Letter Optics Letters 1

Wide-field Broadband EUV Transmission Ptychographyusing a High Harmonic SourcePETER D. BAKSH1,*, MICHAL ODSTRCIL1,2, HYUN-SU KIM1,2, STUART A. BODEN3, JEREMY G. FREY4,AND WILLIAM S. BROCKLESBY1

1Optoelectronics Research Centre, University of Southampton, SO17 1BJ, United Kingdom2 RWTH Aachen University, Experimental Physics of EUV, JARA-FIT, Steinbachstrasse 15, 52074 Aachen, Germany3Zepler Institute, University of Southampton, SO17 1BJ, United Kingdom4School of Chemistry, University of Southampton, SO17 1BJ, United Kingdom*Corresponding author: P.Baksh@orc.soton.ac.uk

Compiled February 11, 2016

High harmonic generation (HHG) provides a labora-tory scale source of coherent radiation ideally suited tolensless coherent diffractive imaging (CDI) in the EUVand X-ray spectral region. Here we demonstrate trans-mission EUV ptychography, a scanning variant of CDI,using radiation at a wavelength around 29 nm from anHHG source. Image resolution is diffraction-limited at54 nm, and fields of view up to ∼100 µm are demon-strated. These results demonstrate the potential forwide-field, high resolution laboratory scale EUV imag-ing using HHG-based sources with potential applica-tion in biological imaging or EUV lithography pellicleinspection.© 2016 Optical Society of America

OCIS codes: 110.0110 Imaging systems, 040.7480 X-rays, softx-rays, extreme ultraviolet (EUV); 110.1650 Coherence imaging

http://dx.doi.org/10.1364/ao.XX.XXXXXX

Coherent Diffraction Imaging (CDI) is a rapidly-evolvingtechnique attractive for imaging at wavelengths where conven-tional lenses become ineffective [1, 2]. CDI methods reconstructthe phase and amplitude of the exit wave field (EWF) from acoherently-illuminated sample using the measured scatteredradiation. Only the spatial intensity profile of the scattered radi-ation is collected by the detector; all phase information is lost.However, if the scatter pattern intensity is sufficiently sampledand additional knowledge (i.e. real space constraints) are pro-vided, the unrecorded phase information can be recovered usingcomputational methods [3, 4]. Rapid development of coherent X-ray sources worldwide has led to development of numerous CDImethods applied to both the physical and biological sciences. Inthe last decade a variation of CDI known as ptychography [5]has advanced the field, allowing the reconstruction of wide-fieldimages of extended samples. In a ptychography experiment,

multiple overlapping regions of the sample are illuminated se-quentially, and the algorithm uses the overlap between adjacentilluminated regions as the real-space constraint for phase re-trieval. The main assumptions of the ptychography method arethat the illumination probe and complex transmission of thesample remain unchanged during the scan, and that the probepositions are known precisely. This additional knowledge, com-bined with sufficient overlap between the scanning positionsand appropriately sampled scatter pattern intensities, providesenough constraints for robust reconstruction of the complex val-ued probe and object, without many of the issues seen in singlediffraction pattern CDI with support constraint methods. [5].

CDI-based methods have found particular use in the X-raycommunity, because of the difficulty in producing high qualityimaging optics in this spectral region. CDI methods generallyrequire a very high level of coherence that can be providedby high brilliance light sources such as 3rd and 4th generationsynchrotron sources or free electron lasers (FELs). The maindrawback of these large facilities is that they are sparse and thebeam time is limited. As a laboratory scale source of coherentEUV radiation, high harmonic generation (HHG) can providethe required spatially coherent light. Single diffraction patternCDI with support constraint, in which a single scatter pattern isrecorded and the real space constraint is provided by expecta-tion of an isolated small sample, has been demonstrated with aHHG based source using binary samples, whose transmissionis either one or zero, giving up to 22 nm resolution [6]. Ptycho-graphic imaging avoids some of the drawbacks of the singlediffraction CDI with support constraint but it places very highdemands on the pointing and intensity stability of the illumi-nation source over the duration of the scan, as it requires theacquisition of multiple scatter patterns with the same illumi-nating probe beam [7, 8]. X-ray ptychography at synchrotronshas been very successful, and in a typical X-ray ptychographyexperiment, thousands of overlapping regions of a sample mightbe illuminated for a single reconstruction. Up until now, theonly demonstration of ptychography using HHG radiation hasbeen reflection-geometry ptychography using a HHG source at29 nm wavelength [9]. The claimed lateral resolution was 40 nmby 80 nm and the field of view achieved was 45 by 65 µm using

Letter Optics Letters 2

198 overlapping illuminated regions.In this letter, we present ptychographic imaging, in a trans-

mission geometry, of extended objects with greater than 100 µmfield of view in a single reconstruction, with high detectionnumerical aperture (NA=0.26), resulting in diffraction-limitedresolution of 54 nm in both axes. The largest field of view was theresult of collection of 967 high dynamic range scatter patterns,demonstrating the extreme stability of the HHG source neededto ensure accurate reconstruction. The field of view is only lim-ited by experimental time taken. The cumulative exposure timewas less than 0.6 s per a position.

