METHOD OF MANUFACTURING A LINEARLY VARIABLE OPTICAL FILTER

Abstract

A method for producing a spectral gradient filter on a substrate including: providing the substrate with a first surface to be coated; providing a shadow mask that includes a bordered coating area with an edge, wherein the geometry of the shadow mask is adjusted to the desired gradient profile of the gradient filter; creating a masked substrate by fixing the shadow mask on the first substrate surface to be coated in such a way that parts of the substrate surface are covered, but the substrate surface is essentially exposed in the coating area; and carrying out PVD coating so that parts of the shadow mask lie directly on the surface of the substrate so that no vapour migration occurs in the area of these parts during the coating process and the shadow mask is detachably fixed to the substrate so that the shadow mask can be used for several coatings.

Claims

1. Method for producing a spectral gradient filter on a substrate comprising the steps of: providing the substrate with a first surface to be coated providing a shadow mask that comprises at least one bordered coating area with an edge, wherein the geometry of the shadow mask is adjusted to the desired gradient profile of the gradient filter creating a masked substrate by fixing the shadow mask on the first substrate surface to be coated in such a way that parts of the substrate surface are covered, but the substrate surface is essentially exposed in the coating area, inserting the masked substrate into a coating system which is based on physical deposition from the gas phase (PVD) carrying out the PVD coating characterized in that at least parts of the shadow mask lie directly on the surface of the substrate so that no vapour migration occurs in the area of these parts during the coating and wherein the shadow mask is fixed to the substrate in a mechanically detachable manner so that the shadow mask can be used for several coatings.

2. The method according to claim 1, characterized in that the edge of the shadow mask is designed to be vertical or wedge-shaped or overhanging.

3. The method according to claim 1, characterized in that the shadow mask comprises several coating areas.

4. The method according to claim 1, characterized in that for providing the shadow mask, test coatings are carried out with test shadow masks of different thicknesses and/or different degrees of edge steepness, wherein the test shadow masks can comprise several coating areas with different degrees of edge steepness.

5. The method according to claim 1, characterized in that the shadow mask has at least one element spaced apart from the edge in the coating area, wherein the element is also spaced from the substrate, when the shadow mask together with the substrate form the masked substrate.

6. The method according to claim 5 characterized in that the at least one element is designed in the form of a web.

7. The method according to claim 1, characterized in that the coating system is a drum system, in which the substrates to be coated are guided past a coating source with a target surface, preferably a sputtering target, wherein the axis of rotation of the drum is arranged parallel to a straight line lying in the target surface and the substrates are mounted on the drum shell in such a way that the plane of the substrates spanned by the substrate surface does not intersect the axis of rotation of the drum.

8. The method according to claim 1, characterized in that the coating system is a system with a turntable, in which the substrates to be coated are guided past a coating source with a target surface, preferably a sputtering target, wherein the axis of rotation of the turntable is perpendicular to the target surface and the substrate surface to be coated is aligned parallel to the target surface at least during the coating process.

9. The method according to claim 1, characterized in that the coating system is a linear system, in which the substrates to be coated are linearly guided past a coating source with a target surface, preferably a sputtering target, and the substrate surface is parallel to the surface formed by the coating source at least during the coating process.

10. A shadow mask with a shadowing area and a coating area, wherein the shadowing area and the coating area are separated by an edge, wherein at least one element spaced apart from the edge is provided in the coating area.

11. The shadow mask according to claim 10 characterized in that the at least one element spaced apart from the edge has a depth which does not exceed half the thickness of the shadow mask.

12. The shadow mask according to claim 11 characterized in that the at least one element spaced apart from the edge is positively connected to one side of the shadow mask.

Description

[0036] The invention will now be explained in detail by means of examples and with the aid of the figures.

[0037] FIG. 1 shows schematically a coating system with a spaced shadow mask according to the prior art.

