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Holographically produced transmission diffraction gratings for soft X-rays
Nuclear Instruments and Methods 172 (1980) 293-296 © North-Holland Publishing Company
HOLOGRAPHICALLY
PRODUCED
TRANSMISSION
DIFFRACTION
GRATINGS
FOR SOFT X-RAYS
*
E.T. A R A K A W A ** and Paul J. CALDWELL *** Oak Ridge National Laboratory, Oak Ridge, Tenn., U.S.A.
A method for producing self-supporting transmission gratings for the soft X-ray region will be discussed. Photoresist is spin-coated onto a glass substrate and then exposed to the interference pattern created by two coherent point sources. The sinusoidal relief pattern which is formed after development is coated at normal incidence with a suitably transparent metal by vacuum evaporation. An additional layer of opaque metal is added obliquely (i.e., to one side of the sir~usoidalprofile) to provide the necessary wavefront amplitude modulation. After coating, a sample holder is glued to the surface of the metal and the photoresist is dissolved, freeing the metal layer from the glass substrate. Self-supported gratings of aluminum with opaque silver strips have been tested at 240, 302,434, and 605 A. At certain angles of incidence, the zeroth order has practically disappeared with most of the intensity appearing in one order.
1. I n t r o d u c t i o n
(fig. 1). A thin layer o f photoresist (Shipley AZ1350J) which had been spin-coated onto a carefully cleaned glass microscope slide [3] was exposed to the interference pattern created by the intersection o f two coherent monochromatic wavefronts. Using the 200 roW, 4579 A emission line o f an Argon ion CW laser, and 25/Jm pinholes, a power density of 1.8 mW cm -2 was obtained at the sample table. No collimating lenses were used to avoid wavefront distortion. With the sample placed about a meter from the pinholes, the wavefronts were approximately planar, giving constant groove spacing. The angle between the intersecting beams was 29 °, resulting in a line density of about 2100 1 m m -1. Samples were exposed for
The advantages, such as high throughput, light weight, and stigmatic imaging properties o f the transmission grating in the soft X-ray region as compared with grazing incidence reflection gratings currently in use make it attractive for some special applications. Transmission gratings o f the type described b y K/illne et al. [1] and Schnopper et al. [2] have been successfully employed in the High Energy Astronomical Observatory (HEAO-B) and the ApoUo Telescope Mount for non-solar cosmic and solar X-ray studies, respectively. These gratings have also been suggested for use in m o n o c h r o m a t o r systems for synchrotron radiation facilities. This paper outlines a new method o f prod/icing transmission gratings and the test results obtained in the soft X-ray region.
Argon Ion Laser
1~
---5, I [ i I
2. Grating production A sinusoidal profile was created on the surface o f photoresist by the standard holographic technique
/'_" :_---- - - - - -
pinhole
14
]
+-~ l
* Research sponsored by the Office of Health and Environmental Research, US Department of Energy, under contract W-7405-eng-26 with the Union Carbide Corporation. ** Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 USA. *** Southern College and University Union participant from Southwestern at Memphis, Memphis, Tennessee 38112 USA.
t ¢
t"~.
/"
|,-.
z= k. x...l~..so~ple table /
/-
/ Fig. 1. Holographic exposure arrangement. 293
VI. PERFORMANCE OF GRATINGS AND MIRRORS
E.T. Arakawa, P.J. Caldwell / Transmission diffraction gratings
294 exposed photoresist
Microscope slide coated,
Dissolve exposed photoresist
with photoresist and
to
expose<~ to interference
surf=ce profile.
leeve sinusoidol
field.
Fig. 2. Holographic production of diffraction gratings in photoresist.
