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A special method to create gratings of variable line density by low voltage electron beam lithography
MICROELECTRONIC ENGINEERING ELSEVIER
Microelectronie Engineering 30 (1996) 49-52
A special method to create gratings of variable line density by low voltage electron beam lithography B.Niemann a, T.Wilhein a, T. Schliebe a, R.Plontke b, O.Fortagne b, I. Stolbergb , M.Zierbock b aGeorg-Augnst-Universit~it G6ttingen,Forschungseinrichtung R6ntgenphysik, FRG, 37073 G6ttingen bJENOPTIK Technologie GmbH, FRG, 07739 Jena XUV radiation and soft X-rays can be focussed using optical elements that represent extremely smooth and planar substrates with diffracting structures on them. Optical elements of this kind can generally be described as reflection gratings with straight or curved lines of variable line density. Such arbitrary line shapes are easily generated with the help of electron beam lithography. Continuous path control applied in conjunction with data fracturing into BI~ZIER curves appears to be particularly suited for this purpose. The method avoids breaking the individual lines down into polygons, which implies an inevitably greater edge roughness. The use of low electron energies allows the individual grid lines to be generated by superpositioning of two path curves without any need to consider the proximity effect. The exposures were carried out on an electron beam lithography system LION-LV11. Measurements performed on BESSY for the first diffraction order showed the resulting optical element to have an efficiency of up to four percent depending on the particular wavelength.
1. I N T R O D U C T I O N Physical quantities of an X-ray emitting plasma source such as its spectrum, line width, effective source diameter, absolute photon flux in a given line are measured in X-ray diagnostics and help to derive other interesting parameters such as temperature, density, electric and magnetic field strengths in the emitting plasma. For use in X-ray plasma diagnostics an off-axis reflecting X-ray zone plate (ORZ) was developed, which can be used as a powerful X-ray spectrometer. The element can also be used as a simple Xray monochromator for use at an undulator X-ray source of an electron storage ring for X-ray photoelectron spectroscopy (XPS) of small specimen areas and in a transmission X-ray microscope (TXM) monochromator. An ealier design of a slitless X-ray monochromator 2 with a spectral resolution of about 600 consisted of a plane holographic grating to disperse the X-rays, followed by a holographic transmission zone plate, which delivered an image of the source with a few micron spatial resolution at a given Xray wavelength3,4, s. An improved version of this monochromator used a holographical grating with variable line
spacin~,s which avoided astigmatism in the X-ray ~mage. 2. T H E N E W X-RAY O P T I C A L A P P R O A C H It is possible to combine the two optical elements in a single one. An exact equivalent to these two is an off-axis transmission zone plate (OTZ), which possesses zones of about 10 mn line width and a zone height of more than 100 nm. Such periodic structures can not be manufactured. However, the projection of such an OTZ on a smooth reflecting plane delivers an ORZ, which is another equivalent of the two element optical arrangement. An ORZ can also be regarded as a reflection grating with curved lines. This grating has a variable line density which is typically in the micrometer range and has a zone height much smaller than its width. This ORZ combines imaging and spectral resolving properties in a single element. Fig. 1 shows a schematic ray diagram of a spectrometer setup using an OP,Z; only rays of the most efficient first diffraction order are shown. The X-rays impinge on the ORZ at a grazing angle c~ of a few degrees. Along the optical axis a series of focal points, each corresponding to a distinct wavelength, is obtained. The detector, a slow scan
0167-9317/96/$15.00 © 1996 - Elsevier Science B.V. All rights reserved. SSDI 0167-9317(95)00192-1
50
B. Niemann et al. I Microelectronic Engineering 30 (1996) 49-52
X-ray CCD camera 7, records the focussed image of a selected wavelength, other wavelengths are also recorded but with less spectral and spatial resolution.
SPECTRUll ON CCB ~ FOCALSPOTS - - - - - - - - - ~
REFLECTING
PlANs ~
o.,cA,
I/
AXIS- , " " / ~ / / / / / / ~,,' /// II/
ZONE PLATE FIGURE
Fig. 1. Schematic ray diagram of an ORZ It has to be stated that the achievable spot size of a focussing X-ray optic which uses curved mirrors is in general limited by the slope error of its surface and the reflectivity is degraded by the surface roughness. In practice monochromators with spot sizes of less than 10 lxm are very difficult to realize and the required mirrrors are extremely expensive. Substrates with low slope errors and little surface roughness can be manufactured best on plane substrates, e.g. from glass and are relatively inexpensive. Such a substrate is necessary to process a metal-on-glass ORZ.
