WO1997027460A1

WO1997027460A1 - Miniaturized optical thin-film waveguide spectrometer

Info

Publication number: WO1997027460A1
Application number: PCT/EP1997/000326
Authority: WO
Inventors: Jörg Müller; Dietmar Sander
Original assignee: Mueller Joerg; Dietmar Sander
Priority date: 1996-01-25
Filing date: 1997-01-24
Publication date: 1997-07-31
Also published as: DE19602584A1; EP0876589A1

Abstract

The invention concerns a spectrometer which comprises: a thin light guide layer (4) in a layered structure (3) produced in thin-film technology between two cover layers (5, 6) with a lower refractive index; a light guide layer end face which has a grid (a, b) and is irradiated through the light guide layer with light to be analyzed; and a projection arrangement for displaying the spectrum on a line-scanning sensor, the grid being formed as a stepped grid with step edges perpendicular to the layer plane by removing regions of the layered structure which project in the layer direction by means of the mask technique. The spectrometer is characterized in that the grid is formed with transparent surfaces (a, b).

Description

Miniaturized optical thin film waveguide spectrometer.

The invention relates to a spectrometer of the type mentioned in the preamble of claim 1.

Such spectrometers then have the very compact structure with dimensions of a few millimeters, the structure consisting essentially of only one layer of dime layers, in particular without moving parts. Such spectrometers are very small and robust.

A spectrometer of the Alt mentioned at the beginning is out

Interdisciplinary Science Reviews, 1993, Vol. 18, No. 3, pages 273-279

known. Figure 6 on page 275 shows the structure of the contaminated construction, in which the grating is formed with reflecting surfaces, that is, it reflects the diffracted light back into the light guide layer. In this construction, the grating is also curved and at the same time serves as a projection device for mapping the spectrum onto the line catcher. A disadvantage of this construction is the step height which is very successful with a reflection grating. The formula applies to a reflection grating

a, = λ m / 2 n _s

with a _r = step height of the reflection grating, λ = light wavelength, m = diffraction order and n _s = refractive index of the layer material of the light guide layer. If the Spectromeler light is to be analyzed in the visible range around the mean wavelength λ = 500 nm (green light) and if n _s = 1.5, as is the case with conventional optical materials, such as glass or plastic, the result is one Step-high _ar of about 200 nm (nanometers).

A step height of 200 nm cannot be produced with conventional lithographic processes using UV light. The resolution of the mask's corner structure becomes too poor. Since the optical properties of a step grating depend very much on the quality of the edge structure, which is predetermined by the corners of the mask, a grating with such a step height produced in the usual way would not be usable.

In the case of the known known construction, therefore, syncluotiOiuöiitgen radiation is used instead of light. The manufacturing effort required for this is currently not acceptable for mass production.

Its original design, also with a reflection grating, is out

IEEE Photonics Technology Letteis, Vol.4, No. 8, 1992, pages 886-887 got it. The problems with the very low slufenli height are avoided by working in the 15th or 20th diffraction pattern. Then a much larger step height is possible, which can be achieved with standard methods. This spectrometer is also used for IR light, which also increases the permissible step height.

Another spectrometer of the same construction is shown in

Optical Society of America, Optics Letteis, Vol. 6, No. 12, pages 639-641

described. However, this construction is not produced in a thin layer structure, but with glass plates. The step grating is produced in a holographic decay, which enables smaller structures than the lithographic mask technique.

Step gratings operated in transmission with transparent surfaces are outside the area of the thin film spectrometer, ie with conventional glass bodies, as are also used as prisms

Applied Optics, Vol. 30, No. 31, pages 4540-4547

known. As mentioned in this document, these constructions are used in spectrographs for cameras or telescopes.

Transmissive grids, that is to say non-mirrored gratings, are also known in thin-film structures, but not as a sliver grating but as a strip load grating, the grating strips not on the end face of the light guide layer, but on the Side surface of a cover layer covering the light guide layer are applied. Such constructions are out

Applied Optics, Vol. 24, No. 2, pages 221-226

and from

Applied Optics, Vol. 23, No. 17, pages 3004-3009

known.

In the case of strip load gratings, the optical properties of the grid are essentially independent of the quality of the edge formation of the strips. Grids with very fine stripes could still be produced with standard mask technology, the selu bad edge structures then not disturbing the optical properties of the grating.

The object of the present invention is to provide a spectrometer of the type mentioned at the beginning which can be used for the analysis of visible light and which can be produced inexpensively in conventional masking technology.

This object is achieved with the features of the key drawing part of claim 1.

