2.1

Coherent EUV Microscopy

The EUV region of the spectrum is well suited for imaging ICs because the short wavelengths allow for high resolution imaging. However, the EUV spectrum lacks suitable refractive lens elements, making it problematic to form images with high-resolution. To overcome this challenge, we use a lensless imaging technique known as ptychographic^[18,19] coherent diffractive imaging (CDI).^[20-22]

In ptychography, a coherent illumination beam is rastered across a sample, and the light scattered from the sample is recorded on a detector. The data is fed into an iterative phase retrieval algorithm, and a complex image of the sample is computationally reconstructed. The amplitude of the complex image yields information about the materials present on the sample, while the phase information primarily encodes topographical information. The resolution of the image is only limited by the wavelength of the illumination, lambda (λ), and the numerical aperture (NA) of the collected scatter patterns. In a recent work, images were obtained using 30nm illumination reflected from the sample, demonstrating 40nm (1.3λ) transverse and 0.6nm axial resolution^[11] (see Fig.1). In a transmission geometry using 13.5 nm illumination, we demonstrated sub-15-nm resolution imaging.^[23] In ptychography, the scanning is area-by-area rather than point-by-point, allowing data to be collected quickly, over large fields-of-view. Each scan position shares about 70% area overlap with adjacent scan positions (except for the areas at the edges of the scan). The overlap in scan positions provides redundant data allowing the ptychographic algorithm to converge quickly, compared with traditional single exposure CDI. Additionally, ptychographic algorithms can be extended to use partially coherent illumination, multiple colors, and orthogonal modes and polarization states.^[13]

Figure 1.

Nanoscale imaging of Ti features patterned on a Si wafer. The (a) amplitude and (b) phase of the ptychographic CDI images are shown to the left and center, respectively. For a comparison, a (c) SEM image of the same sample is shown on the right. With 30 nm illumination wavelength, the ~40 nm resolution represents diffraction limited resolution for the numerical aperture of the scattered light, in this case corresponding to ~42 nm. All the scale bars, upper left, correspond to 10μm. Figure adapted from Zhang et al.^[11]

2.2

Element-specific dynamics in magnetic systems

The Magneto-optic Kerr Effect (MOKE) results in modulation of the reflectivity of a surface depending on the polarization of the incident light with respect to the direction of magnetization of the reflection surface. In the EUV, the MOKE asymmetry is greatly enhanced near the inner-shell absorption edges of the elements. Iron, Nickel, and Cobalt all have absorption edges in the photon energy range of 50-70 eV, making it possible to monitor magnetization in an element specific manner. Since HHG light is intrinsically short-pulse, this provides a method for monitoring dynamics in magnetic systems unique ways. Past work has demonstrated that, for example, in an alloy even through the elements are intermixed, the dynamics of demagnetization when a surface is heated by an ultrashort pulse, are decoupled with a time-energy scale corresponding to the exchange energy for the material.^[9,10,24] In other work, element specificity was used as a method for tagging layers.^[8] The data show that when a multilayer surface is heated with an ultrashort pulse, some of the observed demagnetization results from generation of a very large spin current into the bulk. This can be seen through the transient increase in magnetization of a buried Fe layer due to spin-polarized electron transport from the Ni overlayer.