Masters Theses

Jason Fowlkes

Date of Award

8-1999

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Anthony J. Pedraza

Abstract

A self-organized array of silicon spikes was grown on Si (100) and Si (111) substrates by pulsed KrF laser irradiation. The spike morphology was sensitive to the background gas present during irradiation, the number of laser pulses, the total pressure of the background gas, the partial pressure of gases in a binary gas mixture, and the laser fluence. The spikes that developed during pulsed KrF irradiation in air resembled columns and were so termed "microcolumns". The morphology produced from irradiation in SFe gas were termed "microcones". A spike morphology did not develop on Si wafer when irradiation was carried out in Ar, N2,5% H2 - Ar, and He.

The microcone/microcolumn morphology was characterized using SEM, EDS, X-ray diffraction, AES, and scanning profilometry. The microcolumns were found to consist of silicon in the bulk with a small oxygen concentration at the surface. Both the microcolumn and microcone morphologies grew in the direction of the incident laser irradiation. The microcolumn morphology developed strictly in the fluence range of 2.7 - 3.6 J/cm^2. The morphology first appears following 200 - 300 laser pulses as small surface ridges ~ 1 pm high. The microcolumns grow in height as the number of laser pulses increases. The microcolumns saturate in height between 800 - 1 000 laser pulses at 30 - 35 pm. The maximum microcolumn density of ~ 2x10^6 col/cm^2 is achieved between 1 000 and 1 200 laser pulses. The microcolumns are cylindrical with diameters of ~ 2 - 3 um and have a 2 - 3 um diameter tip at their vertical termination. This tip was the portion of the microcolumn melted by the last laser pulse incident on the Si target. The microcolumns protruded above the initial Si wafer surface by as much as 10 - 20 um.

The microcone morphology, produced during laser irradiation in SF6, developed over the entire fluence range tested of 0.9 - 3.8 J/cm^2. The microcones first developed as surface ridges after 750 - 1 000 laser pulses. The surface ridges produced in SFg were of the same size and morphology as the surface ridges that were produced in air. The microcones protruded by as much as 40 pm above the initial wafer surface after 2 000 - 3 000 laser pulses. Each microcone had a 2 - 3 )xm spherical tip at its vertical termination. The fully developed microcones produced with 2 000 - 3 000 laser pulses had a complex morphology. The growth of small cones ~ 1 - 3 pm long developed on the microcone sides, the cones grew normal to the primary microcone growth direction and grew as the number of laser pulses was increased.

The microcones/microcolumns were found to grow by a mechanism similar to the VLS mechanism of whisker growth described by R.S. Wagner. [41] The microcones/microcolumns grow by deposition of Si from the vapor phase at the 2 - 3 pm tip that is re-melted with each laser pulse. The Si vapor deposits preferentially at the molten spike tips because of the high accommodation coefficient of the liquid phase compared with the solid phase. However, the vapor phase deposition occurs by a "catalyst-free" VLS mechanism. The laser pulse acts to simultaneously produce the flux of Si required for deposition and melt the tips of the microcolumns and microcones providing the deposition location. Scanning profilometry showed the laser etching of Si to be more efficient in gas environments conducive to spike growth. For instance, the etched depth of Si (100) per pulse was 25x greater in air than in He. Thus, more Si vapor is available in the plasma plume during irradiation in air than in He. Supersaturation of the liquid tip by the high pressures of Si in the plasma plume surrounding the tip causes the deposition of Si at the Si(s) - Si(l) interface as the liquid attempts to reestablish equilibrium. Therefore, with each laser pulse the liquid tip rises above the wafer surface on a solid Si pedestal.

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