Polyimide Film Micromachining by Wet-Etching Technology

Transcription

1 Polyimide Film Micromachining by Wet-Etching Polyimide Film Micromachining by Wet-Etching Technology Paper Han Ji-song * Non-member Tan Zhi-yong * Non-member K. Sato * Member M. Shikida * Member We have presented the characteristics of polyimide (PI) film wet-etching using the commercially available and most commonly used Kapton Upilex PI-film as the testing materials and a strong alkaline solution named TPE3000 (from Toray Engineering o., Ltd., Japan) as the etchant. We have introduced the fabrication process of micro through-holes on the PI-film by wet-etching using two types of etching-mask materials, i.e. a photosensitive dry film and thin copper film. We have also developed thermal-type micro sensor device using densely arrayed micro heaters as the sensing elements. The sensing elements were resistors made of sputtered 200-nm-thick platinum film on the thin (50-µm-thick) flexible PI-film substrate. The electrical feed-through was arranged on the different sides of the substrate and interconnected to the micro heater elements via the wet-etched through-holes, inside walls of which were deposited with thin metal film by means of the electroless copper plating technology. The fabricated device was robust, flexible and can be attached to non-flat curved surface. Keywords: polyimide film, wet-etching, micro machining, through-hole wiring, surface treatment, and flexible skins 1. Introduction Polyimide is one of the most widely used polymers in the electronics industry (1). Although polyimide is rarely the best choice for a structural material in MEMS (Micro-Electro-mechanicalsystem), the very low thermal conductivity and the very high mechanical flexibility that it offers are critical advantages to MEMS devices (2). Many papers have been reported to present the use of polyimide as the thermal insulation layer or MEMS structural material (3)~(8). For the thermal insulation layer, polyimide resist is generally used by coating, photolithography and curing process. For the micro structures, there are basically two types of micromachining methods (wet-etching technology and dry etching include the laser technology) can be used directly on the PI-film, besides the method of photolithography technology by means of the polyimide resist curing process on a substrate. Polyimide has a good balance of mechanical, thermal, electrical and chemical properties as well as low dielectric constants, suitable as a packaging and insulating material for electronic devices and circuit boards too (9)~(12). In the thin and compact MEMS packaging, polyimide is playing an important role as a key component. With the dramatic development of mobile machines, the need for PI-film is ever increasing now (13). Densely arrayed micro sensors on the flexible PI-film will be very useful in MEMS applications to obtain the real-time two- or three- * Department of Micro System Engineering, Nagoya University, hikusa, Nagoya, , Japan dimensional distributed measurement of certain physical quantities such as temperature, force, pressure, turbulent flow etc. and micro patterns recognition e.g. fingerprint patterns for the personal identification. In these applications, three-dimensional interconnect technology is required in order to wire the densely arrayed twodimensional sensor elements easily, and make the sensing robust. Here, an important key factor is to fabricate the high-density micro through-holes with a low cost and high productivity for the metal interconnects. The conventional methods to fabricate micro throughholes on the PI-film are by laser, dry etching, mechanical drilling or punching, etc., but because of the expensive equipment, low productivity and a large amount of consuming power, the conventional fabrication methods will inevitably result in the problem of high production cost. Generally, with the methods of mechanical drilling and punching, it is also difficult to generate fine throughholes and fine pitched patterns and easily result in the stress problem or some defects on the periphery of the holes on the flex substrates. Laser drilling