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Fig. 15. Packaging design. A smart flexible interface (a) is obtained by assembling the sensor in different layers of polymeric materials and flexible connections (b). A section of the packaged sensor is shown in (c). The force was applied along the R2-R4 direction, so that, during the loading phase, R4 and R2 are stressed and com- pressed, respectively. The different behaviour of these two piezoresistors can be observed from experimental results, that also confirm that R; and R3 are not significantly influenced. The shear sensitivity of R2 was 0.0323 N-! and the coeffi- cient of determination was 0.9811, showing a high linearity of the sensor response. A flexible packaging has been designed in order to ob- tain a smart interface for biomechanical measurements. Each sensor is considered as an element of an array positioned on a flexible substrate with printed circuits. The packaging must be robust and ensure protection to the silicon structure. At the same time, the packaging should not impair sensor operation

Figure 15 Packaging design. A smart flexible interface (a) is obtained by assembling the sensor in different layers of polymeric materials and flexible connections (b). A section of the packaged sensor is shown in (c). The force was applied along the R2-R4 direction, so that, during the loading phase, R4 and R2 are stressed and com- pressed, respectively. The different behaviour of these two piezoresistors can be observed from experimental results, that also confirm that R; and R3 are not significantly influenced. The shear sensitivity of R2 was 0.0323 N-! and the coeffi- cient of determination was 0.9811, showing a high linearity of the sensor response. A flexible packaging has been designed in order to ob- tain a smart interface for biomechanical measurements. Each sensor is considered as an element of an array positioned on a flexible substrate with printed circuits. The packaging must be robust and ensure protection to the silicon structure. At the same time, the packaging should not impair sensor operation

Related Figures (14)

