The objective of this research project is to gain a fundamental understanding about the physical mechanisms of high speed picosecond laser scribing of multilayer thin film materials and determine optimal processing conditions. Research approaches will include both physics-based computational modeling to better understand the physical processes and an experimental work to characterize the resultant morphological, microstructural and electrical properties. The relationship between process parameters and associated physical mechanisms, such as photo-thermal and photo-mechanical reactions will be developed and optimal process conditions for high speed scribing of multilayer thin film materials will be identified.
The successful completion of this work will provide a scientific understanding of the laser-material interaction under the picosecond regime, high power laser, plasma formation, and its interaction with the target material of various thin film layers used for solar cells. Furthermore, this work will produce fundamental technology advances in the manufacturing of thin film electronics, such as photovoltaic devices, flat panel displays, flexible electronics and advanced semiconductor logic devices and thus will bring a broad impact on manufacturing related issues on these areas. High frequency, ultrashort lasers have many applications from pulsed laser fabrication of thin films to laser micro/nano fabrication. Therefore, improving the understanding of high frequency, ultrashort laser-material interaction can significantly improve the potential applications of picosecond lasers. This project will also bring a positive impact on undergraduate education through the manufacturing classes, undergraduate research fellow programs and undergraduate independent projects.
Intellectual Merits The objective of this research project was to gain a fundamental understanding about the physical mechanisms of high speed picosecond laser scribing of multilayer thin film materials and to determine optimal processing conditions to reduce the thin film solar cell manufacturing cost. Research approaches included both physics-based computational modeling to better understand the physical processes and experimental investigations to characterize the resultant morphological, microstructural and electrical properties after thin film deposition and scribing. The relationship between process parameters and associated physical mechanisms, such as photo-thermal and photo-mechanical reactions, have been developed and optimal process conditions for high speed scribing of multilayer thin film materials have been identified. The overall schematic of the scribing processes is shown in Fig. 1. P1 scribing of Molybdenum layer: It is revealed that the optimum scribing condition for P1 scribing is the overlap ratio of 48% and the laser fluence of 1.72 J/cm2, where a slot with very good quality can be generated (Fig. 2). Therefore, if this laser fluence is achievable at 500 kHz, the overlap ratio of 48% will correspond to scribing speed of 4 m/s. With a higher repetition ratio such as 2 MHz, if such a laser is developed, the scribing speed is likely to reach above 10 m/s. P2 scribing of Copper indium gallium (di)selenide (CIGS) layer: A CIGS layer was deposited on the Mo layer, and the attempt was to scribe the CIGS layer without damaging the Mo layer. According to the study, channels with good quality can be fabricated at the speed as fast as 1.75m/s for 3 J/cm2. The three dimensional profile images of the slot show flat bottom, uniform width, and clean edge. It can be concluded that the maximum scribing speed at the repetition rate of 500 kHz could be as fast as 1.5 m/s. If the repletion rate could be increased to 2 MHz, the scribing speed can be as high as 6 m/s. P3 scribing: A transparent layer of doped ZnO:Al (AZO) with a thickness of 200 nm was deposited on the CIGS layer. The scribing process to remove the AZO and CIGS layer together was studied. The thicknesses of AZO layer and CIGS layer are 200 nm and 1.75 μm, respectively. Within the laser parameters achievable in this experiment, the optimum scribing conditions were determined to be the fluence of 4.5 J/cm2 and an overlap ratio of 87%, corresponding to the scribing speed higher than 1 m/s at 500 kHz. Compositional analysis carried out shows that the slot is cleanly scribed with sharp edge, and the AZO and CIGS layers are completely removed. Numerical simulation shows that the scribing speed could possibly be further increased by increasing the laser fluence more. i-ZnO film by Pulsed Laser Deposition: The i-ZnO film prepared by room temperature pulsed laser deposition (PLD) shows promising electrical, optical and structural properties. Lower resolution top morphological Field Emission Scanning Electron Microscopy (FESEM) images show the deposited film is pretty uniform as expected, while the higher resolution one shows the particle size are in the range of 25 - 40 nm. Cross sectional FESEM images show the thickness is around 50 nm. The deposition rate could be determined to be 1.67 nm/min. Broader impacts This research project has provided a scientific understanding of the ultrashort laser-material interaction under the picosecond regime, high power laser, plasma formation, and its interaction with the target material of various thin film layers used for solar cells. Furthermore, this work produced fundamental technological advances in the manufacturing of thin film electronics, such as photovoltaic devices, flat panel displays, flexible electronics and advanced semiconductor logic devices and thus will bring a broad impact on manufacturing related issues on these areas, High frequency, ultrashort lasers have many applications from pulsed laser fabrication of thin films to laser micro/nano fabrication. Therefore, improving the understanding of high frequency, ultrashort laser-material interaction can significantly improve the potential applications of picosecond lasers, particularly lowering the cost of manufacturing those products and improving the associated quality. This project also brought a positive impact on undergraduate education through the manufacturing classes, undergraduate research fellow programs and undergraduate independent projects. Broad dissemination to the industrial audiences enhanced the public awareness of the developed technology.
