- Email: [email protected]
Wafer level packaging of silicon pressure sensors
Sensors and Actuators 82 Ž2000. 229–233 www.elsevier.nlrlocatersna
Wafer level packaging of silicon pressure sensors H. Krassow ) , F. Campabadal, E. Lora-Tamayo Institut de Microelectronica de Barcelona, CNM-CSIC Campus UniÕersitat Autonoma de Barcelona, Bellaterra 08193, Spain ` ` Received 7 June 1999; accepted 7 October 1999
Abstract In this paper, a new pre-packaging technique for silicon pressure sensors on the wafer level is presented. It is based on the use of UV photopatternable silicone which is deposited over the whole wafer by means of a novel device suitable for low-viscosity material coating and mask alignment. The process consists of the exposure of the deposited layer to UV light leading to cross-linking of the polymer according to the pattern of a chrome mask, which leaves the membrane area as well as the bonding pads free from silicone. After development and dicing, the chips are packaged and conventional wire bonding is performed. The area surrounding the UV photopatternable silicone pattern can then be filled with soft silicone gel for protection of the electrically active components against aggressive media. Results are presented for silicon piezoresistive pressure sensors on which 1.5-mm high octagonal wall structures of silicone elastomer have been patterned. Despite the thickness of the deposited layer, good resolution and aspect ratio are obtained, and the temperature dependence of the packaged pressure sensors is not influenced by the polymer layers. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Wafer level; Packaging; Pressure sensors; Polydimethylsiloxane; Photolithography
1. Introduction The piezoresistive silicon pressure sensor is known as the most prevalent and commercially successful MEMS device w1x. However, the major limiting factor of gaining market share compared to conventional pressure sensors has always been its packaging. It needs to be compatible with a variety of chemical environments to provide pressure and liquid level sensing products for various automotive, industrial and white goods applications even for aggressive media. The most successful method for silicon pressure sensor protection is the packaging in a siliconeoil-filled steel housing. This results, however, in a high price and in a considerable size, which is a limitation for some applications. Another means for providing pressure sensors with some media protection is through the use of barrier coatings. Silicone andror parylene coatings are applied for sensor packaging, one of the most popular
) Corresponding author. Tel.: q34-935-802-625; fax: q34-935-801496. E-mail: [email protected]
examples being a soft gel coating technique used by Motorola w2,3x that may be regarded as the best balance between performance and cost. In the case of pressure sensors with high sensitivity, temperature variations can result in mechanical influences of the gel on the membrane and cause instabilities. Previously, we proposed a new packaging technique w4x which makes use of a UV-patternable silicone Žpolydimethylsiloxane, PDMS. and allows for the protection of metallic areas on the silicon surface and the wire bonds against corrosion, while leaving the pressure sensor membrane free of any additional coating. This packaging technique has been shown to be very promising since the influence of the package on the performance of the sensor regarding thermal stresses is negligible w5x. In addition, it has been applied to the packaging of a pressure-sensor-based water flow meter w6x. Due to the final photolithographic step intrinsic to this method, packaging is done by one device at a time and batch processing would be complicated. In this paper, a batch process for the encapsulation of pressure sensors at the wafer level is proposed. The packaging method developed combines the photolithographic patterning of a flexible silicone rubber with the use of a
0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 3 3 4 - 9
230
H. Krassow et al.r Sensors and Actuators 82 (2000) 229–233
soft silicone gel, so that the sensor and the electrically active components are entirely coated with the exception of the membrane of the sensor. This combination avoids wire failure during thermal cycling that is one of the failure mechanisms when coating electronic devices with silicone rubbers w7x. For the implementation of the proposed packaging technique, a polymer deposition equipment has been designed and fabricated. In the following, this device, together with results obtained for packaged piezoresistive silicon pressure sensors, is presented.
2. Packaging process The proposed packaging procedure is based on the use of a UV- photopatternable elastomer deposited on the entire silicon wafer containing pressure sensors. The first step in the process consists of the design of a mask with the pattern to be transferred to the polymer layer. This mask should leave the sensor membrane as well as the bonding pads free from silicone. As a consequence, the silicon membrane has no additional coating that might influence the temperature performance of the sensor, and the wire bonding process can be carried out. The on-wafer packaging process flow then consists of the following steps: – Deposition of the polymer layer over the whole wafer up to the desired thickness; – Mask alignment and UV exposure; and – Removal of the non-cured polymer. Rinse in DI water and dry. At this stage, the wafer is ready to be diced. Each die is then attached to its housing cavity and the wire bonding is performed. As the last packaging step, the area around the wire bonds can be filled with silicone gel, with the membrane of the sensor being protected by the structure defined during the on-wafer process.
