Photoresist Spray Coating for Resist Film Performance of Deep Silicon Cavities

Transcription

1 Journal of the Korean Physical Society, Vol. 50, No. 6, June 2007, pp Photoresist Spray Coating for Resist Film Performance of Deep Silicon Cavities Duk-Soo Eun, Do-Wok Kim, Chang-Taeg Seo and Jong-Hyun Lee School of Electrical Engineering and Computer Science, Kyungpook National University, Daegu Young-Ho Bae Division of Electronics, Uiduk University, Gyeongju In-Sik Yu Kyungdong College of Techno-Information, Gyeongsan Chang-Gil Suk Culture Education, Ultech Corporation, Daegu (Received 12 September 2006, in final form 28 November 2007) In this paper, we describe a new method for resist performance with a photoresist spray coating system. The system consists of a high-temperature rotational chuck, an ultrasonic spray nozzle module, an angle control module, and a nozzle moving module. The spray coating system utilizes several parameters, including solid content, dispensed volume, the scanning speed of the spray nozzle, and the wafer dimensions. In order to prepare samples for photoresist spray coating, the process starts with a thermal oxidation at a thickness of 1000 Å. Silicon oxide is used as a masking layer for anisotropic wet etching, 50 µm and 100 µm in depth. Photoresist spray coating consumes much less photoresist than conventional spin coating, and the resist uniformity of the deep cavities is more efficient. PACS numbers: Tw, Rs, Sz Keywords: MEMS, Photoresist, Spray coating, Vertical spray, Tilted spray I. INTRODUCTION Recently, there has been a great deal of research interest in chemical sensors, physical sensors and micro electro mechanical systems (MEMS) with regard to deep silicon cavities. Conventional spin coating is an established method using CMOS technology for resist deposition on plane surfaces or on a surface which has a slight unevenness. However, conventional spin coating has problems in deep cavities, with an accumulation of the photoresist at the bottom corner and a reduction of a photoresist at the top corner. As a result, it is not suitable for threedimensional structures. Electro deposition (ED) coating provides a uniform photoresist coating layer and good resolution, but the equipment is expensive, and a separate conductive layer is needed to form a coating layer. Spray coating can reduce photoresist consumption more than 70 % over spin coating, and it can uniformly coat the entire surface of a substrate and does not require an independent film, such as an insulation layer beast22@unitel.co.kr on conductive layer, as ED coating does. Additionally, spray coating can produce a uniform photoresist coating layer, regardless of large-sized display substrates and geometrical shapes. Although, there has been a great deal of research on spray coating systems; however, none has yet been sufficiently developed [1 4]. In this paper, we present a new method for resist performance with a photoresist spray coating system. The system consists of a high-temperature rotational chuck, an ultrasonic spray nozzle module, an angle-control module, and a nozzle-moving module. The spray coating system utilizes several parameters including solid content, dispensed volume, the scanning speed of the spray nozzle, and the wafer dimensions. Photoresist spray coating consumes much less photoresist than conventional spin coating, and the resist uniformity of the deep cavities is more efficient. In order to prepare samples for photoresist spray coating, the process starts with a thermal oxidation of a thickness of 1000 Å. Silicon oxide is used as a masking layer for anisotropic wet etching at depths of 50 µm and 100 µm. The spraying of the photoresist film is performed using an Ultec corporation spray coater.

