WO1996041775A1

WO1996041775A1 - Microstructures in chalcogen-containing glasses

Info

Publication number: WO1996041775A1
Application number: PCT/GB1996/001403
Authority: WO
Inventors: Christopher William Slinger; Peter John Spence Ewen; Abelkarim Zekak
Original assignee: The Secretary Of State For Defence
Priority date: 1995-06-12
Filing date: 1996-06-12
Publication date: 1996-12-27
Also published as: GB9511871D0

Abstract

A method of producing microstructures in chalcogen-containing glasses which exploits the changes induced in such glasses by radiation. The surface of the glass is exposed to a predetermined pattern of radiation which produces regions of varying properties such as refractive index and transmittance. The patterns are selected to produce devices with desirable optical effects such as diffraction gratings.

Description

MICROSTRUCTϋRES IN CHΑLCOGEN-CONTAINING GLASSES.

This invention relates to a process for making surface relief or embedded structures in thin films of chalcogen-containing glasses, or on the surface of bulk components made from these glasses. The height of the structures produced by the process is of the order of 10^_5m or less and they are transmissive to infrared light. The structures are intended mainly for use as infrared optical elements although extension of the principles into other regions of the electromagnetic spectrum is feasible. Typical applications include diffraction gratings, transmission gratings, components and interconnects for integrated optical circuits, Fresnel lenses and microlens arrays. The list is not exhaustive.

The extension of diffractive optical techniques from the visible to the infrared is an important goal since diffractive elements have many potential applications in this waveband, including mirrors, lenses, filters and beam combiners, and may have advantages over conventional refractive/reflective components as regards weight, cost and ease of manufacture. The range of materials, however, which are suitable for the fabrication of infrared elements is limited.

Various techniques already exist for making surface relief and bulk phase gratings and some of these are applicable to infrared-transmitting materials (see for example H Smith (Ed) "Holographic Recording Media", Berlin, Springer Verlag, (1977)) . Some of these techniques have already been applied to chalcogenide glasses with the aim of producing high-frequency gratings for operation in the visible waveband. The two best known families of chalcogen-containing glasses are the chalcogenides and the chalcohalides. Chalcogenide glasses contain one or more of the chalcogen elements, (eg S, Se or Te) in combination with one or more of a variety of other elements, including As, Ge, Si, Tl, Pb, Sb and Bi. As well as binary glasses it is known that multi-component glasses containing 3, 4 or 5 elements can be formed and even more complex chalcogenide systems may be possible.

Examples of chalcogenide glass-forming systems include As-S, As- Se, Ge-S, Ge-Se, As-S-Se, Ge-Sb-Te and As-Ge-Se-Te. If the glass composition includes a halogen as well as chalcogen elements (eg glasses in the As-S-Si system) the glasses are referred to as chalcohalide glasses.

The chalcogen elements themselves, when in amorphous form, can be regarded as special cases of chalcogen-containing glasses. In this specification the term "glass" is used in its broadest sense and signifies a non-crystalline or amorphous solid. It includes not only solids prepared by cooling from a melt, but solid films prepared by techniques such as evaporation, sputtering, spin-coating or plasma deposition.

Chalcogen-containing glasses are well known infrared transmitting materials and have been employed in producing infrared elements such as filters, windows and fibres. Many of these glasses are known to exhibit a wide variety of light induced effects which enable them to be used as optical imaging or storage media in applications such as holography, integrated optics and VLSI lithography. These effects can be used to produce either embedded or surface-relief structures in thin films of these glasses or on the surface of bulk substrates made in these glasses. Hence, because of the transparency of these materials in the infrared spectral region, they can be used to fabricate diffractive elements which are transmissive in the infrared. Generally, when a chalcogen-containing glass is exposed to radiation its properties change as a result of microscopic or macroscopic changes in its composition (A E Owen, A P Firth and P J S Ewen, "Photo-induced structural and physico-chemical changes in amorphous chalcogenide semiconductors", Phil. Mag. B52, 347-362 (1985)). Examples of these effects include photodarkening, photopolymerisation and photodecomposition. In addition to optical illumination, other forms of irradiation, (eg X-rays, ion-beams or electron beams) can induce these changes. These light and radiation induced effects are not fully understood.

It is possible for a chalcogen-containing glass of a given composition to exhibit more than one of these effects, either at the same time or under different conditions, for example, under different illumination conditions or when different sample preparation techniques are used. Throughout this specification the expression "photo-induced change" does not imply a specific underlying mechanism.

The advantages of the elements produced by the proposed technique derive partly from the process used to make them and partly from the materials used.

As well as having the general advantages of low cost and ease of manufacture, the process gives more accurate control over the surface profile; enables a wider variety of structures to be made; has the potential for creating structures on curved surfaces and uses a highly-selective etchant compared with existing processes.