120 s and then developed for 25 s in a 64o-1 dilution of AZ-303A at 21°C and oven dried for 30 min at 95°C (fig. 2). This process created a groove depth of about 1500 A as determined by scanning electron microscopy. The sinusoidal profile formed in the photoresist can be used directly as a transmission grating or coated with metal and used as a reflection grating. For our purposes, this prof'lle was used as a
Support layer normally.
deposited
mold for a metallic transmission grating. As shown in fig. 3, a uniform layer o f aluminum was deposited by vacuum evaporation normal to the surface. This layer served as a support and was fairly transparent in the soft X-ray region. To produce the grating strips, silver was deposited at an angle between 60 ° and 70 ° from the normal such that only one side of the sinusoidal profile was coated. The thickness of the aluminum layer which could support the mass of the silver strips in a free-standing mode was found to be around 1000 A. Gratings with silver strips of ! 0 0 - 7 0 0 A in thickness were tested. Before removing the metallic grating from the surface of the photoresist, a sample holder with a 9 m m diameter hole in the center was glued to the surface. The sample was then immersed in acetone for several hours to dissolve the photoresist and the grating was then lifted vertically from the acetone bath. The hole in the holder provided 64 mm 2 of free-standing metallic grating. Several gratings were made with 80% transmitting screen mesh on the holder to produce a more robust structure.
//// Grating strips
added
obliquely.
PhotoreSist metallic grating
Fig. 3. Metallic transmission grating production.
dissOlved,
transmission removed.
E.T. Arakawa, P.J. Caldwell
Transmission diffraction gratings
295
q~o
A slight distortion of the grating due to the removal process was noted. In one instance, a grating formed with a groove spacing of 4700 A was found to have a groove spacing of 4655 A after removal from the photoresist surface. This distortion differed from grating to grating.
o
~= - 2 0 °
3. Test results Several gratings were tested at soft X-ray wavelengths using a condensed spark source with a 2.2 m grazing incidence monochromator. As illustrated in fig. 4, the experimental chamber allowed the angle of incidence (~0) to be varied about an axis paralel to the grooves and a photomultiplier coated with sodium salicylate to scan the diffracted beams (0). In fig. 5 are shown scans of the dispersion of 320.4 )k radiation by a grating with 2100 1 mm -1 which showed a symmetrical q~= 0 pattern. Changing the angle of incidence to the +~ direction is seen to increase the intensity of the - 1 order while rotation in the -~b direction causes an increase in the +1 order, with a decrease o f the zero order in both cases. The effect of changing incidence angle for a grating which shows a partfal blaze effect at 4)---0 (fig. 6) reveals several interesting features. Of particular interest is the scan for ~b= 50 °. In this case, the zero order is nearly eliminated whereas a substantial
J\ I0
0
J
,
,~,,
-I0
__1
.... +/0
I ....
I ....
~ ....
0
,,,,[ -/o
I
-/o
0(9 Fig. 5. Variation of diffracted intensity with incidence angle
for a grating which showed a symmetrical profile at norms/ incidence.
//oo,~ AI
, zoo/'q3 o
9=0
....
):20 °
'~e
~ -¢
=£0 °
I0
0
-IO
IO
~ . -40"
~/0 °
O*
o
Fig. 4. Schematic of experimental geometry used to test grating dispersion.
-IO
@= 4 9 "
..... I0
0
ii
-IO °
0
-/o
Fig. 6. Variation of diffracted intensity with incidence angle for a grating which showed a non-symmetrical profile at normal incidence. Note extinguishing of zeroth order at ~ = +50 ° .
VI. P E R F O R M A N C E O F G R A T I N G S A N D M I R R O R S
E.T. Arakawa, P.J. Caldwell / Transmission diffraction gratings
296 O.I
/oi
),,= 2 4 0 . ~
- -%
t
•
•
s •
x
o • f
'
,°''
I
v .... I
v-I
oo6t .--,~f/ o 0 W---t,-- . . . . ~. . . . . . F"
O.O3~ |
%, ), ~o2A o.~ ," "'.'I,
,
'\~s
T L
4. Discussion
I
/ .... " o ///
x
o I
attempting to calculate these intensity distributions using scalar electromagnetic theory taking the con, ductivity of the grating into account.
