Method for manufacturing aluminum nitride optical waveguide and aluminum nitride optical waveguide

Abstract

The invention relates to a method for manufacturing an optical waveguide. The method comprises the following steps: providing a substrate; depositing an aluminum nitride waveguide core on the substrate; and arranging a cladding layer on the deposited aluminum nitride waveguide core. The arrangement comprises at least one processing step during which the deposited aluminum nitride waveguide core is subjected to a given maximum temperature. The method is characterized in that depositing the aluminum nitride waveguide core comprises forming nanocrystallites in the aluminum nitride waveguide core, and in that the aluminum nitride waveguide core will need to have a temperature for significantly increasing the size of the formed nanocrystallites above the given maximum temperature during the placement of the cladding layer, wherein the given maximum temperature is about 800 degrees Celsius or higher.

Description

Technical Field

[0001] This application relates to a method for manufacturing an optical waveguide. This application also relates to an optical waveguide preferably manufactured by this method. Background Art

[0002] Integrated photonics has become ubiquitous, and its development has surpassed mature microelectronic technologies by providing superior performance in a wide range of applications. In particular, with the realization of ultra-low loss waveguides, a large number of applications of photonic integrated circuits can be explored, especially in the fields of quantum computing, microwave photonics, biosensing, and nonlinear sources. Among the materials used for integrated photonics, alumina (Al2O3) has emerged as a promising platform material due to its large transparent window, low propagation loss, and high rare earth solubility.

[0003] Another material under investigation is aluminum nitride (AlN), partly because its transparent window covers a larger UV spectrum (e.g., as low as 150 nm, compared to 200 nm for alumina).

[0004] Low losses of 0.04 ± 0.02 dB / cm in the C-band have been demonstrated in amorphous Al2O3 planar slab waveguides deposited by atomic layer deposition (ALD). For near-UV applications, losses below 3 dB / cm have been demonstrated in high confinement single transverse mode ALD-grown and fully etched Al2O3 waveguides. These results demonstrate the wide wavelength range available for Al2O3-based integrated photon platforms.

[0005] When compared with layers of Al2O3 grown using reactive sputtering, the main drawback of ALD-grown Al2O3 layers is the difference in the order of magnitude of the deposition rate. The slab losses of sputtered Al2O3 reported so far are as low as 0.1 dB / cm, and the losses of fully etched waveguides in the C-band are below 0.2 dB / cm. Therefore, it is necessary to reduce the losses of sputtered Al2O3 in order to compete with ALD-grown Al2O3 and more mature silicon nitride-based platforms, which have been shown to have losses as low as 1 dB / m at 1550 nm in high confinement waveguides.

[0006] It is known from the art that a waveguide can be manufactured by providing a substrate, depositing an alumina waveguide core 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.

[0007] It is known from the present art that optical waveguides having an alumina waveguide core made of amorphous alumina tend to exhibit high optical performance and / or low losses. At the same time, it is known that when the deposited amorphous alumina waveguide core is subjected to high temperatures (such as those occurring during annealing), the optical performance deteriorates significantly.

[0008] Those skilled in the art will appreciate that the optical losses in an optical waveguide are determined not only by the losses in the waveguide core but also by the losses in the cladding.

[0009] The optical losses in the cladding can vary between different materials. For example, tetraethyl orthosilicate (TEOS) is known to exhibit relatively low optical losses. However, in order to fabricate such a TEOS cladding, a high-temperature annealing step is typically required. Thus, the problem that arises when combining a TEOS cladding with an amorphous alumina waveguide core is that the loss of the optical performance of the amorphous alumina waveguide core offsets the benefit of the optical quality of the TEOS cladding due to the high temperatures involved in using the TEOS cladding on the amorphous alumina waveguide core.

[0010] There may also be other types of claddings that will be disposed on the deposited alumina waveguide core and require processing steps during which the alumina waveguide core is subjected to high temperatures, resulting in the aforementioned loss of the optical performance of the alumina waveguide core. Hereinafter, a cladding that requires a high-temperature step to be performed at 800 degrees Celsius or higher during or after deposition is referred to as a high-temperature cladding.

[0011] Therefore, there is a problem when combining known alumina waveguide cores with high-temperature claddings. This problem prevents further reduction of the optical losses of the alumina waveguide core. Thus, high-temperature claddings cannot be combined with known amorphous alumina waveguide cores, even though it would seem advantageous to use them in combination because both of these components individually exhibit high optical performance and / or low losses.

[0012] For example, this problem is introduced in the paper "Low-loss integrated photonics for the blue and ultraviolet regime" by WEST, Gavin N. et al., Apl Photonics, 2019, 4.2: 026101. This paper discusses the amorphous alumina film deposited by atomic layer deposition. The alumina film is grown on a bare silicon substrate or on a thick (3.2 μm) silica (also known as thermal oxide) on silicon at a growth temperature of 300 degrees Celsius. For wavelengths of 633 nm and 405 nm, the optical loss in the film was measured using prism coupling (Metricon). Before annealing, the measured optical loss was less than 0.3 decibels per centimeter (dB / cm) or 30 decibels per meter (dB / m).

[0013] The authors pointed out that high-temperature annealing leads to the formation of a dense polycrystalline γ-phase Al2O3. The high temperature is above 800 degrees Celsius. The disclosed alumina film is polycrystalline after annealing at 900 degrees Celsius and 1100 degrees Celsius, and is said to exhibit an optical loss greater than 20 dB / cm or 2000 dB / m.

