CN113687466A - Lithium niobate thin film photon chip based on metal hard mask and processing method thereof - Google Patents
Lithium niobate thin film photon chip based on metal hard mask and processing method thereof Download PDFInfo
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- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/13—Integrated optical circuits characterised by the manufacturing method
- G02B6/136—Integrated optical circuits characterised by the manufacturing method by etching
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- G—PHYSICS
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- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
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- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/122—Basic optical elements, e.g. light-guiding paths
- G02B6/1228—Tapered waveguides, e.g. integrated spot-size transformers
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- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/13—Integrated optical circuits characterised by the manufacturing method
- G02B6/132—Integrated optical circuits characterised by the manufacturing method by deposition of thin films
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Abstract
The invention provides a lithium niobate thin film photon chip based on a metal hard mask and a processing method thereof, wherein the method comprises the following steps: s1, depositing a metal chromium layer on the surface of the lithium niobate film by using a vacuum magnetron sputtering technology; s2, coating an electron beam resist on the metal chromium layer in a spinning mode, and transferring the pattern to be processed to the electron beam resist through electron beam direct writing lithography; s3, etching the metal chromium layer by adopting a metal etching process based on chlorine to form a metal hard mask; and S4, under the dual protection of the metal hard mask and the electron beam resist, etching the lithium niobate thin film through reactive ions to realize the processing of the micron-scale photonic chip. The method has the advantages of high processing precision, high etching selection ratio, good structure depth-to-width ratio, low structure surface pollution level and the like, and can be used for manufacturing high-precision and high-smoothness ridge waveguides, micro-ring resonant cavities and other photonic chips on lithium niobate films.
Description
Technical Field
The invention relates to a lithium niobate thin film photonic chip in the field of photonic chip processing, in particular to a photonic chip for realizing various unspecific micro-nano structures on a lithium niobate thin film based on a metal hard mask and a manufacturing method thereof.
Background
Lithium niobate (LiNbO3) is an optical material with excellent electro-optic and acousto-optic performance effects, has the advantages of good physical and chemical properties, wide low-loss optical window, large electro-optic coefficient, good second-order nonlinear effect and the like, and is mainly used in the application fields of modulators, microwave photonics, nonlinear optics and the like. Meanwhile, the method has the disadvantages of small refractive index difference, weak mode limitation, larger difference between the etching process and the silicon-based chip, more etching residues and the like, and the existing process has certain defects.
The existing lithium niobate film micro-nano chip processing technology mainly comprises two technical routes of a wet method and a dry method. The etching solution used in the wet method has strong toxicity and corrosivity, is influenced by various factors such as reaction temperature and the like, and has certain defects in the accuracy and stability of etching. Dry etching is mainly reactive ion etching and the like, generally needs a soft mask such as a resist/photoresist and the like or a hard mask material such as silicon oxide and the like, has the defects of too low etching selection ratio, difference of etching rates of a fine structure and a large-size structure and the like in the soft mask, and is difficult to realize the preparation of a high-precision and high-aspect-ratio structure; although the silicon oxide-based hard mask has high selectivity, it is difficult to completely remove the mask without damaging the surface of lithium niobate by either a wet method or a dry method, which introduces contamination and is not conducive to integrating metal into a specific location of a chip. The maskless focused ion beam etching technology has high etching precision, but the processing efficiency is too low, which is not beneficial to the manufacture of wafer level chips.
