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Lift-off is a method of patterning a target material (i.e., a metal) using a sacrificial layer (i.e., photoresist) to define the pattern. First, the sacrificial layer is deposited and patterned; then the target material is deposited on top. The final step is the removal of the sacrificial material; the removal of the sacrificial material lifts off the target material.
The main benefit of lift-off is the ability to pattern a wide variety of materials, especially inert and hard to etch metals. It also avoids damage and compatibility issues that can arise with etching. The limitations include difficult lift-off in the spacing between lines, edge effects, and redeposition of material onto the sample. All of these can be mitigated with proper preparation and processing.
Method of operation
- The metal must be thinner than the thickness of the resist.
- For standard resists, ratios larger than 5:1 are preferable.
- LOR, image reversal, or other processes that produce a negative slop or step reduce the required ratio.
- The metal must have good adhesion to the substrate. Poor adhesion causes the removal of the metal attached to the substrate during the lift-off process.
The heating of the resist in a solvent causes the resist to swell and break any connection to the patterns. Ultrasonic breaks up the metal and removes any connections between the metal on the resist and the patterns remaining.
Figures of Merit
Parameters that affect the effect the quality of the lift-off.
Ratio between photoresist and deposited metal thickness
While deposition techniques such as evaporation minimize amount f metal on the sidewalls of the resist, there is still some material on the sidewalls. The thicker the resist the more likely there will be a discontinuity in the coverage of the sidewall. If the sidewall coverage is continuous the material that was attached to the sidewall may still be attached to the metal adhered to the substrate in many locations. This can be seen as rough edges of the metal and can cause shorts between lines.
Angle of photoresist/formation of discontinuous films
The photoresist (PR) profile has a major impact on the quality of your lift-off. To resolve the smallest gaps and have the cleanest edges you need to have a PR pattern that when the metal is deposited it will create a discontinuous metal film so the solvent that is dissolving the PR can cleanly remove the unwanted metal. This is achieved by having either a negative profile (an overhang of the top of the resist relative to the bottom) or a step profile. In the LNF there are two options for creating these profiles:Image reversal and Lift-off resist.
Metal deposition technique
If photoresist is used as the lift-off material, it requires that the metal deposition is completed with the substrate held at near room temperature (<100C) otherwise the sacrificial layer will be compromised. There are two main metal deposition techniques you can use with photoresist liftoff: Evaporation and Sputter deposition. If the sample is held perpendicular to the evaporation source, the deposition is very vertical to the surface, which has minimal to no sidewall coating. This makes it ideal for liftoff as it is easier to create discontinuous films. Sputter deposition has a much higher conformity because it utilizes plasma to randomly knock material from a large target, and the sample is very close to the target. This leads to sidewall coverage and contentious films. To accomplish good lift-off while sputtering an LOR process is required.
Solvent Bench 14
This bench is used for 4" and 6" silicon and glass wafers
Solvent Bench 84
This bench is used for pieces, long-term soaking, and exotic materials
MicroChemicals Lift-off websiteː https://www.microchemicals.com/technical_information/lift_off_photoresist.pdf
LNF wiki page about LORː http://lnf-wiki.eecs.umich.edu/