Direct Photo-Etching of PMMA by Focused EUV ...

Best Poster Award

Direct Photo-Etching of PMMA by Focused EUV Radiation From a Compact Laser Plasma Source Frank Barkusky*, Armin Bayer, Christian Peth, Klaus Mann Laser-Laboratorium Göttingen e.V., Hans-Adolf-Krebs-Weg 1, D-37077 Göttingen, Germany

Abstract In recent years technological developments in the area of extreme ultraviolet lithography (EUVL) have experienced great improvements. So far, there are already intense light sources based on discharge or laser plasmas, light guiding and imaging optics, and detection equipment. Currently, the application of EUV radiation apart from microlithography, such as metrology, high-resolution microscopy, or surface analysis comes more and more into focus. One objective is to make use of the strong interaction between soft x-ray radiation and matter for surface-near probing, modification or structuring techniques. In this contribution, along with first applications, we present a setup capable of generating and focusing EUV radiation, utilizing a table-top laser-induced plasma source. In order to obtain a focal spot of high EUV fluence, a modified Schwarzschild objective consisting of two spherical mirrors with Mo/Si multilayer coatings is adapted to this source. By demagnified (10x) imaging of the source an EUV spot of 30 µm diameter with an energy density of ~100 mJ/cm² is generated. We present first applications of this integrated source and optics system, demonstrating its potential for high-resolution modification and structuring of solid state surfaces. As an example, direct photo-etching of PMMA with resolution up to 130 nm will be displayed. In this context, the influence of so called “out-ofband radiation” to the etching depth of PMMA was determined by an EUV diffraction experiment. Moreover, the fragmentation of PMMA under influence of low-energy EUV radiation was investigated. For this reason the reflectivity of EUV irradiated PMMA was measured around the carbon K-edge using a table-top XUV reflectometer. This modified NEXAFS (near-edge x-ray absorption fine structure) setup in combination with FTIR (fourier transformation infrared) spectroscopy was used to identify changes in the chemical structure of the irradiated PMMA. Keywords: Schwarzschild objective, EUV source, EUV optics, 13.5 nm, laser-induced plasma, micro focus, Mo/Si, Photo-etching, PMMA, NEXAFS

Photon Processing in Microelectronics and Photonics VII, edited by Andrew S. Holmes, Michel Meunier, Craig B. Arnold, Hiroyuki Niino, David B. Geohegan, Frank Träger, Jan J. Dubowski, Proc. of SPIE Vol. 6879, 68791M, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.762553 Proc. of SPIE Vol. 6879 68791M-1 2008 SPIE Digital Library -- Subscriber Archive Copy

INTRODUCTION Extreme ultra-violet lithography (EUVL) can be regarded as the extension of optical lithography to shorter wavelengths and is one option for the semiconductor industry for Next Generation Lithography (NGL) systems.1 Using reflective imaging optics on the basis of multilayer mirrors, electronic devices with structure sizes well below 45 nm could be manufactured at a wavelength of 13.5 nm. For industrial lithographic systems EUV radiation with high average power is required, which can be generated either by laser or discharge based plasma sources. Such EUV sources and corresponding beam steering optics are currently being developed with tremendous effort.2, 3, 4 Besides semiconductor microlithography, there are also other applications of EUV radiation, which can strongly profit from the EUVL source and optics developments. E.g., the generation of color centers in LiF crystals was demonstrated,5 which can be used as laser-active medium,6 for data storage,7 or as a detector for EUV microscopy.8 In this paper we present EUV direct photo-etching obtained with a compact laser-produced EUV plasma source with integrated EUV Schwarzschild objective.9, 10, 11 Imaging of the laser plasma generated in a Xenon gas target leads to a high EUV pulse energy density (approximately 75 mJ/cm2, pulse width 6 ns) far away from the source, capable of structuring or modifying materials with micron resolution. Sample damage due to the influence of the hot plasma can be avoided, and, most important, the observed irradiation effects can be assigned to 13.5 nm radiation exclusively, since the Mo/Si multilayer mirror coatings of the Schwarzschild objective in combination with a zirconium filter are effectively blocking out-of-band radiation in the target plane. Together with a description of the setup and characterization of the integrated EUV source and optics system, results on EUV-induced photo-etching of PMMA are presented. Due to its opto-mechanical and chemical properties PMMA is an interesting candidate for micro-optical components like diffractive elements or waveguide structures12, as well as a photoresist for e-beam and EUV lithography.13

