Electro-optical modulation chip, electro-optical modulator and forming method of electro-optical modulator

Abstract

The invention relates to the technical field of semiconductors, in particular to an electro-optical modulation chip, an electro-optical modulator and a forming method of the electro-optical modulator. A Z-cut film lithium niobate chip plane isotropy and a vertical direction modulation electric field are utilized, the defect of crystal axis inversion before and after waveguide bending in the prior art is overcome, flexible and compact layout is realized, and meanwhile, the characteristic that a vertical electrode structure can be modulated when the waveguide is bent and the structure is changed is utilized, so that the device can be applied to the field of waveguide bending, and the device size is not increased. The length of a modulation area is increased, the modulation efficiency is improved, the defects of phase distortion, amplitude attenuation and the like in an existing modulator are overcome, and the modulator has higher speed and wider bandwidth.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor packaging materials, and in particular to an electro-optic modulation chip, an electro-optic modulator and a molding method thereof. Background Art

[0002] Lithium niobate single crystal film (LNOI) has excellent physical properties, such as wide transparent bandwidth, strong electro-optic coefficient, good thermal stability, etc., and is considered to be an ideal material for manufacturing high-performance electro-optic modulators. In the existing technology: it is still difficult to modulate the absolute refractive index of LNOI using voltage: in order to accumulate π phase shift and provide a reasonable driving voltage, the electro-optic modulator needs a length of millimeters or even centimeters; the device size is difficult to meet the requirements of photonic integrated circuits for device compactness, which is not conducive to improving the integration of photonic integrated circuits.

[0003] In order to solve the above technical problems, the prior art has prepared an X-cut LNOI modulator, but it has the following defects: on the one hand, in order to utilize the maximum linear electro-optic coefficient of the LN material in the Z direction, a horizontally configured GSG electrode is required as a modulation electrode, but the horizontally configured GSG electrode is not conducive to the horizontal folding of the LNOI waveguide; on the other hand, the refractive index of the LN material in the X-cut and Y-cut is anisotropic on the plane, which requires the X-cut and Y-cut (such as Figure 2 The optical waveguide of LN material prepared by X-cut or Y-cut LNOI can only run in a straight line to avoid anisotropy and utilize the electro-optic effect, which limits the further reduction of the volume of X-cut or Y-cut LNOI modulator and makes it difficult to meet the high integration requirements of photonic integrated circuits. At present, the market is in urgent need of an electro-optic modulator with small size and high integration that meets the requirements of photonic integrated circuits. Summary of the invention

[0004] In view of the above analysis, the present invention aims to provide an electro-optic modulation chip, an electro-optic modulator and a forming method thereof, so as to solve at least one of the problems of the prior art, such as the length, large volume and low integration of the electro-optic modulator.

[0005] An electro-optical modulation chip, comprising:

[0006] substrate layer;

[0007] a back electrode formed on top of the substrate layer;

[0008] a bottom dielectric layer formed on top of the back electrode;

[0009] a bottom modulation layer formed on top of the bottom dielectric layer;

[0010] Top waveguide layer: formed on the top of the bottom modulation layer; the top waveguide layer is provided with a plurality of waveguide units connected end to end, and the input optical signal propagates in the waveguide area formed by the waveguide unit and the bottom modulation layer; the waveguide unit and the bottom modulation layer below form a continuous and folded modulation area connected end to end;

[0011] A top dielectric layer formed on the free area of ​​the bottom modulation layer and the top and side of the waveguide unit;

[0012] A top electrode is formed on top of the top dielectric layer.

[0013] Preferably, the bottom modulation layer is Z-cut thin film lithium niobate, and the top waveguide layer is silicon nitride.

[0014] Preferably, the back electrode and the top electrode are arranged horizontally to provide a vertical modulation electric field.

[0015] Preferably, the top dielectric layer is photoresist.

[0016] Preferably, the photoresist is SU8 photoresist.

[0017] Preferably, the thickness of the top waveguide layer is 200 nm to 300 nm; the width of the top waveguide layer is 1 um to 1.5 um; and the distance between adjacent waveguide units is 8 um to 10 um.

[0018] A method for forming an electro-optical modulation chip, used for preparing the electro-optical modulation chip, comprises the following steps:

[0019] Step 1: Grow the top waveguide layer on the bottom modulation layer of the thin-film lithium niobate wafer;

[0020] Step 2: Etch the top waveguide layer to the target shape to prepare a waveguide unit connected end to end;

[0021] Step 3: In a blank area where no waveguide unit is set, etching the bottom modulation layer and the bottom dielectric layer of the thin-film lithium niobate wafer to the back electrode, and obtaining a bare back electrode in the etched area;

[0022] Step 4: Spin-coat photoresist on the upper surface of the preformed chip prepared in step 3 and develop the photoresist, form a top dielectric layer on the top of the bottom modulation layer and the top waveguide layer and the etched area in step 3, and further etch the top dielectric layer in the etched area in step 3 to obtain a bare back electrode;

[0023] Step 5: forming an original electrode layer on the etched area of ​​step 4 and on top of the top dielectric layer;

[0024] Step 6: Etch the original electrode layer to a target shape, form a top electrode on the top of the top dielectric layer, and form an external electrode connected to the back electrode in the exposed area of ​​the back electrode.

[0025] Preferably, after growing the top waveguide in step 1, a metal chromium layer is continuously grown as a protective layer.

[0026] An electro-optic modulator comprises an input coupler, an output coupler, a first modulation arm and a second modulation arm; the first modulation arm and the second modulation arm each comprise an electro-optic modulation chip as described above.

[0027] Preferably, the half-wave voltage of the electro-optic modulator is 4V to 7V, the modulation bandwidth is 19GHz to 28GHz, and the insertion loss is 8dB to 11dB.

[0028] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0029] (1) The present invention adopts a Z-cut thin film lithium niobate chip to prepare a modulator. The Z-cut thin film lithium niobate chip is isotropic in plane, which overcomes the defect of the prior art that the crystal axis is reversed before and after the waveguide is bent, and realizes a flexible and compact layout. At the same time, the Z-cut thin film lithium niobate chip is isotropic in plane, so that it can match the vertical electrode structure and has the characteristics of being able to modulate when the waveguide is bent and the structure changes. It can increase the length of the modulation area without increasing the size of the device, thereby improving the modulation efficiency.

