Method for producing alumina optical waveguide and alumina 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 oxide waveguide core on the substrate; and arranging a cladding layer on the deposited aluminum oxide waveguide core. The arrangement comprises at least one processing step during which the deposited alumina waveguide core is subjected to a given maximum temperature. The method is characterized in that depositing the alumina waveguide core comprises forming nanocrystallites in the alumina waveguide core, and the alumina 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] The present application relates to a method for producing an optical waveguide. The present application also relates to an optical waveguide preferably produced by such a method. Background Art

[0002] Integrated photonics has become ubiquitous, and its development has surpassed mature microelectronics 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, aluminum oxide (Al2O3) has become a popular choice for photonic integrated circuits. 2 O 3 ) emerges as a promising platform material due to its large transparency window, low propagation loss, and high rare earth solubility.

[0003] Another material studied is aluminum nitride (AlN), in part because its transparency window covers more of the UV spectrum (eg, down to 150 nanometers, compared to 200 nanometers for aluminum oxide).

[0004] In the amorphous Al deposited by atomic layer deposition (ALD) 2 O 3 Low losses of 0.04 ± 0.02 dB / cm in the C-band have been demonstrated in planar slab waveguides. For near-UV applications, highly confined single-transverse-mode ALD-grown and fully etched Al 2 O 3 These results demonstrate the potential for Al-based 2 O 3 The broad wavelength range of the integrated photonic platform.

[0005] When using reactive sputtering of Al 2 O 3 Compared with the grown layers, the ALD grown Al 2 O 3 The main disadvantage of the layer is the order of magnitude difference in deposition rate. 2 O 3 The plate loss is as low as 0.1dB / cm, and the loss of the fully etched waveguide in the C band is less than 0.2dB / cm. Therefore, it is necessary to reduce the sputtered Al 2 O 3 loss in order to reduce the loss of Al 2 O 3 and competing with more established silicon nitride-based platforms, which have been demonstrated with losses as low as 1 dB / m at 1550 nm in highly confined waveguides.

[0006] As is known in the art, 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. Here, it is noted that arranging the cladding layer includes at least one processing step during which the deposited aluminum oxide waveguide core is subjected to a given maximum temperature.

[0007] It is known from the prior 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 can deteriorate significantly.

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

[0009] The optical losses in the cladding can vary between different materials. For example, tetraethyl orthosilicate (TEOS) is known to show relatively low optical losses. However, in order to manufacture such a TEOS cladding, a high temperature annealing step is generally required. Therefore, a problem that arises when combining a TEOS cladding with an amorphous alumina waveguide core is that the benefits of the optical quality of the TEOS cladding are offset by the loss of optical performance of the amorphous alumina waveguide core due to the high temperatures involved when using a TEOS cladding on an amorphous alumina waveguide core.

[0010] There may also be other types of cladding layers, which are to be disposed on the deposited alumina waveguide core, requiring processing steps during which the alumina waveguide core is subjected to high temperatures, resulting in the aforementioned loss of optical performance of the alumina waveguide core. In the following, cladding layers that require high temperature steps at 800 degrees Celsius or higher during or after deposition are referred to as high temperature cladding layers.

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

[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. The paper discusses amorphous aluminum oxide films deposited using atomic layer deposition. The aluminum oxide films were grown on a bare silicon substrate or on thick (3.2 μm) silicon dioxide (also called thermal oxide) on silicon at a growth temperature of 300 degrees Celsius. The optical loss in the film was measured using the prism coupling method (Metricon) for wavelengths of 633 nm and 405 nm. Before annealing, the 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 pointed out that high temperature annealing leads to the formation of dense polycrystalline γ-phase Al 2 O 3 The high temperature is above 800 degrees Celsius. The disclosed aluminum oxide films are polycrystalline after annealing at 900 degrees Celsius and 1100 degrees Celsius and are said to exhibit optical losses greater than 20 dB / cm or 2000 dB / m.