EXPERIMENTAL SETUP EUV source

The experimental setup consists of a laser-based EUV source9, 10, 14 and a separate optics chamber adapted to this source (cf.17). EUV radiation is generated by focusing a Nd:YAG laser (Innolas, wavelength 1064 nm, pulse energy 700 mJ, pulse length 6 ns) into a pulsed gas jet produced by a electromagnetically switched valve. The nozzle tip is located in the center of the vacuum chamber, which is evacuated to approximately 10-3 mbar to account for the low mean free path of EUV radiation at atmospheric pressure. The use of a laser-drilled conical nozzle guarantees a high particle density in the pulsed gas jet and, consequently, high quantum efficiency for EUV generation14. Xenon is employed as target gas, accomplishing broadband radiation in the wavelength range from 8 to 15 nm.9 The plasma can be monitored with a pinhole camera, consisting of a CCD chip with an EUV-to-VIS quantum converter and a pinhole (diameter 30 µm) coated with a zirconium foil (thickness approximately 200 nm) for blocking of out-of-band radiation.14 Table 1 shows selected properties of the employed EUV source as used in the described experiments. Wavelength (λ)

8 – 15 nm

Pulse width (τ)

6 ns

Energy (Q)

3 mJ (13.5 nm, 2 %BW, 4π sr)

Plasma diameter (d)

300 µm (FWHM, nearly spherical)

Repetition rate (fp)

1 - 10 Hz

Table 1. Output parameters of the laser-based EUV plasma source

Proc. of SPIE Vol. 6879 68791M-2

Schwarzschild objective The purpose of the employed EUV objective was to achieve a high EUV fluence in the focal plane by demagnified imaging of the source, combined with a high spatial resolution (several lines per micron) and a compact setup. Distance source – image plane

520.5 mm

Magnification (M)

0.102

Numerical aperture (NA)

0.4

Acceptance angle (Ω)

5.33 msr

Reflectivity of Mo/Si mirror coatings (R)

> 60 % per mirror @ 13.5 nm

Number of mirrors (n)

2

Spot size

~ 30 µm (source image) ~ 5 µm (imaging of 50 µm pinhole)

EUV energy density (H)

~ 73 mJ/cm2 (source image) < 37 mJ/cm2 (imaging of 50 µm pinhole)

Table 2. Specifications of EUV mirror objective and resulting focus parameters Table 2 summarizes optical parameters and mechanical dimensions of the mirror design, which has been described previously5. The alignment of the objective was performed at atmospheric pressure, using a Hartmann-Shack wavefront sensor in the visible spectral range15, 16. The dimensions of the entire EUV source and optics system including laser, power supplies, gas bottle and vacuum pump are 100 cm x 60 cm x 145 cm (W x D x H).

Schwarzschild objective

Pinhole caniera with quantum converter

—

EU\T plasma pinhole

sample

/

(for mask-imaging Zirconium foil only)

(spectral filter)

camera (VIS)

Fig. 1: Schematic drawing of the EUV source and optics system. The sample is mounted on a 3-axes translation stage. In order to achieve a flat-top EUV beam profile a 50 µm pinhole was positioned ~2 mm behind the plasma. Optional attenuation and spectral filtering is achieved using a mesh and / or a zirconium foil close to the pinhole.


By imaging the EUV plasma directly, a theoretical energy density of about 73 mJ/cm2 can be evaluated for an EUV source energy of 3 mJ and a source diameter of 300 µm. However, in order to generate a flat-toplike intensity profile, a pinhole (50 µm diameter) was positioned 2 mm behind the plasma and imaged by the objective5. Thus, by translating the sample into the image plane of the pinhole, a writing spot of 5 µm diameter with steep edges is obtained. Due to the reduced fluence on the pinhole, the maximum energy density in the image plane is approximately 2 times smaller than for direct imaging of the plasma. As samples PMMA plates (Goodfellow, thickness 1 mm, standard grade) were used. They were cleaned for five minutes in an ultra-sonic ethanol bath before exposure.

