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

Abstract

This application relates to a method for manufacturing an optical waveguide, the method comprising providing a substrate, depositing an aluminum nitride waveguide core on the substrate, and disposing a cladding layer on the deposited aluminum nitride waveguide core. The disposing comprises at least one processing step in which the deposited aluminum nitride waveguide core is exposed 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 the temperature that the aluminum nitride waveguide core must have during the disposing of the cladding layer to significantly increase the size of the formed nanocrystallites exceeds the given maximum temperature, the given maximum temperature being about 800°C or greater.

Description

[Technical Field]

[0001] The present application relates to a method for manufacturing an optical waveguide, and further to an optical waveguide preferably manufactured by such a method. [Background technology]

[0002] Integrated photonics is becoming ubiquitous as its development opens up opportunities for improved capabilities in applications beyond well-developed microelectronic technologies. In particular, as ultralow-loss waveguides are realized, myriad applications for photonics integrated circuits may be opened up, including quantum computing, microwave photonics, biosensing, and nonlinear sources, among others. Among the materials investigated for integrated photonics, aluminum oxide (Al2O3) has emerged as a promising platform material due to its large transmission window, low propagation loss, and high rare earth solubility.

[0003] Another material that has been investigated is aluminum nitride (AlN), in part due to its transmission window that covers more of the UV spectrum (e.g., up to 150 nanometers compared to 200 nanometers for aluminum oxide).

[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 of less than 3 dB / cm have been demonstrated in highly confined, single transverse mode, ALD-grown and fully etched Al2O3 waveguides. These results demonstrate the broad wavelength range available for Al2O3-based integrated photonics platforms.

[0005] The main drawback of ALD-grown Al2O3 layers is the order of magnitude increase in deposition rate compared to layers grown using reactively sputtered Al2O3. Previously reported slab losses for sputtered Al2O3 are as low as 0.1 dB / cm, and fully etched waveguide losses are known to be less than 0.2 dB / cm at the C-band. Therefore, loss reduction in sputtered Al2O3 is required to compete with both ALD-grown Al2O3 and the more established silicon nitride-based platform, which has demonstrated losses as low as 1 dB / cm at 1550 nm for high-confinement waveguides.

[0006] It is known from the art that a waveguide can be manufactured by providing a substrate, depositing an aluminum oxide waveguide core on the substrate, and arranging a cladding layer on the deposited aluminum oxide waveguide core, where it is noted that arranging the cladding layer includes at least one processing step in which the deposited aluminum oxide waveguide core is exposed to a given maximum temperature.

[0007] It is known in the art that optical waveguides having aluminum oxide waveguide cores made of amorphous aluminum oxide tend to exhibit high optical performance and / or low loss. At the same time, it is known that optical performance degrades significantly when deposited amorphous aluminum oxide waveguide cores are exposed to high temperatures, such as those encountered during annealing.

[0008] Those skilled in the art will recognize that optical loss in an optical waveguide is determined not only by loss in the waveguide core, but also by loss in the cladding layers.

[0009] Optical loss in cladding layers can vary between different materials. For example, tetraethyl orthosilicate (TEOS) is known to exhibit relatively low optical loss. However, fabricating such a TEOS cladding layer typically requires a high-temperature annealing step. Therefore, when combining a TEOS cladding layer with an amorphous aluminum oxide waveguide core, the optical performance of the amorphous aluminum oxide waveguide core is lost due to the high temperatures associated with using a TEOS cladding layer on the amorphous aluminum oxide waveguide core, mitigating the benefits of the optical quality of the TEOS cladding layer.

[0010] There may also be other types of cladding layers placed on the deposited aluminum oxide waveguide core that require processing steps that expose the aluminum oxide waveguide core to high temperatures, resulting in the aforementioned loss in the optical performance of the aluminum oxide waveguide core. Hereinafter, cladding layers that require high temperature steps performed at 800° C. or above during or after deposition will be referred to as high temperature cladding layers.

[0011] Therefore, problems exist when combining known aluminum oxide waveguide cores with high-temperature cladding layers. This problem prevents further reduction of the optical loss of the aluminum oxide waveguide core. As a result, high-temperature cladding layers cannot be combined with known amorphous aluminum oxide waveguide cores, even though both components exhibit high optical performance and / or low loss by themselves and would therefore appear advantageous to use in combination.

[0012] This issue is also seen, for example, in WEST, Gavin N., et al., "Low-loss integrated photonics for the blue and ultraviolet regime," Apl Photonics, 2019, 4.2:026101. This paper discusses amorphous aluminum oxide or alumina films deposited using atomic layer deposition. The alumina films are grown on bare silicon substrates or on thick (3.2 μm) silicon dioxide (also known as thermal oxide) on silicon at a growth temperature of 300°C. Optical loss in the films was measured using a prism coupling method (Metricon) for wavelengths of 633 nanometers and 405 nanometers. Before annealing, optical loss was measured to be less than 0.3 decibels per centimeter (dB / cm) or 30 decibels per meter (dB / m).

[0013] The authors show that high-temperature annealing causes the formation of dense polycrystalline gamma-phase Al2O3. The high temperatures are above 800°C. The disclosed alumina films are said to be polycrystalline and exhibit optical losses above 20 dB / cm or 2000 dB / m after annealing at 900°C and 1100°C.