3. Experimental 3.1. Polymer deposition equipment For the wafer scale polymer deposition, a novel device has been designed and fabricated. This device, as shown in Fig. 1, is similar to an ultralow-pressure injection molding system. It has a wafer support in a PTFE-made mold, on which the wafer to be processed will be placed. The mold is closed via a chrome mask through which the photolithography will be made afterwards. The mask is covered with a Mylar sheet to avoid adhesion of the polymer to the glass of the mask. The wafer support is adjustable in height and allows for infinitely variable distance between the wafer surface and the Mylar sheeted chrome mask according to the desired polymer coating thickness. After optical alignment with the wafer, the chrome mask is clamped airtight to the PTFE mold. The silicone resin is then injected, driven by the pressure of the polymer column in the storage tank. For filling, the device is turned 908 into a vertical position. Thus, deaeration is guaranteed and the mold is slowly filled up beginning from the resin inlet to the deaeration outlet. After filling, the outlet is closed and the UV exposure is done. The device assures a uniform polymer layer thickness of up to several millimetres even for materials of very low viscosity. Any air bubble intrusion is avoided. In the case of low-viscosity materials, neither of these features could be achieved by traditional spin coating. 3.2. Packaging of pressure sensors The proposed on-wafer packaging technique has been applied to the encapsulation of piezoresistive silicon pressure sensors. The wafers are of 100 mm diameter and include two types of sensors with square membrane: one
Fig. 1. Wafer-level polymer deposition equipment.
H. Krassow et al.r Sensors and Actuators 82 (2000) 229–233
Fig. 2. Silicon wafer of pre-packaged pressure sensors.
of 1.46 = 1.46 mm2 and another of 2 = 2 mm2 . In both cases, the bonding pads are arranged in the corners of the die Žsee photograph in the center of Fig. 3.. The total die size is 5380 = 5880 mm2 . Accordingly, a chrome mask has been designed with octagonal patterns, in order to have octagonal wall structures centered around the membrane, with a wall thickness of less than 1 mm. The UV-photopatternable material used is the PDMS Semicosil 948 UV from Wacker Chemie. Before PDMS deposition, the surface of the wafer has been treated with a primer Ž10% MPTS and 90% methanol. in order to improve the PDMS adhesion.
231
Deposition of the PDMS has been carried out by means of the novel device described above. The height of the polymer layer was 1.5 mm and curing was done in 50 s with a conventional mask aligner. After exposure to UV light, only the octagonal patterns are cross-linked. The nonexposed areas of PDMS are removed in the developing step, which consists of soaking the wafer in n-hexane, followed by isopropanol and finally a DI water rinse. In Fig. 2, a photograph of a wafer with pre-packaged pressure sensors is shown. It should be noted that the chrome mask not only features the octagonal patterns, but also includes alignment marks allowing for optical alignment with the silicon wafer, and lines extending from the edge of the wafer that define free paths through the cured PDMS which simplifies the sawing process. Moreover, initial experiments have shown that it is advantageous to minimise the amount of uncured resin by a proper mask layout as this facilitates the removal of liquid residue and the demoulding. Despite the height of the polymer layer being 1.5 mm, the resolution and aspect ratio obtained are excellent, as can be seen in Fig. 3. In addition, very strong adhesion of the polymer to the wafer surface is obtained. It should be pointed out that, in the case of piezoresistive pressure sensors, the membrane formation by anisotropic etching of the backside silicon can be performed either before or after the pre-packaging step. In the case presented, the pre-packaging was done after the wafer processing. When the pre-packaging on the wafer scale is finished, the wafer can be diced. The final step consists of the attachment of the die in the housing and the wire bonding. In the left photograph of Fig. 3, a sensor chip encapsulated
Fig. 3. Packaged pressure sensor with a membrane area of 1.46 = 1.46 mm2 in a conventional TO housing Žon the left.; close detail of a pre-packaged single chip with a sensor membrane of 4 mm2 Žcenter.; and complete packaging of the sensor with the combination of photopatternable silicone rubber and silicone gel Žon the right..
