The following is a list of the abstracts for papers which will be presented in NINTH INTERNATIONAL SYMPOSIUM ON PARTICLES ON SURFACES: DETECTION, ADHESION AND REMOVAL. The listing is alphabetical by presenting author. This list is updated continually to add abstracts as they become available and make appropriate corrections. This list may be conveniently searched by using the editor provided with most popular browsers (e.g. Microsoft Explorer, Netscape, ... etc.)
Particle Removal Challenges and Solutions in Semiconductor Fabrication CMP Processes
Chemical mechanical planarization (CMP) is an important semiconductor process technology used in the fabrication of many high-density IC components (memory, logic, signal processing, etc.). With CMP, semiconductor wafers are processed in contact with a polymeric polishing material and inorganic abrasive slurry in the presence of other chemical additives. Effective post-CMP particle cleanup is critical to achieving good product yields. We shall review several examples of real defects and describe process solutions for each defect. These defects include heavy particle contamination, isolated particle contamination, radial slurry patterns, annular slurry patterns, organic debris, sub-surface debris, microbial particles, water marks, and wafer backside contamination.
Ultrasonics and Particle Extraction
(Abstract not yet available)
*CLEAR & CLEAN Technologische Produkte GmbH, D-23568 Lubeck, GERMANY
The Effect of a Laser-induced Micriroughness of Textile Fibers on Adhesion and Capture of Micrometer-sized Particles
The chemical and physical surface properties of textile fibers govern friction, wetting behavior, adsorption abilities and the adhesion of particles, pastes and pigments not only of the fiber itself, but of the whole woven or non-woven textile structure. Appropriate techniques to secure a certain macroscopic behavior of textiles in further processing as well as in use accordingly form an important part of textile finishing.
A characteristic modification of the surface topography of highly absorbing and orientated synthetic fibers such as PET, PA or aramides was found after the irradiation with pulsed UV excimer lasers. In a range of moderate fluences from below the ablation threshold (about 30 mJ/cm² for PET) up to more than 150 mJ/cm² the originally smooth surface of these fibers changes its morphology to a rather regular roll-like structure in the micrometer scale, which is perpendicular to the fiber axis, after irradiation.
This basic effect has a marked potential to affect technical properties of the fiber, e.g. particle adhesion. Studies in industrial filter facilities with regard to dust filtration as well as in wet-filtration revealed that particles are captured in the grooves of the characteristic surface structure, where much higher adhesion forces act as can be expected from a smooth fiber surface. It is important to note that a large amount of very fine grain, i.e. sub-micrometer dust particles is captured, which would not normally be captured on the smooth fiber surfaces of commercial filters. Based on the same effect on particle adhesion, laser-treated wiping cloth showed a markedly increased wiping efficiency.
Principles and Mechanisms of Sub-Micron Particle Removal By CO2 Cryogenic Technology
The removal of sub-micron particles from surfaces is critical in different cleaning applications, i.e., integrated devices, Microsystems, read/write head, magnetic recording media and other processes requiring precision cleaning. The yield and device performance is based on the efficiency of the cleaning step. The paper investigates the theory and mechanisms of CO2 cryogenic aerosol cleaning and presents experimental data on particle removal in support of the theoretical findings.
Electrodynamic Removal of Particles
(Abstract not yet available)
Particle Removal by Attenuated Total Internal Reflection of Laser Light
This paper reviews the progress and potential of laser removal of particles from semiconductor and dielectric substrates (wafers, masks, blanks, flat panels or windows) by the attenuated total internal reflection of laser light. External laser illumination at near normal incidence removes metal particles from transparent substrates, even when less than 2% of the laser light is absorbed by the particle. By altering the polarization and angle of incidence, greater absorption and cleaning effectiveness have been reported. We report that still greater absorption can be achieved with internal illumination at an angle of incidence greater than the critical angle for total internal reflection. With capillary condensation or steam cleaning, absorptions of up to 80% are predicted. Internal illumination offers additional advantages. Particles on the surface cause a small fraction of the total internal reflected light to leak out of the substrate, so internal illumination may also be used to detect contaminants. Multiple internal reflections cause the beam to search for contaminants at the speed of light, rather than at the speed of scanners or translation stages. In the absence of contaminants, the beam is not attenuated so the laser energy is very efficiently used.
An independent evaluation of cleaning with CO2 -- where is the value?
Cleaning with pressurized and supercritical CO2 has been both successful and unsuccessful. This paper will define in what applications CO2 liquid or fluid cleaning should be attempted, and why.
