CN101001001A - Manufacturing method of low cost DFB laser - Google Patents

Abstract

A method for manufacturing a low-cost DFB laser. The semiconductor laser is composed of an upper cladding layer, a lower waveguide layer, an active layer, an upper waveguide layer, and an upper cladding layer and an electrode contact layer grown sequentially on an InP substrate. There are metal electrodes on the contact layers, and a DFB grating structure exists on the upper waveguide layer or the lower waveguide layer, and the feature is that the DFB grating structure is made by nanoimprinting technology. It can be a grating with an arbitrary phase shift structure, or a grating with an arbitrary sampling structure. The method can produce a laser chip with a uniform periodic grating structure, and can also produce a series of DFB laser chips with different wavelengths on the same epitaxial wafer, or a multi-wavelength DFB laser array on the same chip. The invention has the characteristics of low production cost, high production efficiency and high grating resolution.

Description

Fabrication method of low-cost DFB laser

technical field

The invention relates to a method for manufacturing a DFB grating of an active optoelectronic device, belonging to the technical field of active optical devices.

Background technique

Semiconductor lasers are the most important light sources in optical communication systems. With the rapid development of optical fiber communication in the direction of long distance and large capacity, semiconductor lasers are increasingly required to have high-speed, narrow linewidth and dynamic single longitudinal mode working characteristics. It is generally believed that the laser with distributed feedback Bragg grating (DFB) is the most ideal light source to meet the requirements of long-distance and large-capacity optical fiber communication systems.

The spectrum of the F-P laser with common structure is multi-longitudinal mode, and there is spectral broadening under high-speed modulation, which reduces the transmission bandwidth of the fiber, thus limiting the transmission rate. The DFB laser establishes a Bragg grating inside the semiconductor, and realizes single longitudinal mode selection by light distribution feedback. Moreover, the DFB laser can suppress the mode hopping of the ordinary F-P laser in a wider operating temperature and operating current range, and greatly improve the noise characteristics of the device.

Since the size of the grating pattern of the DFB semiconductor laser is very small (the grid size is about 100nm), the current popular process method is generally realized by the method of double-beam interference exposure of the deep ultraviolet laser (it is difficult to use the best photolithography machine now). Reach the required processing accuracy, and the price is too expensive). However, due to the strict periodicity of the interference pattern, the DFB grating produced will have a completely symmetrical and uniformly distributed structure. The symmetry of the structure leads to the symmetry of the mode distribution, Such a DFB laser will oscillate with two main modes at the same time, that is, a double longitudinal mode phenomenon. Even if there is a single longitudinal mode lasing due to the asymmetry of the laser cleavage plane and the asymmetry of the end face coating, but due to this The phase uncertainty caused by this kind of asymmetry may cause the lasing of the left longitudinal mode, or the lasing of the right longitudinal mode, resulting in the uncertainty of the laser emission wavelength, which cannot meet the requirements of the laser in the DWDM system. Accurate requirements of the lasing wavelength. And in the actual modulation application, this kind of laser with uniform DFB grating may have mode hopping phenomenon, which seriously affects its working performance.

In order to concentrate the radiated power to a dominant mode and increase the threshold gain difference of each oscillation mode, it is necessary to deliberately break the symmetry of the grating structure, thereby disturbing the symmetry of forward and reverse traveling wave feedback. The most effective method at present is to introduce a λ/4 or λ/8 phase shift in the uniformly distributed DFB grating. If there is a λ/4 or λ/8 phase shift at the center of the DFB-LD, regardless of the phase at the laser cavity surface, the laser can work under the condition of a single longitudinal mode, and the lasing of the laser can be precisely controlled wavelength and reduce chirp to improve device performance. But this kind of DFB grating structure with phase shift cannot be realized by the method of interference exposure, and the manufacturing cost of the method of electron beam exposure is too high. The cost of making a two-inch epitaxial wafer (uniformly covered with DFB gratings) by electron beam exposure is about $3,000, and it takes more than ten hours.

Contents of the invention

The purpose of the present invention is to propose a low-cost DFB laser manufacturing method based on nano-imprint technology, which effectively overcomes the shortcomings of the existing semiconductor laser grating using double-beam interference method or electron beam exposure method. The double-beam interferometry method cannot produce phase-shifting gratings, cannot simultaneously produce multi-wavelength DFB lasers for DWDM on the same epitaxial wafer, and cannot produce multi-wavelength DFB laser arrays or DFB-EA arrays on the same chip; at the same time, it also overcomes the shortcomings of electron beam Exposure method has the disadvantage of high mass production cost.

