GB2383644A

GB2383644A - Integrated optical device with non-crystalline light absorbent regions

Info

Publication number: GB2383644A
Application number: GB0130991A
Authority: GB
Inventors: Adrian Petru Vonsovici
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-12-27
Filing date: 2001-12-27
Publication date: 2003-07-02
Also published as: GB0130991D0

Abstract

An integrated optical device formed in an optically conductive substrate 3, the device having one or more light absorbent regions 7, for absorbing unwanted, spurious or stray light in the substrate. The light absorbent regions 7 having a non-crystalline structure, which may be amorphous or polycrystalline silicon, which may also be doped. Also shown in the figure is waveguides 1,2, light sources 4,5 and light sensor 6.

Description

AN INTEGRATED OPTICAL DEVICE This invention relates to an integrated optical device having light absorbent regions for absorbing unwanted or stray light within the device. In the design of the integrated optical circuits there is often a need to deal with light which is not guided by the components forming the circuit or which has entered regions around or adjacent said components. This stray light, which can arise from many sources, such as fibre or laser couplers, scattering from waveguide couplers and leakage from bends, can have a severe impact on the performance of circuits or devices employing these components and may contribute significantly to the background noise in a light sensor so masking the signal to be detected by the sensor.

Conventional methods of dealing with this problem include physically arranging devices on an integrated optical chip such that stray light cannot enter sensitive parts of the chip and the use of isolation trenches to keep stray light away from certain parts of the chip.

The limitation of these approaches is that they do not remove the stray light from the integrated optical chip, but instead attempt to minimise the problem of having stray light within the chip.

US6298178 discloses the use of doped areas to absorb stray light and provides an improvement over such prior art.

The present invention aims to provide an alternative or additional form of light absorbent region for use in an integrated optical device.

According to a first aspect of the invention there is provided an integrated optical device formed in an optically conductive substrate, the device having one or more light absorbent regions for absorbing unwanted or stray light in the substrate, said one or more regions having a non-crystalline structure.

According to a second aspect of the invention, there is provided a method of forming one or more light absorbent regions in an integrated optical device, the method including the step of forming a region having a non-crystalline structure in an optically conductive substrate.

Preferred and optional features of the invention will be apparent from the following description and from the subsidiary claims of the specification.

The invention will now be further described, merely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of a first embodiment of an integrated optical device according to the invention; Figure 2 is a cross-section through the device of Figure 1 along line A-A; and Figure 3 is a schematic plan view of a second embodiment of an integrated optical device according to the invention.

Figure 1 shows a schematic plan view of a pair of substantially parallel waveguides 1 and 2 formed in an optically conductive substrate 3 and extending between optical components 4,5 and 6. Components 4 and 5 may, for instance, be light sources and component 6 may be a light sensor. The substrate 3 is typically formed of a crystalline material, e. g. silicon.

Stray light may enter the substrate 3 from a variety of sources, e. g. from the light sources 4 and 5 themselves, from optical couplings between the light sources 4 and 5 and the respective waveguides 1 and 2 coupled thereto, from the waveguides 1 and 2 themselves (especially if curved) and from optical couplings between the waveguides 1 and 2 and the light sensor 6.

To help reduce the problem caused by stray light in the substrate 3 an absorbent region 7 is provided in the substrate. The absorbent area 7 may be formed in a variety of locations. In the example shown, it extends around each of the light sources 4 and 5 and extends between the waveguides 1 and 2 to the light sensor 6.

The light absorbent region 7 comprises one or more regions having a noncrystalline structure; the defects in the structure providing additional energy states which enable electrons in the material to absorb incoming photons (in some cases of selected wavelengths and in other cases of a range of wavelengths).

In one embodiment of the invention, the light absorbent region 7 is substantially amorphous. A region of the substrate 3 is considered to be amorphous if its structure is without significant crystalline order over dimensions greater than 100nm.