The approach will be to use Hansen Solubility Parameters (HSP) which describe the inter-molecular forces associated with soils and solvents.
HSP have been defined for these systems from thermodynamic relationships and Hildebrand's original definition of solubility parameter. An equation-of-state (EOS) is used to compute the HPSs using these fundamental thermodynamic relationships for both CO2 and many soils.
Agreement of HSP for soils and solvents suggests where solvent cleaning applications can be successful. In this way, applications for CO2 can be defined both mathematically, and graphically.
Using Surfactant Adsorption Information to Predict Particle Removal
It is widely known that surfactants facilitate particle removal. The prediction of surfactant performance in enhanced particle removal under a variety of conditions requires an understanding of surfactant adsorption as well as the effect of adsorption on particle removal. This presentation discusses relevant modeling approaches to predict surfactant adsorption and associated particle removal.
Drying Techniques in the Semiconductor Industry
Throughout the entire manufacturing cycle of integrated circuits in the semiconductor industry, the silicon wafer substrates are subjected to various wet process steps, such as cleaning, etching, etc. Typically, these wet processes end by a final rinsing and drying step. Due to the overall cleanliness requirements in the semiconductor industry, however, each of these steps must be well optimized with respect to surface contamination. In this work, it is shown that for aqueous wet processing steps two important mechanisms for contaminant deposition occur: adsorption during immersion and deposition during evaporation. In this work emphasis is placed on evaporation during the drying step. An experimental methodology is outlined by which the amount of evaporation can be estimated. This is applied to various types of drying techniques, including spin drying and several Marangoni based drying techniques. Additionally, the methodology is used to determine the process window of each of these drying techniques.
Development of an Advanced Water Based Cleaning Technology
Applications of a new water blasting cleaning system are discussed. The system is equipped by a nozzle having an optimal cross section geometry. In the proposed study the nozzles had a rectangular cross section with a wide range of rectangular dimensions. The width of the rectangle changed from 2 too 100 microns while the length ranged from 0.02 ro 50 mm. In the course of the experiments water pressure changed from 50 to 4000 bars and water flow rate from 0.1 to 150 opthamology/min. the investigated nazzle applications involved metal degreasing and depainting, graffiti removal, cleaning of biomedical and electronic materials, etc. the available range of process variables enabled us to optimize the process parameters at each specific application and, thus to improve the system performance. The expected surface cleanliness attained in the course of water blasting by the use of the developed system was investigated. The feasibility of combining the surface decontamination with the control of the surface geometry was estimated. The potential for the improvement of the cleaning technology by the use of this system was determined.
Application of Fine Ice Particles for Precision Cleaning
A device for formation of fine (5 micron and less) ice particles was designed , built and its operation investigated. The particles were generated by freezing water droplets in a nitrogen bath. the size of the particles was controlled by the size of the droplets. The energy and exothermic efficiencies were determined in order to evaluate the energy efficiency of the process. The thermodynamic analysis demonstrated the effectiveness of the direct cooling of drplets by media contact. Various versions of the design of a reactor for particle production were explored. the particles generated were entrained by an air stream and used for blasting contaminated substrates. In order to maintain a desired particle hardness and prevent ice clogging, the air stream was cooled by the nitrogen gas generated in the course of ice formation. The device operated at a wide range of operating conditions. the investigation involved cleaning of highly sensitive substrates, electronic devices and biomaterials. The processes in question included paint removal from a coffee cup. The feasibility of damage free, pollution free processing of sensitive surfaces using ice powder was demonstrated.
Removal of Micron-order Particles by Impinging Air Jet
Removal of solid particulate contaminants using high-speed impinging air jet has been studied with particular attention to the effect of operating conditions on the removal efficiency. The operating conditions investigated in this study are air pressure in the jet nozzle, distance between nozzle tip and the surface on which particles are deposited, jet impinging angle and humidity of removal environment. The effect of the surface material is also discussed. Among them, the effect of the air pressure and the distance can be evaluated by dynamic pressure of air jet estimated by analytical equations. The optimum impinging angle has been decided by the removed particle fraction defined by the removal area and removal efficiency. It is shown that the removal efficiency is significantly affected by the humidity at removal environment and shows a peak at certain humidity (=optimum humidity). The optimum humidity depends on the surface material. In addition, new cleaning methods based on the method has been developed.
