Megasonic Cleaning Research Articles

The Influence of Standing Waves on Cleaning with a Megasonic Nozzle

A non uniform sound field distribution can be a problem in a megasonic cleaning system, since a higher sound intensity can cause damage, while areas exposed to a lower intensity will be insufficiently cleaned. These non uniformities can be the result of sound field reflection, leading to standing waves, and the interference related to the near field. In a single wafer tool with a transducer facing the wafer a small height difference will have a large impact on the cleaning efficiency if standing waves are present. Here we study the impact of the wafer transducer height in a cleaning system using a megasonic nozzle above a rotating wafer.

Megasonic cleaning of wafers in electrolyte solutions: Possible role of electro-acoustic and cavitation effects

Investigations have been conducted on the feasibility of removal of particles from silicon wafers in electrolyte solutions of different ionic strengths irradiated with megasonic waves. Cleaning experiments have been performed using potassium chloride (KCl) as a model electrolyte and silica particles as model contaminant particles. Particle removal efficiency (PRE) increases with KCl concentration and transducer power density and much lower power densities may be used at higher KCl concentration for a comparable level of cleaning. Enhanced cleaning in KCl solutions has been explained as due to two types of electro-acoustic potentials, namely, ionic vibration potential (IVP) and colloidal vibration potential (CVP) that arise when the sound wave propagates through the electrolyte solution. Theoretical computations have shown that the removal forces due to CVP are much larger in magnitude than those due to IVP and are comparable to van der Waals adhesion forces. The effect of ionic strength on cavitation has been investigated through the measurement of acoustic pressure in solutions using a hydrophone. Using Fourier transformation of time dependent pressure data, the size distribution of stable bubbles in KCl solutions of different concentration has been obtained.

Alternative Photoresist Removal Process to Minimize Damage of Low-k Material Induced by Ash Plasma

Dry ashing of photoresist (PR) using oxygen-containing plasma applied subsequently to an etch plasma leads to degradation of porous low-k material. The surface region is substantially depleted in carbon. The low-k film becomes more hydrophilic after being subjected to plasma etch and especially ash process as evidenced by water absorption results. The amount of absorbed water into a 30% porosity film at moisture saturation is estimated to be about 15% of the film volume, which corresponds to 50% of the total pores in the low-k film. A wet, alternative means for PR removal based on dissolution of PR in organic solvents combined with physical forces is presented. Under certain conditions, megasonic cleaning resulted in complete removal of the PR layer without damaging of the dielectric lines. These results suggest that the PR crust is permeable to these solvents and that out-diffusion of dissolved bulk PR also occurred through the crust. Dissolution of bulk PR in organic solvents first makes the PR structure more fragile, then the physical energy helps to remove the remaining crust mechanically without dissolving it. Compared to plasma ashing, solvent strip shows no carbon depletion and no significant increase in k-value.

Global Trend of Precision Cleaning Technology and the Application in Hard Disk Drive Industry

As industry trends for hard disk drive and related electronic devices are moving toward high capacity and miniaturization. A big challenge is to find increasing the efficient ways to clean the parts which are regarded as the most stringent in cleanliness. The critical issues include the methodology for identifying the contaminants and how to choose a correct cleaning chemical and cleaning technology. Today's hard disk media, particularly the perpendicular recording media, is capable of storing up to 300GB per disk under which the distance between the recording head/disk have to be reduced to a few nanometers. It will bring more difficulties for increasing the reliability and disk capacities. On the other hand, all components within the hard disk drive are required to avoid submicron particulate contamination (<0.1μm). Megasonic and ultrasonic cleaning are the most effective and promising technologies to remove such contaminants. This presentation will discuss the technical trend and possible solution of precision cleaning and the strategy for selecting the cleaning chemicals and technologies to meet the cleanliness requirement through a case study.

