Stable test of weak current under high test connection capacitance

A source measurement unit (SMU) is an instrument that can provide current or voltage and measure current and voltage. SMU is used for I-V characterization of various devices and materials. It is designed to measure very sensitive weak currents while providing or sweeping DC voltage. However, in test systems with long cables or other high-capacitance test connections, some SMUs may not be able to tolerate such capacitance on the output, resulting in noisy readings and/or oscillations.
The 4201-SMU medium-power SMU and the 4211-SMU high-power SMU (optional 4200-PA preamplifier) can perform stable low current measurement, including applications with high test connection capacitance, such as the use of very long three Core coaxial cable to connect the application of the device. Compared with other sensitive SMUs, the capacitance indicators of 4201-SMU and 4211-SMU have been improved. These SMU modules are used in the configurable Model 4200A-SCS parameter analyzer and are interactively controlled by Clarius+ software.
This article discusses a variety of application examples in which the 4201-SMU and 4211-SMU can perform stable low current measurement, including testing: OLED pixel devices on flat panel displays, long cable MOSFET transfer characteristics, FETs connected through a switch matrix, and chucks Nano FET IV measurement, capacitor leakage measurement.
Example 1: OLED pixel device test on flat panel display
When measuring the I-V curve of the OLED pixel device on the flat panel display, the SMU is usually connected to the LCD detection station through a switch matrix. At this time, a very long three-core coaxial cable (usually 12-16m) is used. Figure 1 is a typical flat panel display test configuration using the Keithley S500 test system. S500 is an automatic parameter tester, which can be customized and is usually used to test flat panel displays. For the situation shown in the figure, the SMU in the S500 is connected to the detection station through the switch matrix, and then the detection card connects the test signal to the DUT on the glass plate. Due to the use of very long cables for connection, improper use of measurement techniques and instruments can lead to instability in weak current measurement.
"For example, as shown in Figure 2, when a traditional SMU is used to connect to the DUT through a 16m three-core coaxial cable, the saturation curve (orange curve) and the linear curve (blue curve) of the two I-V curves of the OLED device are unstable. However, when using the 4211-SMU to repeat these I-V measurements on the drain terminal of the DUT, the I-V curve stabilized, as shown in Figure 3.

Figure 1. Use Keithley S500 test system to test the configuration of flat panel display image002.jpg Figure 2. OLED saturation and linear I-V curve measured by traditional SMU. 

Figure 3. Saturation and linear I-V curve of OLED measured by 4211-SMU.

Example 2: Long cable nMOSFET transfer characteristics test
Two SMUs can be used to generate the Id-Vg curve of n-type MOSFET. One SMU scans the gate voltage, and the other SMU measures the drain current. Figure 4 is a schematic circuit diagram of a typical test circuit, in which a 20m three-core coaxial cable is used to connect the SMU to the device terminals.

Figure 4. Two SMUs are used to measure the I-V characteristics of MOSFETs.
Figure 5 shows the transfer characteristics measured using two conventional SMUs and using two 4211-SMUs. The blue curve (obtained using two traditional SMUs) shows oscillations in the curve, especially at low currents and changing current ranges. The red curve is the current measurement obtained using two 4211-SMUs, which is very stable.

Figure 5. Id-Vg curve of nMOSFET generated using traditional SMU and 4211-SMU and 20 m three-core coaxial cable.
Example 3: FET test connected through a switch matrix
When testing devices connected through a switch matrix, it may be very challenging because additional cables are required. The three-core coaxial cable is used to connect the SMU to the switch matrix, and then from the switch matrix to the DUT. Figure 6 shows a typical circuit diagram in which two SMUs use remote sensing to connect to a switch matrix. The use of remote sensing (4-wire measurement) instead of local sensing (2-wire measurement) requires two cables to be connected to each SMU. Since the cables are parallel, this doubles the output capacitance of the SMU.
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Figure 6. Simplified schematic diagram of connecting SMU to DUT through the 707B switch matrix.
In this case, the SMU is connected to the row (input) of the switch matrix with a 2m cable; the column (output) of the switch matrix is connected to the distribution frame with a 5m cable. Then use another 1m cable to connect to the probe from the distribution frame, so the total length of the three-core coaxial cable from one SMU to the DUT is: (2 x 2 m) + (2 x 5 m) + (1 m) = 15 m. In addition to the three-core coaxial cable, the switch matrix itself also adds capacitance, which may need to be included when calculating the total capacitance of the test system.
When measuring the output characteristics of FET devices connected through a switch matrix, the results of using two 4211-SMUs are significantly better than using two traditional SMUs. In this test, one of the SMUs was biased with a constant gate voltage, and the other SMU scanned the drain voltage to measure the drain current. Using two traditional SMUs (blue curve) and two 4211-SMU (red curve) to generate the drain current versus drain voltage curve shown in Figure 7. When performing nanoampere measurements, there will be oscillations (as shown by the blue curve) when measuring the drain current with a traditional SMU. When using the 4211-SMU to measure the drain current of the FET connected through the switch matrix, the measurement result is stable (as shown by the red curve).
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Figure 7. Using two traditional SMUs and two 4211-SMUs to measure the Id-Vd curve comparison of FETs connected through a switch matrix.
Example 4: Nano FET with common gate and chuck capacitance
By using 4201-SMU and 4211-SMU, stable low current measurement can be performed on nano-FET and 2D FTE. These FETs and other devices sometimes have a device terminal that contacts the SMU through the inspection station chuck. Figure 8 is a typical circuit diagram of a nano-FET test configuration. In this example, an SMU is connected to the gate terminal via a chuck. The capacitance of the chuck is several nano-Faradays, which can be verified by the detection station manufacturer. In some cases, it may be necessary to use the conductive pad on the top of the chuck to contact the gate.
The SMU can be connected to the chuck using a coaxial cable or a three-core coaxial cable, depending on the manufacturer of the detection station. The coaxial cable chuck is represented as a load capacitance in the test circuit, because this capacitance appears between Force HI and Force LO of the SMU, as shown in the example in the figure. The chuck with a three-core coaxial cable is expressed as cable capacitance.
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Figure 8. Two SMUs are used to test nano-FETs.
When using two conventional SMUs to connect the gate and drain of a 2D FET, a noisy Id-Vg hysteresis curve is generated, as shown in Figure 9. However, when using 4211-SMU to connect the gate and drain of the same device, the resulting hysteresis curve is smooth and stable, as shown in Figure 10.
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Figure 9. 2D FET Id-Vg hysteresis curve measured by traditional SMU.
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Figure 10. Id-Vg hysteresis curve measured by 4211-SMU.
Example 5: Capacitor leakage
When measuring capacitor leakage, it is necessary to apply a fixed voltage to the capacitor under test, and then measure the resulting current. Leakage current decays exponentially over time, so it is usually necessary to apply a voltage for a known period of time before measuring the current. Depending on the device under test, the measured current will generally be very small (usually <10nA). Figure 11 is a circuit diagram for measuring capacitor leakage using SMU. It is recommended to use series diodes in the circuit to reduce measurement noise.
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Figure 11. Use SMU and series diode to measure capacitor leakage.
Figure 12 is a graph of the leakage current of a 100nF capacitor with respect to time measured using 4201-SMU. Due to the improved load capacitance index, the 4201-SMU and 4211-SMU are relatively stable when measuring capacitor leakage, but whether a series diode is required depends on the insulation resistance and amplitude of the capacitor and the current measurement range. This may require some experimentation.
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Figure 12. The leakage current of a 100nF capacitor measured with 4201-SMU versus time.