Test Equipment

Pulse Generator Innovations Respond to Emerging Test Challenges

Electronic equipment and device manufacturers face continuing technical challenges. Developing product designs incorporate ever-increasing numbers of new components and materials, each of which is accompanied by new characterisation and test challenges. Here Todd Stocker, Marketing Manager, Keithley Instruments, Inc. discusses the advantages of pulse testing as an alternative solution with promising results.

To keep pace with fast-changing manufacturing technologies, instrument manufacturers are designing hardware that makes increasing use of new testing techniques, such as pulse testing, which offers advantages unavailable with traditional DC test methods.

What is a pulser?
Pulse testing involves outputting a voltage or current that changes from a low level to a high level at a specified transition rate. Pulses are most commonly used to test for a device’s transient response in order to determine its transfer function. This helps in characterising a material’s electrical behaviour.

Pulse or pattern generators are used in both the research lab and on the production test line. Researchers often need to stimulate a device under test (DUT) with a pulse, a series of pulses, or a prescribed series of data patterns at specified rates in order to characterise device performance. Pulse or pattern generators are often configured into test systems that also include source-measure units (SMUs), digital multimeters, voltmeters, switches, and oscilloscopes.

A variety of factors contribute to the growing need for pulse testing, one of which is the increased speed of electronic circuitry, which highlights the limitations of DC testing of analogue components. A reduction in the size of components is another factor, which leads to an increase in self-heating effects in these smaller, more temperature-sensitive devices.

Key applications for pulse testing include nanotechnology research, semiconductor test and characterisation, memory device testing, clock simulation, functional test of electronic components, and generating streams of digital data of both plain and coded patterns.

Pulse generators have long been considered complicated to understand and use effectively. Today’s research and production test customers are demanding instruments that offer the flexibility to configure a wide range of pulse parameters straightforwardly and quickly.

The flexibility required includes adjustable timing parameters such as repetition rate, rise time, fall time, and delay without impact to the other parameters being held constant. Adjustable amplitude setting with higher offset capabilities are also required. In addition to the simple parameters, today’s applications also require more complicated capabilities, such as combining multiple pulse shapes into a single output. Tight timing and trigger integration and low jitter performance are mandatory.

While researchers and engineers need a broad range of pulsing capabilities, most can’t spare the time necessary to learn how to configure or operate a complex pulse generation instrument. A consistent and intuitive method for changing parameters is critical if users are to learn how to modify pulse outputs accurately. Modern pulse generators typically provide easy-to-understand front panel controls for setting up pulse outputs.

Pulse Testing Applications
Pulse generators are used in a variety of applications. One common use is in thermal analysis, in which a pulse of known parameters is applied to a device intended to heat the material. As heat transfers through the device, another parameter, such as temperature or current, is measured. This technique is widely used in materials research and development.

Device lifecycle or stress testing is another common application for pulse generators. These types of tests are typically performed on Flash, DRAM, SRAM, and PRAM memory devices or new memory technologies. In memory testing, the operator uses the pulse generator to output write/erase pulses to exercise a particular memory location a large number of times. The characteristics of the memory location are measured before and after the stress test to determine the amount of change to the memory cell as a result of the multiple write/erase cycles. This process makes it possible to analyse the device’s reliability, evaluate the manufacturing process, and verify the design of the particular material or component.

Pulse generators are also used as clock or data simulators. Clock and digital data simulation is used to put a device under test into a correct operating state to perform various other DC of RF tests or to determine how a device behaves if the clock or data signal is less than ideal. Here, the pulse stream can be controlled to produce a precise shape designed to test the performance limits of a device. The use of techniques such as a deep memory pattern mode allows the user to configure multiple channels of data to provide multiple bits of data to the device. In embedded devices, the use of serial buses is growing steadily, as is the need to test the behaviour of these buses under real-world conditions. Buses such as the Low Voltage Differential Signalling (LVDS) bus and Inter-Integrated Circuit (I2C) bus are common in these applications.

Another commonly performed test is pulsed current-voltage testing, or pulsed I-V, which is used for device characterisation. Much like traditional I-V testing, pulsed I-V is used to determine the device characteristics as current or voltage increases. Pulses minimise the effect of heat on the device and help ensure the accuracy of measurement results, whereas a DC signal would generate too much heat or would be too slow to give good results.

Charge Pumping
Pulse testing is often used in making chargepumping measurements in semiconductor applications. Charge pumping is widely used to characterise interface state densities in MOSFET devices. Recently, with the development of high dielectric (high κ) gate materials, charge pumping has proven especially useful in characterising charge-trapping phenomena in high κ thin-gate films. In thin-gate films, leakage current is relatively high due to quantum mechanical tunnelling of carriers through the gate. As a result, the traditional technique for extracting interface trap density—collecting simultaneous quasistatic and high frequency C-V measurement data and comparing the difference—can’t be used, because quasistatic C-V is very hard to achieve at the leakage current level.

However, charge-pumping measurements can still be used to extract interface trap density, and the effect of gate leakage can be compensated for by measuring charge-pumping current at lower frequency and subtracting it from measurement results at higher frequencies.

The basic charge-pumping technique involves measuring the substrate current while applying voltage pulses of fixed amplitude, rise time, fall time, and frequency to the gate of the transistor, with the source, drain, and body tied to ground. The pulse can be applied with a fixed amplitude, voltage base sweep or a fixed base, variable amplitude sweep.

In a voltage base sweep, the amplitude and period (width) of the pulse are fixed, while sweeping the pulse base voltage (Figure 3a). At each base voltage, body current can be measured and plotted against base voltage. With this data, the trap density can then be calculated.

A fixed base, variable amplitude sweep has a fixed base voltage and pulse frequency with step changes in voltage amplitude (see Figure 3b). The information obtained is similar to that measured in a voltage base sweep. These measurements can also be performed at different frequencies to obtain a frequency response for the interface traps.

In order to perform these tests, a DC semiconductor characterisation system and a pulse generator are required. The test setup can be seen in Figure 4. A single source-measure unit (SMU) and pulse generator can be used to test a single device or, with the addition of a switch matrix, multiple devices can be tested. This may be convenient if testing is to be performed at the wafer level. The pulse is applied to the gate of the transistor while the current is measured in the well of the device. With this test configuration, the pulse sweep is programmed by changing either the high level parameter (in the case of an amplitude sweep) or by changing the offset level parameter (in the case of a base voltage sweep). At each pulse setpoint, the DC current is measured with the SMU.

Conclusion
Pulse generators are becoming an increasingly important tool for designers developing the next generation of semiconductor and nanotechnology devices and high speed components. Faced with intense budget and time-to-market constraints, they cannot afford to compromise on measurement quality. Pulse generators enable high quality testing and characterisation of the latest semiconductor materials.