Research and Development of Parylene Thin-Film Deposition and Application for Water-Proofing

Transcription

1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Research and Development of Parylene Thin-Film Deposition and Application for Water-Proofing Wen-Cheng Kuo 1,a, Chao-Yang Hsu 1,b 1 Department of Mechanical and Automation Engineering, National Kaohsiung First University of Science and Technology, Taiwan R.O.C. a rkuo@nkfust.edu.tw, b u @nkfust.edu.tw Keywords: mobile phone; nanometer scale thin-film deposition; parylene; water-proof. Abstract. This research presents the development of parylene thin-film deposition from the micrometer scale to nanometer scale. Processes improved film uniformity by 6% in the nanometer scale to meet the requirements of new applications of parylene, such as the delamination layer of organic light-emitting diodes, the dielectric film of through-silicon-via, and water-proofing mobile phones. The application and process of water-proofing mobile phones is also examined. The tested mobile phone was coated with a 0.5 μm parylene coating and functioned properly while submerged underwater. The mobile phone is still operational; thus, nanoscale parylene deposition is a novel application for water-proofing mobile phones. The application does not concern problems associated with parts assembly or damaged rubber material. Introduction Water-proof objects are capable of remaining relatively unaffected by the ingress of water under specific conditions. Electronic products such as cameras and mobile phones risk being short-circuited by water penetration. Traditional methods for water-proofing electronic products include sealing parts with rubber, and spraying epoxy or silicone onto the objects. The advantages of spray-finishing are primarily the ease and speed of application. However, poor film uniformity, void areas, and excessive film limit the application of such an approach for water-proofing consumer electronic products. Parylene treatment has recently attracted significant attention for its properties, such as room temperature CVD stress-free deposition, biocompatibility, and permeability [1]. Parylene is the generic name for a unique polymer series. The chemical structures for commonly used parylenes (parylene-c, parylene-n, and parylene-d) are shown in Fig. 1. Cl Cl n Parylene N n Parylene C n Cl Parylene D Fig. 1. Schematic of chemical structures for the three most commonly employed parylenes. Parylene-N (poly-para-xylylene) is a completely linear, highly crystalline material, and is the most basic member of the series. Parylene-C is produced from the same monomer but substitutes a chlorine atom for aromatic hydrogen, whereas parylene-d similarly substitutes a chlorine atom for two aromatic hydrogens. Parylene-N possesses a low dissipation factor, high dielectric strength, a dielectric constant independent of frequency, and the highest penetrating power of all parylenes. Parylene C is the material of choice for coating critical electronic products because of its combination of electrical and physical properties, exceptionally low permeability to moisture and other corrosive gases, and flawless conformal insulation. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-11/05/16,07:20:33)

2 24 Materials Processing Technology II Table 1 shows the differences in water-proofing properties among parylene-c, N, D, epoxy, and silicone [2]. Table 1. Properties of water-proofing materials. Parylene N C D Epoxy Silicone Tensile Strength (Mpa) ~91 5.6~7 Static NA 1.0 Dynamic NA NA Melting point NA NA Friction Coefficient Water absorption(%24h) <0.1 <0.1 < ~ /7Days Linear Coefficient of thermal expansion 25 E/ ~8 4.5~6.5 25~30 Permeability 37 90%RHg-mil/100in 2 -d The process of parylene deposition [3] is shown in Fig. 2. Parylene is deposited at room temperature at pressures of approximately 20 mtorr, with the mean free path of the molecules on the order of 0.1 cm. The deposition process begins by heating the solid dimer to 150 C for vaporization. The vaporized dimer is then pyrolyzed to cleave the two methylene-methylene bonds and yield stable monomeric radicals (para-xylyene) at approximately 680 C. The monomers are driven into the deposition chamber at room temperature by a pumping system, where they polymerize on the substrate. A cold trap (approximately -100 C) is used to collect the unreacted monomers before they enter the pumping system. Fig. 2. Schematic of parylene deposition process. The traditional technique used to water-proof mobile phones is to seal parts with rubber to resist the ingress of water. The parts require precise dimensions to reduce the tolerance between the assembly and the rubber seal. Furthermore, damage or aging of the rubber material results in leakage. Use of nanoscale thin-film conformal parylene deposition does not require considerations of the tolerance of parts during fabrication and assembly, or the aging of rubber material. Thus, it is a novel method for water-proofing applications. Parylene has generally been applied as a thin film in the micrometer scale. However, thin-film deposition of parylene in the nanometer scale is now required for new applications such as the delamination layer of organic light-emitting diodes (OLED) [4], the dielectric film of through-silicon-via (TSV) [5, 6] and water-proof mobile phones. Deposition chamber design and parameters must be further developed to meet nanoscale thin-film deposition uniformity requirements. An inspection of parylene properties, modification of the parylene deposition chamber, and water-proof testing are discussed to prove the feasibility of parylene coating for water-proofing applications based on nanoscale film deposition requirements.

