Effect of Microbubbles on Ozonized Water for Photoresist Removal

Article pubs.acs.org/JPCC

Effect of Microbubbles on Ozonized Water for Photoresist Removal Masayoshi Takahashi,*,† Hiroaki Ishikawa,‡ Toshiyuki Asano,‡ and Hideo Horibe§ †

National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan Industrial Technology Institute of Ibaraki Prefecture, 3781-1 Nagaoka, Ibaraki, Higashi-Ibaraki, Ibaraki 311-3195, Japan § Kanazawa Institute of Technology, 3-1 Yatsukaho, Hakusan, Ishikawa 924-0838, Japan ‡

ABSTRACT: In this paper, we describe the use of ozone microbubbles in photoresist removal from silicon wafers. Ozonized water has attracted much attention as an environmental friendly cleaning method in semiconductor manufacturing. However, it would be desirable to enhance the oxidative ability of ozonized water for practical application. The existence of microbubbles in ozonized water has been shown to significantly enhance the photoresist removal rate due to an elevated dissolved ozone concentration (approximately 2.5 times that of ordinary ozone bubbling) and a direct effect of the microbubbles (removal rate is approximately 1.3 times faster than water with the same concentration of dissolved ozone without microbubbles). Additionally, the ozone microbubble solution was able to effectively remove a high-dose ion-implanted photoresist, which is extremely resistant to removal by ozonized water and other wet chemicals because of its amorphous carbon-like layer, or “crust”. Electron spin resonance experiments were also performed without the influence of serious metal contamination and indicated the presence of hydroxyl radicals, which are thought to be formed by interaction of ozone with hydroxide ions adsorbed at the gas−water interface upon collapse of the microbubbles. The hydroxyl radicals play an important role in photoresist removal by the ozone microbubble treatment.

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INTRODUCTION Microbubbles are tiny bubbles with diameters of less than 50 μm. When generated in water, the bubbles decrease in size and eventually disappear under water because of their long stagnation and excellent gas dissolution ability.1 The collapse of these bubbles in water has been shown to lead to the generation of activated oxidizers such as hydroxyl radicals. This is thought to arise from the extinction of the gas−water interface, which might trigger dispersion of the elevated chemical potential accumulated at the interface as adsorbed ions.2−4 Similarly, when microbubbles of ozone are generated in water, the dispersion of the chemical potential arising from the extinction of the gas−water interface leads to the decomposition of ozone and the generation of large quantities of hydroxyl radicals, which may provide a new type of advanced oxidation process (AOP).5 Such properties suggest the use of microbubbles as a new method of wastewater treatment. In the production of semiconductor devices, surface contamination of the silicon wafers causes serious problems. As a result, large amounts of chemicals, such as H2SO4, H2O2, HCl, NH4OH, and amine solvents, are required in wet cleaning processes.6,7 For safety and environmental reasons, the use of ozonized water is attracting attention as a viable method for removing organic contaminants, which have been conventionally removed by a high-temperature sulfuric acid−hydrogen peroxide mixture (SPM).8−17 In terms of applications to semiconductor manufacturing, ozonized water cleaning systems are strongly desired as they provide enhanced removal rates for organic contaminants. In addition, the challenge to the amorphous carbon-like damage layer created by ion-implant doses greater than 5E14/cm2 is significantly important to © 2012 American Chemical Society

introduce the ozone cleaning method in the semiconductor manufacturing processes.18−25 In this paper, we investigate the effects of microbubbles on ozonized water for the removal of photoresists. A comparison is made between treatments with and without microbubbles. We also discuss the physicochemical mechanism of ozone microbubbles in enhancing the removal rate, taking into account the electron spin resonance (ESR) results, and describe the treatment of a high-dose ion-implanted photoresist. Ozone Microbubble Generation System Used in Silicon Wafer Cleaning. Figure 1 shows a schematic of the microbubble generation system used in the experiments. For practical application in semiconductor manufacturing, all parts of the system in contact with water are made of metal-free Teflon materials. A bellows cylinder pump (ΣP-15C, Sigma Technology Inc.) realizes low pulsation discharge intakes of water and gas through its intake line, and discharges them through a dissolution tank in which the gas is effectively dissolved in water under a pressure of approximately 0.4 MPa. The water containing the dissolved gas is then released by a dispersing nozzle (Shigen-kaihatu Co., Inc.), and the resulting pressure decreased to ambient conditions causes supersaturation of the dissolved gas. Bubble nuclei generated by turbulent flow at the nozzle grow rapidly into microbubbles under the supersaturated condition. Figure 2 shows the bubble size distribution of generated microbubbles in distilled water measured by a particle-counting spectrometer for liquids Received: February 22, 2012 Revised: May 11, 2012 Published: May 16, 2012 12578