A = 454~,
'/
x rl =-[
oP'"7 ........ ~..... "F">Q: """;40~: ' -40 0 (~(o)
Fig. 7. Variation of diffracted intensity versus incidence angte for three different wavelengths o f a grating with a nonsymmetrical profile at n o r m a l incidence.
increase in the - 1 order is seen over the ¢ = 0 scan intensities. Reference to fig. 4 shows that this geometry corresponds to an incidence angle such that the photons are travelling nearly parallel to the slope of the grating strips. The results for a grating with 500 A thick silver strips on a 1400 A thick aluminum backing are summarized in fig. 7. The absolute intensities transmitted in the 0, +1, and - 1 orders by the grating are shown as a function of incident angle ¢ for three wavelengths. Note that the transmitted intensities for this grating range from ~ I - 1 0 % . Features of particular interest are the asymmetric angular dependence of the n = 0 order and the oscillatory behavior of the n =+-1 orders. We are currently
The limited wavelength scanning capability of the transmission grating for a fixed angle of exit makes its use as a monochromator element restricted unless secondary reflecting elements are used. However, the possibility of its use as a primary order sorting device of greater flexibility than interference filters in existing monochromators makes this kind of grating attractive. It also has great promise in low distortion, fixed wavelength photography. For use in synchrotron storage ring~applications where heat dissipation is an important Copsideration, these gratings may have an advantage over the parallel wire strip gratings since the support layer of aluminum is in complete contact everywhere with the grating strips. A particular advantage of this type of grating is the removal of line density restrictions. Transmission gratings of this type can be made with as many lines per mm as holographic reflection gratings. Finally, for use in different regions of the spectrum, the materials used for the support and grating strips can be easily varied.
References [1] E. K~illne, H.W. Schnopper, J.P. Delvaille, L.P. van Speybroeck and R.Z. Bachrach, Nucl. Instr. and Meth. 152 (1978) 103. [2] H.W. Schnopper, L.P. van Speybroeck, J.P. DeivaiUe, )~. Epstein, E. Kgllne, R.Z. Bachrach, J.H. Dijkstra and L.G. Lantnaard, Appl. Opt. 16 (1977) 1088. [3] S.L. N o r m a n and M.P. Singh, Appl. Opt. 14 (1975) 818.
HOLOGRAPHICALLY
PRODUCED
TRANSMISSION
DIFFRACTION
GRATINGS
FOR SOFT X-RAYS
*
E.T. A R A K A W A ** and Paul J. CALDWELL *** Oak Ridge National Laboratory, Oak Ridge, Tenn., U.S.A.
A method for producing self-supporting transmission gratings for the soft X-ray region will be discussed. Photoresist is spin-coated onto a glass substrate and then exposed to the interference pattern created by two coherent point sources. The sinusoidal relief pattern which is formed after development is coated at normal incidence with a suitably transparent metal by vacuum evaporation. An additional layer of opaque metal is added obliquely (i.e., to one side of the sir~usoidalprofile) to provide the necessary wavefront amplitude modulation. After coating, a sample holder is glued to the surface of the metal and the photoresist is dissolved, freeing the metal layer from the glass substrate. Self-supported gratings of aluminum with opaque silver strips have been tested at 240, 302,434, and 605 A. At certain angles of incidence, the zeroth order has practically disappeared with most of the intensity appearing in one order.
1. I n t r o d u c t i o n
(fig. 1). A thin layer o f photoresist (Shipley AZ1350J) which had been spin-coated onto a carefully cleaned glass microscope slide [3] was exposed to the interference pattern created by the intersection o f two coherent monochromatic wavefronts. Using the 200 roW, 4579 A emission line o f an Argon ion CW laser, and 25/Jm pinholes, a power density of 1.8 mW cm -2 was obtained at the sample table. No collimating lenses were used to avoid wavefront distortion. With the sample placed about a meter from the pinholes, the wavefronts were approximately planar, giving constant groove spacing. The angle between the intersecting beams was 29 °, resulting in a line density of about 2100 1 m m -1. Samples were exposed for
The advantages, such as high throughput, light weight, and stigmatic imaging properties o f the transmission grating in the soft X-ray region as compared with grazing incidence reflection gratings currently in use make it attractive for some special applications. Transmission gratings o f the type described b y K/illne et al. [1] and Schnopper et al. [2] have been successfully employed in the High Energy Astronomical Observatory (HEAO-B) and the ApoUo Telescope Mount for non-solar cosmic and solar X-ray studies, respectively. These gratings have also been suggested for use in m o n o c h r o m a t o r systems for synchrotron radiation facilities. This paper outlines a new method o f prod/icing transmission gratings and the test results obtained in the soft X-ray region.
