GB2079536A - Process for producing an optical network - Google Patents

Abstract

A periodic optical pattern (e.g. a diffraction grating) is produced by ion implantation through a mask of photoresist, exposed e.g. holographically, into a substrate 6,4 e.g. lithium niobate. The implanted regions have lower index of refraction and are more readily chemically etched. The substrate surface 6 may be modified to form a light guide and a metal layer may be interposed between the resin and surface 6 to improve the quality of the pattern produced. <IMAGE>

Description

SPECIFICATION Process for producing an optical network The present invention relates to a process for producing an optical network intended more particularly for use in integrated optics.

In optics a network is a periodic modification of a structure able to diffract light in accordance with a certain number of orders.

The angular position of these orders is entirely defined by the knowledge of the wavelength of the incident light and by the pitch of the network. However, the amplitude of each order is dependent on the profile of the network (shape, depth of the grooves, etc.).

In integrated optics these structures are generally associated with light guides and make it possible to carry out a certain number of interesting functions, such as coupling networks or Bragg reflectors which are well known in the art.

Bragg reflectors play an important part in the construction of beam splitting plates and integrating mirrors, as well as in the production of passive polarization converters.

These networks have extremely fine pitches (between 1500 and 8000A) and one of the presently best known construction methods uses holographic processes using interferences of two laser beams with limited wavelengths.

The standard production of such networks involves depositing a layer of generally positive photosensitive resin on the selected substrate and recording the interference system in said resin. In other known processes recording takes place by contact from a parent network illuminated by an ultraviolet beam or by scanning of an electron beam.

After stripping the network formed in this way in the resin is transferred from the resin used as a mask to the substrate by chemical or ionic etching.

However, this apparently simple operation frequently comes up against a number of technical problems. Thus, for the transfer of the photosensitive resins from the network to the substrate to be possible it is necessary for said resin to constitute a good screen for duplicating the network on the substrate.

Moreover, for the transfer to take place under good conditions no trace of resin must be left behind at the bottom of the grooves of the network.

The latter parameter is in fact impossible to check or control in a simple manner, particularly if the residual height is below 1 ooA. This makes the construction of such networks difficulty reproducible, because a few dozen Angstrums of photosensitive resin left in the grooves is enough to prejudice the transfer phenomena.

The invention relates to a process for producing an optical network which makes it possible to obviate these disadvantages and in particular facilitate the transfer of the photosensitive resin from the network to the substrate.

The present invention therefore relates to a process for producing an optical network obtained by duplication from a network recorded more particularly by holography on a photosensitive resin deposited on a substrate, wherein the network is transferred from the photosensitive resin to the substrate by ion implantation making it possible to create implanted areas in the substrate having a lower index of refraction than the index of refraction of the non-implanted areas, and wherein the resin used as a mask for ion implantation is eliminated.

According to a preferred embodiment of the invention the substrate is made from lithium niobate and the implanted ions can be ions of helium, boron, neon or nitrogen.

It is well known that ion implantation leads to a signigicant reduction of the index of refraction in lithium niobate.

Moreover, the research has shown that ion implantation significantly sensitizes lithium niobate (LiNbO3) to certain chemical etching processes, particularly that using dilute hydrofluoric acid.

This reduction of the index of refraction due to ion implantation and the sensitization of the implanted areas to chemical etching is described in a thesis submitted to the U.S.M.

Grenoble and l.N.P.G. in September 26th 1978 by Mr Destefanis, entitled "Study of the modification of optical properties induced by ion implantation in LiNbO3 -Application to the production of wave guides".

By implanting a very high dose at appropriate energy levels, bearing in mind the thicknesses of the photosensitive resin present after recording the network, it is possible to transfer this network to the substrate and obtain relatively high diffraction efficiencies, bearing in mind the considerable variations in the index of the substrate which can be obtained.

According to a preferred embodiment of the invention before carrying out ion implantation between the photosensitive resin and the substrate is placed at least one layer permitting a good recording of the network in the resin due to the more favourable reflection conditions at the layer-resin interface. This layer is preferably a metal layer.

Moreover, if the said layer is a good thermal conductor it permits a better distribution of the thermal effect over the entire substrate.

This in particular prevents serious thermal constraints which could possibly occur at the implantation doses used.

The invention is described in greater detail hereinafter relative to non-limitative embodiments and with reference to the attached drawings, wherein shown: Figure 1 diagrammatically the process for producing an integrated optical network according to the invention.

Figure 2 diagrammatically the optical network obtained according to Fig. 1.

Figure 3 diagrammatically a variant of the process according to the invention.

Figure 4 for different types of implanted ions, the ion penetration depth (h) (in microns) in the resin as a function of the ion energy (E) (in kilo-electron volt).

Figure 3 variations of the index of refraction can, as a function of the implanted helium dose (number of atoms per square centimetre) at ambient temperature in a lithium niobate substrate.

Fig. 1 shows the different stages in the production process of an optical network.

