Commissioning and aligning the Microfield Exposure Tool (MET), 2003

Commissioning the two-mirror MET, 0.3-NA optic at 13.4-nm wavelength.
Commissioning the MET 0.3-NA, two-mirror optic at 13.4-nm wavelength.
Shown are [Goldberg, Naulleau, Denham]
High-NA null test to calibrate the system geometry. The iamges shows fringes that have a slightly hyberbolic profile due to the planar camera catching a spherical beam.
At high-NA, measurement accuracy is incredibly sensitive to the alignment and geometry. We devised a null-test to provide an absolute measurement of the system geometry for calibration of all other tests.
In a focusing system, the Foucault Test (or Knife-Edge test) is easy to interpret, independent of calibration.
The Foucault Test or (Knife-Edge) test is the easiest to perform, and easiest to interpret. Indepenent of the geometry, the light and dark pattern here reveals the x-derivative of the wavefront error in this moment. The 3rd-order pattern reveals a 4th-order, spherical aberration, which we corrected.
PS/PDI was very difficult to perform owing to the sub-15-nm pinhole sizes. We made it work using a real-time holographic feedback approach that we invented.
Applying our highest-accuracy technique Phase-Shifting Point-Diffraction Interferometry (PS/PDI) was a challenging task since the pinholes were below 15 nm wide and hard to find. We used a real-time holographic feedback approach that we invented, and software I wrote to visualize the data as we aligned the beam. This was a beautiful optic!
Single-grating shearing interferometry at 13.4-nm wavelength.
Shearing interferometry quickly became our favorite approach. Alignment is trivial, the efficiency is high, and the data is easy to analyze. Now, single-grating shearing is used in many beamline applications worldwide.