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Designing a Robust Four-Point and Hot-Point Probe

Designing a Robust Four-Point and Hot-Point Probe. Shawn Adderly and Arica Inglis ECE 445 Group 5 December 2, 2010. Introduction. ECE 444 Fabrication Lab Four-Point Probe Measures sheet resistance and bulk resistivity for use in silicon wafer testing

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Designing a Robust Four-Point and Hot-Point Probe

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  1. Designing a Robust Four-Point and Hot-Point Probe Shawn Adderly and Arica Inglis ECE 445 Group 5 December 2, 2010

  2. Introduction • ECE 444 Fabrication Lab • Four-Point Probe • Measures sheet resistance and bulk resistivity for use in silicon wafer testing • Needs current source and precision voltmeter to operate • Hot Point Probe • Measures the type of doping in the wafer • N-type - majority carriers are electrons • P-type - majority carriers are holes • Needs over 100 heating mechanism and voltmeter to operate

  3. Objective • Replace 4PP/HPP in educational lab (ECE 444) • Reduce cost • Ease of accessibility • Precision

  4. Overview: Block Diagrams Power Supply Mechanical Component Electrical Component

  5. Individual Roles • Shawn: • Fixture • Power Supply • PIC, LCD, and Keypad • Arica: • Voltmeter • Current Sources • Digital Logic • Probe Selection

  6. Power Supply • Primary Winding: 110-115 VAC (RMS) • Secondary Winding: 25 VAC (RMS) • Output: +21.2 VDC, - 21.2VDC, and +5 VDC • Actual: +21.3 VDC, -21.3 VDC

  7. Power Supply

  8. PIC/LCD/Keypad/Digital Logic • Protoboard • Main Processing Unit

  9. PIC Programming Overview Functions • PIC 16F877A • Six functions • 875 lines of code • Powered: + 5 VDC • Clock: 20 MHz crystal Oscillator Main Welcome Keypad Input Current Output Hot Point Four Point

  10. Pseudo code Welcome(){ • Asks user to input wafer size • Calls keypad input } Current_Output() { • Outputs a “1” to the selection lines of the mux • Calls Four_point() } Four_point(){ • Voltage is measured using internal ADC (10-bit) sampled every 20 microseconds • Sheet Resistivity is calculated }

  11. Correction Factors • The thickness of the wafer is between 10-20 mils • a = 0.18 • k = 0.516 • s = 0.1016 cm • t = 0.0508 cm Resistivity (Rho) and Resistance (Rs) Formula: • Rho = (2*pi*k*a*s*(V/I)) Ω-cm • Rs = (2*pi*k*a*s*(V/I)/(t) Ω/sq

  12. PIC Challenges • Recursion and Floating • The PIC has no data stack • Averaging over a large amount of readings would provide a more accurate measurement for (j=0;j<20;j++) { set_adc_channel(0); delay_us(40); voltage=read_adc(); V = voltage * (5.0/1023.0); printf(lcd_putc,"\fV: %0.5lf V\n",V); delay_ms(1000); }

  13. Keypad Button 13 -> asterisk (*) Used for logic on Reset button

  14. Logic for Reset Reset Logic Pin 40 (T4) Pin 36 (T5)

  15. Current Source PCB • Large Board • Many components

  16. Current Source • Four programmableIC current source chips • Constraints on resistor values: • Datasheet specified Rset needed to be minimum of 20kΩ • Testing showed that for stable current output, Vset= Vin-1.5V • Worst-case Vin= 19V

  17. Current Source Resistance Calculated Resistor Values Using equations below: For Vin=19V, when Rset = 1.75MΩ

  18. Current Source Logic/Schematic • Demultiplexer Truth Table • Utilize relays from demux to turn on each current source respectively

  19. Current Source (Relay/PIC) DEMUX to PIC A0->C0 A1->C1 A2->C2 E3->C3

  20. Current Source Testing • Measured voltage across pre-determined resistance Results:

  21. Voltmeter Board • BNCs from 4pp to current source and voltmeter

  22. Voltmeter Where R1=R2=R3=R4

  23. Voltmeter Testing Initial Testing with R=10.2kΩ, 1% Revised Testing, smaller differences, and R=2.7kΩ, 0.1% Revised Testing, small initial voltages with R=2.7kΩ, 0.1%

  24. Fixture Fixture Overview Fixture • The probe can be positioned both the X, Y, Z direction by four linear slide rails • Fixture can accommodate wafers of size 4,6,8 inches • Fabricated by the ECE Machine Shop • Rendered by Sue Feng

  25. Four Point Probe Testing • Tested various wafers on the same pads • Compared measurements against the probe in the 444 lab

  26. Four Point Probe Results 444 Lab Probe Design Probe

  27. Hot Point Probe • Fixture completed with tungsten • Tested with soldering iron • Results inconclusive

  28. Major Part Cost Our Design Commercial Product (Estimation) $2850 for 4PP $2000 for HPP Total: $4,850 • Three ball-bearing linear slides: $1500 • Fourth ball-bearing linear slide: $189 • PIC16F877A: $5.44 • Keypad: $14.34 • 4PP Head: $622 • HPP Head: $60 • TL082 (OP-AMP): $1.99 • Relays: $11.00x4 • Total: $2436.77

  29. Successes • Precision voltmeter • Current source • LCD/PIC/Keypad • 4PP sheet resistance measurements

  30. Challenges • LCD stopped operating several times • Current source delay • Memory (ROM) • Fluke voltmeter became un-calibrated • Soldering issues • Hot Point Probe heating mechanism

  31. Working Relay Spacing Redefining Copper Traces Testing

  32. Recommendations • Use a higher end microcontroller unit such as the MSP430 Series • Redesign the hot probe holder and material • RS232/USB interface to computer • Move from copper PCB Boards to professional ones

  33. Thank you • Mary DeRoo and ECE Staff • Dane Seivers, 444 Lab Engineer • Scott McDonald and Dave Switzer, Machine Shop • Mark Smart and Dan Mast, Parts Shop • Sue Feng, UIUC Industrial Design Student • Professor Sauer • Professor Krein • Dean Umberto Ravioli

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