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Integrating micromachined fast response temperature sensor array in a glass microchannel
Sensors and Actuators A 122 (2005) 189–195
Integrating micromachined fast response temperature sensor array in a glass microchannel Zhenlan Xue, Huihe Qiu ∗ Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China Received 3 December 2004; received in revised form 17 February 2005; accepted 27 April 2005 Available online 6 June 2005
Abstract Total glass microdevices have attracted wide interest in the development of micro total analysis systems (TAS), laser microsurgery, micro heat pipes and microfluidics researches because of the possibility for optical diagnostics and observations. The integration of a detection system, such as integrated sensors into a glass microchannel, however, was difficult due to technology constraints. A microfabrication process integrating a micromachined fast response temperature sensor array inside a glass microchannel is presented. Combining of newly developed two different glass materials anodic bonding, channel etching, fabrication of silicon temperature detectors, makes the fabrication of a glass microchannel with an integrated temperature detector array possible. The implantation process and the channel etching technology were ameliorated to adapt to the new material introduced. The dynamic response of the integrated temperature array has also been calibrated. The response time of the integrated temperature sensor array was examined to be 1.5 s in the glass microchannel. © 2005 Elsevier B.V. All rights reserved. Keywords: Sensor; Anodic bonding; Microelectromechanical system; MEMS technology
1. Introduction Research on rapid fluid dynamics and heat transfer in a microchannel is an emerging area in studying laser ophthalmic microsurgery, pico-liter optical glucose analysis, fabricating and repairing of micro-electronic-mechanical devices, laser deposition of thin liquid film to a specific location in microsystem, micro heat pipes and diffusion of a pollutant liquid, etc. Recent progress in micro- and nanotechnologies has yielded microfluidic devices that involve lots of microchannels in order to carry out assays using very small amounts of fluid. For further development of devices such as lab-on-a-chip and micro total analysis systems (TAS), it is important to understand the physics of transport processes in a microchannel multiphase flow. In the above-mentioned applications, transparent glass microdevices are more attractive because of the possibility for optical observation, treatment and diagnostics, such as the ∗
Corresponding author. Tel.: +852 23587190; fax: +852 23581543. E-mail address: [email protected] (H. Qiu).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.04.019
interfacial film thickness at the gas–liquid interface of a micro bubble/plug in a microchannel [1]. Furthermore, it is indispensable to monitor and optimize the flow conditions such as fluid/wall temperatures, pressure and velocity, etc., which requires the detection systems to be integrated within a glass microchannel. The integration of a detection system, such as integrating a silicon-based temperature sensor array for monitoring the boundary condition of the channel wall into a total glass microchannel (In this paper, total glass microchannel means both the cover and substrate are made of glass.), however, is different due to the lack of suitable microfabrication technology. Several methods were proposed for fabricating semitransparent microchannels which were fabricated on silicon wafer (non-transparent) and anodically bonded with glass cover (transparent) [2,3]. The silicon wafer substrate causes the system being not fully transparent which causes some limits in optical treatment and diagnostics. Although glassto-glass anodic bonding was also demonstrated [4–8], most of such bonding were conducted between Corning 7740, Shchott #8330, and Hoya SD-2, which are commonly used
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for glass-to-silicon anodic bonding. This kind of borosilicate glass contains sodium oxide, which is required for charge separation during the bonding process. Unfortunately, such glass used in anodic bonding does not meet the requirement as a substrate for integrating silicon sensors, because it only aims at forming a microchannel or chamber between the two glass layers, without taking sodium contamination into consideration. If this kind of glass with sodium oxide is used as the substrate, its sodium ion will diffuse and contaminate the sensor layers during the anodic bonding process [9], which may cause malfunction of the devices. It will also contaminate the fabrication facilities such as CVD furnace and consequently spread the ion to other users’ device. To the authors’ knowledge, no literature on silicon-based sensors fabricated on a glass substrate has been reported. Moreover, to bypass the difficulty of anodic bonding caused by the entire intermediate layer, temperature sensors were often fabricated on the backside of the silicon wafer [10]. However, to reach fast temperature response without extra temperature lag caused by the channel wall, it is preferable to integrate temperature sensors inside the microchannel directly. This paper presents a method that integrates a fast response temperature sensor array into a total glass microchannel utilizing a modified anodic bonding technology. In this method two different kinds of glass were used to form a sealed microchannel with micro temperature sensor array integrated in it. Besides, the whole fabrication process of the chip is presented in detail. Several specific problems brought by employment of Corning 1737 (1737 for short below) instead of silicon wafer are described and the solutions of them are introduced. Implantation scheme and annealing temperature and time were modified according to the thermal characteristics of 1737. The microchannel (on the cover) was formed in Corning 7740 (7740 for short below) by wet etching technology. Different sacrifice etching mask was attempted and compared. Finally, an ameliorated Cr + Au + PR (photoresist) masking scheme was adopted. Channels with depth of 60–100 m were obtained with little pinholes and lateral etch. After bonding and slicing, the performance of temperature sensors was calibrated.
