Advanced Integration of Devices Enabled by Laser Crystallization of Silicon
Creators: Lee, Vincent Wing-Ho
Publisher: name: Columbia University
Year: 2012
Subjects: Electrical engineering

description: The push for higher levels of performance drives research and innovation in all areas of electronics. Thus far, shrinking circuit sizes and development of new material systems have satisfied this need. Continued scaling and material improvements have become increasingly difficult; simultaneously, more functionality is needed in smaller spaces. Advanced integration techniques provide a solution by engineering together previously incompatible systems. The fabrication of high-performance devices typically requires high temperature processing steps.  Since fabrication occurs sequentially, the high temperature prevents the direct integration of two high-performance layers, as completed devices cannot withstand the processing temperatures of subsequent steps.  There are significant challenges to integrating process-incompatible systems, and techniques such as wafer bonding, heteroepitaxial growth, and various thin film technologies have shown limited success. In this work, advanced integration is achieved through laser crystallization processes.  Unique to laser methods is the ability to locally heat the surface of a material while keeping the underlying substrate at room temperature.  This property allows for high performance electronic materials to be integrated with substrates of different functionalities.  This thesis focuses on three key components for advanced integration: 1. Laser-crystallized electronic devices, 2. Relevant substrates for integration, and 3. The feasibility of integrating of laser-crystallized devices with low-temperature substrates. Two types of laser-crystallized devices are explored.  Thin-film, laser-crystallized silicon transistors are fabricated at low-temperatures and exhibit high mobilities above 400 cm2 2/Vs.  Vertical structure diodes built from laser-crystallized silicon outperformed epitaxially-grown diodes of the same geometry. Light emitting diode (LED) arrays are fabricated from compound semiconductor substrates and tested for display applications. These LED arrays are envisioned to sit underneath the laser-crystallized devices, enabling new applications where both high brightness and high performance transistors are needed. Substrates of low-κ dielectric material are also of interest, as they are widely used for their low capacitance properties. Preliminary results suggest that laser crystallization of silicon can be successfully performed on a low-κ dielectric. In addition to enabling new device architectures, it is important for laser crystallization methods to leave the underlying layers unaffected.  Simulations of the laser irradiation process predict substrate temperatures to reach only 70C even when the surface reaches the melting temperature of silicon (1400C).  Integration feasibility is further investigated with measurements on conventional front-end field effect transistors.  When comparing properties from wafers with and without laser processing, no changes in transistor characteristics are observed. In all three components of work, proof-of-principle devices and concepts lay out the groundwork for future investigation.  The developed technologies have promising applications in both the microelectronics and display industry.  In particular, the integration of LEDs and laser-crystallized silicon enables a high-brightness microdisplay platform for head-mounted displays, pico projectors, and head-up displays.