RU2841350C1 - Integrated electrooptic modulator - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention relates to photonic integrated circuits and can be used in microwave information processing and transmission devices, as well as in computer circuits and optical radiation direction systems. Integrated electro-optical modulator has substrate 1 on which the following components are arranged in series: optical cladding 2, elongated along Y axis waveguide 3, transparent conducting oxide layer 4, elongated along the X coordinate, perpendicular to the Y coordinate, and crossing mainly at angle of 90° waveguide 3, dielectric layer 5, which coincides in width with transparent conducting layer of oxide 4, leaves free its edges and is located above waveguide 3, metal electrode 6, which coincides in width with dielectric layer 5 and is located above waveguide 3, wherein transparent conducting oxide layer 4, dielectric layer 5 and metal electrode 6 form capacitor 8, wherein on free edge of transparent conducting oxide layer 4 there is metal contact 7, silicon nitride is used as material for waveguide 3.

EFFECT: reduced optical losses of the waveguide circuit of the modulator and possibility of operating in the visible wavelength range.

6 cl, 12 dwg

Description

The invention relates to the field of photonic integrated circuits and can be used in devices for ultra-high-frequency processing and transmission of information, as well as in computing circuits and systems for directing optical radiation.

A semiconductor optical modulator is known which is a silicon ridge waveguide formed on a buried oxide layer. The waveguide is formed by etching silicon from a silicon-on-insulator (SOI) wafer. The waveguide dimensions may range from about 150 nm to about 1000 nm in both transverse dimensions. Thus, the silicon layer on the buried oxide layer has a silicon waveguide protruding from it. A dielectric shell, for example, SiO _2, may surround the semiconductor waveguide on one or more of its sides. The structure is a series of layers forming a capacitor which has an input, an output, an insulator and a resistive material inside which contributes the main resistive function of the element. The entire waveguide section may be doped, i.e. the entire waveguide section inside the modulator may be doped over its entire cross-section. A thin dielectric layer, also known as a "gate dielectric layer", is formed on one or more sides of the waveguide. In embodiments of the invention, the gate dielectric material has a high permittivity, i.e. in the range of about 4-10 or more. In addition, in embodiments of the invention, the gate dielectric material is transparent at the operating wavelengths of the modulator. The dielectric material with a high permittivity of the dielectric layer is located so as to contact, for example, with a coating directly adjacent to a doped semiconductor layer on one or more sides of the waveguide. A transparent conductive layer is located so as to be in contact with the gate dielectric layer, for example, in the form of a coating located on one or more immediately adjacent sides of the waveguide. The transparent conductive layer does not necessarily have to cover the entire gate dielectric layer. In some embodiments of the invention, the transparent conductive layer is on the "upper" side of the waveguide, as well as on its two lateral "sides", but not on its "lower" side, which is the part of the waveguide closest to the silicon layer, the waveguide of which is formed on the buried oxide layer. The first electrical contact, i.e. the so-called "lower" contact, with low resistance to the doped semiconductor layer, which extends in the silicon layer emerging from the waveguide, can be formed on the doped silicon layer near the waveguide. For example, the lower contact can be formed as close as possible to the rest of the modulator structure without forming a short circuit between the lower contact and the rest of the modulator structure, for example, the transparent conductor and/or the second contact [US Patent 10,908,438].

The disadvantage of this device is the high optical losses of the silicon waveguide throughout the photonic circuit and the inability to work with the visible range.

Also known is an integrated electro-optical modulator, comprising a substrate on which an optical shell, a waveguide extended along the Y coordinate, a transparent conductive oxide layer extended along the X coordinate, perpendicular to the Y coordinate, and intersecting the waveguide predominantly at an angle of 90°, a dielectric layer that coincides in width with the transparent conductive oxide layer, leaves its edges free, and is located above the waveguide, a metal electrode that coincides in width with the dielectric layer, and is located above the waveguide, wherein the transparent conductive oxide layer, the dielectric layer and the metal electrode form a capacitor, wherein a metal contact is located on the free edge of the transparent conductive oxide layer. [Patent US 10,908,440 B1]. The disadvantage of this device is the high optical losses of the silicon waveguide throughout the photonic circuit and the inability to work with the visible range.

This device was chosen as a prototype of the proposed solution.

The technical result of the invention consists in reducing the optical losses of the modulator waveguide circuit and the ability to work with the visible range of wavelengths.

