Mechanical-Systems (MEMS) is the integration of mechanical elements (levers, springs, deformable membranes, vibrating structures, etc.), sensors, actuators, and electronics (resistors, capacitors, inductors, etc.) on a common silicon substrate through microfabrication technology. MEMS devices are today considered as one among the most promising technologies of this century, capable to revolutionize the industrial world and the commercial product market. Micro-Electro- MEMS can be considered as “intelligent” systems which combine mechanical and electronic functions in extremely reduced dimension. The dimension of a MEMS device is in the order of the microns and it is manufactured directly on a silicon wafer.
The advanced device and process concepts thrust enables the integration and co-location of actuators, sensors, electronics, and power supplies to merge the functions of compute, communicate and power together with sense, actuate and control to change completely the way people and machines interact with the physical world.
The fabrication of MEMS evolved from the process technology in semiconductor device fabrication, i.e. the basic techniques are deposition of material layers, patterning by photolithography and etching to produce the required shapes.
MEMS fabrication uses many of the same techniques that are used in the integrated circuit domain such as oxidation, diffusion, ion implantation, LPCVD, sputtering, etc., and combines these capabilities with highly specialized micromachining processes. Some of the most widely used micromachining processes are discussed below.
Using an ever-expanding set of fabrication processes and materials, MEMS will provide the advantages of small size, low-power, low-mass, low-cost and high-functionality to integrated electromechanical systems both on the micro as well as on the macro scales.
Bulk micromachining is the oldest paradigm of silicon-based MEMS. The whole thickness of a silicon wafer is used for building the micro-mechanical structures. This technique involves the selective removal of the substrate material in order to realize miniaturized mechanical components. Bulk micromachining can be accomplished using chemical or physical means, with chemical means being far more widely used in the MEMS industry.
A widely used bulk micromachining technique is chemical wet etching, which involves the immersion of a substrate into a solution of reactive chemical that will etch exposed regions of the substrate at measurable rates. Chemical wet etching is popular in MEMS because it can provide a very high etch rate and selectivity. Furthermore, the etch rates and selectivity can be modified by: altering the chemical composition of the etch solution; adjusting the etch solution temperature; modifying the dopant concentration of the substrate; and modifying which crystallographic planes of the substrate are exposed to the etchant solution.
Surface micromachining is another very popular technology used for the fabrication of MEMS devices. Surface micromachining uses layers deposited on the surface of a substrate as the structural materials, rather than using the substrate itself.
There are a very large number of variations of how surface micromachining is performed, depending on the materials and etchant combinations that are used. However, the common theme involves a sequence of steps starting with the deposition of some thin-film material to act as a temporary mechanical layer onto which the actual device layers are built; followed by the deposition and patterning of the thin-film device layer of material which is referred to as the structural layer; then followed by the removal of the temporary layer to release the mechanical structure layer from the constraint of the underlying layer, thereby allowing the structural layer to move.
This MEMS paradigm has enabled the manufacturing of low cost accelerometers for e.g. automotive air-bag systems and other applications where low performance and/or high g-ranges are sufficient.
Wafer bonding is a micromachining method that is analogous to welding in the macroscale world and involves the joining of two (or more) wafers together to create a multi-wafer stack. There are three basic types of wafer bonding including direct or fusion bonding; field-assisted or anodic bonding; and bonding using an intermediate layer.
In direct or fusion wafer bonding, two or more wafers are bonded together that are usually made of silicon or some other semiconductor material; anodic bonding wherein a boron-doped glass wafer is bonded to a semiconductor wafer, usually silicon; thermocompression bonding, wherein an intermediary thin-film material layer is used to facilitate wafer bonding; and eutectic bonding, wherein a thin-film layer of gold is used to bond two silicon wafers. Each of these methods have specific uses depending on the circumstances.
In general, all bonding methods require substrates that are very flat, smooth, and clean, in order for the wafer bonding to be successful and free of voids. The most stringent criteria for wafer bonding is usually the direct fusion wafer bonding since even one or more small particulates can render the bonding unsuccessful. In comparison, wafer bonding methods that use intermediary layers are often far more forgiving.
High-Aspect Ratio MEMS Fabrication Technologies
Both bulk and surface silicon micromachining are used in the industrial production of sensors, ink-jet nozzles, and other devices. But in many cases the distinction between these two has diminished. A new etching technology, deep reactive-ion etching, has made it possible to combine good performance typical of bulk micromachining with comb structures and in-plane operation typical of surface micromachining.
Deep Reactive Ion Etching of Silicon
Deep reactive ion etching or DRIE is a relatively new fabrication technology that has been widely adopted by the MEMS community. This technology enables very high aspect ratio etches to be performed into silicon substrates. The sidewalls of the etched holes are nearly vertical and the depth of the etch can be hundreds or even thousands of microns into the silicon substrate.
