|ALS Scientists Patent Technique To Dramatically Advance Grating-Based Spectroscopy|
However, a new type of ultra-high diffraction grating recently patented by members of the ALS Experimental Systems Group (ESG), working with colleagues from Berkeley Lab’s Center for X-ray Optics, stands to revolutionize the resolution capabilities of soft x-ray spectroscopy. The key to the new technique is the production of a near atomically perfect substrate, using the anisotropic etching of silicon.
“Essentially, we’ve discovered a way to make atomically perfect gratings,” says Howard Padmore, head of ESG. “We’re able to make them with very high line density and to make them diffract in high orders, all of which gets us hugely improved resolution and throughput.”
The most serious imperfection in traditional gratings is micro- and nano-scale roughness, which in a reflection grating leads to background scattered light. The consequence, which has a large impact on spectroscopy capabilities, is that gratings are largely limited to 2D surface structures. Much higher efficiency and vastly improved resolution can be achieved with 3D periodic gratings, but this requires perfect interfaces and therefore a perfect substrate. The 3D structure is made by deposition of high and low Z alternate layers on the grating substrate, so that imperfections in the substrate are transferred and amplified in subsequent layers. A good 3D grating therefore needs as close to an atomically perfect substrate as possible.
Dmitriy Voronov, an ESG scientist involved in the development of the new grating technique, created the patentable, atomically perfect grating by using a perfectly suited substrate material: silicon. Since silicon etches in alkali solutions roughly 1000 times faster in some directions than others, a crystalline silicon grating can be created by cutting the crystal so that the slow etching direction is a few degrees from the surface plane. Etching a lithographically defined pattern results in fast etching along the <111> planes, leaving a perfect sawtooth pattern in the silicon. The residual defects are at the level of 1 atomic plane. This near perfect substrate allows the growth of the 3D grating to the necessary perfection. The result is improved efficiency over today’s gratings, with an increase in resolution of up to a factor of 20 for similar conditions.
These new gratings can be applied across a wide range of x-ray science applications. One of the most promising of these is resonant inelastic x-ray scattering (RIXS), a spectroscopy technique that measures both the energy and momentum change of a scattered photon. Unlike traditional photoelectron spectroscopy, RIXS gives scientists element-specific data. The current generation of spectrometers doesn’t provide the resolution needed to fully exploit RIXS, but a higher resolution, higher efficiency grating would resolve this.
“RIXS is one of the biggest things to happen in x-ray science in the past decade; we need to move from the 100 meV resolution of present instruments to the 5 meV scale of photoemission, and this should be possible using these new techniques” says Padmore.
Voronov says that fabrication of x-ray gratings with the classical ruling technique is quite a slow process — it can take weeks or even months to rule the required number of lines. Moreover, the diamond ruling does not provide superior smoothness of the grooves, which is crucial for 3D gratings. The number of lines and high-order diffraction are key to the technique’s effectiveness since they determine the resolution.
“The anisotropic etch technique we use guarantees almost perfect surface finish of the grooves and a slope of groove’s facets, enabling high-order operation of a diffraction grating,” says Voronov.
“The fact that we can rule these gratings with a very high line density and make them diffract efficiently in a high order means we get higher resolving power,” says Padmore.
With a patent in place, Padmore’s hope is that a company sees the technique as a worthwhile investment and begins commercial production. Until then, the implementation of these gratings at the ALS and other similar facilities will depend on funding. The next steps at ALS will be the integration of the gratings in a real spectrometer and the validation of this new approach. After 200 years of research and application, much still remains to be done to perfect the diffraction grating, one of the most useful and widely used instruments in physical science.
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