- Email: [email protected]
Self-alignment of microchips using surface tension and solid edge
Sensors and Actuators A 139 (2007) 343–349
Self-alignment of microchips using surface tension and solid edge C. Gary Tsai a,∗ , C. Max Hsieh b , J. Andrew Yeh a,b a
Institute of Microelectromechanical System, National Tsing Hua University, Hsinchu, Taiwan, ROC b Institute of Electronics Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC Received 1 August 2006; received in revised form 8 April 2007; accepted 8 April 2007 Available online 19 April 2007
Abstract The purpose of this research is to develop a self-alignment mechanism using surface tension and solid edge for tiny microchips package. The solid edge of a rectangular protrusion replaced the surface treatment, which is usually used in the self-alignment technology. The experiments result showed that the commercial Radio Frequency Identification (RFID) microchip could be accurately auto-aligned on the protrusion. The edge effect, which confined the droplet spreading, is stronger than the boundary of hydrophilic and hydrophobic surface generated by the surface treatment. The standard deviation of aligned position was smaller than 16 m when the surface area of water droplet on protrusion was near to the bottom area of RFID microchip. The self-alignment completed in a tenth of a second. There was no visible position shift on aligned RFID microchip after suffering the simulated motion of transportation. The self-alignment mechanism in this paper can begin a new era of an innovative microchip package process without extra requirement on the microchips. © 2007 Elsevier B.V. All rights reserved. Keywords: Self-alignment; Self-assembly; Surface tension; Solid edge; RFID
1. Introduction RFID system is a powerful technology and has been changing our daily life in many different ways recently. It can replace the barcode system to improve good management or be noncontact tickets and electric-money [1]. Light-emitting diode (LED) is the next generation of light source because of more energy-efficient, controllable light color and small size. Those two kinds of microchips have some similar characteristics: small die size, less connectors, massive amount and higher cost pressure. Therefore, traditional package technologies developed for IC microchips is not suitable for those tiny microchips. If the microchip is too small, conventional “Pick-and-Place” serial package technology will face a serious problem of sticking [2]. When microchips are smaller than a millimeter, the strong adhesive force between the microchip and the manipulator surface is emerging. Electrostatics, Van der Waals forces and surface tension between the microchip and the manipulator will dominate the behavior of microchip in small scale. Traditional vacuum manipulator is not an appropriate tool to pick and place those
∗
Corresponding author. Tel.: +886 3 5715131; fax: +886 3 5745454. E-mail address: [email protected] (C.G. Tsai).
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.04.019
tiny microchips without extra surface treatment or assistant tools because the microchip may adhere to the manipulator. In addition, a complex position control system and a serial process of putting microchips on a specific location of the substrate are required to achieve the precise position control during microchip package process. The package rate is limited by the performance of robotic manipulator system. The package rate and cost are two important issues for microchips production, which usually require a massive amount at low cost. Self-alignment is one of the methods to increase the package rate at low cost. It can eliminate the time-consuming alignment process on Pick-and-Place technology. Various aligning forces used in self-alignment have been proposed by different groups, including electrostatic attraction force [3,4], magnetic attraction force [5], shape-matching [6] and surface tension [7–12]. Those technologies require some extra treatments on microchips to realize self-alignment. In using surface tension as the aligning force, most groups patterned gold pads and coated a layer of selfassembled monolayers (SAMs) on both microchip and substrate surfaces to generate a hydrophobic area on hydrophilic surface in water [7–9] or a hydrophilic area on hydrophobic surface in air [10–12]. Besides of surface property treatment, the microchips used in Alien’s Fluidic Self-Assembly (FSA) process have to etch a specific shape on the bottom of a microchip [1]. Although
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Fig. 1. The process of the self-alignment using surface tension and solid edge: (a) dripping a droplet on the protrusion; (b) putting microchip on the droplet; (c) accomplishing self-alignment.
those self-alignment technologies can shorten the package time, extra treatments on microchips increase its fabrication cost and reduce its yield rate. In this paper, we demonstrated a method to align microchips on the substrate automatically by surface tension and solid edge. The microchips used in our system do not need any modification on its original shape or surface property. This technology is a starting point to develop a parallel or array pick-and-place package technology for those microchips with tiny dimension and less connectors. 2. Self-alignment mechanism A droplet on the protrusion is worked as an apparatus to align microchip on the substrate. The protrusion replaces the surface property treatment and had the same function in the former self-alignment technologies to confine the droplet inside a specific area. The geometric structure of the protrusion depends on the profile of microchip’s bottom for the requirements of selfalignment. IC microchips usually are rectangular die structure so the shape of the protrusion is rectangular and its upper surface is higher than the substrate. Water was chosen as the liquid of the droplet because of high surface tension value in our experiment. But other liquid also can be used for different purposes. The glue can be taken for adhesion purpose and the dielectric liquid can be used for avoiding electrical short circuit. The self-alignment process is illustrated in Fig. 1. In the beginning, a droplet is dripped on the protrusion. After that, a microchip is put on the droplet. Surface tension as aligning force starts to act and then the microchip is dragged to the desired position automatically by the droplet on the protrusion. Finally, the self-alignment is accomplished by surface tension and solid edge. 2.1. Simple model of self-alignment mechanism A simplified static surface tension model is proposed to explain the self-alignment mechanism, drawn in Fig. 2. Some assumptions are made to simplify the static model [11,12]. The droplet on the protrusion is simplified based on the following assumptions: (1) the radius of curvature of the droplet is infinite
Fig. 2. Schematic of a simple static surface tension model.