4. P R A C T I C A L E X P O S U R E SETUP 4.1. Selection of exposure mode Because of the desired grid extension, preferen was given to continuous path control (CPC). contrast to stop-and-go operation, CPC eliminal stitching errors that may affect both periodicity edge smoothness adversely. In CPC mode, the sta is moving with a substrate on it, while the electr. beam remains undeflected in a first approximatic A closed control loop drives the beam tracki~ system to correct for discrepancies between t actual and the nominal position of the stage. As result, the workfield covers virtually the area of t: entire substrate. LION-LV1 provides this exposu mode as an integral feature 2. 4.2. Data format As for the data format, it appears mc reasonable to break up a particular structure iv BI~ZIER curves 8. The particular grid line can described with sufficient accuracy as part o f ; elliptical arc. Ellipses can be represented precise as BI~ZIER functions of the first order. This reduc to seven the number of numerical values required describe one elliptical arc, which saves an enormo amount of data. Based on this input, a dedicat, transputer controller calculates the required sta motion in on-line mode. 4.3. Solution to variable pattern wid requirement Depending on what level of primary energy selected, the e-beam system's probe will have minimum diameter between 2 tun and 5 nm. A Slc of such fineness is both unnecessary and inappr Line width
3.TASK DEFINITION FOR STRUCTURING
as f u n c t i o n o f l i n e d o s e L D = I / v , parameter defocus
900
Based on the above description, a structure is required that corresponds to a section of an elliptical zone plate in the form of a curved grating with grid lines and spaces of varied width. This, in turn, requires that the electron beam lithography tool to be used for structuring must be capable of writing a total of approx. 15,000 curved grid lines each about 1.0 nun long with < 10 nm edge roughness on an area of 3 mm x 8 mm, while concurrently varying the line width between 0.44 ~ma and 0.52 ~un an the line-to-line spacing from 0.49 pan to 0.591am. The tolerance for line width and space variation is not to exceed 10 rim.
600
==300 ° o
3
6
9
Une dose / nC/cm
Fig. 2. Actual line width as function of stage spec v (---~ line dose) and spot size (--->defocus: Each curve corresponds with another spo! size. priate for the given task. Preliminary tests ha' shown that, depending on the energy and the type
B. Niemann et al. / Microelectronic Engineering 30 (1996) 49-52
substrate, controlled defocussing and adaptation of the stage's travelling speed enable technologically processable line widths up to 1 Bm to be manufactured with a stepping of _< 10 nm. In principle, this would provide the conditions for achieving lines of the required width specifications. However, this setup would imply a disadvantage in that the stage's path speed and the defocused status of the electron beam would need to be readjusted for every single grid line. When working with low energy and specially adapted resist thickness, the inter-proximity effect disappears entirely and intra-proximity is much reduced. This, in turn, implies that overexposure will not widen the resulting structure to any significant extent. Accordingly, we decided to compose each individual grid line of two strips written at fixed defocus (0.3 Ima) and stage speed settings, but with different degrees of overlapping so the desired line width is finally achieved. This method simplifies the practical writing procedure significantly.
51
keV) for writing arrays each of 31 x 11 structural elements. As shown in Figure 2, the defocussing and the stage speed were adapted to vary the width which would be written for an individual line in the Y-direction (11 values). Each array element was exposed twice, with the shift between the two exposure runs increasing in steps of 10 nm along X.
Fig. 4. Array detail used for determining the proximity effect's influence (inner circle O 41am) All stra~ghtlined elements were displaced in the Xdirection and all circular elements in the radial direction. Following development, the structure was gold-sputtered and lifted. This ensured that only structures with a safely and entirely exposed resist layer were picked for evaluation. The remaining gold bars were measured for their width again by operating the LION-LV1 in measurement mode with automatic (non-operator) edge anlysis. Figure 5 represents the measured results for both Difference
between
actual
and theoretical
width
Fig. 3 Example for shape alterations necessary when single lines are to be shifted (LION exposure, total length 6.28 Bin) However, for data fractioning one must take into account that the two elliptical arcs required to write one curved grid line represent new functions derived from the initial function by displacing it orthogonally to its tangent. To verify the correctness of these assumptions, we selected various primary energies (2.5 keV, 5.0
E
line
2.5 kev
60 50 40
~
5.0 key
~
toleranoe +
20
~
tolerance
~. ao
-
-20 0
50
100
150
200
250
SHIFT / nm
Fig.5. Line width measurement results of the structures shown in Fig. 4.
