Focal plane array testing

(under development sep 15)

Focal Plane Array testing (FPA testing) is the test engineering process of validation and verification (V&V) of operation of focal plane array imaging devices, device under test (DUT), at various levels of the development and/or production assembly process. V&V can be done internal to the DUT (detector array and readout circuit), such as with Built-In Test Equipment or external, such as with Automatic test equipment. Functional and environmental testing is part of V&V. An FPA is basically composed of a photon or phonon detector array and a readout integrated circuit. There are basically five processing steps performed by these two sub-components, including: getting the electromagnetic energy in the detector; generating a consequent charge in the detector; charge collection; charge to voltage conversion; signal transfer; and digitization. Testing at various levels of the entire process can filter out FPA's with excessive number of unit cell defects before the complete process is carried out.

Overview

FPA testing (automated or semi-automated) utilizes hardware and software to characterize the DUT by measuring parameters such as: signal transfer function; signal transfer function vs differential temperature; spatial noise power spectral density; noise equivalent temperature difference; modulation transfer function; RMS and fixed pattern noise; temporal noise; responsivity and detectivity; spectral response; crosstalk; minimum resolvable temperature difference; intensification gain; field of view; spatial resolution; dynamic range; focus adjustment; harmonization; alignment; distortion.

Software analyses the DUT sensor images in real time, including array failure maps. Source temperature settings and target selection are computer-controlled. The software can integrate a complete test bench comprising collimator, source, optical table and data acquisition system.

DUT examples

*CCD, CID, CMOS, PDA
*Cryocooler/FPA
*Uncooled FPA
*Microbolometers
*SWIR FPA
*Dual-band FPA
*Readout integrated circuits
*Hyperspectral FPA
*Polarimetric FPA
*QWIP FPA
*InGaAs FPA
*HgCdTe FPA

Test equipment

Commercial automatic test equipment for FPA testing is very expensive (prober, parameter analyzer, etc). However, minimal production testing is possible on a boot-strap budget. A relatively low cost test system for testing non-uniformity and signal to noise (S/N) ratio of an uncooled focal plane array can use programmable logic devices to generate the necessary pulses for the DUT and low dropout regulator to obtain low noise bias. A proportional integral derivative (PID) thermoelectric cooler that is microprocessor or microcontroller controlled can stabilize the DUT. A PC-based data acquisition card can then be used as an analog-to-digital converter (ADC) to convert DUT output to digital input for computer analysis. The 12-bit ADC capability provides sufficient accuracy for evaluating the S/N ratio and non-uniformity of 128 x 128 pixels DUT. High level software is used to control test procedures and analyze the signals.

Test considerations

measurement techniques

Targets and collimators...

image resolution

Boundary detection...

responsivity

System response...

noise

Thermal, shot...

transfer functions

Linearity, fourier, modulation...

image quality

Subjective, objective criteria...

automation

Objectivity, productivity, reproducibility, ...

tandardization

Role of industry organizations, SPIE...

tatistical analysis

Objective vs subjective quality...

Testing challenges

*high density (small unit cells and large format arrays)
*small electrical currents (small unit cell)
*probe point impossibilities (flip chip technology)
*pre-binding test required
*conventional probe is time consuming
*MOS off-current calibration (dark current)
*serial vs parallel testing
*built-in current sources

References and external links

*Raytheon Vision Systems [http://www.raytheon.com/businesses/ncs/rvs/Products/index.html]
*Teledyne Imaging Sensors [http://www.teledyne-si.com/imaging/standard_products.html]
*Redstone Technical Test Center [http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.12.9038]