J. Adhesion Sci. Technol., Vol. 18, No. 7, pp. 795 806 (2004) VSP 2004. Also available online - www.vsppub.com Non-contact removal of 60-nm latex particles from silicon wafers with laser-induced plasma IVIN VARGHESE and CETIN CETINKAYA Department of Mechanical and Aeronautical Engineering, Center for Advanced Materials Processing, Wallace H. Coulter School of Engineering, Clarkson University, Potsdam, NY 13699-5725, USA Received in final form 15 March 2004 Abstract In micro- and nano-manufacturing particles are created and circulated as by-products of the processes utilized. The removal of micro- and nano-particles during manufacturing processes is of great importance in many industries, including semiconductors, optics, photonics and microelectromechanical systems (MEMS). One specific application is in the cleaning of silicon wafers. According to the 2002 update of the International Technology Roadmap for Semiconductors, by the year 2006, techniques for the removal of polystyrene latex (PSL)-equivalent particles with size less than 35 nm will be required and currently, according to the roadmap update, there is no known method to remove PSL-equivalent particles smaller than diameter D = 40 nm. The current study reports and demonstrates a non-contact laser-induced plasma method for removal of particles with D = 60 nm. Non-contact techniques are desired to reduce the risk of substrate damage. Chemical usage in traditional cleaning methods is also a serious concern for process cost, workplace safety and environmental conservation. The development of dry, rapid, non-contact and non-damaging nanoparticle removal methods is, therefore, a critical need for various applications. Using a scanning mechanism, the process discussed can be automated for rapid cleaning of large silicon wafers. Keywords: Laser-induced plasma (LIP); nanoparticles; particle removal; nano-manufacturing; micromanufacturing; polystyrene latex (PSL). 1. INTRODUCTION According to the 2002 update of the International Technology Roadmap for Semiconductors (ITRS) [1] for the requirements of surface cleaning, the minimum front surface particle size, designated by the equivalent latex sphere diameter (D), are D 90 nm for the year 2003, D 45 nm for the year 2004 and D 35 nm for the year 2006. The requirement for the back surface particles size of latex sphere To whom correspondence should be addressed. Tel.: (1-315) 268-6514. Fax: (1-315) 268-6438. E-mail: cetin@clarkson.edu
796 I. Varghese and C. Cetinkaya equivalent is D 200 nm for the years 2002 2007. Presently, for cleaning latex particles with D<40 nm, manufacturable solutions are classified as unknown by the ITRS roadmap. For a particular technology node to be achieved, a successful method for removal of particles must exist. Silicon wafer cleaning of nano- and micro-particles is, therefore, of extreme importance in the semiconductor and microelectronics industries. Current techniques, like wet chemical etching, ultrasonic cleaning and brush scrubbing, are limited in efficiency for nano-particle removal. Therefore, one avenue to address the technical challenges to satisfy the ITRS requirements is to develop a fast, dry and efficient method. One proposed solution is the direct laser cleaning in which particles are removed by application of inertial forces (due to rapid thermoelastic expansion of the substrate) at the particles attached to the substrate by means of surface acceleration. Potential damage mechanisms in direct laser cleaning include surface breakage, micro-cracks, and peeling of top layers (due to localized stress) (e.g. see Ref. [2] for stresses generated in the direct laser cleaning). The research objective of the current work was to remove 60 nm size polystyrene latex particles on silicon substrates using dry laser-induced plasma (LIP) removal technique [3, 4]. In the LIP cleaning process, the incident laser beam is converged through a convex lens; therefore, near the focal point of the lens, a plasma core is created by dielectric breakdown of air [3, 5]. This plasma expands resulting in a strong shock wavefront generating a pressure field, which exerts a force on the surface of the particle and causes its removal by rolling, sliding, lifting [6], or a combination of these effects. In the rolling detachment mode, critical moment acting around a point in the adhesion area initiates a crack between the particle and the substrate. Once the adhesion bond is compromised, crack propagation occurs until the particle is free from the surface. In the sliding detachment mode, if the force acting on the particle, parallel to the substrate surface, is greater than the frictional force between the particle and the surface, the particle will be detached. In the lifting detachment mode a force perpendicular to the substrate surface acts on the particle. This mode is the dominant removal mechanism in the direct laser method (see, e.g. Ref. [7]). The lift-off force must be greater than the combined forces of gravity and adhesion in order to lift the particle from the surface. In application, detachment will be achieved by some combination of these three effects [8]. Rolling detachment is the dominant mode for spherical particles on flat substrates; therefore, rolling will be referred to as the principal mechanism in the current study. The Johnson, Kendall and Roberts (JKR) model of particle adhesion approximates the force of adhesion, F A, between a spherical particle and a flat substrate as F A = 3 4 πw AD, (1) where W A is the work of adhesion between the particle and the substrate and D is the diameter of the spherical particle [9]. The radius of the contact circle between
