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nik (IMT), Berlin and the Fraunhofer Institut für Lasertechnik, Aachen, to investigate laser produced1'2'3and pinch plasma4 x-ray sources comparatively, and from the succeeding research program of the Karl SUssCompany, Garching, and the Fraunhofer Institut für Lasertechnik, Aachen, to develop a pinch plasma x-raysource for lithography applications5. Most of this work was previously published, although independentlyas far as the sources are concerned. The discussion is therefore held briefly.

The results referring to full field imaging x-ray microscopy are discussed in more detail. They are ob-tamed in a joint effort of the Forschungseinrichtung Röntgenphysik, Universität Göttingen, the Carl ZeissCompany, Oberkochen, the Fraunhofer Institut (ILl) and the RWTH-Lehrstuhl (LLT) für Lasertechnik,Aachen, to develop an x-ray microscope based on a pinch plasma source, and in a cooperation of the MaxPlanck Institut für Quantenoptik (MPQ), Garching, the Fraunhofer Institut and the RWTFI-Lehrstuhl fürLasertechnik, Aachen, in the field of laser produced plasmas.

A major objective of the latter cooperation is the quantitative comparison of the "integrated spectralbrightness" (see below) of both plasma sources in single x-ray lines, so that the diagnostic equipment istransferred between the laboratories to overcome the uncertainty that is related with a comparison of ex-perimental data obtained using different diagnostic techniques. The corresponding results form the mainpart of this paper.

2. LASER PRODUCED AND PINCH PLASMA X-RAY SOURCES

Both laser generated plasmas and pinch plasmas are widely investigated intense, pulsed x-ray sources.In both, x-rays are generated by radiative recombination and line emission from highly ionized plasmas.The properties of the emission depend on several physical plasma parameters and their temporal and spatialvariations, e.g. the electron density, energy distribution, the used element (Z) as well as the states of ioniza-tion

In optical thick, high atomic-number plasmas, the thermal emission spectrum can be roughly approxi-mated by a black-body spectrum, with some atomic line structure superimposed. The radiation intensityemitted from the plasma can thus be estimated according to Stephan-Boltzmann's law:I = 1.03*i05 W/cm2*(kT/eV)4. Assuming a 100 eV black-body temperature, convenient to shift the emis-sion maximum into the soft x-ray range, the intensity amounts to about i013 W/cm2, so that powerful driv-ers are necessary to generate these plasmas.

2.1 Pinch plasma x-ray source

In pinch plasma devices the plasma is generated by magnetically imploding a low-temperature, cylindri-cal plasma layer. The azimuthal magnetic field is produced by currents of several 100 kA driven by fastcapacitor banks through the plasma. During the implosion the kinetic energy of the plasma layer increasesrapidly due to the pinch effect6. The kinetic energy is thermalized within a few nanoseconds when theplasma stagnates on axis and forms the pinch. The plasma is then ionized to higher ionization states andconverts up to 10% of its energy into (> 1 keV) soft x-rays7. The total K-shell x-ray yield Y scales with thefourth power of the pinch current845. X-ray bursts of up to 100 kJ per pulse are reported to be emitted fromdevices with currents of several megamperes.

2.2 Laser plasma x-ray sources

Laser generated plasmas are produced by illuminating matter, usually a planar target of solid material,with high-power lasers being focused to intensities typically ranging from 1012 - 1015 W/cm2. Under these

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conditions electrical breakdown occurs and forms a plasma at the target surface. The laser liht of wave-length A penetrates into the plasma up to the critical electron density n = io21 (4m)2 cm where mostof the energy is absorbed by heating the electrons. Electron heat conduction transports the energy closer tothe target into regions of higher electron densities where most of the x-rays are generated. The conversionof laser energy to x-ray energy depends on the laser intensity, the laser pulse duration, the laser wavelengthas well as on the target material. With high atomic number targets, conversion efficiencies (total emission)ofmore than 50 % are observed16.

