MEMS processing process is the most important research direction of MEMS technology, MEMS processing process mainly includes body micromachining, surface micromachining and photolithography, electroplating, casting and molding process (LIGA).
Due to the high manufacturing cost of the LIGA process, the most widely used processes are body micromachining and surface micromachining.
1. Methods and materials for MEMS manufacturing processes
MEMS manufacturing technology is based on integrated circuit (IC) manufacturing technology, combined with advances in micromachining technology gradually developed.
Compared with integrated circuit manufacturing, MEMS devices have three-dimensional microstructures and movable parts, and the manufacturing process is more complex.
Due to the functional differences in MEMS devices, structural forms vary, and common features include overhanging structures and high depth-to-width ratios, so there is no single fabrication process that can meet the needs of all types of MEMS devices.
The main fabrication techniques for MEMS include surface micromachining, bulk micromachining and LIGA technology.
Surface micromachining builds microstructures on the substrate layer by layer by thin film deposition, photolithography, etching and other processes, and completes the final form by removing the sacrificial layer.
Body micromachining, on the other hand, is done through top-down etching, commonly used in wet and dry etching, and is mainly used to fabricate MEMS devices with three-dimensional structures.
LIGA technology utilizes synchronous X-ray equipment to batch fabricate microstructures with high depth-to-width ratios.
Currently, surface micromachining and body micromachining are the most widely used and technologically mature methods.
Silicon is the main material for MEMS fabrication, mainly because of the maturity of silicon etching technology and its good mechanical properties.
Through anisotropic etching of silicon, specific mechanical structures can be formed according to the etching rate of different crystallographic directions, while silicon can also be integrated with ICs to build more complex microsystems.
With the continuous development of MEMS technology, new materials are constantly being introduced, such as metals, biocompatible materials and polymers.
In addition to silicon, MEMS processing requires the use of etchants and mask materials.
Different etching methods require different materials, for example, wet etching often uses potassium hydroxide (KOH) solution as the etchant and silicon dioxide (SiO2) as the mask material; while dry etching often uses sulfur hexafluoride (SF6) as the etchant and also uses SiO2 as the mask material.
2. MEMS technology and applications
1) Body Microprocessing Technology
Body micromachining technology refers to the fabrication of MEMS devices on silicon substrates using anisotropic etching technology, which mainly includes wet etching and dry deep etching.
Wet anisotropic etching is the earliest microfabrication technology developed, which etches the silicon material by interacting the etching solution with it through a chemical reaction.
Dry deep etching, on the other hand, is anisotropic etching of silicon by deep reactive ion etching (DRIE) technology, which belongs to the emerging deep etching technology since the 1970s.
a) Wet etching
The materials and equipment required for wet etching include etching solutions, reaction vessels, temperature control devices and cleaning machines.
Commonly used anisotropic etching solutions are potassium hydroxide (KOH) solution and aqueous tetramethylammonium hydroxide (TMAH) solution.
The etch rate of these alkaline solutions on silicon is affected by the crystal surface, and the difference in the etch rate at different crystal surfaces is not clearly explained, but is generally believed to be related to the density of the atomic bonds on the crystal surface.
The higher the molecular density of the crystal surface and the smaller the molecular spacing, the higher the number and strength of the bonds, the greater the energy required for the etching reaction, and therefore the slower the etching speed.
Wet etching is a widely used processing technique in the laboratory, but in practice, problems such as unevenness of the etched surface often occur.
To optimize the etching effect, researchers usually improve the results by adjusting the etching solution formulation and etching conditions, such as adding additives such as isopropyl alcohol to improve the flatness of the silicon surface, controlling the etching temperature, or adjusting the solution cycling rate, etc., which are effective in enhancing the performance of the micro-mechanical structures.
b) Dry deep etching
Dry deep etching has the following characteristics:
Etching rates are high, typically 2 to 15 times higher than wet etching rates;
With a high depth-to-width ratio, it is capable of penetrating the entire wafer;
The etching rate is independent of the direction of the crystal surface, and vertical structures of arbitrary shape can be etched;
The etching is selective and can effectively protect the interface between the etched material and the blocking material.
Dry deep etching utilizes the plasma generated by fluoride gases (e.g., sulfur hexafluoride, SF6) during the discharge process for etching, and at the same time, deep etching is achieved through the introduction of a protective gas that converts the isotropic etching of sulfur hexafluoride into an anisotropic etching.
There are two main processes for dry deep etching: the Bosch process and the low-temperature etching process.
The Bosch process achieves deep etching by alternately energizing an etching gas (SF6) and a protective gas (octafluorocyclobutane, C4F8) at a high frequency in an alternating etching and protection process.
The low-temperature etching process, on the other hand, involves the addition of oxygen while SF6 is energized, and the plasma generated by the oxygen reacts with the inner wall of the etched structure to form a SiOxFy protective layer, thus proceeding simultaneously under the effects of etching and protection.
As dry deep etching has the advantage of high depth-to-width ratio, it is now widely used in body microfabrication technology, capable of utilizing different etching gases and protective atmospheres to etch a wide range of materials, such as polycrystalline silicon, silicon dioxide, metals, etc., which has a high potential for application.
It has been widely used in the fields of microsensors, microactuators, and micro-medical devices.
Compared to wet etching, dry deep etching offers greater processing flexibility.
Different process parameters, such as gas selection, flow rate, and pressure of the reaction environment, can significantly affect the performance of the microdevices and therefore need to be precisely adjusted to the actual requirements.
2) Surface Micromachining Technology
Surface micromachining mainly consists of thin film deposition, photolithography and etching techniques.
Thin film deposition is the physical or chemical deposition of a thin film of nanometer or micrometer thickness on the surface of a substrate;
Photolithography, on the other hand, transfers the graphics on an optical mask plate to the surface of the substrate, using selective exposure to change the chemistry of the photoresist, which in turn dissolves the changed area by means of a developing solution to obtain the desired graphics.
In surface micromachining, the movability of microstructures is achieved by depositing a thin film of the structural layer on a sacrificial layer and then removing the sacrificial layer to release the structural layer.
The main steps include thin film deposition, lithographic patterning, deposition of a sacrificial layer film, patterning of the sacrificial layer, deposition of a mechanically structured layer film, patterning of the mechanically structured layer, and removal of the sacrificial layer (release of the structure).
Surface micromachining technology, which allows fabrication of microstructures up to 10µm and enables the realization of multilayer suspended structures, is an indispensable technology in MEMS manufacturing.
Due to the special characteristics of its processing structure, the mechanical properties of the device are required to be high, and the problems of adhesion, residual stress, friction and driving also need to be solved.
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