Semiconductor device fabrication
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{{Mergefrom|Wafer fabrication|date=May 2007}}
[[Image:Clean room.jpg|thumb|right|280px|NASA's Glenn Research Center [[cleanroom]].]]
'''Semiconductor device fabrication''' is the process used to create chips, the [[integrated circuit]]s that are present in everyday [[electrical]] and [[electronics|electronic]] devices. It is a multiple-step sequence of photographic and chemical processing steps during which electronic circuits are gradually created on a [[wafer (electronics)|wafer]] made of pure [[semiconductor|semiconducting]] material.
[[Silicon]] is the most commonly used semiconductor material today, along with various [[compound semiconductor]]s.
The entire manufacturing process from start to packaged chips ready for shipment takes six to eight weeks and is performed in highly specialized facilities referred to as [[Fab (semiconductors)|fabs]].
==Wafers==
{{main|wafer (semiconductor)}}
A typical [[wafer (semiconductor)|wafer]] is made out of extremely [[silicon#Production|pure silicon]] that is grown into [[single crystal|mono-crystalline]] cylindrical [[ingot]]s ([[boule (crystal)|boule]]s) up to 300 mm (slightly less than 12 inches) in diameter using the [[Czochralski process]]. These ingots are then sliced into wafers about 0.75 mm thick and polished to obtain a very regular and flat surface.
Once the wafers are prepared, many process steps are necessary to produce the desired semiconductor integrated circuit. In general, the steps can be grouped into two areas:
* Front end processing
* Back end processing
==Processing==
In semiconductor device fabrication, the various processing steps fall into four general categories: deposition, removal, patterning, and modification of electrical properties.
*Deposition is any process that grows, coats, or otherwise transfers a material onto the wafer. Available technologies consist of [[physical vapor deposition]] (PVD), [[chemical vapor deposition]] (CVD), electrochemical deposition (ECD), [[molecular beam epitaxy]] (MBE) and more recently, [[atomic layer deposition]] (ALD) among others.
*Removal processes are any that remove material from the wafer either in bulk or selective form and consist primarily of etch processes, both [[wet etching]] and [[dry etching]] such as reactive ion etch (RIE). [[Chemical-mechanical planarization]] (CMP) is also a removal process used between levels.
*Patterning covers the series of processes that shape or alter the existing shape of the deposited materials and is generally referred to as [[lithography]]. For example, in conventional lithography, the wafer is coated with a chemical called a “[[photoresist]]”. The photoresist is exposed by a “[[stepper]]”, a machine that focuses, aligns, and moves the mask, exposing select portions of the wafer to short wavelength light. The unexposed regions are washed away by a developer solution. After etching or other processing, the remaining photoresist is removed by [[plasma ashing]].
*Modification of electrical properties has historically consisted of doping transistor sources and drains originally by diffusion furnaces and later by [[ion implantation]]. These doping processes are followed by [[furnace anneal]] or in advanced devices, by [[rapid thermal anneal]] (RTA) which serve to activate the implanted dopants. Modification of electrical properties now also extends to reduction of dielectric constant in low-k insulating materials via exposure to ultraviolet light in UV processing (UVP).
Many modern chips have eight or more levels produced in over 300 sequenced processing steps.
===Front End Processing===
"Front End Processing" refers to the formation of the [[transistor]]s directly on the [[silicon]]. The raw wafer is engineered by the growth of an ultrapure, virtually defect-free silicon layer through [[epitaxy]]. In the most advanced logic devices, '' prior '' to the silicon epitaxy step, tricks are performed to improve the performance of the transistors to be built. One method involves introducing a "straining step" wherein a silicon variant such as "silicon-germanium" ([[SiGe]]) is deposited. Once the epitaxial silicon is deposited, the crystal lattice becomes stretched somewhat, resulting in improved electronic mobility. Another method, called "[[silicon on insulator]]" technology involves the insertion of an insulating layer between the raw silicon wafer and the thin layer of subsequent silicon epitaxy. This method results in the creation of transistors with reduced parasitic effects.
===Silicon dioxide===
Front end surface engineering is followed by: growth of the [[gate dielectric]], traditionally [[silicon dioxide]] (SiO<sub>2</sub>), patterning of the gate, patterning of the source and drain regions, and subsequent implantation or diffusion of dopants to obtain the desired complementary electrical properties. In memory devices, storage cells, conventionally [[capacitors]], are also fabricated at this time, either into the silicon surface or stacked above the transistor.
===Metal layers===
Once the various semiconductor devices have been created they must be interconnected to form the desired electrical circuits. This "Back End Of Line" (BEOL – the latter portion of the front end of wafer fabrication, not to be confused with "back end" of chip fabrication which refers to the package and test stages) involves creating metal interconnecting wires that are isolated by insulating dielectrics. The insulating material was traditionally a form of SiO<sub>2</sub> or a [[silicate glass]], but recently new [[low-K|low dielectric constant]] materials are being used. These dielectrics presently take the form of SiOC and have dielectric constants around 2.7 (compared to 3.9 for SiO<sub>2</sub>), although materials with constants as low as 2.2 are being offered to chipmakers.
