Megasonic Irradiation Research Articles

Slurry Activation for Enhanced Surface Redox Reactions in CMP

A novel add-on hardware device is placed near the point of slurry dispense that can instantaneously activate slurry performance during polishing via megasonic irradiation. This new technology (Flucto-CMP® is able to overcome the inherent polisher-slurry weaknesses such as wafer-level defects, process vibrations, cost of ownership, slurry waste, remval rate (RR), and RR selectivity. Flucto-CMP® has been successfully applied to various types of CMP slurries resulting in significant increases in the removal rates of copper, SiC, borosilicate hard mask, ILD, TiN, and silicon carbide through chemical alteration of the passivation layer needed for material removal. Specifically, for copper CMP, using Flucto-CMP®, we see an up to 31% boost in RR which is well correlated with the amount of reactive oxidizing species generated through irradiation as well as the measured relative indentation depth of the passivation layer. In addition to boosts in RR, for copper, we see a 50X drop in the variance of shear force and a 5X drop in the variance of normal force when Flucto-CMP® is used. This dramatic reduction in process vibration for copper soft-landing processes ought to reduce wafer-level defects. Much greater rate improvements are observed when polishing SiC, and borosilicate hard masks.

Role of ammonia and carbonates in scavenging hydroxyl radicals generated during megasonic irradiation of wafer cleaning solutions

Chemical formulations used for megasonic cleaning typically contain hydroxides, peroxides, carbonates, and others which can affect particle removal efficiency and feature damage. The current study is focused on investigating the role of carbonates in modulating the oxidation power of megasonic irradiated alkaline solutions through the scavenging of hydroxyl radicals as determined by fluorescence spectroscopy. Although studies exist in literature that report the scavenging of OH by carbonates, these studies have been conducted for waste water treatment applications using advanced oxidation processes. This work focuses on understanding the role of alkaline chemical formulations containing varying levels of carbonates and bicarbonates and solution temperatures on net generation of hydroxyl radicals for applications in semiconductor industry. Investigations were carried out at an acoustic frequency of ∼1MHz and different power densities (2, 4 and 8W/cm2).

Detection of HO<sub>2</sub>•/O<sub>2</sub>•<sup>-</sup> Radicals Formed in Aqueous Solutions Irradiated with Megasonic Waves Using a Cavitation Threshold (CT) Cell Set-Up

Acoustic cavitation, especially transient cavitation, in solutions is accompanied by a number of physical and chemical effects. Due to high temperature and pressure conditions inside bubbles at their collapse, excitation of various species as well as formation of radicals occurs in solution [1-4]. Water molecules excited by megasonic irradiation typically dissociate to hydrogen and hydroxyl radicals (H• and OH•) [5]. The hydroxyl radical is a strong oxidant while the hydrogen radical has reducing properties. In presence of O2 in the solution, H• reacts with O2 to form hydroperoxyl (HO2•) radicals, which act as a reducing as well as a (weak) oxidizing agent [6]. Dissociation of hydroperoxyl radicals result in the formation of superoxide anion radicals (O2•-) as follows [6]:

Measurement of hydroxyl radicals in wafer cleaning solutions irradiated with megasonic waves

Megasonic irradiation of aqueous solutions produces hydroxyl radicals primarily from dissociation of water under extreme transient cavitation temperature conditions. In the current study, the effect of various sound field (∼1MHz) and solution parameters on rate of generation of hydroxyl radicals in alkaline cleaning solutions of interest to semiconductor industry has been investigated. These parameters include transducer power density, liquid temperature, nature of dissolved gases, solution pH, and type of alkali. Terephthalic acid based fluorescence spectroscopy technique is used for the measurement of concentration of hydroxyl radicals.

Evaluation of Very Dilute Alkaline Solutions for Wafer Cleaning with Megasonic Irradiation

In the semiconductor wafer cleaning, ammonium hydroxide based APM (ammonium peroxide mixture) has been widely used to remove particles and organic contaminants [. However as the film thickness and line width of semiconductor structure scales down rapidly, the material losses by etching reaction of alkaline chemicals can cause serious problem in yield loss due to electric failure. The presence of H2O2 could enhance the material loss on silicon wafer. Very dilute alkaline chemicals might be of interest since it could minimize any possible ionic contamination or chemical residues from chemicals as long as we control the surface roughness and particle removal efficiency. Also the characterization of these very dilute alkaline chemicals will be very useful for particle removal in gas dissolved DI water.

