PRODUCTION OF AN INTEGRATED OPTICAL DEVICE

Abstract: A method of manufacturing an integrated optical device (1) on a substrate is described wherein the device (1) comprises first and second areas each containing at least one optical component. The first and second areas are separated by a third relatively featureless area, which provides optical communication between the first and second areas. The method uses a photolithographic stepper to define features of the optical components on the substrate, arranged to define the features of the optical components in the first area within a first exposure field and the features of the optical component(s) in the second area (3) within a second exposure field, the stepper being moved between exposures so that first and second exposure fields are contiguous, and a line of contact or area of overlap between the first and second fields lies within the third, relatively featureless area of the substrate. The method may be used to form an arrayed waveguide grating.

Description: PRODUCTION OF AN INTEGRATED OPTICAL DEVICE FIELD OF THE INVENTION The present invention relates to the field of photonic circuitry, and more specifically to a method of manufacturing an integrated optical device. BACKGROUND ART Integrated optical devices comprising optical components in discrete areas of a substrate are known. An example is an arrayed waveguide grating (hereafter referred to as an AWG) as commonly used for wavelength division multiplexing (WDM) in the field of optical telecommunications. A common application of an AWG is to split light of multiple wavelengths from a single incoming optical fibre and send each wavelength to a different output fibre. In this way, signals may be transmitted on different optical wavelengths in the same fibre and then separated at the destination. In practice, an AWG is usually integrated on a substrate and by providing a series of input waveguides and a series of output waveguides, with an array of curved waveguides therebetween and separated from each series of waveguides by a coupling (or "slab") region (see Fig. 1 described further below). Such an AWG is described further in US5002350. AWGs enable the spatial separation of optical wavelengths and this separation is a result of the optical path differences along each of the curved waveguides in the array. Typically, the lengths of the waveguides in the array are carefully designed so that each one is a constant distance longer than its shorter neighbour. Regardless of whether the increment is constant or varying, all AWGs require the lengths of the waveguides to be accurately calculated, defined and manufactured. If such devices are fabricated poorly, then the errors in each of the waveguide path lengths causes a decrease in the isolation between neighbouring wavelengths afforded by the device. During the manufacture of AWGs it is necessary to transfer the design of the device onto a substrate (typically a silicon wafer) from which it is fabricated. Previously, this has been done using one of the following two methods: a) using a 1 :1 quartz/chromium optical mask and standard ultra-violet photoresist photolithography; or b) directly writing the pattern into a thin layer of resist on the material's surface by using a focussed electron-beam. The present invention aims to provide a further method of manufacturing devices such as AWGs which enable the devices to be made more accurately. DISCLOSURE OF THE INVENTION Thus, according to a first aspect of the present invention there is a method of manufacturing an integrated optical device on a substrate, the device comprising first and second areas each containing at least one optical component with the first and second areas being separated by a third, relatively featureless area which provides optical communication between the first and second areas, the method comprising the use of a photolithographic stepper to define features of the optical components on the substrate, and being arranged to define the features of the optical component(s) in the first area within a first exposure field and the features of the optical component(s) in the second area within a second exposure field, the stepper being moved between exposures so that the first and second exposure fields are contiguous, the arrangement being such that a line of contact or area of overlap between the first and second fields lies within the said third, relatively featureless area of the substrate. By arranging that the line of contact or area of overlap between the first and second fields lies within the third, relatively featureless area of the substrate, it is possible to avoid offset or misalignment in the optical components that would occur if the line of contact or area of overlap lay across the optical component. Such offset, for example, between two parts of a waveguide, would interfere with the operation of the optical device and may, for instance, prevent the transmission of an optical signal through the device or disturb the lengths of optical paths within the device. However, offset within