Thin film coatings are sometimes applied to entire surfaces of substrates, “from wall to wall”, so to speak, in a continuous continuous film. But in many cases, the final shape of any particular material is applied along the pattern, so that it is covered by certain specific areas and exposed in others.
There are two main ways to achieve this effect:
1. Subtractive or Etch Back process - the entire surface is covered and then parts are removed, leaving the desired pattern. A template formation pattern usually includes some form of physical masking agent, and then a suitable type of etching to remove what needs to be removed, and not to damage anything else.
2. Additive or shutdown process - at the first stage a template generation step is created, which, as a rule, usually includes some kind of physical masking agent. Then follows a coating process that is similar to a stencil. Only the desired pattern is applied through the holes in the mask on the actual substrate. The excess ends on top of the mask and is removed when the mask is removed. This type of thin film removal process will be discussed in this article.
An important consideration for choosing a physical vapor deposition (PVD) process for Lift Off is the specification of the pattern. If the template dimensions and tolerances are reliably large, the physical mask, such as a thin sheet metal stencil, can work, and the process can be of almost any type. But for smaller sizes, sharper line resolution and tighter tolerances, the mask should probably be a photoresist. To achieve clean lines, this photoresist is usually exposed and developed, creating a negative slope, “overhanging” edge, so that the deposition can be shaded, which leaves a small gap between the edge of the covered line and the coverage of the photoresist. For this purpose, there are also special two-layer photoresists, which give a stepped canopy instead of a slope.
And in order to take full advantage of the produced ability, which can give good results in micron or smaller sizes, the vapor deposition flow must have a long free path and fall on a masked substrate, perpendicular to its surface. The first requires low pressure in the chamber, usually below 10-4 Torr. And the latter usually requires a relatively long throw - the distance from the source to the substrate.
For both reasons, thermal evaporation is usually a PVD process. The source is usually located in the center of the bottom of the vertical cylindrical chamber. The substrate holder (commonly referred to as instruments) is a dome rotating around a vertical axis, centered above the source, at a typical distance of 18 inches or more. The dome is usually curved, part of the sphere with a certain radius of curvature. For Lift Off, this radius of curvature should be equal to the ejection distance, which is the source for the substrate (dome).
If the source was a true mathematical point source, it would thus be located in the center of an imaginary sphere with radius R, with the actual dome being the topmost part of the indicated sphere. With a working pressure, usually in the range of 10–5–10–6 Torr, the mean free path — the average distance precipitated or the molecule will move in a straight line before colliding with another gas atom or molecule — will be at least comparable to R And when the vapor particles, all moving in straight lines to all points on the dome, each is on a straight radial line and will hit the surface of the dome perpendicular to the plane that will touch the surface at that point.
This condition creates a perpendicular crash on the curved surface of the dome, which is necessary for the best accuracy of the sample - steam flow at an angle that will not be located exactly in the center of the opening of the photoresist (mask), as it was provided. But the substrates are almost always flat, which is a deviation from this ideal curved surface and, therefore, a deviation from a completely perpendicular incidence. A good rule of thumb for Lift Off high-precision patterns is to keep this angle error, the deviation from the perpendicular flow of vapor on the substrate, to less than 5 degrees. And for substrates, such as semiconductor plates in standard instrument domes, the vapor flow is perpendicular to the center of the plate (zero angular error) and increases towards the edges, with the maximum error depending on the diameter of the plate relative to the throw distance.
At a distance of 18 inches, the distance on a 3-inch plate will have a maximum error of 4.8º at the edges, with a 4-inch plate having an error of 6.4º and larger plates with large errors. At a distance of 24 inches, the error of the 4-inch plate will decrease to 4.8 °, and the 6-inch plate will be 7.2 °. Larger plates require longer throw distances for high-resolution Lift Off results, and longer distances also require a longer free path, which means better vacuum pressure.
Another important fact associated with equipping Lift Off, as described, is that with a constant distance from the entire dome, the internal deposition rate from the vapor phase will deviate from the maximum in the center of the dome directly above the source to lower values Approaching the perimeter. In accordance with the Knudsen law, this should follow a theoretical cosine curve to increase the deviation of the evaporation flow angle from zero (vertical) from the center to the maximum at the perimeter of the dome.
This internal irregularity of the deposition thickness must be compensated for by a fixed blocking mask between the source and the dome, working in conjunction with turning the dome, in order to act on part of the heavier central deposition to reduce it to the same level as the perimeter deposition. There are, of course, details in the developed form of this mask, but without it the thickness uniformity will not be optimal. Please note that this uniformity mask is a large-scale fixed blocking mask, behind which the dome rotates, which is very different from the mask of the layout (photoresist) on a small scale that rotates with the dome / substrates.
As with all such things, the details will be developed to create a good workflow for your final product. However, the removal of a thin PVD film using PVD can be a very useful tool available, especially when selective back etching is difficult or impossible due to the use of specific materials that do not have acceptable selective etchants.