The object of the present invention is achieved by adopting the following technical solutions: a method for manufacturing a low-cost DFB laser, the semiconductor laser consists of an upper lower cladding layer, a lower waveguide layer, an active layer, an upper waveguide layer and an upper layer grown sequentially on an InP substrate. The cladding layer and the electrode contact layer are composed of metal electrodes on the substrate and the electrode contact layer respectively, and a DFB grating structure exists on the upper waveguide layer or the lower waveguide layer, and the feature is that the DFB grating structure is made by nanoimprinting technology.

The manufacturing method of the above-mentioned low-cost DFB laser is characterized in that it is manufactured by hot embossing, or by cold embossing (ultraviolet hardening embossing), or by micro-contact embossing.

As above-mentioned low-cost DFB laser production method, it is characterized in that the method for described thermal embossing is:

(1) Make a template with a nano-grating pattern using electron beam direct writing technology; (2) Uniformly coat a layer of thermoplastic polymer photoresist on the upper waveguide layer of the DFB grating to be made; The resist is heated above the glass transition temperature, and the template is pressed into the photoresist layer softened by high temperature by mechanical force, and the high temperature and high pressure are maintained for a period of time, so that the thermoplastic polymer photoresist is filled into the nanostructure of the template; (3 ) After the photoresist is cooled and solidified, release the pressure, and separate the imprint template from the upper waveguide layer; (4) Then perform reactive ion etching (RIE) on the upper waveguide layer to remove the remaining photoresist, and the grating structure can be copied (5) Finally, using the imprinted photoresist as a mask, the required DFB structure is produced on the upper waveguide layer by dry etching or wet etching.

The manufacturing method of the low-cost DFB laser is characterized in that the DFB grating has a uniform periodic structure; or has a λ/4 or λ/8 phase shift structure.

The manufacturing method of the low-cost DFB laser is characterized in that: using electron beam direct writing technology to make multi-wavelength nano grating patterns on the same template.

The present invention has the following advantages:

The invention adopts the nanoimprinting technology to manufacture the grating of the DFB laser, overcomes the above-mentioned shortcoming of the common double-beam interference method, can manufacture the DFB grating with any phase-shift structure, and improves the single-mode yield of the DFB laser.

"Nanoimprinting" is a new method of nano-pattern replication. It is characterized by ultra-high resolution, high output, and low cost. The high resolution is because it has no diffraction phenomenon in optical exposure and scattering phenomenon in electron beam exposure. The throughput is high because it can be processed in parallel like optical exposure, making hundreds or thousands of devices simultaneously. The low cost is because it does not require a complex optical system like an optical exposure machine or a complex electromagnetic focusing system like an electron beam exposure machine. Therefore, nanoimprinting is expected to become an industrial production technology, fundamentally opening up broad prospects for the production of various nano-devices.

Usually, the price of a high-precision electron beam exposure machine is more than 2 million US dollars, while the average price of a nanoimprint equipment capable of processing 50 nanometer line width is 200,000 US dollars, which is only one tenth of the price of an electron beam exposure machine . The cost of making a two-inch epitaxial wafer (uniformly covered with DFB gratings) by electron beam exposure is about 3,000 US dollars, and it takes more than ten hours; while the cost of making gratings on the same size epitaxial wafers by nanoimprinting is much lower. At $50, it's only a fraction of the cost of electron beam exposure, and it only takes a few minutes. At present, due to the low production efficiency and low yield of the DFB laser itself, the cost of the DFB laser in the TO package is about 30 US dollars, which is still too high for the use of FTTH (fiber-to-the-home). Printing technology to make DFB gratings can improve the production efficiency of lasers and increase their yield, thereby reducing costs and finally meeting the requirements of FTTH.

The technology of making gratings by nanoimprinting method, in addition to making ordinary DFB lasers and phase-shifted grating structure DFB lasers, can make multi-channel complete sets of DWDM laser series on the same epitaxial wafer, and can make multi-wavelength DFB lasers on the same chip. Chip arrays, even integrated multi-wavelength DFB-EA arrays or tunable DFB laser arrays can be fabricated on the same chip, which has incomparable advantages over ordinary interference methods or electron beam etching methods. The technology of making DFB gratings by nanoimprinting is expected to become an industrial production technology, fundamentally opening up broad prospects for the production of various nano-devices.

