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Anodic bonding of evaporated glass structured with lift-off technology for hermetical sealing
Sensors and Actuators 83 Ž2000. 150–155 www.elsevier.nlrlocatersna
Anodic bonding of evaporated glass structured with lift-off technology for hermetical sealing S. Sassen ) , W. Kupke, K. Bauer DaimlerChrysler AG, Research and Technology FT2 r M, Microsystems Technology PO Box 800 465, D-81663 Munich, Germany Received 23 June 1999; received in revised form 7 December 1999; accepted 9 December 1999
Abstract This paper reports on an enhanced anodic bonding technology of thin e-beam evaporated glass layers Ž d F 5 mm. for micromachined silicon sensors and actuators. This MOS-compatible technology has been developed for bonding between a silicon wafer with electrical structures and a bulk micromachined silicon wafer. A bonding frame structure can be realized with hermetically sealed metal feedtroughs especially suited for capacitive sensors with a small sensing gap and fast RC-time constants. A lift-off technology for structuring the glass using metal as a sacrificial layer has been developed, because the substrates were heated to about 3008C in order to enhance the quality of the glass layer. A simple model for the current flow during the bonding process is given. The numerically calculated current–voltage behaviour is compared with measured data. An electrostatically excitated silicon resonator is realized to demonstrate the applicability of this technology. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Anodic bonding; Evaporated glass; Hermetical sealing; Metal feedtrough; Capacitive sensors
1. Introduction Bonding technology on wafer scale level is one of the key process steps for bulk micromachining w1–3x. There are four different technologies for wafer scale bonding: silicon direct, anodic, eutectic and adhesive bonding w4–6x. Anodic bonding is often chosen for wafer bonding at moderate temperatures Ž- 5008C., because this technology has been shown to be reliable and reproducible. It is especially essential for capacitive sensors to have small stray capacitances, fast RC-time constants and small capacitive gaps for sensing and actuating. There are commercially available capacitive sensors produced by bonding silicon wafers onto glass-coated wafers w7x. Recently, there have been attempts to reduce the thermal stress of the sensors using thin glass layers — sputtered, evaporated or spin-coated on silicon wafers w8–14x. In this paper we present a technology for structuring thin evaporated glass in order to produce hermetically sealed capacitive sensors with metal feedtroughs and small sens-
) Corresponding author. Tel.: q49-89-607-20284; fax: q49-89-60724001. E-mail address: [email protected] ŽS. Sassen..
ing gaps Ž0.5–5 mm.. The use of evaporated glass has the advantages of reproducible capacitive gap distances and small size of the bonding frame Ž- 300 mm..
2. Glass evaporation We start our process with oxidized single-crystalline 4Y Ž100.-wafers. As a target material for the evaporation we use the glass a8329 from Schott. This glass is especially adapted for e-beam evaporation of layers up to 5 mm thickness. Evaporating the glass at a pressure of 3 = 10y2 Pa results in a rate of about 10 nmrs. The thermal coefficient of the massive glass is 27.5 = 10y7 rK. The substrate wafers are heated to about 3008C in order to guarantee a good layer quality and sufficient adhesion. The composition of the evaporated glass was analysed with energy dispersive X-ray analysis ŽEDX analysis.. The respective EDX plot is shown in Fig. 1. The fluorine content can be traced back to our standard cleaning process after evaporation, which also includes a short dip in HF acid. The titanium content is most probably introduced from chamber or crucible material during evaporation. The exact glass layer composition was determined with atomic absorption spectroscopy ŽAAS.. For reference, we
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 0 0 . 0 0 3 0 0 - 9
S. Sassen et al.r Sensors and Actuators 83 (2000) 150–155
151
Fig. 1. EDX analysis of the evaporated glass layer. Schott glass a8329 was used as the target material.
also analysed the composition of the target material before evaporating onto the substrate wafers. The content of sodium ions is comparable to that in the target material whereas the kalium content is reduced to half the content in the target material. The measurements of the dielectric constant and the refractive index coincide well with those of the target material. These results are summarized in Table 1.