the third, relatively featureless area is of much less consequence, since the third area contains no optical components the operation of which can be substantially disrupted by a small misalignment between the fields. Typically, the fields are rectangular, but any shape of field can be used so long as adjacent fields can be contiguous and so long as they overlap in regions of the device which are relatively featureless. Preferably the fields have an area of at least 400mm 2 , and more preferably at least 750mm 2 . It should be noted that the above field sizes are relatively large for photolithographic stepping purposes and that photolithographic stepper machines required for such stepping, whilst available, are only used for highly specialised applications, such as multi-gigabit DRAM electronic chips and ultra large scale integrated (ULSI) electronic microprocessors. The exposure fields are preferably aligned so that the line of contact, or the longitudinal axis of the area of overlap therebetween, is non-perpendicular to the direction of light transmitted from the first field to the second field. Preferably, the line of contact, or the longitudinal axis of the area of overlap, lies at an angle of at least 45° and more preferably at least 60° to the perpendicular to the direction of light transmitted from the first field to the second field. For example, in Figure 1 , the axis of such a boundary forms an angle of about 30° to the direction of light transmitted from the first field to the second field, i.e. at 60° to the perpendicular to the direction of light transmitted. Such orientation of the boundary helps reduce any effect misalignment of the first and second fields may have on operation of the device. By reducing a series of magnified masks of predetermined pattern down to a predetermined size on the substrate with each exposure, the method of the present invention allows the photolithographical etching to be carried out to a much higher degree of accuracy than the prior art methods referred to above. Preferably the degree of reduction (i.e. demagnification) is by a factor of at least 3 and preferably by a factor of at least 5. Typically, the substrate comprises a silicon wafer and preferably a silicon-on-insulator wafer. Typically the stepper machine achieves a resolution of between 0.25 μm and 0.70 μm, for example 0.35 μm. In yet a further aspect, the present invention provides an integrated optical component obtained by the method described above. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in further detail by way of the following non-limiting example, with reference to the accompanying figures, in which: Figure 1 illustrates the use of a stepper field arrangement according to an embodiment of the invention; and Figure 2 illustrates a photolithographic stepper machine suitable for use in the invention. BEST MODE OF CARRYING OUT THE INVENTION Photolithographic stepper machines are known which contain imaging optics to reduce magnified design patterns, or masks, down to a required size on a material surface, for instance in the fabrication of an integrated electronic circuit. Although such reduction provides a very accurate design image on the material surface, the finite size of the imaging optics imposes a restraint upon the maximum size of area (or "field") which the stepper machine can define in a single exposure. Thus, typically, such machines reduce a series of magnified masks to produce a larger design by way of "stepping" between different fields. The adjacent fields are joined or aligned, known as "stitching", to produce the overall, larger design and although stitching accuracy (often less than 100nm in either direction) is sufficient, to maintain electrical contact between parts of an integrated electronic circuit, a potential misalignment of this magnitude may have substantial adverse effects in an integrated optical circuit as it may, for instance, impair or even destroy the optical communication between two sections of a waveguide or substantially affect the operation of a device by changing the optical path lengths of the device. In Figure 1 , an AWG (1) is shown with waveguide fields manufactured according to a preferred form of the present invention as described above. Thus, a first field containing input waveguides (2) is shown, which overlaps with a second field containing an array of curved waveguides (3), which in turn overlaps with a third field containing output waveguides (4). The second field includes the entire curved array (3), so the curved waveguide array (3) can be formed in a single exposure without the need to stitch two fields across the array. Potential errors due to stitching across the waveguides are thus avoided. Similarly, the input (2) and output (4) waveguides are each defined within single fields and are thus free from any possible stitching error. The regions (5) where the fields overlap lie across the coupler (or "slab") regions between the waveguides. These coupler areas are relatively large and featureless so any slight misalignment of the fields where they