Description of drawings

Figure 1—Schematic diagram of a typical nanoimprint process. Among them, 11 imprint templates, 12 photoresists, and 13 epitaxial wafers.

Fig. 2 - DFB tunable laser with uniform grating in Embodiment 1 of the present invention. Among them, 21 imprint template, 22 photoresist, 23 upper waveguide layer, 24 active area, 25 lower waveguide layer, 26 substrate.

Fig. 3 - DFB tunable laser with a λ/4 phase shift structure according to Embodiment 2 of the present invention. Among them, 31 imprinting mold plate, 32 photoresist, 33 upper waveguide layer, 34 active area, 35 lower waveguide layer, 36 substrate.

Figure 4—is a typical spectrum diagram of the semiconductor laser of the chip in Figure 3.

Fig. 5 - DFB tunable laser with sampling grating according to Embodiment 3 of the present invention. Among them, 51 imprint template, 52 photoresist, 53 upper waveguide layer, 54 active area, 55 lower waveguide layer, 56 substrate, 510 left sampling grating area, 511 active area + phase area, 512 right sampling grating area.

Fig. 6 - Embodiment 4 of the present invention fabricates a multi-wavelength DFB laser chip array on the same chip. Among them, 61 imprint template, 62 photoresist, 63 upper waveguide layer, 64 active area, 65 lower waveguide layer, 66 substrate.

Fig. 7 - Embodiment 5 of the present invention manufactures a multi-channel complete set of DWDM laser chip series on the same epitaxial wafer. Among them, 71 epitaxial wafers.

Fig. 8 - Embodiment 6 of the present invention Fabricate an integrated multi-wavelength DFB-EA array on the same chip. Among them, 81-88 lasers, 810DFB laser arrays, 811 multimode couplers, 812 electroabsorption modulators.

Detailed ways

The invention makes active optoelectronic devices such as DFB gratings of semiconductor lasers by adopting nano imprinting technology. The semiconductor laser grows the lower cladding layer, the lower waveguide layer, the active layer, the upper waveguide layer, the upper cladding layer and the electrode contact layer sequentially on the InP substrate. There are metal electrodes on the substrate and the electrode contact layer respectively, and the upper waveguide layer Or there is a distributed feedback Bragg grating (DFB) structure on the lower waveguide layer. The method for manufacturing DFB gratings by adopting nanoimprinting technology has the characteristics of low manufacturing cost, high production efficiency and high grating resolution.

The scheme of the invention is suitable for making DFB gratings by adopting thermal embossing technology; it is suitable for making DFB gratings by adopting ultraviolet hardening embossing technology; it is suitable for making DFB gratings by adopting micro-contact embossing method.

The scheme of making DFB grating by adopting nanolay embossing technology of the present invention is applicable to the making of all active optoelectronic device gratings; It is suitable for the making of ordinary uniform DFB grating; It is suitable for the making of DFB grating with arbitrary phase shift structure; Fabrication of DFB grating with random sampling structure.

The scheme of making DFB grating by adopting nano-imprinting technology of the present invention is applicable to the making of single-wavelength DFB laser chip; It is suitable for making multi-channel DFB laser chip series of series wavelengths for DWDM at the same time on the same epitaxial wafer; A multi-wavelength DFB laser chip array is produced on the chip; it is suitable for integrated optoelectronic devices, such as DFB-EA externally modulated laser chips and grating fabrication of optoelectronic devices such as tunable semiconductor lasers.

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

Figure 1 is a typical process flow of making DFB gratings using nano-thermal embossing technology. Usually includes a five-step process, (1) use electron beam direct writing technology to make Si or SiO2 template 11 with nano-grating pattern, and uniformly coat a layer of thermoplastic polymer photoresist on the surface of epitaxial wafer 13 to be made DFB grating 12 (such as PMMA material); (2) heat the photoresist 12 on the epitaxial wafer 13 above the glass transition temperature, use mechanical force to press the template 11 into the photoresist 12 layer softened by high temperature, and maintain high temperature and high pressure For a period of time, the thermoplastic polymer photoresist is filled into the nanostructure of the template; (3) after the photoresist 12 is cooled and solidified, the pressure is released, and the imprint template 11 is separated from the epitaxial wafer 13; (4) the epitaxial wafer is then 13 Perform reactive ion etching (RIE) to remove the remaining photoresist, and then the grating structure can be replicated; (5) Finally, using the imprinted photoresist as a mask, dry etching or wet etching method, fabricate the required DFB structure on the epitaxial wafer 13.

