Digital CMOS Abbreviation D Alphabet Dictionary
The Damascene process is a semiconductor manufacturing technique used to pattern copper in microchips. Since copper doesn’t have suitable dry etchants, wet etching cannot be used due to its isotropic nature. Instead, the process involves etching the desired pattern into an oxide layer. Copper is then deposited to fill the patterned trenches, and chemical mechanical polishing (CMP) is used to remove excess copper and create the desired copper interconnects. The double Damascene process allows for the creation of both copper wires and vias simultaneously.
The datapath is a specific section of a digital circuit that contains all the components responsible for digital signal processing. It does not include any control or sequencing logic. The datapath communicates with the controller through status signals and is controlled by the controller through control signals. It essentially handles the computational and arithmetic functions within a circuit.
A decoupling capacitor is a large capacitor placed within a chip or on a PCB between the power supply (Vcc) and ground (GND). Its primary purpose is to mitigate ground and supply voltage fluctuations or “bounce.” Under normal operating conditions, a decoupling capacitor appears as an open circuit for true ground and supply, but during rapid and large current changes, it provides a low-impedance path for current to flow, stabilizing voltage levels. It helps reduce the impact of inductance in the power distribution network.
A defect in the context of semiconductor manufacturing refers to any physical deviation or issue within a semiconductor die that deviates from the expected outcome of the fabrication process. Defects can result from various factors such as misalignment of masks, process variations, temperature variations, electromigration, and more. A defect is considered significant if it affects the behavior or functionality of the circuit.
Defect level, often abbreviated as DL, represents the number of defective chips that are shipped to customers. It plays a crucial role in measuring fault coverage and yield in semiconductor manufacturing. Defect levels are typically expressed in parts per million (ppm). A DL of less than 500 ppm is generally considered acceptable for commercial processes, while values less than 100 ppm are considered very low defect levels. DL values less than 30 ppm are associated with virtually zero defect levels and are often seen in mature processes or specialty applications.
Density of States
The density of states is a function that describes the number of electronic states per unit volume available at a certain energy level (E) in a solid material. This function provides insights into the distribution of energy levels within a material and is particularly relevant in the study of semiconductor physics.
Density rules are specific design rules in semiconductor manufacturing that impose minimum density requirements for features at each layer. These rules are commonly applied to metal layers, especially in the context of copper patterning. Ensuring that wires or features maintain a minimum density is crucial for preventing the formation of deep valleys during oxide deposition using chemical mechanical polishing (CMP). Such valleys can hinder the creation of a smooth surface between metal layers. When density falls below a certain limit, designers may need to insert dummy lines or features to meet the density requirements.
Depletion Load Logic
Depletion load logic is a type of logic family that uses a depletion-mode NMOS transistor as the load element. In this logic family, the driver is typically an NMOS transistor. Unlike enhancement-load logic, depletion load logic does not require diode-connected transistors to function. It offers better performance but has a nonzero logic low value and higher static power dissipation.
Depletion mode refers to a mode of operation in a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) where carriers (electrons or holes) are removed or depleted from the surface region of the semiconductor material. This results in the formation of an insulating depletion layer due to the presence of ionized dopants. The depth of the depletion layer increases with an applied gate voltage until it reaches the threshold for strong inversion.
A depletion MOSFET is a type of MOSFET that has an implanted channel, causing it to be in the “on” state at zero gate voltage. To turn off a depletion MOSFET, a negative gate voltage (for NMOS) needs to be applied. This is because depletion MOSFETs are naturally conductive without a gate voltage, and applying a gate voltage depletes the channel, turning the transistor off
Also known as the space-charge region, the depletion region is an area within a doped semiconductor where the majority of charge carriers have been removed, typically due to the influence of an external or built-in electric field. This region has low charge carrier density, resulting in low conductivity, similar to intrinsic silicon. The presence of dopant ions in the depletion region gives it a net volumetric charge, and it exhibits insulating properties. However, it’s not a perfect insulator due to its moderate bandgap energy. When a suitable electric field exists and charge carriers are present at the depletion region’s edges, current can flow through it.