[0014] Although this problem is illustrated here for an alumina waveguide core, the applicant has found that the same or at least very similar behaviors and problems also exist when manufacturing an aluminum nitride (AlN) waveguide core. Summary of the Invention

[0015] The object of the present invention is to provide a method for manufacturing an optical waveguide including an aluminum nitride waveguide core and a high-temperature cladding layer, which at least partially solves at least one of the above problems.

[0016] According to the present invention, this object is achieved by the method defined in claim 1, which is characterized in that depositing the aluminum nitride waveguide core includes forming nanocrystals in the aluminum nitride waveguide core, wherein during the arrangement of the cladding layer, the aluminum nitride waveguide core will need to have a temperature for significantly increasing the size of the formed nanocrystals exceeding a given maximum temperature, and the given maximum temperature is about 800 degrees Celsius or higher.

[0017] In some embodiments, when the size increases by about 100% or more, the size increase is considered significant. At the same time, any increase in size is harmful, so in a preferred embodiment, when the increase is between about 100% and about 50%, the increase is significant.

[0018] The Applicant has found that by deliberately allowing the formation of nanocrystals during deposition, an aluminum nitride waveguide core having optical properties similar to those of a waveguide core made of amorphous material can be fabricated, while being able to maintain said optical properties until a higher temperature during subsequent processing steps.

[0019] The Applicant has found that Rayleigh scattering plays an important role in the deposited aluminum nitride layer. First, Rayleigh scattering may occur due to differences in the dielectric constant within the aluminum nitride layer. Second, Rayleigh scattering may occur when microcrystals are formed that are at least comparable in size to the wavelength of light in the layer. When the microcrystals have even larger sizes, other optical scattering sources become relevant.

[0020] When deposited at a relatively low temperature, no significant crystallization occurs, but rather an amorphous layer is obtained, and the optical loss of such a layer will be relatively low. However, when such an amorphous aluminum nitride layer is subsequently subjected to a high temperature (e.g., 800 degrees Celsius and above), crystallization will occur. In the amorphous material, microcrystals will begin to grow. Since these microcrystals grow in the amorphous material, they can grow freely, for example, the amorphous material surrounding the microcrystals can be absorbed into the microcrystal with relatively little energy and in each direction. The resulting microcrystals will be relatively large and have dielectric properties different from those of the remaining amorphous material. Therefore, optical scattering including Rayleigh scattering will occur, and the optical properties will deteriorate.

[0021] When deposited at a suitably high temperature, crystallization occurs and microcrystals are formed in the additional amorphous layer. In this case, there will also be local differences in the dielectric constant between the amorphous part and the nanocrystalline part of the layer. Rayleigh scattering associated with these differences will result in a reduction in optical properties. In addition, when the layer is subjected to a high temperature, such as during an annealing step, the nanocrystals present will grow by absorbing the amorphous material surrounding them, thus resulting in relatively large microcrystals and an increase in optical scattering losses.

[0022] When deposited at a relatively high temperature, crystallization will occur to the extent that almost the entire deposited layer is filled with nanocrystals. In this case, local differences in the dielectric constant will be largely absent because the amount of remaining amorphous aluminum nitride is small. The optical scattering in such a layer is relatively low.

[0023] Without being bound by theory, the Applicant notes that the energy required to transform the above-mentioned aluminum nitride layer having a large number of relatively small microcrystals into a layer having a small number of relatively large microcrystals is too high to be reached during the processing steps required for arranging the cladding layer. It can be confirmed that such a waveguide core has been achieved by subjecting the waveguide core to a given maximum temperature. No significant change in optical properties will occur because very little crystal growth will occur within the layer.

[0024] By deliberately allowing the formation of nanocrystals, there is little or no amorphous material present. This means that there are smaller local differences in dielectric properties and thus less optical scattering. Also, the nanocrystals absorb and grow less amorphous material. Thus, in an aluminum nitride waveguide core deposited according to the present invention, existing nanocrystals cannot grow easily and freely, thus significantly limiting their growth and avoiding the aforementioned optical scattering.

[0025] Without being bound by theory, the applicant believes that the energy required for the size growth of nanocrystals mainly surrounded by other nanocrystals is greater than the energy required for the size growth of nanocrystals mainly surrounded by amorphous material. This is another reason why when depositing an aluminum nitride waveguide core such that nanocrystals are formed, the formation of larger microcrystals during subsequent processing steps can be prevented or at least limited. This allows a high-temperature cladding layer to be disposed on the aluminum nitride waveguide core without significantly degrading the optical properties of the aluminum nitride waveguide core. Thus, waveguides having an aluminum nitride waveguide core can be manufactured that exhibit improved optical properties and / or reduced losses.

[0026] The size of the nanocrystals formed during the deposition process strongly affects the amount of optical scattering. The applicant has found that favorable optical properties of the optical waveguide can be obtained by forming nanocrystals such that a given maximum temperature lies within the range between 800 degrees Celsius and 1400 degrees Celsius. A maximum temperature below 800 degrees Celsius will indicate that a relatively large amount of amorphous aluminum nitride is obtained after depositing the waveguide core, and this material will transform into large grains when the cladding layer is disposed. On the other hand, a maximum temperature above 1400 degrees Celsius will pose a risk of degrading the aluminum nitride waveguide core or other components included in the waveguide including the aluminum nitride waveguide core.

