Effect of grain structure and doping on the mechanical properties of polysilicon thin films for MEMS

Yagnamurthy, Naga Sivakumar

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Description

Title

Effect of grain structure and doping on the mechanical properties of polysilicon thin films for MEMS

Author(s)

Yagnamurthy, Naga Sivakumar

Issue Date

2014-01-16T18:17:06Z

Director of Research (if dissertation) or Advisor (if thesis)

Chasiotis, Ioannis

Doctoral Committee Chair(s)

Chasiotis, Ioannis

Committee Member(s)

• Geubelle, Philippe H.
• Lambros, John
• Sottos, Nancy R.

Department of Study

Aerospace Engineering

Discipline

Aerospace Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Polysilicon
• Effect of microstructure
• Effect of doping
• Tensile strength
• Critical stress intensity factor
• Size effects
• Microstructure
• Phosphorus doping

Abstract

Freestanding devices fabricated for Microelectromechanical Systems (MEMS) employ slender polysilicon flexures that are prone to failure due to large operating stresses. Polycrystalline silicon (polysilicon) films with improved mechanical properties to meet demanding applications could be engineered by modification of the material microstructure. Such advances require detailed experimental studies and quantitative understanding of the convoluted effects of the processing methods on the ensuing mechanical properties. This dissertation investigated the role of grain size and doping on the nature and origin of critical flaws that determine the tensile strength and the local resistance to crack initiation in 1-μm thick polysilicon films, as quantified by the effective mode I critical stress intensity factor, KIC,eff. For the purposes of this study microscale polysilicon thin film specimens were fabricated by a custom process at the Sandia National Laboratories. The films were comprised of either columnar grains (grain size 285 nm) or a laminated structure (grain size 125 nm), and were doped with different concentrations of Phosphorus (P). The columnar grain polysilicon typically had 1 - 2 grains across the film thickness, while the laminated polysilicon contained ten grains across the film thickness, each confined in a 100-nm thick layer. The grain structure and doping concentration had no effect on the elastic stiffness of polysilicon: the average Young’s moduli of all polysilicon films were in the narrow range of 153 - 158 GPa. On the other hand, the tensile strength values of undoped columnar grain and laminated polysilicon differed significantly, averaging 1.31±0.09 GPa and 2.44±0.28 GPa, respectively. Heavy doping further impacted the strength of the former type of polysilicon (0.92±0.10 GPa) due to the formation of large sidewall defects at high concentrations of P which, however, had no effect on the tensile strength of laminated polysilicon. The nature and type of the critical sidewall defects were independent of the specimen size: on grounds of the cumulative Weibull probability distribution function, the results of the present experiments predicted quite reasonably the tensile strength of polysilicon specimens that were 180 times smaller in size. The strength of polysilicon films scaled with the sidewall surface area (or equivalently the specimen length), which is also the region where the major critical flaws were identified. Notably, in the absence of the initial critical sidewall defects, the average tensile strength of undoped columnar polysilicon increased by 70%, namely from 1.31±0.09 to 2.2±0.11 GPa, thus approaching the strength of laminated polysilicon. The critical defects in columnar polysilicon were located at the specimen free edges which were defined by reactive ion etching (RIE). These defects were initiated at grain boundaries during RIE and were further exacerbated by the reactions taking place during heavy P-doping in high temperature annealing. Measurements of KIC,eff were used to evaluate the effect of grain structure and doping on the resistance of the two types of polysilicon to crack initiation. The values of KIC,eff for all the polysilicon films were in the range of 0.8 - 1.2 MPa√m. Contrary to the trends in tensile strength values, the KIC,eff of columnar polysilicon was higher than that of laminated polysilicon, but the latter demonstrated a much smaller variability in KIC,eff, which was owed to the averaging effect of its laminated structure. The KIC,eff of columnar polysilicon further increased by 10% as a result of heavy P-doping, which, on the contrary, had no effect on the KIC,eff of laminated polysilicon. Thus, P-doping only modestly increased the fracture resistance of columnar polysilicon, although its effect on the tensile strength was clearly detrimental. Finally, using the measured KIC,eff values and the precise defect geometries determined by Atomic Force and Scanning Electron Microscopy, the tensile strength of different polysilicon films was calculated by linear elastic fracture mechanics models for semi-elliptical surface cracks and quarter elliptical edge cracks. The strength values estimated by a quarter elliptical edge crack analysis agreed fairly well with the values obtained by uniaxial tension experiments, further supporting the electron microscopy observations and the Weibull scaling predictions that the tensile strength of as-fabricated columnar grain polysilicon specimens was governed by sidewall defects. On the other hand, the strength values estimated by an elliptical surface crack analysis agreed fairly well with uniaxial tension experiments with columnar grain polysilicon specimens whose sidewall defects were removed via ion beam milling.

Graduation Semester

2013-12

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Copyright 2013 Naga Sivakumar Yagnamurthy

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