Patent Application
20050105184
This invention relates to thermally tunable devices such as thermo-optically tunable thin film optical filters.
There is a family of devices that are based on thermo-optically tunable, thin-film optical filters. These devices, which are made from amorphous semiconductor materials, exploit what had previously been viewed as an undesirable property of amorphous silicon, namely, its large thermo-optic coefficient. The performance of these devices is based on trying to maximize thermo-optic tunability in thin-film interference structures, instead of trying to minimize it as is often the objective for conventional fixed filters. The devices are characterized by a pass band centered at a wavelength that is controlled by the temperature of the device. In other words, by changing the temperature of the device one can shift the location of the pass band back and forth over a range of wavelengths and thereby control the wavelength of the light that is permitted to pass through (or be reflected by) the device.
The basic structure for the thermo-optically tunable thin film filter is a single cavity Fabry-Perot type filter 10, as illustrated in
To achieve control over the temperature of the device, at least some embodiments include a ZnO or polysilicon heater film 12 integrated into the multilayer structure. The heater film is both electrically conductive and optically transparent at the wavelength of interest (e.g. 1550 nm). Thus, by controlling the current that is passed through the film, one can control the temperature of the filter.
The thermal tuning that is achievable by this thermo-optically tunable filter is illustrated by
In general, in one aspect, the invention features an optical device including: a substrate with a top surface and a bottom surface and a hole extending through the substrate from the top surface to the bottom surface; and a multilayered thin film structure fabricated on the substrate and forming a membrane over the hole, the multilayered thin film structure comprising a thermally tunable thin film optical filter structure at least a portion of which is positioned over the hole.
Other embodiments include one or more of the following features. The multilayered thin film structure is fabricated on the top surface of the substrate. The multilayered thin film structure further includes a heater layer for heating the thermally tunable optical filter structure. The optical device also includes a heater element for heating the thermally tunable optical filter structure. The heater element is formed on the multilayered thin film structure. The thermally tunable optical filter structure is a thermo-optically tunable thin film optical filter structure. The heater element is a trace of resistive material that circumscribes a central region that is located over the hole. The trace of resistive material is a ring-shaped trace of resistive material. The thin film optical filter structure spans the opening. The thin film optical filter structure includes one or more layers comprising amorphous semiconductor, e.g. amorphous silicon. The multilayered thin film structure further includes a layer of silicon supporting the optical filter structure. The layer of silicon is a layer of crystalline silicon. The optical filter structure includes a plurality of thin film interference layers. At least some of the plurality of thin film layers includes amorphous silicon. Each of the layers among the plurality of thin film layers has a thickness that is roughly an integer multiple of λ/4. The hole is circular. The membrane above the hole has an open membrane structure or a closed membrane structure. The thin film optical filter structure includes a stack of multiple Fabry-Perot cavities.
Some of those embodiments also include one or more of the following features. The thin film optical filter structure forms the membrane over the hole and the device further includes an island of silicon attached to the underside of the membrane and positioned within the hole without contacting the substrate in which the hole is formed. The island of silicon is an island of crystalline silicon. The optical device further includes a silicon oxide layer between the island of silicon and the thin film optical filter structure.
In general, in another aspect, the invention features a method of fabricating an optical filter. The method includes: providing a substrate that has a silicon oxide layer on top of an underlying silicon layer; fabricating a thermally tunable thin-film optical filter structure on the substrate; forming a heater element above the oxide layer for heating an operating area of the optical filter structure; and etching into the backside of the substrate and down to the silicon oxide layer to expose a region of the silicon oxide layer that is under the operating area of the optical filter structure.
