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Number 3140
Performing Very Low Frequency Capacitance-Voltage
Application Note Measurements on High Impedance Devices Using the
Series Model 4200-SCS Semiconductor Characterization System
Introduction Capacitor
Capacitance measurements on semiconductor devices are usually
made using an AC technique with a bridge-type instrument.
These AC instruments typically make capacitance and impedance Test Device
Force HI Force HI
measurements at frequencies ranging from megahertz down
to possibly tens of hertz. However, even lower frequency SMU1 SMU2
A A
capacitance measurements are often necessary to derive specific smu_src smu_sense
test parameters of devices such as MOScaps, thin film transistors
(TFTs), and MEMS structures. Low frequency C-V measurements
are also used to characterize the slow trapping and de-trapping Force LO
phenomenon in some materials. Instruments capable of making (Internally Connected)
quasistatic (or almost DC) C-V measurements are often used for
these low frequency impedance applications. However, the Model SMU1 with preamp: SMU2 with preamp:
4200-SCS Semiconductor Characterization System uses a new Outputs DCV with Measures AC current
superimposed ACV and at 0V DC.
narrow-band technique that takes advantage of the low current measures AC voltage.
measurement capability of its integrated source measure unit
(SMU) instruments to perform C-V measurements at specified low
frequencies in the range of 10mHz to 10Hz. This new method is Figure 1. Connections for very low frequency C-V measurements.
called the Very Low Frequency C-V (VLF C-V) Technique.
Basically, while the voltage is forced, voltage and current
The VLF C-V Technique makes it possible to measure very measurements are obtained simultaneously over several cycles.
small capacitances at a precise low test frequency. This patent- The magnitude and phase of the DUT impedance is extracted
pending, narrow-band sinusoidal technique allows for low from the discrete Fourier transform (DFT) of a ratio of the
frequency C-V measurements of very high impedance devices, resultant voltage and current sinusoids. This narrow-band
up to >1E15 ohms. Other AC impedance instruments are usually information can be collected at varying frequencies (10mHz to
limited to impedances up to about 1E6 to 1E9 ohms. The VLF 10Hz) to create a complex, multi-element model of the DUT.
C-V approach also reduces the noise that may occur when The resulting output parameters include the impedance (Z),
making traditional quasistatic C-V measurements. phase angle (), capacitance (C), conductance (G), resistance (R),
The Model 4200-SCS Semiconductor Characterization reactance (X), and the dissipation factor (D).
System comes with preconfigured tests and a user library to Because the very low frequency method works over a limited
perform impedance measurements automatically using this frequency range, the capacitance of the device under test (DUT)
very low frequency technique. Because this approach uses the should be in the range of 1pF to 10nF. Table 1 summarizes
Model 4200-SCS's SMUs, no additional hardware or software the VLF C-V specifications (see Appendix A for complete
is necessary if low current I-V characterization is already specifications).
required. This application note describes the VLF C-V technique,
Table 1. Very Low Frequency C-V specifications.
explains how to make connections to the DUT, shows how to
Measurement Parameters: Cp, Gp, F, Z, , R, X, Cs, Rs, D, time
use the provided software, and describes optimizing VLF C-V Frequency Range: 10mHz to 10Hz
measurements using the Model 4200-SCS. Measurement Range: 1pF to 10nF
Typical Resolution: 3.5 digits, minimum typical 10fF
Very Low Frequency C-V Technique AC Signal: 10mV to 3V RMS