3. BROADBAND SOURCES FOR X-RAY LITHOGRAPHY

Proximity printing XRL has already proved its great process latitude, simple resist processing, excellentstep coverage over topography and its immunity to low atomic number particle contamination for IC fabri-cation at 0.5 .tm design rules17. Every effort is made to push the technology down to 0.25 and 0.12 jim.Large scale production is generally planned around electron storage rings, whose synchrotron radiation pro-vides nearly ideal properties with respect to spectral distribution, beam divergence and brightness. flow-ever, for research goals, to accompany production activities or even for small scale production, compact,low cost x-ray point sources being fully compatible with the manufacturing process at electron storagerings, are required. Spectrum, mean power, spot size and the beam line are of special importance with thisregard.

0.06

Fig. 1. Exposure efficiency, i.e. percentage ofphotons absorbed in the photo resist afterpassing beam line and silicon mask as afunction of wavelength. A 20 jim berylliumfoil is assumed as beam line window. Below0.67 nm wavelength the radiation is mainlyabsorbed by the mask, above 1.2 nm by thebeam line.

The spectral range from 0.67 nm - 1.2 nm enables maximum resolution and stability of the printingprocess (process latitude). It is mainly determined by the mask technology based on 2 jim silicon mem-branes coated with 1 jim absorbing gold structures18. Fig. 1 shows the exposure efficiency as a function ofwavelength, i.e. the percentage of photons absorbed in the photo resist after passing a beam line window oftypical 20 jim beryllium and the silicon mask. Radiation below 0.67 nm wavelength, the K-edge of silicon,is strongly absorbed in the mask which leads to a reduction of resolution due to thermal expansion and de-creasing mask contrast. More severe disadvantages appear above 1.2 nm wavelength. Besides high radia-tion losses due to absorption in the beam line the process latitude of XRL is reduced: the defect densityincreases as dust particles are less transparent and higher absorption in the photo resist reduces the penetra-tion depth and thus the aspect ratio achievable 19,

0.04

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If the spectral properties of a plasma source are matched to XRL there still remains a difference to syn-chrotron radiation, influencing the resolution, i.e. the finite source to wafer distance and the finite spot size.Fig. 2 illustrates the influence of central projection on the resolution. Due to the proximity gap g of several10 im between mask and wafer and the finite diameter of the source the line edge definition is reduced(blurring 13). A proximity gap variation g caused by wafer non-flatness or alignment accuracy in conjunc-tion with the central projection leads to magnification changes (run out p), mainly at the outskirts of theimage field, thus reducing the overlay accuracy.

Fig. 2. Two types of geometricaldistortions are induced by the use ofpoint sources in a distance L fromthe mask:- run out p due to variations ig of

the proximity gap: gp = ig * (R+r)/Lblurring 13 due to the finite spotsize2r: B=g*2r/L

Table 1 shows the typical feature size needed for future complexity levels of ICs. Down to feature sizesof 250 nm excellent process latitude is obtained at about 40,i.im proximity gaps. Scaling to smaller featuresizes requires reduction to 10 tm due to Fresnel diffraction1 . Tolerated run out and blurring is assumed tobe about 1/4 of the feature size. The calculated values for minimum source to wafer distance L and maxi-mum source diameter 2r are calculated assuming an image field of one square inch and a proximity gapaccuracy of 2 m.Tab. 1 Restraints on source to mask distance and source diameter determined by geometrical

distortions. An image field of one square inch and a proximity gap variation of 2 tm areassumed.

256M 1G 4Gfeature size (nm) 600 350 250 180 130

proximity gap (.tm) 40 40 40 20 10

run out / blurring (urn) 130 100 65 45 35

source-mask distance L (cm) 30 40 60 90 115

source diameter 2r (mm) 1 1 1 (2) (4)

The run out criterion determines the minimum source to wafer distance L. L increases with increasingcomplexity of the IC. Using the calculated distances, the maximum source diameter may even exceed1 mm for higher complexity level.

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source window mask resist

R

L ig g

complexity level 16 M 64 M

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3.1 Experimental

The suitability of both laser produced and pinch plasmas for XRL has been investigated in a cooperativework of the Physikalisch Technische Bundesanstalt, Berlin, the Fraunhofer Institut fUr Mikrostrukturtech-nik, Berlin, and the Fraunhofer Institut für Lasertechnik, Aachen.

