WO2000073828A1

WO2000073828A1 - Waveguide junction with silicon nitride

Info

Publication number: WO2000073828A1
Application number: PCT/GB2000/002028
Authority: WO
Inventors: Andrew George Rickman; Arnold Peter Roscoe Harpin
Original assignee: Bookham Technology Plc
Priority date: 1999-06-01
Filing date: 2000-05-26
Publication date: 2000-12-07
Also published as: AU4938600A; GB2350719A; GB9912737D0

Abstract

A junction structure between a semiconductor waveguide region and a first dielectric forming a further waveguide region of lower refractive index than the semiconductor waveguide region. The structure includes a light transmitting semiconductor layer which has an end face at the junction and a substrate below the semiconductor layer which extends beyond the junction and a first dielectric light transmitting layer formed over the extending part of the substrate and extending in alignment with the semiconductor layer to provide the further waveguide region. A second dielectric layer of refractive index below that of the two waveguide regions is formed over the end face of the semiconductor and over the extending part of the substrate. In this way a support layer is formed having a required thickness for the further waveguide region to provide the required alignment of the optical access through the two waveguide regions.

Description

WAVEGUIDE JUNCTION WITH SILICON NITRIDE

The present invention relates to junctions between semiconductor and dielectric waveguides. It relates for example to silicon nitride optical waveguides and in particular to the manufacture of junctions between such waveguides and silicon waveguides, and the use of such junctions, for example, in interferometers.

It is known in the art to provide a silicon on insulator waveguide connected to optical fibres, thus providing a path along which light can propagate. This type of waveguide is useful for many types of optical devices, including interferometers such as a Mach-Zehnder interferometer.

When light is transmitted through a silicon waveguide, some light energy is propagated as an evanescent field outside the geometric boundary of the silicon.

In the case of silicon, the evanescent field is small. The effective refractive index of the waveguide, and hence the phase velocity of any light transmitted through the waveguide, is dependent on the relative values of the refractive index in the material of the waveguide and the refractive index of the surrounding medium in which the evanescent field is propagated. Silicon nitride may also be used for an optical waveguide in the same wavelength ranges as silicon. Silicon nitride has a lower refractive index than silicon and consequently variations in the refractive index of the surrounding medium have a much greater effect on the effective refractive index of a silicon nitride waveguide than is the case for a silicon waveguide. Consequently the use of silicon nitride can be advantageous in applications where sensitivity to variations in surrounding refractive index is required. Such use may be carried out in interferometers and particularly when used to detect the presence of selected external media as may occur in biosensing operations.

In use, one of two branches of the interferometers is immersed in the environment of interest whilst the other branch remains in the control environment . The effect on the refractive index of the immersed branch can be measured by detection of the interference pattern formed with the light passing through the control branch, the degree of interference affecting the light intensity observed. This is because this intensity depends on the proportion of light maintained within the immersed branch, which in turn alters the effective length of the light path, which directly relates to the refractive index. The proportion of light maintained within the immersed branch depends on the relative difference of refractive index between the branch and its environment and therefore may be affected by, for example, a particular chemical .

However, silicon nitride is not readily formed as a thick enough layer for a waveguide having required structural integrity and thickness for accurate optical alignment with connecting optical fibres . Silicon nitride may also provide problems in single mode propagation if the waveguide is curved.

It is an object of the present invention to provide an improved optical waveguide including a silicon/silicon nitride junction.

According to a first aspect of the present invention there is provided a junction structure between a semiconductor waveguide region and a first dielectric forming a further waveguide region of lower refractive index than said semiconductor waveguide region, which structure comprises a light transmitting semiconductor layer having an end face at said junction, and a substrate below the semiconductor layer and extending beyond said junction and a first dielectric light transmitting layer formed over the extending part of the substrate and extending in alignment with the semiconductor layer to provide the further waveguide region, wherein a second dielectric layer of refractive index below that of the two waveguide regions is formed over the end face of the semiconductor and over the extending part of the substrate, thereby forming a support layer of required thickness for the further waveguide region to provide the required alignment of the optical axis through the two waveguide regions.

According to a second aspect of the present invention there is provided an optical interferometer having parallel light 5 transmitting paths, at least one of said paths including a waveguide junction structure as aforesaid.