Photodigital Imaging as a Means of Monitoring Particulate Contamination on Surfaces
A novel photodigital method of monitoring the particle removal efficiency of a cleaning process is discussed. Particles that fluoresce under UV illumination are used in this method. Trace levels of these particles are easily detected by eye, or in photographs, when the surface being examined is illuminated with ultraviolet light (364 nm). Photographs are not only useful for giving a general idea of the cleanliness of the substrate, but can also provide quantitative information on the levels of surface contamination. This is accomplished by digitally analyzing these images on a personal computer with image analysis software. To quantify these observations, a procedure was developed in which photographs of the substrates being processed were captured with a digital camera (Cool-Snap Pro 36 bit color camera) and then digitally analyzed on a personal computer with Media Cybernetics' Image Pro Express image analysis software.
The decontamination factor (DF) associated with a cleaning operation is usually defined as the ratio of the concentration of contaminant present before cleaning (or decontamination) to the concentration of the contaminant present after cleaning. With this method, DF is defined as follows:
DF = (Pixel Intensity of an Contaminated Object - Pixel Intensity of a Blank Object)/ (Pixel Intensity of the Processed Object - Pixel Intensity of a Blank Object)
A major advantages of this method are its simplicity, rapidity, and ability to measure extremely low residual particle concentrations.
Decontamination of Surrogate Pu 238 Legacy Wastes
The United States Department of Energy's Savannah River Site (SRS), near Aiken, South Carolina, stores Pu-238 contaminated transuranic waste. Portions of this waste cannot be shipped to the Waste Isolation Pilot Plant (WIPP) in TRUPACT-II containers due to excessive generation of hydrogen gas as a result of radiolysis of organic constituents in the waste matrix. Decontaminating the organic matrix sufficiently to meet the hydrogen gas concentration limits has the potential of being a simple, safe and effective method of eliminating the need for continued storage at SRS.
A glovebox-sized decontamination system was designed, fabricated and tested with nonradioactive simulants to demonstrate the capability of the SonatolTM process as a means of treating legacy Pu-238 waste. In this process, solid test pieces contaminated with a Pu-238 oxide simulant were subjected to liquid shear in a 1% solution of a highly fluorinated surfactant in perfluoroheptane and then rinsed with perfluoroheptane. Job control waste and debris waste is extremely heterogeneous. The geometry and composition varies widely such that no single method of introducing shear forces may be well suited to the entire range of materials that will be encountered. Hence a cleaning chamber was designed that allows for mechanical agitation of the waste as well as spraying or ultrasonic agitation over a range of ultrasonic frequencies,. The used process liquid was filtered through metallic and ceramic filters to remove suspended particles, and recycled. The rinse liquid was generated by passing some of the recovered surfactant solution through a bed of activated carbon.
Three different combinations of surrogate waste materials were developed to represent divergent Pu-238 waste streams. The organic content of these surrogates varied from 17% to 45%. The surrogates were contaminated with a plutonium oxide simulant that was a mixture of 0.8 micron Osram Sylvania Phosphor 2284C particles, and of 0.3 micron cerium oxide particles. Phosphor 2284C fluoresces intensively at 546 nm (green light) when illuminated with a high intensity ultraviolet (365 nm) light. The cerium oxide (Ce(IV)O2) used in the experiments is a chemical analogue of plutonium oxide. The waste surrogates were contaminated by spraying with dilute (~1000-ppm) suspensions of simulant powders in a volatile, low surface tension liquid (3M's HFE-71-IPA). This liquid has a very low (<1 ppm) non-volatile residue. Evaporation of the liquid leaves behind a uniform powder deposit. The individual samples were analyzed with a high-resolution digital camera under UV light to provide immediate scoring of the decontamination factor (DF). A limitation of the optical system is that the maximum DF that can be measured accurately by this method is 100. Samples were also analyzed by neutron activation analysis (NAA) at the MIT Nuclear Reactor Laboratory. NAA involves the identification of trace quantities of elements by first activating the stable isotope and then counting the energies of the resulting gamma rays. Both simulants can be measured by neutron activation analysis (NAA) by analyzing radionuclides such as Ce-144 in cerium oxide and Zn-65 in Phosphor 2284C
After carrying out some optimization tests, decontamination protocols were developed that resulted in >99% removal of simulated contaminant from a wide range of organic and inorganic test pieces, and >99.9% capture of the simulant on a ceramic filter.