equipment Žcontamination of plasma etching equipment due to the alkaline ions in the glass.. The substrate heating during evaporation prevents the use of polymer resists as sacrificial material for the lift-off. Therefore, we use a metal sacrificial layer Že.g. sputtered aluminum. covered with PECVD-oxide. The oxide is structured with dry etching. Then, wet chemical etching is used to structure the sacrificial metal, resulting in a mask undercut. Fig. 2
3. Lift-off technology and glass annealing The structuring of the glass is done with a lift-off process. This has the advantage of good selectivity to various substrates Žin contrast to wet chemical glass etching using HF. and does not require expensive dry etching Table 1 Comparison between evaporated glass and target material
Na 2 O content Žwt.%. K 2 O content Žwt.%. Dielectric constant Refractive index n mean value between 400 and 1100 nm a
Evaporated layer
Target materiala
3.2%b 0.6%b ; 4.8 1.45 . . . 1.50
3.5% 1.15% 4.7 1.469
Schott data sheet. AAS analysis of evaporated layer.
b
Fig. 2. SEM picture showing the sacrificial metal layer with the evaporated glass before the lift-off process.
152
S. Sassen et al.r Sensors and Actuators 83 (2000) 150–155
Fig. 5. Etch rate of the evaporated glass in BHF. A 30 min annealing process reduces the etch rate up to 25%, indicating a structural change during annealing. Fig. 3. SEM picture showing the evaporated glass layer after the lift-off process.
shows the sacrificial metal and the mask undercut as well as the evaporated glass layer on top. For the lift-off process, the sacrificial metal is removed with wet chemical etchants Žaluminum-etchant.. The result is shown in Fig. 3. After evaporation and lift-off, the glass layers are annealed for 15 min in N2rH 2 atmosphere. We varied the annealing temperature between 3008C and 6008C for different samples. After annealing, the wafer bow and the etch rate of the glass layer in buffered HF were measured. The minimum wafer bow was achieved at an annealing temperature of about 450–5008C. The etch rate decreased with increasing annealing temperature indicating a structural change during annealing. The decrease in etch rate saturates above annealing temperatures of about 5508C. These results are shown in Figs. 4 and 5. For further samples, we applied annealing at 4508C.
constants, a metal feedtrough has advantages compared to implanted feedtroughs. If the sensor has to be sealed hermetically, the metallization of the sensor is covered with a passivation material Že.g. PECVD oxide., which is planarised with standard chemical mechanical polishing ŽCMP. process. Then, in order to ensure the current transport for the anodic bonding, one has to etch contact holes into the passivation. Afterwards the glass for the anodic bonding can be evaporated and structured with the above described lift-off process. The distance of the sensing or actuating capacitor is then related to the layer thickness of the evaporated glass. Fig. 6 schematically shows the cross-section of a capacitive sensor structure with these hermetically sealed metal feedtroughs. A top view photograph of the metal structure and the glass bond frame can be seen in Fig. 7.
5. Anodic bonding process 4. Hermetical sealing of feedtroughs For most sensor applications, it is necessary to have electrical feedtroughs from the inside of the sensor to the wire bondpads. For ease of fabrication and fast RC-time
Fig. 4. Wafer bow after 30 min annealing at given temperature. The optimum annealing temperature is between 4508C and 5008C.
For the anodic bonding process, it is necessary to heat the glass layer to about 450–5008C in order to guarantee a sufficient mobility of the alkaline ions and to soften the glass to overcome the surface roughness of about 5 nm.
Fig. 6. Cross-section of a capacitive sensor fabricated with anodic bonding. The glass is evaporated onto the sensor metal structure, which is passivated and planarised using CMP. The glass is structured with a lift-off process. The metal feedtrough under the bonding frame is surrounded with isolators. There are contact holes through the isolating layers for conducting the bonding current.
