Non-melt selective enhancement of crystalline structure in molybdenum thin films using femtosecond laser pulses

Ayesha Sharif,Mingqing Wang,Rajani K Vijayaraghavan,Patrick J Mcnally,Gerard M O’Connor,Kwang-Leong Choy,Nazar Farid

doi:10.1088/1361-6463/ac3e91

Abstract

It is challenging to crystalize a thin film of higher melting temperature when deposited on a substrate with comparatively lower melting point. Trading such disparities in thermal properties between a thin film and its substrate can significantly impede material processing. We report a novel solid-state crystallization process for annealing of high melting point molybdenum thin films. A systematic investigation of laser induced annealing from single pulse to high pulse overlapping is reported upon scanning at fluences lower than the threshold required for the damage/ablation of molybdenum thin films. The approach allows better control of the grain size by changing the applied laser fluence. Atomic force microscopy surface morphology and x-ray diffraction (XRD) analysis reveal significant improvements in the average polycrystalline grain size after laser annealing; the sheet resistance was reduced by 19% of the initial value measured by a Four-point probe system. XRD confirms the enlargement of the single crystal grain size. No melting was evident, although a change in the close packing, shape and size of nanoscale polycrystalline grains is observed. Ultrashort laser induced crystallinity greatly enhances the electrical properties; Hall measurements reinforced that the overall carrier concentration increases after scanning at different laser fluences. The proposed method, based on the aggregation and subsequent growth of polycrystalline and single crystal-grains, leading to enhanced crystallization, has potential to be applicable in thin film processing industry for their wide applications.

Highlights

In recent years, thin film transistors (TFTs) have become a dominant technology for switches and as drivers in flat panel displays
Atomic force microscopy surface morphology and x-ray diffraction (XRD) analysis reveal significant improvements in the average polycrystalline grain size after laser annealing; the sheet resistance was reduced by 19% of the initial value measured by a Four-point probe system
The key advantage of TFT technology is their ability to be manufactured on large substrates at low cost per unit area at low processing temperatures enabling TFTs to be directly integrated on a variety of flexible substrates without affecting the electronic functionality of the circuit [1,2,3]

Summary

Introduction

Thin film transistors (TFTs) have become a dominant technology for switches and as drivers in flat panel displays. TFTs are fabricated at a low material cost and demonstrated in a wide range of applications such as thin film transistor liquid-crystal displays (TFT-LCDs), organic light emitting displays, flat panel displays, and other electronic device applications. The deposition parameters are optimized for a favourable growth of Mo microstructures to facilitate the requirement of high conductivity for efficient charge transfer in addition to chemical inertness and strong adhesion to the substrate. This is done by using either RF or DC magnetron sputtering techniques or electron beam evaporation followed by an annealing process [11,12,13,14]. It is crucial to develop a general solution by lowering the processing temperature for Mo applications in flexible electronics

Results

Discussion

Conclusion