The Transistor, Fundamental Component of Integrated Circuits

D The transistor, fundamental component of integrated circuits he first transistor was made in (SiO2), which serves as an insulator. The transistor, a name derived from Tgermanium by John Bardeen and In 1958, Jack Kilby invented the inte- transfer and resistor, is a fundamen- Walter H. Brattain, in December 1947. grated circuit by manufacturing 5 com- tal component of microelectronic inte- The year after, along with William B. ponents on the same substrate. The grated circuits, and is set to remain Shockley at Bell Laboratories, they 1970s saw the advent of the first micro- so with the necessary changes at the developed the bipolar transistor and processor, produced by Intel and incor- nanoelectronics scale: also well-sui- the associated theory. During the porating 2,250 transistors, and the first ted to amplification, among other func- 1950s, transistors were made with sili- memory. The complexity of integrated tions, it performs one essential basic con (Si), which to this day remains the circuits has grown exponentially (dou- function which is to open or close a most widely-used semiconductor due bling every 2 to 3 years according to current as required, like a switching to the exceptional quality of the inter- “Moore's law”) as transistors continue device (Figure). Its basic working prin- face created by silicon and silicon oxide to become increasingly miniaturized. ciple therefore applies directly to pro- cessing binary code (0, the current is blocked, 1 it goes through) in logic cir- control gate cuits (inverters, gates, adders, and memory cells). The transistor, which is based on the switch source drain transport of electrons in a solid and not in a vacuum, as in the electron gate tubes of the old triodes, comprises three electrodes (anode, cathode and gate), two of which serve as an elec- transistor source drain tron reservoir: the source, which acts as the emitter filament of an electron gate insulator tube, the drain, which acts as the col- source lector plate, with the gate as “control- gate drain ler”. These elements work differently in the two main types of transistor used channel today: bipolar junction transistors, cutaway view Si substrat which came first, and field effect trans- insulation insulation istors (FET). Lg =gate length Lg Bipolar transistors use two types of charge carriers, electrons (negative Figure. charge) and holes (positive charge), A MOS transistor is a switching device for controlling the passage of an electric current from the source (S) to the drain (D) via a gate (G) that is electrically insulated from the conducting and are comprised of identically doped channel. The silicon substrate is marked B for Bulk. (p or n) semiconductor substrate parts D (next) separated by a thin layer of inversely- the surface, where they attract the few doped semiconductor. By assembling mobile electrons of the semiconduc- two semiconductors of opposite types tor. This forms a conducting channel (a p-n junction), the current can be between source and drain (Figure). made to pass through in only one When a negative voltage is applied to direction. Bipolar transistors, whether the gate, which is electrically insula- n-p-n type or p-n-p type, are all basi- ted by an oxide layer, the electrons are cally current amplifier controlled by a forced out of the channel. As the posi- gate current(1): thus, in an n-p-n trans- tive voltage increases, the channel istor, the voltage applied to the p part resistance decreases, letting pro- controls the flow of current between gressively more current through. the two n regions. Logic circuits that In an integrated circuit, transistors use bipolar transistors, which are cal- together with the other components led TTL (for transistor-transistor logic), (diodes, condensers, resistances) are consume more energy than field effect initially incorporated into a ”chip” with Inc./Bell Labs Technologies Lucent The very first transistor. transistors which present a zero gate more or less complex functions. The current in off-state and are voltage- circuit is built by “sandwiching” layer controlled. upon layer of conducting materials Field effect transistors, most com- and insulators formed by lithography monly of MOS (metal oxide semicon- (Box E, Lithography, the key to minia- ductor) type, are used in the majority turization). By far the most classic of today's CMOS (C for complemen- application of this is the micropro- tary) logic circuits(2). Two n-type cessor at the heart of our computers, regions are created on a p-type sili- which contains several hundred million con crystal by doping the surface. transistors (whose size has been redu- These two regions, also called drain ced 10,000-fold since the 1960s), soon and source, are thus separated by a a billion. This has led to industrial very narrow p-type space called the manufacturers splitting the core of the channel. The effect of a positive cur- processors into several subunits wor- STMicroelectronics rent on the control electrode, natu- king in parallel! 