2 Journal of the Korean Physical Society, Vol. 50, No. 6, June 2007 Fig. 1. Schematic diagram of the spray coating [1]. Fig. 2. Spray pattern for uniform coating: (a) uniform velocity + rectilineal motion and (b) uniform velocity + local rectilineal motion. II. PHOTORESIST SPRAY COATING THEORY Photoresist spray coating produces particles not exceeding 40 µm. They are introduced by spray nozzle from syringe pump on the silicon surface because of the nitrogen pressure. The sprayed photoresist particles are deposited on a silicon surface through an air layer by using the chuck rotation speed of the substrate and the relative movement speed of nozzle to coat the substrate evenly because of the surface tension of substrate. Figure 1 shows a schematic diagram for spray coating [1 5]. Photoresist spray coating influences the uniformity and the thickness of the photoresist layer,depending on variables such as the photoresist viscosity, the nozzle s movement speed, the chuck s rotation speed, the angle of the spray nozzle, the inflow per minute of the photoresist, and temperature control of the substrate. At first, the photoresist viscosity influences the sprayed photoresist particles that are moved by the surface tension of the silicon substrate; other kinds of solvents can be added to control the viscosity. The movement speed of the spray nozzle and chuck s rotation speed have the greatest important influence upon the thickness of the deposited photoresist. The photoresist thickness that can be deposited becomes thinner at higher movement speeds of Fig. 3. Coating layer distribution for various dispensing angles (a) Spin coating (b) Vertical spray coating (c) Tilted- 45 spray coating. the chuck and the nozzle [6 12]. Figure 2 shows the spray pattern for uniform coating according to the movement speed of the spray nozzle and the chuck s rotation. When the spray nozzle s moved uniformly and straight from the substrate edges to the central area, the spray is made at the same distance (Figure 2(a)) in the direction of substrate radius and at an angle of θ in the direction of substrate rotation, regardless of the radius. Therefore, the spray is concentrated at the substrate center, and the photoresist becomes thicker at the substrate center. On the other hand, when the spray nozzle s moved uniformly to a certain area from the substrate edge that is rotated at a specific angular speed, the spray has a spiral path (Figure 2(b)). If the same coating thickness of photoresist is to be maintained, the substrate radius must be divided in proportion to the spray area of the nozzle so that the spray nozzle has different spray times at each part of the substrate. To deposit evenly a substrate that has a threedimensional structure, the spraying should be done effectively on the photoresist not with a vertical spray, but with a tilted spray. When the 3-D step height does not exceed 10 µm, conventional spin coating can coat the photoresist evenly. However, a 3-D structure having

3 Photoresist Spray Coating for Resist Film Performance Duk-Soo Eun et al Fig. 4. Block diagram of the spray coating system. a step height of 10 µm or more can not coat a photoresist at the top, the side, and the bottom of a cavity, as shown in Figure 3 (a). The photoresist was coated by vertical spray on the substrate to indicate the coating thickness, as shown in Figure 3 (b). The formula T p > T bs > T bl was applied; the coating thickness of the photoresist at the top is T p, the coating thickness of the photoresist at the bottom of a small-sized cavity is T bs, and the coating thickness at the bottom of a large-sized cavity is T bl. Then, the formula of T ss > T sl was applied; the coating thickness of the photoresist on the side of a small-sized cavity is T ss, and the coating thickness on the side of a large-sized cavity is T sl. Therefore, a photoresist could not be coated evenly on the top, the bottom, and the side of the 3-D structure. The photoresist coating thickness when using a tilted spray is shown in Figure 3 (c). The tilted spray could create a photoresist coating layer according to the formulae T p = T bs = T bl and T ss = T sl. III. SPRAY COATING SYSTEM The spray coating system consisted of a rotating chuck of controlling the temperature, an ultrasonic spray nozzle, a nozzle movement, a nozzle angle control, and a photoresist supply. It was designed considering the photoresist viscosity, the spray nozzle, the photoresist supply, the spray methods, and other process variables. The silicon substrate was fastened to the chuck for uniform photoresist coating. The chuck of the AC Servo motor was rotated at a uniform angular velocity of rpm, and the speed was controlled exactly. The heating unit of the chuck module was designed to control the volatility and flow of the photoresist and to maintain a substrate temperature of C during spray coating. Figure 4 shows a block diagram of the spray coating system. IV. RESULTS AND DISCUSSION Photoresist spray coating has more variables than spin coating. The coating thickness depends upon the photoresist concentration and the spin speed. In the current study, we controlled the spray nozzle rate, the spray angle of the nozzle, the chuck s rotating rate, and the photoresist inflow to develop a photoresist coating method that is suitable for 3-D micro-structures. The photoresist for coating was AZ 1512 with a viscosity of 20 cp; thickness was measured using a α-step profiler and scanning electron microscope (SEM). A 4 silicon substrate was divided into 9 parts to investigate the photoresist thickness and to verify uniformity, as shown in Figure 5. Figure 6 shows the thickness distribution of the photoresist on the chuck spin speed. The spray nozzle s movement rate and other variables were fixed to maintain a chuck rotation of rpm. The quicker the chuck rotation is, the thinner the photoresist is. A great deal more photoresist was coated on the substrate center than in other areas because the centrifugal force produced by the substrate s rotation speed had less influence on the substrate center. Figures 7 and 8 show the thickness distribution of the photoresist for various nozzle speeds 60 rpm and 30 rpm, respectively. A nozzle movement rate of 2 12 mm/sec produced a thin photoresist. At the same movement speed, the photoresist coating thickness was in inverse