Through search, chinese patent publication No. CN102738339A discloses a lithium niobate substrate having a patterned structure and a method for manufacturing the same, in which: step 1: providing a lithium niobate substrate with a flat surface, and manufacturing a mask pattern on the surface of the lithium niobate substrate; step 2: taking the mask pattern as a mask, and synchronously etching the mask pattern and the lithium niobate substrate by adopting fluorine-based plasma; and step 3: etching the lithium niobate substrate by adopting oxygen plasma to remove lithium fluoride particles formed on the lithium niobate substrate; and 4, step 4: repeating the step 2 to the step 3 for multiple times until the mask patterns completely disappear; and 5: and continuously etching the lithium niobate substrate by adopting fluorine-based plasma, and forming lithium fluoride particles on the surface of the lithium niobate substrate to form the lithium niobate substrate with a patterned structure, wherein the surface of the patterned structure has a nano rough structure. However: the process can form more lithium fluoride deposition, and the use of fluorine-based plasma for simultaneously etching the mask and the structure can cause faster consumption, which is not favorable for preparing the structure with high requirements on the smoothness of the side wall and the etching depth of the lithium niobate.
Therefore, the development of a novel processing method of a lithium niobate thin film photonic chip to realize a manufacturing technology with high processing efficiency, high selectivity and low pollution is a key problem in the field of realizing the lithium niobate photonic chip.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a lithium niobate thin film photonic chip based on a metal hard mask and a processing method thereof, which can realize the manufacturing technology with high processing efficiency, high selection ratio and low pollution, the link of etching lithium niobate is lower for the small size of the mask, and argon plasma is also used in the process of etching lithium niobate, thereby being beneficial to the falling off of lithium fluoride sediments and reducing the surface roughness.
According to one aspect of the invention, a processing method of a lithium niobate thin film photonic chip based on a metal hard mask is provided, which comprises the following steps:
s1, depositing a metal chromium layer on the surface of the lithium niobate film by using a vacuum magnetron sputtering technology;
s2, coating an electron beam resist on the metal chromium layer in a spinning mode, and transferring the pattern to be processed to the electron beam resist through electron beam direct writing lithography;
s3, etching the metal chromium layer by adopting a metal etching process based on chlorine to form a metal hard mask;
and S4, etching the lithium niobate thin film by reactive ions by using argon and fluorine-based gas under the protection of the metal hard mask, thereby realizing the processing of the micron-scale photonic chip.
Optionally, the lithium niobate thin film is etched by reactive ion using argon and fluorine-based gas under the protection of the metal hard mask, wherein the reactive ion etching process is divided into two stages:
the technological parameters of the first stage are argon with the flow rate of 80 sccm-100 sccm, the reaction time is 400-450 s, and the initial reaction temperature is below-10 ℃;
the technological parameters of the second stage are argon with the flow rate of 15 sccm-40 sccm and CHF with the flow rate of 15 sccm-40 sccm3The reaction time is 400 s-480 s, and the initial reaction temperature is below-10 ℃.
Optionally, the reactive ion etching process, wherein:
the average etching rate of the lithium niobate in the first stage is 35 nm/min-42 nm/min;
the average etching rate of the lithium niobate in the second stage is 8 nm/min-12 nm/min.
Optionally, the lithium niobate thin film is etched by reactive ion using argon and fluorine-based gas under the protection of the metal hard mask, wherein: and controlling the surface appearance and the etching effect of the lithium niobate by controlling reaction variables including the thickness of the metal hard mask, the flow of reaction gas, the reaction temperature, the ICP power, the RIE power and the pressure parameter of the reaction cavity. Specifically, the thickness of the metal hard mask is increased, so that the etching depth of the lithium niobate film can be increased; the flow of argon is increased, so that the increase of a side wall angle is facilitated; increasing the flow of fluorine-based gas is beneficial to the reduction of the side wall angle, but increases the adhesion of lithium fluoride; the reaction temperature is increased, so that the etching speed can be accelerated, but the surface roughness and the etching selection ratio can be reduced; ICP and RIE power are improved, the side wall angle is reduced, and the improvement of surface roughness is not facilitated; the surface roughness can be improved by reducing the reaction chamber pressure.
In a second aspect of the present invention, a lithium niobate thin film photonic chip prepared by the above method is provided, wherein the chip is a lithium niobate thin film ridge waveguide, a micro-ring resonant cavity or an inverted cone waveguide.