The essence of the invention lies in the fact that in an integrated electro-optical modulator, containing a substrate on which an optical shell, a waveguide extended along the Y coordinate, a transparent conductive oxide layer extended along the X coordinate, perpendicular to the Y coordinate, and intersecting the waveguide predominantly at an angle of 90°, a dielectric layer that coincides in width with the transparent conductive oxide layer, leaves its edges free, and is located above the waveguide, a metal electrode that is located above the waveguide, wherein the transparent conductive oxide layer, the dielectric layer and the metal electrode form a capacitor, wherein a metal contact is located on the free edge of the transparent conductive oxide layer, silicon nitride is used as the material for the waveguide.

There is a variant in which the waveguide has a rectangular cross-section and is located on the optical cladding.

There is also a variant in which the waveguide has a rectangular cross-section and is recessed into the optical cladding.

There is also a variant in which the waveguide has a broadening along the X coordinate, located under a transparent conducting oxide layer.

There is also a variant in which a transparent conducting oxide layer intersects the waveguide at an angle of 45°.

There is also a variant in which the metal electrode extends beyond the transparent conductive oxide layer on at least one side of the waveguide.

Fig. 1 shows a diagram of an integrated electro-optical modulator (top view).

Fig. 2 shows section A-A in Fig. 1.

Fig. 3 shows a diagram of an integrated electro-optical modulator with a recessed waveguide (section)

Fig. 4 shows a diagram of an integrated electro-optical modulator with trapezoidal elements (top view)

Fig. 5 shows a diagram of an integrated electro-optical modulator with a transparent conductive oxide tilted at 45° (top view)

Fig. 6 shows a diagram of an integrated electro-optical modulator with the upper metal electrode being brought out beyond the transparent conducting oxide and dielectric (top view)

Fig. 7 shows a diagram of an integrated electro-optical modulator with a metal electrode and a metal contact located on both sides of the waveguide (top view)

Fig. 8 shows a diagram of the formation of a protruding waveguide through a resistive mask (section)

Fig. 9 shows a diagram of the formation of a recessed waveguide using the process of chemical-mechanical planarization (section)

Fig. 10 shows a diagram of the formation of a layer of transparent conducting oxide by electron beam evaporation of In ₂ O ₃ or In ₂ O ₃ + SnO from a crucible and the assistance of Ar and O ₂ gas ions through an ion source (section)

Fig. 11 shows a diagram of the formation of dielectric structures (section)

Fig. 12 shows a diagram of the formation of a metal electrode and a metal contact through a resistive mask (section)

The integrated electro-optical modulator comprises a substrate 1 (Fig. 1, Fig. 2) on which an optical cladding 2, a waveguide 3 extended along the Y coordinate, and a transparent conductive oxide layer 4 extended along the X coordinate perpendicular to the Y coordinate are sequentially arranged. Silicon can be used as the substrate 1. The optical cladding 2 can be made of silicon oxide. The length of the waveguide 3 can be in the range from 10 μm to 1 m. The width of the waveguide 3 along the X coordinate can be in the range from 2300 to 3500 nm. The thickness of the waveguide 3 along the Z coordinate can be in the range from 300 to 600 nm. The material of the transparent conductive oxide layer 4 can be indium oxide or indium tin oxide. The length of the transparent conductive oxide layer 4 can be in the range from 25 to 200 μm. The width of the transparent conductive oxide layer 4 along the Y coordinate may be in the range of 1.55 to 15.5 μm. The thickness of the transparent conductive oxide layer 4 along the Z coordinate may be in the range of 10 to 30 nm. In this case, the transparent conductive oxide layer 4 intersects the waveguide 3 predominantly at an angle of 90°. A dielectric layer 5 is located on the transparent conductive oxide layer 4, which coincides with it in width, leaves its edges free, and is located above the waveguide 3. Aluminum oxide or hafnium oxide can be used as the material of the dielectric 5. The length of the dielectric 5 along the X coordinate may be in the range of 4 to 100 μm. The thickness of the dielectric 5 along the Z coordinate may be in the range of 10 to 30 nm. On the dielectric 5, there is a metal electrode 6, which coincides in width with the dielectric layer 5 and is located above the waveguide 3. Gold, aluminum or silver can be used as the material of the metal electrode 6. The length of the metal electrode 6 along the X coordinate can be in the range from 4 to 10 μm. The thickness of the metal electrode 6 along the Z coordinate can be in the range from 40 to 300 nm. The transparent conductive oxide layer 4, the dielectric layer 5 and the metal electrode 6 form a capacitor 8. On the free edge of the transparent conducting oxide layer 4, there is a metal contact 7 made, for example, of gold, aluminum or silver. The dimensions of the metal contact 7 along the X, Y and Z coordinates can be in the ranges from 10 to 80 μm, from 1 to 15 μm, from 40 to 300 nm, respectively. Silicon nitride is used as the material for the waveguide 3.

