A preparation method of a composite coupling DFB laser

Abstract

The application relates to the field of semiconductor laser technology, and particularly provides a preparation method of a composite coupling DFB (distributed feedback) laser, which comprises the following steps: covering a mask layer on the surface of a semiconductor epitaxial wafer, spin-coating photoresist, using i-line to photoetch the patterns of a ridge waveguide and a groove grating on the photoresist; etching the patterns of the ridge waveguide and the groove grating onto the mask layer, etching the structure of the ridge waveguide and the groove grating on the upper limiting layer, and removing the photoresist and the mask layer; covering an insulating layer on the surface of the semiconductor epitaxial wafer, covering the insulating layer with photoresist, using i-line to photoetch the patterns of a metal electrode contact window for the second time, etching the metal electrode contact window on the insulating layer, and removing the photoresist; depositing metal on the metal electrode contact window to form an upper electrode, and polishing the substrate to deposit metal and form a lower electrode. The application adopts twice photoetching and once etching to simultaneously form the ridge waveguide and the groove grating, the groove grating realizes refractive index and gain composite coupling, the side mode suppression ratio is high, the single-mode stability is good, and the application is suitable for mass production.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor laser technology, and particularly relates to a method for fabricating a composite-coupled DFB laser. Background Technology

[0002] With the rapid development of information and communication technologies, coherent optical communication, fiber optic sensing, precision spectral measurement, and lidar, systems are placing higher demands on the output performance of semiconductor lasers, especially requiring stable single-mode output, high side-mode suppression ratio, good temperature stability, and consistent mass production capabilities. Distributed feedback (DFB) semiconductor lasers, by introducing a periodic grating structure inside the device, generate selective feedback at the Bragg wavelength, thereby achieving single-mode oscillation. They offer advantages such as small size, easy integration, and scalable manufacturing, and have become one of the core light sources in optical communication and integrated optical systems.

[0003] Traditional DFB lasers mostly employ refractive index coupling, which couples the forward and backward waves and generates feedback by forming periodic refractive index perturbations (gratings) in the waveguide. While this type of refractive index-coupled DFB laser has a mature structure and relatively straightforward implementation, it is prone to problems such as spatial hole burning and uneven carrier distribution under certain operating conditions (e.g., high output power, high temperature, or high current density). This leads to side-mode enhancement, decreased spectral stability, and even mode switching. To address these issues, gain-coupled DFB lasers have been proposed. By forming periodic gain or loss modulation within the cavity, the coupling mechanism is provided not only by the refractive index perturbation but also by the periodic changes in gain or loss, which contribute to feedback and mode selection. This is expected to reduce spatial hole burning and improve single-mode stability and side-mode suppression. Furthermore, when a device possesses both gain coupling and refractive index coupling mechanisms, a composite-coupled DFB can be formed. This allows for a better balance between feedback strength and mode selection within a certain parameter range, improving the stability and designability of the device output.