dx.doi.org/10.1021/jp301746g | J. Phys. Chem. C 2012, 116, 12578−12583

The Journal of Physical Chemistry C

Article

Figure 3. Change in dissolved ozone concentration over time.

Figure 1. Schematic of the microbubble generation system.

Photoresist Removal by Ozone Microbubbles. The performance of the ozone microbubble solution in photoresist removal was investigated by measuring the change in thickness of the photoresist residue. A 6 in. silicon wafer coated with unexposed high-contrast i-line positive photoresist (THMRiP3300, TOKYO OHKA KOGYO Co. Inc.) was diced into test pieces of 30 mm × 30 mm. The tests were conducted by pouring the microbubble solution onto the diced wafers, as shown in Figure 1. In this test, the flow rate of the microbubble solution was approximately 8 L/min, which was sufficient to cover the entire surface of the diced wafer, ensuring uniform removal of the photoresist. The average thickness of nine distinct measurement points on the wafer was obtained from measurements taken every 30 s for 2 min using a thin film analyzer (F20, Filmetrics Inc.). The dissolved ozone concentration was approximately 15 mg/L, and the water temperature was approximately 30 °C throughout the test. Figure 4 shows the thickness of the photoresist that was removed during the treatment as indicated by the difference in

Figure 2. Size distribution of microbubbles generated by the system.

(LiQuilaz-E20; Particle Measuring Systems Inc.) using a lightobscuration method. The microbubble solution, milky in appearance, exhibits two peaks: a sharp peak around 15 μm in diameter and a broader peak around 40 μm. The change in concentration of dissolved ozone was compared between a glass-bonded diffuser and the microbubble generation system. In both cases, approximately 50 g/m3 of ozone gas was fed at a flow rate of 1 L/min to 5 L of distilled water in a glass container. The dissolved ozone concentration was measured by an ultraviolet ozone analyzer (PL-620A, Ebara Jitsugyo Co., Ltd.). Because the light-obscuration caused by the presence of microbubbles introduced errors in the analysis of the ozone concentration, we accounted for the effect of microbubbles on the analysis by performing a null adjustment for solutions in which air microbubbles were generated by the system. As a result, it was found that there was no significant difference between the indicated ozone concentration in the ozone microbubble measurement using the null adjustment and that in the same ozonized water after the ozone microbubbles had completely dissolved. Figure 3 shows a comparison of the dissolved ozone concentration over time for the microbubble generation system and that obtained under normal bubbling. The supply of ozone by the microbubble generation system resulted in a concentration of dissolved ozone approximately 2.5 times higher than that obtained by normal bubbling. The dissolved ozone concentration of the microbubble solution was also found to linearly increase when the ozone concentration of the source gas supplied to the water was increased.

Figure 4. Thickness of the photoresist that was removed by the ozone microbubble treatment.

thickness from the original value. The negative value at 30 s might be attributed to swelling of the photoresist layer due to action of the ozone microbubbles, and during this initial stage, it was also found that the surface changed from hydrophobic to hydrophilic. After this initial stage, there is an almost linear decrease in the thickness of the photoresist over time. Relation of Photoresist Removal to Ozone Gas Concentration. When applying ozonized water in semiconductor cleaning, an important factor to consider is how the 12579



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rate of photoresist removal could be increased. A determining factor in this process has been shown to be the ozone concentration. Thus, we investigated the effect of ozone concentration of the source gas supplied for microbubble generation on the rate of photoresist removal. The tests were conducted under the same conditions as shown in Figure 1 with the exception that the ozone gas concentration was varied from 30 to 150 g/m3. The removal rate of photoresist was evaluated by measuring the thickness of the photoresist over time by the thin film analyzer. The results of the average removal rate during the initial 3 min are shown in Figure 5 as a