According to a third aspect of the present invention there is provided a method of forming a junction structure between

10 semiconductor and a first dielectric in an aligned region of a waveguide, the method comprising the steps of forming a semiconductor waveguide having an end face at said junction on a substrate below a light transmitting semiconductor layer, such that a substrate extension projects beyond said junction,

15 depositing a layer of first dielectric to form a further waveguide region extending over said substrate extension and prior to depositing the first dielectric, depositing a second dielectric layer of refractive index below that of the semiconductor and the first dielectric so that said second

20 dielectric extends over the end face of the semiconductor waveguide and over the substrate extension, thereby forming a support layer for the first dielectric over the substrate extension, the thickness of the said second dielectric layer providing the required alignment of the optical area through the

25 waveguide .

According to a further aspect of the present invention there is provided a method of biosensing using the interferometer as aforesaid, comprising the steps of; measuring a first 30 interference amplitude with the silicon nitride section exposed to a reference environment, and then exposing the silicon nitride section to a test environment, and measuring a second interference amplitude, and providing a sensing result from selective values of said first and second amplitudes. j-

An embodiment of the present invention will be described by way of example and with reference to the accompanying drawings in which :

Figure 1 shows a sectional structure of a known silicon-on- insulator waveguide; 5 Figure 2 shows a junction between a silicon-on-insulator waveguide and a silicon nitride waveguide in accordance with the invention;

Figure 3 shows a plan view of a modification of the junction of Figure 2, and 10 Figure 4 shows an interferometer using a silicon and silicon nitride waveguide in accordance with the present invention.

In the figures, like reference numerals indicate like parts.

15 Integrated silicon rib waveguides of the type shown in Figure 1 are known. A light transmitting layer of silicon 1 is formed over a buried layer of silicon dioxide 2 on a silicon substrate 3. The silicon layer 1 has an upstanding rib 4 defining an optical transmission path along the waveguide. A layer of

20 silicon dioxide 5 covers the silicon layer 1 and rib 4. The optical profile of light transmitted through the waveguide is shown schematically at 6 although there is an evanescent field which extends outside the silicon rib 4. Further details of this form of waveguide are given in a paper entitled "Low loss single

25 mode optical waveguides with large cross-section in silicon-on- insulator" by J. Schmidtchen et al in Electronic Letters, 27, page 1486, 1999 and in PCT Patent Specification No. WO95/08787.

This form of waveguide provides a single mode, low loss 30 (typically less than 0.2 dB/cm for the wavelength range 1.2 to 1.6 microns) waveguide typically having rib width and height dimensions in the order of 3 to 5 microns which can be coupled to optical fibres and which is compatible with other integrated components . j-i

The speed of light along the waveguide, is dependent on the effective refractive index of the waveguide. This is dependent on the refractive indices of both the silicon of the waveguide and the medium outside the waveguide through which the evanescent field travels . Silicon has a refractive index of about 3.5 whereas silicon nitride has a refractive index of only about 2. Consequently the effective refractive index of a waveguide using silicon nitride as the light transmitting medium is much more responsive to variations in the refractive index of the outside medium than is a silicon waveguide. Consequently, for applications of a waveguide responsive to variation of the external medium, silicon nitride provides advantages over silicon for the light transmitting layer. However, use of low pressure chemical vapour deposition (LPCVD) to deposit silicon nitride may result in a layer of thickness of the order of 0.2 μm whereas a typical silicon waveguide may conveniently have a thickness of about 4 μm. Such thin layers of silicon nitride in a waveguide present problems of structural integrity, single mode operation, as well as accurate alignment with any optical fibre system connected to the waveguide. Furthermore, silicon nitride in such a waveguide does not provide a basis for forming active devices which may respond to application of electric signals to vary the optical transmission in the way that silicon does.

For these reasons, the present embodiment shown in Figures 2 and 3 combines sections of silicon and silicon nitride in an optical waveguide with a junction between silicon and silicon nitride as shown in Figure 2.