The first stage (Fig. 1 a) consists of depositing by any known means (by reel, calendaring, etc.) a generally positive photosensitive resin 2 on a substrate 4, preferably formed from lithium niobate. In the represented case of an integrated optical device the substrate 4 is associated with a light guide 6 which is, for example, made from the substrate by surface modification of its optical properties. The interference system of two laser beams is recorded in resin 2 by holographic processes.

The wavelength of the two laser beams used for recording the interference system is, for example, equal to 4579 . The network 8 (Fig. 1 b) of resin 2, provided with grooves 9 and obtained after stripping by means of a convention development product can be transferred from resin 2 to substrate 4 by duplication.

According to the invention the transfer of the resin 8 to the substrate 4 takes place by ion implantation (Fig. 1c). The ions implanted in the direction indicated by the arrows are constituted, for example, by helium ions of a few hundred KeV and at variable doses. The implanted areas 10 have a lower index of refraction than that of the non-implanted areas. The photosensitive resin 2 serves as a mask for ion implantation. The photosensitive resin 2 can then be eliminated (Fig. 1d) by any known means (chemical etching, etc.).

The thus obtained network 12 can have index differences of the order of 0.1 between the implanted areas 10 and the non-implanted areas.

The implantation energy must be such that the implanted ions are effectively arrested by a height h of resin (cf. Figs. 1b and inc). The choice of the implantation energy E as a function of h for different implanted ions is given in Fig. 4.

Obviously the variation of the index of refraction depends on the dose of implanted ions (number of ions implanted per square centimetre).

The curve shown in Fig. 5 indicates the variation (An) of the index of refraction of lithium niobate as a function of the dose (D) of helium ions implanted therein, said implantation taking place at ambient temperature. A significant difference of the index (an) appears for an implanted does (D) exceeding 1015. To improve the diffraction efficiency of the network obtained (Fig. 1d) by increasing the index difference in the substrate it is possible to carry out a supplementary stage.

This supplementary.stage (Fig. 1e) consists of chemically etching the implanted areas 10, preferably by dilute hydrdfluoric acid. The implanted areas 10 in the lithium niobate are sensitized to such a chamical etching process (cf. the thesis of Mr Destefanis referred to hereinbefore). This chemical etching makes it possible to obtain an integrated network of grooves 14 as shown in Fig. 2. In accordance with the particular applications to the integrated network implanted areas 10 may or may not be chemically etched.

Moreover according to the invention for the purpose of improving the recording of the network 8 in the photosensitive resin 2, it is possible to place between resin 2 and substrate 4 or more specifically between resin 2 and light guide 6, when the latter exists, one or more carefully selected layers. It is of particular interest to interpose a metal layer 16, in the manner shown in Fig. 3, which is made e.g. from aluminium; The advantages of such a metal layer 16 have been defined hereinbefore.

After recording network 8 in the resin, metal layer 16, like the photosensitive resin 2 is eliminated by any known means (chemical etching, etc.).

As has been stated hereinbefore the difficulties encountered in the prior art in producing the integrated network were largely due to the fact that it was very difficult to know whether the grooves 9 were completely free from residues. In the case of ion implantation only the thickness difference between the top and bottom of the groove is important and not the height of photosensitive resin left behind during a conventional transfer process.

This value is also relatively reproducible. To give an idea we will take an initial resin thickness of 1 sooA. After recording and stripping the resin thickness will vary between values h and h' (cf. Figs. 1 b and 1 c), h will be e.g. approximately 1400A and h' below 1 ooh. Thus, the difference between h and h' can vary between 1300 and 1400 in this particular case, i.e. by only 7% in relative values. This variation remains small, despite the relatively large variations of h' (between O and 100 A in this example).

A distinction must be made between the ion implantation conditions used in the invention, which utilize high energy levels of approximately 20 to 100KeV for e.g. lithium niobate and ions of various types and the ionic etching conditions used in the prior art utilizing much lower energy levels of approximately 0.1 to 1 KeV for lithium niobate and ions such as argon and xenon producing a material deficiency in the same way as chemical etching and having the same limitations as the latter.

Claims (10)

1. A process for producing an optical network obtained by duplication from a network recorded more particularly by holography on a photosensitive resin deposited on a substrate, wherein the network is transferred from the photosensitive resin to the substrate by ion implantation making it possible to create implanted areas in the substrate having a lower index of refraction than the index of refraction of the non-implanted areas, and wherein the resin used as a mask for ion implantation is eliminated.

2. A process according to claim 1, wherein the areas sensitized by ion implantation are chemically etched so as to obtain a network of grooves.

3. A process according to claims 1 or 2, wherein prior to carrying out ion implantation at least one layer permitting a good recording of the network in the photosensitive resin is placed between the latter and the substrate.

4. A process according to claim 1, wherein the substrate is made from lithium niobate.

5. A process according to claim 1, wherein the implanted ions are constituted by helium.

6. A process according to claims 2 and 4, wherein chemical etching is performed by dilute hydrofluoric acid.

7. A process according to claim 5, wherein the number of ions implanted per square centimetre exceeds 10'5.

8. A process according to claim 3, wherein the layer is a reflecting layer.

9. A process according to claim 3, wherein the layer is a good thermal conductor.

10. A process according to claims 8 and 9, wherein the layer is metallic.