MEMS sensor

The three most commonly used MEMS manufacturing technologies include bulk micro machining, surface micro machining and Liga.

MEMS refers to micro electromechanical system, which is used to manufacture micro system composed of electronic and mechanical components. The size of the system can range from a few microns or less to a few millimeters. MEMS has been proved to be an efficient and potential technology. Some application examples of this technology include sensors, accelerometers, printer cartridges, robots, etc.

The figure shows the three most commonly used MEMS manufacturing technologies

The three most commonly used MEMS manufacturing technologies include bulk micro machining, surface micro machining and Liga.

1、 Bulk micro machining

In bulk micromachining, most of the substrate, monocrystalline silicon, a very stable mechanical material, is specially removed to form three-dimensional MEMS devices. Three dimensional structure of MEMS for volume micromachining

The top-down manufacturing technology is usually used in the bulk micromachining of micro devices, that is, etching on the prepared silicon wafer to produce three-dimensional MEMS components. It is a subtraction process that uses wet anisotropic etching or dry etching, such as reactive ion etching (RIE), to create large pits, grooves and channels. Materials commonly used for wet etching include silicon and quartz, while dry etching is commonly used for silicon, metals, plastics and ceramics.

·Wet etching

In wet etching, the material (usually silicon wafer) is removed by immersing it in a bath of a chemical etchant. These corrosives can be isotropic (mixture of hna-hf, HNO3 and CH3COOH) or anisotropic (KOH). The etching speed of anisotropic etchant is faster in the preferred direction, and the etching depends on the crystal orientation of the substrate.

·Dry etching

In dry etching, high energy ions accelerate in the plasma phase to provide additional energy for the reaction. The most common form of MEMS is reactive ion etching (RIE), which uses additional energy in the form of radio frequency (RF) power to drive chemical reactions.

·Deep reactive ion etching (DRIE)

Deep reactive ion etching (DRIE) is a high aspect ratio etching method, which includes high-density plasma etching (such as RIE) and protective polymer deposition alternately to obtain higher aspect ratio.

Piezoresistive effect is a widely used transduction mechanism in volume micromachined sensors (such as pressure sensors). In piezoresistive materials, the change of stress causes the change of strain and resistance. Therefore, when the injected piezoresistor is formed at the maximum stress point of the diaphragm (for the pressure sensor), the deflection under the applied pressure will lead to the change of the resistance.

2、 Surface micromachining

In surface micromachining, three-dimensional structure is constructed by adding and removing a series of thin films on the wafer surface, which are called structure layer and sacrificial layer. Sacrificial layers are deposited and then removed to form mechanical spaces or gaps between structural layers. The process steps of surface micromachined cantilever are as follows:

The figure shows the different processing steps of the surface micromachined cantilever

Many surface micromachined sensors use capacitance conversion method to convert the input mechanical signal into equivalent electrical signal. In the capacitance conduction method, the sensor can be considered as a mechanical capacitor in which one plate moves relative to the applied physical stimulation. This changes the gap between the two electrodes and changes the capacitance accordingly. This change in capacitance corresponds to the mechanical stimulation of the input.

3、 Liga

Liga, a German acronym, consists of the letters Li (roentgen lithography, which means X-ray lithography), G (galvanizing, which means electrodeposition) and a (abformung, which means molding other materials into high aspect ratio structures). Therefore, in this technique, thick photoresist is exposed to X-rays to produce a mold that is subsequently used to form a high aspect ratio electroplating three-dimensional structure. Liga process can produce smaller and larger micro parts than traditional process. The process steps of LIGA are shown in the figure below.

Fig. 1 shows the processing steps of Liga

·Fusion welding

In order to form a complex large-scale structure, fusion welding process (body machining and surface micromachining) can be used. It needs to build a structure by atom bonding various wafers. In this case, the cavity is etched as a whole in the bottom wafer. The second wafer is then bonded to form a buried cavity. Drie mask material is then printed on the top wafer. Anisotropic etching is then performed to release the microstructure, and drie masking material is removed to fabricate the final device.

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