Novel Microelectromechanical Systems (MEMS) for the Study of Thin Film Properties and Measurement of Temperatures During Thermal Processing Haruna Tada M.S. Thesis Defense July 21, 1999 Committee Members: Peter Wong and Ioannis Miaoulis, Tufts University Paul Zavracky, Northeastern Univ. / MicroOptical Corp.

Overview Introduction Thin film properties Heat transfer model background & motivation what are T-MEMS? Thin film properties experimental setup numerical model results Heat transfer model T-MEMS radiative properties steady state temperature distribution Evaluation temperature range & resolution proposed modifications effects of high temperature & adhesion Conclusions

Rapid Thermal Processing (RTP) RTP in Microelectronics Industry single wafer processing with radiant heat source high temperatures (up to ~1000 °C) high heating rates (100 °C/sec) short processing times (~seconds) Thermal requirement forecast for the year 2000 uniformity (± 2 °C) over 12" wafer accuracy (± 3 °C) Challenge accurate temperature measurement techniques are needed to meet the requirements

Temperature Measurements in RTP Thermocouples highly intrusive delicate & difficult to handle contact resistance between thermocouple and wafer Pyrometers non-intrusive, optical technique unknown wafer emissivity; changes with temperature and film deposition Alternative methods needed to meet thermal requirements of the microelectronics industry Thermocouple wafer (Sensarray)

MEMS Temperature Sensors Microelectromechanical Temperature Sensors (T-MEMS) small temperature sensors based on MEMS technology ex-situ measurement of maximum process temperature based on differences in thermal expansion coefficients SEM micrograph of T-MEMS Top view by optical microscope

Design & Modeling Behavior of T-MEMS depend on thin film properties Young's modulus, E(T) thermal expansion coefficient, a(T) functions of temperature Previous study of thin film properties Young's modulus of thin films resonance structures tensile testing of micromachined specimen mostly done at room temperature lack in information on thermal expansion coefficient at elevated temperatures

Approach New technique for determining thin film properties of poly-Si and SiO2 use T-MEMS as test structures to find a(T) Evaluate T-MEMS design effect on wafer temperature numerical models for radiative property and temperature distribution performance temperature range & resolution Refine T-MEMS design model beam curvature based on properties found

Study of Thin Film Properties T-MEMS design Experimental setup Numerical model Results

T-MEMS Design Bending T-MEMS array of multilayered cantilevers over Si substrate 6 mm gap by design, ~23 mm in actual sample deflect down at high temperature due to difference in thermal expansion coefficients of layers adhere to substrate at contact LPCVD SiO2 LPCVD poly-Si thermal SiO2 Si substrate 0.19 mm SiO2 0.54 mm poly-Si 1.03 mm SiO2

T-MEMS Design beams are initially curved up due to residual stress 3” wafer changing widths decreasing lengths . . . 1 2 3 . . . 14 100 mm 99 mm . 50 mm die size ~ 4 mm × 4 mm beams are initially curved up due to residual stress

Microscale Curvature Measurement lamp housing CCD camera with telescopic lens fiber optic bundle collimating lens cube beamsplitter quartz plate with Al foil reflector Al reflector thermocouple embedded in Si wafer W-halogen lamp T-MEMS sample quartz rods (support) Si wafer (support) output to computer light source

Microscale Curvature Measurement Imaging System collimated light source illuminating curved sample  only flat portion of beam is seen by the camera Curvature Measurement analyze CCD image to find "apparent length" curvature found through geometric relation between beam curvature and apparent length substrate beam image of beam on camera apparent length

Thermally Induced Curvature Numerical model developed by Townsend (1987) Discretize beam layers into small sub-layers assume no stress gradient within each sub-layer Solve for curvature: constrain interface S Force = 0 S Moment = 0 SiO2 poly-Si t z ti i n .

Curvature Equation Curvature: Thermal strain: Neutral plane: -1 for j < i bij = 0 for j = i 1 for j > i (Townsend, 1987)

Reduction of Variables Curvature at temperture T* is dependent on 4 variables: ESi,ESiO2  at T* aSi, aSiO2  variation from initial temperature to T* E and a appear as a product need to know three before finding the final property Reduction of variables parametric study to find the effect of each variable for T-MEMS, E(T) found to have little influence on K  use literature values as approximation, then find a(T) other film structures can be designed to isolate the effects of E

Piecewise-Linear Approximation of a(T) Low temperature range (< 300 °C) aSiO2 is constant  in general,a of silica glass materials do not vary significantly at temperatures below 300 °C aSi increases linearly up to 300 °C High temperature range (300 ~ 1000 °C) aSi is proportional to specific heat of Si  based on physics principle, verified for bulk crystalline Si aSiO2 increases linearly up to 1000 °C aSi (10-6 °C-1) temperature (°C) a for bulk, crystalline Si

Strategy for Low Temperature Range a25°C a300°C linear fit 1 2 3 4 5 aSi (°C-1) 0 100 200 300 temperature (°C)

