Fracture Mechanisms of SiN x Thin-films on Compliant Substrates
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1078-M14-02
Fracture Mechanisms of SiNx Thin-films on Compliant Substrates Alex Z Kattamis1, Kunigunde H Cherenack2, I-Chun Cheng3, Ke Long4, James C Sturm2, and Sigurd Wagner2 1 Electrical & Semiconductors Practice, Exponent Inc., 420 Lexington Ave #1740, New York, NY, 10170 2 Electrical Engineering and Princeton Insitute for the Science and Technology of Materials, Princeton University, Princeton, NJ, 08544 3 Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan 4 Flexible Display Center, Arizona State University, Tempe, AZ, 85284 Abstract The prospect of large-area electronics on polymers, for flexible applications requires a study of thin film fracture mechanisms. To fabricate thin-film transistor (TFT) backplanes on polymer foils the substrate must first be passivated to protect the polymer substrate from chemicals used during processing and to protect the TFTs from substrate out gassing. Silicon nitride (SiNx) is commonly used for this purpose since it tends to adhere well to polymers and is easily deposited by PE-CVD. When rigid thin films such as SiNx are deposited onto compliant substrates, such as polymer foils, stresses caused by built-in strains and the mismatch in coefficients of thermal expansion can cause fracture. The deposited thin films may fracture, and also the polymer substrate below. Using focused-ion beam milling and scanning electron microscopy we analyzed two distinct thin film fracture morphologies for SiNx films on two different types of polymer substrate. One had a relatively low, the other a relatively high coefficient of thermal expansion. For both SiNx/substrate systems the SiNx was under residual compressive stress and the substrate under tension. In one case the compressive stress in the thin films cause them to debond, buckle, and crack. In the other case the tensile stress in the substrate causes it to tear, followed by the fracture of the SiNx film above. Introduction The strength of a material is described by its Young’s modulus (Y) multiplied by the thickness (d) [1].This paper focuses on a situation where two materials, thin film and substrate, have comparable strengths. For common plastic substrates such as DuPontTM Kapton® E polyimide, Y ≈ 3-6GPa [2] and for Si containing thin films such as silicon nitride (SiNx) Y ≈ 200-300GPa [3], which is about 100 times larger. Since the thickness of the thin films for electronics applications is generally on the order of 100nm and the thickness of plastic substrate is on the order of 10µm, 100 times larger, they have comparable strengths, i.e., Ys·ds ≈ Yf··df. Because of their comparable mechanical strengths, the thin film and substrate affect each other’s strain leading to small substrate curvatures. The total mismatch strain, εM, is the sum of the built-in strain, ε 0 , the thermal mismatch strain, ε th , and the moisture mismatch strain, ε ch , as given below [2]:
ε M = ε 0 + ε th + ε ch .
(1)
The mismatch in the coefficients of thermal expansion (α) between the thin film and the substrate produces εth.
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