Electro-optical device with passivation BTO and method for manufacturing the same

Abstract

The present invention provides devices including electro-optical methods and passivation or protective structures for Pockels materials. These structures are single layers or stacks of several layers of different materials. Typically, these structures are placed between a waveguide containing silicon nitride or the like and a Pockels material such as a BTO (BaTiO3) layer. It has been discovered that the reduction reaction of BTO significantly alters the properties of the layer (primarily increasing conductivity and light absorption).

Description

Background Art

[0001] Related Applications This application claims the benefit of U.S. Provisional Application No. 63 / 502,425, filed May 16, 2023, under 35 U.S.C. § 119(e), the entire disclosure of which is incorporated herein by reference.

[0002] Background of the Invention Silicon photonics has become a platform for high-density and low-cost photonic integrated circuits (PICs) for a wide range of applications that often require high-speed and energy-efficient electro-optical (EO) switches.

[0003] However, silicon modulators have significant limitations. Since the changes in the real and imaginary parts of the refractive index are coupled, high-speed modulation of only the optical phase is impossible. Also, their operating speeds are limited by the charge carrier lifetime in forward-biased or reverse-biased devices. State-of-the-art silicon-based modulators often use different doped regions in the waveguide. Higher doping is required for faster operation, but higher doping increases absorption. Another option is heater-based devices. Here, a heater (often a metal wire) changes the temperature by Joule heating (current), thereby changing the temperature of nearby but sufficiently separated waveguides, so that in the optical mode, the heater conductor with strong optical absorption becomes invisible. Such heater-based devices are slow, power-consuming, and tend to be affected by crosstalk.

[0004] Pockels materials avoid these problems. In such materials, the change in refractive index is induced by the electric field. However, the Pockels effect does not exist in centrifugal crystals such as silicon. Therefore, in order to combine the advantages of bulk Pockels modulators with the low fabrication cost of integrated silicon photonics, it is necessary to integrate materials with a large Pockels coefficient into silicon photonic structures.

[0005] Several approaches exist for integrating silicon-based modulators with materials exhibiting a large effective Pockels effect. For example, the Pockels effect is present in lithium niobate (LiNbOs, LN) single crystals. Lithium niobate is integrated with silicon waveguides, for example, by wafer bonding technology. However, the size mismatch between LN wafers and silicon wafers makes it difficult to scale the integration process to larger substrate sizes, resulting in significantly higher chip costs.

[0006] Barium titanate (BaTiO3, BTO) has emerged to enable Pockels effect-based devices on silicon for several reasons. Firstly, BTO has one of the largest Pockels coefficients. Secondly, BTO has been previously used in thin-film electro-optic modulators on small oxide substrates. Thirdly, BTO can be grown on silicon substrates, which have large wafer sizes and excellent crystal quality. BTO-based photonic electro-optic components on silicon wafers have been demonstrated. [Overview of the Initiative]

[0007] To integrate Pockels materials such as BTO into wafer-scale processes, the materials must be protected from the processes required. For example, the electro-optical properties of BTO (primarily optical loss and leakage current) may change when exposed to certain chemical species in the form of gases, plasmas, and ambient humidity.

[0008] In the current configuration, the BTO layer is often exposed to numerous complex processing steps during product manufacturing. Each of these process steps, particularly those to which the BTO is directly exposed, can degrade electro-optical properties, significantly reduce yield, or make it impossible to manufacture devices with adequate performance.

[0009] In particular, it has been found that the reduction reaction of BTO significantly alters the properties of the layer (mainly increasing conductivity and light absorption). That is, exposure of the BTO layer to hydrogen radicals during the manufacture or operation of BTO-based photonic devices adversely affects the performance of the device. Such exposure can occur, for example, by hydrogen radicals in plasma-based deposition of dielectrics (e.g., silicon nitride (SiN) or silicon dioxide (SiO2)), plasma-based etching of dielectrics or metals, or due to ambient humidity.

