Thin Film Coatings are sometimes applied to entire surfaces of substrates, "wall to wall" so to speak, in a continuous unbroken film. But many times the final form of whatever particular material is being applied is patterned so that it is coated in certain specific areas and bare in others.
There are two principal ways to achieve this effect:
1. Subtractive, or Etch Back process - the entire surface is coated, and then select portions are removed, leaving the desired pattern. The pattern generating step normally involves some form of physical masking agent and then an appropriate type of etching to remove what should be removed and not damage anything else.
2. Additive, or Lift Off process - the pattern generating step, which again will normally involve some sort of physical masking agent, comes first. This is followed by the coating process, which is similar to using a stencil. Only the desired pattern gets applied through the openings in the mask onto the actual substrate. The excess ends up on top of the mask and is removed when the mask is lifted off. This type of Lift Off Thin Film Deposition process will be the subject of this article.
A major consideration for choosing a Physical Vapor Deposition (PVD) process for Lift Off is the pattern specification. If the pattern dimensions and tolerances are relatively large, a physical mask such as a thin sheet metal stencil Film streaming can work and the process can be essentially any type. But for smaller dimensions, sharper line resolution, and tighter tolerances, the mask will probably have to be photoresist. To achieve clean lines, this photoresist is usually exposed and developed to create a negative slope, an "overhanging" edge so that the deposition can be shadowed beneath it leaving a small gap between the edge of the coated line and the photoresist coverage. There are also special dual layer photoresists for this purpose, giving a step overhang instead of a slope.
And to take real advantage of the capability thus afforded, which can give good results in micron or smaller dimensions, the deposition vapor stream must have a long mean free path and impinge on the masked substrate perpendicular to its surface. The former requires low chamber pressure, typically below 10-4 torr. And the latter normally requires a relatively long throw - the distance from source to substrate.
For both of these reasons, Thermal Evaporation is usually the PVD process of choice. The source is normally located in the center of the bottom of a vertical cylindrical chamber. The substrate holder (usually called tooling) is a dome rotating about a vertical axis centered above the source at a typical distance of 18 inches or more. The dome is commonly curved, a portion of a sphere with some radius of curvature. For Lift Off, this radius of curvature should be equal to the throw distance, which is the source to substrate (dome) distance.
