Method for producing rare earth doped optical waveguide and rare earth doped optical waveguide

Abstract

The invention relates to a method for manufacturing an aluminum oxide optical waveguide doped with rare earth ions. The present application further relates to an alumina optical waveguide doped with rare earth ions, preferably manufactured by said method. According to the invention, the method comprises the following steps: providing a substrate; depositing the aluminum oxide waveguide core layer doped with rare earth metal ions on the substrate; and disposing a cladding layer on the deposited waveguide core, the disposing comprising at least one processing step during which the deposited waveguide core is subjected to a given maximum temperature. The method is characterized in that depositing the waveguide core comprises forming nanocrystals in the waveguide core. The minimum temperature at which the percentage quenching of the deposited waveguide core is significantly increased exceeds the given maximum temperature, wherein the given maximum temperature is about 400 degrees Celsius or greater.

Description

[0001] This application relates to a method for manufacturing alumina optical waveguides doped with rare earth ions. This application further relates to a rare earth ion-doped alumina optical waveguide preferably manufactured by the aforementioned method.

[0002] Integrated photonics has become ubiquitous, as its development has provided opportunities to enhance applications within mature and advanced microelectronics technologies. In particular, with the realization of ultra-low-loss waveguides, countless applications of photonic integrated circuits can be explored, including quantum computing, microwave photonics, biosensing, and nonlinear sources. Rare-earth-doped media, in particular, are becoming competitive light source platforms for a wide range of applications, from telecommunications, optical detection and ranging, and LiDAR (Light Detection and Ranging) to environmental and biosensing.

[0003] It is known in the art that optical waveguides can be fabricated by: providing a substrate; depositing an alumina waveguide core doped with rare-earth metal ions on the substrate; and disposing a cladding layer on the deposited alumina waveguide core. Here, it should be noted that disposing the cladding layer includes at least one processing step during which the deposited alumina waveguide core is subjected to a given maximum temperature. For example, such optical waveguides can be used as part of an optical amplifier.

[0004] Optical signals encompass any situation involving one, several, or consecutive streams of signal photons at a specific wavelength. Optical amplifiers can amplify such signals by providing an environment in which these signal photons can induce stimulated emission, the process by which signal photons interact with excited ions, causing the ions to emit the same photons and return to a relaxed state. Specifically, this environment can be provided by doping a medium such as an optical fiber or waveguide with rare-earth ions (i.e., rare-earth metal ions) and introducing pump energy into the medium to excite the rare-earth ions. This pump energy can be provided using a pump laser, but alternatives are also known.

[0005] Those skilled in the art will recognize that the desired stimulated emission processes do not correspond as well as expected in terms of energy migration and energy transfer upconversion. In energy migration, the signal photon, whether a pristine photon or one generated by stimulated emission, is absorbed by a "relaxed" rare-earth ion, thereby exciting it. In energy transfer upconversion, the signal photon, whether a pristine photon or one generated by stimulated emission, is absorbed by an already excited rare-earth ion, thereby further exciting it. These processes are sometimes collectively referred to as quenching. The degree to which quenching occurs can be expressed using any of a number of properties, including quenching percentage, quenching factor, or quenching rate.

[0006] In this application, the quenching percentage will be taken into account.

[0007] Optical waveguides with rare-earth-doped waveguide cores are known in the art. Among materials for integrated photonics research, alumina (Al₂O₃) is a promising platform material due to its large transparency window, low propagation loss, and high rare-earth solubility. Specifically, optical waveguides comprising waveguide cores made of amorphous alumina are known in the art. These waveguides themselves tend to exhibit high optical performance and / or low loss. However, if the doped waveguide core is exposed to high temperatures after deposition, the quenching percentage increases significantly, making it less suitable for use in optical amplifiers.

[0008] Those skilled in the art will recognize that optical losses in an optical waveguide depend not only on the losses in the waveguide core but also on the losses in the cladding layer.

[0009] Optical losses in the cladding can vary depending on the material. For example, tetraethyl orthosilicate (TEOS) is known to exhibit relatively low optical losses. However, the fabrication of TEOS cladding typically requires a high-temperature annealing step. Therefore, when combining TEOS cladding with an amorphous alumina waveguide core doped with rare-earth metal ions, a problem arises: the quenching percentage of the amorphous alumina waveguide core increases significantly due to the high temperatures involved in using the TEOS cladding, thus mitigating the optical quality benefits of the TEOS cladding.

[0010] Other types of cladding layers may also exist, which are disposed on the deposited alumina waveguide core and require processing steps during which the alumina waveguide core is subjected to high temperatures, resulting in an increase in the aforementioned quenching percentage of the alumina waveguide core. In the following text, cladding layers requiring high-temperature steps at 400 degrees Celsius or higher during or after deposition are referred to as high-temperature cladding layers.

[0011] Therefore, there are problems when combining known alumina waveguide cores doped with rare-earth metal ions with high-temperature cladding. This problem hinders further reductions in the optical loss of the alumina waveguide core. Consequently, high-temperature cladding cannot be combined with known amorphous alumina waveguide cores doped with rare-earth metal ions, even though both components themselves exhibit high optical performance and / or low loss, and therefore their combination seems advantageous.

[0012] For example, this question was raised in the article "Characteristics of Er-doped Al₂O₃ Thin Films Deposited by Reactive Co-sputtering" by MUSA, S. et al., IEEE Journal of Quantum Electronics, 2000, 36.9: 1089-1097. This article discusses Er-doped Al₂O₃ thin films deposited by reactive co-sputtering onto thermally oxidized Si wafers. The films were deposited at a substrate temperature of 400 °C to ensure they were amorphous, and the waveguides were not annealed. A relatively broad emission band with a FWHM of 55 nm near a wavelength of 1533 nm was measured. Based on the gain versus pump power curves, a value below 20 × 10⁻⁶ was obtained. -25 m 3 / s upconversion coefficient.

[0013] The authors point out that high-temperature annealing can cause aggregation in ion-implanted materials, leading to higher energy migration at the center of the waveguide core. Therefore, the authors limited the fabrication of Er:Al2O3 films to low-temperature reactive co-sputtering to avoid Er ion aggregation.