[0027] It should be noted that the given maximum temperature is more preferably within the range between 1000 degrees Celsius and 1300 degrees Celsius, and more preferably about 1150 degrees Celsius.

[0028] Disposing the cladding layer may include depositing a cladding layer on the aluminum nitride waveguide core, and the at least one processing step may include annealing the combination of the substrate, the deposited aluminum nitride waveguide core, and the deposited cladding layer at a given maximum temperature.

[0029] After deposition and before annealing, the size of the nanocrystals in the aluminum nitride waveguide core can be between 1 nanometer and 30 nanometers, and preferably between 1 nanometer and about 10 nanometers. In addition, after deposition and before annealing, the nanocrystals in the aluminum nitride waveguide core can account for at least 40 wt% of the aluminum nitride waveguide core, preferably at least 70 wt%, more preferably at least 90 wt%. Additionally or alternatively, the size of the nanocrystals in the aluminum nitride waveguide core after annealing can be between 1 nanometer and 30 nanometers, and preferably between 1 nanometer and 10 nanometers. In addition, the nanocrystals in the aluminum nitride waveguide core can account for at least 50 wt% of the aluminum nitride waveguide core after annealing, preferably at least 75 wt%, more preferably at least 99 wt%.

[0030] The at least one processing step can include depositing a cladding layer on the aluminum nitride waveguide core at a given maximum temperature. In this embodiment, a high-temperature heating step is applied during the deposition of the cladding layer itself. After depositing the cladding layer, the size of the nanocrystals in the aluminum nitride waveguide core can be between 1 nanometer and 30 nanometers, and preferably between 1 nanometer and 10 nanometers. In addition, the nanocrystals in the aluminum nitride waveguide core can account for at least 50 wt% of the aluminum nitride waveguide core after annealing, preferably at least 75 wt%, more preferably at least 99 wt%.

[0031] Any one of reactive sputter deposition, atomic layer deposition, evaporation, or pulsed laser deposition can be used to deposit the aluminum nitride waveguide core. Other methods of arranging the aluminum nitride waveguide core are not excluded.

[0032] The cladding layer can include a high-temperature cladding layer. Such cladding layers are characterized in that these layers are arranged at a relatively high temperature and / or require a heating step after deposition at a relatively high temperature compared to the temperature at which the aluminum nitride waveguide core is deposited. Examples of such cladding layers are TEOS layers, silicon oxynitride layers, aluminum oxide layers, or polymer layers. Such layers have relatively low optical losses compared to low-temperature cladding layers. Any one of plasma-enhanced chemical vapor deposition, low-pressure chemical vapor deposition, evaporation, sputtering, or atomic layer deposition can be used to arrange the high-temperature cladding layer.

[0033] The substrate can include a silicon substrate, a silicon nitride substrate, a silicon thermal oxide substrate, a quartz substrate, or a sapphire substrate.

[0034] The aluminum nitride waveguide core can have a stoichiometric ratio. Additionally or alternatively, the aluminum oxide waveguide core can include AlxNy, where 0.8 < x < 1.1 and 0.9 < y < 1.2, such as x = 0.9 and y = 1.1, preferably x = 1 and y = 1.

[0035] The optical waveguide can be a planar waveguide or a channel waveguide. Other types of waveguides are not excluded in this application.

[0036] In additional embodiments, depositing an aluminum nitride waveguide core on a substrate may further include one or more of the following steps: for example, using chemical mechanical polishing to reduce the surface roughness of the aluminum nitride waveguide core, and using at least one of lithography and etching to define the shape and / or dimensions of the aluminum nitride waveguide core.

[0037] The method may further include the steps of depositing aluminum nitride on a corresponding substrate at a deposition rate of aluminum nitride under varying substrate temperatures and / or varying substrate biases. For each deposited aluminum nitride layer, its optical properties are measured. The method may further include selecting the deposition rate, substrate temperature, and substrate bias used to fabricate the aluminum nitride layer having the best optical properties as the optimal settings. These optimal settings may be used when depositing an aluminum nitride waveguide core for fabricating an optical waveguide as described above.

[0038] According to a second aspect, the present invention provides an aluminum nitride optical waveguide, which includes a substrate, an aluminum nitride waveguide core disposed on the substrate, and a cladding layer disposed on the aluminum nitride waveguide core. According to the present invention, the aluminum nitride waveguide core includes nanocrystals having a size between 1 nanometer and 30 nanometers, preferably between 1 nanometer and 10 nanometers. These nanocrystals account for at least 50% by weight, preferably at least 75% by weight, more preferably at least 99% by weight of the aluminum nitride waveguide core, and the cladding layer includes a high-temperature cladding layer. The high-temperature cladding layer may include at least one of a TEOS layer and a silicon oxynitride layer. Additionally or alternatively, the optical waveguide is a planar waveguide or a channel waveguide. Description of the Drawings

[0039] Next, the present invention will be described with reference to the drawings, where the same reference numerals will be used to refer to the same or similar components, and where:

[0040] Figure 1 A flowchart of a method for manufacturing an optical waveguide according to the present invention is shown;

[0041] Figure 2 A cross-section of a planar waveguide according to the present invention is shown;

[0042] Figure 3 Shows Figure 1 A flowchart of a preferred embodiment of the method of

[0043] Figure 4 An example of a reactive co-sputtering system that can be configured to deposit an alumina waveguide core is shown;

[0044] Figure 5 TEM photographs of the deposited alumina waveguide core at various substrate temperatures are shown;

[0045] Figure 6a And Figure 6bShows AFM photographs of the deposited alumina waveguide core at various substrate temperatures;

[0046] Figure 7 Shows a graph of the deposition temperature of the alumina waveguide core on the X-axis and the measured refractive index on the Y-axis, and for multiple deposition temperatures, a graph of the wavelength of light propagating through the alumina waveguide core deposited at that temperature on the X-axis and the propagation loss measured in that waveguide core on the Y-axis.

