Optical phase shifter using fine lithography defined volumetric junctions

Abstract

In accordance with the present invention, an elongated phase shifting diode is provided for modulating an electrical signal onto an optical wave. Structurally, the phase shifting diode includes a p doped central stripe that extends through a phase shifting length L of a waveguide. P.sup.+ doped finger stripes and N.sup.+ doped finger stripes, which are laterally and axially offset from each other, extend into the waveguide for contact with the p doped central stripe along the length L. In combination, the plurality of N.sup.+ doped finger stripes and the p doped central stripe create a plurality of PN junctions that are structurally aligned along the p doped central stripe to establish electrically parallel phase shifting functions for the elongated diode.

Claims

1. An optical phase shifter which comprises: an optical waveguide having a core surrounded by cladding and a length L, wherein the core has a top surface and a bottom surface with opposed first and second side surfaces therebetween, wherein the side surfaces of the core are at a distance R from each other; a p doped central stripe formed in the waveguide core between the top surface and the bottom surface and centered between the first and second sides of the waveguide core, wherein the p doped central stripe has a width w1 perpendicular to the length L, and wherein w1 is less than R; a plurality of P.sup.+ doped finger stripes, wherein each P.sup.+ doped finger stripe extends laterally into the waveguide in a first direction toward the p doped central stripe to contact the p doped central stripe; a plurality of N.sup.+ doped finger stripes, wherein each N.sup.+ doped finger stripe extends laterally into the waveguide in a second direction toward the p doped central stripe, to contact the p doped central stripe and create a PN junction therewith, wherein the first direction is opposite the second direction, and wherein the N.sup.+ doped finger stripes are laterally and axially offset from the P.sup.+ doped finger stripes for their respective contact with the p doped central stripe; a first bus connected to the plurality of P.sup.+ doped finger stripes; and a second bus connected to the plurality of N.sup.+ doped finger stripes wherein the first bus and the second bus electrically combine the PN junctions, in structural alignment with each other along the length L, to establish an elongated diode body with electrically parallel functions, wherein the elongated diode body includes the entire volume of the p doped central stripe for phase shifting an optical wave as the optical wave passes through the waveguide.

2. The optical phase shifter of claim 1 wherein the first bus and the second bus respectively act as first and second electrodes for the diode, and wherein a depletion region in the volume of the p doped central stripe is modulated by a reverse bias voltage applied between the first and second electrodes to modulate the number of free-carriers therebetween for changing the optical phase of the optical wave.

3. The optical phase shifter of claim 1 wherein the p doped central stripe has a cross section perpendicular to the length L of the waveguide core, and the cross section has a shape selected from the group consisting of a circle, semi-circle, ellipse, truncated ellipse, rectangle, top-hat, octagon, trapezoid, hexagon, triangle, and inverted-triangle.

4. The optical phase shifter of claim 1 wherein the p doped central stripe and the N+ and P+ doped finger stripes have a top view with a shape selected from the group consisting of a broad line, rectangle, trapezoid, zig-zag, circle, circular section, ellipse, elliptical section, ring, annulus, racetrack, oval, curved line, and curved approximation of any polygon.

5. The optical phase shifter of claim 1 wherein a P.sup.+ doped finger stripe has a width w2 and an N.sup.+ doped finger stripe has a width w3, and wherein, with a downstream direction determined by the propagation direction of the optical wave, the P.sup.+ doped finger stripe is separated from an adjacent upstream N.sup.+ doped finger stripe by a distance d32 and is separated from an adjacent downstream N.sup.+ doped finger stripe by a distance d23, to establish a distance equal to d32+w2+d23 between adjacent PN junctions.

6. The optical phase shifter of claim 5 wherein w2 is common for each P.sup.+ doped finger stripe, w3 is common for each N.sup.+ doped finger stripe, the separation distances d32 are common and the separation distances d23 are common along the length L to periodically repeat a pitch equal to w3+d32+w2+d23 between PN junctions along the length L of the waveguide.

7. The optical phase shifter of claim 5 wherein at least one dimension selected from the group consisting of w2, w3, d32 and d23 is variable to irregularly repeat the pitch between PN junctions along the length L of the waveguide.

