- Email: [email protected]
The energy-dispersive reflectometer at BESSY II: a challenge for thin film analysis
Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1077–1080
The energy-dispersive reflectometer at BESSY II: a challenge for thin film analysis U. Pietscha,*, J. Grenzera, Th. Geuea, F. Neissendorferb, G. Brezsesinskic, c . Ch. Symietzc, H. Mohwald , W. Gudatd a
. Potsdam, Am Neuen Palais 10, 14415 Potsdam, Germany . Physik, Universitat Institut fur b Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany c . . Kolloid- und Grenzflachenforschung, MPI fur 14476 Golm, Germany d BESSY GmbH, Albert- Einsteinstrae 15, 12489 Berlin, Germany
Abstract Installed in 1999 the energy-dispersive reflectometer at the 13.2 bending magnet employs the exponentially decaying white X-ray emission spectrum of the 1.7 GeV storage ring of BESSY II outside the vacuum. Using an energy-dispersive detector specular and longitudinal-diffuse reflectivity spectra of thin films can be recorded simultaneously between ( 1 5qz51.2 A ( 1 within a few seconds. 0.2 A The capability of the experiment is demonstrated probing the correlation length of Cd-arachidate and -stearate films at room temperature and its change during annealing. At T=708C we observe an instantaneous decay of specular Bragg peaks accompanied with an increase of the diffuse scattering. This indicates the onset of the melting of 2Dordered acid domains. The vertical diffusion coefficient is estimated to be about 2 1024 m2/s. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Energy-dispersive reflectometry; Synchrotron radiation; X-ray diffuse scattering
1. Introduction A white beam experiment is based on the simplest set-up of a beamline. Emitted from the synchrotron source the incident beam is tailored by different slits without passing another optical element. This highly collimated and brilliant beam can be used to perform an energy-dispersive experiment. In 1999 the authors group installed *Corresponding author. Tel.: +49-331-977-1286; fax: +49331-977-1133. E-mail address: [email protected] (U. Pietsch).
an energy-dispersive reflectometer outside the BESSY II vacuum system. This instrument is an extension of a setup running previously at the wave-length shifter of BESSY I [1]. Now the experiment exploits the exponentially decaying hard X-ray tail of the 1.7 GeV emission spectrum of a bending magnet of BESSY II. The upper limit of useable energy is about 30 keV. The low energy limit at about 4 keV originates from the energydependent absorption of the beryllium exit window. Using a typical slit widths of 0.2 1 mm2 and a beam current of 100 mA the incident beam intensity exceeds 1010 cps. This is similar in
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 7 3 0 - 6
1078
U. Pietsch et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1077–1080
intensity which was available at the wave-length shifter of BESSY I, but the new set-up profits from the higher content of hard X-rays. One of the possible applications of the instrument is the measurement of the X-ray reflectivity from thin films. Here the incident beam strikes the sample surface at a fixed angle a with respect to the sample surface and is recorded at the detector angle 2a Additionally the diffuse component of scattering becomes measurable detuning a by a small offset Da for a fixed 2a. In contrast to an angular-dispersive set-up the entire reflectivity spectrum appears simultaneously which enables in situ inspection of structural variations induced by an external perturbation. In this paper, we demonstrate the capability of the instrument for probing structural variations of thin organic films by annealing in particular the ‘‘lateral melting’’ of 2D-ordered domains [2,3].
2. Experimental realization The experimental set-up used is similar to that published recently [1]. Instead of a Si : Li detector we used a new silicon drift-diode detector which allows detection of photon numbers in the order of 2 105 cps. Using reciprocal space co-ordinates the vector qz is directed parallel to the surface normal, while qx is perpendicularly directed. Both can be expressed by experimental parameters, the incident angle a and the misalignment angle Da qz aE
qx DaaE
tion length L can be determined from the halfwidth Dqx =2p/L using I(Da). Fig. 1 shows typical reflectivity spectra of a Cd-arachidate film at room temperature setting 2a=38. Structural information is provided in the energy range between 4 keV5E530 keV. The intensity of the 1st Bragg peak is reduced due to a thin aluminium foil as absorber attached in front of the detector. The overall reflectivity decays proportional to a function [1] I expð aEÞ expð bE 3 ÞRðE; DÞ:
ð2Þ
The first two terms describe BESSY incident spectrum, the second term considers the absorption by the beryllium window and by air. a and b are fitting parameters (here a=0.4 (keV)1 and b=400 (keV)3). R(E, D) stands for Fresnel’s reflectivity of the film. Fig. 2 shows the variation of the first four Bragg peaks versus qx extracted from the respective longitudinal-diffuse scans shown in ( 1 the decay of Fig. 1. In the range 05qx5104 A intensity is controlled by the energy-dependent instrumental resolution, mainly determined by the ( 1 width of the detector window. For qx>104 A I(qx) represents the true diffuse scattering of the sample. The curves approximately equals for different qz. L is in the order of 5 mm. As a next step we probed the structural variations of Cd-stearate films as a function of annealing temperature. From angular-dispersive experiments it is known that such a multilayer
ð1Þ
( keV). qx,z is given in making use of 4pe/hc 1/(A ( 1 when E is given in keV. A specular scan A measures the intensity distribution along qz for qx=0. For a multilayer sample Bragg peaks appear at whenever qz=n2p/D, where D is its vertical spacing. Longitudinal-diffuse scans run oblique to the qz axis turned by Da. They pass the qx-tails of the Bragg peaks which are originated by the vertical correlation of the interface roughness [3]. The qx-dependence of the Bragg peak can be determined plotting the peak intensities I as a function of Da at fixed E. Depending on the Bragg order n and considering Eq. 1 these curves represent different qx ranges. The lateral correla-
Fig. 1. Specular and diffuse scattering spectra of a Cdarachidate film at room temperature.
