Method of fabricating integrated optical devices by means of field-assisted ion exchange

Abstract: A method of fabricating integrated optical devices by means of field- assisted ion exchange on a starting substrate, wherein the substrate is first prepared to be connected to a source of potential difference, is duly masked in function of the optical circuit to be implemented and is dipped in a melted salt, which is a ion source for the exchange, with another electrode for applying the electric field apt to induce ion exchange.

Description: The present invention concerns technological processes for the fabrication of devices for processing optical signals and more particularly it refers to a method of fabricating integrated optical devices by means of field-assisted ion exchange. There is a growing demand for devices and circuits for processing optical signals, for different applications, such as signal processing, image transmission, printing, and the like. Said circuits, which not long ago used to be discrete devices, namely three-dimensional lenses, glasses, mirrors and the like, are now implemented according to optical integrated circuit criteria, wherein light is confined in a plane whereto different optical components belong, manufactured on a single substrate. The advantages are:reduced encumbrance and circuits wherein all the optical components are perfectly aligned and are not subjected to successive unwanted misalignement, as it might happen in case of discrete components aligned by mechanical supports. Such devices are manufactured by fabricating the guides and the other optical components, in which the optical field remains confined, on the surface of a substrate made of a convenient material by localized increase of its refractive index. One of the methods used to this aim consists in performing an ion exchange among the ions of the zones which have to take up guiding properties with the ions of different materials which have the characteristic of rendering the refractive index higher than that of the surrounding substrate. In this case the exchange can take place by dipping the substrate in a melted salt wherein there are the ions to be incorporated. Some ions of the chosen zones of the substrate interchange with the ions of the melted salt owing to the different concentration, therefore said zones take a different composition and hence a different refractive index from that of the substrate. The concentration profile of the ions inside the substrate obtained from the exchange, and hence the refractive index profile, is chiefly determined by the melted salt temperature and by the exchange duration time. To obtain profiles different from those due to the mere thermal diffusion phenomena, an external electric field is applied capable of inducing a guided ion migration along the force lines. That allows a better control of exchange phenomena and hence a better definition of the geometric characteristics of the waveguides and of the other optical components manufactured on the substrate. According to a method described in the article entitled "One-step field-assisted ion exchange for fabrication of buried multimode optical strip waveguide" by B.G. Pantchev, issued on Electronics Letters, 22 October 1987, pages 1188-1190, the substrate floats on a melted salt, which is the ion source and at the same time the electrode for applying the electric field. The same substrate acts also as bottom wall for an upper box containing also a melted salt, used as the other electrode. In correspondence with the zones wherein the ion exchange is required a thin silver layer is deposited, whose ions penetrate into the substrate under the action of the electric field. This method is not easy to be implemented for two reasons: the difficulty of realizing all the conditions necessary to make the substrate float and the difficulty of laying on the substrate itself the upper box filled with the melted salt, obtaining a stable apparatus, especially taking into account that the upper box is to be connected to the electric supply source. According to another method, a box for melted salt is fabricated by using as walls the various substrates, wherein the ion exchange is to be performed. The external surfaces of the substrates are metallized, so as to form an electrode for applying the electric field, and the internal surfaces are masked, so as to obtain the desired optical geometry. A conductor dipped in the melted salt is the other electrode for applying the electric field. The method requires the building of this box for melted salt whenever an integrated optical device is to be fabricated, taking all the precautions necessary to built a vessel suited to contain a high temperature liquid and to avoid pollutions due to extraneous substances (adhesives and the like), used for fabricating the box or introduced in consequence of manipulations. Hence the method is time consuming and expensive. The disadvantages above are overcome by the method of fabricating integrated optical devices by means of field-assisted ion exchange, provided by the present invention as defined in claim 1, which is easy to implement and does not require the fabrication of proper containers whenever a device is fabricated. The preamble of claim 1 corresponds to the method taught in JP-A-60-237 405. A particular object of the present invention is a method of fabricating integrated optical devices by means of field-assisted ion exchange on a starting substrate, which is characterized in that the substrate surface not involved in the exchange is first metallized, connected to a conductor for the application of the electric field and then insulated by deposition of an insulating-material layer, the surface whereupon ion exchange has to take place is covered with a proper masking layer which allows the required optical circuit to be obtained, then the substrate is dipped in a melted salt, which is an ion source for the exchange, with another electrode for applying the electric field apt to induce ion exchange. The foregoing and other characteristics of the present invention will be made clearer by the following description of a preferred embodiment thereof given by way of a non-limiting example. The melted salt is poured into a conventional container,which does not react with the melted salt and is not an electric conductor, and an electrode is dipped in the salt for applying an electric field. The electrode is made of a metal which does not react with the melted salt, e. g. platinum. The substrate, on whose surface ion exchange is to be performed for fabricating the integrated optical device, is in turn dipped in the melted salt after convenient preparation. More particularly, the surface which is not involved in the exchange is first metallized, connected to a conductor for applying the electric field and then insulated by deposition of a layer of insulating material, e.g. silicon nitrate (Si₃N₄). The requirements of this layer are: lack of porosity; lack of chemichal reactivity with the melted salt; resistance to the process temperatures; convenient thickness, in function of the dielectric strength of the material and of the potential difference applied across the electrodes. On the face wherein ion exchange is to be performed a convenient masking layer is deposited permitting the desired optical circuit to be obtained. This operation can be performed by using one of the known methods, e.g. a continuous titanium layer can be deposited and then the only zones wherein the exchange is to be performed can be removed. It is clear that what has been described has been given only by way of a non limiting example and that variations and modifications are possible without going out of the scope of the claims.

Claims: A method of fabricating an integrated optical device by means of field-assisted ion exchange on a starting substrate, wherein the substrate surface not involved in the exchange is first metallized and connected to a conductor for the application of an electric voltage, wherein: the surface upon which ion exchange is to take place is covered with a masking layer from which the required optical circuit pattern is obtained; and wherein the substrate is then dipped with another electrode in a melted salt which forms an ion source for ion exchange and a voltage causing an electric field within the substrate to induce the ion exchange is applied, characterized in that after the conductor is connected to the metallized surface, said surface is insulated by deposition of an insulating-material layer. A method as in claim 1, characterized in that said insulating material layer is a material which is resistant to the process temperatures, which has a suitable thickness in terms of its dielectric strength and of the potential difference applied across the electrodes, and which is not porous and does not chemically react with the melted salt.

Inventors: MORASCA SALVATORE, MORASCA, SALVATORE
Applicants: CSELT CENTRO STUDI LAB TELECOM, CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1991
Main classification: G02B6
IPC codes: C03C21, G02B6