so the droplet shape become a parallelogram, (2) the thickness of the droplet is uniform, W, (3) the volume of the droplet is fixed, (4) the contact surface between the droplet and the protrusion and microchip is also invariable, and (5) the length of two sides of the droplet is the only variable parameter. Free surface energy is only considered and other energies are neglected in this static surface tension model. The free surface energy is calculated by Eq. (1). Free surface energy = γdA (1) where dA is the change in area and γ is the surface tension value. Because the contact surface between the droplet and the protrusion and the microchip is constant, the surface energy on those surfaces is constant and no influence for the model. Therefore, the value of surface energy only is determined by the change of the two sides. Surface energy is expressed by 2γWS whereS is the side length. S can be calculated by the equation of (H 2 + X2 ) where X is the misaligning distance between the microchip and the protrusion; H is the height of the water droplet on the protrusion. H also can represent the droplet volume. The relationship between free surface energy and misaligning distance are drawn in Fig. 3. The curve has a localized minimum surface energy value, indicating energy well. Based on the principle of minimum free surface energy, the droplet will change its shape to reach the state of the minimum system energy. Therefore, the microchip will be dragged as droplet shape change and reaches the desired position, zero misaligning distance. This model shows that the auto-alignment happens because a droplet trend to adjust itself to the steady position with minimum system energy.
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Fig. 3. The relation between misaligning distance and free surface energy.
The surface tension force has a strong relation with the droplet volume and the misaligning distance from the observation of Fig. 3. Surface tension force is the aligning force in the static model and its value can be calculated from the differential of free surface energy equation, the slope of surface energy curve. The surface tension force will lower to vanish as the misaligning distance approached to zero, see the blue solid line in Fig. 3. Weak surface tension force would reduce the accuracy of selfalignment and increase aligning time. When H is assumed to be zero, indicated the droplet volume is very small, the surface tension force is highest and constant during alignment process, as indicated on the red dash line. When increasing the height of droplet, the surface tension force would lower and the energy well would diminish, as indicated on the green dot line. Based on the static surface tension model, the droplet with small volume has a stronger aligning force and shaper energy well. The measurement result confirmed this observation of model. 2.2. Edge effect model The ability of confining droplet inside the specific area is an important parameter for the accuracy of self-alignment because microchip would be dragged away from the alignment position by unexpected motion of the droplet. In static model, the confined force is regarded as a perfect force so the contact surface between droplet and protrusion is invariable. Sometime, the droplet may spread over the confined area when a microchip was put on top of it because of the impact momentum and gravitation. One of the criterions to estimate the confined force is the maximum contact angle on the edge of specific area. The higher contact angle on the edge means that the edge has a stronger ability to stop the droplet spread over. The solid edge is used to confine the droplet on the protrusion in our self-alignment design. The contact angle on solid edge can be expressed by the Gibbs’ inequality equation: θ0 ≤ θ ≤ (180◦ − ϕ) + θ0
(2)
where θ is the contact angle of the droplet on solid edge, θ 0 the original contact angle on surface and ϕ is the geometrical angle of solid edge. Furthermore, the contact angle on the edge can be expressed by a function of liquid and solid surface properties, structure shape, droplet volume, etc. The equation and further
Fig. 4. Picture of a water droplet on the rectangular protrusion near the critical condition.
research result can be found in the Oliver’s paper [13]. In our system, the geometrical angle of the protrusion is 90◦ and the receding contact angle of water on PMMA plate is about 54–64◦ [14] so the critical contact angle should be about 144–154◦ . Our measurement result had a similar value. Fig. 4 showed a water droplet on a rectangular protrusion when it was close to the critical point. The maximum contact angle approached 140◦ . The contact angle of a water droplet on the edge of a SAMs area is usually about 110◦ [8–11]. This result proved that solid edge had a better capability to confine droplet on a specific area on substrate. There are many advantages to apply solid edge to replace the chemical surface property treatment. There are no extra requirements for microchips. Originally each microchip has a solid edge in periphery. Physical protruded structure on the substrate is more reliable, endurable and reusable than the chemical surface treatment. The protruded structures can easily fabricated by traditional mechanical manufacture technologies, trenching, taping a film, casting the structure and others. The restricting force that confirms liquid on the protrusion is stronger than the boundary of different surface properties. 3. Experiment system The measurement system was composed of a CCD camera with a Zoom XL70 optical image magnification, illustrated in Fig. 5. A protrusion on the PMMA plate was fabricated in the same rectangular shape of a commercial RFID microchip bottom (1 mm × 1 mm). The process of self-alignment guiding by surface tension and solid edge was recorded by the CCD camera. A water droplet was dripped on the protrusion by syringe (see Fig. 6(a)). Then, a commercial RFID microchip was put onto the protrusion and brought into contact with the water droplet by a tweezers. Finally, the position of microchip was aligned automatically due to the surface tension and solid edge (see Fig. 6(b)). The pictures in Fig. 6 were acquired from a recording video clip. The corner positions of the microchip on the pictures were measured using Adobe’s Photoshop software. The calculated pixel size was about 6 m and the estimated measurement error was about ±12 m.
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Fig. 5. Schematic of experiment system.
4. Results and discussion In this section, first, the self-alignment results with different water droplet volume were conducted and their alignment positions were measured. Next, the initial contact area, aligning time and the ability of sustaining the shock and vibration were discussed. 4.1. Effect of the water droplet volume The performance of self-alignment was affected by the volume of water droplet on the protrusion. The ratio of the surface area of water droplet to the upper area of protrusion, SR, was defined as the index of droplet volume before discussion of the experiment result. Fig. 7 shows pictures of the alignment result in three different water droplet volumes. When the surface area of droplet was much greater than the area of microchip, SR = 5.2 (see Fig. 7(c)), the microchip would float on the surface and no self-alignment would happen when it was placed on the water droplet. The water droplet still retained the spherical-like shape which has a minimum free surface energy.