===Interconnect===
Historically, the metal wires consisted of [[aluminium]]. In this approach to wiring often called "subtractive aluminium", blanket films of aluminium are deposited first, patterned, and then etched, leaving isolated wires. Dielectric material is then deposited over the exposed wires. The various metal layers are interconnected by etching holes, called "vias," in the insulating material and depositing [[tungsten]] in them with a [[Chemical vapor deposition|CVD]] technique. This approach is still used in the fabrication of many memory chips such as [[dynamic random access memory]] (DRAM) as the number of interconnect levels is small, currently no more than four.
More recently, as the number of interconnect levels for logic has substantially increased due to the large number of transistors that are now interconnected in a modern [[microprocessor]], the timing delay in the wiring has become significant prompting a change in wiring material from aluminium to [[copper]] and from the silicon dioxides to newer [[high-K]] material. This performance enhancement also comes at a '' reduced cost '' via damascene processing that eliminates processing steps. In damascene processing, in contrast to subtractive aluminium technology, '' the dielectric material is deposited first '' as a blanket film and is patterned and etched leaving holes or trenches. In "single damascene" processing, copper is then deposited in the holes or trenches surrounded by a thin barrier film resulting in filled vias or wire "lines" respectively. In "dual damascene" technology, both the trench and via are fabricated before the deposition of copper resulting in formation of both the via and line simultaneously, further reducing the number of processing steps. The thin barrier film, called Copper Barrier Seed (CBS), is necessary to prevent copper diffusion into the dielectric. The ideal barrier film is effective, but is barely there. As the presence of excessive barrier film competes with the available copper wire cross section, formation of the thinnest yet continuous barrier represents one of the greatest ongoing challenges in copper processing today.
As the number of interconnect levels increases, planarization of the previous layers is required to ensure a flat surface prior to subsequent lithography. Without it, the levels would become increasingly crooked and extend outside the depth of focus of available lithography, interfering with the ability to pattern. CMP ('''C'''hemical '''M'''echanical '''P'''olishing) is the primary processing method to achieve such planarization although dry "etch back" is still sometimes employed if the number of interconnect levels is no more than three.
==Wafer test==
The highly serialized nature of wafer processing has increased the demand for metrology in between the various processing steps. Wafer test metrology equipment is used to verify that the wafers are still good and haven't been damaged by previous processing steps. If the number of dies—the integrated circuits that will eventually become chips—on a wafer that measure as fails exceeds a predetermined threshold, the wafer is scrapped rather than investing in further processing.
==Device test==
{{main|wafer testing}}
Once the Front End Process has been completed, the semiconductor devices are subjected to a variety of electrical tests to determine if they function properly. The proportion of devices on the wafer found to perform properly is referred to as the yield.
The fab [[Wafer testing|tests the chips on the wafer]] with an electronic tester that presses tiny probes against the chip. The machine marks each bad chip with a drop of dye. The fab charges for test time; the prices are on the order of cents per second. Chips are often designed with “testability features” such as "[[built-in self-test]]" to speed testing, and reduce test costs.
Good designs try to test and statistically manage ''corners'': extremes of silicon behavior caused by operating temperature combined with the extremes of fab processing steps. Most designs cope with more than 64 corners.
==Packaging==
{{main|integrated circuit packaging}}
Once tested, the wafer is scored and then broken into individual die. Only the good, unmarked chips go on to be packaged.
Plastic or ceramic packaging involves mounting the die, connecting the die pads to the pins on the package, and sealing the die. Tiny wires are used to connect pads to the pins. In the old days, wires were attached by hand, but now purpose-built machines perform the task. Traditionally, the wires to the chips were gold, leading to a “lead frame” (pronounced “leed frame”) of copper, that had been plated with solder, a mixture of tin and lead. Lead is poisonous, so lead-free “lead frames” are now mandated by [[ROHS]].
Chip-scale package (CSP) is another packaging technology. Plastic packaged chips are usually considerably larger than the actual die, whereas CSP chips are nearly the size of the die. CSP can be constructed for each die ''before'' the wafer is diced [http://www.uic.com/wcms/WCMS2.nsf/index/Resources_26.html].
The packaged chips are retested to ensure that they were not damaged during packaging and that the die-to-pin interconnect operation was performed correctly. A laser etches the chip’s name and numbers on the package.
==List of steps==
This is a list of processing techniques that are employed numerous times in a modern electronic device and do not necessarily imply a specific order.
*Wafer processing
**Wet cleans
**[[Photolithography]]
**[[Ion implantation]] (in which [[dopant]]s are embedded in the wafer creating regions of increased (or decreased) conductivity)
**[[Dry etching]]
**[[Wet etching]]
**[[Plasma ashing]]
**Thermal treatments
***[[Rapid thermal anneal]]
***[[Furnace anneal]]s
***[[Thermal oxidation]]
**[[Chemical vapor deposition]] (CVD)
**[[Physical vapor deposition]] (PVD)
**[[Molecular beam epitaxy]] (MBE)
**Electrochemical Deposition (ECD). See [[Electroplating]]
**[[Chemical-mechanical planarization]] (CMP)
**[[Wafer testing]] (where the electrical performance is verified)
**Wafer backgrinding (to reduce the thickness of the wafer so the resulting chip can be put into a thin device like a [[smartcard]] or [[PCMCIA card]].)
