Experimental Study of Particle Deposition on Semiconductor Wafers

Transcription

1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: Experimental Study of Particle Deposition on Semiconductor Wafers David Y. H. Pui, Yan Ye & Benjamin Y. H. Liu To cite this article: David Y. H. Pui, Yan Ye & Benjamin Y. H. Liu (1990) Experimental Study of Particle Deposition on Semiconductor Wafers, Aerosol Science and Technology, 12:4, , DOI: / To link to this article: Published online: 08 Jun Submit your article to this journal Article views: 366 View related articles Citing articles: 34 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 28 November 2017, At: 09:23

2 Experimental Study of Particle Deposition on Semiconductor Wafers David Y. H. Pui, Yan Ye, and Benjamin Y. H. Liu Particle Technology Laboratory, Mechanical Engineering Department, University of Minnesota, Minneapolis, MN A sensitive method for detecting particle deposition on semiconductor wafers has been developed. The method consisted of generating a monodisperse fluorescent aerosol, depositing the known-size monodisperse aerosol on a wafer in a laminar flow chamber, and analyzing the deposited particles using a fluorometric technique. For aerosol particles in the size range of pm, the mobility classification-inertial impaction technique developed by Romay-Novas and Pui (1988) was used to generate the monodisperse test aerosols. Above a particle diameter of 1.0 pm, monodisperse uranine-tagged oleic acid aerosols were generated by a vibrating-orifice generator. The test wafer was a 3.8-cm diameter silicon INTRODUCTION Particle deposition on semiconductor wafers is an important problem in integrated circuit manufacturing. With the feature size of integrated circuits approaching submicrometer size, particle deposition on wafers is a leading cause of product loss. Aerosol deposition on wafers was first studied theoretically by Liu and Ahn (1987) using the analogy between heat and mass transfer and particle diffusion. Seven other publications concerned with the theory have since been published. Experimental studies, however, are quite limited. Furthermore, none of the existing experimental studies is sufficiently detailed to allow direct comparison with the theories. The objective of this study is to develop methodologies for measuring the deposition velocity of particles on a sernicon- Thls is Particle Technology Laboratory Publication No Aerosol Science and Technology 12: (1990) Elsevier Science Publishing Co., Inc. wafer placed horizontally in a vertical laminar flow chamber which was maintained at a free stream velocity of 20 cm/s. A condensation nucleus counter and an optical particle counter were used to obtain the particle concentration profile in the test cross section and to monitor the stability of aerosol concentration during the experiment. The results show that the measured particle deposition velocities on the wafers agree well with the theory of Liu and Ahn (1987) in the particle size range between 0.15 and 8.0 pm. The deposition velocity shows a minimum around 0.25 pm in particle diameter and increases with both smaller and larger particle size owing to diffusional deposition and gravitational settling, respectively. ductor wafer. The experimental results will then be compared with recent theoretical calculations. PREVIOUS STUDIES Table 1 lists the theoreticql studies of particle deposition on silicon wafers performed by various investigators (compiled from Turner et al., 1989). Two geometrical configurations were studied, namely, an axially symmetric case simulating a free-standing wafer, and a planar case simulating a horizontal wafer placed on a bench. The flow fields used in various studies included a boundary layer approximation, potential flow, and full numerical solution to the Navier-Stokes equation. A variety of methods were used in calculating particle transport properties, including heat and mass transfer analogy, particle equation of motion, and diffusion equation. The mecha /90/$3.50

3 TABLE 1. Theoretical Studies of Particle Deposition on Semiconductor Wafers Author Geometry Flow field Particle transport Mechanismsa Comments Axially symmetric Experimental simulation Mass transfer correlation Liu and Ahn (1987) Liu et al. (1987) Stratmann et al. (1988) Friedlander et al. (1988) Peters et al. (1989) Peterson et al. (1988) Turner and Fissan (1986) Turner et al. (1989) Axially symmetric Planar Planar Axially symmetric Planar Planar Planar Full numerical solution of Navier-Stokes equation Boundary layer approximation Boundary layer approximation Boundary layer approximation Boundary layer approximation and full numerical solution Potential flow Boundary layer approximation Diffusion cquation Particle motion equation Particlc motion equation Particle motion cquation Diffusion equation; particlc motion equation Diffusion equation; particle motion equation Diffusion equation aabbreviations: C, covection D, diffusion; EC, Coulombic force; EI, image force; I, inertia; S, sedimentation; T, thermophorctic force. Compiled from Turner et al. (1989). Superposition of deposition rate for individual mechanism Detailed calculation of fluid streamlines around wafer Dust-free layer above heated surface Diffusive leakage of particle across dust-free layer Monte Carlo simulations of Brownian dynamics Stokes perturbation method to solve particle motion equation Superposition of deposition rate for individual mechanism Turner et al. (1989) also included thermophoresis TABLE 2. Experimental Studies of Particle Deposition on Semiconductor Wafers Particle Wafer diameter potential Author Detecting method Wafer orientation (~m) (v) Particle charge Test condition Hayakawa et al. Image processing system O Not considered Not considered Point source (1986) 30" , 90" 1.09 Donovan et al. Wafer surface scanner Horizontal , >1 Stirred chamber (1987) Boltzmann 0.9 i 5000 rms charge Welker et al. Wafer surface scanner Horizontal > 0.5 With/without Class 100 (1988) ionization system VLF clean room -

