A Novel Method ofAnodic Bonding - Semantic Scholar
Proceedings of the 2009 4th IEEE International Conference on Nano/Micro Engineered and Molecular Systems January 5-8, 2009, Shenzhen, China
A Novel Method ofAnodic Bonding Wei Tang 1, Zhe Chen 2 and Haixia Zhang 3, Member, IEEE lWei Tang (the institute o/microelectronic, Peking University, China) Chen((the institute o/microelectronic, Peking University, China) 3Haixia Zhang((the institute 0/ microelectronic, Peking University, China) Contact Author: Haixia Zhang, [email protected] 2Zhe
Abstract This paper reports a novel method of anodic bonding with 3 intermedia layers, silicon carbide, tungsten and silicon dioxide. The bonding process lasting 10 minutes is in vacuum, with temperature 400°C, pressing force 1000N and voltage 1300V. During the process, Si+ and 0- ions react at the interface of silicon and glass wafers which create Si-O bonds and make bonding stable. After removing off the silicon substrate, a suspended membrane is fabricated. Using this method, membranes with different materials can be fabricated similarly. Compared to the traditional sacrificial layer method, this method can control the depth easily and avoid normal sticking problem.
outstanding mechanical property ensures the device operates well, and the whole process temperature is below 450°C, which make it compatible with CMOS process. First, device structure is described. Then, fabrication process and theory are presented in details. Finally, some experiments results of bonding are discussed. II.
A.
Keywords -MEMS, anodic bonding, Silicon Carbide, intermediate layer I.
INTRODUCTION
Due to the fast development of micro-nano technology, bonding becomes one of the most important processes for MEMS and NEMS fabrications, both for structure fabricating and packaging. There are two traditional bonding methods. One is melting bonding, which is carried out at high temperature to generate Si-O bonds at the interface. The other is anodic bonding, which is done in the vacuum, at a low temperature. Under high pressure and electrostatic force, Si-O bonds can be created. As capacitive pressure sensors become widely used in modem industries, automotive fabrications, aeronautic and medical applications, in order to avoid sticking problem, bonding becomes one important fabrication technology for pressure sensors instead of sacrificial layer technology. A group in Wisconsin University utilized anodic bonding to fabricate a batch-sealed capacitive pressure sensor [1], in which the batch structure on silicon substrate was doped by Boron, and then bonded to the glass wafer. After etching off the silicon substrate, which would stop at the batch structure for it was B-doped, the structure was generated. In another work made by Wen H Ko [2], new material of silicon carbide was successfully bonded to the glass to work as the membrane of a pressure sensor. Melting bonding was adopted, for some potential problems in anodic bonding, such as the distribution of some atoms and electrons, which are essential to the bonding stability. However, B-doped silicon is too stressful to be used as a good mechanical part in [1], and melting bonding is a high-temperature process, which is incompatible with the standard IC fabrication[2]. In this paper, low temperature anodic bonding is utilized to fabricate a suspended diaphragm with SiC as the material, working as the membrane of a capacitive pressure sensor. The
978-1-4244-4630-8/09/$25.00 ©2009 IEEE
FABRICATION
Device's structure
This process is developed during the fabrication of a SiC membrane capacitive pressure sensor [3], as shown in Figure 1. In order to avoid the sticking problem in traditional sacrificial layer technology, we chose anodic bonding, which is based on bulk micromachining.
Figure 1 device structure On the glass wafer, there is a layer of tungsten, serving as the bottom electrode of the pressure sensor. The movable sandwich membrane consists of three layers, which are respectively SiC/W/SiO. W is also the electrode, while SiO is the insulated layer. SiC is the main part. It is 1um-thick. Due to its outstanding mechanical property, it determines the displacement of the membrane under pressure. In this process, the whole complex membrane was suspended over the glass cavity by anodic bonding. Fabrication details are described in the following parts. B.
Fabrication process Figure 2 shows the sketch map of the membranefabricating process. First, shown in (a), on glass substrate, windows were patterned and etched by Advanced Oxide Etching (AOE) to form various diameters cavities ranging from 100um to Imm, with the depth 1-2 um.
739
L_'I
is in the vacuum. When the pressing force and the voltage are applied between two wafers, there is a strong contact occurring. At this time, in the glass, Na+ ions go towards the cathode, with 0- on the surface. While in the silicon wafer, Si+ ions move to the surface. Therefore Si+ and 0- react at the interface. As a result, strong Si-O bonds are generated, which means the bonding is stable.