Results: Curvature Measurements temperature (°C) K (mm-1) tip deflection for 100 mm beam (mm) initial upward curvature due to residual stress minimum curvature limit of system

Results: aSi(T) at Low Temperatures aSi(T) approximated to be linear up to 300 °C temperature (°C) a Si (10-6 °C-1)

Results: a(T) at High Temperatures aSi(T) assumed to be proportional to specific heat aSiO2(T) approximated as linear between 300 ~ 1000 °C temperature (°C) a (10-6 °C-1) poly-Si film bulk crystalline Si SiO2 film

Results: Numerical Fit temperature (°C) D K (mm-1)

Heat Transfer Model Thermal requirements Radiative properties of T-MEMS Steady-state heat transfer model Wafer temperature distributions

Thermal Requirements of T-MEMS Requirement of a non-intrusive temperature sensor: must not affect the heating of wafer temperature of the wafer is same w/ or w/o the sensor Requirement of an accurate temperature sensor: temperature indicated by the sensor is the same as actual wafer temperature local temperature distribution surrounding the sensor is uniform Radiative effects on T-MEMS structures may affect the temperature of the wafer  numerical model was developed to predict the effects

Radiative Effects on a Wafer Properties of silicon wafer varies dramatically with temperature partial transparency at low temperatures wafer becomes opaque at temperatures above 700 °C Thin films (< microns) thin film interference effects at wafer surface Thick films (> microns) incoherent effects; analyzed by raytracing Large 2-D surface patterns averaging by area fill factors raverage = SFiri Fi = Ai / Atotal where area fill factor is: (Abramson, 1998)

Experimental Verification Si wafer at high temperatures partial transparency increase in absorption at high temperatures Single SiO2 films at high temperatures thin film interference Simple patterns (stripes) at high temperatures average area method for 2-D patterns Multilayered film at room temperature thin film interference for multilayered film verify thickness measurement of T-MEMS films

T-MEMS Radiative Properties Five T-MEMS Regions Si partial transparency thin film interference incoherent effects Si substrate  3-films   1-film   3-films & air    1-film & air    2 1 3 4 5 Find net property of T-MEMS die by averaging

Total Radiative Properties of T-MEMS total normal absorptivity total normal emissivity temperature (°C) temperature (°C) Si substrate 3-films 3-films & air T-MEMS average 1-film 1-film & air

Steady-State Heat Transfer Model Simulates a patterned wafer heated radiatively Heat transfer terms: conduction through wafer radiation from lamp radiative heat loss from wafer steady state: Sq = 0 Parameters: heat source: 2200 °C, e = 0.3 flampwafer = 0.1; constant use a and e of wafer at 800 °C kwafer = 30 W/mK 1/4 of wafer modeled due to symmetry no convective term: assumes vacuum 3” wafer 4 mm 1 mm 1.5" (38 mm) temperature profile location thickness 0.35 mm die size 4 mm die spacing 1 mm element size 0.25 mm

Uniform Wafers a e Si 0.664 0.669 3-films 0.603 0.525 Si wafer 3-film wafer position (mm) temperature (°C) Si wafer 3-film wafer a e Si 0.664 0.669 3-films 0.603 0.525

Effect of T-MEMS on 3-Film Wafer position (mm) temperature (°C) T-MEMS wafer uniform wafer a e uniform 0.603 0.525 T-MEMS 0.623 0.580

Effect of T-MEMS on Si Wafer position (mm) temperature (°C) T-MEMS wafer uniform wafer a e uniform 0.664 0.669 T-MEMS 0.623 0.580

Effect of T-MEMS: Other Cases die spacing = 10 mm "packed" die position (mm) temperature (°C) uniform wafer 10mm spacing packed die a e uniform 0.603 0.525 T-MEMS 0.623 0.580 packed 0.619 0.599

Evaluation of T-MEMS Evaluation of original design Proposed design modification Effect of high temperature Comment on adhesion

Performance of Original Design beam length: 50 ~ 100 mm width ratios: 0.2 ~ 0.85 6 mm between Si and beam total of 714 beams on a die Theoretical temperature range 460 to over 2000 °C thermal processing rarely exceeds 1100 °C  large portion of beams will not be used Theoretical resolution varies between 0.1 °C and 9.7 °C in 900 - 1100 °C temperature range

Modified Design Compile a "Wish List" temperature range: 900 ~ 1100 °C resolution: < 0.5 °C die size: as small as possible Beam selection 50 ~ 100 mm in length 0.2 - 1.0 width ratios 6 mm gap total of 867 beams tested selected 97 beams having contact temperature between 900 °C ~ 1100 °C

Evaluation of Modified Design 0.2 ~ 1.0 width ratios 62 ~ 73 mm in length 6 mm gap depth 97 beams, fits on ~1.3 mm square area Resolution vary between 0.1 °C to 9 °C  need to fill in "gaps" in temperature

