# Patent

## Title
Resistive heater for thermo optic device

## Abstract

Resistive heaters formed in two mask counts on a surface of a grating of a thermo optic device thereby eliminating one mask count from prior art manufacturing methods. The resistive heater is comprised of a heater region and a conductive path region formed together in a first mask count from a relatively high resistance material. A conductor formed from a relatively low resistance material is formed directly on the conductive path region in a second mask count. Thermo optic devices formed by these two mask count methods are also described.

## Background
RELATED APPLICATIONS
This application is a Divisional of U.S. application Ser. No. 10/231,898, filed Aug. 29, 2002 now U.S. Pat. No. 7,020,365, which application is incorporated herein by reference.


FIELD OF THE INVENTION
The present invention relates to thermo optic devices, such as optical waveguides. In particular, it relates to a resistive heater for changing an optical characteristic of the thermo optic device. Even more particularly, it relates to an efficiently formed resistive heater.
BACKGROUND OF THE INVENTION
The art of making and developing new uses for thermo optic devices continues to emerge. Presently, thermo optic devices are used as filters, switches, multiplexers, waveguides, and a host of other semiconductor and optical transmission devices.
With reference to FIGS. 1A and 1B, a thermo optic device in accordance with the prior art is shown generally as 110. It comprises a cladding 115 that includes an upper cladding 114 and a lower cladding 112. A core 116 is defined by the cladding and is generally formed of a material having a higher or lower refractive index than that of the cladding. The core 116 may, for example, define an optical waveguide, such as a Y-shaped optical splitter having an input waveguide 122 and two output waveguides 124, 126. The core 116 together with the cladding 115 are sometimes referred to as a grating and are disposed on a substrate 118. The substrate may be formed of silicon but is not required to be. A heater 120 is disposed adjacent to the cladding 115.
During use, a control element (not shown) delivers current to the heater 120, to change an optical characteristic of the thermo optic device. For example, in a Bragg grating formed with a polymer grating, when current is delivered to heater 120, the refractive index of the polymer will change as a result of the thermo optic effect. In turn, this refractive index change affects the wavelength of light that satisfies the known Bragg reflective condition so that a different wavelength is now Bragg reflected in the optical waveguide.
If the process is repeated at another temperature, which is a function of current delivery and heater characteristics, another wavelength will satisfy the Bragg reflective condition. In this manner, the thermo optic device 110 is made tunable. Such a device will normally be operated in a steady state condition so that a single wavelength will satisfy the Bragg reflection condition over a given time interval.
With reference to FIGS. 1C and 1D, a portion of the thermo optic device 110 is shown in greater detail. In particular, the heater 120 is formed with contacts 121, 123 and conductors 131, 133 to, ultimately, connect to the control element during use.
Unfortunately, the heater 120, together with its associated contacts 121, 123 and conductors 131, 133, requires three fabrication masking steps to form with conventional processes, i.e., one masking step to form the heater, one to form the contacts, and one to form the conductors. This unnecessarily complicates manufacturing and wastes resources and finances.
Accordingly, the thermo optic arts desire improved heaters that are cheaper and quicker to produce, e.g., formed by fewer mask counts, without any corresponding sacrifice in quality, reliability or longevity.
SUMMARY OF THE INVENTION
The above-mentioned and other problems become solved by applying the apparatus and method principles and teachings associated with the hereinafter described resistive heater for thermo optic device.
In one embodiment the resistive heater is formed on a surface of a grating of a thermo optic device in two mask counts thereby eliminating one mask count from prior art manufacturing methods. In particular, the resistive heater is comprised of a heater region and a conductive path region formed together in a first mask count from a relatively high resistance material. A conductor, formed from a relatively low resistance material, is formed directly on the conductive path region in a second mask count. Advantageously, manufacturing processes can now be simpler and resource waste can be eliminated.
Thermo optic devices formed by these two mask count methods are also described.
These and other embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a perspective view of a thermo optic device having a Y-shaped optical waveguide in accordance with the prior art;
FIG. 1B is a cross sectional view of the thermo optic device of FIG. 1A taken along line 1B-1B;
FIG. 1C is a more detailed cross sectional view of a heater of the thermo optic device of FIG. 1A;
FIG. 1D is a planar view of the heater of FIG. 1C;
FIG. 2A is a cross sectional view of a grating upon which a resistive heater in accordance with the teachings of the present invention will be formed;