300
52
B. Niemann et al. / Microelectronic Engineering 30 (1996) 49-52
energy settings. The shift between the two exposures is laid off along X, while the difference of the actual line width and the sum of the width of a given single line (0.3 Bm) and its shift is indicated along Y. A zero-shift means that two lines have been written one directly above the other. Consequently, the value plotted for a zero-shift provides a direct clue as to how much the line width increases with double dose. The diagram shows that the tolerance is kept, when the shift is _> 50 nm for 2.5 keV or _> 150 nm for 5.0 keV. The shifts required for exposing an ORZ vary between 0.14Bm and 0.22 pan so a primary energy setting of 2.5 keV appears best suited for this task. Since for an OP,Z layer stack (consisting of 20 nm germanium and 100 nm P M M A on glass, resist structure is transferred by RIE with CBrF3) the electron beam is required to structure a resist layer of only 100 nm thickness, and the electrons' penetration depth is about 180 nm at 2.5 keV, there are no obstacles to working at this energy level. 5. T H E P R O T O T Y P E The processed prototype of such an ORZ was designed for working with 1.5 m source distance and 3 m image distance at 2.4 nm wavelength.
indicates a spectral resolution of X/A)~ > 400 at 9~= 2.88 nm. 1° 6. F U R T H E R P O S S I B L E D E V E L O P M E N T S It is possible to increase the spectral resolutic of the ORZ by choosing different design paramete~ and by increasing the number of processed zor lines. In addition, as the height of the ORZ is only few millimeters, it is possible to put sever~ different ORZs on a single substrate next to eac other. With such an multiple ORZ element, it woul be possible to perform X-ray diagnostics of an ) ray source, which at the same time delivers tt source diameter and a high resolved spectrum different wavelengths on the CCD detector. ACKNOWLEDGEMENTS This work has been supported by the Germa Federal Ministry for Education and Researc (BMBF) under contract No. 13N6146. Data gener~ tion and preparation of exposures were performe by nanoCAD of aiss GmbH in Munich. Tt BI~ZIER algorithms were provided by EQUIcc GmbH in Jena. Assistance with the initial technolc gical investigations was also given by Mrs. C Christink of JENOPTIK Technology GmbH. Tt authors are especially indebted to the group heade by Dr. H. Hertz at the Laser Center of Lund/Swede where this X-ray source was designed, for co~ ducting the experiments on imaging and spectrc metering of a laser-generated plasma X-ray source. REFERENCES 1Bruenger, W. et al. Microelectronic Engineering 27
95) 135 ~ iemarm,B, et al., Applied Optics 15 (1976) 1883
Fig. 6. LION micrograph from the ORZ prototype It can also be used at other wavelengths from about 1 to 4 nm. Tests on the BESSY storage ring show that the prototype possesses up to 4% diffraction efficiency (depending on ix). Preliminary measurements on a droplet target laser-plasma source 9
Schraahl,G. and Rudolph,D, Optik 29 (1969) 577 iemalm,B, et al, Opt.Comm. 12 (1974) 160 ~Schraahl,G. and Rudolph,D. Prog.Optics 14 (1976) 195 6Rudolph,D. et al., X-ray Microscopy, Springer (1984) 192 7 . . . . Wdhem,T. et al., X-ray Microscopy 1V, eds. Erko,A,I. and Arkstov,V.V., Bogorodski Pechatnik Publishing Co., Chemogolovka, Russia, (1995) 8 Hoschek,J. and Lasser,D. tl Grundlagen der geometrischen Datenverarbeitung"(1992) B.G.Teubner StuttgaJ 9Rymell,L.,Berglund,M. and Hertz, I-LM., l.Phys.Lett. 66, 2625 (1995) ilhein,T.,Niemann,B.,Rymell,L, and Hertz,H,M., to be published
~
Microelectronie Engineering 30 (1996) 49-52