4 Particle Deposition on Semiconductor Wafers 797 nisms studied included convection, diffusion, inertia, and sedimentation owing to mechanical forces, as well as those owing to electrical and thermophoretic forces. The theoretical studies on the topic are seen to be quite extensive. Experimental studies on the topic, however, are quite limited. Table 2 summarizes three experimental studies performed to date. The first two studies by Hayakawa et al. (1986) and Donovan et al. (1987) involved using monodisperse (polystyrene latex) PSL particles under controlled laboratory conditions while the third study by Welker et al. (1988) was done in a working clean room using aerosol particles existing in the clean room environment. While qualitative comparison could be made between Welker's work and the theories, a detailed direct comparison was not possible. In these three studies, particle detection on wafer surfaces was performed using either an image processing system or a wafer surface scanner. The wafer was oriented horizontally in two studies but was varied at four different angle positions in the third study. Two studies considered the electrostatic effects while the thrd (Hayakawa et al., 1986) did not control and monitor particle and surface charges. Ths could be the reason that their data were quite scattered and gave deposition velocities significantly higher than the theoretical predictions. Figure 1 compares their experimental data with theoretical deposition curves by Liu and Ahn (1987). The experimental data are seen to give nearly two orders of magnitude higher deposition velocity in one case and to give a reverse trend as the air velocity changes. The study by Donovan et al. (1987) was performed in a stirred chamber. Their results are applicable more for a process equipment environment instead of a laminar flow clean room environment. Therefore, none of the existing experiments is appropriate for direct comparison with the available theories. PARTICLE DIAMETER, pm PARTICLE DIAMETER, pm FIGURE 1. Comparison of the experimental data by Hayakawa et al. (1986) with the theory of Liu and Ahn (1987) for particle deposition on semiconductor wafers.

5 798 Pui et al. EXPERIMENTAL METHODS Figure 2 shows a schematic diagram of the experimental system used in studying particle deposition onto a semiconductor wafer. The system consists of two aerosol generators 'to produce monodisperse test aerosols, a laminar flow test chamber in which the test wafer is placed, and two continuous concentration detectors and a hot-film anemometer to monitor the stability of the test aerosols and to scan the velocity and concentration profile of the test cross section. The details of each of the components are described below. Monodisperse Aerosol Generation Two aerosol generators were used in this study to produce monodisperse test aerosols. For particles in the size range between 0.1 and 1.0 pm, the mobility classification-inertial impaction technique described by Romay-Novas and Pui (1988) was used. As shown in Figure 2, a polydisperse ammonium fluorescein aerosol was first generated by atomizing a solution consisting of fluorescein powder dissolved in dilute aqueous ammonia. The polydisperse aerosol was then classified according to electrical mobility in the differential mobility analyzer (DMA). The "equal mobility" aerosol particles were next neutralized in a 85-Kr radioactive discharger and subsequently mixed with clean dry air before entering a one-stage microorifice impactor. The impactor was used to remove those large particles which, because of their multiple charges, had passed through the DMA in the same mobility channel as the singly charged particles of the desired size. The output was a truly monodisperse aerosol of a known size obtained from the operating conditions of the DMA. The necessity and the effectiveness of the impactor are illustrated in Figures 3 and 4. The top figures give the number size distri- VACUUM - FIGURE 2. Schematic diagram of the experimental system in wafer deposition studies.

6 Particle Deposition on Semiconductor Wafers WITHOUT IMPACTOR WITHOUT IMPACTOR FIGURE 3. Number and mass size distributions of the "equal mobility" particles from the mobility classification technique. butions while the bottom figures give the volume (mass) size distributions. Figure 3 shows that the relatively low number concentration, < 3%, of multiply charged particles can become quite significant, -> 20%, when it is converted to a mass concentration basis. Since the present fluorometric analysis technique is sensitive to mass concentration, the multiply charged particles must be elimi- nated. Figure 4 shows that with the microorifice impactor, the output aerosol stream is mostly free of the multiply charged particles. For particles larger than 1.0 pm in diameter, monodisperse uranine-tagged oleic acid aerosols were generated by the vibrating orifice generator. The aerosol generation and solution preparation techniques were reported by Liu and Pui (1981).

