Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 5M

Lift-off is used with metals that are difficult to etch with plasmas. Lift-off is a patterning process frequently used in MEMS for patterning materials that do not possess a reliable process to etch them (e.g., noble metals). The liftoff process is accomplished via the use of an intermediate layer and deposition process, which has poor step coverage.

Figure below is a schematic of a lift-off process that will deposit and pattern a material on a substrate or underlying layer.

This process involves the following steps:

• Deposit and pattern a thick intermediate layer of a material that is easy to remove (e.g., SiO2 or photoresist) and that will have a slightly reentrant profile.
• Deposit a layer of the material to be patterned utilizing a process that has poor step coverage (e.g., evaporation). The material thickness should be a fraction of the intermediate layer thickness.
• Removal of the intermediate layer will cause the metal layer to fracture due to the stress concentration in the region of poor step coverage.

Alternatively, the lift-off process can involve a process that will explicitly form an undercut metal layer and not rely on the metal layer to fracture at the step. Figure below is a schematic of a process that will involve the explicit development of an undercut region:

• Deposit thick intermediate layer of a material (e.g., SiO2).
• Deposit and pattern a layer of photoresist.
• Undercut the photoresist with a process such as wet chemical etching.
• Deposit a layer of the material to be patterned utilizing a process that has poor step coverage (e.g., evaporation). The material thickness should be a fraction of the photoresist and oxide layers.
• Remove the SiO2 and photoresist, which will leave only the patterned metal layer.