Other embodiments include one or more of the following features. Fabricating the thermally tunable thin-film optical filter structure on the substrate involves fabricating the thermally tunable thin-film optical filter structure on the substrate above the silicon oxide layer. Providing the substrate involves forming the silicon oxide layer on the underlying silicon layer. The method further includes removing the exposed region of the silicon oxide layer. The substrate includes the underlying silicon layer, the silicon oxide layer formed directly on the underlying silicon layer, and a crystalline silicon layer directly on top of the silicon oxide layer. Fabricating the thermally tunable thin-film optical filter structure on the substrate above the silicon oxide layer involves fabricating the thermally tunable thin-film optical filter structure above the crystalline silicon layer. Fabricating the thermally tunable thin-film optical filter structure on the substrate above the silicon oxide layer comprises fabricating the thermally tunable thin-film optical filter structure directly on the crystalline silicon layer. The method also includes forming an oxide on top of the crystalline silicon layer. Fabricating the thermally tunable thin-film optical filter structure on the substrate above the first-mentioned silicon oxide layer involves fabricating the thermally tunable thin-film optical filter structure directly on the oxide layer that is formed on top of the crystalline silicon layer. The method further includes, before etching into the backside of the substrate to expose the region of the first-mentioned silicon oxide layer, etching a trench into the backside of the substrate and down to the first-mentioned silicon oxide layer, wherein the trench circumscribes the region. Etching into the backside of the substrate to expose the region of the silicon oxide layer further involves etching the trench through the first-mentioned silicon oxide layer and down to the silicon oxide layer that is on top of the crystalline silicon layer.
In general, in still another aspect, the invention features a thermally tunable device including: a multilayer structure including a thermally tunable thin film optical filter having an operating region through which an optical signal passes during operation; and a heater fabricated on the multilayer structure for heating the operating region of the optical filter, wherein the heater includes n segments evenly distributed around the operating region of the filter, wherein n is an integer that is greater than 2 and wherein each segment is either linear or curvilinear in shape and has two ends that connect, respectively, to two different voltage supply lines.
Other embodiments include one or more of the following features. The value of n is 4. Each segment represents an arc of a circle. Each segment is made of a resistive material, e.g. platinum. The segments lie on a perimeter with separations between each segment.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
a shows the basic device structure of a thermo-optically tunable thin film filter.
b presents multiple plots of filter transmission characteristics showing the tuning range of a filter with thermo-optic spacer and dielectric mirrors.
a-e illustrate the fabrication of a tunable filter membrane structure.
a-f illustrate the fabrication of a tunable filter membrane structure that includes an underlying crystalline silicon island.
g-i illustrate the fabrication of a tunable filter membrane structure that incorporates a crystalline silicon layer as part of the membrane.
j-l illustrate the fabrication of open membrane structures.
m-n show a top view of two open membrane structures.
It should be understood that the figures are drawn for ease of illustration. The depicted structures are not drawn to scale nor are the relative dimensions intended to be accurate.
Simple Membrane Structure:
The first embodiment is a thermo-optically tunable filter formed as a membrane on a silicon frame. The tunable filter which makes up the membrane is fabricated as described in previously filed applications and published articles. In general, it is a multi-layer thin film structure that includes one or more Fabry-Perot cavities, each of which has two thin film interference mirrors separated by a spacer. The mirrors and the spacers are made of a material that has an index of refraction that is characterized by a relatively high thermal coefficient. In this case, that material is amorphous silicon (a-Si), though other materials could also be used such as amorphous germanium (a-Ge). The resulting optical filter has a optical transmission curve with a band pass located at a wavelength that is determined by the design of the structure, e.g. the thickness of the films that make up the multi-cavity structure. By heating and cooling the optical filter, one can shift that band pass back and forth over a range that is determined by the design of the filter. A resistive ring heater that is formed on the tunable filter provides the mechanism by which the film is heated.
The primary process steps for fabricating this structure are illustrated in
After oxide layers 102 and 104 have been formed, a thermo-optically tunable thin film filter stack 106 is fabricated on oxide 102 that protects the top of the wafer, also referred to herein as topside oxide 102. This multi-layer filter stack 105 is fabricated as described in the following patent applications: U.S. Ser. No. 10/005,174, entitled “Tunable Optical Filter,” filed Dec. 4, 2001; U.S. Ser. No. 10/174,503, entitled “Index Tunable Thin-Film Interference Coatings,” filed Jun. 17, 2002; and U.S. Ser. No. 10/211,970, entitled “Tunable Optical Instruments,” filed Aug. 2, 2002, all of which are incorporated herein by reference. The number of interference films and the number of F-P cavities that are included in the filter depends on the band pass characteristics and other optical requirements that the optical filter must provide in the particular application for which it is being fabricated. In the described embodiment, the filter is a 4 period mirror single cavity (4 quarter wave high index, i.e., 4QW) design with an absentee silicon nitride (2QW) encapsulation layer. The first mirror pair is a 3QW c-Si layer (high index) followed by a 1QW low-index silicon nitride. Slight offsets in 3QW optical thickness are compensated for in the silicon nitride layer, if necessary.
