Switch Matrix Including Optically Non-Linear Elements

The invention relates to a switch matrix composed of a plurality of optically non-linear, e.g., bistable, elements disposed as optically active layers on a common substrate surface to two methods of producing the switch matrix.

Optically non-linear elements may operate as light switches. Simplified, this process can be described as follows.

If the power of a laser beam which irradiates such an element is increased to beyond a certain threshold, a sudden rise in the transmitted light results and a sudden drop in the reflected light. This effect makes it possible to employ such optically non-linear components as switch elements for digital optical data processing. For this use, a two-dimensional arrangement of such switches is of particular interest -- a switch matrix in which the individual switching elements have lateral dimensions in an order of magnitude of 10 .mu.m.times.10 .mu.m and are positioned as closely together as possible.

Such a switch matrix is disclosed in "Optical Bistability III", published by Springer-Verlag, ISBN 3-540-16512-6, pages 39-41. This two-dimensional arrangement of switching elements was realized by means of MBE (molecular beam epitaxy). On a plate-shaped, 0.2 .mu.m thick substrate of Al.sub.0.4 Ga.sub.0.6 As,0 4Ga0 6As, optically active GaAs layers of a size of 9.times.9 .mu.m.sup.2 and a thickness of 1.5 .mu.m are applied at a mutual spacing of 20 .mu.m. Due to the mutual influence, the spaces between the elements cannot be reduced further. Moreover, the manufacturing process is complicated and expensive.

However, optically non-linear switching elements are known which can be produced by precipitating thin layers on a substrate without requiring epitaxial growth on the substrate. A typical example is the material ZnSe which is precipitated on a glass or sapphire substrate. In the past, it has not been attempted to spatially separate individual spots of such materials from one another on the substrate, rather various spots in the same layer are addressed in parallel by spatially limited laser light bundles. The definition of individual switching elements according to this process was thus effected by the spatial expanse of the laser light. Thermal contact and diffusion of charge carriers necessitate a spacing of the illuminated spots operating as switching elements in an order of magnitude of millimeters, instead, as desired, in an order of magnitude of microns, so as to limit crosstalk to permissible values. This solution is unsatisfactory because, due to the required relatively large distance between the individual switching elements, such a switch matrix can accommodate fewer switching elements by orders of magnitude than is desirable.

It is the object of the invention to make available an optical switch matrix having a very high packing density for its switching elements with low mutual influencing (cross-talk) as well as a method of producing the same.

This object is accomplished according to the invention by a switch matrix composed of a plurality of optically non-linear elements disposed as optically active layers on a common substrate surface wherein the substrate surface is a microstructure composed of a plurality of pillars with lateral spacings in a micron range, and the optically active layers of the respective devices are applied to respective frontal faces of free pillar ends in a cross-sectional region of the pillars.

The columnar structure of the substrate according to the invention and its high aspect ratio make it possible to place the individual switching elements in relatively close proximity to one another. This results in a large number and high packing density of the individual elements, with very good mutual decoupling. A structure having the desired very high aspect ratio is made possible by the mentioned LIGA [lithographic-galvanic] process which includes lithographic, galvanic and molding technology manufacturing steps.

The easiest way to produce such substrates resides in the use of plate-shaped X-ray resist material, e.g. PMMA (Plexiglas) which is structured in a brush shape by means of deep-etch X-ray lithography. The production of microstructured bodies according to the LIGA process is described and illustrated, inter alia, in KFK Bericht 3995 (KFK Report) by Kernforschungszentrum Karlsruhe (Nov. 1985). According to this publication, an X-ray radiation sensitive positive resist material, for example, is applied to a metal base plate, is partially irradiated, through a mask, with X-rays and developed so as to produce negative molds of plate-shaped microstructure bodies. The height of the negative mold corresponds to the layer thickness of the resist material; it may be up to 2 mm, depending on the penetration depth of the X-ray radiation. Thereafter, the negative mold is filled by electrodeposition with a metal, while employing the base plate as the electrode, whereupon the remaining resist material is removed by means of a solvent. Molding involves the use of a metal structure produced by deep-etch X-ray lithography and microgalvanic shaping for the repeated production of plastic molds which, in turn, can be filled, for example, by way of the electrodeposition of metals, whereupon the plastic is removed again.

With this technique it is possible to produce extremely accurate and fine structures having lateral dimensions in the micron range with a freely selectable height up to about 2 mm. With somewhat lower heights, it is even possible to realize minimum lateral dimensions in the submicron range.

This method is preferably suitable for the production of structured substrates for the precipitation of optically active layers which do not require epitaxy.