[0014] Although the problem is illustrated here for an aluminum oxide waveguide core, applicants have discovered that the same, or at least very similar, behavior and problems exist when fabricating an aluminum nitride (AlN) waveguide core. Summary of the invention

[0015] It is an object of the present invention to provide a method for manufacturing an optical waveguide comprising an alumina waveguide core and a high temperature cladding layer, which method at least partially solves at least one of the above mentioned problems.

[0016] According to the invention, this object is achieved by a method as defined in claim 1, characterized in that depositing the alumina waveguide core comprises forming nanocrystallites in the alumina waveguide core, wherein the temperature that the alumina waveguide core will need to have during the arrangement of the cladding layer for significantly increasing the size of the formed nanocrystallites exceeds a given maximum temperature, wherein the given maximum temperature is about 800 degrees Celsius or more.

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

[0018] Applicants have discovered that by intentionally allowing the formation of nanocrystallites during deposition, it is possible to produce alumina waveguide cores having similar optical properties to waveguide cores made from amorphous materials, while being able to maintain said optical properties during subsequent processing steps up to higher temperatures.

[0019] Applicants have discovered that Rayleigh scattering plays an important role in deposited aluminum oxide layers. First, Rayleigh scattering can occur due to differences in dielectric constants within the aluminum oxide layer. Second, Rayleigh scattering can occur when crystallites are formed that are at least comparable in size to the wavelength of light in the layer. When the crystallites have even larger sizes, other sources of optical scattering become relevant.

[0020] When deposited at relatively low temperatures, no significant crystallization occurs, but an amorphous layer will be obtained, and the optical loss of such a layer will be relatively low. However, when such an amorphous aluminum oxide layer is subsequently subjected to high temperatures (e.g., 800 degrees Celsius and above), crystallization will occur. In the amorphous material, crystallites will begin to grow. Since these crystallites grow in the amorphous material, they can grow freely, for example, the amorphous material surrounding the crystallites can utilize relatively little energy and be absorbed into the crystallites in each direction. The resulting crystallites 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 performance will deteriorate.

[0021] When depositing at a suitably high temperature, crystallization occurs and crystallites will be formed in the other amorphous layer. In this case, there will also be local differences in the dielectric constant between the amorphous portion and the nanocrystalline portion of this layer. The Rayleigh scattering associated with these differences will cause the optical performance to decrease. In addition, when this layer is subjected to high temperatures, for example during the annealing step, the nanocrystals present will grow by absorbing the amorphous material around them, thus causing relatively large crystallites and optical scattering losses to increase.

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

[0023] Without being bound by theory, the applicant has noticed that the energy required to transform the above-mentioned aluminum oxide layer with a large number of relatively small crystallites into a layer with a small number of relatively large crystallites is too high to be achieved during the processing steps required to arrange the cladding layer. The realization of such a waveguide core can be confirmed by subjecting such a waveguide core to a given maximum temperature. The optical properties will not change significantly because very little crystal growth will occur within the layer.

[0024] By intentionally allowing the nanocrystals to form, there is no or very little amorphous material. This means that there are less local differences in dielectric properties and therefore less optical scattering. Moreover, there is less amorphous material for the nanocrystals to absorb and grow. Therefore, in the alumina waveguide core deposited according to the present invention, the existing nanocrystals cannot grow easily and freely, thereby 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 that are mainly surrounded by other nanocrystals is greater than the energy required for the size growth of nanocrystals that are mainly surrounded by amorphous materials. This is another reason why when the deposition of the alumina waveguide core is performed so that nanocrystals are formed, it is possible to prevent or at least limit the formation of larger crystallites during the subsequent processing steps. This allows the high-temperature cladding layer to be arranged on the alumina waveguide core without significantly reducing the optical performance of the alumina waveguide core. Therefore, waveguides with alumina waveguide cores can be manufactured, which show improved optical performance and / or reduced losses. The size of the nanocrystals formed during the deposition process strongly affects the amount of optical scattering. The applicant has found that by forming nanocrystals so that a given maximum temperature is located within a range between 800 degrees Celsius and 1400 degrees Celsius, favorable optical properties of optical waveguides can be obtained. A maximum temperature below 800 degrees Celsius will indicate that a relatively large amount of amorphous alumina can be obtained after the deposition of the waveguide core, and the material will be transformed into large grains when the cladding layer is arranged. On the other hand, a maximum temperature above 1400 degrees C would risk degradation of the alumina waveguide core. Those skilled in the art will appreciate that around 1400 degrees C the glass transition temperature of alumina begins, so subjecting the alumina waveguide core to this temperature may result in reflow of the structure.

