Microfabrication is the process of fabricating miniature structures of

micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...

scales and smaller. Historically, the earliest microfabrication processes were used for integrated circuit fabrication, also known as "

semiconductor manufacturing Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically integrated circuit (IC) chips such as modern computer processors, microcontrollers, and memory chips such as NAND flash and DRAM that are pres ...

" or "semiconductor device fabrication". In the last two decades microelectromechanical systems (MEMS), microsystems (European usage), micromachines (Japanese terminology) and their subfields,

microfluidics Microfluidics refers to the behavior, precise control, and manipulation of fluids that are geometrically constrained to a small scale (typically sub-millimeter) at which surface forces dominate volumetric forces. It is a multidisciplinary field th ...

/lab-on-a-chip, optical MEMS (also called MOEMS), RF MEMS, PowerMEMS, BioMEMS and their extension into nanoscale (for example NEMS, for nano electro mechanical systems) have re-used, adapted or extended microfabrication methods. Flat-panel displays and solar cells are also using similar techniques. Miniaturization of various devices presents challenges in many areas of science and engineering:

physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...

, chemistry, materials science,

computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (includi ...

, ultra-precision engineering, fabrication processes, and equipment design. It is also giving rise to various kinds of interdisciplinary research.Nitaigour Premchand Mahalik (2006) "Micromanufacturing and Nanotechnology", Springer, The major concepts and principles of microfabrication are microlithography, doping, thin films,

etching Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio (incised) in the metal. In modern manufacturing, other chemicals may be used on other types ...

, bonding, and polishing.

Fields of use

Microfabricated devices include: *

integrated circuits An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...

(“microchips”) (see

) * microelectromechanical systems (MEMS) and microoptoelectromechanical systems (MOEMS) * microfluidic devices ( ink jet print heads) * solar cells * flat panel displays (see AMLCD and

thin-film transistor A thin-film transistor (TFT) is a special type of field-effect transistor (FET) where the transistor is thin relative to the plane of the device. TFTs are grown on a supporting (but non-conducting) substrate. A common substrate is glass, becaus ...

s) * sensors (microsensors) (

biosensor A biosensor is an analytical device, used for the detection of a chemical substance, that combines a biological component with a physicochemical detector. The ''sensitive biological element'', e.g. tissue, microorganisms, organelles, cell rece ...

s, nanosensors) *

power MEMS Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may ...

, fuel cells, energy harvesters/scavengers

Origins

Microfabrication technologies originate from the microelectronics industry, and the devices are usually made on

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...

wafers even though

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling ( quenching ...

, plastics and many other substrate are in use. Micromachining, semiconductor processing, microelectronic fabrication, semiconductor fabrication,

MEMS Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...

fabrication and integrated circuit technology are terms used instead of microfabrication, but microfabrication is the broad general term. Traditional machining techniques such as ''electro-discharge machining'', ''spark erosion machining'', and ''laser drilling'' have been scaled from the

millimeter 330px, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter. The millimetre (American and British English spelling differences#-re, -er, ...

size range to micrometer range, but they do not share the main idea of microelectronics-originated microfabrication: replication and parallel fabrication of hundreds or millions of identical structures. This parallelism is present in various imprint,

casting Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a ''casting'', which is ejected ...

and moulding techniques which have successfully been applied in the microregime. For example, injection moulding of DVDs involves fabrication of submicrometer-sized spots on the disc.

Processes

Microfabrication is actually a collection of technologies which are utilized in making microdevices. Some of them have very old origins, not connected to

manufacturing Manufacturing is the creation or production of goods with the help of equipment, labor, machines, tools, and chemical or biological processing or formulation. It is the essence of secondary sector of the economy. The term may refer to ...

, like lithography or

. 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

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

) 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 Semiconductor memory is a digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memory cells on a sil ...

fabrication there are some 30 lithography steps, 10

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

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 A pattern is a regularity in the world, in human-made design, or in abstract ideas. As such, the elements of a pattern repeat in a predictable manner. A geometric pattern is a kind of pattern formed of geometric shapes and typically repeated li ...