[0038] According to a first exemplary embodiment of the present invention, various shadow masks were tested. On the one hand, the thickness of the masks was varied from 1 mm to 5 mm. On the other hand, the opening of the various coating fields was provided with a vertical edge or with different beveled edges. It was practically determined by means of 3 different coating procedures how the different mask forms affect the course of the layer thickness. The size of the coating openings at 20×20 mm is not optimized for maximum use of the surface, but in such a way that even with the 4 to 5 mm thick masks there are no undesirable shadowing effects from the opposite side.

[0039] FIG. 2 shows a corresponding 5 mm thick shadow mask with 15 coating openings. The 3 openings on the left side have a vertical edge. In a column to the right of it, a wedge of 1 mm width has been realized, wherein the coating opening itself remains the same. The width of the wedge then increases by one millimeter per column, so that wedges with a width of 4 mm are implemented in the rightmost column. For clarification, FIG. 3 shows a section through the shadow mask of FIG. 2.

[0040] Simulations have shown that the thicker the masks, the smaller the increase in layer thickness over the distance, although the distance over which the wavelength profile is approximately linear becomes greater. In addition, the simulations showed that the larger the bevel of the mask, the smaller the increase in layer thickness over the distance.

[0041] It is therefore clear that with the two parameters, 1) thickness of the mask and 2) edge steepness of the coating openings, two degrees of freedom are available that make it possible to generate masks that lead to coating gradients that come as close as possible to customer specifications.

[0042] For a shadow mask with a thickness of 2 mm, FIG. 6 shows the simulated increase in thickness with the distance from the edge of the coating area, specifically for different wedges. “0 mm” is a vertical edge. “1 mm” is a wedge having a width of one millimeter, and so on. It can clearly be seen that the greatest layer thickness gradient can be achieved with a vertical edge. Following this, the dashed line shows an ideally linear course of the increase in layer thickness over a distance of 2 mm.

[0043] In the case of a bandpass produced with a shadow mask having a thickness of 5 mm without a wedge, based on the Fabry-Perot design, the shift of the transmission peak is shown in FIG. 7. The measured transmission curves are shown, with neighboring curves each resulting from a lateral displacement of the substrate by 0.2 mm. As expected, the linearity is only approximated in both cases (FIG. 6 and also FIG. 7). The gradient is larger at the edge of the coating area, whereas the gradient is still present towards the center of the coating area, but is somewhat lower.

[0044] For many applications, this approximation to linearity is sufficient. If this is not the case, however, the shadow mask can be adjusted with thin webs arranged in the coating area in accordance with the masks shown in FIGS. 4 and 5. With reference to FIG. 6, for example, a thin web with a width of only 0.25 mm and a depth of 0.2 mm could be arranged at the location marked with a 1 mm distance.

[0045] In a further example, the invention is explained for a short-pass filter. FIG. 8 shows the layer thicknesses of the filter from a layer system of SiO.sub.2 and Nb.sub.2O.sub.5 as low and high refractive index materials. The layer thicknesses are given in nanometers and the first layer on the substrate is listed in the first line of the table. These specified layer thicknesses apply immediately next to the shadow mask. The shadow mask has a thickness of 5 mm, so that over a distance of 5 mm on the substrate, calculated from the edge of the shadow mask, the shadow mask produces the layer thickness profile shown in FIG. 9 and normalized to the layer thicknesses directly next to the shadow mask. The spectral properties at various positions on the substrate, at a distance of 1 mm from one another from a position directly next to the shadow mask (solid line) up to 5 mm from the edge of the shadow mask (dashed lines), are shown in FIG. 10. In this case, too, the spectral position of the edge changes continuously, almost linearly and within a small area on the substrate.