[0014] Optical waveguides comprising waveguide cores fabricated as single crystals are also known in the art. These optical waveguides exhibit lower quenching percentages but are significantly more expensive and difficult to manufacture, meaning they are not easily commercialized.

[0015] The object of the present invention is to provide a method for manufacturing an optical waveguide comprising an alumina waveguide core and a cladding layer, wherein at least one of the above-mentioned problems is solved in part.

[0016] According to the invention, this objective is achieved by the method defined in claim 1, characterized in that the deposition of the alumina waveguide core includes forming nanocrystals in the alumina waveguide core, wherein the minimum temperature at which the quenching percentage of the deposited waveguide core significantly increases exceeds a given maximum temperature, and wherein the given maximum temperature is about 400 degrees Celsius or higher.

[0017] The applicant has discovered that by intentionally allowing the formation of nanocrystals during deposition, an alumina waveguide core can be manufactured with a lower quenching percentage than that of a waveguide core made of amorphous materials, while maintaining the quenching percentage at higher temperatures during subsequent processing steps.

[0018] When deposited at relatively low temperatures, no significant crystallization occurs, resulting in an amorphous layer doped with rare-earth ions and the formation of a certain amount of ion clusters. However, when this amorphous alumina layer is subsequently subjected to high temperatures, such as 400 degrees Celsius and higher, significantly more aggregation occurs. In amorphous materials, rare-earth ions can migrate towards each other, thereby significantly increasing the quenching percentage.

[0019] When deposited at relatively high temperatures, crystallization will occur to the point that almost the entire deposited layer is filled with nanocrystals.

[0020] Without being bound by theory, the applicant points out that the energy required for the migration and aggregation of rare earth ions in the aforementioned alumina layer with a large number of relatively small grains is too high to be achieved during the processing steps required to lay out the cladding layer.

[0021] This waveguide core was confirmed to have been achieved by subjecting it to a given maximum temperature. The quenching percentage did not increase significantly because almost no aggregation occurred within the layer.

[0022] By intentionally allowing the formation of nanocrystals, almost no amorphous material is present. This means that rare earth ions have much lower mobility and are therefore less likely to aggregate. Thus, the applicant overcomes the technical bias that alumina waveguide cores doped with rare earth ions must be made of amorphous alumina and / or that such cores cannot be annealed.

[0023] The applicant has discovered that by forming nanocrystals, a favorable quenching percentage of the optical waveguide can be obtained at a given maximum temperature between approximately 400 and approximately 1400 degrees Celsius. Maximum temperatures below 400 degrees Celsius will result in a relatively large amount of amorphous alumina after waveguide core deposition, thus allowing for relatively high rare-earth ion mobility during cladding layer placement. On the other hand, maximum temperatures above 1400 degrees Celsius carry the risk of alumina waveguide core degradation. Those skilled in the art will understand that the glass transition temperature of alumina begins at around 1400 degrees Celsius, therefore subjecting the alumina waveguide core to this temperature may lead to structural reflow.

[0024] It should be noted that the given maximum temperature is preferably in the range between about 400 and about 1400 degrees Celsius, preferably between about 500 and about 800 degrees Celsius, and more preferably about 550 degrees Celsius.

[0025] In the context of this application, an increase in the quenching percentage can be considered significant when it is about 20% or more, preferably when it is between about 5% and about 20%.

[0026] Depending on the particular cladding used, arranging the cladding can include depositing the cladding on the deposited waveguide core. At least one processing step for achieving a given maximum temperature includes annealing the combination of the substrate, the deposited waveguide core, and the deposited cladding at the given maximum temperature. In such embodiments, after the deposition of the cladding and before the annealing, the quenching percentage of the alumina waveguide core can be between about 5% and about 35%. Additionally or alternatively, the quenching percentage of the waveguide core after the annealing is between about 0% and about 35%, preferably between about 0% and about 5%.

[0027] For some claddings, the at least one processing step includes depositing the cladding on the alumina waveguide core at the given maximum temperature. In such embodiments, after the deposition of the cladding, the quenching percentage of the alumina waveguide core can be between about 0% and about 35%, preferably between about 0% and about 5%.

[0028] Various materials can be used as specific embodiments of rare earth ions. The rare earth metals can be lanthanides such as erbium (Er3+), ytterbium (Yb3+), thulium (Tm3+), and / or neodymium (Nd3+).

[0029] In a preferred embodiment, the alumina is stoichiometric. Those skilled in the art will understand that this depends on the actual implementation and that this may not always be achievable. Thus, for some embodiments, it can be said that the waveguide core contains AlxOy, where 1.5 < x < 2.5 and 2.5 < y < 3.5, such as x = 1.6 and y = 3.4, and preferably x = 2 and y = 3.

[0030] Those skilled in the art will understand that the waveguide core can be grown using various methods such as any one of reactive sputter deposition, atomic layer deposition, evaporation, or pulsed laser deposition.

[0031] Various types of claddings can also be used, such as TEOS layers, silicon oxynitride layers, or polymer layers. Depending on the desired cladding, one of a variety of application methods can be used. That is, any one of plasma-enhanced vapor deposition, low-pressure chemical vapor deposition, evaporation, sputtering, or atomic layer deposition can be used to arrange the cladding.

[0032] Various types of substrates can also be used, such as silicon substrates, silicon thermal oxide substrates, or quartz substrates.

[0033] The optical waveguide can also take any one of a variety of forms, such as a planar waveguide or a channel waveguide.

[0034] In some embodiments, losses in the manufactured waveguide can be further reduced by including, for example, a step of reducing the surface roughness of the alumina waveguide core using chemical mechanical polishing between depositing the alumina waveguide core and arranging the cladding layer in the manufacturing method.

[0035] In some embodiments, the method further includes defining the shape and / or size of the alumina waveguide core using at least one of, for example, photolithography and etching, before laying the cladding layer.

