm=s for the unbiased and positive bias, respectively. Then, a force balance on the particles


Fig. 9. Number of particles deposited on the wafer placedon the grounded electrode as a function of particle trapmode. The contribution level of particle contaminationfrom individual steps, i.e. during plasma operation andduring the postplasma, is displayed. The gas Kow rate is50, 200, and 350 sccm for the lumping, the winding andthe escaping modes, respectively with a constant rf powerof 100 W.

Fig. 10. The inKuence of dc bias voltage on the numberof particles deposited on the grounded electrode for a rfpower of 100 W and a gas Kow rate of 50 sccm (lumpingmode).

Fig. 11. Particle cloud displacement after extinguishing the plasma for the cases of unbiased and positive bias of 300 V.The gas Kow 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 ofthe particle clouds. Without an applied dc bias, the force balance for particles can be expressed as

mpdvdt

=−Fg + FD; (5)


where v is particle velocity, Fg is the gravitational force, and FD is the drag force. With a dc biasapplied, the force balance on the particles becomes

mpdvdt

=−Fg + FD − FE; (6)

where FE is the Coulomb force on the charged particles due to the applied dc <eld. Because ofthe very short relaxation time of particle, the particle clouds can be assumed to reach their terminalvelocity very rapidly. Combining Eqs. (5) and (6) at the terminal velocity yields

FD(v′t)− FD(vt) = FE; (7)

where FD(v′t) and FD(vt) are the drag forces on the particles at terminal velocity for the cases ofthe positive bias and unbiased, respectively. The drag force can be assumed to follow the Stokesdrag law and is expressed as

FD =3��dp(u− v)

Cc; (8)

where u is the gas velocity.The Coulomb force is given by

FE = peE; (9)

where p is number of elementary unit of charge, e is elementary unit of charge, and E is theintensity of electric <eld. Solving for the number of charges per particle by assuming that the gasvelocity is equal for the case of unbiased and positive bias yields

p=3��dpCceE

(v− v′): (10)

The electric <eld intensity due to a dc bias voltage applied on the grounded electrode in thepostplasma can be assumed to be linear. When the rf power is turned o;, electrons disappear bydi;usion to the walls and attachment to gas molecules in a very short time. The characteristic timeis less than 1 ms (Childs & Gallagher, 2000). Then, the electron density and temperature are too lowfor any signi<cant electrostatic <elds to exist in the postplasma. Therefore, the electric <eld exist inthe postplasma is solely due to the applied dc bias voltage. It takes the value of about 85 V=cm for adc bias voltage of 300 V applied on the grounded electrode. Using Eq. (10), the number of chargesper particle is about 11.4 electron charges. This is in the range of particle charges predicted byCollins et al. (1996) that made an estimate for the residual charge in the postplasma to be 1.4–12.4electron 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 generatedby this method are not monodisperse, but have a distribution on particle size, the predicted chargenumber has uncertainty due to the uncertainty in the particle size. This di;ers from our technique inwhich the particles are injected from the outside so that it enables us to use a well-de<ned particle


size. Therefore, the uncertainty in particle charge prediction due to the uncertainty in particle sizecan be surely eliminated.

4. Conclusions

It has been demonstrated that the proposed method has been used successfully to determine theoccurrence time and the contribution level of particle contamination in individual steps during oper-ation of a PECVD reactor. The proposed method is based on the capability of thermophoresis dueto a temperature gradient to shield the wafer from particle deposition. It is found that the particlecontamination occurs during the postplasma for the lumping and the winding modes whereas it oc-curs during the plasma operation for the escaping mode. The highest level of particle contaminationoccurs in the escaping mode and the lowest level is in the winding mode. Observation of the particlemovement in the presence of a dc <eld using LLS technique and particle deposition on the waferusing SEM gives evidence that most particles in the postplasma retained their negative charge. Theinjection of particles into the plasma from the outside has been proven very useful in studying thee;ect of particle size on particle behavior and transport in a PECVD reactor, both during plasmaoperation and in the postplasma period. We have shown that the uncertainty in the prediction ofresidual particle charge in the postplasma due to the uncertainty in particle size could be reducedusing this experimental technique.

Acknowledgements

This work has been supported in part by Innovation Plaza Hiroshima of the Japan Science andTechnology Corporation (JST). The <nancial support provided by the QUE project, Department ofChemical Engineering, Sepuluh Nopember Institute of Technology (ITS), Indonesia for H. Setyawanis gratefully acknowledged.

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