Session 21: Process Control

Non-Contact Stress Metrology for Flexible Electronics
Thursday, February 15, 2018
2:25 PM - 2:45 PM

We report novel photo-elastic method for measurement of the stress in polyimide (PI) based flat panel displays (FPD) and flexible displays (FD) structures. Method is based on measurement of the change of the state of polarization of the light undergoing reflection (or transmission) in the structures containing PI layer(s). Commonly used FPD and FD contain layers of 5 um or thicker PI layers. PI has stress optic coefficient of about 3.4E-10 Pa^-1 [1], which is almost 100 times larger than glass [2]. This allows easily measurement of stress with sensitivity of the order of 5 MPa and less. We present apparatus for local stress measurements having lateral resolution of 3 cm, and stress resolution of 5 MPa. Presented tool gives promise to become capable of measuring glass panels of the size 1.5 m x 1.85 m does not contained any moving parts. Our tool can be combined with more traditional stress induced deflection based stress measurement [3]. We discuss also methods of numerical analysis of optical data, including stress separation algorithm optimized for this problem, and practical problems related to analysis of the data. [1] Noe, Susan Cunningham. A prism coupling study of optical anisotropy in polymide including moisture, stress, and thickness effects. Diss. Massachusetts Institute of Technology, 1992. [2] Sun, Lan, and Samad Edlou. "Low birefringence lens design for polarization sensitive optical systems." Proc. SPIE. Vol. 6289. 2006. [3] Walecki, Wojtek J Stress Metrology for Flat Panel Displays G6 and Bigger . Abstract submitted to Photonics West 2018 SPIE OPTO (PW18O) .