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 The laser diode chip removed and placed on the eye of a needle for scale A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with e ...

. Microfabrication resembles

multiple exposure In photography and cinematography, a multiple exposure is the superimposition of two or more exposures to create a single image, and double exposure has a corresponding meaning in respect of two images. The exposure values may or may not be id ...

photography, with many patterns aligned to each other to create the final structure.

Substrates

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 wafer In electronics, a wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells. The wafer serv ...

s 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 (see

Thin film deposition A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ap ...

). The purpose of these thin films depends upon 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 In microfabrication, thermal oxidation is a way to produce a thin layer of oxide (usually silicon dioxide) on the surface of a wafer. The technique forces an oxidizing agent to diffuse into the wafer at high temperature and react with it. The ra ...

* Local oxidation of silicon * Chemical vapor deposition (CVD) **

APCVD Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (substra ...

LPCVD Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (substra ...

PECVD Plasma-enhanced chemical vapor deposition (PECVD) is a chemical vapor deposition process used to deposit thin films from a gas state (vapor) to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creat ...

Physical vapor deposition Physical vapor deposition (PVD), sometimes called physical vapor transport (PVT), describes a variety of vacuum deposition methods which can be used to produce thin films and coatings on substrates including metals, ceramics, glass, and polym ...

(PVD) **

Sputtering In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and ca ...

** Evaporative deposition ** Electron beam PVD * Epitaxy

Patterning

It is often desirable to pattern a film into distinct features or to form openings (or vias) in some of the layers. These features are on the micrometer or nanometer scale and the patterning technology is what defines microfabrication. This patterning technique typically uses a 'mask' to define portions of the film which will be removed. Examples of patterning techniques include: * Photolithography *Shadow masking

Etching

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

Microforming is a microfabrication process of microsystem or

microelectromechanical system Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...

(MEMS) "parts or structures with at least two dimensions in the submillimeter range." It includes techniques such as

microextrusion Microextrusion is a microforming extrusion process performed at the submillimeter range. Like extrusion, material is pushed through a die orifice, but the resulting product's cross section can fit through a 1mm square. Several microextrusion proce ...

, 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 A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. Stru ...

(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.

Other

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 Ion implantation is a low-temperature process by which ions of one element are accelerated into a solid target, thereby changing the physical, chemical, or electrical properties of the target. Ion implantation is used in semiconductor device fa ...

* Chemical-mechanical planarization (CMP) *Wafer cleaning, also known as "surface preparation" (see below) *

Wire bonding Wire bonding is the method of making interconnections between an integrated circuit (IC) or other semiconductor device and its packaging during semiconductor device fabrication. Although less common, wire bonding can be used to connect an IC ...

Cleanliness in wafer fabrication

Microfabrication is carried out in

cleanroom A cleanroom or clean room is an engineered space, which maintains a very low concentration of airborne particulates. It is well isolated, well-controlled from contamination, and actively cleansed. Such rooms are commonly needed for scientif ...

s, where air has been filtered of particle contamination and

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

, humidity, vibrations and electrical disturbances are under stringent control.

Smoke Smoke is a suspension of airborne particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-produc ...

dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ho ...

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

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 Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

-peroxide solution removes organic contamination and particles; RCA-2 cleaning in

hydrogen chloride The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...

-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 Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

and

argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as ...

plasma treatments to remove unwanted surface layers, or

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

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 Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

, 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

), or they may have gathered

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

s from wafer boxes, and this might be different depending on wait time. Wafer cleaning and surface preparation work similarly to the machines in a

bowling alley A bowling alley (also known as a bowling center, bowling lounge, bowling arena, or historically bowling club) is a facility where the sport of bowling is played. It can be a dedicated facility or part of another, such as a clubhouse or dwelling ...

: first they remove all unwanted bits and pieces, and then they reconstruct the desired pattern so that the game can go on.

References

External links

Videos and animations on microfabrication techniques and related applications

MicroManufacturing Conference
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