stresses less, but costs more than mechanical drilling. Punching is very rapid but produces large stress on the PI-film. It is very attractive to fabricate high-density micro through-holes on the flexible PI-film by a wet-etching technology, because, wet-etching process has the significant advantage of cost effectiveness in the largevolume production and can avoid the stress problem too. The number of holes or hole sizes in the same wafer does not increase the processing cost. When fabricate a number of same holes or holes of varied sizes on the same wafer, the cost of wet-etching process is significantly lower than that of conventional methods mentioned

2 above. It is well known that polymide film can be etched by a strong alkaline solution and through-holes could be fabricated by PI-film wet-etching. However, to date, few papers have been reported to describe the characteristics of PI-film wet-etching, and the fabrication process of though-holes on the flexible PI-film using wet-etching technology. The widespread utilization of PI-film has not facilitated its micromachining process yet. In order to make effective application of PI-film, it is necessary to understand the characteristics of PI-film including its micromachining properties. In this paper we report the characteristics of PI-film wet-etching using the commercially available and most commonly used Kapton and Upilex PI-film as the testing materials and a strong alkaline solution TPE3000 as the etchant. We also report the fabrication process of micro through-holes on the flexible PI-film using wetetching technology. When using a photosensitive dry film and a thin copper film as the wet-etching mask, the fabricated through-holes showed tapered cross-sections because of the under-cut etching. The etched taper angle was in dry-film mask and in copperfilm mask, because of the adhesion differences of mask material to the substrate. When designing the photomask patterns, it is important to understand the characteristics of PI-film wet-etching in order to select correct process conditions and etching method e.g. singleside etching or double-side etching etc. In this paper, we also discuss the adhesivity of thin metal film deposited on the PI-film surfaces. At last, we present the fabricated micro thermal sensor device, which has three-dimensional through-hole wiring. In our prototype, a densely arrayed one-dimensional heater elements were formed directly on a flexible thin PI-film (50-µm-thick) by means of micromachining technology, arranging the heater elements and electrical feed-through on the different sides of the substrate and interconnected by a through-hole wiring. The through-hole wiring was formed by electroless copper-plating technology. The heater elements were formed by sputtering 200nm-thick platinum using 20-nm-thick titanium as the adhesion-promoting layer between the platinum and the PI-film. Heater elements having a pitch of 200 µm were patterned by lift-off technique. 1-µm-thick electrical feedthrough were formed by the electroless copper plating at the same time with the through-hole wiring process and then patterned by lift-off technique. Micro through-holes for metal interconnects were made by PI-film wet-etching technology. The fabricated device is robust, flexible and can be attached to non-flat surface. 2. Molecular Structures and Material Properties A variety of PI-films have been developed, but there are mainly two types in molecular structures: (1) pyromellitic acid and (2) biphenyl tetra carboxylic acid. ommercially available product Kapton belongs to the former and Upilex to the latter (see Fig. 1). In our study, we used two different types of PI-film (Kapton and Upilex) to examine the characteristics of polyimide micromachining by wetetching technology. Kapton PI-film was provided by Du Pont-Toray o., Ltd., Japan, and Upilex PI-film by UBE Industries, Ltd., Japan. (a) N N Kapton, the most commonly used PI-film, is synthesized by polymerizing an aromatic dianhydride and an aromatic diamine. Upilex, which has higher mechanical strength and thermal stability compared to Kapton, is the product of polycondensation