Acoustic Cavitation Behavior in Isopropyl Alcohol Added Cleaning Solution

As the semiconductor manufacturing technology for ultra-high integration devices continue to shrink beyond 32 nm, stringent measures have to be taken to get damage free patterns during the cleaning process. The patterns are no longer cleaned with the megasonic (MS) irradiation in the advanced device node because of severe pattern damages caused by cleaning. Recently, several investigations are carried out to control the cavitation effects of megasonic to reduce the pattern damages. The mechanism of damage caused by an unstable acoustic bubble motion was mainly attributed to the high sound pressure associated with violent bubble collapse [1]. In order to characterize the dominant factors affecting the cavitation, MS cleaning was conducted with various dissolved gas concentrations in water. It was reported that the cavitation phenomena relating to particle removal efficiency (PRE) and pattern damage were considerably changed with the addition of a specific gas [2]. This changing behavior may be due to the difference in the physical properties of dissolved gases associated with acoustic bubble growth rate as a function of their concentration. In particular, cavitation effects induced during MS cleaning was controlled by adjusting the acoustic bubble growth rate. Also the change of bubble growth rate is well explained by both rectified diffusion for single bubble and bubble coalescence for multi-bubble, respectively. Similarly, it is well-known that surface active solute (SAS) in the ultrasound field plays an important role in controlling the cavitation effects. A detailed explanation of the acoustic bubble growth rate, cavitation threshold and their relationship with various types of SAS and concentration of biomedical and chemical reactions perspective have been reported elsewhere [3,4]. Their studies demonstrated that the change of cavitation effects depends not only on the chain length of alcohol in the solution but also on the physical properties such as surface tension and viscosity of SAS solutions.

Enhanced megasonic processing of wafers in MegPie® using carbonated ammonium hydroxide solutions

Megasonic irradiation of cleaning solutions removes particulate contaminants from wafer surfaces through the mechanisms of acoustic streaming and acoustic cavitation. Uncontrolled cavitation, however, also damages fragile wafer features. It has been hypothesized that damage results primarily from violent transient cavitation while cleaning results from the gentler, stable cavitation and is assisted by substrate etching at alkaline pH. A central challenge in the megasonic cleaning field is to find physical and chemical conditions that would suppress violent cavitation without adversely affecting cleaning efficiency. We have previously reported the strong ability of aqueous CO2 in suppressing sonoluminescence, a measure of transient cavitation, and wafer damage. However, dissolution of CO2 also renders solutions acidic and lowers their ability to remove particles. Here we report the development of two new systems, NH4HCO3/NH4OH and NH4OH/CO2(g) designed to suppress pattern damage and enhance cleaning efficiency. Using megasonic irradiation of bare and line-space patterned wafers in a single wafer spin cleaning tool, MegPie®, we measured the efficiencies of these systems as well as NH4OH, at pH 8.2, in removing SiO2 particles from oxide Si wafers and suppressing damage to wafer features. All systems were found to achieve high particle removal efficiencies but extent of damage was strongly reduced in the newly designed NH4HCO3/NH4OH and NH4OH/CO2(g) compared to NH4OH. This study establishes a means for well controlled generation of CO2(aq) over an extended pH range (4.0–8.5) and its application in strongly reducing wafer damage without compromising megasonic cleaning efficiency.

Effect of Dissolved CO2 in De-Ionized Water in Reducing Wafer Damage During Megasonic Cleaning in MegPie

Particle removal from wafer surfaces can be accomplished by irradiation of cleaning fluid by sound waves in the MHz frequency range. Unfortunately, unless proper cleaning conditions are chosen, megasonic irradiation may also result in damage to fragile wafer features. Here, we demonstrate a strong effect of dissolved CO2 levels on the reduction of wafer damage during megasonic cleaning. Test structures with L/S patterns were irradiated with 0.93 MHz sound waves at varying power densities and dissolved CO2 levels, in a single wafer spin cleaning tool, MegPie®. Dissolution of increasing amounts of CO2 in air saturated DI water caused a significant decrease in the number of breakages to line structures and also decreased the lengths of the line breakages, at all power densities up to 2.94 W/cm2. This ability of dissolved CO2 to protect against feature damage correlates well with its ability to suppress sonoluminescence in sound irradiated DI water.