Non-contact removal of latex particles from silicon wafers with LIP 797 Figure 1. Schematic of experimental set-up for the laser-induced plasma removal technique. d is the distance from the substrate to the center of the plasma core and f l is the focal length of the convex lens. the substrate and the particle at the onset of detachment, a, is determined as ( 3πWA D 2 ) 1/3 a =, (2) 8K with K = 4 [ (1 υ 2 1 ) + (1 υ2 2 ) ] 1, (3) 3 E 1 E 2 υ 1 and E 1 are the Poisson s ratio and the Young s modulus of the particle material, respectively, and υ 2 and E 2 are the Poisson s ratio and the Young s modulus of the substrate material, respectively. The Hamaker constant for the PSL particles on silicon substrate in air [10] is A = 12.2 10 20 J and the work of adhesion is given by, A W A = (4) (12πzo 2 ), where z o is the separation distance between the particle and the substrate (4 Å). In the LIP removal technique, the dominant removal mode is rolling under a transient pressure field generated by a shock wavefront while other removal modes also contribute to the removal (see Fig. 1 for a schematic of the process). Figure 2 depicts a spherical particle on a flat surface under an effective applied pressure field. Due to the small size of the particle, the amplitude of the pressure wave is reasonably approximated to be uniform over the entire particle surface. In order to analyze the rolling mode of removal, a moment balance about point O is applied. The critical
798 I. Varghese and C. Cetinkaya Figure 2. Geometric features of the 60-nm spherical PSL particle attached to a smooth surface under an applied pressure field p. pressure, p c, needed for detachment can be expressed as 2a(F A + mg) p c = A s (D cos θ 2asin θ), (5) where A s is the effective particle surface area perpendicular to the pressure field, m is the particle mass, g the gravitational acceleration, R is the radius of the particle, θ the angle between the force vector and a plane parallel to the substrate surface and α is the contact depth [2] and is given by, α = a2 R. (6) Since the resultant applied force at the center of moment F A is many orders of magnitude higher than mg when dealing with nano-particles, mg can be neglected in equation (5). The adhesion forces and the required pressures for removal can be calculated from these relations. In the case of a 60 nm PSL particle on a silicon substrate, the force of adhesion is 2.85 nn and the required pressure for removal is 45.34 kpa when θ approaches 0 or π. These calculations, coupled with the use of varying particle sizes, could be employed to characterize unknown pressure fields based on observed removal. The smallest particles removed would be used to pinpoint the maximum pressure present. 2. EXPERIMENTAL In the current experiments, reclaimed 6-inch, n-type doped [111] silicon wafers with approximately 1-µm thick thermal oxide layers were used. Due to the geometric constraints associated with the vacuum chamber of the scanning electron microscope (SEM), the wafer was required to be cut into small square pieces of
Non-contact removal of latex particles from silicon wafers with LIP 799 approximately 1.5 cm 1.5 cm, with a diamond scriber. To eliminate initial contamination of the samples, each sample was washed successively with deionized water and methyl alcohol. A diamond shaped reference pattern with an area of approximately 2 mm 2 was inscribed on the sample using the diamond scriber. The diamond shape helps to identify the same location, before and after shooting with the laser. Since the shape of the pattern changes with the application of LIP, the bare sample was cleaned by shooting 5 pulses at the firing distance, d, of 1.4 mm above the substrate before particle deposition. This also ensures that the particles that result while the reference pattern is inscribed, are removed and do not appear in the before cleaning SEM images. Commercially available polystyrene latex microsphere suspensions of mean diameter 60 nm, supplied by Duke Scientific, were used. PSL particles tend to agglomerate very easily. In order to disperse prior agglomerates of the PSL particles, the suspensions were sonicated using a 50 MHz transducer. It is believed that the high frequency excitation is required to create strong short-wavelength shear fields that are needed for breaking the adhesion bonds between the nanometer-sized PSL particles. The colloidal dispersion was then prepared in methanol. The ratio of methanol in the colloidal dispersion was varied depending on the extent of dilution required. To arrest agglomeration further, the ratio of methanol in the colloidal dispersion was increased. As illustrated in Fig. 3, increasing the methanol content