[0004] To achieve gain-coupled or composite-coupled DFB lasers, existing technologies typically revolve around "establishing periodic carrier injection differences, gain differences, or periodic loss differences within the cavity." Typical implementation schemes mainly include the following categories: (1) Scheme using metal gratings: Existing technologies also include schemes that fabricate periodic metal strip gratings (metal gratings) above or to the side of the waveguide to achieve DFB laser feedback or equivalent gain or loss coupling. This scheme is relatively simpler than secondary epitaxy or regeneration route processes, but metal absorption introduces additional internal losses, often leading to increased threshold, decreased efficiency, and more significant thermal effects. At the same time, the coupling between the metal grating and the waveguide is sensitive to the thickness, size, and alignment deviation of the dielectric layer, and the process window is narrow. In addition, its fabrication usually still requires multiple steps such as waveguide or ridge structure, metal grating pattern, and electrodes, often requiring three or more photolithography steps, which is not conducive to further reducing process complexity, cost, and time. (2) Scheme using buried or deeply etched gratings to provide refractive index coupling and superimposed periodic injection or periodic loss to form composite coupling. This type of scheme usually first forms a grating structure with strong refractive index coupling (such as an embedded grating or a deeply etched grating), and then through electrode segmentation, periodic electrode windowing, periodic insulating layer opening, or the introduction of a current blocking layer, the current injection is periodically distributed along the cavity length direction, thereby achieving gain coupling and superimposing it with refractive index coupling to form a composite coupled DFB. This type of scheme can take into account both the feedback strength of refractive index coupling and the mode suppression capability of gain coupling, but usually requires more process modules and multiple patterning steps, and has high alignment and process control requirements. (3) Gain coupling scheme that achieves periodic injection only through electrode or insulating layer patterning. This type of scheme usually, on the basis of a conventional ridge waveguide DFB or waveguide laser, makes periodic metal electrode strips, periodic windowed electrodes, or periodic insulating layer patterns to form a periodic distribution of "strong injection region or weak injection region" along the cavity length direction, thereby generating a gain coupling effect. Compared to solutions requiring secondary epitaxy, regrowth, or the introduction of embedded structures, this type of solution is relatively simpler in overall process, lower in cost, and closer to mass production process routes. However, to achieve key steps such as "ridge waveguide forming + grating or groove structure (or equivalent periodic structure) construction + periodic windowing and metallization of electrodes or insulating layers," existing solutions typically still require at least three photolithography steps and corresponding multiple alignment and etching or deposition processes. The number of process steps remains excessive, leading to increased cumulative errors and hindering further reduction of manufacturing cycle time and improvement of yield consistency. In addition, since periodic injection modulation mainly relies on the upper electrode or insulating structure, the modulation depth and controllability are easily affected by material, resistance, and thermal effects.

[0005] In summary, while existing gain-coupled or composite-coupled DFB laser implementation schemes can improve single-longitudinal-mode stability and side-mode suppression ratio to some extent, they still generally suffer from the following drawbacks: First, some schemes rely on secondary epitaxy, regrowth, or additional complex structures, resulting in long process flows, high costs, and long cycles; second, grating structures and injection modulation structures often require multiple photolithography steps and high-precision alignment, which are cumbersome and difficult to control in terms of yield and consistency; third, even relatively simplified electrode or insulating layer periodic injection schemes usually require three or more photolithography steps to complete device fabrication, which still cannot meet the needs of further reducing process complexity, shortening process time, and reducing costs. Summary of the Invention

[0006] In view of this, the present invention aims to provide a method for fabricating a composite-coupled DFB laser. This invention simultaneously forms a ridge waveguide and a slot grating through two photolithography steps and one etching step. The first photolithography step forms the patterns of the ridge waveguide and slot grating, and the pattern is etched to form the structure of the ridge waveguide and slot grating. The second photolithography step defines the pattern of the metal electrode contact window, and the corresponding insulating layer is etched using the pattern, while retaining the insulating layer in the non-window area. The slot grating achieves a combination of refractive index coupling and gain coupling, resulting in high side-mode suppression ratio, good single-mode stability, and suitability for mass production.

[0007] To achieve the above objectives, the technical solution created by this invention is implemented as follows: This invention provides a method for fabricating a composite-coupled DFB laser, comprising: a semiconductor epitaxial wafer structure of the laser comprising, from bottom to top: a substrate, a lower confinement layer, a lower waveguide layer, an active region, an upper waveguide layer, and an upper confinement layer, including: S1: The surface of the semiconductor epitaxial wafer is covered with a mask layer. The mask layer is covered with photoresist, and the ridge waveguide pattern and the slot grating pattern are first photolithographically etched on the photoresist using i-lines. S2: Etch the ridge waveguide pattern and the slot grating pattern on the photoresist onto the mask layer. Use the pattern on the mask layer to etch the structure of the ridge waveguide and the slot grating on the upper confinement layer. After etching, remove the photoresist and mask layer from the surface of the semiconductor epitaxial wafer. S3: Use insulating material to cover the surface of the semiconductor epitaxial wafer to form an insulating layer. Use photoresist to cover the insulating layer. Use i-line to photo-etch the pattern of the metal electrode contact window on the photoresist of the ridge waveguide for the second time. Etch the pattern of the metal electrode contact window on the photoresist onto the insulating layer. After etching, remove the photoresist. S4: Cover the surface of the metal electrode contact window with metal material to form the upper electrode, polish the substrate of the semiconductor epitaxial wafer, and cover the polished substrate surface with metal material to form the lower electrode.