[0030] (2) The present invention uses a Z-cut thin film lithium niobate chip to prepare a modulator, wherein the electrode direction of the Z-cut thin film lithium niobate crystal is perpendicular to the propagation direction of the light wave, and the electrode has little effect on the light wave, thereby improving the defects of phase distortion and amplitude attenuation in X-cut and Y-cut thin film lithium niobate modulators;

[0031] (3) The present invention uses a Z-cut thin film lithium niobate chip to prepare a modulator, and uses the Z-cut thin film lithium niobate crystal to have a higher nonlinear coefficient and smaller dispersion, thereby obtaining a higher rate and a wider bandwidth. The half-wave voltage of the electro-optic modulator prepared with the Z-cut thin film lithium niobate chip is 4V to 7V, the modulation bandwidth is 19GHz to 28GHz, and the insertion loss is 8dB to 11dB;

[0032] (4) The present invention uses a Z-cut thin film lithium niobate chip to prepare a modulator, and utilizes the high thermal stability and small thermal drift of the Z-cut thin film lithium niobate crystal, which enables the Z-cut thin film lithium niobate modulator to operate in a wider temperature range and have better long-term stability;

[0033] (5) When preparing a Z-cut thin-film lithium niobate chip, the present invention, on the one hand, provides a metal chromium layer before dry etching to reduce damage to each layer caused by dry etching; on the other hand, the metal chromium layer acts as a conductive layer to reduce charging during photolithography, thereby overcoming the defects of reduced modulation bandwidth, increased insertion loss, and decreased electro-optical modulation performance caused by damage;

[0034] (6) When preparing the Z-cut thin-film lithium niobate chip, the present invention performs dry etching on the lithium niobate layer. The argon-based plasma gas contains trifluoromethane. The mixed gas has high selectivity, helps to obtain precise size and smooth surface and good etching feature quality, can increase the modulation bandwidth, reduce the insertion loss, and improve the performance of the electro-optic modulator;

[0035] (7) The present invention selects SU8 photoresist for the top dielectric layer. On the one hand, SU8 photoresist has higher resolution and smaller lateral drift, and a larger refractive index; on the other hand, SU8 photoresist can achieve higher processing accuracy through multiple exposures and developments, and can be processed using an ordinary ultraviolet exposure machine, which helps to further reduce costs.

[0036] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the embodiments of the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0038] Figure 1 is a schematic diagram of the refractive index of anisotropic media;

[0039] Figure 2 Schematic diagram of TE mode and modulation electric field direction when it is X-cut or Y-cut;

[0040] Figure 3 When it is Z-cut, the TM mode is in the direction of the modulated electric field;

[0041] Figure 4 A schematic diagram of a Z-cut thin-film lithium niobate chip in one embodiment of the present invention;

[0042] Figure 5 A schematic diagram of the arrangement of waveguide units in the top waveguide layer of a Z-cut thin-film lithium niobate chip in one embodiment of the present invention;

[0043] Figure 6 A schematic diagram of the structure of a thin-film lithium niobate wafer in one embodiment of the present invention;

[0044] Figure 7 A schematic diagram of the structure of a preformed Z-cut thin-film lithium niobate chip after growing a silicon nitride layer in one embodiment of the present invention;

[0045] Figure 8 It is a schematic diagram of the structure of a preformed Z-cut thin-film lithium niobate chip after the waveguide unit is formed in one embodiment of the present invention;

[0046] Fig. 9 A schematic diagram of the structure of a preformed Z-cut thin-film lithium niobate chip after forming a top dielectric layer in one embodiment of the present invention;

[0047] Fig.10 A schematic diagram of the structure of a preformed Z-cut thin-film lithium niobate chip after etching the top dielectric layer in one embodiment of the present invention;

[0048] Fig.11 A schematic diagram of the structure of a preformed Z-cut thin-film lithium niobate chip after forming an original electrode layer in one embodiment of the present invention;

[0049] Fig.12 A schematic diagram of the structure of a preformed Z-cut thin-film lithium niobate chip after forming a top electrode photoresist protection layer in one embodiment of the present invention;

[0050] Fig.13 A schematic diagram of the structure of a finished Z-cut thin-film lithium niobate chip after forming a top electrode and an external electrode in one embodiment of the present invention;

[0051] Fig.14 Schematic diagram of a Z-cut thin film lithium niobate electro-optic modulator in one embodiment of the present invention.

[0052] Reference numerals

[0053] Input coupler 01; output coupler 02; first modulation arm 03; second modulation arm 04;

[0054] Substrate layer 1; back electrode 2; bottom dielectric layer 3; bottom modulation layer 4; top waveguide layer 5: waveguide unit 501; top dielectric layer 6; top electrode 7; original electrode layer 8; top electrode photoresist 9; external electrode 10. DETAILED DESCRIPTION

[0055] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0056] In order to better illustrate the technical solution of the present invention, the following terms are explained:

[0057] Electro-optic effect: When voltage is applied to an electro-optic crystal, the refractive index of the electro-optic crystal will change, resulting in changes in the characteristics of the light waves passing through the crystal.

[0058] Electro-optical modulation: Based on the electro-optical effect, the phase, amplitude, intensity and polarization state of the optical signal are modulated.

[0059] Half-wave voltage: When light waves propagate in an optical crystal, the voltage required when the optical path difference between the two perpendicular components of the light wave, Ex' and Ey', is half a wavelength (the corresponding phase difference is 180 degrees) is called half-wave voltage; it is related to the electro-optical properties and geometric dimensions of the electro-optical crystal.

[0060] CPW electrode structure:

[0061] The CPW electrode structure belongs to a coplanar waveguide electrode structure, which is a type of dual-conductor transmission line and can transmit transverse electromagnetic waves (TEM waves).

[0062] TE mode: Transverse electric mode, refers to the electric field direction is perpendicular to the propagation direction; there is a magnetic field component but no electric field component in the propagation direction, which is called a transverse electric wave; in a waveguide (closed cavity structure), the electromagnetic field component has E y , H x , H z The propagation direction is z direction and its propagation mode is TE mode.

[0063] TM mode: Transverse magnetic mode, refers to the magnetic field direction is perpendicular to the propagation direction. There is an electric field component but no magnetic field component in the propagation direction, which is called a transverse magnetic wave; in a waveguide (closed cavity structure), the electromagnetic field component has H y 、E x 、E z The propagation direction is the z direction and its propagation mode is the TM mode.

