Development of clean technology in wafer drying processes

Transcription

1 Journal of Cleaner Production 9 (2001) Development of clean technology in wafer drying processes In-Soung Chang a,*, Jae-Hyung Kim b a Department of Environmental Engineering, Hoseo University, Asan, Chung-Nam, , South Korea b Super-Tech, Songkok-Dong, Ansan, Kyungki-Do, , South Korea Received 8 February 2000; accepted 15 June 2000 Abstract In the aqueous processing of silicon wafers, drying of wafers must take place after each processing step. However, the drying processes currently used are not environmentally sound, i.e., great amounts of solvents and energy are consumed. To fulfill environmental and economic requirements, a new wafer drying system based on concepts of clean technology was investigated. To reduce isopropyl alcohol (IPA) and energy consumption and to improve drying performance, a new dryer consisting of two separate chambers (IPA-chamber and drying-chamber) was developed. On-site operation with this new configuration was performed to compare its characteristics with those of other widely used dryers. It showed that IPA and energy consumption were smaller than those of the conventional dryers. Drying performance of the new dryer was also better than that of the conventional dryer. Consequently, the new dryer provides an environmentally friendly alternative to wafer drying Published by Elsevier Science Ltd. Keywords: Drying; Isopropyl alcohol; Semiconductor; Surface tension; Wafer 1. Introduction Semiconductors are electrical devices that perform information processing and display, power handling, and conversion between light energy and electrical energy. Semiconductors are used in computers, communication equipment, electrical control devices, robots, and numerous other electrical products. The semiconductor manufacturing industry is expanding to fulfill desires to process information faster and more efficiently. A sizable industry expansion is also expected in this century [1]. Semiconductor manufacturing is a complex operation involving several physico-chemical processes. The general process is shown in Fig. 1. As a first step, a silicon wafer is produced by the crystal growth (1st stage). To give it semiconductor characteristics, the wafer processing (2nd stage) includes many steps such as oxidation, photoresist spreading, UV exposure, etching, ion implementation and chemical vapor deposition (CVD). Finally, the chip is produced through cutting and fabrication (3rd stage). * Corresponding author. Tel.: , fax: address: cis@office.hoseo.ac.kr (I.-S. Chang). Fig. 1. Block diagram of the wafer manufacturing process (bold rectangles denote processes requiring a cleaning step). Every wafer-processing step is a potential source of contamination such as production of particles, inorganic and organic residues, which may lead to defect-formation and device failure. Numerous processes aimed at performing wafer cleaning have been developed [2,3]. Commonly used wet-cleaning techniques are still dominant because of their overall higher cleaning strength. The last steps in every wet-cleaning process are rinsing and drying. Both are critical steps because clean sur /01/$ - see front matter 2001 Published by Elsevier Science Ltd. PII: S (00)00055-X

2 228 I.-S. Chang, J.-H. Kim / Journal of Cleaner Production 9 (2001) faces become recontaminated easily if not processed properly. After wet-cleaning, rinsing is conducted with flowing high purity de-ionized (DI) water. Finally, the drying step is required to prepare wafer surfaces for subsequent processing. Whereas the use of diluted chemicals and powerful ultrasonic has led to huge improvements in the wet-cleaning process, the final drying still represents a performance bottleneck for cleaning equipment. Thus, it is recognized as a critical step of semiconductor manufacturing [4]. The most commonly practiced drying process is spin dry (SD). However, for today s deep-submicron devices, it often fails due to the immense stress on the wafers. It has been suggested that this method recontaminates the wafer surface with particles and aerosols caused by mechanical motion of the dryer itself [5]. Another drying method is solvent vapor drying (VD), typically using isopropyl alcohol (IPA, 2-propanol) [6]. IPA strips the water from the wafer surface by the feature of the low surface tension of IPA. It is well known that VD using IPA provides several functional advantages over SD. But tremendous amounts of alcohol and energy are wasted, which raises costs of consumables and of environmental protection. Additionally, it causes safety concerns due to the high operating temperature [7]. Most conventional VD systems employ only one chamber where both IPA-vaporization and drying are carried out simultaneously. Since the operation of carrying wafers to and from the dryer is performed in the chamber where the liquid IPA is heated and vaporized, the chamber cannot be sealed [8]. The wet wafer surface can be exposed to oxygen so easily that this system creates a large quantity of water-marks (or water-spots ) on the wafer surface [9]. The reaction between the wafers, water and oxygen is Si+H 2 O+O 2 H 2 SiO 3. Water-marks pose a significant challenge in the drying of silicon wafers because they can cause electrical failure due to locally thicker oxide and prevent proper adhesion of films [10]. This conventional VD system loses much thermal energy as well as IPA vapor because it is not isolated from the surrounding environment. In addition, it is not free from the risk of fire due to IPA vapor leakage. Therefore, this system is not considered to be an environmentally friendly process. In this study, we solve these problems by providing a closed process chamber. With this configuration, the IPA loss is minimized and the explosive hazardous conditions inherent in IPA processing are eliminated. This new system also makes it possible to minimize the thermal energy loss. Accordingly, this new drying system could be considered as a cleaner technology compared to the conventional drying systems. The goal of this research is to develop an environmentally benign process, which provides an alternative to wafer drying. 2. Experimental 2.1. System configuration and operation Fig. 2(a) shows a schematic diagram of the new closed type dryer; it consists of two separate chambers (the IPA-chamber and the drying-chamber). The IPAchamber where generation of processing vapor (IPA vapor) takes places, is isolated from the atmosphere and is connected to the drying chamber. The samples used in this study were 6 in. diameter wafers put on the cassette and placed in the drying-chamber. After vaporization reaches steady state in the IPAchamber, i.e., vaporized IPA is in equilibrium with the liquid IPA, nitrogen gas is delivered into the IPAchamber to carry the IPA vapor into the drying-chamber for 2 3 min. Subsequently, the wafers are exposed to N 2 without IPA vapor for 4 5 min. This blow-drying by pure N 2 is important in removing the IPA on the wafers Evaluation of drying performance Drying was conducted for 2 min exposure of the IPA vapor and for 5 min exposure of N 2 without IPA vapor. Drying efficiency was estimated by monitoring either drying time or IPA amounts delivered to the dryingchamber. Drying time corresponds to the time required to dry the wafer surface in the drying-chamber. The drying efficiency is inversely proportional to the drying time. The IPA amounts were measured after collecting the Fig. 2. (a) Schematics of the new wafer drying system. (b) System modification aiming at measuring IPA concentration.