[0036] Those skilled in the art will understand that the precise setup required to manufacture an optical waveguide will always depend on the specific machine used, the batch of resources / materials used, etc. In order to determine the ideal precise setup for providing an optical waveguide according to the invention, in some embodiments, the method further includes: - Deposit rare earth ion-doped alumina layers on the corresponding substrates at different substrate temperatures and / or different substrate biases at the alumina deposition rate. - For each deposited alumina layer, measure its quenching percentage; - Select the deposition rate, substrate temperature, and substrate bias voltage as optimal settings to produce the alumina layer with the lowest quenching percentage; and - The optimal setup is used when depositing the alumina waveguide core to manufacture an optical waveguide according to any of the preceding claims.

[0037] According to another aspect of this application, an optical waveguide core is provided, comprising a substrate, an alumina waveguide core, and a cladding layer. The alumina waveguide core is doped with rare earth metal ions and disposed on the substrate. The cladding layer is disposed on the waveguide core. The alumina waveguide core comprises nanocrystals, and the quenching percentage of the optical waveguide is 5% or less. The cladding layer comprises a high-temperature cladding layer.

[0038] The waveguide core as a whole can be considered nanocrystalline or polycrystalline, depending on the grain size and the proportion of this form in the waveguide core. In a particular embodiment, the size of the nanocrystals can be between about 1 nanometer and about 30 nanometers, preferably between about 1 nanometer and about 10 nanometers. In a particular embodiment, the nanocrystals form at least 50% by weight of the alumina waveguide core, preferably at least 75% by weight, and more preferably at least 99% by weight.

[0039] Various coating layers can be used. For example, a high-temperature coating layer can include at least one of a TEOS layer or a silicon oxynitride layer.

[0040] Optical waveguides can take any of the following forms, including planar waveguides or channel waveguides.

[0041] The optical waveguides discussed above are preferably manufactured using one of the methods mentioned above.

[0042] The invention will now be described with reference to the accompanying drawings, wherein the same reference numerals will be used to refer to the same or similar components, and wherein: Figure 1 A flowchart of a method for manufacturing an optical waveguide according to the present invention is shown; Figure 2 A cross-section of a planar waveguide according to the present invention is shown; Figure 3 It shows Figure 1 A flowchart of a preferred embodiment of the method; Figure 4 An example of a reactive co-sputtering system according to the present invention, which can be configured to deposit an alumina waveguide core, is shown. Figure 5 TEM images of alumina waveguide cores deposited at different substrate temperatures are shown. Figure 6a and 6b AFM images of alumina waveguide cores deposited at different substrate temperatures are shown. Figure 7 The graph shows the relationship between the deposition temperature of the alumina waveguide core on the X-axis and its measured refractive index on the Y-axis, as well as the relationship between the wavelength of light propagating through the alumina waveguide core deposited at several deposition temperatures on the X-axis and the propagation loss measured on the Y-axis.

[0043] Figure 8a The graph shows the relationship between the temperature of the deposited waveguide core on the X-axis and the grain size formed in the alumina waveguide core on the Y-axis. Figure 8b The graph shows the relationship between the temperature of the deposited waveguide core on the X-axis and the weight percentage of the waveguide core in a specific phase (especially amorphous (A) or crystalline (C)) on the Y-axis. Figure 8c The graph shows the relationship between the temperature of the deposited waveguide core on the X-axis and the optical loss achieved in the waveguide core on the Y-axis. Figure 9a and 9b Both diagrams show the relationship between the distance light travels through the waveguide core on the X-axis and the intensity of the light on the Y-axis.

[0044] Figure 10 A test setup for measuring the gain of an optical waveguide is shown.

[0045] In the context of this application, the measure of the amount of quenching that occurs in an optical waveguide, or more specifically, if the waveguide is to be used for optical amplification and / or to be included in an optical amplifier, is a quenching percentage. Those skilled in the art will recognize that this can be derived in a variety of ways.

[0046] In any case, according to a definition followed in the context of this invention, the quenching percentage can refer to the percentage of rare-earth ions that the signal photons cannot reach. This may be because the decay rate constant is too fast.

[0047] This percentage can be obtained by inducing steady-state excitation in the doped waveguide core, i.e., by providing pump energy to the waveguide core until it saturates, and measuring the non-radioactive decay after ceasing the supply of said pump energy. Based on the assumed distribution of the excited states, i.e., known to those skilled in the art and depending on the assumptions of the rare earth ions in question and the total non-radioactive decay measured, the percentage or fraction of rare earth ions that are quenched and not quenched can be derived.

[0048] Figure 1 A flowchart including steps S1-S3 is shown for one embodiment of a method for manufacturing an optical waveguide according to the present invention. Figure 2 A schematic cross-section of an optical waveguide, for example, that can be manufactured using the method according to the invention, is shown. Figure 3 A flowchart illustrating a preferred embodiment of the method is shown.

[0049] In step S1, a substrate 10 is provided. In the next step S2, an alumina waveguide core 11 doped with rare-earth metal ions is deposited on the substrate. Specifically, the alumina core 11 is deposited such that nanocrystals are formed therein. In the next step S3, a cladding layer 12 is disposed on the alumina waveguide core 11. During at least one processing step required to dispose of the cladding layer 12, the alumina waveguide core 11 is subjected to a given maximum temperature, which is about 400 degrees Celsius or higher, and preferably between about 400 degrees Celsius and about 1400 degrees Celsius. The minimum temperature at which the quenching percentage of the deposited waveguide core significantly increases exceeds the given maximum temperature.

[0050] Raising the maximum temperature above approximately 1400 degrees Celsius may risk degrading the alumina waveguide core 11. Those skilled in the art will understand that the glass transition temperature of alumina begins at around 1400 degrees Celsius, so exposing the waveguide core to this temperature may cause structural reflow. Furthermore, depending on the materials used, this maximum temperature may also degrade the performance of other components of the waveguide. The substrate 10 may be made of silicon, which begins to melt at around 1400 degrees Celsius. The substrate 10 may also contain silicon dioxide, where localized density fluctuations may form upon exposure to such temperatures, leading to scattering and thus degrading performance.

[0051] In a particular embodiment, the substrate is made of thermal SiO2 with a thickness of about 6.0 μm, the waveguide core is made of Al2O3:Er3+ with a width of about 1.6 μm and a thickness of 0.78 μm, and the cladding layer is made of CVD SiO2 with a thickness of about 8.0 μm.