[0026] It is 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.

[0027] Arranging the cladding layer may include depositing the cladding layer on the alumina waveguide core, and the at least one processing step may include annealing the combination of the substrate, the deposited alumina waveguide core and the deposited cladding layer at a given maximum temperature.

[0028] After deposition and before annealing, the size of the nanocrystals in the alumina waveguide core can be between 1 nanometer and 30 nanometers, and preferably between 1 nanometer and about 10 nanometers. Additionally, after deposition and before annealing, the nanocrystals in the alumina waveguide core can account for at least 40% by weight of the alumina waveguide core, preferably at least 70%, more preferably at least 90%. Additionally or alternatively, the size of the nanocrystals in the alumina waveguide core after annealing can be between 1 nanometer and 30 nanometers, and preferably between 1 nanometer and 10 nanometers. Further, the nanocrystals in the alumina waveguide core after annealing can account for at least 50% by weight of the alumina waveguide core, preferably at least 75%, more preferably at least 99%.

[0029] The at least one processing step can include depositing a cladding layer on the alumina 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 alumina waveguide core can be between 1 nanometer and 30 nanometers, and preferably between 1 nanometer and 10 nanometers. Additionally, the nanocrystals in the alumina waveguide core after annealing can account for at least 50% by weight of the alumina waveguide core, preferably at least 75%, more preferably at least 99%.

[0030] Any one of reactive sputter deposition, atomic layer deposition, evaporation, or pulsed laser deposition can be used to deposit the alumina waveguide core. Other methods of arranging the alumina waveguide core are not excluded.

[0031] 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 alumina waveguide core is deposited. Examples of such cladding layers are TEOS layers, silicon oxynitride 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.

[0032] The substrate can include a silicon substrate, a silicon nitride substrate, a silicon thermal oxide substrate, or a quartz substrate.

[0033] The alumina waveguide core can have a stoichiometric ratio. Additionally or alternatively, the alumina waveguide core can include AlxOy, where 1.5 < × < 2.5 and 2.5 < y < 3.5, such as x = 1.6 and y = 3.4, preferably x = 2 and y = 3.

[0034] The optical waveguide can be a planar waveguide or a channel waveguide. Other types of waveguides are not excluded in this application.

[0035] In other embodiments, depositing the aluminum oxide waveguide core on the substrate may also include one or more of the following steps: for example, using chemical mechanical polishing to reduce the surface roughness of the aluminum oxide waveguide core, and using at least one of lithography and etching to define the shape and / or size of the aluminum oxide waveguide core.

[0036] The method may further comprise the steps of depositing aluminum oxide on the respective substrates at a deposition rate of aluminum oxide, at a varying substrate temperature and / or a varying substrate bias voltage. For each deposited aluminum oxide layer, its optical properties are measured. The method may further comprise selecting as optimal settings the deposition rate, substrate temperature and substrate bias voltage used to produce the aluminum oxide layer with the best optical properties. These optimal settings may be used when depositing an aluminum oxide waveguide core for producing an optical waveguide as described above.

