UV detection applications have been restricted by the lack of available sensors that have both high responsivity in the UV and also efficient out-of-band rejection of sunlight. A technology has been developed for producing UV absorption filters of organic molecules embedded in UV transparent polymer films that achieves transmission levels of 10% or better in the UVc and absorbing levels much higher than 12 OD rejection from the UVb through the near IR. The filters can be easily optimized for the spectral characteristics of individual sensors. An important advantage of the UV bandpass filters is the exclusive use of absorption type components, enabling wide angle detection.

When combined with ultra-low noise, UV optimized, back-illuminated CCD sensors, which have been demonstrated to have better than 70 percent quantum efficiency, and readout noise levels of less than 3 electrons at moderate bandwidths, this new filtering technology allows, for the first time, "megapixel" UV solar blind sensors with unparalleled sensitivity.

UV solar blind detectors have a broad range of applications that include, but are not limited to; flame detection and characterization, plume detection, high voltage power line corona monitoring, oil and gas pollutant molecules, and charge particle emissions from nuclear fuel.

1.1 Discussion of Problem and Opportunity

Lacking a practical array detector, the UV spectral region is an unexploited wavelength band. Commercial applications for this waveband include semiconductor mask and reticle inspection, power line corona discharge monitoring, UV wind shear measurement monitoring, pollution monitoring, and non-proliferation monitoring of nuclear fuel.

Typical optimal bandpass filters are basically Fabry-Perot interferometers incorporating metal/dielectric thin films. Efficient bandpass interference filters with very high rejection out of the bandpass are difficult to implement in the UV. This is due to the limited choice of non-absorbing high refractive index dielectric materials and the high UV light absorbing power of the metal layers (aluminum or silver). In order to obtain high out of band rejection, multiple Fabry-Perot cavities are required, which usually result in very low transmission within the band pass. Sharp rejection slopes and out of band rejection levels of 10 OD and more required for outdoor solar blind detection applications are not achievable by interference filter technology. Another serious disadvantage of interference filters is the dependence of their transmission on the angle of incidence. The absorption of a UV interference filter and a the proposed UV bandpass filter are shown in Figure 1. As can be seen in the figure, the innovation results in approximately 8 orders of magnitude greater out-of-band rejection.

In order to obtain UV bandpass filters with sharp rejection slopes and out of band rejection levels exceeding 10 OD filter architectures described by Childs in the earlier days of ultraviolet research and development for space applications have been reconsidered. These employed absorption type filter components. The potential of using molecular light absorbers for the production and band transmission filters for the ultraviolet was originally demonstrated by M. Kasha.

A new technology developed by Ofil Ltd (Rehovot, Israel) has been demonstrated, and PixelVision, Inc. and Ofil have entered into an exclusive teaming agreement to develop and commercialize the technology in North American markets. The UV filters consist of organic molecules embedded in UV transparent polymer films. A selection of molecules has been developed, each with a characteristic absorption band.

In order to extend and improve the potential, dyes of different chromophores with enhanced absorption in the UV filters were produced by embedding these dyes into polymer matrices. High rejection levels are very sharp rejection slopes of up to 1 OD/nm on the long wavelength side of the transmission bands have been demonstrated. The absorption spectra of typical solar blind absorption and interference type bandpass filters are compared in Figure X.

The multiple molecular absorbers can be embedded with different concentrations in order to shape the spectral characteristics of a filter so that it can match the requirements of application and the characteristics of the sensor. The available collection of synthesized molecular absorbers allows tailoring of narrow band (15-25 nm FWHM) UV filters with transmission maximum n the 250nm – 340 nm range.

Linear dependence of the absorption on concentration of embedded molecules has been verified. This permits precise spectral design of the UV component of the filters. The absorption spectra of films containing multiple absorbers can therefore be calculated as a linear superposition of the absorption spectra of the individual absorbing molecule. One of the most common complaints digital camera users express is lack of UV/blue response. Front-illuminated CCD sensors experience poor response in the blue region of the visible spectrum due to absorption by the polysilicon gate electrodes.

The polysilicon gate electrodes, used to clock out the charge from the imaging area, strongly absorb light in the blue spectral region. As pixel geometries get smaller, this problem is exacerbated. The sensor’s blue response decreases rapidly with pixel size. The reasons for this are discussed below.

Thinning and back-illuminating a CCD allows for light to enter the back surface of the CCD. Thinned, back-illuminated CCDs (BCCDs) overcome the performance limits of the conventional front-illuminated CCD by illuminating and collecting charge through the back surface away from the polysilicon electrodes. When the CCD is mounted, face down on a substrate and the bulk silicon is removed, only a thin layer of silicon containing the circuit’s device structures, remains. Without having to pass through the polysilicon gate electrodes, the image’s photons enter the CCD back surface unobstructed. By illuminating the CCD through the back surface in this manner, quantum efficiencies greater than 90% are achieved.

The spectral conversion efficiency curves for a conventional CCD and a back-illuminated, thinned CCD are shown in Figure 2. As the first link in the optical chain, the responsivity is the most important feature in determining system performance. The advantages of the back-illumination are readily apparent. The ninety- percent quantum efficiency of the devices allows for a near "ideal" detector in the sense that it converts nearly every incident photon into an electron in the CCD well.

Because the back surface of a thinned CCD is of uniform composition, anti-reflection (AR) coatings can be employed (AR coatings cannot be used on frontside illuminated sensors). This is an important advantage given the high refraction index of silicon (>3.5 at visible and near-IR wavelengths). This high index would otherwise produce large reflection losses of more than 30%, resulting in lower QE. With even the simplest single layer AR coating the QE can be pushed close to 100% at the device’s peak response wavelength (which can be tailored by adjusting the thickness of the coating). This results in larger signals, and hence, improved signal-to-noise performance. In low-light applications, this feature can be used to generate superior-quality data and/or to increase frame rate.

In the ultraviolet (and electron sensing regions), the measured quantum efficiency of a CCD is the product of three important quantities: the transmission coefficient, the quantum yield, and the internal quantum efficiency of the CCD. The transmission coefficient accounts for reflection from the surface and absorption in the native oxide, the quantum yield accounts for the statistically-averaged number of electron-hole pairs produced at the energy of the incident photon, and the internal quantum efficiency accounts for internal losses in the CCD, such as recombination of electron-hole pairs at the back surface of the CCD.