[0008] Preferably, the mask layer and insulating layer covering the surface of the semiconductor epitaxial wafer are made of silicon-based materials.

[0009] Preferably, the pattern of the ridge waveguide in the first photolithography is also used to determine the lateral mode and lateral spot size of the ridge waveguide.

[0010] Preferably, the slot grating can also be disposed on the top, sidewall, and adjacent area of ​​the ridge waveguide; the slot shape of the slot grating can be a rectangular slot, trapezoidal slot, rounded corner slot, or multi-level slot; the slot grating can also adopt locally continuous slots and periodic notches.

[0011] Preferably, the mask layer is removed by wet etching or dry etching.

[0012] Preferably, the metal electrode is formed by electron beam evaporation or magnetron sputtering.

[0013] Preferably, the pattern of the first photolithography also includes alignment marks or process inspection structures for alignment in the second photolithography.

[0014] Preferably, the ridge waveguide etching depth extends to the upper confinement layer or the upper waveguide layer.

[0015] Preferably, the etching depth of the slot grating is consistent with the height of the ridge waveguide.

[0016] Preferably, the fabrication is carried out using a method for fabricating a composite-coupled DFB laser.

[0017] Compared with the prior art, the present invention can achieve the following beneficial effects: Existing gain-coupled or composite-coupled DFB laser implementation schemes generally suffer from the following drawbacks: First, the process flow is complex and involves many steps, with some schemes relying on secondary epitaxy, regrowth, or additional functional layers, requiring high process window precision; second, the number of photolithography steps is high and the alignment relationships are complex, often requiring step-by-step completion of gratings, waveguides, and injection modulation or electrode structures, typically requiring three or more photolithography steps accompanied by multiple high-precision alignments, resulting in large cumulative errors; third, this leads to high manufacturing costs and long processing times, hindering rapid iteration and large-scale mass production; fourth, the multiple processes and the superposition of alignment errors increase the dispersion of key device parameters, making it difficult to control mass production consistency and yield; in addition, schemes such as metal gratings may introduce additional internal losses and thermal effects, causing increased threshold voltage, decreased efficiency, and poorer temperature stability. To address these drawbacks, this invention provides a technical solution for fabricating composite-coupled DFB lasers without secondary epitaxy or regrowth, requiring only two photolithography steps. This invention employs a two-step photolithography fabrication route. The first photolithography and first etching complete the structure of the ridge waveguide and slot grating, while the second photolithography defines the window for the metal electrode. This eliminates the need for secondary epitaxy, reducing the number of critical steps and alignment steps, significantly lowering processing costs and shortening the process cycle. The invention retains an insulating layer as an electrical isolation layer, which helps suppress leakage current and process fluctuations. By simultaneously introducing refractive index coupling caused by etching within the cavity through the slot grating, and gain coupling generated by the interaction of the electrode and injection path, this invention achieves a composite coupling mechanism without additional photolithography. This enhances single-mode selectivity and side-mode suppression, and improves output stability. The composite coupling strength of this invention can be designed and adjusted using geometric parameters such as the slot depth, slot width, duty cycle, and slot position of the slot grating. Simultaneously, the electrode windowing and coverage relationship can be adjusted, thereby achieving comprehensive optimization of gain coupling and refractive index coupling without increasing process complexity. This facilitates device parameter customization for different application requirements. Attached Figure Description

