Many of the products in microsystem technology are based on silicon, and most of the processing techniques used in the fabrication of microsystems are borrowed from the microelectronics industry. There are several important reasons why silicon is a desirable material in MST: (1) the microdevices in MST often include electronic circuits, so both the circuit and the microdevice can be fabricated in combination on the same substrate; (2) in addition to its desirable electronic properties, silicon also possesses useful mechanical properties, such as high strength and elasticity, good hardness, and relatively low density (3) the technologies for processing silicon are well-established, owing to their widespread use in microelectronics; and (4) use of single-crystal silicon permits the production of physical features to very close tolerances.
Microsystem technology often requires silicon to be fabricated along with other materials in order to obtain a particular microdevice. For example, microactuators often consist of several components made of different materials. Accordingly, microfabrication techniques consist of more than just silicon processing.
Microfabrication is actually a collection of technologies which are utilized in making microdevices. Some of them have very old origins, not connected to manufacturing, like lithography or etching. Polishing was borrowed from optics manufacturing, and many of the vacuum techniques come from 19th century physics research. Electroplating is also a 19th-century technique adapted to produce micrometer scale structures, as are various stamping and embossing techniques.
To fabricate a microdevice, many processes must be performed, one after the other, many times repeatedly. These processes typically include depositing a film, patterning the film with the desired micro features, and removing (or etching) portions of the film. Thin film metrology is used typically during each of these individual process steps, to ensure the film structure has the desired characteristics in terms of thickness (t), refractive index (n) and extinction coefficient (k), for suitable device behavior. For example, in memory chip fabrication there are some 30 lithography steps, 10 oxidation steps, 20 etching steps, 10 doping steps, and many others are performed. The complexity of microfabrication processes can be described by their mask count. This is the number of different pattern layers that constitute the final device. Modern microprocessors are made with 30 masks while a few masks suffice for a microfluidic device or a laser diode. Microfabrication resembles multiple exposure photography, with many patterns aligned to each other to create the final structure.
Microfabricated devices are not generally freestanding devices but are usually formed over or in a thicker support substrate. For electronic applications, semiconducting substrates such as silicon wafers can be used. For optical devices or flat panel displays, transparent substrates such as glass or quartz are common. The substrate enables easy handling of the micro device through the many fabrication steps. Often many individual devices are made together on one substrate and then singulated into separated devices toward the end of fabrication.
Deposition or Growth
Microfabricated devices are typically constructed using one or more thin films. The purpose of these thin films depends on the type of device. Electronic devices may have thin films which are conductors (metals), insulators (dielectrics) or semiconductors. Optical devices may have films which are reflective, transparent, light guiding or scattering. Films may also have a chemical or mechanical purpose as well as for MEMS applications. Examples of deposition techniques include:
- Thermal oxidation
- chemical vapor deposition (CVD)
- Physical vapor deposition(PVD)
- evaporative deposition
- Electron beam PVD
It is often desirable to pattern a film into distinct features or to form openings in some of the layers. These features are on the micrometer or nanometer scale and the patterning technology is what defines microfabrication. The patterning technique typically uses a ‘mask’ to define portions of the film which will be removed. Examples of patterning techniques include:
- Shadow Masking
Etching is the removal of some portion of the thin film or substrate. The substrate is exposed to an etching (such as an acid or plasma) which chemically or physically attacks the film until it is removed. Etching techniques include:
- Dry etching (Plasma etching) such as Reactive-ion etching (RIE) or Deep reactive-ion etching (DRIE)
- Wet etching or Chemical Etching
Microforming is a microfabrication process of microsystem or microelectromechanical system (MEMS) “parts or structures with at least two dimensions in the submillimeter range.” It includes techniques such as microextrusion, microstamping, and microcutting. These and other microforming processes have been envisioned and researched since at least 1990, leading to the development of industrial and experimental-grade manufacturing tools. However, as Fu and Chan pointed out in a 2013 state-of-the-art technology review, several issues must still be resolved before the technology can be implemented more widely, including deformation load and defects, forming system stability, mechanical properties, and other size-related effects on the crystallite (grain) structure and boundaries:
In microforming, the ratio of the total surface area of grain boundaries to the material volume decreases with the decrease of specimen size and the increase of grain size. This leads to the decrease of grain boundary strengthening effect. Surface grains have lesser constraints compared to internal grains. The change of flow stress with part geometry size is partly attributed to the change of volume fraction of surface grains. In addition, the anisotropic properties of each grain become significant with the decrease of workpiece size, which results in the inhomogeneous deformation, irregular formed geometry and the variation of deformation load. There is a critical need to establish the systematic knowledge of microforming to support the design of part, process, and tooling with the consideration of size effects.
A wide variety of other processes for cleaning, planarizing, or modifying the chemical properties of microfabricated devices can also be performed. Some examples include:
- Doping by either thermal diffusion or ion implantation
- Chemical-mechanical planarization (CMP)
- Wafer cleaning, also known as “surface preparation”
- Wire bonding
Cleanliness in wafer fabrication
Microfabrication is carried out in cleanrooms, where air has been filtered of particle contamination and temperature, humidity, vibrations and electrical disturbances are under stringent control. Smoke, dust, bacteria and cells are micrometers in size, and their presence will destroy the functionality of a microfabricated device.
Cleanrooms provide passive cleanliness but the wafers are also actively cleaned before every critical step. RCA-1 clean in ammonia-peroxide solution removes organic contamination and particles; RCA-2 cleaning in hydrogen chloride-peroxide mixture removes metallic impurities. Sulfuric acid-peroxide mixture (a.k.a. Piranha) removes organics. Hydrogen fluoride removes native oxide from silicon surface. These are all wet cleaning steps in solutions. Dry cleaning methods include oxygen and argon plasma treatments to remove unwanted surface layers, or hydrogen bake at elevated temperature to remove native oxide before epitaxy. Pre-gate cleaning is the most critical cleaning step in CMOS fabrication: it ensures that the ca. 2 nm thick oxide of a MOS transistor can be grown in an orderly fashion. Oxidation, and all high temperature steps are very sensitive to contamination, and cleaning steps must precede high temperature steps.
Surface preparation is just a different viewpoint, all the steps are the same as described above: it is about leaving the wafer surface in a controlled and well known state before you start processing. Wafers are contaminated by previous process steps (e.g. metals bombarded from chamber walls by energetic ions during ion implantation), or they may have gathered polymers from wafer boxes, and this might be different depending on wait time.
Wafer cleaning and surface preparation work a little bit like the machines in a bowling alley: first they remove all unwanted bits and pieces, and then they reconstruct the desired pattern so that the game can go on.
Fields of Use
Microfabricated devices include:
- Fabrication of integrated circuits (“microchips”)
- Microelectromechanical systems (MEMS), MOEMS,
- microfluidic devices (ink jet print heads)
- solar cells
- Flat Panel Displays
- Sensors (micro-sensors) (biosensors, nanosensors)
- Power MEMSs, fuel cells, energy harvesters/scavengers
- Fu, M.W.; Chan, W.L. (2014). “Chapter 4: Microforming Processes”. Micro-scaled Products Development via Microforming: Deformation Behaviours, Processes, Tooling and its Realization. Springer Science & Business Media. pp. 73–130. ISBN 9781447163268.
Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, 5th Edition, Mikell P. Groover
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