by Felix Beyeler, Ph.D.
CEO, FemtoTools
If you are working in academic research, the field of MEMS development can be very exciting, but it can also be quite stressful. You will spend long hours in the cleanroom and your professor is constantly pushing you to finalize the prototype fabrication in order to start writing scientific publications. You will probably see no sunlight for a while. When developing a new MEMS fabrication process, the first few wafers (or wafer batches) will usually not yield working devices. This is normal -- a PhD thesis would be much shorter otherwise. Depending on the complexity and the novelty of the process, it takes several weeks, months or even years to get a handful of good chips.
You may ask yourself the question -- how does one make MEMS process development more efficient? My advice is to take the time to inspect all process steps in detail. This sounds trivial, but the inspection part is often neglected. In some case, people continue to process wafers even though all structures are faulty already. Also, you may think that you’ve finally created working devices, but after dicing, gluing and wire bonding you are finally realizing that none of the chips work.
Many fabrication steps can be easily inspected with an optical microscope. This usually takes only a few minutes and helps to identify MEMS fabrication problems. However, the tricky part is to also catch problems that cannot be identified with a microscope. Below you will find a list of the top eight problems where optical inspection will not work. For each problem, I suggest alternative inspection methods.
1. Inaccurate layer thickness of MEMS structure
Many processes rely on the deposition of materials (e.g. via PVD, CVD or electroplating) to create mechanical structures or electrical components. The thicknesses of these material layers are important for a proper functionality and cannot be seen with an optical microscope.
Common inspection method/equipment:
- Stylus profilometer
- Ellipsometer
- Cleave the wafer and inspect with a scanning electron microscope (destructive test)
- Probe-based micromechanical testing
2. Bad sidewall profile
The sidewall of microstructures can have a large impact on the device performance. When looking at your structures with a microscope, the sidewalls cannot be seen very well. Especially under-etching and notching is not usually visible. However, those geometrical variations will significantly change the mechanical properties of springs and flexures.
Common inspection method/equipment:
- Cleave the wafer and inspect with a scanning electron microscope (destructive test)
- Probe-based micromechanical testing
3. Adhesion problems
The adhesion between two layers may be too weak for further processing or for the actual MEMS device. Signs of delamination may be seen with the microscope, but weakly bonded layers cannot be observed.
Common inspection method/equipment:
- Acoustic microscopy
- Probe-based micromechanical testing (destructive test)
4. Internal stress and stress gradients
Internal stress is a common problem when using thin films. Stress introduced in the fabrication process may lead to a reduction in device yield and performance, as well as delamination or cracking of deposited films.
Common inspection method/equipment:
- Optical wafer curvature measurement
- Test structures on wafer in combination with a microscope or a white light interferometer
- Test structures on wafer in combination with probe-based micromechanical testing
5. Cracks
Most cracks can be seen with an optical microscope. However, in some cases, thin “hairline” cracks cannot be seen due to the optical resolution limitations.
Common inspection method/equipment:
- Electrical testing using a probe station
- Acoustic microscopy
- Probe-based micromechanical testing
6. Unsuccessful release
A so-called “release” process is often performed to allow movement of the mechanical part of the MEMS device. This is usually done by under-etching the material which connects the mechanical part to the underlying substrate. In cases of unsuccessful release, it is important to find the point where most of the structure is released but the anchor area is not.
Common inspection method/equipment:
- Break-off device layer of a single chip or a test structure (destructive test)
- Probe-based micromechanical testing
7. Stiction
Mechanical structures such as cantilevers, membranes or shuttles may stick to the underlying substrate upon release of the structure, resulting in a permanent failure of the device. If the distance between the MEMS structure and the substrate is small, the curvature may not be visible with a microscope. However, you want to select good chips only for the time consuming packaging step.
Common inspection method/equipment:
- Electrical testing using a probe station (e.g. in case of capacitive transducers)
- Probe-based micromechanical testing
8. Inaccurate material properties
Novel materials show a great potential for future MEMS devices. However, thin-film materials often show different properties than bulk materials. Especially when using polymers, the mechanical properties (e.g. Young’s Modulus, linearity, hysteresis, etc.) heavily depend on the processing parameters. Inaccurate or unexpected material properties may reduce device performance or even lead device failure.
Common inspection method/equipment:
- Probe station (for electrical properties)
- EDX (stoichiometric analysis)
- Probe-based micromechanical testing (for mechanical properties)
*********************************************
This article is a part of MEMS Journal's ongoing market research project in the area of MEMS testing tools, techniques, and equipment. If you would like to receive our comprehensive market research report on this topic, please contact Dr. Mike Pinelis at [email protected] for more information about rates and report contents.
Copyright 2015 MEMS Journal, Inc.
Comments