MEMS : Fabrication Techniques

MEMS devices use semiconductor processing technologies to produce 3D mechanical structures.

      The three most used fabrication technologies include Bulk Micro Machining, Surface Micro Machining and LIGA

·         BULK MICROMACHINING

In bulk micromachining, the bulk of the substrate, i.e., single crystal silicon, a very stable mechanical material, is specifically removed to form three-dimensional MEMS devices.

The bulk micromachining manufacture of micro devices generally uses top-down fabrication techniques of etching deep into prepared silicon wafers to create three-dimensional MEMS components. It is a subtractive process that uses wet anisotropic etching or a dry etching method such as reactive ion etching (RIE), to create large pits, grooves and channels. Materials typically used for wet etching include silicon and quartz, while dry etching is typically used with silicon, metals, plastics and ceramics.

·         Wet Etching

In Wet etching, the material is removed through the immersion of a material (typically a silicon wafer) in a liquid bath of a chemical etchant. These etchants can be isotropic (HNA – mixture of HF, HNO3 and Ch3COOH) or anisotropic (KOH). Anisotropic etchants etches faster in a preferred direction; etching is dependent on the crystal orientation of the substrate.

·         Dry Etching

In dry etching, energetic ions are accelerated towards the material to be etched within a plasma phase supplying the additional energy needed for the reaction. The most common form for MEMS is reactive ion etching (RIE) which utilizes additional energy in the form of radio frequency (RF) power to drive the chemical reaction.

·         Deep Reactive Ion Etching(DRIE)

Deep Reactive Ion Etching (DRIE) is a much higher-aspect-ratio etching method that involves an alternating process of high-density plasma etching (as in RIE) and protective polymer deposition to achieve greater aspect ratios

 

The transduction mechanism widely used in bulk micromachined sensors, e.g., pressure senor is the piezoresistive effect. In piezoresistive materials, the change in the stress causes a strain and a corresponding change in the resistance. Thus, when implanted piezoresistors are formed at the maximum stress points of the diaphragm (in case of pressure sensor), the deflection under the applied pressure causes a change in the resistance.

 

·         SURFACE MICROMACHINING

In surface micromachining, the 3-D structure is built up by the orchestrated addition and removal of a sequence of thin film layers to/from the wafer surface called structural and sacrificial layers, respectively.  Sacrificial layers are deposited and then removed to form the mechanical spaces or gaps between the structural layers.  The process steps for surface micromachined cantilever are shown below:

      Many of the surface micro-machined sensors use the capacitive transduction method to convert the input mechanical signal to the equivalent electrical signal. In the capacitive transduction method, the sensor can be considered a mechanical capacitor in which one of the plates moves with respect to the applied physical stimulus. This changes the gap between the two electrodes with a corresponding change in the capacitance. This change in capacitance is the electrical equivalent of the input mechanical stimulus.

 

·         LIGA

      LIGA is a German acronym consisting of the letters LI (Roentgen Lithography, meaning X-ray lithography), G (Galvanik, meaning electrodeposition) and A (Abformung, meaning molding of other materials into high aspect ratio structures  ). Accordingly, in this technique thick photoresists are exposed to X-rays to produce molds that are subsequently used to form high-aspect ratio electroplated 3-D structures.  The LIGA process can build microparts that are smaller than conventional machining processes and also bigger than surface micromachined parts. The process steps for LIGA are shown in following figure.

·         Fusion Bonding

To form complex & large structures, the process of fusion bonding (uses both bulk and surface micromachining) may be used. It entails building up a structure by atomically bonding various wafers. In this case, cavity is bulk etched in the bottom wafer. Then, second wafer is then bonded forming buried cavity. This is followed by patterning of DRIE masking material on the top wafer. Anisotropic etching is then carried out to release the microstructure, followed by removal of DRIE masking material to produce the final device.

 

MEMS-MATERIALS

The choice of a good material for MEMS application depends on its properties, but not so much on carrier mobility as in microelectronics. Actually, materials are selected more on mechanical aspect; small or controllable internal stress, low processing temperature, compatibility with other materials, possibility to obtain thick layer, patterning possibilities, etc. In addition, depending on the fieeld of application, the material often needs to have extra properties. RF MEMS requires material to have with small loss tangent, , optical MEMS may need a transparent substrate, BioMEMS will need bio-compatibility, sensing application may need materials to have piezoresistivity/piezoelectricity, etc. Commonly used Materials for MEMS are Si, SiO₂, SiN, PolySi, Glass, Gold, Aluminium, etc.