Figure 6. Change in photoresist thickness by the ozone microbubble treatment.

over time, and the photoresist was completely removed by 9.6 min. Because the thickness of the photoresist was 1.20 μm, the average photoresist removal rate was calculated to be 0.125 μm/min in this case. Figure 7 shows a comparison of the

Figure 5. Effect of ozone gas concentration on photoresist removal rate.

function of ozone gas concentration. It can be seen that that there is a linear relationship between the ozone gas concentration and the photoresist removal rate. In terms of the photoresist removal rate, the increase in ozone gas concentration has the same effect in the microbubble treatment as that in the ordinary ozone treatment. Comparison of Photoresist Removal Rate between Ozonized Water and Ozone Microbubble Solution. To evaluate the effect of microbubbles on photoresist removal in more detail, further tests were conducted to compare the removal rates obtained using ozonized water and an ozone microbubble solution. A six-inch silicon wafer coated with unexposed g-line positive photoresist (OFPR820-23CP, TOKYO OHKA KOGYO Co., Inc.) was diced into test pieces of 30 mm × 30 mm. The thickness of the unexposed photoresist on the wafer was approximately 1.20 μm. Because the photoresist includes sulfur as an ingredient, removal of the photoresist could be evaluated by measuring the amount of sulfur present in the photoresist layer without being affected by unexpected swelling or shrinking of the layer caused by the ozone microbubbles. The tests were conducted by pouring ozonized water, with or without microbubbles, onto the diced wafers. In both cases, the water flow rate was approximately 2 L/min, and the temperature was controlled at approximately 22 °C by a cooling system. The sulfur content of the photoresist layer on the silicon wafer was measured by energy dispersive X-ray spectroscopy (EDX-800HS, SHIMADZU Co. Inc.). The test was continued until the sulfur content reached 0%, indicating complete removal of the photoresist. Figure 6 shows the thickness of the photoresist evaluated as the change in sulfur content of the wafer during application of a microbubble solution with a dissolved ozone concentration of 8.7 mg/L. The photoresist thickness decreased almost linearly

Figure 7. Photoresist removal rates for ozonized water with or without microbubbles.

photoresist removal rate obtained for ozonized water with or without microbubbles. It can be seen that the presence of microbubbles in the ozonized water enhances the removal rate by more than 30% in comparison to the ozonized water without microbubbles. Removal of High-Dose Ion-Implanted Photoresist by Ozone Microbubble Water. The previous tests demonstrated enhancement in the photoresist removal rate by treatment with ozone microbubbles. However, in terms of practical application in semiconductor manufacturing, it would also be useful to remove photoresists damaged during manufacturing processes. One of the most challenging targets for the industry in terms of cleaning is the treatment of highdose ion-implanted photoresists. Cleaning such photoresists is a considerable challenge due to an amorphous carbon-like damage layer (crust) caused by ion-implantation at doses higher than 5E14/cm2, and conventionally a two-step process is used for photoresist removal: low-pressure plasma ashing in a single-wafer tool followed by SPM-based wet removal in a batch immersion tool. For advanced semiconductor fabrication processes, single-wafer tools are preferable as they prevent cross-contamination. Therefore, we then conducted tests on implementation of the ozone microbubble generation system to a conventional single-wafer spin cleaning tool, as shown in Figure 8. 12580



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the existence of microbubbles in ozonized water enhanced the photoresist removal rate for a backed photoresist and could effectively remove a high-dosed ion-implant photoresist. To clarify the mechanism by which the ozone microbubble solution acts on the photoresist, we performed ESR measurements by the spin-trap method. 5,5-Dimethyl-1-pyrroline-Noxide (DMPO) was used as the spin-trap reagent because it has been widely used to identify oxygen-centered radicals such as superoxide and the hydroxyl radical.26−31 Ozonized water with or without microbubbles was tested by the ESR spin-trap method. Distilled water was used after the addition of ozone for the test. The microbubble generation system was used for the ozone microbubble solution, while the glass-bonded diffuser was used for the ozonized water without microbubbles. The electrical conductivity of the waters was approximately 1.0 μS/cm, and the dissolved ozone concentration was approximately 8 mg/L in both cases. DMPO was mixed with the samples to a concentration of approximately 40 mM under gentle stirring, and ESR spectra were measured at room temperature using an ESR spectrometer (ESRX-10SA-v4, KEYCOM Co., Ltd.). The hyperfine splitting constant was calibrated using Mn2+ as an external standard. Figure 10 shows

Figure 8. Schematic of the single-wafer spin cleaning tool and microbubble generation system.