8 nanometre transistor developed by the Crolles2 Alliance bringing together rally called the gate, positioned over STMicroelectronics, Philips and Freescale the semiconductor forces the holes to Semiconductor. (1) This category includes Schottky transistors or Schottky barrier transistors which are field effect transistors with a metal/semiconductor control gate that, while more complex, gives improved charge-carrier mobility and response times. (2) Giving MOSFET transistor (for Metal Oxide Semiconductor Field Effect Transistor). E Lithography, the key to miniaturization ptical lithography (photoli- quality, increase in numerical Othography) is a major appli- aperture). cation in the particle-matter Over the years, the increasing interaction, and constitutes the complexity of the optical sys- classical process for fabrica- tems has led to resolutions ting integrated circuits. It is a key actually below the source wave- step in defining circuit patterns, length. This development could and remains a barrier to any not continue without a major future development. Since reso- technological breakthrough, a lution, at the outset, appears to huge step forward in wave- be directly proportional to wavelength. For generations of inte- length, feature-size first pro- grated circuits with a lowest gressed by a step-wise shorte- resolution of between 80 and ning of the wavelength λ of the 50 nm (the next “node” being radiation used. Artechnique at 65 nm), various different The operation works via a Photolithography section in ultra-clean facilities at the approaches are competing to STMicroelectronics unit in Crolles (Isère). reduction lens system, by the offer particle projection at ever- exposure of a photoresist film shorter wavelengths. They use to energy particles, from the ultravio- The next step for high-volume produc- either “soft” X-rays at extreme ultra- let (UV) photons currently used through tion was expected to be the F2 laser violet wavelength (around 10 nm), “hard” to X photons, ions, and finally electrons, (λ = 157 nm), but this lithography tech- X-rays at wavelengths below 1 nm, ions all through a mask template carrying a nology has to all intents and purposes or electrons. pattern of the desired circuit. The aim been abandoned due to complications The step crossing below the 50 nm bar- of all this is to transfer this pattern onto involved in producing optics in CaF2, rier will lead towards low-electron- a stack of insulating or conducting layers which is transparent at this wavelength. energy (10 eV)-enabled nanolithogra- that make up the mask. These layers While the shortening of wavelengths in phy with technology solutions such as will have been deposited previously (the exposure tools has been the driving fac- the scanning tunnelling microscope and layering stage) on a wafer of semicon- tor behind the strong resolution gain molecular beam epitaxy (Box C) for pro- ductor material, generally silicon. After already achieved, two other factors have ducing “superlattices”. this process, the resin dissolves under nevertheless played key roles. The first exposure to the air (development). The was the development of polymer-lat- exposed parts of the initial layer can tice photoresists with low absorbance then be etched selectively, then the resin at the wavelengths used, implementing is lifted away chemically before depo- progressively more innovative input sition of the following layer. This litho- energy reflection/emission systems. The graphy step can take place over twenty second was enhanced optics reducing times during the fabrication of an inte- diffraction interference (better surface grated circuit (figure). In the 1980s, the microelectronics indus- try used mercury lamps delivering near- source UV (g, h and i lines) through quartz optics, with an emission line of 436 nanometres (nm). This system was able to etch structures to a feature-size of 3 microns (µm). This system was used spin-coating centrifuging, of the resin curing, baking mask through to the mid-90s, when it was replaced by excimer lasers emitting far- deposition of projection UV light (KrF, krypton fluoride at 248 nm, the next layer optics then ArF, argon fluoride at 193 nm, with the photons thus created generating several electronvolts) that were able to reach a resolution of 110 nm, pushed to under 90 nm with new processes. resin lift-off etching development step-and-repeat In the 1980s, the CEA's Electronics and exposure Information Technology Laboratory (Leti) pioneered the application of lasers in Figure. The various phases in the lithography process are designed to carve features out lithography and the fabrication of inte- of the layers of conducting or insulating materials making up an integrated circuit. The sequences grated circuits using excimer lasers, and of the operation are laying of a photoresist, then projecting the pattern on a mask using a reduction optics system, which is followed by dissolution of the resin that is exposed to the light beam even the most advanced integrated cir- (development).

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