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Post-CMOS and Post-MEMS compatible flexible skin technologies

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Karanvir Singh
Karanvir Singh

This ppt reviews flexible skin technologies that enable monolithic integration of silicon CMOS circuits and high temperature MEMS devices on flexible substrates. The monolithic integration is achieved by fabricating CMOS circuits or MEMS sensors on silicon wafers first and then forming flexible skins by post-processing. In this sense, these flexible skin technologies are termed as post-CMOS and -MEMS compatible. Most flexible devices developed using these technologies share a common structuresilicon islands connected by flexible cables.

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POST-CMOS AND POST-MEMS COMPATIBLE FLEXIBLE SKIN TECHNOLOGIES: A REVIEW Karanvir Singh PH14M003 Dept. of Electrical Engg IIT Madras Post-CMOS and Post-MEMS Compatible Flexible Skin Technologies: A Review by Yong Xu, Senior Member, IEEE IEEE SENSORS JOURNAL, VOL. 13, NO. 10, OCTOBER 2013
Outline Introduction Different flexible skin technologies Important issues of post-CMOS and post-MEMS compatible flexible skin technologies Comparison with other technologies Conclusion
Introduction MEMS devices and CMOS circuits traditionally fabricated on rigid silicon wafers. However, flexible electronics and sensors are required for a wide variety of applications. The monolithic integration is achieved by fabricating CMOS circuits or MEMS sensors on silicon wafers first and then forming flexible skins by post-processing. Most flexible devices developed using these technologies share a common structuresilicon islands connected by flexible cables.
Different flexible skin technologies Flexible Skin Technology based on Silicon Island structure Silicon Islands connected by Silicon cables Flexible Skin Technology based on frontside etching Other post-CMOS compatible Flexible Skin Technologies
Flexible Skin Technology based on Silicon Island structure Basic structure of the flexible skin is a silicon island array connected by flexible cables. Cables are usually made of a polymer like Polyimide, Parylene C. MEMS devices and CMOS circuits are fabricated on the silicon islands before the formation of the skin structure.
Flexible Skin Technology based on Silicon Island structure Simplified Process Flow: a) Deposit/Coat and pattern a polymer layer on the front side. b) Thin down and etch through the wafer from the back side to form the silicon islands. c) Deposit/Coat another polymer layer on the back side to encapsulate the silicon islands.
Flexible Shear-Stress Sensor Skins The shear-stress sensors, were fabricated on the front side of the wafer using high temperature MEMS processes such as LPCVD. After a polyimide layer was coated on the front side, the wafer was thinned down and etched from the backside by DRIE to form the silicon island structure. Another polyimide layer was spin-coated and cured to sandwich the silicon island arrays. This was developed for flow separation detection along the leading edge of Unmanned Aerial Vehicles.
Silicon Islands connected by Silicon cables The two silicon islands are connected by very thin silicon cables. Process involves deep boron diffusion, which limits its capability to use commercial CMOS foundry services. CMOS integrated neural probe One silicon island hosts CMOS circuitry and bonding pads. The other carries connection logic and penetrating probes.
Flexible Skin Technology based on frontside etching This involves only frontside processes. Simplified Process Flow: a) Boron diffusion. b) Patterning the device layer and removing the exposed BOX layer. c) Al deposition and patterning to form traces and pads. d) First 3 亮m parylene deposition.
Flexible Skin Technology based on frontside etching e) Patterning the parylene openings and etching away underneath metal traces. f) XeF2 etching to release the devices. g) Second 10 亮m parylene deposition. h) Patterning the parylene layer and releasing the device.
Other post-CMOS compatible Flexible Skin Technologies Simplified Process Flow: a) Take a SOI wafer. b) Deposition of a parylene layer on the front side of the SOI wafer. c) Bonding of the SOI wafer to a carrier wafer. d) Completely removing the handle silicon of the SOI wafer. e) Deposition of another parylene layer. f) Transfer of the wafer to another carrier substrate. g) Opening contact pads. h) De-bonding of the resulting flexible layer from the carrier wafer.