-1-
---------gs-l~
....... S
heater
Figure 4 traditional anodic bonding
c::::::::Jglass
_
W -
Si
_SiC _Si02
In this work, there is a complex membrane consisting of 3 layers between Si and Glass wafers, which is distinguish to the original one. However, the principle is similar.
Figure 2 fabrication process Second, shown as (b), 1 urn-thick, low-stress«10MPa) amorphous SiC was deposited on the silicon substrate by PECVD[4]. Afterwards, 2000A-thick conductive layer W, which serves as the upper electrode, and 5000A-thick dielectric layer SiO2 as the insulated layer in capacitors were deposited on silicon substrate. The cross-section view of the complex intermediate layers is shown in Figure ~.
Figure 3 the cross-section view of the complex intermediate layers Third, shown as (c)-(d), silicon and glass wafers were bonded. During this step, wafers should be cleaned to remove particles on the surface of the wafer. After that the two wafers were aligned by the microscope and then put into a box, which would be pumped to the vacuum then. Bonding was carried out at the temperature of 380°C. There was a pressing force around 1000N, and voltage 1000V. The whole process lasted 10 minutes. After it was done, silicon substrate was removed off in KOH, leaving the SiC membrane suspended over the glass cavity.
c.
Theory analysis As we know, traditional anodic bonding is applied for the immediate bonding of glass and silicon wafers. Figure 4 shows the sketch map of this principle. The whole equipment
Figure5 theory of complex bonding Since Silicon carbide layer is deposited by PECVD here, it is amorphous and its conductivity is low as the insulator. When silicon wafer is connected with the anode, in this layer, negative ions go to the interface close to silicon substrate, while positive ones go to the other. In the SiO layer, 0- ions gather at the interface close to tungsten due to positive ions in SiC layer, and Si+ ions move towards the other side. On the glass wafer, with the high negative voltage applied, 0- ions move to the surface. Thus, 0- ions react with the Si+ ions generating Si-O bonds. After about 10 minutes, bonding is done. That is the main theory of this bonding. D. Experiment results According to this principle, we did a series of bonding experiments, shown as Figure 6. The glass is connected to the cathode, and silicon to the anode, with different materials as the intermedia layer. In group No.1 (Figure 6(1)), intermedia layer was tungsten and Si02 on the silicon wafer. Group No.2 (Figure 6(2)) added a layer of SiC between silicon substrate and W. The two groups were successfully bonded. In other cases, Si02 or tungsten was deposited on the glass, as shown in No.3 (Figure 6(3)) or 4 (Figure 6(4)), compared with No.1. But they all failed. That's because there was no Si-O bond generated at the interface. This confirmed our theory above in another aspect. Furthermore, when we reduced the thickness and the area of the tungsten on glass (Figure 6(5,6)), the result turned successful. Bonding results are in table 1.
740
Testing experiments of the bonding intensity were done as shown in Figure 8.
(I)
(3)
Figure 8 bonding intensity experiment
(5)(4)
The bonded wafers were attached to the two parts. With the huge pulling force, wafers were separated from the parts, without any influence on the bonding surface. It implied that bonding intensity was better than the adhesion of the glueo
~4000AW
(6)
E.
~OOOAW CJglas5
-
-SiC
W -
Si
-SiOz
Figure 6 bonding experiments
Group no. 1 2 3 4 5 6
T able 1 b ond'Ing expenmen s results cathode anode Result Glass Glass Glass/SiO GlassIW GlassIW(4000A) (partly covered) GlassIW(2000A) (partly covered)
Si/W/SiO Si/SiC/WISiO Si/SiC/W Si/SiC/WISiO Si/SiC/WISiO Si/SiC/W/SiO
Discussions
Generally, at the interface of bonding area, Si02 supplies Si+ ions with Si substrate connected to the anode, and Glass supplies 0- ions with glass substrate connected to the cathode. Under huge pressure and 380°C, Si-O bonds generate. After removing off the silicon substrate, the suspended membrane could be obtained, no matter whether it is conductive or not. Thus, this method can be used widely in the MEMS fabrication, due to its low temperature and simple process. Figure 9 presents the SEM picture of our SiC capacitive pressure sensor. Moreover, it can be applied not only for silicon and glass substrate, but also for silicon and silicon, glass and glass, for example, the fabrication of SOl wafers is using this bonding way.
success success failure failure Failure partly success
The results imply that when 0- ions and Si+ ions react at the interface, bonding will be done. In addition, the roughness should be less than 4000A. The final top-view of the device is shown in Figure 7. As we can find out that around the membrane, bonding is completed well. The SEM photo of cross-section of the membrane is in Fi re 3.