reaction between biphenyltetracarboxylic dianhydride (BPDA) and diamine. Upilex is relatively stronger in mechanical properties, and shows higher chemical resistance and dimensional stability over a wide range of temperatures than Kapton PI-film, while Kapton is stronger in folding endurance, and shows higher electrical resistance and lower thermal conductivity than Upilex PI-film. Table 1 shows the compared results of the mechanical, thermal, chemical and electrical properties of Kapton and Upilex PI-film. Table 2 shows the compared results of the material properties for H-type and EN-type Kapton PIfilm. Both types possess polyimide inherent outstanding characteristics and have similar values, but the EN-type shows relatively higher dimensional stability. Strong alkaline solution TPE3000 was used as the PI-film wet etchant. It can be provided from Toray Engineering o., Ltd., Japan. TPE3000 is composed of 20wt% KH solution and 20~40wt% aliphatic amine compound 2H7N, and can hydrolyze polyimide and polyester compounds. When PI-film is immersed in TPE3000 solution, the imide ring of the polyimide molecule is opened, and polymer is converted to amide that is soluble to the solution (1). 3. haracteristics of PI-Film Wet-Etching 3.1 Etching Rate We studied the etching rate of Kapton and Upilex PI-film in TPE3000 solution by experiment. As shown in Fig. 2, etching rates of the PI-films are strongly temperature dependent: (1) the increase of etching rates is remarkable at temperatures higher than 80 ; (2) the etching rate of Upilex PIfilm is remarkably lower than that of Kapton PI-film. With an increase in etching temperature, the etching rate differences increase. It might be derived from the strong molecular structure of Upilex which built- N Fig. 1. PI-film molecular structure: (a) Kapton PI-film (10) ; Upilex PI-film (12). N R n n

3 Polyimide Film Micromachining by Wet-Etching Table1. Material properties of Kapton and Upilex polyimide film (data from (7)~(12) ). Mechanical properties Thermal properties Items Kapton-H Upilex-25S Film thickness (µm) Tensile strength (MPa) Young's modulus (GPa) Stress at 5% elongation (MPa) Ultimate elongation (%) Folding endurance (cycles) Density (g/cm 3 ) , >,000 Melting point None None Thermal coefficien of Linear expansion (ppm/k) Thermal conductivity (W/m Κ) Specific heat (J/g Κ) Test method ASTM D882 ASTM D2176 ASTM D1505 ASTM E ASTM D ASTM F Differential calorimetry hemical properties Electrical properties Water absorption (%) Water PH=1.0 Water PH=10.0 Strength retained [%] Elongation retained [%] Modulus retained [%] Strength retained [%] Elongation retained [%] Modulus retained [%] Dielectric strength (kv) Dielectrical constant Volume resistivity (Ω cm) Surface resistivity (Ω) Equilibrium at 50% RH / Immersion in water 23 /24h Immersion ( /2 weeks) Immersion ( /4 days) ASTM D > ASTM D150 ASTM D257 ASTM D257 ASTM D570 ASTM D882 Table 2. Main material properties of H-type and EN-type of Kapton polyimide film (data from (9), (10) ). Iterms H (25 µm) EN (25 µm) Strength (MPa) Elongation (%) Young's modulus (GPa) Dielectric strength (kv/mm) Heat shrinkage (%) Thermal coefficien of linear expansion (ppm/κ) Humidity expansion coefficient (ppm/%rh) Water absorption (%) Thermal conductivity (W/m K) Folding endurance (cycles) Volume resistivity (Ω cm) , , Test method JIS 2318 IP No (200 ) 50~200 (temperature increments (10 /min)) 3~90% RH Immersion in water for 24hr Model T-0 comparing method JIS P 8115 JIS

4 up its resistance to hydrolysis and inhibited itself to be etched away. At 87, the etching time of Upilex for per unit thickness is about 7~9 times longer than that of Kapton PI-film; (3) in Kapton PI-film, the etching rate of EN-type is a little higher (about 1.3 times) than that of H-type. 3.2 Patterned Structure Depending on the etching-mask material and the mask opening area, the wet-etched through-hole became different. We studied the PI-film etching characteristics in TPE3000, using photosensitive dry film and thin copper film as the etching-mask. The rubber-based photosensitive resist can also be used as the wet-etching mask, but because of the possibility