[0064] Fundamental mode and higher-order mode: The wavelength of light in the optical fiber has first-order, second-order and higher-order modes. Light of different modes is transmitted differently in the optical fiber, at different transmission speeds, and with different optical fiber intrinsic equations. The transmitted light can only be transmitted separately in certain modes according to the optical fiber equation.

[0065] The present invention discloses an electro-optical modulation chip, such as Figure 4 , Figure 5 As shown, including:

[0066] Substrate layer 1;

[0067] A back electrode 2 formed on the top of the substrate layer 1;

[0068] A bottom dielectric layer 3 formed on the top of the back electrode 2;

[0069] A bottom modulation layer 4 formed on the top of the bottom dielectric layer 3;

[0070] Top waveguide layer 5: formed on the top of the bottom modulation layer 4; the top waveguide layer 5 is provided with a plurality of waveguide units 501 connected end to end, and the input optical signal propagates in the waveguide region formed by the waveguide unit 501 and the bottom modulation layer 4; the waveguide unit 501 and the bottom modulation layer 4 below form a continuous and folded modulation region connected end to end;

[0071] A top dielectric layer 6 formed on the free area of ​​the bottom modulation layer 4 and the top and side surfaces of the waveguide unit 501;

[0072] The top electrode 7 is formed on the top of the top dielectric layer 6 .

[0073] Regarding the modulation principle of the electro-optic modulator, it should be explained that: a light beam of a specific wavelength is transmitted to the top waveguide layer through an external coupler, and the top waveguide layer and the bottom modulation layer are an upper and lower structure. The light in the top waveguide layer can be coupled to the bottom modulation layer through evanescent coupling, and then modulated in the bottom modulation layer; after modulation, the light oscillates and propagates between the bottom modulation layer and the top waveguide layer along the extension direction of the waveguide unit.

[0074] Specifically, modulation in the bottom modulation layer includes: when the top electrode and the back electrode are loaded with reverse modulation electrical signals, the propagation speed, refractive index and propagation phase of the light beam in the bottom modulation layer change with the change of the modulation electrical signal, and the propagation phase of the light beam after passing through the modulation area also changes accordingly, thereby realizing electro-optical modulation of the light beam.

[0075] It should be noted that the top waveguide layer is configured as a plurality of waveguide units connected end to end. Preferably, the waveguide units are connected end to end and arranged in parallel, forming a continuous and folded modulation area connected end to end with the bottom modulation layer below, thereby realizing highly integrated modulation in a limited space.

[0076] Preferably, the bottom modulation layer may be made of lithium niobate.

[0077] It can be understood that thin-film lithium niobate has excellent physical properties, such as wide transparent bandwidth, strong electro-optic coefficient, good thermal stability, etc., and the electro-optic response time of lithium niobate material is in the femtosecond range, indicating that its intrinsic bandwidth is in the terahertz range, which is much higher than the intrinsic bandwidth of silicon-based modulators based on the plasma dispersion effect in the prior art.

[0078] Preferably, the top waveguide layer is silicon nitride.

[0079] It is understandable that silicon nitride material has a similar refractive index to lithium niobate material, and has ultra-low propagation loss, low second-order nonlinearity, a small thermo-optic coefficient, and extremely high power handling capability.

[0080] Further preferably, in order to meet the requirements of high integration and folding of the modulation area, a vertical modulation electric field is selected, and the back electrode and the top electrode are arranged horizontally.

[0081] Further preferably, in order to adapt to the high integration of the modulation area, the folding requirements and the vertical modulation electrode, the thin film lithium niobate uses Z-cut thin film lithium niobate, which has the same refractive index in all directions of the material on the xy plane and is isotropic. There is no problem of crystal axis inversion before and after the waveguide is bent, allowing a flexible and compact layout. At the same time, the vertical electrode structure can be modulated even when the waveguide is bent and the structure changes. The length of the modulation area can be increased without increasing the size of the device, thereby improving the modulation efficiency.

[0082] In order to better illustrate the contribution of the present invention over the prior art, the following further describes the electro-optic modulation of thin-film lithium niobate and the influence of different tangential directions of thin-film lithium niobate on the electro-optic modulation performance:

[0083] The electro-optic effect refers to the phenomenon that the dielectric constant of a crystal, that is, its refractive index, changes under the action of an external electric field; and lithium niobate materials use the linear electro-optic effect to modulate the light field. The electro-optic effect analysis of lithium niobate materials can be realized based on the non-simple harmonic oscillator model. Under the action of an external electric field E, the refractive index n(E) of the medium satisfies:

[0084] n(E)=n+aE+bE 2

[0085] Among them, a represents the primary electro-optic effect, also known as the Pockels linear electro-optic effect; b represents the secondary electro-optic effect, also known as the Kerr effect; E represents the external electric field. When there is no external electric field E, n is the refractive index in the general sense.

[0086] It should be noted that lithium niobate crystal is an anisotropic material without reverse symmetry, and has Pockels linear electro-optic effect. In lithium niobate crystal, linear Pockels electro-optic effect is dominant, so Kerr effect can be ignored. When there is no external electric field in lithium niobate crystal, the standard refractive index ellipsoid equation in its principal axis coordinate system satisfies:

[0087]

[0088] The above formula is based on Figure 1 The refractive index ellipsoid shown is used to represent the refractive index of the anisotropic medium. Here, n x , n y , n z They are the lengths of the three axes of the ellipsoid, called the principal axis refractive index, and x, y, and z are the principal axis directions of the medium, that is, the electric displacement vector D and the electric field intensity E along these directions in the crystal are parallel. After determining the direction of light propagation, a plane perpendicular to the transmission direction and passing through the origin of the refractive index ellipsoid is selected. The semi-major axis and semi-minor axis in the plane are the two refractive indices corresponding to the selected propagation direction.

[0089] Specifically, for lithium niobate crystals, n x =n y =n o , n z =n e , n o is the ordinary refractive index, n e is the extraordinary refractive index, and n o >n e , so the lithium niobate crystal is a negative uniaxial crystal, and its refractive index ellipsoid equation satisfies:

[0090]

[0091] The maximum value of the electro-optic coefficient determines the change in the refractive index after applying an external electric field, and plays the greatest role in the change of the refractive index. When an arbitrary electric field E is applied, the refractive index ellipsoid will be disturbed, causing the main axis to produce an angular deviation. Therefore, in order to avoid the birefringence effect, the polarization direction of the optical carrier is usually along the Z axis of the crystal, and the external electric field is also parallel to the Z axis. It has been measured that when the wavelength of the communication light is 1550nm, n o and n e They are 2.214 and 2.138 respectively. When the polarization direction is along the Z axis of the crystal and the external electric field is also parallel to the Z axis, the maximum value of the measured electro-optic coefficient is 31pm / V.