[0052] The applicant has discovered that, in addition to alumina, other host materials that allow for the formation of nanocrystals and have sufficiently high rare-earth solubility can be used. One such alternative host material is aluminum nitride. Those skilled in the art will understand how the following manufacturing process specifications for alumina waveguide cores can and / or should be adapted for the manufacture of other host materials, such as aluminum nitride waveguide cores.

[0053] Other rare earth metals can also be used. For example, lanthanides can be used. Some specific examples are erbium (Er). 3+ ), Yb 3+ ), Thulium (Tm 3+ ) and / or neodymium (Nd) 3+ Those skilled in the art will recognize how the following erbium manufacturing process specifications can and / or should be adapted for the manufacture of waveguide cores doped with other rare earth ions.

[0054] The concentration of rare earth ions can be between approximately 0.25 × 10⁻⁶. 20 ions / cm 3 With approximately 1×10 21 ions / cm 3 Any value between, preferably between approximately 0.25 × 10 20 ions / cm 3 With approximately 4×10 20 ions / cm 3 between.

[0055] The method according to the invention ensures that no clusters are formed, or at least very few clusters are formed. A corresponding positive statement is that the method according to the invention provides a waveguide core in which rare earth metal ions are uniformly and / or evenly distributed. However, those skilled in the art will understand that the localization of rare earth ions will never be perfectly uniform or even. However, this is also unnecessary. In the context of this application, uniform or even distribution should be understood as a distribution that exhibits an approximately uniform distribution when the distribution of ions throughout the waveguide core is described as a random process.

[0056] Now for reference Figure 3Step S3, which involves depositing the cladding layer, may include step S31, which involves depositing the cladding layer 12 on the alumina waveguide core 11. Step S31 may be a processing step during which the alumina waveguide core 11 is subjected to a given maximum temperature. For example, this may be applicable to embodiments that deposit polymer-based cladding layers.

[0057] Alternatively, the high-temperature step is not part of the coating deposition, but rather part of a subsequent heating step. Figure 3 The text provides an example of such a subsequent step, such as step S32, in which the combination of the substrate 10, the deposited alumina waveguide core 11, and the arranged cladding layer 12 is annealed, thereby subjecting the alumina waveguide core 11 to a given maximum temperature. This is applicable, for example, to an embodiment where a TEOS cladding layer is deposited.

[0058] It should be noted that various other steps can be performed between the deposition of the alumina waveguide core 11 and the arrangement of the cladding layer 12. For example, the deposited alumina layer can be subjected to chemical mechanical polishing to reduce surface roughness, and to electron beam lithography and reactive etching steps to define a channel waveguide or other types of waveguides.

[0059] The following is for reference. Figures 4 to 7 This paper discusses a first possible implementation scheme for depositing alumina waveguide core 11 to form nanocrystals. Combined with... Figure 8a -c provides a conceptual explanation of the formed nanocrystals.

[0060] Al2O3:Er used in the embodiments of the present invention 3+ Thin films can be deposited onto silicon wafers with an 8-micron oxide buffer layer via reactive sputtering. The advantage of reactive sputtering utilized here is the energy available to each adsorbed atom on the substrate. The adsorbed atoms attached to the substrate exhibit high mobility, resulting in a high-density layer morphology at relatively low substrate temperatures and high deposition rates. This allows for the deposition of high-density Al₂O₃:Er₂ at CMOS-compatible wafer temperatures. 3+ The layer has a planar waveguide propagation loss of less than 0.1 dB / cm at 1550 nm.

[0061] Although the achievement of low quenching Al2O3:Er has been discussed 3+ Several methods exist for optical waveguides, but none have been demonstrated to be related to the morphology of alumina in the prior art. The applicant recognizes that, given the complexity of reproducible reactive sputtering deposition processes, understanding the morphology of alumina can contribute to improved reproducibility and layer quality.

[0062] Al2O3:Er deposited by reactive sputtering 3+The morphology of the layer is primarily determined by the available energy (EPA) of each adsorbed atom and the material properties of the deposited layer. The material properties that determine the morphology are the activation energy and diffusion constant that determine the diffusion length of the adsorbed atom at a given available kinetic energy per atom, and the critical nucleation size that determines the critical diffusion length required for stable nucleation. Although Al2O3:Er 3+ The material properties of the layer are given, but the energy of each adsorbed atom is the ratio of the deposition rate to the total energy contribution during deposition.

[0063] For reactive sputtering, the total energy flux is a linear combination of different contributions. The contribution of the sputtering process to the energy flux toward the substrate can be divided into at least four groups.

[0064] The contributions of atoms and molecules attached to the substrate required for Al2O3 layer formation should be considered first. Adsorbed atoms accelerated from the target contribute their kinetic energy upon adsorption onto the substrate. Additionally, the kinetic energy of adsorbed oxygen molecules also contributes. Even if atoms or molecules are not adsorbed, some of their kinetic energy can still be transferred when they collide with the substrate. This contribution can be particularly significant when gas molecules gain energy through collisions with higher-velocity ions. Another form of kinetic energy is related to the substrate temperature. Furthermore, besides the contribution of kinetic energy to layer formation, the formation of Al2O3:Er 3+ The potential energy released by the exothermic chemical reaction is also an important contribution.

[0065] The other three energy contributions are radiation from the plasma, electrons incident on the substrate, and ions accelerated toward the substrate. It should be noted that a substrate bias voltage can be applied. This bias voltage can be increased and / or decreased to increase or decrease the electron and ion bombardment on the substrate.

[0066] All energy contributions increase the available energy for each adsorbed atom, thus affecting the layer morphology and the resulting propagation losses.

[0067] In this embodiment, an AJA ATC 1500 RF reactive co-sputtering system 100 can be used to deposit Al2O3:Er on a 10 cm silicon wafer with an 8-micron-thick thermal oxide buffer layer. 3+ layer.

