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36-525: Non-Quasi Static model ( NQS ) is a transistor model used in analogue /mixed signal IC design . It becomes necessary to use an NQS model when the operational frequency of the device is in the range of its transit time. Normally, in a quasi-static (QS) model, voltage changes in the MOS transistor channel are assumed to be instantaneous. However, in an NQS model voltage changes relating to charge carriers are delayed. This electronics-related article

72-424: A mask set . A curvilinear photomask has patterns with curves, which is a departure from conventional photomasks which only have patterns that are completely vertical or horizontal, known as manhattan geometry. These photomasks require special equipment to manufacture. For IC production in the 1960s and early 1970s, an opaque rubylith film laminated onto a transparent mylar sheet was used. The design of one layer

108-429: A beam of light without causing an optical path shift due to its small film thickness. In 1978, Shea et al. at IBM patented a process to use the "pellicle" as a dust cover to protect a photomask or reticle. In the context of this entry, "pellicle" means "thin film dust cover to protect a photomask". Particle contamination can be a significant problem in semiconductor manufacturing. A photomask is protected from particles by

144-407: A defined pattern. Photomasks are commonly used in photolithography for the production of integrated circuits (ICs or "chips") to produce a pattern on a thin wafer of material (usually silicon ). In semiconductor manufacturing, a mask is sometimes called a reticle . In photolithography, several masks are used in turn, each one reproducing a layer of the completed design, and together known as

180-730: A pellicle – a thin transparent film stretched over a frame that is glued over one side of the photomask. The pellicle is far enough away from the mask patterns so that moderate-to-small sized particles that land on the pellicle will be too far out of focus to print. Although they are designed to keep particles away, pellicles become a part of the imaging system and their optical properties need to be taken into account. Pellicles material are Nitrocellulose and made for various Transmission Wavelengths. Current pellicles are made from polysilicon, and companies are exploring other materials for high-NA EUV and future chip making processes. The SPIE Annual Conference, Photomask Technology reports

216-430: A strong impact on photomask requirements. The commonly used attenuated phase-shifting mask is more sensitive to the higher incidence angles applied in "hyper-NA" lithography, due to the longer optical path through the patterned film. During manufacturing, inspection using a special form of microscopy called CD-SEM (Critical-Dimension Scanning Electron Microscopy) is used to measure critical dimensions on photomasks which are

252-406: A transistor. These include: Scattering parameters, or S parameters, can be measured for a transistor at a given bias point with a vector network analyzer . S parameters can be converted to another parameter set using standard matrix algebra operations. Photomask A photomask (also simply called a mask ) is an opaque plate with transparent areas that allow light to shine through in

288-438: A very large tooling cost, primarily for the photomasks used to create the devices, and there is a large economic incentive to get the design working without any iterations. Complete and accurate models allow a large percentage of designs to work the first time. Modern circuits are usually very complex. The performance of such circuits is difficult to predict without accurate computer models, including but not limited to models of

324-402: Is a stub . You can help Misplaced Pages by expanding it . Transistor model The modern transistor has an internal structure that exploits complex physical mechanisms. Device design requires a detailed understanding of how device manufacturing processes such as ion implantation , impurity diffusion , oxide growth , annealing , and etching affect device behavior. Process models simulate

360-475: Is that a linear model is easier to think about, and helps to organize thought. A transistor's parameters represent its electrical properties. Engineers employ transistor parameters in production-line testing and in circuit design. A group of a transistor's parameters sufficient to predict circuit gain , input impedance , and output impedance are components in its small-signal model . A number of different two-port network parameter sets may be used to model

396-422: Is they can be solved directly, while large signal nonlinear models are generally solved iteratively, with possible convergence or stability issues. By simplification to a linear model, the whole apparatus for solving linear equations becomes available, for example, simultaneous equations , determinants , and matrix theory (often studied as part of linear algebra ), especially Cramer's rule . Another advantage

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432-507: The SEMATECH Mask Industry Assessment which includes current industry analysis and the results of their annual photomask manufacturers survey. The following companies are listed in order of their global market share (2009 info): Major chipmakers such as Intel , Globalfoundries , IBM , NEC , TSMC , UMC , Samsung , and Micron Technology , have their own large maskmaking facilities or joint ventures with