There is a variant in which the waveguide 3 (Fig. 2) has a rectangular cross-section and is located on the optical shell 2.

There is also a variant in which the waveguide 3 (Fig. 3) has a rectangular cross-section and is recessed into the optical shell 2.

There is also a variant in which the waveguide 3 (Fig. 4) has a broadening 9 along the X coordinate, located under the transparent conducting oxide layer 4. The dimensions of the broadening 9 along the X and Y coordinates can be in the ranges from 2900 to 4000 nm, from 10 to 30 µm, respectively. The broadening 9 can begin as shown in Fig. 4 and extend to its full width under the capacitor 8.

There is also a variant in which the transparent conducting oxide layer 4 (Fig. 5) intersects the waveguide 3 at an angle of 45°. This variant is optimal, but there may be other angles of the location of the transparent conducting oxide layer 4 relative to the waveguide 3.

There is also a variant in which the metal electrode 6 extends beyond the layer of transparent conductive oxide 4 (Fig. 6), at least on one side of the waveguide 3. In this case, the outgoing part of the metal electrode 6 can have a C-shape with two outputs (as indicated in Fig. 6), but there can also be other shapes, for example, an L-shape with one output.

There is also a variant in which the metal electrode 6 extends beyond the layer of transparent conductive oxide 4 (Fig. 7) on both sides of the waveguide 3. In this case, the outgoing parts of the metal electrode 6 can have C-shaped forms with two outputs (as indicated), but there can also be other forms, for example, L-shaped with one output.

The method for manufacturing an integrated electro-optical modulator may consist of the following operations. An optical shell of silicon oxide 2, several micrometers thick, is applied to a silicon substrate 1 by thermal oxidation in water vapor. Then, a layer of silicon nitride is applied by low-pressure chemical vapor deposition. Next, a waveguide 3 made of silicon nitride is formed using electron-beam lithography and plasma-chemical etching (PECE) through the first resistive mask 10 (Fig. 8) [for more details, see Liu, J., Huang, G., Wang, R.N. et al. Nat Commun 12, 2236 (2021)]. Broadenings 9 can also be formed at this stage. In the case of a design with a recessed waveguide 3 in the optical shell 2, the sequence of operations can be as follows. After applying the optical shell 2, rectangular grooves for future waveguides are formed in it using lithography and PCT methods; then a silicon nitride film is applied using the low-pressure vapor deposition method; then the silicon nitride surface is planarized right up to the walls of the optical shell using the chemical-mechanical planarization method with a grinding pad 11 so that only the waveguide 3 remains in the grooves (Fig. 9) [for more details, see Churaev M.et al.Nat Commun. 14, 1 (2023)]. Then, operations for forming capacitor 8 are carried out. Through the previously formed second resistive mask 12, a layer of transparent conducting oxide 4 made of indium tin oxide or indium oxide is deposited, for example, by the method of electron beam evaporation from crucible 13 with the assistance of oxygen and argon ions from ion source 14 (Fig. 10) [for more details, see Lotkov, E.S., Baburin, A.S., Ryzhikov, I.A. et al. Sci Rep 12, 6321 (2022)]. Next, aluminum oxide or hafnium oxide is applied by the method of atomic layer deposition, followed by wet etching of the dielectric layer 5 through the third resistive mask 15 (Fig. 11). Then, the structures of the metal electrode 6 and the metal contact 7 are applied by vacuum deposition through the fourth resistive mask 16 prepared by electron beam lithography (Fig. 12) [for more details, see Amin, R., Maiti, R., Gui, Y. et al. Sci Rep 11, 1287 (2021)]. For all the described designs, the operations for forming the layers of the capacitor 8 will be similar. The signal can be input into the end or vertical-lattice elements manufactured at the stage of forming the waveguide 3 and, if necessary, by polishing the surface of the end of the waveguide 3 [For details, see Mu, X., Wu, S., Cheng, L., & Fu, H. Y. Applied Sciences, 10(4), 1538, (2020)].

