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Making Femtofarad (1e-15F) Capacitance Measurements

with the 4215-CVU Capacitance Voltage Unit

APPLICATION NOTE

Introduction

Typical semiconductor capacitances are in the picofarad

(pF) or nanofarad (nF) ranges. Many commercially available

LCR or capacitance meters can measure these values using

proper measurement techniques including compensation.

However, some applications require very sensitive

capacitance measurements in the femtofarad (fF), or 1e-15,

range. These applications include measuring metal-to-metal

capacitance, interconnect capacitance on a wafer, MEMS

devices such as switches, or capacitance between terminals

on nano devices. These very small capacitances are very

difcult to measure without using the proper instrumentation

and measurement techniques.

Using a tool such as the Keithley 4200A-SCS Parameter

Analyzer equipped with the optional 4215-CVU Capacitance

Voltage Unit (CVU) enables the user to measure a wide range

of capacitances, including very low values of capacitance,

<1pF. The CVU is designed with unique circuitry and is

controlled by the Clarius+ software to support features

and diagnostic tools that ensure the most accurate results.

Using this CVU with proper techniques can enable the user

to achieve very low capacitance measurements with tens of

attofarads (1e-18F) noise levels.

This application note explains how to make femtofarad

capacitance measurements using the 4215-CVU Capacitance

Voltage Unit. This includes making proper connections and

using the proper test settings in the Clarius software for the

best results. Further information on making capacitance

measurements, including cabling and connections, timing

settings, guarding, and compensation, can be found in the

Keithley application note, Making Optimal Capacitance

and AC Impedance Measurements with the 4200A-SCS

Parameter Analyzer.

Making Connections to the Device

Making the proper connections to the device under test

(DUT) is crucial for making sensitive low capacitance

measurements.

For the best results, use only the supplied red SMA cables for

making connections from the CVU to the DUT. The red SMA

cables have characteristic impedance of 100 Ω. Two 100Ω

cables in parallel have characteristic impedance of 50Ω,

which is standard for high frequency sourcing and measuring

applications. The supplied accessories allow connecting

to a test xture or prober with BNC or SMAs connections.

Using the supplied torque wrench, tighten the SMA cable

connections to ensure good contacts.

The CVU congured for two-wire sensing is shown in

Figure1. The HCUR and HPOT terminals are connected to a

BNC tee to form CVH (HI), and the LCUR and LPOT terminals

are connected to form CVL (LO).

Figure 1. CVU connections for two-wire sensing.

An example of four-wire sensing to the DUT is shown in

Figure 2. In this case, the HCUR and HPOT terminals are

connected to one end of the device, and the LPOT and

LCUR terminals are connected to the other end of the device.

Four-wire connections to the device facilitate sensitive

measurements by measuring the voltage as close as possible

to the device.