Imaging in the ultraviolet is full of complications – more and more optical materials stop transmitting as the wavelengths get shorter until, finally, only reflectance works. But, because more and more image sensors are capable of UV imaging and because UV imaging reveals many interesting industrial, medical and scientific phenomena, the need for high-performance, cost-effective UV camera designs in steadily increasing. Successful execution of these designs requires careful consideration of the characteristics of optics, sensors and the radiation itself.
What is UV?
Ultraviolet is the range of electromagnetic radiation occupying the wavelength range from just shorter than the visible (about 400 nm) to about 10 nm, where the photons begin to penetrate materials enough to be called soft x-rays. The UV range is further subdivided by certain practical effects five bands.
The wavelength boundaries between these bands are based on photobiological activity and may vary in different references.
- UVA is tanning and blacklight ultraviolet. It passes through most optical materials like visible wavelengths and can generally be detected by visible light detectors. Although it is invisible to humans, many insects use UVA images to identify certain plant materials, which are highly reflective in this band. UVA above 340 nm is commonly used to stimulate visible fluorescence in paints and the like.
- UVB is the sunburn and “solar blind” ultraviolet. Most UVB is blocked by ozone in the earth’s atmosphere. In small does, it produces vitamin D but more exposure causes sunburn and other skin damage. Any intense UVB must come from local sources. Solar blind imagers, which respond only to the UVB band, permit monitoring of local sources like hydrogen flames and electrical corona in daylight. Some local sources, like quartz halogen bulbs, unfiltered mercury lamps, and short wave LEDs, can emit enough UVB to pose significant UV skin hazards.
- UVC is the industrial and germicidal ultraviolet. Solar UVC is almost entirely blocked by ozone but large amounts of UVC are produced artificially, primarily by mercury lamps at emitting at 253.7 nm. This UV is used extensively for curing, sterilization and erasing EPROMS. Glasses largely block UVC so quartz or other materials are needed in optics for this band.
- Vacuum UV begins below 200 nm, where water vapor and oxygen begin to absorb ultraviolet strongly, relegating use of such wavelengths to evacuated systems. Absorption of similar magnitude continues down to about 10 nm by one or more atmospheric gases so this entire band is called vacuum UV.
- Extreme UV is the designation of the below-100 nm band to indicate that there are no transmissive optical materials useful below 100 nm.
Mirroring the infrared, as UV wavelengths move farther from the visible, fewer and fewer optical materials are available and managing the radiation becomes increasingly difficult and expensive.
Sensing UV images
Fortunately, silicon can be made to work well as an ultraviolet detector across all of the UV bands. However, as the wavelength drops, so does the characteristic penetration distance of the radiation into the silicon before absorption occurs. Ultimately, the distance gets so short that surface effects on the silicon can draw a substantial proportion of the charge towards the surface instead of allowing it to travel inward to the collection point. The minimum 1/e distance, about 25 nm, occurs around the 254 nm wavelength. Treatments have been developed to provide an electric field in a favorable direction at the surface of the silicon to force the charge away from the surface where it is generated to a junction where it can be collected. These treatments can use very thin silver layers or chemisorption techniques but only work where the unoxidized surface of the silicon can be presented for treatment. As a result, they can generally only be applied to backside thinned scientific imagers.
Careful design of photodiodes can extend useful response below 200 nm for applications in spectroscopy but these design techniques are used primarily in line scan devices where the process constraints are generally more relaxed. Frontside area imagers with useful UV response below 350 nm are rare. In typical CCD imagers, the photosites are covered with polysilicon electrodes needed to move the charge. The polysilicon is thick enough to absorb essentially all radiation below 400 nm before it reaches the photodiode. In frontside CMOS imagers, the photodiodes are buried under multiple layers of silicon dioxide and plastics that severely attenuate the UV. Techniques have been devised to provide electrodes without covering the entire photosite or to replace polysilicon with less UV-absorptive materials to improve UV response. With these, the response can be pushed below 400 nm.
One time-tested technique for detecting UV radiation that cannot reach the silicon surface directly has been to coat the sensor with a transparent phosphor film that converts incoming UV into visible radiation that the sensor can readily detect. This technique can extend the response of the typical sensor below the nominal 400 nm useful lower cutoff to below 200 nm. In general, however, these coatings will absorb some incident visible radiation, reducing overall quantum efficiency slightly and increasing response nonuniformity.
The high index of refraction of silicon causes the reflectance of uncoated surfaces to rise to more than 60% in the 200-300 nm range resulting in low external quantum efficiency.
Generally, the surface of the silicon is covered by silicon dioxide, but that helps little because it has an index of refraction too low to form a matching layer. Hafnium oxide (HfO2) is a higher-index material that works well but is practical only in large area applications to backside-thinned sensors. With both field generation and antireflection applied, sensors with external QE in excess of 50% at 300 nm can be routinely accomplished. Multilayer coatings can increase this to over 90% in devices where the bare silicon surface is accessible for coating.
Around 340 nm the photon energy equals the pair creation energy in silicon so that at shorter wavelengths two electron-hole pairs are generated per absorbed photon and the internal QE exceeds 1. A second extra carrier pair is generated at 170 nm and an additional pair for each incremental 3.7 eV of photon energy. Since this extra charge does not improve the shot noise statistics, it can use up well capacity in sensors without delivering the S/N expected of strong signals. As a result, well capacity must be carefully considered for imaging in the VUV.
UV Imaging Applications
The most widespread applications for UV in industry are in the production and inspection of semiconductor masks and devices. Essentially all exposure of photolithographic resists used in defining patterns on semiconductor wafers use ultraviolet illumination. In the past, the 254 nm mercury arc lines was used, followed by KrF laser output at 248 nm for higher intensity. Currently, the vast majority of semiconductor production lines use 193 nm ArF laser sources. A substantial effort was made to implement systems using the 157 nm F2 laser for finer geometries but the optical problems proved overwhelming. Even though the optical problems are much worse for even shorter wavelengths, it is still believed that a switch to the 13.4 nm wavelength from Xenon plasma must be accomplished to allow continuing reductions in device geometry. These same sources are also used for the production and inspection of the exposure masks and for inspection of fabricated wafers for proper geometries.
More generally, UV is used to image materials that fluoresce. Of course, in such applications the UV serves only as an illumination source and the actual images are made using the fluorescent output, usually in the visible, so no special optics are needed except perhaps a UV blocking filter. Images of UV emissions are valuable primarily in monitoring of dangerous situations such as the presence of hydrogen flames and the occurrence of electrical corona discharge from high-voltage transmission lines. Special-purpose cameras that provide UV images of the hazards and overlaid visual images for context have been developed for these applications.