3 I.-S. Chang, J.-H. Kim / Journal of Cleaner Production 9 (2001) condensed IPA vapor into a receiving bottle using the apparatus shown in Fig. 2(b). In this case, chemical oxygen demand (COD) of the water in the receiving bottle was measured first. It was converted to the IPA concentration using the calibration curve plotted formerly. COD analyses were carried out according to Standard Methods [11]. 3. Results and discussion 3.1. Principle of the new drying process The drying mechanism in the drying-chamber is illustrated in Fig. 3. Water on the wafer surface forms a positive meniscus before IPA vapor inflow [Fig. 3(a)]. q 1 denotes the contact angle between the water droplets and the wafer surface before drying. After IPA vapor inflow, IPA vapor begins to condense on the wafer surface because IPA has a great solubility in water and a low surface tension; the solubility of IPA is g/100 g H 2 O, and the surface tension of IPA at 25 C is N/m, whereas that of water is N/m [12,13]. As the amounts of the condensed IPA on the wafer surface are increasing continuously, the water droplets on the wafer surface have a lower surface tension, causing a lower contact angle, q 2 [Fig. 3(b)]. As the contact angle, q 2, becomes smaller, water droplets spread on the wafer surface. Accordingly, the water on the wafer surface flows under the influence of gravity [Fig. 3(c)]. The remaining IPA condensed on the wafer surface is removed by the blow-drying of N 2 [Fig. 3(d)]. Consequently, wafer drying has been completed Effects of temperature and N 2 flow rate on wafer drying Fig. 4 shows the effects of temperature in the IPAchamber on drying. Drying time decreased as the temperature increased. Since the amounts of vaporized IPA became large as temperature increased, effective drying was achievable at high temperature. However, further increments of drying efficiency were not observed as the temperature exceeded the boiling point of IPA, 82.5 C (at 1 bar). Fig. 5 shows the relationship between N 2 flow rate and drying time. As the flow rate increased, the drying time decreased. However, as the N 2 flow rate exceeded 50 l/min the drying time did not decrease further, suggesting that the evaporation rate did not surpass the nitrogen flow rate. The amount of nitrogen at that flow rate was enough to carry the vaporized IPA into the drying-chamber, i.e., a further supply of nitrogen was not necessary to carry the IPA vapor Quantitative determination of IPA evaporation Total amounts of IPA carried into the drying-chamber were measured using the apparatus in Fig. 2(b). Fig. 6 shows the plot of the evaporation rate of IPA (ml IPA/s) vs processing time (s). The evaporation rate was changed from 1.03 to 3.56 ml IPA/s according to the processing time. In Fig. 6, the profile of evaporation rate could be divided into three stages; (1) abrupt increase of the rate within the initial certain period of time ( 10 s); (2) gradual decline of the rate (10 90 s); (3) steady state ( 90 s). The evaporation rate of IPA depends on the pressure and the temperature in the IPA-chamber, respectively. Before the nitrogen flowed into the IPA-chamber, the static pressure and the temperature in the IPA-chamber were 0.6 bar (gauge pressure) and 80 C, respectively. After the nitrogen swept the IPA vapor (1st stage began), the pressure in the IPA-chamber decreased to 0.2 bar because the static pressure driven by the IPA vapor was released. In this period, temperature change was not observed. The decreased pressure in the IPA-chamber made it possible to reduce the boiling point of IPA. Therefore, the initial abrupt increase of IPA evaporation was attributed to the reduced boiling point of IPA. For the second stage, the temperature in the IPA- Fig. 3. Drying principle of the new IPA vapor dryer.