[0046] An essential aspect of the present invention is that very good system utilization can be achieved and that it is possible to produce filters with identical properties. FIG. 11 shows a shadow mask optimized for system utilization. A plan substrate with dimensions of 110 mm×160 mm is covered with a shadow mask of the same external dimensions. For the specific geometry of the required filter (5 mm×5 mm outer dimensions with a linearly variable filter area in the center of 2 mm×2 mm), the shadow mask can be covered with 4 open areas that extend almost over the entire height of the substrate. A variable filter area is created on each side of each opening so that 8 rows of filters are produced on the substrate. There is space for 30 filters in each row, so that 240 filters can be produced on each substrate, which filters can be obtained by separating them from the large substrate after the coating process. Before the separation, further coatings, such as an anti-reflective coating, can be applied to the back of the substrate.

[0047] So far, only shadow masks with vertical edges or with a wedge have been shown in the figures. In addition, shadow masks with overhanging edges, i.e. edges with a recess, can be interesting.

[0048] So far, only those gradients have been considered whose gradient property is due to only one edge of the shadow mask, i.e. an increase in layer thickness with a distance increasing perpendicular to this edge.

[0049] If two edges that are at an angle to one another influence the change in layer thickness with increasing distance from these edges, as is the case, for example, in the corners of the shadow masks shown in FIGS. 2 to 5, then the layer thickness will depend on two coordinates on the substrate. One could speak of a two-dimensional gradient here. The present invention also relates to such two-dimensional gradient filters and their manufacture.

[0050] A method has been disclosed for producing a spectral gradient filter on a substrate, comprising the steps of: [0051] providing the substrate with a first surface to be coated [0052] providing a shadow mask that comprises at least one bordered coating area with an edge, wherein the geometry of the shadow mask is adjusted to the desired gradient profile of the gradient filter [0053] creating a masked substrate by fixing the shadow mask on the first substrate surface to be coated, in such a way that parts of the substrate surface are covered, but the substrate surface is essentially exposed in the coating area, [0054] inserting the masked substrate into a coating system based on physical deposition from the gas phase (PVD) [0055] carrying out the PVD coating

[0056] characterized in that

[0057] at least parts of the shadow mask lie directly on the surface of the substrate so that no vapour migration occurs in the area of these parts during the coating and wherein the shadow mask is fixed to the substrate in a mechanically detachable manner so that the shadow mask can be used for several coatings.

[0058] In the process, the edge of the shadow mask can be designed to be vertical or wedge-shaped or overhanging.

[0059] The shadow mask can comprise several coating areas.

[0060] To provide the shadow mask, test coatings can be carried out with test shadow masks of different thicknesses and/or different degrees of edge steepness, wherein the test shadow masks can comprise several coating areas with different degrees of edge steepness.

[0061] The shadow mask can have at least one element spaced apart from the edge in the coating area, wherein the element is also spaced from the substrate, when the shadow mask together with the substrate form the masked substrate. The at least one element can be designed in the form of a web.

[0062] The coating system can be a drum system, in which the substrates to be coated are guided past a coating source with a target surface, preferably a sputtering target, wherein the axis of rotation of the drum is arranged parallel to a straight line lying in the target surface and the substrates are mounted on the drum shell in such a way that the plane of the substrates spanned by the substrate surface does not intersect the axis of rotation of the drum.

[0063] The coating system can also be a system with a turntable, in which the substrates to be coated are guided past a coating source with a target surface, preferably a sputtering target, wherein the axis of rotation of the turntable is perpendicular to the target surface and the substrate surface to be coated is aligned parallel to the target surface at least during the coating.

[0064] Furthermore, the coating system can be, for example, a linear system, in which the substrates to be coated are guided linearly past a coating source with a target surface, preferably a sputtering target, and at least during the coating process the substrate surface is parallel to the surface formed by the coating source.

[0065] A shadow mask with a shadowing area and a coating area has been disclosed, wherein the shadowing area and the coating area are separated by an edge, wherein at least one element spaced apart from the edge is provided in the coating area. The at least one element spaced apart from the edge can have a depth which does not exceed half the thickness of the shadow mask. The at least one element spaced apart from the edge can be positively connected to one side of the shadow mask.