The integrated electro-optical modulator operates as follows:

When an electric potential is applied between the metal electrode 6 and the metal contact 7, a charge accumulation layer of a few nanometers thick is formed at the boundary of the dielectric layer 5 and the transparent conducting oxide 4 in the capacitor 8, where the electron concentration increases significantly compared to the initial one. In the charge accumulation region, the transparent conducting oxide layer 4 changes the real and imaginary part of the permittivity due to the dispersion effect of free charge carriers and, consequently, the real and imaginary part of the refractive index. The dependence of the permittivity on the electron concentration in transparent conducting oxides is determined by the Drude-Lorentz model. The optical mode, which enters the capacitor 8 from the waveguide 3, is localized in the layers of the transparent conducting oxide layer 4 and the dielectric layer 5, so that its volume intersects the volume of the charge accumulation layer. Therefore, it is absorbed or changes phase depending on the initial characteristics of the layer of transparent conductive oxide layer 4, and can also change the localization position. Thus, it is possible to control both the intensity and the phase of the radiation passing through the modulator.

The fact that in an integrated electro-optical modulator containing a substrate 1 on which an optical cladding 2, a waveguide 3 extended along the Y coordinate, a transparent conductive oxide layer 4 extended along the X coordinate perpendicular to the Y coordinate and intersecting the waveguide 3 predominantly at an angle of 90°, a dielectric layer 5 which coincides in width with the transparent conductive oxide layer 4, leaves its edges free and is located above the waveguide 3, a metal electrode 6 which coincides in width with the dielectric layer 5 and is located above the waveguide 3, wherein the transparent conductive oxide layer 4, the dielectric layer 5 and the metal electrode 6 form a capacitor 8, wherein a metal contact 7 is located on the free edge of the transparent conductive oxide layer 4, silicon nitride is used as the material for the waveguide 3, provides a reduction in the optical losses of the modulator waveguide circuit from no more than 0.1 dB/cm up to no more than 0.01 dB/cm due to the absence of free charge carriers and impurities in silicon nitride and, consequently, a reduced absorption coefficient of the material and the ability to work with a wavelength range from 400 to 8000 nm.

The fact that waveguide 3 has a rectangular cross-section and is located on optical cladding 2 makes it possible to localize radiation as much as possible inside waveguide 3, improve mode coupling between waveguide 3 and capacitor 8 and, consequently, reduce optical losses introduced by coupling.

The fact that waveguide 3 has a rectangular cross-section and is deepened into optical shell 2 eliminates the possibility of the appearance of optical modes of other polarizations inside capacitor 8 due to the absence of conditions for propagation on the side walls of waveguide 3, which leads to a reduction in optical losses introduced by polarization violation and an increase in the efficiency of the device due to an increase in the degree of localization of the useful optical mode.

The fact that waveguide 3 has a broadening 9 along the X coordinate, located under the transparent conducting oxide layer 4, makes it possible to increase the effective refractive index of the optical mode inside waveguide 3 and improve the mode coupling between waveguide 3 and capacitor 8 and, consequently, reduce the optical losses introduced by coupling.

The fact that the transparent conductive oxide layer 4 intersects the waveguide 3 at an angle of 45° makes it possible to reduce the capacitance of the capacitor 8 due to a decrease in the overlap area of the metal electrode 6 and the transparent conductive oxide layer 4 and, thereby, to increase the speed of the device due to a decrease in the charging time.

The fact that the metal electrode 6 extends beyond the layer of transparent conductive oxide 4, at least on one side of the waveguide 3, makes it possible to obtain the required value of the impedance of the metal electrode 6 and the metal contact 7, to reduce parasitic reflections of the high-frequency signal from the metal electrode 6 and the metal contact 7, and, consequently, to increase the modulation efficiency at high frequencies due to the increased proportion of the useful signal.

Claims (6)

1. An integrated electro-optical modulator comprising a substrate on which an optical cladding, a waveguide extended along the Y coordinate, a transparent conductive oxide layer extended along the X coordinate perpendicular to the Y coordinate and intersecting the waveguide predominantly at an angle of 90°, a dielectric layer which coincides in width with the transparent conductive oxide layer, leaves its edges free and is located above the waveguide, a metal electrode which is located above the waveguide, wherein the transparent conductive oxide layer, the dielectric layer and the metal electrode form a capacitor, wherein a metal contact is located on the free edge of the transparent conductive oxide layer, characterized in that silicon nitride is used as the material for the waveguide. 2. The device according to item 1, characterized in that the waveguide has a rectangular cross-section and is located on the optical shell. 3. The device according to item 1, characterized in that the waveguide has a rectangular cross-section and is recessed into the optical shell. 4. The device according to item 1, characterized in that the waveguide has a broadening along the X coordinate, located under the transparent conductive oxide layer. 5. The device according to item 1, characterized in that the transparent conductive oxide layer intersects the waveguide at an angle of 45°. 6. The device according to claim 1, characterized in that the metal electrode extends beyond the layer of transparent conductive oxide on at least one side of the waveguide.