3 Advanced Materials Research Vols Research process 1. Parylene property inspection The parylene property inspection uses the Thermogravimetric Analyzer (TGA) to analyze the dimer evaporation rate at different temperatures to improve deposition uniformity and quality. 2. Modification of the parylene deposition chamber Fig. 3 shows a traditional parylene deposition chamber. The parylene monomers diffuse into the chamber from the inlet pipe before being deposited on the object placed on on the rotating platform. The unreacted monomers are driven to the outlet by pumps and condensing equipment. The uniformity of parylene film is in the micrometer scale. Fig. 4 shows the required outlet nozzle pipe attached to the chamber to meet uniformity requirements in the nanometer scale. The CFD simulation shows that the flow field is symmetrical during the inlet and outlet, as shown in Fig. 5. Fig. 3. Traditional parylene deposition chamber design. Fig. 4. Modified parylene deposition chamber design. Fig. 5. CFD simulation of parylene deposition chamber.

4 26 Materials Processing Technology II 3. Thickness measurement of parylene films Fig. 6 shows a cage with four heights of holders and four locations of glass slides on each, created to measure the variations in film inside the chamber before and after modifications. The surface profiler is used to measure film es on glass slides following deposition. Experimental results Fig. 6. Setup of the samples for parylene film measurement. 1. Measurement results of parylene dimer TGA inspection Fig. 7 shows the results of TGA inspection of pyrolysis temperature of parylene-c. The rate of weight loss is stable when the temperature is kept between 90 C and 140 C. The rate of weight loss increases dramatically above 150 C, until the dimer is evaporated completely at C. Therefore, vaporization temperature must be maintained under 140 C. This ensures that the vaporization rate is stable, and that any variation in film will be insignificant. Fig. 7. Parylene-C TGA inspection results. 2. The measurement results of parylene thin film Table 2 shows the measurement results of the microscale parylene film without the outlet nozzle shown in Fig. 3. The evaporation temperature was set at 140 C, pyrolysis temperature at 680 C, chamber pressure at 20 mtorr, chamber temperature at 20 C, and the rotary table revolution rate at 80 rev/s. The maximum deviation was 1.03 μm, and uniformity was 13.7%. Table 2. Thickness measurement results of microscale parylene film. Layer No. per layer (μm) (μm) Max deviation (μm) Uniformity (%) %

5 Advanced Materials Research Vols Table 3 shows the nanoscale parylene film measurement results with the outlet nozzle shown in Fig. 4. The evaporation temperature was set at 140 C, pyrolysis temperature at 680 C, chamber pressure at 20 mtorr, chamber temperature at 20 C, and the rotary table revolution rate at 80 rev/s. The maximum deviation was 40 nm, and the uniformity was 6%. Table 3. Thickness measurement results of nanoscale parylene film. Layer No. per layer (nm) (nm) Max deviation (nm) Uniformity (%) % 3. Water-proof testing of a mobile phone The mobile phone was disassembled into major parts (Fig. 8) to maximize the parylene coating process. Parts were placed inside the parylene coater to be coated with a 0.5 μm parylene film. Parts were reassembled into and tested for functionality following the coating process. The mobile phone was turned on and gradually placed into the water for testing (Figs. 9 and 10). The mobile phone continued working while submerged without short-circuiting. Fig. 8. Illustrations of major mobile phone parts. Fig. 9. Water-proof testing (Mobile phone is gradually submerged in water while operating). Fig. 10. Water-proof testing (The mobile phone is still operational).

6 28 Materials Processing Technology II Conclusion This study presents improvements on the uniformity of parylene deposition in the nanoscale. The parylene TGA inspection, CFD simulation, and modification of parylene deposition chamber and water-proof testing improved uniformity by 6% in the nanometer scale to meet new applications of parylene, such as the delamination layer of OLED, TSV, and water-proofing mobile phones. This study also presented the application and process of water-proofing mobile phones. The tested mobile phone was coated with a 0.5 μm parylene coating and functioned properly while submerged under water. The mobile phone is still operational; thus, nanoscale parylene deposition is a novel application for water-proofing mobile phones. The application does not concern problems associated with parts assembly or damaged rubber material. Acknowledgment The authors would like to thank the technical supports of La-Chi Enterprise Company and National Science Council of Taiwan for the financial support (NSC E CC3). References [1] Y.-C. Tai: 2005 IEEE international Conference on Robotics and Biomimetics (ROBIO2005), pp [2] L.-Q. Wu: Electro-Mechanical Engineering, 20 (2004) pp (in Chinese) [3] V. S. Kale and T. J. Riley: IEEE Transactions on Parts, Hybrids, and Packaging, PHP-13 (1977) pp [4] L. Ke, R. S. Kumar, K. Zhang, S. J. Chua, and A. T. S. Wee: Synthetic Metals, 140 (2004), pp [5] B. Majeed, N. P. Pham, D. S. Tezcan, and Eric Beyne: Electronic Components and Technology Conference (2008), pp [6] M. Ji, Y. Zhu, S. Ma, X. Sun, Min Miao, and Y. Jin: International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP 2009), pp

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