2. Fabrication technology 2.1. Selection of channel materials Fabrication of micromachined fast response temperature sensors on a silicon substrate has been described in [11,12]. However, while using transparent substrate, it imposes more restrictions on the design and manufacturing. The most often used transparent materials in MEMS are quartz and commercially available glass material named Corning 7740 and 1737. Glass 7740 is widely used in the glass–silicon anodic bonding to form microchannels. However, to integrate silicon-based microsensors on 7740, the potential contamination problem must be addressed [10]. Especially, when the substrate is
Table 1 Comparison of thermal expansion coefficient between 1737 and 7740 7740 0–300 ◦ C 25 ◦ C–set point 515 ◦ C
32.5 × 10−7 /◦ C 35.0 × 10−7 /◦ C
1737 0–300 ◦ C 25 ◦ C–set point 671 ◦ C
37.6 × 10−7 /◦ C 42.0 × 10−7 /◦ C
also using 7740, the sodium ion migration by the electrical field during the bonding process may contaminate the sensors which are built closely at the glass–polysilicon interface. Such problem never exists in silicon-to-glass anodic bonding, because the substrate is silicon wafer and no sodium ion migration problem. Furthermore, due to the potential contamination to the devices and the fabrication system, in many microfabrication laboratories, 7740 is banned in many facilities such as CVD furnace and dry etching devices, which are needed to share with other users, such as CMOS users. Therefore, an ion-free material is preferable to be selected as the substrate. Glass 1737 is mostly used as substrates for flat panel displays. Because it contains very little metal ion (near zero), it will not cause contamination to the sensors, devices, and the fabrication systems that are shared with other users. Moreover, the thermal expansion coefficient of 1737 is at the same order of 7740 (Table 1), which facilitates successful anodic bonding under a relatively high temperature at around 250–450 ◦ C. While quartz’s thermal expansion coefficient is about 5.9 × 10−7 /◦ C thus at different order from that of 7740, which makes it difficult to bond quartz with 7740. As a result, glass 1737 was proposed as the substrate where the temperature sensor can be built. To form a total glass microchannel, a glass cover channel is needed. Because 1737 does not contain required sodium ion, glass–glass thermal fusion bonding will be an option to bond two 1737 together. However, because thermal fusion bonding needs high temperature (about 800 ◦ C) [13], the bonding temperature may cause problems for the processing steps previously performed (e.g. the resistor diffusion). Therefore, anodic bonding utilizing glass 7740 was selected as the channel cover material. Anodic bonding is the most suitable for this case, because it works under comparatively low temperature and would not cause damage to glass wafer and the devices. Furthermore, the demands on surface roughness and contamination are much lower than those for the fusion bonding processes. Bonding at low temperatures, with less critical demands for surface flatness, and the possibility of metalized electrical feed through will offer more process flexibility in the fabrication of sensors and actuators. Because the sensors are built on the substrate surface, no direct contact between the sensors and the top channel cover is formed. The contamination problem can, therefore, be eliminated. An intermediate polysilicon film was deposited that provides the bonding mechanism as shown in Fig. 1. The double-side deposited polysilicon film takes place of sensor machining as well as the anode of bonding process.
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Fig. 1. Glass-to-glass anodic bonding setup.
It makes the substrate 1737 practically “semi-conducting” and renders anodic bonding theoretically possible. 2.2. Low temperature annealing In the fabrication, some non-standard MEMS technologies were employed at the first time. One of them is the fabrication of integrated temperature sensor array on the glass (1737) substrate. To build the sensor on the glass (1737), polysilicon was deposited on the glass substrate first. The sensor array was then fabricated with polysilicon layer. However, there was problem associated with implantation and annealing process. Normally, an impurity process of a polysilicon can be either conducted by diffusion or implantation. According to the thermal properties of 1737, it cannot withstand high temperature that diffusion requires. Therefore, implantation technology was employed to dope impurity to polysilicon. Usually, the process conditions for phosphorous impurity of silicon wafers require implantation energy of 120 keV, 950 ◦ C annealing temperature and 60 min in a diffusion furnace. However, the 950 ◦ C annealing temperature is definitely too high for 1737 that claims a maximum processing temperature of 620 ◦ C. Therefore, the annealing temperature was set to 620 ◦ C for 1737, which is the same as polysilicon deposition temperature in chemical vapor deposition (CVD). Although this temperature is much lower than the standard annealing temperature of IC fabrication, it is still sufficient to activate most of the impurity [14]. One hold back of using low temperature annealing is that there is little possibility to redistribute the doped impurity because the diffusion (penetration) of the implanted phosphorous is a function of annealing temperature. To solve this problem, multiple doping processes with different implantation energies and concentrations were introduced. Fig. 2 is the simulation result of two equivalent implantation schemes with SUPREM: • one time dope: 1.5E15/cm2 at 120 keV • three times dope: 5E14/cm2 at 80, 110, 140 keV, respectively. The total doses of these two schemes are the same. From Fig. 2, the three times dope scheme gets more uniform distribution of the impurity even if other conditions remain
Fig. 2. Results of simulation with two dope schemes.
unchanged. After annealing, the sheet resistance of three times dope of test wafers was measured. For five wafers in one batch, the measured Rs are listed in Table 2 which demonstrates that the sheet resistance with three times dope under low temperature annealing scheme is close to that with once dope of silicon wafer. 2.3. Mask for 7740 etching The other technology developed was the etching of 7740. It is crucial to find a material that can work as the etching mask of 7740. Furthermore, the etching rate should be calibrated to get a certain etching depth. Many techniques have been used to structure 7740. Deep etching in borosilicate glass using a polysilicon film deposited by low pressure chemical vapor deposition (LPCVD) has been reported [15]. The LPCVD polysilicon mask can be used to etch through a 500 m thick pyrex glass wafer. The problem with this method is that during the high temperature LPCVD deposition process, the sodium ions diffuse out of the glass and contaminate the furnace. A similar method using plasma enhanced chemical vapor deposition (PECVD) silicon carbide as mask has also been reported [16]. Etch masks of Cr + Au + resist are often used [17–19]. But the mask cannot be used for very large etch depths since pinholes appear after a while. Moreover, a large lateral under-etch of chromium is present. Deep etching of borosilicate glass using anodically bonded silicon substrate as mask has also been reported [20]. Although this method allows etch depths up to 500 m with no surface damage to the silicon mask, the process is costly and time-consuming. Other methods like laser structuring, electro-chemical discharge drilling or sand blasting of straight wall in glass, but Table 2 Rs of polysilicon of different schemes Wafer 1 Rs of 1737 wafer (k) Rs of silicon wafer (k)
2
0.883 0.838 0.820 0.791
3
4
5
0.791 0.845
0.879 0.756
0.784 0.837
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Fig. 3. Comparison of different masks of glass etching: (a) polysilicon; (b) Cr + Au + PR.