The design choice of using a support chip bonded to the sensing element is related to the fact that the physical in- teraction with the outer world occurs at one side, while the electrical connections must be performed at the opposite side of the sensible structure. The combination of a carrier chip and the flip-chip technology allows to have horizontal electri- Concerning the state of the art of three- axial or shear force silicon-based sensors, they have been developed mainly us- ing the piezoresistive and capacitive principles. Yao et al. [8] developed a three-axial force sensor integrating eight piezoresistors in a silicon bossed-diaphragm structure. Chu et al. [9] reported a three-axial tactile sensor based on the differential capacitive principle. Wang and Beebe [10] devel- oped a normal and shear force sensitive device by integrating four piezoresistors in a silicon square diaphragm equipped with an EPON SU-8 mesa and using them as independent strain gauges. Butefisch et al. [11] built a square boss silicon membrane with 24 embedded piezoresistors and a stylus for mechanical characterisation of micromaterials. Kane et al. [12] reported a shear-stress sensor array (64 x 64 elements capable of high-resolution imaging. The sensors are devel- oped with a fully CMOS compatible fabrication process and consist of a shuttle plate suspended over an etched pit by four beams in which polysilicon piezoresistors are integrated. Bartsch et al. [13] adapted the tactile microsensor reported by Kane et al. to a millimetre-scale silicon micromachined force sensor to measure the ground reaction microforces pro- duced by a cockroach in order to obtain useful parameters for biomimetic robot design. Jin and Mote [14] integrated 12 piezoresistors in a polysilicon cross-beam to implement a three-component microforce sensor. Jiang et al. [15] fab- ricated a shear stress sensor skin to perform 2D profiling in aerodynamics, and Xu et al. [16] recently reported improve- ments of this technology for the application in unmanned aerial vehicles. Moreover, a shear-stress type sensor has been reported [17] for the AK prosthesis application. It consists of a 3mm x 3mm x 300m square diaphragm, built by bulk micromachining, with two implanted piezoresistors and a 1.2mm x 1.2mm x 3 pm silicon dioxide protuberance.
The design choice of using a support chip bonded to the sensing element is related to the fact that the physical in- teraction with the outer world occurs at one side, while the electrical connections must be performed at the opposite side of the sensible structure. The combination of a carrier chip and the flip-chip technology allows to have horizontal electri- Concerning the state of the art of three- axial or shear force silicon-based sensors, they have been developed mainly us- ing the piezoresistive and capacitive principles. Yao et al. [8] developed a three-axial force sensor integrating eight piezoresistors in a silicon bossed-diaphragm structure. Chu et al. [9] reported a three-axial tactile sensor based on the differential capacitive principle. Wang and Beebe [10] devel- oped a normal and shear force sensitive device by integrating four piezoresistors in a silicon square diaphragm equipped with an EPON SU-8 mesa and using them as independent strain gauges. Butefisch et al. [11] built a square boss silicon membrane with 24 embedded piezoresistors and a stylus for mechanical characterisation of micromaterials. Kane et al. [12] reported a shear-stress sensor array (64 x 64 elements capable of high-resolution imaging. The sensors are devel- oped with a fully CMOS compatible fabrication process and consist of a shuttle plate suspended over an etched pit by four beams in which polysilicon piezoresistors are integrated. Bartsch et al. [13] adapted the tactile microsensor reported by Kane et al. to a millimetre-scale silicon micromachined force sensor to measure the ground reaction microforces pro- duced by a cockroach in order to obtain useful parameters for biomimetic robot design. Jin and Mote [14] integrated 12 piezoresistors in a polysilicon cross-beam to implement a three-component microforce sensor. Jiang et al. [15] fab- ricated a shear stress sensor skin to perform 2D profiling in aerodynamics, and Xu et al. [16] recently reported improve- ments of this technology for the application in unmanned aerial vehicles. Moreover, a shear-stress type sensor has been reported [17] for the AK prosthesis application. It consists of a 3mm x 3mm x 300m square diaphragm, built by bulk micromachining, with two implanted piezoresistors and a 1.2mm x 1.2mm x 3 pm silicon dioxide protuberance.
“The sensor is based on the piezoresistive effect and the fractional change in resistance is given by [22] Fig. 2. Mechanical structure of the sensor where the applied forces are indi- cated with F, for the normal force, while F, and Fy are the two components of the shear load.
“The sensor is based on the piezoresistive effect and the fractional change in resistance is given by [22] Fig. 2. Mechanical structure of the sensor where the applied forces are indi- cated with F, for the normal force, while F, and Fy are the two components of the shear load.
Fig. 3. FEA performed by means of ANSYS® 5.7 with an applied load of 0.329 MPa.
Fig. 3. FEA performed by means of ANSYS® 5.7 with an applied load of 0.329 MPa.
Fig. 4. Detail drawing of the sensor with dimensions. sensor, since the areas of maximum stress remain the same due to the sensor cross shape. De es |e, ee, Se ee. Ne ne, a Pe: eee See Sse
Fig. 4. Detail drawing of the sensor with dimensions. sensor, since the areas of maximum stress remain the same due to the sensor cross shape. De es |e, ee, Se ee. Ne ne, a Pe: eee See Sse
The piezoresistors dimensions have been chosen much smaller than the tethers dimensions so that stresses can be considered constant in the piezoresistors. A low doping con- centration is used in order to achieve a high piezoresistive coefficient. Being for p-type piezoresistors 2 = =744/2 where 44 =138 x 10—'! Pa for low doping levels [26,27]; Sr becomes Fig. 5. Schematization of the sensor main fabrication steps. (a) Device layer process: after implantation of the piezoresistors, fabrication of Al intercon- nections and Au pads electrochemical growth, 100 .m thick tethers are fab- ricated by advanced silicon etch (ASE™)., (b) Handle layer process: a silicon step of 325 ~m (1) and one of 200 ym (2) are fabricated with the double- level ASE™ process that yields the 525 jm high cylindrical mesa emerging 200 ~m from the rest of the structure. At first the device layer is processed after thermal oxi- dation of the wafer and resist is used to protect the layer of thermal oxide on the handle layer. The p-type piezoresis- tors are obtained by ion implantation of Boron with a dose of 1.5 x 10!°cm~°, energy of 40kV and a subsequent ion activation process at 900°C of 30 min in an atmosphere of
The piezoresistors dimensions have been chosen much smaller than the tethers dimensions so that stresses can be considered constant in the piezoresistors. A low doping con- centration is used in order to achieve a high piezoresistive coefficient. Being for p-type piezoresistors 2 = =744/2 where 44 =138 x 10—'! Pa for low doping levels [26,27]; Sr becomes Fig. 5. Schematization of the sensor main fabrication steps. (a) Device layer process: after implantation of the piezoresistors, fabrication of Al intercon- nections and Au pads electrochemical growth, 100 .m thick tethers are fab- ricated by advanced silicon etch (ASE™)., (b) Handle layer process: a silicon step of 325 ~m (1) and one of 200 ym (2) are fabricated with the double- level ASE™ process that yields the 525 jm high cylindrical mesa emerging 200 ~m from the rest of the structure. At first the device layer is processed after thermal oxi- dation of the wafer and resist is used to protect the layer of thermal oxide on the handle layer. The p-type piezoresis- tors are obtained by ion implantation of Boron with a dose of 1.5 x 10!°cm~°, energy of 40kV and a subsequent ion activation process at 900°C of 30 min in an atmosphere of
Fig. 6. SEM picture of structures at the wafer front-side presenting a silicon step of 100 um. The sensing and the carrier chips have been designed in order to be bonded via flip-chip. A conductive polymeric glue (H37MP, Epotek Technology, Billerica, MA, USA) is used to perform the bond between the corresponding pads of the two chips; a robotic station devoted to dispense the glue and to manipulate and to precisely position the chips has been set-up.
Fig. 6. SEM picture of structures at the wafer front-side presenting a silicon step of 100 um. The sensing and the carrier chips have been designed in order to be bonded via flip-chip. A conductive polymeric glue (H37MP, Epotek Technology, Billerica, MA, USA) is used to perform the bond between the corresponding pads of the two chips; a robotic station devoted to dispense the glue and to manipulate and to precisely position the chips has been set-up.
Fig. 8. Cross-section at one tether after the last ASE™ step.
Fig. 8. Cross-section at one tether after the last ASE™ step.
Fig. 7. SEM picture of the sensor structure.
Fig. 7. SEM picture of the sensor structure.
Fig. 10. A schematization of the characterization apparatus.
Fig. 10. A schematization of the characterization apparatus.
Fig. 9. Diagram of the signal conditioning circuit.
Fig. 9. Diagram of the signal conditioning circuit.
Fig. 14. Tangential loading response of the senso!
Fig. 14. Tangential loading response of the senso!
Fig. 16. A flexible circuit housing a 2 x 2 array of sensors.
Fig. 16. A flexible circuit housing a 2 x 2 array of sensors.
Related topics:
Materials EngineeringMechanical EngineeringIon ImplantationForce SensorSensor ArrayElectrical and Electronic Engineering
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