Metastable strained ferroelectric devices
By using the metastable tetragonal phase of BTO and a tensile strain initiation layer on the SiO2 underlayer, the high loss and speed limitation of silicon photonic modulators were solved, realizing a high-efficiency and low-cost electro-optic modulation device.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- LUMIPHASE AG
- Filing Date
- 2024-10-31
- Publication Date
- 2026-06-02
AI Technical Summary
Existing silicon photonic modulators suffer from high insertion loss, limited operating speed, and high power consumption. Furthermore, the size mismatch between integrated Pockels materials such as LiNbO3 and silicon waveguides leads to high chip costs and limited Pockels effect values.
By employing the metastable tetragonal phase of BTO and using direct wafer bonding technology on the SiO2 underlayer, combined with a tensile strain initiation layer such as SrHfO3, an electro-optic device with high Pockels coefficient and low dielectric loss is formed, avoiding structural domain boundaries and achieving efficient electro-optic modulation.
This has enabled electro-optic devices with high Pockels coefficients, reduced dielectric loss and leakage current, improved operating speed and energy efficiency, and lowered chip costs.
Smart Images
Figure CN122139153A_ABST
Abstract
Description
[0001] Related applications
[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 594,982, filed November 1, 2023, pursuant to 35 USC 119(e), which is incorporated herein by reference in its entirety. Background Technology
[0003] For a wide range of applications, silicon photonics has become a platform for dense and low-cost photonic integrated circuits (PICs) that require fast and energy-efficient electro-optic (EO) switches in many cases.
[0004] However, silicon modulators have significant limitations. When the refractive index is modulated, changes in both the real and imaginary parts are affected, resulting in high insertion loss. Furthermore, their operating speed is limited by the charge carrier mobility in forward-biased or reverse-biased devices. State-of-the-art silicon-based modulators are often based on regions within the waveguide that are doped in different ways. Higher speeds require higher doping, but higher doping increases absorption. Another option is heater-based devices. Here, the heater (often a metal wire) changes temperature via Joule heating (current), and thus changes the temperature in the nearby waveguide. However, the heater must be far enough away that strong optical absorption is not visible in the heater conductor. Such heater-based devices tend to be slow, have high power consumption, and are susceptible to crosstalk.
[0005] Pockels materials avoid these problems. In such materials, the change in refractive index is caused by an electric field. However, the Pockels effect does not exist in centrosymmetric crystals such as silicon. Therefore, materials with a fairly large Pockels coefficient must be integrated onto silicon photonic structures to combine the benefits of bulk Pockels modulators with the low manufacturing cost of integrated silicon photonics.
[0006] Several approaches exist for integrating materials with a large effective Pockels effect in silicon-based modulators. For example, the Pockels effect exists in lithium niobate (LiNbO3, LN). Furthermore, lithium niobate has been integrated with silicon waveguides, for example, via wafer bonding. However, the size mismatch between LN wafers and silicon wafers makes it difficult to scale the integration process to large substrate sizes (resulting in considerably high chip costs), and the magnitude of the Pockels effect in LN is limited to 20-30 pm / V.
[0007] Barium titanate (BaTiO3, BTO) has emerged to enable the realization of Pockels-based devices on silicon for several reasons. First, BTO has one of the largest Pockels coefficients. Second, it has previously been used in thin-film EO modulators on small-size oxide substrates. Third, BTO can be grown on silicon substrates with large wafer sizes and excellent crystal quality. In fact, BTO-based photonic electro-optic components on silicon wafers have been demonstrated.
[0008] BTO exhibits different phases with varying electro-optic properties. The cubic phase is stable and paraelectric at high temperatures (above Tc = 120°C). However, it lacks spontaneous polarization and does not exhibit any Pockels effect. Below Tc and down to room temperature, BTO has a tetragonal structure. This phase is ferroelectric, meaning it possesses spontaneous polarization that can be reversed by applying an external electric field. The Pockels effect is large, and the electro-optic properties are well-defined relative to the direction of the polarization vector. At even lower temperatures, BTO undergoes transitions to orthorhombic and rhombohedral phases, which are also ferroelectric and exhibit large Pockels effects.