they do not meet the desired standard batch process requirements and present problems of accuracy [21]. In this study, annealed Cr + Au + PR mask for glass etching has been proposed. For using the annealed Cr + Au + PR mask, because RTA process only takes several minutes and the metal etching is also a several minutes’ job, this scheme saves more time than the “anodically bonded wafer mask” scheme which is reported to obtaining good result in deep glass etching. Therefore, to etch a relatively deep 7740 channel (10–102 m), the conventional Cr + Au mask is the most feasible except the pinholes, which are believed to be caused by imperfection of metal sputtering, as well as the high cost of employing gold as a sacrifice layer. It has been known that chrome is also resistive to hydrofluoric acid (HF). Chrome evaporated lithography mask was tested in HF solution (HF:H2 O = 1:3) and it can stand 5–6 h with few pinholes. But since it was not easy to obtain such
˚ gold was good qualify of chrome film, a thin layer of 500 A ˚ sputtered on the top of 1500 A chrome. After that, a 5 min of 400 ◦ C rapid thermal annealing (RTA) was followed to repair the defect of the metal films. This annealing process was important for eliminating the pinholes’ damage. PR207 was employed as the mask of metal etching. Au was etched by wet etching in solution of KI:I2 :H2 O = 20 g:5 g:400 ml under room temperature, and the etching rate was about ˚ 1000 A/min. Cr was also etched in CEP-200 micro chrome etchant that is commonly used to etch chrome lithography ˚ mask, and etching rate is about 1000 A/min. After metal etching, PR was remained as an extra protection of glass etching. So it is actually an ameliorated Cr + Au + PR etching scheme. Only small quantities of gold were used, which makes this scheme more cost-effective, and the RTA process cured the imperfection of metal film to avoid serious pinhole problem. Fig. 3 shows etching results of polysilicon mask scheme and
Fig. 4. Process flow of a microchannel integrated with temperature sensors.
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Table 3 Etching rates of glass in different solution Etchant
Glass 7740 (m/min)
Glass 1737 (m/min)
HF (49%) Buffered HF (1:3) BOE (1:6)
6.6 0.28 0.034
9.8 1.53 0.1
ameliorated Cr + Au + PR scheme. In the latter picture, almost no pinholes can be found. The etchant solution used is buffered HF solution. Fast etching rate and little damage to etching mask are preferred. Different solutions and different concentrations were evaluated and the etching rate of glass 7740 and glass 1737 was measured. Table 3 indicates that etching rate of glass 1737 is always larger than glass 7740. And for 49% HF, the etching rate is far larger than buffered solutions. Therefore, 49% HF is suitable for deep etching while the other two can work better in shallow etching since it is easier to control the etching depth precisely.
Fig. 5. Microscopic picture of sensors array pattern.
2.4. System integration Based on the above-developed processes, the whole fabrication process is shown in Fig. 4. First, 1737 wafers were ˚ polysilicon with CVD furnace. cleaned and deposited 4000 A After that, a Varian CF3000 Ion Implanter is used to implant phosphorus into the wafer. An annealing was followed to activate the impurity and remove the damage. Then polysilicon was patterned by dry etching in AME8110. Heavy implantation was conducted to facilitate good contact between sensors ˚ gold was sputtered on 300 A ˚ Ti/W and and electrodes. 1500 A ˚ PECVD silidefined the electrodes pattern by lift off. 2000 A con oxide was deposited on the surface of wafer to protect the sensors and electrodes from shortcut. The oxide on contact pads was etched for wire bonding. Fig. 5 is a microscopic picture of the sensor array pattern. The microchannel was fabricated on 7740, following the above-mentioned specific etching method. After that, anodic bonding was performed with Karl Suss Bonder. After the fabricated glass wafer was placed in the chamber, it was pumped to a vacuum of
Fig. 6. 3D sketch of bonded pair.
10−4 mbar, purged with 980 mbar nitrogen and then heated to 375 ◦ C. The chamber was then brought back up to atmosphere pressure and cooled. Then the chamber was brought to 1500 mbar and 1000 V was applied for about 6–7 min until the current dropped down to 10% of the peak value. The bonded channel was then evaluated under microscope and the sodium diffused to the outside surface of the channel was cleaned. The bonding condition can be referred to Wei et al. ˚ [22] and Joseph’s suggestions [4]. Although about 1800 A
Fig. 7. Overview photo of a bonded sample.
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˚ silicon oxide intermetal electrodes topography and 2000 A mediate layer exist, the bonding still occurred. Voids in some areas also can be seen in small partition of the whole area, which is caused by particles or trapped air. So it is important that the wafer surface is clean enough and the vacuum of bonding chamber should be higher to get better bonding result. Figs. 6 and 7 show 3D sketch and the real photo of the bonded sample, respectively.
