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Design and manufacture of sputtered multilayers for applications to soft X-ray optics
Ph. Houdy, P. Boher
To cite this version:
Ph. Houdy, P. Boher. Design and manufacture of sputtered multilayers for applications to soft X-ray optics. Journal de Physique III, EDP Sciences, 1994, 4 (9), pp.1589-1598. �10.1051/jp3:1994226�.
�jpa-00249209�
J Phys. III France 4 (1994) 1589-1598 SEPTEMBER 1994, PAGE 1589
Classification Physic-s Abstiacts
85.15C 78.90
Design and manufacture of sputtered multilayers for
applications to soft X-ray optics
Ph. Houdy and P. Boher
Universitd d'Evry, Boulevard des Coquibus, 91025 Evry Cedex, France (*) (Received J9 No»ember J993, iei>ised II March /994. accepted J6 Marc-h J994)
Rksumk. Des muticouches nanomdtriques ont 6t6 d6pos6es par pulv6risation diode rf ultravide dans une chambre 6quip6e d'ellipsombtres in situ. L'influence de la composition, de la rugosit6, de la pr6sence d'une couche d'interface et du nombre de pdriodes a 6t6 estim6e afin d'optimiser (es empilements pour la r6flexion de rayons X mous. Le comportement de ces structures sous recuit
thermique a dt6 observ6. Enfin des r6seaux ont 6t6 r6alis6s avec succbs.
Abstract.- Nanometer scale multilayers has been deposited using high vacuum diode rf
sputtering chamber equipped with in situ kinetic ellipsometers. The influence of the composition, the roughness, the interface layer and the number of periods have been studied in order to optimize the stacks for soft X-ray reflection. The behaviour of the structures under thermal annealing has been observed. At last, gratings have been sucessfully manufactured.
1. Introduction.
The last several years have seen growing interest in material structured at the nanometer scale, for many applications such as magneto-optical recording media or magnetic heads, such as
supraconductors and also for neutron optics and soft X-ray mirrors in the optic field.
Multilayers alternating materials at atomic scale have been manufactured by many authors to be used as soft X-ray mirrors [1-3] with high efficiencies. We present in this paper, a summary of the results we have obtained in that field. We have used diode rf sputtering to deposit the
multilayers and we will focuse on the interface formation, observed using ellipsometry [4].
Thermal stability of the multilayers will be described, for carbon based multilayers and silicon based multilayers and advantage of silicon/silicon nitride and silicon/silicon oxide multilayers
will be discussed.
(*) The results presented in this paper have been obtained at Laboratoire d'Electronique Philips 94, France.
1590 JOURNAL DE PHYSIQUE III N° 9
2. Deposition process.
The multilayers [5, 6] have been deposited in a high vacuum (10-9 Torrs) chamber using diode rf,sputtering. The deposition is made at room temperature under low argon pressure (a few m Torrs), low rf power (a few hundred W) on a 12 cm diameter target. The target-sample
distance is 10 cm and good homogeneity is obtained for samples with 5 cm diameter. The
substrates used are float glass or silicon wafers in any case with very low surface roughness (0.2 nm). As can be seen in figure I, the argon pressure is regulated as well as the rf power. For layers such as silicon nitride or silicon oxide, an argon plasma is maintained under the Si target and a special pipe at the top of the sample is used to introduce the reactive gas. The partial
pressure of reactive gas is regulated using a mass quadrupolar analyser in an aside chamber
(Fig. I).
aF sysTEw
aUMmPQLAR
AWALYZER AUTQWAS
p VMVE
TARGET
Wf@JLATW
» «ET
FAST
mm-
~w~nm To ~yw
Fig. I. Schematic view of the high vacuum diode rf sputtering system the regulations for argon pressure and rf power can been seen on the right part, those for reactive gas on the left part.
This system allows for the deposition of many materials and of their oxides, nitrides, or carbides with growth rates in the order of 0. nm/s. The thickness accuracy is in the order of 0.01nm and the composition accuracy in the order of I §b.
Ellipsometers have been installed on the sputtering chamber in order to study, in situ, the
growth of the layers and their interface formation. Ellipsometry [7] consists in the analysis of
the reflection (0,
=
18°) of polarized light (632 nm) (Fig. 2).
The parameters recorded are tg #~ =R~/R, and cosA =cos(6~- 6~), where R~, R~, 6~, and 8, are respectively the p wave and s wave reflectivities and dephasing. The curves,
represented in the (tg #~, cos A plane, allow for the determination of the thickness and of the
optical index of the growing layer with a thickness accuracy of a few hundredth nanometers and a composition accuracy of a few percents. First experiments are made on thick layers
loo rim) in order to calibrate the growth and to determine deposition rate and optical index. In
N° 9 SPUTTERED MULTILAYERS 1591
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Fig. 2. Schematic drawing of the ellipsometers installed on the high vacuum sputtering chamber.
a second time multilayers are deposited with a special attention to the interface formation as
explained in the next section.