Compared with the prior art, the invention solves the technical problems of high precision, high efficiency and low pollution of the submicron-level lithium niobate film photonic chip. Compared with the prior art, the method has the advantages of at least one of the following:
1. the chromium film is deposited by adopting vacuum magnetron sputtering, and compared with the processes such as evaporation and the like, the chromium film has the advantages of high density and good film uniformity;
2. the electron beam direct writing lithography is adopted to replace the ultraviolet lithography, so that the method has the advantage of low processing line width, can realize the processing precision of 300nm level, and can realize a more fine chip structure;
3. the chromium metal hard mask is used for replacing a resist/photoresist or silicon oxide, and compared with measures such as a conductive adhesive and the like, the method can increase the conductivity and improve the accuracy and the pattern stability of the electron beam direct writing lithography. Because the reaction conditions for etching the metal and the lithium niobate medium are obviously different, the damage to the lithium niobate film during the mask preparation can be effectively avoided, the mask loss during the lithium niobate etching can be reduced, and the etching selection ratio and the depth-to-width ratio of the structure can be improved. In addition, the mask on the lithium niobate thin film can be completely removed after the processing is finished, thereby being beneficial to the processes of rear-end bonding, packaging and the like;
4. in the reactive ion etching, the physical etching based on argon and the physical chemical etching based on argon and fluorine-based gas are carried out in stages, the advantages of high etching rate and low residue of the physical etching are combined, and the advantages of high fineness and high etching selection ratio of the physical chemical etching are combined, so that the depth-to-width ratio of the structure is increased, the deposition of etching byproducts is reduced, and the smoothness of the side wall is improved;
5. the design and manufacture of devices with different depths and side wall angles can be realized by finely controlling indexes such as pressure, temperature, gas flow, radio frequency power, ion power, reaction time and the like in the reaction process.
Drawings
Other features, objects and advantages of the invention will become more apparent upon reading of the detailed description of non-limiting embodiments with reference to the following drawings:
FIG. 1 is a flow chart of a method according to an embodiment of the present invention;
FIG. 2 is a schematic illustration of the processing of a preferred embodiment of the present invention;
in the figure: 1 is a silicon oxide substrate; 2 is a single crystal lithium niobate film; 3 is chromium deposited by vacuum magnetron sputtering; 4 is electron beam resist; 5 is electron beam resist pattern after electron beam direct writing lithography; 6 is electron beam resist left after metal etching; 7 is a metal hard mask formed after metal etching; and 8, forming a lithium niobate structure after reactive ion etching and mask removal.
Detailed Description
The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that variations and modifications can be made by persons skilled in the art without departing from the spirit of the invention. All falling within the scope of the present invention.
In order to realize the manufacturing technology with high processing efficiency, high selection ratio and low pollution in the field of lithium niobate photonic chips, the embodiment of the invention provides a novel processing method of a lithium niobate thin film photonic chip. Specifically, the method comprises the following steps:
fig. 1 is a flowchart of a method according to an embodiment of the present invention, and referring to fig. 1, the method for processing a lithium niobate thin film photonic chip based on a metal hard mask according to the embodiment includes:
s1, forming a compact metal chromium layer for protecting the waveguide on the lithium niobate thin film by using a vacuum magnetron sputtering technology, so as to protect the photon pattern of the lithium niobate thin film, enhance the surface conductivity and improve the function of electron beam direct writing lithography; of course, the lithium niobate thin film can be sufficiently cleaned before the vacuum magnetron sputtering is carried out. The chromium film is deposited by adopting vacuum magnetron sputtering, so that the density is high and the film uniformity is high.
S2, an electron beam resist is spin-coated on the chrome layer, and electron beam direct write lithography is performed to transfer the predetermined pattern to the electron beam resist layer. And by adopting electron beam direct writing photoetching, the processing linewidth is low, higher processing precision can be realized, and a more fine chip structure can be realized.
