Making Micro-Machines with Lasers: Learning How to do it Better

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Source: http://internetofthingsagenda.techtarget.com/definition/micro-electromechanical-systems-MEMS

Chances are, if you took apart your cell phone, you would of course find a bunch of very small parts. What might not be so obvious, though, is that your cell phone contains parts that are even too small for you to see without using a microscope. These parts include what are called “micro-electrical-mechanical systems,” of MEMS for short. MEMS are found in your cell phone, but also in countless other mechanical and electrical devices. Making them requires work on a microscopic level. In everything from cars and televisions to blood pressure sensors used in hospitals, MEMS have paved the way for new frontiers in technology. These tiny devices can be made to sense pressure, act as valves to control the flow of gas or liquid, form micromirror arrays for digital displays, and serve many other purposes that are almost unimaginable for something so small.

 

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An example of just some of the places we might find MEMS in our daily life. Source: https://www.intechopen.com/books/air-quality-new-perspective/microscopy-and-spectroscopy-analysis-of-mems-corrosion-used-in-the-electronics-industry-of-the-baja-

The potential applications for MEMS might seem endless, except at least one important problem is keeping MEMS from reaching their full potential. One such challenge is that silicon, the material most commonly used material for creating these microscopic contraptions, isn’t the most durable. When exposed to high temperatures or harsh environments, it loses its strength. Therefore, MEMS made out of simple silicon can’t be used for everything. There is great potential for MEMS to be used in outer space, missile control systems, and oil well drilling equipment, but in many of these uses the MEMS would be exposed to high heat, lots of pressure, or other extreme environments. Silicone might not hold up in these scenarios. To fix this problem, some MEMS are now being constructed from a similar, but much stronger material called silicon carbide. Silicon carbide is one of the hardest materials known, and is very resistant to damage. There are, therefore, many possible applications for silicon carbide MEMS that cannot be filled by standard silicon MEMS. The potential of silicon carbide MEMS, though, is still being realized, partly because they are more difficult to construct than plain silicon MEMS.

 

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How small are MEMS? While you can see many MEMS with just your eyes (such as the small panel in the photograph at the start of this article), MEMS contain parts that are truly microscopic. This photo shows a microscopic mite approaching a set of microscopic gears. These gears, like most components of MEMS, are shaped by laser drilling. Image source: http://www.sandia.gov/mstc/mems_info/movie_gallery.html

Silicon carbide is a great choice for MEMS construction because of its durability; unfortunately, that durability makes it extremely difficult to manufacture on a microscopic level. In order to create a MEMS, the silicone or silicone carbide must be carved into shape. The tools available for this job are limited. Etching (the process of selectively carving into a surface using chemical or physical processes), used commonly to shape silicon, goes very slowly in silicon carbide, making it an inefficient and unfavorable option. Another option is to use lasers to remove material.

Lasers are basically very focused beams of light. Some lasers can be used to “cut” into material by heating it so that it either melts or vaporizes. Not all lasers are capable of this, of course – common laser pointers are very different from the advanced and extremely precise lasers used to make MEMS and are nearly harmless. In MEMS manufacturing, however, extremely focused lasers are used to make microscopic alterations to the surface of silicon carbide by heating and either melting or vaporizing the material. When the laser beam hits the silicon carbide, energy from the laser beam is absorbed by the silicon carbide, thereby heating it up. One of the main challenges in working with these lasers is that the melting of silicon carbide can leave imperfections. Some tiny amount of debris may be left behind, or the heat affected zone might be deformed in other ways. Because MEMS are so small, there isn’t much room for error, and even one microscopic crack or deformity might ruin the device.

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Nd:YAG laser drilling Source: https://www.netl.doe.gov/research/oil-and-gas/project-summaries/completed-ep-tech/de-fc26-00nt40917-

