Effect of surfactant on removal of particle contamination on Si wafers in ULSI
TAN Bai-mei(檀柏梅), LI Wei-wei(李薇薇), NIU Xin-huan(牛新环),
WANG Sheng-li(王胜利), LIU Yu-ling(刘玉岭)
Institute of Microelectronics, Hebei University of Technology, Tianjin 300130, China
Received 10 April 2006; accepted 25 Apirl 2006
Abstract: 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.
Key words: megasonic cleaning; nonionic surfactant; particle removal; silicon wafer; adsorption
1 Introduction
For ULSI fabrication, the particle contamination on silicon wafer critically impacts final yield. Atoms of wafer surface form dangling-bonds because the chemical bond vertical to cutting direction is destroyed, its freedom force field near the surface is easy to adsorb particles. The particles are adsorbed on wafer quickly and form the chemical bond which is extremely difficult to remove. During the diffusion process of p-n junction, the particles will form nail model, cause lower punchthrough, pipeline punchthrough[1]. During the lithography process, the particles adhering to photomasks or wafer would cause short circuit or open circuit after transferring the desired circuit. During wafer bonding, adsorbed particle is one of the main sources of cavity. All these factors seriously influence device the performance and yield [2, 3]. Moreover, with the continue reduction of the feature size (0.05 mm by the year 2010), the limited size of particles will be rigorous inducing circuit imperfect. The particle size larger than 0.12 μm should be less than 500 per square meter [4].
At present, brush scrubbing [5] and wet cleaning [6, 7] are usually used in silicon wafer cleaning. In brush scrubbing cleaning the wafer is scrubbed with two-sided brush slices machine within 2 h after polishing, other- wise the particle forms the chemical bond that is difficult to be removed. The equipment is expensive and the efficiency is low (scrub slices sequentially, per unit need many set); easy to cause second damage (the reprocessing rate is high). In wet cleaning method, RCA cleaning is effective for silicon wafer cleaning when the feature size is larger than 0.25 μm, but many kinds of reagents are used in RCA cleaning process such as NH4OH, HCl, H2O2 and HF that will pollute the environment and when the feature size is smaller than 0.13 μm, the removal of micro-particles is important, but RCA cleaning is inefficient. Megasonic cleaning is the improved wet cleaning which can remove the particle adsorbed on wafer in the physical state with sound wave spreading in cleaning liquid, but can’t remove the particle adsorbed on wafer in the chemical state. In this paper, the complex surfactant was studied as megasonic cleaning liquid to change the adsorption state and remove the particle.
2 Experimental
The silicon wafers (P type, crystal orientation [111],d100 mm) were cleaned by megasonic cleaning machine of Verteq Sunburst S600-41 with the deionized water or nonionic surfactant as megasonic cleaning liquid separately after polishing or grinding. The temperature was kept at 50-55 ℃. After that the wafers were rinsed with deionized water. The amount of particle adsorbed on the wafer surface was measured before and after the megasonic cleaning by laser detecting equipment of LS-5000 type. The surfactant was confected FA/O complex nonionic surfactant and the resistance rate of deionized water was 18 MΩ·cm.
3 Results and discussion
3.1 Effect of surfactant
Silicon wafers after polishing were divided into two groups (24 slices each group), then were cleaned in the deionized water or 1% nonionic surfactant respectively with megasonic, the temperature was 50°C. According to the different time each group was divided into 6 batches (4 slices each batch). The cleaning time was 2, 4, 6, 10, 14, 20 min, respectively.
The result is shown in Fig.1. The removal rate of particles was calculated as: (Particle amount before cleaning-particle amount after cleaning)/particle amount before cleaning.
Fig.1 Comparison of cleaning efficiency with deionized water and surfactant
It can be seen that cleaning liquid with surfactant can facilitate the particle removal efficiency notably. When the cleaning time is longer than 10min, the removal rate reaches 96%.
Particle deposits on the wafer surface by interactions, including the Van der Waals forces and electrostatic forces mainly, As shown in Fig.2, the electrostatic forces is largely predominant with respect to the Van der Waals forces exception the region very close to the surface. The mechanism that the particle in the solution deposited on the surface can be divided into two steps[8]: 1) the gravitation existing between wafer and particles nearby makes these particles adsorbed rapidly; 2) this deposition creates a concentration gradient which leads to diffusion of the particles to the wafer surface. The zeta potential of silicon wafer is kept negative, and the results of theory and experiments show that only the particle with positive zeta potential deposits on silicon wafers. The Van der Waals force is weak attraction. Therefore the electrostatic interactions should be controlled to prevent the deposition. In order to suppress particles depositing on the wafer surface, it is important that the wafer surface and the particles must have the same polarity of the zeta potential. According to colloid chemistry, surfactants can prevent the particle from adsorbing on silicon wafer [9].
Fig. 2 Force between particles and wafer surface
There are a lot of kinds of surfactant, the reason that we chose the nonionic surfactant is as follows：1) nonionic surfactant can not ionize in water and does not cause the ion pollution; 2) the molecule of nonionic surfactant is larger, so it can be adsorbed on the wafer surface by dispersion force, which is easy to remove; 3) nonionic surfactant can be adsorbed on the surface of the general electriferous solid and increase the stability of system doubly. Because the hydrophilic group of nonionic surfactant nearly accounts for 2/3-4/5, its nonpolar carbon-hydrogen group contacts solid, but the hydrophilic polyethylene chain stretches to water mostly to form thicker protected layer which is benefit to improve the removal rate of particles; 4) the nonionic surfactant can not be affected by acid, alkali and electrolyte, and has strong osmosis.