U. Pietsch et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1077–1080
1079
Fig. 2. qx-dependence of Bragg peaks as shown in Fig. 2. The lines are drawn as a guide for the eyes.
Fig. 3. Time-evolution of the 2nd order specular and diffuse Bragg peaks of a Cd-stearate films at T=708C for different Dqx .
structure undergoes two subsequent phase transitions before melting, the first one at about 708C and the second one between 1008C and 1108C [4,5]. For fixed temperatures and 2a=3.38 a set of specular and longitudinal-diffuse scans was recorded subsequently for increasing Da using a counting time of 60 s each. All scans were normalised to the primary beam intensity measured by a monitor straight behind the beryllium exit window. Before starting the next cycle of scans the specular scattering condition was readjusted to compensate irregularities during the experiment. The procedure provided a time resolution of about 10–20 min for inspecting both the specular and diffuse scattering. Caused by radiation damage we observed a tiny decrease of specular intensities already at about 508C but no change of the diffuse scattering. The behaviour changes instantaneously increasing the temperature to 708C. Fig. 3 shows the time evolution of the second order Bragg peak for different Dqx. Within the time of inspection the specular intensity is reduced by about one order of magnitude accompanied by an increase of the amount of the diffuse scattering. The latter one corresponds to a decrease of L down to 2 mm indicating a ‘‘melting’’ of 2D-ordered domains. Between 05t5108 min the specular intensity decays with a time constant of ( 1 this corresponds t=0.026 min1. At q=0.24 A
to a vertical diffusion coeffiecient D=(q2t)–1 2 1024 m2/s.
3. Discussion As shown in Fig. 1 the intensities of the first Bragg peaks exceed 107 cps. A counting time of one second is sufficient to record a scattering spectrum between 4 keV5E520 keV. Considering the time necessary for data read-out, time dependent measurements can be realised on the time scale of a few seconds per spectrum. For the determination of the lateral correlation function it is advantageous to measure high order Bragg peaks. This can be realised by attenuation of the low order Bragg peaks using absorbers accompanied with a higher flux provided by increasing slit widths. This is in contradiction with the resolution requirements which limits the tolerable beam divergence. For the present experiment we used receiving slits of about 100 and 200 mm before and after the sample corresponding to an angular divergence of Dai 6 mrad and Daf 0.4 mrad. Due to the small slit widths the experiment is highly sensitive for any fluctuation of beam. In future the measurement are planned to perform under protecting atmosphere to prevent radiation damage (ozone production) at low sample temperatures.
1080
U. Pietsch et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1077–1080
Acknowledgements The authors thanks BESSY for the possibility to install the beamline. One of us (FN) thanks HSPIII for financial support.
References . [1] F. Neissendorfer, U. Pietsch, G. Brezesinski, H. Mohwald. Meas. Sci. Technol. 10 (1999) 354.
. [2] Th. Geue, M. Schultz, U. Englisch, R. Stommer, U. Pietsch, K. Meine, D. Vollhardt, J. Chem. Phys. 110 (1999) 8104– 8111. . [3] R. Stommer, U. Englisch, U. Pietsch, V. Holy, Physica B 221 (1996) 284. [4] A. Bolm, U. Englisch, F. Penacorada, M. Gerstenberg, U. Pietsch, Supramol. Sci. 4 (1997) 229. . [5] W. Mahler, T.A. Barberka, U. Pietsch, U. Hohne, H.J. Merle, Thin Solid Films 256 (1995) 198.
