Fabrication Process

MEMS support the principle of miniaturisation, multiplicity, and microelectronics. Miniaturisation is preferred in order to achieve faster response times and less space, whereas multiplicity is essential in order to advocate batch fabrication. Batch processing is a processing technique that allows for thousands of components to be simultaneously manufactured in order to significantly reduce the cost of the device. Microelectronics embeds the real part of the MEMS device. These can be sensors, actuators, signal conditioning, signal processing or even logic circuits.

The design process involved for IC manufacturing is called microfabrication. The sequences of microfabrication include film growth, doping, lithography, etching, dicing and packaging. A polished silicon wafer is mainly used as the substrate. A thin film is grown on the substrate. Then the properties of the layer are modulated by appropriately introducing doped material in a controllable manner. The doping can be achieved by thermal diffusion. The subsequent process is called lithography, which refers to the creation of a masking pattern. The pattern on a mask is transferred to the film by means of a photoresist. A mask usually consists of a glass plate, which is coated with a patterned layer, which is usually chromium film. The subsequent process is called etching. Etching is a process of removing the portions of material from an insulating base by chemical or electrolytic means. The two types of etching are wet and dry etching. In wet etching the material is dissolved by immersing it in a chemical solution. On the other hand, in dry etching, the material is dissolved by using reactive ions or a vapor phase etchant (Vittorio 2001). The finished wafer has to be segmented into a small dice-like structure. Finally, the individual sections are packaged. Packaging is a complex process that involves physically locating, connecting, and protecting a component or whole device. MEMS design also considers all the process sequences employed for microfabrication, however in this case it is considered as an extension of the IC fabrication process. The following methods are common as far as manufacturing of MEMS designs are concerned:

• Bulk micromachining

• Surface micromachining

• Micromolding

Bulk micromachining makes micromechanical devices by etching deeply into the silicon wafer. It is a subtractive process that involves the selective removal of the wafer materials to form the micro structure, which may include cantilevers, holes, grooves, and membranes.

The majority of currently used MEMS processes involve bulk etching. In light of newly introduced dry etching methods, which are compatible with complementary metaloxidesemiconductors, it is unlikely that bulk micromachining will decrease in popularity in the near future (Kovacs 1998). The available etching methods fall into three categories in terms of the state of the etchant: wet, vapor, and plasma. The etching reactions rely on the oxidation of silicon to form compounds that can be physically removed from the substrate (Kovacs, 1998). Conversely, surface micromachining technology makes thin micromechanical devices on the surface of a silicon wafer.

The surface micromachining sequences are as follows (Madou www. mmadou.eng.uci.edu/Classes/MSE621/MSE62101(12).pdf),

• Wafer cleaning

• Blanket n+ diffusion of Si substrate

• Passivation layer formation

• Opening up of the passivation layer for contacts

• Stripping of resist in piranha

• Removal of thin oxide through BHF etchant systems

• Deposition of a base, spacer or sacrificial layer using phosphosilicate glass (PSG)

• Densification at 950°C for 30-60 min in wet oxygen

• Base window etching in BHF for anchors

• Deposition of structural material deposition (e.g., poly-Si using CVD method at about 600°C, 100 Pa and 125 sccm at about 150 A/min)

• Anneal of the poly-Si at 1050°C for 1 hour to reduce stress in the structure

• Doping: in-situ, PSG sandwich and ion implantation

• Release step, selective etching of spacer layer.

The micromolding process involves use of molds to define the deposition of the structural layer. In this case, the structural material is deposited only in those areas constituting the microdevice structure. This is apparently in contrast to both bulk and surface micromachining processes. Feature blanket deposition of the structural material followed by etching to realise the final device geometry is done in one step. Once the structural layer deposition is over the mold is dissolved by using a chemical etchant. Note that the etchant does not corrugate the structural material. One of the most widely used micromolding processes is the LIGA process. LIGA is a German acronym standing for lithographie, galvanoformung and abformung, or lithography, electroplating, and molding. Photosensitive polyimides are mostly used for fabricating plating molds.

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