The surfactant molecule is adsorbed on the wafer surface to form a molecule layer with the hydrophile group inward and the hydrophobic group outward, out of the layer another layer is formed with the hydrophobic group inward and the hydrophile group outward. The complex surfactant consists of osmotic agent and dis-
persant, the molecule of osmotic agent can permeate and expand between the wafer surface and absorbate, which plays a splitting role as throwing into a “wedge” to the interface (Fig.3) The dispersant molecule holds up the particle and replaces it to be adsorbed on the wafer surface to prevent the particle from deposition again. The particle is surrounded by a layer of complex surfactant molecules at the same time [10] (Fig.4).
Fig. 3 Strong osmosis of surfactant
Fig.4 Particle in solution surrounded by surfactant
3.2 Influence of cleaning temperature on surfactant cleaning liquid
Silicon wafers after polishing were divided into two groups (24 slices each group), then were cleaned in 1% nonionic surfactant with mega sonic, the temperature was 40, 45, 50, 55, 60 and 65 ℃, respectively. According to different temperature each group was divided into 6 batches (4 slices each batch). The cleaning result is shown in Fig.5.
Fig.5 Removal rate of particle at different temperatures
It can be seen that the cleaning temperature has effect on the particle removal with the same concentration of surfactant obviously. The removal rate is improved when the temperature changes from 40 ℃ to 50 ℃, but when the temperature is over 55°C the removal rate is decreased. The removal rate is higher when the temperature is 50-55 ℃. Because when the temperature is lower, the complex surfactant can be dissolved with water absolutely[11], the dissociation adsorption is an endothermal process, the particle adsorbed on the wafer gets more energy with temperature increasing, at the same time osmosis effect of the osmotic agent is strengthened and adsorption of the surfactant on the solid-liquid interface is increased with the temperature increasing, so the removal particle is improved rapidly. But the energy of hydrogen bond between hydrophilic groups of nonionic surfactant with water is low, when the temperature increases to a certain value, the heat motion of surfactant molecule is enhanced to break the hydrogen bond, and the solubility of surfactant in water will be declined. The surfactant separates out from water and the liquid becomes opacities that means the cloud point. When the temperature exceeds the cloud point [12], the effect of surfactant is lowered. The cloud point of FA/O nonionic surfactant is 55 ℃, so the cleaning temperature should be lower than 55 ℃. The best temperature is 50-55 ℃.
3.3 Effect of surfactant concentration on particle removal
The silicon wafers after polishing were divided into five groups; the density of surfactant was 0.05%, 0.1% 0.5%, 1%, 2%, respectively. The wafers were washed in nonionic surfactant liquid with the megasonic. The result is shown in Fig.6.
Fig.6 Removal rate with different concentration of surfactant
With the same cleaning time (15 min), the density of 1% is the best from the results. The concentration of surfactant is too low to form the covering layer on silicon wafer and surround the particles; but if the concentration of surfactant is high enough (higher than critical micelle concentration) to form the big micelle or the micelle group which is unfavorable to remove particles. The concentration of surfactant should be adjusted properly in application according to batch size and actual process condition.
The amounts of different size particles on silicon wafer after cleaning with cleaning liquid which the density of surfactant is 1% are shown in Table 1. It can be seen that all the amounts of the particles which size is larger than 0.2 μm are less than 10 per wafer and reach the SEMI standard.
Table 1 Amount and size of particle on wafer after cleaning
3.4 Comparison of surfactant with traditional alkaline cleaning liquid
The surface of silicon wafer after grinding may adsorb a large amount of silicon particles, abrasive Al2O3 and other particles and needs to be cleaned.
The silicon wafers after grinding were divided into two groups (each 24 slices) to wash for 15-20 min, with traditional alkaline liquid (pH =11-13) and nonionic surfactant liquid (pH=9) as megasonic cleaning liquid, then they were examined by the parallel light. The qualified ratio of silicon wafers that washed in the traditional alkaline liquid is under half, but the silicon wafers washed in nonionic surfactant are all qualified.
The main reaction in traditional alkaline liquid is alkali with silicon and Al2O3. The silicon atom is removed by chemical peeling off, it is difficult to remove the particle that is not dissolved and will adsorb again, but surfactant forms physical adsorption film that is benefit to remove particle on the wafer surface, the particle is taken away by the liquid.
4 Conclusions
With complex nonionic surfactant as megasonic cleaning liquid, the particle removal efficiency is improved. The nonionic surfactant can be adsorbed on silicon wafer in physical state that is easy to clean and the surfactant molecule can permeate and expand between the wafer surface and absorbate and surround the particle into water. The particle can be prevented from depositing on wafer surface again. The wafer surface just processed keeps in physical adsorption state without forming the chemical bond. The best concentration and temperature of surfactant cleaning solution is confirmed by the experiment. The damage induced by increasing megasonic frequency in cleaning experimental to improve cleaning efficiency can be avoided with FA/O complex nonionic surfactant as megasonic cleaning liquid.
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(Edited by LI Xiang-qun)
Corresponding author: TAN Bai-mei; Tel: +86-22-26564424; Fax: +86-22-26556455; E-mail:tanbaimei@eyou.com