FIG. 2B is a cross sectional view in accordance with the teachings of the present invention of a first layer deposited upon the grating of FIG. 2A;
FIG. 2C is a cross sectional view in accordance with the teachings of the present invention of a first mask used upon the first layer of FIG. 2B;
FIG. 2D is a cross sectional view in accordance with the teachings of the present invention of a patterned first layer formed after the first mask application of FIG. 2C;
FIG. 2E is a cross sectional view in accordance with the teachings of the present invention of a second mask used upon the patterned first layer of FIG. 2D;
FIG. 3A is a planar view of a representative embodiment of a patterned first layer in accordance with the teachings of the present invention;
FIG. 3B is a planar view in accordance with the teachings of the present invention of a second mask used upon the representative embodiment of the patterned first layer of FIG. 3A;
FIG. 3C is a planar view of one embodiment of a resistive heater in accordance with the teachings of the present invention;
FIG. 4A is a planar view of another representative embodiment of a patterned first layer in accordance with the teachings of the present invention;
FIG. 4B is a planar view of another embodiment of a resistive heater in accordance with the teachings of the present invention;
FIGS. 5A and 5B are planar views of a plurality of cascaded resistive heaters in accordance with the teachings of the present invention;
FIG. 5C is a planar view of a plurality of cascaded resistive heaters connected electrically in serial in accordance with the teachings of the present invention;
FIGS. 6A and 6B are planar views of a plurality of grouped resistive heaters in accordance with the teachings of the present invention;
FIG. 6C is a planar view of a plurality of cascaded resistive heaters connected electrically in parallel in accordance with the teachings of the present invention;
FIG. 7 is a block diagram of a system having a thermo optic package comprising resistive heaters formed in accordance with the teachings of the present invention; and
FIG. 8 is an alternative embodiment of a thermo optic package comprising resistive heaters formed in accordance with the teachings of the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration, specific embodiments in which the inventions may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that process, electrical or mechanical changes may be made without departing from the scope of the present invention. As used herein, the term substrate includes any base semiconductor structure, such as silicon-on-sapphire (SOS) technology, silicon-on-insulator (SOI) technology, thin film transistor (TFT) technology, doped and undoped semiconductors, epitaxial layers of a silicon supported by a base semiconductor structure, as well as other semiconductor structures well known to one skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and their equivalents.
The following description and figures use a reference numeral convention where the first digit of the reference numeral corresponds to the figure and the following two digits correspond to like elements throughout the specification. For example, the grating of a thermo optic device of the present invention has a reference number of 200, 300, 400, etc. corresponding to the grating X00 in FIGS. 2, 3, 4, etc., where X is the number of the figure in which the reference numeral appears.
A resistive heater for use with a thermo optic device will now be described that, advantageously, has less mask counts than any prior art heaters. With reference to FIG. 2A, a thermo optic grating in accordance with the present invention is shown generally as 200. The grating 200 comprises a cladding 202 and a core layer 208. It is formed on a substrate 210 preferably formed of silicon. The substrate, however, could be any variety of well known materials for supporting a thermo optic grating.
The cladding 202 includes an upper cladding 204 and a lower cladding 206 that define the shape of the core layer 208. The materials selected for the cladding 202 and core layer 208 are selected in such a way that they have different indices of refraction, either higher or lower than one another. As is known, the core layer acts to propagate light by reflecting light at the boundaries between the core layer and the cladding. In a preferred embodiment, the upper and lower claddings 204, 206 are formed of silicon oxides while the core layer is formed of a silicon oxynitride.
The core layer, in one embodiment, forms an optical waveguide. It could be similar in shape to the Y-shaped optical waveguide with an input and two output waveguides as shown in FIG. 1A. It could also be an X-shaped waveguide, a continually shaped section of waveguide, or other waveguide structure now known or hereinafter developed.
In one embodiment, the grating is formed by depositing the lower cladding, depositing the core layer, photo patterning the core layer, etching the core layer and depositing the upper cladding. In another embodiment, the lower cladding 206 is a grown layer with the core layer and upper cladding being formed in the same manner.
