doclecture 20 On-wafer sensors
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lecture 20 On-wafer sensors
Spanos & Poolla EE290H F03 In-chamber and on-wafer sensors A Paradigm Shift Lecture 20: On -Wafer Sensors 1 Spanos & Poolla EE290H F03 Overview • Exact chamber environment control is relatively new • Various sensors (pressure, gas flow, gas composition, temperature) are needed to accomplish it. • An interesting transition to “on-wafer” sensors holds much promise... Lecture 20: On -Wafer Sensors 2 1 Spanos & Poolla EE290H F03 Thermocouples • operating principle Peltier-Seebeck effect, up to 3000o C T gradient along wires of different materials develop different emf emf measures junction T platinum rhodium alloy, or silicon based sensitivity 100-200µV /oK • problems big problems with shield design radiative effects low signal -- need amplifiers or use thermopile invasive gas T measurement is very hard, especially < 10-4 torr • comments inexpensive, low drift accuracy ~+/- 5oC at low bandwidth 800oC where do you want to measure T ? Lecture 20: On -Wafer Sensors 3 Spanos & Poolla EE290H F03 Acoustic Wave sensors • operating principle – acoustic wave is transmitted through body – surface and internal waves propagate through body at T dependent speed – interference with source gives beats – beat frequency determines T • issues – implementation difficulty – invasive – calibration Lecture 20: On -Wafer Sensors 4 2 Spanos & Poolla EE290H F03 Pyrometry • operating principle – hot objects radiate – radiation is wavelength dependent – radiation model for black bodies (Planck's Law) Rλ = 37418 14388/ λT λ (e 5 − 1) λ in microns, T in oK, Rλ – for non-black bodies need to account for emissivity • issues – – – – – surface properties affect radiation multiple internal reflections emissivity is wavelength and geometry dependent can change during processing calibrations via thermocouples, difficult Lecture 20: On -Wafer Sensors 5 Spanos & Poolla EE290H F03 Pressure Sensors • direct gauges – displacement of a solid or liquid surface – capacitance manometer, McLeod pressure transducer • indirect gauges – measurement of a gas related property – momentum transfer, charge generation • huge range of available sensors – cost – sensitivity – range Lecture 20: On -Wafer Sensors 6 3 Spanos & Poolla EE290H F03 Capacitance manometer • basic idea – pressure differential causes displacement of diaphragm – sense capacitance change between diaphragm and fixed electrode – resolution 10-2 % at 2 hertz and 10-3 torr Lecture 20: On -Wafer Sensors 7 Spanos & Poolla EE290H F03 Gas flow meters • differential pressure meters • thermal mass flow meters – – – – – mass flow = K / (T1 - T2) K depends on specific heat of gas etc. must be calibrated for different gases accuracy ~ 1 sccm at flows of 40 sccm low bandwidth because of thermal inertia Lecture 20: On -Wafer Sensors 8 4 Spanos & Poolla EE290H F03 Mass Spectrometers • two types – flux analyzers : sample gas through aperture – partial pressure sensors : analysis in exhaust stack • issues – recombination in mass spec tube changes – indistinguishable species : (ex: CO, N2 and Si have same amu (28)) – pressure measurements are removed from processing chamber Lecture 20: On -Wafer Sensors 9 Spanos & Poolla EE290H F03 RGA • basic idea special kind of mass spectrometer measures gas compositions works at low vacuum < 10-5 torr ion beam is produced from gas sample by e-bombardment beam is collimated by electric fields q/m ratio of ions determines bending in B field detection of ions via a Faraday cup • issues quadrupole (magnetless design) very noisy !! good for diagnostics can withstand 500 o C can also be used at higher pressures with differential pumps mass range 50 amu, resolution 2 amu, Lecture 20: On -Wafer Sensors 10 5 Spanos & Poolla EE290H F03 How about placing sensors on the wafer??? Sensarray products Lecture 20: On -Wafer Sensors 11 Spanos & Poolla EE290H F03 Calibration is an issue... Lecture 20: On -Wafer Sensors 12 6 Spanos & Poolla EE290H F03 Long Term Reliability also an Issue... Lecture 20: On -Wafer Sensors EE290H F03 13 Spanos & Poolla On-Wafer Etch Rate by Resonant Structure IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. 