232
H. Krassow et al.r Sensors and Actuators 82 (2000) 229–233
in a conventional TO header is shown. In the case of applications where aggressive media will be in contact with the sensor, protection of the metal bond pads and wires will be needed. Then, a simple process of filling the cavity left between the housing and the octagonal wall structure with gel will prevent metal corrosion. In our case, the hydrophobic silicone gel Sylgard 527 w A & B from Dow Corning is used. After curing, the gel is extremely soft and has self-healing characteristics, allowing the gel to flow around the wires without damaging them. In addition, this gel links chemically to the UV-sensitive silicone at the interface, sealing any path for the corrosive media to reach the metal areas. In Fig. 3c, the complete sensor packaging is shown.
4. Pressure sensor characterisation In order to assess the influence of the packaging on the performance of the devices, piezoresistive pressure sensors were encapsulated following the proposed technique and their characteristics were analysed. The sensors have a square membrane of 1.46 = 1.46 mm2 with a thickness of 15 mm. They feature a full Wheatstone bridge of ion-implanted piezoresistors and were designed for a differential pressure range of 100 mbar. Before the on-wafer silicone deposition, a 1-mm thick glass substrate ŽPyrex a7740. was anodically bonded at a temperature of 4008C. The measured sensitivity of the sensors is 126 mV Vy1 bary1 with a terminal non-linearity of less than 1%, at a temperature of 258C. To investigate the influence of the polymer packaging, the sensor characteristics over the temperature range 25–808C were measured for both sensors with the silicone encapsulation and without any additional coating. The results obtained are shown in Figs. 4 and 5, for the offset voltage when biasing the Wheatstone bridge at 5 V DC and for the sensitivity shift with respect to the sensitivity at T s 258C, respectively. Comparing the pressure sensors with and without the silicone layers, it is clear that the package does not influence the performance of the sensor
Fig. 5. Sensitivity shift of the pressure sensors with and without silicone packaging as a function of the temperature.
significantly. As a consequence, it can be concluded that the encapsulating layers do not exert thermomechanical stress on the membrane.
5. Conclusions The packaging method presented in this paper allows for the patterning of thick protection layers at the wafer level. When applied to piezoresistive silicon pressure sensor, protection against aggressive media is achieved while leaving the membrane of the sensor free from additional coatings. The main advantage of the proposed method is the batch processing, leading to a reduced cost for the packaging. In addition, a novel device has been developed which can be useful not only in this application but also when the low viscosity of the material or the height of the structures to be deposited make conventional spin coating unsuitable. The packaging procedure has been applied to piezoresistive pressure sensors, and the analysis of the obtained results has shown that the combination of photopatternable silicone rubber with silicone gel does not influence the sensor performance. In addition, the proposed technique can be readily used for any other sensors in which a direct path to the external world is necessary but protection of other areas is also compulsory. In microsystem technology in general, it can be useful for defining polymer canals or alike. Another interesting possibility could be the deposition of multilayers to achieve more complex 3D structures. As a conclusion, this novel technique offers a wide range of possibilities in the field of MEMS.
Acknowledgements
Fig. 4. Offset voltage of the pressure sensors with and without silicone packaging as a function of the temperature.
This work has been partially supported by the Comision ´ Interministerial de Ciencia y Tecnologıa ´ under project no. 95-0196-OP.