S. Sassen et al.r Sensors and Actuators 83 (2000) 150–155
153
Fig. 7. Photograph of the metal feedtrough and the glass bond frame, which is patterned using lift-off process.
Then, applying a bond voltage of about 100 V results in the characteristic bonding current. This leads to a depletion of the alkaline ions close to the bond interface and allows the anodic oxidation to perform the bonding process. Fig. 8 shows a simple electrical model to derive the bonding current–voltage behaviour. The glass layer is expected to show ohmic behaviour. The variable capacitance C Ž Q . describes the depletion of the alkaline ions at the bond interface. A leakage current through the depleted layer is neglected. It is assumed that the depletion depth d is proportional to the accumulated depleted charge Q. Therefore, it follows: C Ž Q . s C0 Q0
1 Q
Ž 1.
The product of the initial capacitance C0 and the initial depleted charge Q0 is independent of the initial depletion depth d 0 : C0 Q0 s ´´ 0 A2 n Na q
Ž 2.
with the dielectric constant ´ s 4.8 for glass, the bonding area A, the amount of sodium ions per volume n Na and the electron charge q. The applied bond voltage V bond is equal to the sum of the voltage drop across the series resistance VR , which depends on the glass layer thickness, on the mobility of the alkaline ions, and the voltage drop across the depletion zone VC : V bond s VR q VC
Fig. 8. Simple electrical model for the bonding process.
Fig. 9. Measured current–voltage behaviour for a complete bonding of a 4 in. wafer compared to numerically simulated data.
Inserting Eq. Ž1. gives the following nonlinear differential equation for the depletion charge QŽ t .: V bond Ž t . s RQ˙ q
Q2 C0 Q0
Ž 4.
In general, this equation can only be solved numerically. Fig. 9 shows the numerical solution of Eq. Ž4. in comparison with a measured bonding current-voltage behaviour. The bond voltage was raised to 100 V in 30 s and then it was kept constant. The parameters are adapted for the bonding of the total area of a 4 in. wafer. It is assumed that all sodium ions take part in the current transport. The simulation fits closely to the measured data for a resistance R s 100 k V. This resistance is about 100 times higher compared to that of an anodic bond of a similar glass layer evaporated directly onto the silicon substrate. This can be explained, considering that the layer structure for hermetical sealing ŽFig. 10. employs lateral current transport for the bonding process. The remaining deviation between measurement and simulation indicates a non-ohmic behaviour of the current transport during bonding. Especially at the contact holes, the current concentration results in an accumulation of
Ž 3.
Fig. 10. Numerical simulation of the accumulated depleted charge for an anodic bonding process. The applied voltage is also shown.
154
S. Sassen et al.r Sensors and Actuators 83 (2000) 150–155
was 5 = 5 mm2 and they were spaced from each other by 300 mm.
6. Device application Using this anodic bonding technology, we fabricated an electrostatically driven resonator. The resonator structure is made out of single-crystalline silicon Žthickness 100 mm. using deep RIE etching. This structure is anodically bonded with the previously described wafer containing the hermetically embedded metal feedtroughs and evaporated glass layer. Fig. 12 shows a schematic of this structure, a photograph of a top view and a cross-section of the embedded feedtrough. Fig. 11. Photograph of a contact hole after an anodic bonding process. The concentration of the bonding current at the contact results in a visible degradation of the glass.
sodium ions with increasing bond time. Fig. 11 shows the degradation of the evaporated glass around a contact hole after a bonding process. We used an evaporated glass layer with a thickness of 1.5 mm. The size of the contact holes
7. Conclusions A lift-off process using sacrificial metal has been developed for low cost patterning of evaporated glass layers with very high selectivity to the substrate material. This technology allows the evaporation onto heated substrates in order to enhance the glass layer quality. Hermetically embedded metal feedtroughs are realized. In combination
Fig. 12. Silicon resonator structure Žthickness 100 mm. anodically bonded together with an electrical preprocessed wafer with electrodes for actuation and detection. The metal feedtroughs are hermetically sealed as it is shown in the right photograph.