A Nd laser (laser energy 3 J at X = 1064 nm wavelength, 1 .2 J at X = 532 nm wavelength, pulse length15 ns, Berlin) has been used for laser produced plasma experiments1. Iron, nickel, copper and brass havebeen investigated as target materials with respect to conversion efficiency and emission of plasma debris.Best results were obtained with iron targets in agreement with results published by other authors20. A studyof the resist exposure revealed that mainly x-rays at about A = 1 .4 nm contribute to the printing process.The conversion efficiency of laser energy to x-ray energy is about 3 % (up to 15 mJ/pulse/sr) with 1064 nmlaser 1iht (intensit1: i013 W/cm2) and about 5 % (up to 10 mJ/pulse/sr) with 532 nm laser light(5*101 W/cm2)1'2' .The x-ray source diameter is about 0.1 mm.

The emission of pinch plasmas was studied at Aachen using a plasma focus type device capable to drivecurrents up to 400 kA through the plasma. In agreement with other published results21'22 neon gas provedto have the best properties concerning reproducible operation of the gas discharge and the emission spec-trum. Resist exposure is mainly by recombination radiation of hydrogen like and helium like neon ions be-tween 0.7 nm and 1 .1 nm. About 800 mJ/pulse/sr of x-rays were emitted into this wavelength range from asource diameter of about 0.4 mm for a single pulse and a mean diameter of about 0.9 mm integrated overseveral pulses.

3.2 Exposure performanceIn a succeeding research program of the Karl SUss company, Garching and the ILT, Aachen a plasma

source for use with commercial x-ray steppers has been developed. The pinch plasma system has been se-lected for two reasons. First, the exposure efficiency is about a factor of 4 higher due to the shorter wave-length range of the emission according to Fig. 1. Second, the emitted energy per pulse exceeds that of thelaser produced plasma by a factor 50. In total the repetition rate of the laser produced plasma had to be afactor of 200 higher than that of the pinch plasma device to achieve comparable exposure times.

The upgraded pinch plasma device (foot print: 1 m 2 m) is powered by a fast 5U capacitor bank5'23'24capable of operation at 2 Hz repetition rate. A three window beam line of about 15 % transmission providesboth protection from erosion products and plasma debris as well as a plane exit window of one square inchagainst atmospheric pressure. About 100 .tW/cm2 of x-rays are provided at the resist surface in 40 cm dis-tance from the source. Reproduced 0.2 j.tm structures in 1 im thick 60 mJ sensitivity resist (RAY-PF)within the full exposure field of one square inch reveal that the process performance is close to conditionsat electron storage rings25'26.

4. LINE EMITTERS FOR X-RAY MICROSCOPY

Like in XRL the large majority of high resolution XRM experiments take place at electron storagerings2732 where x-ray imaging of biological specimen in their wet natural state is demonstrated with55 nm33 (sub optical) resolution. Further improvements of the resolution to less than 20 nm are to be ex-pected in the near future along with the rapid progress in the manufacture of zone plate optics in particular.If radiation within the water window wavelength range (fig. 4) is applied to make use of high transmissionthrough water and of natural contrast34, several tm thick, wet biological samples can be studied under at-mospheric conditions and with little preparational effort35.

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CCD

Microzone plate

- -

Specimen

Mirror ________condenser ________

Plasma source —_________

Fig. 3. Set up of a full field imaging x-raymicroscope with a pulsed plasma x-ray source. Amirror condenser provides the illumination of the

specimen, and a high resolution, high efficiencymicro zone plate forms the image.

Fig. 4. Absorption of water and protein as afunction of the wavelength. Close above theK-edge of oxygen at 2.33 nm the absorption ofwater is considerably lower than of protein,allowing for investigation of wet specimen.Nitrogen and carbon have the strongest K-shellemission lines within the water window.