[0092] In the design of lithium niobate waveguide electro-optical modulator, considering the maximum electro-optic coefficient, it is critical to correctly select the orientation of lithium niobate wafers. Usually the wafer orientation is represented by a "cut" in a certain direction, such as Z-cut represents that the Z axis of the crystal is perpendicular to the smooth surface of the crystal, and the same is true for X-cut and Y-cut. The smooth surface is the plane for preparing optical waveguides and electrodes. The relationship between the crystal cut and the applied electric field of lithium niobate electro-optical modulator is as follows: Figure 2 and Figure 3 As shown:

[0093] Figure 2 Indicates: When X-cut or Y-cut, the TE mode and the modulated electric field (E ‖ ) Direction diagram; Figure 3 Indicates: When Z is cut, the TM mode and the modulated electric field (E ⊥ ) direction; the crystal thickness direction is the X axis, the width direction is the Z axis, and the field is assumed to be unchanged in the Z direction, and the propagation direction is the Y axis. Through analysis, it can be seen that the TE mode in the crystal has only three field components E z , H x and H y , and satisfy:

[0094]

[0095]

[0096] The field components of the TM mode are Hz, Ex and Ey, and satisfy:

[0097]

[0098]

[0099] Among them, H x , H y , Hz represent the components of the magnetic field at x and y respectively, E x 、E y 、E z They represent the components of the electric field at x, y, and z respectively, β represents the transmission constant, ω represents the angular frequency of the signal, ε0 represents the dielectric constant in a vacuum, μ0 represents the magnetic permeability in a vacuum, and n represents the refractive index of the dielectric material.

[0100] It can be seen from the above formula that in order to utilize the maximum electro-optic coefficient of the crystal, the transmission mode of the X-cut crystal waveguide should be TE mode, and the electric field direction should be parallel to the crystal surface; the transmission mode of the Z-cut crystal waveguide should be TM mode, and the electric field direction should be perpendicular to the crystal surface.

[0101] In the prior art, when a compact modulator is manufactured using a CPW electrode structure with a horizontal electric field and X-cut thin film lithium niobate, in order to solve the problem of crystal axis inversion before and after waveguide bending, a polarization process or a cross-waveguide structure needs to be used, which undoubtedly increases the complexity of the modulator manufacturing process.

[0102] Compared with the existing technology, the modulator is prepared by using Z-cut thin film lithium niobate chip. The planar isotropy of Z-cut thin film lithium niobate chip is utilized to overcome the defect of crystal axis inversion before and after waveguide bending in the existing technology, and realize flexible and compact layout. At the same time, the planar isotropy of Z-cut thin film lithium niobate chip makes it match the vertical electrode structure, and has the characteristic of being able to modulate when the waveguide is bent and the structure changes. It can increase the length of the modulation area and improve the modulation efficiency without increasing the size of the device.

[0103] In addition, in X-cut and Y-cut thin-film lithium niobate modulators, the direction of the electrodes is parallel or perpendicular to the direction of light wave propagation. Due to design defects, the electrodes will affect the light waves, thereby reducing the performance of the modulator, thus causing phase distortion, amplitude attenuation and other problems.

[0104] Compared with the prior art, the present invention adopts Z-cut thin film lithium niobate chip to prepare the modulator, wherein the electrode direction of the Z-cut thin film lithium niobate crystal matches the light wave propagation direction and the isotropic plane of the chip, and the electrode has little effect on the light wave, thereby improving the defects of phase distortion, amplitude attenuation and the like in X-cut and Y-cut thin film lithium niobate modulators.

[0105] In addition, the Z-cut thin film lithium niobate crystal has a higher nonlinear coefficient and smaller dispersion, which can achieve higher speed and wider bandwidth.

[0106] In addition, the Z-cut thin film lithium niobate crystal has higher thermal stability and smaller thermal drift, which allows the Z-cut thin film lithium niobate modulator to operate in a wider temperature range and have better long-term stability.

[0107] Specifically, a silicon substrate is selected as the substrate layer.

[0108] Specifically, a gold electrode is selected as the external electrode of the back electrode.

[0109] It can be understood that gold has excellent conductivity and chemical stability, which can improve the performance and stability of the modulator.

[0110] Specifically, the bottom dielectric layer may be silicon dioxide.

[0111] It is understandable that silicon dioxide has excellent electrical properties and chemical stability, which can improve the quality and stability of devices; in addition, silicon dioxide has a high refractive index and can form a high refractive index difference waveguide with lithium niobate, which is conducive to compact design; at the same time, silicon dioxide has good optical transparency and can be used as an optical waveguide material.

[0112] Specifically, the bottom modulation layer has a thickness of 400 nm to 600 nm.

[0113] It can be understood that the bottom modulation layer and the top waveguide layer form a dielectric-loaded hybrid waveguide, with part of the optical mode distributed in the top waveguide layer and part in the bottom modulation layer. The hybrid waveguide size should meet the conditions of a single-mode waveguide, so that most of the power of the light is kept in the fundamental mode during transmission, reducing the transfer of power from the fundamental mode to the higher-order mode, which helps to reduce the transmission loss of the light.

[0114] It should be noted that, as an example, when lithium niobate is selected for the bottom modulation layer, the thickness of the thin film lithium niobate wafer used as the raw material is between 400nm and 600nm. The thicker lithium niobate film requires a higher ion injection capability during intelligent ion cutting of the lithium niobate material. Therefore, the lithium niobate film after annealing and peeling has more surface defects and poorer quality. At the same time, when the thickness of the bottom modulation layer is less than 400nm, the power proportion of the light distributed in the lithium niobate material is reduced, resulting in a decrease in the modulation efficiency of the device.

[0115] Specifically, the thickness of the top waveguide layer is 200nm-300nm; the width of the top waveguide layer is 1um-1.5um; and the distance between adjacent waveguide units is 8um-10um.