In order to avoid these imperfections and make the process of manufacturing MEMS as efficient and effective as it can be, scientists want to know why these imperfections happen. Think of it like trying to bake bread without a recipe. If you had never done it before, you might first start by mixing ingredients you think probably go into bread, like flour and salt and maybe yeast, then setting the oven to some common baking temperature and baking it until it looks done. But what if the bread doesn’t rise and comes out of the oven too hard to eat. What went wrong? Was your baking temperature wrong? Did you use the wrong ingredients, or the wrong amount of an ingredient? The best way to find out might be to change each thing you did one at a time and try baking a new loaf each time. You might see that if you add more yeast, the bread rises more, but if you bake it too hot, it doesn’t have time to rise. You want to figure out the best combination of techniques to make the ideal bread. This is similar to the process that researchers are using to figure out how to make the best silicon carbide MEMS. For example, a 2017 study published in the Journal of Applied Physics looked at different ways that lasers (specifically, lasers called Nd:YAG lasers) can be used to drill into silicon carbide. The study looked at how using different wavelengths of light in the laser beam, different laser beam profiles, and submerging the laser and silicon carbide in water while drilling could create different effects in the final products. Its goal was to gather information that could be used to determine the best combination of these parameters depending on what the final product should be able to do.

Beam Profile

This study tested two different beam profiles, the Gaussian beam and the Bessel beam. Gaussian beams are considered the “conventional” method, but they have relatively large spot size and therefore limit the precision of drilling that can be achieved, much like trying to draw a very detailed picture with a dull pencil. Bessel beams are more precise, but they tend not to be able to drill as deep as Gaussian beams.

Wavelength

Light travels in waves. These waves “look” (although we can’t actually see the waves in light, we just see the light itself) like ocean waves, moving up and down as they travel over some distance. The “wavelength” of light is determined simply by how long each wave is – that is, the distance, measured in nanometers, from the peak of one wave to the peak of the next one. Different wavelengths of light also have different energy levels and affect silicon carbide slightly differently (recall that the energy from the light beam is what causes the silicon carbide to heat up and melt). Thus, while one wavelength is not necessarily better than another, a balance must be achieved between wavelength and other conditions, such as beam profile. The study compared near-infrared light with a wavelength of 1064 nanometers to visible light with a wavelength of 532 nanometers. It found that when a Bessel beam profile was paired with a longer wavelength, a thick layer of re-solidified silicon that limited the depth that the laser could drill. However, pairing the longer wavelength with the Gaussian beam was more effective. It is important to note, though, that the Gaussian beam was also used successfully with shorter wavelength light, especially in an air environment.

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Diagram showing the different wavelengths of light. Source: https://www.khanacademy.org/science/biology/photosynthesis-in-plants/the-light-dependent-reactions-of-photosynthesis/a/light-and-photosynthetic-pigments

Ambient Conditions

To control the effects of heat on the silicon carbide, many researchers have suggested using water to cool the silicon carbide. However, water also made deep drilling more difficult. The study found that, although submerging the drill site in water did help to reduce the defects and debris left by melting, it also caused cracks to form around the hole.

What does it all mean?

The use of lasers to manufacture microscopic electrical and mechanical systems is an evolving area of research. While this study did provide important information about the effects that light wavelength, beam profile, and ambient conditions have on silicon carbide drilling, actually deciding what the best combination of these parameters is for different applications is a topic for future researchers to address. It is exciting to think, though, what MEMS could do once we perfect how they are made.

 

Sources:

Paschotta, Dr. Rüdiger. “YAG Lasers.” Encyclopedia of Laser Physics and Technology – YAG lasers, Nd:YAG laser, Yb:YAG, yttrium aluminum garnet. February 20, 2017. Accessed May 12, 2017. https://www.rp-photonics.com/yag_lasers.html.

“Laser Drilling Services – Laserage Technology Corporation.” Laserage. Accessed May 12, 2017. http://www.laserage.com/laser-drilling/.

Kim, Byunggi, Ryoichi Iida, Duc Hong Doan, and Kazuyoshi Fushinobu. “Mechanism of nanosecond laser drilling process of 4H-SiC for through substrate vias.” Applied Physics A 123, no. 6 (2017). doi:10.1007/s00339-017-0986-2.

“Introduction to Microelectromechanical Systems (MEMS).” Introduction to Microelectromechanical Systems (MEMS) | Compliant Mechanisms. Accessed May 12, 2017. https://compliantmechanisms.byu.edu/content/introduction-microelectromechanical-systems-mems.

Sarro, Pasqualina M. “Silicon carbide as a new MEMS technology.” Sensors and Actuators A: Physical 82, no. 1-3 (2000): 210-18. doi:10.1016/s0924-4247(99)00335-0.

“Band gap.” Energy Education. Accessed May 12, 2017. http://energyeducation.ca/encyclopedia/Band_gap.S