Improving Resolution Customized beam designs with specific target temperature are needed to fill in gaps in resolution Proposed design: varying bottom layer length adjusting the bottom layer length will give full control of contact temperature can be modeled by simple geometry

Effects of High Temperature Effect of long-time exposure to high temperatures (~850°C) room-temperature tip deflection decrease with time Possible reason: thermal oxide growth on top layer T-MEMS may be annealed to have zero initial curvature K (mm-1) tip deflection (mm) total time (min)

Adhesion Adhesion between bottom layer (SiO2) and substrate (Si) is a necessity for T-MEMS Preliminary testing with loose beams on Si wafer beams on plain Si wafer, heated to ~ 600 °C test adhesion strength lightly rubbed by cotton swab after cooling adhesion was confirmed under microscope adhesion stregth at room temperature is stronger than fracture strength of beams

Conclusions Thin Film Properties T-MEMS used as testing structures for finding properties developed experimental apparatus for measuring microscale curvature at very high temperatures thermal expansion coefficient of poly-Si and SiO2 found for high temperatures T-MEMS as Temperature Sensors theoretical evaluation of original design design modification to target specific temperature ranges thermally non-intrusive when used on Si wafer beam adhesion confirmed in preliminary study

Future Work: Thin Film Properties Modify beam design to target other properties Extend study to other materials SiNx (silicon nitride) on SiO2 beams Modify experimental setup view larger curvatures reduce uncertainty Verify results with alternative methods resonance method for E(T) wafer curvature measurement for the product Ea SEM micrograph of SiNx-on-SiO2 beams

Future Work: Temperature Sensors Finalize design modifications define target temperature range temperature resolution optimize die size Fabrication, testing & calibration of modified design experimental testing with thermocouples Verify adhesion using 6-mm gap Model temperature gradient during transient state

Acknowledgements Committee Members: Professors Peter Wong & Ioannis Miaoulis, Tufts Univ. Professor Paul Zavracky, Northeastern Univ. / MicroOptical Corp. Graduate Students: Seth Mann & Alexis Abramson, Tufts Univ. Patricia Nieva, Northeastern Univ. Undergraduate Researchers: Amy Kumpel, Rich Lathrop, John Slanina (REU 99 T-MEMS Group) Emilie Nelson & Melissa Bargman This work is supported by the National Science Foundation under grant number DMI-9612058

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T-MEMS Fabrication Process ~1 mm thermal SiO2, ~0.6 mm LPCVD poly-Si, ~0.2 mm LPCVD SiO2 deposited on single-sided 3” Si wafer apply photoresist (PR) to pattern top layer LPCVD low thermal SiO2 LPCVD poly-Si thermal SiO2 Si substrate photoresist etch top layer (LTO) etch bottom layer (poly-Si), remove PR

Fabrication Process (continued) grow thin thermal SiO2 layer to protect poly-Si layer during final etch LPCVD low thermal SiO2 LPCVD poly-Si thermal SiO2 Si substrate photoresist apply PR to pattern bottom layer pattern bottom layer (thermal SiO2), remove PR release structure by etching Si substrate

E(T) of Poly-Silicon From Kahn, et.al, 1998; using lateral resonance structures Varies from ~168 GPa at room temp. to ~163 GPa at 500 °C temperature (°C) E (GPa) Comparison: ~ 6 GPa higher than crystalline Si values; similar temperature-dependence

Beam Curvature Geometry q/2 R L q C A B By geometry: beam Curvature: R radius of curvature of beam L apparent length of beam from CCD image q cone angle of imaging system; found at room temperature

Reflectivity Measurement reference port sample port focusing mirror monochromator Si or PbS detector (on top) diffraction gratings 8° integrating sphere Order-sorting filters Chopper collimator fiber optics W-Hg lamp SR510 lock-in amplifier chopper controller RS-232 interface focusing mirror PC RS-232 interface

Reflectivity Measurement high temperature modification 45° aluminum ramp cooling systems detector light source sample mount cooling system heater 45° ramp sphere wall sample transmitted light reflected light incident

Spectral Reflectivity of 3-Film Region temperature (°C) rl

Spectral Reflectivity of Silicon rl 500 °C 600 °C 1000 °C temperature (°C)

Spectral Reflectivity of Stripes at 500 °C temperature (°C) rl

Radiative Effects in a Wafer Radiative effects through a wafer coherent effects: thin film interference scattering diffraction from small patterns (<microns) incoherent effects: partial transparency large patterns (>microns) thick layers (>microns) incident radiation thin films substrate coherent effects incoherent effects

Control Volume Sq = 0 qrad,in = awafer f elamp s Tlamp4 A z d qrad,in qcond,1 qcond,4 qcond,2 qcond,3 qrad,out bottom top qrad,in = awafer f elamp s Tlamp4 A qrad,out = ewafer Twafer4 A qcond,i = kwafer Ac (Ti-T) / d Sq = 0 at steady state d = 0.25 mm