GB2379994A

GB2379994A - Integrated optical waveguide with isotopically enriched silicon

Info

Publication number: GB2379994A
Application number: GB0122662A
Authority: GB
Inventors: Paul Gideon Huggett
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-09-20
Filing date: 2001-09-20
Publication date: 2003-03-26
Also published as: GB0122662D0

Abstract

An integrated optical waveguide is formed in part by isotopically enriched silicon in which at least one isotope is present in a higher proportion than in naturally occurring silicon. The waveguide can have a rib 1 portion projecting from a slab portion 2 where the slab comprises <SP>28</SP>Si or silicon dioxide. The waveguide may also be in the form of an arrayed waveguide including a p-i-n diode 10, 11 for attenuating a signal passing therethrough. Methods of manufacturing isotopically enriched silicon layers 2 on waveguides are also included. In another embodiment the integrated optical waveguide is formed in part by an isotopically enriched optically conductive material other than silicon in which at least isotope of that material is in a higher proportion than in naturally occurring forms of that material in order to enhance optical properties of that material, such material may include InP, Ge, SiGe, SiC and GaAs.

Description

AN INTEGRATED OPTICAL WAVEGUIDE This invention relates to an integrated optical waveguide and, in particular, a waveguide formed in silicon.

Silicon optical waveguides are known and are used in integrated optical devices fabricated on silicon chips, particularly on silicon-on-insulator chips which comprise a silicon layer separated from a substrate, typically also of silicon, by a light confining layer, typically silicon dioxide.

The present invention aims to provide silicon waveguides with improved properties over the known waveguides.

According to a first aspect of the invention, there is provided an integrated optical waveguide at least part of which is formed of isotopically enriched silicon in which at least one isotope thereof is present in a higher proportion than in naturally occurring silicon.

An arrayed waveguide grating comprising an array of such waveguides may be provided.

According to a second aspect of the invention, there is provided a method of fabricating an optical waveguide comprising the steps of: forming a layer of isotopically enriched silicon in which at least one isotope thereof is present in a higher proportion than in naturally occurring silicon; and forming a waveguide in the isotopically enriched silicon layer.

According to a third aspect of the invention, a method of fabricating an optical waveguide comprising the steps of:

mounting or forming a component of isotopically enriched silicon in which at least one isotope thereof is present in a higher proportion than in naturally occurring silicon on a substrate or component formed of silicon comprising some other combination of isotopes or formed of some other material.

According to another aspect of the invention, there is provided an integrated optical waveguide at least part of which is formed of an isotopically enriched optically conductive material, other than silicon, in which at least one isotope thereof is present in a higher proportion than in naturally occurring forms of the material whereby the optical properties of the material are enhanced.

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 perspective view of a rib waveguide formed on a silicon-oninsulator chip ; and Figure 2 is a cross-sectional view of a p-i-n diode modulator formed across a silicon rib waveguide such as that shown in Figure 1.

Silicon, an abundantly available element in nature, is widely used for fabricating electrical and electro-optical devices and is now also being used to form integrated optical circuits and devices. In its naturally occurring form, silicon is primarily composed of three isotopes of silicon, namely 92. 2% 28Si, 4.7% 29Si, and 3. 1 % 30Si, which is also roughly the composition of the silicon crystals used by the silicon device industry.

Electrical devices fabricated on single-crystal silicon have performance characteristics that are governed by the electrical and physical properties of

the silicon itself. Some of the important properties of the single-crystal silicon which affect device characteristics are carrier mobilities, energy band gap, effective mass of the carriers, and thermal conductivity. Carrier mobilities, for example, govern signal transit times and thus place a limit on device speed.

Thermal conductivity, on the other hand, governs power dissipation which, in turn, places an upper limit on the packing densities achievable for devices on a chip or the amount of power that can safely be generated in the circuit without significantly degrading circuit performance.

As described in US 5144409, the content of which is incorporated herein, it is known to fabricate electrical devices such as metal-oxide-semiconductor field effect transitions (MOSFETs) and NPN bipolar transistors in isotropically enriched silicon having a higher proportion of the 28Si isotope than is present in naturally occurring silicon. Such devices, and the other types of devices mentioned in US 5144409, take advantage of the higher carrier mobility and/or higher thermal conductivity of 28Si. Higher carrier mobility means that devices fabricated from the isotopically enriched silicon will exhibit faster device speeds and higher frequency performance than conventional compositions of silicon. Higher thermal conductivity means that the isotopically enriched silicon devices will exhibit better heat dissipation so device packing densities can be increased within integrated circuit chips and to increase power output per unit area of power devices. The thermal conductivity of isotopically enriched silicon can be up to 6 times that of naturally occurring silicon.