[0014] And although this problem is illustrated here for aluminum oxide waveguide cores, applicants find that the same, or at least very similar, behavior and problems occur during the manufacture of aluminum nitride (AlN) waveguide cores.

[0015] An optical waveguide similar to at least the preamble of claim 1 is known from the paper "Polycrystalline low-loss aluminum oxide waveguides" by W. Hendriks et al. (SPIE Smart Structures and Materials+Non-Destructive Evaluation and Health Monitoring, 2005, vol. 11689, 5 March 2021, pp. 116890Z-116890Z).

[0016] Further optical waveguides are described in the papers "Rare-earth ion-doped Al2O3 for active integrated photonics" by W. Hendriks et al. (Advances in Physics: X, vol. 6, no. 1, 14 December 2020), "High refractive index low-loss aluminum oxide waveguides" by W. Hendriks et al. (Eur. Conf. Integr. Opt. (I), 2-4, 25 June 2020), "Alumina Thin Films as Optical Waveguides" by Stadler et al. (Journal of the American Ceramic Society, vol. 78, no. 12, 1 December 1995, pp. 3336-3344), and "Microstructure, morphology and their annealing behaviors of alumina films synthesized by ion beam assisted deposition" by Zhang et al. (Nuclear Instruments & Methods in Physics Research, section B: Beam interactions with Materials and Atoms, vol. 206, 1 May 2003, pp. 357-361).

[0017] An optical waveguide at least similar to the preamble of claim 1 is also known from US Pat. No. 6,650,816 (B2). Summary of the Invention

[0018] It is an object of the present invention to provide a method for manufacturing an optical waveguide including an aluminum nitride waveguide core and a high-temperature cladding layer, in which at least one of the above-mentioned problems is at least partially solved.

[0019] According to the invention, this object is achieved by a method according to claim 1, characterized in that depositing an aluminum nitride waveguide core comprises forming nanocrystallites in the aluminum nitride waveguide core, and in that the temperature that the aluminum nitride waveguide core needs to have during the deposition of the cladding layer in order to significantly increase the size of the formed nanocrystallites exceeds a given maximum temperature, the given maximum temperature being equal to or greater than about 800°C.

[0020] In some embodiments, an increase in size is considered significant when it is about 100% or greater. At the same time, any increase in size is detrimental; therefore, in preferred embodiments, an increase is significant when it is between about 100% and about 50%.

[0021] Applicants have discovered that by intentionally forming nanocrystallites during deposition, it is possible to produce aluminum nitride waveguide cores with optical performance similar to those made from amorphous materials, while maintaining that optical performance up to higher temperatures during subsequent processing steps.

[0022] The applicant has found that Rayleigh scattering plays an important role in deposited aluminum nitride layers. First, Rayleigh scattering can occur as a result of differences in the dielectric constant inside the aluminum nitride layer. Second, Rayleigh scattering can occur when crystallites with a size at least comparable to the wavelength of light in the layer form. Another cause of light scattering becomes relevant when the crystallites have a larger size.

[0023] When deposited at relatively low temperatures, no significant crystallization occurs, resulting in an amorphous layer with relatively low optical loss. However, if such an amorphous aluminum nitride layer is subsequently exposed to high temperatures, e.g., 800°C or higher, crystallization occurs. Crystallites begin to grow in the amorphous material. Because these crystallites grow in the amorphous material, they are free to grow; for example, the amorphous material surrounding them can absorb light with relatively little energy and in all directions. The resulting crystallites are relatively large and have different dielectric properties than the rest of the amorphous material. This causes light scattering, including Rayleigh scattering, which degrades optical performance.

[0024] When deposited at moderately high temperatures, crystallization occurs, and crystallites form in the otherwise amorphous layer. In this case, there are also localized differences in dielectric constant between the amorphous and nanocrystalline portions of the layer. Rayleigh scattering associated with these differences leads to degraded optical performance. Furthermore, if this layer is exposed to high temperatures, for example during an annealing step, the existing nanocrystallites will grow by absorbing the amorphous material surrounding them, thus causing larger crystallites and increased light scattering losses.

[0025] Deposition at relatively high temperatures leads to crystallization to the extent that the deposited layer is almost entirely filled with nanocrystallites. In this case, the amount of remaining amorphous aluminum nitride is so small that local variations in the dielectric constant are negligible. Light scattering in such layers is relatively low.

[0026] Without being bound by theory, the applicant notes that the energy required to convert the aluminum nitride layer described above, which has a large amount of relatively small crystallites, into a layer with a small amount of relatively large crystallites is too high to be achieved during the processing steps required to deposit the cladding layer. The achievement of such a waveguide core can be confirmed by subjecting it to a given maximum temperature. Since little crystal growth occurs inside the layer, no significant change in optical performance occurs.

[0027] By intentionally forming nanocrystallites, there is little or no amorphous material. This means there are fewer local differences in dielectric properties and therefore less light scattering. There is also less amorphous material for the nanocrystallites to absorb and grow. Therefore, in aluminum nitride waveguide cores deposited according to the present invention, the existing nanocrystallites cannot grow easily and freely, thereby significantly limiting their growth and avoiding the aforementioned light scattering.