Referring to
To form the suspended membrane structure, the silicon below the filter is removed. This is accomplished by patterning the backside oxide layer to define an opening 116 (e.g. a circular opening) through oxide 116 in those areas in which the silicon is to be removed. After the openings are formed, the silicon is etched away until a well 118 is being formed reaches topside oxide 102 located just under filter structure 106. This can be done by using either a wet etch process or a dry etch process. In the described embodiment, a deep reactive ion etch (DRIE) process is used. The advantage of the DRIE process is that it more easily produces straight sidewalls on the well that is being etched and the SiO2 layer under the filter stack serves as very effective etch stop layer that prevents the etch from going deeper than desired. After well 118 is formed, the portion of oxide layer 102 that is exposed by well 118 and that protects the backside of filter stack 106 is removed using an appropriate wet etch, e.g. a buffered HF solution (see
This process forms a composite structure having a very small thermal mass and supported by a surrounding silicon frame. When the optical filter is thermo-optically tunable, as described above, this yields fast, uniform, and efficient heating of the tunable optical filter element.
Earlier designs of the thermo-optic tunable thin film optical filters have used a doped polysilicon heater layer that was deposited on top of a transparent fused silica substrate (typically about 500 um thick) and the filter stack was on top of the polysilicon layer. One problem encountered when using the earlier design was thermal non-uniformity across the XY-plane of the heater. This was a result of the implementation of a sheet heater which tends to result in temperatures that are hotter at the center than at the edges. The non-uniformity in temperature translated into a tuning gradient across the filter itself which degraded its optical performance. Additionally, doped poly-silicon heaters have been known to exhibit resistance drift when exposed to high temperature over long periods of time. To counteract this problem, the drift was measured during an initial calibration process, and compensated for during signal processing. Stabilizing the heater resistance by using the structure disclosed herein removes the need for drift compensation in software.
The approach described above has multiple benefits over the earlier design. First of all, it uses fewer processing steps. In addition, the membrane device structure improves the optical performance of a thermo-optic tunable filter by providing more uniform heating and less optical scattering. It also provides a stable heating element whose resistance can be used to calibrate filter temperature and therefore wavelength. Additionally, it simplifies processing since this filter structure requires no anti-reflection coating.
In the above-described embodiment, the frame that supports the thermo-optic tunable filter element is made of silicon. Materials other than silicon can also be used. In addition, the membrane that was described above is a closed membrane, meaning that it is attached at all points around its periphery. One could alternatively have fabricated an open membrane that is attached to the frame only at discrete points about its periphery. That would produce even greater thermal isolation of the thermo-optically tunable element. In addition, instead of fabricating the heater as a ring heater formed on top of the filter stack, one could incorporate it into the membrane itself as a doped layer that is heated by passing current across it.
Membrane Structure Fabricated Using SOI Wafer:
An improvement on the design described above includes a single crystal silicon layer beneath the filter stack, either as an island on the underside of the membrane (see
The fabrication of the structure with the crystalline silicon island will first be described followed by a description of the fabrication of a membrane which incorporates the crystalline silicon layer as part of the membrane.
The primary process steps for fabricating this structure are illustrated in
By using, for example, a wet oxidation process, SiO2 layer 206 and 208 are grown, respectively, on top of device layer 204 and on the backside of the wafer. As before, oxide layer 206, also referred to as topside oxide 206, will serve as a stop layer for deep etches that are later performed from the backside of the substrate. The optical filter structure 210 is then fabricated over the entire wafer on top of oxide 206. Since oxide 206 will be in the optical path of filter structure 210, its thickness needs to be carefully controlled so it acts either as an absentee layer or a reflection layer which forms part of the optical filter.
Next, a metal layer is deposited on top of filter structure 210 and, using standard fabrication techniques, that metal layer is then patterned and etched to form a ring heater 212 with contact pads.
Referring now to
If the silicon island (device layer) thickness is such that it can be incorporated into the filter stack design (e.g. its thickness is roughly equal to an integer multiple of a quarter-wavelength making it either an absentee layer or an interference layer), then an AR coat may not necessary. Otherwise, at this point, an AR coat needs to be deposited on the backside of the island.