Because the disturbance came from the RFID microchip was too small, compared with the overall surface of the water droplet and in resulted the state of minimum free surface energy was only changed slightly. In Fig. 7(b), the RFID microchip was aligned automatically when the surface ratio was greater than one, SR = 1.6. However, the microchip had a notable inclination angle due to asymmetric droplet surface on the side and therefore the aligning position error was high. Based on the principle of minimum free surface energy, there are many possible steady locations for microchips only if the sum of surface area achieved the requirement of minimum free surface energy. Fig. 7(a) shows the result of putting a microchip on a small volume of droplet. The accuracy of auto-alignment was highest when the surface area ratio is close to one, SR = 0.9. Those results verified the observation of static surface tension model: the aligned force is large and the distribution curve of free surface energy near minimum valve is shape when the droplet volume is small. There were no overflow and friction force generated from the solid contact between microchip and substrate in our experiment. In Sato’s experiment, they have mentioned, droplets overflow the
Fig. 6. Pictures were intercepted from the record video clip: (a) dripping a droplet on the protrusion; (b) accomplishing self-alignment.
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Fig. 7. Pictures of the commercial RFID tag microchip when being put on the droplet in different conditions of volume: (a) SR is 0.9. (b) SR is 1.6. (c) SR is 5.2.
confined area and solid contact friction between microchip and substrate were the two main influences on the self-alignment accuracy in their experiment [11]. However, we have cleverly employed solid edge on protrusion as the confined force, the solid edge could efficiently stop the droplet spreading over the edge and avoid overflow. The microchip was elevated by the protrusion could avoid the solid contact between microchip and substrate. Therefore, a droplet on the protrusion can avoid these two phenomena reported by Sato. The self-alignment experiment was done more than twenty times using the same RFID microchip. The measurement data lists on Table 1 sorted by different volume of water droplet. The condition of SR 1 was eliminated due to ineffectiveness of self-alignment. The standard deviation (Std) of the corner positions was an important parameter to value the accuracy of self-alignment. The standard deviation meant that position difference of aligning microchip in every time. The small volume droplet on the protrusion had the higher accuracy of self-alignment. This result verified the prediction of the simple static surface tension model and the observation of acquired pictures. The standard deviations for the case of 0.8 ≤ SR < 1.3 had the best alignment performance, where it is smaller than 16 m. Most values already approached the estimated measured error, indicating that the commercial RFID microchips were aligned automatically and precisely at the same position each time by surface tension and solid edge. The standard deviations in the condition of 0.4 ≤ SR < 0.8 had increased
slightly because the droplet may not fill up the overall gap between the microchip and the protrusion when the droplet volume was too small and therefore generate solid contact friction between microchip and protrusion surface. The normalized alignment accuracy was about 2% compared with the dimension of the RFID microchip, one mm square. The auto-aligning result in the conditions of 0.4 ≤ SR < 0.8 and 0.8 ≤ SR < 1.3 were good enough for the alignment requirement of this commercial RFID microchip because the diameter of solder ball is about 80 m. When the SR was little larger than one, the auto-aligning result may cause misalignment between solder and pad due to the large aligning error. The volume of droplet on the protrusion must be controlled roughly for the yield ratio of package process. Fortunately, a droplet on the protrusion did not need to control the water volume precisely for getting precise alignment result. For getting acceptable self-alignment result, only one rule had to be achieved which was the surface of droplet on the protrusion had to close to the area of microchips. 4.2. Effect of transportation In our design, the PMMA plate is a transportation apparatus. The solder balls on the aligned RFID microchips will contact with the golden pads on the RFID tag flexible film. Then, reflow process will be applied to bond the microchip onto the film via the solder. To test the position shift after the aligned microchips
Table 1 The measurement data in different conditions of droplet volume Occupied area Sample number
0.4 SR < 0.8 6
0.8 SR < 1.3 7
1.3 SR < 2 8
Format (unit) P1 (X,Y) P2 (X,Y)
(348 ± 18, 267 ± 18)a (381 ± 19, 1263 ± 9)
(367 ± 13, 250 ± 16) (399 ± 13, 1256 ± 13)
(379 ± 22, 240 ± 60) (406 ± 21, 1116 ± 62)
a
Values are in (average ± Std, average ± Std) (m).
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Fig. 8. The pictures of contact area was smaller than the half area of RFID microchip: (a) putting microchip onto the droplet; (b) accomplishing self-alignment.
suffer transportation, the PMMA plate was put on a shock equipment to simulate the transporting motion. The PMMA plate was accelerated to 2 m/s in a half second and then decelerated to the rest state in 0.1 s. This motion repeated continuously for 5 s. This simulated experiment was repeated several times and each time the corner positions are listed on Table 2. Those measured data shows that the microchip aligned on the protrusion was held strongly during the simulated harsh transportation condition. In the experiment the standard derivation of position shift was smaller than the pixel resolution which meant that the microchip was stated in the same position. This result gave us confidence to develop following package processes. A protrusion array can be installed on the plate and transferred microchips to the substrate in the way of array type. 4.3. Effect of contact area and alignment time The initial contact area is another important parameter for self-alignment. If self-alignment technology required that the contact area have to be large enough, the position of putting the microchip on the PMMA plate must be precisely controlled. That means the precision requirement of the position control system cannot be eased and the self-alignment we proposed become meaninglessness. From the experiment observation, the water droplet on the protrusion as auto-alignment device had a large tolerance of initial contact area between the microchip and the water droplet. Fig. 8 shows that the self-alignment still happened even when the initial contact area was small. Because the protruTable 2 The position shift of an aligned microchip after transportation simulation Simulated transportation times
Origin Once
Twice Third
Fourth Std
Sample Position 1
X1 Y1 X2 Y2
336 312 412 1288
348 302 410 1278
350 314 412 1288
336 306 404 1286
2
X1 Y1 X2 Y2
358 292 344 1258
364 298 350 1262
366 286 352 1258
358 288 354 1262
Unit: m.