Preferred deposition techniques for these, and after described layers include, but are not limited to, any variety of chemical vapor depositions (CVD), physical vapor depositions (PVD), epitaxy, evaporation, sputtering or other similarly known techniques. Preferred CVD techniques include low pressure (LP) ones, but could also be atmospheric pressure (AP), plasma enhanced (PE), high density plasma (HDP) or other. Preferred etching techniques include, but are not limited to, any variety of wet or dry etches, reactive ion etches, etc.
In the following figures, a resistive heater will be described that is formed on a surface 212 of the grating 200. It will be appreciated that this surface 212 is the top of the upper cladding 204.
In a first step after formation of the grating 200, a first layer 220 is deposited on surface 212 as shown in FIG. 2B. Preferably, the first layer is a poly silicon layer. Even more preferably, it is a poly silicon layer doped with an impurity such as arsenic, phosphorous or boron. In one embodiment, the first layer is a phosphorous doped poly silicon having a resistance of about 40 Ω/cm2. The first layer, however, only needs to be a material that has a relatively high resistance. As will be described later, the first layer will form a heater region portion of the resistive heater and needs to resist current flow and produce heat. Accordingly, the first layer could be any variety of other relatively high resistance materials, such as representative chromium, indium or other silicon arranged materials.
In one embodiment, the thickness of the first layer is deposited to a thickness of at least about 100 Å thick. In another embodiment, the thickness is about 4000 Å of the phosphorous doped poly silicon. The thickness is primarily dictated according to the heating requirements of the thermo optic device. Thicker depositions provide more heating capabilities while thinner depositions provide less. The deposition technique for the first layer is LPCVD but could be any of the other foregoing described techniques.
With reference to FIG. 2C, the first layer 220 is acted upon in a photomasking step. The first layer is masked with a first mask 224 (mask count 1) and a portion thereof is photo impacted 226. In this embodiment, the photo impacting is accomplished with an ultraviolet light from a photolithography device well known to those skilled in the art. The photo impacting, however, should not be limited to such an embodiment and may alternatively include X-rays or other light sources.
Thereafter, the photo impacted first layer is etched so that a patterned first layer 230 on surface 212 of the grating remains as shown in FIG. 2D. In one embodiment, the etch is a reactive ion etch with a plasma containing fluorinated chlorine. The etch, however, could be any well known etch described above or any etch hereinafter developed.
As depicted, it will be appreciated that the patterned first layer 230 is a photomasking island, but could have been produced as a photomasking hole. The particular embodiment, island or hole, depends upon whether the configuration of the first mask is a clear-field or dark-field mask as those terms as well understood in the art. In either event, both embodiments are embraced by the scope of this invention.
With reference to FIG. 3A, a representative patterned first layer 330 is shown in a top-down planar view as it is formed on surface 312 of grating 300 after the process step shown in FIG. 2D. In this embodiment, the patterned first layer comprises a heater region 331 and a conductive path region 333 on either sides thereof.
In the embodiment shown, the heater region 331 has planar dimensions Lh×Wh, with L being length, W being width, and h being the heater region. These dimensions are of no particular size provided they fit within the geographic confines of surface 312 of grating 300. The size is dictated by how much heat is required by the thermo optic device during use.
In a similar manner, the dimensions LC1×WC and LC2×WC, where subscript C is the conductive path region and subscript 1 is the left and 2 is the right conductive path regions, respectively, are not of a particular size. In fact, the dimensions of both conductive path regions could be the same or very dissimilar to one another or the heater region. The size of the conductive path regions is a function of design according to many parameters such as where the heater region is to be connected to an external control system for the delivery of current, how much current is to flow, etc.
The shapes of both the heater region and the conductive path regions, while shown as generally rectangular, could be circular, serpentine, polygonal, triangular, square, or any other geometric shape(s) that could be fabricated with the first mask. As is taught herein, the size and dimension is a function of, among other things, how much heat is required to be generated by the heater region.
Even further, the planar (x-y plane) positioning of the patterned first layer, including the heater region and conductive path regions, as it is formed on surface 312 is similarly a function of the thermo optic device and in what application the thermo optic device will be used.
