Patent Description:
It has become increasingly common for display systems, in particular head or helmet-mounted display systems and head-up display systems, to use waveguides incorporating diffractive elements. Such waveguides may serve the multiple purposes of: conveying light from an image source to a line of sight to a viewer; of expanding the pupil of the image-bearing light in one or two dimensions as the light propagates through the waveguide, providing for a greater range of eye positions from which a user may view an image; and to act as a combiner in transparent displays so that the image to be displayed may be viewed overlain on the user's view of the outside world as seen through the transparent waveguide.

Two or three different diffraction gratings may be embedded within a waveguide or provided on or close to the surface of a waveguide to couple collimated light into and out of the waveguide and to cause expansion of the pupil of light. However, the fabrication of such waveguides and diffraction gratings to the tolerances required to achieve high image quality can be challenging, in particular when a large waveguide having large diffraction gratings is required.

<CIT> discloses methods of manufacturing an optical waveguide where a binary master relief grating is provided and then modified to render a first and second blazed relief structure. Two grating structures are selectively coated and laminated together to form non-overlapping grating structures within the waveguide.

Embodiments of the present invention will now be described in more detail, by way of example only, with reference to the accompanying drawings of which:.

An example of a known method for fabricating a transparent waveguide incorporating two main diffraction gratings will be described briefly with reference to <FIG>.

Referring initially to <FIG>, an example shown in a sectional view, not to scale, is a representation of a master grating tool <NUM> that has been made for use in the fabrication of a waveguides having two diffraction grating regions with different grating profiles. The grating tool <NUM> comprises two different master gratings <NUM>, <NUM>, which would typically be fabricated separately and mounted upon a single grating tool substrate <NUM>. To enable the two master gratings to be fabricated and then mounted on the tool substrate <NUM>, they need to have a not insignificant thickness. Their thickness necessarily results in the two master gratings having edges which, in particular, form a gap <NUM> between the master gratings when fixed to the single section of tool substrate <NUM>.

<FIG> also shows, in a sectional view, a representation of the result of one example method for replicating the master gratings <NUM>, <NUM> comprising imprinting the master grating tool <NUM> into a "replication layer" <NUM> of a UV-curable polymer that had been applied to a glass base layer <NUM>. <FIG> shows the replication layer <NUM> after UV curing of the polymer and removal of the master grating tool <NUM>, leaving replicas <NUM>, <NUM> of the master grating profiles <NUM>, <NUM> respectively imprinted into the replication layer <NUM>. A protrusion <NUM>, one of three protrusions in this example, remains in the replication layer <NUM> corresponding to the gap <NUM> between the master gratings <NUM>, <NUM> of the master grating tool <NUM>. Besides imprinting, other methods are known for replicating a master grating profile <NUM>, <NUM>, including nano-imprint lithography. However, the protrusions, including the central protrusion <NUM>, would remain as features in the resultant replication layer <NUM>.

Referring to <FIG>, a representation of a completed waveguide structure is shown, in a sectional view, in which respective conformal layers <NUM>, <NUM> of a dielectric material have been applied to the imprinted grating profiles <NUM>, <NUM>. A 'lamination layer" <NUM> made of the same or a similar UV-curable polymer to that used for the replication layer <NUM> is applied to cover the replication layer <NUM>, and another glass layer <NUM> is applied to the lamination layer <NUM>, under some pressure to ensure that the lamination layer <NUM> fully conforms to the profile of the gratings <NUM>, <NUM> with their dielectric coatings <NUM>, <NUM>, leaving no gaps. The UV-curable polymer of the lamination layer <NUM> is then cured to result in the structure shown in <FIG>.

In practice, the depth of the replication layer <NUM>, and hence the depth of the protrusion <NUM> in the replication layer, are of the order of <NUM>-<NUM>. However, the effect of such protrusions, such as the protrusion <NUM>, on light propagating through the waveguide structure shown in <FIG> may be significant, as will now be explained with reference to <FIG>.