The structure that is formed, like the earlier described structure shown in
When viewed from above, as shown in
In the example illustrated by
As noted earlier, an alternative design to using a silicon island beneath the filter stack is to incorporate the crystalline silicon layer directly into the membrane itself as a continuous, unpatterned sheet. This structure trades off some thermal isolation for reduced processing complexity. Such a structure is fabricated as follows. Instead of defining trench regions 218 as shown in
In the structure depicted in
As was also noted above, to eliminate the need for an AR coating requires that the device layer thickness, d, be roughly an integer multiple of quarter wavelengths (note that compensation layers can be grown to offset slight variations in the thickness of this layer). To achieve this level of thickness control, the “smart cut” process is used to fabricate the SOI wafer.
The “smart cut” process uses two polished Si wafers, wafer A and wafer B. An oxide is thermally grown on wafer A, after which hydrogen is implanted through the oxide layer and into the underlying silicon to a predetermined depth. Wafer A is then hydrophilicly bonded to wafer B under the application of pressure and a temperature of about 400-600° C. During a subsequent heat treatment, the hydrogen ion implantation acts as an atomic scalpel enabling a thin slice of crystalline film (of thickness d) to be cut from wafer A (i.e., the donor wafer) and transferred on top of wafer B (i.e., the receiving wafer).
The bond is strengthened by a second, subsequent anneal at about 1100° C. In the resulting structure, the thin crystalline Si film (generally referred to as the “device” layer) is bonded to the oxide film which is now firmly bonded to wafer B (also referred to as the “handle” layer). The device layer is typically 300-500 nm thick with high accuracy (about +30-40 nm). A final light polish of the exposed Si-film surface is then carried out to ensure a very smooth surface.
Wafers that are made by this process are commercially available from S.O.I.TEC Silicon On Insulator Technologies (Soitec) of Bemin, France.
Modified “Ring” Heater Design:
A modification to the design of the ring heater that produces a further reduction in the thermal gradients and stresses that occur across the membrane is shown in
In addition, in ring heater 400 includes four separate heating elements 404a-d (or segment) that are electrically connected together at points that are located away from a central region 410 that is being heated by the heater. Each heating element includes a segment of a circular ring that spans about 90° of the circle. On the circle which the group of segments form, each segment is separated from its neighbor by a gap 407. And at each end of the segment there is a conductive path 406 leading radially away from central region 410 and towards an associated contact pad to which it is connected. In other words, the segments are electrical connected to each other at locations that are radially outside of the central heating ring (in this particular example it is at the contact pads but it could be before that). Moving the electrical connection points between the segments to locations that are outside of the central heating elements also appears to improve the uniformity of the heating of the working portion of the tunable filter.
This heater design improves the mechanical performance of the membrane which in turn improves the optical performance of the filter, specifically reducing the stress-induced polarization dependent loss (PDL) to below 0.2 dB at 0.5 dB passband. Low PDL is required in most telecom applications and is difficult to achieve in most tunable filters unless there is some additional compensation scheme. This structure makes it possible to more easily achieve that level of performance. In addition, such a structure also permits open membrane designs.
The segments shown here are curvilinear (more specifically, segments of a circle) but they could also be linear. In many applications, the supply voltage that is available for the heater is down around 5 volts which places a serious limitation on the design of the heater especially if control over a wide temperature range is desired. The overall resistivity must be quite low meaning that the path length must be short thus limiting one to straight or curved segments, as opposed to serpentine structures.
The structures described above have particular usefulness in connection with the thermo-optically tunable thin film optical filters. But these structures would also be useful for other devices in which a heater with excellent electrical stability, high resistance to delamination and rupture, and/or good transparency in the IR without scattering is required.
In the embodiments described above, the optical filter was fabricated on top of the substrate or the crystal silicon layer prior to etching the well under the filter. It is also possible to first fabricate a membrane, e.g. an SiO2 or crystal silicon layer, etch the well, and fabricate the filter in the well on the backside of the membrane.
Though the descriptions presented above generally focused on the fabrication of an individual device on a wafer substrate, in reality there will be many such devices fabricated on a single wafer and they will later be separated into individual components by cutting and dicing the wafer to produce many individual die.
Other embodiments are within the following claims.
This application claims the benefit of U.S. Provisional Application No. 60/509,379, filed Oct. 7, 2003; and U.S. Provisional Application No. 60/509,200, filed Oct. 7, 2003.
| Number | Date | Country | |
|---|---|---|---|
| 60509200 | Oct 2003 | US | |
| 60509379 | Oct 2003 | US |