[0047] Figure 8a Shows a graph of the waveguide core deposition temperature on the X-axis and the crystallite size formed in the alumina waveguide core on the Y-axis;

[0048] Figure 8b Shows a graph of the waveguide core deposition temperature on the X-axis and the weight percentage of the waveguide core of a specific phase (specifically amorphous (A) or crystalline (C)) on the Y-axis;

[0049] Figure 8c Shows a graph of the waveguide core deposition temperature on the X-axis and the optical loss achieved in the waveguide core on the Y-axis;

[0050] Figure 9a and Figure 9b Both show a graph of the distance that light propagates through the waveguide core on the X-axis and the intensity of the light on the Y-axis. Detailed Description

[0051] Figure 1 Shows a flowchart of an embodiment of a method for manufacturing an optical waveguide according to the present invention, the method including steps S1 to S3. Figure 2 Shows a schematic cross-section of an optical waveguide that can be manufactured, for example, using the method according to the present invention. Figure 3 Shows a flowchart of a preferred embodiment of the method.

[0052] In step S1, a substrate 10 is provided. In the next step S2, an alumina waveguide core 11 is deposited on the substrate. The alumina waveguide core 11 can be deposited directly on the substrate. Embodiments are also conceivable where the alumina waveguide core 11 is deposited on the substrate with other layers therebetween. An exemplary embodiment is the embodiment of stacking waveguide cores. In another exemplary embodiment, the alumina waveguide core 11 is deposited on an alumina layer, which in turn is deposited on the substrate. In another exemplary embodiment, the substrate is a sapphire substrate, preferably a single-crystal sapphire substrate, on which a SiO2 layer is arranged and on which the alumina waveguide core is deposited.

[0053] Specifically, an alumina waveguide 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 for disposing the cladding layer 12, the alumina waveguide core 11 is subjected to a given maximum temperature, which is about 800 degrees Celsius or higher, and preferably between 800 degrees Celsius and 1400 degrees Celsius.

[0054] During the disposition of the cladding layer, the alumina waveguide core would need to have a temperature for significantly increasing the size of the formed nanocrystals that exceeds the given maximum temperature. In other words, the minimum temperature for significantly increasing the size of the formed nanocrystals exceeds the given maximum temperature. Thus, the alumina waveguide core 11 can have the said maximum temperature without a significant increase in the said size.

[0055] Increasing the maximum temperature to more than 1400 degrees Celsius risks deteriorating the alumina waveguide core 11. Those skilled in the art will understand that around 1400 degrees Celsius, the glass transition temperature of alumina begins, so subjecting the waveguide core to this temperature may cause backflow of the structure. Additionally, such a maximum temperature may also deteriorate the performance of other components of the waveguide, depending on the materials used. The substrate 10 can be made of silicon, which can start to melt at around 1400 degrees Celsius. The substrate 10 can also include silica, and when exposed to such temperatures, local density fluctuations may form in the silica, and these local density fluctuations cause scattering, thus reducing performance.

[0056] When the grain size increases by about 100% or more, preferably when it increases between about 100% and about 50%, the increase can be considered significant.

[0057] Now referring to Figure 3 , the step S3 of disposing the cladding layer can include a step S31 of depositing the cladding layer 12 on the alumina waveguide core 11. Step 31 can be a processing step of subjecting the alumina waveguide core 11 to a given maximum temperature. This can be applicable, for example, in embodiments of depositing a polymer-based cladding layer.

[0058] Alternatively, the high-temperature step is not part of depositing the cladding layer, but rather part of a subsequent heating step. In Figure 3 , an example of such a subsequent step is provided as step S32, where the combination of the substrate 10, the deposited alumina waveguide core 11, and the disposed cladding layer 12 is annealed, thereby subjecting the alumina waveguide core 11 to a given maximum temperature. This is applicable, for example, in embodiments of depositing a TEOS cladding layer.

[0059] 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 lithography and etching steps to define channel waveguides or other types of waveguides.

[0060] For any embodiment related to the alumina waveguide core 11 (e.g., method and / or device), alternative embodiments can be considered, where the waveguide core is alternatively made of aluminum nitride (AlN). For example, the embodiments Figures 1 to 3 explained can also be applied to the fabrication of waveguides including an aluminum nitride waveguide core. Those skilled in the art will realize how the following description of the fabrication process of the alumina waveguide core can and / or should be adjusted for the fabrication of an aluminum nitride waveguide core.

[0061] The Al2O3 thin film used in the embodiments of the present invention can be deposited onto a silicon wafer having an 8-micron oxide buffer layer by reactive sputtering. The advantage of reactive sputtering utilized here is due to the energy available to each adsorbed atom on the substrate. The adsorbed atoms landing on the substrate have a high mobility, resulting in a high-density layer morphology that can be obtained at relatively low substrate temperatures and high deposition rates. This allows for the deposition of a high-density amorphous Al2O3 layer at CMOS-compatible wafer temperatures, with a slab waveguide propagation loss below 0.1 dB / cm at 1550 nm.