are stitched together in these areas is of little or no consequence. In the regions (5) where the fields overlap, it may be desirable to incorporate alignment devices (such as Vernier scales) to assist in or verify the accurate alignment of adjacent fields. The stepping procedure described above can be carried out using a photolithographic stepping machine as illustrated in Figure 2, for example the PAS 5500/100D produced by ASM Lithography. This is a high-throughput, ii-line stepper with a standard 5 x lens which achieves 0.4 μm resolution with an alignment accuracy of <60nm . Such a stepper has a high speed stage (6) that steps the wafer precisely with respect to the imaging optics, moving the distances necessary to exactly repeat the image field in a Cartesian grid and thus fill the wafer surface. The stage (6) travels in the horizontal plane underneath a fixed, vertically mounted lens (7). The stage (6) can also be moved vertically by a small distance. Once the wafer is placed on an exposure chuck and stepped under the lens (7), it is aligned by automatic systems that detect wafer targets optically and move the stage (6) in small increments to correct the wafer position with respect to the ideal image field. De-magnified images of the mask pattern are projected onto the wafer surface by the microlithographic lens (7) mounted only a few millimetres above the wafer surface. An illumination system (8) exposes light through a chrome-on-glass reticle (9) that has the mask pattern etched into the chrome. An integrated optical circuit such as the AWG described above comprising optical components separated by a relatively featureless area can be fabricated by stepping consecutively through the respective fields containing the components, e.g. the input waveguides (2), then the array (3), and lastly the output waveguides (4), making sure that stitching only occurs in the relatively featureless areas between the respective components. Thus, the present invention allows appropriate integrated optical devices, such as AWGs, to be manufactured with greater accuracy than conventional fabrication methods used in this field without introducing errors in the device due to the process used.

Claims: CLAIMS 1. A method of manufacturing an integrated optical device on a substrate, the device comprising first and second areas each containing at least one optical component with the first and second areas being separated by a third, relatively featureless area which provides optical communication between the first and second areas, the method comprising the use of a photolithographic stepper to define features of the optical components on the substrate, and being arranged to define the features of the optical component(s) in the first area within a first exposure field and the features of the optical component(s) in the second area within a second exposure field, the stepper being moved between exposures so that the first and second exposure fields are contiguous, the arrangement being such that a line of contact or area of overlap between the first and second fields lies within the said third, relatively featureless area of the substrate. 2. A method as claimed in Claim 1 , wherein the fields are substantially rectangular. 3. A method as claimed in either Claim 1 or Claim 2, wherein the fields have an area of at least 400mm 2 and preferably at least 750mm 2 . 4. A method as claimed in Claim 1 , 2 or 3, wherein the rectangular fields are aligned so that the line of contact, or a longitudinal axis of the area of overlap therebetween, is non-perpendicular to the direction of light transmitted from the first field to the second field. 5. A method as claimed in claim 4 in which the line of contact, or longitudinal axis of the area of overlap is at an angle of at least 45° and preferably at least 60° to the perpendicular to the direction of light transmitted from the first field to the second field. 6. A method as claimed in any preceding claim, wherein the stepper de-magnifies the features of the optical components during each exposure such that the degree of de-magnification is by a factor of at least 3 and preferably by a factor of at least 5. 7. A method as claimed in any preceding claim, wherein the substrate comprises a silicon wafer. 8. A method as claimed in Claim 7, where the substrate comprises a silicon-on- insulator wafer. 9. A method as claimed in any preceding claim, wherein the stepper achieves a resolution of between 0.25μm, and 0.70μm, and preferably about 0.35 μm. 10. An integrated optical component obtained by the method of any preceding claim. 11. An integrated optical component as claimed in Claim 10 comprising an arrayed waveguide grating. 12. An integrated optical component as claimed in Claim 10 or 11 fabricated with a dimensional accuracy of between 0.25μm, and 0.70μm, and preferably about 0.35 μm.

Inventors: ROBERTS STEPHEN WILLIAM, ROBERTS, STEPHEN, WILLIAM
Applicants: BOOKHAM TECHNOLOGY PLC, ROBERTS STEPHEN WILLIAM, ROBERTS, STEPHEN, WILLIAM
Application year: 2000
Main classification: G03F7
IPC codes: G02B6, G03F7