Eight-inch silicon wafers, spin-coated with a 0.129 μm i-line positive photoresist (TDMR AR87LB-18G, TOKYO OHKA KOGYO Co. Inc.) and patterned by an exposure tool, were used as the test samples after phosphorus-ion implantation at a dose of 1E15/cm2. The ion energy was 60 keV. The tests were conducted by pouring the ozone microbubble solution onto the wafers, which were rotating at a spin rate of 200 rpm. Ultrapure water was used with an electrical resistance of approximately 17 MΩ/cm. The dissolved ozone concentration was approximately 60 mg/L, and the water temperature was approximately 22 °C throughout the test. Figure 9 shows

Figure 9. Removal of high-dose ion-implanted photoresist by ozone microbubbles. Figure 10. ESR spectrum of ozone microbubbles showing DMPO− OH. The four peaks in the midsection (marked by circles) are signals of the DMPO−OH adduct, and the two peaks at each end are signals of Mn2+ (external reference).

photographs of the wafer at 0, 5, and 10 min after the start of treatment, which show that the high-dose ion-implanted photoresist was being removed from the outer edge to the center of the wafer. We also conducted the same test for an ozone solution without microbubbles. Significantly, the solution of dissolved ozone without microbubbles (also with a concentration of 60 mg/L) did not remove any of the photoresist crust. Figure 9 indicates another important factor to take into account when considering removal of the photoresist by the ozone microbubble solution. Because the lateral movement of the spinning wafer increases away from the central axis, the movement of the ozone microbubbles relative to the photoresist on the wafer also increases toward the outer edge of the wafer. The rapid relative movement of the water generates a turbulent flow beneath the surface. In fact, the observed trend of photoresist removal from the edge to the center may indicate that turbulent flow plays a significant role in removal of the photoresist. However, at present we do not have sufficient information to clarify the mechanism, and can only state that it may be a result of accelerated mass transfer of ozone to the surface of the wafer, or an effect of turbulent flow in collapsing of the microbubbles. Determination of Hydroxyl Radical Generation by ESR Measurement. The above experiments demonstrate that

the ESR spectrum of the ozone microbubble solution, which contains four lines (hyperfine splitting constants of AN = AH = 14.9 G), consistent with the spectra of DMPO−OH, indicating the formation of hydroxyl radicals (·OH).29−31 In the absence of microbubbles, we did not observe any spectral indications of hydroxyl radicals. This finding is significant, particularly in relation to removal of the high-dose ion-implanted photoresist, because hydroxyl radicals have a much higher standard redox potential (2.80 V) than ozone (2.07 V), and exhibit immediate and nonselective reactivity with the majority of organic compounds. In terms of mechanism, the previous work has demonstrated that interfacial ions accumulated at the gas−water interface might play a role in the generation of free radicals during collapse of the microbubbles.

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DISCUSSION Generation of free radicals is one of the most important features of microbubble treatment methods in applications such as disinfection and water treatment. The present work has 12581



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cations, that is, protons in this case, and generates an electrical double layer as shown in Figure 11. Because the interior gases