And S3, etching the metal chromium by using a chlorine-based dry etching process, and transferring a preset pattern onto the chromium layer to form a chromium metal hard mask. The chromium metal hard mask can increase the conductivity, improve the accuracy and the pattern stability of the electron beam direct writing lithography, effectively avoid damaging the lithium niobate film when preparing the mask, reduce the loss of the mask when etching the lithium niobate, and improve the etching selection ratio and the depth-to-width ratio of the structure.
And S4, under the double protection of the resist and the metal hard mask, etching the lithium niobate film protected by the chromium metal hard mask by adopting a reactive ion etching process to realize the etching of the film photon structure. Further, in this step, the plasma reactive ion etching process may be performed using argon and fluorine-based gas, and the plasma reactive ion may be performed using an inductively coupled plasma process.
After the above steps are completed, the residual chromium metal hard mask can be further washed away, and the lithium niobate thin film photon structure is obtained.
The embodiment of the invention has the advantages of high processing precision, high etching selection ratio, good structure depth-to-width ratio, low structure surface pollution level and the like, and can be used for manufacturing high-precision and high-smoothness ridge waveguides, micro-ring resonant cavities and other photonic chips on a lithium niobate film. In addition, the mask on the lithium niobate thin film can be completely removed after the processing is finished, and the processes of rear-end bonding, packaging and the like are facilitated.
In the above examples, the lithium niobate thin film was a Z-cut single crystal lithium niobate thin film deposited on a silicon oxide substrate, sufficiently cleaned with acetone and isopropyl alcohol, and surface-activated with oxygen ions.
In the above embodiments, the chromium metal hard mask may have a thickness of 60nm to 100 nm.
In the above examples, the electronic resist was a positive resist AR-P6200 or Zep520A, and the post spin-bake temperature was 130 deg.C hotplate heating for 60 s.
In the above embodiment, the metal etching process based on chlorine gas has a flow ratio of chlorine gas to oxygen gas of 1:7 during the reaction, and the reaction time is 125 s.
In the above embodiment, the process of etching the lithium niobate thin film by reactive ions using argon and fluorine-based gases, and the reactive ion etching process is divided into two stages. The first stage is physical etching with technological parameters of 80sccm of argon, reaction time of 450s and initial reaction temperature of-10 ℃. The second stage is a physical-chemical etch with process parameters of 20sccm argon, 20sccm CHF3, a reaction time of 400s, and an initial reaction temperature of-10 ℃. The average etching rate of the lithium niobate in the first stage is 40nm/min level, and the average etching rate of the lithium niobate in the second stage is 12 nm/min. By adopting the argon and fluorine-based gas-based physical chemical etching to be carried out in stages, the advantages of high etching rate and low residue of the physical etching and the advantages of high fineness and high etching selection ratio of the physical chemical etching are combined, thereby being beneficial to increasing the depth-to-width ratio of the structure, reducing the deposition of etching byproducts and improving the smoothness of the side wall.
In the above embodiment, the control of the surface morphology and the etching effect of the lithium niobate can be realized by controlling the reaction variables including the metal hard mask thickness, the reaction gas flow rate, the reaction temperature, the ICP power, the RIE power, the reaction chamber pressure, and other parameters by using the process of etching the lithium niobate thin film by reactive ions with the argon and fluorine-based gases. For example, under the protection of a 60nm chromium hard mask, the height of optical structures such as ridge waveguides, micro-ring resonant cavities, inverted cone waveguides and the like obtained by processing on a 600nm thick Z-cut monocrystal lithium niobate thin film on a silicon oxide substrate is 400 nm.
To better illustrate the above technical solutions, the following description is given in connection with the preparation of specific products, but the following examples are not intended to limit the present invention.
As shown in fig. 2, an embodiment of the present invention relates to a method for manufacturing a photonic chip with multiple unspecified micro-nano structures on a lithium niobate thin film based on a metal hard mask, which includes the following specific steps:
the preparation process comprises the following steps: a500 um lithium niobate (not shown) having a size of 10mm × 12mm and a 600nm thick Z-cut single crystal lithium niobate thin film 2 on a 2 μm silicon oxide 1 substrate were taken and washed with acetone, isopropyl alcohol and deionized water in this order. And after fully drying, treating the film by using oxygen plasma to remove organic residues and improve the surface adhesion.
