# Patent

## Title
Low loss contact structures for silicon based optical modulators and methods of manufacture

## Abstract

The present invention provides silicon based thin-film structures that can be used to form high frequency optical modulators. Devices of the invention are formed as layered structures that have a thin-film dielectric layer, such as silicon dioxide, sandwiched between silicon layers. In one aspect of the invention an electrical contact structure is provided. The electrical contact structure comprises a connecting portion that electrically connects an active region of at least one of the silicon layers to a contact portion of the electrical contact structure.

## Background
This application claims the benefit of U.S. Provisional Application Ser. No. 60/554,457, filed Mar. 18, 2004, entitled “Silicon Based Optical Modulators and Methods of Manufacture,” which disclosure is incorporated herein by reference in its entirety for all purposes.


TECHNICAL FIELD
The present invention relates to silicon based optical modulators for optical transmission systems. More particularly, the present invention relates to electrical contact structures for silicon based thin-film structures for use in optical modulators and methods of making such structures.
BACKGROUND
The state of the art in optical communication networks, particularly that related to photonics based components for use in such networks, has advanced rapidly in recent years. Present applications require, and future application will demand, that these communication systems have the capability to reliably transfer large amounts of data at high rates. Moreover, because these networks need to be provided in a cost efficient manner, especially for “last mile” applications, a great deal of effort has been directed toward reducing the cost of such photonic components while improving their performance.
Typical optical communications systems use fiber optic cables as the backbone of the communication system because fiber optics can transmit data at rates that far exceed the capabilities of wire based communication networks. A typical fiber optic based communication network uses a transceiver based system that includes various types of optoelectronic components. Generally, a transceiver includes a light source, means to convert an electrical signal to an optical output signal, and means to convert an incoming optical signal back to an electrical signal. A laser is used to provide the source of light and a modulator is used to turn the light source into an information bearing signal by controllably turning the light on and off. That is, the modulator converts the light from the laser into a data stream of ones and zeroes that is transmitted by a fiber optic cable. The incoming optical signal can be converted back to an electrical signal by using components such as amplifiers and photodetectors to process the signal.
Commercially used optical modulators are either lithium niobate based devices or compound semiconductor based devices such as the III-V based devices that use gallium arsenide or indium phosphide material systems. Additionally, silicon based devices have been developed. However, silicon based optical modulator technology has not been able to provide a device that can perform like the commercially available products and many problems need to be solved before such silicon based devices can compete with the commercially available lithium niobate and compound semiconductor devices.
Lithium niobate devices rely on an electrooptic effect to provide a modulating function. That is, an electric field is used to change the refractive index of the material through which the light is traveling. These devices are usually provided as a Mach-Zehnder interferometer. In this type of modulator, an incoming light source is divided and directed through two separate waveguides. An electric field is applied to one of the waveguides, which causes the light passing through it to be out of phase with respect to the light in the other waveguide. When the light emerges from both waveguides and recombines, it interferes destructively, effectively turning the light off.
In contrast, compound semiconductor based devices rely on an electroabsorption effect. In this type of modulator an applied electric field is also used, but not to vary the refractive index of the material through which the light is propagating. In a compound semiconductor material, an electric field can be used to shift the absorption edge of the material so that the material becomes opaque to a particular wavelength of light. Therefore, by turning the electric field on and off, the light can be turned on and off.
One problem with lithium niobate based modulators is that as the data transfer rate increases for these devices, so must the size of the device itself. This requires more material, which can increase cost. These modulator devices are often integrated into packages with other components where the demand for smaller package sizes is continually increasing. Therefore, modulator size is a concern. Another problem with lithium niobate based devices is that the drive voltage can be somewhat high as compared to compound semiconductor devices. Accordingly, because a large voltage change between the on and off state is more difficult to produce than a lower voltage swing, the drive electronics required to provide such large voltage changes are typically relatively expensive and can introduce more cost to the systems.
Compound semiconductor modulator devices can be made extremely small and are not limited by the size restrictions of lithium niobate based devices. Moreover, these devices can handle high data transfer rates at relatively low drive voltages. However, current compound semiconductor based modulators, such as those fabricated from the indium phosphide material system, have certain limitations. In particular, these devices can suffer from problems related to coupling losses and internal absorption losses, which are generally not present in lithium niobate based devices.
As an additional concern, the processing and manufacture of compound semiconductor based devices is expensive when compared to silicon based devices, for example. One reason for this is that many of the base materials used for compound semiconductor processing are expensive and difficult to handle. For example, indium phosphide wafers are presently limited in size and the largest wafers are expensive. This makes low cost high volume manufacturing difficult as compared to that which can potentially be obtained in the manufacture of silicon based devices.
Regarding silicon based technology, a silicon based modulator can be designed to function in a manner that is similar to the way a lithium niobate based device functions in that it changes the phase of the light passing through a waveguide. This phase change can be used in a Mach-Zehnder type device to form a modulator. More particularly, a silicon based device generally operates on the principle that a region of high charge concentration can be used to shift the phase of light in the waveguide. Importantly, the magnitude of the phase shift is proportional to the charge concentration and the length of the charged region in a direction in which the light travels. Thus, the ability to create a region of sufficient charge density to interact with the light is essential to be able to induce a phase change, especially one that can shift the phase by an amount suitable for use in a Mach-Zehnder type device.