[0062] Although several low-loss propagation slab waveguide results for Al2O3 optical waveguides have been achieved, the link to the layer morphology has not been demonstrated in the prior art. The present applicant recognizes that, considering the complexity of reproducing the reactive sputtering deposition process, the understanding of the morphology and the corresponding propagation loss can facilitate improved reproducibility and layer quality.

[0063] The morphology of the Al2O3 layer deposited by reactive sputtering is mainly 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 the diffusion constant, which determine the diffusion length of the adsorbed atoms at a given kinetic energy available to each adsorbed atom; and the critical nucleation size, which determines the critical diffusion length required for stable nucleation. Although the material properties are given for the Al2O3 layer, the energy of each adsorbed atom is the ratio of the deposition rate to the total energy contribution during deposition.

[0064] For reactive sputtering, the total energy flux is a linear combination of different contributions. The contributions of the sputtering process to the energy flux towards the substrate can be classified into at least four groups.

[0065] First, the contributions of atoms and molecules that attach to the substrate required to form the Al2O3 layer should be considered. The adsorbed atoms accelerated from the target contribute their kinetic energy when adsorbed onto the substrate. Additionally, when oxygen molecules are adsorbed, their kinetic energy is also contributed. Even if the atoms or molecules are not adsorbed, part of their kinetic energy can still be transferred when they collide with the substrate. In particular, when gas molecules gain energy by colliding with faster ions, this contribution can become significant. Another form of kinetic energy is related to the temperature of the substrate. Furthermore, in addition to the kinetic energy contribution to layer formation, the potential energy released by the exothermic chemical reaction forming Al2O3 is also an important contribution.

[0066] Three other sets of energy contributions are radiation from the plasma, electrons incident on the substrate, and ions accelerated towards the substrate. Note that a substrate bias can be applied. The bias can be increased and / or decreased to increase or decrease the electron and ion bombardment of the substrate.

[0067] All the energy contributions increase the available energy per adsorbed atom and thus affect the layer morphology and the resulting propagation loss.

[0068] In one embodiment, an Al2O3 layer can be deposited on a 10 cm silicon wafer with an 8 - micron - thick thermally oxidized buffer layer using an AJA ATC 1500RF reactive co - sputtering system 100.

[0069] In Figure 4 The system 100 schematically shown in includes a target 101 containing aluminum with a purity of 99.9995% disposed above a cathode 102. Opposite the target 101, a substrate 10 is disposed on an anode 103, which is electrically connected to a chamber 103A. RF power is applied between the anode 103 and the cathode 102. This causes the generation of a plasma 104, in which the supplied Ar atoms 105 are ionized into Ar ions 106 and electrons 107. The Ar ions 106 are accelerated towards the target 101 under the influence of a self - generated DC bias. At the target 101, these Ar ions will collide with Al atoms, resulting in an Al atom stream 108 towards the substrate 10. At the substrate 10, the Al atoms 108 that have been deposited onto the substrate 10 will react with oxygen molecules 109 to form aluminum oxide.

[0070] The main deposition chamber is evacuated to a base pressure of 0.1 micro - torr through an inlet 110 to prevent hydroxide ions from binding to the Al2O3 layer, which cause absorption losses near 750 nm, 970 nm, and 1400 nm.

[0071] To maintain magnetron discharge, a balance needs to be maintained between the rate of secondary electron emission from target 101 under ion bombardment and the rate of electrons 107 escaping from plasma 104. Although the RF power source does not directly apply a DC potential difference between cathode 102 and anode 103, electrons 107 in plasma 104 absorb RF energy significantly more effectively than the heavier argon ions 106. The high electron mobility causes electrons 107 to be collected on the electrodes. The self-generated DC bias is the result of the asymmetry between target 101 and chamber 103A of sputtering system 100.

[0072] A magnetic field is applied using a permanent magnet below the target to increase the electron density in plasma 104, thereby increasing the argon ionization rate and reducing the required discharge voltage. In addition, this magnetic field greatly increases the sputtering yield by enhancing ionization and thus the bombardment rate of the target.

[0073] Sputtering system 100 also includes heating means, such as infrared heater 111, which is used to directly heat substrate 10 or heat the substrate through anode 103 on which substrate 10 is disposed.

[0074] Exemplary process conditions for depositing an alumina waveguide core are listed in the following table.

[0075]

[0076] Considering the dependence of layer morphology on the available energy of each adsorbed atom, the substrate temperature can be varied to change the available energy of each adsorbed atom, as this parameter is almost independent of other parameters in this process. This enables us to study how the layer morphology changes with substrate temperature and the corresponding optical propagation loss in the layer.

[0077] The substrate temperature is the set temperature measured on the substrate holder and thus is not the exact temperature of the substrate. Calibration of the substrate temperature as a function of the set temperature can be provided.

[0078] Figure 5 TEM photographs of the deposited alumina waveguide layer at various substrate temperatures are shown. The morphology of the layer is amorphous at the lowest selected temperature of 420 degrees Celsius. When the substrate temperature is increased to 460 degrees Celsius, nanocrystals begin to form, and their density increases significantly at 500 degrees Celsius and 540 degrees Celsius.