RESULTS Determination of ablation rates For determination of etch rates as a function of EUV energy density the surface of a PMMA sample (1 cm x 1 cm) was irradiated by different EUV energies and pulse numbers17. The depths of the generated ablation craters were determined by atomic force microscopy (Nanoscope III) in tapping mode. At lower densities (below 10 mJ/cm2) the crater depth is about proportional to the pulse number, whereas at higher densities nonlinearities are observed. These can, at least partly, be ascribed to deformations on the bottom of the crater, leading to an incorrect depth determination. The vaulted crater bottom can be explained by several different mechanisms. One is radiation-induced cross-linking of polymer chains, which could reduce the EUV etch rate of PMMA as discussed by Mocek et al.18. Another possibility is the attenuation of the radiation by the gaseous fragments of PMMA, leading to absorption of the trailing edge of the incoming nanosecond EUV pulse.

Fig. 2: PMMA etch rate as a function of EUV energy density. To obtain comparable energy densities the measurement without Zr filter was performed with a stainless steel attenuator mesh (25 µm wire diameter, 50 µm period, transmission 25 %). In order to investigate the influence of still existing out-of-band radiation, the experiment was repeated using an additional zirconium filter, leading to 13.5 nm radiation in the image plane exclusively (FWHW of 2 %, limited by the reflectance of the Mo/Si mirrors). With this setup, EUV energy densities between 2 and 13 mJ/cm2 can be obtained. Results are shown in Fig. 2. Both graphs show a nearly linear trend. Other than for corresponding excimer laser results19, there is obviously no indication of a threshold level in energy density. However, the etch rate for unfiltered radiation shows a larger slope than the zirconium filtered curve, which can be ascribed to the out-of-band radiation (cf. following paragraph).


EUV diffraction experiment The influence of out-of-band radiation was investigated in detail by conducting a diffraction experiment. In order to generate a diffraction pattern in the target plane, a steel mesh with 25 µm wire diameter and a period of 50 µm was positioned 5 mm in front of the objective. For this plane, the spatial coherence length of the EUV radiation emitted from the source can be calculated from the Van Zittert-Zernike theorem20. For a pinhole radius of 25 µm and a distance between pinhole and mesh of 385 mm, the spatial coherence length (1/e decay) is approximately 85 µm. Therefore, interference patterns originating from two mesh periods should be visible in the etch profile of an irradiated PMMA sample. The actual experiment was performed both with Zr filtered and unfiltered radiation. In Fig. 3, left, an AFM image of the PMMA surface topography is shown after applying 2000 EUV pulses (pulse energy 1.02 µJ @ 13.5 nm, unfiltered radiation). Clearly, a diffraction pattern is observed: besides the 0th order, also the orders 1, 1.1 and even faint 3rd order spots are visible, whereas the 2nd diffraction order is suppressed. The lattice constant of the periodic etch profile was determined to approximately 10.9 µm.

I! Fig. 3: Diffraction of EUV radiation from stainless steel mesh (20 lines/mm) positioned in the beam path (cf. Fig. 1). Left: AFM measurement of PMMA site after irradiation with 2000 pulses. Right: simulated intensity distribution in the image plane (cf. text). In order to simulate this experimental result, standard diffraction theory 21 was used, approximating the incident wavefront as a plane wave for simplification. The corresponding computed diffraction pattern is shown in Fig. 3 (right), indicating good qualitative agreement with the experiment, as documented in particular by the absence of 2nd order diffraction patterns. Taking the magnification of the mirror objective into account, the theoretical spacing between diffraction orders is 9.7 µm for 13.5 nm radiation. This fits well with the period obtained experimentally, proving the fact that the higher diffraction orders are indeed caused by EUV radiation. For a quantitative determination of the influence of out-of-band radiation on PMMA photo-etching, the depths of the generated diffraction orders obtained for filtered and unfiltered radiation are compared with the simulated intensities, assuming a linear relation between etch depth and the number of applied pulses (cf. Fig. 2). The result is shown in Fig. 4 (all patterns normalized to first diffraction order). Obviously, the diffraction pattern generated by filtered radiation correlates very well with the simulated intensity profile. The pattern generated without Zr filter only fits for the higher orders, which are generated by 13.5 nm radiation exclusively due to the much higher diffraction angles for (higher wavelength) out-of-band radiation. In contrast, the measured 0th order (which contains the whole incident spectrum) is approximately 25 % higher than the simulated one. Obviously, despite of spectral filtering of the incident radiation by the Mo/Si multilayer mirrors of the Schwarzschild objective, out-of-band radiation contributes by ~25% to the etch depth of PMMA. This could be explained by local heating of the sample due to out-of band-radiation, which leads to increased etch rates as demonstrated by A. Bartnik for etching of PTFE 22.