[0018] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a structural diagram of a semiconductor epitaxial wafer provided according to an embodiment of the present invention; Figure 2 This is a structural diagram of a composite-coupled DFB laser after the first photolithography and etching according to an embodiment of the present invention; Figure 3 This is a structural diagram of the insulating layer covering a composite-coupled DFB laser according to an embodiment of the present invention. Figure 4 This is a structural diagram of the metal electrode contact window of a composite-coupled DFB laser provided according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the carrier distribution, gain, or electric field intensity modulation changes caused by periodic electrical injection formed under the guidance of a slot grating, according to an embodiment of the present invention.

[0019] The reference numerals in the figures include: Substrate 1, lower confinement layer 2, lower waveguide layer 3, active region 4, upper waveguide layer 5, upper confinement layer 6, ridge waveguide 7, slot grating 8, metal electrode contact window 9, insulating layer 10. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and do not constitute a limitation thereof. Similar elements in different embodiments are referred to by associated similar element reference numerals. In the following embodiments, many details are described to facilitate a better understanding of the invention. However, those skilled in the art will readily recognize that some features may be omitted in different situations, or may be replaced by other elements, materials, or methods. In some cases, some operations related to the invention are not shown or described in the specification. This is to avoid obscuring the core parts of the invention with excessive description. For those skilled in the art, detailed description of these related operations is not necessary; the relevant operations can be fully understood based on the description in the specification and general technical knowledge in the art.

[0021] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined to form various implementations. Furthermore, the order of the steps or actions in the method description can be changed or adjusted in a manner readily apparent to those skilled in the art. Therefore, the various orders in the specification and drawings are merely for the clear description of a particular embodiment and do not imply a mandatory order, unless otherwise stated that a particular order must be followed.

[0022] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0023] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0024] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0025] Please see Figure 1 In one embodiment of the present invention, a method for fabricating a composite-coupled DFB laser is provided, comprising: a semiconductor epitaxial wafer structure of the laser comprising, from bottom to top: a substrate, a lower confinement layer, a lower waveguide layer, an active region, an upper waveguide layer, and an upper confinement layer, including: S1: The surface of the semiconductor epitaxial wafer is covered with a mask layer. The mask layer is covered with photoresist, and the ridge waveguide pattern and the slot grating pattern are first photolithographically etched on the photoresist using i-lines. S2: Etch the ridge waveguide pattern and the slot grating pattern on the photoresist onto the mask layer. Use the pattern on the mask layer to etch the structure of the ridge waveguide and the slot grating on the upper confinement layer. After etching, remove the photoresist and mask layer from the surface of the semiconductor epitaxial wafer. S3: Use insulating material to cover the surface of the semiconductor epitaxial wafer to form an insulating layer. Use photoresist to cover the insulating layer. Use i-line to photo-etch the pattern of the metal electrode contact window on the photoresist of the ridge waveguide for the second time. Etch the pattern of the metal electrode contact window on the photoresist onto the insulating layer. After etching, remove the photoresist. S4: Cover the surface of the metal electrode contact window with metal material to form the upper electrode, polish the substrate of the semiconductor epitaxial wafer, and cover the polished substrate surface with metal material to form the lower electrode.

[0026] In this embodiment of the invention, the semiconductor epitaxial wafer used to fabricate a DFB laser comprises, from bottom to top: a substrate 1, a lower confinement layer 2, a lower waveguide layer 3, an active region 4, an upper waveguide layer 5, and an upper confinement layer 6. The active region 4 can employ a multiple quantum well structure. The semiconductor epitaxial wafer can be grown using metal-organic chemical vapor deposition or molecular beam epitaxy. Before fabricating the DFB laser using the semiconductor epitaxial wafer, the surface of the wafer needs to be cleaned and decontaminated, and then dried to ensure consistency in subsequent thin film deposition, photolithography, and etching.