3. Simulation of the mirrors efficiency.
The reflectivity and the selectivity of the multilayers are the main parameters to optimize for soft X-ray optics. Study of these characteristics <,eisus the structural parameters of the multilayers (number of periods, thickness ratio, roughness, thickness drift and interface layer)
have been done. It has been shown that the number of layers has to be so large that all X-rays
are absorbed (about 70 bilayers for 4.47 nm C/~V multilayer at 4.47 nm). The roughness has a strong effect on reflectivity (0.5 nm roughness decreases the reflectivity by a factor of 2) but a
light one on selectivity. The thickness drift has a strong effect on both reflectivity and
selectivity (0.2 % thickness drift decreases the reflectivity by a factor 2 and increases the bandwidth two times). The presence of a mixed layer at the interface is also responsible for the reflectivity decrease. One can see in figure 3 the influence of the interface layer on the reflectivity of Rh/C multilayers : the loss in reflectivity due to the presence of a 0.5 nm
rhodium carbide layer at the interface can be estimated to be 20fb. The control of the
formation of the interface layer (alloy, roughness) at atomic scale is a crucial point.
One of the solutions to control the quality of the stack, in case of diffusion or reaction of the two compounds at the interface, is to introduce a thin anti-diffusive layer of a third material,
between the two materials. If the third material is well adjusted (opposite in the
(#, T) plane of the Fig. 4), we can observe an increase of the reflectivity especially due to an
optical effect consisting of the displacement of the maximum amplitude of the waves in the less absorbent material [8]. This trilayer optical effect can be observed in figure 5. For Rh/C multilayers the occurrence of a thin tungsten layer (0.4 nm) at the interface increases the
reflectivity of few percents. In this case it is possible to avoid the Rh/C inter-diffusion and inter-reaction and to get a reflectivity increase.
JULIRN~L DC P»h~lQU~ III T 4 N'9 'EPTEMBER l~~'>4
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1592 jOURNAL DE PHYSIQUE III N° 9
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Fig. 3. Experimental and calculated reflectivities of Rh/C multilayers in the range I-I1.4 nm. The
maximum reflectivities are obtained for a perfect infinite multilayer. The other curves show reflectivities
for realistic multilayers with and without interface.
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Fig. 4. Reflectivity maxima of bilayer stacks in the (~, T) plane for carbon as spacer (light element)
~ Im (Fj F~)/Im (F~), T
=
Re (Fj F~)/Im (F~ F~).
4. Carbon based multilayers.
Carbon is a very good candidate as light element for many soft X-ray multilayers [9]. Due to its
nanographite structure the roughness of the multilayers is low (0.4 nm) and the formation of carbide at the interface avoids strong diffusion. A comparative study has been done on the influence of the interface on thermal stability in metal/carbon multilayers. One can see in
figure 6 ellipsometric trajectories for different carbon based multilayers. It appears that for the
No 9 SPUTTERED MULTILAYERS 1593
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Fig. 5. Potential performances of different multilayers alternating tungsten, rhodium and carbon in the range I-1.14 nm.
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Fig. 6. Ellipsometric trajectorie~ recorded during the growth of carbon based multilayers.
1594 JOURNAL DE PHYSIQUE III N° 9 element with positive carbide free energy formation at room temperature (Rh, Ni, Co, Fe) a
strong interdiffusion is observed by ellipsometry (trajectories are followed back) corresponding
to 0.7 nm in Rh case [10]. For the element with negative carbide free energy formation (Cr, Mo, W, Ti), no interdiffusion is observed : ellipsometry stagnant points, when depositing
carbon on tungsten, corresponding to the formation of an equivalent monolayer carbide. In these cases, the diffusion process is locked and when heating the multilayers, the disappear-
ance of the multilayer structure occurs at higher temperature (600 °C for W instead of 300 °C for Rh) as can be seen in figure 7.
f CARBON-BASED MULTILAYERS
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Fig. 7. Thermal stability of carbon based multiiayers : intensity of the Third Bragg peak and relative variation of the periodicity.
5. Silicon based multilayers.
Silicon is an other very good candidate as light element in the soft X-ray mirrors, particularly
for the high wavelength (12-32nm). Mo/Si multilayers are used in this case with high reflectivity but poor selectivity and low thermal stability. To get better selectivity and thermal
stability it is possible to use silicon/silicon oxide or silicon/silicon nitride multilayers I Ii- The silicon nitride layers have been obtained using nitrogen pipe at the top of the sample during
silicon sputtering under argon plasma. One can see in figure 8 the ellipsometric trajectories
N° 9 SPUTTERED MULTILAYERS 1595
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Fig. 8. Ellipsometric trajectories of SiN, layers ior different ratio nitrogen pressure/total pressure.