7 Pui et al. WITH IMPACTOR WITH IMPACTOR FIGURE 4. Number and mass size distributions of monodisperse particles from the mobility classification technique when a micro-orifice impactor is used to remove the doubly charged particles. Laminar Flow Test Chamber The test chamber was in the form of a vertical laminar flow tunnel. A high-efficiency particulate air (HEPA) filter was placed at each end of the tunnel. A blower was used to maintain a constant air velocity of 20 cm/s in the tunnel. The velocity profile was mea- sured using a hot-film anemometer and was shown to have a flat profile in the tunnel cross section. The 3.8-cm diameter test wafer was suspended in the center of the chamber. In order to expose the test wafer to the hlghest possible concentration of monodisperse

8 Particle Deposition on Semiconductor Wafers Position, cm FIGURE 5. Particle concentration profile at the test section of the wafer deposition experiment. aerosol from the aerosol generators, the aerosol was injected into the chamber using an 4.4-cm I.D. isokinetic injection probe with a sharp edge above the wafer surface. This was done to insure that the aerosol remained undiluted in the chamber. The matched air velocity from the probe and the free stream was measured by the hot-film anemometer. The particle concentration profile was measured by a condensation nucleus counter. Figure 5 gives the measured concentration profile showing that the wafer was exposed to a uniform concentration of the test aerosol. To obtain the free stream aerosol concentration to which the wafer was exposed, an isokinetic sampler consisting of a 47-mm filter holder with a 3.8-cm diameter sampling probe was placed in the same position where wafer was located. The free stream concentration was measured before and after each deposition period to obtain the average aerosol concentration. Continuous Aerosol Monitors The deposition period typically required between 2 and 10 h. Consequently, two contin- uous aerosol monitors, a TSI model 3760 condensation nucleus counter (TSI Inc., St. Paul, MN) and a PMS model ASAS-300 laser particle counter (Particle Measuring System, Inc., Boulder, CO) were used to monitor the chamber concentration continuously. Both instruments were connected to an IBM-AT microcomputer which recorded the chamber concentration during the deposition period. RESULTS AND DISCUSSION The particle deposition mechanisms for the present experiment are expected to be convection, Brownian diffusion, sedimentation, and inertial impaction. Since the wafer was electrically grounded and monodisperse aerosol particles were neutralized by a radioactive source, the electrostatic effect can be neglected. The Stokes number for 8-pm particles, the largest-size particles used in the experiment, is estimated to be < under the present experimental conditions. Consequently, inertial impaction is considered to be less important compared to other deposition mechanisms. The flow field presented by the experimental system is axially

9 Pui et al. symmetric. The deposition velocity measured in this experiment is the mean deposition velocity over the entire wafer instead of at a small area around the stagnation point. All of these experimental conditions match the assumptions used in Liu and Ahn's theory. The results are presented giving the particle deposition velocity as a function of particle size. The deposition velocity Vd of particles on the wafer is defined as the ratio of the particle flux to the wafer (number deposited per unit area and unit time) J to the airborne particle concentration in the bulk medium above the wafer N, i.e., In a typical run, monodisperse aerosol was produced by one of the two aerosol generators and transported to the isokinetic injection probe. The aerosol concentration exiting the injection probe was first collected by an isokinetic filter sampler. The test wafer was then placed below the injection probe and exposed to the uniform concentration of monodisperse test aerosol. At the completion of the deposition period, another isokinetic filter sample was taken. The collected fluo- 10 O Theoretical (Liu & Ahn, 1987) rescein-tagged particles from both the filters and the wafer were then dissolved in separate washing solutions. For the ammonium fluorescein particles, 0.1 N NH,OH was used as a washing solution; for oleic acid particles tagged with uranine, N NaOH was used as a washing solution. The fluorescence concentrations of the wash solutions were then measured by a sensitive fluorometer (Aminco, American Instrument Co., Silver Spring, MD). Using the fluorometer readings, the mass of fluorescein material collected on the wafer and filter can be calculated accurately. Since the mass of fluorescein is proportional to the mass of particles, the particle deposition velocity on the wafer can be determined as where M, and Mf are the mass of fluorescein collected on the wafer and filters, respectively; t, and t, are the sampling periods for collecting particles on the wafer and the filters; Q, is the sampling flow rate of the isokinetic sampling probe; and A, is the area of the wafer. The measurement uncer- Particle Diameter, pm FIGURE 6. Comparison of the first sets of wafer deposition data with the theory of Liu and Ahn (1987).