Cleaning of semiconductor substrates by controlled cavitation

The continuing downscaling of device geometries in the semiconductor industry is driving the requirements for both process and contamination control. Historically, the physical and the chemical processes required for contamination control were evolutionarily scaled with device geometry. However, today's tailored wet‐chemical cleaning approaches must strive to meet stringent requirements to assure a minimal material loss and no damage to extremely fragile structures. While chemical solutions exist for the control of molecular‐organic and metallic ion contamination, the physico‐chemical solutions for the removal of nanosized particulate contamination to critical diameters below 20 nm are still undetermined. Therefore, the potential and the limitations of megasonic cleaning, which is mainly based on cavitation, are carefully balanced and a detailed understanding of the ongoing physical mechanisms is necessary to maintain a stable window of operation. The relevant active mechanisms present in such a cavitatio...

In‐situ monitoring of megasonic cleaning

Acoustic agitation is used in ultrasonic cleaning to induce a physical force to remove nanoparticles in semiconductor processing. The performance of cleaning tools is commonly quantified ex‐situ by the particle removal efficiency η (i.e. the ratio of the remaining to the original particle concentration). The resulting η ‐wafer maps often reveal spatial nonuniformities related to tool design. The local study of the cleaning dynamics, quantified by the removal frequency (fR[s‐1]) also indicates that the density of cleaning events over a wafer can be strongly non homogeneous. Because the sound frequencies used in industrial tools are in the MHz range, visualization of the cavitating bubbles (resonant size 3 μm) is highly challenging, both in time and in spatial resolution. Considering that the cleaning effect of a cavitating bubble is permanent a method is presented here to determine the acoustic cleaning event size (Aevent[μm2]) and the event flux (φ[μm‐2s‐1]) through visualization techniques. The method consists of time‐resolved imaging of the removal of a nanoparticle layer deposited on a transparent substrate. The microscopic cleaning activity can then be correlated to the actual removal frequencies fR through the relation fR=φAevent to further clarify the cleaning mechanisms in industrial tools.

The Removal of Silica Particles from Micron Wide Trenches by Megasonic Cleaning

In order to obtain high yields during IC manufacturing, particles - added during layer deposition, etching … - have to be removed. In order to meet the stringent requirements set by the ITRS roadmap, this cleaning has to occur with minimal substrate etching. This necessitates the use of physically-assisted particle-removal techniques, e.g. megasonic cleaning. These methods are usually evaluated on blanket wafers. However, many cleaning steps occur on patterned wafers. The goal of this paper is to investigate the particle removal efficiency (PRE) for patterned substrates compared to blanket wafers. A full-wafer contamination and detection protocol was developed to evaluate the removal efficiencies of micron-wide trenches. The student-t test reveals a significantly lower PRE for 1μm wide by 2.2 μm deep trenches versus a blanket wafer for megasonic cleaning. This is relevant for STI cleaning and cleaning of dielectric trenches in damascene patterning.

Impact of Re-Gasified Water on Megasonic Cleaning

Megasonic cleaning using de-gassed water (less than 2ppm N2, O2, CO2) in a 300mm batch immersion tool often does not give optimal particle performance, with particle streaks and clusters added onto the wafer, and low particle removal efficiency (PRE). When water was re-gasified with N2, the resultant stable cavitation activity reduced particle adders and increased PRE. With N2 concentration increased to just above 5ppm, number of particle adders decreased by three folds. Optimal particle performance could be obtained by operating at an N2 level close to saturation.

Acoustic Field Analysis of a T Type Waveguide in Single Wafer Megasonic Cleaning and its Effect on Particle Removal

T type megasonic waveguide was analyzed by finite element method (FEM), acoustic pressure measurements and particle removal efficiency for the single wafer cleaning application. Compared to conventional longitudinal waves, a transverse waves were generated in a T type waveguide. Not like longitudinal waves, transverse waves showed changes of direction and phase which increased the cleaning efficiency.