The experimental setup is shown in Fig. 1. The laser gener-ates pulses at 800 nm wavelength with 50 fs pulse length and1 kHz repetition rate. The generated IR pulses of energy 1.8 mJare focused down into a 3 mm long gas-cell by a 75 cm focallength lens. The gas cell is filled with argon gas at a pressure of80 mbar. The EUV and the fundamental IR beam are separatedby a single 200 nm thick aluminium filter. The generated EUVis spectrally filtered and focused onto a sample by a sphericalB4C/Si mirror with radius of curvature of 40 cm. The fold angleof the mirror is ∼ 7°, resulting in a significant astigmatism ofthe focused beam. The EUV beam is spatially constrained bya pinhole placed near to the beam focus. The structured illu-mination at the sample plane is therefore defined by near-fieldpropagation of the cropped EUV beam. The sample is movedrelative to the pinhole by 3-dimensional nanoprecision piezostages (Smaract 3D SLC-1740). The scattered radiation from thesample is collected in the far-field regime by a 1024 × 1024 EUVsensitive CCD camera (Andor DV434) placed 2.55 cm behindthe sample. A key feature, essential to the success of the experi-

AM

L

C1C2

G F

XM

PS

XC

Fig. 1. A schematic of the experimental design. After the am-plifier, the IR laser is reflected off a first stabilisation mirror(M), before passing through a stabilised focusing lens (L). Themain beam is then reflected into the gas cell whilst a small frac-tion of the beam passes through the back of the dielectric mir-ror towards two stabilisation CMOS cameras (C1 and C2). Thebeam path between amplifier and gas cell is approximately6 m. The main beam passes through the gas cell (G) where theEUV is generated, and is attenuated by an aluminium filter (F),which transmits the EUV beam. A multilayer spherical mirror(XM) is used to focus the EUV onto the aperture (P) and sam-ple (S). The diffraction pattern is measured in the far field by acooled CCD (XC)

ments, is the careful attention to all factors that can influence thebeam stability. Beam pointing stability was improved using bothpassive and active techniques. The fast positional fluctuationscaused mainly by air fluctuations and mechanical vibrationswere reduced by sealing the beam path between the amplifierand the vacuum system, and mechanical oscillations were min-imised. Slow positional variations with frequency below 1 Hz,originating from the laser system, and small air temperaturevariations in the laboratory during the experiment can adversely

affect the stability of the probe illumination. These slower move-ments were compensated using a custom-built active stabilisa-tion system based on a PID controller and two CMOS cameras(DCC1545M) (Fig. 1) driving Picomotor actuators that can pro-vide angular precision better than 0.2 µrad. The measured EUVstability is approximately five times worse than the IR stabil-ity, due to extra experimental instabilities after the gas cell thatcannot be corrected for with active stabilisation.

The dynamic range of the collected scatter patterns was in-creased by exposing the same sample region using differentexposure times [5]. To reduce total readout time, only the cen-tral regions were read out in the shorter exposures. The imagestitching was done by following equation

I(k) =∑N

i Wi(k)Ii(k)∑i Wi(k)ti

tN (1)

where Ii(k) denotes background subtracted diffraction intensityat the position k, ti is the i-th exposure time, tN is the maximalexposure time and Wi(k) are weighting factors. Weighting factorWi is zero for over-saturated regions and inversely proportionalto the expected noise level elsewhere. No mechanical beam blockwas used in our setup. Exposure times ti were corrected to avoidsystematic errors caused by finite shutter opening and closingtime. This method allows the extension of the dynamic range by3 orders of magnitude without losing the low spatial frequencyinformation. The total readout time was increased by a factor ofapproximately two compared to a single exposure. This methodwas used instead of more common methods such as multi imageaccumulation [9], which is slow, or using a static beam stop [10],which results in large missing regions in the collected diffractionpatterns. In the presented experiment, the high dynamic rangemethod provides a dynamic range of 106 using exposure timesbetween 5 ms and 0.5 s. All scatter patterns were corrected forthe consequences of the curvature of the Ewald sphere and theuse of a flat detector [11].