[0068] like Figure 4As schematically shown, system 100 includes a target 101 disposed above cathode 102, the target comprising aluminum with a purity of 99.9995%. System 100 further includes a second target 101B (not shown), the second target comprising a rare earth metal, such as erbium with a purity of 99.95% or ytterbium with a purity of 99.9%. The second target may be disposed on the same cathode 102 or on a separate cathode adjacent to said cathode. Opposite to targets 101 and 101B, a substrate 10 is disposed on an anode 103, which is electrically connected to chamber 103A. RF power is applied between anode 103 and cathode 102. The two targets 101 and 101B can be powered by their own RF power supplies, thus having separate bias voltages. This causes the generation of plasma 104, in which supplied Ar atoms 105 are ionized into Ar ions 106 and electrons 107. Ar ions 106 are accelerated toward target 101 under the influence of a self-generated DC bias voltage. At target 101, they will collide with Al atoms, thereby generating an Al atom flow 108 toward substrate 10. At substrate 10, the Al atoms 108 deposited on substrate 10 will react with oxygen molecules 109 to form aluminum oxide. At target 101B, Ar ions 106 collide with Er atoms, thereby generating Er atoms (especially Er) toward substrate 10. 3+ Ion flow.

[0069] The main deposition chamber was evacuated to a baseline pressure of 0.1 μT through inlet 110 to prevent hydroxide ions from incorporating into Al2O3:Er 3+ In the layer, hydroxide ions cause absorption losses near 750 nm, 970 nm and 1400 nm.

[0070] To maintain the magnetron discharge, a balance needs to be maintained between the rate at which secondary electrons emitted from the target 101 under ion bombardment and the rate at which electrons 107 escape from the plasma 104. Although the RF power supply does not directly apply a DC potential difference between the cathode 102 and the anode 103, the electrons 107 in the plasma 104 absorb RF energy much more efficiently than the heavier argon ions 106. This high electron mobility allows the electrons 107 to be collected on the electrodes. The self-generated DC bias is a result of the asymmetry between the target 101 and the chamber 103A of the sputtering system 100.

[0071] A magnetic field is applied using a permanent magnet beneath the target to increase the electron density in plasma 104, thereby increasing the argon ionization rate and reducing the required discharge voltage. Additionally, the magnetic field significantly improves the sputtering rate by increasing ionization, thus enhancing the target bombardment rate.

[0072] The sputtering system 100 further includes a heating device, such as an infrared heater 111, for heating the substrate 10 directly or via an anode 103 on which the substrate 10 is disposed.

[0073] The table below lists exemplary process conditions for depositing alumina waveguide cores.

[0074]

[0075] Given the dependence of layer morphology on the available energy of each adsorbed atom, the substrate temperature can be varied to alter the available energy of each adsorbed atom, as it is largely independent of other parameters in the process. This allows for the study of layer morphology, since the layer morphology varies with the substrate temperature and the corresponding light propagation loss within the layer.

[0076] The substrate temperature is the set temperature measured on the substrate holder and is therefore not the precise temperature of the substrate. Calibration of the substrate temperature, which varies with the set temperature, can be provided.

[0077] Figure 5 TEM images of alumina waveguide layers deposited at different substrate temperatures are shown. At the lowest selected temperature of 420°C, the morphology of the layer is amorphous. As the substrate temperature increases to 460°C, nanocrystals begin to form, and their density increases significantly at 500°C and 540°C.

[0078] As the temperature increased from 500°C to 580°C, the surface roughness increased, with even greater waviness observed. Although the surface waviness of the layer deposited at 580°C remained, a clear transition had occurred from a predominantly amorphous layer with nanocrystals to a predominantly polycrystalline morphology. The waviness began to disappear from a temperature of 620°C as more and more polycrystalline morphologies with slightly columnar growth profiles were observed. No significant difference in morphology was observed as the temperature further increased to 700°C.

[0079] Figure 6a and 6b AFM measurements of alumina waveguide layers deposited at different substrate temperatures are presented. The AFM measurements show that waviness appears at 500 degrees Celsius (…). Figure 6a (bottom left), the amplitude increases at 540 degrees Celsius ( Figure 6a (bottom right), and decreases and disappears at 620 degrees Celsius ( Figure 6b (Top right). Besides the variability, at 500 degrees Celsius ( Figure 6a (bottom left) to 580 degrees Celsius ( Figure 6b Layers grown at temperatures above the left (top left) exhibit reduced refractive index and thickness uniformity.

[0080] Figure 7 The bottom section shows the refractive index of the alumina layer measured at 1550 nm using elliptic polarization at different deposition temperatures.

[0081] The optical propagation loss of each alumina layer was investigated using a Metricon 2010 / M41 with an optical fiber loss module. These losses are shown in the figure above. Clearly, for layers grown at a substrate temperature of 700°C, the loss decreases with increasing deposition temperature, dropping to 1.57 dB / cm at 377 nm and to 0.84 dB / cm at 403 nm.

[0082] In an alternative embodiment, when depositing the aluminum nitride waveguide core, the exact deposition temperature may be related to... Figure 5 Unlike the temperature mentioned in point 6, the applicant did indeed observe the same behavior. The same morphological changes and / or the same trend in refractive index and thickness uniformity were observed in the aluminum nitride waveguide core when deposited at different temperatures. Figure 4-7 The teachings derived herein, while explained based on the alumina waveguide core embodiment, can also be applied to the aluminum nitride waveguide core embodiment.

[0083] Figure 8a -c Each graph shows the relationship between the deposition temperature of the deposited alumina waveguide core 11 and various properties of the core 11. The deposition temperature is shown on the X-axis of each graph. Those skilled in the art will understand that the precise temperature value for depositing a particular waveguide core is highly machine-dependent; for example, the "deposition temperature" value given by the machine may deviate from the actual temperature of the waveguide core, which is practically difficult to know. However, given a specific machine and the achievable deposition rate, temperature scanning can be performed to determine the occurrence of... Figure 8a -c shows the precise temperature value for the behavior shown in the figure.

[0084] In this field, it is clearly preferred to fabricate amorphous Al2O3 waveguide cores doped with rare-earth ions because these cores exhibit low optical loss. Figures 8a-8c In this context, the deposition temperature required to achieve this type of waveguide is referred to as temperature P1. If the deposition temperature is decreased starting from P1, the deposited layer becomes less dense, and voids appear in the amorphous alumina. These voids can act as scattering objects and may cause losses. If the deposition temperature is increased starting from P1, nanocrystals form in the amorphous material. These nanocrystals can also act as scattering objects and may cause losses.