[0009] In BTO thin films, metastable phases can exist under specific conditions. Metastable phases are not the most thermodynamically stable phases of the material for a given temperature range, but they can be stabilized in the presence of external parameters. In thin films, the exact nature of this specific additional parameter can vary, but the known pressure and resulting material strain lead to the stabilization of the metastable phase. This stabilization process can result in the acquisition of a bulk phase outside their typical temperature range (e.g., tetragonal BTO stable above Tc = 120°C), or it can lead to the stabilization of structures not present in their bulk form. Summary of the Invention
[0010] High-efficiency EO devices using BTO require high Pockels coefficients, low dielectric loss, and low leakage current to achieve high performance. In other words, current BTO technology is limited by a direct trade-off between dielectric loss and the achieved Pockels coefficient.
[0011] Some metastable phases of BTO offer better trade-offs. Metastable tetragonal crystalline phases of BTO, for example, are predicted to emerge upon application of biaxial tensile strain. Bulk BTO tetragonal phase exhibits one long axis and two short axes of equal length, with polarization along the long axis. Metastable BTO tetragonal phase has one short axis and two long axes of equal length. Several benefits can be achieved. Tensile strain in BTO leads to higher bulk polarization, thus favoring a higher Pockels coefficient. Furthermore, the metastable tetragonal phase will not exhibit the typical ac crystallographic domains expected for bulk tetragonal BTO films. Fewer domain boundaries are beneficial, providing lower leakage and dielectric losses. It can also favor the formation of larger ferroelectric domains, which in turn is beneficial for reducing dielectric losses associated with domain wall motion.
[0012] Based on this understanding, metastable phases of BTO are used to fabricate EO devices. The metastable tetragonal phase of BTO is preserved by using direct wafer bonding on a thick SiO2 underlayer for optical confinement in active BTO.
[0013] Metastable BTO phases are obtained by employing tensile strain, for example, using stress generated by adjacent crystal layers with larger lattice units, or using stress generated by bonding processes or any other process steps. This can form metastable phases with higher Pockels effects and no in-plane structural domains. The present invention produces a high-Pockels, low-loss layer in which efficient confinement is achieved in an active BTO layer exhibiting the Pockels effect.
[0014] Generally speaking, according to one aspect, the present invention is characterized by an electro-optic device comprising a strained Pockels material stabilized in a metastable structure bonded to an underlying optical cladding.
[0015] In the current example, the Pockels material is BTO.
[0016] A tensile strain initiation layer, such as SrHfO3, is preferably used to induce tensile strain in Pockels materials.
[0017] Generally speaking, according to one aspect, the present invention is characterized by a method for manufacturing an electro-optic device, the method comprising forming a Pockels material on a tensile strain initiation layer that induces tensile strain, transferring the Pockels material to a lower cladding layer, and forming an electro-optic device comprising the Pockels material.
[0018] The above-described features and other features, including various novel combinations of construction details and components, of the present invention will now be described more specifically with reference to the accompanying drawings and pointed out in the claims. It will be understood that the specific methods and devices for carrying out the invention are shown by way of illustration and not by way of limitation. The principles and features of the invention may be employed in various and numerous embodiments without departing from the scope of the invention. Attached Figure Description
[0019] In the accompanying drawings, reference numerals are used throughout the different views to denote the same parts. The drawings are not necessarily drawn to scale; rather, the focus is on illustrating the principles of the invention. In the accompanying drawings:
[0020] Figure 1A , Figure 1B , Figure 1C and Figure 1D This is a cross-sectional view illustrating the process of forming a metastable phase in the BTO layer of an EO device; and
[0021] Figure 2A , Figure 2B , Figure 2C and Figure 2D This is a cross-sectional view illustrating another process for forming a metastable phase in the BTO layer of an EO device. Detailed Implementation
[0022] The invention will now be described more fully with reference to the accompanying drawings, which illustrate illustrative embodiments of the invention. However, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided to make this disclosure thorough and complete, and to fully convey the scope of the invention to those skilled in the art.
[0023] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items. Furthermore, all conjunctions used should be understood in the most inclusive sense possible. Therefore, unless the context explicitly requires it, the word “or” should be understood as having the definition of logical “or” rather than logical “mutually exclusive or”. Additionally, unless explicitly stated otherwise, the singular form and the articles “a” and “the” are also intended to include the plural form. It will be further understood that the terms “includes,” “comprises,” “including,” and / or “comprising”, when used in this specification, specify the presence of the stated 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. Furthermore, it will be understood that when an element comprising a component or subsystem is mentioned and / or shown as connected or coupled to another element, it may be directly connected or coupled to that other element, or there may be an intermediate element present.