Important issues of post-CMOS and post-MEMS compatible flexible skin technologies A. Materials Single crystal silicon has been the dominant substrate for flexible skins because it is used for fabrication of CMOS and many MEMS devices and there are well developed methods of etching. SOI wafer can be used as buried oxide layer provides an intrinsic etch stop. The main materials used to make the interconnection cables are polymers, including polyimide and parylene C. Parylene films are vapor phase deposited and very conformal.
Important issues of post-CMOS and post-MEMS compatible flexible skin technologies B. Robust Interconnection Cables Interconnection cables connecting affect the flexibility and robustness of the flexible skins. Anisotropic etching is used to form silicon islands to avoid thin peripheries on these islands. Even then, the edge of the silicon island remains a stress concentration area. An innovative cushion structure has been proposed to minimize the stress concentration at the silicon island edge.
This structure is realized by XeF2 gas phase isotropic silicon etching and parylene conformal coating. SiO2 underneath the metal was removed in the flexible cable area to avoid cracking. The channel needs to be further prolonged under the silicon oxide layer to avoid a short circuit between the metal trace and the silicon.
C. Releasing methods Earlier HNA was used but it resulted in thin peripheries which crack. DRIE is a more convenient method to release silicon islands. Silicon islands can also be released from the front side by using XeF2 isotropic etching. A parylene layer can be conformally coated to encapsulate the released skin but resulting skin is thinner which leads to greater mechanical strain during bending and folding.
Comparison with other technologies Direct fabrication on flexible substrates is limited to low temperature processes. Organic FETs have excellent flexibility. However, in terms of performance, still lag far behind silicon based MOSFETs. Hybrid approaches have been developed to integrate MEMS and CMOS on flexible substrates but this process has a limitation in terms of the size/density of the bonding pads. The major advantage of post-MEMS and post-CMOS compatible flexible skin technologies is that MEMS devices and CMOS electronics can be fabricated on the silicon wafer before the formation of the skin using mainstream technologies.
Conclusion Flexible skin technologies enable monolithic integration of silicon-based CMOS and MEMS. CMOS circuits and MEMS devices can first be fabricated on silicon wafers without temperature limitation. The skin structures are fabricated after the high temperature CMOS and MEMS processes. The flexible cables can be made from silicon, polyimide and parylene C but polymer cables are more robust. Flexible skin technologies will play crucial roles in the development of advanced medical implants, wearable sensors, and intelligent textiles.
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Post-CMOS and Post-MEMS compatible flexible skin technologies

  • 1. POST-CMOS AND POST-MEMS COMPATIBLE FLEXIBLE SKIN TECHNOLOGIES: A REVIEW Karanvir Singh PH14M003 Dept. of Electrical Engg IIT Madras Post-CMOS and Post-MEMS Compatible Flexible Skin Technologies: A Review by Yong Xu, Senior Member, IEEE IEEE SENSORS JOURNAL, VOL. 13, NO. 10, OCTOBER 2013
  • 2. Outline Introduction Different flexible skin technologies Important issues of post-CMOS and post-MEMS compatible flexible skin technologies Comparison with other technologies Conclusion
  • 3. Introduction MEMS devices and CMOS circuits traditionally fabricated on rigid silicon wafers. However, flexible electronics and sensors are required for a wide variety of applications. The monolithic integration is achieved by fabricating CMOS circuits or MEMS sensors on silicon wafers first and then forming flexible skins by post-processing. Most flexible devices developed using these technologies share a common structuresilicon islands connected by flexible cables.
  • 4. Different flexible skin technologies Flexible Skin Technology based on Silicon Island structure Silicon Islands connected by Silicon cables Flexible Skin Technology based on frontside etching Other post-CMOS compatible Flexible Skin Technologies
  • 5. Flexible Skin Technology based on Silicon Island structure Basic structure of the flexible skin is a silicon island array connected by flexible cables. Cables are usually made of a polymer like Polyimide, Parylene C. MEMS devices and CMOS circuits are fabricated on the silicon islands before the formation of the skin structure.
  • 6. Flexible Skin Technology based on Silicon Island structure Simplified Process Flow: a) Deposit/Coat and pattern a polymer layer on the front side. b) Thin down and etch through the wafer from the back side to form the silicon islands. c) Deposit/Coat another polymer layer on the back side to encapsulate the silicon islands.
