LIFT-OFF PROCESSING FOR FORMATION OF ISOLATION REGIONS IN LASER DIODE STRUCTURES
A method of fabricating a laser diode structure is provided where a photolithographic process is utilized to form at least a portion of an axially extending waveguide structure such that a patterned photoresist remnant resides over the axially extending waveguide structure following the photolithographic process. A patterned isolated opening and a lift-off photoresist portion are formed in the patterned photoresist remnant by subjecting the patterned photoresist remnant to an additional photolithographic process such that the lift-off photoresist portion remains in residence over the axially extending waveguide structure following the additional photolithographic process. An insulating layer is formed over the patterned isolated opening and the lift-off photoresist portion.
1 . A method of fabricating a laser diode structure comprising a semiconductor substrate, an axially extending waveguide structure, and an insulating layer positioned over the semiconductor substrate, wherein the method comprises:
utilizing a photolithographic process to form at least a portion of the axially extending waveguide structure such that a patterned photoresist remnant resides over the axially extending waveguide structure following the photolithographic process;
forming a patterned isolated opening and a lift-off photoresist portion in the patterned photoresist remnant by subjecting the patterned photoresist remnant to an additional photolithographic process such that the lift-off photoresist portion remains in residence over the axially extending waveguide structure following the additional photolithographic process;
forming the insulating layer over the patterned isolated opening and the lift-off photoresist portion; and
subjecting the insulating layer and underlying lift-off photoresist portion to a lift-off process to leave a patterned isolation region of the insulating layer in residence over the axially extending waveguide structure.
2 . A method as claimed in claim 1 wherein the insulating layer comprises silicon nitride and is formed over the patterned isolated opening and the lift-off photoresist portion at a temperature that does not exceed a hard bake temperature of the lift-off photoresist portion.
3 . A method as claimed in claim 1 wherein the insulating layer comprises silicon nitride and is formed over the patterned isolated opening and the lift-off photoresist portion at a temperature that does not exceed 200° C.
4 . A method as claimed in claim 1 wherein the insulating layer comprises silicon nitride presented in the form of Si 3 N 4 .
5 . A method as claimed in claim 1 wherein the insulating layer comprises silicon oxide and is formed over the patterned isolated opening and the lift-off photoresist portion at a temperature that does not exceed a hard bake temperature of the lift-off photoresist portion.
6 . A method as claimed in claim 1 wherein the insulating layer comprises silicon oxide and is formed over the patterned isolated opening and the lift-off photoresist portion at a temperature that does not exceed 200° C.
7 . A method as claimed in claim 1 wherein the insulating layer comprises silicon oxide presented in the form of SiO 2 .
8 . A method as claimed in claim 1 wherein a waveguide ridge including the patterned photoresist remnant and a least a portion of the axially extending waveguide structure is formed in the semiconductor substrate.
9 . A method as claimed in claim 8 wherein the waveguide ridge is formed by etching the semiconductor substrate.
10 . A method as claimed in claim 1 wherein:
the laser diode structure further comprises a control element extending over a limited axial portion of the waveguide structure; and
at least a portion of the control element is formed over the patterned isolation region in residence over the axially extending waveguide structure.
11 . A method as claimed in claim 10 wherein the control element comprises a heating element extending over the limited axial portion of the waveguide structure and heater pads conductively coupled to the heating element.
12 . A method as claimed in claim 1 wherein the laser diode structure comprises a plurality of functional regions and the patterned isolation region is formed over one of the functional regions of the laser diode structure to isolate electrically a gain section of the laser diode structure.
13 . A method as claimed in claim 1 wherein the laser diode structure comprises a laser facet and the isolation region is formed near the laser facet as an unpumped window section of the laser diode.
14 . A method as claimed in claim 1 wherein the laser diode structure comprises a ridge waveguide.
15 . A method as claimed in claim 1 wherein the laser diode structure comprises a double heterostructure laser, a quantum well laser, a quantum cascade laser, a DBR semiconductor laser, a DFB semiconductor laser, or an external cavity laser.
16 . A method as claimed in claim 1 wherein the laser diode structure comprises a DBR semiconductor laser and the patterned isolation region is formed over a wavelength selective DBR portion of the laser.
17 . A laser diode structure comprising a semiconductor substrate, an axially extending waveguide structure, a control element extending over a limited axial portion of the waveguide structure; and a patterned isolation region of an insulating layer, which region lies in residence over the axially extending waveguide structure, wherein:
at least a portion of the control element is formed over the patterned isolation region in residence over the axially extending waveguide structure;
the patterned isolation region, the control element, and a waveguide portion of the insulating layer reside substantially contiguously over the waveguide structure along a limited axial dimension of the waveguide structure.
18 . A laser diode structure as claimed in claim 17 wherein:
the limited axial dimension of the waveguide structure corresponds to a wavelength selective portion of the laser diode structure; and
the control element is configured to control a wavelength selective characteristic of the wavelength selective portion of the laser diode structure.
19 . A laser diode structure as claimed in claim 18 wherein the control element comprises a heating element extending over the limited axial portion of the waveguide structure.
20 . A laser diode structure as claimed in claim 17 wherein:
the limited axial dimension of the waveguide structure corresponds to a wavelength selective portion of the laser diode structure; and
the laser diode structure further comprises one or more unpumped window sections that are isolated electrically from the control element by the patterned isolation region.