In order to provide a charged region that can be used for phase shifting, these devices are known to use injection of electrons or depletion of holes in a diode or triode type device. In operation, a concentration of charge carriers can be provided in an active portion of a guiding region of a waveguide in these devices. One parameter that is important in a silicon based optical modulator is the speed in which a charged region can be created and subsequently dissipated. More particularly, the speed at which charge carriers can be generated as well as the speed at which charge carries can be removed (by recombination, for example) affects the speed at which modulation can be performed. These generation and recombination processes are directly related to the mobility of the charge carriers in the particular material. Because these devices use both single crystal silicon and non-single crystal silicon and because the mobility of charge carriers in non-single crystal silicon is significantly lower than the mobility of charge carriers in single crystal silicon, the low mobility non-single crystal material unfortunately limits the rate at which the device can modulate light.
SUMMARY
The present invention thus provides silicon based thin-film structures that are capable of rapidly creating and removing a charged region for shifting the phase of light passing through the structure. In accordance with the present invention, high frequency optical modulators can be formed using silicon-insulator-silicon thin-film structures. In order to provide a charged region for phase shifting, in accordance with the present invention, devices of the present invention are preferably formed as layered structures that have an insulator layer, such as silicon dioxide, sandwiched between silicon layers. A concentration of charge carriers can be provided in a region adjacent to each silicon/oxide interface by applying an electrical bias across the silicon layers. This effectively moves charge carriers from the bulk silicon material toward the oxide layer so they build up in a region near the interface.
One parameter that is important in this type of device is the speed in which a charged region can be created. This speed is directly related to the mobility of the charge carriers in the particular material. Therefore, in one embodiment, a high performance device can be provided when both silicon layers comprise high-mobility silicon such as crystalline silicon. High mobility material is particularly preferred for the active portion of the waveguide of the device. Moreover, to function as an optical modulator, such devices preferably include structure of appropriate materials for rapidly altering the free carrier concentration across the optical path of a waveguide and preferably the structure is defined to confine and guide the light through the waveguide without degrading or attenuating the signal.
Another parameter that is important in this type of device relates to the efficiency of the device. In particular, losses due to absorption in any part of the device can affect the efficiency of this type of optical device. Optical absorption can occur where metal is used in the device such as at an electrical contact. High efficiency devices are preferred for many reasons such as for providing devices that can be operated at low voltages. Accordingly, in one embodiment of the present invention, electrical contact structures are provided wherein contact regions that use a metal(s) are spaced apart or away from an active region of the device in order to control possible optical losses.
The present invention provides silicon-insulator-silicon structures for optical modulators having first and second silicon layers with each preferably comprising active regions comprising high free carrier mobility silicon. Such silicon-insulator-silicon structures are desirable for high speed optical signal modulation (greater than 1×109 hertz, for example). Preferably, silicon that has a bulk free carrier mobility of at least 500 centimeters2/volt-second (cm2/V-s) at room temperature (if n-type silicon is used) and at least 200 cm2/V-s at room temperature (if p-type silicon is used) is provided as starting material to form a thin-film optical modulator structure in accordance with one aspect of the present invention. That is, this high mobility silicon may be provided and further doped to form an active region of a silicon layer for an optical modulator structure, which doping can change, and typically lowers, the mobility of the active region from the initial value. Preferably, such an active region is doped to a level sufficient to achieve the desired modulation performance. In any case, the doped active region is considered to have a high free carrier mobility in accordance with the present invention if it is formed from a material that has a free carrier mobility as set forth above.
It has been estimated that speeds in excess of 1×109 hertz and as high as or greater than 10×109 hertz can be realized when the active region of the second silicon layer has a mobility that is at least 20%–25% of the mobility of the active region of the first silicon layer. Accordingly, the second silicon layer is preferably formed from silicon material that has a mobility that is at least 20%–25% of the mobility of the material that is used to form the first silicon layer. To further improve the modulation performance, it is preferable to have the second layer mobility at about 50%, and most preferably close to 100%. Thus, the initial silicon material for forming the active region of the second silicon layer preferably has a mobility of about 50%, and most preferably close to 100% of the initial silicon material for the active region of the first silicon layer.
Accordingly, in one aspect of the present invention, a silicon based thin-film structure for an optical modulator is provided. The thin-film structure preferably comprises a waveguide. The waveguide preferably includes a guiding region comprising a silicon-insulator-silicon thin-film structure. The silicon-insulator-silicon structure preferably comprises a dielectric device layer sandwiched between first and second silicon device layers. The thin-film structure also preferably comprises an electrical contact structure for at least one of the first and second silicon device layers. The electrical contact structure preferably comprises a contact portion spaced apart from the guiding region of the waveguide.
In another aspect of the present invention, a silicon based thin-film structure for an optical modulator is provided. The thin-film structure generally comprises a substrate, first and second silicon device layers, a thin-film dielectric device layer, a waveguide, and an electrical contact structure for the first silicon layer. The first silicon device layer is preferably formed on the substrate and the thin-film dielectric device layer is preferably formed on the first silicon device layer. The thin-film dielectric device layer preferably has a predetermined thickness. A second silicon device layer is preferably formed on the thin-film dielectric device layer. The waveguide preferably comprises a guiding region and at least a portion of the first silicon device layer, the thin-film dielectric device layer, and the second silicon device layer. The electrical contact structure for the first silicon device layer preferably includes a contact portion spaced apart from the guiding region of the waveguide.
In yet another aspect of the present invention, a method of forming a silicon based thin-film structure for an optical modulator is provided. The method generally includes steps of forming a silicon-insulator-silicon thin-film structure, forming a waveguide, and forming an electrical contact structure. Forming the silicon-insulator-silicon structure preferably comprises providing a substrate comprising a first silicon layer, depositing a thin-film dielectric layer on the first silicon layer of the substrate, and depositing a second silicon layer on the thin-film dielectric layer. The waveguide preferably includes a guiding region and at least a portion of the first silicon layer, the thin-film dielectric layer, and the second silicon layer. The electrical contact structure preferably includes a contact portion spaced apart from the guiding region of the waveguide.



BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:
FIG. 1 is a schematic cross-sectional view of an exemplary layered thin-film silicon-insulator-silicon structure in accordance with the present invention that can be used to form an optical modulator;
FIG. 2 is a schematic cross-sectional view of a silicon-on-insulator substrate that can be used to form a layered thin-film silicon-insulator-silicon structure in accordance with an embodiment of the present invention such as the layered thin-film silicon-insulator-silicon structure shown in FIG. 1;
FIG. 3 is a schematic cross-sectional view of the silicon-on-insulator substrate of FIG. 2 showing in particular a thin-film etch stop layer that has been provided on a silicon layer of the silicon-on-insulator substrate to form a layered structure;
FIG. 4 is a schematic cross-sectional view of the layered structure of FIG. 3 showing in particular channels that are provided to isolate a portion of the silicon layer of the silicon-on-insulator structure in accordance with an aspect of the present invention;
FIG. 5 is a schematic cross-sectional view of the layered structure of FIG. 4 with a thin-film dielectric layer covering the thin-film etch stop layer and filling the channels;
FIG. 6 is a schematic cross-sectional view of the layered structure of FIG. 5 after partial removal of the covering dielectric layer and removal of the thin-film etch stop layer, thus leaving a first electrically isolated silicon device layer having an exposed surface;
FIG. 7 is a schematic cross-sectional view of the layered structure of FIG. 6 with a functional thin-film dielectric device layer provided over the first silicon device layer;
FIG. 8 is a schematic cross-sectional view of the layered structure of FIG. 7 showing a second silicon device layer overlying part of the thin-film dielectric layer and the first silicon device layer to form a thin-film silicon-insulator-silicon structure that, in accordance with the present invention, can be used to form an optical modulator;
FIG. 9 is a schematic top view of a layered structure that can be used to form an optical modulator such as the optical modulator shown in FIG. 8 with first and second contact structures that can be used to position contact regions away from an active region of the layered structure;
FIG. 10 is a schematic cross-sectional side view of the layered structure of FIG. 9 showing a thin-film dielectric device layer positioned between first and second silicon device layers; and
FIG. 11 is a schematic top view of another layered structure that can be used to form an optical modulator such as the optical modulator shown in FIG. 8 with first and second contact structures that can be used to position contact regions away from an active region of the layered structure wherein the first and second contact structures are provided at an angle with respect to a direction of light propagation through the layered structure.