In another embodiment, the light absorbent material has a poly-crystalline structure, the dimensions of individual crystals thereof being significantly less than the dimensions of the optical components of the device. For example, the waveguides 1 and 2 typically have cross-sectional dimensions of a few microns whereas the individual crystals of the poly-crystalline region preferably have dimensions of less than 0.1 microns.

Amorphous silicon has a high absorption coefficient for some wavelengths of light, particularly light of a wavelength in the range 1. 3-1. 55 microns, the wavelengths commonly used in optical telecommunications. The absorption coefficient at 1.55 microns is, for instance, around 800cm-1. This is around 40 times greater than that of crystalline silicon from which the waveguides 1 and 2 are preferably formed. Amorphous regions can thus be formed in the substrate 3 to absorb stray light in the substrate.

Amorphous silicon can be formed in a variety of ways. One method is by ion implantation. Ion implantation bombards the crystalline structure of the substrate 3 and thus breaks up the crystalline nature of the substrate. The degree by which the crystalline structure is broken up depends on the intensity and duration of the bombardment. Conventional implantation apparatus is capable of forming an amorphous layer up to 1-2 microns deep.

The ions implanted may be the same as those of the substrate, e. g. in this example Si+ ions, or any other type of ions.

If the ion implantation uses dopant atoms, such as phosphorus or boron, the absorbent region 7 will be doped as well as having a non-crystalline structure. If conventional doped regions are formed in this way, they are subsequently heat treated to re-crystallise the silicon. In the present invention, this step is omitted.

If the non-crystalline region is also doped, its absorption coefficient typically increases by an order of magnitude or more.

Another method of forming amorphous silicon is by deposition of silicon on an amorphous layer, e. g. on a layer of Si02, SiN or SiON. The silicon layer grown on such an amorphous layer naturally assumes an amorphous structure (and would require subsequent heat treatment if it were desired to have a crystalline structure).

If the absorbent region 7 has a poly-crystalline structure it will absorb some of the stray light but a poly-crystalline structure is not as absorbent as an amorphous structure. A poly-crystalline structure is thus preferably also doped so the combination of the absorption due to the structure of the region and the absorption by free carrier scattering (as described in US6298178 referred to above) provides an increased level of absorption.

The absorbent region 7 preferably has dimensions of at least 100 microns, e. g. the width of the region 7 (perpendicular to the length of the waveguides 1 and 2) is preferably at least 100 microns so the majority of the stray light incident therein is absorbed and very little, if any, is transmitted through the region to emerge into the substrate on the other side thereof.

The waveguides 1 and 2 are preferably rib waveguides comprising ribs 1A and 2A projecting from slab regions 18 and 2B formed in the substrate 3, as shown in Figure 2. As indicated above, the substrate 3 is preferably of silicon.

In the example shown, the substrate comprises a silicon layer 3 separated from a supporting substrate 8 (typically also) of silicon, by a light confinement layer 9 (typically of silicon dioxide). Such a structure is conveniently provided by a silicon-on-insulator chip. In this case, the light absorbing region preferably extends through the silicon layer 3 to the underlying oxide layer 9.

If the light absorbent region 7 comprises amorphous silicon, it may either be formed by ion implantation or may be formed by depositing silicon on the oxide layer 9 (after first etching away the crystalline silicon in this region).

Virtually no light penetrates through the oxide layer 9 so the structure of the supporting substrate 8 is not of importance. However, if the substrate is of some other nature without an optical confinement layer such as the oxide layer 9, it may be advantageous for the substrate beneath the optical components to also have an amorphous structure.

Figure 3 illustrates another application of an amorphous or poly-crystalline region 10 as a light absorber. In this case, the absorbent region 10 is located to receive light from the end of a waveguide 11 so as to act as a beam dump.

Preferably, the absorbent region is surrounded by features, e. g. trenches 12, which confine the light within the region enclosed thereby. Thus, light entering the enclosure will be repeatedly reflected around the interior of the enclosure until it has all been absorbed by the light absorbent region 10.

