How do you reduce the stress and deformation of MEMS structures during fabrication?

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MEMS, or microelectromechanical systems, are devices that combine mechanical and electrical components at the microscale. They have applications in sensors, actuators, biomedical devices, and more. However, MEMS fabrication poses many challenges, such as stress and deformation of the structures due to thermal, mechanical, and chemical processes. How do you reduce the stress and deformation of MEMS structures during fabrication? Here are some tips and techniques that can help you achieve high-quality and reliable MEMS devices.

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Optimize deposition parameters:

By carefully controlling the gas ratios, temperature, and pressure during deposition processes, you can minimize residual stress in MEMS structures. It’s a precision game where attention to detail pays off in durability.
Exposure level testing:

When working with lithography, test various levels of light exposure to find the sweet spot. This can greatly improve feature quality while accounting for factors like surface reflectivity.

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1 Choose appropriate materials

The choice of materials for MEMS structures affects their performance, compatibility, and durability. Different materials have different thermal expansion coefficients, Young's moduli, residual stresses, and chemical properties. These factors can cause mismatched strains, warping, cracking, or delamination of the structures during fabrication. Therefore, you should choose materials that have similar or compatible properties, or use buffer layers, adhesives, or coatings to reduce the stress and deformation.

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Coleman Murray

Founder & Biotech Innovator | Scalable & Automated Biotools | New Venture Launch
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Material compatibility is a critical aspect to control. Thermal expansion is particularly critical since a typical MEMS fab process requires multiple temperature cycling steps. Another feature is in understanding wettability of different materials. For example, glass will not have optimal wettability for most photoresists which can result in poor thin film deposition and film adhesion. Often surface modification is needed (I.E. silane or HMDS treatment for glass)

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2 Optimize the process parameters

The process parameters for MEMS fabrication, such as temperature, pressure, time, flow rate, etching rate, and deposition rate, can also influence the stress and deformation of the structures. For example, high temperature can cause thermal stress and diffusion, high pressure can cause mechanical stress and bending, and high etching rate can cause undercutting and roughness. Therefore, you should optimize the process parameters to minimize the stress and deformation, or use techniques such as annealing, stress relief, or planarization to mitigate the effects.

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Jay Cech
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Most deformations are caused by deposited film stress, and changes in that film stress during thermal process steps later in the flow. Careful optimization of stress by control of the deposition parameters (gas ratios, temperature, pressure ) for CVD and sputtering processes, as well as controlling the process sequence, meaning the sequence of high temperature steps, and their temperature/time can minimize residual stress in fabricated structures.

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Coleman Murray

Founder & Biotech Innovator | Scalable & Automated Biotools | New Venture Launch
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Lithography requires significant optimization. In the past when I designed a new device, I would develop a ladder of light exposure levels to test then observe the lithography feature quality for each exposure. Typically I target +/- 15% of the calculated exposure time. Surface reflectivity is a key factor here, for example, if you are doing lithography on a highly reflective surface this will affect your exposure time.

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Shu Xiang

Thoughtful regulatory service for small to medium sized medical device innovators
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I also would like to add that for certain polymers (e.g. SU-8), the residual stress can gradually change with time. Freshly processed SU-8 can go through a slow relaxation process over a few days to weeks.

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3 Design for manufacturability

The design of MEMS structures can also affect their stress and deformation during fabrication. For example, complex geometries, sharp corners, thin films, or large aspect ratios can increase the stress concentration, fracture risk, or stiction probability. Therefore, you should design for manufacturability, which means considering the fabrication constraints and limitations, and avoiding features that can cause stress and deformation. You can also use simulation tools, such as finite element analysis, to predict and evaluate the stress and deformation of your design.

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Coleman Murray

Founder & Biotech Innovator | Scalable & Automated Biotools | New Venture Launch
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No matter what, start your process on wafer based formats, preferably 4inch. I wasted a lot of time trying to do processing on small slides. Try to integrate as much of your process on wafer level and leverage the automated tools.

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4 Test and characterize the structures

The final step to reduce the stress and deformation of MEMS structures during fabrication is to test and characterize them. You can use various techniques, such as scanning electron microscopy, atomic force microscopy, interferometry, or Raman spectroscopy, to measure the stress and deformation of the structures, and compare them with the design specifications and expectations. You can also use reliability tests, such as thermal cycling, mechanical shock, or humidity exposure, to assess the stability and durability of the structures under different conditions.

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How do you reduce the stress and deformation of MEMS structures during fabrication?

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3

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1 Choose appropriate materials

2 Optimize the process parameters

3 Design for manufacturability

4 Test and characterize the structures

5 Here’s what else to consider

MEMS

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