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HAL Id: jpa-00227306

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Submitted on 1 Jan 1987

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REFLECTIVITY MEASUREMENTS OF SYNTHETIC MULTILAYERS IN THE 1-5 nm WAVELENGTH

RANGE

J. Bonnet, M. Suraud, M. Chassevent, M. Bonnefoy, Agnès Fleury, Fabrice Bonnet, S. Bliman

To cite this version:

J. Bonnet, M. Suraud, M. Chassevent, M. Bonnefoy, Agnès Fleury, et al.. REFLECTIVITY MEA-

SUREMENTS OF SYNTHETIC MULTILAYERS IN THE 1-5 nm WAVELENGTH RANGE. Journal

de Physique Colloques, 1987, 48 (C9), pp.C9-117-C9-121. �10.1051/jphyscol:1987919�. �jpa-00227306�

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REFLECTIVITY MEASUREMENTS OF SYNTHETIC MULTILAYERS IN THE 1-5 nm WAVELENGTH RANGE

J.J. BONNET, M.G. SURAUD*, M. CHASSEVENT, M. BONNEFOY, A. FLEURY, F. BONNET and S. BLIMAN*

Laboratoire de Physique des Collisions Atomiques,

CNAM, 292,

Rue

Saint-Martin,

F-75141 Paris Cedex

03,

France 'DRF-LAGRIPPA-CENG, BP

85X,

F-38041 Grenoble Cedex, France

RESUME - La mesure de la rbflectivite intbgrhe R de deux multicouches constituees de Tungsthe et de Silicium a 6t6 effectuee dans le domaine des longueurs d'onde s'bten- dant de 1 B 5 nm.

Dans ce travail deux types d'6mission X ont kt6 utilisees :

-

les raies caractbristiques produites par un tube X B anticathode gazeuse ; - les raies de la sbrie de Lyman correspondant B la dbsexcitation d'un ion hydro- gknoPde produit par &change de charge.

Un tr&s bon accord a bte trouve entre les deux series de mesures.

Nous comparons les valeurs obtenues de R B celle d'un film de Langmuir-Blodgett (Myristate de Plomb) et d'un cristal organique (Phtalate acide de Rubidium).

ABSTRACT - Reflectivity measurements of two Layered Synthetic Microstructures (LSM) of Tungsten-Silicium has been made in the 1-5 nm wavelength.

In this work two kinds of soft X-ray radiations have been used :

-

characteristic X-rays emitted from an open-window X-ray tube ;

- soft X-ray radiation accompanying the decay of hydrogen-like ions produced after charge-exchange.

A very good agreement has been found between the two sets of measurements. We compa- re these latter R values with integrated reflectivity of other diffracting elements, namely :

-

a Langmuir-Blodgett film (Lead Myristate) - an organic crystal (Rubidium acid Phtalate : Rb AP).

Over thepastseveral years there has been a considerable regrowth of interest in providing instrumentation for the absolute measurement of X-ray emission in the 1-10 nm region. Part of this interest has centered for provision of large d spacing dif- fracting structures. The three principal types of diffracting structures are : (i) two-dimensional lattice representing a crystal with Bragg planes (Rb AP for

example)

.

(ii) thin organic films deposited by the classical Langmuir-Blodgett technique (Lead Myristate for example).

(iii) Layered Synthetic Microstructure (LSM) composed of n-alternating layers of high-Z elements of thickness dl, and low-Z elements of thickness d2 (W-Si with dl t d2 = 5,84 and 4,16 nm respectively in this work).

This paper describes some of the work that has been done in our laboratory to provi- de reliable reflectivity values with typical LSM's.

Comparisons are made with traditional diffracting structures (Rb AP 2d = 2,6 nm and Lead Myristate 2d = 8 nm) indicating improvements of one order of magnitude in inte- grated reflectivity.

I

-

EXPERIMENTAL SET-W AND MEASUREMENT PROCEDURE

The X-ray photon emission was observed in a direction perpendicular to that of the incident (electron or ion) beam by a plane crystal Bragg spectrometer /I/. In this paper we report on two commercially / 2 / available W-Si LSM, an organic Rb AP crystal

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987919

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JOURNAL

DE

PHYSIQUE

and a Langmuir-Blodgett film (Lead Myristate).

For the measurements described here, two kinds of soft X-ray radiations have been used :

-

characteristic photons emitted from a small home-made X-ray tube /I/ : a gaseous target (Neon, Oxygen, Nitrogen or SFg) is excited by an electron beam of 3 mm in diameter (typical current around 2 mA) ;

-

soft X-ray radiation accompanying the decay of excited states of ~+(q-l)* ions formed in A+q(4 KeV a.m.u.1-1 + (H2, He) collisions. (6 5 q 5 8 and A stands for C,N,O). At this low incident energy, a selective electron capture occurs in the n=4 level. So only three Lyman X-ray lines can be observed.