[0027] In step S1, a silicon-based material is deposited on the surface of the semiconductor epitaxial wafer as a hard mask layer. The hard mask layer can be one or more of silicon dioxide, silicon nitride, or amorphous silicon. The deposition method can be plasma-enhanced chemical vapor deposition or other equivalent processes. The hard mask layer is used to improve the etching selectivity and size retention of subsequent pattern transfer.

[0028] Please see Figure 2 A photoresist is spin-coated onto the surface of a hard mask layer and soft-baked. An i-line exposure and development process is then used to form the first photolithographic pattern. The first photolithographic pattern includes at least: a strip pattern corresponding to the ridge waveguide 7 and a periodic pattern corresponding to the slot grating 8. The strip pattern corresponding to the ridge waveguide 7 is used for subsequent formation of the ridge waveguide 7, achieving lateral mode confinement and lateral spot size control. The periodic pattern of the slot grating 8 is periodically distributed along the resonant cavity direction and is used for subsequent formation of the slot grating 8. The period Λ of the slot grating 8 is determined according to the DFB laser mode selection design, for example, satisfying the Bragg condition. Optionally, the first photolithographic pattern may also simultaneously include alignment marks or process inspection structures for alignment in the second photolithography step.

[0029] In step S2, using the pattern on the photoresist as a mask, the hard mask layer is first etched using inductively coupled plasma to etch the patterns of the ridge waveguide 7 and the slot grating 8 onto the hard mask layer. Then, using the hard mask layer as a mask, the upper confinement layer 6 or the upper waveguide layer 5 of the underlying semiconductor epitaxial wafer is dry-etched using inductively coupled plasma. The etching depth is above the active region 4 and can be adjusted according to the performance requirements of the laser. The ridge waveguide 7 structure and the slot grating 8 structure are formed simultaneously. After etching, the photoresist is cleaned off, and the hard mask layer is removed using selective wet etching or dry etching, thereby exposing the semiconductor surface and providing a clean surface for the subsequent deposition of the insulating layer 10.

[0030] Please see Figure 3 In step S3, a silicon-based material is redeposited on the surface of the semiconductor epitaxial wafer as an insulating layer 10. The deposition method can be plasma-enhanced chemical vapor deposition. This layer is used for surface passivation, sidewall protection, and electrical isolation between the metal electrode and the semiconductor, and serves as a dielectric layer for subsequent metal electrode contact openings.

[0031] Please see Figure 4 Photoresist is spin-coated onto the surface of insulating layer 10 and soft-baked. The pattern of metal electrode contact window 9 is formed on the photoresist of ridge waveguide 7 by i-line exposure and development. The pattern of the window is located on the top of ridge waveguide 7.

[0032] Reactive ion etching is used to etch and open the insulating layer 10 corresponding to the pattern of the metal electrode contact window 9. The etching passes through the insulating layer to expose the semiconductor contact layer area, forming an ohmic contact window between the metal electrode and the semiconductor. After etching, the photoresist is cleaned off. The insulating layer 10 is not removed entirely; only the insulating layer 10 in the area of ​​the metal electrode contact window 9 is removed, while the insulating layer 10 in the remaining areas is completely retained. This is used to suppress leakage current, reduce surface recombination, and stabilize the current injection path.

[0033] In step S4, a metal layer is deposited on the surface of the metal electrode contact window 9 using electron beam evaporation or magnetron sputtering as the upper electrode. The upper electrode metal system can be selected according to the material system, such as titanium, platinum, or gold, and annealing is performed as needed to reduce contact resistance. Since the insulating layer 10 from step 3 is completely preserved in the non-windowed area, it plays an electrical isolation role between the metal electrode and the semiconductor, which helps to reduce leakage current and improve device stability and consistency.