A special method to create gratings of variable line density by low voltage electron beam lithography B.Niemann a, T.Wilhein a, T. Schliebe a, R.Plontke b, O.Fortagne b, I. Stolbergb , M.Zierbock b aGeorg-Augnst-Universit~it G6ttingen,Forschungseinrichtung R6ntgenphysik, FRG, 37073 G6ttingen bJENOPTIK Technologie GmbH, FRG, 07739 Jena XUV radiation and soft X-rays can be focussed using optical elements that represent extremely smooth and planar substrates with diffracting structures on them. Optical elements of this kind can generally be described as reflection gratings with straight or curved lines of variable line density. Such arbitrary line shapes are easily generated with the help of electron beam lithography. Continuous path control applied in conjunction with data fracturing into BI~ZIER curves appears to be particularly suited for this purpose. The method avoids breaking the individual lines down into polygons, which implies an inevitably greater edge roughness. The use of low electron energies allows the individual grid lines to be generated by superpositioning of two path curves without any need to consider the proximity effect. The exposures were carried out on an electron beam lithography system LION-LV11. Measurements performed on BESSY for the first diffraction order showed the resulting optical element to have an efficiency of up to four percent depending on the particular wavelength.
1. I N T R O D U C T I O N Physical quantities of an X-ray emitting plasma source such as its spectrum, line width, effective source diameter, absolute photon flux in a given line are measured in X-ray diagnostics and help to derive other interesting parameters such as temperature, density, electric and magnetic field strengths in the emitting plasma. For use in X-ray plasma diagnostics an off-axis reflecting X-ray zone plate (ORZ) was developed, which can be used as a powerful X-ray spectrometer. The element can also be used as a simple Xray monochromator for use at an undulator X-ray source of an electron storage ring for X-ray photoelectron spectroscopy (XPS) of small specimen areas and in a transmission X-ray microscope (TXM) monochromator. An ealier design of a slitless X-ray monochromator 2 with a spectral resolution of about 600 consisted of a plane holographic grating to disperse the X-rays, followed by a holographic transmission zone plate, which delivered an image of the source with a few micron spatial resolution at a given Xray wavelength3,4, s. An improved version of this monochromator used a holographical grating with variable line
spacin~,s which avoided astigmatism in the X-ray ~mage. 2. T H E N E W X-RAY O P T I C A L A P P R O A C H It is possible to combine the two optical elements in a single one. An exact equivalent to these two is an off-axis transmission zone plate (OTZ), which possesses zones of about 10 mn line width and a zone height of more than 100 nm. Such periodic structures can not be manufactured. However, the projection of such an OTZ on a smooth reflecting plane delivers an ORZ, which is another equivalent of the two element optical arrangement. An ORZ can also be regarded as a reflection grating with curved lines. This grating has a variable line density which is typically in the micrometer range and has a zone height much smaller than its width. This ORZ combines imaging and spectral resolving properties in a single element. Fig. 1 shows a schematic ray diagram of a spectrometer setup using an OP,Z; only rays of the most efficient first diffraction order are shown. The X-rays impinge on the ORZ at a grazing angle c~ of a few degrees. Along the optical axis a series of focal points, each corresponding to a distinct wavelength, is obtained. The detector, a slow scan
0167-9317/96/$15.00 © 1996 - Elsevier Science B.V. All rights reserved. SSDI 0167-9317(95)00192-1
50
B. Niemann et al. I Microelectronic Engineering 30 (1996) 49-52
X-ray CCD camera 7, records the focussed image of a selected wavelength, other wavelengths are also recorded but with less spectral and spatial resolution.
SPECTRUll ON CCB ~ FOCALSPOTS - - - - - - - - - ~
REFLECTING
PlANs ~
o.,cA,
I/
AXIS- , " " / ~ / / / / / / ~,,' /// II/
ZONE PLATE FIGURE
Fig. 1. Schematic ray diagram of an ORZ It has to be stated that the achievable spot size of a focussing X-ray optic which uses curved mirrors is in general limited by the slope error of its surface and the reflectivity is degraded by the surface roughness. In practice monochromators with spot sizes of less than 10 lxm are very difficult to realize and the required mirrrors are extremely expensive. Substrates with low slope errors and little surface roughness can be manufactured best on plane substrates, e.g. from glass and are relatively inexpensive. Such a substrate is necessary to process a metal-on-glass ORZ.