[0028] Without being bound by theory, applicants believe that the energy required to grow the size of nanocrystallites that are primarily surrounded by other nanocrystallites is greater than the energy required to grow the size of nanocrystallites that are primarily surrounded by amorphous material. This is a further reason why, when deposition of an aluminum nitride waveguide core is performed in such a way that nanocrystallites are formed, the formation of larger crystallites during subsequent processing steps can be prevented or at least limited. This allows for the placement of high-temperature cladding layers on the aluminum nitride waveguide core without significantly degrading the optical performance of the aluminum nitride waveguide core. As a result, waveguides having aluminum nitride waveguide cores that exhibit improved optical performance and / or reduced loss can be fabricated.

[0029] The size of the nanocrystallites formed during deposition has a strong influence on the amount of light scattering. Applicants have found that advantageous optical performance of optical waveguides can be obtained by forming the nanocrystallites at a given maximum temperature in the range of 800-1400°C. A maximum temperature lower than 800°C would result in a relatively large amount of amorphous aluminum nitride being available after deposition of the waveguide core, which would convert to large crystallites when the cladding layer is deposited. On the other hand, a maximum temperature higher than 1400°C would risk degradation of the aluminum nitride waveguide core or other components included in the waveguide, including the aluminum nitride waveguide core.

[0030] It is noted that the maximum temperature given is more preferably in the range of 1000-1300°C, more preferably about 1150°C.

[0031] Disposing the cladding layer may include depositing the 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.

[0032] The nanocrystallites in the aluminum nitride waveguide core after deposition and before annealing may be 1 nanometer to 30 nanometers in size, preferably 1 nanometer to about 10 nanometers in size. Furthermore, the nanocrystallites in the aluminum nitride waveguide core after deposition and before annealing may form at least 40% by weight of the aluminum nitride waveguide core, preferably at least 70%, and more preferably at least 90%. Additionally or alternatively, the nanocrystallites in the aluminum nitride waveguide core after annealing may be 1 nanometer to 30 nanometers in size, preferably 1 nanometer to 10 nanometers. Furthermore, the nanocrystallites in the aluminum nitride waveguide core after annealing may form at least 50% by weight of the aluminum nitride waveguide core, preferably at least 75%, and more preferably at least 99%.

[0033] At least one processing step may include depositing a cladding layer on the aluminum nitride waveguide core at a given maximum temperature. In this embodiment, the high-temperature heating step is applied during the deposition of the cladding layer itself. The nanocrystals in the aluminum nitride waveguide core after depositing the cladding layer can be sized from 1 nanometer to 30 nanometers, preferably from 1 nanometer to 10 nanometers. Further, the nanocrystals in the aluminum nitride waveguide core can form at least 50 wt%, preferably at least 75%, more preferably at least 99% of the aluminum nitride waveguide core after annealing.

[0034] The aluminum nitride waveguide core can be deposited using any one of reactive sputter deposition, atomic layer deposition, evaporation, or pulsed laser deposition. Other means of disposing the aluminum nitride waveguide core are not excluded.

[0035] The cladding layer may include a high-temperature cladding layer. Such cladding layers are characterized in that these layers are disposed 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 when compared to low-temperature cladding layers. The high-temperature cladding layer can be disposed using any one of plasma-enhanced evaporation, low-pressure chemical vapor deposition, evaporation, sputtering, or atomic layer deposition.

[0036] The substrate may include a silicon substrate, a silicon nitride substrate, a silicon thermal oxide substrate, a quartz substrate, or a sapphire substrate.

[0037] The aluminum nitride waveguide core can be stoichiometric. Additionally or alternatively, Nitriding The aluminum waveguide core can include AlxNy, where 0.8 < x < 1.1 and 0.9 < y < 1.2, for example, x = 0.9 and y = 1.1, preferably x = 1 and y = 1.

[0038] The optical waveguide may be a slab waveguide or a channel waveguide, although the present application does not exclude other types of waveguides.

[0039] In further embodiments, depositing the aluminum nitride waveguide core on the substrate may further include one or more of the following steps: reducing a surface roughness of the aluminum nitride waveguide core using, for example, chemical mechanical polishing; and defining the shape and / or size of the aluminum nitride waveguide core using at least one of lithography and etching.

[0040] The method may further include depositing aluminum nitride on each substrate at various substrate temperatures and / or various substrate bias voltages at various aluminum nitride deposition rates. For each deposited aluminum nitride layer, its optical performance is measured. The method may further include selecting the deposition rate, substrate temperature, and substrate bias voltage that produced the aluminum nitride layer with the best optical performance as optimal settings. These optimal settings can be used when depositing aluminum nitride waveguide cores to fabricate optical waveguides as described above.