(a) cross-section view
(b) top view
Figure 9 pictures of a SiC capacitive pressure sensor Now silicon-silicon anodic bonding is carrying out in our group, with IC circuits on one silicon wafer and MEMS capacitive pressure sensors on another silicon wafer. It also overcomes the sealing problem when it comes to the connection with the electrode in the vacuum, and it is more compatible with the standard IC fabrication.
F.
Conclusions
A novel method of anodic bonding with 3 intermedia layers, silicon carbide, tungsten and silicon dioxide is presented in this paper. The bonding process lasts 10 minutes is in vacuum, with
Figure 7 top view of the device
741
temperature 400°C, pressing force IOOON and voltage 1300V. During the process, Si+ and 0- ions react at the interface of silicon and glass wafers which create Si-O bonds and make bonding stable. After removing off the silicon substrate, a suspended membrane is fabricated. Using this method, membranes with different materials can be fabricated similarly. Compared to the traditional sacrificial layer method, this method can control the depth easily and avoid normal sticking problem.
III.
ACKNOWLEDGMENT
Thanks the institute of microelectronics in Peking University. Thanks the professor Zhang Guobing. Thanks for National Pre-Research Funding support, No. 9140C790108070C7903 and 9140A08080206JW0201. REFERENCES [1] [2] [3] [4] [5] [6] [7]
J.-S. Park, "A Low Cost Batch-Sealed Capacitive Pressure Sensor" 1999 IEEE. Jiangang Du, "Poly-SiC Capacitive Pressure Sensor Made by Wafer Bonding"2005 IEEE. Wei Tang, Zhe Chen, Dayu Tian and Haixia Zhang "Fabrication of SiC MEMS Pressure Sensor by Anodic Bonding", proceedings of MicroNano08 Zhe Chen, Dayu Tian, Guobing Zhang, Haixia Zhang. "Investigation of PECVD SiC nano film" 2007 7th IEEE International Conference on Nanotechnology, Hong Kong, Sep,2007. A Berthold, "Glass to Glass Anodic Bonding with Standard IC Technology Thin Films as Intermediate Layers," Sensors and Actuators 82(2000) 224-228. Qingan Huang «Silicon micromachining technology» Science press, 1996 M Esashi, "Anodic Bonding for Integrated Capacitive Sensor", Micro Electro Mechanical System'92
742
A Novel Method ofAnodic Bonding Wei Tang 1, Zhe Chen 2 and Haixia Zhang 3, Member, IEEE lWei Tang (the institute o/microelectronic, Peking University, China) Chen((the institute o/microelectronic, Peking University, China) 3Haixia Zhang((the institute 0/ microelectronic, Peking University, China) Contact Author: Haixia Zhang, [email protected] 2Zhe
Abstract This paper reports a novel method of anodic bonding with 3 intermedia layers, silicon carbide, tungsten and silicon dioxide. The bonding process lasting 10 minutes is in vacuum, with temperature 400°C, pressing force 1000N and voltage 1300V. During the process, Si+ and 0- ions react at the interface of silicon and glass wafers which create Si-O bonds and make bonding stable. After removing off the silicon substrate, a suspended membrane is fabricated. Using this method, membranes with different materials can be fabricated similarly. Compared to the traditional sacrificial layer method, this method can control the depth easily and avoid normal sticking problem.
outstanding mechanical property ensures the device operates well, and the whole process temperature is below 450°C, which make it compatible with CMOS process. First, device structure is described. Then, fabrication process and theory are presented in details. Finally, some experiments results of bonding are discussed. II.
A.
Keywords -MEMS, anodic bonding, Silicon Carbide, intermediate layer I.
