Process for making optical waveguides

Details Process for making optical waveguides Process for making optical waveguides

1999-04-05

2001-05-15

Alanko, Anita (Department: 1746)

Etching a substrate: processes

Forming or treating optical article

C216S002000

Reexamination Certificate

active

06231771

ABSTRACT:

FIELD OF INVENTION
The present invention relates to a process for making optical waveguides.
BACKGROUND OF THE INVENTION
Optical fibre communication systems and optical fibre based instruments and devices often require the accurate alignment and reliable attachment of optical fibres with integrated optical devices such as waveguides integrated on a substrate. One important consideration in the design of such optical connections is that good alignment is achieved between the waveguide and the optical fibre. A typical structure of such an optical connection is that of a fibre set within a V-groove to a waveguide integrated on a silicon substrate. Such a structure based on silicon rib or ridge waveguides integrated on silicon insulator wafer is described in PCT GB96/01068. In order to achieve a good connection, the fibre should be brought to within a gap of 5&mgr;m or less of the waveguide facet. Since the V-groove does not have an end face perpendicular to the base of the groove, but rather it is set at an angle towards the base, it is desirable to undercut the waveguide to form a waveguide structure overhanging the angled face like a “diving board”. Such a concept can be found in PCT/GB96/01068. Whilst this particular concept is a desirable way of achieving the appropriate alignment features, processing tolerances can cause the overhanging structure to exhibit an unwanted “shelf” of buried oxide extending beyond the end facet of the waveguide after the V-groove has been formed. When a nitride layer is deposited over the end face containing the oxide shelf, a small nitride “shelf” can likewise be formed. If left, the shelves would distort the exit of optical waves from the waveguide. It is an aim of this invention to improve the surface quality of the waveguide facet to provide an improved optical connection to an optical fibre.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a process for treating a waveguide structure comprising: a silicon substrate with an integrally formed rib waveguide, the waveguide comprising an end portion with a facet, the end portion overhanging the silicon substrate and having an oxide layer on its underside protruding from the facet of the waveguide, and a nitride layer extending over the upper surface of the waveguide and the facet, the process comprising:
i) carrying out an oxide etch step to remove the oxide layer from the underside;
ii) carrying out an oxide growth step to form a new oxide layer on exposed silicon on the underside, said new oxide layer terminating at the facet;
iii) carrying out a nitride etch step to remove the nitride layer; and
iv) depositing a new nitride layer extending over the upper surface and facet without protruding beyond the facet, such that silicon nitride is formed on the facet.
According to another aspect of the invention there is provided a process for making a waveguide structure in a silicon-on-insulator wafer comprising a silicon substrate, a layer of oxide on top of the silicon substrate and a layer of epitaxial silicon on top of the layer of oxide, the process comprising: defining a rib waveguide in the epitaxial layer; etching through the epitaxial layer on either side of the rib waveguide at an end portion thereof to expose the buried oxide layer; depositing oxide and nitride layers successively on the rib waveguide; undercutting the end portion to form a V-groove in the silicon substrate for aligning an optical fibre to the waveguide structure, said undercutting step leaving an unwanted part of the buried oxide layer extending beyond an end facet of the rib waveguide; and treating the waveguide structure by a process as defined hereinabove.
The etch steps and deposition steps (i) to (iv) above can be carried out as blanket processes, without the need for masking. Therefore, the process is quite simple to implement. The thickness of the new nitride layer can be controlled to control the optical properties of the silicon nitride formed on the facet.
For a better understanding of the present invention and to show how the same may be carried into effect reference will now be made by way of example to the accompanying drawings.


REFERENCES:
patent: 5444805 (1995-08-01), Mayer
patent: 5641612 (1997-06-01), Lee et al.
patent: 5700382 (1997-12-01), Splett
patent: 5787214 (1998-07-01), Harpin et al.
patent: 6063299 (2000-05-01), Drake et al.
patent: WO 97/42534 (1997-11-01), None
UK Search Report dated Jul. 15, 1999 re British Patent Application No. 9827504.3.

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