Adhesion and Friction of Single Micron-sized Particles
An "Particle Interaction Apparatus" for measuring interparticle forces based on the AFM technique has been developed. It has been used to measure the adhesion forces between individual pairs of silica, polystyrene, gold and iron particles (radius 0.5 - 10 µm) in air. For the silica particles, a linear dependence of adhesion on reduced radius was found as expected from the JKR and DMT theory. For the other materials, adhesion also had a clear tendency to increase with particle radius, but the more inhomogeneous surfaces or these materials caused a much higher scatter of the experimental results. For gold and polystyrene, increasing the contact time from 1 s to 100 s lead to an increase of the average adhesion force by roughly 30%.
Sliding friction was measured between silica particles (diameter 5 µm) and silicon wafers. The variation in friction force with applied normal force was found to follow the variation in contact area as predicted by the JKR theory.
Commercial AFMs were used to create quantitative "adhesion maps" (i.e. spatial distributions of the adhesion force) of particle surfaces to correlate surface topography with variations in adhesion.
Advanced Front End of the Line Clean for Post CMP Processes
Wafer surface preparation has become more critical to today's sub 90 nm high yield devices. Numerous techniques have established a post CMP cleaning procedure that focused mainly on reducing the defects (particles) on the wafer surface. Most of these standard methods typically use brush scrubbers and are based on an RCA type process . These procedures have proven to be costly and do not necessarily produce the desired wafer surface characteristics. In this paper, we propose an advanced front end of line (AFEOL) clean as an alternative to high cost scrubbing wafer-cleaning systems. The results show that the AFEOL process is a viable clean that meets the tight manufacturing low particle requirements.
Laser Cleaning of Fibrous Substrates
Biogenetic fibrous substrates such as paper and parchment consist of cellulose and protein. Their conservation is of major importance for the preservation of the human cultural and technological heritage. Their cleaning of condensed film and particulate contaminants with pulse lasers offers the advantage of a contact-less and dry process [1-3]. A common contaminant is particulate dust intermixed into the biogenetic fiber structure. The influence of various laser wavelengths (355nm, 532nm and 1064nm) and the aging status of modern paper test systems is reviewed. The fundamental understanding of the involved processes on fibrous substrates is still rudimentary and will be discussed in analogy to first models of laser-induced particle separation from well-defined flat surfaces [4-6].
1. W. Kautek, S. Pentzien, P. Rudolph, J. Krüger, and E. König, Appl. Surf. Sci 127-129 (1998) 746.
2. J. Kolar, M. Strlic, S. Pentzien, W. Kautek, Appl. Phys. A 71 (2000) 87.
3. W. Kautek, S. Pentzien, D. Müller-Hess, K. Troschke, R. Teule,
Laser Techniques and Systems in Art Conservation, SPIE 4402 (2001) 130.
4. R. Oltra, E. Arenholz, P. Leiderer, W. Kautek, C. Fotakis, M. Autric, C. Afonso, P. Wazen, SPIE Proceedings, "High-Power Laser Ablation II" 3885 (2000) 499.
5. Y.W. Zheng, B.S. Luk'yanchuk, Y.F. Lu, W.D. Song, Z.H. Mai, J. Appl. Phys. 90 (2001) 2135.
6. N. Arnold, Appl. Surf. Sci. 197 (2002) 904.
Removal of Particles Using the Combined Effect of Laser-induced Shock Wave and Explosive Vaporization of Liquid
The laser-shock cleaning (LSC) method based on the photomechanical effect of a high-power laser pulse is one of the promising techniques to remove micro/nanoscale contaminants. In the dry LSC technique, laser-induced breakdown of the ambient gas creates a strong shock wave and impingement of the wave onto a solid surface removes particles on the surface. In this work, a novel process combining the laser shock cleaning process with the conventional laser steam cleaning process is introduced.
The dry LSC process is first described, with emphasis on removal of submicron particulates from solid surfaces. The dynamics of the laser-induced plasma/shock wave and their effect on particle-removal process are experimentally analyzed for various process conditions. Discussions are made on the cleaning mechanisms based on the experimental observations. Finally, the novel process based on the combined effect of laser-induced shock impingement and explosive vaporization of a thin liquid film is introduced. It is shown that the novel process leads to substantial enhancement of particle-removal efficiency by synchronizing the shock arrival timing and the onset of explosive vaporization of the liquid film.
Recent Developments in Imaging and Analysis of Micro and Nanosize Particles
Imaging and analysis of micro and nanosize particles are crucial in understanding and developing micro and nanostructures and their interactions. Because of this importance, many advances and new developments have been made in characterization techniques for very small particles. For example, aberration-free imaging with sub-Œngstrom resolution has been successfully demonstrated in HRTEM (high-resolution transmission electron microscopy). Examples of other developments include combination of techniques such as SPM (scanning probe microscopy) with SNOM (scanning near-field optical microscopy) and other chemically-sensitive sensors, in-situ wet TEM, high-resolution NMR (nuclear magnetic resonance) spectroscopy, radio-frequency single-electron transistor, scanning thermal microscopy, grazing incidence small-angle x-ray scattering and high-resolution 3D x-ray microscopy. It is now possible to completely characterize particles from 0.1 to 100 nm. Some of these recent developments will be described.