Study of the Dynamics of Local Particle Removal Efficiencies Using Localized Haze Maps

The local particle removal efficiency (PRE) of nano particles in megasonic cleaning experiments is studied. This approach makes it possible to quantify non uniform cleaning effects over the wafer and to look into the dynamics of particle removal at different areas on the wafer. A direct correlation between PRE and megasonic induced damage of device structures demonstrates that a considerable amount of damage is already formed at less efficiently cleaned areas of the wafer.

Local Removal Frequency of Nano Particles in Megasonic Cleaning

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Damage Clustering and Damage-Size Distributions After Megasonic Cleaning

In this work the damage added on gate stack features after batch megasonic cleaning is identified and classified by SEM review. To understand if damaging is a random or a clustered process, damage clustering distributions are compared with a theoretical random distribution of damage sites. In the damaged areas, defect clusters are observed that we explain by the presence of so-called weak spots in the neighborhood of a damaging event. Only a limited influence of the megasonic cleaning settings was found on the damage-size distributions. This suggests that, for the cleaning conditions investigated in this work, the number of added defects varied with the amount of the damage events but the event strength remains the same.

Local Removal Frequency of Nano Particles in Megasonic Cleaning

In single wafer megasonic cleaning non homogeneous cleaning efficiencies across the wafer are often observed. By studying the local cleaning dynamics, which is quantified by the particle removal frequency fR, it will be shown that the most important factor generating these non homogeneities is a difference in dwell time of the sound field above different spots of the wafer. The impact of the dwell time is studied for 2 different tool designs: a megasonic nozzle and a rod system. For the megasonic nozzle system, adapting the scan speed of the nozzle above the wafer can eliminate most of the variation in removal frequency across the wafer. For the megasonic rod system the attenuation of the sound field in the rod results in a linear dependence of the removal frequency on the wafer radius, thereby cleaning fastest at the spots with the shortest dwell time.

Particle Removal from Micrometer-Sized Trenches Using High-Velocity-Aerosol Cleaning and Comparison with Megasonic Tank Cleaning

Cleaning technologies employing physical forces (such as megasonic and high-velocity-aerosol cleaning) are usually evaluated by comparing the particle removal efficiency on blanket wafers with damage formation on fragile structures, typically gate stack type structures. However patterned wafers provide a much more realistic cleaning challenge. Particle removal efficiencies from trenches in micrometer dimension have been evaluated using high-velocity-aerosol cleaning. When using short cleaning times, a large decrease in cleaning efficiency is found for even modest pattern dimension. By extending the cleaning time, cleaning efficiencies comparable to blanket-wafer tests can be reached. Moreover, batch megasonic cleaning shows a similar decrease in performance as high-velocity-aerosol cleaning.

Shock wave emission upon spherical bubble collapse during cavitation-induced megasonic surface cleaning

When a gas bubble in a liquid interacts with an acoustic wave near a solid surface, the bubble first expands and then collapses. In this paper, a mathematical framework combining the Gilmore model and the method of characteristics is presented to model the shock wave emitted at the end of the bubble collapse. It allows to describe the liquid velocity at the shock front as a function of the radial distance to the bubble center in the case of spherical bubble collapse. Numerical calculations of the liquid velocity at the shock front have shown that this velocity increases with the acoustic amplitude and goes through a maximum as a function of the initial bubble radius. Calculations for different gas state equations inside the bubble show that the Van der Waals law predicts a slightly higher liquid velocity at the shock front than when considering a perfect gas law. Finally, decreasing the value of the surface tension at the bubble/liquid interface results in an increase of the liquid velocity at the shock front. Our calculations indicate that the strength of the shock waves emitted upon spherical bubble collapse can cause delamination of typical device structures used in microelectronics.