.-=(
$..
-.
Fig. 9. TEM micrograph of a Silsio multilayer the quality of the interface is visible even for the top layers.
1596 JOURNAL DE PHYSIQUE III N° 9
corresponding to the deposition of silicon for different nitrogen pressure ratio. A saturation of the silicon films occurs around 20 % corresponding to a complete nitridation of the silicon
depositing layer.
It is then possible to realize silicon/silicon nitride multilayers by introducing nitrogen at the top of the sample or not. The multilayers are very well defined, and the interfaces very sharp.
Only a light nitridation of the top of the silicon layer is observed when starting the introduction of nitrogen, increasing the silicon nitride layer thickness by 0.5 nm.
We can see in figure 9 a TEM micrograph of a silicon/silicon oxide multilayer showing the quality of the stack. With such structures, it is possible to increase the selectivity by a factor of 4 compared to Mo/Si multilayers, and to get higher thermal stability as can be seen in
figure 10. The silicon/silicon nitride multilayer keeps its structure up to 800 °C while Mo/Si is
destroyed around 400 °C.
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N° 9 SPUTTERED MULTILAYERS 1597
6. Gratings.
To get better selectivity for high resolution applications in the EUV-soft X-ray range it is possible to use multilayer gratings [12]. Mo/Si and Rh/C multilayers have been deposited to fabricate these gratings. In the Mo/Si case, the multilayers have been etched down to the substrate (using electron beam lithography and fluorinated plasma), in the Rh/C case the multilayers have been deposited on gratings. We can see in figures II and 12 the comparison
of the reflectivities of a Mo/Si multilayer mirror and the same multilayer etched as grating (5~Lm pitch, 4.5~Lm multilayer). Soft X-ray reflectances have been measured using
HULTILAYER 97eV
45
4@
~ 35
~ 30
(C
26 28 38
BRAGGANGLE(DEG.) Fig. ii- Experimental rocking curve of a Mo/Si muitiiayer at 97 ev.
DETECTOR SCAN
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+1 -1
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WI
)
#m
3
<
S
fl
5 24. 5 26. 5 28. 5 3a. 5 32. 5
DATRACIION ANGLE 0 (dog).
Fig. 12. Detector scan of the grating realized with the same multilayer (recorded at o
= 28°).
1598 JOURNAL DE PHYSIQUE III N° 9
synchrotron radiation, near 30° incidence for the multilayer (0-2 0 scan), at 27.5° incidence
scanning the detector for the grating. The reflectivity of the multilayer is around 45 % when the
reflectivity of the grating is in the order of 13 %. The selectivity is about 20 times better for the
grating with 9 orders visible. Others gratings have been realized (l0~Lm pitch, I ~Lm
multilayer), allowing up to a number of 50 orders. The etching process have given better results than the deposition on gratings, probably because of their poor surface quality.
7. Conclusion.
Since some years, it is possible to get high efficiency soft X-ray mirrors. Deposition processes
have been improved and multilayers with controlled structure can be realized. Low interfacial
roughnesses can be reached, interracial diffusion processes can be locked or avoid and a great number of period can be deposited. Next years could see the development of more modulated
structures allowing high speed deposition of multifunctionnal multilayers on large surface. We will especially focuse our research on titanium based multilayers for optical applications such
as neutron mirrors and also for tribological applications. We will use the special reactive gas
pipe in order to control with high accuracy the composition of nanomultilayers, and we will compare multilayers properties with equivalent alloy properties.
References ii Barbee T. W., SP/E 563 (1985) 2.
[2] Spiller E., SP/E 563 (1985) 135.
[3] Fernandez F. E., Falco C.. SP/E563 (1985) 195.
[4] Aspnes D., f Phi'.<. Cr)lloq. Flame 44 (1983) C10-610.
[5] Houdy Ph.. Chauvineau J. P.. Le vide [es couches minces 245 (1989) 69.
[6] Houdy Ph., Material Scieme Fr)rum 59 & 60 (1990) 58i.
[71 Houdy Ph.. Ret. Phi'.< Appt. 23 (1988) 165~.
[8] Boher P., SPfE1345 j1090) 198.
[9] Houdy Ph. et al.. SP/E 684 (1988 95.
[10] Boher P. et al.. SP/E 1742 (1992) 331.
ii ii Boher P, et al., Opt. Erg. 3 (1991) 1049.
[12] Bac S. et al., f Opt. 24 (1993) 88.
Pioofs not <.rile<.ted bj' the aiithoi.v.