3,0

3,0 simulated intensity depth, with Zr filter depth, without Zr filter

2,5

2,5

2,0

'

1,5 .d I-

V a) 1,0

1,0 a.

. 05-

0,5

a) V

U)

2

3

diffraction order Fig. 4: Measured etch depths and simulated intensity of observed diffraction orders (both normalized to the first order). With Zr filter: only 13.5 nm radiation in sample plane (± 2 % FWHM, 8000 pulses, pulse energy 0.47 µJ), without Zr filter: radiation attenuated by stainless steel mesh, spectral filtering only by Mo/Si mirrors (EUV pulse energy 1.02 µJ, 2000 pulses).

XUV Analysis of irradiated PMMA samples In order to investigate the interaction of EUV radiation with PMMA, test samples were irradiated at EUV energy densities of 0.5 mJ/cm2 with different pulse numbers. The radiation induced changes were analyzed afterwards using NEXAFS in reflection mode as well as by FTIR (Fourier Transformation Infrared) spectroscopy. For the FTIR measurements a Bruker IFS 88 IR-spectrometer was used in ATR-mode (attenuated total reflectance). In this case the penetration depth of the evanescent wave is in the range of 0.5 – 2.5 micrometers for PMMA 23, in contrast to 15-30 nm for the NEXAFS measurement. For the experiments, a PMMA plate (Goodfellow, thickness 1mm) was cut into 10 x10 mm2 pieces and cleaned in ethanol. The samples were positioned 90 mm next to an EUV plasma (4 mJ, 2 % BW, 4π sr) and were irradiated with different numbers of EUV pulses. In order to block non-EUV radiation, a zirconium foil (thickness 220 nm) could be positioned in front of the sample, transmitting radiation between 6 nm and 20 nm (14, transmission approximately 50% at 13.5 nm). The experimental setup for the generation and characterization of soft x-ray radiation emitted from laser plasmas in the “water window” is shown schematically in Fig. 5. The XUV source is based on the same principle as the EUV source, using Krypton gas instead of Xenon, accomplishing broadband radiation (Kr XXV – Kr XXXVI) in the spectral range of the “water window”. Due to the small mean free path of the soft X-ray radiation at atmospheric pressure the target vacuum chamber is evacuated to approximately 104 mbar.


Fig. 5: Experimental setup of the laser-plasma XUV source used for NEXAFS experiments. The setup can be used in a transmission and reflection geometry. The insert on the left shows a picture of the krypton plasma taken by the pinhole camera. An XUV spectrometer (1 – 5 nm) was used both for the spectral investigation of the plasma source and the NEXAFS experiments. The spectrometer consists of a 100 µm entrance slit, an aberration corrected flatfield grating (Hitachi, 2400 lines/mm) and a back-side illuminated CCD camera (Roper Scientific, pixel size 13 µm). The resolution of this spectrometer was experimentally determined to be λ/∆λ ≈ 200 @ 2.87 nm. It was mounted 90° to the laser beam and opposite to the pinhole camera. To block visible radiation from the plasma and scattered laser light a titanium foil (200 nm thickness) was positioned between plasma source and sample. For adjustment in the XUV beam the samples were mounted on a rotary / linear motion stage. The distance between plasma source and sample is about 220 mm and the distance between sample and the entrance slit of the spectrometer is about 425 mm. The setup can be used for NEXAFS experiments both in transmission as well as in reflection mode. For the latter grazing incidence conditions are chosen (θ = 2°). NEXAFS measurements in reflection mode where carried out on the EUV irradiated samples after checking that the chemical structure of the sample is not changed during the NEXAFS experiments by recording several NEXAFS spectra of the same sample subsequently. A comparison of the energy dependent reflectivity for PMMA samples irradiated with different EUV pulse numbers is shown in Fig. 6. The characteristic peaks of the reflectivity can be assigned to C=C and C=O double bonds. It can be seen that with an increasing pulse number of EUV light the peak height of the C=O bond in the NEXAFS spectra decreases. This clearly indicates a loss of the carbonyl functional group that is easily radiation damaged24. Furthermore, the appearance of a new feature at 285 eV can be observed with increasing radiation dose. That energy corresponds to the C 1s → π* C=C transition indicating the formation of a C=C double bond, which is not part of the chemical structure of PMMA. This is an indication of crosslinking upon irradiation 24. It can be explained by the removal of the ester side chain, that was shown to have a linear stoichiometric relationship (1:1) with the formation of a C=C double bond25. The shift of the reflectivity minimum with increasing pulse number to lower photon energies can be attributed to a loss of oxygen and hydrogen atoms from the PMMA molecule that is consistent with the process leading to crosslinking 24.


