incoming radiation reflected radiation thin film substrate coherent effects incoherent effects Project Description • Heat transfer in rapid thermal processing modeled • microscale radiative effects on patterned wafers • Need for innovative temperature measurement • develop Microelectromechanical Systems (MEMS) for high temperature processing
MEMS Development • MEMS temperature sensors (T-MEMS) • temperature measurement in rapid thermal processing • current techniques (thermocouples of pyrometry) inadequate in rapid thermal processing • Thermophysical Properties of Thin Films • Young's modulus (E), thermal expansion coef. (a), fracture strength (sF) varies w/ temperature (~1000°C) • thin film properties not well known or understood • innovative techniques needed for high-temperature measurements
Recent Advances • Thermophysical Properties of Thin Films • high temperature properties (up to ~1000°C) • temperature dependence • effects of fabrication process parameters • fracture mechanics • interaction between different materials • Experimental Methods • new techniques to measure the thermophysical properties of thin films at high temperatures • "bending structures" • "breaking structures"
Experimental Apparatus Rapid radiant heater with imaging system for the visualization of T-MEMS as they heat to ~1000°C CCD camera with telescopic lens collimated light source for measuring curvature of bending T-MEMS reflectors to enhance radiative heating single W-halogen lamp as heat source
Schematic of Apparatus CCD camera collimated light source pellicle beam splitter quartz plate Al reflector thermocouple Si wafer quartz rod W-halogen lamp T-MEMS sample lamp housing
SiNx bridge suspended over Si substrate. Bridge breaks at high temperature due to tensile stress in film (aSiNx< aSi) 2L ~ 100mm 1082°C 1148°C 1124°C 1100°C 1106°C 1172°C 1190°C 1208°C SiNx smallest device: highest temperature 620°C 748°C Si substrate largest device: lowest temperature increasing length, L increasing temperature Design: "Breaking" T-MEMS
Bilayered cantilevers of SiNx or poly-Si on SiO2 suspended on Si substrate. Beams bend down at high temperature due to low aSiO2. At certain temperature, beam will adhere to substrate. SiNx or poly-Si SiO2 L Si substrate 750°C 1100°C increasing length L=100mm L=50mm increasing temperature Design: "Bending" T-MEMS
Breaking T-MEMS • Breaking temperature is a function of: • E and a of film & substrate • sF of film • film geometry SEM micrograph of SiNx breaking T-MEMS
Bending T-MEMS • Beam curvature is a function of: • a of poly-Si (or SiNx) and SiO2 films • beam geometry • E of film SEM micrograph of Si-on-SiO2 bending T-MEMS. Note the initial upward curvature caused by residual stress.
(film shown in yellow) Best Breaking T-MEMS: Innovations • Fabrication process was refined to achieve minimum damage to SiNx from residual stress (deposition temperature of 840°C) • Of the four geometries tested (shown below), the best-behaving geometry was identified • Method for finding a and sF was outlined: • fabricate structures on 2 substrates (silicon & sapphire) • compare results from 2 cases to find a and sF
Bending T-MEMS: Innovations • Simple technique for in-situ measurement of beam curvature • Custom-designed beams for isolation of material properties in simple experiments • Si-on-SiO2 beams sensitive only to variations in ESi • numerically find ESi (T) by assuming linear variation w/ temperature and "guessing" the initial values Bending T-MEMS under an optical microscope
Results from Breaking T-MEMS SEM micrograph of breaking structures taken at room temperature. Larger structures broke during fabrication due to residual stresses in film. one thermometer increasing length
3000 2000 1000 0 curvature (m-1) -1000 -2000 -3000 -4000 -5000 0 100 200 300 400 500 600 700 800 900 1000 temperature (°C) Results from Bending T-MEMS Curvature of Si-on-SiO2 beams as they heat to 1000°C (compilation of data from 2 heating/cooling cycles)
900 Geometry of the Breaking T-MEMS tested, seen under an optical microscope. 800 700 600 500 Temperature (°C) 400 300 1st set 200 2nd set 100 0 120 140 160 180 200 Breaking T-MEMS length (mm) Results from Breaking T-MEMS Temperature at which T-MEMS broke during heating from room temperature to 1000°C (compilation of data from 2 thermometers; geometry shown below)