[0010] At the same time, it is often necessary to allow oxygen to enter the BTO layer during annealing and other processes.

[0011] The present invention includes the introduction of a passivation or protective structure for Pockels materials. This structure is a single layer or a stack of several layers of different materials. Typically, this structure is placed between a waveguide containing silicon nitride, etc., and a Pockels material such as a BTO (BaTiO3) layer. In the current configuration, in one design, a BTO-based ridge waveguide uses a planar layer of BTO for vertical light confinement and a layer of structured silicon nitride on top of the BTO. The silicon nitride layer enables lateral light confinement.

[0012] In other embodiments, other variations of the Pockels material are used, such as various possible compositions of (B,S)TO((Ba,Sr)TiO3) containing possible doping elements.

[0013] In this configuration, the passivation structure can block the entry of hydrogen radicals into the Pockels material layer during and / or subsequent ambient conditions, storage conditions, and operating conditions.

[0014] Furthermore, the passivation structure can significantly reduce the intrusion of hydrogen radicals into the Pockels material layer when exposed to a plasma or gas containing hydrogen radicals during manufacturing at a temperature range from room temperature to several hundred degrees Celsius (typically the CMOS BEOL processing temperature) (0 to 400 degrees Celsius). The passivation structure can also help in the gas release of hydrogen radicals from the Pockels material and the oxidation of the Pockels material (oxygen intrusion) at higher annealing temperatures such as 300 to 800 degrees Celsius.

[0015] In many ways, including such a passivation structure is counterintuitive. At first glance, it appears that, for example, the electro-optic properties of a SiN / BTO ridge waveguide would be degraded. If the protective layer is placed between the SiN and BTO layers in the core of the waveguide where the optical power is highest, the protective layer reduces the overlap of optical modes with the BTO layer, and consequently reduces the effective electro-optic response of the waveguide (which should ideally be maximized).

[0016] However, by using an appropriate passivation structure (material, layer, thickness), it is possible to adjust the diffusion characteristics of hydrogen radicals while simultaneously significantly reducing the adverse effects on mode overlap with BTO (electro-optic efficiency).

[0017] The passivation structure must be sufficiently thin and preferably have an effective refractive index lower than that of SiN and BTO. The "effective refractive index" corresponds to the refractive index of the homogeneous material and can replace the layer stack, resulting in identical (almost identical) optical mode characteristics. Furthermore, the passivation structure must contain a material with low hydrogen permeability.

[0018] The slight decrease in electro-optic efficiency caused by the presence of the passivation structure is largely compensated for by the associated mitigation of leakage degradation during manufacturing and subsequent optical loss.

[0019] In many embodiments, the passivation structure includes a thin layer of silicon dioxide (SiO2) or a stack containing alternating SiO2 / SiN / SiO2 layers. In the present configuration, the passivation structure further includes an STO layer, specifically an STO layer with a thickness of several nanometers, e.g., 4 nanometers, between the BTO and the SiO2 cap. In other embodiments, the BTO is in direct contact with the passivation stack.

[0020] Furthermore, the passivation structure offers several additional advantages. It can protect the BTO surface from etching defects in the plasma etching of the SiN waveguide and in wet chemistry (etching solution, solvent). Also, the passivation structure can act as an etching stop to prevent the BTO layer from being over-etched at the locations of highest optical power when patterning the SiN waveguide (reducing optical overlap).

[0021] The above and other features and other advantages of the present invention, including various novel details of component configurations and combinations, are described in more detail with reference to the accompanying drawings and are set forth in the claims. It should be understood that specific methods and devices embodying the present invention are shown as examples and are not limiting to the invention. The principles and features of the present invention can be used in various embodiments without departing from the scope of the invention. [Brief explanation of the drawing]

[0022] In the attached drawings, reference numerals indicate the same parts throughout the different drawings. The drawings are not necessarily to scale, and instead the focus is on illustrating the principles of the present invention.

