Quaternary GaInAsSb 2.0-2.5 micron back-illuminated focal plane array for blood glucose monitoring

ABSTRACT A focal plane array detector sensitive from 2.0-2.5 µ m and consisting of 32, 1.0 mm x 50 µ m pixels, all functional, is demonstrated. Mean room-temperature R 0 A is found to be 1.0 2-cm , limited by sidewall leakage. The focal plane array is fabricated from an MBE-grown homojunction p-i-n GaInAsSb grown on an n-type GaSb substrate. Back-illumination geometry is compared to front-illumination geometry and is found to be favorable, particularly the improved responsivity (1.3 A/W at 2.35 µ m corresponding to 68% quantum efficiency) due to reflection of light off the metal contact. Further, back-illumination is the most convenient geometry for mounting the array onto a compact blood glucose sensor because both contacts can be mounted on one side, while detector illumination occurs on the other. Keywords: Focal plane array, GaInAsSb infrared detector, back-illumination, MBE growth, wet-etching, dry-etching 1. INTRODUCTION There is a great deal of interest in near-infrared detectors for the 2.0-2.5 µ m wavelength region for diverse applications from medical and environmental sensing to astronomy. We are particularly interested in detector focal plane arrays optimized for mounting onto an integrated optical sensor for blood glucose monitoring. Such a sensor would pass light from a spectrally broadband bright source such as an LED through interstitial fluid, a fluid found under the skin that has glucose levels correlating closely to those in the blood; spectrally resolve the light; and detect the different wavelengths in a focal plane array. Using MEMS techniques, such a sensor has the potential to be extremely compact. To be useful, the optical probe must cover wavelengths at which glucose specific absorption features occur, but at the same time avoid wavelengths at which water absorption is high. Water can strongly attenuate optical probe transmission and degrade the S/N of the measurement. The 2.0-2.5 µ m wavelength region, known as the combination region, satisfies both criteria. In addition to glucose, other complex molecules in the interstitial fluid matrix, such as lactate, also have absorption features in the combination spectral region. Fortunately, multivariate calibration algorithms have shown that glucose concentrations in the complex matrix can be very effectively extrapolated [1,2]. The key requirement for such models to work is optical measurements with a sufficiently high S/N ratio. Thus for a blood glucose sensor, the focal plane array must have high S/N ratio over the 2.0-2.5 µ m combination spectral region. Furthermore, if the sensor is to be implantable sub- or trans-cutaneously, it should operate at room temperature. For maximal convenience in mounting the focal plane array onto the compact sensor, the array should be fabricated in a back-illuminated geometry, so that all electrical contacts can be on one side of the detector, while illumination of the array occurs on the other side. Another advantage of back-illumination compared to a front-illumination geometry is that it allows flip-chip bonding. Flip-chip bonding has a number of advantages over more conventional wire-bonding, an advantage that grows as the number of pixels in the array increases. In particular, by flipping the chip onto a custom header, electrical connections can be made to all pixels at once rather than individually wire-bonding each pixel.