The wafer of Figure 2 comprises the silicon-on-insulator waveguide 11 joined to a silicon nitride waveguide 12 at the junction 10 depicted as a sectional view in Figure 2. In this figure the direction of light travel is left to right. It may also be operated with light travel from right to left. The required optical alignment is achieved, which means that the centre of the light mode travelling through the silicon waveguide 11 is aligned with the centre of the light mode travelling through the silicon nitride waveguide 12. This achieves maximum light energy transfer across the junction. Approximate mode field matching is achieved despite the large difference in size in the vertical direction between the waveguides due to the difference in refractive indices between the guides which compensates for the former effects. The optimum alignment position for the silicon nitride waveguide 12 is just below the centreline of the silicon waveguide 11 as detailed below.

The junction as shown in Figure 2 depicts a standard silicon-on- insulator waveguide 11 having an end face 13. A layer 14 of silicon nitride which acts as an anti-reflective coating covers oxide 5 on top of the rib 4. It also covers the end face 13 and the part of silicon oxide layer 2 which is not covered by the silicon waveguide 11 and forms an extension with the substrate 3 beyond the end face 13. On top of this layer is a deposited layer of silicon dioxide 16, and on top of this is a layer 17 of silicon nitride. The centre of the optical intensity profile through the silicon waveguide 11 and the silicon nitride waveguide 12 is indicated as a dotted line labelled reference numeral 18.

The formation of the junction and the alignment of the two waveguides will now be explained.

The silicon rib waveguide 11 is formed in the normal way according to the prior art but includes exposing an extension of the silicon substrate 3 and oxide layer 2 beyond the end 13 of the waveguide. Photolithography is used to selectively remove the oxide 5 from the waveguide end face 13. An anti-reflective silicon nitride layer 14 is deposited over the entire structure at a suitable thickness for maximum effectiveness at the transmission wavelengths required. In this embodiment the thickness is approximately 0.16 μm. The next step is to measure the height of the silicon waveguide 11 to obtain the height of the centreline above the buried oxide layer 2. In this embodiment, the silicon waveguide has a height of 4.3 μm above the layer 2, which means the centreline would be at 2.15 μm.

However, the optimum height of maximum light energy in the waveguide 11 is slightly lower than this at 1.84 μm. For other embodiments, the optimum height could be determined by simulation or testing. The next step is to deposit over the whole structure a layer of silicon dioxide 16 by PECVD . The thickness of this oxide 16 is carefully controlled to build up a bed over the extended substrate 3 and oxide 2 which has a thickness of 1.74 μm. This means that when the silicon nitride 18 is deposited at 0.2 μm thickness, also by PECVD, it forms a silicon nitride waveguide 12 with a centreline exactly in line with the height of maximum light energy within the silicon waveguide 11 at 1.84 μm above the buried oxide layer 2. Thus the maximum light energy is at line 18 throughout the junction and into the silicon nitride waveguide 12. The silicon nitride waveguide 12 can be patterned lithographically if desired, eg into a ridge or stripe structure.

Figure 3 shows a modified form of the junction in Figure 2 and similar reference numerals have been used for similar parts. In this case the junction is curved to form a convex lens structure at the end of the silicon waveguide 11 and a concave face on the silicon nitride 17 at the junction. The nitride layer 14 and oxide layer 16 each form a curved layer interposed between the convex end face of the silicon layer and the concave end face of the silicon nitride 17. The refractive index of the nitride layer 14 is less than that of the silicon layer 1 so that the curved interface acts as a composite cylindrical collimating lens focusing light in the horizontal plane.

In an alternative embodiment, the silicon oxide 16 and the silicon nitride 18 can be formed using thermal oxidation and LPCVD respectively. The heights of the layers could also be varied. A different dielectric could be used in place of the silicon oxide 16, such as silicon oxynitride or a polymer.

A silicon/silicon nitride waveguide of the type shown in Figure 2 or Figure 3 may be used in an interferometer, such as a Mach-

Zehnder interferometer as shown in Figure 4. Such an interferometer provides two parallel light paths 40 and 41 between a light source 42 and a light detector such as a photodiode 43. If a phase shift is introduced into the light transmitted by one of the paths relative to the other, either destructive or constructive re-combination may occur or be detected. The path 40 may be provided with a test window 44 for introducing a phase shift dependent on the external refractive index relative to the refractive index within the waveguide forming the part of the path adjacent the window 40.