3. Evaluation of the integrated system 3.1. Verification of water tightness It is important to verify the water tightness of the microfluidic chip. A verification test was conducted. The inlet and outlet of the channel were connected to two mini pipes utilizing epoxy. The pipe of the inlet was connected to a pressurized water tank with a pressure gage installed between the control valve and the channel. Water was flowing into the microchannel while the valve was opened. The test was started at the pressure of 0.5 bar where the water flow in the microchannel was already observable. The flow passes through the microchannel and the readings of the pressure gage were monitored simultaneously. No leakage was found even the pressure reach to 1.2 bar with a reasonable flow rate. It seems that the anodic bonding with oxide and metal electrodes intermediate layers was mechanically successful. The water tightness at the electrode position may be resulted from the oxide layer which was used to insulate and protect the electrodes. 3.2. Evaluation of temperature sensors To evaluate the performance of the integrated temperature sensor array, the performance of sensors was tested with HP 4145B semiconductor parameter analyzer before packaging. Under room temperature, the tested sensor I–V curve is as Fig. 8. The R–T curve is shown in Fig. 9. A quite good linearity can be observed between the R and T relationship. Some
Fig. 9. R–T curve of sensor.
Fig. 10. Response time of temperature sensor.
small derivation in the R–T curve may possibly come from the measurement uncertainty. The response time of the sensor is also a crucial characteristic of temperature sensor, which was tested by a laser impulse method [1]. Fig. 10 shows the response of sensor induced by laser pulse heating. And the calculated response time of the whole instrument is about 1.5 s, which can be further improved if a fast (broadband) amplifier is utilized.
4. Conclusion
Fig. 8. I–V curve under room temperature.
A novel method integrating micromachined fast response temperature sensor array into a total glass microchannel was developed. This method utilizes a modified anodic bonding technique for bonding two different glass materials (Corning 1737 and 7740) with polysilicon intermediate layer. The implantation process was ameliorated to meet the temperature characteristics of 1737. The experiments indicate that RTA effectively helps Cr + Au + PR mask to work better in glass etching, and anodic bonding can still occur even if ˚ electrodes layer plus 2000 A ˚ silicon oxide are sand1800 A wiched between glass 7740 and polysilicon. This fabrication technology can also be used to integrate other types of siliconbased sensors in glass microdevices with full transparency.
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The performance of the integrated fast response temperature sensor array was evaluated and the response time of 1.5 s was achieved.
Acknowledgement This research is supported by the Hong Kong Government under Research Grants Council (RGC), grant no. (HKUST 6214/01E and 6230/02E).
References [1] X. Wang, H.-H. Qiu, Fringe probing of liquid film thickness of a plug bubble in a micropipe, Meas. Sci. Technol. 16 (2004) 1374–1380. [2] J. Lichtenberg, N.F. de Rooij, E. Verpoorte, A microchip electrophoresis system with integrated in-plane electrodes for contactless conductivity detection, Electrophoresis 23 (2002) 3769–3780. [3] T. Fujii, K. Hosokawa, T. Nojima, S. Shoji, A. Yostumoto, I. Endo, A microfabricated reactor for cell-free protein synthesis, in: Proceedings of the 19th International Conference on IEEE/EMBS, Chicago, IL, USA, October 30–November 2 1997, pp. 2572–2574. [4] T. Joseph, Wafer bonding for forming microscale fluidic channels, National Nanofabrication Users Network, 1999, pp. 74–75. [5] A. Berthold, L. Nicola, P.M. Sarro, M.J. Vellekoop, Glass-to-glass bonding with standard IC technology thin films as intermediate layers, Sens. Actuators 82 (2000) 224–228. [6] D.J. Lee, B.K. Ju, Y.H. Lee, J. Jang, M.H. Oh, Glass-to-glass anodic bonding for high vacuum packaging of microelectronics and its stability, in: Proceedings of the Thirteenth Annual International Conference on Micro Electro Mechanical Systems 2000 (MEMS 2000), 23–27 January 2000, pp. 253–258. [7] V.G. Kutchoukov, F. Laugere, W. Van der Vlist, L. Pakula, Y. Garini, P.F.A. Alkemade, Fabrication of nanochannels using glass to glass anodic bonding, in: Proceedings of the 12th Innational Conference on TRANSDUCERS, Solid-State Sensors, Actuators and Microsystems, vol. 2, 8–12 June 2003, pp. 1327–1330. [8] A. Berthold, F. Laugere, H. Schellevis, C.R. de Boer, M. Laros, R.M. Guijt, P.M. Sarro, M.J. Vellekoop, Fabrication of a glass-implemented microcapillary electrophoresis device with integrated contactless conductivity detection, Electrophoresis 23 (2002) 3511–3519. [9] K. SchjØlberg-Henriksen, G.U. Jensen, A. Hanneborg, H. Jakobsen, Sodium contamination of SiO2 caused by anodic bonding, J. Micromech. Microeng. 13 (2003) 845–852. [10] M. Lee, Y.Y. Wong, M. Wong, Y. Zohar, Size and shape effects on two-phase flow patterns in microchannel forced convection boiling, J. Micromech. Microeng. 13 (2003) 155–164. [11] L. Jiang, M. Wong, Y. Zohar, Micromachined polycrystalline thin film temperature sensors, Meas. Sci. Technol. 10 (1999) 653.