[0085] like Figures 8a-8c As shown, P1 is a local minimum that strikes a balance between reducing porosity and preventing nanocrystal formation. At P1, relatively low loss can be achieved. Known alumina optical waveguides are based on alumina layers deposited at a temperature corresponding to P1. However, such layers suffer from the aforementioned drawback: they are susceptible to the effects of high-temperature processing steps following alumina layer deposition.

[0086] The applicant recognizes that in alumina optical waveguides deposited at deposition temperatures around P1, the greatest loss is caused by localized differences in dielectric properties. Both voids and grains possess dielectric properties different from those of amorphous alumina. More voids and / or more grains mean that these localized differences occur more frequently, and optical losses also increase.

[0087] The applicant further recognizes that this localized variation most frequently occurs when the alumina layer contains a significant amount of amorphous material and nanocrystals, and therefore the resulting losses can be the highest. For example, this can be seen in... Figure 8b As seen in the diagram, line A describes how much alumina (expressed as a weight percentage) in the layer is an amorphous phase, and line C describes how much alumina (expressed as a weight percentage) is a crystalline phase. Those skilled in the art will understand that the amount of amorphous material and / or the number of nanocrystals can also be expressed using other units and / or measures.

[0088] The applicant further realized that as the deposition temperature increases, nanocrystals will surpass amorphous materials at a certain temperature, becoming the dominant material in the deposited layer. Due to the reduced amount of amorphous alumina, the discontinuity between amorphous and crystalline alumina decreases, resulting in a reduction in optical loss observed above temperature P2. Figure 8b In this context, for illustrative purposes only, temperature P2 is chosen to be the temperature at which the amorphous and crystalline alumina contents are the same.

[0089] The reduction in optical loss continues until virtually all the alumina in the waveguide core is in the form of nanocrystals and almost none of the alumina is in the amorphous phase at the deposition temperature. This point can be referred to as P3. Figure 8c The rectangle R is further indicated, which shows the relationship with... Figure 7 The temperature range corresponding to the temperature changes in the above figure.

[0090] In other words, for the deposition temperatures between P2 and P3, the scattering can be attributed to the occasional presence of amorphous material blocks between the nanocrystalline alumina grains. Therefore, as the deposition temperature is further increased, less and / or smaller amorphous material is formed, the frequency of local differences decreases, and losses are reduced.

[0091] The applicant further recognizes that, although an increasing number of alumina particles exhibit a nanocrystal shape within the P1 and P3 ranges, the size of individual nanocrystals has not increased significantly. This is conceptually reflected in... Figure 8aIn this study, only when deposition temperatures above P3 are used can the size of individual grains potentially increase significantly, while the percentage of alumina included in the grains remains constant. It should be noted that for deposition temperatures below P1, no size indication is given because nanocrystals are virtually nonexistent. Raising the temperature above P3 causes nanocrystals to coalesce into larger grains. These relatively large grains lead to increased light scattering, thereby increasing optical losses in the layer. Therefore, at temperature P3, a local minimum in terms of loss can be observed, comparable to the minimum at temperature P1. However, unlike alumina layers deposited at temperature P1, alumina layers deposited at temperature P3 are much less sensitive to subsequent heating steps, such as annealing steps used to process the deposited coating.

[0092] The morphology obtained at deposition temperature P3 can also be described as polycrystalline alumina saturated with nanocrystals. Here, nanocrystals refer to grains that are relatively small in size compared to the waveguide through which light passes. This morphology is advantageous because: a) Nanocrystals are so small that they do not cause significant Rayleigh scattering due to their size; b) The saturation of the core with nanocrystals ensures that the dielectric properties of the entire core are almost unaffected, thus limiting Rayleigh scattering; c) The saturation of the core with nanocrystals also means that there is almost no amorphous material around the nanocrystals that can be absorbed and grown. d) Growth that occurs because nanocrystals align with each other and form a single larger grain only happens at much higher temperatures.

[0093] Figure 8a The diagrams in -c conceptually illustrate the deposition of alumina layers at different substrate temperatures. While each diagram indicates points P1, P2, and P3, the associated behavior may not necessarily be exactly the same. Figure 8a Each property described in -c occurs at the same temperature. Furthermore, although... Figure 8c This indicates that local minima at P1 and P3 allow for the same low loss, but for various reasons, implementations of the method according to the invention may result in different levels of loss at local minima P1 and P3.

[0094] The applicant has discovered that subjecting the alumina waveguide core 11, in which nanocrystals are formed, to temperatures of 550 degrees or higher (e.g., 800 degrees or higher) may even be advantageous and / or reduce losses. The latter, for example, in… Figure 9a and 9b As shown in the image.

[0095] When the alumina waveguide core 11 is exposed to this temperature, although the growth of nanocrystals is very limited, the growth will still consume all or at least most of the amorphous alumina that may have formed during the deposition of the alumina waveguide core 11. After being subjected to this temperature, if there is less amorphous alumina, the frequency of local differences in dielectric properties occurs less, thus reducing scattering.

[0096] Those skilled in the art will understand that it can be confirmed that an alumina waveguide core with nanocrystals can be achieved by depositing an alumina waveguide core free of rare earth ions and subjecting it to a temperature of about 800 degrees or higher (preferably between about 800 and about 1400 degrees). Optical performance will not change significantly because almost no crystal growth occurs within the undoped layer. The increase is considered significant when the grain size increases by about 100% or more, preferably when the increase is between about 100% and about 50%. This must be tested in the absence of rare earth ions, because at temperatures below 800 degrees (e.g., 400 degrees), rare earth ions accumulate in the amorphous material, leading to a decrease in optical performance.

[0097] for Figure 9a The alumina waveguide core under discussion is deposited to form nanocrystals within the alumina. No further processing steps are performed, during which the deposited alumina waveguide core is subjected to temperatures of 800 degrees Celsius or higher. The achieved loss is 1 + / - 0.5 dB / cm.

[0098] for Figure 9b , Figure 9a The alumina waveguide core was subjected to approximately 1150 degrees Celsius for about four hours in a nitrogen atmosphere. The achieved loss was 0.7 + / -0.2 dB / cm.

