CV Measurement Techniques, Tips and Pitfalls - Common CV Measurement Errors II

Cable length compensation is an often overlooked correction technique. It is the correction of phase offsets for certain special cables provided by the instrument manufacturer. It takes a certain amount of time for an AC signal to propagate along a cable, and the resulting phase offset in the measurement is proportional to the cable length and propagation delay. Note that an AC impedance meter actually measures impedance and phase, so any phase offset introduced by the cable will appear as a direct error in the measurement. Most AC impedance meters allow for compensation for a set of predetermined cable lengths, such as 0m, 1.5m, or 3m.
CV measurements become increasingly sensitive to phase error as the test frequency increases, especially at frequencies above 1MHz. Different switch matrices[1], probe station cables, and probes all have different path lengths that must be accounted for and corrected for. Different cables also have different propagation delays and therefore different phase errors. It is not a good idea to just pick a coaxial cable off the shelf, as it may not match the propagation delay specified by the manufacturer's capacitance meter. Fortunately, some newer CV meters, such as Keithley's 4210-CVU[2], can measure and adjust for phase error to compensate for different cable systems and different path lengths.
 
Some probe station chucks have long cables that connect to the probe station, which may not match the type of cables that connect to the manipulator. This can cause problems with chuck-based measurements. If possible, the solution is to use dual top-side contacts. If this is not possible, try to match the cables that connect to the chuck to the cables that connect to the manipulator. Most AC impedance meters are best used with a set of Kelvin cables, one for force and one for sense, but sometimes it is not practical to use both cables in the system. In this case, the Kelvin leads can be connected together in a T-connection, with only one line connected to the device. This method can introduce gain errors, especially for large capacitors.
 
The chuck itself is a very heavy and complex load that can affect measurement accuracy. The best results are obtained by connecting the AC excitation terminal (usually called the high current terminal) of the AC impedance meter to the chuck and the current detection terminal (usually called the low current terminal) to the manipulator.
 
Different cable types and cable impedances [3] can also cause problems. For example, some CV meters are 100 ohm systems and some are 50 ohm systems. It is usually best to use the cables recommended by the AC meter manufacturer. The switch matrix creates long, sometimes uncontrollable impedance paths. Reducing the test frequency generally improves switch matrix based measurements. The probes add contact resistance in series with the measurement, which can be compensated for using short circuit correction.
Table 1 summarizes typical CV measurement error sources and provides corresponding correction suggestions.
 
Table 1.

Error Sources	result	Correction Processing
Cable length	Gain error at high frequencies	Use appropriate cable length Perform software correction on cable length Reduce test frequency
Length of probe arm and manipulator	Gain error at high frequencies	· Perform software correction for cable length · Reduce test frequency
The chuck cable and the detection cable do not match	Gain error at high frequencies Offset capacitance	· Use dual top contact · Software correction for cable length · Reduce test frequency
Cable length after Kelvin point	Impedance gain/offset error	Use shorter cables Use open/short correction
The chuck is a heavy and complex load	Gain error at high frequencies Measured C is too low Measurement noise is high	· Use double top contact · Connect the high current terminal to the chuck · Increase the AC excitation level · Reduce the test frequency · Jumper the chuck ground wire and the cable ground wire together
Cable Type and Impedance	Impedance gain/offset errors that cannot be corrected by cable length correction	Use cables recommended by the manufacturer . Pay attention to the cable impedance (50 ohms, 75 ohms, 100 ohms).
Switch Matrix	Impedance gain/offset error, large measurement noise	Shorten the cable length Reduce the test frequency
Probe contact resistance [4]	Poor measurement repeatability	· Replace the probe · Use open/short correction

 
Conclusion
The accuracy of semiconductor CV measurements depends on high-precision test instruments, carefully designed wiring structures, and an accurate understanding of these underlying measurement principles. After mastering these aspects, you can design hardware and wiring structures that meet the needs of your test application.