Tunable Deep UV light source
Light sources for Deep UV
To build a tunable light source for deep UV one first needs a continuous light source of reasonable power that covers this wavelength range. There are only a few options available at present. Supercontinuum lasers typically end at 400nm. There are some devices that enter the UV but with very low power and there seems to be nothing available for DUV at the moment. Another option would be, to use a tunable laser. The advantage here is that you don’t need a monochromator to cut out a wavelength, the downside is the very high price of such a device. What’s left are the well-known thermal sources.
Laser-driven xenon lamp
To produce DUV light with a thermal source, high temperatures around 5000K are needed. A very frequently used lamp is the xenon arc lamp, which in the blue and UV range exhibits a line-free thermal spectrum. The company ENERGETIQ™ offers a laser-driven xenon lamp where the plasma is ignited electrically but maintained and powered by a focused IR laser. This allows the achievement of very high luminance with a low plasma diameter (<200 µm), and at the same time high spatial stability and – due to the reduction of burn-up almost to zero – very long lifetime. The spectrum extends to DUV and even vacuum UV. From the perspective of optics, these light sources are already very close to offering a point source which makes then ideal to build a tunable light source.
Principle of a laser-driven light source
The Hyperchromator
We have developed a monochromator especially for the EQ-99X laser-driven light source that we call Hyperchromator. The traditional method to couple the EQ-99X with a monochromator would be to map the plasma to the entrance slit. The optical elements needed to do this, would cause loss of energy, particularly in the UV and DUV range. We instead directly take the plasma as our “entrance slit”. This can be done because the plasma is very small and bright and everything else in the lamp is dark. To collected as much light as possible at reasonable expense, an off-axis parabolic mirror with f-number 1.5 is used to collimate the light to a 50x50mm² rotatable diffractive grating. A second off-axis parabolic mirror maps the fraction of wavelengths selected by the grating to an adjustable exit silt or directly to a multimode fiber (see figure below). Typical fiber diameters are between 100 and 400µm.
Schematic illustration of the Hyperchromator
The Hyperchromator
Configuration example for DUV
There are various options to choose your grating. For this example, we choose a holographic grating with 1200 l/mm and blaze angle 250nm. With a 400 µm fiber, the bandwidth is around 2.7nm FWHM.
When looking at the output of the Hyperchromator set to wavelength λ with a spectrometer, you see a single line with a gaussian-like shape at λ. The height of a peek corresponds to the output power per Nanometer. The figure below shows this spectral output for several wavelength.
Spectral output of the Hyperchromator
The next figure gives the output power for the configuration example with logarithmic scale. Although the power goes down when going from 300 to 200nm, it doesn’t drop below 8.0 µW.
Output power of the Hyperchromator
The Hyperchromator comes as a single- or dual-grating device. Dual-grating allows operation of two gratings for a wider usable wavelength range or for using two different bandwidths. We can extend the wavelength range of our configuration with a second grating with 1200 l/mm and blaze wavelength 1.0 µm up to 1600 nm. The resulting output power is shown in the following figure.
