A method of producing optical waveguides

Abstract: The invention relates to a method of producing optical waveguides, by an ion exchange technique, in a glass substrate (1), the ion exchange between a source of ions (such as AgNO₃) and the glass substrate (1) taking place within selected areas of the glass substrate, as determined by an ion exchange mask, such ion exchange only taking place within surface areas of the glass substrate over which the ion exchange mask does not extend. In the method of the invention, the ion exchange mask is formed by depletion areas (5A, 5B) formed in the surface of the glass substrate.

Description: The invention relates to a method of producing optical waveguides by ion-exchange technique in a glass substrate, wherein the ion-exchange between a source of ions and the surface of the glass substrate takes place within glass substrate surface areas having no ion-exchange mask. Optical waveguides produced by ion-exchange technique in a glass substrate are promising for use in optical signal processing applications in optical communications, sensors, and other related areas. In ion-exchange optical waveguides are formed in a glass substrate when the refractive index of the glass substrate is modified (increased) locally by exchanging ions originally present in glass (such as sodium or Na⁺ ions) by diffusion with other ions from an external source of ions, such as Cs⁺, Rb⁺, Li⁺, K⁺, Ag⁺ or Tl⁺. The source of ions generally consists of a molten salt solution or a silver film deposited on the surface of the glass. For the principles of the ion-exchange technique, the following article is referred to: [1] Ion-Exchanged Glass Waveguides: A Review, R.V. Ramaswamy, Journal of Lightwave Technology, Vol. 6, No. 6, June 1988. Traditionally, a metal film acting as a mask layer is deposited on the surface of the glass substrate. Openings or windows corresponding to the optical waveguides are then patterned in the metal film by lithography technique. Ions are diffused through the openings or windows into the glass at elevated temperature. As a result of the ion exchange the refractive index of the glass substrate is increased at the mask openings, so that optical waveguides or channels are formed in the glass. When the source of ions consists of molten salt, it is today customary to use two-layer masks (e.g. Al/Al₂O₃) as film masks to prevent the formation of pin holes, that is, spotlike corrosions penetrating through the mask. In general, molten salt solutions, however, corrode all metal masks to such an extent that deep optical channels requiring long diffusion times are difficult to produce. Examples of the use of masks in connection with molten salt solutions can be found in the articles: [2] Index profiles of Multimode Optical Strip Waveguides by Field-Enhanced Ion Exchange in Glasses, H.J. Lilienhof, Optics Communications, Volume 35, No. 1, October 1980. [3] Planar Gradient-Index Glass Waveguide and its Applications to a 4-port Branched Circuit and Star Coupler, Eiji Okuda, Applied Optics, Vol. 23, No. 11, June 1984. Thin films of silver can be used in place of molten salt sources. The silver film (Ag) is evaporated on the surface of a glass substrate, so that the ion-exchange takes place under the influence of an electric field connected between the silver film and a cathode film positioned on the other side of the substrate. To produce optical channels, the anode side (facing the silver film) has to be patterned. The pattern can be formed directly in the silver film. A technique slightly easier to carry out is to use a patterned aluminium mask film between the glass and the silver film because the lithography technique is more applicable to aluminium. The lithography steps can be avoided by using commercial Cr mask glasses comprising a mask film of chrome on a high-quality glass substrate. The manufacturer patterns the chrome films by a lithography technique used in the production of integrated circuits with a very high precision and reliability. These chrome masks cannot be used with molten salt solutions because these corrode chrome. Examples of the use of masks with silver films can be found in the articles: [4] Ion Exchange Process for Fabrication of Waveguide Couplers for Fiber Optic Sensor Applications, S. Honkanen, Journal of Applied Physics 61, p. 52-56, January 1987. [5] Ion Exchange Processes in Glass for Fabrication of Waveguide Couplers, A. Tervonen, SPIE, Vol. 862 -Optical Interconnections (1987), p. 32-39. As the silver film is intended to touch the glass substrate through the openings in the mask film, a step or an edge in the mask disturbs the control of the ion exchange when forming deep optical channels and using thick silver films. Reference [5] discloses a method in which potassium-containing zones formed in the surface of the glass are used in place of a metal film, thus obtaining an even surface on which the silver film is formed. A problem with this method is that even though the potassium ions within the mask zones have a low mobility at the process temperature (about 200°C), part of them are nevertheless replaced by silver ions, which alters the refractive index also within the mask areas. For this reason, the potassium-containing zones do not act sufficiently efficiently as a mask when deep optical channels requiring long diffusion times are produced. In addition, this may cause crosstalk between the different channels in certain applications. The object of the invention is to provide a new method of producing optical waveguides, which avoids the above-described problems. This is achieved by means of a method of the type described in the preamble, which is characterized in that the ion exchange mask is formed by depletion areas formed in the surface of the glass substrate. The basic idea of the invention is that an electric field is connected at elevated temperature between a metal film mask (anode) formed on one side of the glass substrate and a metal film (cathode) formed on the opposite side. Positive ions (such as sodium ions) under the metal film then move away from the vicinity of the surface and form so-called depletion areas. As used herein, the term depletion area refers to glass substrate areas comprising little if any sodium ions. When the metal mask is removed, a depletion area pattern is left on the glass substrate. This pattern is a "copy" of the mask pattern. A conventional ion exchange process is then carried out by using a silver film or a molten salt solution as a source of ions. The ion exchange takes place only within areas between the depletion areas (at the openings of the original metal mask). The advantages of the invention include the following: Pin holes contained in the original metal mask are blocked within the depletion area, which is of great importance in long ion exchange processes utilizing an electric field, As distinct from a metal mask, the depletion area mask will not wear nor corrode by molten salt solutions, not even in long processes, The depletion area mask can penetrate relatively deep into the glass substrate, thus restricting sideward ion exchange in the direction of the surface of the substrate. This enables the production of highly symmetrical uniform optical waveguides. In addition, it is possible to provide optical channels even narrower than the openings of the original metal mask, which reduces the standard required from applied lithography. The invention enables direct use of commercial Cr mask glasses as substrates with molten salt solutions. A completely planar surface is obtained which facilitates long ion exchanges from thick silver films. If the ion exchange is carried out in a vacuum during the evaporation of the silver film, the silver films adhere better to the glass than when using a mask patterned in the surface. Embodiments of the invention are described below by way of example with reference to the accompanying drawings, in which:- FIGURES 1 and 2 are schematic sectional views illustrating successive steps in a method embodying the invention, FIGURES 3A and 4A are schematic sectional views illustrating further successive steps in the method, FIGURES 3B and 4B are schematic sectional views illustrating alternative steps to those illustrated in Figure 3A and 4A respectively, and FIGURE 5 is a schematic sectional view illustrating the end result of the method. In a method embodying the invention, a thin film 2 of metal is deposited, e.g. by sputtering, on one planar surface of a preferably sheetlike glass substrate 1. Openings 4 are patterned in the metal film by a lithography technique, as shown in Figure 1. The thin-film mask used forms a so-called positive mask pattern in which the openings 4 correspond to the desired optical waveguides and the thin-film areas 2 correspond to areas in which no ion exchange and resulting formation of optical waveguides should take place. Alternatively, it is also possible to use such commercial mask plates as described in the above reference [5], that is, a thin film of chrome deposited on a glass substrate. In the second operational step of the method, the temperature of the glass substrate 1 is raised (e.g. 350°) and an electric field is connected between the mask film 2A, 2B and a metal film 3 formed on the opposite side of the glass substrate 1 by connecting the more positive terminal of the source of voltage to the mask film 2A, 2B and the more negative terminal to the metal film 3. Under the influence