[0037] According to a second aspect, the present invention provides an alumina optical waveguide comprising a substrate, an alumina waveguide core arranged on said substrate, and a cladding layer arranged on the alumina waveguide core. According to the present invention, the alumina waveguide core comprises nanocrystallites having a size between 1 nm and 30 nm, preferably between 1 nm and 10 nm. These nanocrystallites account for at least 50%, preferably at least 75%, more preferably at least 99% of the weight of the alumina waveguide core, 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 DESCRIPTION OF THE DRAWINGS

[0038] Next, the present invention will be described with reference to the accompanying drawings, wherein the same reference numerals will be used to refer to the same or similar components, and wherein:

[0039] Figure 1 A flow chart showing a method for manufacturing an optical waveguide according to the present invention;

[0040] Figure 2 shows a cross section of a slab waveguide according to the present invention;

[0041] Figure 3 Shows Figure 1 A flow chart of a preferred embodiment of the method;

[0042] Figure 4 An example of a reactive co-sputtering system that may be configured to deposit an aluminum oxide waveguide core is shown;

[0043] Figure 5 shows TEM images of the deposited alumina waveguide core at various substrate temperatures;

[0044] Figure 6a and Figure 6bshows AFM images of the deposited alumina waveguide core at various substrate temperatures;

[0045] Figure 7 A graph showing the relationship between the deposition temperature of the aluminum oxide waveguide core on the X-axis and the measured refractive index on the Y-axis, and a graph showing the relationship between the wavelength of light propagating through the aluminum oxide waveguide core deposited at the temperature on the X-axis and the propagation loss measured in the waveguide core on the Y-axis for multiple deposition temperatures.

[0046] Figure 8a showing a graph of waveguide core deposition temperature on the X-axis versus the size of crystallites formed in the alumina waveguide core on the Y-axis;

[0047] Figure 8b A graph showing the waveguide core deposition temperature on the X-axis versus the weight percentage of the waveguide core of a particular phase (specifically amorphous (A) or crystalline (C)) on the Y-axis;

[0048] Figure 8c showing a graph of waveguide core deposition temperature on the X-axis versus optical loss achieved in the waveguide core on the Y-axis;

[0049] Figure 9a and Figure 9b Both show a graph of the distance light is propagated through the waveguide core on the X-axis versus the intensity of said light on the Y-axis. DETAILED DESCRIPTION

[0050] Figure 1 A flow chart showing an embodiment of a method for manufacturing an optical waveguide according to the present invention is shown, the method comprising steps S1 to S3. Figure 2 Shown is a schematic cross section of an optical waveguide which can be produced, for example, using the method according to the invention. Figure 3 A flow chart showing a preferred embodiment of the method is shown.

[0051] 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. It is also conceivable that the aluminum oxide waveguide core 11 is deposited on the substrate with other layers in between. An exemplary embodiment is an embodiment in which the waveguide cores are stacked. In another exemplary embodiment, the aluminum oxide waveguide core 11 is deposited on an aluminum oxide layer, which in turn is deposited on the substrate. In another exemplary embodiment, the substrate is a sapphire substrate, preferably a crystalline sapphire substrate, on which SiO is arranged. 2 layer and wherein the SiO 2 An alumina waveguide core is deposited on the layer.

[0052] Specifically, the aluminum oxide waveguide core 11 is deposited so that nanocrystallites are formed therein. In the next step S3, the cladding layer 12 is arranged on the aluminum oxide waveguide core 11. During at least one processing step required for arranging the cladding layer 12, the aluminum oxide 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.

[0053] The alumina waveguide core will need to have a temperature exceeding a given maximum temperature during the deposition of the cladding layer in order to significantly increase the size of the formed nanocrystallites.

[0054] In other words, the lowest temperature at which the size of the formed nanocrystallites increases significantly exceeds a given maximum temperature. Thus, the alumina waveguide core 11 can have said maximum temperature without said significant increase in size occurring.