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[12] W.S. Wong, H.H. Qiu, A fast response temperature sensor for molten metal microdrop solidification research, senior year project report, MECH, HKUST, 2000. [13] N.F. Raley, J.C. Davidson, J.W. Balch, Examination of glass–silicon and glass–glass bonding techniques for microfluidic systems, Proc. SPIE 2639 (1995) 40–45. [14] S.M. Sze, VLSI Technology, McGraw-Hill, New York, 1988. [15] M.A. Gretillat, F. Paoletti, P. Thiebaud, S. Roth, M. Koudelka-Hep, N.F. de Rooij, A new fabrication method for borosilicate glass capillary tubes with lateral inlets and outlets, Sens. Actuators A 60 (1997) 219–222. [16] A.F. Flannery, N.J. Mourlas, C.W. Storment, S. Tsai, S. Tan, PECVD silicon carbide for micromachined transducers, in: Proceedings of the Transducers ’97, pp. 217–220. [17] T. Diepold, E. Obermeier, Smoothing of ultrasonically drilled holes in borosilicate glass by wet chemical etching, in: Proceedings of the MME, Copenhagen, 1995, pp. 35–38. [18] S. Shoji, H. Kikuchi, H. Torigoe, Anodic bonding below 180 ◦ C for packaging and assembling of MEMS using lithium alminosilicate-quartz glass–ceramic, in: Proceedings of the MEMS, 1997, pp. 482–487. [19] T. Corman, P. Enoksson, G. Stemme, Gas damping of electrostatically excited resonators, Sens. Actuators A 61 (1997) 249–255. [20] T. Corman, P. Enoksson, G. Stemme, Deep wet etching of borosilicate glass using an anodically bonded silicon substrate as mask, J. Micromech. Microeng. 8 (1998) 84–87. [21] D.C.S. Bien, P.V. Rainey, S.J.N. Mitchell, H.S. Gamble, Characterization of masking materials for deep glass micromachining, J. Micromech. Microeng. 13 (2003) S34–S40. [22] J. Wei, H. Xie, M.L. Nai, C.K. Wong, L.C. Lee, Low temperature wafer anodic bonding, J. Micromech. Microeng. 13 (2003) 217– 222.
Biographies Zhenlan Xue is a PhD candidate in the Department of Mechanical Engineering, Hong Kong University of Science and Technology. She received her bachelor and master degree in thermal energy engineering in 2000 and 2003, respectively, from Tianjin University. Her research interests include MEMS fabrication, micro fluid flow and optical diagnostics including PDA and PIV. Huihe Qiu received his BS and MS degrees from Department of Precision Instruments at Tianjin University in 1982 and 1985, respectively. He received his PhD degree from LSTM at the University of ErlangenN¨urnberg, Germany, in 1994. Prof. Qiu joined the faculty of the Department of Mechanical Engineering at the Hong Kong University of Science and Technology in 1994. He is the recipient of the Best Paper Award of Institute of Physics (IOP) in 1994 and the State Scientific and Technological Progress Award in 1989 (SSTPA). His current research interests include microfabrication, sensors and actuators, optical diagnostics and transport phenomena in microscale multiphase flows.
Integrating micromachined fast response temperature sensor array in a glass microchannel Zhenlan Xue, Huihe Qiu ∗ Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China Received 3 December 2004; received in revised form 17 February 2005; accepted 27 April 2005 Available online 6 June 2005
Abstract Total glass microdevices have attracted wide interest in the development of micro total analysis systems (TAS), laser microsurgery, micro heat pipes and microfluidics researches because of the possibility for optical diagnostics and observations. The integration of a detection system, such as integrated sensors into a glass microchannel, however, was difficult due to technology constraints. A microfabrication process integrating a micromachined fast response temperature sensor array inside a glass microchannel is presented. Combining of newly developed two different glass materials anodic bonding, channel etching, fabrication of silicon temperature detectors, makes the fabrication of a glass microchannel with an integrated temperature detector array possible. The implantation process and the channel etching technology were ameliorated to adapt to the new material introduced. The dynamic response of the integrated temperature array has also been calibrated. The response time of the integrated temperature sensor array was examined to be 1.5 s in the glass microchannel. © 2005 Elsevier B.V. All rights reserved. Keywords: Sensor; Anodic bonding; Microelectromechanical system; MEMS technology
1. Introduction Research on rapid fluid dynamics and heat transfer in a microchannel is an emerging area in studying laser ophthalmic microsurgery, pico-liter optical glucose analysis, fabricating and repairing of micro-electronic-mechanical devices, laser deposition of thin liquid film to a specific location in microsystem, micro heat pipes and diffusion of a pollutant liquid, etc. Recent progress in micro- and nanotechnologies has yielded microfluidic devices that involve lots of microchannels in order to carry out assays using very small amounts of fluid. For further development of devices such as lab-on-a-chip and micro total analysis systems (TAS), it is important to understand the physics of transport processes in a microchannel multiphase flow. In the above-mentioned applications, transparent glass microdevices are more attractive because of the possibility for optical observation, treatment and diagnostics, such as the ∗
Corresponding author. Tel.: +852 23587190; fax: +852 23581543. E-mail address: [email protected] (H. Qiu).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.04.019
interfacial film thickness at the gas–liquid interface of a micro bubble/plug in a microchannel [1]. Furthermore, it is indispensable to monitor and optimize the flow conditions such as fluid/wall temperatures, pressure and velocity, etc., which requires the detection systems to be integrated within a glass microchannel. The integration of a detection system, such as integrating a silicon-based temperature sensor array for monitoring the boundary condition of the channel wall into a total glass microchannel (In this paper, total glass microchannel means both the cover and substrate are made of glass.), however, is different due to the lack of suitable microfabrication technology. Several methods were proposed for fabricating semitransparent microchannels which were fabricated on silicon wafer (non-transparent) and anodically bonded with glass cover (transparent) [2,3]. The silicon wafer substrate causes the system being not fully transparent which causes some limits in optical treatment and diagnostics. Although glassto-glass anodic bonding was also demonstrated [4–8], most of such bonding were conducted between Corning 7740, Shchott #8330, and Hoya SD-2, which are commonly used