Typical X-ray spectra obtained with W-Si LSM are illustrated on figure 1.

200 W -Si

rnultilayers

100

Figure 1 : Typical X-ray Lyman spectra emitted in +'N + Hp collision and analysed with two different LSM diffracting elements. It must be noted that within 20 % uncertainty, this emission can be considered as a monochromatic one around 2.5 nm.

In both excitation cases, the X-rays were detected by a gas flow proportional coun- ter supplied with a Formvar window (nominal thickness 0.4 pm). The counter gas was propane at a regulated pressure of 2.1 x

104

Pa.

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where S is the signal rate measured with the proportional counter, Q the solid angle, L the interaction length viewed by the counter, I/qe the number of incident elec- tron (q=l) or ions, N the target atom density, T(A) the transmission of the propor- tional counter window and

R(X)

is the integrated reflectivity of the multilayer.

S2L has been measured in situ using Ar L X-rays produced in ~ r + + Ar collisions 131.

The transmission of each window was measured using a technique described by Schartner et a1 / 4 / . The emission cross section oem(X) is directly measured with a gas flow proportional counter. In electron impact excitation of Ne and 02, the measured value agree within 10 % with the emission cross sections of Tawara et a1 / 5 / .

If many lines are present in the spectrum, the relative intensity x;(hi) of each line was measured by using conventional diffracting elements like Rb AP, Lead Stearate, etc,..., whose relative efficiency is well-known / I / . Then, using only a gas flow proportional counter viewing the interaction region, the signal rate S' was measured and therefore the total and partial X-ray cross sections can be obtained from :

Using this method, many X-ray emission cross sections have been obtained with an uncertainty of 20 %, finally, in this non monochromatic case, the integrated reflec- tivity R(Xi) can be obtained from relation

(I).

I1

-

RESULTS

Integrated reflectivity R measurements concerning the two LSM diffracting elements studied here are illustrated on figure 2. The estimated uncertainty is varying from 10 to 30 %. A good agreement is found between R values obtained with incident elec- tron excitation (full circles) and ion beam impact (open circles) : this can be con- sidered as a consistency check of the experiment procedure described here.

On figure 2, integrated reflectivity measurements of Rb AP and Lead Myristate (see triangles) are four to ten times lower than LSM reflectivity.

The total intensity that may be measured with a diffracted spectral line is propor- tional the integrated reflectivity, R, for a given analyser. If a gaussian shape is assumed for the peak diffraction pattern, the integrated reflectivity R is related to percent peak reflectivity P and to full width at half maximum w (in radian) of the Bragg peak by the relation :

So, it is important to note that R(X) is an intrinsic characteristic of a particular LSM whereas P and

w

depend on the experimental set-up used for the measurement.

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JOURNAL DE PHYSIQUE

", A , A& M YRISTAT E I

1 2 3 . 4 5

Figure 2 : First order integrated reflectivity of two W-Si multilayers as a func- tion of photon wavelength (2d = 5.84 nm with dw = (0,7 ?r 0.1) nm ; dsi = (2,2 f 0,l) nm,200 layer pairs ; 2d = 4,16 nm with dW = (0,4

*

0 , l ) nm, dSi = (1,7 ? 0,l) nm,

250 layer pairs). Full and open circles concern efficiency measurements made with electron and ion beam respectively. The nature of X-ray emissions used here are directly indicated on the figure. Rb AP (2d = 2,61 nm) and Lead Myristate(2d = 8 nm) measurements respectively. Full lines have been drawn to guide the eyes.

The values of R measured here are in good agreement with first order integrated re- flectivities of typical multila~er structures and crystals published by Underwood and Barbee 161.

CONCLUSION

Integrated reflectivity measurements of two LSM of Tungsten-Silicium in the 1-5 nm wavelength range indicate improvement of one order of magnitude compare to tradi- tional diffracting element (Rb AP,Langmuir-Blodgett film). Computation of the inten- sity of X-ray reflection from an LSM at Bragg angle, or for any other angle of inci-

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Laboratory.

The high integrated reflectivity of LSM structures offergreat potential in many areas of soft X-ray research and instrumentation.

REFERENCES

/ I / A. Fleury et al, Nucl. Instrum. Methods, (1986) B14 353.

/ 2 / Ovonic Synthetic Materials Company 1788 Northwood Drive Troy. Michigan 48084.

1 3 1 H. Tawara et al. Phys. Lett. (1973) 43A 266.

/ 4 / K.H. Schartner et al. J. Phys. B (1974) 7L 111.

/ 5 / H. Tawara et al. Physica Scripta (1973) 63-351.

/ 6 / J.H. Underwood and T.W. Barbee Jr (1981) AIP Conference proceedings No 75-170.

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