Sensor vendor discusses SSoC at Test Vision 2020

Consumer, communications, industrial, medical, and automotive markets are all targets for ams AG, according to Thomas Burger, the company’s director of test development and senior manager for test technology, speaking at last week’s Test Vision 2020 workshop, held in conjunction with SEMICON West.

Sensors are analog, he said, with ams having a strong analog background, but now the term “sensor” is being applied to anything that includes a “real sensor or transducer” inside, along with a digital interface, power-management function, microcontroller or microprocessor, and connectivity. He calls such a device a sensor system on chip, or SSoC.

Sensors, he said, present tough test challenges, but they often need trimming, so test can be considered a value-added process.

ams, he said, offers sensors for light, gesture, color, and other properties. Cognitive lighting presents major light sensor opportunities, he said, with ams providing dynamic management combining LED control and sensor technology. The company also offers audio MEMS microphones for analog active noise cancellation (ANC), for which low noise at low cost and low power is key.

Other markets ams serves include industrial automation and control (with 2D and 3D position encoders), building automation, security, and HVAC. The company also provides seismic sensor interfaces. Other applications include medical computed tomography, digital X-ray, mammography, ultrasound, personal health devices, and robotic surgical solutions.

Sensor test challenges include the need for nonelectrical signals, he said. The semiconductor test world typically deals with electrical signals, and often temperature, but sensors present the need to deal with light, acoustics, pressure, gases, humidity, flow, magnetics, acceleration, movement, and more. And most of these signals are analog. (Consequently, Burger noted, ams has adopted the slogan “Analog makes sense.)

Nonelectrical signals are not really new for the industry in general, Burger said, but the rapid inclusion of the MEMS technologies, continuous developments in feature size, the use of advanced packaging, and market trends like the IoT are increasingly yielding integrated SSoCs, “…and we have to adapt to deal with it.

The test industry, he said, is used to handling wafers and devices of many package types, moving them to electrical contactors, and providing a well-defined temperature environment, but traditional semiconductor robotics equipment is not designed to provide other physical conditions and stimuli. And test equipment can deal with low and high voltages, RF, and high power, for example, but it is not designed to provide or measure other physical stimuli. Testers are not designed to accommodate a light source with optical path to the device under test controlled by the current source of the tester with a chamber to provide a controllable magnetic environment, for example.

To overcome these limits, Burger said, cooperation between customers and suppliers is essential to develop the necessary dedicated test cells. “We need to get suppliers to provide a complete solution to us,” he said. And the resulting integrated test cell should support trimming and calibration, and there need to be methods to determine long-term drift, reliability, and aging effects.

He added that in the case of sensors, trimming might not be enough, suggesting a multidimentional calibration process using math processing to generate regression models. It’s necessary to question whether to take such steps, and if so, perhaps they should take place at the customer side during system integration. Other questions center on where to store and manage the data generated during calibration and how to optimize test calibration times and cost.

Burger addressed additional issues as well. On-chip self-test is becoming more important. High-end modeling can help correlate relevant nonelectrical parameters with electrical parameters to support the skipping or sampling of nonelectrical test. Packaging presents a particular conundrum. Typical packages are sealed to prevent corrosion, for instance. SSoC packages, however, often need to provide openings to permit the sensing of the environment, which may be harsh.

Burger said he sees an overall trend for SSoCs to employ WLCSP variants with some means of coating or over molding for multicomponent systems, making use of 3D integration, perhaps using wire-bonding stacking and SiP techniques.

He reiterated his request for test vendors to take an active role in modular, scalable, integrated test cells. He also asked that EDA firms integrate the mixed-signal test development into their design flow/tools.

He concluded with the intriguing idea that sensor devices could self-test and self-calibrate in the field using the various references and data points available in the IoT ecosystem. An IoT device could perhaps become more accurate after a month in the field.