Argon Ion Laser
1~
---5, I [ i I
2. Grating production A sinusoidal profile was created on the surface o f photoresist by the standard holographic technique
/'_" :_---- - - - - -
pinhole
14
]
+-~ l
* Research sponsored by the Office of Health and Environmental Research, US Department of Energy, under contract W-7405-eng-26 with the Union Carbide Corporation. ** Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 USA. *** Southern College and University Union participant from Southwestern at Memphis, Memphis, Tennessee 38112 USA.
t ¢
t"~.
/"
|,-.
z= k. x...l~..so~ple table /
/-
/ Fig. 1. Holographic exposure arrangement. 293
VI. PERFORMANCE OF GRATINGS AND MIRRORS
E.T. Arakawa, P.J. Caldwell / Transmission diffraction gratings
294 exposed photoresist
Microscope slide coated,
Dissolve exposed photoresist
with photoresist and
to
expose<~ to interference
surf=ce profile.
leeve sinusoidol
field.
Fig. 2. Holographic production of diffraction gratings in photoresist.
120 s and then developed for 25 s in a 64o-1 dilution of AZ-303A at 21°C and oven dried for 30 min at 95°C (fig. 2). This process created a groove depth of about 1500 A as determined by scanning electron microscopy. The sinusoidal profile formed in the photoresist can be used directly as a transmission grating or coated with metal and used as a reflection grating. For our purposes, this prof'lle was used as a
Support layer normally.
deposited
mold for a metallic transmission grating. As shown in fig. 3, a uniform layer o f aluminum was deposited by vacuum evaporation normal to the surface. This layer served as a support and was fairly transparent in the soft X-ray region. To produce the grating strips, silver was deposited at an angle between 60 ° and 70 ° from the normal such that only one side of the sinusoidal profile was coated. The thickness of the aluminum layer which could support the mass of the silver strips in a free-standing mode was found to be around 1000 A. Gratings with silver strips of ! 0 0 - 7 0 0 A in thickness were tested. Before removing the metallic grating from the surface of the photoresist, a sample holder with a 9 m m diameter hole in the center was glued to the surface. The sample was then immersed in acetone for several hours to dissolve the photoresist and the grating was then lifted vertically from the acetone bath. The hole in the holder provided 64 mm 2 of free-standing metallic grating. Several gratings were made with 80% transmitting screen mesh on the holder to produce a more robust structure.
//// Grating strips
added
obliquely.
PhotoreSist metallic grating
Fig. 3. Metallic transmission grating production.
dissOlved,
transmission removed.
E.T. Arakawa, P.J. Caldwell
Transmission diffraction gratings
295
q~o
A slight distortion of the grating due to the removal process was noted. In one instance, a grating formed with a groove spacing of 4700 A was found to have a groove spacing of 4655 A after removal from the photoresist surface. This distortion differed from grating to grating.
o
~= - 2 0 °
3. Test results Several gratings were tested at soft X-ray wavelengths using a condensed spark source with a 2.2 m grazing incidence monochromator. As illustrated in fig. 4, the experimental chamber allowed the angle of incidence (~0) to be varied about an axis paralel to the grooves and a photomultiplier coated with sodium salicylate to scan the diffracted beams (0). In fig. 5 are shown scans of the dispersion of 320.4 )k radiation by a grating with 2100 1 mm -1 which showed a symmetrical q~= 0 pattern. Changing the angle of incidence to the +~ direction is seen to increase the intensity of the - 1 order while rotation in the -~b direction causes an increase in the +1 order, with a decrease o f the zero order in both cases. The effect of changing incidence angle for a grating which shows a partfal blaze effect at 4)---0 (fig. 6) reveals several interesting features. Of particular interest is the scan for ~b= 50 °. In this case, the zero order is nearly eliminated whereas a substantial
J\ I0
0
J
,
,~,,
-I0
__1
.... +/0
I ....