8. The phase shifter of claim 1 wherein each P.sup.+ doped finger stripe extends inwardly toward the p doped central stripe to a respective location short of the p doped central stripe for overlap and contact with a p doped lateral protrusion extending from the p doped central stripe toward the first side of the waveguide core.

9. The phase shifter of claim 1 wherein each N.sup.+ doped finger stripe extends inwardly toward the p doped central stripe to a respective location short of the p doped central stripe for overlap with a p doped lateral protrusion extending from the p doped central stripe toward the second side of the waveguide core, and wherein the overlap between the N.sup.+ doped finger stripe and a respective p doped lateral protrusion counter-dopes the p doped lateral protrusion and creates a PN junction in a volume external to the p doped central stripe.

10. The optical phase shifter of claim 1 wherein each N.sup.+ doped finger stripe overlaps the p doped central stripe to create a counter-doped N region within the p doped central stripe.

11. The optical phase shifter of claim 1 wherein each N.sup.+ doped finger stripe extends beyond the p doped central stripe to create a counter-doped N region having opposed PN junctions within the p doped central stripe.

12. An elongated diode for shifting the phase of an optical signal, wherein the elongated diode includes a plurality of diode units and each diode unit comprises: a portion of an optical waveguide, wherein the waveguide has an axis, has a phase shifting length L, and includes a p doped central stripe extending along the length L of the waveguide; a PN junction, wherein the PN junction is created by contacting the p doped central stripe of the diode unit with an N.sup.+ doped finger stripe; a P.sup.+ doped finger stripe contacting the p doped central stripe at a contact point, wherein the diode units are consecutively aligned together with the PN junction of each N.sup.+ doped finger stripe being straddled between adjacent P.sup.+ doped finger stripes and their respective contact points with the p doped central stripe along the length L, and wherein the contact point for each P.sup.+ doped finger stripe is at a respective distance from each adjacent PN junction with a portion of the p doped central stripe positioned between each contact point and the PN junction; a first bus connected to the plurality of P.sup.+ doped finger stripes; and a second bus connected to the plurality of N.sup.+ doped finger stripes wherein the first bus and the second bus electrically combine the PN junctions, in structural alignment with each other along the length L, to establish an elongated diode body with electrically parallel functions, wherein the elongated diode body includes the entire volume of the p doped central stripe for phase shifting an optical wave as the optical wave passes through the waveguide.

13. The elongated diode of claim 12 wherein the p doped central stripe has a width w1 along the length L, wherein the P.sup.+ doped finger stripes and the N.sup.+ doped finger stripes are uniform in doping profile through the depth h of the upper layer of semiconductor and respectively extend laterally into the waveguide toward the p doped central stripe from opposite directions to contact the p doped central stripe, and wherein each P.sup.+ doped finger stripe has a width w2 and each N.sup.+ doped finger stripe has a width w3, and wherein, with a downstream direction determined by the propagation direction of the optical wave, the P.sup.+ doped finger stripe is separated from an adjacent upstream N.sup.+ doped finger stripe by a distance d32 and is separated from an adjacent downstream N.sup.+ doped finger stripe by a distance d23, to establish a distance equal to d32+w2+d23 between adjacent PN junctions.

14. The elongated diode of claim 13 wherein w2 is common for each P.sup.+ doped finger stripe, w3 is common for each N.sup.+ doped finger stripe, the separation distances d32 are common and the separation distances d23 are common to periodically repeat a pitch equal to w3+d32+w2+d23 between PN junctions along the length L of the waveguide.

15. The elongated diode of claim 13 wherein at least one dimension selected from the group consisting of w2, w3, d32 and d23 is variable to irregularly repeat the pitch between PN junctions along the length L of the waveguide.

16. The elongated diode of claim 13 wherein each P.sup.+ doped finger stripe extends inwardly toward the p doped central stripe to a respective location short of the p doped central stripe for overlap and contact with a p doped lateral protrusion extending from the p doped central stripe toward the first side of the waveguide core.

17. The elongated diode of claim 13 wherein each N.sup.+ doped finger stripe overlaps the p doped central stripe to create a counter-doped N region within the p doped central stripe.

