In electronics, a self-aligned gate is a transistor manufacturing feature whereby a refractory gate electrode region of a MOSFET transistor is used as a mask for the doping of the source and drain regions. This technique ensures that the gate will slightly overlap the edges of the source and drain.
Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. The identification of the electron in 1897, along with the invention of the vacuum tube, which could amplify and rectify small electrical signals, inaugurated the field of electronics and the electron age.
A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.
The metal-oxide-semiconductor field-effect transistor is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. A metal-insulator-semiconductor field-effect transistor or MISFET is a term almost synonymous with MOSFET. Another synonym is IGFET for insulated-gate field-effect transistor.
The use of self-aligned gates is one of the many innovations that led to the large increase in computing power in the 1970s. Self-aligned gates are still used in most modern integrated circuit processes.
An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny transistors into a small chip results in circuits that are orders of magnitude smaller, cheaper, and faster than those constructed of discrete electronic components. The IC's mass production capability, reliability and building-block approach to circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.
The self-aligned gate is used to eliminate the need to align the gate electrode to the source and drain regions of a MOS transistor during the fabrication process.With self-aligned gates the parasitic overlap capacitances between gate and source, and gate and drain are substantially reduced, leading to MOS transistors that are faster, smaller and more reliable than transistors made without them. After the early experimentation with different gate materials (aluminum, molybdenum, amorphous silicon) the industry almost universally adopted self-aligned gates made with polycrystalline silicon, the so-called Silicon Gate Technology (SGT), which had many additional benefits over the reduction of parasitic capacitances. One important feature of SGT was that the silicon gate was entirely buried under top quality thermal oxide (one of the best insulators known), making it possible to create new device types, not feasible with conventional technology or with self-aligned gates made with other materials. Particularly important are charge coupled devices (CCD), used for image sensors, and non-volatile memory devices using floating silicon-gate structures. These devices dramatically enlarged the range of functionality that could be achieved with solid state electronics.
Innovations that made Self-Aligned Gate Technology possible
Certain innovations were required in order to make self-aligned gates:
Prior to these innovations, self-aligned gates had been demonstrated on metal-gate devices, but their real impact was on silicon-gate devices.
A metal gate, in the context of a lateral Metal-Oxide-Semiconductor MOS stack, is just that—the gate material is made from a metal.
The aluminum-gate MOS process technology, developed in the mid-sixties, started with the definition and doping of the source and drain regions of MOS transistors, followed by the gate mask that defined the thin-oxide region of the transistors. With additional processing steps, an aluminum gate would then be formed over the thin-oxide region completing the device fabrication. Due to the inevitable misalignment of the gate mask with respect to the source and drain mask, it was necessary to have a fairly large overlap area between the gate region and the source and drain regions, to ensure that the thin-oxide region would bridge the source and drain, even under worst-case misalignment. This requirement resulted in gate-to-source and gate-to-drain parasitic capacitances that were large and variable from wafer to wafer, depending on the misalignment of the gate oxide mask with respect with the source and drain mask. The result was an undesirable spread in the speed of the integrated circuits produced, and a much lower speed than theoretically possible if the parasitic capacitances could be reduced to a minimum. The overlap capacitance with the most adverse consequences on performance was the gate-to-drain parasitic capacitance, Cgd, which, by the well-known Miller effect, augmented the gate-to-source capacitance of the transistor by Cgd multiplied by the gain of the circuit to which that transistor was a part. The impact was a considerable reduction in the switching speed of transistors.
In 1966 Dr. Bower realized that if the gate electrode was defined first, it would be possible not only to minimize the parasitic capacitances between gate and source and drain, but it would also make them insensitive to misalignment. He proposed a method in which the aluminum gate electrode itself was used as a mask to define the source and drain regions of the transistor. However, since aluminum could not withstand the high temperature required for the conventional doping of the source and drain junctions, Dr. Bower proposed to use ion implantation, a new doping technique still in development at Hughes Aircraft, his employer, and not yet available at other labs. While Bower’s idea was conceptually sound, in practice it did not work, because it was impossible to adequately passivate the transistors, and repair the radiation damage done to the silicon crystal structure by the ion implantation, since these two operations would have required temperatures in excess of the ones survivable by the aluminum gate. Thus his invention provided a proof of principle, but no commercial integrated circuit was ever produced with Bower’s method. A more refractory gate material was needed.
In 1967 John C. Sarace and collaborators at Bell Labs replaced the aluminum gate with an electrode made of vacuum-evaporated amorphous silicon and succeeded in building working self-aligned gate MOS transistors. However, the process, as described, was only a proof of principle, suitable only for the fabrication of discrete transistors and not for integrated circuits; and was not pursued any further by its investigators.
