The lift-off process in microstructuring technology is a method of creating structures of a target material on the surface of a substrate using a sacrificial material. It is an additive technique as opposed to more traditional subtracting technique like etching. The scale of the structures can vary from the nanoscale up to the centimeter scale or further, but are typically of micrometric dimensions.
Process
An inverse pattern is first created in the sacrificial stencil layer, deposited on the surface of the substrate. This is done by etching openings through the layer so that the target material can reach the surface of the substrate in those regions, where the final pattern is to be created. The target material is deposited over the whole area of the wafer, reaching the surface of the substrate in the etched regions and staying on the top of the sacrificial layer in the regions, where it was not previously etched. When the sacrificial layer is washed away, the material on the top is lifted-off and washed together with the sacrificial layer below. After the lift-off, the target material remains only in the regions where it had a direct contact with the substrate.
Substrate is prepared
Sacrificial layer is deposited and an inverse pattern is created
Target material is deposited. This layer covers the remaining resist as well as parts of the wafer that were cleaned of the resist in the previous developing step.
The rest of the sacrificial material is washed out together with parts of the target material covering it, only the material that was in the "holes" having direct contact with the underlying layer stays
Advantages
Lift-off is applied in cases where a direct etching of structural material would have undesirable effects on the layer below. Lift-off is a cheap alternative to etching in a research context, which permits a slower turn-around time. Finally, lifting off a material is an option if there is no access to an etching tool with the appropriate gases.
Disadvantages
There are 3 major problems with lift-off: ;Retention ;Ears If the ears remain on the surface, the risk remains that these ears will go through different layers put on top of the wafer and they might cause unwanted connections. ;Redeposition
Use
Lift-off process is used mostly to create metallic interconnections. There are several types of lift-off processes, and what can be achieved depends highly on the actual process being used. Very fine structures have been used using EBL, for instance. The lift-off process can also involve multiple layers of different types of resist. This can for instance be used to create shapes that will prevent side walls of the resist being covered in the metal deposition stage.