revealed that microbubbles are very effective in photoresist removal because of the generation of hydroxyl radicals and an elevated concentration of dissolved ozone. Furthermore, the newly developed, metal-free microbubble generation system can produce microbubbles without serious contamination, enabling us to investigate the mechanism of transformation of ozone into hydroxyl radicals. Our experimental research into radical generation by collapsing microbubbles has revealed the following results:1−5,32 (1) The microbubbles decrease in size at an increasing rate and eventually disappear because of dissolution of the interior gas into solution. (2) ζ potential measurements reveal that the microbubbles are electrically charged owing to the adsorption of H+ and OH− ions at the gas−water interface. The pH level of the water significantly affects the ζ potential of the microbubble, and the interface is negatively charged over a wide range of pH conditions. Counter ions are consequently attracted to the gas−water interface, resulting in the formation of an electrical double layer. (3) In the collapse process, the increasing rate of shrinking of the gas−water interface leads to the accumulation of ions near the interface, resulting in a rapid increase in the absolute value of the ζ potential. The drastic environmental change caused by the extinction of the gas−water interface triggers radical generation via dispersion of the elevated chemical potential that has accumulated around the interface. (4) Low pH conditions are thought to be unfavorable for the generation of hydroxyl radicals in the case of a dissolved ozone solution, but in the case of an ozone microbubble solution, a significant amount of hydroxyl radicals are generated under strongly acidic conditions. From a mechanistic point of view, it is important to clarify the relationship between radical generation and the ions accumulated at the gas−water interface. As described, two types of ions exist at the interface of the collapsing microbubble: adsorbed ions and those electrically attracted as counterions. The counterions are mainly electrolytes in typical cases. For microbubble solutions in deionized water, the adsorbed ions are mainly hydroxide ions while the counterions are protons (hydronium ions).32 Li et al. revealed that copper significantly catalyzes the generation of hydroxyl radicals from microbubbles under strongly acidic conditions.4 Iron ions have also been found to play an important role in catalytic reaction in the wellknown Fenton reaction, although hydrogen peroxide is used here as an oxidizer instead of ozone.33,34 While there is insufficient data to discuss the mechanism in detail, it might be considered that the positively charged metallic ions are attracted to the microbubble interface, thus playing an important catalytic role in the generation of hydroxyl radicals. Meanwhile, the present study suggests that hydroxide ions adsorbed at the gas−water interface may also have a catalytic role in the chain reaction of ozone decomposition. In pure water, H+ and OH− exclusively exit as ions, which are generated by the dissociation of water molecules. In a microbubble solution, these ions tend to be adsorbed at the gas−water interface, with OH− tending to be more effectively adsorbed at the interface than H+, giving rise to a negative charge at the interface under a wide range of pH conditions. The negatively charged gas−water interfaces then attracts

Figure 11. Generation of hydroxyl radicals at the gas−water interface of ozone microbubbles.

of the microbubbles are effectively dissolved into the surrounding water through the interface region, ozone molecules encounter accumulated hydroxide ions at the interface. This is almost the same situation for ozone as being dissolved in a high pH aqueous solution, and the radical chain that produces hydroxyl radicals is stimulated by the chemical reaction between ozone and hydroxide ions.

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CONCLUSION Ozonized water has attracted much attention as an environmentally friendly cleaning method in semiconductor manufacturing, but its oxidative ability must be enhanced for practical application. Ozone microbubbles generated from a metal-free microbubble generator were investigated in the removal of photoresists from silicon wafers. It was found that the existence of microbubbles in ozonized water significantly enhances the photoresist removal rate due to the elevated dissolved ozone concentration and the collapse of the microbubbles. Moreover, the ozone microbubble solution was able to effectively remove a high-dose ion-implanted photoresist that is extremely resistant to removal by ozonized water and other wet chemicals. ESR experiments revealed that the hydroxide ions adsorbed at the gas−water interface of the collapsing microbubbles may initiate the transformation of ozone into hydroxyl radicals, which play an important role in photoresist removal by the ozone microbubble treatment.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partly supported by the Japan Science and Technology Agency (JST) and the Ministry of Economy, Trade and Industry (METI). The authors also acknowledge Hitachinaka Techno Center, Inc., Renesas Electronics Corporation, Hitachi, Ltd., Toshiba Mitsubishi-Electric Industrial Systems Corporation, and Seto Engineering Co., Ltd. for a wide 12582



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(34) Burbano, A. A.; Dionysiou, D. D.; Suidan, M. T.; Richardson, T. L. Water Res. 2005, 39, 107−118.

variety of technical support. We are also very grateful to T. Nishimoto for assisting in experimental work.

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Effect of Microbubbles on Ozonized Water for Photoresist Removal

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