While it is common in surface micromachining to have structural layer thickness in the range of 2 µm, in HAR silicon micromachining the thickness can be from 10 to 100 µm. The materials commonly used in HAR silicon micromachining are thick polycrystalline silicon, known as epi-poly, and bonded silicon-on-insulator (SOI) wafers although processes for bulk silicon wafer also have been created (SCREAM). Bonding a second wafer by glass frit bonding, anodic bonding or alloy bonding is used to protect the MEMS structures. Integrated circuits are typically not combined with HAR silicon micromachining.
Another popular high aspect ratio micromachining technology is called LIGA, which is a German acronym for “LIthographie Galvanoformung Adformung.” This is primarily a non-silicon based technology and requires the use of synchrotron generated x-ray radiation. Because LIGA requires a special mask and a synchrotron (X-ray) radiation source for the exposure, the cost of this process is relatively expensive.
Lasers can generate an intense amount of energy in very short pulses of light and direct that energy onto a selected region of material for micromachining. Among the many types of lasers now in use for micromachining include: CO2, YAG, excimer, etc. Each has its own unique properties and capabilities suited to particular applications. Factors that determine the type of laser to use for a particular application include laser wavelength, energy, power, and temporal and spatial modes; material type; feature sizes and tolerances; processing speed; and cost.
3D Printing Offers Low-Cost Solution to Small-Volume MEMS Production
Although MEMS technologies can be efficiently produced at high volumes using large-scale semiconductor manufacturing techniques, the manufacture of MEMS components in small and medium-size batches is challenging, due to the high startup cost of manufacturing process development and device design optimizations. As a result, engineers often must choose between suboptimal off-the-shelf MEMS devices or economically unviable startup costs, according to professor Frank Niklaus, who led the research. “The costs of manufacturing process development and device design optimizations do not scale down for lower production volumes,” he said.
Researchers at KTH Royal Institute of Technology have developed a 3D-printing technique for specialized manufacturing of microelectromechanical systems (MEMS). The approach offers a cost-efficient way to manufacture custom-designed MEMS devices, in small volumes, that could be used as sensors in robotics, navigation, and more.
Using a two-photon polymerization process in combination with metal evaporation to form strain gauge transducers, the team demonstrated a 3D-printed functional MEMS accelerometer. It characterized the responsivity, resonance frequency, and stability of the accelerometer over time and confirmed its successful operation. The results suggest that the 3D-printing method could enable the efficient manufacture of a variety of custom-designed MEMS devices.
The process of two-photon polymerization produces high-resolution objects that are as small as a few hundred nanometers, but that are not capable of sensing. To form the transducing elements, the researchers used a shadow-masking technique that worked in way similar to a stencil. On the 3D-printed structure, the researchers fabricated features with a T-shaped cross-section that functioned like an umbrella. When they deposited metal from a point above the 3D-printed structure, the sides of the T-shaped features, protected by this “umbrella,” were not coated with the metal. The metal on the top of the T was electrically isolated from the rest of the structure.
Using this method, the researchers manufactured about 12 custom-designed MEMS accelerometers in just a few hours using commercial manufacturing tools. This 3D-printing method could be used for prototyping MEMS devices and for manufacturing small- and medium-size batches of a few thousand to tens of thousands of MEMS sensors per year in an economical way.
“This is something that has not been possible until now, because the startup costs for manufacturing a MEMS product using conventional semiconductor technology are on the order of hundreds of thousands of dollars and the lead times are several months or more,” Niklaus said. “The new capabilities offered by 3D-printed MEMS could result in a new paradigm in MEMS and sensor manufacturing.”
The approach could be used for devices that require costly customization, including accelerometers for aircraft as well as vibration sensors for industrial machinery. It could also be applied to a variety of MEMS sensors, such as pressure sensors, gyroscopes, and flow sensors. Other low-volume products that could benefit from the technique include motion and vibration control units for robots, industrial tools, and wind turbines.
A 3D-printed MEMS unit is seen next to a 2 cent Euro coin. A photonics-based 3D printing approach for MEMS sensors supports the ability to integrate the sensors efficiently and in a cost effective way for robotics, navigation, medicine, and other applications. The first sensor type that the developing team created was an accelerometer. It used a two-photon polymerization process in combination with metal evaporation to form strain gauge transducers to make the sensor. Courtesy of Simone Pagliano.
Moreover, 3D printing could enable complex device geometries for new MEMS sensors that are not currently possible to achieve using conventional silicon micromachining. The strategy used by the researchers to selectively functionalize the surfaces of the 3D-printed MEMS structure by integrating shadow-masking elements in combination with directional material deposition is versatile, and facilitated innovative designs and the integration of a variety of transducer elements.
The quick turn-around between the design and the fabrication of small batches of 3D-printed MEMS accelerators allowed the researchers to assess the performances of the devices and optimize them in a matter of a few hours. From an industrial perspective, according to the researchers, this could dramatically reduce the startup cost for manufacturing custom MEMS devices for small- and medium-volume applications, compared to standard microfabrication techniques.
“Scalability isn’t just an advantage in MEMS production; it’s a necessity,” Niklaus said. “This method would enable fabrication of many kinds of new, customized devices.”
The research was published in Nature: Microsystems & Nanoengineering (www.doi.org/10.1038/s41378-022-00440-9).
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