The Improvement of Wet Anisotropic Etching with Megasonic Wave

A new method to improve the wet etching characteristics is described. The anisotropic wet-etching of (100) Si with megasonic wave has been studied in KOH solution. Etching characteristics of p-type (100) 6inch Si have been explored with and without megasonic irradiation. It has been observed that megasonic irradiation improves the characteristics of wet etching such as the etch rate, etch uniformity, surface roughness. The etching uniformity was less than ±1% on the whole wafer. The initial root-mean-squre roughness(Rrms) of single crystal silicon is 0.23nm [1]. It has been reported that the roughnesses with magnetic stirring and ultrasonic agitation were 566nm and 66nm [3]. But with megasonic irradiation, the Rrms of 1.7nm was achieved for the surface of 37µm depth. Wet etching of silicon with megasonic irradiation can maintain nearly the original surface roughness during etching. The results have verified that the megasonic irradiation is an effective way to improve the etching characteristics - the etch rate, etch uniformity and surface roughness.

Preparation of Oleic Acid/Water Emulsions in Surfactant-Free Condition by Sequential Processing Using Midsonic−Megasonic Waves

Emulsifying action of high-frequency acoustic waves was investigated on surfactant-free oleic acid/water mixtures using megasonic irradiation (200 kHz and 1 MHz) in baths manufactured for precision cleaning. While the droplet size distribution was bimodal for single irradiation at 40 kHz or 1 MHz, a unimodal distribution was obtained at 200 kHz irradiation. The whole amount of dispersed oil was evaluated with the total organic carbon measurement. The largest dispersed amount was obtained for the 40 kHz treatment, while the amount was quite small for 200 kHz or 1 MHz treatment. Then, a tandem emulsification method was devised for oleic acid/water emulsion, in which the first irradiation was made at 40 kHz, followed by the second one at 200 kHz over varying exposure times. A primary peak appeared at 100 nm droplet size accompanying with a subpeak at 400 nm in the first dispersion. The latter population decreased with increase in exposure time in the second dispersion until it disappeared. This indicates that the large droplets are disrupted to form or release the smaller ones as a result of sonic acceleration. The averaged droplet size showed a preliminary increase before being stabilized within 2 days in the 40 kHz → 200 kHz and 40 kHz → 200 kHz → 1 MHz sequential processing. Contrary to a low stability for the emulsion obtained at 40 kHz exposure, the turbidity for these emulsions could be observed for 1−2 years. A slight rise in the ζ potential was noticed for the droplet at the early stage. These results demonstrate that sequential processing with megasonic waves has a high potential to improve characteristics of unstable emulsions in surfactant-free condition.

계면활성제가 첨가된 DHF의 Post-Oxide CMP 세정 공정에의 적용 연구

Post-Oxide CMP(Chemical-Mechanical Polishing) 결과 실리콘 웨이퍼를 오염 시키고 있는 슬러리 입자의 세정 가능성을 조사하기 위하여DHF(Diluted HF)에 비이온성 계면 활성제인 PAAE(Polyoxyethylene Alkyl Aryl Ether), 비양성자성 용제인 DMSO(Dimethylsulfoxide) 와 초순수의 혼합물인 새로운 세정액을 제조하였다. 세정력을 평가하기 위해서 세정제 내에서 각각 다른 제타 포텐셜을 갖는 실리카(<TEX>$SiO_2$</TEX>), 알루미나(<TEX>$Al_2O_3$</TEX>)와 PSL(polystylene latex) 입자를 실리콘 웨이퍼 표면의 산화막에 인위적으로 오염시킨 후 실험에 이용하였다. 초음파하에서 세정액의 성능 평가 결과 본 세정기술은 효과적인 입자의 세정능력과 금속이온에 대한 세정 능력을 나타내고 있음을 확인하였다. 즉 기존의 APM(<TEX>$NH_4OH,\;H_2O_2$</TEX>와 D.I.W의 혼합물)과 달리 상온에서 세정이 가능하고 세정과정이 단축 되었으며, 낮은 농도의 HF를 사용함으로써 최소의 에칭에 의하여 표면 거칠기를 감소시킬 수 있음을 보여주고 있다. 또한 주요 CMP 금속 배선 물질들에 대한 낮은 부식력으로 기존의 CMP 후 세정공정에 뿐만 아니라 차세대CMP 공정으로 각광 받고 있는 Copper CMP 에 대한 Brush 세정 공정의 보조 세정제로 본 세정제가 적용될 가능성이 있음을 확인하였다. In order to remove particles on surface of post-oxide CMP wafer, new cleaning solution was prepared by mixing with DHF (Diluted HF), nonionic surfactant PAAE (Polyoxyethylene Alkyl Aryl Ether), DMSO (Dimethylsulfoxide) and D.I.W.. Silicone wafers were intentionally contaminated by silica, alumina and PSL (polystylene latex) which had different zeta potentials in cleaning solution. This cleaning solution under megasonic irradiation could remove particles and metals simultaneously at room temperature in contrast to traditional AMP (mixture of <TEX>$NH_4OH,\;H_2O_2$</TEX> and D.I.W) without any side effects such as increasing of microroughness, metal line corrosion and deposition of organic contaminants. This suggests that this cleaning solution would be useful future application with copper CMP in brush cleaning process as well as traditional post CMP cleaning process.