Results from Bending T-MEMS • good fit for a0 between 1.0 and 1.5×10-6 K-1 • a varies from 1 to ~12×10-6 K-1between 25 and 1000ºC
Conclusions from Breaking T-MEMS • Data shows large scatter • Possible sources of instability: • structure behavior depends on fabrication process • non-uniformities during fabrication • film geometry affects the behavior of breaking T-MEMS • data from some shapes showed less scatter than others (data shown is for the most consistent) • fundamental effects at small length scales • fracture of thin films is a statistical process
Conclusions from Bending T-MEMS • Thermal expansion coefficient of Si have large variation with temperature • the values found are much greater than corresponding values for bulk crystalline Si • uncertainty of method must be improved to verify results • Repeatability of curvature indicates that there was no relaxation of residual stress
Next Steps for Breaking T-MEMS • Study geometries, fabrication processes, and nanoscale effects to understand sources of inaccuracies: • why do different geometries behave differently? • what is causing the scatter in data? • how can film fracture be made more consistent? • Fabricate T-MEMS on sapphire (Al2O3) substrate • use the results from two substrates to find a and sfracture of SiNx film • Redesign T-MEMS for improved accuracy
Next Steps for Bending T-MEMS • Improve uncertainty of experimental technique • Design beams which are responsive to other properties to be studied • Study high-temperature bonding phenomena between SiO2 and Si • adapt studies done on direct wafer bondingto adhesion on the microscale level • Use alternative methods to find E(T) • measure resonance frequency of cantilevers • wafer curvature technique
Publications • A. R. Abramson, I. N. Miaoulis, P. Y. Wong, P. Nieva, and P. Zavracky, "Partial Transparency Effects of Silicon During Rapid Thermal Processing," Materials Research Society Symposium Proceeding, Vol.525, pp.15-20, 1998. • H. Tada, A. R. Abramson, I. N. Miaoulis, P. Y. Wong, P. Nieva, and P. Zavracky, "MEMS as Temperature Sensors During High Temperature Processing," presented and to be published in Proceedings of the Symposium on Microelectromechanical Structures for Materials Research, Materials Research Society, MRS Spring Meeting, San Francisco, CA, 1998. • H. Tada, A. R. Abramson, I. N. Miaoulis, and P. Y. Wong, "Effect of Surface Patterning in Thin Film Structures on the Thermal Radiative Properties During Rapid Thermal Processing," American Society of Mechanical Engineers Conference Proceedings, HTD-Vol. 361-2, pp.93-98, 1998. • H. Tada, I. N. Miaoulis, and P. Y. Wong, "Numerical Simulation of Radiant Thermal Processing of Bilayer Microcantilevers," American Society of Mechanical Engineers Conference Proceedings, DSC-Vol.66, pp.37-44, 1998. • H. Tada, P. Nieva, P. Zavracky, I. N. Miaoulis, and P. Y. Wong, "Determining the High Temperature Properties of Thin Films Using Bi-Layered Cantilevers," presented and to be published in the Proceedings of the Symposium on Materials Science of Microelectromechanical System (MEMS) Devices, Materials Research Society, MRS Fall Meeting, Boston, MA, 1998. • P. Nieva, H. Tada, P. Zavracky, G. Adams, I. N. Miaoulis, and P .Y. Wong, "Mechanical and Thermophysical Properties of Silicon Nitride Thin Films at High Temperatures," presented and to be published in the Proceedings of the Symposium on Materials Science of Microelectromechanical System (MEMS) Devices, Materials Research Society, MRS Fall Meeting, Boston, MA, 1998. • P. Nieva, P. Zavracky, G. Adams, H. Tada, A. R. Abramson, I. N. Miaoulis, and P. Y. Wong, "Temperature Measurements During Rapid Thermal Annealing Using MEMS," to be presented and published at the 5th ASME/JSME Joint Thermal Engineering Conference, San Diego, CA, 1999. • A. R. Abramson, P. Nieva, H. Tada, P. Zavracky, I.N. Miaoulis, and P. Y. Wong, "Effect of Doping Level During Rapid Thermal Processing of Multi-Layer Structures," submitted for consideration to be published in the Journal of Materials Research, 1998. • A. R. Abramson, H. Tada, I. N. Miaoulis, and P. Y. Wong, "The Cumulative Effects of Partial Transparency and Thin-Film Interference on Radiative Properties of Silicon Wafers," submitted for consideration to be published in Optical Engineering, 1998.