[0055] Increasing the maximum temperature to over 1400 degrees Celsius risks degrading the alumina waveguide core 11. Those skilled in the art will appreciate that at around 1400 degrees Celsius the glass transition temperature of alumina begins, so subjecting the waveguide core to this temperature may result in reflow of the structure. In addition, 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 may begin to melt at around 1400 degrees Celsius. The substrate 10 may also include silicon dioxide, in which local density fluctuations may form when exposed to such temperatures, and these local density fluctuations cause scattering, thus degrading performance.

[0056] The increase in grain size is considered significant when the increase is about 100% or more, preferably when the increase is between about 100% and about 50%.

[0057] Reference now Figure 3 , the step S3 of arranging the cladding layer may comprise a step S31 of depositing the cladding layer 12 on the alumina waveguide core 11. Step 31 may be a treatment step of subjecting the alumina waveguide core 11 to a given maximum temperature. This may be applicable, for example, in embodiments where a polymer-based cladding layer is deposited.

[0058] Alternatively, the high temperature step is not part of depositing the coating layer, but is part of a subsequent heating step. Figure 3 In the embodiment, an example of such a subsequent step is provided as step S32, wherein the combination of the substrate 10, the deposited alumina waveguide core 11 and the arranged cladding layer 12 is annealed, so that the alumina waveguide core 11 is subjected to a given maximum temperature. This applies, for example, to embodiments in which a TEOS cladding layer is deposited.

[0059] It should be noted that various other steps may be performed between the deposition of the alumina waveguide core 11 and the placement of the cladding layer 12. For example, the deposited alumina layer may be subjected to chemical mechanical polishing to reduce surface roughness, as well as photolithography and etching steps to define channel waveguides or other types of waveguides.

[0060] For any embodiment (e.g., method and / or apparatus) related to an aluminum oxide waveguide core 11, alternative embodiments are contemplated in which the waveguide core is instead made of aluminum nitride (AlN). Figures 1 to 3 The explained embodiments are also applicable to the manufacture of waveguides comprising an aluminium nitride waveguide core.A person skilled in the art will appreciate how the following description of the manufacturing process of an aluminium oxide waveguide core can and / or should be adapted for the manufacture of an aluminium nitride waveguide core.

[0061] The Al used in the embodiments of the present invention 2 O 3 The thin films can be deposited by reactive sputtering onto silicon wafers with an 8 micron oxide buffer layer. The advantage of reactive sputtering exploited here is due to the energy available per adatom on the substrate. The adatoms landing on the substrate have high mobility, resulting in a high density layer morphology that can be obtained at relatively low substrate temperatures and high deposition rates. This allows the deposition of high density amorphous Al at CMOS compatible wafer temperatures. 2 O 3 layer, the slab waveguide propagation loss is less than 0.1dB / cm at 1550nm.

[0062] Although Al 2 O 3 Several low-loss propagation slab waveguide results for optical waveguides, but a link to layer morphology has not been demonstrated in the prior art. The applicants have recognized that, given the complexity of reproducing reactive sputtering deposition processes, an understanding of the morphology and corresponding propagation losses can facilitate improved reproducibility and layer quality.

[0063] Al deposited by reactive sputtering 2 O 3 The morphology of the layer is mainly determined by the available energy EPA per adatom 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 adatoms for a given kinetic energy available to each adatom; and the critical nucleation size, which determines the critical diffusion length required for stable nucleation. Although for Al 2 O 3 The layers give the material properties, but the energy per adatom 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 the 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, we should consider the attachment to form Al 2 O 3 The contribution of atoms and molecules on the substrate required for the layer. Adsorbed atoms accelerated from the target contribute their kinetic energy when adsorbed on the substrate. In addition, when oxygen molecules are adsorbed, their kinetic energy is also contributed. Even if atoms or molecules are not adsorbed, part of their kinetic energy can still be transferred when colliding with the substrate. This contribution can become significant, especially when gas molecules gain energy by colliding with higher-speed ions. Another form of kinetic energy is related to the temperature of the substrate. In addition, in addition to the kinetic energy contribution to layer formation, the kinetic energy generated by the formation of Al 2 O 3 The potential energy released by the exothermic chemical reaction is also an important contribution.