DETAILED DESCRIPTION
In FIG. 1, a first embodiment of an optical modulator 10 in accordance with the present invention is schematically illustrated in cross-section. As shown, the optical modulator 10 includes a substrate 12, preferably silicon, an insulator that preferably comprises buried oxide layer 14, and a first silicon layer 16. Preferably, the first silicon layer 16 and the buried oxide layer 14 are provided as a silicon-on-insulator structure, as conventionally known. However, the modulator 10 does not require use of silicon-on-insulator technology and may be formed by other techniques including in particular those described below. Silicon-on-insulator structures are preferred because of their compatibility with conventional complementary metal oxide semiconductor (CMOS) processing. As such, the optical functionality of an optical modulator (which itself is an electro-optical device) can be integrated with the electrical functionality of devices such as transistors, resistors, capacitors, and inductors on the same substrate. These electro-optical and electrical devices can be formed by using the common processing techniques to provide optical circuits that are integrated with electrical circuits. Moreover, silicon-on-insulator technology provides an easy way to provide a high quality single crystal layer and to electrically isolate plural devices that can be formed in the silicon layer from each other.
The modulator 10 also preferably includes a thin-film dielectric layer 18 sandwiched between the first silicon layer 16 and a second silicon layer 20. In one preferred embodiment, the thin-film dielectric layer 18 comprises a silicon dioxide layer. Also, the first silicon layer 16 preferably comprises an electrically isolated layer. That is, the silicon layer 16 is preferably surrounded by an insulating material in order to laterally and vertically isolate the silicon layer 16. Preferably, as described in more detail below, a silicon-on-insulator substrate is used and a conventionally known shallow trench isolation process can be used to laterally isolate the silicon layer 16 from other adjacent devices formed on the same substrate. The buried oxide layer of the silicon-on-insulator structure can thus provide vertical isolation. As such, the silicon layer 16 (or silicon island) can be structurally isolated by the thin-film dielectric layer 18 and a surrounding oxide filled trench 21, which includes portions 22 and 23 that can be seen in cross-section.
Although preferred, silicon-on-insulator technology does not need to be used. Any conventionally known or future developed technique capable of functioning in the same manner to sufficiently isolate device structures for forming high frequency optical modulators in accordance with the present invention can be used. In particular, such structures generally require sufficient lateral as well as horizontal isolation to functionally isolate devices from each other. For example, it is contemplated that conventionally known techniques such as deep trench isolation or local oxidation of silicon (LOCOS) can be used to laterally isolate device structures. Regarding vertical isolation, any technique that is conventionally known or future developed for sufficiently vertically isolating the silicon layer 16 in accordance with the present invention may be used.
An oxide layer 24 is also preferably provided, as illustrated, and is preferably designed in order to at least partially define a waveguide 30 that extends for a predetermined distance in a direction of propagation of an electromagnetic field through the waveguide 30. That is, the oxide 24 preferably assists to confine light in the waveguide 30. A propagating electromagnetic field is also referred to as light herein. As shown, the waveguide 30 is preferably at least partially defined by the thin-film dielectric 18 and the first and second silicon layers 16 and 20, respectively.
The waveguide 30 functions to confine and guide light propagating through a guiding region 32 of the waveguide 30. In order to illustrate this guiding and confining functionality, a mode 34 of an electromagnetic field that can propagate through the guiding region 32 of the waveguide 30 is illustrated schematically. More specifically, the waveguide 30 is preferably designed for single mode transmission. That is, the waveguide 30 is preferably designed so that the lowest order bound mode (also called the fundamental guided mode or trapped mode) can propagate at the wavelength of interest. For typical optical communications systems, wavelengths in the near infra-red portion of the electromagnetic spectrum are typically used. For example, wavelengths around 1.55 microns are common.
Thin-film structures and techniques for designing such structures for optical waveguides are well known and any structure capable of confining and guiding light in accordance with the present invention can be used. For example, such waveguides may include interfaces between thin-film materials having different refractive index, which interfaces can be used in order to guide and confine light in accordance with the present invention. Moreover, graded index regions, such as can be formed by controllably varying the composition of a material, may be used to guide and confine light as is well known.
The first silicon layer 16, the thin-film dielectric layer 18, and the second silicon layer 20 are also preferably designed to be capable of modulating light that is traveling through the guiding region 32 of the waveguide 30. More specifically, at least a portion of the guiding region 32 of the waveguide 30 is preferably designed to include an active region. Accordingly, the first silicon layer 16 and the second silicon layer 20 are preferably operatively doped to form active regions (or doped regions) in the first and second silicon layers 16 and 20. Preferably, the first and second silicon layers, 16 and 20, are doped in a region or area where it is desired to rapidly alter the free carrier concentration across the optical path of the light propagating through the guiding region 32 of the waveguide 30. As described below, the silicon layer 16 can be p-type and the silicon layer 20 can be n-type or vice versa. Preferably, the active regions are sufficiently doped in order to achieve a desired modulation or switching speed.
Also, the modulator 10 preferably includes a first electrical contact 26 for providing a contact to the silicon layer 20 and a second electrical contact 28 for providing a contact to silicon layer 16. As shown the contacts 26 and 28 are preferably spaced apart from the active portion of the guiding region 32 in order to minimize optical losses that can be caused by metals as are preferably used in such contacts. Low loss contact structures and methods of forming such contacts that can be used in accordance with the present invention are described in more detail below.
Preferably, the contacts 26 and 28 are formed as low resistance ohmic contacts wherein current varies linearly with applied voltage. Accordingly, the second silicon layer 20 preferably includes a highly doped region 36 that at least partially forms the contact 26. For example, a highly doped region is preferably doped to between 5×1017/cm3 and 2×1018/cm3. Likewise, the first silicon layer 16 preferably includes a highly doped region 38 that at least partially forms the contact 28. The regions 36 and 38 are preferably doped to correspond with the doping of the respective silicon layer. That is, if the first silicon layer 16 is p-type, the region 38 is also preferably p-type. Similarly, if the second silicon layer 20 is n-type, the region 36 is also preferably n-type. Any contacts capable of providing an electrical bias to the guiding region 30, thereby forming an active region in accordance with the invention, are contemplated and can be used. Such ohmic contacts are well known by those in the art of complementary metal oxide semiconductor (CMOS) processing.
In operation, a phase shift can be produced in light passing through the waveguide 30 by applying an electrical bias across the structure. This bias activates the doped first and second silicon layers, 16 and 20, which thereby causes charge carriers to move toward the dielectric layer 18. In particular, contacts 26 and 28 can be used to provide a bias across the first and second silicon layers, 16 and 20, as the mode 34 passes through the guiding region 32 of the waveguide 30. Charge carriers in the active (doped) regions of the first and second silicon layers, 16 and 20, move toward the dielectric layer 18 and build up so as to provide regions of charge concentration that together can produce a phase shift in the light. This phase shift can be used to modulate light where the modulator 10 is provided as an arm of a Mach-Zehnder interferometer, for example.
