Bench Talk

(Source: Martin - stock.adobe.com)

Microelectromechanical systems (MEMS) have become a mainstay technology in many industries and their market size only continues to grow. MEMS are small devices that use both mechanical and electronic parts. These devices have been around for decades and have since become a multibillion-dollar industry. One of the defining features of MEMS is that it is not a single device or a single product; rather, the concept of MEMS is a portfolio of fabrication techniques and design processes that can create different miniature systems. Another key feature of MEMS is that they are not an off-the-shelf product, but they are instead designed and customized to their intended application. Here, we look at how different MEMS are created using these processes.

MEMS are a combination of electronic and mechanical components that work together to create a small integrated device with a specific function. MEMS come in many shapes and sizes and can have many different materials and components within the system. For these systems to be classified as a ‘micro’ system, the individual components of the MEMS must be on the micron scale. However, depending on how many components are built together to create a functional device, the sizes of a complete MEMS device can range from a few micrometers all the way to millimeter-sized devices.

While there are many specific devices using MEMS, all MEMS devices contain some combination of mechanical microstructures, microsensors, microactuators, and microelectronicsall of which are integrated onto silicon chips. Inside of these general component areas, there are many specific components behind a MEMS device—including levers, gears, pistons, and motors—that give it its overarching properties. These individual components are made from many different types of materials, depending on the intended function, and commonly include the following:

• Silicon
• Silicon-based materials like silicon oxide, silicon nitride, and silicon carbide
• Thin metal films
• Polymers
• Glass
• Fused quartz substrates
• Diamond
• Gallium arsenide
• Other group III-V compound semiconductors and shape-memory alloys

The combination of components possible with MEMS means that the devices created can sense, control, and actuate at the micro level but show operational effects at the macro level. Keep in mind that the macro output is the sum of the individual micro components working together to perform specific functions. Because there are so many different components that make up a MEMS device, the application scope of MEMS is large, but regularly includes different types of sensors, actuators, and transducers across a range of technical and scientific industries.

Between the wide range of potential materials and the sheer number of components that can be integrated into a MEMS device, there are numerous fabrication methods possible for building these devices. Generally speaking, the electronic components are created using integrated circuit batch processing methods, while the mechanical parts are fabricated using micromachining methods. Even so, there are a range of advanced fabrication techniques now available thanks to advancements in process tooling and high precision microfabrication methods, including both lithography and non-lithography-based methods.

Micromachining methods used to create MEMS are similar to conventional machining methods because they define specific features into a material. However, there are some differences between the two, with micromachining being able to simultaneously fabricate thousands of identical features onto the same wafer, as well as being able to process many wafers at the same time. The other main difference is that micromachining methods can create much smaller features in materials than conventional machining methods―at least one order of magnitude smaller.

As far as the common and more basic micromachining tools go, there are several methods used. For example, epitaxy, sputtering, evaporation, chemical-vapor deposition, and spin-on fabrication methods are all used to deposit thin layers of semiconductors, metals, insulators, and polymers.

On the other hand, lithography methods are used for printing photosensitive polymer layers on top of the MEMS components, so that they can be etched away at the microscale to create specific patterns and features. When it comes to the etching process itself, both wet and dry etching methods are used in the production of MEMS devices. Dry etching is more popular with electrochemical etching, with plasma etching, deep reactive ion etching, and isotropic wet etching also being used to selectively remove material.

There are also a number of advanced machining methods that don’t require lithography techniques to create specific features, patterns, and geometries in the different MEMS materials and components. One such method is ultraprecision mechanical machining that can mill silicon and other metals into specific shapes with features below one micron. This is an important non-lithographic method, as it can create shapes, such as retrograde undercuts with flat sidewalls, that are not possible with lithography methods.

Another advanced machining method is laser machining, which can be used to create silicon chips, but is also used to ablate material off metal, ceramic, and plastic materials and/or create holes in them. On the other hand, ultrasonic machining methods require hard and brittle materials such as glass, ceramics, and diamond, while using ultrasonic waves, whereas electro-discharge machining uses electrical discharges to erode away small pieces of material, but only for conductive materials. Beyond these methods, there are other lesser-used advanced machining and material removal techniques that introduce small features and patterns, including microcontact printing, nanoimprint lithography, and hot embossing.

Aside from machining-based methods, there are many other advanced fabrication methods used in specific scenarios. For example, anodic bonding methods are used to join silicon wafers to glass substrates, but silicon direct bonding is used to fuse two silicon materials together. Sol-gel deposition methods are used to coat MEMS components with optical absorption or index-graded antireflective coatings, Additionally, electroplating methods are used to build thin metal layers, including those made of gold, copper, nickel, and nickel-iron.

MEMS are an established set of microscale devices that have been used for many years. Naturally, as with any established technology, there are a number of manufacturing processes used to create MEMS devices. The scope of manufacturing and fabrication in the production of MEMS is vaster than a lot of other small-scale devices because there are so many different materials and components that go into creating them and making them functional. It is this combination of different fabrication methods that have enabled MEMS to be used in a range of applications. As fabrication methods get more advanced and their resolutions get narrower, smaller and smaller features—as well as smaller components—are being created within MEMS devices.