An amorphous or crystalline region can also be used as a light absorber in other arrangements. Further examples are given in the UK patent application No.............. entitled"An Integrated Optical Arrangement"filed on the same day as the present application. This discloses the use of trenches arranged so as to direct stray light in a substrate into a light trap formed by trenches arranged around a light absorbent region so that light that enters the light trap (through one or more gaps between the trenches) is repeatedly reflected around the interior of the trap until absorbed by the light absorbent region.

This light absorbent region may be an amorphous or poly-crystalline region as discussed above.

An advantage of using a non-crystalline region as a light absorber is that it can be easily fabricated with the rest of the device and, when used without dopant, avoids the need to apply further thermal stress to the device (eg to diffuse the dopant to the desired locations). Fabrication of an amorphous region by ion implantation or a poly-crystalline region by deposition can be carried out at low temperatures, eg at room temperature. Even when a dopant is used in conjunction with a non-crystalline structure, the level of dopant required is reduced so that thermal stress is correspondingly reduced.

Fabrication of a non-crystalline region also does not require a trench to be etched in the substrate (this often being required with doped regions) A non-crystalline region can be fabricated close to other components of the optical device as its boundaries are relatively sharp (compared to those of a doped region). The location of a non-crystalline region can also be accurately determined. A non-crystalline region may, for example, be formed on top of the rib of a rib waveguide or over the top of the entire waveguide.

A non-crystalline region can be formed over large areas of a device, even over the entire surface thereof in some cases.

The above examples relate to optical devices formed in a silicon substrate but it will be appreciated that non-crystalline regions can be used as light absorbers in other optically conductive substrates. A non-crystalline region may, for example, be formed adjacent a silica waveguide or other waveguide comprising a core surrounded by a cladding layer to absorb stray light leaking from the waveguide into adjacent areas of the substrate.

Claims

CLAIMS 1. An integrated optical device formed in an optically conductive substrate, the device having one or more light absorbent regions for absorbing unwanted or stray light in the substrate, said one or more regions having a non-crystalline structure.
2. A device as claimed in claim 1 in which said one or more regions are poly-crystalline, the maximum dimensions of the individual crystals thereof being significantly less than the dimensions of optical components formed in the optically conductive crystalline substrate.
3. A device as claimed in claim 1 in which said one or more regions are substantially amorphous.
4. A device as claimed in claim 3 in which said one or more regions have a structure without significant crystalline order over dimensions greater than 100 nm.
5. A device as claimed in any preceding claim comprising a rib waveguide formed in the substrate, said one or more regions being formed in the substrate adjacent the rib waveguide.
6. A device as claimed in any of claims 1 to 4 in which said one or more regions substantially surround a feature from which stray light may emanate.
7-. A device as claimed in any of claims 1 to 4 in which said one or more regions are located within a light trap.
8. A device as claimed in any preceding claim in which the non-crystalline material is amorphous silicon or polycrystalline silicon.

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9. A device as claimed in any preceding claim in which the optically conductive substrate is silicon or silica.
10. An integrated optical device substantially as hereinbefore described with reference to and/or as shown in one or more of the accompanying drawings.
11. A method of fabricating a light absorbent region in an integrated optical device, the method including the step of forming a region having a non- crystalline structure in an optically conductive substrate.
12. A method as claimed in claim 9 in which said region is formed by ion implantation.
13. A method as claimed in claim 9 in which said region is formed by deposition of material onto an amorphous layer.
14. A method as claimed in any of claims 9-11 in which said region is also doped.
15. A method as claimed in any of claims 9-12 in which the optically conductive substrate is silicon.
16. A method as claimed in any of claims 9-13 in which said region comprises amorphous or poly-crystalline silicon.
17. A method of fabricating a light absorbent region in an integrated optical device substantially as hereinbefore described.