In thermal IR sensors (bolometers), absorptive coatings convert IR power into heat. Absorptive coating gold-black has been used for many decades for IR bolometers, particularly for array detectors used in night vision for defense and security. Gold-black is a nanocrystalline deposit formed when an inert gas causes gold atoms to collide and bind with each other to form a web-like structure. While the material achieves near-unity absorption from visible to far IR, it is extremely fragile and cannot currently be patterned to selectively coat only the sensing elements of IR array detectors. This patent addresses issues related to patterning and stability of gold-black.
This gold-black coating for infrared sensors from UCF establishes a means of protecting the fragile layer of gold-black with an overcoat of evaporated dielectric, and of coating individual pixels to improve the sensitivity of micro-bolometers. For IR array detectors, only the sensing element should be coated leaving the spaces in between the pixels clear in order to avoid thermal and electrical bridging. Laser ablation has been used in the past to remove coatings deposited between the pixels, but this slow process is unsuited for mass production. This new method solves that problem by providing a means to selectively coat the active regions of individual pixels.
This new method of patterning gold-black reaches at least 90% absorption, but more often approaches 100%, in the mid-IR range using standard photolithography and metal lift-off. In the application process, a photoresist pattern is coated on the device, followed by a gold-black deposition using thermal evaporation in an inert gas at 0.3-1 Torr (depending on the deposition chamber).
The gold-black is encapsulated using SiO2 as a protection layer by electron beam evaporation. The photoresist then dissolves in acetone, resulting in submillimeter-size gold-black patterns that retain >90% and >70% IR absorption in the 3-5 and 8-12 μm wavelength range, respectively, and exhibit good mechanical stability as well.
Figure 1. A schematic diagram of perspective views of steps in making a patterned gold-black IR-absorber.
Figure 2. Reflectance spectra extracted from six different pixels shown above (gold substrate was used, hence transmission ~ 0%).
Characterization responsivity for a multiple gold-black-coated prototype device showed improvement by 47% in a wide range of wavelengths, 0.6-40 μm, since no filter was applied in front of the black body.
Figure 3. Measured voltage responsivity and noise voltage over a range of detectors, displayed as a function of detector resistance. Measurements are made at 80 Hz chopping frequency with 1 V applied bias and no optical filter. A 300°C blackbody temperature was used. The noise measurements had a degree of uncertainty, which is indicated by error bars. Uncertainty in responsivity was less than the symbol size.
- More than ~90% and ~70% absorption in 3-5 and 8-12 μm wavelength range, respectively. Hence, this invention can be enabled in multi-band imaging devices and detectors.
- Improved sensitivity
- Night vision for defense and security