The materials have wide pass-bands in the infrared, so that diffractive elements produced could operate over a wide wavelength range: for example certain compositions could yield devices operating over both the (3 - 5) x Kr'rn and (8 - 12) x 10^"6m thermal bands simultaneously. The materials also have a low thermal coefficient of refractive index so the diffractive elements produced are significantly less sensitive to temperature changes over the range 0 - 90°C than other types of element.

Over 50 chalcohalide glass forming systems exist and many are known to exhibit photo-induced structural changes. There is therefore considerable scope for optimising specific characteristics of the diffractive elements produced.

According to this invention, a method of manufacturing optical elements from a chalcogen containing glass includes the step of exposing selected areas of the glass surface to electromagnetic radiation so that a change in the properties of the material so exposed is induced.

In a preferred embodiment the glass comprises a film, produced by suitable deposition techniques, on a suitable substrate.

In a further preferred embodiment the glass comprises the bulk material.

A further preferred embodiment includes the step of chemically etching the glass using an etchant whose effects on the exposed and unexposed areas are different.

The structural changes brought about by irradiation of a chalcogen containing glass are accompanied by changes in the refractive index of the glass. This effect, along with the low absorbtion losses exhibited by these glasses, gives rise to their ability to form efficient, phase modulated diffraction gratings, useable at wavelengths from visible red to beyond 15 x 10^" !. Both transmission and reflective gratings may be formed.

In addition to modification of the optical properties of the glass, the chemical properties of the glass are also changed. This allows selective removal of either exposed or unexposed areas by chemical etching. The invention will now be described, by example only, with reference to the following figures in which:-

Figure 1 illustrates the change in refractive index brought about in a chalcogen containing glass by irradiation;

figure 2 illustrates the change in transmission brought about in a chalcogen containing glass by irradiation;

figure 3 illustrates the effects on a chalcogen containing glass of the current invention and;

figure 4 further illustrates the process of the current invention.

Referring to figure 1, the variation of refractive index of unexposed 1 and exposed 2 (ie photodarkened) As₂S₃ with wavelength is shown. The results were derived from spectrometer measurements on 2xl0^_6m thick films, evaporated on to NaCl substrates. Exposure was carried out with a UV lamp for 3 hours, at an intensity of 21mWcm"². For any given wavelength, a marked increase in refractive index on exposure to radiation is shown.

Referring to figure 2, the variation in interference free transmission for unexposed 3 and exposed 4 As₂S₃ with wavelength is shown. The results were obtained under similar conditions to those detailed for the results shown in figure 1. For any given wavelength, a marked decrease in transmission on exposure to radiation is shown.

Referring to figure 3a, a photoresist mask 5 (for example a binary or greyscale material) , defining the desired profile, is applied to a chalcogen containing glass 6. The glass may take the form of a thin film deposited on a suitable substrate or may constitute the bulk substrate itself. Referring to figure 3b, after exposure to suitable radiation an removal of the mask, a structure with periodically varying microstructure is obtained. This gives rise to areas n_x and n₂ o differing refractive index. The dimensions of these areas n_x and n₂ are chosen to achieve a desired optical effect (eg diffraction) at a given wavelength.

Areas of different microstructure may have different chemical properties and different susceptibilities to chemical etching. Thus a periodically undulating surface profile, as represented in figure 3c, may be obtained by exposure of the structure represented in figure 3b to suitable etchant.

As an alternative to a physical mask which is applied to the glass, the pattern of exposure may be determined by other standard techniques, for example holographic techniques.

Referring to figure 4, where the desired structure is to be incorporated in a thin film, the process begins with cleaning o a suitable substrate followed by deposition of the film.

The surface to be treated, either a deposited film or a bulk substrate, is then masked to provide the required pattern (as described with reference to figure 3) before exposure takes place. This produces a bulk phase microstructure which may the be further processed by chemical etching to produce a surface relief microstructure.

The following conditions were used to produce a square-wave profile surface relief grating with a depth of lOOOnm and a period of 12000nm.

FILM DEPOsmoN The source material used was As₂S₃ fragments wi a deposition rate of 0.1 - 0.5nm s^"1. A film thickness of lOOOn was produced.

EXPOSDKE OOiDIT αNS A 200W Hg lamp was used as an ultraviolet light source. The mask period was I2000nm and the exposure tim was 1 hour. CHEMICAL DEVELOPMENT The etchant used comprised 0.05M NaOH in methanol. This gave rise to an etching rate (unexposed area) of 20 nm s^*1.

GRATING PERFORMANCE A square profile was obtained with a diffraction efficiency of 40% at a wavelength of 632.8nm.

Claims

Claims,

1. A method of manufacturing optical elements from a chalcogen containing glass which includes the step of exposing selected areas of the glass surface to radiation so that a change in the properties of the material so exposed is induced.

2. The method of claim 1 where the glass comprises a film, produced by suitable deposition techniques, on a suitable substrate.

3. The method of claim 1 where the glass-' comprises the bulk material.

4. The method of any of the preceding claims and further including the step of chemically etching the glass using an etchant whose effects on the exposed and unexposed areas of the glass are different.