Full field imaging microscopes make use of two x-ray optical components. A condenser provides theillumination of the specimen and an x-ray objective forms the image (fig. 3). If grazing incidence mirrorsand zone plates are considered as x-ray optical components, the first are more efficient and do not limit thespectral bandwidth of radiation, whereas higher imaging resolution is reported with zone plates. The micro-scope shown in Fig. 3 utilizes a mirror condenser and a zone plate objective to benefit from a combinationof the mentioned features. However, the use of a zone plate implies that radiation must be narrowband, asthese diffractive x-ray lenses exhibit chromatic aberrations. The resolution limit & = 1.22 ôr (5r is thewidth of the zone plates outermost zone) can be reached in x-ray imaging, if the relative reciprocalbandwidth of the radiation fulfills X/& =n 36, 37, where n specifies the number of zones. A high resolutionzone plate of a several hundred .tm focal length at wavelengths around 2.5 nm typically consists of a fewhundred zones 38,39,40 In contrast to experiments at electron storage rings, a monochromator stage is not

SPIE Vol. 2015/37

Research directed to smaller and cheaper radiation sources forms an important part of the current devel-opment with the objective to promote XRM to routine like optical and electron microscopies. Laser pro-duced and pinch plasmas are of special interest, because of their potential to form the x-ray image in onesingle pulse, before motion within the specimen or radiation damage could interfere. The requirements ontheir radiative properties depend on the specific microscopy technique, i.e. x-ray microradiography, scan-ning x-ray microscopy or full field imaging microscopy, and on the x-ray optics used.

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2.0 3.0 4.0 5.0WAVELENGTH hn]

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necessarily required in case of laser produced and pinch plasma x-ray sources, if these are optimized toemit line radiation, however, without additional emission lines or continuous radiation in the close spectralneighborhood41.

K-shell lines are advantageous over L- or M-shell lines because of their larger spectral separation. Espe-cially the first resonant lines, i.e. the is - 2p line and the is2 12p line, for hydrogen and helium like ions,respectively, are most intense because they are of highest oscillator strength within the line series. Theselines are located in the water window wavelength range for the elements nitrogen and carbon (fig. 4). Threefacts favor the use of nitrogen over carbon. Two emission lines around 2.5 nm wavelength, the N VII is -2p and the N VII is2 is2p lines, coincide within a reciprocal relative bandwidth X/&. > 2i0, so that they

can be used simultaneously to enhance the intensity contributing to image formation, at a resolution thatcorresponds to the resolution limit of a zone plate of 210 zones. In addition, their wavelength is close to theoxygen K-edge where transmission losses by absorption in wet specimen are the lowest (fig. 4). Finally, thequantum efficiency of CCDs increases with photon energy43. These image sensors may be expected to be-come most popular for microscopy experiments because of their high sensitivity44'45 and the rapid avail-ability and handling of image data.

Ideally, the x-ray image is formed before motion or radiation damage reduce the resolution limit. There-fore, a sufficient number of photons has to be emitted in a single pulse. Furthermore, these photons have tobe emitted from a source with a diameter determined by the image field size in the microscopes objectplane in combination with the demagnification of the source profile by the condenser stage. As a conse-quence, the spectral brightness integrated over the pulse duration and the line profile is the figure of merit.This number corresponds to the yield per unit source surface, per solid angle and per pulse in the line ofinterest, and will be referred to as integrated spectral brightness in the following.

4.1 Experimental

A frequency-doubled Nd laser (wavelength X = 532 nm, i pulse/iO mm, io m2 footprint, MPQGarching) with a pulse duration of 3 ns and a pulse energy up to iO J was used for the laser producedplasma experiments. Planar boron nitride targets were chosen to produce the N VII is - 2p and N VI is2 -is3p lines. This target choice simultaneously avoids additional x-ray lines within the water window wave-

length range from target elements other than nitrogen. The laser was focused at normal incidence and theemission was observed at an angle of 450 to the laser axis.

On the other hand, a plasma focus device46 with 2.5 kJ electrical storage energy and 350 kA peak cur-rent (i pulse/20 sec., 2 m2 foot print, ILT/LLT Aachen) was used for the pinch plasma experiments. Thissystem is especially dedicated to an x-ray microscope48 under development, the set up of which is depictedin Fig. 3. Nitrogen gas is used as discharge load to generate the N VII is - 2p and N VI is2 - is3p lines.The emission was observed in axial direction, with respect to the plasma column.