[0116] It should be noted that the thickness of the top waveguide layer is 200nm to 300nm. When the thickness of the top waveguide layer is greater than this range, in order to maintain the single-mode condition of the hybrid waveguide, the top waveguide layer needs to have a smaller width, and the device manufacturing is extremely difficult. At the same time, as an example, when silicon nitride is selected as the top waveguide layer and lithium niobate is selected as the bottom modulation layer, the thickness of the thin film lithium niobate wafer used as the raw material is between 400nm and 600nm. Silicon nitride and lithium niobate materials form a hybrid waveguide. The thickness and width of silicon nitride need to meet the single-mode condition of the hybrid waveguide, so that most of the power of light during transmission is maintained in the fundamental mode state, reducing the transfer of power from the fundamental mode to the higher-order mode, and reducing the transmission loss of light.

[0117] When the thickness of the top waveguide layer is greater than this thickness range, the proportion of optical power in the lithium niobate material will be reduced, which will reduce the performance of the device. When the thickness of the top waveguide layer is less than this thickness range, the limiting effect of the waveguide on light will be weakened, causing the light mode distributed in the lithium niobate to evolve into a planar mode, which will increase the transmission loss of light.

[0118] Similarly, if the width of the top waveguide layer is greater than 1.5um, the size of the hybrid waveguide will not meet the single-mode condition, and the power will be transferred from the fundamental mode to the higher-order mode during light transmission, increasing the transmission loss of light. If the width of the top waveguide layer is less than 1.0um, the limiting effect of the ridge waveguide on light will be weakened, causing the distributed mode in the hybrid waveguide to evolve into a slab mode.

[0119] Similarly, if the spacing between adjacent waveguide units is greater than 10um, the size of the entire device will be larger. The larger device size cannot meet the requirements of photonic integrated circuits for device compactness, which is not conducive to improving the integration of the device; if the spacing between adjacent waveguide units is less than 8um, evanescent wave coupling will occur between two adjacent waveguides, which will have an adverse effect on device performance, and it will be more difficult to manufacture waveguides with too small a spacing.

[0120] Specifically, the top dielectric layer may be a photoresist.

[0121] Preferably, the top dielectric layer is SU8 photoresist.

[0122] It should be noted that, on the one hand, SU8 photoresist can be spin-coated on top of the bottom modulation layer to produce a highly uniform surface with a relatively large thickness, and the thickness can be conveniently controlled by the spin-coating speed; on the other hand, SU8 photoresist has high processing precision and can produce more complex structures.

[0123] Compared with the prior art, SU8 photoresist has higher resolution and smaller lateral drift; SU8 photoresist has good transparency, and its refractive index at 1550nm is 1.57, which is greater than 1.46 of silicon dioxide; SU8 photoresist can achieve higher processing accuracy through multiple exposures and developments, and can be processed using ordinary ultraviolet exposure machines. In the prior art, silicon dioxide is often used as the top dielectric layer, but the processing cost of silicon dioxide is high, and expensive equipment is required for processing, such as exposure machines and deep etching equipment. It is not only difficult to control the processing accuracy, but also costly.

[0124] Preferably, the top waveguide layer has a rectangular cross-section.

[0125] It should be noted that the cross-section of the waveguide layer is rectangular, which can reduce the overflow of light in all directions of the cross-section, better confine the light in the waveguide layer, and reduce the loss of light during the light propagation process.

[0126] Preferably, the thin-film lithium niobate chip is provided with a groove for external connection of the back electrode, and the groove can be prepared by etching, in which a bare back electrode is provided for external connection of the back electrode.

[0127] During implementation, the two poles of the external electrode are respectively connected to the top electrode and the back electrode in the groove, and an external voltage is applied to the top electrode and the back electrode for chip modulation.

[0128] On the other hand, the present invention discloses a method for forming an electro-optical modulation chip, such as Figure 6-Figure 13 As shown, the following steps are included:

[0129] Step 1: growing a top waveguide layer 5 on a bottom modulation layer 4 of a thin film lithium niobate wafer;

[0130] Step 2: Etching the top waveguide layer 5 to a target shape to prepare a waveguide unit 501 connected end to end;

[0131] Step 3: In a blank area where no waveguide unit is set, etching the bottom modulation layer 4 and the bottom dielectric layer 3 of the thin-film lithium niobate wafer to the back electrode 2, and obtaining a bare back electrode 2 in the etched area;

[0132] Step 4: Spin-coat photoresist on the upper surface of the preformed chip prepared in step 3 and develop the photoresist, form a top dielectric layer 6 on the top of the bottom modulation layer 4 and the top waveguide layer 5 and the etched area in step 3, and further etch the top dielectric layer 6 in the etched area in step 3 to obtain a bare back electrode;

[0133] Step 5: forming an original electrode layer 8 on the etched area of ​​step 4 and on top of the top dielectric layer 6;

[0134] Step 6: Etching the original electrode layer 8 to a target shape, forming a top electrode 7 on the top of the top dielectric layer 6 , and forming an external electrode 10 connected to the back electrode 2 in the exposed area of ​​the back electrode 2 .

[0135] It should be noted that the target shape of the etching of the silicon nitride layer is a plurality of silicon nitride waveguide units connected end to end, and each waveguide unit is arranged in parallel and connected end to end.

[0136] It should be noted that, as an example, a typical commercially available thin film lithium niobate wafer structure, such as Figure 6 As shown, including:

[0137] Substrate layer 1;

[0138] A back electrode 2 formed on the top of the substrate layer 1;

[0139] A bottom dielectric layer 3 formed on the top of the back electrode 2;

[0140] The bottom modulation layer 4 is formed on the top of the bottom dielectric layer 3 .

[0141] As an example, a typical lithium niobate wafer has: the substrate layer is a silicon substrate, the back electrode is a gold electrode, the bottom dielectric layer is silicon dioxide, and the bottom modulation layer is a thin film lithium niobate.

[0142] Preferably, the bottom modulation layer is Z-cut thin film lithium niobate.

[0143] Specifically, the top waveguide layer is silicon nitride.

[0144] Specifically, after the top waveguide layer is grown in step 1, a metal chromium layer is continuously grown as a photolithography protection layer.

[0145] In step 1, the top waveguide layer is grown, including:

[0146] S101: depositing a top waveguide layer using a reduced pressure chemical vapor deposition technique;

[0147] S102: forming a first metal chromium layer on the top waveguide layer;

[0148] S103: Spin-coat a negative photoresist, and obtain a soft mask of a target shape after exposure and development.