Referring to <FIG> some example light paths <NUM>, <NUM>, <NUM> of light propagating through the waveguide of <FIG> are shown. Light following either of the paths <NUM>, <NUM> are diffracted by the second grating <NUM>, <NUM> to emerge from the waveguide substantially at right-angles to the surface of the glass layer <NUM>, as intended. However, light following the path <NUM> passes through the protrusion <NUM> in the replication layer and, due to slight differences in the refractive index of the materials used in the replication layer <NUM> and the lamination layer <NUM>, the light eventually emerges from the glass layer <NUM> at an oblique angle to the surface of the glass layer <NUM>, causing a viewer to see a secondary image. It has been shown by modelling and by experimentation that a difference of as little as +/-<NUM> in the refractive indices of the materials of the replication <NUM> and lamination layer <NUM> can cause a deviation of <NUM>. 5mR or more in the light emerging from the waveguide, being sufficient deviation for a viewer to discern a secondary image. A person of ordinary skill in the relevant art would consider this a surprising result and would recognise that achieving a match in the refractive indices of UV-curable polymers for example, even when nominally the same material is used for the replication <NUM> and lamination layer <NUM>, is difficult to achieve in practice. Differences in refractive index may arise for example due to slight differences in operating temperature of the replication and lamination layers, causing differences in refractive index of +/-<NUM> per °C. Moreover, for a waveguide with an overall thickness of for example <NUM>, each protrusion in the replication layer due to an edge on the master grating tool <NUM> causes <NUM>% to <NUM>% of the light propagating through the waveguide to become diverted. An improved method for fabricating large waveguides is therefore required.

According to the present disclosure, in a first improvement, a different method not being part of the claimed invention has been devised for making a single master grating tool, for example a master grating tool having master grating profiles for two different diffraction gratings. The method will now be described in an example with reference to <FIG> and to <FIG>.

Referring to <FIG>, a method for fabricating a single master grating tool is represented in five stages by <FIG>.

At a first stage, represented in <FIG>, a single master grating tool substrate <NUM> is coated with a photoresist layer <NUM>.

At a second stage, represented in <FIG>, a mask is used to cover all except a first area <NUM> of the photoresist layer <NUM>. A fringe pattern <NUM> for a first diffraction grating is recorded in the exposed first area <NUM> of the photoresist layer <NUM>, for example using a laser-derived interference pattern, while the remaining area <NUM> of the photoresist layer <NUM> remains covered by the mask.

At a third stage, represented in <FIG>, a different mask is used to expose a second area <NUM> of the photoresist layer <NUM>. A fringe pattern <NUM> for a second diffraction grating is recorded in the exposed second area <NUM> of the photoresist layer <NUM>, for example using a laser-derived interference pattern, while the remaining area <NUM> of the photoresist layer <NUM> remains covered by the mask.

At a fourth stage, represented in <FIG>, the photoresist layer <NUM> is developed and photoresist in the first and second areas <NUM>, <NUM> is removed according to where the fringe patterns <NUM>, <NUM> for the first and second gratings respectively were recorded, exposing a corresponding pattern of underlying tool substrate100. First and second master grating profiles <NUM>, <NUM> are then etched into the master tool substrate <NUM> in the areas <NUM>, <NUM> respectively, following the grating patterns <NUM>, <NUM>, respectively, where the photoresist <NUM> has been removed. The master grating profiles <NUM>, <NUM> may be etched using for example ion beam etching. If necessary, several stages of exposure and etching may be required to create the required first and second grating profiles <NUM>, <NUM> in the tool substrate <NUM>.

At a fifth stage, represented in <FIG>, any remaining photoresist <NUM> is removed to leave the etched first and second grating profiles <NUM>, <NUM> formed in the tool substrate <NUM>. The principle advantage of this technique is that any edges to the grating profiles are very small so that when the master grating tool is used to imprint the first and second grating profiles <NUM>, <NUM> into a replication layer of UV-curable polymer, no significant protrusions remain in the replication layer. The problem described above with reference to <FIG> is therefore avoided.

Referring to <FIG>, an example of a waveguide not being part of the claimed invention that has been fabricated using the single master grating tool shown in <FIG>, is represented in a sectional view. As can be seen in <FIG>, the first and second grating profiles <NUM>, <NUM> are replicated in a replication layer <NUM>, for example by embossing in a layer <NUM> of UV-curable polymer applied to a first outer glass layer <NUM> of the waveguide. Unlike the prior art waveguide shown in <FIG> and <FIG>, in a waveguide according to the present disclosure made by replication from the single master grating tool shown in <FIG>, here are no significant protrusions in the replication layer <NUM> caused by edges of the first and second grating profiles <NUM>, <NUM>.

After coating the first and second grating profiles <NUM>, <NUM> with respective dielectric coatings <NUM>, <NUM>, a lamination layer <NUM> of substantially the same UV-curable polymer material as used for the replication layer <NUM> is applied to cover the first and second gratings <NUM>, <NUM>. The lamination layer <NUM> of UV-curable polymer is firstly applied to a second outer glass layer <NUM> and the combination is then pressed against the replication layer <NUM> so that the UV-curable polymer contacts the entire surface of the coated first and second grating profiles <NUM>, <NUM> conformably, leaving no gaps. The UV-curable polymer of the lamination layer <NUM> is then cured with UV light.

According to the present disclosure, in a second improvement, a different method has been devised to make a waveguide incorporating two or more diffraction gratings. This method will now be described with reference to <FIG> and to <FIG>.

