The 5½-digit Model 6487 Picoammeter/Voltage Source improves on the measurement capability of the award-winning Model 6485, and adds a high resolution 500V source. It provides higher accuracy and faster rise times than the 6485, as well as a damping function for use with capacitive devices. With eight current measurement ranges and high speed autoranging, this cost-effective instrument can measure currents from 20fA to 20mA, take measure ments at speeds up to 1000 readings per  second, and source voltage from 200µV to 505V.

The Model 6487’s 10fA resolution, superior sensitivity, voltage sweeping, and Alternating Voltage resistance measurements make it well suited for characterizing low current devices. Using the latest current measurement technology, it is significantly less expensive than other instruments that  perform similar functions, such as optical power meters, tera-ohmmeters, competitive picoammeters, or user-designed solutions. With a price that’s comparable to a high-end DMM, the Model 6487 makes picoamp-level measurements affordable for virtually any laboratory or production floor.

Low Voltage Burden and Higher Accuracy

While DMMs typically employ shunt ammeter circuitry to measure current, the Model 6487 is a feed-back picoammeter. This design reduces voltage burden by several orders of magnitude, resulting in a voltage burden of less than 200µV on the lower measurement ranges. The low voltage burden makes the Model 6487 function much more like an ideal ammeter than a DMM, so it can make current measurements with high accuracy, even in circuits with very low source voltages.

Successor to the Model 487

The Model 6487 builds on the strengths of one of Keithley’s most popular picoammeters, the Model 487, offering an additional 20mA measurement range, as well as much higher measurement speeds, up to 1000 readings per second. It simplifies device characterization with built-in voltage sweeping capability and the Alternating Voltage method for high resistances. A time-stamped 3000-reading data buffer provides minimum, maximum, and standard deviation statistics. A built-in emulation mode makes it possible to control the Model 6487 with any custom code written to control the Model 487.

Features that Expand Test and Measurement Flexibility

• Direct resistance measurements. Optimized for resistances from 50Ω to 5×10 14 Ω using the Source Voltage/Measure Current method.
• Alternating Voltage method resistance measurements. This method improves resistance measurements on devices with high background current or high noise. It extends the measurable resistance range up to 10 16 Ω.
• 500V overload protection. This high overload protection and a robust design let the Model 6487  tolerate abusive overflows, including accidentally shorting the voltage source directly into the ammeter.
• Rear panel triax input. This allows the picoammeter to be used in floating operation, up to 500V. When not floating, the addition of a triax to BNC adapter allows inexpensive, easy-to-use BNC cables to be employed, rather than more expensive triaxial cables.
• RS-232 and IEEE-488 interfaces. These interfaces make it easy to integrate the Model 6487 into automated test and measurement systems.
• Scaled voltage analog output. This output allows the Model 6487 to transmit measurement results to devices like DMMs, data acquisition cards, oscilloscopes, or strip chart recorders.
• Built-in Trigger Link interface. The Trigger Link interface simplifies synchronizing the Model 6487 with other instruments and voltage sources. This interface combines six independent selectable trigger lines on a single connector for simple, direct control over all instruments in a system.
• Display on/off switch. For research on light-sensitive components, such as measuring the dark currents of photodiodes or I-V measurements on unpackaged semiconductors, the front panel display can be switched off to avoid introducing light that could significantly reduce the accuracy of the results.
• One-touch front panel design. Functions can be configured easily with the push of a button, without complicated function menus.

A Broad Range of Low Current Applications

Wafer-Level Photodiode Testing

The Model 6487 Picoammeter/Voltage Source can be paired with a calibrated light source and a probing fixture to create a cost-effective photodiode test system. Multiple Model 6487s can be connected to the DUT’s probe pads to provide photocurrent readings or, with the addition of a switch matrix, one pico ammeter can take current measurements from multiple pads. In the first step of the measurement  process, performed in total darkness, the Model 6487 produces a voltage sweep and then measures the resulting dark current. In the second step, a voltage bias is applied and the resulting photocurrent is meas ured while the light level is increased in calibrated steps. The same basic test configuration can be used for testing positive intrinsic negative (PIN) and avalanche photodiodes (APDs). The 6487’s high resolution on the 10V source range provides superior sweeping and biasing when small biases are required. The 500V source capability is necessary to bias APDs.

Monitoring and Control of Focused Ion Beam Currents

In semiconductor fabrication, focused ion beam systems are often used for nanometer-scale imaging, micromachining, and mapping. Careful monitoring of the magnitude of the beam current with an ion detector is critical. The ion detector generates a secondary current that’s proportional to the current of the primary ion beam. When this secondary current is measured, it can be used to control the intensity of the primary beam. However, this secondary current is very low, often just a few picoamps, so the instrumentation measuring it must provide high measurement accuracy and repeatability, as well as sub-picoamp resolution. The Model 6487’s wide measurement range and 5½-digit resolution make it ideal for this application. Signal connections to the Model 6487 are made through the instrument’s triax connector. Often, a detector may require high voltage to attract ions, making the 6487’s 500V source a necessity.

High Resistance Measurements

The Model 6487 Picoammeter can be used to measure high resistances (>1GO) in applications such as insulation resistance testing. A constant voltage is placed in series with the unknown resistance and the picoammeter. The voltage drop across the picoammeter is negligible, so all the voltage appears across the unknown resistance. The resulting current is measured by the picoammeter and the resistance is calculated using Ohm’s Law (R = V/I). To prevent generated current due to electrostatic interference, the unknown resistance is housed in a shielded test fixture. A small series resistor may be added to reduce noise if the un known resistor has high stray capacitance across it.

Applications:

• Resistance/resistivity measurements
• Beam monitoring and radiation monitoring
• Leakage current testing in insulators, switches, relays, and other components
• Galvanic coupling measurements
• I-V characterization on semiconductor and optoelectronic devices
• Fiber alignment
• Circuit test and analysis in DCLF circuits
• Sensor characterization
• Capacitor leakage