S. Sassen et al.r Sensors and Actuators 83 (2000) 150–155
with the glass lift-off process, we demonstrated a bonding technology between an electrically preprocessed wafer with electrodes for electrostatic actuation and capacitive detection and a bulk-micromachined wafer with mechanical resonator structures.
References w1x Q.-Y. Tong, U. Gosele, Semiconductor Wafer Bonding, Wiley, New ¨ York, 1999. w2x W.K. Ko, Bonding Techniques for Microsensors, Micromachining and Micropackaging of Transducers, Elsevier, Amsterdam, 1985. w3x L. Ristic, Sensor Technology and Devices, Artech House, Norwood, MA, 1994. w4x S. Middelhoek, S.A. Audet, Silicon Sensors, Academic Press, London, 1989. w5x G. Klink, B. Hillerich, Wafer bonding with an adhesive coating, SPIE Conf. on Micromachined Devices and Components, Santa Clara, California, SPIE 3512 Ž1998. 50–61. w6x G. Wallis, D.I. Pomerantz, Field assisted glass-metal sealing, J. Appl Phys. 40 Ž1969. 3946–3949. w7x VTI Product Catalog, http:rrwww.vti.fi w8x P. Krause, M. Sporys, E. Obermeier, K. Lange, S. Grigull, Silicon to silicon anodic bonding using evaporated glass, Transducers ’95 1 Ž1995. 228–231. w9x S. Weichel, R. Reus, M. Lindahl, Silicon to silicon wafer bonding using evaporated glass, Sens. Actuators, A 70 Ž1998. 179–184. w10x M. Esashi, A. Nakano, S. Shoji, H. Hebiguchi, Low-temperature silicon to silicon anodic bonding with intermediate low melting point glass, Sensors and Actuators A 21–23 Ž1990. 931–934. w11x W.-B. Choi, B.-K. Ju, Y.-H. Lee, J.-W. Jeong, M.R. Haskard, N.-Y. Lee, M.-Y. Sung, M.-H. Oh, Experimental analysis on the anodic bonding with an evaporated glass layer, J. Micromech. Microeng. 7 Ž1997. 319–322.
155
w12x A. Dorst, S. Scherbaum, G. Klink, M. Feil, Anodic bonding with sputtered pyrex glass layers, in: Proc. Micro Material ’97, Berlin, April 1997, pp. 933–937. w13x H.-J. Quenzer, C. Dell, B. Wagner, Silicon–silicon anodic-bonding with intermediate glass layers using spin-on glasses, in: Proc. Micro Electro Mechanical, Systems, MEMS ’96 San Diego, 1996, pp. 272–276. w14x R. Reus, M. Lindahl, Si to Si wafer bonding using evaporated glass, in: Transducers ’97, 1997, pp. 661–664.
Biographies Stefan Sassen was born in Munich, Germany, in 1969. He received the Diploma degree in Physics in 1996 from the Technical University of Munich. He is currently working on the PhD degree in electrical engineering at the DaimlerChrysler, Research Laboratory for Microsystems Technology in Munich, Germany. His research work concentrates on the development of silicon bulk-micromachined inertial sensors Žaccelerometers and gyroscopes. for automotive applications. The aim is to develop concepts for multi-axial sensors and a cost-effective technology for precision inertial sensors. Winfried Kupke received his Diploma in Physics from the Technical University Braunschweig, Germany. He started as an industrial research scientist at Messerschmitt-Bolkow-Blohm. Currently, he is a research ¨ scientist at the DaimlerChrysler Laboratory for Microsystems Technology in Munich, Germany. Karin Bauer received her MS in Physics from the University of Washington in Seattle in 1986 and completed her German Diploma in 1988 and her PhD in 1991 at the Institute of Applied Physics of the University of Regensburg. Her research was on the structure and transport properties of disordered materials and later in theoretical and applied statistical physics. Since 1994, she works in the field of microsystems in silicon and is now mainly concerned with conceiving, modelling and evaluating microsensors and sensor assemblies and relating them to applications.