[0034] The bottom surface of the semiconductor epitaxial wafer is thinned and polished to reduce series resistance and improve heat dissipation. Subsequently, the bottom surface is cleaned and surface oxidation is removed. A lower electrode (N electrode) metal layer is deposited on the bottom surface to form an ohmic contact. The lower electrode metal system can be selected according to the material system, such as indium phosphide or gallium arsenide. The system can be gold germanium or nickel or gold, or an equivalent metal system can be used. After the bottom surface is metallized, alloying annealing is performed to obtain a stable N-type ohmic contact with low contact resistance.

[0035] The semiconductor epitaxial wafer, after completing the above steps, is cleaved to form individual laser chips. Antireflection coatings or high-reflection coatings can be deposited on the resonant cavity end faces as needed to optimize output performance.

[0036] The DFB laser structure prepared in this embodiment includes: a substrate 1, a lower confinement layer 2, a lower waveguide layer 3, an active region 4, an upper waveguide layer 5, an upper confinement layer 6, a ridge waveguide 7, a slot grating 8, a metal electrode contact window 9, and an insulating layer 10.

[0037] The ridge waveguide 7 is located on the upper part of the semiconductor epitaxial wafer and extends along the direction of the resonant cavity. The ridge waveguide 7 is etched through the upper confinement layer 6 to expose the upper waveguide layer 5. The etching depth is above the active region 4 and can be adjusted according to the performance requirements of the laser. The width and etching depth of the ridge waveguide 7 are used to confine the transverse optical field distribution, thereby realizing transverse single-mode or low-order transverse mode operation, achieving the purpose of limiting the transverse spot size and improving transverse mode control.

[0038] The slot grating 8 is located on the ridge waveguide 7 and is periodically arranged along the direction of the resonant cavity. The slot grating 8 is formed by etching to create a periodic groove structure, and the period Λ of the slot grating 8 satisfies the Bragg condition. The geometry of the slot grating 8 (including slot depth, slot width, duty cycle, etc.) is preset and adjusted according to the required gain coupling strength and refractive index coupling strength.

[0039] The metal electrode contact window 9 is located on top of the ridge waveguide 7. The second photolithography opens the window only in the non-slot region, retaining the insulating layer 10 in the slot region. Gain coupling is achieved without additional processes, and it is superimposed with the refractive index coupling brought by the slot grating 8 itself, forming a high-performance composite coupled DFB laser.

[0040] Except for the metal electrode contact window 9, the remaining positions are covered with an insulating layer 10. The insulating layer 10 is made of silicon-based material, specifically silicon dioxide, silicon nitride, or a combination of both. The deposition method is plasma-enhanced chemical vapor deposition.

[0041] The upper electrode (P-side electrode) is disposed on the metal electrode contact window 9, forming an ohmic contact with the semiconductor through the metal electrode contact window 9. The metal system of the upper electrode can be titanium, platinum, or gold, and is formed by electron beam evaporation or magnetron sputtering deposition.

[0042] The lower electrode (N electrode) is located on the bottom surface of the semiconductor epitaxial wafer. During fabrication, the bottom surface of the semiconductor epitaxial wafer is thinned and polished, and then a metal layer is deposited to form an ohmic contact. The metal system of the lower electrode can be gold-germanium, nickel, gold, or other alloy systems suitable for the corresponding material system.

[0043] Please see Figure 5 In this embodiment, the composite-coupled DFB laser, during operation, introduces periodic perturbations to the effective refractive index of the waveguide through the periodic etching morphology of the slot grating 8, forming refractive index coupling. Simultaneously, the morphology of the slot grating 8, in conjunction with the retained insulating layer 10 and the metal electrode contact window 9, guides current to form periodic injection differences along the cavity length when the device is forward biased (the non-slot region is the strong injection region, and the slot region is the weak injection region), thereby forming gain coupling. The combined effect of refractive index coupling and gain coupling constitutes composite coupling, effectively suppressing the spatial hole burning effect and improving single-longitudinal-mode stability and side-mode suppression ratio.