of the electric field, sodium or Na⁺ ions in the surface of the glass substrate 1 under the mask films 2A, 2B move toward the metal film 3 (cathode) away from the vicinity of the surface. As shown in Figure 2, so-called depletion areas 5A and 5B containing little if any sodium ions are formed below the mask films 2A, 2B (anode), penetrating into a predetermined depth in the glass substrate 1. At the openings 4 of the mask film, no depletion of sodium ions occurs, so the amount of sodium ions is normal within areas 6 remaining at the openings 4 in the surface of the substrate 1. Subsequently, the metal films 2A, 2B and 3 are removed as shown in Figure 3A, whereby the depletion areas 5A and 5B formed as described above remain permanently in the surface of the glass substrate 1, forming a "copy" of the original mask pattern, in which the depletion areas correspond to areas previously covered with the metal mask. The area 6 between the formed depletion areas 5A and 5B is preferably constricted slightly narrower than the original opening 4 in the metal film 2. This lowers the standard required from the lithography technique. The glass substrate masked according to the invention with depletion areas can now be used as such in connection with conventional ion exchange techniques. In Figure 4A, for example, a molten salt solution, such as AgNO₃ acting as a source of ions is allowed to influence the masked surface of the glass substrate 1 at elevated temperature (e.g. 250°C). In practice, the glass substrate is immersed in the salt solution. Ion exchange between the salt solution AgNO₃ and the glass takes place only within the unmasked areas 6 containing sodium ions but not within the masked depletion areas 5A and 5B because they do not contain any sodium ions which could be replaced with Ag⁺ ions. When the ion exchange continues for a sufficiently long period of time, a great amount of Ag⁺ ions is gathered within the area 6. The Ag⁺ ions increase the glass refractive index within this particular area, forming an optical guide 8, as shown in Figure 5. Other widely used molten salt solutions include KNO₃, CsNO₃, TlNO₃ and NaNO₃. Lateral ion exchange is not possible between the area 6 and the depletion areas 5A and 5B, which enables the production of more symmetrical uniform optical waveguides. Alternatively, as illustrated in Figure 3B, after the stage of Figure 2, the metal film 3 can be retained, with only the original mask film 2 being removed after the formation of the depletion areas 5A and 5B. Thereafter, as illustrated in Figure 4B a silver film 7 acting as a source of ions can be formed directly on the surface of the glass substrate. An electric field is connected at elevated temperature between the silver film 7 and the metal film 3 provided on the opposite side of the substrate 1. Ion exchange then takes place only within the previously unmasked areas 6 similarly as described in connection with Figure 4A. As a result, an optical waveguide 8 of Figure 5 is obtained.

Claims: A method of producing optical waveguides (8) by ion exchange technique in a glass substrate (1), wherein the ion exchange between a source of ions and the surface of the glass substrate (1) takes place within glass substrate surface areas having no ion exchange mask, characterized in that the ion exchange mask is formed by depletion areas (5A, 5B) formed in the surface of the glass substrate (1). A method according to claim 1, characterized in that the depletion areas (5A, 5B) are formed by depositing a metal film (2A, 2B) on the surface of the glass substrate (1); removing the metal film from areas in which the optical waveguides are to be formed; depositing another metal film (3) on the opposite side of the glass substrate; connecting an electric field between the first and the second metal film so that ions in the surface of the glass substrate under the first metal film move away from the vicinity of the surface, a depletion area being formed in the surface; and removing said metal films. A method according to claim 1 or 2, characterized in that the ions of the glass substrate are sodium ions. A method according to claim 1, 2 or 3, characterized in that the source of ions consists of a molten salt solution. A method according to claim 1, 2 or 3, characterized in that the source of ions consists of a silver film formed directly on the surface of the glass substrate. An ion exchange mask, characterized in that the mask comprises depletion areas (5A, 5B) formed in a surface of a glass substrate (1).

Inventors: HONKANEN SEPPO, TAMMELA SIMO, TERVONEN ARI, HONKANEN, SEPPO, TAMMELA, SIMO, TERVONEN, ARI
Applicants: NOKIA OY AB, OY NOKIA AB
Application year: 1990
Main classification: G02B6
IPC codes: G02B6, C03C21