In this master thesis, the strain of plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiNx) microtensile specimens has been evaluated using two dimensional gratings. Physical vapor deposition (PVD) of 20 nm thick gold as two dimensional gratings with period vary from 4 µm to 18 µm were deposited on the top surface of mechanical structures and silicon nitride specimens. Both lateral and axial strains of four silicon nitride specimens can be measured by examining the laser interference pattern of the gratings.

Silicon Nitride speciment with gratings on it (click to enlarge all images in this page)

Two dimensional grating provides more information for referencing in the devices alignment in measurement setup than one dimensional grating. Improvement of alignment decreases the uncertainty and increase both precision and accuracy. Laser vibrometer with feedback and 632.8 nm wavelength made sure it illuminates perpendicular to a silicon nitride specimen with gratings producing interference patterns at two CCD cameras. A tensile force applied by a piezo actuator to the mechanical structure and measured by a load cell extend the silicon nitride specimen and gratings periods which lead to a change of the interference pattern period captured. The strain of specimens corresponds to interference pattern period.

The working principle of this measurement method (left) and Gratings interference pattern (right).

The measurement setup for this measurement simplifies the measurement process. Combination of high precision motor with piezo actuator, high quality cameras and laser vibrometer that is put on the stabilized table and inside the clean room would maximize the measurement accuracy and pecision. The following figures are the photograph of measurement setup and the simulation of the diffraction pattern.

Tensile measurement setup (left) and Simulation of the diffraction from gratings (right).

Advanced signal processing using powerful software for data analysis improves the reliability of interference patterns evaluation. Non-contact measurement minimizes the influence to the measurement result. Special mechanical structures designed with dimension 13 mm x 10.4 mm provide simple alignment of specimens and actuator. Four silicon nitride specimens in the single mechanical structure increase the information in single measurement process.

Tensile test structure fabrication

I have developed software for controlling the actuators, measurement device and capturing the interference pattern from the camera. The software was developed using Visual Basic that combined with Mathematica. This combination improves the speed of measurement, accuracy and reduce the uncertainty. The following figure is the screenshot of the software that I have developed.

Tensile test software user interface

Two CCD cameras with one-dimension of 3000 pixels sensor array are driven by high accuracy and precision actuators. This setup produces two dimensional interference pattern images that useful for reference in setup alignment. The strains were obtained by fitting the peaks position of the interference pattern to the general equation. This experiment has successfully extracted the material properties of silicon nitride, such as the Young’s Modulus is 178.4 ± 4 GPa and fracture strength is 1.443 ± 0.03 GPa. The resolution of optical setup achieved 2.133 nm/pixel (1 pixel = 7 µm) or in strain dimension is 152.4 ± 3 µ/pixel. The maximum strain before fracture is 0.8435 % ± 1.8 % of the initial length.