In 1968 the MOS industry was prevalently using aluminum gate transistors with high threshold voltage (HVT) and desired to have a low threshold voltage (LVT) MOS process in order to increase the speed and reduce the power dissipation of MOS integrated circuits. Low threshold voltage transistors with aluminum gate demanded the use of  silicon orientation, which however produced too low a threshold voltage for the parasitic MOS transistors (the MOS transistors created when aluminum over the field oxide would bridge two junctions). To increase the parasitic threshold voltage beyond the supply voltage, it was necessary to increase the N-type doping level in selected regions under the field oxide, and this was initially accomplished with the use of a so-called channel-stopper mask, and later with ion implantation.
The SGT was the first process technology used to fabricate commercial MOS integrated circuits that was later widely adopted by the entire industry in the 1960s. In late 1967, Tom Klein, working at the Fairchild Semiconductor R&D Labs, and reporting to Les Vadasz, realized that the work function difference between heavily P-type doped silicon and N-type silicon was 1.1 volt lower than the work function difference between aluminum and the same N-type silicon. This meant that the threshold voltage of MOS transistors with silicon gate could be 1.1 volt lower than the threshold voltage of MOS transistors with aluminum gate fabricated on the same starting material. Therefore, one could use starting material with  silicon orientation and simultaneously achieve both an adequate parasitic threshold voltage and low threshold voltage transistors without the use of a channel-stopper mask or ion implantation under the field oxide. With P-type doped silicon gate it would therefore be possible not only to create self-aligned gate transistors but also a low threshold voltage process by using the same silicon orientation of the high threshold voltage process.
In February 1968, Federico Faggin joined Les Vadasz’s group and was put in charge of the development of a low-threshold-voltage, self-aligned gate MOS process technology. Faggin's first task was to develop the precision etching solution for the amorphous silicon gate, and then he created the process architecture and the detailed processing steps to fabricate MOS ICs with silicon gate. He also invented the ‘buried contacts,’ a method to make direct contact between amorphous silicon and silicon junctions, without the use of metal, a technique that allowed a much higher circuit density, particularly for random logic circuits.
After validating and characterizing the process using a test pattern he designed, Faggin made the first working MOS silicon gate transistors and test structures by April 1968. He then designed the first integrated circuit using silicon gate, the Fairchild 3708, an 8-bit analog multiplexer with decoding logic, that had the same functionality of the Fairchild 3705, a metal-gate production IC that Fairchild Semiconductor had difficulty making on account of its rather stringent specifications.
The availability of the 3708 in July 1968 provided also a platform to further improve the process during the following months, leading to the shipment of the first 3708 samples to customers in October 1968, and making it commercially available to the general market before the end of 1968. During the period, July to October 1968, Faggin added two additional critical steps to the process:
With silicon gate, the long-term reliability of MOS transistors soon reached the level of bipolar ICs removing one major obstacle to the wide adoption of MOS technology.
By the end of 1968 the silicon gate technology had achieved impressive results. Although the 3708 was designed to have approximately the same area as the 3705 to facilitate using the same production tooling as the 3705, it could have been made considerably smaller. Nonetheless, it had superior performance compared with the 3705: it was 5 times faster, it had about 100 times less leakage current, and the on resistance of the large transistors making up the analog switches was 3 times lower.[ citation needed ] The silicon gate technology (SGT) was also adopted by Intel at its founding (July 1968), and within a few years became the core technology for the fabrication of MOS integrated circuits worldwide, lasting to this day. Intel was also the first company to develop non-volatile memory using floating silicon gate transistors.
The self-aligned gate design was patented in 1969 by the team of Kerwin, Klein, and Sarace.It was independently invented by Robert W. Bower (U.S. 3,472,712, issued October 14, 1969, filed October 27, 1966). The Bell Labs Kerwin et al. patent 3,475,234 was not filed until March 27, 1967, several months after R. W. Bower and H. D. Dill had published and presented the first publication of this work at the International Electron Device Meeting, Washington, D.C. in 1966.
However, in a legal action involving Bower and Dill, the Third Circuit Court of Appeals determined that Kerwin, Klein and Sarace were the true inventors of the self-aligned silicon gate transistor. On that basis, they were awarded the basic patent US 3,475,234 (the US patent system awards the basic patent to the party that first makes the invention, not the party that first files a patent application, per rules at that time). Bower's work described the self-aligned-gate MOSFET, made with both aluminum and polysilicon gates. It used both ion implantation and diffusion to form the source and drain using the gate electrode as the mask to define the source and drain regions. The Bell Labs team attended this meeting of the IEDM in 1966, and they discussed this work with Bower after his presentation in 1966. Bower believed he had first made the self-aligned gate using aluminum as the gate and, before presentation in 1966, made the device using polysilicon as the gate. However, he was not able to prove it to the appellate court, who sided with the Bell Labs team.
