Next-generation metrology solutions in various technology areas require to image sample areas at the nanoscale. Coherent diffractive imaging based on ptychography is the route towards EUV imaging of nanostructures without lenses. A key component in a table-top EUV beamline is a high-brightness high-harmonic generation (HHG) source. Since our research is mainly directed towards wafer metrology for lithography in the semiconductor industry, we adhere to a reflection setup: the EUV light is scattered by the nanostructures at the surface of the sample, and is reflected towards a CCD camera, where a far-field diffraction pattern is recorded. A data-set comprising a multitude of these diffraction patterns is generated for partially overlapping positions of the focused probe on the sample. This provides the necessary redundancy for phase retrieval of the complex-valued field of the sample. Recent advancements in both hardware and software for computation enable the development of advanced algorithms. In particular, the benefits of automatic differentiation are exploited in order to cope with a drastic growth in model complexity. Our computational imaging algorithms realize wavelengthmultiplexed reconstruction and a modal approach for the spatial coherence of the source.
Ptychography as a means of lensless imaging is used in wafer metrology applications using Extreme Ultraviolet (EUV) light, where use of high quality optics is out-of-scope. To obtain sufficient diffraction intensity, reflection geometries with shallow (ca. 20 degrees) grazing incidence angles are used, which require re-sampling the diffraction data in a process called tilted plane correction (TPC). The tilt angle used for TPC is conventionally obtained through either experimentally tricky calibration, manual estimation based on diffraction pattern symmetry, although computational approaches are emerging. In this work we offer an improved numerical optimization approach as an alternative to TPC, where we use the flexibility offered by our Automatic Differentiation (AD)-based ptychography approach to include the data resampling into the forward model to learn the tilt angle. We demonstrate convergence of the approach across a range of incidence angles on simulated and experimental data obtained on an EUV beamline with either a high-harmonic generation (HHG)-based or a visible light source.
High harmonic generation at high repetition rate is realized with a high average power 100W, 600kHz fiber laser system. Optimization is done for two different operation regimes. At 69-75eV the source delivers a world-record photon flux of >10^11photons/s/harmonic when using argon gas jets. The use of neon gas allows for operation at significantly shorter wavelength. The important 93eV harmonic can be generated at 5·10^9 photons/s/(1% bandwidth), while even higher values of >10^10 photons/s/(1% bandwidth) are achieved between 115-140eV. The HHG source provides excellent long-term power stability of ~1% RMS for each of the operation regimes.
We report a highly sensitive ultrasound sensor based on an integrated photonics silicon Mach-Zehnder interferometer (MZI). One arm of the MZI is located on a thin membrane, acting as the sensing part of the device. Ultrasound waves excite the membrane’s vibrational mode, thus inducing modulation of the MZI transmission. The measured sensor transfer function is centered at 0.47 MHz and has a −6 dB bandwidth of 21.2%. For 1.0 mW optical input power, we obtain a high sensitivity of 0.62 mV/Pa, a low detection limit of 0.38 mPa/Hz1/2 at the resonance frequency and a large dynamic range of 59 dB. In preliminary ultrasound imaging experiments using this sensor, an image of a wire phantom is obtained. The properties of this sensor and the generated image show that this sensor is very promising for ultrasound imaging applications.
Coherent Fourier Scatterometry (CFS) is a scanning optical technique that is particularly suitable for nanoparticle detection. Inspection of wafer surfaces is one of the critical bottle-necks for high yield in the production of semiconductor chips. Ideally, inspection systems are required to work fast, be sensitive, and should not thermally damage the samples with an excess of illuminating light power. The sensitivity of detection of nanoparticles, attributed to the smallest size of the scatterer that can be detected, is severely limited by noise. The optical readout of the scatterometer consists of a bi-cell (a split photodetector) that collects the scatterred light from the surface to be inspected while the latter is scanned in the lateral direction (2D scan). The difference voltage signal resulting from integrating and subtracting the two halves of the bi-cell is recorded as a function of the lateral scanning position of the sample surface. The bi-cell has two functions: first, it allows us to acquire signals in a fast manner, and second, it eliminates effects due to substrate spurious reflections, which is usually a big issue in dark field based particle detection systems. In this paper, we present an extension of the original CFS detection system by incorporating a heterodyne technique to the detection system. We show the implementation of the new detector system as well as a comparative signal-to-noise ratio (SNR) gain studies that are used to determine the suitable frequencies and waveforms for both modulation and reference signals. We demonstrate the detection of polystyrene nanoparticles with a diameter of 80 nm, which were deposited on top of a silicon wafer, with high SNR at low illuminating light power. The experiments were performed with a diode laser at the wavelength of 405 nm. In this particular particle size, we have observed an improvement of the SNR of about 45 dB as compared to the original detection system of the CFS. Although the proposed heterodyne CFS technique already shows excellent performance for detection of polystyrene nanoparticles on silicon wafer, there is still room for improving the sensitivity towards even smaller particles, as discussed in the outlook and conclusions section.
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