of its cancerous toxic, in our experiments we get rid of it from the options. PI film etching rate (µm/sec) Etchant: TPE3000 Kapton H-type Kapton EN-type Upilex s-type Etchant Temperature ( ) Fig. 2. PI-film wet-etching rate versus etching temperature in TPE3000 solution. Figure 3 shows the SEM (scanning electron microscope) images of the wet-etched through-holes patterned on the PI-film using metal mask (a) and photosensitive dry film mask. In (a), the mask aperture size is Φ50 µm and the starting material is 50-µm-thick of EN-type Kapton PI-film sandwiched by 5-µm-thick copper film (named as Metaloyal and provided by Toyo Metallizing o., Ltd., Japan). PI-film is etched at 80 in TPE3000 from one side of the substrate using the 5-µm-thick copper film as the etching-mask. Thin copper film on the Metaloyal is formed by electrical plating following to an adhesion-promoting layer of 3-nm-thick sputtered chrome film. Good adhesion is realized and during the polyimide wet-etching no peeling was observed. The etched taper angle θis 30~35. In, mask aperture size is Φ150 µm and the substrate is 25-µm-thick of EN-type Kapton PI-film. 15-µm-thick photosensitive dry film RY is provided and laminated by Hitachi hemical o., Ltd., Japan, and to be used as the wet-etching-mask. PI-film wet-etching is performed from the double sides of the substrate at 80 in TPE3000. The etched taper angle θis 44~49. Photosensitive dry film is not as strong as the copper film and its adhesivity to the PIfilm is not so strong as that of copper film. Peeling was found in TPE3000 when the etching time is longer than 15 minutes. So, when fabricating through-holes on the thicker PI-film, we recommend to use thin copper film as the wet-etching mask instead of RY Etched tapers are supposed to be by the under-cut of the PI-film during the wet-etching, and the differences in taper angles are derived from the difference in adhesivity between the PI-film and the etchingmask layer. We investigated the through-hole shape change compared to the (a) t θ t θ µm Fig. 3. SEM pictures of wet-etched through-hole geometrical shapes on the PI-film. In (a): mask aperture size is Φ50 µm and the starting material (named as Metaloyal) is 50-µm-thick of EN-type Kapton polyimide film sandwiched by 5-µm-thick copper film. Polyimide film is etched from single side using the 5-µm-thick copper film layer as the etching mask; In : mask aperture size is Φ150 µm and the substrate is 25-µm-thick of EN-type Kapton polyimide film. Polyimide film is etched from the double sides of the surface using 15-µm-thick of laminated photosensitive dry film RY-3215 as the etching mask

5 Polyimide Film Micromachining by Wet-Etching Increase in diameter (µm): (etched dia.)-(mask size) Increase in diameter (µm): (etched dia.)-(mask size) 50 µm Fig. 4. ptical microscopy image of wet-etched throughhole on the Kapton PI-film. Wet-etching was carried out from the double sides using a square mask aperture of 50 µm 50 µm (a) Etching time (s) Etching time (s) Fig. 5. Relationship between the etching time and overetching at different etching temperature. The starting material was u-plated Metaloyal (u/kapton PI-film/u: 5/50/5 µm) using the 5 µm thick u layer as the etching mask, and etched from a single side. The mask aperture size was (a) Φ µm and Φ2000 µm. mask aperture shape during the wet-etching process. Although we used a square-shaped 50 µm 50 µm mask aperture, the etched through-hole shape became round as shown in Fig. 4. This is because TPE3000 shows quite isotropic etching characteristics. This is unlike to some other etchant e.g. 35% of TEAH (tetraethyl-ammoniumhydroxide) which shows anisotropic characteristics in polyimide etching (14). We also carried out some experiments to study the relationship between the overetching and the etching time at different etching temperature, as shown in Fig. 5. The starting material is same to that of in Fig. 3-(a), and etched from a single side. The mask aperture sizes are Φ µm and Φ2000 µm respectively in Fig. 5 (a) and. From Fig. 5, it is known that the overetching increases with the etching time and the temperature rise, regardless of