Nine: New SMU overcomes the tricky test challenges of low-current capacitive settings

When a long cable or capacitive chuck is used in the test setup, the output capacitance of the test instrument will increase, resulting in inaccurate or unstable measurement, especially for very sensitive weak current measurements, because it also provides or scans DC voltage. To solve these challenges, Keithley, a subsidiary of Tektronix Technology, has introduced two new source measurement unit (SMU) modules for Keithley 4200A-SCS, which can perform stable weak current measurements even in applications with high test connection capacitance.

As designers continue to reduce current levels to save energy, this measurement challenge is growing. This is the case for testing large LCD panels, which will eventually be used in smart phones or tablets. Other applications that may have high capacitance test connection problems include: Nano FET IV measurements on chucks, transfer characteristics of mosfets with long cables, FET tests through switch matrices, and capacitor leakage measurements.

Supported capacitance increased by 1000 times

Compared with other sensitive SMUs, the newly introduced Keithley 4201-SMU medium power SMU and 4211-SMU high power SMU (optional 4200-PA preamplifier) have greatly improved the maximum load capacitance index. In the lowest current range supported, the system capacitance that 4201-SMU and 4211-SMU can supply and measure is 1,000 times higher than that of today’s systems. For example, if the current level is between 1 and 100 pA, the Keithley module can handle loads up to 1 µF (micro Farads). In contrast, competing products with the largest load capacitance at this current level can only tolerate 1,000 pF before the measurement accuracy deteriorates.

These two new modules provide important solutions for customers facing these problems, saving the original debugging time and saving the cost of reconfiguring test settings to eliminate additional Capacitors. When a test engineer or scientific researcher notices a measurement error, they must first find the source of the error. This in itself takes hours of work, and they usually have to investigate many possible sources before they can narrow the scope. Once they find that the measurement error originates from the system capacitance, they must adjust the test parameters, cable length, and even re-arrange the test settings. This is far from ideal.

So how does the latest SMU module work in practice? Let’s look at several key applications in the research of flat panel displays and nano-FETs.

Example 1: OLED pixel driver circuit on flat panel display

The OLED pixel driver circuit is printed next to the OLED device on the flat panel display. In order to measure its DC characteristics, it is usually connected to the SMU through a switch matrix, and then connected to the LCD detection station using a 12-16 meter long triaxial cable. Since the connection requires a very long cable, it is common for the weak current measurement to be unstable. When using a traditional SMU to connect to a DUT (as shown in the figure below) for measurement, this instability is displayed in the two IV curves of the OLED driver circuit, that is, the saturation curve (orange curve) and the linear curve (blue curve).

Saturation and linear IV curve of OLED measured using traditional SMU.

However, when using the 4211-SMU to repeat these IV measurements on the drain terminal of the DUT, the IV curve is stable, as shown in the figure below, the problem is solved.

Saturation and linear IV curves of OLED measured using Keithley’s latest 4211-SMUs.

Example 2: Nano FET with common gate and chuck capacitance

Nano-FETs and 2D FETs testing requires the use of a device terminal to contact the SMU through the probe station chuck. The capacitance of the chuck may be as high as a few nanofarads, and in some cases, it may be necessary to use a conductive land on the top of the chuck to contact the gate. The coaxial cable adds extra capacitance. To evaluate the latest SMU module, we connected two conventional SMUs to the gate and drain of the 2D FET to obtain a noisy Id-Vg hysteresis curve, as shown in the figure below.

Noisy Id-Vg hysteresis curve of 2D FET measured using traditional SMUs.

However, when we connect two 4211-SMUs to the gate and drain of the same device, the hysteresis curve obtained is smooth and stable, as shown in the figure below, which solves the main problem that researchers have been solving.

Smooth and stable Id-Vg hysteresis curves measured using two 4211-SMUs.

The 4201-SMU and 4211-SMU can be pre-configured into the 4200A-SCS when ordering to provide a comprehensive parameter analysis solution; they can also be upgraded on-site in existing units.