4 230 I.-S. Chang, J.-H. Kim / Journal of Cleaner Production 9 (2001) Fig. 4. Effect of temperature in the IPA-chamber on drying time. Fig. 5. Effect of N 2 flow rate on drying time. chamber was changed from 80 to 75 C whereas the pressure (0.2 bar) was kept constant. So the sluggish gradual decline of the evaporation rate was due to the decreased temperature. After 90 s (3rd stage), the pressure and temperature were kept constant (0.2 bar, 75 C). This may be the reason why the evaporation rate had the smallest value in the whole process Economic analysis for the new dryer This new dryer was applied to one of the semiconductor manufacturing industries in Korea. To qualify the new dryer economically, a number of criteria have been tested in the laboratory as well as at the plant. On-site data about the characteristics of the new and the conventional dryer were obtained. In most cases, the new dryer was directly compared to a spin dryer (SD) and a conventional vapor dryer (VD). Particle numbers on the wafer surface were detected (Table 1). The new dryer showed a superior performance, i.e., it leaves very few particles on a wafer surface. Reduction of particles on a wafer surface is crucial for cost-effectiveness in semiconductor manufacturing. The economic balance sheet analyzing the costs of the VD and the new dryer is summarized in Table 2. It was found that the IPA consumption of the new dryer is only 50% of the conventional VD. The IPA consumption cost of the conventional VD was calculated by using on-site data. So the estimated IPA consumption for each dryer is US$12,820 and US$25,641 per year, respectively. In the VD process, additional expenses to reduce particles to the levels of the new process have to be considered. These extra expenditures are estimated to be US$34,189/year. This calculation is based on the conventional wet-cleaning cost. The new process requires less IPA than the VD process, so that the organic loading to the wastewater treatment plant can be reduced. A new dryer is able to reduce the wastewater treatment costs by US$90 per year. Because the new dryer is isolated from the surrounding environment, it consumes less electrical energy than the conventional VD. One dryer gives a saving of US$142

5 I.-S. Chang, J.-H. Kim / Journal of Cleaner Production 9 (2001) Fig. 6. Variation of evaporation rate during the process. Table 1 Counting of particles for 1 month New Dryer VD Type SD Type Particles a (number/wafer) a Average of 30 measurements. Table 2 Economic balance for wafer drying (US$/dryer, year) New Dryer IPA consumption 12,820 25,641 Additional cost for particle removal 0 34,189 Wastewater treatment cost Energy (electricity cost) Nitrogen consumption Investment costs for safety & health Total 18,077 65,449 Conventional VD per year. In addition, the costs of the safety and health facility, such as fire-extinguisher system, fireproof equipment, anti-explosion system and IPA-ventilation, can be reduced. On the other hand, the new dryer consumes nitrogen gas. This additional cost is about US$4870 per year. The total costs of the new dryers are only 28% of the conventional VD. From the view-point of environmental protection and clean technology, it is verified that the new drying system has many advantages over the conventional drying system. 4. Conclusions An innovative cleaner drying technology was developed and applied in the semiconductor manufacturing industry. The new dryer consisted of two separate chambers (IPA-chamber and drying-chamber) in order to reduce IPA and energy consumption as well as to improve drying performance. After optimum operating conditions, such as nitrogen flow rate and temperature in the IPA-chamber were identified, on-site operation with this new dryer was performed to compare its benefits with widely used conventional dryers. It gave a lower IPA and energy consumption. It also offered a better drying performance than the conventional dryer did. Consequently, this dryer provides an environmentally friendly and economically beneficial alternative for the wafer drying. Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) and through the Semiconductor Equipment Research Center at Hoseo University in Korea. References [1] Gilles DG, Loehr RC. Waste generation and minimization in semiconductor industry. J Environ Eng 1994;120: [2] Olim M. Liquid-phase processing of hydrophilic features on a silicon wafer. J Electrochem Soc 1997;144: [3] Heyns M, Meterns P, Ruzyllo J, Lee M. Advanced wet and dry

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