in the colloidal dispersion reduced agglomeration. The silicon wafer sample was attached to an aluminium stud using a light adhesive tape. The colloidal dispersion was then excited using the transducer, after which it was deposited onto the sample using the drop-agitation technique. In the drop-agitation technique, a drop of the colloidal dispersion is deposited on the sample which is mechanically vibrated at 60 Hz by a base exciter, during which the methanol present evaporates which helps in achieving good dispersion of the particles. The objective of deposition is to achieve a uniform distribution of colloidal particles with a proper density to facilitate analysis of the target area of the sample. The surface vibrations produced restrict particle agglomeration while the methanol evaporates from the surface. The colloidal dispersion was applied to the central area of the sample, where the reference pattern was inscribed. The sample was allowed to completely dry following deposition. The initial cleaning and deposition were conducted in a class-10 cleanroom. After the deposition was completed and the colloidal dispersion had dried completely, the sample was analyzed in a Jeol Scanning Electron Microscope by acquiring before LIP images of the target area. Images were acquired at 5000 magnification in order to observe clearly the uniformity of PSL 60 nm particle distribution present throughout the inscribed reference pattern. Images were acquired at different locations of the reference pattern for the analysis. These were the same locations where the after LIP images were to be taken after the application of LIP. A Q-switched Nd:YAG laser was employed for the application of LIP. The laser characteristics were as follows: fundamental wavelength of 1064 nm (Quantel Brilliant series Q44) with
800 I. Varghese and C. Cetinkaya (a) (b) (c) (d) (e) Figure 3. SEM images at 10 000 showing different levels of agglomerates and deposition densities of 60-nm PSL particles. The approximate size of an individual PSL particle is identified in each of the images. PSL volume content in methanol-based colloidal dispersion decreases progressively in the images from 0.35% in (a), 0.287% in (b), 0.224% in (c), 0.161% in (d), 0.098% in (e) and to 0.035% in (f). (f)
Non-contact removal of latex particles from silicon wafers with LIP 801 pulse energy of 370 mj, pulse length of 5 ns, repetition rate of 10 Hz and a beam diameter of 5 mm. A 25-mm-diameter, 100-mm focal length convex lens with an antireflective coating was used to converge the beam. The sample was placed on a linear translation stage with a resolution of 20 ± 10 µm. Horizontal translation was achieved in two dimensions using sliding posts with millimeter markings. Vertical translation controls the critical parameter, the firing distance d, depicted in Fig. 1, i.e. the distance between the substrate and the center of the generated laser plasma core. A firing distance of 1.4 mm was utilized as this was found to give maximum removal efficiency with minimum mechanical substrate damage (observed in SEM images). A He-Ne laser was employed for positioning of the lens and vertical alignment of the sample with the Nd:YAG beam. A diode laser was used to mark the horizontal position of the plasma and to align the sample. The target test area where LIP was to be applied was identified with the diode laser. Previous experimental results showed that a single pulse affected a circular area with a diameter of about 2 3 mm [3]. As the test area was approximately 2 mm 2 in area, a single pulse was sufficient to cover the test area. Thus after the sample had been positioned to be shot at the target area, no more translation was required in any direction. 10 pulses were imparted to the target area in this experiment. The sample was analyzed using the Jeol Scanning Electron Microscope, by acquiring after LIP images of the target area. Images were acquired at the same magnifications and at the same locations as the before LIP images so that comparisons could be made and conclusions could be drawn. The LIP transient pressure (P ) response was obtained at different firing distances (d) using a PVDF poly(vinylidene fluoride) line transducer [11]. Using the JKR model, the minimum size of the PSL particle (D) that could be removed, corresponding to the pressure available was computed. These two results are combined in Fig. 4. Experimentally at a firing distance of d = 1.3 mm, substrate damage was observed. Therefore, the damage threshold is marked on the graph at d = 1.3 mm, and the corresponding particle removal limit is also marked. 3. RESULTS From Fig. 4, it is observed that, on decreasing the firing distance higher pressures are available and, therefore, smaller particles could be removed. Further it is observed that 60-nm PSL particles can be removed at a firing distance d of 2 mm. Figures 5, 6 and 7 present a set of SEM images of the substrate before and after the application of the LIP removal technique at three different locations on the inscribed reference pattern. It is evident that a vast majority of the deposited particles have been removed. The SEM images clearly show that 60-nm particles are present in the before LIP images and a vast majority of them have been removed in the after LIP images. Figure 7 shows that even large agglomerates of PSL particles can be removed. Though the SEM images reveal no damage in the test area, there is some