The production method of cold embossing (ultraviolet hardening imprinting) is as follows: firstly, a stencil with a nano-grating pattern is made, and the stencil material must use quartz that can allow ultraviolet rays to penetrate; Lay a layer of low-viscosity, ultraviolet-sensitive liquid polymer photoresist; after aligning the stencil and the epitaxial wafer, press the template into the photoresist layer and irradiate ultraviolet light to make the photoresist polymerize and harden; Then remove the mold, use RIE etching to remove the remaining photoresist; finally use the imprinted photoresist as a mask, and use dry etching or wet etching to produce the required DFB structure on the epitaxial wafer .

The manufacturing method of micro-contact imprinting: first make a template with a nano-grating pattern; then coat a layer of liquid on the surface of the template; contact the template coated with liquid with the surface of the epitaxial wafer to be made DFB grating, so that the epitaxial wafer and A self-assembled monolayer film is formed where the template contacts; finally, the required DFB structure is etched on the epitaxial wafer using the self-assembled monolayer film as a mask.

Fig. 2 is a schematic diagram of a semiconductor laser with a uniform structure DFB grating fabricated by nanoimprinting technology. This semiconductor laser grows the lower cladding layer and the lower waveguide layer 25, the active layer 24, the upper waveguide layer 23 and the upper cladding layer and the electrode contact layer (not shown in the figure) successively on the InP substrate 26, and the substrate 26 and the electrode There are metal electrodes on the contact layers respectively, and distributed feedback Bragg gratings exist on the upper waveguide layer 23 . The grating is made by nanoimprinting method and has a uniform structure.

Fig. 3 is a schematic diagram of a semiconductor laser with a λ/4 phase shift structure DFB grating fabricated by nanoimprinting technology. This semiconductor laser grows the lower cladding layer and the lower waveguide layer 35, the active layer 34, the upper waveguide layer 33, and the upper cladding layer and the electrode contact layer (not shown in the figure) successively on the InP substrate 36. There are metal electrodes on the layers respectively, and there is a distributed feedback Bragg grating on the upper waveguide layer 33 . The grating is fabricated by nanoimprinting method, and has a λ/4 phase shift structure in the center of the laser grating. Regardless of the phase at the laser cavity facet, the laser can work under the condition of single longitudinal mode. Figure 4 is the lasing spectrum of a semiconductor laser with a λ/4 phase-shift structure grating fabricated by nanoimprinting technology.

Fig. 5 is a schematic diagram of fabricating a sampling grating of a tunable semiconductor laser using nanoimprint technology. This tunable semiconductor laser grows the lower cladding layer and the lower waveguide layer 55, the core layer 54, the upper waveguide layer 54, and the upper cladding layer and the electrode contact layer (not shown in the figure) successively on the InP substrate 56. There are metal electrodes respectively on the contact layers. The tunable semiconductor laser includes a left sampling grating region 510 , an active region+phase region 511 and a right sampling grating region 512 from left to right. The grating structures in the left and right grating areas are all sampling gratings, and the sampling periods are d1 and d2 respectively (d1<d2 in the figure), and the gratings are all made by nanoimprinting method.

Fig. 6 is a schematic diagram of fabricating a multi-wavelength DFB laser array on the same chip by nanoimprinting technology. In addition to the nanoimprinting technology of the present invention can be used to make ordinary DFB lasers with uniform gratings, DFB lasers with phase-shifted grating structures, and tunable laser gratings with sampling periods, another important application is It can be conveniently used to make a single-chip multi-wavelength DFB array. In the figure, there are n (n≥2) DFB lasers on the same chip, and the grating period of each laser is different. The period of the first laser is Λ1 (the corresponding lasing wavelength is λ1), and the i-th laser The period of the nth laser is Λi (the corresponding lasing wavelength is λi), the period of the nth laser is Λn (the corresponding lasing wavelength is λn), and Λ1>Λ2>...Λn, each laser has a different central excitation Radiation wavelength, and λ1>…λi>…>λn.