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

[0067] All energy contributions increase the available energy per adatom, thus affecting the layer morphology and the consequent propagation losses.

[0068] In one embodiment, an AJA ATC 1500RF reactive co-sputtering system 100 can be used to deposit Al on a 10 cm silicon wafer with an 8 μm thick thermal oxide buffer layer. 2 O 3 layer.

[0069] exist Figure 4 The system 100 schematically shown in comprises a target 101 comprising aluminum having a purity of 99.9995% arranged above a cathode 102. Opposite to the target 101, the substrate 10 is arranged 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, thereby generating a stream 108 of Al atoms 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 was evacuated through inlet 110 to a base pressure of 0.1 μTorr to prevent the incorporation of hydroxide ions into the Al2 O 3 In the layer, hydroxide ions cause absorption losses near 750nm, 970nm and 1400nm.

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

[0072] 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 greatly increases the sputtering yield by increasing ionization and thus increasing the bombardment rate of the target.

[0073] The sputtering system 100 further includes a heating device, such as an infrared heater 111, for directly heating the substrate 10 or heating the substrate through an anode 103 on which the substrate 10 is arranged.

[0074] Exemplary process conditions for depositing an aluminum oxide waveguide core are listed in the table below.

[0075] parameter value unit Basic pressure <0.3 Micro-support Argon flow 30 Sccm Oxygen flow 3.0±0.2 Sccm Oxygen partial pressure 0.2±0.1 mTorr pressure 3.5±0.1 mTorr Pump flow 50±2 l / s Substrate set temperature 420 460 500 540 580 620 660 700 ℃ Target-substrate distance 15.2 cm power 200 W

[0076] Given the dependence of the layer morphology on the available energy per adatom, the substrate temperature can be varied to change the available energy per adatom, since this parameter is almost independent of the other parameters in the process. This allows us to study the variation of the layer morphology with substrate temperature and the corresponding light propagation losses in the layer.

[0077] The substrate temperature is the 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 may be provided.

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

[0079] As the temperature increases from 500°C to 580°C, the surface roughness increases with increasing waviness. Although the layer deposited at 580°C still has surface waviness, a clear transition from a predominantly nanocrystalline amorphous layer to a predominantly polycrystalline morphology has occurred. This waviness disappears 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 differences in morphology are observed.

[0080] Figure 6a and Figure 6b The AFM measurement results of the deposited aluminum oxide waveguide layer at various substrate temperatures are shown. The AFM measurement shows that the ripples are Figure 6a , lower left) appears, and its amplitude increases at 540 degrees Celsius ( Figure 6a , lower right), and decreases and disappears at 620 degrees Celsius ( Figure 6b , top right). In addition to the ripples, at 500 °C ( Figure 6a , lower left) to 580 degrees Celsius ( Figure 6b Layers grown at temperatures below 20 °C (upper left) show reduced refractive index and thickness uniformity.

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

[0082] The light propagation losses for 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. Clearly, for the layers grown at a substrate temperature of 700 degrees Celsius, the losses decrease with increasing deposition temperature, down 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 related to Figure 5 Deposition temperatures differ from those mentioned in FIG6 , however, Applicants did find that the same behavior occurs. The same morphology changes and / or the same refractive index and thickness uniformity trends can be observed in the aluminum nitride waveguide core when deposited at varying temperatures. Figures 4 to 7 The derived teachings, although explained based on an aluminum oxide waveguide core embodiment, are also applicable to an aluminum nitride waveguide core embodiment.