[0023] [Figure 1] This is a cross-sectional view showing the fabrication of an electro-optical device according to the present invention.

[0024] [Figure 2]A cross-sectional view showing alternative fabrication steps of an electro-optical device according to the present invention.

[0025] [Figure 3] A cross-sectional view showing another embodiment of an electro-optical device according to the present invention.

Mode for Carrying Out the Invention

[0026] Hereinafter, the present invention will be more fully described with reference to the accompanying drawings showing exemplary embodiments of the present invention. However, the present invention can be implemented in many different forms and should not be construed as limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and the scope of the present invention will be fully conveyed to those skilled in the art.

[0027] As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. Also, all conjunctions used should be understood in the broadest possible sense. Thus, the word "or" should be understood to have the definition of logical "or" rather than logical "exclusive or" unless the context clearly requires otherwise. Further, unless otherwise specified, the singular forms and the articles "a", "an", and "the" are intended to include the plural forms as well. As used herein, the terms "comprising", "including", "containing", and / or "having" define the presence of the recited features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Further, when an element including a component or subsystem is referred to as being connected or coupled to another element, and / or shown, it will be understood that it may be directly connected or coupled to the other element, or intervening elements may be present.

[0028] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as generally understood by those skilled in the art to which this invention pertains. Furthermore, terms such as those defined in commonly used dictionaries should be interpreted as having the same meaning as their meanings in the context of the relevant art, and it will be understood that they should not be interpreted ideally or excessively formally unless explicitly defined.

[0029] Figures 1A, 1B, 1C, 1D, 1F, and 1G are cross-sectional views illustrating the fabrication of electro-optical devices such as active phase shifters.

[0030] More specifically, Figure 1A shows the initial stages of device manufacturing. The Pockels material 110 is patterned on a lower cladding layer 112. Both of these layers are supported on a wafer 114, such as a silicon wafer.

[0031] Typically, the Pockels material 110 is first introduced as a blanket film and then patterned by wet etching or dry etching. Generally, Pockels materials are materials that exhibit a change in refractive index (Pockels effect) when an electric field is applied. In the current configuration, the Pockels material is BTO or BSTO (barium strontium titanate) ((Ba,Sr)TiO3)). In the current configuration, the Pockels material layer 110 has a refractive index of approximately 2.3 and a thickness of 200-250 nm.

[0032] Figure 1B shows the next step in the fabrication process. A passivation structure 150 is deposited to encapsulate the Pockels layer 110. Its thickness may range from a few nanometers to 100 nanometers (nm), such as 2 nanometers. In the current configuration, this structure has a thickness of 25 nm, and is typically in the range of 5 to 50 nm. In any case, it is usually less than 50% of the thickness of the subsequent waveguide 116 W. The effective refractive index is in the range of 1.44 to 2.1.

[0033] Generally, the passivation structure 150 should include one or more layers and satisfy the characteristics of having low light absorption in the operating wavelength range of the electro-optical device, low hydrogen radical transmittance, and preferably having an effective refractive index lower than that of the waveguide layer (SiN) and the Pockels material 110.

[0034] The passivation structure 150 always includes one or more of the following: SiO2, SiN, Si, Al2O3, STO, CaTiO3, SrO, BTO / STO superlattices, and / or epitaxial oxides (SrHfO3, BaHfO3, SrZrO3, BaZrO3). The passivation structure may also include other layers capable of trapping hydrogen, with low optical loss and low optical index, such as tungsten oxide, chromium oxide, boron nitride, aluminum nitride, and zirconium nitride. See Vincenc Nemanic, "Hydrogen permeation barriers: Basic requirements, materials selection, deposition methods, and quality evaluation" (Nuclear Materials and Energy, Vol. 19, May 2019, pp. 451-457).