Anodic bonding of evaporated glass structured with lift-off technology for hermetical sealing S. Sassen ) , W. Kupke, K. Bauer DaimlerChrysler AG, Research and Technology FT2 r M, Microsystems Technology PO Box 800 465, D-81663 Munich, Germany Received 23 June 1999; received in revised form 7 December 1999; accepted 9 December 1999
Abstract This paper reports on an enhanced anodic bonding technology of thin e-beam evaporated glass layers Ž d F 5 mm. for micromachined silicon sensors and actuators. This MOS-compatible technology has been developed for bonding between a silicon wafer with electrical structures and a bulk micromachined silicon wafer. A bonding frame structure can be realized with hermetically sealed metal feedtroughs especially suited for capacitive sensors with a small sensing gap and fast RC-time constants. A lift-off technology for structuring the glass using metal as a sacrificial layer has been developed, because the substrates were heated to about 3008C in order to enhance the quality of the glass layer. A simple model for the current flow during the bonding process is given. The numerically calculated current–voltage behaviour is compared with measured data. An electrostatically excitated silicon resonator is realized to demonstrate the applicability of this technology. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Anodic bonding; Evaporated glass; Hermetical sealing; Metal feedtrough; Capacitive sensors
1. Introduction Bonding technology on wafer scale level is one of the key process steps for bulk micromachining w1–3x. There are four different technologies for wafer scale bonding: silicon direct, anodic, eutectic and adhesive bonding w4–6x. Anodic bonding is often chosen for wafer bonding at moderate temperatures Ž- 5008C., because this technology has been shown to be reliable and reproducible. It is especially essential for capacitive sensors to have small stray capacitances, fast RC-time constants and small capacitive gaps for sensing and actuating. There are commercially available capacitive sensors produced by bonding silicon wafers onto glass-coated wafers w7x. Recently, there have been attempts to reduce the thermal stress of the sensors using thin glass layers — sputtered, evaporated or spin-coated on silicon wafers w8–14x. In this paper we present a technology for structuring thin evaporated glass in order to produce hermetically sealed capacitive sensors with metal feedtroughs and small sens-
) Corresponding author. Tel.: q49-89-607-20284; fax: q49-89-60724001. E-mail address: [email protected] ŽS. Sassen..
ing gaps Ž0.5–5 mm.. The use of evaporated glass has the advantages of reproducible capacitive gap distances and small size of the bonding frame Ž- 300 mm..
2. Glass evaporation We start our process with oxidized single-crystalline 4Y Ž100.-wafers. As a target material for the evaporation we use the glass a8329 from Schott. This glass is especially adapted for e-beam evaporation of layers up to 5 mm thickness. Evaporating the glass at a pressure of 3 = 10y2 Pa results in a rate of about 10 nmrs. The thermal coefficient of the massive glass is 27.5 = 10y7 rK. The substrate wafers are heated to about 3008C in order to guarantee a good layer quality and sufficient adhesion. The composition of the evaporated glass was analysed with energy dispersive X-ray analysis ŽEDX analysis.. The respective EDX plot is shown in Fig. 1. The fluorine content can be traced back to our standard cleaning process after evaporation, which also includes a short dip in HF acid. The titanium content is most probably introduced from chamber or crucible material during evaporation. The exact glass layer composition was determined with atomic absorption spectroscopy ŽAAS.. For reference, we
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 0 0 . 0 0 3 0 0 - 9
S. Sassen et al.r Sensors and Actuators 83 (2000) 150–155
151
Fig. 1. EDX analysis of the evaporated glass layer. Schott glass a8329 was used as the target material.
also analysed the composition of the target material before evaporating onto the substrate wafers. The content of sodium ions is comparable to that in the target material whereas the kalium content is reduced to half the content in the target material. The measurements of the dielectric constant and the refractive index coincide well with those of the target material. These results are summarized in Table 1.