[0079] When the temperature increases from 500 °C to 580 °C, the surface roughness increases with increasing waviness. Although the deposited layer at 580 °C still has surface ripples, an obvious transition has occurred from an amorphous layer mainly composed of nanocrystals to a mainly polycrystalline morphology. The ripples disappear starting from a temperature of 620 °C, as a gradually increasing polycrystalline morphology with a slightly columnar growth profile is observed. When the temperature is further increased to 700 °C, no significant difference in morphology is observed.

[0080] Figure 6a and Figure 6b shows the AFM measurement results of the deposited alumina waveguide layer at various substrate temperatures. The AFM measurements show that ripples appear at 500 °C ( Figure 6a , bottom left), their amplitude increases at 540 °C ( Figure 6a , bottom right), and decreases and disappears at 620 °C ( Figure 6b , top right). In addition to the ripples, the layers grown at temperatures from 500 °C ( Figure 6a , bottom left) to 580 °C ( Figure 6b , top left) exhibit reduced refractive index and thickness uniformity.

[0081] Figure 7 The lower part specifically shows the refractive index of the alumina layer measured at 1550 nm by ellipsometry for different deposition temperatures.

[0082] The optical propagation losses of each alumina layer were studied using a Metricon 2010 / M41 with a fiber loss module. These losses are shown in the upper part of the figure. Apparently, for the layers grown at a substrate temperature of 700 °C, the losses decrease with increasing deposition temperature, dropping to 1.57 dB / cm at 377 nm and 0.84 dB / cm at 403 nm.

[0083] In an alternative embodiment, when depositing an aluminum nitride waveguide core, the exact deposition temperature may be different from those mentioned with respect to Figure 5 and FIG. 6. However, the applicant has indeed found that the same behavior occurs. When depositing at varying temperatures, the same morphological changes and / or the same trends in refractive index and thickness uniformity can be observed in the aluminum nitride waveguide core. The teachings derived from Figures 4 to 7 are applicable to the aluminum nitride waveguide core embodiment although they are explained based on the alumina waveguide core embodiment.

[0084] Figures 8a to 8cEach shows a graph of the deposition temperature during the deposition of the alumina waveguide core 11 versus various properties of the core 11. The deposition temperature is shown on the X-axis of each of these graphs. Those skilled in the art will understand that the precise temperature value for depositing a particular waveguide core depends to a large extent on the machine. For example, the "deposition temperature" value given by the machine may deviate from the actual temperature of the waveguide core (which is actually difficult to know). However, considering a particular machine and the achievable deposition rate, a temperature scan can be performed to determine the precise temperature value at which the behavior shown in Figures 8a to 8c occurs.

[0085] In the art, there is a clear preference for fabricating dense amorphous Al2O3 waveguide cores because these cores exhibit lower optical losses. In Figures 8a to 8c , the deposition temperature at which such a waveguide is achieved is referred to as temperature P1. If the deposition temperature is decreased starting from P1, the deposited layer will become less dense and there will be voids in the amorphous alumina. The voids can act as scattering objects and may cause losses. If the deposition temperature is increased starting from P1, nanocrystals will form in the amorphous material. The nanocrystals can act as scattering objects and may cause losses.

[0086] As Figures 8a to 8c shown, P1 is a local minimum where a balance is achieved between reducing the amount of voids and preventing the formation of nanocrystals. At P1, relatively low losses can be achieved. Known alumina optical waveguides are based on alumina layers deposited at a temperature corresponding to P1. However, such layers have the aforementioned drawback of being susceptible to high-temperature processing steps after the deposition of the alumina layer.

[0087] The applicant recognizes that in alumina optical waveguides deposited at deposition temperatures within the range near P1, the maximum loss is caused by local differences in dielectric properties. Both voids and microcrystals have dielectric properties different from those of amorphous alumina. More voids and / or more microcrystals mean that such local differences occur more frequently and the optical loss will increase.

[0088] The applicant also recognizes that such local differences will occur most frequently when the alumina layer contains a comparable amount of amorphous material and nanocrystals, and thus the losses caused thereby may be at their highest level. This can be seen, for example, in Figure 8b , where line A describes how much alumina (expressed as a weight percentage) in the layer is in the amorphous phase and line C describes how much alumina (expressed as a weight percentage) in the layer is in the 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 metrics.

[0089] The applicant has also recognized that when increasing the deposition temperature, nanocrystals will surpass amorphous materials as the main material in the deposited layer at some temperature. As the number of discontinuities between amorphous aluminum oxide and crystalline aluminum oxide decreases due to the reduction in the amount of amorphous aluminum oxide, a decrease in optical loss can be observed above temperature P2. Figure 8b In the example, the temperature P2 is selected as the temperature at which the contents of amorphous aluminum oxide and crystalline aluminum oxide are the same for illustrative purposes only.

[0090] The reduction in optical loss continues until the deposition temperature at which substantially all of the aluminum oxide in the waveguide core is in nanocrystalline form and little to no aluminum oxide is in the amorphous phase. This point may be referred to as P3. Figure 8c Also indicated is a rectangle R showing the Figure 7 The temperature changes in the upper part correspond to the temperature range.

[0091] In other words, for deposition temperatures between P2 and P3, the occasional amorphous material in the originally nanocrystalline aluminum oxide can be considered to be responsible for the scattering. Therefore, when the deposition temperature is further increased, less and / or smaller volumes of amorphous material are formed, local differences occur less frequently, and losses are reduced.