[0099] The light intensity given on the Y-axis is an estimate derived from scattered light measured over the propagation length of the waveguide core. Those skilled in the art, unwilling to be bound by theory, will understand that, due to the limitations of this estimate, the light intensity may appear to increase, but it can still be said with certainty that losses do occur. These losses are estimated by fitting the measured data to a log-linear model using the maximum likelihood estimator sample consistency MLESAC algorithm. The given error tolerance is determined by fitting different portions of the total propagation. Those skilled in the art will understand that other methods can also be used to estimate the light intensity within the waveguide core and derive the average loss from the measured data.

[0100] In alternative embodiments, the temperature values ​​of P1, P2, or P3 may differ from those found for alumina waveguide cores when depositing aluminum nitride waveguide cores, but the applicant has indeed observed the same behavior. Similar changes in the phase of aluminum nitride (e.g., amorphous or (nano)crystalline), a similar increase in (nano)grain size, and a conceptually comparable loss profile can be identified in aluminum nitride waveguide cores when deposited at different temperatures. Figure 8a The teachings explained in -c and 9a-b, although based on the alumina waveguide core embodiment, can also be applied to the aluminum nitride embodiment.

[0101] The following discusses a second possible implementation scheme for depositing alumina waveguide core 11 to form nanocrystals. Regarding... Figure 8a The conceptual explanation of nanocrystals given by -c also applies to this implementation scheme.

[0102] To deposit the waveguide core, reactive magnetron sputtering was used to deposit an alumina film. A 786 nm thick Al₂O₃:Er₂ film was deposited using an O₂ flow rate of 2.8 sccm, a deposition rate of 3.74 nm / min, and a stage temperature of 760 °C. 3+ The layer has a refractive index of 1.739 at 1030 nm, as measured using variable-angle spectral elliptic polarization (VASE). The erbium concentration of the layer is known to be approximately 3.9 × 10⁻⁶ using a calibrated sputtering power based on Rutherford backscattering measurements (RBS). 20 ions / cm 3 Samples were stored in an N2 environment between manufacturing steps to avoid OH- contamination, a known source of recombination centers in erbium-doped amplifiers. Electron beam lithography (EBL) was used at 1000 μC / cm². 2The dosage of negative resist was used to pattern the waveguide as an etch mask to define the spiral, ring resonator, and straight waveguide for signal enhancement and background loss characterization. Reactive ion etching (RIE) was performed at a chamber pressure of 3 mTorr and an RF power of 25 W, using BCl3 and HBr gas flows of 25 and 10 sccm, respectively, to define the waveguides. Plasma-enhanced chemical vapor deposition (PECVD) was used to deposit SiO2 cladding layers at a stage temperature of 300 °C and a power of 60 W, utilizing a chamber pressure of 650 mTorr and a deposition rate of 37 nm / min, using SiH4 / N2 and N2O, respectively. The chips were diced and annealed in a tube furnace under N2 atmosphere at 550 °C. To reduce coupling loss (ac), the chip sidewalls were polished using a Flex waveguide polisher from KrellTech to reduce scattering loss. The polisher has multiple polishing pads with various roughnesses of 3.0, 1.0 and 0.3 μm, wetted with demineralized water.

[0103] Using this specific instance, Al2O3:Er can be manufactured. 3+ The waveguide has a gain of 33.5 dB at 1532 nm, and the on-chip power of the 12.9 cm amplifier is 475 mW (26.1 dBm).

[0104] For example, this can be achieved by using, such as Figure 10 The setup shown includes a bidirectional pump amplifier using an off-chip WDM and a filter for the spectrum below 1500 nm. In this setup, two signal sources can be used to switch from a low-power signal state to a high-power signal state. The obtained signal can be monitored using an OSA spectrometer. Measurements were taken for different pump and signal powers using helices of varying lengths (see...). Figure 11 It exhibits a peak on-chip gain of 3.5 dB / cm per unit length at 1532 nm in an amplifier with a length of 4.9 cm.

[0105] Reactive sputtering of Al2O3:Er 3+ The erbium concentration in the waveguide amplifier is 3.9 × 10⁻⁶. 20 ions / cm3, and using bidirectional pumping at 1480 nm, a gain of over 30 dB can be achieved at 1532 nm, such as based on Figure 10 The setup shown can be observed. The chip output power shown here exceeds 120 mW, demonstrating the advantages achieved by the optical waveguide core manufactured according to the method of the present invention.

[0106] The present invention typically relates to providing a waveguide core with minimal quenching. This can be achieved according to the method of appended claim 1.

[0107] Quenching can be more or less a result of rare-earth ion aggregation. If most ions are part of a cluster, photons emitted spontaneously are more likely to be emitted within the cluster, meaning they are more likely to interact with other nearby ions within the cluster. Therefore, as an alternative, the method according to the invention may be characterized in that the deposition of the waveguide core includes forming nanocrystals in the waveguide core, wherein the percentage of rare-earth ions contained in the cluster is significantly increased at a minimum temperature exceeding a given maximum temperature, and the given maximum temperature is about 400 degrees Celsius or higher.

[0108] Larger clusters mean that photons emitted by ions due to spontaneous emission are more likely to interact with other nearby ions, even if only because there are more nearby ions in a larger cluster. Therefore, as an alternative, the method according to the invention may be characterized in that the deposition of the waveguide core includes forming nanocrystals in the waveguide core, where the average size of the ion clusters is significantly increased at a minimum temperature exceeding a maximum temperature, and the given maximum temperature is about 400 degrees Celsius or higher.

[0109] In the field of crystallography, clusters are sometimes described based on the number of ions they contain. For example, a monomer refers to an isolated ion in a host material that is at least locally homogeneous. A dimer refers to a cluster of two, and a trimer refers to a cluster of three. Therefore, the absence of a rare-earth ion cluster can be definitively stated as the presence of rare-earth ions being monomers. Therefore, as an alternative, the method according to the invention may be characterized in that the deposition of the waveguide core includes forming nanocrystals in the waveguide core, wherein the minimum temperature at which the number of monomer ions is significantly reduced exceeds a given maximum temperature, and the given maximum temperature is approximately 400 degrees Celsius or higher.