*[[Die preparation]]
**Wafer mounting
**Die cutting
*[[IC packaging]]
**[[Die attachment]]
**IC Bonding
***[[Wire bonding]]
***[[Flip chip]]
***Tab bonding
**[[IC encapsulation]]
***Baking
***[[Plating]]
***Lasermarking
***Trim and form
* [[Semiconductor fabrication#Device Test|IC testing]]
== Hazardous materials note==
Many toxic materials are used in the fabrication process. These include:
*poisonous elemental [[dopants]] such as [[arsenic]], [[boron]], [[antimony]] and [[phosphorus]]
*poisonous compounds like [[arsine]], [[phosphine]] and [[silane]]
*highly reactive liquids, such as [[hydrogen peroxide]], fuming [[nitric acid]], [[sulfuric acid]] and [[hydrofluoric acid]]
It is vital that workers not be directly exposed to these dangerous substances. The high degree of automation common in the IC fabrication industry helps to reduce the risks of exposure of this sort. Most fabrication facillities employ exhaust management systems, such as wet scrubbers, combustors, heated absorber cartridges etc, to control the risk to workers and also the environment if these toxic materials are released into the atmosphere. Some of these toxic materials, if released into the atmosphere, can remain there for thousands of years and contribute to [[global warming]].
== History ==
When feature widths were far greater than about 10 [[micrometre]]s, purity was not the issue that it is today in device manufacturing. As devices became more integrated, [[cleanroom]]s became even cleaner. Today, the fabs are pressurized with filtered air to remove even the smallest particles, which could come to rest on the wafers and contribute to defects. The workers in a semiconductor fabrication facility are required to wear [[cleanroom suit]]s to protect the devices from human contamination.
In an effort to increase profits, semiconductor device manufacturing has spread from [[Texas]] and [[California]] in the 1960s to the rest of the world, such as [[Republic of Ireland|Ireland]], [[Israel]], [[Japan]], [[Taiwan]], [[Korea]], [[Singapore]] and [[China]]. It is a global business today.
The leading semiconductor manufacturers typically have facilities all over the world. [[Intel]], the world's largest manufacturer, has facilities in Europe and Asia as well as the U.S. Other top manufacturers include [[STMicroelectronics]] (Europe), [[Analog Devices]] (US/Asia), [[Atmel]] (US/Europe), [[Freescale Semiconductor]] (US), [[Samsung]] (Korea), [[Texas Instruments]] (US), [[Advanced Micro Devices]] (AMD) (US) see [http://www.amd.com], [[Toshiba]] (Japan), [http://www.necel.com/index.html NEC Electronics] (Japan), [[Infineon]] (Europe), [[Renesas]] (Japan), [[TSMC|Taiwan Semiconductor Manufacturing Company]] (Taiwan, see [http://www.tsmc.com/ TSMC web site]), [[Sony]](Japan), [[NXP Semiconductors]] (Europe), [[Hynix]] (Korea) and SMIC (China, see [http://www.smics.com/ SMIC web site]).
==See also==
*[[Atomic layer deposition]]
*[[Cleanroom]]s
*[[Electronic design automation]]
*[[Foundry (electronics)]]
*[[GDS-II]]
*[[Microfabrication]]
*[[OASIS (standard)|OASIS]]
*[[Semiconductor Equipment and Materials International|SEMI]] — The semiconductor industry trade association
== External links ==
*[http://www.siliconfareast.com/manufacturing.HTM Semiconductor Manufacturing] - www.SiliconFarEast.com
*[http://intel.com/education/makingchips Intel's Animated step-by-step process]
*[http://www.necel.com/v_factory/en/index.html NEC Electronics' Virtual Factory Tour]
*[http://www.semiconductorglossary.com Semiconductor Glossary]
*[http://www.logitech.uk.com/semicon.asp Semiconductor materials processing]
*[http://www.lelandstanfordjunior.com/thermaloxide.html Calculator for Silicon thermal oxidation]
*[http://www.ece.byu.edu/cleanroom/ BYU Cleanroom - semiconductor properties, calculators, processes, etc.]
*[http://www.omron-semi-pv.eu/ Omron An introduction to Application Expertise - Semiconductor, Photo Voltaic & Electronics Industry]
[[Category:Semiconductor device fabrication]]
[[bg:Планарна технология]]
[[de:Halbleitertechnologie]]
[[es:Fabricación de circuitos integrados]]
[[fr:Procédés de fabrication des dispositifs à semi-conducteurs]]
[[id:Fabrikasi semikonduktor]]
[[it:Fabbricazione dei dispositivi a semiconduttore]]
[[ja:半導体工学]]
[[zh:半导体器件制造]]