Papers by Keyword: Megasonic Cleaning

Abstract: Due to the emergence of sub-7 nm technologies, next generation CMP slurry formulations have continued to increase in additive (nanoparticle and chemistry) complexity to meet stringent device specifications. Therefore, it is essential to probe the molecular level interactions at the nanoparticle/slurry chemistry/substrate interface and in turn correlate them to key performance metrics such as removal rate, post CMP defects, and planarization efficiency. This work will address key interactions through a series of case studies focusing on the role of supramolecular structure and cleaning method (i.e. contact vs. non-contact) during STI post-CMP cleaning, probing the impact of supramolecular structure and mode of cleaning relevant to Cu post-CMP, and development of a biomimetic matrix with chemical activity to act as a brush in STI post-CMP cleaning processes. Results show in both BEOL and FEOL post-CMP cleaning there is a strong correlation to the delivery and “soft” nature of the chemistry to allow for effective particle removal at low mechanical force and prevent further defect formation. Furthermore, this work shows a clear correlation between supramolecular structure and particle removal efficiency under both contact and non-contact post-CMP processes.

Abstract: Megasonic cleaning is one of the promising technologies to remove the particles during semiconductor processing. Acoustic bubble cavitation plays a key role in removing the particles. In this work, the effect of an anionic surfactant sodium dodecyl sulfate (SDS) on a bubble in the presence of hydrogen dissolved DIW water was studied. The bubble dynamics were observed using a high-speed camera. It was found that with the increase of surfactant the bubble characteristics were changed very significantly. Several parameters affecting the bubble dynamics were investigated.

Abstract: The removal of nanoparticles from patterned wafers is one of the main challenges facing the semiconductor industry. As the size of structures shrinks with each new generation of devices, it becomes more difficult to remove nanoscale particles. Nanostructures (specially, poly silicon lines) were found to be vulnerable to damage as a result of cavitation when megasocnic cleaning is utilized. Megasonics utilizes acoustic streaming to reduce the acoustic boundary layer and utilize the generated pulsating flow to remove nanoscale particles from trenches and other structures on the wafer. Although Megasonics is believed to be a solution for many of these cleaning challenges, it has been shown to cause damage to nanoscale device structures such as poly-silicon lines.

Abstract: The continuous miniaturization of electronic building blocks in the semiconductor industry imposes more stringent requirements on the different cleaning processes. Purely chemical particle cleaning is based on weak etching of the substrate. This etching reduces the attractive van de Waals interaction between particle and substrate and at the same time, electrostatic repulsion ensures the removal of particles. This technique is not applicable anymore for future technology nodes, since an unacceptable large substrate loss (up to 3 nm) is necessary to obtain high particle removal efficiencies with pure chemical cleaning alone [. As a result, an additional physical force has to be considered to overcome this limitation. Several physical cleaning techniques exist, but all of them suffer from too much damage creation when fragile structures are cleaned. Currently, the industry is more focusing on spray cleaning and state-of-the-art spray tools show a high control over droplet size and droplet velocity [2]. Despite all of the advancements in spray cleaning, damage creation of fragile elements remains an issue, which could partially be attributed to the chaotic behavior of the water layer on the wafer surface [3]. Therefore, megasonic cleaning is still considered as a possible alternative to reduce damage formation during a physical cleaning process. Recently, it has been shown that the acoustic pressure amplitude can be reduced while maintaining the same particle removal efficiency level. This is achieved by (1) using pulsed acoustic fields which makes it possible to control the average bubble size and maximize the number of resonant bubbles, by (2) increasing the dissolved gas concentration which facilitates bubble nucleation and, finally, by (3) introducing traveling waves to transport bubbles to the wafer surface which needs to be cleaned. These conditions are briefly discussed and are applied during the investigation of the influence of dissolved CO₂ on bubble activity. Dissolved CO₂ is particularly interesting since it has been reported that sonoluminescence (i.e. strong bubble collapse) as well as damage formation is reduced when CO₂ is added to the cleaning liquid [4,5]. Here, it is shown that also particle removal efficiencies (PREs) diminish with increasing CO₂ concentrations.

