GB2138966A

GB2138966A - An arrangement of thin layers

Info

Publication number: GB2138966A
Application number: GB08409958A
Authority: GB
Inventors: Hermann Flad
Original assignee: Balzers AG
Current assignee: OC Oerlikon Balzers AG
Priority date: 1983-04-18
Filing date: 1984-04-17
Publication date: 1984-10-31
Also published as: SE8402167L; NL8401158A; FR2544505A1; SE8402167D0; JPS59202408A; DE3404736A1; GB8409958D0

Abstract

To reduce the formation of hairline cracks in systems containing alternate optically active layers of MgF2 and TiO2 (3-6) of average optical thickness greater than 100 nm at least one MgF2 and one TiO2 layer (2) of thickness less than 1 DIVIDED 5 and of the average thickness of the optically active layers (& preferably smaller than 1 DIVIDED 40) are inserted as auxiliary layers between the system and the base (1). Applications are for thermal filters, beam splitters and highly reflective metal-free mirrors. <IMAGE>

Description

SPECIFICATION An arrangement of thin layers The invention relates to an arrangement of thin layers comprising an optically active multi-layer system carried by a base and consisting of alternate layers of MgF2 and TiC2 wherein the average optical thickness of the individual layers is at least 100 nm.

In this specification the term "optical thickness" is intended to mean the product of actual thickness and the refractive index of the layer substance. Multilayer systems of MgF2/Ti02 are used for thermal filters, beam splitters, metal-free highly refractive mirrors etc.

Nearly all thin layers irrespective whether they were deposited on substrates by evaporation, cathode sputtering or by a chemical method, are subject to more or less pronounced tensile and compression stresses which manifest themselves by the formation of cracks in the layer or by chipping off. In the literature are discussed various reasons for the stresses in the layers and numerical values are given for various layer substances; a summary presentation can be found in chapter 12 ofthe Handbook of Thin Films, McGraw-Hill Inc 1970.

It can be said, in general, that experts agree that the stresses in layers depend on various factors and that even small changes in the manufacturing conditions may result in widely different stresses. Apart from the chemical nature of the layer substance and the coefficient of thermal expansion resulting therefrom impurities are primarily believed to be the reason for the variable degree of stress in the same layer substance. Stresses can lead not only to the damage of the layer by hairline cracks and separation from the base or of the layers from each other, but further also to the deformation of optical surfaces, which have adverse effects in applications in precision optics e.g. in interference systems.This deformation forms even the basis of a measuring method for the determination of layer stress, which is mostly given as force per cm2 which acts in the layer cross-section perpendicularly to the layer surface.

For MgF2 layers deposited by evaporation is known from literature (A. E. Ennos, Applied Optics vol 5, page 51, January 1966) that A/4 layers, which are very often used in optical technology, and which have (according to the reference wavelength) a layer thickness of 100 nm or more, have stresses of up to 5000 kp per cm2 (=490 MPa), which, when the layer is exposed to air humidity, later drop to 3400 kp per cm2 (333MPa). Naturally absorption of water by the layers should be avoided as far as possible because it results in an absorption band which is particularly disturbing in infra-red applications. In any case MgF2 layers, as mentioned, are subject to high inner stresses.

As regards TiC2 layers it is known that they have also high stresses of the order of 3600 kp per cm2 (=353 MPa) (W. Heitman, Applied Optics, volume 10, No. 12, page 2685, December1971). It is recommended in both the literature references that one should attempt to build the layer systems of individual layers such that the tensile stresses in one layer are compensated for by the compression stresses in the other layer substance. In view of the limited choice of layer substances usuable for any one application, this problem is difficult to solve and particularly for MgF2/TiO2 layer systems it seems insoluble, because these two substances, as is apparent from the cited literature, suffer from high tensile stresses which cannot therefore compensate each other.Systems with alternate layersofTiO2 and MgF2 are therefore unsatisfactory (particularly in arrangements with a large number of layers) because they have many hairline cracks (H.Anders, "Duenne Schichten fuer die Optik", Stuttgart, 1965, page 164) and due to light diffused on the cracks they have a cloudy appearance. Due to their otherwise good properties, it is desirable to use these particular two layer substances for interference filters.

The aim of the present invention is to devise a new arrangement for a system comprising alternate layers of MgF2 and TiO2, in which a much smaller number of hairline cracks is formed, i.e. which is practically without cracks and does not chip off the base, even when the system is composed of many individual layers, e.g. more than ten layers. (Up to now cracks have usually appeared when ten layers were used).