equipment Žcontamination of plasma etching equipment due to the alkaline ions in the glass.. The substrate heating during evaporation prevents the use of polymer resists as sacrificial material for the lift-off. Therefore, we use a metal sacrificial layer Že.g. sputtered aluminum. covered with PECVD-oxide. The oxide is structured with dry etching. Then, wet chemical etching is used to structure the sacrificial metal, resulting in a mask undercut. Fig. 2
3. Lift-off technology and glass annealing The structuring of the glass is done with a lift-off process. This has the advantage of good selectivity to various substrates Žin contrast to wet chemical glass etching using HF. and does not require expensive dry etching Table 1 Comparison between evaporated glass and target material
Na 2 O content Žwt.%. K 2 O content Žwt.%. Dielectric constant Refractive index n mean value between 400 and 1100 nm a
Evaporated layer
Target materiala
3.2%b 0.6%b ; 4.8 1.45 . . . 1.50
3.5% 1.15% 4.7 1.469
Schott data sheet. AAS analysis of evaporated layer.
b
Fig. 2. SEM picture showing the sacrificial metal layer with the evaporated glass before the lift-off process.
152
S. Sassen et al.r Sensors and Actuators 83 (2000) 150–155
Fig. 5. Etch rate of the evaporated glass in BHF. A 30 min annealing process reduces the etch rate up to 25%, indicating a structural change during annealing. Fig. 3. SEM picture showing the evaporated glass layer after the lift-off process.
shows the sacrificial metal and the mask undercut as well as the evaporated glass layer on top. For the lift-off process, the sacrificial metal is removed with wet chemical etchants Žaluminum-etchant.. The result is shown in Fig. 3. After evaporation and lift-off, the glass layers are annealed for 15 min in N2rH 2 atmosphere. We varied the annealing temperature between 3008C and 6008C for different samples. After annealing, the wafer bow and the etch rate of the glass layer in buffered HF were measured. The minimum wafer bow was achieved at an annealing temperature of about 450–5008C. The etch rate decreased with increasing annealing temperature indicating a structural change during annealing. The decrease in etch rate saturates above annealing temperatures of about 5508C. These results are shown in Figs. 4 and 5. For further samples, we applied annealing at 4508C.
constants, a metal feedtrough has advantages compared to implanted feedtroughs. If the sensor has to be sealed hermetically, the metallization of the sensor is covered with a passivation material Že.g. PECVD oxide., which is planarised with standard chemical mechanical polishing ŽCMP. process. Then, in order to ensure the current transport for the anodic bonding, one has to etch contact holes into the passivation. Afterwards the glass for the anodic bonding can be evaporated and structured with the above described lift-off process. The distance of the sensing or actuating capacitor is then related to the layer thickness of the evaporated glass. Fig. 6 schematically shows the cross-section of a capacitive sensor structure with these hermetically sealed metal feedtroughs. A top view photograph of the metal structure and the glass bond frame can be seen in Fig. 7.
5. Anodic bonding process 4. Hermetical sealing of feedtroughs For most sensor applications, it is necessary to have electrical feedtroughs from the inside of the sensor to the wire bondpads. For ease of fabrication and fast RC-time
Fig. 4. Wafer bow after 30 min annealing at given temperature. The optimum annealing temperature is between 4508C and 5008C.
For the anodic bonding process, it is necessary to heat the glass layer to about 450–5008C in order to guarantee a sufficient mobility of the alkaline ions and to soften the glass to overcome the surface roughness of about 5 nm.
Fig. 6. Cross-section of a capacitive sensor fabricated with anodic bonding. The glass is evaporated onto the sensor metal structure, which is passivated and planarised using CMP. The glass is structured with a lift-off process. The metal feedtrough under the bonding frame is surrounded with isolators. There are contact holes through the isolating layers for conducting the bonding current.
S. Sassen et al.r Sensors and Actuators 83 (2000) 150–155
153
Fig. 7. Photograph of the metal feedtrough and the glass bond frame, which is patterned using lift-off process.
