# Patent

## Title
Thermo-optical device

## Abstract

A polymer well may be formed over a thermal oxide formed over a semiconductor substrate in one embodiment. The well may include a waveguide and a pair of heaters adjacent the waveguide. Each heater may be mounted on a platform of insulating material to reduce heat loss through the substrate and the thermal oxide, in one embodiment.

## Background
This application is a divisional of U.S. patent application Ser. No., 10/609,839, filed on Jun. 30, 2003 now U.S. Pat. No. 7,071,492.


BACKGROUND
This invention relates generally to thermo-optical devices.
A thermo-optical device is an optical device whose optical characteristics can be modified by heating. For example, the refractive index of a material may be selectively changed by heating. Examples of thermo-optic devices include tunable filters, lasers, variable optical amplifiers, and switches, to mention a few examples.
It is desirable that thermo-optic devices supply the least amount of power for heating the thermo-optic device to change its optical characteristics. Commonly, a waveguide is provided in a polymer filled region. The refractive index of the polymer filled region is changed by a heater. The tuning range is highly dependent on the effectiveness and efficiency of the heat delivered by the nearby heater.
Generally these schemes use a heater which is positioned over a polymer filled region. After long term thermal cycling, the heater may be subject to delamination due to the thermal mismatch between the polymer and the heater. The tuning range may also be unpredictable due to uncontrollable heat delivered to the polymer.
Thus, there is a need for other ways to provide thermo-optic devices.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an enlarged, cross-sectional view of one embodiment of the present invention;
FIG. 2 is an enlarged, cross-sectional view of the embodiment shown in FIG. 1 in an early stage of manufacture in one embodiment of the present invention;
FIG. 3 is an enlarged, cross-sectional view of the embodiment shown in FIG. 2 at a subsequent stage of manufacture in one embodiment of the present invention;
FIG. 4 is an enlarged, cross-sectional view of the embodiment shown in FIG. 3 at a subsequent stage of manufacture in accordance with one embodiment of the present invention;
FIG. 5 is an enlarged, cross-sectional view of the embodiment shown in FIG. 4 at a subsequent stage of manufacture in accordance with one embodiment of the present invention; and
FIG. 6 is an enlarged, cross-sectional view of the embodiment shown in FIG. 5 at a subsequent stage of manufacture in accordance with one embodiment of the present invention.



DETAILED DESCRIPTION
Referring to FIG. 1, a tunable thermo-optic device 10 may be any device whose optical characteristics are altered by heating. For example, some materials may change refractive index in response to heating. These materials may be used to form tunable filters, tunable lasers, variable optical amplifiers, and switches, to mention a few examples.
A silicon substrate 12 may be covered by a thermal oxide 14 in one embodiment. The thermal conductivity of the thermal oxide 14 may be very low to reduce heat transfer to the substrate 12. Above the thermal oxide 14 is an upper cladding 16. In one embodiment the cladding 16 may be borophosphosilicate glass (BPSG). The upper cladding 16 may have an aperture that is filled with a polymer well 18. Suitable polymers include high molecular weight polymers with covalent bonds that exhibit a high thermo-optic coefficient. An example of such polymers is deuterated poly(methylmethacrylate).
Also positioned within the polymer well 18 is a waveguide 22. A pair of heaters 26a and 26b may be positioned over insulator platforms 24a and 24b. In one embodiment, the insulator platforms 24a and 26b may be formed of polyimide. Polyimide is widely used as an ultraviolet sensitive photoresist and its optical characteristics change in response to heating.
The layer 20 may be used to protect the well 18. It may be formed of an insulating material in one embodiment.
Thus, the heat loss path from the heaters 26 to the relatively thermally conductive substrate 12 goes through the thermal oxide 14 and the insulator platforms 24a and 24b. In some cases, the thermal conductivity of the thermal oxide 14 is much higher than that of the polymer well 18. As a result, heat loss may be controlled. In addition, cross talk may be reduced and optical performance may be less affected by unwanted sources of heat.
In one embodiment, the waveguide 22 may be formed of germanium doped silicon, deposited patterned and etched, for example, by reactive ion etching (RIE) as shown in FIG. 2. An etch stop layer 28, such as a metal or dielectric, is formed as shown in FIG. 3. The waveguide 22 may be covered with a BPSG top cladding 16 in one embodiment shown in FIG. 4. The top cladding 16 may be RIE etched down to the etch stop layer 28 to form the polymer well 18, as shown in FIG. 5. The etch stop layer 28 may thereafter be removed.
Polyimide is coated, patterned, and cured at 300° C. to form the platforms 24 for the heaters 26 in one embodiment, shown in FIG. 6. Polyimide may have a thermal conductivity comparable to that of the polymer well 18 and is processable by patterning and hard bake curing. Thereafter, the heaters 26 may be formed to fabricate the structure shown in FIG. 1.
Through the use of the platforms 24 better heating efficiency may enable lower power tuning in some embodiments. Achieving lower power tuning may reduce cross-talk and improve performance.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.

## Claims

1. A method comprising:
forming an upper cladding over a semiconductor substrate;
forming a well in said upper cladding;
forming a waveguide in said well;
forming a thermo-optic device including a heater in said well;
locating the heater over the substrate on a platform in said well; and
covering said heater with a polymer material.

2. The method of claim 1 wherein forming a substrate includes forming a thermal oxide over silicon.

3. The method of claim 1 including forming two heaters in said well.

4. The method of claim 3 including forming each heater on a platform over the substrate.

5. The method of claim 1 including depositing a polymer within said well.

6. The method of claim 1 including forming said platform by depositing a layer of material and patterning said material.

7. The method of claim 6 including depositing a thermal insulating material to substantially fill said well and forming said platform of a material different than the thermal insulating material deposited to substantially fill said well.

8. The method of claim 7 including forming said platform of a material having substantially the same thermal conductivity as said thermal insulating material.