18. The elongated diode of claim 13 wherein each N.sup.+ doped finger stripe extends inwardly toward the p doped central stripe to a respective location short of the p doped central stripe for overlap with a p doped lateral protrusion extending from the p doped central stripe toward the second side of the waveguide core, and wherein the overlap between the N.sup.+ doped finger stripe and a respective p doped lateral protrusion counter-dopes the p doped lateral protrusion and creates a PN junction in a volume external to the p doped central stripe.

19. A method for manufacturing an optical phase shifter having a plurality of structurally aligned contiguous diodes with electrically parallel functions, wherein the method comprises the steps of: providing a workpiece, wherein the workpiece includes an upper layer of semiconductor material positioned over a lower layer of a buried oxide, wherein the upper layer of semiconductor material has an exposed upper surface and a lower surface adjacent the layer of buried oxide with opposed first and second sides extending therebetween; p doping a central stripe with a defined longitudinal axis together with a first plurality of lateral protrusions and a second plurality of lateral protrusions, wherein the p doped central stripe and its longitudinal axis are aligned, wherein the lateral protrusions in the first plurality are distanced from each other and extend away from the p doped central stripe toward the first side, wherein each p doped lateral protrusion in the second plurality is axially positioned between a pair of p doped lateral protrusions in the first plurality and extends away from the p doped central stripe toward the second side; P.sup.+ doping a plurality of P.sup.+ doped finger stripes into the workpiece from the first side toward the p doped central stripe with at least a portion of each P.sup.+ doped finger stripe contacting a respective p doped lateral protrusion in the first plurality; N.sup.+ doping a plurality of N.sup.+ doped finger stripes into the workpiece to individually extend inwardly from the second side into contact with at least a portion of each N.sup.+ doped finger stripe contacting a respective p doped lateral protrusion in the second plurality, to counter-dope the p doped lateral protrusion and create a PN junction therewith; and etching into the upper layer of semiconductor material to create a waveguide core, wherein the waveguide core includes the p doped central stripe, has a width R and a height h, and wherein the etching step removes a portion of semiconductor material from the workpiece external to the waveguide core.

20. The method of claim 19 wherein the p doping step is accomplished using a first implant mask to perform fine lithography on the workpiece to create the p doped central stripe as a narrow continuous stripe along the length L with a width w1 less than R, and wherein the P.sup.+ doping step is accomplished using a second implant mask to perform fine lithography on the workpiece, wherein the lateral P+ doped finger stripes have a width w2, and wherein the N.sup.+ doping step is accomplished using a third implant mask to perform fine lithography on the workpiece to create the PN junctions and to provide the N.sup.+ lateral finger stripes with a width w3; and wherein the etching step is accomplished using an etching mask.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:

(2) FIG. 1 is a perspective view of a plurality of phase shifting diode units axially aligned for collective use as an elongated diode, to shift the phase of an optical signal;

(3) FIG. 2 is a top view of an implantation scheme for the present invention, showing doped regions for a phase shifting diode, and exemplary dimensions for the doped regions, in accordance with the present invention;

(4) FIG. 3 is a preferred embodiment for the implantation scheme shown in FIG. 2; and

(5) FIG. 4 shows sequential top views of implantation steps, with corresponding cross-sectional views (A-A, B-B), needed to create doped regions for phase shifting diode units used by the present invention (note: implantation steps are not necessarily presented in a required order).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) An optical phase shifter in accordance with the present invention is shown in FIG. 1 and is generally designated 10. As shown, the phase shifter 10 is elongated and it has a longitudinal axis 12. The phase shifter 10 includes an upper layer 14 that is made of a semiconductor material, such as silicon, and it includes a lower layer 16 that is made of a dielectric, such as buried oxide. FIG. 1 also shows that the upper layer 14 is etched to form a waveguide core 18 having a width R. FIG. 1 also shows that the phase shifter 10 will extend along the axis 12 through a phase shifting length L. Functionally, the waveguide core 18 establishes an optical waveguide that extends along the length L. As will be appreciated by further disclosure below, the exact shape of the waveguide core 18 may be varied. The rectangular presentation shown in FIG. 1 is only exemplary.

(7) With reference to FIG. 2, it is to be appreciated that this figure is primarily intended to identify dimensions for specific components of the present invention. Specifically, this is done to generally show their positional and structural relationship with other components. Further, it is to be appreciated that the various dimensions shown in FIG. 2, and the respective configurations for different components, can be varied somewhat, as desired. Their functionality, however, will remain the same.

