Optics: measuring and testing – Plural test
Reexamination Certificate
2001-09-25
2003-12-02
Evans, F. L. (Department: 2877)
Optics: measuring and testing
Plural test
C356S301000, C356S369000
Reexamination Certificate
active
06657708
ABSTRACT:
This invention relates to an apparatus for optically characterising thin layered material.
Such characterising that is non-destructive and can therefore be used in situ during manufacture or for checking finished products, enables knowing at least certain elements making up the analysed matter and possibly their concentration. It can also enable accessing the thickness of thin layers.
To perform characterising, we have known until now, on the one hand Raman spectroscopy devices, on the other hand reflectometers, photometers or ellipsometers, that may be spectroscopic.
We know that the Raman effect is caused by a sample lit at a given wavelength &lgr;
e
diffusing a Raman luminous beam at a wavelength &lgr;
r
close to &lgr;
e
whose intensity is very small with respect to that of the Rayleigh light which is diffused at the same wavelength &lgr;
e
as the lighting beam. In case when the Raman spectrometer is coupled with a microscope, the lighting beam is currently under a normal incidence with respect to the sample and the Raman diffusion is measured by its intensity and by its spectrum in a solid wide angle.
We also know reflectometry characterising. The purpose is then to light the sample under an angle that is often small (vastly different from the normal to the sample) and to analyse the light that is reflected specularly by the sample. We are then more particularly interested in the luminous intensity in the case of photometry and in the amplitude of the various components of the polarised light in the case of ellipsometry.
We know, in particular, phase-modulated ellipsometry in which a modulator acts on the polarisation state of the incident luminous beam, the spectroscopic ellipsometry in which the wavelength spectrum of the reflected light is analysed and the modulated reflectometry that takes into account the effect of the modulation generated by a periodic external excitation, for example electric or optic excitation, acting on the sample. A modulated spectroscopic ellipsometer is for instance described in the European patent EP-0.663.590 to which reference can be made.
Each one of these major alternatives of optical characterising of a sample exhibits its own advantages.
Generally speaking, macroscopic response of a thin layered material to an electromagnetic excitation by a tensor &egr;(&ohgr;) where &ohgr; is the frequency of the electromagnetic excitation. In the case of an isotropic solid, this tensor &egr;(&ohgr;) is reduced to a scalar and we obtain the relationship D=&egr;
o
&egr;E where &egr;
o
is electric permittivity of vacuum, D is the electric displacement vector and E is the applied electric field.
Polarisability &agr; is then defined on the base of the local bipolar moment p (per atom or group of atoms). Indeed, p is linked with the local electric field E
loc
(itself function of the external electric field E by the relationship: p=&agr;&egr;
o
E
loc
.
The macroscopic dipolar moment per volume unit or polarisation vector is given by the formula: P=Np where N represents the space density of dipoles. Polarisation is linked with the other macroscopic quantities by the relation: P=&egr;
o
(&egr;−1) E.
Reflectometry, more particularly, spectroscopic ellipsometry, enables accessing the dielectric function &egr;(&ohgr;). In the range of wavelengths from ultraviolet to the visible, absorption is often dominated by electronic transitions (it is for example the case in semiconductors). In the infrared range, ellipsometry is sensitive to vibration absorption, i.e. dipole excitation. The thickness probed may vary considerably in relation to the wavelength as in the case of semiconductors that are generally very absorbing in the ultraviolet and quasi transparent in the infrared.
The Raman diffusion is, for its own part, sensitive to polarisability variations in the presence of excitations &Dgr;&agr;(&ohgr;).
It can be noted that, as regards the determination of related physical values, ellipsometry and Raman diffusion are techniques of different natures. In particular, from the viewpoint of quantum mechanics, the efficient sections of certain vibrations could be very different in one case and in the other.
Reflectometric measurement is conducted in a specular and elastic fashion (conservation of the wavelength), generally in reflection. Consequently, it is sensitive to interference phenomena that enable measuring thicknesses of thin layers. More generally, ellipsometry is well suited for characterising a multilayer material (which exhibits thickness divergences). In usual applications of ellipsometry, the angle of incidence varies between 55 and 80° approximately, which corresponds to the Brewster angles of most materials and provides optimal sensitivity. Two wavelength ranges are used generally: the first is said ‘visible ultraviolet’, extending from the near ultraviolet (0.25 &mgr;m) to the near infrared IR (1.7 &mgr;m) and the second, called ‘infrared’, in the more remote infrared from 2.5 to 12 or 16 &mgr;m approximately. Measurements with higher wavelengths are difficult because of experimental limits imposed by the sources and the detectors.
&egr;(&ohgr;) is represented by a complex number whose determination calls generally for the measurement of two independent parameters as this can be made in ellipsometry. However, photometry, pending the use of so-called Kramers-Konig relationships, can also enable measuring &egr;(&ohgr;). Modulated reflectometry techniques measure the variation of &egr;(&ohgr;) in the presence of an external excitation, which brings complementary information. In particular, in semiconducting materials, the modulated external excitation generates loaded carriers that concur to that measurement.
Conversely, the Raman diffusion is inelastic. The measurement is then generally conducted in normal incidence; whereas a laser that emits a ray in the ultraviolet, the visible or the near infrared range provides the excitation. The Raman photons are collected under a solid wide angle at wavelengths close to those of the incident light. We therefore measure spectroscopically a positive or negative difference in wavelengths between the exciting ray and the Raman spectrum. By comparison, the remote reflectometry infrared corresponds in Raman to the wavelengths closest to the exciting wavelength. Such measurements are therefore technically easier in Raman spectrometry. It should be underlined that the characterised thickness is linked with the absorption of the material at the wavelength of the incident light that is hardly modifiable in Raman for a given material.
Thin films have been subject to Raman spectroscopy surveys as of the end of the sixties. It has been suggested first of all to study thin layers deposited on a metal surface. Lit under an angle of incidence of 70°, the Raman flux obtained exhibited a maximum intensity around 60°. It has then been proposed to use a thin layer as an optic wave-guide to which a light flux was coupled by means of a prism or of a grating. Under strict conditions of angle of incidence and of polarisation, one or several electric or magnetic transverse modes can be propagated in the film while creating therein a Raman flux whose intensity can reach up to two thousand times the flux intensity usually generated by backscattering. In such a case, the minimum thickness of the film, linked with the excitation wavelength, cannot be smaller than a few nanometres. We can go below this limit only while resorting to multilayer structures. Anyway, all these methods call for particular preparation of the film or of the thin layer, on a specific support, which implies tooling and adjustments incompatible with the survey of industrial materials and even more with in situ or real-time measurements, during the implementation of a method of manufacture.
These presentations of the measurements by Raman effect on the one hand, and by reflectometry on the other hand, ellipsometric or photometric reflectometry, as they outline that we obtain different effects, hence different sources of knowledge
da Silva Edouard
Drevillon Bernard
Ramdane Benferhat
Evans F. L.
Geisel Kara
Jobin Yvon S.A.
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