Process variation (semiconductor)
Encyclopedia
Process variation is the naturally occurring variation the attributes of transistors (length, widths, oxide thickness) when integrated circuit
s are fabricated. It becomes particularly important 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.
Process variation causes measurable and predictable variance in the output performance of all circuits but particularly analog circuits due to mismatch. If the variance causes the measured or simulated performance of a particular output metric (bandwidth, gain, rise time, etc) to fall below or rise above the specification for the particular circuit or device it reduces the overall yield for that set of devices.
, the co-inventor of the transistor, in his 1961 analysis of junction breakdown.
An analysis of systematic variation was performed by Schemmert and Zimmer in 1974 with their paper on threshold-voltage sensitivity. This research looked into the effect that the oxide thickness and implantation energy had on the threshold voltage
of MOS devices.
The most basic approach is for designers to increase the size of devices which are sensitive to mismatch.
techniques are used.
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...
s are fabricated. It becomes particularly important 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.
Process variation causes measurable and predictable variance in the output performance of all circuits but particularly analog circuits due to mismatch. If the variance causes the measured or simulated performance of a particular output metric (bandwidth, gain, rise time, etc) to fall below or rise above the specification for the particular circuit or device it reduces the overall yield for that set of devices.
History
The first mention of variation in semiconductors was by William ShockleyWilliam Shockley
William Bradford Shockley Jr. was an American physicist and inventor. Along with John Bardeen and Walter Houser Brattain, Shockley co-invented the transistor, for which all three were awarded the 1956 Nobel Prize in Physics.Shockley's attempts to commercialize a new transistor design in the 1950s...
, the co-inventor of the transistor, in his 1961 analysis of junction breakdown.
An analysis of systematic variation was performed by Schemmert and Zimmer in 1974 with their paper on threshold-voltage sensitivity. This research looked into the effect that the oxide thickness and implantation energy had on the threshold voltage
Threshold voltage
The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer and the substrate of the transistor. The purpose of the inversion layer's forming is to allow the flow of electrons through the gate-source junction...
of MOS devices.
Characterization
Semiconductor foundries run analyses on the variability of attributes of transistors (length, width, oxide thickness, etc) for each new process node. These measurements are recorded and provided to customers such as fabless semiconductor companies. This set of files are generally referred to as "model files" in the industry and are used by EDA tools for simulation of designs.Statistical Analysis
Designers using this approach run from tens to thousands of simulations to analyze how the outputs of the circuit will behave according to the measured variability of the transistors for that particular process. The measured criteria for transistors are recorded in model files given to designers for simulating their circuits before simulation.The most basic approach is for designers to increase the size of devices which are sensitive to mismatch.
Patterning Techniques
To reduce roughness of line edges, advanced lithographyPhotolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
techniques are used.