H. Krassow et al.r Sensors and Actuators 82 (2000) 229–233
233
References
Biographies
w1x J.W. Knutti, Finding markets for microstructures, Proc. of the SPIE Conf. on Micromachining and Microfabrication Process Technol. IV, SPIE Vol. 3511, Santa Clara, CA, USA, Sept. 1998, pp. 17–23. w2x G. Bitko, D.J. Monk, Th. Maudie, D. Stanerson, J. Wertz, J. Matkin, S. Petrovic, Analytical techniques for examining reliability and failure mechanisms of barrier coating encapsulated silicon pressure sensors exposed to harsh media, Proc. SPIE 2882 Ž1996. 248–258. w3x Th. Maudie, D.J. Monk, D. Zehrbach, D. Stanerson, Sensor media compatibility: issues and answers, Proc. Sensors Expo, Anaheim, Ca, USA, April 1996, pp. 215–229. w4x H. Krassow, D. Heimlich, F. Campabadal, E. Lora-Tamayo, Novel packaging technique and its application to a wetrwet differential pressure silicon sensor, Proc. of the 9th Int. Conference on Solid-State Sensors and Actuators, Transducers ’97, Chicago, IL, USA, June 1997, pp. 275–278. w5x H. Krassow, F. Campabadal, E. Lora-Tamayo, Photolithographic packaging of silicon pressure sensors, Sensors and Actuators A 66 Ž1998. 279–283. w6x H. Krassow, F. Campabadal, E. Lora-Tamayo, The smart-orifice meter: a mini head meter for volume flow measurement, Flow Meas. Instrum. 10 Ž1999. 109–115. w7x J.W. Balde, The effectiveness of silicone gels for corrosion prevention of silicon circuits, IEEE Trans. Compon., Hybrids, Manuf. Technol. 14 Ž1991. 352–365.
Heiko Krassow received his BS and MS in Mechanical Engineering from ŽGermany, 1994. and the PhD in the Technische Universitat ¨ Munchen ¨ Electronic Engineering from the Univesitat Autonoma de Barcelona ` ŽSpain, 1999.. His research at the Centro Nacional de Microelectronica ´ ŽCNM. in Barcelona focuses on design and development in sensor packaging and sensor systems. He has applied for two patents. Dr. Francesca Campabadal received the PhD degree in Physics in 1986 from the Universitat Autonoma de Barcelona. In 1987, she joined CNM, ` working on thin oxide technology and characterisation. Since 1992, her research activities have been also in the field of technologies for the monolithic integration of mechanical sensors and CMOS circuitry. Emilio Lora-Tamayo was born in 1950. He obtained degrees in Physics ŽMadrid, 1972., DEA ŽToulouse, 1973. and PhD ŽMadrid, 1977.. He is a full Professor of Microelectronics at the Autonomous University of Barcelona and now Vice-President of Scientific Research at Spanish Council of Science ŽCSIC.. He has worked in more than 30 projects, is author of more than 40 papers, and has made 90 contributions to Scientific Conferences.
Wafer level packaging of silicon pressure sensors H. Krassow ) , F. Campabadal, E. Lora-Tamayo Institut de Microelectronica de Barcelona, CNM-CSIC Campus UniÕersitat Autonoma de Barcelona, Bellaterra 08193, Spain ` ` Received 7 June 1999; accepted 7 October 1999
Abstract In this paper, a new pre-packaging technique for silicon pressure sensors on the wafer level is presented. It is based on the use of UV photopatternable silicone which is deposited over the whole wafer by means of a novel device suitable for low-viscosity material coating and mask alignment. The process consists of the exposure of the deposited layer to UV light leading to cross-linking of the polymer according to the pattern of a chrome mask, which leaves the membrane area as well as the bonding pads free from silicone. After development and dicing, the chips are packaged and conventional wire bonding is performed. The area surrounding the UV photopatternable silicone pattern can then be filled with soft silicone gel for protection of the electrically active components against aggressive media. Results are presented for silicon piezoresistive pressure sensors on which 1.5-mm high octagonal wall structures of silicone elastomer have been patterned. Despite the thickness of the deposited layer, good resolution and aspect ratio are obtained, and the temperature dependence of the packaged pressure sensors is not influenced by the polymer layers. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Wafer level; Packaging; Pressure sensors; Polydimethylsiloxane; Photolithography
1. Introduction The piezoresistive silicon pressure sensor is known as the most prevalent and commercially successful MEMS device w1x. However, the major limiting factor of gaining market share compared to conventional pressure sensors has always been its packaging. It needs to be compatible with a variety of chemical environments to provide pressure and liquid level sensing products for various automotive, industrial and white goods applications even for aggressive media. The most successful method for silicon pressure sensor protection is the packaging in a siliconeoil-filled steel housing. This results, however, in a high price and in a considerable size, which is a limitation for some applications. Another means for providing pressure sensors with some media protection is through the use of barrier coatings. Silicone andror parylene coatings are applied for sensor packaging, one of the most popular