As mentioned above, naturally occurring silicon contains predominantly 28Si but also contains around 8% of other isotopes. This concentration of imperfections in the isotope make-up far exceeds the density of imperfections usually found in silicon crystals. A typical silicon crystal may comprise: 1015 - 1020 dopant atoms 1010 -1011 heavy metals

1017 -1018 oxygen 4 x 1021 29Si and 30Si Thus, by removing or reducing the minority isotopes, 28Si crystals will have a more perfect crystal lattice which generates less heat and electromagnetic noise when an electrical current passes therethrough and has a higher thermal conductivity to more efficiently dissipate heat that is generated.

These improvements are due to reduced phonon-phonon and phononelectron interactions in the crystal lattice.

The present invention takes this further and stems from the realisation that as a crystal lattice fabricated from isotopically enriched silicon will be less distorted and more crystallographically perfect that this gives rise to small but materially different optical characteristics which can provide improved optical properties in silicon waveguides and other optical devices compared to optical devices fabricated in naturally occurring silicon. These differences arise as in naturally occurring silicon slight differences in interatomic distances and interactions give rise to small geometrical strains in the crystal structure and electronic imperfections in the crystal structure. These differences give rise to what is known as the isotope effect. Thus, in addition to improved electronic and thermal proportions, isotopically enriched silicon has improved optical properties. A more perfect crystal lattice will, for example, have a more uniform refractive index in all directions, i. e. be less birefringent, and the extinction coefficient will be improved as the absorption coefficient will be reduced due to the decrease in electromagnetic noise within the crystal.

By forming optical waveguides and other optical devices from isotopically enriched silicon, it has been realised that in addition to the advantages described in US 5144409, improvements are provided in the optical properties of the material which may, for example, help reduce problems due to insertion losses, back reflection, polarisation dependent losses (PDL) and polarisation dependent frequency (PDF). For instance, polarisation dependent losses

arise, at least in part, due to the birefringence effect of silicon lattices under strain (known as the photoelastic effect). By making the silicon lattice more crystallographically perfect, these strains are reduced and hence these effects are reduced so polarisation dependent losses are reduced.

6 PDF is due to an apparent frequency shift or differential produced between light sent into a device at one frequency band. Because of birefringence of the device, light differently polarised appears to the device to have different frequencies. Thus, in the extreme, light of one polarisation sent into a grating device will not emerge. PDF is measured as a delta frequency but manifests itself as an intensity/amplitude reduction. The problems associated with PDF arise from changes in the band gap due to distortion of the crystal lattice so by using a more perfect crystal lattice, with less distortion, by fabricating the waveguide from isotopically enriched silicon, these problems are reduced.

Thus, an optical waveguide formed in isotopically enriched silicon has improved optical properties compared to a waveguide formed in naturally occurring silicon.

Figure 1 shows a rib waveguide which may be formed in isotopically enriched silicon. The rib waveguide comprises a rib 1 etched in a silicon layer 2. The silicon layer 2 is separated from a supporting silicon substrate 3 by a silicon dioxide layer 4 which acts as an optical confinement layer. In practice, a layer of silicon dioxide (not shown) is usually provided over the rib 1 and over at least adjacent parts of the silicon layer 2. As is well known, an optical mode (shown by dashed lines 5 in Figure 1) transmitted by such a waveguide is guided by the rib but also lies partially in the slab region from which the rib projects.

A rib waveguide formed of isotopically enriched silicon can be exactly the same as a rib waveguide formed of naturally occurring silicon apart from the different proportions of the isotopes it is made up from.

Optical waveguides are used in a wide variety of integrated optical circuits and devices and these can all benefit from the improvements described herein. Particular benefits, however, arise in devices in which waveguides are designed to have precisely known lengths and where the function of the device is highly dependent upon the properties of the waveguides used therein. One example of this is an arrayed waveguide grating (AWG) which comprises an array of waveguides positioned adjacent each other and are constructed so that the effective optical path length of each waveguide is designed to have a precisely known relationship with that of the adjacent waveguide (s). An AWG fabricated from isotopically enriched silicon will have improved properties compared to an AWG fabricated from naturally occurring silicon. This is due not only to the improved optical qualities of the individual waveguides as discussed above, but also because of the enhanced thermal properties of the isotopically enriched silicon.