In this example, the waveguide extending along the window 44 is formed of silicon nitride with a junction at each end 45,46 to a silicon waveguide as described with reference to Figure 2. The silicon waveguide at each end of the window, and that forming the path 41, may be joined in known manner to optical fibres leading to the detector 43 and from the source 42. A silicon waveguide with a 4 μm rib can be aligned with an optical fibre by undercutting the material which the waveguide is formed on to allow the optical fibre to be brought closer to the end of the waveguide as is disclosed in our US Patent 5787214. This would not be satisfactory with a waveguide of only 0.2 μm height and so the interposition of the silicon waveguides between the silicon nitride region and the optical fibres overcomes this problem.

The refractive index of silicon nitride is only 2, compared to 3.5 for silicon. This, together with the geometry used, results in a smaller fraction of the light being maintained within the waveguide. In the case of an interferometer for biosensing this is a useful property, because the waveguide is extremely sensitive to small concentrations of test substances around the sensing window. Furthermore, silicon nitride can be coated with various substances, so that the immediate surrounding of the sensing window can be used to induce a change in the effective refractive index. This is achieved by pre-coating the sensing window with a substance which is known to or expected to react with the substance under test . A typical example would be to coat it with a protein which is known to form a strong bond with a particular antibody under test, before immersion in the test substance. If a bonding reaction takes place, the resulting biochemical structure has such a different structure from the protein coating alone that its refractive index is often very different from that of the protein coating alone. Therefore, the proportion of light energy transmitted outside the waveguide changes with the result that a different interference pattern is detected at the right-hand end of the waveguide. This indicates that the particular antibody is present in the test substance. Furthermore, the rate of change of refractive index can be monitored to give an indication of the speed of the bonding reaction. It is of course possible that the refractive index will be changed as soon as the sensing branch is immersed in the test substance even if no bonding reaction takes place, but in general the change is more pronounced if a reaction does take place .

Several of the structures described above can be formed on a single chip which could then be used to test several different substances at once. For example, the window 44 of each structure could be coated with a different protein but then each be immersed in a solution containing different antibodies in order to determine which antibodies are contained in the solution and at what concentration.

The silicon waveguide used in the reference path 41 may include a modified active region (eg a PIN diode) in which a variable electrical potential is applied to vary the amplitude of light transmitted through the waveguide. In this way, the electric field applied may be varied to maintain a constant light amplitude detectable by the light detector as the relative phases in the two paths varies . The light level selected may be one at which the detected amplitude variation with phase change has the steepest gradient thereby making the interferometer with maximum sensitivity to detection of phase change.

It will be understood that in the manufacture of the structure shown in Figures 2 and 3 the steps of depositing the silicon nitride layers 14 and 17 involve blanket depositions over the entire area without the need for masking specific areas. Similarly the formation of the oxide layer 16 is a blanket operation over the entire area without the need for masking.

The nitride layer may be left as a blanket layer if the junction is curved to act as a lens as described above . Alternatively the nitride layer may be patterned to form a rib or ridge or a flat stripe.

Claims

CLAIMS :

1. A junction structure between a semiconductor waveguide region and a first dielectric forming a further waveguide region of lower refractive index than said semiconductor waveguide region, which structure comprises a light transmitting semiconductor layer having an end face at said junction, and a substrate below the semiconductor layer and extending beyond said junction and a first dielectric light transmitting layer formed over the extending part of the substrate and extending in alignment with the semiconductor layer to provide the further waveguide region, wherein a second dielectric layer of refractive index below that of the two waveguide regions is formed over the end face of the semiconductor and over the extending part of the substrate, thereby forming a support layer of required thickness for the further waveguide region to provide the required alignment of the optical axis through the two waveguide regions.

2. A junction structure between silicon and silicon nitride in a waveguide, which structure comprises a light transmitting silicon layer having an end face at said junction, and a substrate below the silicon layer and extending beyond said junction and a silicon nitride light-transmitting layer formed over the extending part of the substrate and extending as a further waveguide region in alignment with the silicon layer, wherein a dielectric layer of refractive index below that of silicon and silicon nitride is formed over the end face of the silicon between the silicon and the silicon nitride and over the extending part of the substrate, thereby forming a support layer of required thickness for the silicon nitride to provide the required alignment of the optical axis through the silicon nitride with the optical axis through the silicon.

3. A junction structure according to claim 1 or 2 , wherein the dielectric layer below the further waveguide region is silicon dioxide .