Step 1: and depositing a chromium film 3 with the thickness of 70nm on the surface of the lithium niobate by using vacuum magnetron sputtering equipment.
Step 2: a layer 4 of positive resist AR-P6200 of 400nm was spin-coated on the surface of the chromium layer using a spin-coating apparatus, and then the resist was pre-baked on a hot plate at 150 ℃ for 120 seconds to cure the resist.
And step 3: the exposure was performed using an electron beam direct write lithography apparatus, and developed in methyl isobutyl ketone (MIBK) for 75s, and fixed in isopropyl alcohol for 30s, at which time the pattern was transferred onto the resist layer 4. The hot plate is baked for 60s at 130 ℃ to improve the adhesion of the resist layer 4.
And 4, step 4: using a metal etching machine, and setting the gas flow ratio of chlorine to oxygen to be 1: rf power was set at 50W and 125s was etched under the protection of the resist layer 4 to form a patterned metal hard mask 7. The cooled sample was then purged with 100sccm of Ar.
And 5: a first stage of physical etching was performed under the protection of the remaining resist layer 6 and metal hard mask 7 with parameters of argon flow 80sccm, reaction time 450s, ICP power 600W, initial reaction temperature-10 ℃ and cooling with helium purge. After the reaction is finished, the temperature is reduced to-10 ℃, and the second-stage physical chemical etching is executed, wherein the parameters are argon flow rate of 20sccm and CHF3The flow rate is 20sccm, the ICP power is 100W, the reaction time is 400s, and after the reaction is finished, the etched lithium niobate thin film 8 is formed and has the thickness of 400 nm.
Step 6: the chip was washed with N-methylpyrrolidone, acetone, and isopropanol in that order, and the organic resist residue was washed away. And (3) washing away the residual chromium metal mask by using a chromium cleaning solution prepared from ammonium ceric nitrate and nitric acid, and cleaning by using deionized water to obtain the lithium niobate photonic chip structure with high precision (100 nm-level resolution), low pollution (no significant mask residue), low side wall roughness (10 nm-level) and high aspect ratio (400nm:300 nm).
In another embodiment of the present invention, a method for manufacturing a lithium niobate thin film ridge waveguide is provided, which specifically comprises the following steps:
the preparation process comprises the following steps: the minimum line width of the lithium niobate thin film ridge waveguide to be processed is 300 nm. A500 um lithium niobate (not shown) having a size of 10mm × 12mm and a 600nm thick Z-cut single crystal lithium niobate thin film 2 on a 2 μm silicon oxide 1 substrate were taken and washed with acetone, isopropyl alcohol and deionized water in this order. And after fully drying, treating the film by using oxygen plasma to remove organic residues and improve the surface adhesion.
Step 1: and depositing a 60nm thick chromium film 3 on the surface of the lithium niobate by using vacuum magnetron sputtering equipment.
Step 2: a layer 4 of positive resist AR-P6200 of 400nm was spin-coated on the surface of the chromium layer using a spin-coating apparatus, and then the resist was pre-baked on a hot plate at 150 ℃ for 120 seconds to cure the resist.
And step 3: the exposure was performed using an electron beam direct write lithography apparatus, and developed in methyl isobutyl ketone (MIBK) for 75s, and fixed in isopropyl alcohol for 30s, at which time the pattern was transferred onto the resist layer 4. The hot plate postbaking is carried out for 60s at 125 ℃, so that the adhesion of the resist layer 4 is improved.
And 4, step 4: using a metal etching machine, and setting the gas flow ratio of chlorine to oxygen to be 1: rf power was set at 50W and etched 120s under the protection of the resist layer 4 to form a patterned metal hard mask 7. The cooled sample was then purged with 100sccm of Ar.