  • 7. Flexible Shear-Stress Sensor Skins The shear-stress sensors, were fabricated on the front side of the wafer using high temperature MEMS processes such as LPCVD. After a polyimide layer was coated on the front side, the wafer was thinned down and etched from the backside by DRIE to form the silicon island structure. Another polyimide layer was spin-coated and cured to sandwich the silicon island arrays. This was developed for flow separation detection along the leading edge of Unmanned Aerial Vehicles.
  • 8. Silicon Islands connected by Silicon cables The two silicon islands are connected by very thin silicon cables. Process involves deep boron diffusion, which limits its capability to use commercial CMOS foundry services. CMOS integrated neural probe One silicon island hosts CMOS circuitry and bonding pads. The other carries connection logic and penetrating probes.
  • 9. Flexible Skin Technology based on frontside etching This involves only frontside processes. Simplified Process Flow: a) Boron diffusion. b) Patterning the device layer and removing the exposed BOX layer. c) Al deposition and patterning to form traces and pads. d) First 3 亮m parylene deposition.
  • 10. Flexible Skin Technology based on frontside etching e) Patterning the parylene openings and etching away underneath metal traces. f) XeF2 etching to release the devices. g) Second 10 亮m parylene deposition. h) Patterning the parylene layer and releasing the device.
  • 11. Other post-CMOS compatible Flexible Skin Technologies Simplified Process Flow: a) Take a SOI wafer. b) Deposition of a parylene layer on the front side of the SOI wafer. c) Bonding of the SOI wafer to a carrier wafer. d) Completely removing the handle silicon of the SOI wafer. e) Deposition of another parylene layer. f) Transfer of the wafer to another carrier substrate. g) Opening contact pads. h) De-bonding of the resulting flexible layer from the carrier wafer.
  • 12. Important issues of post-CMOS and post-MEMS compatible flexible skin technologies A. Materials Single crystal silicon has been the dominant substrate for flexible skins because it is used for fabrication of CMOS and many MEMS devices and there are well developed methods of etching. SOI wafer can be used as buried oxide layer provides an intrinsic etch stop. The main materials used to make the interconnection cables are polymers, including polyimide and parylene C. Parylene films are vapor phase deposited and very conformal.
  • 13. Important issues of post-CMOS and post-MEMS compatible flexible skin technologies B. Robust Interconnection Cables Interconnection cables connecting affect the flexibility and robustness of the flexible skins. Anisotropic etching is used to form silicon islands to avoid thin peripheries on these islands. Even then, the edge of the silicon island remains a stress concentration area. An innovative cushion structure has been proposed to minimize the stress concentration at the silicon island edge.
  • 14. This structure is realized by XeF2 gas phase isotropic silicon etching and parylene conformal coating. SiO2 underneath the metal was removed in the flexible cable area to avoid cracking. The channel needs to be further prolonged under the silicon oxide layer to avoid a short circuit between the metal trace and the silicon.
  • 15. C. Releasing methods Earlier HNA was used but it resulted in thin peripheries which crack. DRIE is a more convenient method to release silicon islands. Silicon islands can also be released from the front side by using XeF2 isotropic etching. A parylene layer can be conformally coated to encapsulate the released skin but resulting skin is thinner which leads to greater mechanical strain during bending and folding.
  • 16. Comparison with other technologies Direct fabrication on flexible substrates is limited to low temperature processes. Organic FETs have excellent flexibility. However, in terms of performance, still lag far behind silicon based MOSFETs. Hybrid approaches have been developed to integrate MEMS and CMOS on flexible substrates but this process has a limitation in terms of the size/density of the bonding pads. The major advantage of post-MEMS and post-CMOS compatible flexible skin technologies is that MEMS devices and CMOS electronics can be fabricated on the silicon wafer before the formation of the skin using mainstream technologies.
  • 17. Conclusion Flexible skin technologies enable monolithic integration of silicon-based CMOS and MEMS. CMOS circuits and MEMS devices can first be fabricated on silicon wafers without temperature limitation. The skin structures are fabricated after the high temperature CMOS and MEMS processes. The flexible cables can be made from silicon, polyimide and parylene C but polymer cables are more robust. Flexible skin technologies will play crucial roles in the development of advanced medical implants, wearable sensors, and intelligent textiles.