[0084] Figures 8a to 8cEach shows a graph of the deposition temperature during the deposition of an 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. One skilled in the art will appreciate that the exact temperature value for depositing a particular waveguide core is highly machine dependent, e.g., the "deposition temperature" value given by the machine may deviate from the actual temperature of the waveguide core (which is difficult to know in practice). However, a temperature scan may be performed to determine, taking into account the particular machine and achievable deposition rate, the deposition temperature. Figures 8a to 8c The exact temperature at which the indicated behavior occurs.

[0085] In the field, there is a clear trend towards making dense amorphous Al 2 O 3 waveguide cores, as these cores exhibit lower optical losses. Figures 8a to 8c In the embodiment of the present invention, the deposition temperature at which such a waveguide is realized is referred to as temperature P1. If the deposition temperature is reduced from P1, the deposited layer will become less dense and voids will exist in the amorphous alumina. The voids can act as scattering objects and may cause losses. If the deposition temperature is increased from P1, nanocrystals will form in the amorphous material. The nanocrystals can act as scattering objects and may cause losses.

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

[0087] Applicants have recognized that in alumina optical waveguides deposited at deposition temperatures in the range around P1, the maximum losses are caused by local differences in dielectric properties. Both voids and crystallites have different dielectric properties than amorphous alumina. More voids and / or more crystallites means that such local differences occur more frequently and optical losses will increase.

[0088] The applicant has also realised that such local differences will occur most often when the aluminium oxide layer contains comparable amounts of amorphous material and nanocrystallites, and that the losses caused thereby are therefore likely to be at their highest level. Figure 8b , where line A describes how much aluminum oxide (expressed as a weight percentage) is in the amorphous phase in the layer, and line C describes how much aluminum oxide (expressed as a weight percentage) is in the crystalline phase in the layer. Those skilled in the art will appreciate that the amount of amorphous material and / or the number of nanocrystallites may 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 the dielectric properties of the entire core have almost no fluctuations, thus also limiting Rayleigh scattering;

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

[0097] d) Growth due to alignment of nanocrystallites with each other and formation of a single larger crystallite will only occur at much higher temperatures.

[0098] Figures 8a to 8c The graphs in FIG. 1 conceptually illustrate the deposition of aluminum oxide layers at various substrate temperatures. Although points P1, P2, and P3 are indicated in each figure, for Figures 8a to 8c Each property described in , where the relevant behavior is manifested, does not necessarily occur at exactly the same temperature. In addition, although Figure 8c It is shown that the local minima at P1 and P3 allow achieving the same low loss, but a specific implementation of the method according to the invention may result in the local minima P1 and P3 achieving different levels of loss due to various reasons.

[0099] The applicant has found that subjecting the alumina waveguide core 11 in which nanocrystallites are formed to a temperature of 800 degrees or more may even be beneficial and / or reduce losses. Figure 9a and Figure 9b Shown in.

[0100] When the alumina waveguide core 11 is exposed to such temperatures, although the growth of nanocrystallites 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 such temperatures, if there is less amorphous alumina, local differences in dielectric properties appear less and therefore scattering is reduced.

[0101] for Figure 9a The alumina waveguide core in question was deposited such that nanocrystallites were formed in the alumina. No further processing steps were performed during the time the deposited alumina waveguide core was subjected to temperatures of 800 degrees Celsius or higher. The loss achieved was 1 + / - 0.5 dB / cm.

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

[0103] The light intensity given on the Y-axis is an estimate derived from the scattered light measured over the propagation length of the waveguide core. Without wishing to be bound by theory, one skilled in the art will appreciate that due to the limitations of this estimate, the light intensity appears to increase, but it is still safe to say that losses do occur. These losses are estimated by fitting the measured data to a log-linear model using a Maximum Likelihood Estimation Sample Consensus (MLESAC) algorithm. The given error ranges are determined by fitting different portions of the total propagation. One skilled in the art will appreciate that other methods may be used to estimate the light intensity within the waveguide core and derive the average losses from the measured data.