According to the invention, the grating is formed with transparent surfaces on the end face of the light guide layer, which is the optical waveguide of a layer structure forms. Dami does not apply the above-mentioned formula for reflection gratings, but the formula

a _t = λ m / (n _s - n _w )

with a ^ = step height of the transmission grating, λ = light wavelength, n _s - refractive index of the layer material, m = diffraction order and n _w = refractive index of the further material beyond the grating.

If there is air beyond the grating, then n _w = 1. With the usual optical material of the light guide layer with n _s = 1.5, the Nemier of the break is 0.5. In the calculation example for green light cited at the beginning, this results in a step height of 1 μm. The step height is therefore 5 times higher and can be easily produced with standard lithographic methods using masking technology with usable edge structuring. The manufacturing costs are thereby reduced by several orders of magnitude. Conventional systems for thin-film production, such as are available in the semiconductor industry, can be used.

The features of claim 2 are advantageously provided. In this case, n _w would be the refractive index of the material of the second light guide layer which continues to fill. The Nemier in the above-mentioned formula for a _j kaim dami can be made very small, so that much larger slufen heights are possible, which are even easier to manufacture. First, the layer structure is produced with the first light guide layer, the grating edge is made, and the second light guide layer with its layer structure is applied.

The features of claim 3 are advantageously provided. The second light guide layer that fills the grating can be a flat cylindrical be formed layered lens, which forms the piojection device with its transition to the wide-flowing third light guide layer and images the spectium onto the line sensor to be attached to the end face of the light guide layer.

The features of claim 4 are advantageous. If the light guide - pushes out sufficiently - its thickness is to be selected depending on the wavelength range used, so it only guides light into a propagation mode. Diffraction incisions due to the alternating influence of several light propagation modes are thus eliminated.

In the drawings, the egg invention is shown for example and schematically.

1 shows a di perpendicular to the layer plane on a first embodiment of a spectiometer according to the invention,

2 shows a three-dimensional cast position of the layer structure of FIG. 1 on the section plane according to line 2-2 in FIG. 1,

FIG. 3 shows a top view according to FIG. 1 of a spectiometry according to the invention in a second embodiment in and

Fig. 4 a stai viigiösseiten Scluiitt in dei layer height according to line

4 - 4 in Fig. 3

The spectiometer part shown in FIG. 1 in a top view and in FIG. 2 in three-dimensional staikei Ausschnittsveigrössei ungi ti on a commercially available, made of silicon subsluit 1 a layered structure 3 with a light guide Layer 4 between a lower cover layer 5 and an upper cover layer 6. The layers 4, 5 and 6 consist of SiO ₂ . The thickness of the lower cover layer 5 is, for example, 3 μm, while the layers 4 and 6 each have a thickness of approximately 0.6 μm. With a thickness of approximately 0.6 μm, the light guide layer 4 is so thin that katui only propagate into its light in a propagation mode. The cover layers 5 and 6 have a refractive index which differs from the refractive index of the light guide layer 4 in order to effect the light film in the light guide layer 4.

An end surface 7 running through all three layers 4, 5 and 6 of the layer structure 3, which therefore also represents the end surface of the light guide layer 4, is, as the enlargement of FIG. 2 shows, step-shaped with a regular step width b and slit height a . The steps are arranged at right angles so that the end surface 7 is inclined overall. The end face 7 is straight overall. So it is a non-focusing grating.

Light to be analyzed is parallelized by a light source 8 via a cylindrical lens 9 and irradiated into the light guide layer 4 via an end surface 10, which lies opposite the end surface 7, and runs in the arrow glow parallel to the step grating formed on the end surface 7.

The step grating formed with transparent surfaces a, b diffracts the light radiated by the light source 8, which is shown in FIG. 1 consisting of two lines. By diffraction at the step grating, the light is broken down into two lines and emitted into the air beyond the step grating. A cylindrical lens I 1 as a projection device images the spectrum with two lines on a line sensor 12. For the analysis of visible light in the range around the wavelength 500 nm (green light) and in the preferred first order (maximum intensity), that is to say with a blaze wavelength of 500 mn, a transmission stage grating results in a slaving line height a of eg 1 μm and b (corresponding to the lattice constant) of eg 1 to 2 μin.

Such a grating can be produced in a conventional masking technique using lithographic methods using UV light with sufficient quality of the edges.

For this purpose, the layer structure is first applied in succession to the substrate 1. This is preferably done using LPCVD technology (Low Pressure Chemical Vapor Deposition) at 930 ° C. The Si0 ₂ is deposited from SiCl ₂ H ₂ . Optically high-quality silicon oxide layers with a desired refractive index in the range of 1.5 can thereby be produced.