4. A junction structure according to claim 2 or claim 3, wherein the substrate comprises a layer of silicon dioxide and a layer of silicon.

5. A junction structure according to any of claims 2 to 4 , wherein the dielectric layer also extends over the light transmitting semiconductor layer.

6. A junction structure according to claim 5, wherein the silicon nitride layer also overlies the dielectric layer over the light transmitting semiconductor layer.

7. A junction structure according to any of claims 2 to 6 , wherein there is provided an anti-reflective layer over said end face.

8. A junction structure according to claim 7, wherein the anti- reflective layer is silicon nitride.

9. A junction structure according to claim 7 or claim 8, wherein the light transmitting semiconductor layer is directly covered by a layer of silicon dioxide on the side remote from the substrate .

10. A junction structure according to any of claims 2 to 9 , wherein the layer forming the further waveguide region is patterned.

11. A junction structure according to any of claims 2 to 10, wherein the waveguide regions are in the form of rib waveguides.

12. A junction structure according to any of claims 2 to 11, wherein the silicon nitride layer is of sub-micron thickness and is less than one tenth the thickness of the silicon layer.

13. A junction structure according to any one of the preceding claims in which the said end face of the semiconductor waveguide at the junction is curved and forms a lens to direct transmitted light into the adjacent waveguide section.

14. An optical interferometer having parallel light transmitting paths, at least one of said paths including a waveguide junction structure as claimed in any one of claims 1 to 13.

15. An optical interferometer according to claim 14 in which said one of the paths includes a silicon waveguide having a section of its length in which the light transmitting layer is formed of a region of silicon nitride, a silicon to silicon nitride junction being formed at each end of said section and aligned with an optical path through the waveguide.

16. An interferometer according to claim 15, wherein each of the said paths includes a silicon waveguide formed by a light - transmitting layer of silicon on an insulating layer.

17. An interferometer according to claim 15 or claim 16, wherein the or each silicon waveguide is a rib waveguide formed from a silicon-on-insulator wafer.

18. An interferometer according to any of claims 15 to 17, wherein the insulating layer is silicon dioxide.

19. A method of forming a junction structure between semiconductor and a first dielectric in an aligned region of a waveguide, the method comprising the steps of: forming a semiconductor waveguide having an end face at said junction on a substrate below a light transmitting semiconductor layer, such that a substrate extension projects beyond said junction; depositing a layer of first dielectric to form a further waveguide region extending over said substrate extension and prior to depositing the first dielectric, depositing a second dielectric layer of refractive index below that of the semiconductor and the first dielectric so that said second dielectric extends over the end face of the semiconductor waveguide and over the substrate extension, thereby forming a support layer for the first dielectric over the substrate extension, the thickness of the said second dielectric layer providing the required alignment of the optical area through the waveguide .

20. A method according to claim 19, wherein the semiconductor is silicon, the first dielectric is silicon nitride and said second dielectric layer is silicon dioxide.

21. A method according to claim 20, wherein the substrate comprises a layer of silicon dioxide over a layer of silicon.

22. A method according to claim 20 or 21, wherein the second dielectric layer and the silicon nitride layer are deposited such that they also extend over the top surface of the semiconductor waveguide .

23. A method according to any of claims 19 to 22, wherein an anti-reflective coating is deposited over the end face of the semiconductor waveguide before the second dielectric layer is deposited.

24. A method according to claim 20, wherein the anti-reflective coating is silicon nitride.

25. A method according to claim 23 or 24 in which the end face is curved.

26. A method according to any of claims 20 to 25, wherein the first dielectric layer is patterned.

27. A method according to any of claims 20 to 26, wherein the semiconductor and first dielectric form a common integrated waveguide device .

28. A method of biosensing using the interferometer of claim 15, comprising the steps of: measuring a first interference amplitude with the silicon nitride section exposed to a reference environment, and then exposing the silicon nitride section to a test environment, and measuring a second interference amplitude, and providing a sensing result from selective values of said first and second amplitudes .

29. A method according to claim 28 comprising the further step Of: prior to measuring the said first interference amplitude, coating the silicon nitride section with a substance comprising a first protein, wherein if the test environment comprises a second protein, the second interference amplitude is dependent on the number of bonds formed between the first protein and the second protein.

30. A method according to claim 29, wherein one of the first protein and the second protein is an antibody.