[0041] According to a second aspect, the present invention provides an aluminum nitride optical waveguide comprising 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 comprises nanocrystallites having a size of 1 nanometer to 30 nanometers, preferably 1 nanometer to 10 nanometers. The nanocrystallites form at least 50% by weight of the aluminum nitride waveguide core, preferably at least 75%, and more preferably at least 99%, and the cladding layer comprises a high-temperature cladding layer. The high-temperature cladding layer may comprise at least one of a TEOS layer and a silicon oxynitride layer. Additionally or alternatively, the optical waveguide is a slab waveguide or a channel waveguide. [Brief explanation of the drawings]

[0042] The present invention will now be described with reference to the accompanying drawings, wherein like reference numerals are used to refer to like or similar components. [Figure 1] 2 shows a flow chart of a method for manufacturing an optical waveguide according to the present invention. [Figure 2] 1 shows a cross section of a slab waveguide according to the present invention. [Figure 3] 2 shows a flow chart of a preferred embodiment of the method of FIG. 1; [Figure 4] 1 illustrates an example of a reactive co-sputtering system that can be configured to deposit aluminum oxide waveguide cores. [Figure 5] 1 shows TEM images of deposited aluminum oxide waveguide cores for various substrate temperatures. [Figure 6a] 10 shows AFM pictures of deposited aluminum oxide waveguide cores for various substrate temperatures. [Figure 6b] 10 shows AFM pictures of deposited aluminum oxide waveguide cores for various substrate temperatures. [Figure 7] 1 shows a graph relating the deposition temperature of an aluminum oxide waveguide core on the X-axis to its measured refractive index on the Y-axis, as well as a graph relating, for several deposition temperatures, the wavelength of light propagating through an aluminum oxide waveguide core deposited at that temperature on the X-axis to the measured propagation loss therein on the Y-axis. [Figure 8a] 1 shows a graph relating the temperature at which a waveguide core is deposited on the X-axis to the size of the crystallites formed within said aluminum oxide waveguide core on the Y-axis. [Figure 8b] 1 shows a graph relating the temperature at which the waveguide core is deposited on the X-axis to the weight percentage of the waveguide core that is in a particular phase, specifically amorphous (A) or crystalline (C), on the Y-axis. [Figure 8c] 1 shows a graph relating the temperature at which a waveguide core is deposited on the X-axis to the optical loss achieved in said waveguide core on the Y-axis. [Figure 9a]1 shows a graph relating the distance light has propagated through a waveguide core on the X-axis to the intensity of that light on the Y-axis. [Figure 9b] 1 shows a graph relating the distance light has propagated through a waveguide core on the X-axis to the intensity of that light on the Y-axis. DETAILED DESCRIPTION OF THE INVENTION

[0043] Fig. 1 shows a flow chart including steps S1 to S3 of one embodiment of a method for producing an optical waveguide according to the present invention. Fig. 2 shows a schematic cross-sectional view of an optical waveguide that can be produced, for example, using the method according to the present invention. Fig. 3 shows a flow chart of a preferred embodiment of the method.

[0044] In step S1, a substrate 10 is provided. In the next step S2, an aluminum oxide waveguide core 11 is deposited on the substrate. The aluminum oxide waveguide core 11 can be deposited directly on the substrate. Embodiments are also contemplated in which the aluminum oxide waveguide core 11 is deposited on the substrate with other layers in between. An exemplary embodiment is one in which the waveguide cores are stacked. A further exemplary embodiment is one in which the aluminum oxide waveguide core 11 is deposited on an aluminum oxide layer, which is then deposited on the substrate. A further exemplary embodiment is one in which the substrate is sapphire, preferably a crystalline sapphire substrate, a SiO2 layer is disposed on the substrate, and the aluminum oxide waveguide core is deposited on the SiO2 layer.

[0045] Specifically, the aluminum oxide waveguide core 11 is deposited such that nanocrystallites are formed therein. In a next step S3, a cladding layer 12 is disposed on the aluminum oxide waveguide core 11. During at least one processing step required to deposit the cladding layer 12, the aluminum oxide waveguide core 11 is exposed to a given maximum temperature of about 800°C or higher, preferably between 800°C and 1400°C.

[0046] The temperature that the aluminum oxide waveguide core must have to reach in order to significantly increase the size of the nanocrystallites formed during the deposition of the cladding layer exceeds this given maximum temperature. Alternatively, the minimum temperature at which the size of the formed nanocrystallites increases significantly exceeds the given maximum temperature. Thus, the aluminum oxide waveguide core 11 can have this maximum temperature without causing a significant increase in size.

[0047] Increasing the maximum temperature beyond 1400°C risks degradation of the aluminum oxide waveguide core 11. Those skilled in the art will understand that the glass transition temperature of aluminum oxide begins at approximately 1400°C, and therefore, exposing the waveguide core to this temperature may cause the structure to reflow. Furthermore, such maximum temperatures may also degrade the performance of other components of the waveguide, depending on the materials used. The substrate 10 may be made of silicon, which begins to melt at approximately 1400°C. The substrate 10 may also contain silicon dioxide, and when exposed to such temperatures, local density variations may form, causing scattering and therefore degrading performance.

[0048] The stated increase in crystallite size can be considered significant when it is greater than or equal to about 100%, preferably between about 100% and about 50%.

[0049] 3, the step S3 of disposing a cladding layer may include a step S31 of depositing a cladding layer 12 on the aluminum oxide waveguide core 11. Step S31 may be a processing step in which the aluminum oxide waveguide core 11 is exposed to a given maximum temperature. This may apply, for example, to embodiments in which a polymer-based cladding layer is deposited.

[0050] Alternatively, the high-temperature step is not part of the deposition of the cladding layer, but rather part of a subsequent heating step. An example of such a subsequent step is provided in Figure 3 as step S32, in which the combination of substrate 10, deposited aluminum oxide waveguide core 11, and disposed cladding layer 12 is annealed, thereby exposing aluminum oxide waveguide core 11 to a given maximum temperature. This is true, for example, in embodiments in which a TEOS cladding layer is deposited.

