Source Measure Unit / SMU

Source Measure Unit / SMU

Source Measure Units (SMU) instruments are specialized test instruments capable of sourcing current and simultaneously measuring voltage, or sourcing current and simultaneously measuring voltage with high speed and accuracy.

The typical SMU provides the following four functions

• Measure voltage
• Measure current
• Source voltage
• Source current

These functions can be used separately or they can be used together in the following combinations

• Simultaneously source voltage and measure current, or
• Simultaneously source current and measure voltage

General Advantages of SMUs

• Adding current and voltage sourcing capabilities to a measuring instrument provides an extra degree of versatility for many low level measurement applications. For example, very high resistance values can be determined by applying a voltage across a device and measuring the resulting current
• Avoid separate power supplies and DMMs. The added sourcing functions also make a SMU more convenient and versatile than using separate instruments for such applications as generating I-V (current-voltage) curves of semiconductors and other types of devices, also called I-V Characterization
• SMUs have a number of electrometer-like characteristics that make them suitable for low level measurements. The input resistance is very high (typically 100TΩ or more), minimizing circuit loading when making voltage measurements from high impedance sources. The current measurement sensitivity is also similar to that of the electrometer picoammeter—typically as low as 10fA
• Another important advantage of many source-measure units is their sweep capability. Either voltage or current can be swept across the desired range at specified increments, and the resulting current or voltage can be measured at each step. Built-in source-delay-measure cycles allow optimizing measurement speed while ensuring sufficient circuit settling time to maintain measurement integrity

Keithley SourceMeter®
Keithley pioneered SMU development over 20 years ago with their SourceMeter® Instruments. The SourceMeter instrument is very similar to the source-measure unit in many ways, including its ability to source and measure both current and voltage and to perform sweeps. In addition, a SourceMeter instrument can display the measurements directly in resistance, as well as voltage and current.

The typical SourceMeter instrument does not have as high an input impedance or as low a current capability as a source-measure unit. The SourceMeter instrument is designed for general-purpose, high speed production test applications. It can be used as a source for moderate to low level measurements and for research applications.
Unlike a DMM, which can make a measurement at only one point, a SourceMeter instrument can be used to generate a family of curves, because it has a built-in source. This is especially useful when studying semiconductor devices and making materials measurements.

When used as a current source, a SourceMeter instrument can be used in conjunction with a nanovoltmeter to measure very low resistances by automatically reversing the polarity of the source to correct for offsets. A single Source-Measure Unit (SMU) channel is required when testing two-terminal devices such as resistors or capacitors. Three- and four-terminal devices, such as BJTs and FETs, may require two or more SMU channels. Dual-channel System SourceMeter instruments provide two SMUs in a half-rack instrument. Their ease of programming, flexible expansion, and wide coverage of source/measure signal levels make them ideal for testing a wide array of discrete components.

Keithley SourceMeter® Software
SourceMeter instruments each use a powerful on-board test sequencer known as the Test Script Processor (TSP™). The TSP is accessed through the instrument communications port, most often, the GPIB. The test program, or script, is simply a text file that contains commands that instruct the instrument to perform certain actions. Scripts can be written in many different styles as well as utilizing different programming environments.

SMU Test Fixtures
A test fixture can be used for an external test circuit. The test fixture can be a metal or nonmetallic enclosure, and is typically equipped with a lid. The test circuit is mounted inside the test fixture.

Selection Considerations for Source Measurement Units (SMUs)

• Number of Channels
• Current Maximum and Minimum
• Voltage Maximum and Minimum
• Maximum number of readings per second
• Resolution and Accuracy
• Communications Interfaces
• Is semiconductor characterization software needed?
• Are precise measurement or characterization of rise/fall times required?
• Is touchscreen preferred?

 

Background on Switch and Semiconductor Test Systems

Switch and Semiconductor Test Systems are electronic test systems that use relay switching to connect multiple Devices Under Test (DUTs) to sources and measurement instruments. In some cases, multiple sources and measuring instruments are connected to a single device. Switching allows automating the testing of multiple devices, thereby reducing error and cost.

Designing the switching for an automated test system demands an understanding of the signals to be switched and the tests to be performed. Test requirements can change frequently, so automated test systems must provide the flexibility needed to handle a variety of signals. Even simple test systems often have diverse and conflicting switching requirements. Given the versatility that test systems must offer, designing the switching function may be one of the most complex and challenging parts of the overall system design.

As a signal travels from its source to its destination, it may encounter various forms of interference or sources of error. Each time the signal passes through a connecting cable or switch point, the signal may be degraded. When calculating the overall system accuracy, the engineer must include not only the effects of the switch but all the switching hardware in the system.
The quality of a switch system depends in large part on its ability to preserve the characteristics of the test signals routed through it. For example, when the test signal is a low voltage, the switching system must minimize errors such as offset voltage and IR drops. Leakage current may be a problem for high resistance and low current switching applications. Depending on the type of test signal involved, specific switching techniques must be used to maintain signal integrity through the switching system.

Examples of Switching Types

• Voltage Switching. Applications such as testing batteries, circuit assemblies, thermocouples

• Low Voltage Switching. Millivolts range or less
• High Voltage Switching. Applications such as insulation resistance of cables, printed circuit boards, hi-pot testing
• High Impedance Voltage Switching. Applications such as monitoring electrochemical cells and measuring semiconductor resistivity
• Current Switching. Applications such as testing of power supplies, insulation resistance, capacitor leakage, resistivity of materials, batteries, and semiconductors
• High Current Switching
• Low Current Switching
• Low Current Matrix Switching. Switching of several source measure units (SMUs) to multipin devices or wafer level semiconductor measurements
• Resistance Switching. Applications include measuring the insulation resistance of materials, continuity testing of cables and connectors, contact resistance measurements, and measuring components such as resistors, thermistors, and potentiometers
• Low Resistance Switching. Applications such as contact resistance measurements and cable continuity testing
• High resistance Switching. Applications such as measuring capacitor leakage, multi-conductor cable insulation resistance, and pin-to-pin leakage connectors
• RF and Microwave Switching. Applications include testing of components making up communication systems such as RF integrated circuits (RFICs) and microwave monolithic integrated circuits (MMICs). Typically, these are tested at gigahertz (GHz) frequencies or higher. The main components of a typical test system may include a DC bias source, DC measurement instruments, RF power meter, network analyzer, etc. Automating the test process and improving test efficiency demands integrating RF/microwave and low frequency switching systems into the test system
• Digital Switching. High speed digital signals exhibit RF behavior, which creates need for testing

• Multi-pin devices and components
• Nanotechnology Devices
• Power Supplies
• Semiconductor Devices
• Solar Cells
• Temperature Sensors
• Wafer Level Testing
• Wireless Devices