10 Particle Deposition on Semiconductor Wafers Theoretical (Liu & Ahn, 1987) Experimental V = 20 cmls Particle Diameter, prn FIGURE 7. Comparison of the final. deposition data with the theory of Liu and Ahn (1 987). tainty results primarily from the accuracy of fluorescence measurement and the drift in particle concentration during deposition. It is estimated that the uncertainty in the determination of the deposition velocity is approximately 20%. Figure 6 shows the results of the first set of experiments obtained with thls system. The theoretical curve is calculated using the theory of Liu and Ahn (1987) for 3.8-cm diameter wafer and for a free stream velocity of 20 cm/s. One measurement difficulty encountered in this experiment was that the impactor substrates could be loaded with particles in a short time. Although the substrate surfaces were coated with grease to prevent particle bounce, the heavy particle loading would soon provide solid surfaces which promote particle bounce-off. This effect is demonstrated in Figure 6. Runs 2 and 3 were obtained by rotating the impactor nozzles with respect to the substrates in 20 and 10 min, respectively. Run 4 featured a continuous nozzle-substrate rotation using an electric motor. The data shows that- as the rotation of nozzle substrate is made more frequent, particle bounce is reduced due to the availability of fresh greased surface. The results based on the continuous rotating noz- zle substrate agree well with the theoretical prediction. Figure 7 shows data obtained with the mobility classifier and the vibrating-orifice generator. The impactor nozzle was rotated continuously using a built-in electric motor. It is seen that the experimental data agree well with the theoretical curve by Liu and Ahn (1987). The deposition curve has a minimum around 0.25 pm in diameter and increases with both smaller and larger particle size due to diffusional deposition and gravitational settling, respectively. CONCLUSIONS A new methodology has been successfully developed to measure particle deposition on semiconductor wafers. This will allow a data base to be obtained for comparison with various theoretical models. Experimental results show that the measured deposition data give good agreement with the theory of Liu and Ahn (1987). Plans are now being made to measure the deposition rates taking into account the effects of particle charge and electric field, as well as thermal gradient.

11 Pui et al. This research is supported by the Particulate Contamination Control Research Consortium at the University of Minnesota. Members of the Consortium include: Au Products and Chemicals, Applied Materials, Donaldson Co., Inc., IBM Corp., Magnetic Peripherals, Inc., Millipore Corporation, Nupro Company, Texas Instruments, Inc., The BOC Group, and TSI, Inc. The support of the Consortium is gratefully acknowledged. REFERENCES Donovan, R. P., Clayton, A. C., and Ensor, D. S. (1987). In Proceedings, IES 33rd Annual Technical Meeting, San Jose, CA, pp Friedlander, S. K., Fernandez de la Mora, J., and Gokoglu, S. (1988). J. Colloid Interjuce Sci. 126: Hayakawa, I., Fujii, S., and Kim, K. Y. (1986). In Proceeding IES 32nd Annual Technical Meeting, Dallas, TX, pp Liu, B. Y. H., and Ahn, K. H. (1987). Aerosol Sci. Technol. 6: Liu, B. Y. H., Fardi, B., and Ahn, K. H. (1987). Proceedings, IES 33rd Annual Technical Meeting, San Jose, CA, pp Liu, B. Y. H., and Pui, D. Y. H. (1981). Atmos. Enoiran: 15: Peters, M. H., Cooper, D. W., and Miller, R. J. (1989). J. Aerosol Scr. 20: Peterson, T. W., Sannes, K. M., Stratmann, F., and Fissan, H. J. (1988). J. Aerosol Sci. 19: Romay-Novas, F., and Pui, D. H. Y. (1988). Aerosol Sci. Technol. 9: Sparrow, E. M., and Geiger, G. T. (1985). J. Heut Trunsfer 127: Stratmann, F., Friedlander, S. K., Fissan, H. J., and Papperger, A. (1988). Aerosol Sci. Technol. 9: Turner, J. R., and Fissan, H. J. (1986). Presented at the Annual Meeting of the Fine Particle Socicty, San Francisco, CA. Turner, J. R., Linguras, D. K., and Fissan, H. J. (1989). J. Aerosol Sci. 20: Wclker, R. W. (1988). Proceedings, IES 34th Annual Technical Meeting, King of Prussia, PA, pp Received December 9, 1988; accepted February 17, 1989.