Experimental and Numerical Investigation of Nanoparticle Removal Using Acoustic Streaming and the Effect of Time

The removal of nanoparticles is becoming increasingly challenging as the minimum linewidth continues to decrease in semiconductor manufacturing. In this paper, the removal of nanoparticles from flat substrates using acoustic streaming is investigated. Bare silicon wafers and masks with a silicon cap layer are cleaned. The silicon-cap films are used in extreme ultraviolet masks to protect Mo–Si reflective multilayers. The removal of polystyrene latex (PSL) particles from these substrates is conducted using single-wafer megasonic cleaning. The results show higher than 99% removal of PSL nanoparticles. The results also show that dilute SC1 provides faster removal of particles, which is also verified by the analytical analysis. Particle removal from the Si-cap substrate is slightly more difficult as compared to bare silicon wafers. The experimental results show that the removal of nanoparticles takes a relatively long removal time. Numerical simulations showed that the long time is due to particle oscillatory motion and redeposition, and that this phenomenon is not observed in the removal of sub- or larger size particles.

Effect of surfactant on removal of particle contamination on Si wafers in ULSI

The adsorption mechanism of particle on the surface of silicon wafer after polishing or grinding whose surface force field is very strong was discussed, and the removal method of particle was studied. Particle is deposited on the wafer surface by interactions, mainly including the Van der Waals forces and static forces. In order to suppress particles depositing on the wafer surface, it is essential that the wafer surface and the particles should have the same polarity of the zeta potential. According to colloid chemistry and lots of experiments, this can be achieved by adding surfactants. Nonionic complex surfactant was used as megasonic cleaning solution, and the adsorptive state of particle on Si wafers was effectively controlled. The efficiency and effect of megasonic particle removal is greatly improved. A perfect result is also obtained in wafer cleaning.

Megasonics: A Cavitation Driven Process

In this paper attention is given to different parameters playing a role in megasonic cleaning, which is based on gaseous cavitation such as gas type and concentration, acoustic drive pressure and their influence on sonoluminescence, the motion of a void, particle removal efficiency, uniformity and damaging. The cavitation process is a non-uniform process in itself, but also larger scaled non-uniformities can be observed for SL and PRE. Single bubble simulations show that only a limited bubble size distribution will be active in a megasonic field and that the main parameters to consider are the power and the bubble size affected by the saturation level of gas. Depending on the megasonic system and process conditions, a positive relation between SL and PRE is not always found but damage always increases along with PRE. In fact not the type of gas but the level of saturation is determining the success of the particle removal in a megasonic process.

Evaluation of Megasonic Cleaning for Sub-90nm Technologies

Cleaning of nanoparticles (&lt; 50nm ) is becoming a major challenge in semiconductor manufacturing and the future use of traditional methods, such as megasonic cleaning, is questioned. In this paper the capability of megasonic cleaning to remove nanoparticles without inflicting damage to fragile structures is investigated. The role of dissolved gas in cleaning efficiency indicates that cavitation is the main cleaning mechanism. Consequently gas mass-balance analyses are needed to optimize the performance of cleaning tools. When gas is dissolved in the cleaning present tools can remove nanoparticles down to about 30 nm using dilute chemistries at low temperature. Ultimate performance is limited by cleaning uniformity, which depends on tool design and operation. However no tool reached the target of high particle removal efficiency andlow damage. Significantly lower damage could only be obtained by decreasing the power, at the cost of a lower cleaning efficiency for nanoparticles. The development of damage-free megasonic is discussed.

Damage-Free Cleaning of Sub-50 nm Devices Using Directed Megasonics Technology in a Single Wafer Processor

Introduction It has been well established that single wafer cleaning tools can achieve reduced cycle times and improved cleaning performance relative to batch tools. Recently, a unique immersion-based single wafer processor, the Emersion, was shown to achieve damage-free megasonic cleaning of sub-50 nm device structures with a corresponding high particle removal efficiency (PRE) [1]. In this paper, we investigate the mechanisms of cleaning and damage-free processing with the Emersion multiple transducer chamber. We present particle removal efficiency, sonoluminescence imaging data, and megasonics damage results for a wide range of operating conditions. These results are used to propose a model for the operation of the multiple transducers in the chamber.