190
Z. Xue, H. Qiu / Sensors and Actuators A 122 (2005) 189–195
for glass-to-silicon anodic bonding. This kind of borosilicate glass contains sodium oxide, which is required for charge separation during the bonding process. Unfortunately, such glass used in anodic bonding does not meet the requirement as a substrate for integrating silicon sensors, because it only aims at forming a microchannel or chamber between the two glass layers, without taking sodium contamination into consideration. If this kind of glass with sodium oxide is used as the substrate, its sodium ion will diffuse and contaminate the sensor layers during the anodic bonding process [9], which may cause malfunction of the devices. It will also contaminate the fabrication facilities such as CVD furnace and consequently spread the ion to other users’ device. To the authors’ knowledge, no literature on silicon-based sensors fabricated on a glass substrate has been reported. Moreover, to bypass the difficulty of anodic bonding caused by the entire intermediate layer, temperature sensors were often fabricated on the backside of the silicon wafer [10]. However, to reach fast temperature response without extra temperature lag caused by the channel wall, it is preferable to integrate temperature sensors inside the microchannel directly. This paper presents a method that integrates a fast response temperature sensor array into a total glass microchannel utilizing a modified anodic bonding technology. In this method two different kinds of glass were used to form a sealed microchannel with micro temperature sensor array integrated in it. Besides, the whole fabrication process of the chip is presented in detail. Several specific problems brought by employment of Corning 1737 (1737 for short below) instead of silicon wafer are described and the solutions of them are introduced. Implantation scheme and annealing temperature and time were modified according to the thermal characteristics of 1737. The microchannel (on the cover) was formed in Corning 7740 (7740 for short below) by wet etching technology. Different sacrifice etching mask was attempted and compared. Finally, an ameliorated Cr + Au + PR (photoresist) masking scheme was adopted. Channels with depth of 60–100 m were obtained with little pinholes and lateral etch. After bonding and slicing, the performance of temperature sensors was calibrated.
2. Fabrication technology 2.1. Selection of channel materials Fabrication of micromachined fast response temperature sensors on a silicon substrate has been described in [11,12]. However, while using transparent substrate, it imposes more restrictions on the design and manufacturing. The most often used transparent materials in MEMS are quartz and commercially available glass material named Corning 7740 and 1737. Glass 7740 is widely used in the glass–silicon anodic bonding to form microchannels. However, to integrate silicon-based microsensors on 7740, the potential contamination problem must be addressed [10]. Especially, when the substrate is
Table 1 Comparison of thermal expansion coefficient between 1737 and 7740 7740 0–300 ◦ C 25 ◦ C–set point 515 ◦ C
32.5 × 10−7 /◦ C 35.0 × 10−7 /◦ C
1737 0–300 ◦ C 25 ◦ C–set point 671 ◦ C
37.6 × 10−7 /◦ C 42.0 × 10−7 /◦ C
also using 7740, the sodium ion migration by the electrical field during the bonding process may contaminate the sensors which are built closely at the glass–polysilicon interface. Such problem never exists in silicon-to-glass anodic bonding, because the substrate is silicon wafer and no sodium ion migration problem. Furthermore, due to the potential contamination to the devices and the fabrication system, in many microfabrication laboratories, 7740 is banned in many facilities such as CVD furnace and dry etching devices, which are needed to share with other users, such as CMOS users. Therefore, an ion-free material is preferable to be selected as the substrate. Glass 1737 is mostly used as substrates for flat panel displays. Because it contains very little metal ion (near zero), it will not cause contamination to the sensors, devices, and the fabrication systems that are shared with other users. Moreover, the thermal expansion coefficient of 1737 is at the same order of 7740 (Table 1), which facilitates successful anodic bonding under a relatively high temperature at around 250–450 ◦ C. While quartz’s thermal expansion coefficient is about 5.9 × 10−7 /◦ C thus at different order from that of 7740, which makes it difficult to bond quartz with 7740. As a result, glass 1737 was proposed as the substrate where the temperature sensor can be built. To form a total glass microchannel, a glass cover channel is needed. Because 1737 does not contain required sodium ion, glass–glass thermal fusion bonding will be an option to bond two 1737 together. However, because thermal fusion bonding needs high temperature (about 800 ◦ C) [13], the bonding temperature may cause problems for the processing steps previously performed (e.g. the resistor diffusion). Therefore, anodic bonding utilizing glass 7740 was selected as the channel cover material. Anodic bonding is the most suitable for this case, because it works under comparatively low temperature and would not cause damage to glass wafer and the devices. Furthermore, the demands on surface roughness and contamination are much lower than those for the fusion bonding processes. Bonding at low temperatures, with less critical demands for surface flatness, and the possibility of metalized electrical feed through will offer more process flexibility in the fabrication of sensors and actuators. Because the sensors are built on the substrate surface, no direct contact between the sensors and the top channel cover is formed. The contamination problem can, therefore, be eliminated. An intermediate polysilicon film was deposited that provides the bonding mechanism as shown in Fig. 1. The double-side deposited polysilicon film takes place of sensor machining as well as the anode of bonding process.
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Fig. 1. Glass-to-glass anodic bonding setup.