1) Novellus Systems, Inc., 300 N 56th Street, Chandler, Arizona 85226
2) Novellus Systems, Inc., 4000 N 1st Street, San Jose, California 95134
Development of a Non-Contact Post-CMP Cleaning Process for Copper
A post-CMP (chemical mechanical planarization) cleaning process using megasonic energy supplied by a single wafer transducer is found to be effective for nano-size particle removal. Superior results are demonstrated using a non-contact approach compared to traditional cleaning methods, such as brushes. A unique cleaning chemistry and surface conditioning sequence combined with the appropriate treatment of the wafer immediately following polishing is developed. For blanket Cu films, typical cleaning results show defect densities of < 0.15 defects/cm2 measured by KLA Tencor AIT II and < 0.08 defects/cm2 measured by KLA Tencor SP-1 at 0.24 um. A reduction of defects compared with traditional brush cleaning methods for patterned Cu and TEOS is demonstrated. The scanning electron microscopy (SEM) defect analysis for three polished and cleaned 300mm Cu wafers exhibited an average defect density of 0.064 defects/cm2. Of these defects < 5% of the total defects are attributed to cleaning. The other defects are attributed to the incoming copper film or from the CMP process. This efficient cleaning technique can also be extended to other post-CMP applications.
1) Department of Chemistry and Physics, Arkansas State University, Jonesboro, Arkansas 72467-0419, USA
2) Department of Mechanical Engineering, University of Memphis, Memphis, TN 38152-3180
Particle-ETM-substrate interaction in Steam Laser Cleaning
Steam laser cleaning (SLC) of critical surfaces involves the deposition of an energy-transfer medium (ETM) on the particle-contaminated surface. Subsequent irradiation by a short pulse laser explosively evaporates the ETM, which then leaves the surface, carrying the contaminating particles with it. The ETM thus provides an optomechanical coupling of the laser pulse energy to these particles through sub-nanosecond explosive boiling and lift-off of the thin ETM layer. As has been shown in our recent SLC experiments, the mechanical interaction between ETM and particles is determined by the inertial force of ETM explosion underneath the particulates and the viscous drag force applied to them when the ETM lifts off as an initially intact liquid layer. SLC efficiency and threshold appear to be affected by the level of particle-substrate, particle-ETM and ETM-substrate interactions, depending strongly on the molecular properties of participating particles, substrate and ETM, while the ETM-particulate and ETM-substrate interactions may become critically important under the dynamic conditions of SLC. Experimental facts demonstrating different aspects of ETM influence on SLC parameters are presented.
A Study in Generating Ozonated DI Water for Wafer Cleaning
Ozonated DI water (DIO3) is a chemical solution for semiconductor wafer cleaning in integrated circuit manufacturing. A study of DIO3 generation was carried out by bubbling O3 gas into DI water. It was found that the maximum of the DIO3 concentration varied from place to place, due to the difference of DI water treatments. It was found that the concentration was always lower if DI water was pre-treated by 185nm wavelength ultraviolet light than without the pre-treatment, because the light excites DI water to generate hydroxyl radicals and the radicals decompose ozone molecules. In this case bubbling carbon dioxide (CO2) gas into the DI wafer neutralized the radicals, resulting in rebounding the concentration back up. Ozone molecules in DI water was monitored by absorbance at 254nm ultraviolet light, but was not hydroxyl radicals monitored. Both ozone molecules and hydroxyl radicals are highly reactive oxidants. In either case of high ozone molecular concentration with CO2 bubbling or of low ozone molecular concentration without CO2 bubbling, the DIO3 performed no difference in wafer cleaning. A result, for example, showed that particle removal efficiency by 80 ppm DIO3 with CO2 bubbling as the same by 6 ppm DIO3 without CO2 bubbling.