Then, applying a bond voltage of about 100 V results in the characteristic bonding current. This leads to a depletion of the alkaline ions close to the bond interface and allows the anodic oxidation to perform the bonding process. Fig. 8 shows a simple electrical model to derive the bonding current–voltage behaviour. The glass layer is expected to show ohmic behaviour. The variable capacitance C Ž Q . describes the depletion of the alkaline ions at the bond interface. A leakage current through the depleted layer is neglected. It is assumed that the depletion depth d is proportional to the accumulated depleted charge Q. Therefore, it follows: C Ž Q . s C0 Q0
1 Q
Ž 1.
The product of the initial capacitance C0 and the initial depleted charge Q0 is independent of the initial depletion depth d 0 : C0 Q0 s ´´ 0 A2 n Na q
Ž 2.
with the dielectric constant ´ s 4.8 for glass, the bonding area A, the amount of sodium ions per volume n Na and the electron charge q. The applied bond voltage V bond is equal to the sum of the voltage drop across the series resistance VR , which depends on the glass layer thickness, on the mobility of the alkaline ions, and the voltage drop across the depletion zone VC : V bond s VR q VC
Fig. 8. Simple electrical model for the bonding process.
Fig. 9. Measured current–voltage behaviour for a complete bonding of a 4 in. wafer compared to numerically simulated data.
Inserting Eq. Ž1. gives the following nonlinear differential equation for the depletion charge QŽ t .: V bond Ž t . s RQ˙ q
Q2 C0 Q0
Ž 4.
In general, this equation can only be solved numerically. Fig. 9 shows the numerical solution of Eq. Ž4. in comparison with a measured bonding current-voltage behaviour. The bond voltage was raised to 100 V in 30 s and then it was kept constant. The parameters are adapted for the bonding of the total area of a 4 in. wafer. It is assumed that all sodium ions take part in the current transport. The simulation fits closely to the measured data for a resistance R s 100 k V. This resistance is about 100 times higher compared to that of an anodic bond of a similar glass layer evaporated directly onto the silicon substrate. This can be explained, considering that the layer structure for hermetical sealing ŽFig. 10. employs lateral current transport for the bonding process. The remaining deviation between measurement and simulation indicates a non-ohmic behaviour of the current transport during bonding. Especially at the contact holes, the current concentration results in an accumulation of
Ž 3.
Fig. 10. Numerical simulation of the accumulated depleted charge for an anodic bonding process. The applied voltage is also shown.
154
S. Sassen et al.r Sensors and Actuators 83 (2000) 150–155
was 5 = 5 mm2 and they were spaced from each other by 300 mm.
6. Device application Using this anodic bonding technology, we fabricated an electrostatically driven resonator. The resonator structure is made out of single-crystalline silicon Žthickness 100 mm. using deep RIE etching. This structure is anodically bonded with the previously described wafer containing the hermetically embedded metal feedtroughs and evaporated glass layer. Fig. 12 shows a schematic of this structure, a photograph of a top view and a cross-section of the embedded feedtrough. Fig. 11. Photograph of a contact hole after an anodic bonding process. The concentration of the bonding current at the contact results in a visible degradation of the glass.
sodium ions with increasing bond time. Fig. 11 shows the degradation of the evaporated glass around a contact hole after a bonding process. We used an evaporated glass layer with a thickness of 1.5 mm. The size of the contact holes
7. Conclusions A lift-off process using sacrificial metal has been developed for low cost patterning of evaporated glass layers with very high selectivity to the substrate material. This technology allows the evaporation onto heated substrates in order to enhance the glass layer quality. Hermetically embedded metal feedtroughs are realized. In combination
Fig. 12. Silicon resonator structure Žthickness 100 mm. anodically bonded together with an electrical preprocessed wafer with electrodes for actuation and detection. The metal feedtroughs are hermetically sealed as it is shown in the right photograph.