The first and second silicon layers, 16 and 20, and the dielectric layer 18 form an optical device capable of providing a desired phase shift to light passing through the structure. The structure also functions like a capacitor, at least in the sense that charge carriers move toward the dielectric layer in response to an applied electric field. The structure is preferably not designed for use as a charge storage device (an electrical capacitor) because such a storage device may require contact metal positioned near the active portion of the device, which metal can cause optical losses. Moreover, the structure of the present invention preferably comprises a sandwich structure comprising active silicon (highly doped) layers rather than a body layer as used in a typical transistor.
As noted above, the first silicon layer 16 is preferably formed from a layer of a silicon-on-insulator structure because such silicon-on-insulator structures are readily available commercially with a top silicon layer that comprises a single crystal silicon layer, which can have a high free carrier mobility. For example, a high free carrier mobility is generally greater than at least 500 cm2/V-s at room temperature for n-type silicon and at least 200 cm2/V-s at room temperature for p-type silicon. It is noted that these values for free carrier mobility represent preferred values for the bulk material (or initial starting material) that is used to form the first silicon layer 16. That is, as described below, the first silicon layer 16 is preferably formed from an initial silicon material that has a high free carrier mobility and is subsequently doped to provide a functional device layer or active region for an optical modulator, which doping may change, and typically lowers, the free carrier mobility of a portion of the initial silicon layer. Accordingly, an active region of a device layer of an optical modulator in accordance with the present invention may have a free carrier mobility that is different from another portion of the same device layer. In any case, it is preferably to start with a silicon material that has a high free carrier mobility, as set forth above, and form an active region from (or in) that initial material.
In accordance with the present invention, the silicon layer 20 is preferably provided such that the silicon layer 20 has a free carrier mobility that is as close to the free carrier mobility of the silicon layer 16 as possible (as measured before doping to form active device layers for an optical modulator). For example, in one embodiment of an optical modulator, the silicon layer 20 can be formed as an amorphous silicon layer that is subsequently crystallized at least partially within the functional optical zone of such layer to provide improved free carrier mobility. In another embodiment of the invention, the silicon layer 20 can be formed with an effective crystal structure by selective epitaxial growth wherein extended lateral overgrowth of such selective epitaxial growth is used to provide the silicon layer 20 as such overlies at least some of the thin-film dielectric layer 18. Another technique for providing the silicon layer 20 is by using a bonding process whereby any of the necessary layers can be provided as desired and bonded together to create a desired structure.
Optical modulators in accordance with the present invention, such as the optical modulator 10 shown in FIG. 1, can be made as described below. Preferably, conventional CMOS processing techniques can be used although any other known or developed techniques can be used instead of or in combination. Referring to FIG. 2, a silicon-on-insulator structure 40 that includes a substrate 42 (typically silicon), buried oxide layer 44, and first silicon layer 46 is illustrated. Such silicon-on-insulator substrates are commercially available. The thickness of the buried oxide layer 44 and the thickness of the silicon layer 46 are preferably selected by considering certain desired properties of the particular optical modulator to be made, such as the dimensions and/or structure of the device, as well as the processing techniques to be used. Also, the silicon layer 46 preferably has a high free carrier mobility. Preferably, if n-type silicon is used, the silicon layer 46 has a free carrier mobility of at least 500 cm2/V-s at room temperature. If p-type silicon is used, the silicon layer 46 preferably has a free carrier mobility of at least 200 cm2/V-s at room temperature.
Preferably, a portion of the silicon layer 46 is electrically isolated to form a first device layer 48 as shown in FIG. 6. Such electrical isolation facilitates the formation of plural devices on the same substrate. In the illustrated embodiment, an oxide filled trench 49 is used to define the first device layer 48 by forming a border of dielectric material around the device perimeter. As shown in cross-section, the oxide trench 49 includes oxide portions 50 and 52 as part of the surrounding trench 49 that isolates the first device layer 48 from the remainder of silicon layer 46. The buried oxide layer 44 isolates the first device layer 48 from below so that an electrically isolated island of silicon can be created as the first device layer 48. This can be done, for example, by shallow trench isolation (STI) or local oxidation of silicon (LOCOS) procedures as known in conventional CMOS processing.
In a typical trench isolation process, an etch stop layer 54, (see FIG. 3) such as a silicon nitride layer, is first deposited on the silicon layer 46. Next, conventionally known masking and etching processes are used to form trench 49 as shown in FIG. 4 (trench portions 56 and 58 are shown in cross-section). The buried oxide layer 44, as shown, defines the depth of the trench 49. Accordingly, the trench 49 preferably defines the first device layer 48 as an island. An oxide layer 60 is then provided to fill the trench 49 as shown in FIG. 5, which step also may cover the etch stop layer 54, as shown. Next, the oxide layer 60 over the first silicon device layer 48 and silicon layer 46 of the structure shown in FIG. 5 can be planarized, such as by using chemical mechanical processing (CMP). Then, the etch stop layer 54 (which would preferably be relatively very thin) can be removed, such as by, for example, an acid bath that can selectively remove the etch stop layer 54 from the silicon layer 46 and the first device layer 48. The structure shown in FIG. 6 can thus be provided wherein the first device layer 48 is electrically isolated from the silicon layer 46 by the oxide trench 49 and the buried oxide layer 44. While shallow trench isolation with silicon-on-insulator technology is preferred, other techniques as described above, can be used. Moreover, any known or developed methods for planarizing or removing materials are contemplated, and such processes may be conducted by any number of combined steps of multiple varieties.
Preferably, the first device layer 48 is doped to form a p-type active region for an optical modulator but the first device layer 48 may be doped to form an n-type active region if desired. Such doping can be done before the trench 49 is formed or after the trench is formed. However, such doping is preferably performed in a manner that minimizes the possibility of undesirable thermal diffusion of dopant species. Dopants such as boron can be used to form p-type regions and dopants such as arsenic, phosphorus, and antimony can be used to form n-type active regions. Preferably, the first device layer 48 is doped sufficiently to provide a p-type material suitable for use in optical modulation. Conventionally known photolithography and ion implantation processes may be used, for example to perform the material doping. It is noted that such doping may change the free carrier mobility of the first device layer 48 such that it is different from the free carrier mobility of the initial silicon layer 46. In any case, it is preferred to start with a high mobility material such as the silicon layer 46 to form the first device layer 48. Additionally, as mentioned above, the first device layer is preferably sufficiently doped to form an active region capable of achieving the desired modulation performance. For example, by starting with the silicon layer 46 with the above noted free carrier mobility, an active region can be formed that is capable of achieving high modulation performance (greater that 1×109 hertz).