[0035] There are several examples of passivation structure 150. Before bonding, SiO2 can be deposited or grown (e.g., thermal oxidation). A SiO2 / SiN / SiO2 trilayer, acting like a "mini" humidity cap, can be used. A SiO2 / Al2O3 / SiO2 trilayer is expected to be very good at blocking hydrogen. Si(amorphous)+SiO2 is another option. Yet another option is to add SrTiO3(STO) on top of BTO. In the current configuration, BTO is grown on Si, first a 4nm STO layer is grown on Si, and then a thicker BTO layer is grown on top. As a result, after wafer bonding, the 4nm STO layer will be placed on top of the BTO layer. Thus, in passivation layer or stack and waveguide material, the resulting stack will be bottom cladding, BTO, 4nm STO, passivation stack, SiN waveguide. However, the role of STO itself in hydrogen blocking is not clear.

[0036] Figure 1C shows the deposition of the waveguide layer 116. In the next step shown, the waveguide layer 116 is patterned into waveguide 116W, as shown in Figure 1D. The passivation structure is used to protect the Pockels material during the fabrication of this waveguide 116W, for example, during the etching process for patterning the waveguide layer 116 into waveguide 116W. Generally, waveguide 116W is an amorphous dielectric material whose refractive index is higher, lower, or equal to that of the Pockels material 110. In the current configuration, waveguide 116W is about 1 micrometer wide to support optical modes (single modes of the C-band and O-band).

[0037] Figure 1E shows the deposition of the upper cladding layer 118. This layer has a refractive index of approximately 1.45 and a thickness ranging from 50 nm to 3000 nm.

[0038] Figure 1F shows the openings of the contact vias 120. In this embodiment, these contact openings extend downward through the upper cladding layer 118 and the passivation layer 150.

[0039] In Figure 1G, the metal contact 122 is formed within the contact opening 120. The metal extends to or near the Pockels layer 110. In some cases, passivation is left beneath the contact, such as a tunnel contact. This can mitigate the reaction between the metal and BTO.

[0040] The refractive index of the passivation structure 150 is important because it occupies the center of the optical modes extending between the waveguide 116W and the Pockels layer. The adverse effects of the passivation structure 150 are reduced when its effective refractive index is lower than that of the Pockels material 110 and the waveguide 116W.

[0041] Figures 2A, 2B, 2C, and 2D are cross-sectional views showing the fabrication of an electro-optic BTO device, such as an active phase shifter, according to another embodiment, using a transfer process.

[0042] Figure 2A shows the results of the initial steps. A seed passivation structure 150 is deposited or grown on a wafer 114. Then, the Pockels material 110 is deposited or transferred onto the passivation structure 150 by other means.

[0043] In a specific embodiment of the current configuration, a thin (approximately 2-4 nm) seed SiOx interface exists between the wafer substrate and the Pockels material 110 after epitaxy.

[0044] Although not shown in the diagram, STO can be used with any additional interface layer beneath BTO. More specifically, in the current configuration, a thin layer of STO is first grown on Si, and then BTO is deposited. The deposition of the BTO film begins to oxidize the interface between Si and STO, thereby forming a 2-4 nm SiO2 layer between the Si substrate and the STO layer. This SiO2 layer can then be thickened by annealing the Si / STO / BTO stack in high-temperature oxygen immediately after the BTO epitaxial process. As a result, the 2-4 nm SiO2 interface layer can be grown to 25-30 nm or more.

[0045] Figure 2B shows the annealing of a seed passivation structure to generate a new layer 150A. Specifically, the grown wafer is annealed in an oxidizing atmosphere (air / oxygen / Ar+water vapor) to oxidize the silicon near the interface and form a high-quality thermal silicon oxide. The thickness can be controlled by temperature / time / atmosphere, and annealing is usually performed until the thickness reaches 5-20 nm, but it can be increased to a maximum thickness of 100 nm or more.

[0046] Simultaneously, the lower cladding layer 112 is deposited onto the handle or transfer wafer 130.

