Method for the production of multi-layer systems

Details Method for the production of multi-layer systems Method for the production of multi-layer systems

2001-04-17

2002-11-19

Mai, Huy (Department: 2873)

Optics: measuring and testing

Dimension

Thickness

C356S503000, C356S632000

Reexamination Certificate

active

06483597

ABSTRACT:

CROSS REFERENCE
This application claims the foreign priority benefit of German Application No. 100 19045.6-51, filed Apr. 18, 2000 for Method For The Production Of Multi-Layer Systems.
FIELD OF THE INVENTION
The invention relates to a method for the production of multi-layer systems with N layers having predetermined thicknesses, especially for the production of multi-layer systems for wavelength ranges in the extreme ultraviolet and soft X-ray wavelength range, in which N layers are deposited and if need be one or more layer are partially removed after deposition and in which at the same time as deposition and/or removal of layers, the layers' reflectivity dependent on layer thickness is measured.
BACKGROUND OF THE INVENTION
Artificially adjusted multi-layer systems which are used as interference systems have been known for decades. In the visible wavelength range, they play an important role in optical products such as cold light mirrors, filters etc. It is a question, within a given wavelength range, of achieving as complete a reflection, or possibly in some circumstances also transmission, as possible, on the multi-layer system, however achieving a negligible transmission/reflection in the wavelength ranges outside, as transition-free as possible. In order, for example, to produce multi-layer X-ray mirrors with high reflectivity, a large number of thin layers must be applied, in such a way that all the layers with an essentially equal phase contribute towards the reflected beam.
As a rule the layering consists of a number of alternating layers of two or more materials, which have different refractive indices. For specific wavelengths an additional reflection due to interference occurs on all pairs of layers with periodic structure, which in a first approximation is described by the Bragg equation: 2·d sin &thgr;
m
=m·&lgr;, where d=d
A
+d
B
is the sum of the individual layer thicknesses of the materials A and B, &lgr; the wavelength, &thgr;
m
the angle of incidence and m the order of diffraction.
The maximum reflectivity can be optimised by two opposite possibilities. One possibility consists of using materials whose refractive indices differ from each other as much as possible. The layers all have the same optical thickness. In the case of an angle of incidence of 90° the layer thickness corresponds to a quarter of the wavelength which is reflected in phase. The other possibility consists of reproducing an “ideal Bragg crystal”, in which layers of absorbent material, which are as thin as possible, preferably atomic mono-layers, alternate with thick layers of non-absorbent material. Losses due to absorption are minimised, by disposing the thin absorbent layers in the nodal planes of the standing waves, which are produced by the incident and reflected beam. X-ray mirrors are used for example in X-ray fluorescence analysis and X-ray astronomy. A commercially highly significant deposition of interference mirrors in the X-ray and ultraviolet range is semi-conductor lithography. For this deposition optical elements made from multi-component multi-layer systems are used primarily to reproduce semi-conductor circuit structures on wafers.
Deviations from the theoretical reflectivity of an interference system result primarily from flaws in the thickness of the individual layers. This problem becomes even more serious if the number of layers is very high. Also the roughness of the interfaces between the individual layers reduces reflectivity as the intensity is reduced by scatter on the surface roughnesses.
The patent specification DE 27 50 421 C2 describes a measuring method and measuring device for the production of multi-layer systems for the visible wavelength range. In this, alternating layers with high and low refractivity are applied to transparent substrates. At the same time the layers are each applied to one test glass per material, the test glasses being exposed to the same coating material flow as the substrates.
During coating, the transmission and/or reflection behaviour of the layers is continuously registered. For this purpose the test glass just being coated is impinged upon by a monochromatic light beam and the proportion of light reflected or passing through is measured. To obtain the layer thickness from the reflectivity, advantage is taken of the fact that the light beam which is reflected on the upper surface of the layer and the light beam which is reflected on the undersurface of the layer, i.e. in the case of test glasses the interface between layer material and glass, are superimposed. Depending on the relative retardation or phase difference of these two beams, negative or positive interference phenomena will be obtained.
Ideally, the reflectivity depends sinusoidally on the layer thickness. The reftectivity measurement result will thus be used for definite interruption of the coating process. The multi-layer systems produced have constant layer thicknesses of &lgr;/4. &lgr; is the wavelength which will be the wavelength in the case of later use, as well as the wavelength with which the reflectivity is measured. The coating process is always interrupted either at maximum or minimum reflectivity. The extrema can be registered manually or automatically for the visible wavelength range sufficiently accurately without great expense.
The “minimum-maximum method” is also applied in the U.S. Pat. No. 5,551,587, the U.S. Pat. No. 5,151,295 and the JP 61296305 A. As a rule, the thickness of the deposited layer is parametrised by the deposition time. According to the JP 61296305 A, two beams of different wavelengths are used to measure the transmission dependent on the time during the deposition of the layers. This improves the accuracy of the determination of the peak values of the transmission. The actual deposition stop occurs slightly after the respective peak values.
The special feature of the U.S. Pat. No. 5,551,587 is to manufacture a multilayer mirror by stimulation of the penetration of one of the materials into the layer with the other material after which the original layer thickness of the first material is removed by etching.
The JP 63028862 A improves the accuracy of the “minimum-maximum method” by fitting the measured transmittivity or reflectivity data by quadratic regression. The target vapor deposition time of the layer is based on the value obtained.
At the beginning of the 1980s the principle of layer thickness control by means of reflectivity measurements was also successfully transferred to the production of interference systems for the extreme ultraviolet and the soft X-ray range. In this connection see also the publications of E. Spiller et al., Appl. Phys. Lett. 37 (11), pp 1048-1050 (1980), E. Spiller, Proc. SPIE 563, 367 (1985), M. P. Bruijn, Dissertation, University of Amsterdam (1986) and J. Verhoeven et al., Vacuum 39, pp 711-716 (1989).
The terms “extreme ultraviolet” and “soft X-ray range” should here be understood to mean the wavelengths of an order of magnitude between one-tenth and ten nm. Also in the case of production of interference systems for this short-wave wavelength range, the reflectivity is measured in situ, essentially using light of a wavelength that is comparable with that of the final deposition (i.e. soft X-ray range or extreme ultraviolet range), the thickness of the top layer is deduced from the reflectivity and one changes over between the coating with one or other material at the extrema.
In the aforementioned 1985 publication of E. Spiller, the layer thickness is monitored not only via the reflectivity, but also via a quartz crystal monitor. It is a matter of special interference systems in which the absorbing layer is too thin for its thickness to be determined by means of reflectivity measurements. Only the deposition of the less absorbent layer is controlled by means of the reflectivity measurement.
To change over from deposition of the less absorbent layer to deposition of the strongly absorbent layer, a maximum point in the development of the reflectivity is selected. By using the

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