The self-aligned gate typically involves ion implantation, another semiconductor process innovation of the 1960s. The histories of ion implantation and self-aligned gates are highly interrelated, as recounted in an in-depth history by R.B. Fair.
The first commercial product using self-aligned silicon-gate technology was the Fairchild 3708 8-bit analog multiplexor, in 1968, designed by Federico Faggin who pioneered several inventions in order to turn the aforementioned non working proofs of concept, into what the industry actually adopted thereafter.
The importance of self-aligned gates comes in the process used to make them. The process of using the gate oxide as a mask for the source and drain diffusion both simplifies the process and greatly improves the yield.
The following are the steps in creating a self-aligned gate:
These steps were first created by Federico Faggin and used in the Silicon Gate Technology process developed at Fairchild Semiconductor in 1968 for the fabrication of the first commercial integrated circuit using it, the Fairchild 3708
Complementary metal–oxide–semiconductor (CMOS) is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for several analog circuits such as image sensors, data converters, and highly integrated transceivers for many types of communication. Frank Wanlass patented CMOS in 1963 while working for Fairchild Semiconductor.
An EPROM, or erasable programmable read-only memory, is a type of memory chip that retains its data when its power supply is switched off. Computer memory that can retrieve stored data after a power supply has been turned off and back on is called non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in digital circuits. Once programmed, an EPROM can be erased by exposing it to strong ultraviolet light source. EPROMs are easily recognizable by the transparent fused quartz window in the top of the package, through which the silicon chip is visible, and which permits exposure to ultraviolet light during erasing.
The threshold voltage, commonly abbreviated as Vth, of a field-effect transistor (FET) is the minimum gate-to-source voltage VGS (th) that is needed to create a conducting path between the source and drain terminals. It is an important scaling factor to maintain power efficiency.
A power MOSFET is a specific type of metal oxide semiconductor field-effect transistor (MOSFET) designed to handle significant power levels.
In integrated circuits, depletion-load NMOS is a form of digital logic family that uses only a single power supply voltage, unlike earlier nMOS logic families that needed more than one different power supply voltage. Although manufacturing these integrated circuits required additional processing steps, improved switching speed and the elimination of the extra power supply made this logic family the preferred choice for many microprocessors and other logic elements.
Capacitance–voltage profiling is a technique for characterizing semiconductor materials and devices. The applied voltage is varied, and the capacitance is measured and plotted as a function of voltage. The technique uses a metal–semiconductor junction or a p–n junction or a MOSFET to create a depletion region, a region which is empty of conducting electrons and holes, but may contain ionized donors and electrically active defects or traps. The depletion region with its ionized charges inside behaves like a capacitor. By varying the voltage applied to the junction it is possible to vary the depletion width. The dependence of the depletion width upon the applied voltage provides information on the semiconductor's internal characteristics, such as its doping profile and electrically active defect densities., Measurements may be done at DC, or using both DC and a small-signal AC signal, or using a large-signal transient voltage.
The floating-gate MOSFET (FGMOS) is a field-effect transistor, whose structure is similar to a conventional MOSFET. The gate of the FGMOS is electrically isolated, creating a floating node in DC, and a number of secondary gates or inputs are deposited above the floating gate (FG) and are electrically isolated from it. These inputs are only capacitively connected to the FG. Since the FG is completely surrounded by highly resistive material, the charge contained in it remains unchanged for long periods of time. Usually Fowler-Nordheim tunneling and hot-carrier injection mechanisms are used to modify the amount of charge stored in the FG.
Charge Trap Flash (CTF) is a semiconductor memory technology used in creating non-volatile NOR and NAND flash memory. The technology differs from the more conventional floating-gate MOSFET technology in that it uses a silicon nitride film to store electrons rather than the doped polycrystalline silicon typical of a floating gate structure. This approach allows memory manufacturers to reduce manufacturing costs five ways:
Hot carrier injection (HCI) is a phenomenon in solid-state electronic devices where an electron or a “hole” gains sufficient kinetic energy to overcome a potential barrier necessary to break an interface state. The term "hot" refers to the effective temperature used to model carrier density, not to the overall temperature of the device. Since the charge carriers can become trapped in the gate dielectric of a MOS transistor, the switching characteristics of the transistor can be permanently changed. Hot-carrier injection is one of the mechanisms that adversely affects the reliability of semiconductors of solid-state devices.