) Corresponding author. Tel.: q34-935-802-625; fax: q34-935-801496. E-mail: [email protected]
examples being a soft gel coating technique used by Motorola w2,3x that may be regarded as the best balance between performance and cost. In the case of pressure sensors with high sensitivity, temperature variations can result in mechanical influences of the gel on the membrane and cause instabilities. Previously, we proposed a new packaging technique w4x which makes use of a UV-patternable silicone Žpolydimethylsiloxane, PDMS. and allows for the protection of metallic areas on the silicon surface and the wire bonds against corrosion, while leaving the pressure sensor membrane free of any additional coating. This packaging technique has been shown to be very promising since the influence of the package on the performance of the sensor regarding thermal stresses is negligible w5x. In addition, it has been applied to the packaging of a pressure-sensor-based water flow meter w6x. Due to the final photolithographic step intrinsic to this method, packaging is done by one device at a time and batch processing would be complicated. In this paper, a batch process for the encapsulation of pressure sensors at the wafer level is proposed. The packaging method developed combines the photolithographic patterning of a flexible silicone rubber with the use of a
0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 3 3 4 - 9
230
H. Krassow et al.r Sensors and Actuators 82 (2000) 229–233
soft silicone gel, so that the sensor and the electrically active components are entirely coated with the exception of the membrane of the sensor. This combination avoids wire failure during thermal cycling that is one of the failure mechanisms when coating electronic devices with silicone rubbers w7x. For the implementation of the proposed packaging technique, a polymer deposition equipment has been designed and fabricated. In the following, this device, together with results obtained for packaged piezoresistive silicon pressure sensors, is presented.
2. Packaging process The proposed packaging procedure is based on the use of a UV- photopatternable elastomer deposited on the entire silicon wafer containing pressure sensors. The first step in the process consists of the design of a mask with the pattern to be transferred to the polymer layer. This mask should leave the sensor membrane as well as the bonding pads free from silicone. As a consequence, the silicon membrane has no additional coating that might influence the temperature performance of the sensor, and the wire bonding process can be carried out. The on-wafer packaging process flow then consists of the following steps: – Deposition of the polymer layer over the whole wafer up to the desired thickness; – Mask alignment and UV exposure; and – Removal of the non-cured polymer. Rinse in DI water and dry. At this stage, the wafer is ready to be diced. Each die is then attached to its housing cavity and the wire bonding is performed. As the last packaging step, the area around the wire bonds can be filled with silicone gel, with the membrane of the sensor being protected by the structure defined during the on-wafer process.
3. Experimental 3.1. Polymer deposition equipment For the wafer scale polymer deposition, a novel device has been designed and fabricated. This device, as shown in Fig. 1, is similar to an ultralow-pressure injection molding system. It has a wafer support in a PTFE-made mold, on which the wafer to be processed will be placed. The mold is closed via a chrome mask through which the photolithography will be made afterwards. The mask is covered with a Mylar sheet to avoid adhesion of the polymer to the glass of the mask. The wafer support is adjustable in height and allows for infinitely variable distance between the wafer surface and the Mylar sheeted chrome mask according to the desired polymer coating thickness. After optical alignment with the wafer, the chrome mask is clamped airtight to the PTFE mold. The silicone resin is then injected, driven by the pressure of the polymer column in the storage tank. For filling, the device is turned 908 into a vertical position. Thus, deaeration is guaranteed and the mold is slowly filled up beginning from the resin inlet to the deaeration outlet. After filling, the outlet is closed and the UV exposure is done. The device assures a uniform polymer layer thickness of up to several millimetres even for materials of very low viscosity. Any air bubble intrusion is avoided. In the case of low-viscosity materials, neither of these features could be achieved by traditional spin coating. 3.2. Packaging of pressure sensors The proposed on-wafer packaging technique has been applied to the encapsulation of piezoresistive silicon pressure sensors. The wafers are of 100 mm diameter and include two types of sensors with square membrane: one
Fig. 1. Wafer-level polymer deposition equipment.
