ASIC interview questions & answers
ASIC interview questions
What is Body effect ?
The threshold voltage of a MOSFET is affected by the voltage which is applied to the back contact. The voltage difference between the source and the bulk, VBS changes the width of the depletion layer and therefore also the voltage across the oxide due to the change of the charge in the depletion region. This results in a difference in threshold voltage which equals the difference in charge in the depletion region divided by the oxide capacitance, yielding:.
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What are standard Cell's?
In semiconductor design, standard cell methodology is a method of designing Application Specific Integrated Circuits (ASICs) with mostly digital-logic features. Standard cell methodology is an example of design abstraction, whereby a low-level VLSI-layout is encapsulated into an abstract logic representation (such as a NAND gate). Cell-based methodology (the general class that standard-cell belongs to) makes it possible for one designer to focus on the high-level (logical function) aspect of digital-design, while another designer focused on the implementation (physical) aspect. Along with semiconductor manufacturing advances, standard cell methodology was responsible for allowing designers to scale ASICs from comparatively simple single-function ICs (of several thousand gates), to complex multi-million gate devices (SoC).
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What are Design Rule Check (DRC) and Layout Vs Schematic (LVS) ?
Design Rule Check (DRC) and Layout Vs Schematic (LVS) are verification processes. Reliable device fabrication at modern deep submicrometre (0.13 µm and below) requires strict observance of transistor spacing, metal layer thickness, and power density rules. DRC exhaustively compares the physical netlist against a set of "foundry design rules" (from the foundry operator), then flags any observed violations.
LVS is a process that confirms that the layout has the same structure as the associated schematic; this is typically the final step in the layout process. The LVS tool takes as an input a schematic diagram and the extracted view from a layout. It then generates a netlist from each one and compares them. Nodes, ports, and device sizing are all compared. If they are the same, LVS passes and the designer can continue. Note: LVS tends to consider transistor fingers to be the same as an extra-wide transistor. For example, 4 transistors in parallel (each 1 um wide), a 4-finger 1 um transistor, and a 4 um transistor are all seen as the same by the LVS tool. Functionality of .lib files will be taken from spice models and added as an attribute to the .lib file.
What is Antenna effect ?
The antenna effect, more formally plasma induced gate oxide damage, is an efffect that can potentially cause yield and reliability problems during the manufacture of MOS integrated circuits. Fabs normally supply antenna rules, which are rules that must be obeyed to avoid this problem. A violation of such rules is called an antenna violation. The word antenna is somewhat of a misnomer in this context—the problem is really the collection of charge, not the normal meaning of antenna, which is a device for converting electromagnetic fields to/from electrical currents. Occasionally the phrase antenna effect is used this context but this is less common since there are many effects and the phrase does not make clear which is meant.
What are steps involved in Semiconductor device fabrication ?
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.
Ion implantation (in which dopants are embedded in the wafer creating regions of increased (or decreased) conductivity)
Rapid thermal anneal
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.)
Trim and form
What is Clock distribution network ?
In a synchronous digital system, the clock signal is used to define a time reference for the movement of data within that system. The clock distribution network distributes the clock signal(s) from a common point to all the elements that need it. Since this function is vital to the operation of a synchronous system, much attention has been given to the characteristics of these clock signals and the electrical networks used in their distribution. Clock signals are often regarded as simple control signals; however, these signals have some very special characteristics and attributes.
Clock signals are typically loaded with the greatest fanout, travel over the greatest distances, and operate at the highest speeds of any signal, either control or data, within the entire synchronous system. Since the data signals are provided with a temporal reference by the clock signals, the clock waveforms must be particularly clean and sharp. Furthermore, these clock signals are particularly affected by technology scaling (see Moore's law), in that long global interconnect lines become significantly more resistive as line dimensions are decreased. This increased line resistance is one of the primary reasons for the increasing significance of clock distribution on synchronous performance. Finally, the control of any differences and uncertainty in the arrival times of the clock signals can severely limit the maximum performance of the entire system and create catastrophic race conditions in which an incorrect data signal may latch within a register.
The clock distribution network often takes a significant fraction of the power consumed by a chip. Furthermore, significant power can be wasted in transitions within blocks, even when their output is not needed. These observations have lead to a power saving technique called clock gating, which involves adding logic gates to the clock distribution tree, so portions of the tree can be turned off when not needed.
What is Clock Gating ?