[0051] It should be noted that various other steps can be performed between the deposition of the aluminum oxide waveguide core 11 and the placement of the cladding layer 12. For example, the deposited aluminum oxide layer can be subjected to chemical mechanical polishing to reduce surface roughness, and can be subjected to lithography and etching steps to define channel or other types of waveguides.

[0052] For any embodiment (e.g., method and / or device) relating to an aluminum oxide waveguide core 11, alternative embodiments may be considered in which the waveguide core is instead made of aluminum nitride (AlN). For example, the embodiments described in connection with FIGS. 1-3 may also be applied to fabricate waveguides including aluminum nitride waveguide cores. Those skilled in the art will recognize how the following specifications of a fabrication process for an aluminum oxide waveguide core can and / or should be adapted to fabricate an aluminum nitride waveguide core.

[0053] The Al2O3 thin films used in embodiments of the present invention can be deposited by reactive sputtering on silicon wafers with an 8 micrometer oxide buffer layer. The advantage of reactive sputtering utilized here is a result of the energy available per adatom on the substrate. The adatoms that land on the substrate have high mobility, resulting in a dense layer morphology that can be utilized at relatively low substrate temperatures and high deposition rates. This allows for the deposition of dense amorphous Al2O3 layers at CMOS-compatible wafer temperatures with slab waveguide propagation losses of less than 0.1 dB / cm at 1550 nm.

[0054] While some low-loss propagation slab waveguide results have been achieved for Al2O3 optical waveguides, the prior art has not demonstrated a correlation with layer morphology. Applicants have recognized that, given the complexity of reproducing reactive sputter deposition processes, understanding the morphology and corresponding propagation loss can facilitate improved reproducibility and layer quality.

[0055] The morphology of Al2O3 layers deposited by reactive sputtering is primarily determined by the available energy per adatom (EPA) and the material properties of the deposited layer. The material properties that determine the morphology are the activation energy and diffusion constant, which determine the adatom diffusion length at a given kinetic energy available per adatom, and the critical nucleation dimension, which determines the critical diffusion length required for stable nucleation. While the material properties are given for Al2O3 layers, the energy per adatom is the ratio of the deposition rate to the total energy contribution during deposition.

[0056] In 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 divided into at least four groups:

[0057] The contribution of atoms and molecules adsorbed onto the substrate, which is necessary for the formation of the Al2O3 layer, must first be considered. Adatoms accelerated from the target contribute their kinetic energy when they adsorb onto the substrate. Oxygen molecules also contribute their kinetic energy when they adsorb. Even if atoms or molecules are not adsorbed, a portion of their kinetic energy can still be transferred when colliding with the substrate. This contribution can be significant, especially when gas molecules are excited by collisions with faster ions. Another form of kinetic energy is related to the temperature of the substrate. In addition to the contribution of kinetic energy to layer formation, the potential energy released by the exothermic chemical reaction that forms Al2O3 is also a significant contributor.

[0058] The other three groups of energy contributions are radiation from the plasma, electrons incident on the substrate, and ions accelerated towards the substrate. Note that a substrate bias may be applied. Increasing and / or decreasing the bias can be done to increase or decrease electron and ion bombardment on the substrate.

[0059] All energy contributions add to the available energy per adatom and therefore affect the layer morphology and the resulting propagation loss.

[0060] In one embodiment, the Al2O3 layer can be deposited using an AJA ATC 1500 RF reactive co-sputtering system 100 on a 10 centimeter silicon wafer with an 8 micrometer thick thermal oxide buffer layer.

[0061] The system 100, shown schematically in FIG. 4, includes a target 101 containing 99.9995% pure aluminum positioned above a cathode 102. Opposite the target 101, a substrate 10 is positioned on an anode 103 electrically connected to a chamber 103A. RF power is applied between the anode 103 and the cathode 102. This generates a plasma 104 in which supplied Ar atoms 105 are ionized into Ar ions 106 and electrons 107. The Ar ions 106 are accelerated toward the target 101 under the influence of a self-generated DC bias. At the target 101, they collide with Al atoms, thereby generating a flow of Al atoms 108 toward the substrate 10. At the substrate 10, the Al atoms 108 deposited on the substrate 10 react with oxygen molecules 109 to form aluminum oxide.

[0062] The main deposition chamber is evacuated to a base pressure of 0.1 microTorr through inlet 110 to prevent incorporation of hydroxide ions into the Al2O3 layer, which would induce absorption losses at approximately 750 nm, 970 nm, and 1400 nm.

[0063] Sustaining the magnetron discharge requires maintaining a balance between secondary electron emission from the target 101 under ion bombardment and the velocity of electrons 107 escaping 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. High electron mobility causes the electrons 107 to collect on the electrode. In this case, the self-generated DC bias voltage is the result of an asymmetry between the target 101 and the chamber 103A of the sputtering system 100.

[0064] A magnetic field is applied using a permanent magnet below the target to increase the electron density in the plasma 104, thereby increasing the argon ionization rate and reducing the required discharge voltage. In addition, the magnetic field significantly increases the sputter yield by increasing the ionization and therefore the bombardment velocity of the target.

