Multimode Sensing for Roll-to-Roll Flexible Electronics Print Process Control

Microcontact(µCP) printing holds the promise of large-scale, high rate surface micro-patterning. We address the real time monitoring, metrology and control issues for the scale-up process using a roll-to-roll(R2R) continuous printing method. 

Web handling: Precise web tension measurement is critical for web handling and process control in R2R manufacturing systems. Calculating web tension by out-of-plane vibration measurement offers a contactless solution to tension measurement during the R2R process, which is suitable for situations where contact with the web is not possible. We employ a contactless measurement using movement speed and out-of-plane vibration to determine the tension in real time.  First, a universal prediction method based on artificial neural network was trained to derive web tension from web moving speed and natural frequency. A real-time frequency estimation algorithm based on the Fourier series solves the frequency resolution problem that exists in real-time applications when conventional FFT is then applied. The experimental results on the testbed show that the proposed method can predict the web tension with an error less than 10% for various tension values and track the tension changes within one analysis widow of 50 ms.

Print quality monitoring: A serious limitation of the present R2R µCP process is the lack of inline metrology of the nano-thin, nanoscale or micron-scale monolayer patterns in the large area of a continuous moving web. We present a real-time imaging technique for inspecting the online R2R µCP pattern using condensation figures (CFs). Due to the wettability contrast, water droplets are more likely nucleated and grown on the hydrophilic regions than hydrophobic regions. The formed condensation figures are thus correlated to the patterns of the surface which can be segmented under optical microscopes. The real-time desirable CFs are achieved through a controlled open condensation chamber and a synchronous imaging process. In this chamber, the nucleation process has a vapor flow rate around the target surface as a control parameter. Consistent droplet size is maintained as the output of a closed loop feedback system by fine-tuning the balance of vaporization and evaporation of droplets on the surface by the vapor flow rate. The droplet size can be controlled under 5 um. In our experimental results, the distribution of droplets of water recorded by a high-speed camera on an Hexadecanethiol (HDT)-gold surface is clustered and segmented for metrology. Our metrology method can achieve pattern resolution up to 0.7 µm and a web moving speed of 3 in/s. The metrology results can be promisingly used in combination with a pattern “truth table” for online R2R print quality monitoring and in-process control.