Using high-power digital source meter to build a multi-source measurement unit (SMU) system - Part 4

Ensure that the cables used during testing meet the maximum voltage rating of the test system. Use cables that are capable of the performance required for high-voltage, low-current testing often encountered during off-state characterization of power devices.

During high voltage testing, ensure adequate insulation and minimize the effects of leakage current and system capacitance.

 

Proper insulation

 

Use cables with a withstand voltage rating of at least the maximum voltage of the test system. To achieve low current measurements, use high-quality insulators in the test fixture. The insulation resistance is in parallel with the resistance of the device under test, which will introduce measurement errors (see Figure 1). With the Model 2657A source measure unit (SMU), voltages up to 3kV may appear in the test circuit, so the current produced through these insulators is large relative to the current measured through the device under test. To get good measurements, make sure the insulation resistance is several orders of magnitude higher than the resistance of the device under test.

 

Figure 1. Currents induced in an insulator affect the measurement of the DUT current. To minimize measurement errors, make sure the insulator resistance (RL) is much higher than the DUT resistance (RDUT).

 

Leakage Current and System Capacitance

 

Guarding can be used to minimize the effects of insulators in the test circuit. Guarding is a technique that forces a low-impedance node in the circuit to be at approximately the same potential as the high-impedance input node. In Figure 1, even with high-quality insulators, current leakage from the insulator still exists. This leakage can be problematic when measuring currents in the nanoamp range. Note how guarding improves the measurement, see Figure 2. The leakage current will flow out through the high-impedance measurement node (HI), so it is not included in the measurement.

 

Figure 2. Guarding reduces the voltage of the insulator in the circuit to nearly 0V, thereby reducing leakage current. Any remaining leakage that occurs during measurement will flow out through the high-resistance measurement node.

 

Since the guard end and the high-resistance end are at the same potential, the guard voltage is a dangerous voltage. Therefore, triaxial cables are used to lay out protection circuits and protect operators from the risk of electric shock. In triaxial cables, the high-resistance end is connected to the center conductor, the inner shield is guarded, and the outer shield is grounded. Figure 3 shows a cross-sectional diagram of a triaxial cable.

Guarding also minimizes the effects of system capacitance. System capacitance affects the settling of voltage sources and current measurements. The test setup must allow for charging of the capacitance and settling of currents at or below the expected device measurement noise floor. The high impedance nature of these setups will inevitably result in long settling times. Figure 3 illustrates how guarding reduces the effects of cable capacitance. Common triaxial cable capacitance is about 40pF/ft. For a two- to three-meter cable, the capacitance is on the order of several hundred picofarads, and the voltage settling time is tens of milliseconds, depending on the maximum current of the test setup. Placing the guard on the inner shield of the triaxial cable means that there is no voltage drop across the cable insulation. Therefore, the capacitance of this insulation does not need to be charged. Under steady-state conditions, the guard voltage at the high impedance end (HI) is within 4mV according to the performance specifications of the Model 2657A source measure unit (SMU). Keithley's Model HV-CA-554 is a triaxial high voltage cable that safely transmits signals with guard voltages up to 3280V. Keithley's Model HV-CA-554 cable meets the needs of 3kV voltage and low current measurement systems. To minimize settling time and leakage current, the source measure unit (SMU) guards are connected directly to the device pins. This avoids the need to charge other capacitors in the system. Since the guard voltage can be as high as 3kV, it is important to ensure that the guard terminal is a safe distance away from other conductors.

 

Figure 3 When VHI ≈ VG, the voltage drop across the capacitor and resistor is 0 V. In fact, the protection avoids leakage currents due to the cable insulation and minimizes the response time because the cable capacitance does not need to be charged.

In some systems, a conversion to a coaxial connection is necessary. The SHV is the industry standard coaxial connector for high voltage testing. Keithley offers the SHV-CA-553 cable set to allow conversion from high voltage triaxial to SHV. These cable sets use triaxial cable so that protection can be implemented as far as possible before connecting to the SHV. Using a coaxial connection will result in degraded performance because the benefit of protection is lost from the guard cutoff point. This means that the remaining cable capacitance and test system capacitance must be charged.

When designing a test fixture, users may take steps to minimize capacitance by shortening trace lengths and device connections after the triax-to-coax conversion.

The impact of switching to coaxial connections can be even greater on a probe station, where the cables and connections depend on the size of the wafer and the device orientation (portrait or landscape). If the cable capacitance is taken into account, the capacitance in the probe station can easily reach nanofarads, resulting in long capacitor charging times and measurement settling times.