Next, as shown in FIG. 7, a thin-film dielectric layer 60 is preferably deposited over the first device layer 48 and the oxide filled trench 49, as illustrated. The thin-film dielectric layer 60 provides a functional device layer portion over at least part of the first silicon device layer 48 for electro-optically creating an optical modulator. For example, silicon dioxide having a thickness that is preferably less than 100 angstroms may be thermally grown or deposited using conventional low pressure chemical vapor deposition (LPCVD). Moreover, the thin-film dielectric layer 48 may be formed from or include other dielectric materials, or combination thereof, such as silicon nitride, aluminum oxide, aluminum nitride, as well as those material generally characterized as titanates. Any deposition techniques may be used such as those including chemical vapor deposition, physical vapor deposition, and the like.
After the thin-film dielectric layer 60 is deposited to a desired functional thickness, an amorphous silicon layer (or polycrystalline silicon layer) is then preferably deposited, patterned, doped, and at least partially crystallized to form a second silicon device layer 62, and to eventually (after structural processing define a structure such as shown in FIG. 8 in accordance with an aspect of the present invention. In particular at least a portion of second silicon device layer 48 that comprises a part of a functional optical modulator (where the first and second device layers 48 and 68 overlap with thin-film dielectric layer 60 in between) is crystallized as an effective single crystal structure for increased carrier mobility as described below in more detail. The second device layer 62 can be provided by any desired technique including deposition, crystallization, and bonding processes. For example, the second device layer 62 can by provided by using a deposition and/or crystallization process as described in commonly owned co-pending U.S. patent application Ser. No. 10/915,299, entitled SILICON-INSULATOR-SILICON THIN-FILM STRUCTURES FOR OPTICAL MODULATORS AND METHODS OF MANUFACTURE, filed on even date herewith, the entire disclosure of which is fully incorporated herein by reference for all purposes. Also, the second device layer 62 can be provided by using a bonding process as described in commonly owned co-pending U.S. patent application Ser. No. 10/915,081, entitled BONDED THIN-FILM STRUCTURES FOR OPTICAL MODULATORS AND METHODS OF MANUFACTURE, filed on even date herewith, the entire disclosure of which is fully incorporated herein by reference for all purposes. Patterning may be done by any known or developed silicon etching or removal technique to preferably create an island of silicon as the second device layer 62. Doping of the second device layer 62 may be done in any manner, such as described above with respect to first device layer 48 in order to achieve a desired modulation performance.
Additionally, a cap layer 63, such as silicon dioxide or the like is preferably formed over the patterned second device layer 62 as shown. The patterning, doping, and crystallizing steps can be performed in any desired order although the second device layer 62 is preferably at least partially crystallized before doping to minimize any potential diffusion effects of the dopant. Where the first device layer 48 is p-type, the second device layer 62 is therefore n-type, and vice versa. Preferably, the second device layer 62 is doped sufficiently to provide an n-type material suitable for use in optical modulation.
The silicon material (amorphous or polycrystalline) for the second device layer 62 can be deposited by any technique such as low pressure chemical vapor deposition, for example. The second device layer 62 is preferably patterned to create a structure wherein a portion of the first device layer 48 can be accessed for forming a contact 64 to the first device layer 48 such as illustrated. The contact 64 is preferably an ohmic contact and can be formed by conventionally known techniques that may include forming an opening 65 through the cap layer 63 and the dielectric layer 60 to provide access to a surface of the first device layer 48. The contact 64 is preferably created at a point sufficiently spaced from a guiding region 68 to minimize potential absorption related loss effects that can be caused by metal materials. The second device layer 62 is also preferably patterned to create a structure so that a contact 66 can be provided to the second device layer 62 and such that the contact 66 is also sufficiently spaced from the guiding region 68 to minimize potential absorption related loss effects that can be caused by such contacts. Structure in accordance with an aspect of the present invention that can be used to provide low loss contacts are described in further detail below. The contact 66 is preferably an ohmic contact and can be formed by conventionally known techniques that may include forming an opening 66 in the cap layer 63 to provide access to the second device layer 62. Formation of such a contact may also include any doping and/or deposition steps in order to provide the contact with any desired electrical properties.
In accordance with an aspect of the present invention, the second device layer 62, if provided as an amorphous or polycrystalline material, is preferably thermally processed such as by using a furnace, epi reactor, rapid thermal processor, heated element, or laser system to at least partially crystallize the second device layer 62. Alternatively, the second device layer 62 can be provided in accordance with a bonding process as described in commonly owned co-pending U.S. patent application Ser. No. 10/915,081, entitled BONDED THIN-FILM STRUCTURES FOR OPTICAL MODULATORS AND METHODS OF MANUFACTURE, filed on even date herewith, the entire disclosure of which is fully incorporated herein by reference for all purposes. Preferably, the second device layer 62 is at least partially crystallized so that a crystalline or polycrystalline region with enhanced carrier mobility can be provided in at least an active portion of the guiding region 68. Accordingly, the second silicon layer 62 is preferably formed from silicon material that has a mobility that is at least 20%–25% of the mobility of the material that is used to form the first device layer 48 (the silicon layer 46). To further improve the modulation performance, it is preferable to have the second silicon layer 62 mobility at about 50%, and most preferably close to 100%. Thus, the initial silicon material for forming the active region of the second silicon layer 62 preferably has a mobility of about 50%, and most preferably close to 100% of the initial silicon material for the active region of the first silicon layer 48.
Any process can be used that is capable of at least partially crystallizing a silicon layer, such as an amorphous silicon layer, to provide a desired mobility. Such crystallization can be done at any time after the second silicon layer is formed. Moreover, any process capable of improving the free carrier mobility of a silicon material, whether crystalline or not, may be used. Moreover, such a technique can be used to improve the crystallinity, such as by reducing defects or the like, of a crystalline, polycrystalline or partially crystalline silicon layer for the purpose of improving free carrier mobility. For example, crystallization of deposited silicon films by furnace, lamp, and laser techniques at a sufficient temperature and time to achieve a desired degree of crystallization can be used.
As described above, a waveguide structure that comprises a silicon-insulator-silicon thin-film structure that can guide and confine light through the waveguide structure can be provided in accordance with the present invention. Additionally, in accordance with the present invention, contact structures for controlling and minimizing losses in the optical path are provided. Preferably, a contact structure in accordance with one aspect of the present invention includes a contact portion that is spaced apart from the guiding region of the waveguide. Such a contact structure also preferably includes a connecting portion that connects an active region of the waveguide to the contact portion. Preferably, as described in more detail below, such a connecting structure comprises a finger-like structure that extends from the guiding region of the waveguide such as from a guiding edge of a layer of the waveguide. By providing a finger-like structure, the contact portion of the contact structure can be spaced apart from the guiding region of the waveguide to minimize potential optical losses as described in more detail below. Moreover, using such a finger-like structure also provides the ability to space the contact portion away from the guiding region of the waveguide without compromising the guiding function of an edge of the waveguide as described in more detail below.
