If you're committed to academic research, the field of MEMS sensor development can be incredibly exciting, but it's also incredibly stressful. You'll spend long hours in cleanrooms, possibly without sunlight for extended periods, and your supervisor will constantly push you to complete prototype manufacturing in order to write and publish academic papers. When developing a new MEMS sensor manufacturing process, the first few wafers typically won't produce working devices in large quantities. Depending on the complexity and innovation of the process, it can take weeks, months, or even years to obtain a limited number of good chips.
You might ask yourself: how can we make MEMS sensor manufacturing processes more efficient? My personal suggestion is to spend some time and effort carefully inspecting all process steps. It sounds simple, but this inspection is often overlooked. In some cases, people continue processing wafers even when all the structures are flawed. Similarly, you might think you've built a working device, but after slicing, gluing, and bonding, you find that not a single chip works.
Under an optical microscope, many manufacturing steps can be easily observed, often within minutes, to help identify problems in MEMS sensor manufacturing. However, the most challenging issues are those that the microscope cannot help identify. Below are eight key problems beyond the reach of an optical microscope lens, with specific inspection methods provided for each.
1. Inaccurate MEMS sensor structure layer thickness
Many process methods (such as physical vapor deposition, chemical vapor deposition, or electroplating) rely on deposited materials to construct mechanical structures or electronic components, and the thickness of the material layer, which is not visible under an optical microscope, has a significant impact on performance.
Common inspection methods/equipment:
- Contouring
- Elliptic Compass
- Cut the wafer and observe it using a scanning electron microscope (a destructive test).
- Probe-based micromechanical testing
2. Poor sidewall profile
The sidewalls of microstructures have a significant impact on device performance. When examining the structure under an optical microscope, the sidewalls are not clearly visible. In particular, under-etching and trenches are often invisible. However, these geometric deformations can significantly alter the mechanical properties of springs and flexible plates.
Common inspection methods/equipment:
- Cut the wafer and observe it using a scanning electron microscope (a destructive test).
- Probe-based micromechanical testing
3. Adhesion issues
The adhesion between the layers within a MEMS sensor structure may be very small. An optical microscope might show signs of delamination, but the tiny adhesive layers would not be observable.
Common inspection methods/equipment:
- Acoustic microscope
- Probe-based micromechanical testing (destructive testing)
4. Internal stress and stress gradient
Internal stress is a common problem when using thin films. Stress generated during the manufacturing process can lead to reduced device yield and performance, as well as delamination and cracking of the deposited film.
Common inspection methods/equipment:
-Optical wafer surface measurement
- Combine microscopy or white light interferometer to test wafer structure
- Probe-based micromechanical testing of wafer structures
5. Cracks
Most cracks can be seen under an optical microscope; however, in some cases, fine "hairline" cracks are invisible due to resolution limitations.
Common inspection methods/equipment:
- Probe station electrical testing
- Acoustic microscope
- Probe-based micromechanical testing
6. Failed release process
The so-called release process typically involves incomplete etching of the material connecting the mechanical parts to the substrate to create movable mechanical parts in MEMS devices. When release fails, it's crucial to identify the areas where most of the release structures successfully released, but the anchor points failed to release properly.
Common inspection methods/equipment:
- Destructive testing of a single chip or a test structure.
- Probe-based micromechanical testing
7. Viscosity
Mechanical structures like cantilever beams, thin films, and shuttle valves may permanently fail due to adhesion between the structure and the underlying substrate after release. If the distance between the MEMS sensor structure and the substrate is very small, the curvature will be invisible under a microscope. If you want high-quality chips, you'll likely have to select them during the packaging and testing phase.
Common inspection methods/equipment:
- Probe station electrical testing (e.g., for capacitive sensors)
- Probe-based micromechanical testing
8. Inaccurate material properties
Novel materials have shown great potential in MEMS sensor devices. However, thin film materials exhibit more diverse properties than bulk materials. Especially when using polymers, mechanical properties such as Young's modulus, linearity, and hysteresis are heavily dependent on process parameters. Inaccurate or undesirable material properties can degrade device performance or even lead to device failure.