Spectra of the nitrogen pinch plasma and the boron nitride laser produced plasma are shown in Fig. 5and Fig. 6, respectively. They were recorded by means of a 2 m grazing incidence VUV spectrograph49with a i i52 grooves/mm gold coated grating (fig. 5) and a flat field grazing incidence spectrograph with anaberration corrected concave grating50 (fig. 6). The resolution limit of either spectrograph is about

= 300. Both spectra consist mainly of emission lines and recombination continua of hydrogen andhelium like ions. The N VII is - 2p line at 2.478 nm and the N VI is2 - is3p line at 2.49 nm, which coin-cide within a reciprocal relative bandwidth of X/EX = 2i0, are resolved in Fig. 5, whereas the lower valueof X/& = i70 deduced from Fig. 6 reveals that line broadening has to be considered in the laser producedplasma. At an electron density around 1021 - i022 cm3 and an extension of the emitting region of a few

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Fig. 5. Emission spectrum of a nitrogen pinchplasma. The N VII is - 2p line at 2.478 nm andthe N VI is2 - ls3p line at 2.49 nm are resolved.(resulting bandwidth X/&. = 200).

>.I-.C1)zIjJ0C.)IQ.

C,

0I0.

Transmission pinhole (50 .tm diameter) grating spectrographs51'52 with a CCD detector have been usedto measure the line to continuum ratio 'L"C• Higher order diffraction efficiencies are negligible in compari-son to the first order diffraction efficiency for these gratings (ratio gap width to grating period: 0.5).Addi-tionally, the CCD is a detector of high linear response over a wide dynamic range. The spectral resolutionof these spectrographs strongly depends on the source size. To compensate for the larger pinch plasmasource, two gratings of 10000 lines/mm and 2000 lines/mm have been used to achieve a comparable resolu-tion of X/LV = 15; sufficient to resolve the line pair of interest at 2.5 nm wavelength from neighbored lines.

Two differences appear in the corresponding spectra of the pinch plasma (fig. 7) and the laser producedplasma (fig. 8). First, the continuum intensity of the pinch plasma drops with increasing wavelength below2.3 nm. This is due to absorption by oxygen (fig. 4) gas in the spectrograph. It was used to preventreabsorption of the line pair of interest by the (neutral) nitrogen discharge gas. Second, the continuum in-tensity of the laser produced plasma exceeds that of the pinch plasma at wavelengths above 2.3 nm. This ison one hand attributed to increased line wing intensitIes caused by opacity broadening. On the other handintensity contributions due to free-free transitions are more significant than in the case of pinch plasmas,according to the higher density. Radiative recombination of boron, in contrast, is of minor importance inthe close spectral neighborhood of the line pair.

The analysis of the spectrum from the pinch plasma shown in Fig. 7 gives a ratio of line intensity overcontinuum intensity of 'L"C = 80 within X/&. = 30, or 'L'C = 500 if extrapolated to the actual bandwidthx/& = 200 (fig. 5). In contrast, 'L''C amounts to 2.7 (fig. 8) for the laser produced plasma, withinXI&t = 30 and to 'L'C = within the broadened line profile (fig. 6) of X/& = 170. The latter is consider-ably low in view of the requirements by x-ray imaging with zone plates41. As a result of transport mecha-nisms in combination with the temporal development of the plasma temperature, the source size generallydepends on wavelength. For this reason, measurements with simultaneous spectral and lateral resolution arenecessary to determine the integrated spectral brightness of the line pair. The above mentioned pinhole

10 jim, both Stark broadening and opacity broadening in resonant lines are possible candidates.

0.4

0.3

I:1.5 2.0 2.5 3.0

WAVELENGTH mm] WAVELENGTH (nm]

Fig. 6. Emission spectrum of a boron nitride laserproduced plasma. Due to line broadening theN VII is - 2p and the N VI is2 - ls3p are notresolved. The reciprocal relative bandwidth isreduced to XfEs) = 170.

