Application of Nanoindentation to Characterize Fracture in ILD Films Used in the BEOL.
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Application of Nanoindentation to Characterize Fracture in ILD Films used in the BEOL. Eva E. Simonyi, E. Liniger, M. Lane, Q. Lin, C. D. Dimitrakopoulos, C. Tyberg IBM T. J. Watson Research Center, 1101 Kitchawan Rd., Yorktown Heights, NY 10463, U.S.A.
Abstract It is of importance to understand cracking behavior in low dielectric constant, low modulus materials. Nanoindentation method is presented as a tool to estimate the critical film thickness, thickness above which spontaneous cracking could occur, for ILD films used in the BEOL. The critical film thickness was then used to calculate cohesive energies and fracture toughness of the films. Materials were investigated using nanoindentation combined with AFM imaging. The results were compared to data acquired by four point bend methods.
Introduction The drive in the semiconductor industry for ever decreasing feature sizes in the BEOL drives the requirement for materials with ever decreasing dielectric constants. The materials with the lowest dielectric constants exhibit poor mechanical characteristics: low modulus and significant cracking propensity. Due to the large number of materials that are evaluated for the manufacturing process, there is a need to investigate the mechanical characteristics with a fast turnaround time in order to support the efforts of optimizing these materials to achieve low dielectric constants with robust mechanical characteristics. Traditionally cohesive energy of thin dielectric films is measured with 4 point bend(1) measurement setup, stress with a beam bending rig(2), and modulus and hardness by nanoindentation. The motivation for developing this method was to have a measurement method, which does not require time consuming sample preparation as is required for 4 point bend measurements, and will give data that agree with 4 point bend.
Experimental Hardness and modulus were measured with a Nanoindenter XP system (Nano Instruments Innovation Center) fitted with the dynamic contact modulus (DCM) head. The DCM head provides the XP system with an overall miniaturization, allowing to perform indentations at maximum indentation forces of .01 to 12mN. The DCM machine uses a Berkovitch indenter. This is a 3 sided pyramid with 65.3° between vertical axis and face. The instrument was operated using the continuous stiffness measurement option (CSM). This method superimposes a small oscillating force on the applied load, allowing a continuous measurement of the hardness and modulus during the indentation process. Tip calibration was based on the Oliver –Pharr (3) method. The indentation was done maintaining a constant strain rate. Surface was detected by a
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stiffness change of 4. The stress was measured with a beam bending apparatus - applying Stoney(4) equation- The measurement error is about 10 percent. This method requires the measurement cracks created by indentation at a given load. This load is not arbitrary: the lowest value is about 10 percent higher than the force necessary to initiate a crack, and less than the for
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