The EUV photon flux incident on the sample is ∼ 6 ×108 photons/s, equivalent to a flux of 2.7 mW/cm2 incident onthe sample. The use of a single mirror results in polychromaticillumination of the sample (see Fig 2b). The EUV bandwidth islimited by the reflectivity spectrum of the B4C/Si mirror, withpeak reflectivity around 30 nm and by the absorption edge of ar-gon gas, which strongly attenuates wavelengths above ∼30 nm.The experimental geometry (Fig. 1) introduces a significantastigmatism into the beam, therefore the aperture required forptychography was placed at the "circle of least confusion" (CLC)between the two foci to maximise EUV flux through the aperture.The sample was positioned approximately 0.1 mm behind thepinhole. It is advantageous to keep this distance in the near-fieldregime (i.e. Fresnel number � 1) in order to produce sharpfeatures in the illumination probe. Furthermore, any increasein beam size arising from propagation would result in lowerphoton flux density on the sample and lower oversampling ofthe scatter pattern, which would in turn increase demands onbeam stability and coherence.

For the datasets presented in this letter, aperture diametersof 7 µm and 10 µm were used. The step size of the scan waschosen to produce 70∼80% linear overlap to ensure sufficientptychographic sampling [12], resulting in either 1.5 or 3 micronsteps depending on the pinhole size.

We present results from two samples. Both samples wereprepared on a 50 nm SiN membrane. The first sample (Fig. 3)is an aperiodic high-contrast grid pattern produced by electron

Letter Optics Letters 3

beam lithography and coated with 100 nm of gold. Figure 3(b)shows an SEM image of the aperiodic grid sample. The secondsample consisted of 400 nm diameter PMMA spheres depositedonto a 50 nm silicon nitride membrane to produce an extendedsample with a random distribution and feature size similar tothose found in biological cells [13].

103

104

105

106

θ [rad]

Cou

nts

[-]

Measured broadband diffractionSimulated monochromatic diffraction

0 0.1 0.20.05 0.15 0.25 22 24 26 28 30 32 34Wavelength [nm]

Spec

tral

Inte

nsi

ty

Fig. 2. A line profile of the far field diffraction pattern of a dou-ble slit placed at the CLC position. The profile is comparedto that from a simulated monochromatic spectrum to demon-strate the blurring effect caused by the polychromatic sourceillumination. Right plot shows the reconstructed EUV spec-trum derived from the double slit diffraction pattern [14].

10 µm

a)

100

102

104

106

Num

ber of photons

c)e)

d)

b)π 0

Fig. 3. (a) shows amplitude and phase image of the grid sam-ple. Here, the image hue represents relative phase and imageintensity shows the amplitude of the reconstructed complextransmission function. The inset of (a) shows the complex re-constructed probe field on the same scale as the sample (b)shows an SEM image of the sample. (c) and (d) show the com-plex electric field distribution of the probe incident on thesample, and after back propagation to the plane of the pinholeaperture. (e) is an example of a single collected scatter pattern.

The EUV spectrum incident on the sample was measuredby a Young’s slits-based spectrometer [14] in order to obtainthe EUV spectrum under the same experimental conditions thatwere used in the imaging experiments. A double slit with 4 µmseparation and 1 µm width was milled by an FIB into 200 nmAu-coated silicon nitride and placed in the CLC position. A lineprofile of the collected high dynamic range far-field diffractionpattern is shown in the Fig 2. The spectral resolution (Fig. 2)was diffraction limited to approximately 0.5 nm FWHM. How-ever, thanks to a relatively long driving pulse length (50 fs), the

produced HHG spectral peaks are very narrow. Despite us-ing polychromatic illumination, sharp speckles are visible evenwhen measured using NA>0.4 in a previous experiment. Themain advantage of using the polychromatic spectrum is the in-creased coherent photon flux density incident on the sample [15]due to the use of a single mirror, necessary because of the lowreflectivity (peak reflectivity <30%) of multilayer mirrors in thisspectral region.

Reconstructions were performed using the extended ptycho-graphic iterative engine (ePIE) algorithm [5] in combinationwith the application of relaxation of several constraints. We haveused the PolyCDI method [16] to relax for broadband illumina-tion using the reconstructed EUV spectrum. PolyCDI is a validapproximation if the sample can be assumed to be sufficientlyachromatic. Otherwise a full multicolour ptychographic methodmust be implemented [17]. Reduced visibility due to finite co-herence length and fast beam fluctuations was relaxed using aconvolution based correction method [18]. Finally, intensity andreadout noise relaxations were implemented as in Ref. [19]. Thepixel size was estimated from the experimental geometry andmore precisely refined during the reconstruction process by across-correlation method [8].

0.4 0.5 0.6 0.7 0.8 0.9 1 1.10

0.2

0.4

0.6

0.8

1

(a) (b)ReconstructionRefocusedERFC fit

Distance [µm]

Ampl

itude

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1280 140 93 70 54

Four

ier r

ing

corr

elat

ion

Spatial frequency / Nyquist

Half period resolution [nm]

FRC verticalFRC horizontal1 bit threshold

Fig. 4. (a) shows a zoomed region of the reconstruction shownin Fig. 3 and one example of intensity values around a sharpedge and an error function fit to a numerically-refocused sharpedge profile. The white line indicates the region of the recon-struction where the shown line profile was obtained.The starsrepresent reconstructed data points and the circles denote datapoints after numerical refocusing of the image. (b) shows theFourier ring correlation with 1-bit resolution criterion.

