Test and Measurement
Fundamentals of
semiconductor C-V
measurements
C-V measurements provide a wealth of information about device
and material characteristics. Lee Stauffer from Keithley
Instruments explains.
C
apacitance-voltage are extremely important to product and fabrication (see Figure 1). Although
(C-V) testing is yield enhancement engineers, who are these devices may be used in actual
widely used to responsible for improving processes circuits, they are typically integrated
determine and device performance. Reliability into fabrication processes as a test
semiconductor engineers use these measurements to structure. Since they are simple
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parameters, particularly in MOSCAP qualify material suppliers, monitor structures and their fabrication is easy
and MOSFET structures. However, other process parameters, and analyse failure to control, they are a convenient way to
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types of semiconductor devices and mechanisms. evaluate the underlying processes.
.eur technologies can also be characterised With appropriate methodologies, The metal/polysilicon layer shown
oasiasemiconductor
with C-V measurements, including instrumentation, and software, a in Figure 1 is one plate of the capacitor,
bipolar junction transistors (BJTs), JFETs, multitude of semiconductor device and and silicon dioxide is the insulator. Since
III-V compound devices, photovoltaic material parameters can be derived. the substrate below the insulating layer
cells, MEMs devices, organic TFT This information is used all along the is a semiconducting material, it is not by
displays, photodiodes, carbon production chain beginning with itself the other plate of the capacitor. In
nanotubes (CNTs), and many others. evaluation of epitaxially grown crystals, effect, the majority charge carriers
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The fundamental nature of these including parameters such as average become the other plate. Physically,
measurements makes them useful in a doping concentration, doping profiles, capacitance, C, is determined from the
wide range of applications and and carrier lifetimes. In wafer processes, variables in the following equation:
square4
Issue I 2009
disciplines. They are used in the C-V measurements can reveal oxide
research labs of universities and thickness, oxide charges, mobile ions C = A (κ/d), where
semiconductor manufacturers to (contamination), and interface trap
evaluate new materials, processes, density. These measurements continue A is the area of the capacitor,
devices, and circuits. C-V measurements to be used after other process steps, κ is the dielectric constant of the
such as lithography, etching, cleaning, insulator, and
dielectric and polysilicon depositions, d is the separation of the two
and metallisation. After devices are fully plates.
fabricated on the wafer, C-V is used to
characterise threshold voltages and Therefore, the larger A and κ are,
other parameters during reliability and and the thinner the insulator is, the
basic device testing and to model the higher the capacitance will be. Typically,
performance of these devices. semiconductor capacitance values
range from nanofarads to picofarads, or
The physics of smaller.
Figure 1. C-V measurement circuit for a semiconductor capacitance The procedure for taking C-V
MOSCAP structure formed on a P-type A MOSCAP structure is a fundamental measurements involves the application
substrate device formed during semiconductor of DC bias voltages across the capacitor
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