Department of Electrical Engineering, the University of Nebraska-Lincoln, NE 68588-0511
Laser-Assisted Nanofabrication Using Micro and Nanoparticles
Two laser-assisted approaches to fabricating nanoscale structures much smaller than the diffraction limit of the laser wavelength have been investigated. In the first approach, monolayers of particles, with different diameters ranging from 10 nm to 5 µm, were deposited on silicon substrates by self-assembly. A quartz plate was tightly placed on the surface of each sample. A KrF excimer laser beam (248 nm) was irradiated on the quartz / nanoparticle / silicon structure. The silica particles were imprinted into silicon substrates by the quartz due to the transient Si surface melting during the laser pulse. Ultrasonic cleaning and hydrofluoric-acid (HF) solution were then used to remove the silica particles. 2-D and 3-D photonic bandgap structures with nanoscale features were formed on Si substrate surfaces. In the second approach, optical resonance in spherical particles was used to "focus" an incident laser beam to a range much smaller than the particle size and the laser wavelength. Spherical silica particles with a size of 1 µm were self-assembled on Si substrates. After laser irradiation with the excimer laser, holes with a size of 100 nm were obtained at the original positions of the particles. Mechanism of the formation of the sub-wavelength structures were investigated and found to be the near-field optical resonance effect induced by particles on surface. The nanofabrication methods using laser beam in combination with self-assembled nanoparticles have potential applications in nanolithography, fabrication of nanophotonics devices, tribology and biology.
Allentown, PA 18195, USA
Particle Removal in Dense Phase Fluids Using Entrainers and Ultrasonic
Precision cleaning of semiconductor substrates during integrated circuit fabrication is typically performed using wet chemical processing. Such methods present significant disadvantages as circuit dimensions and device density advance. Among the alternatives under consideration are dense phase fluids, including supercritical fluids (SCFs) or compressed liquids. While SCFs retain densities similar to liquids they have viscosities and diffusivities similar to gases, they retain densities similar to liquids. Therefore, these fluids can penetrate small recesses in surfaces, and have been known to dissolve/extract contaminants from such recessesd with high efficiency. However, while SCF mixtures may effectively dissolve and remove some films, organic particulate or molecular contaminants, other inert particulate material may remain on the surface. A proposed solution to this problem would combine ultrasonic (>20 KHz) energy and dense phase fluid SCF cleaning in the same tool. In this process the semiconductor substrate would be exposed to ultrasonic energy while immersed in SCF mixtures containing dissolved solvents, chelating agents and entrainers. The ultrasonic energy dislodges the inert particles, while dissolved entrainers prevent re-attachment to the cleaned surface. A likely SCF candidate for this process is CO2 due to its high solvent capabilities and the ease with which it can be brought to supercritical conditions. The SCF state is created through heating and pressurization of the substrate cleaning chamber. Tests performed in SCF and compressed liquid CO2 have shown promising results on patterned post-ash substrates.
Adhesion of Particles on Surfaces: A New Thermodynamic Consequence
Above absolute zero, regardless of their phase of matter, atoms/molecules have significant thermal motion. Consequently, they have appreciable thermal kinetic energy and momentum. Thermodynamics has always accounted for the thermal kinetic energy. But in 1905, to treat the special problem of Brownian motion, Einstein introduced a monumental advancement into that statistical theory: thermal momentum. Since the 1960's, Einstein's theory has been generalized by this author to interfacial systems. The new thermodynamic theory has revealed that whenever certain parameters vary across a surface or any other interface, the first and second laws require the existence of significant electric charges at such sites. Consequently, between a particle and a surface, crucial electric dipole charges can form that induce mutually attractive Coulomb forces. The primary objective of this paper is to review the relevant theory; and to describe corroborative experiments involving the detection of charges on surfaces of different materials. Theory suggests, and experiments confirm that the effects of surface charges extend to centimeters, rather than to atomic distances, as the van der Waals theory predicts. To be cited are some theoretical and experimental investigations that could further the quantitative understanding of the mechanism of adhesion, and how to minimize its forces.
TNO TPD, P.O. box 155, 2600 JA Delft, THE NETHERLANDS
Removal of Sub-100 Nm Particles from Structured Substrates
For 157 nm and EUV lithography no suitable pellicle material is available for protection of the mask against particles. If such a mask is contaminated by particles as small as a quarter of the CD, print defects are likely to occur. Unless a suitable cleaning method is available, a contaminated mask can considered to be a write-off. A cleaning method should be able to remove particles as small as 30 to 50 nm and overcome the additional difficulty of removal from the trenches of the structures.
Particle removal by CO2 snow was investigated under water free and controlled exposure conditions. Structured substrates were obtained by writing dense lines and spaces with EBPG. These substrates were soiled with PSL and SiO2 particles. Particles were counted prior to and after cleaning with SEM.