[0149] Specifically, the raw material gas and its intake amount for the reduced pressure chemical vapor deposition in S101 satisfy: SiH4 (8 sccm~12 sccm): NH3 (4 sccm~8 sccm): N2 (250 sccm~350 sccm).

[0150] For example, the SiH4 flow rate can be: 8 sccm, 8.5 sccm, 9 sccm, 9.5 sccm, 10 sccm, 10.5 sccm, 11 sccm, 11.5 sccm and 12 sccm.

[0151] For example, the NH3 flow rate can be: 4 sccm, 4.5 sccm, 5 sccm, 5.5 sccm, 6 sccm, 6.5 sccm, 7 sccm, 7.5 sccm and 8 sccm.

[0152] For example, the N2 flow rate may be: 250 sccm, 260 sccm, 270 sccm, 280 sccm, 290 sccm, 300 sccm, 310 sccm, 320 sccm, 330 sccm, 340 sccm, and 350 sccm.

[0153] Specifically, the chamber pressure of the reduced pressure chemical vapor deposition in S101 is 50Pa-150Pa, and the temperature is 280°C-320°C.

[0154] For example, the chamber pressure of the reduced pressure chemical vapor deposition is 50Pa, 60Pa, 70Pa, 80Pa, 90Pa, 100Pa, 110Pa, 120Pa, 130Pa, 140Pa or 150Pa.

[0155] For example, the temperature of the reduced pressure chemical vapor deposition is 280°C, 290°C, 300°C, 310°C, or 320°C.

[0156] It is understandable that the chamber pressure of the reduced pressure chemical vapor deposition is selected to be 50Pa to 150Pa because: if the chamber pressure of the reduced pressure chemical vapor deposition is less than 50Pa, insufficient chamber pressure will lead to poor film growth rate. On the one hand, it may cause a decrease in the concentration of reactants near the substrate surface, resulting in slow or incomplete chemical reaction and film deposition; on the other hand, it will cause uneven distribution of reactants, resulting in uneven film thickness on the entire substrate surface, which will have a negative impact on the ideal properties and functions of the deposited film; at the same time, low chamber pressure may allow contaminants, such as air or moisture, to penetrate into the deposition chamber. These impurities will have an adverse effect on the quality of the film, leading to the introduction of defects or changes in film-forming composition; if the chamber pressure of the reduced pressure chemical vapor deposition is greater than 150Pa, on the one hand, the larger gas velocity and turbulence will destroy the deposition process, resulting in uneven film thickness, rough surface, or the formation of unnecessary by-products; on the other hand, high pressure conditions may prevent effective molecular interactions and reduce the bonding force between the film and the substrate surface, thereby hindering the proper adhesion between the deposited film and the substrate; at the same time, too high pressure will produce mechanical pressure on the chemical vapor deposition equipment and chamber, which may cause equipment failure, leakage or reduced process stability.

[0157] It is understandable that the reason why the temperature of reduced pressure chemical vapor deposition is selected to be 280℃~320℃ is: when the temperature of reduced pressure chemical vapor deposition is less than 280℃, on the one hand, insufficient temperature makes it difficult to start and maintain the chemical vapor deposition reaction process, which will lead to slow or incomplete chemical reaction between the precursor gas of the reduced pressure chemical vapor deposition reaction and the surface of the substrate, resulting in poor film growth rate and insufficient film coverage; on the other hand, insufficient temperature will lead to reduced desorption of volatile by-products and impurities, thereby causing them to be integrated into the deposited film, which has a negative impact on the quality, purity and required performance of the film; when the temperature of reduced pressure chemical vapor deposition is greater than 320℃, on the one hand, too high a temperature will lead to the chemical vapor deposition reaction Thermal decomposition of the precursor gas before reaching the substrate surface results in the formation of unwanted by-products or the loss of required precursor raw materials, affecting the composition and quality of the film; on the other hand, high temperature can increase the surface mobility of atoms or molecules, leading to excessive surface diffusion, resulting in uneven film growth, rough surface, and difficulty in controlling film thickness and morphology; in addition, high temperature can damage the base material or the previously deposited layer structure, causing substrate melting, mutual penetration, or chemical reaction between the substrate and the deposited film, thereby damaging the adhesion and integrity of the film; at the same time, operating at excessively high temperatures will bring huge thermal stress to the vapor deposition equipment, resulting in equipment degradation, reduced reliability, and potential safety hazards.

[0158] Specifically, in S102 , the thickness of the metal chromium layer is 60 nm to 100 nm, and the metal chromium is deposited on the silicon nitride layer to act as a conductive layer to reduce charging during photolithography.

[0159] Specifically, the negative photoresist in S103 is AR-N 7520.18 negative photoresist, and the electron beam exposure technology is used for exposure.

[0160] Compared with the prior art, the present invention can reduce charge accumulation of the silicon nitride layer and the like during dry etching and reduce etching damage by providing a metal chromium layer.

[0161] Specifically, in step 2, the top waveguide layer is etched to a target shape, with a silicon nitride layer as the top waveguide layer and a thin film lithium niobate as the bottom modulation layer as an example, including:

[0162] S201: Based on the first soft mask corresponding to the target shape after etching the silicon nitride layer, performing a first dry etching on the first metal chromium layer to obtain a first metal chromium mask corresponding to the developed shape of the soft mask, wherein the first dry etching satisfies the etching depth to the silicon nitride layer, and the etched area forms a bare silicon nitride layer;

[0163] S202: Based on the metal chromium mask, performing a second dry etching on the exposed silicon nitride layer in the etching area in S201 to obtain a silicon nitride layer corresponding to the shape developed by the first metal chromium mask, wherein the second dry etching satisfies the etching depth to the lithium niobate layer, and a bare lithium niobate layer is formed in the etching area;

[0164] S203: performing a third dry etching on the preformed chip after etching in S202 to remove the remaining photoresist; removing the first metal chromium layer by selective wet etching to obtain a waveguide unit 501 (such as Figure 5 shown).

[0165] Specifically, in step S201, the first dry etching is chlorine-based inductively coupled plasma dry etching, and the first metal chromium layer can be etched according to the soft mask to form a first metal chromium mask without damaging the silicon nitride layer under the chromium layer.

