Process corners
Encyclopedia
In semiconductor
manufacturing, a process corner is an example of a design-of-experiments (DoE)
technique that refers to a variation of fabrication parameters used in applying an integrated circuit
design to a semiconductor wafer
. Process corners represent the extremes of these parameter variations within which a circuit that has been etched onto the wafer must function correctly. A circuit running on devices fabricated at these process corners may run slower or faster than specified and at lower or higher temperatures and voltages, but if the circuit does not function at all at any of these process extremes the design is considered to have inadequate design margin.
In order to verify the robustness of an integrated circuit design, semiconductor manufacturers will fabricate corner lots, which are groups of wafers that have had process parameters adjusted according to these extremes, and will then test the devices made from these special wafers at varying increments of environmental conditions, such as voltage, clock frequency, and temperature, applied in combination (two or sometimes all three together) in a process called characterization. The results of these tests are plotted using a graphing technique known as a shmoo plot
that indicates clearly the boundary limit beyond which a device begins to fail for a given combination of these environmental conditions.
Corner-lot analysis is most effective in digital electronics because of the direct effect of process variations on the speed of transistor switching during transitions from one logic state to another, which is not relevant for analog circuits, such as amplifiers. Analog techniques rely more on over-design and simulation software tools such as SPICE
, or Monte Carlo analysis.
integrated circuit
microprocessor
design and semiconductor fabrication
, a process corner represents a three or six sigma variation
from nominal doping
concentrations (and other parameters) in transistors on a silicon wafer. This variation can cause significant changes in the duty cycle
and slew rate
of digital signals, and can sometimes result in catastrophic failure
of the entire system.
Variation may occur for many reasons, such as minor changes in the humidity or temperature changes in the clean-room when wafers are transported, or due to the position of the die
relative to the center of the wafer.
(NMOS) corner, and the second letter refers to the P channel (PMOS) corner. In this naming convention, three corners exist: typical, fast and slow. Fast and slow corners exhibit carrier mobilities that are higher and lower than normal, respectively. For example, a corner designated as FS denotes fast NFETs and slow PFETs.
There are therefore five possible corners: typical-typical (TT) (not really a corner of an n vs. p mobility graph, but called a corner, anyway), fast-fast (FF), slow-slow (SS), fast-slow (FS), and slow-fast (SF). The first three corners (TT, FF, SS) are called even corners, because both types of devices are affected evenly, and generally do not adversely affect the logical correctness of the circuit. The resulting devices can function at slower or faster clock frequencies, and are often binned
as such. The last two corners (FS, SF) are called "skewed" corners, and are cause for concern. This is because one type of FET will switch much faster than the other, and this form of imbalanced switching can cause one edge of the output to have much less slew than the other edge. Latching devices may then record incorrect values in the logic chain.
In addition to the FET
s themselves, there are more on-chip variation (OCV) effects that manifest themselves at smaller technology nodes. These include process, voltage and temperature (PVT) variation effects on on-chip interconnect, as well as via structures.
or BSIM
simulation
models for all (or, at the least, TT, FS, and SF) process corners, which enables circuit designers to detect corner skew
effects before the design is laid out
, as well as post-layout (through parasitics extraction), before it is taped out.
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
manufacturing, a process corner is an example of a design-of-experiments (DoE)
Design of experiments
In general usage, design of experiments or experimental design is the design of any information-gathering exercises where variation is present, whether under the full control of the experimenter or not. However, in statistics, these terms are usually used for controlled experiments...
technique that refers to a variation of fabrication parameters used in applying an integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
design to a semiconductor wafer
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...
. Process corners represent the extremes of these parameter variations within which a circuit that has been etched onto the wafer must function correctly. A circuit running on devices fabricated at these process corners may run slower or faster than specified and at lower or higher temperatures and voltages, but if the circuit does not function at all at any of these process extremes the design is considered to have inadequate design margin.
In order to verify the robustness of an integrated circuit design, semiconductor manufacturers will fabricate corner lots, which are groups of wafers that have had process parameters adjusted according to these extremes, and will then test the devices made from these special wafers at varying increments of environmental conditions, such as voltage, clock frequency, and temperature, applied in combination (two or sometimes all three together) in a process called characterization. The results of these tests are plotted using a graphing technique known as a shmoo plot
Shmoo plot
In electrical engineering, a shmoo plot is a graphical display of the response of a component or system varying over a range of conditions and inputs. Often used to represent the results of the testing of complex electronic systems such as computers, ASICs or microprocessors...
that indicates clearly the boundary limit beyond which a device begins to fail for a given combination of these environmental conditions.
