Thin Film Deposition For Lift Off: Essential Basics

Essential

Thin Film Coatings are sometimes applied to entire surfaces of substrates, “wall to wall” as we say, in a continuous unbroken film. But a lot of times the final kind of whatever particular material will be employed is patterned so it is coated using specific places and bare in the others.

There are two main ways to achieve this result:

Inch. Subtractive, or Etch Back process – the entire surface has been coated, and pick components are removed, leaving the desired design. The pattern generating step generally entails some type of physical masking representative after which an appropriate type of etching to remove what should be removed also not damage other things.

2. Additive, or Lift Off process – the pattern manufacturing step, which may typically entail some type of physical masking agent, includes. That is followed closely with the coat procedure, which is much like using a stencil. Only the desirable pattern gets applied through the openings at the mask on the actual substrate. The excess eventually ends up in addition to the mask and is removed when the mask is lifted off. This type of Lift Off Thin Film Deposition process is going to be the main topic of this article Regarder film streaming.

A important concern for choosing a Physical Vapor Deposition (PVD) process for Lift Off is the design specification. In case the pattern measurements and tolerances are rather large, then a physical mask such as a thin sheet metal stencil can perhaps work and the procedure can be essentially any type. To achieve clean lines, this photoresist is usually exposed and developed to create a negative incline, an “overhanging” border therefore that the deposition can be shadowed underneath it leaving a little gap between the boundary of the coated line and the photoresist policy. Additionally, there are special dual layer photoresists for this purpose, giving a step over hang rather than a slope.

And also to take real advantage of the capability thus afforded, that may give fantastic results in micron or smaller dimensions, the deposition vapor flow needs to have a long mean free path and impinge on the masked substrate perpendicular to its surface. The former requires low space pressure, typically below 10-4 torr. And the latter normally takes a relatively long throw – the distance from source to substrate.

For both reasons, Thermal Evaporation is usually the PVD means of preference. The origin is usually situated in the middle at the underside of a vertical cylindrical chamber. The substrate holder (usually called tooling) is just a dome rotating about a vertical axis based above the source at a regular distance of 18 inches or even more. The dome is usually curved, a part of a sphere with some radius of curvature. For Lift Off, this radius of curvature needs to be corresponding to the throw distance, that’s the source to substrate (do me) space.

If the source were a true mathematical point source, it might thus be located at the center of the imaginary universe of radius R, with the actual dome function as topmost portion of said audience. With process pressure typically from the 10-5 to 10-6 torr range, the mean free path – the typical distance an disappeared atom or molecule will travel in a straight line before
colliding

with another gas atom or molecule – will be at least equal to R. And with the vapor contaminants all travel in straight lines to each of points onto the terrace, each is about an immediate radial line and will strike the surface of the dome perpendicular to the plane that would be tangent to the surface in the point.

This affliction produces perpendicular incidence on the curved dome surface, and this is essential for the best design accuracy – a vapor stream arriving at a angle won’t deposit exactly in the middle of their photoresist (hide) opening as had been intended. However, substrates are almost always flat, and it is actually a deviation from this perfect curved surface and so a deviation from completely vertical prevalence. A fantastic rule of thumb for high accuracy Lift Off patterns is to keep this error, the deviation from perpendicular vapor stream impingement on the substrate, to less than 5 degrees. And, for substrates such as semiconductor wafers in ordinary tooling domes, the vapor flow is perpendicular at the middle of the wafer (zero angular error) and rises toward the edges, with the maximum error being reliant upon the wafer diameter in relation to the throw distance.

At a 24-inch throw space, the 4 inch wafer’s error would reduce 4.8º using a 6 inch wafer being 7.2º.

Another essential fact associated with Lift Off tooling as described is that, with the throw space being constant over the whole dome, the intrinsic vapor residue rate will fall away out of the maximum at the dome’s center directly above the source to reduce worth coming the perimeter. In keeping with Knudsen’s Law this should stick to a theoretical cosine curve for its increasing deviation of evaporant stream angle from zero (vertical) in the center to its maximum at the do me perimeter.

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