[0024] Unless otherwise defined, all terms used herein (including technical and scientific terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It will also be understood that terms (such as those defined in common dictionaries) shall be interpreted as having the same meaning as they have in the context of the relevant field and shall not be interpreted in an idealized or overly formal sense unless expressly defined herein.
[0025] Figure 1A , Figure 1B , Figure 1C and Figure 1D This is a cross-sectional view showing the process for forming an electro-optic device with a metastable BTO layer.
[0026] like Figure 1A As shown, a BTO layer 110 is formed on a SrHfO3 layer 112, which in turn is formed on a sacrificial silicon wafer 114.
[0027] SrHfO3 is a cubic perovskite structured crystal used as the tensile strain initiation layer in BTO layer 110, and it also acts as the interface with silicon 114. To achieve this, SrHfO3 is grown in a relaxed manner. SrHfO3 has a larger lattice than BTO. Therefore, BTO attempts to adapt to this larger lattice, thereby initiating tensile strain.
[0028] like Figure 1B As shown, the tensile strain is maintained in the BTO.
[0029] Then, as Figure 1C As shown, in the next step of the manufacturing process, the BTO is flipped and the wafer is bonded to a silicon dioxide under-cladding 116, which is, for example, supported on a silicon-processed wafer 118.
[0030] In this transfer, the tensile strain in BTO 110 is enhanced by utilizing the different coefficients of thermal expansion of silicon and BTO, thereby maintaining the desired metastable tetragonal phase in BTO.
[0031] Following this transfer, the sacrificial silicon 114 is removed. Furthermore, in different embodiments, SrHfO3 112 is removed (see [reference]). Figure 1C and Figure 1D ) or be maintained (see Figure 2A , Figure 2B , Figure 2C and Figure 2D ).
[0032] In typical subsequent steps, the upper package structure 120 and / or electrode 122 are added to complete the EO device.
[0033] While the invention has been particularly shown and described with reference to preferred embodiments thereof, those skilled in the art will understand that various changes in form and detail may be made therein without departing from the scope of the invention as covered by the appended claims.
Claims
1. An electro-optic device, comprising: Stressed Pockels materials with metastable structures; The lower optical cladding, and possibly the upper cladding and / or electrodes.
2. The device of claim 1, wherein the Pockels material is BTO.
3. The device of claim 1, further comprising a tensile strain initiation layer for initiating tensile strain in the Pockels material.
4. The device of claim 3, wherein the tensile strain initiating layer comprises SrHfO3.
5. The device of claim 3, wherein the tensile strain initiating layer comprises a material having a larger lattice constant than BTO.
6. The device of claim 3, wherein the tensile strain in the Pockels material is caused by lattice mismatch, difference in thermal expansion coefficients, or a combination thereof.
7. The device of any one of claims 1, wherein the metastable structure is a metastable tetragonal phase of BTO, the metastable tetragonal phase of BTO being stabilized at a temperature above 120°C using a tensile strain initiation layer.
8. The electro-optic device of claim 1, further comprising an electrode arrangement configured to apply an electric field in a direction parallel to the polarization vector of the metastable phase of the BTO, wherein the applied field enhances the effective electro-optic response.
9. The device of claim 1, wherein the electro-optic device further comprises a waveguide layer comprising silicon or silicon nitride optically coupled to a strained BTO material.
10. The device of claim 1, wherein the upper cladding layer comprises a low-loss dielectric material selected from the group consisting of silicon dioxide, silicon nitride, and aluminum oxide.
11. A method for manufacturing an electro-optical device, comprising: Pockels material is formed on a tensile strain initiation layer, which induces tensile strain in the Pockels material. Transferring Pockels material to the lower cladding layer; and Forming an electro-optic device comprising Pockels material and possibly an upper cladding and / or electrodes.
12. The method of claim 11, wherein the Pockels material is BTO.
13. The method of claim 11, further comprising removing the tensile strain initiation layer.
14. The method of claim 11, further comprising maintaining the tensile strain initiation layer.
15. The method of claim 11, wherein the tensile strain initiating layer comprises SrHfO3.
16. The method of claim 11, wherein tensile strain is maintained during bonding by controlling temperature and pressure conditions to optimize lattice alignment between the Pockels material and the cladding.