Process for producing a low-loss optical waveguide in an epitaxi

Details Process for producing a low-loss optical waveguide in an epitaxi Process for producing a low-loss optical waveguide in an epitaxi

1992-03-02

1993-10-12

Chaudhuri, Olik

Fishing, trapping, and vermin destroying

437162, 148DIG30, 148DIG35, H01L 2120

Patent

active

052525147

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention relates to a process for the production of a low-loss optical waveguide in an epitaxial silicon film of a silicon structural element with integrated electronic components in a silicon substrate.
Such a method is described in "Optics Letters," Vol. 13, No. 2, 1988, pages 175 to 177. In this known method for the production of an optical waveguide in an epitaxial silicon film of a silicon structural element, an insulating layer of SiO.sub.2 or Al.sub.2 O.sub.3 is provided between the silicon substrate and the epitaxial silicon film; the epitaxial film is formed of a "silicon-on-insulator" (SOI) material. The optical waveguide is made complete by the fact that in the known method, a so-called rib waveguide is formed by etching, as described in detail in "IEEE Journal of Quantum Electronics," Vol. QE-22, No. 10, 1986, pages 873 to 879. Lateral waveguiding is achieved in this manner. An advantage of an optical waveguide produced in this way is that interactions between an electrical field and an optical field are possible by means of charge carrier injections. In addition, the waveguide is very low-loss, in advantageous manner, because its attenuation is less than 1 dB/cm. However, there are difficulties in the production of such an optical waveguide, in that the SOI material, which is rather rare until now, must be used to produce it.


SUMMARY OF THE INVENTION

Furthermore, a process for the production of a waveguide in a lithium-niobium substrate is known, in which a photoresist layer is applied to partial areas of the surface of the substrate. Subsequently, a thin titanium coating is applied to these partial areas, and to the areas of the substrate surface which are free of the layer, and after removal of the photoresist layer, this coating remains only at those locations on the substrate where the photoresist layer was not present before. Subsequently, the remaining titanium coating is diffused into the substrate, forming the waveguide.
The invention provides a method for the production of a low-loss, optical waveguide in an epitaxial silicon film of a silicon structural element, with which an optical waveguide with low attenuation can be produced in comparatively inexpensive manner, using an integrated construction method.
To accomplish this task, according to the invention, in a process of the type indicated at the beginning, the weakly doped epitaxial silicon film is applied to the silicon substrate, and a substance containing germanium and having a refractive index with a real component higher than that of silicon, is applied to the epitaxial silicon film and diffused in under the effect of heat.
An advantage of the process according to the invention consists of the fact that due to the use of widely available substances, it can be carried out with little expenditure, and makes it possible to obtain an optical waveguide which has the property, in addition to low attenuation, that an interaction between an electrical field and an optical field can be achieved by means of charge carrier injection. In this way, additional losses due to free charge carriers are avoided, by using the substance containing germanium. The weak doping of the epitaxial silicon film <10.sup.16 /cm.sup.3 results in sufficiently low attenuations in the optical waveguide.
In the process according to the invention, germanium itself can be used as the substance. However, it must be taken into consideration in this that due to the low diffusion constant of germanium in silicon, a high process temperature of approximately 1200.degree. C. is required, which is higher than the melting temperature of germanium; the melting temperature of germanium is 937.degree. C. Accordingly, non-homogeneities might form due to germanium-silicon alloy drops, which can lead to an absorption of the optical wave in the waveguide.
It is viewed as particularly advantageous in this connection if in another embodiment of the process according the invention, a Ge.sub.x Si.sub.(1-x) alloy is used as the substance. In t

REFERENCES:
patent: 4199386 (1980-04-01), Rosnowski et al.
Optics Letters, vol. 13, No. 2, Feb. 1988, Optical Society of America; B. N. Kurdi et al.: Optical Waveguides in oxygen-implanted buried-oxide silicon-on-insulator structures, pp. 175-177.
IEEE Journal of Quantum Electronics, vol. QE-22, No. 6, Jun. 1986, R. A. Soref et al.: All-Silicon Active and Passive Guided-Wave Components for .lambda.=1.3 and 1.6 .mu.m, pp. 873-879.

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