I ....
~ ....
0
,,,,[ -/o
I
-/o
0(9 Fig. 5. Variation of diffracted intensity with incidence angle
for a grating which showed a symmetrical profile at norms/ incidence.
//oo,~ AI
, zoo/'q3 o
9=0
....
):20 °
'~e
~ -¢
=£0 °
I0
0
-IO
IO
~ . -40"
~/0 °
O*
o
Fig. 4. Schematic of experimental geometry used to test grating dispersion.
-IO
@= 4 9 "
..... I0
0
ii
-IO °
0
-/o
Fig. 6. Variation of diffracted intensity with incidence angle for a grating which showed a non-symmetrical profile at normal incidence. Note extinguishing of zeroth order at ~ = +50 ° .
VI. P E R F O R M A N C E O F G R A T I N G S A N D M I R R O R S
E.T. Arakawa, P.J. Caldwell / Transmission diffraction gratings
296 O.I
/oi
),,= 2 4 0 . ~
- -%
t
•
•
s •
x
o • f
'
,°''
I
v .... I
v-I
oo6t .--,~f/ o 0 W---t,-- . . . . ~. . . . . . F"
O.O3~ |
%, ), ~o2A o.~ ," "'.'I,
,
'\~s
T L
4. Discussion
I
/ .... " o ///
x
o I
attempting to calculate these intensity distributions using scalar electromagnetic theory taking the con, ductivity of the grating into account.
A = 454~,
'/
x rl =-[
oP'"7 ........ ~..... "F">Q: """;40~: ' -40 0 (~(o)
Fig. 7. Variation of diffracted intensity versus incidence angte for three different wavelengths o f a grating with a nonsymmetrical profile at n o r m a l incidence.
increase in the - 1 order is seen over the ¢ = 0 scan intensities. Reference to fig. 4 shows that this geometry corresponds to an incidence angle such that the photons are travelling nearly parallel to the slope of the grating strips. The results for a grating with 500 A thick silver strips on a 1400 A thick aluminum backing are summarized in fig. 7. The absolute intensities transmitted in the 0, +1, and - 1 orders by the grating are shown as a function of incident angle ¢ for three wavelengths. Note that the transmitted intensities for this grating range from ~ I - 1 0 % . Features of particular interest are the asymmetric angular dependence of the n = 0 order and the oscillatory behavior of the n =+-1 orders. We are currently
The limited wavelength scanning capability of the transmission grating for a fixed angle of exit makes its use as a monochromator element restricted unless secondary reflecting elements are used. However, the possibility of its use as a primary order sorting device of greater flexibility than interference filters in existing monochromators makes this kind of grating attractive. It also has great promise in low distortion, fixed wavelength photography. For use in synchrotron storage ring~applications where heat dissipation is an important Copsideration, these gratings may have an advantage over the parallel wire strip gratings since the support layer of aluminum is in complete contact everywhere with the grating strips. A particular advantage of this type of grating is the removal of line density restrictions. Transmission gratings of this type can be made with as many lines per mm as holographic reflection gratings. Finally, for use in different regions of the spectrum, the materials used for the support and grating strips can be easily varied.
References [1] E. K~illne, H.W. Schnopper, J.P. Delvaille, L.P. van Speybroeck and R.Z. Bachrach, Nucl. Instr. and Meth. 152 (1978) 103. [2] H.W. Schnopper, L.P. van Speybroeck, J.P. DeivaiUe, )~. Epstein, E. Kgllne, R.Z. Bachrach, J.H. Dijkstra and L.G. Lantnaard, Appl. Opt. 16 (1977) 1088. [3] S.L. N o r m a n and M.P. Singh, Appl. Opt. 14 (1975) 818.