[0092] The applicant also realized that although more and more aluminum oxide takes on the shape of nanocrystals in the range of P1 to P3, the size of individual nanocrystals does not increase significantly. This is conceptually reflected in Figure 8a Only when a deposition temperature higher than P3 is used does the size of the individual crystallites increase significantly, while the percentage of aluminum oxide contained in the crystallites itself remains unchanged. It should be noted that for deposition temperatures below P1, no size is indicated, since nanocrystallites are almost absent. Raising the temperature above P3 will result in the nanocrystallites combining into large crystallites. These relatively large crystallites cause an increase in optical scattering and thus increase the optical losses in the layer. Therefore, a local minimum of the losses can be observed at temperature P3, which is comparable to the minimum at temperature P1. However, unlike the deposited aluminum oxide layer deposited at temperature P1, the aluminum oxide layer deposited at temperature P3 is significantly less sensitive to subsequent heating steps, such as annealing steps for treating the deposited coating layer.

[0093] The morphology obtained at the deposition temperature P3 can also be described as polycrystalline aluminum oxide full of nanocrystallites. Here, nanocrystallites refer to crystallites having relatively small sizes compared to the waveguide of the light to be passed through the optical waveguide. This morphology is advantageous because:

[0094] a) Nanocrystallites are so small that they do not cause significant Rayleigh scattering due to their size;

[0095] b) Filling the core with nanocrystals ensures that there are few fluctuations in the dielectric properties throughout the core, thereby also limiting Rayleigh scattering;

[0096] c) Filling the core with nanocrystals also means that the existing nanocrystals have little or no amorphous material around them to absorb and grow;

[0097] d) Growth that occurs due to the alignment of nanocrystals with each other to form a single larger microcrystal only occurs at much higher temperatures.

[0098] Figures 8a to 8c The graph in [reference] conceptually shows the deposition of an alumina layer at various substrate temperatures. Although points P1, P2, and P3 are indicated in each figure, for Figures 8a to 8c each property described in [reference], the associated behavior does not necessarily occur at exactly the same temperature. Additionally, although Figure 8c shows that the local minima at P1 and P3 allow for the same low loss, the specific implementation of the method according to the present invention may result in different levels of loss at local minima P1 and P3 due to various reasons.

[0099] The applicant has found that subjecting the alumina waveguide core 11 in which nanocrystals are formed to a temperature of 800 degrees or higher may even be beneficial and / or reduce losses. This is shown, for example, in Figure 9a and Figure 9b .

[0100] When the alumina waveguide core 11 is exposed to such a temperature, although the growth of the nanocrystals is very limited, the growth will still consume all or at least most of the amorphous alumina that may have been formed during the deposition of the alumina waveguide core 11. After being subjected to the temperature, if there is less amorphous alumina, fewer local differences in the dielectric properties occur, and thus scattering is reduced.

[0101] For Figure 9a , the alumina waveguide core under discussion is deposited such that nanocrystals are formed in the alumina. During the period when the deposited alumina waveguide core is subjected to a temperature of 800 degrees Celsius or higher, no further processing steps are performed. The achieved loss is 1 + / - 0.5 dB / cm.

[0102] For Figure 9b , Figure 9a the alumina waveguide core of [reference] is subjected to a high temperature of approximately 1150 degrees Celsius in a nitrogen environment for approximately 4 hours. The achieved loss is 0.7 + / - 0.2 dB / cm.

[0103] The optical intensity given on the Y-axis is an estimate derived from scattered light measured over the propagation length of the waveguide core. Without being bound by theory, those skilled in the art will understand that due to the limitations of such an estimate, the optical intensity may appear to increase, but it can still be said with certainty that losses do occur. These losses are estimated by fitting the measurement data to a log-linear model using the Maximum Likelihood Estimation Sample Consensus (MLESAC) algorithm. The given error range is determined by fitting different parts of the total propagation. Those skilled in the art will understand that other methods can also be used to estimate the optical intensity within the waveguide core and to derive the average losses from the measurement data.

[0104] In an alternative embodiment, when depositing an aluminum nitride waveguide core, the temperature values of P1, P2, or P3 may be different from those found for an alumina waveguide core, yet the applicant has indeed found that the same behavior occurs. When deposited at varying temperatures, similar phase transitions (e.g., amorphous or (nano)crystalline) of aluminum nitride can be identified in the aluminum nitride waveguide core, similar (nano)crystallite size increases, and conceptually equivalent loss distributions. The teachings explained in Figures 8a to 8c and Figures 9a to 9b Although based on an alumina waveguide core embodiment, can also be applied to an aluminum nitride embodiment.

[0105] Above, the present invention has been explained using detailed embodiments of the present invention. However, it should be apparent to those skilled in the art that various modifications can be made to these embodiments without departing from the scope of the present invention as defined by the appended claims and their equivalents.