Clock gating is one of the power-saving techniques used on many synchronous circuits including the Pentium 4 processor. To save power, clock gating refers to adding additional logic to a circuit to prune the clock tree, thus disabling portions of the circuitry where flip flops do not change state. Although asynchronous circuits by definition do not have a "clock", the term "perfect clock gating" is used to illustrate how various clock gating techniques are simply approximations of the data-dependent behavior exhibited by asynchronous circuitry, and that as the granularity on which you gate the clock of a synchronous circuit approaches zero, the power consumption of that circuit approaches that of an asynchronous circuit.
What is Netlist ?
Netlists are connectivity information and provide nothing more than instances, nets, and perhaps some attributes. If they express much more than this, they are usually considered to be a hardware description language such as Verilog, VHDL, or any one of several specific languages designed for input to simulators.
Most netlists either contain or refer to descriptions of the parts or devices used. Each time a part is used in a netlist, this is called an "instance." Thus, each instance has a "master", or "definition". These definitions will usually list the connections that can be made to that kind of device, and some basic properties of that device. These connection points are called "ports" or "pins", among several other names.
An "instance" could be anything from a vacuum cleaner, microwave oven, or light bulb, to a resistor, capacitor, or integrated circuit chip.
Instances have "ports". In the case of a vacuum cleaner, these ports would be the three metal prongs in the plug. Each port has a name, and in continuing the vacuum cleaner example, they might be "Neutral", "Live" and "Ground". Usually, each instance will have a unique name, so that if you have two instances of vacuum cleaners, one might be "vac1" and the other "vac2". Besides their names, they might otherwise be identical.
Nets are the "wires" that connect things together in the circuit. There may or may not be any special attributes associated with the nets in a design, depending on the particular language the netlist is written in, and that language's features.
Instance based netlists usually provide a list of the instances used in a design. Along with each instance, either an ordered list of net names are provided, or a list of pairs provided, of an instance port name, along with the net name to which that port is connected. In this kind of description, the list of nets can be gathered from the connection lists, and there is no place to associate particular attributes with the nets themselves. SPICE is perhaps the most famous of instance-based netlists.
Net-based netlists usually describe all the instances and their attributes, then describe each net, and say which port they are connected on each instance. This allows for attributes to be associated with nets. EDIF is probably the most famous of the net-based netlists.
What Physical timing closure ?
Physical timing closure is the process by which an FPGA or a VLSI design with a physical representation is modified to meet its timing requirements. Most of the modifications are handled by EDA tools based on directives given by a designer. The term is also sometimes used as a characteristic, which is ascribed to an EDA tool, when it provides most of the features required in this process.
Physical timing closure became more important with submicrometre technologies, as more and more steps of the design flow had to be made timing-aware. Previously only logic synthesis had to satisfy timing requirements.
With present deep submicrometre technologies it is unthinkable to perform any of the design steps of placement, clock-tree synthesis and routing without timing constraints. Logic synthesis with these technologies is becoming less important. It is still required, as it provides the initial netlist of gates for the placement step, but the timing requirements do not need to be strictly satisfied any more.
When a physical representation of the circuit is available, the modifications required to achieve timing closure are carried out by using more accurate estimations of the delays.
What Physical verification ?
Physical verification of the design, involves DRC(Design rule check), LVS(Layout versus schematic) Check, XOR Checks, ERC (Electrical Rule Check) and Antenna Checks.
This step involves comparing two layout databases/GDS by XOR operation of the layout geometries. This check results a database which has all the mismatching geometries in both the layouts. This check is typically run after a metal spin, where in the re-spin database/GDS is compared with the previously taped out database/GDS.
Antenna checks are used to limit the damage of the thin gate oxide during the manufacturing process due to charge accumulation on the interconnect layers (metal, polysilicon) during certain fabrication steps like Plasma etching, which creates highly ionized matter to etch.
The antenna basically is a metal interconnect, i.e., a conductor like polysilicon or metal, that is not electrically connected to silicon or grounded, during the processing steps of the wafer. If the connection to silicon does not exist, charges may build up on the interconnect to the point at which rapid discharge does take place and permanent physical damage results to thin transistor gate oxide. This rapid and destructive phenomenon is known as the antenna effect.
The Antenna ratio is defined as the ratio between the physical area of the conductors making up the antenna to the total gate oxide area to which the antenna is electrically connected.