One example of a layered silicon-insulator-silicon structure 80 in accordance with the present invention that can be used for minimizing optical losses is shown schematically in FIGS. 9 and 10. In FIG. 9 a top schematic view is shown and in FIG. 10 a side cross-sectional schematic view is shown. A waveguide 81 is shown that preferably includes first layer 82, thin-film dielectric layer 84, and second layer 86. The waveguide 81 is preferably designed so that it can confine and guide light in a propagation direction, generally indicated by reference numeral 88 in FIG. 9. The layers 82, 84, and 86 preferably form a silicon-insulator-silicon structure such as those described above. That is, the structure 80 can be integrated with the structures described above and formed by the same techniques in accordance with the present invention.
The layered structure 80 preferably includes a contact structure 83 for the layer 82 and a contact structure 87 for the layer 86. As shown, the contact structure 83 includes a contact portion 85 and a connecting portion 92 that connects the contact portion 85 to an active region 89 of the layer 82. Also, as shown, the contact structure 87 includes a contact portion 91 and a connecting portion 90 that connects the contact portion 91 to an active region 93 of the layer 86. The contact portions 85 and 91 preferably comprise highly doped regions and can be formed in accordance with the invention as described above.
As shown, the waveguide 81 includes guiding edges that function to guide light propagating through the waveguide 81 in the propagation direction 88. In particular, an edge 95 of the layer 82 and an edge 96 of the layer 86 work together to provide a guiding function on one side of the waveguide 81. A guiding function is provided on the other side of the illustrated waveguide 81 by an edge 97 of the layer 82 and an edge 98 of the layer 86.
As illustrated, the connecting portions 90 and 92 preferably extend laterally from the edges 98 and 97, respectively. In particular, the connecting portions 90 and 92 are preferably designed as finger-like structures that electrically connect the contact regions 91 and 85 that are positioned away from the active portion of the waveguide 81 to the respective active regions 93 and 89. By spacing the contact regions 91 and 85 away from the active regions, coupling between light passing though the waveguide 81 and the contact regions 91 and 85 can be minimized thereby minimizing optical losses and providing efficient devices in accordance with an aspect of the present invention. Because such a finger-like extension interrupts the guiding edge from which it extends, the connecting portions 90 and 92 are preferably designed so that sufficient guiding is provided by the particular guiding edge. For example, referring to the layer 82, the guiding edge 97 is interrupted by the connecting portion 92 of the contact structure 83. In particular, the connecting portion 92 has a width 99 that effectively interrupts the guiding edge 97. Preferably, the width 99 of the connecting portion 92 is chosen so that the benefits of spacing the contact region 85 away from the active region 89 of the waveguide 81 can be realized without compromising guiding of light through the waveguide 81. The contact structure 87 can be designed in the same manner so that both of the contact regions 91 and 85 can be spaced apart from the active region of the waveguide and sufficient guiding within the waveguide 81 can be provided. That is, the contact structures 83 and 87 are preferably designed so that the edges 97 and 98 provide sufficient guiding and confining of light passing through the waveguide 81.
Preferably, the contact structure 83 is spaced apart along the propagation direction 88 from the contact structure 87 and at different levels, but the contact structures 83 and 87 can overlap if desired. Moreover, any number of contact structures can be used for the layers 82 and 86. The contact structures can be positioned on the same side of the waveguide 81, as shown, or may be positioned on opposite sides of the waveguide 81. That is, any number of number of contact structures provided on one or both sides of the waveguide 81 can be used as long as a desired guiding function can be provided by the waveguide 81. Also, the connecting portions 90 and 92 may follow any desired path to connect the respective device layer to the contact portions, 91 and 85, and do not need to follow a linear path as is shown in FIG. 9. In any case, a contact structure of the present invention is preferably designed to provide an electrical connection to an electrically active region of a layer of a silicon-insulator-silicon thin-film structure that controls potential optical losses by spacing a contact region of the contact structure away from the active region.
As mentioned, the contact portions 85 and 91 are spaced apart from the waveguide 81 in order to help to minimize optical losses that can be caused by metal that is often used in the contact portions 85 and 91 or metal that is used to form conducting lines in an integrated circuit. That is, by spaced apart it is meant that the contact portions 85 and 91 do not overlap with any portion of the active regions 89 and 93. Also, the contact portions, 85 and 91, are preferably positioned away from the active regions, 89 and 93, so that interconnecting circuit lines can be routed to minimize or eliminate optical losses from such lines. In one preferred embodiment, the contact regions 85 and 91 may be formed about 1 micron or more away from the active portion of the guiding region to prevent coupling between a metal in the contact regions 85 and 91 and the optical signal. However, the contact regions 85 and 91 can be spaced apart from the active portion of the guiding region by any distance sufficient to control losses due to coupling between light in the active portion of the device and metal in a contact region. Moreover, the contact regions 85 and 91 can be spaced apart from the active portion of the device by the same or different distances.
As illustrated in the FIG. 9 embodiment, the connecting portions 90 and 92 extend from the edge 95 at an angle of about 90 degrees (normal to the propagation direction 88). Also, one or both of the connecting portions 90 and 92 may extend from the edge 95 at an angle less than 90 degrees (an oblique angle with respect to the propagation direction 88). For example, in FIG. 11, a layered thin-film structure 100 is shown that includes a waveguide 102 formed from a dielectric device layer 104 sandwiched between first and second silicon layers 106 and 108. The thin-film structure 100 may be provided as described above with respect to the thin-film structure 80. As shown, the layered structure 100 includes contact structures 110 and 112 that comprise contact regions 114 and 116, respectively, as well as connecting portion 118 and 120, respectively. The connecting portions 118 and 120 of the contact structures 110 and 112 can be designed, as shown, to extend from a propagation direction 122 at an angle that is less than 90 degrees (an oblique angle with respect to the propagation direction). By forming the connecting portions 118 and 120 at an oblique angle with respect to the propagation direction 122, it is generally more difficult for light traveling in the waveguide to reach any of the contact regions 114 and 116, thereby reducing the possibility of optical losses due to absorption.
The present invention has now been described with reference to several embodiments thereof. The entire disclosure of any patent or patent application identified herein is hereby incorporated by reference. The foregoing detailed description and examples have been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. It will be apparent to those skilled in the art that many changes can be made in the embodiments described without departing from the scope of the invention. Thus, the scope of the present invention should not be limited to the structures described herein, but only by the structures described by the language of the claims and the equivalents of those structures.