8 6 4 5 364 288 354 1264
4 5 4 3
sion was higher than the plate, the microchip would not touch the plate surface even half of the microchip hanging outside. The microchip would not touch the plate and generated adhesive force between both surfaces. When applying a droplet on the protrusion as self-alignment device, the precise motion and position feedback control can be removed from the Pick-andPlace machine. The parallel or array Pick-and-Place machine may achieve in a simple way for packaging microchips with tiny size and less connector. The package through-rate would increase dramatically and save package cost. The alignment time by surface tension and solid edge was very fast. For the water droplet with small volume, the autoalignment would be accomplished in a tenth of second. When the volume was large than one, the microchip would take longer time to adjust the horizontal position because of weak surface tension force. Again the droplet with small volume had better result for the auto-alignment time. 5. Conclusion A droplet on protrusion as a self-alignment device had been researched in this paper. Solid Edge replaced the traditional chemical surface treatment used in the self-assembly technology to provide the boundary of restricting the droplet spread and alignment position. The protrusion can be easily fabricated by trenching, taping a film, casting the structure and others manufacture technologies. The commercial RFID microchip can be aligned and transported without extra requirements. The experiment result showed that the microchip can align to the edge by surface tension and the self-alignment accuracy researched the micrometer. The micrometer accuracy meets the package requirement. Because, under the advantage of the protrusion, it could avoid overflow and solid friction, which were the two reasons of influencing the alignment accuracy reported by other researchers. The self-alignment guided by surface tension and solid edge can be easily achieved without precise volume control system for dripping water, and complex robotic manipulator and feedback control for adjusting the microchip position. Therefore, a novel package technology with high throughput rate in short production time and low package cost has been developed for tiny microchips with less connector based on a simple self-alignment mechanism.
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Acknowledgments This research was financially supported by the center of Aerospace and Systems Technology, Industrial Technology Research Institute, Taiwan (Grant Number: C34C033120) and National Science Council, Taiwan. References [1] http://www.alientechnology.com/index.php. [2] R.S. Fearing, Survey of sticking effects for micro-parts, in: IEEE/RSJ International Conference on Robotics and Intelligent Systems (IROS), Pittsburg, PA, 1995, pp. 212–217. [3] M.B. Cohn, Assembly techniques for microelectromechanical systems, Ph.D. dissertation, University of California at Berkeley (1997). [4] K.-F. B¨ohringer, K. Goldberg, M. Cohn, R.T. Howe, A.P. Pisano, Parallel microassembly with electrostatic force fields, in: International Conference on Robotics & Automation, Leuven, Belgium, 1998, pp. 1204–1211. [5] B. Vikramaditya, B.J. Nelson, G.E. Yang, E.T. Enikov, Microassembly of hybrid magnetic MEMS, J. Micromech. 1 (2) (2001) 99–116. [6] H.-J.J. Yeh, J.S. Smith, Fluidic self-assembly for the integration of GaAs light-emitting diodes on Si substrates, IEEE Photonics Technol. Lett. 6 (6) (1994) 706–708. [7] G.M. Whitesides, B. Grzybowski, Self-assembly at all scales, Science 295 (5564) (2002) 2418–2421. [8] E.U. Srinivasan, D. Liepmann, R.T. Howe, Microstructure to substrate selfassembly using capillary forces, J. MEMS 10 (1) (2001) 17–24. [9] X. Xiong, Y. Hanein, et al., Controlled multibatch self-assembly of microdevices, J. MEMS 12 (2) (2003) 117–127. [10] J. Fang, S.-H. Liang, et al., Self-assembly of flat micro components by capillary forces and shape recognition, in: Conference on Foundations of Nanoscience: Selfassembled Architectures and Devices (FNANO), Snowbird, UT, 2005. [11] K. Sato, K. Ito, et al., Self-alignment of microparts using liquid surface tension—behavior of micropart and alignment characteristics, Precis. Eng. 27 (2003) 42–50. [12] J.-M. Kin, K. Yasuda, K. Fujimoto, Resin self-alignment processes for self-assembly systems, J. Electron. Packag. 127 (2005) 18–24. [13] J.F. Oliver, C. Huh, S.G. Mason, Resistance to spreading of liquids by sharp edges, J. Colloid Interface Sci. 59 (3) (1977) 568–581. [14] H.Y. Erbil, G. McHale, S.M. Rowan, M.I. Newton, Determination of the receding contact angle of sessile drops on polymer surface by evaporation, Langmuir 15 (1999) 7378–7385.
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Biographies C. Gary Tsai was born in Taiwan. He studied power mechanical engineering at National Tsing Hua University, Hsinchu, Taiwan, where he received the BS degree in 1997 and the MS degree in 1999. After being a thermal engineer several years, he decided back to school and currently is pursuing the PhD degree in the Institute of Microelectromechanical System at National Tsing Hua University. Currently, he is engaged in the development of actuator used in the tunable liquid lens. His research interests include the RF switch, bistable phenomenon and surface energy. C. Max Hsieh received the BS degree in power mechanical engineering from the National Tsing Hua University (NTHU), Hsinchu, Taiwan, ROC, in 2005, and is currently working toward the MS degree in electronics engineering at the NTHU. One of his current research interests is to study properties of nanostructures such as optical, dielectric and heat transfer characteristics and he tries to use these particular characteristics to realize novel electronic devices. Another of his current research interests is in the aspect of optical micro-electro-mechanical system (MEMS) design. In July and August of 2006, he is a research assistant of Taiwan semiconductor manufacturing company (TSMC) Ltd. and devotes himself to process analysis and development of digital light processing (DLP) MEMS devices. J. Andrew Yeh is an associate professor at the Institute of Micro-Electro-Mechanical Systems at National Tsing Hua University in Taiwan where his interests are optical microsystems, nanophotonics and sensors. He is currently a member of the steering committee in the IEEE/LEOS Optical MEMS conference. In early 2000, he co-founded an optical MEMS company, AIP Networks, Inc. In 1999, he was a post-doctoral associate at Cornell University, NY, USA. He received a BS degree in mechanical engineering from National Taiwan University, Taiwan in 1992, and Master degrees in mechanical engineering and in electrical engineering from Cornell University in 1996 and 1997, respectively. He received a PhD degree in electrical engineering from Cornell University in 1999.