Abstract: Removal of particulate residues represents a very challenging step in current CMOS-technology nodes. The continued miniaturization and the introduction of novel materials in the semiconductor industry have resulted in very stringent requirements for device fabrication steps such as cleaning processes [. Physical forces, acting directly on the surface to be cleaned, are currently employed for delicate particle removal as an alternative to more aggressive chemistries [2]. High frequency ultrasounds (500 kHz 4 MHz), or megasonics, rely on the action of oscillating bubbles created during the ultrasonic agitation of the cleaning liquid. Strongly oscillating gas bubbles are able to generate shear forces, which are considered to be responsible for cleaning [3]. However, collapsing bubbles close to a surface can also produce water jets and shockwaves which lead to damage of fragile structures. Fundamental research is needed in order to overcome these issues by improving the understanding of the physical parameters playing a role in the acoustic cavitation of bubbles. This study reports the effects of lowering the surface tension of the liquid bulk on the bubble activity in the MHz range. A lower surface tension (45 mN/m) with respect to water (72 mN/m) is obtained by adding a non-ionic surface-active agent (TritonX-100). After fully characterizing its wettability, a cleaning solution containing surfactant is investigated under pulsed and continuous acoustic fields, for different acoustic amplitudes and gas concentrations. The aim is to increase bubble activity while reducing the strength of the bubble collapse. The results obtained can be useful in tuning megasonic cleaning systems towards more efficient processes.

Abstract: An important problem in megasonic cleaning is the nucleation process of bubbles, which act as the cleaning agents. A fundamental understanding of this nucleation process will help to optimize the cleaning parameters for future applications to achieve damage free cleaning. In this work, we use quantitative stroboscopic Schlieren imaging to study the interaction of nucleating bubbles with a travelling acoustic wave. The advantage of this method is that it is non-interfering, meaning that it does not disturb the bubble nucleation. It is revealed that nucleation mechanism is a 2 step process, where a regime of slow bubble growth due to rectified diffusion is subsequently followed by a transient cavitation cycle, where bubbles grow explosively. The latter is accompanied by broadband acoustic emission and enhanced thermal dissipation, leading to the occurrence of thermal convection visible in the Schlieren images.

Abstract: Light emission in sound-irradiated liquids, known as Sonoluminescence (SL), is associated with the phenomenon of cavitation that affects wafer damage during megasonic processing of wafers. It has been shown that the intensity of SL can be substantially decreased through the dissolution of carbon dioxide in deionized water. However, such dissolution decreases the pH to roughly 4.0, which is not very desirable for the removal of contaminant particles. This paper reports two chemical systems that are capable of taking advantage of the effect of CO₂ while allowing the use of slightly higher pH values. Specifically, NH₄OH/CO₂ and NH₄HCO₃/dilute HCl systems have been shown to be capable of well controlled reduction in SL at pH 5.7 or 7.0. In order to test whether the free radical scavenging ability of CO₂ may be responsible for its strong SL-inhibitory effect, the effect of a well known free radical scavenger, dimethyl sulfoxide (DMSO), on SL produced in DI water has been investigated.

Abstract: An improved fundamental understanding of the megasonic cleaning process is necessary to optimize cleaning efficiency and minimize the unwanted damage to fragile structures. Argon sonoluminescence (SL) measurements are done to achieve an improved insight in the collapse threshold and behavior of microbubbles. This paper reports on acoustic cavitation by means of Ar Sonoluminescence (SL) investigation achieved with a dedicated test cell, a photomultiplier tube (PMT) and a gasification system. The results show an increase in SL signal as a function of the applied acoustic power density. An increase in Ar concentration results in a decrease in SL signal. Furthermore, a clear hysteretic behavior in the SL signal is identified when ramping the acoustic power up and down. This hysteresis effect can be attributed to the nucleation of bubbles during the increasing branch of the power loop. Finally the time evolution of SL light after the switching on of the acoustic transducers revealed the existence of a delay time.

Abstract: The megasonic cleaning efficiency is evaluated as a function of the angle of incidence of acoustic waves on a Si wafer. Acoustic Schlichting streaming alone is not able to remove nanoparticles smaller than 400 nm. It is shown that oscillating or collapsing behavior of bubbles are responsible for removing nanoparticles smaller than 400 nm during a cleaning process with ultrasound. Optimal particle removal efficiency is obtained around the angle of acoustic transmission of the silicon wafer.

Abstract: 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.