The arrangement according to the invention is characterised in that between the base and the optically active multi-layer system is situated a system of superimposed auxiliary layers consisting of at least one auxiliary layer of MgF2 and one auxiliary layer ofTiO2, the thickness of the individual auxiliary layers being smaller than 1/5 of the average thickness of the individual layers of the optically active system.

Preferably the system of auxiliary layers comprises more than two auxiliary layers and is formed by alternately arranged auxiliary layers of MgF2 and TiO2. The average thickness of an individual layer of the auxiliary layers is preferably selected to be at the most /40 of the reference wavelength of the optically active MgF2/TiO2 multi-layer system. The term "reference wavelength" as used herein is intended to mean that wavelength which serves as a basis for the calculation of the system to obtain in the region of this wavelength according to the intended use maximum reflection of transmission.

The step forward achieved by the invention is probably obtained because the very thin auxiliary layers allow "sliding" of the optically active system of thicker layers, which rests on the auxiliary layers without thereby weakening the bond of the system on the base. This hypothesis could explain that the system of the optically active thin layers, when the arrangement according to the invention is used, adheres very well due to the system of auxiliary layers on the base, as if the system were directly connected to the base, on the other hand, however, due to the possibility of sliding no cracks appear, as would be the case of it were rigidly connected to the base.

The invention will now be explained in greater detail with reference to examples. In the drawings: Figure 1 shows an arrangement of thin layers consisting of four layers of optically active thickness and a system of auxiliary layers consisting of ten much thinner individual layers; and Figure 2 shows a second embodiment in which a system of auxiliary layers is incorporated between optically active layers, while the glass carrier together with the first part of the optically active layers forms the base for the attached system of auxiliary layers and a further optically active system according to Claim 1.

The references in Figure 1 have the following meaning: 1 base 2 packet of auxiliary layers consisting of ten alternate TiO2 and MgF2 layers (the optical thickness of the individual layers is Xl40, the reference wavelength X is 550 nm).

3 a TiO2 layer of optical thickness X/4 4 an MgF2 layer of the same optical thickness 5 a TiO2 layer of the same optical thickness 6 an MgF2 layer of the same optical thickness The references in Figure 2 have the following meanings: 7 base 8 and 10 a TiO2 layer of optical thickness X/4 9 and 11 a MgF2 layer of the same optical thickness 12 a layer packet consisting of eight alternate layers ofTiO2 and MgF2 (the optical thickness the individual layers in this case is X/50) 13 and 15 a Tio2 layer of optical thickness X/4 14 and 16 an MgF2 layer of the same optical thickness In the dashed parts (between layers 4 and 5 in Figure 1 and layers 9 and 10 and also 14 and 15 in Figure 2) are arranged further MgF2 and TiO2 layers of optical thickness X/4, depending on the optically active multi-layer system used in a particular case.

The refractive indices of the TiO2 and MgF2 layer substances are in the two examples 2.3 and 1.38 respectively. It should be noted that the layer thicknesses could not be drawn in the correct scale with respect to the base and to each other.

The layers can be applied by known methods such as coating in vacuum or cathode sputtering. The method of deposition does not form part of this invention: there is a vast specialist literature which could be consulted.

Claims

1. An arrangement of thin layers comprising an optically active multi-layer system carried by a base and consisting of alternate layers of MgF2 and TiO2 in which the average optical thickness of the individual layers is at least 100 nm, wherein between the base and the optically active multi-layer system is situated a system of superimposed auxiliary layers consisting of at least one auxiliary layer of MgF2 and one auxiliary layer of TiO2, the thickness of the individual auxiliary layers being smaller than 1/5 of the average thickness of the individual layers of the optically active system.

2. An arrangement according to Claim 1, wherein the system of auxiliary layers comprises more than two auxiliary layers and is formed by alternately arranged auxiliary layers of MgF2 and TiO2.

3. An arrangement according to Claim 1 or 2, wherein the average thickness of the individual auxiliary layers is at the most X/40 of the reference wavelength of the optically active multi-layer system.

4. An arrangement according to any one of Claims 1 to 3, wherein the base consists of a carrier to which have already been applied layers.

5. An arrangement according to any one of Claims 1 to 4 wherein the optically active multi-layer system consists of at least ten individual layers.

6. An arrangement of thin layers constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, the accompanying drawings.

7. A device comprising an arrangement of thin layers according to any one of Claims 1 to 6.