SONOS, short for "silicon–oxide–nitride–oxide–silicon", more precisely, "polycrystalline silicon"—"silicon dioxide"—"silicon nitride"—"siicon dioxide"—"silicon", is a cross sectional structure of MOSFET, realized in late 70's. This structure is often used for non-volatile memories, such as EEPROM and flash memories. It is sometimes used for TFT LCD displays. It is one of CTF (charge trap flash) variants. It is distinguished from traditional non-volatile memory structures by the use of silicon nitride (Si3N4 or Si9N10) instead of "polysilicon-based FG (floating-gate)" for the charge storage material. A further variant is "SHINOS" ("silicon"—"hi-k"—"nitride"—"oxide"—"silicon"), which is substituted top oxide layer with high-κ material. Another advanced variant is "MONOS" ("metal–oxide–nitride–oxide–silicon"). Companies offering SONOS-based products include Cypress Semiconductor, Macronix, Toshiba, United Microelectronics Corporation and Floadia.
P-type metal-oxide-semiconductor logic uses p-channel metal-oxide-semiconductor field effect transistors (MOSFETs) to implement logic gates and other digital circuits. PMOS transistors operate by creating an inversion layer in an n-type transistor body. This inversion layer, called the p-channel, can conduct holes between p-type "source" and "drain" terminals.
Robert W. Bower is an American applied physicist. Immediately after receiving his Ph.D. from The California Institute of Technology in 1973, he worked for over 25 years in many different professions: Engineer, Scientist, Department Head at University of California, Davis, and as president and CEO of Device Concept Inc. He also served as the President of Integrated Vertical Modules, which focused on three-dimensional, high-density structures. His most notable contribution, however, is his field-effect device with insulated gates—also known as a self-aligned-gate MOSFET, or SAGFET. Bower patented this design in 1969 while working at the Hughes Research Laboratories in Malibu, California. He has also published over 80 journals and articles, patented over 28 inventions, and authored chapters in 3 different books.
In electronics, a native transistor is a variety of the MOS field-effect transistor that is intermediate between enhancement and depletion modes. Most common is the n-channel native transistor.
Process variation is the naturally occurring variation in the attributes of transistors when integrated circuits are fabricated. The amount of process variation becomes particularly pronounced at smaller process nodes (<65 nm) as the variation becomes a larger percentage of the full length or width of the device and as feature sizes approach the fundamental dimensions such as the size of atoms and the wavelength of usable light for patterning lithography masks.
In field effect transistors (FETs), depletion mode and enhancement mode are two major transistor types, corresponding to whether the transistor is in an ON state or an OFF state at zero gate–source voltage.
Polysilicon depletion effect is the phenomenon in which unwanted variation of threshold voltage of the MOSFET devices using polysilicon as gate material is observed, leading to unpredicted behavior of the electronic circuit. Polycrystalline silicon, also called polysilicon, is a material consisting of small silicon crystals. It differs from single-crystal silicon, used for electronics and solar cells, and from amorphous silicon, used for thin film devices and solar cells.
The field-effect transistor (FET) is an electronic device which uses an electric field to control the flow of current. This is achieved by the application of a voltage to the gate terminal, which in turn alters the conductivity between the drain and source terminals.
Bernard A Yurash was a significant contributor to the creation of the first commercially viable CMOS integrated circuits by finding the sources of mobile sodium ions coming from the manufacturing process. Today, virtually all digital electronics use CMOS circuitry. Bernard worked at Fairchild Semiconductor in Silicon Valley from 1958, through the buyouts of the company by Schlumberger and National Semiconductor, and finally retiring in 1990. In the 1960s Fairchild Semiconductor, a division of Fairchild Camera and Instrument Corp., and Texas Instruments, revolutionized electronics by employing the first integrated circuit technology. Fairchild's Robert Noyce filed for this patent using deposited (printed) metal lines and Jean Hoerni's Planar Process. At the time virtually all the devices were of the bipolar type which were used to construct RTL and DTL type circuits, which unfortunately drew more power than was desired, and eventually lost ground to Texas Instruments' TTL (Transistor-Transistor-logic). The next great technological leap in computer chips would be CMOS transistors, which promised significantly lower power and greater circuit density than the Bipolar circuitry. Although Frank Wanlass first filed for the CMOS patent in 1963, Fairchild could not produce the devices for commercial output for many years because of the mystery of the mobile ions degrading their performance. Much research time and money was expended in 1967 and 1968 at Fairchild on trying to manufacture the highly promising technology, the MOS SGT circuits utilizing the field effect from the "gate" on the conducting "channel" from source to drain.