468-399: The abovementioned companies. The worldwide photomask market was estimated as $ 3.2 billion in 2012 and $ 3.1 billion in 2013. Almost half of the market was from captive mask shops (in-house mask shops of major chipmakers). The costs of creating new mask shop for 180 nm processes were estimated in 2005 as $ 40 million, and for 130 nm - more than $ 100 million. The purchase price of

504-418: The absorber film will need to become thinner, and hence less opaque. A 2005 study by IMEC found that thinner absorbers degrade image contrast and therefore contribute to line-edge roughness, using state-of-the-art photolithography tools. One possibility is to eliminate absorbers altogether and use "chromeless" masks, relying solely on phase-shifting for imaging. The emergence of immersion lithography has

540-477: The conceptual stages of circuit design (to decide between alternative design ideas before computer simulation is warranted) and using computers. A small-signal model is generated by taking derivatives of the current–voltage curves about a bias point or Q-point . As long as the signal is small relative to the nonlinearity of the device, the derivatives do not vary significantly, and can be treated as standard linear circuit elements. An advantage of small signal models

576-484: The device behavior modeled in this way was very simple – mainly drift plus diffusion in simple geometries – today many more processes must be modeled at a microscopic level; for example, leakage currents in junctions and oxides, complex transport of carriers including velocity saturation and ballistic transport, quantum mechanical effects, use of multiple materials (for example, Si-SiGe devices, and stacks of different dielectrics ) and even

612-428: The device designer whether the device process will produce devices with the electrical behavior needed by the circuit designer, and is used to inform the process designer about any necessary process improvements. Once the process gets close to manufacture, the predicted device characteristics are compared with measurement on test devices to check that the process and device models are working adequately. Although long ago

648-436: The device. Such models are slow to run and provide detail not needed for circuit design. Therefore, faster transistor models oriented toward circuit parameters are used for circuit design. Transistor models are used for almost all modern electronic design work. Analog circuit simulators such as SPICE use models to predict the behavior of a design. Most design work is related to integrated circuit designs which have

684-493: The devices used. The device models include effects of transistor layout: width, length, interdigitation, proximity to other devices; transient and DC current–voltage characteristics ; parasitic device capacitance, resistance, and inductance; time delays; and temperature effects; to name a few items. Nonlinear , or large signal transistor models fall into three main types: Small-signal or linear models are used to evaluate stability , gain , noise and bandwidth , both in

720-452: The dimensions of the patterns on a photomask. EUV photomasks work by reflecting light, which is achieved by using multiple alternating layers of molybdenum and silicon . Leading-edge photomasks (pre-corrected) images of the final chip patterns are magnified by four times. This magnification factor has been a key benefit in reducing pattern sensitivity to imaging errors. However, as features continue to shrink, two trends come into play:

756-414: The entire wafer, thus increasing productivity. Features 150 nm or below in size generally require phase-shifting to enhance the image quality to acceptable values. This can be achieved in many ways. The two most common methods are to use an attenuated phase-shifting background film on the mask to increase the contrast of small intensity peaks, or to etch the exposed quartz so that the edge between

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792-597: The etched and unetched areas can be used to image nearly zero intensity. In the second case, unwanted edges would need to be trimmed out with another exposure. The former method is attenuated phase-shifting , and is often considered a weak enhancement, requiring special illumination for the most enhancement, while the latter method is known as alternating-aperture phase-shifting , and is the most popular strong enhancement technique. As leading-edge semiconductor features shrink , photomask features that are 4× larger must inevitably shrink as well. This could pose challenges since

828-480: The fab or be used as master-photomask to produce the final actual working photomasks. As feature size shrank, the only way to properly focus the image was to place it in direct contact with the wafer. These contact aligners often lifted some of the photoresist off the wafer and onto the photomask and it had to be cleaned or discarded. This drove the adoption of reverse master photomasks (see above), which were used to produce (with contact photolithography and etching)

864-458: The first is that the mask error factor begins to exceed one, i.e., the dimension error on the wafer may be more than 1/4 the dimension error on the mask, and the second is that the mask feature is becoming smaller, and the dimension tolerance is approaching a few nanometers. For example, a 25 nm wafer pattern should correspond to a 100 nm mask pattern, but the wafer tolerance could be 1.25 nm (5% spec), which translates into 5 nm on