H. Krassow et al.r Sensors and Actuators 82 (2000) 229–233
Fig. 2. Silicon wafer of pre-packaged pressure sensors.
of 1.46 = 1.46 mm2 and another of 2 = 2 mm2 . In both cases, the bonding pads are arranged in the corners of the die Žsee photograph in the center of Fig. 3.. The total die size is 5380 = 5880 mm2 . Accordingly, a chrome mask has been designed with octagonal patterns, in order to have octagonal wall structures centered around the membrane, with a wall thickness of less than 1 mm. The UV-photopatternable material used is the PDMS Semicosil 948 UV from Wacker Chemie. Before PDMS deposition, the surface of the wafer has been treated with a primer Ž10% MPTS and 90% methanol. in order to improve the PDMS adhesion.
231
Deposition of the PDMS has been carried out by means of the novel device described above. The height of the polymer layer was 1.5 mm and curing was done in 50 s with a conventional mask aligner. After exposure to UV light, only the octagonal patterns are cross-linked. The nonexposed areas of PDMS are removed in the developing step, which consists of soaking the wafer in n-hexane, followed by isopropanol and finally a DI water rinse. In Fig. 2, a photograph of a wafer with pre-packaged pressure sensors is shown. It should be noted that the chrome mask not only features the octagonal patterns, but also includes alignment marks allowing for optical alignment with the silicon wafer, and lines extending from the edge of the wafer that define free paths through the cured PDMS which simplifies the sawing process. Moreover, initial experiments have shown that it is advantageous to minimise the amount of uncured resin by a proper mask layout as this facilitates the removal of liquid residue and the demoulding. Despite the height of the polymer layer being 1.5 mm, the resolution and aspect ratio obtained are excellent, as can be seen in Fig. 3. In addition, very strong adhesion of the polymer to the wafer surface is obtained. It should be pointed out that, in the case of piezoresistive pressure sensors, the membrane formation by anisotropic etching of the backside silicon can be performed either before or after the pre-packaging step. In the case presented, the pre-packaging was done after the wafer processing. When the pre-packaging on the wafer scale is finished, the wafer can be diced. The final step consists of the attachment of the die in the housing and the wire bonding. In the left photograph of Fig. 3, a sensor chip encapsulated
Fig. 3. Packaged pressure sensor with a membrane area of 1.46 = 1.46 mm2 in a conventional TO housing Žon the left.; close detail of a pre-packaged single chip with a sensor membrane of 4 mm2 Žcenter.; and complete packaging of the sensor with the combination of photopatternable silicone rubber and silicone gel Žon the right..
232
H. Krassow et al.r Sensors and Actuators 82 (2000) 229–233
in a conventional TO header is shown. In the case of applications where aggressive media will be in contact with the sensor, protection of the metal bond pads and wires will be needed. Then, a simple process of filling the cavity left between the housing and the octagonal wall structure with gel will prevent metal corrosion. In our case, the hydrophobic silicone gel Sylgard 527 w A & B from Dow Corning is used. After curing, the gel is extremely soft and has self-healing characteristics, allowing the gel to flow around the wires without damaging them. In addition, this gel links chemically to the UV-sensitive silicone at the interface, sealing any path for the corrosive media to reach the metal areas. In Fig. 3c, the complete sensor packaging is shown.
4. Pressure sensor characterisation In order to assess the influence of the packaging on the performance of the devices, piezoresistive pressure sensors were encapsulated following the proposed technique and their characteristics were analysed. The sensors have a square membrane of 1.46 = 1.46 mm2 with a thickness of 15 mm. They feature a full Wheatstone bridge of ion-implanted piezoresistors and were designed for a differential pressure range of 100 mbar. Before the on-wafer silicone deposition, a 1-mm thick glass substrate ŽPyrex a7740. was anodically bonded at a temperature of 4008C. The measured sensitivity of the sensors is 126 mV Vy1 bary1 with a terminal non-linearity of less than 1%, at a temperature of 258C. To investigate the influence of the polymer packaging, the sensor characteristics over the temperature range 25–808C were measured for both sensors with the silicone encapsulation and without any additional coating. The results obtained are shown in Figs. 4 and 5, for the offset voltage when biasing the Wheatstone bridge at 5 V DC and for the sensitivity shift with respect to the sensitivity at T s 258C, respectively. Comparing the pressure sensors with and without the silicone layers, it is clear that the package does not influence the performance of the sensor
Fig. 5. Sensitivity shift of the pressure sensors with and without silicone packaging as a function of the temperature.
significantly. As a consequence, it can be concluded that the encapsulating layers do not exert thermomechanical stress on the membrane.