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grating (2000 lines/mm) which forms a dispersed imae of the plasma source was exploited for this pur-pose. Its diffraction efficiency (10% into first order51'5") as well as the spectral response of the CCD detec-tor are well known so that the integrated spectral brightness can be measured absolutely. The source tograting and grating to CCD distances were chosen to achieve a simultaneous spectral and lateral resolutionof A/AX = 10 and & = 125 tim, sufficient to resolve the line pair of interest from neighbored lines accord-ing to Fig. 7. Laterally resolved spectra were recorded with single pulses. However, the lateral resolution isnot sufficient for laser produced plasmas, the size of which was expected to be close to the focal spot of thelaser (50 tim). In this case, calibrated condenser zone plates (KZP 5; 10% efficiency53'54) were used toimage the plasma, exploiting the wavelength dependence of the focal length to achieve a spectral resolutionof X/EsX = 15. The spatial resolution of & = 13 .tm was determined by the finite pixel size of the CCD atthe chosen magnification of 5.5. Spectrally resolved images of the source were also obtained with singlepulses. This zone plate spectrometer is not appropriate in case of the larger pinch plasma, as its spectralresolution decreases with increasing source size.

Fig. 7. Emission spectrum of a nitrogen pinchplasma measured with a 10000 lines/mmtransmission grating. Recombination radiationand emission lines below 2.33 nm are filtered byoxygen gas in the beam line. The line tocontinuum intensity ratio is 'L"C = 80 withinX/AX = 30.

Fig. 9 shows the integrated spectral brightness in the N VII is - 2p and the N VI is2 is3p line pair as afunction of radial position. The source profile of the pinch plasma corresponds to the highest average valueof 0.42 .iJ/.tm2/sr/pulse, that is reproducible within a standard deviation of less than 20%. It was obtainedat a pinch current of about 250 kA. A factor of 2 higher values can be achieved at the same device, how-ever, less reproducible. In contrast, the source profile for the laser produced plasma shows the highest inte-grated spectral brightness (0.28 tJ4im2/sr/pulse) that was observed throughout the experiments, at a laserpulse energy (20% reproducibility) of 9 J. According to these results, an integrated spectral brightness thatequals that of the pinch plasma (250 kA) will be achieved with comparable reproducibility at a laser pulseenergy of about 15 J. The conversion efficiency may be assumed to remain constant with the correspondent

to

0.$

line

continuum

NV11s2-1.2p

tO 2.0 3.0 4.0WAVELENGTH [nm]

5.0

20.0

.1

I::50

0.0

Fig. 8. Emission spectrum of a laser producedboron nitride plasma measured with a2000 lines/mm transmission grating. The line tocontinuum intensity ratio is = 2.7 withinXIEX = 30.

to 2.0 3.0 4.0 5.0WAVELENGTH (nm]

40/SPIE Vol. 2015

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increase of laser intensity, as will be discussed below.

The uncertainty which is related to the relative comparison of the integrated spectral brightness of bothsources is essentially determined by the CCD which served as detector standard in these experiments. Itamounts to about 10 % and includes systematical deviations caused by the different techniques to image thelaser produced and pinch plasma sources with simultaneous spectral resolution, i.e. the zone plate and thepinhole grating, respectively.

I °. Fig. 9. Integrated spectral brightness in theN VII is - 2p and the N VI is2 13p lines at

'. OA2.5 tim as a function of radial position for the

0.3 nitrogen pinch plasma (measured in axial_ direction of the pinch column) and for the laser

: produced boron nitride plasma (measured at an

. 0.1angle of 45° to the incident laser beam).

o.o.1000.0 400.0 0.0 500.0RADIAL POSITiON [ p.m]

As the diameter of the laser produced plasma source (70 j.tm) is slightly larger than the focal spot of50 im on the target, it is concluded that the plasma volume emitting the nitrogen line pair expands at avelocity of about iO° cm/s during the laser pulse. The total yield in the line pair is evaluated by integratingthe two dimensional source profiles and amounts to i .2 mJ/sr/pulse for the laser produced and to55 mJ/sr/pulse for the pinch plasma. This result agrees well (< 5% deviation) with the absolute yield ofi.3 mi/sr/pulse evaluated from the spectrum in Fig 8, which was recorded with the 2000 1/mm grating in-stead of the zone plate, at a laser pulse energy of 10.5 J. Similar to the broadband emission discussed insection 3. i, the yield from the pinch plasma exceeds that of the laser produced plasma by a factor of 45;here, mainly because of the larger emission profile of 400 .im diameter.