It makes the substrate 1737 practically “semi-conducting” and renders anodic bonding theoretically possible. 2.2. Low temperature annealing In the fabrication, some non-standard MEMS technologies were employed at the first time. One of them is the fabrication of integrated temperature sensor array on the glass (1737) substrate. To build the sensor on the glass (1737), polysilicon was deposited on the glass substrate first. The sensor array was then fabricated with polysilicon layer. However, there was problem associated with implantation and annealing process. Normally, an impurity process of a polysilicon can be either conducted by diffusion or implantation. According to the thermal properties of 1737, it cannot withstand high temperature that diffusion requires. Therefore, implantation technology was employed to dope impurity to polysilicon. Usually, the process conditions for phosphorous impurity of silicon wafers require implantation energy of 120 keV, 950 ◦ C annealing temperature and 60 min in a diffusion furnace. However, the 950 ◦ C annealing temperature is definitely too high for 1737 that claims a maximum processing temperature of 620 ◦ C. Therefore, the annealing temperature was set to 620 ◦ C for 1737, which is the same as polysilicon deposition temperature in chemical vapor deposition (CVD). Although this temperature is much lower than the standard annealing temperature of IC fabrication, it is still sufficient to activate most of the impurity [14]. One hold back of using low temperature annealing is that there is little possibility to redistribute the doped impurity because the diffusion (penetration) of the implanted phosphorous is a function of annealing temperature. To solve this problem, multiple doping processes with different implantation energies and concentrations were introduced. Fig. 2 is the simulation result of two equivalent implantation schemes with SUPREM: • one time dope: 1.5E15/cm2 at 120 keV • three times dope: 5E14/cm2 at 80, 110, 140 keV, respectively. The total doses of these two schemes are the same. From Fig. 2, the three times dope scheme gets more uniform distribution of the impurity even if other conditions remain
Fig. 2. Results of simulation with two dope schemes.
unchanged. After annealing, the sheet resistance of three times dope of test wafers was measured. For five wafers in one batch, the measured Rs are listed in Table 2 which demonstrates that the sheet resistance with three times dope under low temperature annealing scheme is close to that with once dope of silicon wafer. 2.3. Mask for 7740 etching The other technology developed was the etching of 7740. It is crucial to find a material that can work as the etching mask of 7740. Furthermore, the etching rate should be calibrated to get a certain etching depth. Many techniques have been used to structure 7740. Deep etching in borosilicate glass using a polysilicon film deposited by low pressure chemical vapor deposition (LPCVD) has been reported [15]. The LPCVD polysilicon mask can be used to etch through a 500 m thick pyrex glass wafer. The problem with this method is that during the high temperature LPCVD deposition process, the sodium ions diffuse out of the glass and contaminate the furnace. A similar method using plasma enhanced chemical vapor deposition (PECVD) silicon carbide as mask has also been reported [16]. Etch masks of Cr + Au + resist are often used [17–19]. But the mask cannot be used for very large etch depths since pinholes appear after a while. Moreover, a large lateral under-etch of chromium is present. Deep etching of borosilicate glass using anodically bonded silicon substrate as mask has also been reported [20]. Although this method allows etch depths up to 500 m with no surface damage to the silicon mask, the process is costly and time-consuming. Other methods like laser structuring, electro-chemical discharge drilling or sand blasting of straight wall in glass, but Table 2 Rs of polysilicon of different schemes Wafer 1 Rs of 1737 wafer (k) Rs of silicon wafer (k)
2
0.883 0.838 0.820 0.791
3
4
5
0.791 0.845
0.879 0.756
0.784 0.837
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Fig. 3. Comparison of different masks of glass etching: (a) polysilicon; (b) Cr + Au + PR.
they do not meet the desired standard batch process requirements and present problems of accuracy [21]. In this study, annealed Cr + Au + PR mask for glass etching has been proposed. For using the annealed Cr + Au + PR mask, because RTA process only takes several minutes and the metal etching is also a several minutes’ job, this scheme saves more time than the “anodically bonded wafer mask” scheme which is reported to obtaining good result in deep glass etching. Therefore, to etch a relatively deep 7740 channel (10–102 m), the conventional Cr + Au mask is the most feasible except the pinholes, which are believed to be caused by imperfection of metal sputtering, as well as the high cost of employing gold as a sacrifice layer. It has been known that chrome is also resistive to hydrofluoric acid (HF). Chrome evaporated lithography mask was tested in HF solution (HF:H2 O = 1:3) and it can stand 5–6 h with few pinholes. But since it was not easy to obtain such
˚ gold was good qualify of chrome film, a thin layer of 500 A ˚ sputtered on the top of 1500 A chrome. After that, a 5 min of 400 ◦ C rapid thermal annealing (RTA) was followed to repair the defect of the metal films. This annealing process was important for eliminating the pinholes’ damage. PR207 was employed as the mask of metal etching. Au was etched by wet etching in solution of KI:I2 :H2 O = 20 g:5 g:400 ml under room temperature, and the etching rate was about ˚ 1000 A/min. Cr was also etched in CEP-200 micro chrome etchant that is commonly used to etch chrome lithography ˚ mask, and etching rate is about 1000 A/min. After metal etching, PR was remained as an extra protection of glass etching. So it is actually an ameliorated Cr + Au + PR etching scheme. Only small quantities of gold were used, which makes this scheme more cost-effective, and the RTA process cured the imperfection of metal film to avoid serious pinhole problem. Fig. 3 shows etching results of polysilicon mask scheme and
Fig. 4. Process flow of a microchannel integrated with temperature sensors.
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Table 3 Etching rates of glass in different solution Etchant
Glass 7740 (m/min)
Glass 1737 (m/min)
HF (49%) Buffered HF (1:3) BOE (1:6)
6.6 0.28 0.034
9.8 1.53 0.1
ameliorated Cr + Au + PR scheme. In the latter picture, almost no pinholes can be found. The etchant solution used is buffered HF solution. Fast etching rate and little damage to etching mask are preferred. Different solutions and different concentrations were evaluated and the etching rate of glass 7740 and glass 1737 was measured. Table 3 indicates that etching rate of glass 1737 is always larger than glass 7740. And for 49% HF, the etching rate is far larger than buffered solutions. Therefore, 49% HF is suitable for deep etching while the other two can work better in shallow etching since it is easier to control the etching depth precisely.