[0044] As an optional embodiment, the slot grating 8 can be disposed on the sidewall of the ridge waveguide 7 or in the region adjacent to the ridge waveguide 7. The slot shape can be a rectangular slot, a trapezoidal slot, a rounded corner slot, or a multi-level slot; the slot grating 8 can also adopt equivalent structures such as locally continuous slots and periodic notches to achieve equivalent refractive index periodic perturbation and periodic injection modulation.

[0045] As an alternative embodiment, an alternative implementation of gain modulation is provided. In addition to directly modulating the current spread using the slot grating 8, local high-resistance or low-resistance paths can be introduced in the slot region. For example, through local ion implantation compensation, local annealing, local oxidation, or local dielectric thickness differences, a periodic distribution of strong or weak implantation regions can be formed. Alternatively, without changing the principle of two photolithography steps, equivalent periodic implantation differences can be achieved by changing the window area, metal coverage length, or dielectric thickness, thereby forming gain coupling.

[0046] As an alternative embodiment, an alternative to the hard mask or dielectric material and process is provided. The hard mask layer used for transferring the pattern of the ridge waveguide 7 and the slot grating 8 in the first photolithography can be one or more combinations of silicon dioxide, silicon nitride, and amorphous silicon; the insulating layer 10 covering the surface of the semiconductor epitaxial wafer can be one or more combinations of silicon dioxide, silicon nitride, and aluminum oxide; the hard mask etching can be replaced by dry or wet methods; the semiconductor epitaxial wafer etching can be replaced by inductively coupled plasma or reactive ion etching.

[0047] As an alternative embodiment, an alternative to the metal electrode formation and ohmic contact process is provided. The upper electrode can be formed by a peeling method or a metal etching method. The metal system can be selected according to the material system, such as titanium or platinum or gold, chromium or gold, nickel or gold, etc. The ohmic contact can be achieved by different annealing conditions or different contact layer structures. The lower electrode can adopt a back metallization or a mesa or lateral ohmic contact equivalent structure (which can be combined with the second photolithography), all of which do not affect the purpose of composite coupling.

[0048] As an alternative embodiment, an alternative to resonant cavity and phase control is provided. The resonant cavity can be designed with different cavity lengths and different end face reflectivities (AR / HR film), or a phase shift region (such as λ / 4 phase shift) can be introduced to optimize single-mode output. The phase control can achieve equivalent phase shift by changing the parameters of the local slot grating, or the local period or duty cycle, without changing the core idea of ​​introducing composite coupling in the slot structure.

[0049] As an alternative embodiment, a replacement for a DFB laser with a similar single-longitudinal-mode feedback structure is provided. Without changing the refractive index perturbation and periodic injection modulation introduced by the slot grating 8, the device can also be replaced with a single-longitudinal-mode feedback laser form such as a DBR (distributed Bragg reflector laser) or a DFB-DBR (DFB laser-distributed Bragg reflector laser) hybrid structure. As long as the periodic structure of the slot grating 8 and the injection modulation are still achieved by two photolithography processes, the same inventive purpose can be achieved.

[0050] As an alternative embodiment, an alternative material system and waveband application is provided. The substrate 1 of the present invention can be applied to different material systems such as indium phosphide, gallium arsenide, and gallium nitride, as well as different operating wavebands. The active region 4 can be MQW (multiple quantum well), QD (quantum dot), or other equivalent gain structures. The doping type and concentration of the waveguide layer and confinement layer can be adjusted according to the material system, without affecting the core purpose of achieving composite coupling through two photolithography steps.

[0051] As an alternative embodiment, an alternative process sequence is provided, wherein the deposition of the insulating layer 10 can be performed immediately after the first etching or after surface cleaning; bottom thinning and lower electrode metallization can be performed before or after the upper electrode fabrication; and end-face coating can be performed after cleaving or after dicing. This change in sequence does not alter the essential inventive elements of two i-line lithography steps, the simultaneous formation of the ridge waveguide 7 and the slot grating 8 in the first lithography step, and the formation of the ohmic contact window between the metal electrode and the semiconductor in the second lithography step, while retaining the dielectric layer isolation.