4.2 Microscopy performance

Using the pinch plasma source, images with sub-optical resolution of dehydrated specimen have beentaken with a single pulse40. From our comparative measurements no restraint appears to obtain the sameresult using the laser produced plasma, except the low line to continuum intensity ratio. To achieve higherresolution of wet specimen further improvements of the optical system in the microscope and the integratedspectral brightness of the sources are necessary. In this context, the question arises about limitations of bothplasma sources.

Relevant quantities in pinch plasma devices are the pinch current which determines the total amount ofenergy fed to the plasma, and the gas pressure which determines the number of emitters. If current andpressure are matched, the intgrated spectral brightness is expected to scale proportional to the pressure un-til opacity causes saturation4 . Fig. 10 shows the integrated spectral brightness as a result of a correspond-ing experimental optimization. The decrease above 600pascal (open circles) is due to the ignition mode ofthe gas discharge which turns from a homogeneous to a filamentary one46. As the saturation regime is not

SPIEVo!. 2015/41

1000.0

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yet reached, a further increase of the integrated spectral brightness with gas pressure is expected.

400.0 000.0

Pnftrog.n [P1

Fig. 10. Increase of the integrated spectralbrightness of the nitrogen pinch plasma with gaspressure. The drop of the brightness above 600 pa(open circles) is due to a disturbed gas discharge.

Fig. 11. Increase of the integrated spectralbrightness of laser produced boron nitrideplasmas with the laser pulse energy at constantpulse duration (3 ns) and constant spot diameter(50 .tm) on the target. The corresponding laserintensity incident on the target varies from 2* i0'to 2*10' W/cm2.

Fig. 1 1 shows the integrated spectral brightness of the laser produced plasma as a function of the inten-sity. The diameter of the focal spot on the target and the laser pulse duration were kept constant in thisexperiment, whereas the pulse energy was varied up to the maximum value of 10 J limited by the lasersystem. The integrated spectral brightness increases linearly with laser intensity in the accessible parameterrange. According to this result, a further improvement of the brightness with the laser pulse energy is to beexpected.

5. SUMMARY

In this paper, the two types of x-ray sources were evaluated from the viewpoint of two specific applica-tions. Both sources exhibit the potential to be optimized for the generation of broadband radiation as wellas for narrowband line emission with high integrated spectral brightness.

The comparison of the pinch plasma (250 kA) and the laser produced plasma (10 J pulse energy) withrespect to the integrated spectral brightness even gave similar results. Due to the larger source size both thetotal pulse yield and the pulse yield into one single line from the pinch plasma exceeded that of the laserproduced plasma by nearly two orders of magnitude. This fact, that higher yield is achieved with devices ofsimilar size and complexity, may be of importance for certain applications.

However, with respect to the decision about industrial applications, technical and financial aspects,rather than physical aspects, play the major role.

42 ISP1E Vol. 2015

T

0.5 NVII1s2p:X—25nrn .• o.o

0.4 ' 0.4

0.3 1

102 () 1020.1 • S-STABLE DISCHARGE MODE 0.1

0 DISTURBED DISCHARGE STABIUTY

0.00.0 200.0

I

000.0 1000.0 0.0

• NVII1s-2p:X'2.5nm

Laser: X - 532 nm, t - 3 nsFocal Spot: 50 jim

0.0 2.0 4.0 60 8.0 10.0

LASERB'IERGV EJI

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6. ACKNOWLEDGMENTS

The cooperation with the Karl SUss KG, Garching, the Carl Zeiss company, Oberkochen and theForschungseinrichtung Röntgenphysik, Universität Gottingen is gratefully acknowledged. We wish to ex-press our special thanks to the GOttingen group for providing calibrated zone plates and the 10000 I/mmtransmission grating, and to M. Krumrey and F. Scholze of the Physikalisch Technische Bundesanstalt(PTB Berlin) for their assistance and the possibility to calibrate the CCD detector at the BESSY electronstorage ring facility. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under con-tract number lie 1292/1-2, the Bundesministerium für Forschung und Technologie under contract numbers13N53290, 13N5680 and 13N5838, and the Commission of the European Community in the framework ofthe Association Euratom/IPP.

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