Depth of Focus
Depth of focus refers to the acceptable range within which a semiconductor wafer must be positioned to ensure that light from a lens can be projected onto the wafer with acceptable focus. It is a critical parameter in semiconductor lithography processes, where precise alignment and focus are necessary for accurate pattern transfer.
Design Rule Check (DRC)
DRC is one of the final steps in the ASIC (Application-Specific Integrated Circuit) design flow. During DRC, the layout of the chip is rigorously examined against the design rules provided by the semiconductor manufacturing vendor. Any violations of these design rules are reported as DRC violations. Most semiconductor manufacturers require that layouts be free of DRC violations before proceeding with fabrication.
Design rules are a set of guidelines and specifications that dictate various aspects of semiconductor chip layout. These rules cover parameters such as minimum feature width, spacing between features, feature density, and layer-to-layer alignment. Design rules are essential to ensure manufacturability, prevent issues like misalignment or short circuits, and enhance yield.
Development, in the context of photoresist used in semiconductor fabrication, is the process of removing the soluble portions of the photoresist material from a semiconductor wafer. It is achieved by immersing the wafer in a developer solution. Successful development depends on precise control of factors like baking, exposure, and the developer’s solution to achieve the desired pattern.
A die is a square or rectangular area on a semiconductor wafer that contains all the essential components and functionalities of an integrated circuit (IC). Each die on a wafer is later separated and mounted in a package to create individual ICs. It represents the core functional unit of an IC.
Diffusion is a physical phenomenon where particles move from regions of higher concentration to regions of lower concentration. In semiconductor manufacturing, it refers to the process of introducing dopant atoms into a semiconductor material by creating a high concentration gradient. This process is used for purposes such as doping semiconductor wafers to create specific electrical characteristics.
This term is an alternative name for the active layer in semiconductor devices. It is worth noting that despite the name, diffusion is not used in building the thin oxide or in doping the source and drain regions.
In photolithography, diffusion refers to a process where particles are inserted into a feature on a semiconductor wafer due to a high concentration gradient. This process can result in inaccuracies in feature size and dopant concentration. It is commonly used for well doping in semiconductor fabrication.
Diffusion current is the electric current that arises from the movement of charge carriers (either electrons or holes) through a crystalline structure due to diffusion processes.
Direct tunneling refers to a tunneling phenomenon that occurs when charge carriers move across the entire thickness of an insulating material. It is a significant source of leakage current in highly scaled CMOS (Complementary Metal-Oxide-Semiconductor) devices.
Domino logic is a design methodology used in digital circuits where the fundamental building block is a dynamic logic gate followed by a small static CMOS inverter. This approach helps prevent charge loss associated with cascading dynamic CMOS stages, improving circuit performance and speed.
In semiconductor physics, a donor is an intentional dopant atom that is added to a semiconductor material to increase its conductivity. Donors typically belong to a higher atomic group than the host semiconductor. In elemental semiconductors like silicon, donors are often from group 15 (such as phosphorus). Donors provide additional electrons to the semiconductor lattice without creating corresponding holes, significantly increasing the concentration of free electrons in the material.
A dopant is an intentional impurity atom introduced in very low concentrations into a semiconductor material during its manufacturing process. Dopants are used to alter the fundamental electrical and optical properties of the semiconductor. In elemental semiconductors like silicon, dopants can come from groups 13 and 15 of the periodic table.
The dot operator is a mathematical operator used in computer architecture and digital design. It combines two sets of group generates and propagates into a larger group generate-propagate. It consists of two operators, one acting on “generates” and the other on “propagates.” These operators are typically implemented as simple complementary metal-oxide-semiconductor (CMOS) gates. The dot operator is not commutative and is particularly significant when applied to continuous or overlapping ranges. It helps in designing high-speed adders.
In a metal-oxide-semiconductor field-effect transistor (MOSFET), the drain is one of the four terminals. In NMOS, the drain is heavily doped n-type, while in PMOS, it is heavily doped p-type. The drain is responsible for collecting the charge carriers (either electrons or holes) that flow from the source when the transistor is in operation.