[0166] Specifically, the second dry etching in step S202 uses a fluorine-based inductively coupled plasma etching technique to transfer the metal chromium mask target pattern to the silicon nitride film without damaging the lithium niobate layer.

[0167] Specifically, the second dry etching gas raw materials and their intake volumes in step S202 satisfy: SF6 (2 sccm to 7 sccm): C4F8 (2 sccc to 7 sccc): Ar (70 sccm to 110 sccm).

[0168] Preferably, the second dry etching in step S202 is reduced pressure etching, and the chamber pressure is 0.5Pa to 2.0Pa.

[0169] Preferably, the etching rate of the second dry etching in step S202 is 2.0 nm / sec to 3.2 nm / sec.

[0170] Specifically, in step S203, the third dry etching is performed using oxygen plasma technology to remove the residual glue while having little impact on the silicon nitride layer and the lithium niobate layer.

[0171] Preferably, the third dry etching in step S203 includes cleaning in a solution of sulfuric acid and hydrogen peroxide at a mass ratio of 2 to 4:1 at 60° C. to 80° C. for 1 to 10 minutes to further remove residual photoresist.

[0172] Specifically, after the third dry etching in step S203, the remaining metal chromium layer is removed and an oxygen plasma cleaning step is performed.

[0173] Preferably, the remaining metallic chromium layer is removed by selective wet etching.

[0174] Preferably, the lithium niobate thin film is etched by inductively coupled reactive ion etching (ICP-RIE).

[0175] Existing technologies for etching lithium niobate films include: wet etching, reactive ion etching, ion beam etching, plasma etching and laser ablation. Among these technologies, wet etching has a slow etching speed, requires a longer processing time, and has limited control over the etching profile and sidewall roughness; ion beam etching (IBE) requires expensive equipment and has limited wafer output; large-area processing of laser ablation technology is limited by output, the deep etching process is relatively slow, the cost is high, and the etching efficiency is heavily dependent on the laser wavelength and energy.

[0176] Compared with other etching methods in the prior art, ICP-RIE can better control the etching depth and profile uniformity. The ICP-RIE used in the present invention has the following advantages:

[0177] 1. High etching rate:

[0178] ICP-RIE is able to achieve high etch rates, which allows for faster processing compared to wet etching or other plasma etching techniques, and is suitable for processing large area substrates or when time-sensitive manufacturing processes are required;

[0179] 2. Anisotropic etching:

[0180] ICP-RIE provides excellent anisotropic etching capabilities, resulting in well-defined vertical sidewalls, facilitating precise pattern transfer and the fabrication of high-aspect ratio structures with sharp features;

[0181] 3. Selectivity and control:

[0182] ICP-RIE offers high selectivity, allowing for the selective removal of specific materials while preserving others, useful when etching complex multi-layer structures or when it is necessary to etch specific layers without damaging underlying or adjacent materials.

[0183] Specifically, obtaining the exposed back electrode in step 3 includes:

[0184] S301: forming a second metal chromium layer on the lithium niobate layer of the waveguide unit chip formed in step 2;

[0185] S302: Spin-coat a negative photoresist, and obtain a soft mask of a target shape after exposure and development;

[0186] S303: based on the soft mask, dry-etching the second metal chromium layer to obtain a second metal chromium mask corresponding to the developed shape of the soft mask, wherein the dry etching satisfies the etching depth to the lithium niobate layer, and the etched area forms a bare lithium niobate layer;

[0187] S304: Based on the second metal chromium mask, dry-etching the lithium niobate layer exposed in the etching area in S303 to obtain a bottom dielectric layer corresponding to the development shape of the second metal chromium mask, wherein the dry etching satisfies the etching depth to the bottom dielectric layer, and the etching area forms an exposed bottom dielectric layer;

[0188] S305: dry-etching the bottom dielectric layer exposed in the etching area of ​​S304 to the back electrode to obtain an exposed back electrode.

[0189] Specifically, S304 uses argon-based plasma gas to dry-etch the lithium niobate layer.

[0190] Preferably, the argon-based plasma gas includes: a mixed gas of trifluoromethane and argon, with an inlet molar ratio of 1:5 to 10; the etching rate is between 20 nm / min and 30 nm / min, and the etching selectivity to Cr is about 1:10.

[0191] It should be noted that the present invention uses a mixed gas of trifluoromethane and argon to etch lithium niobate, which has the following advantages:

[0192] 1. Trifluoromethane chemicals, such as CHF3, exhibit high selectivity when etching LiNbO3, that is, the etching process mainly removes LiNbO3 materials and has little reaction to other materials in the substrate or underlying layer; this selectivity enables precise pattern transfer and helps maintain the integrity of surrounding structures;

[0193] 2. A mixture of CHF3 and Ar, when used in an inductively coupled plasma reactive ion etching (ICP-RIE) system, can anisotropically etch LiNbO3, i.e., preferentially remove material in the vertical direction, resulting in well-defined vertical sidewalls, which facilitates the preparation of precise pattern transfer and high aspect ratio structures;

[0194] 3. In ICP-RIE etching, the combination of CHF3 and Ar mixed gas tends to result in a smoother etch profile with minimal sidewall roughness, which helps to obtain precise dimensions and smooth surfaces, helping to improve modulator performance;

[0195] 4. In ICP-RIE etching, the LiNbO3 etching rate can be controlled by adjusting the flow rate of CHF3 and the ratio to Ar to achieve the desired etching depth while maintaining high selectivity and preserving the quality of the etched features.

[0196] Specifically, in S305 CF4 gas is used to etch the bottom dielectric layer to the back electrode.

[0197] Specifically, the CF4 gas intake amount in S305 is 10 sccm to 100 sccm.

[0198] It should be noted that the advantage of using CF4 gas is that CF4 gas exhibits high selectivity when etching SiO2: CF4 is a chemically stable gas, which means that it is inert and does not easily react with other materials under normal etching conditions. It preferentially reacts with silicon dioxide and removes it, while its reactivity with other materials is minimal; this selectivity allows precise pattern transfer and preserves the integrity of the underlying or adjacent structures, thereby ensuring controllable and predictable etching and reducing the possibility of unnecessary side reactions or contamination.

[0199] Specifically, oxygen plasma technology is selected for dry etching in S305 to remove residual glue while having little impact on the silicon nitride layer and the lithium niobate layer.