[0065] The sputtering system 100 further comprises a heating means such as an infrared heater 111 for heating the substrate 10 directly or via the anode 103 on which the substrate 10 is placed.

[0066] Exemplary process conditions for depositing an aluminum oxide waveguide core are listed in the table below.

[0067] [Table 1]

[0068] Considering the dependence of the layer morphology on the available energy per adatom, which is largely independent of other parameters in the process, it is possible to vary the substrate temperature in order to vary the available energy per adatom, which allows for the investigation of the layer morphology as it changes with the substrate temperature and the corresponding optical propagation loss within the layer.

[0069] The substrate temperature is a set temperature measured on the substrate holder and is therefore not the exact temperature of the substrate. A calibration of the substrate temperature as a function of the set temperature can be provided.

[0070] Figure 5 shows TEM images of the deposited aluminum oxide waveguide layer for various substrate temperatures. At the lowest selected temperature of 420°C, the layer morphology is amorphous. As the substrate temperature increases to 460°C, nanocrystallites begin to form, and at 500°C and 540°C, the density of the nanocrystallites increases significantly.

[0071] As the temperature increases from 500°C to 580°C, the surface roughness increases and the waviness becomes more pronounced. While the surface waviness is still present for the layer deposited at 580°C, a clear transition occurs from a mostly amorphous layer with nanocrystallites to a mostly polycrystalline morphology. From a temperature of 620°C, the waviness disappears as a polycrystalline morphology is increasingly observed, with a slight columnar growth profile. Further increasing the temperature to 700°C, no significant difference in morphology is observed.

[0072] Figures 6a and 6b illustrate AFM measurements of deposited aluminum oxide waveguide layers for various substrate temperatures. The AFM measurements show that waviness appears at 500°C (Figure 6a, bottom left), increases in amplitude at 540°C (Figure 6a, bottom right), decreases, and disappears at 620°C (Figure 6b, top right). In addition to waviness, layers grown at temperatures between 500°C (Figure 6a, bottom left) and 580°C (Figure 6b, top left) exhibit a decrease in refractive index and thickness uniformity.

[0073] Specifically, the bottom of Figure 7 shows the refractive index of aluminum oxide layers measured by ellipsometry at 1550 nm for various deposition temperatures.

[0074] The optical propagation loss of each aluminum oxide layer was investigated using a Metricon 2010 / M41 with a fiber loss module. These losses are shown at the top of the figure. Clearly, the loss decreases with increasing deposition temperature, from 1.57 dB / cm at 377 nm to 0.84 dB / cm at 403 nm for layers grown at a substrate temperature of 700 °C.

[0075] In alternative embodiments, when depositing aluminum nitride waveguide cores, the exact deposition temperature may differ from that described in connection with Figures 5 and 6, but applicants have found that the same behavior occurs. The same changes in morphology and / or trends in refractive index and thickness uniformity can be observed for aluminum nitride waveguide cores when deposited at various temperatures. The teachings derived from Figures 4-7, although described based on aluminum oxide waveguide core embodiments, can also be applied to aluminum nitride waveguide core embodiments.

[0076] 8a-8c each show graphs relating the deposition temperature at which an aluminum oxide waveguide core 11 is deposited to various properties of that core 11. On the x-axis of each of these graphs, the deposition temperature is shown. Those skilled in the art will understand that the exact temperature value at which a particular waveguide core is deposited is highly machine-dependent, and that, for example, the value given by the machine for "deposition temperature" may deviate from the actual, practically very difficult-to-know, temperature of the waveguide core. However, given the particular machine and the deposition rate that can be achieved, a temperature sweep can be performed to determine the exact temperature value at which the behavior described in the graphs shown in FIGS. 8a-8c occurs.

[0077] It is clearly desirable in the art to fabricate high-density amorphous Al2O3 waveguide cores, as these have demonstrated low optical loss. In Figures 8a-8c, the deposition temperature at which such a waveguide is achieved is referred to as temperature P1. Starting from P1, decreasing the deposition temperature results in a lower density of the deposited layer, resulting in the presence of voids in the amorphous aluminum oxide. These voids can act as scatterers and cause losses. Starting from P1, increasing the deposition temperature results in the formation of nanocrystallites in the amorphous material. These nanocrystallites can act as scatterers and cause losses.

[0078] As shown in Figures 8a-8c, P1 is a minimum value at which a balance is struck between reducing the amount of voids and preventing the formation of nanocrystallites. At P1, relatively low losses can be achieved. Known aluminum oxide optical waveguides are based on aluminum oxide layers deposited at temperatures corresponding to P1. However, such layers suffer from the aforementioned drawback of being vulnerable to high-temperature processing steps following the deposition of the aluminum oxide layer.

[0079] Applicants have recognized that in aluminum oxide optical waveguides deposited at deposition temperatures in the range around P1, the greatest loss is caused by local differences in dielectric properties. Both voids and crystallites have different dielectric properties than amorphous aluminum oxide. More voids and / or more crystallites mean that such local differences occur more frequently, increasing optical loss.