Drain capacitance refers to the capacitance observed at the drain terminal of a MOSFET. This capacitance primarily arises from the depletion capacitance across the reverse-biased PN junction to the body. Depending on the MOSFET’s operating region and gate-drain overlap, it may also include an oxide capacitance component associated with the gate.
Drain-Induced Barrier Lowering (DIBL)
DIBL is a phenomenon observed in MOSFETs, particularly in shorter channel lengths. It occurs when the drain potential exerts a significant influence on the channel, causing a reduction in the gate’s control over the channel. DIBL can result in a lowered threshold voltage for shorter channels or an increase in the subthreshold swing. It is a critical short-channel effect that can lead to increased leakage current, posing challenges in semiconductor device design.
DRAM Soft Errors
DRAM (Dynamic Random-Access Memory) soft errors occur when the contents of random memory locations within a DRAM module are altered due to external factors, such as alpha particle radiation from the environment. To address this reliability issue, many DRAM arrays incorporate error correction coding mechanisms to detect and correct soft errors.
DRAM, 1 Transistor
In a 1-transistor DRAM cell, each cell comprises a single transistor, which serves as an access device, and a storage capacitor. The capacitor stores the data, and the transistor is used to access it. This type of DRAM cell typically requires an additional fabricated capacitor because the parasitic capacitance is insufficient for data storage. It is often embedded in CMOS processes but has limitations such as destructive reads and the need for frequent refreshing due to leakage.
DRAM, 3 Transistor
In a 3-transistor DRAM cell, each cell consists of three NMOS transistors. Data storage takes place dynamically at the gate of one of the transistors. These cells have non-destructive reads, do not require an additional storage capacitor, and are relatively fast. They are suitable for embedded memory applications and do not require non-traditional fabrication steps.
DRAM, 4 Transistor
A 4-transistor DRAM cell comprises four transistors. It is essentially an SRAM (Static Random-Access Memory) cell stripped of its PMOS transistors. Data storage occurs dynamically at one node. However, the cell contains an unnecessary feedback path, making it less efficient and less commonly used compared to 3-transistor DRAM cells.
Drift is a physical phenomenon in which free charge carriers (electrons or holes) move in a semiconductor material under the influence of an electric field. If this motion occurs through a crystalline structure, the velocity of the carriers is typically constant.
Drift current is the electric current that results from the movement or drift of charge carriers (e.g., electrons or holes) through a crystalline semiconductor structure under the influence of an electric field.
Drift velocity refers to the relatively small but directional velocity attained by charge carriers (electrons or holes) as they drift through a crystalline semiconductor structure under the influence of an electric field. This velocity oscillates between a null and a maximum value due to carrier collisions with imperfections in the crystal.
Dynamic CMOS is a logic family where the number of transistors and the delay characteristics are comparable to ratioed logic families. However, the voltage transfer characteristic (VTC) and static power consumption are similar to standard complementary metal-oxide-semiconductor (CMOS) logic. Dynamic CMOS gates may have at least one output state where the output node is in a high-impedance state, making them susceptible to signal integrity issues.
A dynamic hazard is a logical hazard that occurs when a combinational output makes a transition from “0” to “1” or from “1” to “0.” Dynamic hazards typically indicate the presence of at least three paths for the same signal that converge in a single AND or OR gate. These hazards are associated with specific logic structures and can be resolved by addressing the underlying static hazard.
A dynamic latch is a latch where data storage takes place on a high-impedance capacitive node, often at the input of an inverter. Dynamic latches are more compact than static latches but may suffer from signal integrity issues, particularly related to leakage if they are not updated frequently.
Dynamic power refers to the power dissipated in resistors while capacitors are charging or discharging. It is a significant source of power dissipation in semiconductor devices, especially in long-channel MOSFETs and FinFETs.
A dynamic register is a register constructed using dynamic latches. These registers are typically much more compact than static registers but may face signal integrity challenges due to their dynamic nature.