The energy-dispersive reflectometer at BESSY II: a challenge for thin film analysis U. Pietscha,*, J. Grenzera, Th. Geuea, F. Neissendorferb, G. Brezsesinskic, c . Ch. Symietzc, H. Mohwald , W. Gudatd a
. Potsdam, Am Neuen Palais 10, 14415 Potsdam, Germany . Physik, Universitat Institut fur b Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany c . . Kolloid- und Grenzflachenforschung, MPI fur 14476 Golm, Germany d BESSY GmbH, Albert- Einsteinstrae 15, 12489 Berlin, Germany
Abstract Installed in 1999 the energy-dispersive reflectometer at the 13.2 bending magnet employs the exponentially decaying white X-ray emission spectrum of the 1.7 GeV storage ring of BESSY II outside the vacuum. Using an energy-dispersive detector specular and longitudinal-diffuse reflectivity spectra of thin films can be recorded simultaneously between ( 1 5qz51.2 A ( 1 within a few seconds. 0.2 A The capability of the experiment is demonstrated probing the correlation length of Cd-arachidate and -stearate films at room temperature and its change during annealing. At T=708C we observe an instantaneous decay of specular Bragg peaks accompanied with an increase of the diffuse scattering. This indicates the onset of the melting of 2Dordered acid domains. The vertical diffusion coefficient is estimated to be about 2 1024 m2/s. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Energy-dispersive reflectometry; Synchrotron radiation; X-ray diffuse scattering
1. Introduction A white beam experiment is based on the simplest set-up of a beamline. Emitted from the synchrotron source the incident beam is tailored by different slits without passing another optical element. This highly collimated and brilliant beam can be used to perform an energy-dispersive experiment. In 1999 the authors group installed *Corresponding author. Tel.: +49-331-977-1286; fax: +49331-977-1133. E-mail address: [email protected] (U. Pietsch).
an energy-dispersive reflectometer outside the BESSY II vacuum system. This instrument is an extension of a setup running previously at the wave-length shifter of BESSY I [1]. Now the experiment exploits the exponentially decaying hard X-ray tail of the 1.7 GeV emission spectrum of a bending magnet of BESSY II. The upper limit of useable energy is about 30 keV. The low energy limit at about 4 keV originates from the energydependent absorption of the beryllium exit window. Using a typical slit widths of 0.2 1 mm2 and a beam current of 100 mA the incident beam intensity exceeds 1010 cps. This is similar in
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 7 3 0 - 6
1078
U. Pietsch et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1077–1080
intensity which was available at the wave-length shifter of BESSY I, but the new set-up profits from the higher content of hard X-rays. One of the possible applications of the instrument is the measurement of the X-ray reflectivity from thin films. Here the incident beam strikes the sample surface at a fixed angle a with respect to the sample surface and is recorded at the detector angle 2a Additionally the diffuse component of scattering becomes measurable detuning a by a small offset Da for a fixed 2a. In contrast to an angular-dispersive set-up the entire reflectivity spectrum appears simultaneously which enables in situ inspection of structural variations induced by an external perturbation. In this paper, we demonstrate the capability of the instrument for probing structural variations of thin organic films by annealing in particular the ‘‘lateral melting’’ of 2D-ordered domains [2,3].
2. Experimental realization The experimental set-up used is similar to that published recently [1]. Instead of a Si : Li detector we used a new silicon drift-diode detector which allows detection of photon numbers in the order of 2 105 cps. Using reciprocal space co-ordinates the vector qz is directed parallel to the surface normal, while qx is perpendicularly directed. Both can be expressed by experimental parameters, the incident angle a and the misalignment angle Da qz aE
qx DaaE
tion length L can be determined from the halfwidth Dqx =2p/L using I(Da). Fig. 1 shows typical reflectivity spectra of a Cd-arachidate film at room temperature setting 2a=38. Structural information is provided in the energy range between 4 keV5E530 keV. The intensity of the 1st Bragg peak is reduced due to a thin aluminium foil as absorber attached in front of the detector. The overall reflectivity decays proportional to a function [1] I expð aEÞ expð bE 3 ÞRðE; DÞ:
ð2Þ
The first two terms describe BESSY incident spectrum, the second term considers the absorption by the beryllium window and by air. a and b are fitting parameters (here a=0.4 (keV)1 and b=400 (keV)3). R(E, D) stands for Fresnel’s reflectivity of the film. Fig. 2 shows the variation of the first four Bragg peaks versus qx extracted from the respective longitudinal-diffuse scans shown in ( 1 the decay of Fig. 1. In the range 05qx5104 A intensity is controlled by the energy-dependent instrumental resolution, mainly determined by the ( 1 width of the detector window. For qx>104 A I(qx) represents the true diffuse scattering of the sample. The curves approximately equals for different qz. L is in the order of 5 mm. As a next step we probed the structural variations of Cd-stearate films as a function of annealing temperature. From angular-dispersive experiments it is known that such a multilayer
ð1Þ
( keV). qx,z is given in making use of 4pe/hc 1/(A ( 1 when E is given in keV. A specular scan A measures the intensity distribution along qz for qx=0. For a multilayer sample Bragg peaks appear at whenever qz=n2p/D, where D is its vertical spacing. Longitudinal-diffuse scans run oblique to the qz axis turned by Da. They pass the qx-tails of the Bragg peaks which are originated by the vertical correlation of the interface roughness [3]. The qx-dependence of the Bragg peak can be determined plotting the peak intensities I as a function of Da at fixed E. Depending on the Bragg order n and considering Eq. 1 these curves represent different qx ranges. The lateral correla-
Fig. 1. Specular and diffuse scattering spectra of a Cdarachidate film at room temperature.