The complex reconstruction (i.e amplitude and phase) of thegrid sample is presented in Fig. 3(a). The reconstruction of thecomplex illumination probe field is shown in Fig. 3(c). Figure3(d) shows back-propagation of the illumination probe onto theaperture plane where a saddle-shaped wave front cropped by acircular aperture is clearly visible, as expected. The pinhole tosample distance was estimated to be 55 µm using angular spec-trum model (ASM) [20] based propagation code. The highestreconstructed spatial frequency was limited by the experimentalgeometry to 54 nm. There is large attenuation of 29 nm light bythe 100 nm thick Au layer, so no phase variation is observed.Due to insufficient flatness of the sample, the majority of theobserved phase variation, i.e. fringes around some of the sharpedges, are diffraction effects. The depth of focus (DOF), given as

DOF = ±12

λ

(NA)2 (2)

is approximately 200 nm in this experimental geometry. Thus thesample to pinhole distance must vary by less than 200 nm overthe whole scanning region in order to have the whole reconstruc-tion in focus. This is a serious limitation, particularly for higherNA short wavelength imaging. This effect originates from the

Letter Optics Letters 4

nature of ptychography, which searches for a single optimalillumination probe for the whole dataset. However, becausefeatures in our aperiodic grid sample are sparse, and flatnessis close to the limit given by Eq. 2, it is possible to numerically“refocus” the reconstruction by the ASM method at differentregions of the sample. An example of the numerical refocusingeffect is demonstrated in Fig. 4(a). Figure 4(a) shows an exampleof a cross section of the reconstructed EWF taken at a region ofthe sample containing a sharp edge. The reconstructed complexdata points (stars) were numerically refocused (circles) usingthe ASM to show a very flat profile on either side of the risingedge, as expected from the nature of the sample. This knife edgeanalysis was applied to multiple sample regions to estimate theresolution. The example of the refocused data in Fig. 4(a) agreeswell with the error function complement (ERFC) fit. The knifeedge analysis shows that the 10 − 90% distance, 58 ± 5 nm, isapproximately equal to the pixel size of 54 nm, indicating thatthe resolution is limited mainly by the experimental geometryand not by the reconstruction process or insufficient EUV signal.

The repeatability of our measurement was evaluated fromtwo independent datasets using Fourier ring correlation (FRC) asshown in Fig. 4. We have used the 1-bit threshold criterion [21]to estimate the resolution. Our aperiodic grid sample possessesa high degree of asymmetry, and the distribution of scatteredintensity is significantly anisotropic (see Fig. 3(e)). Thus the FRCin the vertical direction is reduced at higher spatial frequenciescompared to the horizontal direction. The resolution in the verti-cal direction was estimated from the FRC to 100 nm and in thehorizontal direction it is limited by the experimental geometryto 56 nm, in good agreement with the knife-edge estimates.

105 μm

π 0

Fig. 5. An image reconstruction in phase and amplitude of anextended sample of PMMA spheres placed on 50 nm thickSi3N4 support. Pixel size is 54 nm and the field of view is105 µm. The illumination probe is presented on the same scaleas the reconstructed image.

A reconstruction of the second extended sample is presentedin Fig. 5. The field of view here is ∼ 105 µm, achieved by col-lecting 976 scatter patterns with a probe aperture diameter of10 µm, moved in steps of 3 µm. The pixel size is again limited bythe geometry to 54 nm. The total dose deposited into the samplewas approximately 105 Gy.

We have presented diffraction-limited, wide-field transmis-sion ptychography using a high harmonic generation basedtable-top source. We have shown that broadband radiation canbe acceptable and even beneficial if it provides significantlylarger flux for illumination. High resolution and large field of

view reconstructions of extended samples are now possible, de-spite the lower stability and flux of table-top laboratory EUVsources, by application of a variety of appropriate CDI relaxationtechniques. As HHG research progresses towards generationof sufficient coherent flux for imaging in the water window, theHHG-based ptychographic microscope should become a highlyvaluable tool for high-contrast, high-resolution quantitative mi-croscopy.

M.O. acknowledges financial support from the EU FP7 Eras-mus Mundus Joint Doctorate Programme EXTATIC under frame-work partnership agreement FPA-2012-0033. The K5200 graph-ical card used for this research was donated by the NVIDIACorporation. The data for this work is accessible through theUniversity of Southampton Institutional Research Repository(DOI:10.5258/SOTON/382320).

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