The temperature of an AWG and of free space regions at each end thereof also has to be carefully controlled to maintain the desired relationships between the optical path lengths of the waveguides thereof (which may be disturbed by temperature fluctuations). The enhanced thermal conductivity of isotopically enriched silicon helps ensure an even temperature across the AWG so avoiding hot spots, or cooler spots, and enables a finer control of the temperature. Thus, whereas in the prior art the enhanced thermal properties are used to improve dissipation of heat from heat generating devices, in this case the enhanced thermal properties are used to improve the temperature control of a region of the silicon which is heated to a desired temperature.

As is well known, a p-i-n diode can be formed across a waveguide in order to inject charge carriers into the waveguide to alter its refractive index and hence the effective optical path length thereof and/or to attenuate an optical signal transmitted along the waveguide. Such a device may thus be used as a modulator and/or as a variable optical attenuator (VOA). Such devices may

be formed in a variety of arrangements and a typical arrangement is shown in Figure 2.

Figure 2 shows a cross-sectional view of a rib waveguide such as that shown in Figure 1, with p-and n-doped regions 10 and 11 formed in the silicon layer 2 on opposite sides of the rib 1. An electrical potential is applied between the p-and n-doped regions 10 and 11 by electrical contacts (not shown) provided thereon, to cause change carriers to be injected into the rib waveguide in the region through which the optical mode passes.

If such a device is fabricated in isotopically enriched silicon, its properties are improved. The optical properties of the waveguide will be improved as described above in relation to Figure 1. In addition, the electrical properties, and in particular their interaction with the optical function of the device, will be improved. In a device formed of isotopically enriched silicon, the carrier lifetime will be longer and the series resistance of the diode will be reduced due to a decrease in crystal defects producing changes in the band-gap, the switching speed and the extinction coefficient are improved due to the higher carrier mobility, and power dissipation is improved due to the enhanced thermal conductivity of the silicon.

Thus, whilst some of these advantages correspond to those known in electrical devices as described in US 5144409, they are used in conjunction with the enhanced optical properties described above and they are used to improve optical characteristics of the device, i. e. variation of the refractive index and/or attenuation properties, as opposed to its electrical properties.

The above components can be combined to provide a variety of different circuits and devices. One example is a variable multiplexer (or demultiplexer). This comprises an array of input waveguides each receiving signals of a given wavelength band and each having a VOA formed across part of the waveguide to attenuate the signal therein. The outputs of the array

of input waveguides are directed across a free space region to an AWG. The outputs from the AWG interfere in a further free space region and are directed into a single output waveguide which thus provides a multi-wavelength output signal. Such an arrangement may be used in the opposite direction as a variable de-multiplexer.

A variable multiplexer is known but the performance of the device is improved by fabricating the device from isotopically enriched silicon as it benefits from all of the advantages described above in relation to the individual components of the device.

The Figures iliustrate rib waveguides formed in silicon but other types of waveguide can also be made from isotopically enriched silicon, including channel waveguides and slab waveguides. Indeed, any silicon structure which is arranged to guide light in at least one dimension can take advantage of the improved optical properties of isotopically enriched silicon, and the term waveguide is intended to include such structures.

The entire silicon chip, or just the silicon layer used to conduct light, may be fabricated from isotopically enriched silicon. Alternatively, the chip and/or the light conducting layer may be fabricated from natural, isotopically mixed, silicon and isolated isotopically enriched regions formed or provided in one or more selected areas. This is preferably achieved, for example, by mounting a component or device formed of isotopically enriched silicon on a substrate formed of naturally occurring silicon, or indeed, some other form of substrate. This may be achieved by a direct bonding technique in which surfaces of the components to be joined are prepared so as to be atomically smooth and flat and pressing the surfaces together so they are bonded together by van der Waal's forces between the atoms thereof. The whole waveguide or structure may be formed from isotopically enriched silicon or just part thereof.

The waveguides described above will, in use, be connected to one or more light sources and one or more light detectors. The light source may be a light emitter such as a laser diode mounted on the chip or an optical fibre mounted on the chip leading to a light source off the chip. The light detector may be a photodiode mounted on the chip or an optical fibre mounted on the chip leading to a light detector off the chip.

Optical components and devices may be formed from isotopically enriched silicon by a variety of methods, some of which are described in US5144409.

Isotopically enriched silicon is also available from Isonics Corporation of the USA who provide epitaxial wafers with a layer of 28Si up to 25 microns in thickness, 28Si silane gas and 28Si tetrafluorid.