Self-alignment of microchips using surface tension and solid edge C. Gary Tsai a,∗ , C. Max Hsieh b , J. Andrew Yeh a,b a
Institute of Microelectromechanical System, National Tsing Hua University, Hsinchu, Taiwan, ROC b Institute of Electronics Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC Received 1 August 2006; received in revised form 8 April 2007; accepted 8 April 2007 Available online 19 April 2007
Abstract The purpose of this research is to develop a self-alignment mechanism using surface tension and solid edge for tiny microchips package. The solid edge of a rectangular protrusion replaced the surface treatment, which is usually used in the self-alignment technology. The experiments result showed that the commercial Radio Frequency Identification (RFID) microchip could be accurately auto-aligned on the protrusion. The edge effect, which confined the droplet spreading, is stronger than the boundary of hydrophilic and hydrophobic surface generated by the surface treatment. The standard deviation of aligned position was smaller than 16 m when the surface area of water droplet on protrusion was near to the bottom area of RFID microchip. The self-alignment completed in a tenth of a second. There was no visible position shift on aligned RFID microchip after suffering the simulated motion of transportation. The self-alignment mechanism in this paper can begin a new era of an innovative microchip package process without extra requirement on the microchips. © 2007 Elsevier B.V. All rights reserved. Keywords: Self-alignment; Self-assembly; Surface tension; Solid edge; RFID
1. Introduction RFID system is a powerful technology and has been changing our daily life in many different ways recently. It can replace the barcode system to improve good management or be noncontact tickets and electric-money [1]. Light-emitting diode (LED) is the next generation of light source because of more energy-efficient, controllable light color and small size. Those two kinds of microchips have some similar characteristics: small die size, less connectors, massive amount and higher cost pressure. Therefore, traditional package technologies developed for IC microchips is not suitable for those tiny microchips. If the microchip is too small, conventional “Pick-and-Place” serial package technology will face a serious problem of sticking [2]. When microchips are smaller than a millimeter, the strong adhesive force between the microchip and the manipulator surface is emerging. Electrostatics, Van der Waals forces and surface tension between the microchip and the manipulator will dominate the behavior of microchip in small scale. Traditional vacuum manipulator is not an appropriate tool to pick and place those
∗
Corresponding author. Tel.: +886 3 5715131; fax: +886 3 5745454. E-mail address: [email protected] (C.G. Tsai).
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.04.019
tiny microchips without extra surface treatment or assistant tools because the microchip may adhere to the manipulator. In addition, a complex position control system and a serial process of putting microchips on a specific location of the substrate are required to achieve the precise position control during microchip package process. The package rate is limited by the performance of robotic manipulator system. The package rate and cost are two important issues for microchips production, which usually require a massive amount at low cost. Self-alignment is one of the methods to increase the package rate at low cost. It can eliminate the time-consuming alignment process on Pick-and-Place technology. Various aligning forces used in self-alignment have been proposed by different groups, including electrostatic attraction force [3,4], magnetic attraction force [5], shape-matching [6] and surface tension [7–12]. Those technologies require some extra treatments on microchips to realize self-alignment. In using surface tension as the aligning force, most groups patterned gold pads and coated a layer of selfassembled monolayers (SAMs) on both microchip and substrate surfaces to generate a hydrophobic area on hydrophilic surface in water [7–9] or a hydrophilic area on hydrophobic surface in air [10–12]. Besides of surface property treatment, the microchips used in Alien’s Fluidic Self-Assembly (FSA) process have to etch a specific shape on the bottom of a microchip [1]. Although
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C.G. Tsai et al. / Sensors and Actuators A 139 (2007) 343–349
Fig. 1. The process of the self-alignment using surface tension and solid edge: (a) dripping a droplet on the protrusion; (b) putting microchip on the droplet; (c) accomplishing self-alignment.
those self-alignment technologies can shorten the package time, extra treatments on microchips increase its fabrication cost and reduce its yield rate. In this paper, we demonstrated a method to align microchips on the substrate automatically by surface tension and solid edge. The microchips used in our system do not need any modification on its original shape or surface property. This technology is a starting point to develop a parallel or array pick-and-place package technology for those microchips with tiny dimension and less connectors. 2. Self-alignment mechanism A droplet on the protrusion is worked as an apparatus to align microchip on the substrate. The protrusion replaces the surface property treatment and had the same function in the former self-alignment technologies to confine the droplet inside a specific area. The geometric structure of the protrusion depends on the profile of microchip’s bottom for the requirements of selfalignment. IC microchips usually are rectangular die structure so the shape of the protrusion is rectangular and its upper surface is higher than the substrate. Water was chosen as the liquid of the droplet because of high surface tension value in our experiment. But other liquid also can be used for different purposes. The glue can be taken for adhesion purpose and the dielectric liquid can be used for avoiding electrical short circuit. The self-alignment process is illustrated in Fig. 1. In the beginning, a droplet is dripped on the protrusion. After that, a microchip is put on the droplet. Surface tension as aligning force starts to act and then the microchip is dragged to the desired position automatically by the droplet on the protrusion. Finally, the self-alignment is accomplished by surface tension and solid edge. 2.1. Simple model of self-alignment mechanism A simplified static surface tension model is proposed to explain the self-alignment mechanism, drawn in Fig. 2. Some assumptions are made to simplify the static model [11,12]. The droplet on the protrusion is simplified based on the following assumptions: (1) the radius of curvature of the droplet is infinite
Fig. 2. Schematic of a simple static surface tension model.