Accordingly, another representative embodiment of a patterned first layer 430 with a heater region 431 and conductive path regions 433 on a surface 412 of a grating 400 is shown in FIG. 4A. In this Figure, the heater region is formed between the conductive path regions with neck down areas 441 to depict a much smaller heater region in comparison to the conductive path regions. Bond pad regions 437 are also formed to, ultimately, facilitate electrical connection the heater region 431. Again, no particular, size, shape, positioning of the patterned first layer is required and all embodiments are embraced herein.
In all figures, it should be appreciated that both the heater and conductive path regions are formed of the relatively high resistance material in a first mask count even though the heater and conductive path regions have different functions as will be described subsequently.
With reference to FIG. 2E, in another photomasking step, a second mask 232 (mask count 2) is arranged with respect to the patterned first layer 230 to isolate the heater region from the conductive path regions. In FIG. 3B, the second mask 332 is exaggeratedly shown over heater region 331 to isolate the heater region from each of the conductive path regions 333 on either side thereof. Like the first mask, the second mask can be configured as a clear-field or dark-field mask to isolate the heater region from the conductive path region and both embodiments are embraced herein.
Thereafter, the areas of the conductive path regions are plated with a relatively low resistance material to form conductors 235 as shown in FIG. 2E.
In a preferred embodiment, the conductive path regions are electroless plated with nickel by submergence of at least the conductive path regions in a liquid bath. In another embodiment, the conductors 235 are formed with any of tungsten, titanium, tantalum, molybdenum or cobalt. In still another embodiment, multiple layer stacks such as TiSix/TiN/W may be used to reduce contact resistance between conductor and heater layers. The material selected for the conductors, it should be appreciated, need only be a material having a relatively low resistance so that current can flow relatively well. As such, still other metals, combinations of metals or other materials can be used in forming the conductors.
With reference to FIG. 3C, a representative resistive heater in accordance with the present invention is shown generally as 340. It comprises a heater region 331 and conductors 335 formed over the conductive path regions. The resistive heater is formed on surface 312 of grating 300 of a thermo optic device.
In FIG. 4B, the representative resistive heater of the present invention is shown generally as 440. It includes a heater region 431 and conductors 435 formed over conductive path regions 433 (FIG. 4A). The bond pad regions 437, after plating, form bond pads 439. As with the other embodiments, the resistive heater 440 is formed on surface 412 of grating 400 of a thermo optic device, which in turn, is formed on a substrate.
It will be appreciated that the foregoing described resistive heater is formed with two mask counts which is at least one less mask count than any known prior heaters. Advantageously, the practice of this invention yields quicker formed resistive heaters for use with thermo optic devices. Since photomasking machinery, and therefore masking operations, is extremely expensive in relation to other types of process machines, such as CVD machines, the practicing of the present invention will also result in financial savings.
During use, the resistive heaters of the present invention are connected to a control element (perhaps via the bond pads) via the conductors to deliver current to the resistive heater to heat the thermo optic device thereby changing an optical characteristic thereof. It will be appreciated that the conductors will flow current because they are formed of low resistance material while the heater region will inhibit current flow because it is formed of a high resistance material. The heater region, in turn, will heat up the thermo optic grating while the conductors will not.
During use, the resistive heaters of the present invention may be used with thermo optic devices in a variety of applications. For example, the thermo optic device may be an amplifier, an optical waveguide, a switch, a modulator, or any other optical transmission device.
Heretofore, the resistive heaters of the present invention have been shown as singular resistive heater on a grating. The present invention also embraces pluralities of resistive heaters depending upon application requirements for heating a thermo optic device. For example, a plurality of resistive heaters 540 may be cascaded together across a surface 512 of grating 500 as shown in FIGS. 5A and 5B. In FIG. 5C, the cascaded resistive heaters are electrically connected in series across surface 512 of grating 500. In particular, a plurality of heater regions 531 are serially interconnected with a plurality of conductive path regions 530 on either side of each heater region. In this manner, a single control element may be connected to terminal ends 571 of the conductive path regions and thereby flow a single current through the plurality of heater regions 531 to heat them simultaneously. It will be appreciated that the heater regions, while shown as similar devices, may all individually have unique shapes and sizes thereby being capable of delivering various dissimilar regions of heating to the thermo optic device during use.