[0080] Applicant further recognized that such local differences occur most frequently, and thus the losses caused thereby may be highest, when the aluminum oxide layer contains comparable amounts of amorphous material and nanocrystallites. This can be seen, for example, in Figure 8b, where line A describes how much of the aluminum oxide in the layer is in the amorphous phase, expressed as a percentage by weight, and line C describes how much of the aluminum oxide is in the crystalline phase, expressed as a percentage by weight. Those skilled in the art will understand that the amount of amorphous material and / or the number of nanocrystallites may also be expressed using other units and / or metrics.

[0081] Applicant further recognized that as the deposition temperature increases, at a certain temperature, nanocrystallites overtake amorphous material as the dominant material in the deposited layer. A decrease in the amount of amorphous aluminum oxide reduces the number of discontinuities between amorphous and crystalline aluminum oxide, so that above temperature P2, a reduction in optical loss can be observed. In Figure 8b, temperature P2 is selected for illustrative purposes only as the temperature at which the amorphous and crystalline aluminum oxide contents are identical.

[0082] The reduction in optical loss continues until a deposition temperature is reached where substantially all of the aluminum oxide in the waveguide core is in the form of nanocrystalline crystals, with very little or no amorphous phase present. This point can be referred to as P3. Figure 8c further shows a rectangle R illustrating the temperature range corresponding to the temperature variations above in Figure 7.

[0083] Similarly, for deposition temperatures between P2 and P3, a possible source of scattering is the sporadic presence of volumes of amorphous material among the otherwise nanocrystalline aluminum oxide. Thus, further increasing the deposition temperature results in the formation of fewer and / or smaller volumes of amorphous material, resulting in less frequent localized differences and reduced losses.

[0084] Applicant further recognized that in the ranges P1 and P3, while an increasing amount of aluminum oxide takes on the form of nanocrystallites, the size of individual nanocrystallites does not increase significantly. This is conceptually reflected in Figure 8a. Only when deposition temperatures above P3 are used can there be a significant increase in the size of individual crystallites, while the percentage of aluminum oxide contained within the crystallites themselves remains the same. Note that sizes are not shown for deposition temperatures below P1, since nanocrystallites are nearly absent. As the temperature is increased beyond P3, the nanocrystallites combine to form larger crystallites. These relatively large crystallites cause increased light scattering, thereby increasing optical loss in the layer. Therefore, at temperature P3, a loss minimum comparable to the minimum at temperature P1 can be observed. However, unlike the aluminum oxide layer deposited at temperature P1, the aluminum oxide layer deposited at temperature P3 is much less susceptible to subsequent heating steps, such as annealing steps to treat the deposited cladding layer.

[0085] The morphology achieved at deposition temperature P3 can also be described as polycrystalline aluminum oxide saturated with nanocrystallites, where nanocrystallites are crystallites that are relatively small compared to the waveguide of the light propagating through the optical waveguide. This morphology is advantageous for the following reasons: a) Nanocrystallites are so small that they do not cause significant Rayleigh scattering due to their size. b) Saturation of the core with nanocrystallites ensures that there is little or no variation in dielectric properties throughout the core, thereby also limiting Rayleigh scattering. c) Saturation of the core with nanocrystallites also means that the existing nanocrystallites have little or no amorphous material around them to absorb and grow. d) Growth, which occurs when nanocrystallites align with one another to form a single larger crystallite, occurs only at much higher temperatures.

[0086] The graphs in Figures 8a-8c conceptually illustrate the deposition of an aluminum oxide layer at various substrate temperatures. Although points P1, P2, and P3 are shown on each graph, the associated behavior need not occur at exactly the same temperature for each characteristic depicted in Figures 8a-8c. Also, while Figure 8c suggests that minima at P1 and P3 allow the same low loss to be achieved, implementation of the method according to the present invention may result in minima P1 and P3 achieving different levels of loss for a variety of reasons.

[0087] Applicant has found that exposing the aluminum oxide waveguide core 11 in which the nanocrystallites are formed to temperatures of 800° C. or higher can be advantageous and / or reduce losses. This is shown, for example, in Figures 9a and 9b.

[0088] When the aluminum oxide waveguide core 11 is exposed to such temperatures, nanocrystallite growth is very limited, but such growth will still consume all or at least most of the amorphous aluminum oxide that may have formed during deposition of the aluminum oxide waveguide core 11. If less amorphous aluminum oxide is present after exposure to such temperatures, local differences in dielectric properties occur less frequently, and therefore scattering is reduced.

[0089] With reference to Figure 9a, the aluminum oxide waveguide core in question was deposited such that nanocrystallites formed in the aluminum oxide. No further processing steps were performed in which the deposited aluminum oxide waveguide core was exposed to temperatures above 800°C. The loss achieved is 1 + / - 0.5 dB / cm.

[0090] For Figure 9b, the aluminum oxide waveguide core of Figure 9a was exposed to about 1150°C in a nitrogen environment for about 4 hours. The loss achieved is 0.7 + / - 0.2 dB / cm.

[0091] The light 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 limitations of this estimate, it is still safe to say that although the light intensity appears to increase, losses occur. These losses are estimated by fitting the measured data to a log-linear model using a maximum likelihood estimator sample consensus (MLESAC) algorithm. A given error margin is determined by fitting different segments of the total propagation. Those skilled in the art will understand that other techniques for estimating the light intensity inside the waveguide core and deriving the average loss from the measured data can also be used.