4. P R A C T I C A L E X P O S U R E SETUP 4.1. Selection of exposure mode Because of the desired grid extension, preferen was given to continuous path control (CPC). contrast to stop-and-go operation, CPC eliminal stitching errors that may affect both periodicity edge smoothness adversely. In CPC mode, the sta is moving with a substrate on it, while the electr. beam remains undeflected in a first approximatic A closed control loop drives the beam tracki~ system to correct for discrepancies between t actual and the nominal position of the stage. As result, the workfield covers virtually the area of t: entire substrate. LION-LV1 provides this exposu mode as an integral feature 2. 4.2. Data format As for the data format, it appears mc reasonable to break up a particular structure iv BI~ZIER curves 8. The particular grid line can described with sufficient accuracy as part o f ; elliptical arc. Ellipses can be represented precise as BI~ZIER functions of the first order. This reduc to seven the number of numerical values required describe one elliptical arc, which saves an enormo amount of data. Based on this input, a dedicat, transputer controller calculates the required sta motion in on-line mode. 4.3. Solution to variable pattern wid requirement Depending on what level of primary energy selected, the e-beam system's probe will have minimum diameter between 2 tun and 5 nm. A Slc of such fineness is both unnecessary and inappr Line width
3.TASK DEFINITION FOR STRUCTURING
as f u n c t i o n o f l i n e d o s e L D = I / v , parameter defocus
900
Based on the above description, a structure is required that corresponds to a section of an elliptical zone plate in the form of a curved grating with grid lines and spaces of varied width. This, in turn, requires that the electron beam lithography tool to be used for structuring must be capable of writing a total of approx. 15,000 curved grid lines each about 1.0 nun long with < 10 nm edge roughness on an area of 3 mm x 8 mm, while concurrently varying the line width between 0.44 ~ma and 0.52 ~un an the line-to-line spacing from 0.49 pan to 0.591am. The tolerance for line width and space variation is not to exceed 10 rim.
600
==300 ° o
3
6
9
Une dose / nC/cm
Fig. 2. Actual line width as function of stage spec v (---~ line dose) and spot size (--->defocus: Each curve corresponds with another spo! size. priate for the given task. Preliminary tests ha' shown that, depending on the energy and the type
B. Niemann et al. / Microelectronic Engineering 30 (1996) 49-52
substrate, controlled defocussing and adaptation of the stage's travelling speed enable technologically processable line widths up to 1 Bm to be manufactured with a stepping of _< 10 nm. In principle, this would provide the conditions for achieving lines of the required width specifications. However, this setup would imply a disadvantage in that the stage's path speed and the defocused status of the electron beam would need to be readjusted for every single grid line. When working with low energy and specially adapted resist thickness, the inter-proximity effect disappears entirely and intra-proximity is much reduced. This, in turn, implies that overexposure will not widen the resulting structure to any significant extent. Accordingly, we decided to compose each individual grid line of two strips written at fixed defocus (0.3 Ima) and stage speed settings, but with different degrees of overlapping so the desired line width is finally achieved. This method simplifies the practical writing procedure significantly.
51
keV) for writing arrays each of 31 x 11 structural elements. As shown in Figure 2, the defocussing and the stage speed were adapted to vary the width which would be written for an individual line in the Y-direction (11 values). Each array element was exposed twice, with the shift between the two exposure runs increasing in steps of 10 nm along X.
Fig. 4. Array detail used for determining the proximity effect's influence (inner circle O 41am) All stra~ghtlined elements were displaced in the Xdirection and all circular elements in the radial direction. Following development, the structure was gold-sputtered and lifted. This ensured that only structures with a safely and entirely exposed resist layer were picked for evaluation. The remaining gold bars were measured for their width again by operating the LION-LV1 in measurement mode with automatic (non-operator) edge anlysis. Figure 5 represents the measured results for both Difference
between
actual
and theoretical
width
Fig. 3 Example for shape alterations necessary when single lines are to be shifted (LION exposure, total length 6.28 Bin) However, for data fractioning one must take into account that the two elliptical arcs required to write one curved grid line represent new functions derived from the initial function by displacing it orthogonally to its tangent. To verify the correctness of these assumptions, we selected various primary energies (2.5 keV, 5.0
E
line
2.5 kev
60 50 40
~
5.0 key
~
toleranoe +
20
~
tolerance
~. ao
-
-20 0
50
100
150
200
250
SHIFT / nm
Fig.5. Line width measurement results of the structures shown in Fig. 4.