INTRODUCTION
Due to the fast development of micro-nano technology, bonding becomes one of the most important processes for MEMS and NEMS fabrications, both for structure fabricating and packaging. There are two traditional bonding methods. One is melting bonding, which is carried out at high temperature to generate Si-O bonds at the interface. The other is anodic bonding, which is done in the vacuum, at a low temperature. Under high pressure and electrostatic force, Si-O bonds can be created. As capacitive pressure sensors become widely used in modem industries, automotive fabrications, aeronautic and medical applications, in order to avoid sticking problem, bonding becomes one important fabrication technology for pressure sensors instead of sacrificial layer technology. A group in Wisconsin University utilized anodic bonding to fabricate a batch-sealed capacitive pressure sensor [1], in which the batch structure on silicon substrate was doped by Boron, and then bonded to the glass wafer. After etching off the silicon substrate, which would stop at the batch structure for it was B-doped, the structure was generated. In another work made by Wen H Ko [2], new material of silicon carbide was successfully bonded to the glass to work as the membrane of a pressure sensor. Melting bonding was adopted, for some potential problems in anodic bonding, such as the distribution of some atoms and electrons, which are essential to the bonding stability. However, B-doped silicon is too stressful to be used as a good mechanical part in [1], and melting bonding is a high-temperature process, which is incompatible with the standard IC fabrication[2]. In this paper, low temperature anodic bonding is utilized to fabricate a suspended diaphragm with SiC as the material, working as the membrane of a capacitive pressure sensor. The
978-1-4244-4630-8/09/$25.00 ©2009 IEEE
FABRICATION
Device's structure
This process is developed during the fabrication of a SiC membrane capacitive pressure sensor [3], as shown in Figure 1. In order to avoid the sticking problem in traditional sacrificial layer technology, we chose anodic bonding, which is based on bulk micromachining.
Figure 1 device structure On the glass wafer, there is a layer of tungsten, serving as the bottom electrode of the pressure sensor. The movable sandwich membrane consists of three layers, which are respectively SiC/W/SiO. W is also the electrode, while SiO is the insulated layer. SiC is the main part. It is 1um-thick. Due to its outstanding mechanical property, it determines the displacement of the membrane under pressure. In this process, the whole complex membrane was suspended over the glass cavity by anodic bonding. Fabrication details are described in the following parts. B.
Fabrication process Figure 2 shows the sketch map of the membranefabricating process. First, shown in (a), on glass substrate, windows were patterned and etched by Advanced Oxide Etching (AOE) to form various diameters cavities ranging from 100um to Imm, with the depth 1-2 um.
739
L_'I
is in the vacuum. When the pressing force and the voltage are applied between two wafers, there is a strong contact occurring. At this time, in the glass, Na+ ions go towards the cathode, with 0- on the surface. While in the silicon wafer, Si+ ions move to the surface. Therefore Si+ and 0- react at the interface. As a result, strong Si-O bonds are generated, which means the bonding is stable.
-1-
---------gs-l~
....... S
heater
Figure 4 traditional anodic bonding
c::::::::Jglass
_
W -
Si
_SiC _Si02
In this work, there is a complex membrane consisting of 3 layers between Si and Glass wafers, which is distinguish to the original one. However, the principle is similar.
Figure 2 fabrication process Second, shown as (b), 1 urn-thick, low-stress«10MPa) amorphous SiC was deposited on the silicon substrate by PECVD[4]. Afterwards, 2000A-thick conductive layer W, which serves as the upper electrode, and 5000A-thick dielectric layer SiO2 as the insulated layer in capacitors were deposited on silicon substrate. The cross-section view of the complex intermediate layers is shown in Figure ~.
Figure 3 the cross-section view of the complex intermediate layers Third, shown as (c)-(d), silicon and glass wafers were bonded. During this step, wafers should be cleaned to remove particles on the surface of the wafer. After that the two wafers were aligned by the microscope and then put into a box, which would be pumped to the vacuum then. Bonding was carried out at the temperature of 380°C. There was a pressing force around 1000N, and voltage 1000V. The whole process lasted 10 minutes. After it was done, silicon substrate was removed off in KOH, leaving the SiC membrane suspended over the glass cavity.
c.
Theory analysis As we know, traditional anodic bonding is applied for the immediate bonding of glass and silicon wafers. Figure 4 shows the sketch map of this principle. The whole equipment
Figure5 theory of complex bonding Since Silicon carbide layer is deposited by PECVD here, it is amorphous and its conductivity is low as the insulator. When silicon wafer is connected with the anode, in this layer, negative ions go to the interface close to silicon substrate, while positive ones go to the other. In the SiO layer, 0- ions gather at the interface close to tungsten due to positive ions in SiC layer, and Si+ ions move towards the other side. On the glass wafer, with the high negative voltage applied, 0- ions move to the surface. Thus, 0- ions react with the Si+ ions generating Si-O bonds. After about 10 minutes, bonding is done. That is the main theory of this bonding. D. Experiment results According to this principle, we did a series of bonding experiments, shown as Figure 6. The glass is connected to the cathode, and silicon to the anode, with different materials as the intermedia layer. In group No.1 (Figure 6(1)), intermedia layer was tungsten and Si02 on the silicon wafer. Group No.2 (Figure 6(2)) added a layer of SiC between silicon substrate and W. The two groups were successfully bonded. In other cases, Si02 or tungsten was deposited on the glass, as shown in No.3 (Figure 6(3)) or 4 (Figure 6(4)), compared with No.1. But they all failed. That's because there was no Si-O bond generated at the interface. This confirmed our theory above in another aspect. Furthermore, when we reduced the thickness and the area of the tungsten on glass (Figure 6(5,6)), the result turned successful. Bonding results are in table 1.