(8) In FIG. 2 it will be seen that a diode unit 20 is identified. In accordance with further disclosure provided below, the diode unit 20 serves as a basic building block for the optical phase shifter 10. Thus, it will be appreciated that a plurality of contiguous diode units 20 can be aligned along the length L. Preferably, the diode units 20 will all define a same pitch for the optical phase shifter 10.

(9) Identifiable components for the optical phase shifter 10 include a p doped central stripe 22 that extends along the entire length L of the optical phase shifter 10. Also, a plurality of P.sup.+ doped finger stripes 24, of which the P.sup.+ doped finger stripes 24a and 24b are exemplary, extend between a first bus 26 and the p doped central stripe 22. Similarly, a plurality of N.sup.+ doped finger stripes 28, of which the N.sup.+ doped finger stripes 28a and 28b are exemplary, extend between a second bus 30 and the p doped central stripe 22.

(10) For the arrangement of stripes 22, 24 and 28 shown in FIG. 2, the p doped central stripe 22 will have a width w1, the P.sup.+ doped finger stripes 24 will have a width w2, and the N.sup.+ doped finger stripes 28 will have a width w3. Further, using the arrow 32 to indicate the direction of propagation for an optical signal passing through the optical phase shifter 10, FIG. 2 also shows that an upstream N.sup.+ doped finger stripe 28a will be separated from its adjacent downstream P.sup.+ doped finger stripe 24a by a distance d32. Similarly, the P.sup.+ doped finger stripe 24a will be separated from its next adjacent downstream N.sup.+ doped finger stripe 28b by a distance d23. In this arrangement, w3 may or may not equal w2, and the distance d32 may or may not equal the distance d23. In any event, a pitch within each diode unit 20 will be equal to w3+d32+w2+d23. By varying the value of at least one variable from one diode unit 20 to another value in an adjacent diode unit 20, it can also be appreciated that the pitch can be randomized.

(11) For a preferred embodiment of the optical phase shifter 10, cross reference FIG. 2 and FIG. 3. There it will be seen that the first bus 26 includes a plurality of electrical anode contacts 34. Similarly, the second bus 30 includes a plurality of electrical cathode contacts 36. Also shown in FIG. 3 are a series of first-side lateral protrusions 38, which are attached to the p doped central stripe 22. As shown, the first-side lateral protrusions 38 extend laterally from the p doped central stripe 22 toward the first bus 26. Likewise, a series of second-side lateral protrusions 40, which are also attached to the p doped central stripe 22, extend laterally from the p doped central stripe 22 toward the second bus 30. FIG. 3 also shows that the P.sup.+ doped finger stripes 24 include a first-side fan-out connection 42 that individually interconnects each P.sup.+ doped finger stripe 24 with the first bus 26, and that the N.sup.+ doped finger stripes 28 include a second-side fan-out connection 44 that individually interconnects each N.sup.+ doped finger stripe 28 with the second bus 30.

(12) The doping scheme for the present invention is best appreciated with reference to FIG. 3. There, it is to be appreciated that the central stripe 22 is p doped, as well as both lateral protrusions 38 and 40. It is also to be appreciated that the P+ finger stripes 24, the first bus 26 and the first-side fan-out connections 42 are all P.sup.+ doped. Additionally, the N.sup.+ finger stripes 28, the second bus 30 and the second-side fan-out connections 44 are all N.sup.+ doped. An important consequence here is that the N.sup.+ doped finger stripes 28 collectively establish a plurality of PN junctions axially aligned along the p doped central stripe 22.

(13) Within the doping scheme disclosed above, the first bus 26 establishes an anode for the optical phase shifter 10 and the second bus 30 establishes a cathode for the optical phase shifter 10. Thus, a voltage source (not shown) can be electrically connected via anode contacts 34, cathode contacts 36, and buses 26 and 30 with the optical phase shifter 10 to provide an electrical signal that will modulate the number of free carriers in the volume of the p doped central stripe 22 between the P.sup.+ doped finger stripes 24 and the N.sup.+ doped finger stripes 28. In accordance with the present invention, this modulation is coordinated through the entire elongated diode body due to the electrically parallel functions of all diode units. In one electrical signal state, the depletion region 46 minimally impinges into the p doped central stripe in the vicinity of the plurality of PN junctions. In a second electrical signal state, the depletion region 46 grows to almost completely occupy the volume of the p doped central stripe 22.