## Claims

1. A silicon based thin-film structure for an optical modulator, the thin-film structure comprising:
a waveguide having a guiding region comprising a silicon-insulator-silicon thin-film structure comprising a dielectric device layer sandwiched between first and second silicon device layers, at least one of the first and second silicon device layers comprising a guiding edge that at least partially defines the guiding region; and
an electrical contact structure for at least one of the first and second silicon device layers, the electrical contact structure comprising a contact portion and a connecting portion, the contact portion comprising a metal region spaced apart from the guiding region of the waveguide and the connecting portion extending from the guiding edge of the at least one of the first and second silicon device layers and electrically connecting an active region of the at least one of the first and second silicon device layers to the contact portion.

2. The thin-film structure of claim 1, wherein the guiding region of the waveguide is at least partially defined by the active region of the first silicon device layer.

3. The thin-film structure of claim 2, wherein the guiding region of the waveguide is at least partially defined by an active region of the second silicon device layer.

4. The thin-film structure of claim 1, wherein the connecting portion of the electrical contact structure extends laterally from a direction of propagation of light through the guiding region of the waveguide.

5. The thin-film structure of claim 4, wherein the connecting portion extends laterally from a direction of propagation of light through the guiding region of the waveguide at an angle normal to the direction of propagation of light through the guiding region.

6. The thin-film structure of claim 4, wherein the connecting portion extends laterally from a direction of propagation of light through the guiding portion of the waveguide at an oblique angle with respect to the direction of propagation of light through the guiding region.

7. The thin-film structure of claim 1, comprising a first electrical contact structure for the first silicon device layer having a first connecting portion extending from a guiding edge of the first silicon layer and connected to a first contact portion comprising a metal region spaced apart from the guiding region of the waveguide and a second electrical contact structure for the second silicon device layer having a second connecting portion extending from a guiding edge of the second silicon layer and connected to a second contact portion comprising a metal region spaced apart from the guiding region of the waveguide.

8. A silicon based thin-film structure for an optical modulator, the thin-film structure comprising:
a substrate comprising a first silicon device layer;
a thin-film dielectric device layer on the first silicon device layer of the substrate, the thin-film dielectric device layer having a predetermined thickness to function within a waveguide of an optical modulator;
a second silicon device layer on the thin-film dielectric device layer;
a waveguide having a guiding region, the waveguide comprising at least a portion of the first silicon device layer, the thin-film dielectric device layer, and the second silicon device layer, the first silicon device layer comprising a guiding edge that at least partially defines the guiding region; and
an electrical contact structure for the first silicon device layer, the electrical contact structure having a contact portion and a connecting portion, the contact portion spaced apart from the guiding region of the waveguide and the connecting portion extending from the guiding edge of the first silicon device layer and electrically connecting an active region of the first silicon device layer to the contact portion.

9. The thin-film structure of claim 8, wherein the connecting region of the electrical contact structure extends laterally from a direction of propagation of light through the guiding portion of the waveguide.

10. The thin-film structure of claim 9, wherein the connecting portion extends laterally from a direction of propagation of light through the guiding region of the waveguide at an angle normal to the direction of propagation of light through the guiding region.

11. The thin-film structure of claim 9, wherein the connecting portion extends laterally from a direction of propagation of light through the guiding portion of the waveguide at an oblique angle with respect to the direction of propagation of light through the guiding region.

12. The thin-film structure of claim 8, further comprising a second electrical contact structure for the second silicon device layer, the second electrical contact structure having a contact portion spaced apart from the guiding region of the waveguide.

13. The thin-film structure of claim 12, wherein the second electrical contact structure for the second silicon device layer comprises a connecting portion that connects an electrically active region of the second silicon device layer to the contact portion of the second electrical contact structure.

14. The thin-film structure of claim 13, wherein the connecting portion of the second electrical contact structure extends laterally from a direction of propagation of light through the guiding region of the waveguide.

15. The thin-film structure of claim 14, wherein the connecting portion of the second electrical contact structure extends laterally from a direction of propagation of light through the guiding region of the waveguide at an angle normal to the direction of propagation of light through the guiding region.

16. The thin-film structure of claim 14, wherein the connecting portion of the second electrical contact structure extends laterally from a direction of propagation of light through the guiding portion of the waveguide at an oblique angle with respect to the direction of propagation of light through the guiding region.

17. The thin-film structure of claim 8, wherein the substrate comprises a silicon-on-insulator structure, the silicon-on-insulator structure having a first silicon layer and a buried oxide layer.

18. The thin-film structure of claim 17, wherein the first silicon device layer of the substrate comprises the silicon layer of the silicon-on-insulator structure.

19. The thin-film structure of claim 8, wherein the first silicon device layer comprises a single crystal layer.

20. The thin-film structure of claim 8, wherein the second silicon device layer is at least partially crystallized in at least a portion of the guiding region of the waveguide.

21. The thin-film structure of claim 8, wherein the thin-film dielectric device layer comprises silicon dioxide.

22. A method of making a silicon based thin-film structure for an optical modulator, the method comprising the steps of:
providing a substrate comprising a first silicon layer;
depositing a thin-film dielectric layer on the first silicon layer of the substrate, the thin-film dielectric layer having a predetermined thickness;
depositing a second silicon layer on the thin-film dielectric layer;
forming a waveguide having a guiding region, the waveguide comprising at least a portion of the first silicon layer, the thin-film dielectric layer, and the second silicon layer, the first silicon layer comprising a guiding edge that at least partially defines the guiding region; and
forming an electrical contact structure for the first silicon layer by providing a contact portion of the contact structure at a predetermined distance from the guiding region of the waveguide and electrically connecting the contact portion of the contact structure to an active region of the first silicon layer with a connecting portion of the contact structure by extending the connecting portion from the guiding edge of the first silicon layer.

23. The method of claim 22, wherein the step of forming an electrical contact structure for the first silicon layer comprises photolithographically patterning the first silicon layer to form the connecting portion that connects the active region of the first silicon layer and the contact portion of the electrical contact structure.

24. The method of claim 22, further comprising the step of forming an electrical contact structure for the second silicon layer, the electrical contact structure for the second silicon layer having a contact portion comprising a metal region spaced apart from the guiding region of the waveguide.

25. The method of claim 22, wherein the step of forming an electrical contact structure for the second silicon layer comprises photolithographically patterning the second silicon layer to form a connecting portion that electrically connects an active portion of the second silicon layer and the contact portion of the electrical contact structure for the second silicon layer.