[0104] In alternative embodiments, when depositing aluminum nitride waveguide cores, the temperature values ​​of P1, P2 or P3 may differ from those found for aluminum oxide waveguide cores, however Applicants did find that the same behavior occurs. Similar phase transitions of aluminum nitride (e.g., amorphous or (nano)crystalline), similar increases in (nano)crystallite size, and conceptually comparable loss profiles can be identified in aluminum nitride waveguide cores when deposited at varying temperatures. Figures 8a to 8c and Figure 9a to Figure 9b Although the teachings explained in are explained based on an aluminum oxide waveguide core embodiment, they can also be applied to aluminum nitride embodiments.

[0105] In the 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 may 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 include: providing a substrate; depositing an alumina waveguide core on the substrate; as well as arranging a cladding layer on the deposited alumina waveguide core, said arranging comprising at least one processing step during which the deposited alumina waveguide core is subjected to a given maximum temperature; The method is characterized in that depositing the aluminum oxide waveguide core includes forming nanocrystals in the aluminum oxide waveguide core; wherein the temperature that the alumina waveguide core will need to have during the disposition of the cladding layer in order to significantly increase the size of the formed nanocrystallites exceeds the given maximum temperature, wherein the given maximum temperature is about 800 degrees Celsius or higher; And the method is characterized in that the alumina waveguide core comprises nanocrystallites having a size between 1 nm and 30 nm, preferably between 1 nm and 10 nm, wherein the nanocrystallites account for at least 50%, preferably at least 75%, more preferably at least 99% of the weight of the alumina waveguide core, and wherein the cladding layer comprises a high temperature cladding layer.

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

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

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

5. The method of claim 4, wherein after said depositing of said cladding layer and before said annealing, the size of said nanocrystallites in said alumina waveguide core is between 1 nanometer and 30 nanometers, and preferably between 1 nanometer and about 10 nanometers.

6. The method of claim 5, wherein after said depositing and before said annealing, said nanocrystallites in said alumina waveguide core account for at least 40%, preferably at least 70%, more preferably at least 90% of the weight of said alumina waveguide core.

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

8. The method of claim 7, wherein after said annealing, said nanocrystallites in said alumina waveguide core account for at least 50%, preferably at least 75%, more preferably at least 99% of the weight of said alumina waveguide core.

9. The method according to claim 1, 2 or 3, wherein the at least one processing step comprises depositing the cladding layer on the alumina 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 alumina 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 alumina waveguide core account for at least 50% by weight of the alumina waveguide core, preferably at least 75%, more preferably at least 99%.

12. The method according to any one of the preceding claims, wherein any one of reactive sputter deposition, atomic layer deposition, evaporation or pulsed laser deposition is used to grow the alumina waveguide core.

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

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

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 or a quartz substrate.

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

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: reducing the surface roughness of the alumina waveguide core, for example using chemical mechanical polishing, between depositing the alumina waveguide core and arranging the cladding layer.

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

20. The method according to any one of the preceding claims, comprising: depositing an alumina layer on a corresponding substrate at a deposition rate of alumina at a varying substrate temperature and / or a varying substrate bias; measuring the optical properties of each deposited alumina layer; selecting the deposition rate, the substrate temperature and the substrate bias used to fabricate the alumina layer having the best optical properties as the optimal settings; using the optimal settings when depositing the alumina waveguide core for manufacturing an optical waveguide according to any one of the preceding claims.

21. An alumina waveguide preferably manufactured according to any one of the preceding claims, comprising: a substrate; an alumina waveguide core arranged on the substrate; and a cladding layer disposed on the alumina waveguide core; The alumina waveguide is characterized in that the alumina waveguide core comprises nanocrystallites having a size between 1 nm and 30 nm, preferably between 1 nm and 10 nm, wherein the nanocrystallites account for at least 50%, preferably at least 75%, more preferably at least 99% of the weight of the alumina waveguide core, and wherein the cladding layer comprises a high temperature cladding layer.

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

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

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