And 5: a first stage of physical etching was performed under the protection of the remaining resist layer 6 and metal hard mask 7 with parameters of argon flow 80sccm, reaction time 400s, ICP power 600W, initial reaction temperature-10 ℃ and cooling with helium purge. After the reaction is finished, the temperature is reduced to-10 ℃, and the second-stage physical chemical etching is executed, wherein the parameters are argon flow rate of 30sccm and CHF3The flow rate is 30sccm, the ICP power is 100W, the reaction time is 375s, and the etched lithium niobate thin film 8 is formed after the reaction is finished and has the thickness of 300 nm.
Step 6: the chip was washed with N-methylpyrrolidone, acetone, and isopropanol in that order, and the organic resist residue was washed away. The residual chromium metal mask was washed away using a chromium rinse solution formulated from ammonium ceric nitrate-nitric acid and washed with deionized water to obtain a lithium niobate ridge waveguide structure with high precision (100nm resolution), low contamination (no significant mask residue), low sidewall roughness (10nm level), and high aspect ratio (300nm:300 nm).
In another embodiment of the present invention, a method for manufacturing a lithium niobate thin film micro-ring resonant cavity is provided, which specifically comprises the following steps:
the preparation process comprises the following steps: the minimum line width of the lithium niobate thin film micro-ring resonant cavity to be processed is 2000 nm. A500 um lithium niobate (not shown) having a size of 10mm × 12mm and a 600nm thick Z-cut single crystal lithium niobate thin film 2 on a 2 μm silicon oxide 1 substrate were taken and washed with acetone, isopropyl alcohol and deionized water in this order. And after fully drying, treating the film by using oxygen plasma to remove organic residues and improve the surface adhesion.
Step 1: and depositing a 60nm thick chromium film 3 on the surface of the lithium niobate by using vacuum magnetron sputtering equipment.
Step 2: a layer 4 of positive resist AR-P6200 of 400nm was spin-coated on the surface of the chromium layer using a spin-coating apparatus, and then the resist was pre-baked on a hot plate at 150 ℃ for 120 seconds to cure the resist.
And step 3: the exposure was performed using an electron beam direct write lithography apparatus, and developed in methyl isobutyl ketone (MIBK) for 75s, and fixed in isopropyl alcohol for 30s, at which time the pattern was transferred onto the resist layer 4. The hot plate postbaking is carried out for 60s at 125 ℃, so that the adhesion of the resist layer 4 is improved.
And 4, step 4: using a metal etching machine, and setting the gas flow ratio of chlorine to oxygen to be 1:7.5, set to 50W rf power, etch 120s under the protection of the resist layer 4, forming a patterned metal hard mask 7. The cooled sample was then purged with 100sccm of Ar.
And 5: a first stage of physical etching was performed under the protection of the remaining resist layer 6 and metal hard mask 7 with parameters of argon flow rate of 86sccm, reaction time 440s, ICP power 600W, initial reaction temperature of-10 ℃ and cooling with helium purge. After the reaction is finished, the temperature is reduced to-10 ℃, and the second-stage physical chemical etching is executed, wherein the parameters are argon flow rate of 20sccm and CHF3The flow rate is 25sccm, the ICP power is 100W, the reaction time is 375s, and the etched lithium niobate thin film 8 is formed after the reaction is finished and has the thickness of 300 nm.
Step 6: the chip was washed with N-methylpyrrolidone, acetone, and isopropanol in that order, and the organic resist residue was washed away. And (3) washing away the residual chromium metal mask by using a chromium cleaning solution prepared from ammonium ceric nitrate and nitric acid, and cleaning by using deionized water to obtain the lithium niobate micro-ring resonant cavity structure with high precision (100 nm-level resolution), low pollution (no significant mask residue), low sidewall roughness (10 nm-level) and high aspect ratio (300nm:300 nm).