ERC (Electrical rule check)
ERC (Electrical rule check) involves checking a design for all well and substrate areas for proper contacts and spacings thereby ensuring correct power and ground connections. ERC steps can also involve checks for unconnected inputs or shorted outputs.
What is Stuck-at fault ?
A Stuck-at fault is a particular fault model used by fault simulators and Automatic test pattern generation (ATPG) tools to mimic a manufacturing defect within an integrated circuit. Individual signals and pins are assumed to be stuck at Logical '1', '0' and 'X'. For example, an output is tied to a logical 1 state during test generation to assure that a manufacturing defect with that type of behavior can be found with a specific test pattern. Likewise the output could be tied to a logical 0 to model the behavior of a defective circuit that cannot switch its output pin.
What is Different Logic family ?
Listed here in rough chronological order of introduction along with their usual abbreviations of Logic family
* Diode logic (DL)
* Direct-coupled transistor logic (DCTL)
* Complementary transistor logic (CTL)
* Resistor-transistor logic (RTL)
* Resistor-capacitor transistor logic (RCTL)
* Diode-transistor logic (DTL)
* Emitter coupled logic (ECL) also known as Current-mode logic (CML)
* Transistor-transistor logic (TTL) and variants
* P-type Metal Oxide Semiconductor logic (PMOS)
* N-type Metal Oxide Semiconductor logic (NMOS)
* Complementary Metal-Oxide Semiconductor logic (CMOS)
* Bipolar Complementary Metal-Oxide Semiconductor logic (BiCMOS)
* Integrated Injection Logic (I2L)
What is Different Types of IC packaging ?
IC are packaged in many types they are:
* Ball grid array
* Ceramic ball grid array
* Die attachment
* Dual Flat No Lead
* Dual in-line package
* Flat pack
* Flip chip
* Flip-chip pin grid array
* Land grid array
* Leadless chip carrier
* Low insertion force
* Micro FCBGA
* Micro Leadframe Package
* Multi-Chip Module
* Package on package
* Pin grid array
* Plastic leaded chip carrier
* Quadruple in-line package
* ROM cartridge
* Shrink Small-Outline Package
* Single in-line package
* Small-Outline Integrated Circuit
* Staggered Pin Grid Array
* Surface-mount technology
* Thin small-outline package
* Through-hole technology
* Zig-zag in-line package
What is Substrate coupling ?
In an integrated circuit, a signal can couple from one node to another via the substrate. This phenomenon is referred to as substrate coupling or substrate noise coupling.
The push for reduced cost, more compact circuit boards, and added customer features has provided incentives for the inclusion of analog functions on primarily digital MOS integrated circuits (ICs) forming mixed-signal ICs. In these systems, the speed of digital circuits is constantly increasing, chips are becoming more densely packed, interconnect layers are added, and analog resolution is increased. In addition, recent increase in wireless applications and its growing market are introducing a new set of aggressive design goals for realizing mixed-signal systems. Here, the designer integrates radio frequency (RF) analog and base band digital circuitry on a single chip. The goal is to make single-chip radio frequency integrated circuits (RFICs) on silicon, where all the blocks are fabricated on the same chip. One of the advantages of this integration is low power dissipation for portability due to a reduction in the number of package pins and associated bond wire capacitance. Another reason that an integrated solution offers lower power consumption is that routing high-frequency signals off-chip often requires a 50O impedance match, which can result in higher power dissipation. Other advantages include improved high-frequency performance due to reduced package interconnect parasitics, higher system reliability, smaller package count, smaller package interconnect parasitics, and higher integration of RF components with VLSI-compatible digital circuits. In fact, the single-chip transceiver is now a reality.
What is Latchup ?
A latchup is the inadvertent creation of a low-impedance path between the power supply rails of an electronic component, triggering a parasitic structure, which then acts as a short circuit, disrupting proper functioning of the part and possibly even leading to its destruction due to overcurrent. A power cycle is required to correct this situation.
The parasitic structure is usually equivalent to a thyristor (or SCR), a PNPN structure which acts as a PNP and an NPN transistor stacked next to each other. During a latchup when one of the transistors is conducting, the other one begins conducting too. They both keep each other in saturation for as long as the structure is forward-biased and some current flows through it - which usually means until a power-down. The SCR parasitic structure is formed as a part of the totem-pole PMOS and NMOS transistor pair on the output drivers of the gates.