In this blog post, we’ll explain the concept of micro-electro-mechanical systems (MEMS) and its key fabrication processes—lithography and etching—in simple terms.
From cartoon to reality: tiny machines are all around us
Anyone who has watched “The Magic School Bus” will remember the children learning about science whilst riding on a bus that could change size at will. In particular, the scene in the ‘Exploring the Human Body’ episode, where the school bus shrinks to an infinitesimally small size to travel inside the human body and explain its science, captured the imagination of many.
However, it is difficult to dismiss this science-fiction-like imagination as completely far-fetched. Today, engineers are actually manufacturing mechanical structures so small they are hard to imagine, and as a result, ultra-fine sensors are widely used in everyday devices such as smartphones.
For example, accelerometers detect the tilt and movement of a mobile phone, making mobile games and screen transitions more dynamic, whilst magnetic sensors use the Earth’s magnetic field to determine location and provide navigation via maps. Touchscreens can also be seen as a distillation of sensor technology.
What is MEMS: A general term for creating tiny machines
MEMS stands for Micro Electro Mechanical Systems and is a general term for various manufacturing processes used to create mechanical structures on the order of micrometres (μm). Although the name sounds complicated, it essentially refers to a collection of manufacturing technologies that create conventional mechanical devices on a much smaller scale.
Commercially available micro-sensors typically range in size from a few micrometres to several tens of micrometres (μm). Considering that a human hair is approximately 60 μm thick, one can begin to appreciate just how small MEMS devices are.
The materials primarily used in the manufacture of MEMS include metals and semiconductors (particularly silicon). Whilst the processes and materials employed vary greatly depending on the purpose of the device and the required performance, two technologies are fundamental to almost all processes: lithography (optical patterning) and etching (a process of shaping materials by removing material).
Lithography and Etching: The Manufacturing Process Explained Through the Analogy of Printmaking
It is easier to understand lithography if you think of it as printmaking. Just as there is a process of drawing a sketch on a wooden block before printing, lithography is akin to the stage of drawing the outline of the pattern to be created on a silicon wafer.
In this context, light acts as the ‘pencil’. Light consists of particles called photons. When light is shone onto a silicon wafer coated with a photosensitive material called photoresist, the physical properties of the areas exposed to light change, allowing a pattern of the desired shape to be formed. Lenses are used to reduce the original pattern to a microscopic size and project it onto the wafer, whilst the positioning of the pattern is carried out with precision using autofocus mechanisms and computer control.
Once the outline (pattern) is complete, the process moves on to etching, the stage where the material is actually removed. Just as a engraving knife carves various shapes into a plate in printmaking, there are various types of ‘knives’ used in etching. One of the most representative methods is Reactive Ion Etching (RIE).
In RIE, plasma ions are directed at the desired location and collide with the silicon surface; the material is then removed through chemical and physical reactions between the ions and the silicon atoms. By etching layer by layer in this manner, three-dimensional microstructures are created.
Just as printing a print onto paper produces a two-dimensional work, the master plate itself possesses thickness and a sense of depth. The master plate (the structure on the wafer) obtained through the MEMS process is three-dimensional, and this master plate forms the very core of various sensors and micro-mechanical devices.
Although lithography and etching form the basis of MEMS manufacturing, just as there are various types of plate materials and ‘carving tools’, there are numerous actual processes and variations used. The final device is completed through a combination of numerous subsequent processes, such as deposition, doping and chemical mechanical polishing.
Current Status and Outlook: The Challenge of a Smaller World
Even at this very moment, countless researchers and engineers are striving to make MEMS technology more precise and efficient. Thanks to their research, the technology is expanding beyond the sensors found in smartphones into a wide range of applications.
In the not-too-distant future, we cannot rule out the possibility of technologies such as miniature robots—which, like the ‘Mystery School Bus’, move freely through blood vessels to diagnose diseases or assist in treatment—becoming a reality. Advances in MEMS processes are the key to turning such imaginings into actual devices.
In conclusion, MEMS is not merely a ‘machine made small’, but a foundational technology that enables the operating principles of many electronic devices we use in our daily lives. Understanding basic processes such as lithography and etching will allow us to better comprehend the engineering magic unfolding in the ultra-micro world.