900-441: The manufacturing steps and provide a microscopic description of device "geometry" to the device simulator . "Geometry" does not mean readily identified geometrical features such as a planar or wrap-around gate structure, or raised or recessed forms of source and drain (see Figure 1 for a memory device with some unusual modeling challenges related to charging the floating gate by an avalanche process). It also refers to details inside

936-679: The needed many actual working photomasks. Later, projection photo-lithography meant photomask lifetime was indefinite. Still later direct-step-on-wafer stepper photo-lithography used reticles directly and ended the use of photomasks. Photomask materials changed over time. Initially soda glass was used with silver halide opacity. Later borosilicate and then fused silica to control expansion, and chromium which has better opacity to ultraviolet light were introduced. The original pattern generators have since been replaced by electron beam lithography and laser -driven mask writer or maskless lithography systems which generate reticles directly from

972-469: The original computerized design. Lithographic photomasks are typically transparent fused silica plates covered with a pattern defined with a chromium (Cr) or Fe 2 O 3 metal absorbing film. Photomasks are used at wavelengths of 365 nm , 248 nm, and 193 nm. Photomasks have also been developed for other forms of radiation such as 157 nm, 13.5 nm ( EUV ), X-ray , electrons , and ions ; but these require entirely new materials for

1008-414: The pattern in the photomask is projected and shrunk by four or five times onto the wafer surface. To achieve complete wafer coverage, the wafer is repeatedly " stepped " from position to position under the optical column or the stepper lens until full exposure of the wafer is achieved. A photomask with several copies of the integrated circuit design is used to reduce the number of steppings required to expose

1044-399: The photomask. The variation of electron beam scattering in directly writing the photomask pattern can easily well exceed this. The term "pellicle" is used to mean "film", "thin film", or "membrane." Beginning in the 1960s, thin film stretched on a metal frame, also known as a "pellicle", was used as a beam splitter for optical instruments. It has been used in a number of instruments to split

1080-409: The reticle commonly contains only one copy, also called one layer of the designed VLSI circuit. (However, some photolithography fabrications utilize reticles with more than one layer placed side by side onto the same mask, used as copies to create several identical integrated circuits from one photomask). In modern usage, the terms reticle and photomask are synonymous. In a modern stepper or scanner,

1116-532: The statistical effects due to the probabilistic nature of ion placement and carrier transport inside the device. Several times a year the technology changes and simulations have to be repeated. The models may require change to reflect new physical effects, or to provide greater accuracy. The maintenance and improvement of these models is a business in itself. These models are very computer intensive, involving detailed spatial and temporal solutions of coupled partial differential equations on three-dimensional grids inside

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1152-548: The structure, such as the doping profiles after completion of device processing. With this information about what the device looks like, the device simulator models the physical processes taking place in the device to determine its electrical behavior in a variety of circumstances: DC current–voltage behavior, transient behavior (both large-signal and small-signal), dependence on device layout (long and narrow versus short and wide, or interdigitated versus rectangular, or isolated versus proximate to other devices). These simulations tell

1188-420: The substrate and the pattern film. A set of photomasks , each defining a pattern layer in integrated circuit fabrication , is fed into a photolithography stepper or scanner , and individually selected for exposure. In multi-patterning techniques, a photomask would correspond to a subset of the layer pattern. Historically in photolithography for the mass production of integrated circuit devices, there

1224-491: Was a distinction between the term photoreticle or simply reticle , and the term photomask . In the case of a photomask, there is a one-to-one correspondence between the mask pattern and the wafer pattern. The mask covered the entire surface of the wafer which was exposed in its entirety in one shot. This was the standard for the 1:1 mask aligners that were succeeded by steppers and scanners with reduction optics. As used in steppers and scanners which use image projection,

1260-464: Was cut into the rubylith, initially by hand on an illuminated drafting table (later by machine ( plotter )) and the unwanted rubylith was peeled off by hand, forming the master image of that layer of the chip, often called "artwork". Increasingly complex and thus larger chips required larger and larger rubyliths, eventually even filling the wall of a room, and artworks were to be photographically reduced to produce photomasks (Eventually this whole process

1296-415: Was replaced by the optical pattern generator to produce the master image). At this point the master image could be arrayed into a multi-chip image called a reticle . The reticle was originally a 10X larger image of a single chip. The reticle was by step-and-repeater photolithography and etching used to produce a photomask with image-size the same as the final chip. The photomask might be used directly in

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