300
52
B. Niemann et al. / Microelectronic Engineering 30 (1996) 49-52
energy settings. The shift between the two exposures is laid off along X, while the difference of the actual line width and the sum of the width of a given single line (0.3 Bm) and its shift is indicated along Y. A zero-shift means that two lines have been written one directly above the other. Consequently, the value plotted for a zero-shift provides a direct clue as to how much the line width increases with double dose. The diagram shows that the tolerance is kept, when the shift is _> 50 nm for 2.5 keV or _> 150 nm for 5.0 keV. The shifts required for exposing an ORZ vary between 0.14Bm and 0.22 pan so a primary energy setting of 2.5 keV appears best suited for this task. Since for an OP,Z layer stack (consisting of 20 nm germanium and 100 nm P M M A on glass, resist structure is transferred by RIE with CBrF3) the electron beam is required to structure a resist layer of only 100 nm thickness, and the electrons' penetration depth is about 180 nm at 2.5 keV, there are no obstacles to working at this energy level. 5. T H E P R O T O T Y P E The processed prototype of such an ORZ was designed for working with 1.5 m source distance and 3 m image distance at 2.4 nm wavelength.
indicates a spectral resolution of X/A)~ > 400 at 9~= 2.88 nm. 1° 6. F U R T H E R P O S S I B L E D E V E L O P M E N T S It is possible to increase the spectral resolutic of the ORZ by choosing different design paramete~ and by increasing the number of processed zor lines. In addition, as the height of the ORZ is only few millimeters, it is possible to put sever~ different ORZs on a single substrate next to eac other. With such an multiple ORZ element, it woul be possible to perform X-ray diagnostics of an ) ray source, which at the same time delivers tt source diameter and a high resolved spectrum different wavelengths on the CCD detector. ACKNOWLEDGEMENTS This work has been supported by the Germa Federal Ministry for Education and Researc (BMBF) under contract No. 13N6146. Data gener~ tion and preparation of exposures were performe by nanoCAD of aiss GmbH in Munich. Tt BI~ZIER algorithms were provided by EQUIcc GmbH in Jena. Assistance with the initial technolc gical investigations was also given by Mrs. C Christink of JENOPTIK Technology GmbH. Tt authors are especially indebted to the group heade by Dr. H. Hertz at the Laser Center of Lund/Swede where this X-ray source was designed, for co~ ducting the experiments on imaging and spectrc metering of a laser-generated plasma X-ray source. REFERENCES 1Bruenger, W. et al. Microelectronic Engineering 27
95) 135 ~ iemarm,B, et al., Applied Optics 15 (1976) 1883
Fig. 6. LION micrograph from the ORZ prototype It can also be used at other wavelengths from about 1 to 4 nm. Tests on the BESSY storage ring show that the prototype possesses up to 4% diffraction efficiency (depending on ix). Preliminary measurements on a droplet target laser-plasma source 9
Schraahl,G. and Rudolph,D, Optik 29 (1969) 577 iemalm,B, et al, Opt.Comm. 12 (1974) 160 ~Schraahl,G. and Rudolph,D. Prog.Optics 14 (1976) 195 6Rudolph,D. et al., X-ray Microscopy, Springer (1984) 192 7 . . . . Wdhem,T. et al., X-ray Microscopy 1V, eds. Erko,A,I. and Arkstov,V.V., Bogorodski Pechatnik Publishing Co., Chemogolovka, Russia, (1995) 8 Hoschek,J. and Lasser,D. tl Grundlagen der geometrischen Datenverarbeitung"(1992) B.G.Teubner StuttgaJ 9Rymell,L.,Berglund,M. and Hertz, I-LM., l.Phys.Lett. 66, 2625 (1995) ilhein,T.,Niemann,B.,Rymell,L, and Hertz,H,M., to be published
~