CO2 snow proved to be very effective for particle removal from structured substrates. 60-70 % of all 50 nm PSL particles were removed from trenches between 100-250 nm. When the angle between snow jet and substrate increased, the particle removal diminished. Silica particles (30 - 100 nm) were nearly completely removed from flat areas and trenches. Ageing of the silica particles did not affect the removal.
1) Fa. Clear u. Clean Technologische, Produkte GmbH, Niels-Bohr-Ring 36, D-23568 Lübeck, GERMANY
Evaluating Wiping Materials used in Cleanrooms and other Controlled Environments
Wiping materials used in cleanroom have to fulfill a delicate job. Inseparable from the idea to create better or more efficient Hi-Tech-wiping materials for Hi-Tech-applications is the need of methods to evaluate the performance of such wipers. Since the early 70s when the first cleanrooms were installed in the United States, cleanroom wipers were demanded and available. The first tests to evaluate the quality of these cloths were also developed during this time. Most of the methods are still in use but should be questioned because they asses the wiping material paying no attention to the actual wiping process.
One of the main tasks of cleanroom wipers is to guarantee particle free surfaces. A dust particle with a size of a few microns size placed on a silicon wafer will destroy the produced micro-chips. Cleanroom wipers should therefore release and leave very low amounts of these particles during and after use as well in the environment as on the cleaned surface.
Established testing specifications are compared to alternative methods, discussing advantages and disadvantages of these methods. The aim is to demonstrate missunderstandings in the methods to evaluate the best cleanroom wiper.
Primary and Secondary Effect of Laser Cleaning for Nano-Sized Particulate Contamination
(Abstract not yet available)
Removal of Nano-Particles and Structural Damage in Megasonic Cleaning of Silicon Wafers
In semiconductor manufacturing, as features sizes are scaling down below 100 nm, particles with a diameter of a few tens of nanometers need to be considered as potential killer defects. For reasons related to substrate consumption budget, present critical cleans make use of megasonic agitation and diluted chemistries with low etching capability to remove contaminant particles. As particle size decreases, the ratio of adhesion force over cleaning force increases, thereby potentially compromising the particle removal efficiency (PRE). On the other hand, wafer surfaces may present fine structures with fairly high aspect ratios, such as gate electrodes, that become vulnerable to sideward impact by physical forces. The combination of these trends results in a collapse of the process window to the extent that cleaning of nano-particles is becoming a major challenge in production and the future use of traditional cleaning methods is questioned. In this study a survey was conducted on a representative set of megasonic cleaning tools. Results will be presented on parameters affecting PRE of small silica and silicon nitride particles in relation to damaging of poly-on-gate lines and the cleaning mechanisms with megasonics.
Single Wafer Megasonics Configurations: Parallel and Perpendicular to the Wafer Surface
Despite its long history with semiconductor industry, megasonics assisted wafer cleaning still remains the most effective method of particle removal in semiconductor wet process applications. However, rapidly advancing semiconductor technology demands more improvement in this process area. With introduction of 300mm wafer fabs, use of single wafer wet processing becomes an advantage due to its short development cycle time. The transformation from bath-type megasonic cleaning to a single wafer type has its challenges. The main challenge for single wafer cleaning is it must be fast and reliable to be competitive with wet-bench systems. Effectiveness of megasonics is typically the most critical part of process reliability because of the short cycle time. This requires that all causes and effects in the megasonic process are well understood. Since there are some gray areas in understanding the physical mechanisms involved in particle removal, in this work we have attempted to test the mechanism of cavitation and its role in particle removal. We will also review different configurations for single wafer megasonics: parallel and perpendicular to the wafer surface.
Modeling Particle Defect Transport During Sputter Deposition
(Abstract not yet available)
Laser Inactivation and Detection of Bacteria
With the ever increasing change towards higher technology services, processes and products, there is set to be continued increase in clean room demand for the foreseeable future. Access to clean room environments is critical for sustained industrial competitiveness; the ease with which companies can build and sustain such environments is therefore of major importance for their success. This is particularly true for economies with strategic emphasis on microelectronics, optoelectronics, nanotechnology, biotechnology, chemistry and pharmaceutical industries. Running clean rooms is expensive; it is not uncommon for a Class 10 facility to cost in excess of $500,000 p.a. Improvement in the delivery of clean room technology that has the potential to reduce running costs and improve performance overall may, therefore, play a pivotal role in encouraging and sustaining a country's growth and creative output and shifting its technology base towards higher value, advanced technology. One common theme that remains a problem is bacterial or spore infection as source of contamination in clean room production processes, reducing yield and thus profit margins. Lasers have been increasingly used to reduce particulate numbers and clean surfaces, however, there use at inactivating microorganisms remains less well known. The potential use of high power CO2 and Nd:YAG lasers as a sterilization tool for clean room applications will be investigated. Further work will be described where low power He-Ne laser based optical systems have been used to detect microorganism.