U. Pietsch et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1077–1080
1079
Fig. 2. qx-dependence of Bragg peaks as shown in Fig. 2. The lines are drawn as a guide for the eyes.
Fig. 3. Time-evolution of the 2nd order specular and diffuse Bragg peaks of a Cd-stearate films at T=708C for different Dqx .
structure undergoes two subsequent phase transitions before melting, the first one at about 708C and the second one between 1008C and 1108C [4,5]. For fixed temperatures and 2a=3.38 a set of specular and longitudinal-diffuse scans was recorded subsequently for increasing Da using a counting time of 60 s each. All scans were normalised to the primary beam intensity measured by a monitor straight behind the beryllium exit window. Before starting the next cycle of scans the specular scattering condition was readjusted to compensate irregularities during the experiment. The procedure provided a time resolution of about 10–20 min for inspecting both the specular and diffuse scattering. Caused by radiation damage we observed a tiny decrease of specular intensities already at about 508C but no change of the diffuse scattering. The behaviour changes instantaneously increasing the temperature to 708C. Fig. 3 shows the time evolution of the second order Bragg peak for different Dqx. Within the time of inspection the specular intensity is reduced by about one order of magnitude accompanied by an increase of the amount of the diffuse scattering. The latter one corresponds to a decrease of L down to 2 mm indicating a ‘‘melting’’ of 2D-ordered domains. Between 05t5108 min the specular intensity decays with a time constant of ( 1 this corresponds t=0.026 min1. At q=0.24 A
to a vertical diffusion coeffiecient D=(q2t)–1 2 1024 m2/s.
3. Discussion As shown in Fig. 1 the intensities of the first Bragg peaks exceed 107 cps. A counting time of one second is sufficient to record a scattering spectrum between 4 keV5E520 keV. Considering the time necessary for data read-out, time dependent measurements can be realised on the time scale of a few seconds per spectrum. For the determination of the lateral correlation function it is advantageous to measure high order Bragg peaks. This can be realised by attenuation of the low order Bragg peaks using absorbers accompanied with a higher flux provided by increasing slit widths. This is in contradiction with the resolution requirements which limits the tolerable beam divergence. For the present experiment we used receiving slits of about 100 and 200 mm before and after the sample corresponding to an angular divergence of Dai 6 mrad and Daf 0.4 mrad. Due to the small slit widths the experiment is highly sensitive for any fluctuation of beam. In future the measurement are planned to perform under protecting atmosphere to prevent radiation damage (ozone production) at low sample temperatures.
1080
U. Pietsch et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1077–1080
Acknowledgements The authors thanks BESSY for the possibility to install the beamline. One of us (FN) thanks HSPIII for financial support.
References . [1] F. Neissendorfer, U. Pietsch, G. Brezesinski, H. Mohwald. Meas. Sci. Technol. 10 (1999) 354.
. [2] Th. Geue, M. Schultz, U. Englisch, R. Stommer, U. Pietsch, K. Meine, D. Vollhardt, J. Chem. Phys. 110 (1999) 8104– 8111. . [3] R. Stommer, U. Englisch, U. Pietsch, V. Holy, Physica B 221 (1996) 284. [4] A. Bolm, U. Englisch, F. Penacorada, M. Gerstenberg, U. Pietsch, Supramol. Sci. 4 (1997) 229. . [5] W. Mahler, T.A. Barberka, U. Pietsch, U. Hohne, H.J. Merle, Thin Solid Films 256 (1995) 198.