4 Journal of the Korean Physical Society, Vol. 50, No. 6, June 2007 Fig. 5. Dispense region of the silicon wafer. Fig. 8. Thickness distribution of the photoresist for various nozzle speeds- The chuck spin speed is 30 rpm. Fig. 6. Dependence of the photoresist thickness on the chuck spin speed for various positions on the wafer. Fig. 9. Photoresist thickness distribution for vertical dispensing. Fig. 7. Dependence of the photoresist thickness of on the nozzle speed for various positions on the wafer- The chuck spin speed is 60 rpm. Fig. 10. Cross section of the sample vertically dispensed by using a spray nozzle. proportion to the nozzle movement speed and the chuck rotation speed. The photoresist was divided into three areas, top, side and bottom, to measure the thickness, which varied depending upon spray nozzle angle. Figures 9 and 10 shows the photoresist thickness of silicon with a 100-µm cavity for a vertical spray. The spray nozzle sprayed vertically to coat the edge wall of the substrate at a thickness of 3.5 µm more thickly than top; this was because the vertically sprayed photoresist was moved from top to bottom by the surface tension of the edges. Figure 11 shows a photograph of silicon with a 100 µm cavity produced by using a tilted 45 spray coating. As Figure 12 shown, the inclined-spraycoated photoresist edge wall and top of the substrate

5 Photoresist Spray Coating for Resist Film Performance Duk-Soo Eun et al Fig. 11. Photoresist thickness distribution for tilted dispensing. The current study investigated a new method for resist performance with a photoresist spray coating system. The experiment adjusted important variables of the spray coating system, including the chuck s rotation speed, the nozzle s movement speed, the photoresist inflow, and the nozzle s spray angle, to determine the thickness and the uniformity of photoresist and to make the photoresist thickness proportional to the viscosity and the spray of photoresist and to make it inversely proportion to the nozzle s movement speed and the chuck s rotation speed. Additionally, the tilted spray could get a more uniform photoresist coating than the vertical spray did. Especially, the experiment with the tilted spray achieved a coating layer error of under 1 µm. It is likely that such technology can be used for the PR thickness uniformity even in deep and complex structures as in MEMS element production. ACKNOWLEDGMENTS This work was supported in part by the BK21 program in Korea. This research was financially supported by the Ministry of Commerce, Industry and Energy (MOCIE) and Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Regional Innovation. REFERENCES Fig. 12. Cross section of the dispensed at 45 by using a spray nozzle (a) Convex corner (b) Bottom. had thicknesses of 2.87 µm and 2.15 µm, respective and a bottom thickness of 2.68 µm, which were less influenced by the surface tension than by the vertical spray. V. CONCLUSIONS [1] N. P. Pham, J. N. Burghartz and P. M. Sarro, Electronics Packaging Technology Conference (Singapore, 2003). [2] S. Linder, H. Baltes, F. Gnaedinger and E. Doening, IEEE International Workshop on Micro Electro Mechanical System, MEMS 96 (SanDiego, 1996). [3] V. K. Singh, M. Sasaki. K. Hane, Y. Watanabe, M. Kawakita and H. Hayashi, Transducers 05 (Seoul 2005). [4] V. K. Singh, M. Sasaki, J. H. Song and K. Hane, Microprocesses and Nanotechnology Conference (Tokyo, 2002). [5] N. P. Pham, E. Boellaard, J. N. Burghartz and P. M. Sarro, J. MEMS 13, 491 (2004). [6] K. Fischer and R. S vss, IEEE/CPMT/SEMI 29th International Electronics Manufacturing Technology Symposium (San Jose, 2004). [7] V. K. Singh, M. Sasaki, J. H. Song and K. Hane, Transducers 03 (Boston, 2003). [8] M. Ichiki, L. Zhang, Z. Yang, T. Ikehara and R. Maeda, Transducers 03 (Boston, 2003). [9] G. Percin and B. T. Khuri-Yakub, IEEE Trans. Semicond. Manufact. 16, 452 (2003). [10] D. S. Eun, J. H. Lee, Y. M. Kim, S. H. Kong and J. H. Lee, J. Korean Phys. Soc. 45, 763 (2004). [11] D. S. Kim, E. J. Lee, J. Kim and S. H. Lee, J. Korean Phys. Soc. 46, 1208 (2005). [12] H. Park, H. J. Hyun, D. H. Kah, H. D. Kang, H. J. Kim, Y. I. Kim, Y. J. Kim, D. H. Shim, Y. I. Choi, Y. K. Choi, J. Lee, J. M. Park and K. S. Park, J. Korean Phys. Soc. 49, 1401 (2006).