[0092] In alternative embodiments, when depositing an aluminum nitride waveguide core, the temperature values ​​for P1, P2, or P3 may differ from those found for an aluminum oxide waveguide core, but Applicants have found that the same behavior occurs. Similar changes in the phase that the aluminum nitride has (e.g., amorphous or (nano)crystalline), similar increases in the size of the (nano)crystalline, and conceptually equivalent loss profiles can be identified in aluminum nitride waveguide cores when deposited at various temperatures. The teachings described in Figures 8a-8c and 9a-9b are described based on an aluminum oxide waveguide core embodiment, but can also be applied to aluminum nitride embodiments.

[0093] Although the present invention has been described using detailed embodiments thereof, 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 as defined by the appended claims and their equivalents.

Claims

1. 1. A method for manufacturing an optical waveguide, comprising: Providing a substrate; depositing an aluminum nitride waveguide core on the substrate; disposing a cladding layer on the deposited aluminum nitride waveguide core, said disposing including at least one processing step in which the deposited aluminum nitride waveguide core is exposed to a given maximum temperature; depositing the aluminum nitride waveguide core includes forming nanocrystallites in the aluminum nitride waveguide core; the temperature that the aluminum nitride waveguide core must have during deposition of the cladding layer to significantly increase the size of the formed nanocrystallites exceeds the given maximum temperature, the given maximum temperature being about 800°C or greater; and 1. A method according to claim 1, wherein an aluminum oxide waveguide core comprises nanocrystallites having a size of 1 nanometer to 30 nanometers, preferably 1 nanometer to 10 nanometers, said nanocrystallites forming at least 50% by weight of said aluminum oxide waveguide core, preferably at least 75%, more preferably at least 99%, and wherein said cladding layer comprises a high temperature cladding layer.

2. 10. The method of claim 1, wherein the increase in size of the formed nanocrystallites is significant when it is about 100% or greater, preferably about 100% to about 50%.

3. 3. The method of claim 1 or 2, wherein the given maximum temperature is in the range of 800 to 1400°C, preferably 1000 to 1200°C, more preferably about 1150°C.

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

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

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

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

8. 8. The method of claim 7, wherein the nanocrystallites in the aluminum nitride waveguide core form at least 50% by weight of the aluminum nitride waveguide core after the annealing, preferably at least 75%, more preferably at least 99%.

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

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

11. 10. The method of claim 9, wherein the nanocrystallites in the aluminum nitride waveguide core form at least 50% by weight of the aluminum nitride waveguide core after depositing the cladding layer, preferably at least 75%, more preferably at least 99%.

12. The method of any one of claims 1 to 11, 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 of any one of claims 1 to 12, wherein the cladding layer comprises a TEOS layer, a silicon oxynitride layer, an aluminum oxide layer, or a polymer layer.

14. The method of any one of claims 1 to 13, wherein the cladding layer is deposited using any one of plasma enhanced deposition, low pressure chemical vapor deposition, evaporation, sputtering, or atomic layer deposition.

15. The method of any one of claims 1 to 14, wherein the substrate comprises a silicon substrate, a silicon nitride substrate, a silicon thermal oxide substrate, a quartz substrate, or a sapphire substrate.

16. 16. The method of any one of claims 1 to 15, wherein the aluminium nitride waveguide core is stoichiometric and / or the aluminium nitride waveguide core comprises AlxNy, where 0.8<x<1.1 and 0.9<y<1.2, for example x=0.9 and y=1.1, preferably x=1 and y=1.

17. The method according to any one of claims 1 to 16, wherein the waveguide is a slab waveguide or a channel waveguide.

18. 18. The method of any one of claims 1 to 17, further comprising reducing a surface roughness of the aluminum nitride waveguide core, for example using chemical mechanical polishing, between depositing the aluminum nitride waveguide core and disposing the cladding layer.

19. 19. The method of any one of claims 1 to 18, further comprising defining the shape and / or size of the aluminum nitride waveguide core, for example using at least one of lithography and etching, before depositing the cladding layer.

20. depositing an aluminum nitride layer on each substrate at a deposition rate of aluminum nitride at various substrate temperatures and / or various substrate bias voltages; measuring the optical performance of each deposited aluminum nitride layer; selecting as optimal settings the deposition rate, substrate temperature and substrate bias voltage at which the aluminum nitride layer with the best optical performance was produced; and using said optimum settings when depositing said aluminum nitride waveguide core to produce an optical waveguide according to any one of claims 1 to 19.

21. Preferably, an aluminum nitride waveguide manufactured according to any one of claims 1 to 20, A substrate; an aluminum nitride waveguide core disposed on the substrate; a cladding layer disposed on the aluminum nitride waveguide core; 1. An aluminum nitride waveguide, characterized in that the aluminum nitride waveguide core comprises nanocrystallites having a size of 1 nanometer to 30 nanometers, preferably 1 nanometer to 10 nanometers, the nanocrystallites forming at least 50% by weight of the aluminum nitride waveguide core, preferably at least 75%, more preferably at least 99%, and the cladding layer comprises a high temperature cladding layer.

22. 22. The aluminum nitride optical waveguide of claim 21, wherein the high temperature cladding layer comprises at least one TEOS layer or silicon oxynitride layer.

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

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