5. Conclusions The packaging method presented in this paper allows for the patterning of thick protection layers at the wafer level. When applied to piezoresistive silicon pressure sensor, protection against aggressive media is achieved while leaving the membrane of the sensor free from additional coatings. The main advantage of the proposed method is the batch processing, leading to a reduced cost for the packaging. In addition, a novel device has been developed which can be useful not only in this application but also when the low viscosity of the material or the height of the structures to be deposited make conventional spin coating unsuitable. The packaging procedure has been applied to piezoresistive pressure sensors, and the analysis of the obtained results has shown that the combination of photopatternable silicone rubber with silicone gel does not influence the sensor performance. In addition, the proposed technique can be readily used for any other sensors in which a direct path to the external world is necessary but protection of other areas is also compulsory. In microsystem technology in general, it can be useful for defining polymer canals or alike. Another interesting possibility could be the deposition of multilayers to achieve more complex 3D structures. As a conclusion, this novel technique offers a wide range of possibilities in the field of MEMS.
Acknowledgements
Fig. 4. Offset voltage of the pressure sensors with and without silicone packaging as a function of the temperature.
This work has been partially supported by the Comision ´ Interministerial de Ciencia y Tecnologıa ´ under project no. 95-0196-OP.
H. Krassow et al.r Sensors and Actuators 82 (2000) 229–233
233
References
Biographies
w1x J.W. Knutti, Finding markets for microstructures, Proc. of the SPIE Conf. on Micromachining and Microfabrication Process Technol. IV, SPIE Vol. 3511, Santa Clara, CA, USA, Sept. 1998, pp. 17–23. w2x G. Bitko, D.J. Monk, Th. Maudie, D. Stanerson, J. Wertz, J. Matkin, S. Petrovic, Analytical techniques for examining reliability and failure mechanisms of barrier coating encapsulated silicon pressure sensors exposed to harsh media, Proc. SPIE 2882 Ž1996. 248–258. w3x Th. Maudie, D.J. Monk, D. Zehrbach, D. Stanerson, Sensor media compatibility: issues and answers, Proc. Sensors Expo, Anaheim, Ca, USA, April 1996, pp. 215–229. w4x H. Krassow, D. Heimlich, F. Campabadal, E. Lora-Tamayo, Novel packaging technique and its application to a wetrwet differential pressure silicon sensor, Proc. of the 9th Int. Conference on Solid-State Sensors and Actuators, Transducers ’97, Chicago, IL, USA, June 1997, pp. 275–278. w5x H. Krassow, F. Campabadal, E. Lora-Tamayo, Photolithographic packaging of silicon pressure sensors, Sensors and Actuators A 66 Ž1998. 279–283. w6x H. Krassow, F. Campabadal, E. Lora-Tamayo, The smart-orifice meter: a mini head meter for volume flow measurement, Flow Meas. Instrum. 10 Ž1999. 109–115. w7x J.W. Balde, The effectiveness of silicone gels for corrosion prevention of silicon circuits, IEEE Trans. Compon., Hybrids, Manuf. Technol. 14 Ž1991. 352–365.
Heiko Krassow received his BS and MS in Mechanical Engineering from ŽGermany, 1994. and the PhD in the Technische Universitat ¨ Munchen ¨ Electronic Engineering from the Univesitat Autonoma de Barcelona ` ŽSpain, 1999.. His research at the Centro Nacional de Microelectronica ´ ŽCNM. in Barcelona focuses on design and development in sensor packaging and sensor systems. He has applied for two patents. Dr. Francesca Campabadal received the PhD degree in Physics in 1986 from the Universitat Autonoma de Barcelona. In 1987, she joined CNM, ` working on thin oxide technology and characterisation. Since 1992, her research activities have been also in the field of technologies for the monolithic integration of mechanical sensors and CMOS circuitry. Emilio Lora-Tamayo was born in 1950. He obtained degrees in Physics ŽMadrid, 1972., DEA ŽToulouse, 1973. and PhD ŽMadrid, 1977.. He is a full Professor of Microelectronics at the Autonomous University of Barcelona and now Vice-President of Scientific Research at Spanish Council of Science ŽCSIC.. He has worked in more than 30 projects, is author of more than 40 papers, and has made 90 contributions to Scientific Conferences.