INDUSTRY COATINGS
Enhancing coating uniformity for electron beam lift-off
To form the best films, users of electron beam evaporators must understand how uniformity is influenced by the number of axes for substrate rotation, the insertion of shadow masks and the distance between source and substrate.
BY PHILIP GREENE FROM FERROTEC, TEMESCAL DIVISION
IN VERY GENERAL TERMS, compound semiconductor devices are all fabricated in the same way. Whether it is LEDs, lasers, solar cells or electronics, production always begins with deposition of epitaxial layers onto a substrate, followed by processes that add metal films and contact wires to construct a chip that is housed in a suitable package.
A standard technique for depositing a metal on a compound semiconductor is electron beam evaporation. It may be performed as part of a lift-off process, which involves deposition on a patterned sacrificial layer that is then removed to leave a metallic film on part of the surface.
Strengths of electron-beam evaporation include a high effective deposition rate, which enables acceptable deposition times, and a highly directional source
Figure 2. In a single-axis rotating dome, deposition without a mask produces variations in metal film thickness of +/-12.8 percent to +/-24 percent, depending on material
that limits detrimental sidewall coverage. Very high-quality films are possible by carrying out electron beam evaporation under a high vacuum: This leads to very low levels of incorporation of background gas contaminants, while scattering of the evaporated materials is minimised, thereby maintaining the highly directional nature of the evaporated species.
Figure 1. Growth rates of metals in an electron- beam evaporator decrease with distance from the source to the substrate, and vary with angle
The directional nature limits photoresist via sidewall coverage when the substrate is oriented perpedicularly to the evaporant flux. However, a substrate oriented perpendicularly to the evaporant flux only receives truly normal incident flux at its centre, with some angular variation of the incident flux across the width of the substrate. This can be kept within a desired range by placing it at an appropriate distance from the source. The greater the distance, the smaller the
deviation from normal incidence, with a lower deposition rate as one price to pay. Although the rate can be cranked up by increasing the source power density, there are material-based limits to what is practical. Unfortunately, the nature of the process is such that when an array of perpendicularly oriented substrates are placed at a fixed distance from the source, the deposition thickness will not be inherently uniform, but have some variation in thickness across and between substrates at different locations.
At Temescal, a division of Ferrotec based in Livermore, California, we have devoted much effort to exploring the critical interrelationships between electron beam system architecture, material behaviour and process methodology in order to devise approaches that lead to higher uniformities for electron beam
March 2014
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