In another embodiment of the present invention, a method for manufacturing a lithium niobate thin film inverted cone waveguide is provided, which specifically comprises the following steps:
the preparation process comprises the following steps: the minimum line width of the lithium niobate thin film reverse taper waveguide to be processed is 1600 nm. A500 um lithium niobate (not shown) having a size of 10mm × 12mm and a 600nm thick Z-cut single crystal lithium niobate thin film 2 on a 2 μm silicon oxide 1 substrate were taken and washed with acetone, isopropyl alcohol and deionized water in this order. And after fully drying, treating the film by using oxygen plasma to remove organic residues and improve the surface adhesion.
Step 1: and depositing a 60nm thick chromium film 3 on the surface of the lithium niobate by using vacuum magnetron sputtering equipment.
Step 2: a layer 4 of positive resist AR-P6200 of 400nm was spin-coated on the surface of the chromium layer using a spin-coating apparatus, and then the resist was pre-baked on a hot plate at 150 ℃ for 120 seconds to cure the resist.
And step 3: the exposure was performed using an electron beam direct write lithography apparatus, and developed in methyl isobutyl ketone (MIBK) for 75s, and fixed in isopropyl alcohol for 30s, at which time the pattern was transferred onto the resist layer 4. The hot plate postbaking is carried out for 60s at 125 ℃, so that the adhesion of the resist layer 4 is improved.
And 4, step 4: using a metal etching machine, and setting the gas flow ratio of chlorine to oxygen to be 1: rf power was set at 50W and etching was carried out for 130s under the protection of the resist layer 4 to form a patterned metal hard mask 7. The cooled sample was then purged with 100sccm of Ar.
And 5: a first stage of physical etching was performed under the protection of the remaining resist layer 6 and metal hard mask 7 with parameters of argon flow 80sccm, reaction time 400s, ICP power 600W, initial reaction temperature-10 ℃ and cooling with helium purge. After the reaction is finished, the temperature is reduced to-10 ℃, and the second-stage physical chemical etching is executed, wherein the parameters are argon flow rate of 30sccm and CHF3The flow rate is 35sccm, the ICP power is 100W, the reaction time is 375s, and the etched lithium niobate thin film 8 is formed after the reaction is finished and has the thickness of 300 nm.
Step 6: the chip was washed with N-methylpyrrolidone, acetone, and isopropanol in that order, and the organic resist residue was washed away. And (3) washing away the residual chromium metal mask by using a chromium cleaning solution prepared from ammonium ceric nitrate and nitric acid, and cleaning by using deionized water to obtain the lithium niobate inverted cone-shaped waveguide structure with high precision (100 nm-level resolution), low pollution (no significant mask residue) and low sidewall roughness (10 nm-level).
In summary, the link of etching lithium niobate in the embodiment of the invention is lower for a small mask, and when the lithium niobate is etched, the argon plasma is also used in addition to the fluorine-based plasma, so that the falling of the lithium fluoride deposit is facilitated, the surface roughness is reduced, and the method has the advantages of high processing precision, high etching selection ratio, good structure depth-to-width ratio, low structure surface pollution level and the like, and can be used for manufacturing photonic chips on a lithium niobate film with structures such as ridge waveguides, micro-ring resonant cavities and the like with high precision, low roughness and high light constraint force.
The foregoing description of specific embodiments of the present invention has been presented. It is to be understood that the present invention is not limited to the specific embodiments described above, and that various changes and modifications may be made by one skilled in the art within the scope of the appended claims without departing from the spirit of the invention. The above-described preferred features may be used in any combination without conflict with each other.
Claims (10)
1. A lithium niobate thin film photonic chip processing method based on a metal hard mask is characterized by comprising the following steps:
s1, depositing a metal chromium layer on the surface of the lithium niobate film by using a vacuum magnetron sputtering technology;
s2, coating an electron beam resist on the metal chromium layer in a spinning mode, and transferring the pattern to be processed to the electron beam resist through electron beam direct writing lithography;
s3, etching the metal chromium layer by adopting a metal etching process based on chlorine, and transferring a preset pattern onto the metal chromium layer to form a metal hard mask;
and S4, under the dual protection of the metal hard mask and the electron beam resist, etching the lithium niobate thin film through reactive ions to realize the processing of the micron-scale photonic chip.