the mask aperture size. In addition, at the etching temperature higher than 75 the overetching became extremely increased. This corresponds to the results of Fig PI-Film Micromachining by Wet-Etching 4.1 High-density Micro Through-holes Two sets of PIfilm wet-etching process using different etching-mask materials are introduced here. ne of the etching-mask is a photosensitive dryfilm (> 10-µm-thick) and the other one is a thin u-film (> 4-µmthick) Dry-Film as an Etching-Mask Material In dryfilm mask process, RY-3215 was laminated on to the both sides of the PI-film surfaces at the condition of 110 /(0.29~0.49 MPa) and to be used as the PI-film wet-etching mask. RY-3215 can be exposed under at 65mJ/cm2 of ultraviolet (UV), followed by PEB (post exposure bake) in the oven (90 /15min). After PEB, dry-film development was performed in 1wt% of Na23 solution at 30 followed by rinse in deionized water. After the etching-mask patterns were formed by dry-film photolithography, the PI-film wet-etching was carried out in TPE3000 solution to fabricate the micro throughholes. To make through-holes on the 25~50-µm-thick Kapton PIfilm, 4~5 minutes of etching time was necessary at 80. After the PI-film wet-etching, the etching-mask RY-3215 was removed by 3wt% of NaH solution at 50. We first examined the patterning resolution of dry film in photolithography. We made a mask aperture patterns having diameters of 20, 30, 50, and -µm on the same photomask. ontact type exposure system was used. The developed dry film mask patterns are shown in Fig. 6. Mask aperture size of Φ50 µm was successfully developed, but that of Φ30 µm was always irregular shape (Fig. 6- b), and by that of Φ20 µm aperture could not be opened (Fig. 6-c). PI-film (25-µm-thick) is wet-etched from the double sides after dryfilm lithography. Through-hole sizes fabricated by PI-film wetetching are slightly lager than the etching-mask aperture. At the condition of 80 /4min, when the mask apertures are Φ30 µm and Φ50 µm, the etched through-holes became Φ33 µm (in Fig. 7-a) and Φ53 µm (in Fig. 7-b) respectively. In the case of the mask aperture Φ µm, we found the etched through-hole is Φ108 µm. We further found that, when a pitch distance of the mask aperture is large, the increasement in through-hole diameters became lager although the mask aperture diameter is not changed, as shown in Fig. 7. The etched through-hole in Fig. 7-(c) is Φ120 µm, while that of in Fig. 7-(d) is Φ108 µm, though mask aperture diameter are both µm. The reason is supposed to be that the bigger pitch distance allows the etched products to be diffused from the surface of the substrate easily,

6 (a) Photo mask: Φ50 µm (c) Photo mask: Φ30 µm Photo mask: Φ20 µm Fig. 6. SEM images of developed micro holes formed by RY-3215 photolithography: (a) mask aperture of Φ50 µm is successful; mask aperture of Φ30 µm can be penetrated but the geometrical shape is irregular; (c) mask aperture of Φ20 µm could not be penetrated after dry film lithgraphy. (a) 110 µm 130 µm Photo mask: Φ30 µm Photo mask: Φ50 µm (c) (d) 240 µm 150 µm Photo mask: Φ µm Photo mask: Φ µm Fig. 7. ptical microscopy images of wet-etched micro through-holes using 25-µm-thick Kapton PI-film as the substrate and etched from both sides. Etching mask is dry film RY Photomask aperture diameters are 30, 50 and µm. so that the substrate has more chance to be reacted with the fresh etchant. So, when design the photomask patterns, it is important to consider that the differences between the mask aperture size and the wet-etched real size depend not only upon the mask aperture size but also upon the pitch distance even under the same process conditions opper Film as an Etching-Mask In Fig. 8, the starting material is u-plated Metaloyal (u/kapton PI-film/u: 5/ 50/5-µm). Dry film or photosensitive resist can be used as the u film etching-mask. u film is patterned by 39wt% of Fel3 solution. PI-film wet-etching is carried out in TPE3000 solution from the single side of the substrate. After the micro through-holes are fabricated, u film is removed by 39wt% of Fel3 solution. Arrayed micro through-holes with the pitches of µm and µm are