Alternatively, the resistive heaters of the present invention may be arranged as rows or groupings of resistive heaters 640 in rows A or B on surface 612 of grating 600 in FIG. 6. In FIG. 6C, the row A of resistive heaters are electrically connected in parallel. In particular, a plurality of heater regions 631 each having a conductive path region 630 on either side thereof is electrically connected in parallel via a plurality of inter-bridging conductive paths 673 having substantially the same electrical resistance as the conductive path regions. In this manner, a single control element may be connected to terminal ends 671 of the conductive path regions and thereby flow a plurality of currents through the plurality of heater regions 631 to heat them simultaneously. It will be appreciated that the heater regions, while shown as similar devices, may all individually have unique shapes and sizes thereby being capable of delivering various dissimilar regions of heating to the thermo optic device during use.
Those skilled can envision still other arrangements of resistive heaters formed in accordance with the teachings of this invention without departing from the spirit or scope of the defined claims.
It will be appreciated that during use, the resistive heaters of the present invention may be used with thermo optic devices in a variety of applications. For example, the thermo optic device may be an amplifier, modulator, gate, filter, time delay element, switch, multiplexer, or other.
With reference to FIG. 7, a system, having as part thereof a resistive heater formed in accordance with the teachings of the present invention, is shown generally as 741. The system may be an exclusively fiber optic system or may be a system having other software and hardware devices, as indicated by the dashed line 745, operably coupled to at least one fiber optic component thereof.
In either system, a light source 743 will be provided as the source for propagating light signals along at least one fiber optic line 747. Well known light sources include, but are not limited to, laser light sources. In the embodiment shown, the system 741 includes a plurality of fiber optic lines 747.
Coupleable to the fiber optic lines via a plurality of input fiber optic ports 751 is a thermo optic package 749. Contained within the thermo optic package is at least one thermo optic device 753 having at least one resistive heater formed in accordance with the present invention. In the embodiment shown, the thermo optic device 753 is coupled to the input fiber optic port 751 via an input connector 755 while an output connector 757 couples the thermo optic device to an output fiber optic port 759. In turn, the output fiber optic port 759 is coupled to another fiber optic line 747 of system 741.
During use, a system user merely needs to couple fiber optic lines 747 to the input and output fiber optic ports of the package 749 to readily achieve a thermo optic device having the advantages offered by the resistive heaters of the present invention.
With reference to FIG. 8, an alternative embodiment of a thermo optic package 849 is shown having a thermo optic device 853 with a single input connector 855 and a plurality of output connectors 857. The input connector 855 connects with input fiber optic port 851 which is readily matable with a fiber optic line 847 of a system. The output connectors 857 of thermo optic device 853 are each matable with an output fiber optic port 859.
In another embodiment, the single input connector of the thermo optic device 853, having a resistive heater formed in accordance with the present invention, may alternatively be replaced with two or more input connectors while the two output connectors may be replaced with one or more output connectors depending upon the type and desired use of the thermo optic device 853.
Conclusion
The above structures and fabrication methods have been described, by way of example, and not by way of limitation, with respect to resistive heaters for thermo optic devices.
In particular, resistive heaters formed in two mask counts on a surface of a grating of a thermo optic device have been described that eliminate at least one mask count from prior art manufacturing methods. The resistive heater is comprised of a heater region and a conductive path region formed together in a first mask count from a relatively high resistance material. A conductor formed from a relatively low resistance material is formed directly on the conductive path region in a second mask count. Thermo optic devices formed by these two mask count methods are also described.
As a result, resistive heaters of this invention can be formed quicker and cheaper without any corresponding sacrifice in quality, reliability or longevity.
The present invention has been particularly shown and described with respect to certain preferred embodiment(s). However, it will be readily apparent to those of ordinary skill in the art that a wide variety of alternate embodiments, adaptations or variations of the preferred embodiment(s), and/or equivalent embodiments may be made without departing from the intended scope of the present invention as set forth in the appended claims. Accordingly, the present invention is not limited except as by the appended claims.