2. The method for processing the lithium niobate thin film photonic chip based on the metal hard mask as claimed in claim 1, wherein in S1, the thickness of the metal chromium layer is 60nm to 100 nm.
3. The method for processing the lithium niobate thin film photonic chip based on the metal hard mask as claimed in claim 1, wherein the electron beam resist is positive resist AR-P6200 or Zep520A, and the baking temperature after spin coating is 120-160 ℃ for 55-70 s by hot plate heating.
4. The method for processing the lithium niobate thin film photonic chip based on the metal hard mask as claimed in claim 1, wherein the metal chromium layer is etched by the metal etching process based on chlorine gas, wherein: the flow ratio of chlorine to oxygen is 1: 5-1: 7.5, and the reaction time is 117-135 s.
5. The method for processing the lithium niobate thin film photonic chip based on the metal hard mask as claimed in claim 1, wherein the lithium niobate thin film is etched by reactive ion using argon and fluorine-based gas under the protection of the metal hard mask, wherein the reactive ion etching process is divided into two stages:
the technological parameters of the first stage are argon with the flow rate of 80 sccm-100 sccm, the reaction time is 400-450 s, and the initial reaction temperature is-10 ℃ or below;
the technological parameters of the second stage are argon with the flow rate of 15 sccm-40 sccm and CHF with the flow rate of 15 sccm-40 sccm3The reaction time is 400 s-480 s, and the initial reaction temperature is below-10 ℃.
6. The method for processing the lithium niobate thin film photonic chip based on the metal hard mask as claimed in claim 5, wherein the reactive ion etching process, wherein:
the average etching rate of the lithium niobate in the first stage is 35 nm/min-42 nm/min;
the average etching rate of the lithium niobate in the second stage is 8 nm/min-12 nm/min.
7. The method for processing the lithium niobate thin film photonic chip based on the metal hard mask as claimed in claim 1, wherein the lithium niobate thin film is etched by reactive ions using argon and fluorine-based gases under the protection of the metal hard mask, wherein:
the control of the surface appearance and the etching effect of the lithium niobate is realized by controlling reaction variables;
the reaction variables include one or more of metal hard mask thickness, reaction gas flow, reaction temperature, ICP power, RIE power, reaction chamber pressure parameters.
8. The method for processing the lithium niobate thin film photonic chip based on the metal hard mask as claimed in claim 7, wherein the control of the surface morphology and the etching effect of the lithium niobate is realized by controlling reaction variables, wherein:
the thickness of the metal hard mask is increased, so that the etching depth of the lithium niobate film can be increased;
the flow of argon is increased, so that the increase of a side wall angle is facilitated;
increasing the flow of fluorine-based gas is beneficial to the reduction of the side wall angle, but increases the adhesion of lithium fluoride;
the reaction temperature is increased, the etching speed can be accelerated, but the surface roughness and the etching selection ratio are reduced;
ICP and RIE power are improved, and the side wall angle is reduced, but the improvement of surface roughness is not facilitated;
the pressure of the reaction cavity is reduced, and the surface roughness can be improved.
9. The method of processing a lithium niobate thin film photonic chip based on a metal hard mask as claimed in claim 7, wherein the reaction variables are controlled, wherein:
under the protection of a 60nm chromium hard mask, processing an optical structure of one of a ridge waveguide, a micro-ring resonant cavity and an inverted cone waveguide, which is obtained on a 600nm thick Z-cut monocrystal lithium niobate thin film on a silicon oxide substrate, wherein the height of the optical structure is 400 nm.
10. A lithium niobate thin film photonic chip prepared by the method of any one of claims 1 to 9, wherein the chip is a lithium niobate thin film ridge waveguide or a micro-ring resonator or an inverted cone waveguide.
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