Fig. 5. Microscopic picture of sensors array pattern.
2.4. System integration Based on the above-developed processes, the whole fabrication process is shown in Fig. 4. First, 1737 wafers were ˚ polysilicon with CVD furnace. cleaned and deposited 4000 A After that, a Varian CF3000 Ion Implanter is used to implant phosphorus into the wafer. An annealing was followed to activate the impurity and remove the damage. Then polysilicon was patterned by dry etching in AME8110. Heavy implantation was conducted to facilitate good contact between sensors ˚ gold was sputtered on 300 A ˚ Ti/W and and electrodes. 1500 A ˚ PECVD silidefined the electrodes pattern by lift off. 2000 A con oxide was deposited on the surface of wafer to protect the sensors and electrodes from shortcut. The oxide on contact pads was etched for wire bonding. Fig. 5 is a microscopic picture of the sensor array pattern. The microchannel was fabricated on 7740, following the above-mentioned specific etching method. After that, anodic bonding was performed with Karl Suss Bonder. After the fabricated glass wafer was placed in the chamber, it was pumped to a vacuum of
Fig. 6. 3D sketch of bonded pair.
10−4 mbar, purged with 980 mbar nitrogen and then heated to 375 ◦ C. The chamber was then brought back up to atmosphere pressure and cooled. Then the chamber was brought to 1500 mbar and 1000 V was applied for about 6–7 min until the current dropped down to 10% of the peak value. The bonded channel was then evaluated under microscope and the sodium diffused to the outside surface of the channel was cleaned. The bonding condition can be referred to Wei et al. ˚ [22] and Joseph’s suggestions [4]. Although about 1800 A
Fig. 7. Overview photo of a bonded sample.
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˚ silicon oxide intermetal electrodes topography and 2000 A mediate layer exist, the bonding still occurred. Voids in some areas also can be seen in small partition of the whole area, which is caused by particles or trapped air. So it is important that the wafer surface is clean enough and the vacuum of bonding chamber should be higher to get better bonding result. Figs. 6 and 7 show 3D sketch and the real photo of the bonded sample, respectively.
3. Evaluation of the integrated system 3.1. Verification of water tightness It is important to verify the water tightness of the microfluidic chip. A verification test was conducted. The inlet and outlet of the channel were connected to two mini pipes utilizing epoxy. The pipe of the inlet was connected to a pressurized water tank with a pressure gage installed between the control valve and the channel. Water was flowing into the microchannel while the valve was opened. The test was started at the pressure of 0.5 bar where the water flow in the microchannel was already observable. The flow passes through the microchannel and the readings of the pressure gage were monitored simultaneously. No leakage was found even the pressure reach to 1.2 bar with a reasonable flow rate. It seems that the anodic bonding with oxide and metal electrodes intermediate layers was mechanically successful. The water tightness at the electrode position may be resulted from the oxide layer which was used to insulate and protect the electrodes. 3.2. Evaluation of temperature sensors To evaluate the performance of the integrated temperature sensor array, the performance of sensors was tested with HP 4145B semiconductor parameter analyzer before packaging. Under room temperature, the tested sensor I–V curve is as Fig. 8. The R–T curve is shown in Fig. 9. A quite good linearity can be observed between the R and T relationship. Some
Fig. 9. R–T curve of sensor.
Fig. 10. Response time of temperature sensor.
small derivation in the R–T curve may possibly come from the measurement uncertainty. The response time of the sensor is also a crucial characteristic of temperature sensor, which was tested by a laser impulse method [1]. Fig. 10 shows the response of sensor induced by laser pulse heating. And the calculated response time of the whole instrument is about 1.5 s, which can be further improved if a fast (broadband) amplifier is utilized.
4. Conclusion
Fig. 8. I–V curve under room temperature.
A novel method integrating micromachined fast response temperature sensor array into a total glass microchannel was developed. This method utilizes a modified anodic bonding technique for bonding two different glass materials (Corning 1737 and 7740) with polysilicon intermediate layer. The implantation process was ameliorated to meet the temperature characteristics of 1737. The experiments indicate that RTA effectively helps Cr + Au + PR mask to work better in glass etching, and anodic bonding can still occur even if ˚ electrodes layer plus 2000 A ˚ silicon oxide are sand1800 A wiched between glass 7740 and polysilicon. This fabrication technology can also be used to integrate other types of siliconbased sensors in glass microdevices with full transparency.
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The performance of the integrated fast response temperature sensor array was evaluated and the response time of 1.5 s was achieved.
Acknowledgement This research is supported by the Hong Kong Government under Research Grants Council (RGC), grant no. (HKUST 6214/01E and 6230/02E).
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Biographies Zhenlan Xue is a PhD candidate in the Department of Mechanical Engineering, Hong Kong University of Science and Technology. She received her bachelor and master degree in thermal energy engineering in 2000 and 2003, respectively, from Tianjin University. Her research interests include MEMS fabrication, micro fluid flow and optical diagnostics including PDA and PIV. Huihe Qiu received his BS and MS degrees from Department of Precision Instruments at Tianjin University in 1982 and 1985, respectively. He received his PhD degree from LSTM at the University of ErlangenN¨urnberg, Germany, in 1994. Prof. Qiu joined the faculty of the Department of Mechanical Engineering at the Hong Kong University of Science and Technology in 1994. He is the recipient of the Best Paper Award of Institute of Physics (IOP) in 1994 and the State Scientific and Technological Progress Award in 1989 (SSTPA). His current research interests include microfabrication, sensors and actuators, optical diagnostics and transport phenomena in microscale multiphase flows.