740
Testing experiments of the bonding intensity were done as shown in Figure 8.
(I)
(3)
Figure 8 bonding intensity experiment
(5)(4)
The bonded wafers were attached to the two parts. With the huge pulling force, wafers were separated from the parts, without any influence on the bonding surface. It implied that bonding intensity was better than the adhesion of the glueo
~4000AW
(6)
E.
~OOOAW CJglas5
-
-SiC
W -
Si
-SiOz
Figure 6 bonding experiments
Group no. 1 2 3 4 5 6
T able 1 b ond'Ing expenmen s results cathode anode Result Glass Glass Glass/SiO GlassIW GlassIW(4000A) (partly covered) GlassIW(2000A) (partly covered)
Si/W/SiO Si/SiC/WISiO Si/SiC/W Si/SiC/WISiO Si/SiC/WISiO Si/SiC/W/SiO
Discussions
Generally, at the interface of bonding area, Si02 supplies Si+ ions with Si substrate connected to the anode, and Glass supplies 0- ions with glass substrate connected to the cathode. Under huge pressure and 380°C, Si-O bonds generate. After removing off the silicon substrate, the suspended membrane could be obtained, no matter whether it is conductive or not. Thus, this method can be used widely in the MEMS fabrication, due to its low temperature and simple process. Figure 9 presents the SEM picture of our SiC capacitive pressure sensor. Moreover, it can be applied not only for silicon and glass substrate, but also for silicon and silicon, glass and glass, for example, the fabrication of SOl wafers is using this bonding way.
success success failure failure Failure partly success
The results imply that when 0- ions and Si+ ions react at the interface, bonding will be done. In addition, the roughness should be less than 4000A. The final top-view of the device is shown in Figure 7. As we can find out that around the membrane, bonding is completed well. The SEM photo of cross-section of the membrane is in Fi re 3.
(a) cross-section view
(b) top view
Figure 9 pictures of a SiC capacitive pressure sensor Now silicon-silicon anodic bonding is carrying out in our group, with IC circuits on one silicon wafer and MEMS capacitive pressure sensors on another silicon wafer. It also overcomes the sealing problem when it comes to the connection with the electrode in the vacuum, and it is more compatible with the standard IC fabrication.
F.
Conclusions
A novel method of anodic bonding with 3 intermedia layers, silicon carbide, tungsten and silicon dioxide is presented in this paper. The bonding process lasts 10 minutes is in vacuum, with
Figure 7 top view of the device
741
temperature 400°C, pressing force IOOON and voltage 1300V. During the process, Si+ and 0- ions react at the interface of silicon and glass wafers which create Si-O bonds and make bonding stable. After removing off the silicon substrate, a suspended membrane is fabricated. Using this method, membranes with different materials can be fabricated similarly. Compared to the traditional sacrificial layer method, this method can control the depth easily and avoid normal sticking problem.
III.
ACKNOWLEDGMENT
Thanks the institute of microelectronics in Peking University. Thanks the professor Zhang Guobing. Thanks for National Pre-Research Funding support, No. 9140C790108070C7903 and 9140A08080206JW0201. REFERENCES [1] [2] [3] [4] [5] [6] [7]
J.-S. Park, "A Low Cost Batch-Sealed Capacitive Pressure Sensor" 1999 IEEE. Jiangang Du, "Poly-SiC Capacitive Pressure Sensor Made by Wafer Bonding"2005 IEEE. Wei Tang, Zhe Chen, Dayu Tian and Haixia Zhang "Fabrication of SiC MEMS Pressure Sensor by Anodic Bonding", proceedings of MicroNano08 Zhe Chen, Dayu Tian, Guobing Zhang, Haixia Zhang. "Investigation of PECVD SiC nano film" 2007 7th IEEE International Conference on Nanotechnology, Hong Kong, Sep,2007. A Berthold, "Glass to Glass Anodic Bonding with Standard IC Technology Thin Films as Intermediate Layers," Sensors and Actuators 82(2000) 224-228. Qingan Huang «Silicon micromachining technology» Science press, 1996 M Esashi, "Anodic Bonding for Integrated Capacitive Sensor", Micro Electro Mechanical System'92
742