so the droplet shape become a parallelogram, (2) the thickness of the droplet is uniform, W, (3) the volume of the droplet is fixed, (4) the contact surface between the droplet and the protrusion and microchip is also invariable, and (5) the length of two sides of the droplet is the only variable parameter. Free surface energy is only considered and other energies are neglected in this static surface tension model. The free surface energy is calculated by Eq. (1). Free surface energy = γdA (1) where dA is the change in area and γ is the surface tension value. Because the contact surface between the droplet and the protrusion and the microchip is constant, the surface energy on those surfaces is constant and no influence for the model. Therefore, the value of surface energy only is determined by the change of the two sides. Surface energy is expressed by 2γWS whereS is the side length. S can be calculated by the equation of (H 2 + X2 ) where X is the misaligning distance between the microchip and the protrusion; H is the height of the water droplet on the protrusion. H also can represent the droplet volume. The relationship between free surface energy and misaligning distance are drawn in Fig. 3. The curve has a localized minimum surface energy value, indicating energy well. Based on the principle of minimum free surface energy, the droplet will change its shape to reach the state of the minimum system energy. Therefore, the microchip will be dragged as droplet shape change and reaches the desired position, zero misaligning distance. This model shows that the auto-alignment happens because a droplet trend to adjust itself to the steady position with minimum system energy.
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Fig. 3. The relation between misaligning distance and free surface energy.
The surface tension force has a strong relation with the droplet volume and the misaligning distance from the observation of Fig. 3. Surface tension force is the aligning force in the static model and its value can be calculated from the differential of free surface energy equation, the slope of surface energy curve. The surface tension force will lower to vanish as the misaligning distance approached to zero, see the blue solid line in Fig. 3. Weak surface tension force would reduce the accuracy of selfalignment and increase aligning time. When H is assumed to be zero, indicated the droplet volume is very small, the surface tension force is highest and constant during alignment process, as indicated on the red dash line. When increasing the height of droplet, the surface tension force would lower and the energy well would diminish, as indicated on the green dot line. Based on the static surface tension model, the droplet with small volume has a stronger aligning force and shaper energy well. The measurement result confirmed this observation of model. 2.2. Edge effect model The ability of confining droplet inside the specific area is an important parameter for the accuracy of self-alignment because microchip would be dragged away from the alignment position by unexpected motion of the droplet. In static model, the confined force is regarded as a perfect force so the contact surface between droplet and protrusion is invariable. Sometime, the droplet may spread over the confined area when a microchip was put on top of it because of the impact momentum and gravitation. One of the criterions to estimate the confined force is the maximum contact angle on the edge of specific area. The higher contact angle on the edge means that the edge has a stronger ability to stop the droplet spread over. The solid edge is used to confine the droplet on the protrusion in our self-alignment design. The contact angle on solid edge can be expressed by the Gibbs’ inequality equation: θ0 ≤ θ ≤ (180◦ − ϕ) + θ0
(2)
where θ is the contact angle of the droplet on solid edge, θ 0 the original contact angle on surface and ϕ is the geometrical angle of solid edge. Furthermore, the contact angle on the edge can be expressed by a function of liquid and solid surface properties, structure shape, droplet volume, etc. The equation and further
Fig. 4. Picture of a water droplet on the rectangular protrusion near the critical condition.
research result can be found in the Oliver’s paper [13]. In our system, the geometrical angle of the protrusion is 90◦ and the receding contact angle of water on PMMA plate is about 54–64◦ [14] so the critical contact angle should be about 144–154◦ . Our measurement result had a similar value. Fig. 4 showed a water droplet on a rectangular protrusion when it was close to the critical point. The maximum contact angle approached 140◦ . The contact angle of a water droplet on the edge of a SAMs area is usually about 110◦ [8–11]. This result proved that solid edge had a better capability to confine droplet on a specific area on substrate. There are many advantages to apply solid edge to replace the chemical surface property treatment. There are no extra requirements for microchips. Originally each microchip has a solid edge in periphery. Physical protruded structure on the substrate is more reliable, endurable and reusable than the chemical surface treatment. The protruded structures can easily fabricated by traditional mechanical manufacture technologies, trenching, taping a film, casting the structure and others. The restricting force that confirms liquid on the protrusion is stronger than the boundary of different surface properties. 3. Experiment system The measurement system was composed of a CCD camera with a Zoom XL70 optical image magnification, illustrated in Fig. 5. A protrusion on the PMMA plate was fabricated in the same rectangular shape of a commercial RFID microchip bottom (1 mm × 1 mm). The process of self-alignment guiding by surface tension and solid edge was recorded by the CCD camera. A water droplet was dripped on the protrusion by syringe (see Fig. 6(a)). Then, a commercial RFID microchip was put onto the protrusion and brought into contact with the water droplet by a tweezers. Finally, the position of microchip was aligned automatically due to the surface tension and solid edge (see Fig. 6(b)). The pictures in Fig. 6 were acquired from a recording video clip. The corner positions of the microchip on the pictures were measured using Adobe’s Photoshop software. The calculated pixel size was about 6 m and the estimated measurement error was about ±12 m.
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Fig. 5. Schematic of experiment system.
4. Results and discussion In this section, first, the self-alignment results with different water droplet volume were conducted and their alignment positions were measured. Next, the initial contact area, aligning time and the ability of sustaining the shock and vibration were discussed. 4.1. Effect of the water droplet volume The performance of self-alignment was affected by the volume of water droplet on the protrusion. The ratio of the surface area of water droplet to the upper area of protrusion, SR, was defined as the index of droplet volume before discussion of the experiment result. Fig. 7 shows pictures of the alignment result in three different water droplet volumes. When the surface area of droplet was much greater than the area of microchip, SR = 5.2 (see Fig. 7(c)), the microchip would float on the surface and no self-alignment would happen when it was placed on the water droplet. The water droplet still retained the spherical-like shape which has a minimum free surface energy.