The layer structure is applied to substrate 1 a little longer, that is to say projecting beyond end surface 7. Then a wolf ram or molybdenum layer is wildly applied to the layer structure, which is coated with photoresist, exposed with the pattern of the stair tunnels of the lattice and then washed off beyond the stair-shaped end surface 7. The metal mask is now removed there and then in a plasma etching process preferably with pure CHF ₃ plasma with beam direction perpendicular to the layer plane dei entire layer structure beyond the step-shaped edge of the metal mask, so that the step-shaped end face 7 shown in FIG. 2 is formed. The metal film vvu d is then cleaned using known methods 3 shows a spectrometer in another embodiment. The layer structure 3 is in turn provided on the substrate 1 between the end face 10 on the irradiation side and the step-shaped end face 7 carrying the grating, which end face is constructed exactly as shown in FIG. 2. Subsequent to the layer structure 3, a further, second layer structure 3 'is provided, in which the light guide layer 4', which consists of a different material, adjoins the light guide layer 4 of the first layer structure 3 in abutting manner and at the same height.

The light to be analyzed which runs through the first light guide layer 4 thus continues after diffraction at the Gilter of the end face 7 in the second light guide layer 4 ', as indicated in FIG. 4. The diffracted light running in the second light guide layer 4 'runs up to an end surface 13. This end surface 13, as shown in FIG. 3, is continuously curved in the form of a lens. At the end face 13, a second light guide layer 4 ″ adjoins the second light guide layer 4 ′, which sits in a third layer construction 3 ″. The line sensor 12 is arranged on the opposite end face 14 of the third light guide layer 4 ″.

As shown in Figs. 3 and 4, the length L of the overall arrangement came e.g. 30 mm, the width B of the layer structure is approximately 5 mm and the thickness D of the layer structure is approximately 4 μm.

3 and 4, the projection device for imaging the spectrum on the line sensor 12 is designed as a cylindrical layer lens between the light guide layers 4 'and 4 ". This results in a completely integrated spectrometer in a solid structure.

The materials used in the embodiment of FIGS. 3 and 4 for the light conductor layers 4. 4 'and 4 "can be chosen such that on the there is a selu small difference in the bending index of the step grating provided on the end face 7.

1 and 2, the difference in the refractive index was approximately 0.5, since the material of the light guide layer 4 has a refractive index of 1.5 and air has a refractive index of 1. The essential difference in the bend index for the Slufenli height is 0.5.

3 and 4, the difference in the refractive index at the grating can be very much smaller. With a suitable choice of material, it can e.g. be 0.2 or less. As a result, the usable step height a (FIG. 2) becomes larger, so that the formation of the slits in the end face 7 can be produced more easily with conventional tecluiology.

The materials used in the embodiment of FIGS. 3 and 4 for the materials of the light guide layers 4, 4 'and 4 "can be, for example, Si ₃ N ₄ for the light guide layer 4 with a refractive index of 2, Si0 ₂ with a refractive index of 1.46 for the light guide layer 4 'and again Si ₃ N ₄ for the light guide layer 4 ". At each of the end surfaces 7 and 13 serving as interfaces between these layers, there is a difference in the refractive index of approximately 0.5.

The layer structure of the construction of FIGS. 3 and 4 can be produced in such a way that the layer structure of FIG. 2 is first produced. The layer structure 3 'is then applied, for example continuously also over the layer structure 3 (where it is subsequently removed). In a corresponding manner, as described in the embodiment in FIG. 2, the end face 13 is produced using masking technique and then the Layer structure 3 "produced in the same way. As shown in FIG. 4, the lower one can also be produced Cover layer 5 can be formed continuously, just like the upper cover layer 6, only the light guide layers 4, 4 'and 4 "being made to abut one another in mask technology.

Claims

PATENT CLAIMS:

1. Spectrometer with a thin light guide layer (4) in a layer structure (3) produced in thin-layer technology between two cover layers (5, 6) of lower refractive index, with an end face (7) having a grating (a, b) the light guide layer, which is irradiated through the light guide layer with light to be analyzed (light source 8), and with a piojection device (1 1, 4 ', 13, 4 ") for imaging the spectrum onto a line sensor (12), wherein the grating is designed as a step grating (a, b) with step channels perpendicular to the layer plane by ablation using masking technology from regions of the layer structure protruding in layer clearing, characterized in that the grating is formed with transparent surfaces (a, b).

2. Spectrometer according to claim 1, characterized in that a butt adjoining the end face (7) of the layer (4) in a further layer structure (3 '), a second light guide layer (4') is arranged with a different Bie¬ chung index.

3. Spectrometer according to claim 2, characterized in that the second light guide layer (4 ') with an end surface (13) which is continuously curved in the layer plane merges into a third light guide layer (4 ") adjoining butt.

4. Spectrometer according to claim 1, characterized in that the Lichtleiter¬ layer (4) is so thin in adaptation to the wavelength range of the light to be analyzed that it only allows single-mode light propagation.