S. Sassen et al.r Sensors and Actuators 83 (2000) 150–155
with the glass lift-off process, we demonstrated a bonding technology between an electrically preprocessed wafer with electrodes for electrostatic actuation and capacitive detection and a bulk-micromachined wafer with mechanical resonator structures.
References w1x Q.-Y. Tong, U. Gosele, Semiconductor Wafer Bonding, Wiley, New ¨ York, 1999. w2x W.K. Ko, Bonding Techniques for Microsensors, Micromachining and Micropackaging of Transducers, Elsevier, Amsterdam, 1985. w3x L. Ristic, Sensor Technology and Devices, Artech House, Norwood, MA, 1994. w4x S. Middelhoek, S.A. Audet, Silicon Sensors, Academic Press, London, 1989. w5x G. Klink, B. Hillerich, Wafer bonding with an adhesive coating, SPIE Conf. on Micromachined Devices and Components, Santa Clara, California, SPIE 3512 Ž1998. 50–61. w6x G. Wallis, D.I. Pomerantz, Field assisted glass-metal sealing, J. Appl Phys. 40 Ž1969. 3946–3949. w7x VTI Product Catalog, http:rrwww.vti.fi w8x P. Krause, M. Sporys, E. Obermeier, K. Lange, S. Grigull, Silicon to silicon anodic bonding using evaporated glass, Transducers ’95 1 Ž1995. 228–231. w9x S. Weichel, R. Reus, M. Lindahl, Silicon to silicon wafer bonding using evaporated glass, Sens. Actuators, A 70 Ž1998. 179–184. w10x M. Esashi, A. Nakano, S. Shoji, H. Hebiguchi, Low-temperature silicon to silicon anodic bonding with intermediate low melting point glass, Sensors and Actuators A 21–23 Ž1990. 931–934. w11x W.-B. Choi, B.-K. Ju, Y.-H. Lee, J.-W. Jeong, M.R. Haskard, N.-Y. Lee, M.-Y. Sung, M.-H. Oh, Experimental analysis on the anodic bonding with an evaporated glass layer, J. Micromech. Microeng. 7 Ž1997. 319–322.
155
w12x A. Dorst, S. Scherbaum, G. Klink, M. Feil, Anodic bonding with sputtered pyrex glass layers, in: Proc. Micro Material ’97, Berlin, April 1997, pp. 933–937. w13x H.-J. Quenzer, C. Dell, B. Wagner, Silicon–silicon anodic-bonding with intermediate glass layers using spin-on glasses, in: Proc. Micro Electro Mechanical, Systems, MEMS ’96 San Diego, 1996, pp. 272–276. w14x R. Reus, M. Lindahl, Si to Si wafer bonding using evaporated glass, in: Transducers ’97, 1997, pp. 661–664.
Biographies Stefan Sassen was born in Munich, Germany, in 1969. He received the Diploma degree in Physics in 1996 from the Technical University of Munich. He is currently working on the PhD degree in electrical engineering at the DaimlerChrysler, Research Laboratory for Microsystems Technology in Munich, Germany. His research work concentrates on the development of silicon bulk-micromachined inertial sensors Žaccelerometers and gyroscopes. for automotive applications. The aim is to develop concepts for multi-axial sensors and a cost-effective technology for precision inertial sensors. Winfried Kupke received his Diploma in Physics from the Technical University Braunschweig, Germany. He started as an industrial research scientist at Messerschmitt-Bolkow-Blohm. Currently, he is a research ¨ scientist at the DaimlerChrysler Laboratory for Microsystems Technology in Munich, Germany. Karin Bauer received her MS in Physics from the University of Washington in Seattle in 1986 and completed her German Diploma in 1988 and her PhD in 1991 at the Institute of Applied Physics of the University of Regensburg. Her research was on the structure and transport properties of disordered materials and later in theoretical and applied statistical physics. Since 1994, she works in the field of microsystems in silicon and is now mainly concerned with conceiving, modelling and evaluating microsensors and sensor assemblies and relating them to applications.