I ,O

•

I

I

I

I

I

U,ö V

a)

0,4

0,2 — PMMA(unirradiated) 10.000 pulses (EUV only)

—20.000pulses(EUVonIy) •

OH, I

I

• 00 —2O.OOOpuIses(vthoutfiItering) '270 275 280 285 290 Photon energy [eV]

L

—in

•

295

300

Fig. 6: Measured XUV reflectivity as a function of photon energy for PMMA, irradiated with different numbers of EUV pulses (angle of incidence to the surface 2°).

Direct writing in PMMA In order to demonstrate the possibility of direct writing of small structures in PMMA, the Schwarzschild objective was used in the mask imaging mode, with a 10 µm pinhole behind the plasma. By translating the sample laterally with a piezo-driven manipulator, letters were written with an EUV spot of 1 µm diameter (cf. Fig. 9). The sample was moved approximately 1 µm between two positions, each site on the sample was exposed to 100 EUV pulses. The ablated area was visualized with the help of an AFM microscope.

Fig. 7: Direct writing of structures in PMMA by direct photo-etching with EUV radiation (AFM image). The EUV writing spot has a diameter of 1 µm. Each site was exposed to 100 pulses of approximately 25 mJ/cm². In order to test the maximum resolution of the setup, a mesh (period approximately 13 µm) was imaged onto a PMMA-based photo-resist. The mesh was positioned 5 mm behind the plasma to generate an object


with sharp edges. By placing a sample with a spin-coated photo-resist into the image plane of the mesh behind the objective, a structure with 130 nm steep edges could be generated (Fig. 8).

Fig. 8: Resolution test in photo-resist (AFM-micrograph). A mesh was placed behind the plasma to generate an object with sharp edges. The resist was exposed by 50 EUV pulses with energy densities of approximately 20 mJ/cm2. The section (right) shows an edge width of 130 nm, which indicates that the possible resolution of the setup is better than 150 nm.

CONCLUSION The observed PMMA surface modification presented in this paper can be assigned to 13.5 nm radiation exclusively due to the effective blocking of out-of-band radiation in the employed optical setup. For this reason, as to our knowledge, the results represent the first systematic investigations on photo-induced etching in the EUV spectral range. Etch rates were determined to be in the range of 1 angstrom per EUV pulse at energy densities of 13 mJ/cm2. In contrast to excimer laser ablation of PMMA with nanosecond pulses, neither an energy threshold nor the need for incubation pulses was observed. This can be attributed to the much higher photon energy (92 eV for λ=13.5 nm as compared to 5 eV for λ=248 nm), by far exceeding the bonding energy within the PMMA polymer (several eV). Thus, a single EUV photon is capable of breaking a large number of PMMA bonds. In order to analyze the contribution of “out-of-band” radiation (especially above a wavelength of 100 nm) to PMMA etching, rates were determined both with and without zirconium filtering. For unfiltered radiation the rates were found to be about 20 - 30 % higher due the influence of out-of-band radiation. This result was confirmed by a diffraction experiment; here, the spectral discrimination was not achieved by filtering, but by diffraction of the incident radiation from a periodic mesh positioned in front of the Schwarzschild objective. This leads to the formation of a pronounced diffraction pattern in the surface topography of the ablated PMMA sample. While the 0th order of this structure is caused by light of the entire wavelength spectrum, the higher orders can be attributed to 13.5 nm radiation exclusively, as documented by the excellent agreement with the simulation. NEXAFS measurements on EUV irradiated PMMA using a table top XUV source indicate that the radiation causes a reduction of the C=O double bond. Furthermore, an increase of the C=C bond could be observed at higher doses, which is a strong indication for cross-linking in the irradiated polymer.

ACKNOWLEDGEMENT The authors like to thank the Bundesministerium für Bildung und Forschung for financial support within the project “KOMPASS” (Kompakte Strahlquelle hoher Brillanz für den weichen Röntgen-


Spektralbereich), and the Deutsche Sonderforschungsbereich SFB755.

Forschungsgemeinschaft

for

support

within

the

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