A layer of epitaxial silicon may be grown on a conventional SOI wafer (comprising a thin epitaxial layer of silicon over the oxide layer) using a source of 28Si enriched gas, e. g. 28Si C14, 28Si HCts or 28SiH4 and waveguides fabricated in the epitaxial silicon layer by conventional methods. The epitaxial layer thus comprises a thin layer, e. g. 0.2 microns thick, of mixed isotopes and a layer of isotopically enriched silicon, e. g. 2-10 microns thick, formed over this (the thin layer of mixed isotopes acting as a seed layer for the epitaxial growth of the isotopically enriched silicon layer).

An AWG may, for example, be manufactured from a conventional substrate wafer and all silicon addition steps carried out using isotopically enriched silicon. Such steps may include epitaxial growth across the whole wafer, the whole waveguide being formed in this material, or in localised regions as described in GB2355312A which describes a method of fabricating a second feature, e. g. a tapered portion of a waveguide, on top of a first feature, e. g. a parallel-sided waveguide, in two separate steps. In the latter case, the wedge may thus be formed on a waveguide formed of isotopically mixed silicon or a waveguide of isotopically enriched silicon.

The isotopically enriched silicon preferably comprises 28Si as the highest proportion as this isotope is the predominate constituent of naturally occurring silicon. However, the advantages described above are not dependent upon

which isotope predominates so, it could equally be 29Si or 30Si. of The isotopically enriched silicon preferably comprises at least 94% of one isotope, e. g. 28Si, and most preferably at least 98% thereof.

The enhanced optical properties of isotopically enriched silicon can be extrapolated to other optically conductive semi-conductors. As for silicon, greater perfection in the crystal lattice as a result of a more uniform composition of isotopes, can be expected to lead to enhanced optical properties. Examples include : ! nP, Ge, SiGe, SiC and GaAs.

Claims

1. An integrated optical waveguide at least part of which is formed of isotopically enriched silicon in which at least one isotope thereof is present in a higher proportion than in naturally occurring silicon.

2. A waveguide as claimed in claim 1 in which the said at least one isotope is 28Si.

3. A waveguide as claimed in claim 2 in which the silicon comprises at least 94% 28Si.

4. A waveguide as claimed in claim 3 in which the silicon comprises at least 98% 28Si.

5. A waveguide as claimed in any preceding claim comprising a rib portion projecting from a slab portion.

6. A waveguide as claimed in any preceding claim formed on a silicon chip which comprises a layer of isotopically enriched silicon.

7. A waveguide as claimed in any preceding claim in which the waveguide or the said part thereof is formed in an isotopically enriched piece of silicon which is mounted or formed on a substrate or component formed of silicon comprising some other combination of isotopes or formed of some other material.

8. A waveguide as claimed in any preceding claim with a p-i-n diode formed across the waveguide.

9. A waveguide as claimed in any preceding claim connected to one or more light sources and one or more light receivers.

10. An arrayed waveguide grating comprising an array of waveguides each of which comprises a waveguide as claimed in any preceding claim.

11. An arrayed waveguide grating as claimed in claim 10 arranged to receive light from one or more input waveguides and to transmit light to one or more output waveguides, one or more of the input or output waveguides comprising a p-i-n diode for attenuating the optical signal passing therethrough.

12. A waveguide as claimed in any preceding claim formed on a silicon-on- insulator chip comprising a layer of optically conductive silicon separated from a supporting substrate by an optical confinement layer.

13. An integrated optical waveguide at least part of which is formed of an isotopically enriched optically conductive material, other than silicon, in which at least one isotope thereof is present in a higher proportion than in naturally occurring forms of the material whereby the optical properties of the material are enhanced.

14. A waveguide as claimed in Claim 13 in which said material is selected from: InP, Ge, SiGe, SiC and GaAs.

15. An integrated optical waveguide substantially as hereinbefore described with reference to and/or as shown in one or more of the accompanying drawings.

16. A method of fabricating an optical waveguide comprising the steps of: forming a layer of isotopically enriched silicon in which at least one isotope thereof is present in a higher proportion than in naturally occurring silicon ; and forming a waveguide in the isotopically enriched silicon layer.

17. A method of fabricating an optical waveguide comprising the steps of: mounting or forming a component of isotopically enriched silicon in which at least one isotope thereof is present in a higher proportion than in naturally occurring silicon on a substrate or component formed of silicon comprising some other combination of isotopes or formed of some other material.

18. A method of fabricating an optical waveguide substantially as hereinbefore described with reference to one or more of the accompanying drawings.