[0047] Figure 2C shows the transfer of the lower cladding layer 112 onto the Pockels material 110. Specifically, a donor wafer 114 having the Pockels layer 110 is inverted and placed on the lower cladding material 112, and the wafer is bonded. Then, in another processing step, the donor wafer 114 is removed using one or a combination of lapping, grinding, chemical mechanical polishing (CMP), and wet etching. The etching solution is selected to be selective for the wafer, and the thermal SiOx passivation structure acts as an etching stop.

[0048] Figure 2D shows the removal of the donor wafer 114. This protects the Pockels layer 110 from the passivation layer 150A. However, in this embodiment, the Pockels material is not encapsulated as shown in Figure 1B.

[0049] Subsequently, as shown in Figures 1C to 1G, further processing can be performed to deposit the waveguide 116W and additional cladding layers along with the formation of metal contacts.

[0050] Figure 3 shows another embodiment, where the phase shifter further includes a second lower waveguide 116L located below or within the BTO layer and parallel to the upper waveguide 116W. The addition of the second lower waveguide improves optical overlap by "pulling down" the optical modes toward the lower waveguide 116L.

[0051] Although the present invention has been shown and described in particular with reference to preferred embodiments, it will be understood by those skilled in the art that various modifications in form and detail can be made without departing from the scope of the invention as encompassed by the appended claims.

Claims

1. Waveguides and A Pockels material adjacent to and / or including the waveguide, The upper cladding layer above the waveguide and An electro-optical device comprising a passivation structure between the waveguide and the Pockels material.

2. The electro-optical device according to claim 1, wherein the waveguide is made of silicon nitride.

3. The electro-optical device according to claim 1 or 2, wherein the passivation structure has a lower refractive index than the waveguide and / or the Pockels material.

4. The electro-optical device according to any one of claims 1 to 3, wherein the passivation structure includes silicon dioxide.

5. The electro-optical device according to any one of claims 1 to 4, wherein the passivation structure comprises silicon dioxide and silicon nitride.

6. The electro-optical device according to any one of claims 1 to 5, wherein the passivation structure has a thickness of 2 to 100 nm.

7. The material of the aforementioned Pockels material is barium titanate (BaTiO ３ An electro-optical device according to any one of claims 1 to 6, including ).

8. The material of the aforementioned Pockels material is barium strontium titanate ((Ba,Sr)TiO ３ An electro-optical device according to any one of claims 1 to 6, including ).

9. The electro-optical device according to any one of claims 1 to 8, wherein the Pockels material is one or more layers.

10. The electro-optical device according to any one of claims 1 to 9, further comprising a lower cladding layer below the Pockels material and / or an upper cladding layer above the Pockels material.

11. The electro-optical device according to any one of claims 1 to 10, further comprising a second waveguide lower cladding layer below the Pockels material.

12. The steps of forming a waveguide, The steps of arranging the Pockels material adjacent to and / or including the waveguide, The steps include providing an upper cladding layer above the waveguide, A method for fabricating an electro-optical device, comprising the step of placing a passivation structure between the waveguide and the Pockels material to protect the Pockels material from a reduction reaction during fabrication.

13. The method according to claim 12, further comprising the step of protecting the Pockels material during the etching process for forming the waveguide.

14. The method according to claim 12 or 13, wherein the passivation structure has a lower refractive index than the waveguide and / or the Pockels material.

15. The method according to any one of claims 12 to 14, wherein the passivation structure comprises silicon dioxide.

16. The method according to any one of claims 12 to 15, wherein the passivation structure comprises silicon dioxide and silicon nitride.

17. The method according to any one of claims 12 to 16, wherein the passivation structure has a thickness of 2 to 100 nm.

18. The material of the aforementioned Pockels material is barium titanate (BaTiO ３ The method according to any one of claims 12 to 17, including )

19. The material of the aforementioned Pockels material is barium strontium titanate ((Ba,Sr)TiO ３ The method according to any one of claims 12 to 17, including )

20. The method according to any one of claims 12 to 19, further comprising the step of forming a second waveguide in the lower cladding layer below the Pockels material.

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