Because the disturbance came from the RFID microchip was too small, compared with the overall surface of the water droplet and in resulted the state of minimum free surface energy was only changed slightly. In Fig. 7(b), the RFID microchip was aligned automatically when the surface ratio was greater than one, SR = 1.6. However, the microchip had a notable inclination angle due to asymmetric droplet surface on the side and therefore the aligning position error was high. Based on the principle of minimum free surface energy, there are many possible steady locations for microchips only if the sum of surface area achieved the requirement of minimum free surface energy. Fig. 7(a) shows the result of putting a microchip on a small volume of droplet. The accuracy of auto-alignment was highest when the surface area ratio is close to one, SR = 0.9. Those results verified the observation of static surface tension model: the aligned force is large and the distribution curve of free surface energy near minimum valve is shape when the droplet volume is small. There were no overflow and friction force generated from the solid contact between microchip and substrate in our experiment. In Sato’s experiment, they have mentioned, droplets overflow the
Fig. 6. Pictures were intercepted from the record video clip: (a) dripping a droplet on the protrusion; (b) accomplishing self-alignment.
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Fig. 7. Pictures of the commercial RFID tag microchip when being put on the droplet in different conditions of volume: (a) SR is 0.9. (b) SR is 1.6. (c) SR is 5.2.
confined area and solid contact friction between microchip and substrate were the two main influences on the self-alignment accuracy in their experiment [11]. However, we have cleverly employed solid edge on protrusion as the confined force, the solid edge could efficiently stop the droplet spreading over the edge and avoid overflow. The microchip was elevated by the protrusion could avoid the solid contact between microchip and substrate. Therefore, a droplet on the protrusion can avoid these two phenomena reported by Sato. The self-alignment experiment was done more than twenty times using the same RFID microchip. The measurement data lists on Table 1 sorted by different volume of water droplet. The condition of SR 1 was eliminated due to ineffectiveness of self-alignment. The standard deviation (Std) of the corner positions was an important parameter to value the accuracy of self-alignment. The standard deviation meant that position difference of aligning microchip in every time. The small volume droplet on the protrusion had the higher accuracy of self-alignment. This result verified the prediction of the simple static surface tension model and the observation of acquired pictures. The standard deviations for the case of 0.8 ≤ SR < 1.3 had the best alignment performance, where it is smaller than 16 m. Most values already approached the estimated measured error, indicating that the commercial RFID microchips were aligned automatically and precisely at the same position each time by surface tension and solid edge. The standard deviations in the condition of 0.4 ≤ SR < 0.8 had increased
slightly because the droplet may not fill up the overall gap between the microchip and the protrusion when the droplet volume was too small and therefore generate solid contact friction between microchip and protrusion surface. The normalized alignment accuracy was about 2% compared with the dimension of the RFID microchip, one mm square. The auto-aligning result in the conditions of 0.4 ≤ SR < 0.8 and 0.8 ≤ SR < 1.3 were good enough for the alignment requirement of this commercial RFID microchip because the diameter of solder ball is about 80 m. When the SR was little larger than one, the auto-aligning result may cause misalignment between solder and pad due to the large aligning error. The volume of droplet on the protrusion must be controlled roughly for the yield ratio of package process. Fortunately, a droplet on the protrusion did not need to control the water volume precisely for getting precise alignment result. For getting acceptable self-alignment result, only one rule had to be achieved which was the surface of droplet on the protrusion had to close to the area of microchips. 4.2. Effect of transportation In our design, the PMMA plate is a transportation apparatus. The solder balls on the aligned RFID microchips will contact with the golden pads on the RFID tag flexible film. Then, reflow process will be applied to bond the microchip onto the film via the solder. To test the position shift after the aligned microchips
Table 1 The measurement data in different conditions of droplet volume Occupied area Sample number
0.4 SR < 0.8 6
0.8 SR < 1.3 7
1.3 SR < 2 8
Format (unit) P1 (X,Y) P2 (X,Y)
(348 ± 18, 267 ± 18)a (381 ± 19, 1263 ± 9)
(367 ± 13, 250 ± 16) (399 ± 13, 1256 ± 13)
(379 ± 22, 240 ± 60) (406 ± 21, 1116 ± 62)
a
Values are in (average ± Std, average ± Std) (m).
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Fig. 8. The pictures of contact area was smaller than the half area of RFID microchip: (a) putting microchip onto the droplet; (b) accomplishing self-alignment.
suffer transportation, the PMMA plate was put on a shock equipment to simulate the transporting motion. The PMMA plate was accelerated to 2 m/s in a half second and then decelerated to the rest state in 0.1 s. This motion repeated continuously for 5 s. This simulated experiment was repeated several times and each time the corner positions are listed on Table 2. Those measured data shows that the microchip aligned on the protrusion was held strongly during the simulated harsh transportation condition. In the experiment the standard derivation of position shift was smaller than the pixel resolution which meant that the microchip was stated in the same position. This result gave us confidence to develop following package processes. A protrusion array can be installed on the plate and transferred microchips to the substrate in the way of array type. 4.3. Effect of contact area and alignment time The initial contact area is another important parameter for self-alignment. If self-alignment technology required that the contact area have to be large enough, the position of putting the microchip on the PMMA plate must be precisely controlled. That means the precision requirement of the position control system cannot be eased and the self-alignment we proposed become meaninglessness. From the experiment observation, the water droplet on the protrusion as auto-alignment device had a large tolerance of initial contact area between the microchip and the water droplet. Fig. 8 shows that the self-alignment still happened even when the initial contact area was small. Because the protruTable 2 The position shift of an aligned microchip after transportation simulation Simulated transportation times
Origin Once
Twice Third
Fourth Std
Sample Position 1
X1 Y1 X2 Y2
336 312 412 1288
348 302 410 1278
350 314 412 1288
336 306 404 1286
2
X1 Y1 X2 Y2
358 292 344 1258
364 298 350 1262
366 286 352 1258
358 288 354 1262
Unit: m.