fabricated. It is possible to get the targeted value of patterns by control and adjusting the process parameters, such as etching-mask material, mask aperture size, etching temperature as well as etching time etc. In order to get the precise process parameters further experiments are necessary. When using u film as the etching-mask, the etched taper angle θ is by 15 smaller than that of the dry film etchingmask. In the case of the same etching-mask, etching from the double sides could form much smaller size of through-holes. By double sides etching, the diameter of fabricated through-hole is about the value of {(PI film thickness) tangent (θ)} smaller than that of single side etching. So, in order to get densely arrayed micro through-holes on the PI-film, it is useful to etch from both sides of the PI-film using u film as the etching-mask, although the process became longer and a little complicated comparing to that of the dry film. 5. PI-Film Surface Treatment for Improving Metal Film Adhesivity It is known that, PI-film shows poor adhesivity against the metal thin film. Surface modification techniques such as chemical etching, plasma treatments and ultraviolet irradiation of polymer surface, as well as thermal treatment of metal-polymer system, have shown to improve the adhesivity of metal thin films to polyimide, measured by the peeling test (15). hemical etching is a good choice of low cost method and can be used without big investment. In the following copper plating process, we applied surface modification as the pretreatment by means of the chemical light etching on the PI-film substrate. In order to realize three-dimensional metal interconnects on the fabricated through holes, we performed electroless copper plating process. In this process, we used a plating solution named as MELPLATE U-5 (supplied by Meltex o., Ltd, Japan). By using MELPLATE U-5, about 1-µm-thick copper film could be formed on the substrate in 15 minutes at Without pretreatment, the adhesivity of copper film to the polyimide substrate is very poor as shown in Fig. 9. The adhesion effects are confirmed by some peeling tests with an adhesive tape. The peeling test results show that, after chemical etching pretreatment (by TPE3000 light etching) the adhesivity can be greatly improved. Figure 10 shows a good adhesivity after peeling test. The improved adhesivity probably derived from the PI-film wet-etching mechanism. In wet-treat, the reaction not only etches away some of the polyimide but also converts some into an amide that would adhere better to metals (1). Surface modification studies of polyimide indicate the formation of carbon radicals due to carbonyl oxygen losses. These radicals can either react with each other, resulting in crosslink formation among the polyimide chains, or with the evaporated metal atoms (15)

7 Polyimide Film Micromachining by Wet-Etching (a) 10 µm 200 µm Fig. 8. SEM and optical microscopy images of arrayed micro holes fabricated on the 50-µm-thick polyimide film by wet-etching technology using thin copper film (5 µm) as the etching mask, and etched from single side of the surface: (a) pitch distance is µm; pitch distance is µm. Fig. 9. SEM image of plated copper film after peeling test. The peeling test was performed with adhesive tape, after electroless copper plating process. The electroless copper plating was performed without pre-treatment on the polyomide film surface. Fig. 10. SEM images of successfully adhered copper film after pealing test. Before the electroless copper plating process, pre-treatment was performed to the polyimide film surface by light wet-etching in TPE3000 solution to improve the adhesivity. 6. Arrayed Micro Heater Elements Having 3-D Metal Interconnects Here, we present the fabricated micro thermal sensor device, which has three-dimensional through-hole wiring using 50-µm-thick PIfilm as the substrate as shown in Fig. 11. In Fig. 11: (a) the crosssectional view of the sample; the enlarged top view of the throughhole wiring structure; (c) the top view of the arrayed heater elements include through-hole wiring; (d) the enlarged bottom view of the through-hole wiring structure; and (e) the bottom view of the electrical feed-through. In this device, the heater elements on the top surface and the electrical feed-through on the backside are interconnected by 1-µm-thick metal film via the wet-etched through-holes. The through-hole wiring is formed by the electroless copper-plating technology. The heater elements are formed by sputtering 200-nmthick platinum using 20-nm-thick titanium as the adhesion-promoting layer between the platinum and the PI-film. Heater elements having a pitch of 200 µm are patterned by lift-off technique. The electrical