Claims

1. A method for manufacturing an optical waveguide, the method comprising: Provide a substrate; Deposit an aluminum nitride waveguide core on the substrate; And Dispose a cladding layer on the deposited aluminum nitride waveguide core, the disposing including at least one processing step during which the deposited aluminum nitride waveguide core is subjected to a given maximum temperature; The method is characterized in that depositing the aluminum nitride waveguide core includes forming nanocrystals in the aluminum nitride waveguide core; Wherein during disposing the cladding layer the aluminum nitride waveguide core will need to have a temperature for significantly increasing the size of the formed nanocrystals exceeding the given maximum temperature, wherein the given maximum temperature is about 800 degrees Celsius or higher; And the method is characterized in that the aluminum oxide waveguide core includes nanocrystals having a size between 1 nanometer and 30 nanometers, preferably between 1 nanometer and 10 nanometers, wherein the nanocrystals account for at least 50 wt%, preferably at least 75 wt%, more preferably at least 99 wt% of the aluminum oxide waveguide core, and wherein the cladding layer includes a high-temperature cladding layer.

2. The method according to claim 1, wherein when the size of the formed nanocrystals increases by about 100% or more, preferably when the increase is between about 100% and about 50%, the increase is significant.

3. The method according to claim 1 or 2, wherein the given maximum temperature is in the range between 800 degrees Celsius and 1400 degrees Celsius, preferably in the range between 1000 degrees Celsius and 1200 degrees Celsius, and more preferably about 1150 degrees Celsius.

4. The method according to claim 1, 2 or 3, wherein arranging the cladding layer includes depositing the cladding layer on the aluminum nitride waveguide core, and wherein the at least one processing step includes annealing the combination of the substrate, the deposited aluminum nitride waveguide core and the deposited cladding layer at the given maximum temperature.

5. The method according to claim 4, wherein after the deposition of the cladding layer and before the annealing, the size of the nanocrystals in the aluminum nitride waveguide core is between 1 nanometer and 30 nanometers, and preferably between 1 nanometer and about 10 nanometers.

6. The method according to claim 5, wherein after the deposition and before the annealing, the nanocrystals in the aluminum nitride waveguide core account for at least 40% by weight of the aluminum nitride waveguide core, preferably at least 70% by weight, more preferably at least 90% by weight.

7. The method according to any one of claims 4, 5 or 6, wherein after the annealing, the size of the nanocrystals in the aluminum nitride waveguide core is between 1 nanometer and 30 nanometers, and preferably between 1 nanometer and 10 nanometers.

8. The method according to claim 7, wherein after the annealing, the nanocrystals in the aluminum nitride waveguide core account for at least 50% by weight of the aluminum nitride waveguide core, preferably at least 75% by weight, more preferably at least 99% by weight.

9. The method according to claim 1, 2 or 3, wherein the at least one processing step includes depositing the cladding layer on the aluminum nitride waveguide core at the given maximum temperature.

10. The method according to claim 9, wherein after depositing the cladding layer, the size of the nanocrystals in the aluminum nitride waveguide core is between 1 nanometer and 30 nanometers, and preferably between 1 nanometer and about 10 nanometers.

11. The method according to claim 9, wherein after depositing the cladding layer, the nanocrystals in the aluminum nitride waveguide core account for at least 50% by weight of the aluminum nitride waveguide core, preferably at least 75% by weight, more preferably at least 99% by weight.

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

13. The method according to any one of the preceding claims, wherein the cladding layer comprises a TEOS layer, a silicon oxynitride layer, an alumina layer, or a polymer layer.

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

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

16. The method according to any one of the preceding claims, wherein the aluminum nitride waveguide core has a stoichiometric ratio, and / or wherein the aluminum nitride waveguide core comprises AlxNy, where 0.8 < x < 1.1 and 0.9 < y < 1.2, such as x = 0.9 and y = 1.1, preferably x = 1 and y = 1.

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

18. The method according to any one of the preceding claims, further comprising: Between depositing the aluminum nitride waveguide core and disposing the cladding layer, for example, chemical mechanical polishing is used to reduce the surface roughness of the aluminum nitride waveguide core.

19. The method according to any one of the preceding claims, further comprising: Before disposing the cladding layer, at least one of lithography and etching, for example, is used to define the shape and / or size of the aluminum nitride waveguide core.

20. The method according to any one of the preceding claims, comprising: Deposit an aluminum nitride layer on a corresponding substrate at a deposition rate of aluminum nitride at a varying substrate temperature and / or a varying substrate bias; For each deposited aluminum nitride layer, measure its optical properties; Select the deposition rate, the substrate temperature, and the substrate bias used for manufacturing the aluminum nitride layer having the best optical properties as the optimal settings; When depositing the aluminum nitride waveguide core for manufacturing an optical waveguide according to any one of the preceding claims, use the optimal settings.

21. An aluminum nitride waveguide preferably manufactured according to any one of the preceding claims, the aluminum nitride waveguide comprising: A substrate; An aluminum nitride waveguide core disposed on the substrate;And A cladding layer disposed on the aluminum nitride waveguide core; The aluminum nitride waveguide is characterized in that the aluminum nitride waveguide core includes nanocrystals having a size between 1 nanometer and 30 nanometers, preferably between 1 nanometer and 10 nanometers, wherein the nanocrystals account for at least 50 wt%, preferably at least 75 wt%, more preferably at least 99 wt% of the aluminum nitride waveguide core, and wherein the cladding layer includes a high-temperature cladding layer.

22. The aluminum nitride optical waveguide according to claim 21, wherein the high-temperature coating layer comprises at least one TEOS layer or silicon oxynitride layer.

23. The aluminum nitride waveguide according to claim 21 or 22, wherein the waveguide is a planar waveguide or a channel waveguide.

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