For optically active layers that must be produced by epitaxial growth, the use of crystals, e.g. Si, is proposed which obtained their columnar structure by anisotropic etching.

The invention will now be described below with reference to an embodiment and of FIGS. 1 to 6. Wherein:

FIG. 1 illustrates the function principle of an optically non-linear switching element in the low transmission switching state;

FIG. 2, illustrates the function principle of an optically non-linear switching element in the high transmission switching state;

FIG. 3 is a schematic representation of the mutual influencing between two adjacent optical switching elements;

FIG. 4 is a schematic representation of a possible improvement by way of structuring the optically active layer;

FIG. 5 illustrates the structuring of the substrate into pillars having a low aspect ratio; and

FIG. 6 illustrates the structuring of the substrate into pillars having a high aspect ratio.

FIG. 1 is a simplified illustration of the manner of operation of an optically non-linear switching element on a transparent substrate, with slight absorption losses being neglected. The length of the drawn arrows is intended to symbolize the intensity of the beams. In this case, switching element 3, is composed of a transparent substrate 7 onto whose front surface 1 an optically active layer 4 has been applied. The intensity of a holding beam 8 lies below a certain threshold at which the sudden change from a low transmission switching state to a high transmission switching state would take place. This results in a reflected beam 9 of the intensity of the holding beam 8 reduced by a loss component which leaves substrate 7 as a transmitted beam 10.

According to FIG. 2, a relatively low intensity signal beam 11 now causes the threshold to be exceeded and causes a transition of the layer 4 to the high transmission switching state. The reflected beam 11 disappears except for an insignificant remainder and the intensities of holding beam 8 and signal beam 9 produce the intensity of transmitted beam 10.

FIG. 3 is a cross-sectional view of a switch matrix which is defined by a first zone 14 of interaction with a first beam bundle 12 and a second zone 15 of interaction with a second beam bundle 13. The smallest possible distance between the two switching elements 3 is defined by their annoying interaction 16 due to thermal contact and the diffusion of charge carriers through optically active layer 4 and substrate 7. Therefore, thoughts have been directed toward spatially separating the individual switching elements, if they are spaced relatively closely laterally, by the introduction of troughs between them. Such troughs have not yet been fabricated for this purpose. The use of a diamond saw or a laser beam as cutting tool is contemplated. If such troughs are ever to become a reality, they will in any case have an aspect ratio of less than about 5, i.e. the depth of a trough will be greater at most by a factor of 5 than its width. For effective decoupling, deep troughs having a substantially higher aspect ratio are desirable. For the same reason, the use of dry etching processes known from the manufacture of microelectronic circuits is not appropriate for such structuring, insofar as the materials and substrates available for this purpose permit them at all.

FIG. 4 shows a possible improvement by structuring the optically active layer 4 so as to prevent at least the annoying interaction 16 from optically active layer 4.

FIG. 5 is a sectional view of the structuring according to the invention of substrate surface 1 in which pillars 2 reduce the annoying interaction 16 from substrate 7.

Substantial decoupling of the two switching elements 3 is realized by structuring substrate surface 1 for a high aspect ratio as shown in FIG. 6. According to the LIGA process, it is possible to produce transparent and non-transparent substrates having aspect ratios of 100 and sufficient stability. Typical dimensions are: a=5 .mu.m, b=500 .mu.m and c=10 .mu.m to 30 .mu.m. The easiest way to produce such substrates is by employing an X-ray resist material, e.g. PMMA (Plexiglas) as the substrate material and deep-etch X-ray lithography. Precipitation of optically active layers 4 on the end faces 5 of pillars 2 causes individual switching elements 3 to be produced there which can be addressed separately from one another and in parallel by means of light beams. A partial lateral vapor-deposition on the pillars and on the bottom area between them does not interfere with the operation of these elements. If necessary, several possibilities exist for interrupting the deposition of layers between the pillars.

For the economical mass production of such substrates, the LIGA process permits the use of plastic molds. Thus other transparent substrate materials can also be employed. If it should become possible to perform sintering processes within LIGA structures, glasses and ceramics would also be suitable.

The use of transparent substrates for optical switching elements is necessary only if the transmitted light serves as the information carrier. For a series of uses under discussion, the reflected light is sufficient as the information carrier. In this case, the substrate need not be transparent and it is possible to employ the entire rear side of the substrate for temperature control purposes. Very suitable are brush-shaped metal structures, e.g. of Ni or Cu, produced by the LIGA process onto which may be applied, if necessary, a layer to electrically insulate the optical switching elements and to absorb the radiation transmitted by the switching elements.