8 Through-hole wiring (face side) (c) Micro heater array (Pt/Ti, pitch: 200 µm) PI film (50 µm) (a) ross-sectional view of the sample Wire-bonding (Al: 25 µm) (µm3) (d) Through-hole wiring (underside) (e) Electrical feed-through (u) Fig. 11. Fabricated prototype of arrayed micro heater elements with a vertically interconnected through-hole wiring: (a) cross-sectional view of the sample; top view of the through-hole wiring structure; (c) top view of the arrayed heater elements include through-hole wiring; (d) bottom view of the through-hole wiring structure;(e) bottom view of the electrical feed-through. (a) (c) Fig. 12. Fabricated sensor samples with Kapton PI-film as the substrate. Densely arrayed micro sensing elements were formed on the substrate by sputtering thin platinum film and lift-off patterning. The sensing elements were arranged on the face-side surface, and electrical feed through were formed by u lift-off and arranged on the backside surface. Sensing elements and electrical feed through were interconnected by through-hole wiring. Through-holes were fabricated by PI-film wet-etching and through-hole wiring were realized by electroless copper plating: (a)-(c) fabricated sensor wafer is flexible and can be attached to non-planar surface

9 Polyimide Film Micromachining by Wet-Etching feed-through are formed by 1-µm-thick electroless copper plating at the same time with the through-hole wiring process and then patterned by lift-off technique. The experimental results about this sensor will be reported in another paper (now in the peer review in Journal of Micromechanics and Microengineering), in which its thermal electrical properties are described in detail for the application of the thermal type micro fingerprint sensor. Figure 12 shows the fabricated prototype with the flexible PI-film as the substrate. The sensor wafer is flexible and can be attached to non-planar surface. The presented technology in this paper will be useful to develop densely arrayed distributed sensors to obtain real-time 2- or 3- dimensional physical information such as temperature, force and pressure. This technology will also be useful to develop flexible skins that can be easily taped or glued on non-planar surfaces applicable to the aerodynamics study and the sensitive robot skins. 7. onclusions The characteristics of PI-film wet-etching were presented. By a åstrong alkaline solution TPE3000, some micromechanical structures can be achieved isotropically on the PI-film. The fabrication process of the micro through-holes by PI-film wet-etching was introduced in detail using two types of etching-mask materials, i.e. photosensitive dry film RY-3215 and thin copper film. The wet-etched taper angle of through-holes were when using a copper film etchingmask and when using dry film etching-mask. PI-film wet treat is a simple and effective way to improve the adhesivity of the metal film onto the PI-film surface. Three-dimensional through-hole wiring was realized by depositing thin metal film to the walls of the wet-etched PI-film through-holes using electroless copper plating process. Densely arrayed one-dimensional micro sensor device having vertically interconnected through-hole wiring was fabricated using the flexible Kapton PI-film (50-µm-thick) as the substrate. The fabricated device was robust, flexible and can be attached to non-flat curved surface. These technologies will be useful when developing applications of distributed sensors such as micro temperature distribution sensors, micro flow sensors and thermaltype micro fingerprint