8 6 4 5 364 288 354 1264
4 5 4 3
sion was higher than the plate, the microchip would not touch the plate surface even half of the microchip hanging outside. The microchip would not touch the plate and generated adhesive force between both surfaces. When applying a droplet on the protrusion as self-alignment device, the precise motion and position feedback control can be removed from the Pick-andPlace machine. The parallel or array Pick-and-Place machine may achieve in a simple way for packaging microchips with tiny size and less connector. The package through-rate would increase dramatically and save package cost. The alignment time by surface tension and solid edge was very fast. For the water droplet with small volume, the autoalignment would be accomplished in a tenth of second. When the volume was large than one, the microchip would take longer time to adjust the horizontal position because of weak surface tension force. Again the droplet with small volume had better result for the auto-alignment time. 5. Conclusion A droplet on protrusion as a self-alignment device had been researched in this paper. Solid Edge replaced the traditional chemical surface treatment used in the self-assembly technology to provide the boundary of restricting the droplet spread and alignment position. The protrusion can be easily fabricated by trenching, taping a film, casting the structure and others manufacture technologies. The commercial RFID microchip can be aligned and transported without extra requirements. The experiment result showed that the microchip can align to the edge by surface tension and the self-alignment accuracy researched the micrometer. The micrometer accuracy meets the package requirement. Because, under the advantage of the protrusion, it could avoid overflow and solid friction, which were the two reasons of influencing the alignment accuracy reported by other researchers. The self-alignment guided by surface tension and solid edge can be easily achieved without precise volume control system for dripping water, and complex robotic manipulator and feedback control for adjusting the microchip position. Therefore, a novel package technology with high throughput rate in short production time and low package cost has been developed for tiny microchips with less connector based on a simple self-alignment mechanism.
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Acknowledgments This research was financially supported by the center of Aerospace and Systems Technology, Industrial Technology Research Institute, Taiwan (Grant Number: C34C033120) and National Science Council, Taiwan. References [1] http://www.alientechnology.com/index.php. [2] R.S. Fearing, Survey of sticking effects for micro-parts, in: IEEE/RSJ International Conference on Robotics and Intelligent Systems (IROS), Pittsburg, PA, 1995, pp. 212–217. [3] M.B. Cohn, Assembly techniques for microelectromechanical systems, Ph.D. dissertation, University of California at Berkeley (1997). [4] K.-F. B¨ohringer, K. Goldberg, M. Cohn, R.T. Howe, A.P. Pisano, Parallel microassembly with electrostatic force fields, in: International Conference on Robotics & Automation, Leuven, Belgium, 1998, pp. 1204–1211. [5] B. Vikramaditya, B.J. Nelson, G.E. Yang, E.T. Enikov, Microassembly of hybrid magnetic MEMS, J. Micromech. 1 (2) (2001) 99–116. [6] H.-J.J. Yeh, J.S. Smith, Fluidic self-assembly for the integration of GaAs light-emitting diodes on Si substrates, IEEE Photonics Technol. Lett. 6 (6) (1994) 706–708. [7] G.M. Whitesides, B. Grzybowski, Self-assembly at all scales, Science 295 (5564) (2002) 2418–2421. [8] E.U. Srinivasan, D. Liepmann, R.T. Howe, Microstructure to substrate selfassembly using capillary forces, J. MEMS 10 (1) (2001) 17–24. [9] X. Xiong, Y. Hanein, et al., Controlled multibatch self-assembly of microdevices, J. MEMS 12 (2) (2003) 117–127. [10] J. Fang, S.-H. Liang, et al., Self-assembly of flat micro components by capillary forces and shape recognition, in: Conference on Foundations of Nanoscience: Selfassembled Architectures and Devices (FNANO), Snowbird, UT, 2005. [11] K. Sato, K. Ito, et al., Self-alignment of microparts using liquid surface tension—behavior of micropart and alignment characteristics, Precis. Eng. 27 (2003) 42–50. [12] J.-M. Kin, K. Yasuda, K. Fujimoto, Resin self-alignment processes for self-assembly systems, J. Electron. Packag. 127 (2005) 18–24. [13] J.F. Oliver, C. Huh, S.G. Mason, Resistance to spreading of liquids by sharp edges, J. Colloid Interface Sci. 59 (3) (1977) 568–581. [14] H.Y. Erbil, G. McHale, S.M. Rowan, M.I. Newton, Determination of the receding contact angle of sessile drops on polymer surface by evaporation, Langmuir 15 (1999) 7378–7385.
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Biographies C. Gary Tsai was born in Taiwan. He studied power mechanical engineering at National Tsing Hua University, Hsinchu, Taiwan, where he received the BS degree in 1997 and the MS degree in 1999. After being a thermal engineer several years, he decided back to school and currently is pursuing the PhD degree in the Institute of Microelectromechanical System at National Tsing Hua University. Currently, he is engaged in the development of actuator used in the tunable liquid lens. His research interests include the RF switch, bistable phenomenon and surface energy. C. Max Hsieh received the BS degree in power mechanical engineering from the National Tsing Hua University (NTHU), Hsinchu, Taiwan, ROC, in 2005, and is currently working toward the MS degree in electronics engineering at the NTHU. One of his current research interests is to study properties of nanostructures such as optical, dielectric and heat transfer characteristics and he tries to use these particular characteristics to realize novel electronic devices. Another of his current research interests is in the aspect of optical micro-electro-mechanical system (MEMS) design. In July and August of 2006, he is a research assistant of Taiwan semiconductor manufacturing company (TSMC) Ltd. and devotes himself to process analysis and development of digital light processing (DLP) MEMS devices. J. Andrew Yeh is an associate professor at the Institute of Micro-Electro-Mechanical Systems at National Tsing Hua University in Taiwan where his interests are optical microsystems, nanophotonics and sensors. He is currently a member of the steering committee in the IEEE/LEOS Optical MEMS conference. In early 2000, he co-founded an optical MEMS company, AIP Networks, Inc. In 1999, he was a post-doctoral associate at Cornell University, NY, USA. He received a BS degree in mechanical engineering from National Taiwan University, Taiwan in 1992, and Master degrees in mechanical engineering and in electrical engineering from Cornell University in 1996 and 1997, respectively. He received a PhD degree in electrical engineering from Cornell University in 1999.