[0110] The invention has been explained above using detailed embodiments. However, it will be apparent to those skilled in the art that various modifications can be made to these embodiments without departing from the scope of the invention, which is defined by the appended claims and their equivalents.

Claims

1. A method for manufacturing an optical waveguide, the method comprising: Provide a base; An alumina waveguide core layer doped with rare earth metal ions is deposited onto the substrate; A cladding layer is arranged on the deposited waveguide core. The feature is that depositing the alumina waveguide core includes forming nanocrystals in the alumina waveguide core, wherein the size of the nanocrystals is between about 1 nanometer and about 30 nanometers, preferably between about 1 nanometer and about 10 nanometers, and wherein the nanocrystals form at least 50% by weight of the alumina waveguide core, preferably at least 75% by weight, and more preferably at least 99% by weight, and wherein the cladding layer is a high-temperature cladding layer.

2. The method of claim 1, wherein arranging the cladding layer comprises at least one processing step, during which the deposited waveguide core is subjected to a given maximum temperature, wherein the given maximum temperature is about 400 degrees Celsius or higher.

3. The method of claim 2, wherein the deposition of the alumina waveguide core is such that the quenching percentage of the deposited waveguide core does not increase significantly due to the subsequent arrangement of the cladding layer.

4. The method of claim 2 or 3, wherein the minimum temperature at which the quenching percentage of the deposited waveguide core significantly increases exceeds the given maximum temperature.

5. The method according to claim 3 or 4, wherein the increase in the quenching percentage is significant when the increase is about 20% or more, preferably when the increase in the quenching percentage is between about 5% and about 20%.

6. The method according to any one of claims 2-5, wherein the given maximum temperature is in the range between about 400 and about 1400 degrees Celsius, preferably between about 500 and about 800 degrees Celsius, and more preferably about 550 degrees Celsius.

7. The method according to any one of claims 2-6, wherein arranging the cladding layer comprises depositing the cladding layer on the deposited waveguide core, and wherein the at least one processing step comprises annealing the combination of the substrate, the deposited waveguide core and the deposited cladding layer at the given maximum temperature.

8. The method of claim 7, wherein the quenching percentage of the alumina waveguide core is between about 5% and about 35% after the deposition of the cladding layer and before the annealing.

9. The method according to claim 7 or 8, wherein the quenching percentage of the waveguide core after the annealing is between about 0% and about 35%, preferably between about 0% and about 5%.

10. The method according to any one of claims 2-6, wherein the at least one processing step comprises depositing the cladding layer on the alumina waveguide core at the given maximum temperature.

11. The method of claim 10, wherein after the deposition of the cladding layer, the quenching percentage of the alumina waveguide core is between about 0% and about 35%, preferably between about 0% and about 5%.

12. The method according to any one of the preceding claims, wherein the rare earth metal is a lanthanide element, such as erbium, preferably Er. 3+ Ytterbium, preferably Yb 3+ Thulium, preferably Tm 3+ ; and / or neodymium, preferably Nd 3+ .

13. The method according to any one of the preceding claims, wherein the alumina is stoichiometric, and / or wherein the waveguide core comprises Al. x O y , where 1.5 < x < 2.5 and 2.5 < y < 3.5, such as x = 1.6 and y = 3.4, and preferably x = 2 and y = 3.

14. The method according to any one of the preceding claims, wherein the waveguide core is grown using any one of reactive sputtering deposition, atomic layer deposition, evaporation, or pulsed laser deposition.

15. The method according to any one of the preceding claims, wherein the coating layer comprises a TEOS layer, a silicon oxynitride layer, or a polymer layer.

16. The method according to any one of the preceding claims, wherein the coating layer is arranged using any one of plasma-enhanced vapor deposition, low-pressure chemical vapor deposition, evaporation, sputtering, or atomic layer deposition.

17. The method according to any one of the preceding claims, wherein the substrate comprises a silicon substrate, a silicon thermal oxide substrate, or a quartz substrate.

18. The method according to any one of the preceding claims, wherein the optical waveguide is a planar waveguide or a channel waveguide.

19. The method according to any one of the preceding claims, further comprising, between depositing the alumina waveguide core and arranging the cladding layer, for example using chemical mechanical polishing to reduce the surface roughness of the alumina waveguide core.

20. The method according to any one of the preceding claims, further comprising defining the shape and / or size of the alumina waveguide core using, for example, at least one of photolithography and etching, prior to arranging the cladding layer.

21. The method according to any one of the preceding claims, comprising: Alumina layers doped with rare earth ions are deposited on the corresponding substrates at different substrate temperatures and / or different substrate biases at the alumina deposition rate. For each deposited alumina layer, its quenching percentage was measured; The deposition rate, substrate temperature, and substrate bias are selected as optimal settings to produce the alumina layer with the lowest quenching percentage. The optimal setup is used when depositing the alumina waveguide core to manufacture an optical waveguide according to any of the preceding claims.

22. An optical waveguide comprising: Base; An alumina waveguide core doped with rare earth metal ions, the alumina waveguide core being disposed on the substrate; and A cladding layer is disposed on the waveguide core; Its features are, The alumina waveguide core comprises nanocrystals, the quenching percentage of the optical waveguide is 5% or lower, and the cladding layer comprises a high-temperature cladding layer. The size of the nanocrystals is between about 1 nanometer and about 30 nanometers, preferably between about 1 nanometer and about 10 nanometers, and the nanocrystals form at least 50% by weight of the alumina waveguide core, preferably at least 75% by weight, and more preferably at least 99% by weight.

23. The optical waveguide of claim 22, wherein the high-temperature cladding layer is a cladding layer in which the process of arranging the high-temperature cladding layer includes at least one processing step, during which the alumina waveguide core is subjected to a temperature equal to or greater than 400 degrees Celsius.

24. The optical waveguide according to any one of claims 22 or 23, wherein the high-temperature cladding layer comprises at least one of a TEOS layer or a silicon oxynitride layer.

25. The optical waveguide according to any one of claims 22, 23 or 24, wherein the waveguide is a planar waveguide or a channel waveguide.

26. The optical waveguide according to any one of claims 22 to 25, wherein the optical waveguide is manufactured by the method according to any one of claims 1 to 21.

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