Particle Removal Mechanism of Hydrogenated Ultrapure Water with Megasonic Irradiation

Particle Removal Mechanism of Hydrogenated Ultrapure Water with Megasonic Irradiation

Advanced Alkali Cleaning Solution for Simplification of Semiconductor Cleaning Process

ABSTRACTNH4OH/H2O2/H2O (called APM or SC–1) cleaning combined with megasonic irradiation is found to feature outstanding removal efficiency for various types of particulate contaminant. The conventional APM cleaning, however, allows metallic impurity in solution to adhere onto substrate surface, and it must be followed by acid cleaning such as HCI/IH2O2/H2O (called HPM or SC–2) cleaning to remove metallic impurity from substrate. The advanced APM cleaning using MC–1 which is alkali cleaning agent containing chelating agent has been developed, and this new cleaning is found capable for preventing various metallic impurities including Al in solution from contaminating substrate surface. Besides, with cleaning conditions optimized, the advanced APM cleaning using MC–1 can also remove metallic impurity from substrate surface. In short, this modified APM cleaning is capable for removing particle and metallic impurity at the same time, which is not possible with the conventional cleaning technology. The cleaning process of semiconductor manufacturing process can be simplified if HPM cleaning is eliminated by introducing the advanced APM cleaning using MC–1. This leads to drastic reduction of cleaning cost and improvement of throughput of the cleaning process.

Megasonic Irradiation Induced Chemical Reaction in the Solution for Silicon Wafer Cleaning

ABSTRACTWe have already proposed chemical vapor free and room temperature wet cleaning (UCT cleaning) for Si substrate surface, instead of conventional RCA cleaning, which consumes very little amount of chemicals and ultrapure water compared to that of RCA cleaning. This new wet cleaning has been developed by scientifically understanding contaminants removal mechanism as follows;(1) Particles can be removed by simultaneously satisfying following two conditions,(a) particles and substructure surface must have same polarity of zeta potential in the cleaning solution to exhibit repulsive electric coulomb force with each other.(b) adhered particles must be lifted off from substrate surface by slight etching to make electric repulsive force greater than van der Waals force,(2) Redox potential of the cleaning solution must be larger than a critical value to enable removal of electrons from adhered metals in order to dissolve them into the cleaning solution as positive ions and to decompose adhered organic molecules to CO2, H2O etc.Megasonic irradiation is very essential in UCT cleaning, particularly to remove particles by lifting them off from substrate surface. In this study, chemical reaction in the cleaning solution induced by megasonic irradiation is mainly discussed. Irradiation of ultrasonic having frequencies higher than a few hundred KHz (Megasonic) to ultrapure water has been confirmed to be able to decompose H2O molecules to H radicals and OH radicals. Decomposition efficiency of H20 molecules is strongly dependent on remaining gas components and their volume in the ultrapure water. When the remaining gas volume is decreased to less than 0.2 ∼0.3ppm, H2O molecules decomposition to H radicals and OH radicals has not been observed. Generated OH radicals have been confirmed to produce H2O2, and NH4+, NO2−, NO3− ions by reacting. with remaining N2 gas. Thus the cleaning capability of the cleaning solutions can be controlled by irradiating megasonic.

Parallel Down Flow Rinsl : Water Saving Technology in Wafer Rinse

AbstractWe had proposed parallel downflow rinse which was capable of draining out chemical carried over to rinsing bath together with wafer about twice as fast as overflow rinse. In an attempt to further improve rinsing efficiency of PDR, we have investigated (1) the effect of wafer carrier shape, and (2) removal of boundary layer on wafer surface. Boat‐type wafer holder is preferable to conventional cassette. Three stirring techniques adequate for PDR, namely, megasonic irradiation. N2 bubbling, liquid level shifting, are found effective to remove chemical ion remaining boundary layer in measuring conductivity and resistivity.