Surface Micromachining - Lift-Off Process Kit

(Formerly the Lift-off Kit)

Description

This kit supports the MTTC Pressure Sensor Process Learning Module.

Using this kit, participants execute and observe the lift-off process that is used to fabricate various MEMS components. Participants follow a procedure that results in selective removal of two pre-deposited layers: chrome and gold. The chrome and gold which remain on the chip form a Wheatstone bridge circuit, the sensing circuit used in a MEMS pressure sensor.


The kit contains seven chips pre-deposited with chrome and gold and patterned for a Wheatstone bridge circuit, plus a chip holder and process flask.  Also included is the MTTC Pressure Sensor Process Learning Module – Book 1 and 2, which provides background material and kit-specific activities. 

Because this kit requires the use of acetone, all chemical safety rules must be observed.  Participants must study the Material Safety Data Sheet for acetone prior to completing this activity in order to learn the potential hazards of acetone as well as safe handling procedures. For example, acetone is a severe irritant on the eyes and should not be ingested. Also, proper gloves are required when handling acetone. Due to shipping regulations, the instructor is required to supply the acetone, available at home hardware stores.    See more
 

Kit refills are available which consist of replacement, patterned chips ready for lift-off.
 
 

Where can I use this kit?

This kit can be used by itself or as part of a series of hands-on experiences to bring microsystems fabrication to the classroom.
 
If you teach any of the following STEM topics, this kit can add an exciting hands-on element to your students' learning:
  1. Chemistry - This hands-on activity allows students to see the actual lifting off of chrome/gold.  The sacrificial photoresist layer is removed in a wet etch process leaving the desired pattern on the chip. This process can be used to teach concepts on evaporation (deposition of metal) and dissolution.  Students enjoy seeing the thin flakes of gold lifting off and floating away in the beaker.  One can also investigate the role of temperature and concentration on the dissolution of the sacrificial, photoresist layer.  This has been successfully used in not only college tech classes but also in AP chemistry classes. 
  2. MEMS (Microsystems) Fabrication - There are many devices requiring the deposition and patterning of thin metal layers.  Typically, metal is deposited on a substrate and a pattern is subsequently etched into this material.  The pattern is transferred to the metal by 1) coating it with a thin film of photoresist, a photoactive compound, 2) exposing the resist to a UV light pattern, 3) removing the resist which was exposed to UV light, and 4) etching the metal which is no longer protected by the photoresist.  The resist is resistant to etch and acts to protect what is under it.  Lift-off is a process not often taught in semiconductor process classes.  In the case of the liftoff process, the process is stood on its head.  Two layers of resist are first coated on the substrate - 1) LOR or Lift Off Resist followed by 2) PR or photo resist.  The photoresist is patterned with light and developed exposing the substrate where you want the metal to be deposited.  A blanket deposition of metal is performed resulting in metal being coated on top of the exposed substrate and the remaining photoresist.  During lift-off the remaining resist is "lifted off" along with the metal which is on top of the resist leaving behind the desired metal pattern. Here the patterned resist is a sacrificial material; it is removed along with the metal that was deposited on top.  The lift-off process is relatively common in making MEMS devices including pressure sensors and hard drive heads. 


This kit along with the anisotropic etch kit, the rainbow wafer kit, and the pressure sensor process kit allows the educator/trainer to bring these processes into the classroom to teach the fundamentals of making MEMS devices in a cleanroom.  The images below are of an actual lift-off process.

 

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