Referring to <FIG> and to <FIG>, a method for fabricating a waveguide <NUM> incorporating first and second diffraction gratings <NUM>, <NUM> respectively, as shown in a sectional view in <FIG>, is represented in <FIG> as a four-stage process.

At a first stage, represented in <FIG>, a first master grating tool <NUM> and a second master grating tool <NUM> are fabricated on respective tool substrates <NUM>, <NUM> using a technique as described for example in a published paper: <NPL>). By this published technique, or by other known techniques, grating profiles may be formed over a relatively large area, in particular over an area sufficiently large to enable first and second master grating profiles <NUM>, <NUM> to be formed over substantially the whole area of a surface of the respective tool substrates <NUM>, <NUM>. The tool substrates <NUM>, <NUM> have an area of at least the area of a surface of the waveguide <NUM> to be fabricated.

According to one such technique, similar to that described above with reference to <FIG>, a layer of a photoresist is applied firstly over substantially all of a surface of each of the first and second tool substrates <NUM>, <NUM>. A laser-derived interference pattern forming a first grating pattern corresponding to the first master grating profile <NUM> is generated and recorded over substantially the whole of the area of the photoresist applied to the first tool substrate <NUM>, for example by scanning according to the above-referenced paper. Similarly, a laser-derived interference pattern forming a second grating pattern corresponding to the second master grating profile <NUM> is generated and recorded over substantially the whole of the area of the photoresist applied to the second tool substrate <NUM>, for example by the same technique. The photoresists are then developed, removing photoresist according to the first and second grating patterns to cause corresponding patterns of exposure of the underlying first and second tool substrates <NUM>, <NUM>, respectively. An etching technique, for example ion-beam etching, is then used to etch the first and second master grating profiles <NUM>, <NUM> into the exposed first and second grating patterns of the underlying first and second tool substrates <NUM>, <NUM>, respectively. Any remaining photoresist is then removed from the first and second tool substrates <NUM>, <NUM> to complete the fabrication of the first and second master grating tools <NUM>, <NUM>.

At a second stage, represented in <FIG>, the first master grating profile <NUM> of the first master grating tool <NUM> is replicated in a first replication layer <NUM> applied to a first outer glass layer <NUM> of the waveguide <NUM>, for example using one of the techniques described above with reference to <FIG>. In one such technique, the grating profile <NUM> of the first master grating tool <NUM> may be replicated across the whole area of the first replication layer <NUM>, comprising a layer <NUM> of UV-curable polymer applied to the first outer glass layer <NUM>, by embossing. Similarly, the second master grating profile <NUM> of the second master grating tool <NUM> is replicated across the whole area of a second replication layer <NUM>, for example a layer <NUM> of UV-curable polymer applied to a second outer glass layer <NUM> of the waveguide <NUM>, for example using the same technique as for the first replication layer <NUM>, by embossing.

At a third stage, represented in <FIG>, an area <NUM> of the first grating profile <NUM>', corresponding to the intended area of the first diffraction grating <NUM>, is coated with a layer <NUM> of dielectric material. Similarly, an area <NUM> of the second grating profile <NUM>', corresponding to the intended area of the second diffraction grating <NUM>, is coated with a layer <NUM> of dielectric material.

At a fourth stage, the waveguide is assembled by applying a lamination layer <NUM> of a UV-curable polymer, substantially the same as that used for the first and second replication layers <NUM>, <NUM>, to cover one or both of the grating profiles <NUM>', <NUM>' formed in the first and second replication layers <NUM>, <NUM>. The assemblies of first replication layer <NUM> and first outer glass layer <NUM> and of the second replication layer <NUM> and second outer glass layer <NUM> are then brought together, under pressure, thereby to sandwich the lamination layer <NUM> of UV-curable polymer between the first and second replication layers <NUM>, <NUM>. This ensures that the layer <NUM> of UV-curable polymer fills the space between the two replication layers <NUM>, <NUM> leaving no gaps. The polymer forming the lamination layer <NUM> is then cured and fabrication of the waveguide <NUM> is substantially complete.

Those regions of the first and second grating profiles <NUM>', <NUM>' that were not coated in a dielectric material form a direct interface between the materials of the respective replication layer <NUM>, <NUM> and the lamination layer <NUM>. Due to the substantially matching refractive indices of the polymers used in the replication and lamination layers <NUM>, <NUM>, <NUM>, this interface would have almost no diffractive effect on light propagating through the waveguide <NUM>. The diffractive efficiency of the regions coated by the dielectric layers <NUM>, <NUM>, intended to form the first and second diffraction gratings <NUM>, <NUM> respectively, is of a much higher order.

