Inherent residual stresses during material deposition can have profound effects on the functionality and reliability of fabricated MEMS devices. Residual stress often causes device failure due to curling, buckling, or fracture. Typically, the material properties of thin films used in surface micromachining are not very well controlled during deposition. The residual stress, for example, tends to vary significantly for different deposition conditions; experiments were carried out to study the polysilicon and silicon nitride deposited by Low Pressure Chemical Vapor Deposition (LPCVD) method at wide range of process conditions. High temperature annealing effects on stress in case polysilicon are also reported. The reduced residual stress levels can significantly improve device performance, reliability, and yield as MEMS devices become smaller. 1. Introduction The deposition of thin films is an important field of Micro Electro Mechanical Systems (MEMS) or micro system technology. Most of the thin films exhibit stress after deposition. This stress has many different causes. Most films are deposited at elevated temperature. If the thermal expansions of the film and substrate are not identical there will be stress between them after cooling. Other sources are lattice mismatch, crystallization, atomic peening, incorporation of foreign atoms, microscopic voids, variation of interatomic spacing with crystal size, crystallite coalescence at grain boundaries, phase transformations, and texture effect. Sometimes this stress is called internal stress or residual stress. This stress may cause problem for thin film technology. It changes the behavior of the thin films often in an uncontrolled manner (Figure 1 shows the one of the structure under stress), reducing the yields and long term durability and sometimes causing fracture. Many researchers had investigated the mechanical response of thin films, for example. Frequently, each particular investigation involving MEMS tends to be device dependent; type of film used and deposition methods adopted, and introduces new fundamental questions. Progress in this field has leaned towards providing more specific technological solutions rather than generating a basic understanding of mechanical behavior. Figure 1: Effect of residual stress on free standing structure. In the recently developed technology of microsystem, more and more standing thin film structures are used, for example, resonator, movable parts in surface micromachining, thin film membranes, and cantilever beams in transduction [1, 2]. For these applications, the
References
[1]
J. G. Korvink and O. Paul, MEMS: a Practical Guide to Design, Analysis, and Applications, Springer, Berlin, Germany, 2006.
[2]
O. Paul, J. Gaspar, and P. Ruther, “Advanced silicon microstructures, sensors, and systems,” IEEJ Transactions on Electrical and Electronic Engineering, vol. 2, no. 3, pp. 199–215, 2007.
[3]
Y. C. Huang, S. Y. Chang, and C. H. Chang, “Effect of residual stresses on mechanical properties and interface adhesion strength of SiN thin films,” Thin Solid Films, vol. 517, no. 17, pp. 4857–4861, 2009.
[4]
S. Wolf and R. N. Tauber, Silicon Processing, vol. 1, Lattice Press, 1986.
[5]
S. M. Sze, VLSI Technology, McGraw-Hill, New York, NY, USA, 1988.
[6]
J. Woloszyn, “A method for controlling residual film stress in LPCVD polysilicon films for surface micromachined MEMS,” in Proceedings of the IEEE/SEMI Advanced Semiconductor Manufacturing Conference and Workshop, pp. 399–403, May 2004.
[7]
P. J. French, “Polysilicon: a versatile material for microsystems,” Sensors and Actuators A, vol. 99, no. 1-2, pp. 3–12, 2002.
[8]
J. G. E. Gardeniers, H. A. C. Tilmans, and C. C. G. Visser, “LPCVD silicon-rich silicon nitride films for applications in micromechanics, studied with statistical experimental design,” Journal of Vacuum Science and Technology A, vol. 14, no. 5, pp. 2879–2892, 1996.
[9]
T. P. Ma, “Making silicon nitride film a viable gate dielectric,” IEEE Transactions on Electron Devices, vol. 45, no. 3, pp. 680–690, 1998.
[10]
H. Amjadi and G. M. Sessler, “Charge storage in APCVD silicon nitride,” in Proceedings of the Annual Conference on Electrical Insulation and Dielectric Phenomena, pp. 64–67, Minneapolis, Minn, USA, October 1997.
[11]
J. M. Olson, “Analysis of LPCVD process conditions for the deposition of low stress silicon nitride. Part I. Preliminary LPCVD experiments,” Materials Science in Semiconductor Processing, vol. 5, no. 1, pp. 51–60, 2002.
[12]
G. Roman, Process Characterization of LPCVD Silicon Nitride and the Consequential Fabrication of Low Stress Micro cantilevers, National Nanofabrication Users Network.
[13]
N. Sharma, M. Hooda, and S. K. Sharma, “Concept of modeling and fabrication of test structures for thin film qualification for MEMS/NEMS structure,” in Proceedings of the National Seminar on Nanomaterials and their Applications (NANOMAT '2011), pp. 220–229, Allied Publisher, 2011.
[14]
X. Zhang, T. Y. Zhang, and Y. Zohar, “Measurements of residual stresses in thin films using micro-rotating-structures,” Thin Solid Films, vol. 335, no. 1-2, pp. 97–105, 1998.
[15]
Y. B. Gianchandani, M. Shinn, and K. Najafi, “Impact of long, high temperature anneals on residual stress in polysilicon,” in Proceedings of the International Conference on Solid-State Sensors and Actuators, pp. 623–624, Chicago, Ill, USA, June 1997.
[16]
L. B. Freund and S. Suresh, Thin Film Materials, Cambridge University, Cambridge, UK.
[17]
W. D. Nix and B. M. Clemens, “Crystallite coalescence: a mechanism for intrinsic tensile stresses in thin films,” Journal of Materials Research, vol. 14, no. 8, pp. 3467–3473, 1999.
[18]
F. Spacemen, “Interfaces and stresses in thin films,” Acta Materialia, vol. 48, no. 1, pp. 31–42, 2000.
[19]
S. Kamiya, J. H. Kuypers, A. Trautmann, P. Ruther, and O. Paul, “Process temperature-dependent mechanical properties of polysilicon measured using a novel tensile test structure,” Journal of Microelectromechanical Systems, vol. 16, no. 2, pp. 202–212, 2007.
[20]
Y. Hwangbo, J. M. Park, W. L. Brown, J. H. Goo, H. J. Lee, and S. Hyun, “Effect of deposition conditions on thermo-mechanical properties of free standing silicon-rich silicon nitride thin film,” Microelectronic Engineering, vol. 95, pp. 34–41, 2012.
[21]
A. G. Noskov, E. B. Gorokhov, G. A. Sokolova, E. M. Trukhanov, and S. I. Stenin, “Correlation between stress and structure in chemically vapour deposited silicon nitride films,” Thin Solid Films, vol. 162, pp. 129–143, 1988.
