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Design of Capacitive Displacement Sensors for Chip Alignment

Design of Capacitive Displacement Sensors for Chip Alignment. Jose Medina Professor N. McGruer. Outline. Introduction Displacement sensors Capacitive sensors FEM simulations Readout circuit Scaled models Experiments Conclusions. Introduction. Approach Assemble Alignment Transfer.

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Design of Capacitive Displacement Sensors for Chip Alignment

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  1. Design of Capacitive Displacement Sensors for Chip Alignment Jose Medina Professor N. McGruer

  2. Outline • Introduction • Displacement sensors • Capacitive sensors • FEM simulations • Readout circuit • Scaled models • Experiments • Conclusions

  3. Introduction Approach • Assemble • Alignment • Transfer 2

  4. Introduction • Requirements • Accuracy to nm • Cost effective • Fast • Compatible • Fabrication • Electronics • Actuator (nanopositioner) • Variable gap • Connections to only one side

  5. Displacement Sensors A. A. Kuijpers, ‘Micromachined Capacitive Ling-Range Displacement Sensor for Nano-positioning of Microactuator Systems’, PhD thesis, Universiteit Twente

  6. Capacitive sensors Capacitive sensor literature • Widely used • Drug delivery, temperature/humidity sensors, automotive, positioners • No sensor moves in two dimensions ‘Modeling and Optimization of a Fast Response Capacitive Humidity Sensor’, Tetelin ‘Perspectives on MEMS in Bioengineering: A Novel Capacitive Position Microsensor’, Pedrocci

  7. Capacitive sensors Electrodes on substrate and template

  8. Capacitive sensor ~ C=Q/V=f(geometry) Chip alignment: connections only on one electrode

  9. Capacitive sensor Complete system Equivalent circuit

  10. Capacitive sensor Cantor set geometry • First level • Second level • Third level

  11. Capacitive sensor Central fractal geometry • First level • Second level • Third level

  12. FEM simulations Numerical method A. Hiekes, SIEMENS; Baxter, Capacitive Sensors

  13. FEM simulations Modeling scenarios • Closed system • Open boundary • Natural boundary condition • Trefftz domain • Infinite elements ANSYS, Inc

  14. FEM simulations

  15. FEM Simulations Models • Doped Si substrates • Glass top substrate • Glass bottom substrate

  16. Readout Circuit Converter • transforms a signal to another more convenient Voltage applied at capacitor

  17. Readout circuit Alignment precision and converter performance Circuits • Oscillator • AC-bridge • Transimpedance amplifier • Switched-capacitor • Sigma-Delta modulator

  18. Readout circuit Transimpedance amplifier Synchronous demodulator Low pass filter

  19. Readout circuit φ1 φ2 Switched-Capacitor Amplifier

  20. Readout circuit Sigma-Delta modulator Cap-to-digital converter based on SC modulator

  21. Scaled Models Scaled models • How do and C scale with geometry?

  22. Scaled Models Theoretical accuracy Scaled models

  23. Experiments Two PCBs • Large w/g ratio • Max accuracy? • Small w/g ratio • Geometry performance?

  24. Experiments Setup • Stage • PCBs • Readout circuit • AD7745 • Connectors, wires • Computer

  25. Experiments Results: large feature board • Experiments greater capacitance • Sim and experiments same profile • Experiments different results • Accuracy • 5fF (specs 4fF) • 0.1mm (calculations 2 μm) • Sim/exp results further for long displacements

  26. Experiments Results: small feature board • Cap increases with displacement! • Similar profiles • Good performance at short gap • Min largest gap 3mm (u=0.762 mm)

  27. Conclusions Simulations • DC capacitance • Ground close than at infinite • 5 electrodes + stage • Sim/exp further for large gaps • Cap to stage dominant • Variations between experiments • Plates not parallel, gap varies • Increase cap with displacement • C13 and C23 decrease • C12 dominant, board ‘perturbs’ E

  28. Conclusions Sensor design suggestion • Central fractal geometry • Width depends upon min/max gap • Min: g/w < 1/3 for last level to take over, ideally <1/10 • Max: g/w < 1 to avoid instabilities • Capacitor width w, #levels u • Chip 15x15 mm sq  sensor 0.5x0.5 mm sq

  29. Conclusions

  30. Thank you for you attention Acknowledgments • Advisor: Professor McGruer • Professors: Adams, Busnaina, Muftu, Papageorgious, Sun • Students: Prashanth, Juan Carlos Aceros, Peter Ryan, Andy Pamp, Siva, Harris Mussolis

  31. Capacitive sensors Definition Capacitance Conductor a + + + + + + + + + + + + e- + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Vs - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Conductor b

  32. FEM Simulations Convergence

  33. Switched-Capacitor amplifier φ1 φ2 • Charge-transfer phase (φ1 off, φ2 on) • Sampling phase (φ1 on, φ2 off)

  34. Correlated Double Sampling • Processing phase • Sampling phase

  35. Simulations switched-capacitors V (volts) Time (ms)

  36. Bandwidth switched-capacitors

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