11, NO. 2, MAY 1998 A Novel In Situ Monitoring Technique for Reactive Ion Etching Using a Surface Micromachined Sensor Michael D. Baker, Frances R. Williams, Student Member, IEEE, and Gary S. May, Senior Member, IEEE Lecture 20: On -Wafer Sensors 14 7 Spanos & Poolla EE290H F03 Remote reading of resonant sensor Lecture 20: On -Wafer Sensors 15 Spanos & Poolla EE290H F03 Noise is the biggest problem... On the bench... Lecture 20: On -Wafer Sensors In the chamber... When plasma is on... 16 8 Spanos & Poolla EE290H F03 But it works! (almost) Innovative noisy intrusive may contaminate... Lecture 20: On -Wafer Sensors 17 Spanos & Poolla EE290H F03 Our Vision In-situ sensor array, with integrated power and telemetry Applications: process control, calibration, diagnostics & monitoring, process design Lecture 20: On -Wafer Sensors 18 9 Spanos & Poolla EE290H F03 Issues • Sensor arrays – – – – inexpensive, modular environmentally isolated transparent to wafer handling robotics on-board power & communications • Operating mode – no equipment modifications !! – Smart “dummy” wafer for in-situ metrology Lecture 20: On -Wafer Sensors 19 Spanos & Poolla EE290H F03 Test Case: Etch Rate • Onboard etch-rate sensor for plasma etch – – – – – many sensor points on a wafer accurate film thickness measurement real-time data available etch-friendly materials wired power and communications (for now) Lecture 20: On -Wafer Sensors 20 10 Spanos & Poolla EE290H F03 Transduction Scheme - Etch Rate Van der Pauw structure: ln 2 I t = ρ π V Lecture 20: On -Wafer Sensors 21 Spanos & Poolla EE290H F03 Current Design • • • • • • • • Integrated Sensor Wafer Test Design 57 etch-rate sensors on a 4” wafer Full-wafer addressing of each sensor from a single die Redundant interconnect to enhance yield Four styles of sensor, selectable from a single die On-board current-sourcing Wired power and communications (at first) Expandable to allow wireless power and communication Lecture 20: On -Wafer Sensors 22 11 Spanos & Poolla EE290H F03 Experimental Procedure • Bond wires to wafer – solder wires to “strip header” – glue header to wafer edge – wire bond from header to wafer’s bond pads • Verify operation on bench • Place wafer in XeF2 Chamber – Measure film-thickness / etch-rate in real time – Calibrate using Nanospec thickness measurements Lecture 20: On -Wafer Sensors 24 12 Spanos & Poolla EE290H F03 Pictures Lecture 20: On -Wafer Sensors 25 Spanos & Poolla EE290H F03 Pictures Lecture 20: On -Wafer Sensors 26 13 Spanos & Poolla EE290H F03 Pictures Lecture 20: On -Wafer Sensors 27 Spanos & Poolla EE290H F03 Pictures Lecture 20: On -Wafer Sensors 28 14 Spanos & Poolla EE290H F03 Results • Individual circuit elements work perfectly • Overall circuit doesn’t work – Most likely due to flaw in decoder circuit, either due to yield problems or design flaw • Individual (disconnected) sensors still work ⇒ Wire directly to sensors Lecture 20: On -Wafer Sensors 29 Spanos & Poolla EE290H F03 Pictures Lecture 20: On -Wafer Sensors 30 15 Spanos & Poolla EE290H F03 Pictures Lecture 20: On -Wafer Sensors 31 Spanos & Poolla EE290H F03 Results • 8 sensors (in a row) wired together in series • Everything works perfectly! • In-Situ XeF2 test performed – XeF2 etch rate much too fast (~0.2µm/sec) – Sensor structure only 0.45 µm thick, gone in 2 sec – Sensors wired in series so when one etches through, all measurements stop ⇒ Data collected during etch, but no calibration available Lecture 20: On -Wafer Sensors 32 16 Spanos & Poolla EE290H F03 Data - Etch #1 Polysilicon Etch-Rate vs. Time for Experiment #1 Filtered Etch-Rate (A/sec) 500 400 300 200 100 0 0 5 10 15 Time (sec) 20 25 30 25 30 Polysilicon Thickness vs. Time for Experiment #1 Measured Thickness (A) 5000 4000 3000 2000 1000 0 0 5 10 15 Time (sec) 20 Lecture 20: On -Wafer Sensors 33 Spanos & Poolla EE290H F03 Data - Etch #2 Polysilicon Etch-Rate vs. Time for Experiment #2 Filtered Etch-Rate (A/sec) 3500 3000 2500 2000 1500 1000 500 0 0 2 4 6 Time (sec) 8 10 12 10 12 Polysilicon Thickness vs. Time for Experiment #2 Measured Thickness (A) 5000 4000 3000 2000 1000 0 0 2 4 6 8 Time (sec) Lecture 20: On -Wafer Sensors 34 17 Spanos & Poolla EE290H F03 Data - Etch #3 Polysilicon Etch-Rate vs. Time for Experiment #3 Filtered Etch-Rate (A/sec) 2000 1500 1000 500 0 0 1 2 3 4 5 Time (sec) 6 7 8 9 10 8 9 10 Polysilicon Thickness vs. Time for Experiment #3 Measured Thickness (A) 5000 4000 3000 2000 1000 0 0 1 2 3 4 5 Time (sec) 6 7 Lecture 20: On -Wafer Sensors 35 Spanos & Poolla EE290H F03 Plan • Design new sensor wafer with no onboard electronics, only sensors • Simple process ⇒ one week turnaround time instead of one year • Add several features – Polysilicon “guard ring” around sensors to reduce XeF2 etch rate by “loading” the etcher – Larger sensors to allow in-situ reflectometry – Clip-on wires to decrease time-to-experiment – Parallel connection of sensors, for better reliability Lecture 20: On -Wafer Sensors 36 18 Spanos & Poolla EE290H F03 How about completely wireless??? Lecture 20: On -Wafer Sensors 38 19 Spanos & Poolla EE290H F03 Lecture 20: On -Wafer Sensors 39 Spanos & Poolla EE290H F03 “Smart dummy” developed in 1998 • Developed and tested at the UC Berkeley Microfabrication Laboratory. • 4 sensors, wafer covered with layer of epoxy • LED used for real-time, one-way transmission Lecture 20: On -Wafer Sensors 40 20 Spanos & Poolla EE290H F03 First Test results in plasma, 1999 13.56MHz, 100W, 0.76 Torr, O2 Lecture 20: On -Wafer Sensors 41 Spanos & Poolla EE290H F03 An Update on OnWafer Sensors • OnWafer technologies Inc, a Berkeley startup, was founded in 2000. • Today OnWafer products are in use in most of the major fabs around the world, and by most the major tool makers (LAM, Applied, TEL, Nikon). Lecture 20: On -Wafer Sensors 42 21 Spanos & Poolla EE290H F03 Present OnWafer System SensorWafer Base Station IR Dongle Shipper Lecture 20: On -Wafer Sensors 43 Spanos & Poolla EE290H F03 The Approach processing equipment OnWafer feedback process control wafers to be processed Lecture 20: On -Wafer Sensors data base station 44 22 Spanos & Poolla EE290H F03 PlasmaTemp SensorWafer • • • • • • 42 sensors/wafer, 1Hz 0.5 °C accuracy Rechargeable. Functional up to 140 °C, several kW RF Suitable for oxide/poly plasma etch Non-contaminating, cleanable and reusable Lecture 20: On -Wafer Sensors Spanos & Poolla EE290H F03 Example - Process Monitoring of 200mm Poly Etching Low He main etch pre-etch 45 over etch Lecture 20: On -Wafer Sensors Increased He de-chuck 46 23 Spanos & Poolla EE290H F03 Example - Gas flow trouble in TEL DRM Etcher “before” “before”data data isis hotter, hotter, further, further, the the prepreetch etchstep stepisissignificantly significantly less less uniform… uniform… Lecture 20: On -Wafer Sensors 47 Spanos & Poolla EE290H F03 Example - Comparison between 8 PEB plates on a 193nm wafer track Worst! Best! 11 23 12 24 13 14 21 22 Data Data collection collection in in two two 10-minute 10-minute cassette-to-cassette cassette-to-cassette missions missions Lecture 20: On -Wafer Sensors 48 24 Spanos & Poolla EE290H F03 On-Wafer PEB / CD Analysis Six Six TEL TEL ACT ACT 88 plates plates used used for for 90nm 90nm CD CD lines lines (193nm (193nm Lithography) Lithography) P1 P2 P3 P4 P5 P6 Lecture 20: On -Wafer Sensors 49 Spanos & Poolla EE290H F03 Analyzing PEB Plates using BakeInfo Overshoot Steady Heating Cooling 200mm ArF 90nm 130o C 60sec Chill Lecture 20: On -Wafer Sensors 50 25