Figure 7 is a schematic diagram of a complete set of DFB laser series for DWDM with multiple channels fabricated on the same epitaxial wafer by nanoimprinting technology. Nanoimprint technology has incomparable superiority in the production of DWDM lasers compared to ordinary interference methods. In DWDM system applications, all lasers are required to have different wavelengths with equal intervals. The DFB grating made by the double-beam interference method has a uniform grating structure on the entire epitaxial wafer, and all lasers work at the same wavelength. Therefore, it is necessary to use multiple epitaxial wafers to fabricate different grating periods, in order to obtain the required series of wavelength laser chips. In the present invention, nano-imprinting technology is adopted to make imprinting templates into different periods, and a complete set of 40-channel DWDM laser chip series with an interval of 100 GHz can be produced on the same epitaxial wafer 71. The lasing frequency of each laser is The wavelengths are λ1, λ2, ... λ39, λ40; even a complete set of DWDM laser chip series with 80 channels at 50 GHz intervals can be obtained on the same epitaxial wafer by one imprinting process.

FIG. 8 is a schematic diagram of an integrated multi-wavelength DFB-EA chip array (such as an 8-wavelength array) fabricated on the same chip using nanoimprint technology. Using nanoimprint technology to fabricate gratings in highly integrated active optoelectronic devices, such as DFB-EA chips, tunable DFB laser chips, etc., has incomparable advantages over ordinary interference methods or electron beam etching methods. A set of DFB gratings with multiple wavelengths can be fabricated simultaneously on the same chip by nanoimprinting technology, and a λ/4 or λ/8 phase shift can be easily added to each grating. The integrated device in the figure is composed of three terminals. The first terminal is a DFB laser array 810, on which there are eight lasers 81-88. The DFB grating is made by nanoimprinting technology. The central lasing wavelength of each laser is All are different. The second end is a multimode coupler 811, which couples the laser light from the laser array into a single laser beam. The third terminal is an electroabsorption modulator 812, which uses electrical signals to modulate the laser beam at high speed.

Claims (6)

1. A method for manufacturing a low-cost DFB laser. The semiconductor laser consists of a lower cladding layer, a lower waveguide layer, an active layer, an upper waveguide layer, and an upper cladding layer and an electrode contact layer grown sequentially on an InP substrate. There are metal electrodes on the contact layer with the electrodes, and there is a DFB grating structure on the upper waveguide layer or the lower waveguide layer, and the feature is that the DFB grating structure is made by nanoimprinting technology. 2. The manufacturing method of low-cost DFB laser as claimed in claim 1, characterized in that: it is made by hot embossing method, or it is made by cold embossing method of ultraviolet hardening embossing, or it is made by micro-contact embossing method . 3. The manufacturing method of low-cost DFB laser according to claim 2, characterized in that the method of thermal embossing is: (1) Make a template with a nano-grating pattern using electron beam direct writing technology; (2) Uniformly coat a layer of thermoplastic polymer photoresist on the upper waveguide layer of the DFB grating to be made; The resist is heated above the glass transition temperature, and the template is pressed into the photoresist layer softened by high temperature by mechanical force, and the high temperature and high pressure are maintained for a period of time, so that the thermoplastic polymer photoresist is filled into the nanostructure of the template; (3 ) After the photoresist is cooled and solidified, release the pressure, and separate the imprint template from the upper waveguide layer; (4) Then perform reactive ion etching (RIE) on the upper waveguide layer to remove the remaining photoresist, and the grating structure can be copied (5) Finally, using the imprinted photoresist as a mask, the required DFB structure is produced on the upper waveguide layer by dry etching or wet etching. 4. The manufacturing method of low-cost DFB laser according to claim 1, 2 or 3, characterized in that the DFB grating has a uniform periodic structure; or has a λ/4 or λ/8 phase shift structure. 5. The manufacturing method of low-cost DFB laser according to claim 1, 2 or 3, characterized in that: using electron beam direct writing technology to make multi-wavelength nano-grating patterns on the same template. 6. The method for manufacturing low-cost DFB lasers as claimed in claim 4, characterized in that: using electron beam direct writing technology to make multi-wavelength nano-grating patterns on the same template.

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