(14) As envisioned for the present invention, an optical phase shifter 10 in accordance with the present invention can be manufactured using well known fine lithography techniques. Accordingly, the present invention envisions that dimensions of components used for the present invention can be accurate within the range of a few to tens of nanometers. Moreover, the present invention envisions that with this accuracy, various configurations can be incorporated into the present invention to augment its electro-optical advantages. By way of example, the p doped central stripe 22 may be created with simple Gaussian or Pearson doping profiles or complex combinations of doping profiles that can approximate a cross section shape, perpendicular to the length L of the waveguide core, that may be either a circle, semi-circle, ellipse, truncated ellipse, rectangle, top-hat, octagon, trapezoid, hexagon, triangle, or inverted-triangle. Also, the p doped central stripe 22 and P+ doped finger stripes 24 and N+ doped finger stripes 28 can be formed with a top view having the shape of a broad line, rectangle, trapezoid, zig-zag, circle, circular section, ellipse, elliptical section, ring, annulus, racetrack, oval curved line, or curved approximation of any polygon.

(15) A method for manufacturing an optical phase shifter 10 in accordance with the present invention requires providing a standard workpiece 48. The workpiece 48 will include an upper layer of semiconductor material 14 that is positioned over a lower layer of a dielectric 16. The upper layer of semiconductor material 14 will have an exposed upper surface and its lower surface will be integrated with the lower layer dielectric 16. The workpiece 48 will have an origin and printed fiducials on the exposed upper surface that define a longitudinal axis 12.

(16) The doping scheme mentioned above for the optical phase shifter 10 will be best appreciated with reference to FIG. 4, and with cross reference to FIG. 2 and FIG. 3. Using a first implant mask (not shown), fine lithography can be employed on the workpiece 48 to create the p doped central stripe 22 having a length L and width w1. Also created with the first implant mask will be the plurality of p doped first-side lateral protrusions 38. Additionally, a plurality of p doped second-side lateral protrusions 40 are created into the workpiece 48 on the side of the p doped central stripe 22 that is opposite the first-side lateral protrusions 38. Importantly, the p doped central stripe 22 is aligned and extends axially along the longitudinal axis 12. The p doped first-side lateral protrusions 38 are separated from each other by the diode unit pitch w3+d32+w2+d23, and the second-side lateral protrusions 40 on the opposite side of the p doped central stripe 22 are axially offset from the p doped first-side lateral protrusions 38 by a distance w2+d23.

(17) A second implant mask (not shown) is used for P.sup.+ doping a plurality of lateral P.sup.+ doped finger stripes 24 into the workpiece 48 and to overlay the p doped first-side lateral protrusions 38. The second implant mask is aligned to the longitudinal axis 12 and workpiece origin so that the line-ends of the P+ doped finger stripes 24 are positional with the desired extension distance x2 relative to the p doped central stripe 22. Also, the first-side fan-out connections 42, along with the first bus 26 are P.sup.+ doped in this step.

(18) N.sup.+ doping a plurality of lateral N.sup.+ doped finger stripes 28 into the workpiece 48 is accomplished using a third mask (not shown). The third implant mask is aligned to the longitudinal axis and workpiece origin so that the N+ doped finger stripes 28 are positioned with the desired extension distance x3 relative to the p doped central stripe 22 and overlay the p doped second-side lateral protrusions 40. Also, the second-side fan-out connections 44 and second bus 30 are N+ doped in this step.

(19) As a final step, an etching mask (not shown) is used to etch into the upper layer of semiconductor material 14 in the workpiece 48 to create the waveguide core 18. The etching mask is aligned to the longitudinal axis 12 and workpiece origin. In detail, the etched waveguide core 18 will be centered on the p doped central stripe 22, it will have a width R that is greater than w1, and the after-etch height of the upper layer semiconductor outside the waveguide core 18 will be less than a full depth h of the upper layer of semiconductor material inside the waveguide core 18. This etching will extend beyond the length L.

(20) While the particular Optical Phase Shifter Using Fine Lithography Defined Volumetric Junctions as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.