Thin Film Deposition For Lift Off: Essential Basics
But many situations the last kind of whatever particular material is being employed is stained so that it is coated in certain specific places and bare in others.
There are two main ways to achieve this particular effect:
Inch. Subtractive, or Etch Back process – that the entire surface is coated, and then pick components are removed, leaving the required pattern. The pattern generating step generally entails some type of physical masking agent and then an proper sort of etching to remove everything should be removed and not hurt anything else.
2. Additive, or Lift Off process – that the pattern generating step, which will normally involve some sort of physical hiding representative, comes. This is followed closely by the coat process, that is similar
to Regarder film streaming
having a stencil. Only the desired pattern gets applied through the openings at the mask onto the true substrate. The excess ends in addition to the mask and is removed while the mask has been lifted off. Such a Lift Off Thin Film Deposition process is going to soon be the subject of this article.
In the event the pattern measurements and tolerances are rather large, then a physical mask like a thin sheetmetal stencil can perhaps work and the process can be any type. But for more compact measurements, 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 generate a negative slope, an “over hanging” border therefore that the deposition may be shadowed beneath it leaving a little gap between the boundary of their coated line and also the photoresist coverage. There are also special dual layer photoresists for this purpose, giving a step over hang instead of a slope.
And also to take advantage of this capability thus afforded, that may give good success in micron or smaller dimensions, the deposition vapor flow must have a long mean free path and also impinge on the hidden substrate perpendicular to its surface. The previous requires low chamber pressure, typically below 10-4 torr. And the latter normally takes a relatively long throw – that the distance from source to substrate.
For both of these causes, Thermal Evaporation is typically the PVD process of choice. The origin is usually positioned in the center of the base of a vertical cylindrical chamber. The substrate holder (usually called tooling) is just a dome rotating about a vertical axis based above the foundation at a common distance of 18 inches or longer. The dome is often curved, a part of a world with some radius of curvature. To get Lift Off, this radius of curvature should be equal to this throw distance, which is the cause of substrate (do me) space.
If the foundation were a true mathematical point source, it would thus be located at the center of the imaginary world of radius R with the true dome being the lightest part of said world. With process pressure typically in the 10-5 to 10-6 torr range, the mean free path – the average space an disappeared atom or molecule can travel in a direct line before colliding with another gas atom or molecule – will probably soon be equal to R. And with the vapor contaminants all travel in straight lines to all things onto the terrace, each is about an immediate radial line and also certainly will strike the top of dome perpendicular to the plane which would be tangent to the surface at the time.
This condition creates vertical incidence on the curved ribbon surface, which is necessary for the best pattern accuracy – and – vapor flow arriving at a angle will not deposit exactly in the center of the photoresist (hide) opening as was intended. But substrates are always flat, and it is really a deviation from the great curved surface and therefore a deviation from completely vertical prevalence. A fantastic rule of thumb for high accuracy Lift Off routines is to keep this angular error, the deviation from perpendicular vapor flow impingement on the substrate, to less than 5 degrees. And, for substrates such as semiconductor wafers in standard tooling domes, the vapor stream is vertical at the middle of the wafer (zero angular error) and rises following the borders, with the maximum error being dependent on the wafer diameter in relation to the throw distance.
In an 18 inch throw space, a 3 inch wafer would hence have a 4.8º maximum mistake at its edges, with a 4 inch wafer using a 6.4º mistake and larger wafers having larger errors. At a 24 inch throw distance, the four inch wafer’s mistake would reduce 4.8º using a 6 inch wafer being 7.2º. Bigger wafers require longer throw distances for high resolution Lift Off results, and more distances additionally require a longer mean free path which means better vacuum pressure.
One other important fact correlated with Lift Off tooling as clarified is that, with the throw distance being constant across the whole dome, the inherent vapor deposition rate will fall away out of its highest in the dome’s centre directly above the foundation to lower values coming the perimeter. In line with Knudsen’s Law, this should adhere to with a theoretical cosine curve for its rising deviation of evaporant flow angle in zero (vertical) in the centre to its maximum at the do me perimeter.
