What is Bulk Micro manufacturing – Types Of Bulk Micro machining

Bulk Micromachining
Bulk micromachining is the earliest and best-characterized method of producing micromachined devices. Its principle consists of etching deeply into the silicon wafer. Although various different materials can be used as the substrate for micromachined structures, silicon is being used for that purpose in most cases because of the greater level of experience with this material, achieved through the production of semiconductor devices. Additionally, silicon offers the best characteristics with respect to cost, metallization and machinability. Alternatives to Si include ceramic, plastic or glass materials.

The first instances of etching a substrate reach back to the fifteenth century, when etching and masking techniques were used to decorate armors, an action which traditional engraving tools were too soft. For this purpose, mostly wax masks were patterned, using scribing tools to remove the masking material in the areas that should be etched later in an acid-based etchant. Later as photosensitive chemicals were invented (around 1820), the process of chemically patterning some kind of substrate grew more and more important, since structures were made possible by this means that could not be otherwise produced. Also the introduction of printed circuit boards in the electronic industries in the 1940s and 50s resulted in major advancements in this technique.

There are several ways to etch the silicon wafer.

Anisotropic etching uses etchants that etch different crystallographic directions at different rates. Certain crystallographic planes etch extremely slowly, therefore being called stop planes. Anisotropic etching usually produces V grooves, pyramids, and channels into the surface of the silicon wafer.

Isotropic etching etches all directions in the silicon wafer with nearly the same rate, regardless of the crystalline structure. Thus it produces rounded depressions on the surface of the wafer that usually resemble hemispheres and cylinders.

Reactive Ion Etching (RIE) uses a plasma to etch straight walled structures on the wafer and provides a means for dry etching silicon. Since this kind of etching has not been part of the project worked on during the described practical training semester, attention will be kept on the wet etching techniques in this report.

  • Bulk micromanufacturing technique involves creating 3-D components by removing materials from thick substrates (silicon or other materials) using primarily etching method.
  • Etching – dry or wet etching is the principal technique used in bulk micro manufacturing.

Substrates that can be etched in bulk micro manufacturing include:-

1) Silicon 2) SiC 3) GaAs 4) Special polymers

Wet etching involves the use of chemical solvents (called etchants ) Dry etching uses plasma to remove materials at the desired locations on a substrate.

micro machining
Etched grooves using (a) anisotropic etchants, (b) isotropic etchants, (c) Reactive Ion Etching (RIE)

Figure shows the differences between the various sorts of etching procedures. The thin layer visible between the photoresist and the silicon itself is silicon dioxide (SiO2). It serves as an etch mask for the Si etch, since its etch rate in most of the acidic etchants is considerably lower than that of Si, whereas photoresists do not stand up to the strong attack of most of the etchants. It is either a naturally grown oxide of approximately 20 Å (oxidation of silicon is nearly unavoidable when exposed to an atmosphere containing oxygen) or is thermally grown to a distinctive thickness. This is usually done by heating the wafer to temperatures between 600 and 1200°C when surrounded by steam or either wet or dry oxygen/nitrogen mixtures. A common etchant for patterning SiO2 is buffered hydrofluoric acid (BHF), attacking it at 1000 Å/min but leaving photoresist untouched for a reasonable stretch of time.

Reference : http://terpconnect.umd.edu/~sandborn/research/JPL_MEMS/microeng_bulk.html

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