Piezoresistive Sensors by Deep RIE Technology

1. Piezoresistive Sensors by Deep RIE Technology Young-Tae LeeAndong National University

2. Flow sensor “Flow sensor” is a device for sensing fluid flow • Thermal type flow sensors • Anemometer • Calorimetric flow sensors • Time of flight sensors • b. Mechanical type flow sensors • Drag force type • Pressure difference type

3. Flow sensor Gas Anemometer(heat loss) Laminar flow bypass Calorimetric flow sensors (thermotransfer) Time of flight sensors (time delay) Thermal type flow sensor

4. Drag force type Cantilever Gas Pressure difference type Piezoresistor Orifice Diaphragm Gas Pressure drop ΔP Mechanical type flow sensor

5. Flow sensors • Sensor structures • Design • Fabrication process • Results • Discussion • Conclusion

6. Cantilever Si Paddle Si SiO2 SiO2 Piezoresistor Diaphragm Orifice Si SiO2 Sensor structure

7. σ σ F F Piezoresistive effect Metal L A Silicon T F F T Piezoresistance coefficient Shear stress Normal stress

8. Cantilever 30 10 170 Piezoresistor 10 Electrode 50 30×30 120 60 60 Sensing element • Piezoresistor(Normal stress)

9. Piezoresistor Diaphragm Pad(Al) Supply voltage V Diaphragm 25×25 y 60 x 50 x’ y’ 10×10 α 50 L W (2) Single-element four-terminal(Shear stress)

10. Flow sensors • Sensor structures • Design • Fabrication process • Results • Discussion • Conclusion

11. Bonding pad (200×200) Align mark ANSYS simulation (1) Stress distribution 1000 2000 Scribe line(500) 2000 Paddle(B:500×100×7, P:300×600×7) Cantilever(800×100×7㎛3) (2) Shear stress distribution Diaphragm with orifice Diaphragm(1000×1000×7 ㎛3, 1kPa pressure)

12. Beam length 800 Paddle ⓐ ⓑ ANSYS simulation (paddle type) Beam length=500 Beam length=300 Beam length=200 Beam length=400

13. Cantilever(800×100) Paddle(300×600) Bonding pad (200×200) Align mark 200 400 1000㎛ 1000 2000 Beam size(500×100) Orifice (300㎛) Diaphragm Scribe line(500) 2000 1000㎛ Sensor size

14. Flow sensors • Sensor structures • Design • Fabrication process (dry etching with etch stop technology) • Results • Discussion • Conclusion

15. Single crystal silicon Insulator Silicon substarte What is the SOI(silicon-on-insulator) • SDB - Silicon Direct Bonding • SOS - Silicon on Sapphire • SIMOX – Separation by Implantation of Oxygen

16. Insulator (etch stop layer) Silicon dioxide(SiO2) Sapphire(Al2O3) Silicon nitride Si Etching Diaphragm Etching selectivity SiO2:Si = 1:3700 (Wet, TMAH) SiO2:Si = 1:220 (Dry, SF6) Etch stop layer : SiO2 Etch stop

17. Piezoresistor 2nd Al2O3(Dielectric isolation) 1st Al2O3(etch stop) Si High temperature pressure sensor using single SOI structure Case 1 : pressure sensor Diaphragm size: 1080×360×5 ㎛3 Piezoresistor: 60×60 ㎛2 Reference: Sensors and Actuators A, Vol.43, Issues 1-3, 1994, pp59-64.

18. SiO2(etch stop) Wet etching (TMAH) Si Dry etching(RIE) Si Glass Case 2 : accelerometer Mass size: 1380×1380×230 ㎛3 Beam size: 310×120×5 ㎛3 Reference: Proceeding of the 12th Sensor Symposium, Japan, 1994, pp223-236.

19. ip i Rp p ir n Rsub Dielectric isolation P-N Junction PN junction Resistance shift ip i p Rp 120~150℃ Insulator Rsub Temperature n SOI structure Dielectric isolation

20. High dose radiation Insulator Silicon - - - - - - - - - + + + + + + + + + Electron Hole High dose radiation environment

21. About 1.4 GPa About 0.27 GPa SiO2/Si and SiO2/SI/SiO2 structure Al2O3/Si and Al2O3/SI/Al2O3 structure Residual stress (ANSYS simulation results)

22. SiO2 150×150×3 ㎛3 SiO2 （a）Oxidation （a）Oxidation （b）RIE (b) Deep Silicon RIE 150 ㎛ SiO2(etch stop layer) 150 ㎛ Cantilever (c) Deep Silicon RIE SiO2(etch stop layer) 100㎛ 800㎛ (d) Cantilever Formation In this research : Deep RIE with etch stop technology (c) Diaphragm Formation Etching selectivity SiO2:Si = 1:220 (Dry, SF6)

23. Resist Electrode Piezoresistor Si （a) Piezoresistor formation (Boron = 1×1019 cm-3) （b) Electrode formation (Aluminum 0.6㎛) （c) Front-side RIE (about 7㎛) Electrode Piezoresistor SiO2 (etch stop layer) Diaphragm (f) Resist remove (e) Etch-stop (etching selectivity of Si and SiO2 is about 220:1) (d) Back-side RIE (about 230㎛) Fabrication process

24. B A T 55° B = A + 1.4T SiO2 Silicon (100) Advantage (1) Etching process becomes very simple the protection of already fabricated circuit part can be possible just by coating photo-resist (2) Accurate thickness control (3) Sensor can be fabricated smaller

25. Diaphragm Paddle Scribe line 2000㎛ 1000㎛ 500㎛ Cantilever Orifice Photography of the flow sensors

26. Flow sensors • Sensor structures • Design • Fabrication process • Results • Discussion • Conclusion

27. Gas Pressure gauge Sensor Control valve MFC Sensor Gas Measurement system Flow Pressure Flow Pressure

28. Paddle Piezoresistor Beam 300 Bumper 600 Front Back 800 7 Paddle size=600×300 ㎛2 Beam size=500×100 ㎛2 Drag force type (paddle) 12mm

29. Cantilever Stress concentration Bumper (stress absorption) Frame Frame Stress absorption Stress concentration Cantilever Frame Frame Bumper Bumper?

30. Gas Pressure gauge Sensor Control valve Characteristics (paddle)

31. Cantilever Pad Piezoresistor Bumper 100 Front Back 800 12mm 800×100×7 ㎛3 Drag force type (cantilever)

32. Gas Pressure gauge Sensor Control valve Characteristics (cantilever)

33. Piezoresistor Pad Front Back Orifice Diaphragm 1000 12mm Orifice size : 300 ㎛ Diaphragm Pressure difference type (diaphragm with orifice)

34. Gas Pressure gauge Sensitivity : 1.5 mV/V·kPa Nonlinearity : 0.5%F.S. Resolution : 20~30 Pa Sensor MFC Sensor Control valve Gas Characteristics (diaphragm with orifice)

35. Piezoresistor Pad Back Diaphragm 1000 12mm 1000×1000×7 ㎛3 Diaphragm Pressure difference type (diaphragm)

36. Gas Pressure gauge Sensor Control valve Sensitivity : 3.5 mV/V·kPa Nonlinearity : 0.5%F.S. Resolution : 20Pa Characteristics (diaphragm)