Picture perfect
To verify CMOS image sensors, one company developed a tester consisting of a Eurocard chassis connected to a computer.
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 3/1/2005
CMOS image sensors. The sensors may have analog or digital outputs with up to 300 connections. Capture speeds range from a few images to thousands of images per second. Develop a flexible system to verify 25–30 new designs per year. Measure parameters such as dark nonuniformity, dark noise, current consumption, spectral response, and light-to signal ratio. If a sensor includes an on-chip analog-to-digital converter (ADC), then the system must measure integral nonlinearity, differential nonlinearity, and conversion speed.
PROJECT DESCRIPTION
FillFactory, a division of Cypress Semiconductor (Mechelen, Belgium, www.FillFactory.com), designs and manufactures CMOS image sensors. To verify sensor designs, the engineers, including staff product engineer Dirk Van Aken, developed an automated test system consisting of a Eurocard chassis that connects to a computer through an IEEE 1394 link. The chassis holds custom analog, digital, and power-supply boards connected through a 100-MHz backplane. By adding a lens to the chassis, the engineers can effectively use the DUT as though it is embedded in a camera.
An automated test system measures the signal output of CMOS image sensors during design verification.
Each sensor model requires a unique analog board, which holds the DUT, capacitors, and bias resistors. For sensors that include on-chip ADCs, the analog board passes the DUT's digital output to the digital board through the backplane.
High-speed sensors may have eight 14-MHz analog outputs. These devices need an analog board that includes a 14-bit ADC and a multiplexer. The multiplexer scans analog outputs from each sensor pixel for the ADC to digitize. Analog boards also include test points.
The digital board—common to all DUTs—consists of a field-programmable gate array that provides the DUT with control logic and clock signals. Onboard memory holds image data from a sensor. Some sensors produce thousands of images per second. The data-capture rate of high-speed sensors exceeds the rate of link between the digital board and a host PC. The system can't operate in real time, but it doesn't have to because the engineers can analyze data after capturing images.
A power-supply board provides power for the DUT from two power supplies. Through the card, an electrometer measures the DUT's current consumption while in the dark or illuminated.
Using imaging libraries, FillFactory engineers capture thousands of images and transfer them to a PC for analysis. Tests begin with the DUT in a dark enclosure where engineers measure the "dark" output (noise level) of the device with an oscilloscope. They also measure the DUT's pixel-to-pixel uniformity.
The engineers then illuminate the DUT with LEDs and measure its spectral response (signal level vs. wavelength). They use the oscilloscope to measure the sensor's signal output as a function of light intensity and resolution. They look for uniformity in pixel outputs and search for defective pixels. To measure crosstalk between pixels, the engineers project a fixed-edge image on the sensor's test pixel and then move the sensor in the x and y directions by 1 µm. By measuring the output of pixels adjacent to the illuminated one, they can determine crosstalk.
The test system performs standard ADC tests such as differential nonlinearity, integral nonlinearity, and conversion speed. A function generator creates the analog test signals.
RESULTSVan Aken said that building a tester saved FillFactory money and provided more flexibility. than buying an ATE system. For each new DUT, the engineers just design a new analog board.