[0052] In summary, the above description is merely a preferred embodiment of this specification and is not intended to limit the scope of protection of this specification. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this specification should be included within the scope of protection of this specification.

[0053] The systems, apparatuses, modules, or units described in one or more of the above embodiments may be implemented by a computer chip or entity, or by a product having a certain function. A typical implementation device is a computer. Specifically, a computer may be, for example, a personal computer, a laptop computer, a cellular phone, a camera phone, a smartphone, a personal digital assistant, a media player, a navigation device, an email device, a game console, a tablet computer, a wearable device, or any combination of these devices.

[0054] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0055] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on describing the differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments.

Claims

1. A method for fabricating a composite-coupled DFB laser, wherein the semiconductor epitaxial wafer structure of the laser comprises, from bottom to top: The substrate, lower confinement layer, lower waveguide layer, active region, upper waveguide layer, and upper confinement layer are characterized by comprising: S1: The surface of the semiconductor epitaxial wafer is covered with a mask layer, and the mask layer is covered with photoresist. The i-line is used to photo-etch the ridge waveguide pattern and the slot grating pattern on the photoresist for the first time. S2: Etch the ridge waveguide pattern and the slot grating pattern on the photoresist onto the mask layer, and use the pattern on the mask layer to etch the structure of the ridge waveguide and the slot grating on the upper confinement layer. After etching, remove the photoresist and mask layer from the surface of the semiconductor epitaxial wafer. S3: Cover the surface of the semiconductor epitaxial wafer with insulating material to form an insulating layer, cover the insulating layer with photoresist, and use i-line to photo-etch the pattern of the metal electrode contact window on the photoresist of the ridge waveguide for the second time. Etch the pattern of the metal electrode contact window on the photoresist onto the insulating layer. After etching, remove the photoresist. S4: Cover the surface of the metal electrode contact window with metal material to form an upper electrode, polish the substrate of the semiconductor epitaxial wafer, and cover the polished substrate surface with metal material to form a lower electrode.

2. The method for fabricating a composite-coupled DFB laser according to claim 1, characterized in that, The mask layer and insulating layer covering the surface of the semiconductor epitaxial wafer are made of silicon-based materials.

3. The method for fabricating a composite-coupled DFB laser according to claim 1, characterized in that, The pattern of the ridge waveguide obtained from the first photolithography is also used to determine the lateral mode and lateral spot size of the ridge waveguide.

4. The method for fabricating a composite-coupled DFB laser according to claim 1, characterized in that, The slot grating can also be disposed on the top, sidewall, and adjacent area of ​​the ridge waveguide; the slot shape of the slot grating can be a rectangular slot, trapezoidal slot, rounded corner slot, or multi-level slot; the slot grating can also employ locally continuous slots and periodic notches.

5. The method for fabricating a composite-coupled DFB laser according to claim 1, characterized in that, The mask layer is removed by wet etching or dry etching.

6. The method for fabricating a composite-coupled DFB laser according to claim 1, characterized in that, The metal electrode is formed by electron beam evaporation or magnetron sputtering.

7. The method for fabricating a composite-coupled DFB laser according to claim 1, characterized in that, The pattern created in the first photolithography also includes alignment marks or process inspection structures for alignment in the second photolithography.

8. The method for fabricating a composite-coupled DFB laser according to claim 1, characterized in that, The ridge waveguide is etched to the upper confinement layer or the upper waveguide layer.

9. The method for fabricating a composite-coupled DFB laser according to claim 1, characterized in that, The etching depth of the slot grating is consistent with the height of the ridge waveguide.

10. A DFB laser, characterized in that, The composite-coupled DFB laser is prepared using the fabrication method described in any one of claims 1 to 9.

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