Clean Then Assemble Versus Assemble Then Clean: Several Comparisons
In precision assemblies, the dominant strategy is to clean all of the components individually and then assemble them in a cleanroom. It can be shown that many subassemblies can be assembled outside of a cleanroom and then cleaned and dried. This results in considerable savings: reduction in the amount of cleanroom floorspace, reduction in the number of individual parts that must be racked and cleaned, and reduction in the cost of operating the cleaner, etc. In addition, it can be shown that the assemble then clean strategy results in a cleaner part because contamination accumulated during assembly can be effectively reduced.
Several case studies will be presented.
Effect of Entropic Force on the Adsorption of Particles on Surfaces
The addition of non-absorbing smaller particles into stable larger particle suspension results in a depletion attraction between these larger particle pairs.1 The physical origin of this induced attraction is both simple and general. For each large particle that may be a colloid , a protein or a bacteria DNA in a cell, there surrounds a crust that is unreachable for the center of the added particles. De-mixing or self-organization of large particles will overlap their crusts and result in extra free volumes for polymer latex to translate and contribute extra translation entropy of the polymer latex, although at the expense of lowering the entropy of mixing. This "attraction by repulsion" underlies a wide spectrum of entropically driven phase separation and assembly phenomena and has been extensively studied theoretically, experimentally and by computer simulations.2-3 Kaplan et al.2 discovered that the instability arising binary particle mixtures was phase separation into ordered surface phase near a hard wall and a disordered bulk liquid phase. Ordered and disordered surface phases can also be understood using simple excluded-volume entropy arguments, in this case between the large particle and the surface of the wall. We studied the geometry of the particles adsorbed on metal surfaces by surface enhanced Raman scattering spectroscopy. And a depletion interaction was proposed to understand the self-aggregation of particles and the molecular orientation on surfaces, as shown in Figure 1.4
(1) Colloid Physics, Proceedings of the Workshop on Colloid Physics, University of Konstanz, Germany, 1995 (Physica A 1997, 235).
(2) Yodh, A. G.; Lin, K. H.; Crocker, J. C.; Dinsmore, A. D.; Verma, R.; Kaplan, P. D. Phil. Trans. R. Soc. Lond. A 2001 359, 921.
(3) Asakura, S.; Oosawa, F. J. Chem. Phys. 1954, 22, 1255.
(4) Zhou, D.; Zhang, J.; Li, L.; Xue, G. J. Am. Chem. Soc., 125(39) 11774, 2003.
Figure 1 Depletion effect of disk-like and hard sphere particles on the surfaces
(Left) Schematic of the depletion effect in of disk-like particles on the wall. The dark shaded region represents the gain of small-sphere excluded volume (entropy).
(Right) The centers of small spheres are excluded from a depletion zone (hashed regions) outside the large spheres and corrugated walls. The large particles are preferentially drawn to interior corners. The cartoon is inspired by Yodh, A. G. et al., [Phil. Trans. R. Soc. Lond. A 2001 359, 921]
1) Department of Mechanical Engineering, New Jersey Institute of Technology, Newark, NJ 07102
2) Chilworth Technology, Monmouth Junction, NJ 08852
Modeling of Flow-influenced Surface Deposition of Suspended and Charged Particulate in Confined Chambers
Flow patterns and trajectories of charged particles in confined chambers are important to the optimum design and operation of industrial devices such as in electrostatic precipitation and ink-jet printers. In this study, modeling of flow fields and particle trajectories of dilute gas-solid two-phase flows with charged particles in confined chambers is performed. The dilute gas-solid two-phase flows are simulated by use of a hybrid Eulerian - Lagrangian approach with the one-way coupling between the gaseous phase and particle phase. There is a strong coupling between the charged particles and the induced electric filed. The equation of motion of charged particles strongly depends on the electric field induced by charged particles. The electric field with space charge distribution in turn depends on the particle trajectories that determine the space charge distribution. Examples of flow pattern and particle trajectories in confined chambers with different charges are illustrated. Surface deposition of charged particles can be determined from the change of resonance module of a suspended sample plate. A modeling approach for this resonance change with various particle depositions is developed.