Corner-lot analysis is most effective in digital electronics because of the direct effect of process variations on the speed of transistor switching during transitions from one logic state to another, which is not relevant for analog circuits, such as amplifiers. Analog techniques rely more on over-design and simulation software tools such as SPICE
SPICE
SPICE is a general-purpose, open source analog electronic circuit simulator.It is a powerful program that is used in integrated circuit and board-level design to check the integrity of circuit designs and to predict circuit behavior.- Introduction :Unlike board-level designs composed of discrete...
, or Monte Carlo analysis.
Significance to digital electronics
In Very-Large-Scale Integration (VLSI)Very-large-scale integration
Very-large-scale integration is the process of creating integrated circuits by combining thousands of transistors into a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed. The microprocessor is a VLSI device.The first semiconductor...
integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
design and semiconductor fabrication
Semiconductor fabrication
Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer...
, a process corner represents a three or six sigma variation
Standard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...
from nominal doping
Dopant
A dopant, also called a doping agent, is a trace impurity element that is inserted into a substance in order to alter the electrical properties or the optical properties of the substance. In the case of crystalline substances, the atoms of the dopant very commonly take the place of elements that...
concentrations (and other parameters) in transistors on a silicon wafer. This variation can cause significant changes in the duty cycle
Duty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....
and slew rate
Slew rate
In electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit.Limitations in slew rate capability can give rise to non linear effects in electronic amplifiers...
of digital signals, and can sometimes result in catastrophic failure
Catastrophic failure
A catastrophic failure is a sudden and total failure of some system from which recovery is impossible. Catastrophic failures often lead to cascading systems failure....
of the entire system.
Variation may occur for many reasons, such as minor changes in the humidity or temperature changes in the clean-room when wafers are transported, or due to the position of the die
Die (integrated circuit)
A die in the context of integrated circuits is a small block of semiconducting material, on which a given functional circuit is fabricated.Typically, integrated circuits are produced in large batches on a single wafer of electronic-grade silicon or other semiconductor through processes such as...
relative to the center of the wafer.
Types of corners
One naming convention for process corners is to use two-letter designators, where the first letter refers to the N-channel MOSFETMOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
(NMOS) corner, and the second letter refers to the P channel (PMOS) corner. In this naming convention, three corners exist: typical, fast and slow. Fast and slow corners exhibit carrier mobilities that are higher and lower than normal, respectively. For example, a corner designated as FS denotes fast NFETs and slow PFETs.
There are therefore five possible corners: typical-typical (TT) (not really a corner of an n vs. p mobility graph, but called a corner, anyway), fast-fast (FF), slow-slow (SS), fast-slow (FS), and slow-fast (SF). The first three corners (TT, FF, SS) are called even corners, because both types of devices are affected evenly, and generally do not adversely affect the logical correctness of the circuit. The resulting devices can function at slower or faster clock frequencies, and are often binned
Product binning
In semiconductor device fabrication, product binning is the process of sorting manufactured products based on tested levels of performance. Large variances in performance are condensed into a smaller number of marketed designations...
as such. The last two corners (FS, SF) are called "skewed" corners, and are cause for concern. This is because one type of FET will switch much faster than the other, and this form of imbalanced switching can cause one edge of the output to have much less slew than the other edge. Latching devices may then record incorrect values in the logic chain.
In addition to the FET
Fet
Fet is a municipality in Akershus county, Norway. It is part of the Romerike traditional region. The administrative centre of the municipality is the village of Fetsund.Fet was established as a municipality on 1 January 1838...
s themselves, there are more on-chip variation (OCV) effects that manifest themselves at smaller technology nodes. These include process, voltage and temperature (PVT) variation effects on on-chip interconnect, as well as via structures.
Accounting for corners
To combat these variation effects, modern technology processes often supply SPICESPICE
SPICE is a general-purpose, open source analog electronic circuit simulator.It is a powerful program that is used in integrated circuit and board-level design to check the integrity of circuit designs and to predict circuit behavior.- Introduction :Unlike board-level designs composed of discrete...
or BSIM
BSIM
BSIM refers to a family of MOSFET transistor models for integrated circuit design. Accurate transistor models are needed for electronic circuit simulation, which in turn is needed for integrated circuit design...
simulation
Simulation
Simulation is the imitation of some real thing available, state of affairs, or process. The act of simulating something generally entails representing certain key characteristics or behaviours of a selected physical or abstract system....
models for all (or, at the least, TT, FS, and SF) process corners, which enables circuit designers to detect corner skew
Skew
Skew may refer to:In mathematics:* Skew lines, lines that are neither parallel nor intersecting* Skew-symmetric matrix, a square matrix whose transpose is also its negative...
effects before the design is laid out
Layout
Layout may refer to:* Page layout** Comprehensive layout** In computer graphics, another name for a scene used to render 2D/3D graphics or animation* Layout , software that automatically calculates the positions of objects...
, as well as post-layout (through parasitics extraction), before it is taped out.