## Claims

1. A method, comprising:
providing a grating having a surface;
forming a patterned polysilicon first layer on the surface with a first mask; and forming a resistive heater on the surface from the patterned first layer with a second mask.

2. The method according to claim 1, wherein the forming the patterned first layer further includes depositing a first layer on the surface.

3. The method according to claim 2, wherein the forming the patterned first layer further includes photo impacting the first layer.

4. The method according to claim 1, wherein the forming the resistive heater further includes plating a conductive path region of the patterned first layer.

5. The method according to claim 1, wherein the forming the resistive heater further includes masking a heater region of the patterned first layer.

6. The method according to claim 1, further including forming the grating on a substrate.

7. An apparatus formed in accordance with the method of claim 1.

8. A method, comprising:
providing a grating having a surface;
forming a polysilicon heater region and a conductive path region on the surface with a first mask; and
masking the heater region with a second mask to form a resistive heater on the surface.

9. The method according to claim 8, wherein the forming the heater region and the conductive path region includes depositing a first layer on the surface.

10. The method according to claim 8, wherein the masking the heater region with the second mask to form the resistive heater further includes plating the conductive path region.

11. The method according to claim 8, further including forming the grating on a substrate.

12. An apparatus formed in accordance with the method of claim 8.

13. The method of claim 8, wherein forming the heater region includes forming the heater region to a thickness of about 100 Angstroms.

14. The method of claim 8, wherein forming the heater region includes depositing polysilicon.

15. A method, comprising:
providing a grating having a surface;
forming a patterned polysilicon first layer on the surface with a first mask; and forming a neck down, resistive heater on the surface from the patterned first layer with a second mask.

16. The method according to claim 15, wherein the forming the patterned first layer further includes depositing a first layer on the surface, and photo impacting the first layer.

17. The method according to claim 16, wherein the forming the resistive heater further includes plating a conductive path region of the patterned first layer.

18. The method according to claim 17, wherein the forming the resistive heater further includes masking a heater region of the patterned first layer to a smaller dimension than the conductive path region.

19. The method according to claim 15, wherein providing the grating includes forming a lower cladding on a substrate and forming an upper cladding on the lower cladding, wherein the upper cladding and lower cladding have different indexes of refraction.

20. The method according to claim 19, wherein forming a lower cladding includes growing the lower cladding on a substrate.

21. A method, comprising:
providing a grating having a surface;
forming a patterned, polysilicon\metal first layer on the surface with a first mask; and
forming a resistive heater on the surface from the patterned first layer with a second mask.

22. The method of claim 21, wherein the first layer includes one of chromium, indium and silicon.