The ongoing challenge for today’s semiconductor test engineers is to identify and create new test solutions that can offer significantly lower test costs as well as address the need for configurable, open-architecture, flexible test solutions that can provide comparable features to proprietary ATE platforms. In particular, for test requirements with low to moderate volumes — such as pilot production, verification, and focused production test applications — the need for flexible and cost-effective ATE solutions is particularly acute. For these applications, test engineers historically have relied upon legacy test systems that have a low acquisition cost, but high operating costs or in-house designed rack-and-stack solutions. However, semiconductor test system solutions based on the PXI platform have made significant advancements in functionality and performance over the past three or four years, offering test engineers a viable alternative for both current and future test needs.

Figure 1. System configuration with PMU, digital instrument, and switching matrix.
For digital and system-on-chip test applications, today’s PXI test systems offer moderate to high digital channel count capability with parametric measurement unit (PMU) per pin functionality. However, existing PXI digital subsystems have been largely limited to verifying DC and functional test characteristics of a device due to their limited timing engine capabilities. To fully meet the test requirements found in “big ATE,” a PXI-based test system (and its digital subsystem) must also be capable of testing a device’s AC characteristics such as setup and hold times — a capability that has not been easily achieved by the current generation of PXI instruments. The latest generation of PXI systems and instrumentation offers many of the features found in proprietary ATE systems, including an advanced timing engine and software tools for importing test vectors and developing test programs.

Semiconductor Test Requirements

The basic test needs for digital and mixed-signal devices include DC/AC parametric and functional tests. For the DC tests, all of a device’s pins must be characterized, which requires a PMU. A PMU, which can source voltage/measure current or source current/measure voltage, must be able to access all of the device’s pins, which requires some type of switching/multiplexer if a single PMU is used. Once DC characterization is completed, functional/AC parametric testing of the device can be performed. In this case, a digital instrument with sufficiently deep memory, per-channel programmability (voltage, loads, and direction), programmable edge placement, and real-time compare provides the key features for testing AC parametrics and functionality. A basic setup that addresses these capabilities is shown in Figure 1.

Figure 2. 3U PXI, 32-channel digital I/O with per-pin PMU architecture.
The configuration shown is not practical for even moderate pin-count ATE systems. Today’s PXI test systems incorporate a PMU per-pin or channel architecture, offering high channel count configurations and superior test performance (both for speed and measurement accuracy). Figure 2 details the architecture of a digital instrument that includes a PMU per-pin configuration. PXI test systems offer a high-channel-count digital and mixed-signal test system in a small, compact, single PXI chassis.

Performing DC Parametric Tests

As noted, a PMU can be used in one of two modes to perform DC characterization tests on the input and output pins of digital devices:

  • Force voltage and measure current. With this method, the PMU applies a constant voltage, and using its onboard measurement capability, it measures the current being drawn by the device/pin being tested. The voltage being supplied by the PMU can also be measured.
  • Force current and measure voltage. With this method, the parametric measurement unit either forces a constant current across a device, or sinks a constant current from a device pin and then measures the resultant voltage. The PMU sink/source current also can be measured.

By combining a PMU per channel with digital test capabilities in one instrument, performing a range of DC tests on digital and mixed signal devices is significantly simplified. Common DC tests performed on digital devices include input voltage levels (VIH/VIL), output voltage levels (VOL/VOH), input leakage, and output short circuit current tests.

Performing Input Leakage Test and V-I Tests

Figure 3. Input leakage test using the digital instrument’s PMU capability.
Testing a device’s inputs includes leakage current testing as well as characterizing the protection diodes present on each input of the device under test (DUT). These tests are performed by applying a constant voltage in steps over a specified test voltage range to the DUT input pin and measuring the input current at each step (Figure 3). As leakage currents are often in the uA range, the PMU should be set to its more sensitive current ranges to achieve more accurate measurements.