802 I. Varghese and C. Cetinkaya Figure 4. The maximum LIP pressure (P ) exerted on the surface at several firing distances (d)andthe corresponding PSL particle diameter (D) that can be removed with the pressure level P. The damage threshold is due to the physical contact between the plasma and the surface [11]. substrate deformation observed on the sides of the inscribed diamond reference pattern. This damage can be attributed to the fact that, since a force was applied on the substrate by the diamond scriber while making the reference pattern, the surface roughness of the substrate increased, thus resulting in a weaker substrate. Such damage is of no concern as in the actual industrial application there would be no need for a reference pattern to be inscribed, as the entire wafer mounted on a scanning system will be cleaned. It is noteworthy that no re-deposition of the particles is observed during the experiments. As the after LIP images indicate no ablated substrate or particles, we can conclude that no thermal damage of the substrate or the particles occurs. During SEM examinations, no mechanical damage on the surface of the wafer was observed either. 4. CONCLUSIONS AND REMARKS The detachment and removal of 60-nm polystyrene latex (PSL) particles from silicon substrates with the laser induced plasma (LIP) technique has been demonstrated. Utilizing a high frequency ultrasonic method at a 50 MHz level, the PSL particles were well dispersed and deposited on a vibrating substrate using the dropagitation technique. Using LIP, 60-nm PSL particles were removed, to a large extent, as was evident from the set of images presented. No substrate damage incurred in the regions of interest at the shooting distance of 1.4 mm with 10 pulses. The
Non-contact removal of latex particles from silicon wafers with LIP 803 (a) (b) Figure 5. 5000 magnification of (a) before LIP and (b) after LIP SEM images at one location. The approximate size of an individual PSL particle is identified in each of the images. The dashed lines indicate the boundaries between the cleaning zones and location markings.
804 I. Varghese and C. Cetinkaya (a) (b) Figure 6. 5000 magnification of (a) before LIP and (b) after LIP SEM images at another location. The approximate size of an individual PSL particle is identified in each of the images. The dashed lines indicate the boundaries between the cleaning zones and location markings.
Non-contact removal of latex particles from silicon wafers with LIP 805 (a) (b) Figure 7. 5000 magnification of (a) before LIP and (b) after LIP SEM images at another location. PSL agglomerate, size identified in (a), has been removed in (b). The dashed lines indicate the boundaries between the cleaning zones and location markings. Removal of large agglomerates is illustrated.
806 I. Varghese and C. Cetinkaya shockwave imparts enough energy to the particles to overcome the adhesion forces existing between the PSL particles and the silicon substrate. If the number of pulses is increased, a better removal can be attained, but the shooting distance is constrained by possible substrate damage. Based on the results presented, LIP can be concluded to be an efficient method for the removal of PSL particles 60 nm and larger. The LIP process can be applied on extended areas to clean full-scale wafers. Using a scanning mechanism, the entire process can be automated for rapid cleaning of large wafers (e.g. 300 mm), which would have significant industrial relevance. Thus, we conclude that the LIP removal technique has serious potential as a nearfuture cleaning technique. Experimental work to investigate the removal of 30-nm PSL particles from silicon surfaces is currently underway. Acknowledgements The authors acknowledge the National Science Foundation (Nanoscale Exploratory Research Program, Award ID 0210242), the New York State Science and Technology Foundation and the Center for Advanced Materials Processing (CAMP) for their partial financial supports. REFERENCES 1. The International Technology Roadmap for Semiconductors 2002 Update, accessible at http://public.itrs.net/files/2002update/2002update.pdf (2002). 2. J. Lin and C. Cetinkaya, J. Adhesion Sci. Technol. 17, 91 113 (2003). 3. C. Cetinkaya, R. Vanderwood and M. Rowell, J. Adhesion Sci. Technol. 16, 1201 1214 (2002). 4. J. M. Lee and K. G. Watkins, J. Appl. Phys. 89, 6496 6500 (2001). 5. R. Vanderwood and C. Cetinkaya, J. Adhesion Sci. Technol. 17, 129 147 (2003). 6. M. Soltani and G. Ahmadi, J. Adhesion 44, 161 175 (1994). 7. C. Cetinkaya, C. Li and J. Wu, J. Sound Vibr. 231, 195 217 (2000). 8. T. Hooper, Jr. and C. Cetinkaya, J. Adhesion Sci. Technol. 17, 763 776 (2003). 9. K. L. Johnson, K. Kendall and A. D. Roberts, Proc. Roy. Soc. London A 324, 301 313 (1971). 10. F. Zhang, A. A. Busnaina, M. A. Fury and S. Wang, J. Electr. Mater. 29, 199 204 (2000). 11. C. Cetinkaya and M. D. Murthy Peri, Nanotechnology 15, 435 440 (2004).