sensors, etc. Acknowledgements We would like to thank Hitachi hemical o., Ltd. for their kind help with dry-film laminating process. We also thank Toray Engineering o., Ltd., Toyo Metallizing o., Ltd. and Meltex o., Ltd. for their help in supplying the samples and advice with the process. (Manuscript received Mar. 1, 2004, revised Sep. 21, 2004) thermal probe using a low temperature polyimide-based micromachining process, in Proc. IEEE Int. onf. Micro Electro Mechanical Systems (MEMS 00), Miyazaki, Japan, pp (Jan. 2000) ( 3 ) Ji-song Han, Zhi-yong Tan, K Sato, and M. shikida: Threedimensional interconnect technology on a flexible polyimide film, J. Micromech. Microeng.14, pp (2004) ( 4 ) F. Jiang, G.B. Lee, Y.. Tai, and.m. Ho: A flexible micromachinebased shear-stress sensor array and its application to separation-point detection, Sensors and Actuators 79, pp (2000) ( 5 ) J. Engel, J. hen, and. Liu: Development of a multi-modal, flexible tactile sensing skin using polymer micromachining, Transducers 03, The 12th international conference on solid state sensors, actuators and microsystems, Boston, June 8-12, pp (2003) ( 6 ) V. J. Lumelsky, M. S. Shur, and S. Wagner: Sensitive Skin, IEEE Sensors Journal, Vol. 1, No. 1, pp (June 2001) ( 7 ) Stemme G. : A monolithic gas flow sensor with polyimide as thermal insulator, IEEE Trans. on Electron Devices Vol. 33, pp (1986) ( 8 ) G. Stemme : An integrated gas flow sensor with puls-modulated output Proc. Transducers 87 (Tokyo, Japan, June 2-5) pp (1987) ( 9 ) (10) atalogue, Kapton polyimide film, Du Pont-Toray o., Ltd., Japan. (11) (12) atalogue, Upilex polyimide film, Ube Industries, Ltd., Japan. (13) Electronic Packaging Technology Vol. 19. No.2 (2003) (14) N. Peixoto, F. javier Ramirez-Fernandez: Wet anisotropic etching of polyimide, Proceedings of 8th International Meeting on hemical Sensors, IMS 2000, Basel-Suica, Julho 2000, p. 280 (Julho 2000) (15) Marta M.D. Ramos: Theoretical study of metal-polyimide interfacial properties, Vacuum 64, pp (2002) Han Ji-song (Non-member) received his B. Eng. and M. Eng. from Jilin University of Technology, hina, in 1984 and 1987 respectively. Since 1987, he had been working in Automobile Institute of Technology hang-chun, hina. He is now a researcher in Dept. of Micro System Engineering, Nagoya University, Japan. Tan zhi-yong (Non-member) received the B.S. degree in mechanical engineering in 1998 from Shenyang Institute of Aeronautical Engineering, hina and the M.S. degree in micro system engineering in 2003 from Nagoya University, Japan. Presently he is a doctoral course student in Dept. of Micro System Engineering, Nagoya University, Japan. References ( 1 ) D. Kim and Y.R. Shen: Study of wet treatment of polyimide by sumfrequency vibrational spectroscopy, Appl. Phys. Lett., Vol. 74, pp (1999) ( 2 ) M. -H Li, J. Wu, and YB Gianchandani: High performance scanning

10 Kazuo Sato (Member) received his B.S. degree (mechanical engineering) from Yokohama National University in 1970 and a Ph.D. from the University of Tokyo in In 1970, he joined Hitachi Ltd., Tokyo. He has been engaged in researching micromachining technologies and their applications since Since 1994, he has been a professor at Nagoya University. Dr. Sato is a member of the Japan Society of Mechanical Engineers, the Japan Society for Precision Engineering, the Institute of Electrical Engineers of Japan, and the Japan Society for Technology of Plasticity. Mitsuhiro Shikida (Member) received his B.S. and M.S. degrees in electrical engineering from Seikei University, Tokyo, in 1988 and 1990, respectively. He received a Ph.D. from Nagoya University in From 1990 to 1995, he worked at Hitachi, Ltd., Tokyo. In 1995, he joined the Department of Micro-System Engineering at Nagoya University as a research associate. He has been an assistant professor since 1998, and joined the Research enter for Advanced Waste and Emission Management at Nagoya University in His research interests include microactuators, microfabrication, and micromechanical structures. Dr. Shikida is a member of the Institute of Electrical Engineers of Japan and the Japan Society of Mechanical Engineers