As for the first example according to the present disclosure, described above with reference to <FIG> and <FIG>, no part of the replication layers <NUM>, <NUM> protrudes significantly into the lamination layer <NUM>, so avoiding the problem with prior art waveguides described above with reference to <FIG>.

One advantage of the method for fabricating a waveguide <NUM> according to <FIG> and <FIG>, as compared with that described above with reference to <FIG> and <FIG>, is that the same master grating tools <NUM>, <NUM> may be used in fabricating waveguides with the same grating profiles <NUM>', <NUM>' but with other diffraction grating configurations. It is only when the dielectric layers <NUM>, have been applied to selected areas <NUM>, <NUM> of the replicated grating profiles <NUM>, <NUM> that the diffraction grating regions <NUM>, <NUM> are defined. That is, diffraction gratings <NUM>, <NUM> of different sizes, shapes and positions within a waveguide <NUM> may be fabricated using grating profiles <NUM>', <NUM>' replicated from the same master grating tools <NUM>, <NUM>, simply by applying dielectric coatings <NUM>, <NUM> to different areas of the replicated grating profiles <NUM>', <NUM>' before laminating the two replicated grating structures <NUM>, <NUM>, <NUM>, <NUM> together.

In some examples, not being part of the claimed invention, a method for fabricating a waveguide master grating imprint tool is described. The method comprises: coating a substrate with at least one photoresist layer; selectively exposing a first diffraction grating master profile onto a first area of the at least one photoresist layer; selectively exposing a second diffraction grating master profile onto a second area of the at least one photoresist layer; and processing the substrate to form the first diffraction grating master profile and the second diffraction grating master profile, wherein each of the first diffraction grating profile and the second diffraction grating profile comprises an edge between the substrate and the respective grating profile that is substantially perpendicular to the substrate surface and each of the edges is substantially the same height as a maximum depth of the first diffraction grating master profile and the second diffraction grating master profile.

In some examples, the method may further comprise etching the substrate to form the first diffraction grating master profile and the second diffraction grating master profile; and removing the at least one photoresist layer from the substrate.

In some examples, the first diffraction profile master pattern may be different from the second diffraction grating master profile.

In some examples, not being part of the claimed invention, a master grating imprint tool for fabricating a waveguide is described. The master grating tool comprising: a substrate; a first diffraction grating profile etched into a first area of the substrate; a second diffraction grating profile etched into a second area of the substrate; and wherein each of the first diffraction grating profile and the second diffraction grating profile comprises an edge between the substrate and the respective grating profile that is substantially perpendicular to the substrate surface, and each of the edges is substantially the same height as a maximum depth of the first diffraction grating master profile and the second diffraction grating master profile.

In some examples, the edge between the substrate and the respective grating profile is less than <NUM> millimetres.

In some examples, not being part of the claimed invention, a method to fabricate a waveguide comprising a least two diffraction grating profiles is described. The method comprising: using the waveguide master grating imprint tool according to the previous two paragraphs to replicate the first diffraction grating master profile and second diffraction grating master profile to form a first diffraction grating profile and second diffraction grating profile, wherein the first diffraction grating master profile and second diffraction grating master profile are imprinted in the same process step; applying at least one dielectric layer over the first diffraction grating pattern and second diffraction grating pattern.

In some examples, the first diffraction grating profile and second diffraction grating profile comprise at least one of an input grating and an output grating.

Claim 1:
A method for fabricating a waveguide (<NUM>), the method comprising:
(i) fabricating a first master grating tool (<NUM>) comprising a first tool substrate (<NUM>) having a surface with an area corresponding at least to the area of a surface of the waveguide and having a first grating profile (<NUM>) formed over substantially all of the surface of the first tool substrate;
(ii) fabricating a second master grating tool (<NUM>) comprising a second tool substrate (<NUM>) having a surface with an area corresponding at least to the area of the surface of the waveguide and having a second grating profile (<NUM>) formed over substantially all of the surface of the second tool substrate;
(iii) using the first master grating tool to replicate the first grating profile over substantially all of a surface of a first waveguide substrate;
(iv) using the second master grating tool to replicate the second grating profile over substantially all of a surface of a second waveguide substrate;
(v) applying a first dielectric layer (<NUM>) over a selected area (<NUM>) of the first grating profile (<NUM>') replicated on the surface of the first waveguide substrate;
(vi) applying a second dielectric layer (<NUM>) over a selected area (<NUM>) of the second grating profile (<NUM>') replicated on the surface of the second waveguide substrate; and
(vii) applying a layer of laminating material to at least one of the surfaces of the first and second waveguide substrates and bringing the surfaces of the first and the second waveguide substrates together thereby to join the first and second waveguide substrates together by an intermediate lamination layer (<NUM>).