[0200] Preferably, the dry etching in S305 includes cleaning in a solution of sulfuric acid and hydrogen peroxide in a mass ratio of 2 to 4:1 at 60° C. to 80° C. for 1 to 10 minutes to further remove residual photoresist.

[0201] Specifically, in step 4, photoresist is spin-coated on the upper surface of the preformed chip prepared in step 3 and developed, and the top dielectric layer of the etched area in step 3 is further etched to obtain an exposed back electrode, including:

[0202] S401: Spin-coat photoresist using an automatic spin-coating machine.

[0203] S402: exposing and developing the photoresist to form a top dielectric layer on the top of the lithium niobate layer and the silicon nitride layer and in the etched area of ​​step 3;

[0204] S403: etching the top dielectric layer in the etching area of ​​step 3 to the back electrode layer to expose the back electrode.

[0205] Specifically, the photoresist in S401 is SU8 photoresist, with a thickness of 0.5 μm to 1.5 μm, and a spin coating rate of 1200 rpm / s to 1800 rpm / s.

[0206] In S403, chlorine-based inductively coupled plasma dry etching is selected for etching the top dielectric layer in the etching area of ​​step 3.

[0207] Specifically, in step 5, electron beam evaporation deposition is selected to form the original electrode layer.

[0208] Specifically, etching the original electrode layer to a target shape in step 6 includes:

[0209] S601: Spin-coating a top electrode photoresist 9 on the original electrode layer, and obtaining a soft mask of a target shape after exposure and development; the top electrode photoresist 9 is a negative photoresist;

[0210] S602: Based on a soft mask, the original electrode layer 8 is dry-etched to form a top electrode 7 on the top of the top dielectric layer 6, and an external electrode 10 connected to the back electrode 2 is formed in the exposed area of ​​the back electrode 2; wherein, the original electrode layer is dry-etched to a depth of the back electrode 2, and the external electrode 10 is connected to the substrate layer 1 and the bottom dielectric layer 3 through the back electrode 2.

[0211] Specifically, the negative photoresist in S601 is AR-N 7520.18 negative photoresist, and the electron beam exposure technology is used for exposure.

[0212] The present invention discloses an electro-optic modulator, such as Fig.14 As shown, it includes an input coupler 01, an output coupler 02, a first modulation arm 03 and a second modulation arm 04; the first modulation arm and the second modulation arm each include one of the above-mentioned thin film lithium niobate chips;

[0213] The input coupler 01 is coupled to one end of the waveguide layer on the top of the thin-film lithium niobate chip in the first modulation arm 03 and the second modulation arm 04 .

[0214] Optionally, the output coupler 02 is coupled to the other end of the waveguide layer on the top of the thin film lithium niobate chip in the first modulation arm 03 and the second modulation arm 04 .

[0215] The optical signal to be modulated is divided into two optical signals at the input coupler 01 and enters the first modulation arm 03 and the second modulation arm 04 for modulation respectively.

[0216] In a feasible implementation manner, the wavelength and / or phase of the optical signal after modulation changes compared to before modulation.

[0217] In a feasible implementation manner, the first modulation arm 03 and the second modulation arm 04 output optical signals with different wavelengths and / or phases.

[0218] In a feasible implementation manner, the optical signals modulated by the first modulation arm 03 and the second modulation arm 04 are combined at the output coupler 02 to obtain a mixed optical signal composed of optical signals with different wavelengths and / or phases.

[0219] In order to further illustrate the improvement of the present invention, the following examples and comparative examples are provided:

[0220] Example 1

[0221] This embodiment discloses a typical commercially available thin film lithium niobate wafer structure, such as Figure 6 As shown, including:

[0222] Substrate layer 1;

[0223] A back electrode 2 formed on the top of the substrate layer 1;

[0224] A bottom dielectric layer 3 formed on the top of the back electrode 2;

[0225] The bottom modulation layer 4 is formed on the top of the bottom dielectric layer 3 .

[0226] As an example, a typical lithium niobate wafer has: the substrate layer is a silicon substrate, the back electrode is a gold electrode, the bottom dielectric layer is silicon dioxide, and the bottom modulation layer is a Z-cut thin film lithium niobate.

[0227] This embodiment discloses a method for forming an electro-optical modulation chip. Figure 6-Figure 13 As shown, the following steps are included:

[0228] Step 1:

[0229] S101: When the chamber pressure is 50 Pa, the temperature is 280° C., and the raw material gas and its intake amount are SiH4 (8 sccm): NH3 (4 sccm): N2 (250 sccm), a silicon nitride layer is deposited on the lithium niobate layer of the lithium niobate wafer by using a reduced pressure chemical vapor deposition technology;

[0230] S102: forming a first metal chromium layer on the silicon nitride layer, the thickness of the metal chromium layer is 60 nm;

[0231] S103: Spin-coat a negative photoresist, and obtain a soft mask of a target shape after exposure and development.

[0232] Step 2:

[0233] S201: based on the first soft mask, performing a first dry etching on the first metal chromium layer to obtain a first metal chromium mask corresponding to the developed shape of the soft mask, wherein the first dry etching satisfies the etching depth to the silicon nitride layer, and the etched area forms an exposed silicon nitride layer;

[0234] S202: Based on the first metal chromium mask, dry-etching the silicon nitride layer when the gas raw materials and their intake volumes satisfy: SF6 (2 sccm): C4F8 (2 sccc): Ar (70 sccm); the chamber pressure is 0.5 Pa, and the etching rate is 2.0 nm / sec;

[0235] S203: performing a third dry etching on the preformed chip etched in S202 to remove the remaining photoresist; removing the first metal chromium layer by selective wet etching to obtain a waveguide unit connected end to end.

[0236] Step 3: In a blank area where no waveguide unit is set, the lithium niobate layer and the bottom dielectric layer of the thin-film lithium niobate wafer are etched to the back electrode to obtain a bare back electrode in the etched area;

[0237] S301: forming a second metal chromium layer on lithium niobate;

[0238] S302: Spin-coat AR-N 7520.18 negative photoresist, and obtain a soft mask of target shape after exposure and development;

[0239] S303: based on the soft mask, performing a fourth dry etching on the second metal chromium layer to obtain a second metal chromium mask corresponding to the developed shape of the soft mask, wherein the fourth dry etching satisfies the etching depth to the lithium niobate layer, and the etched area forms a bare lithium niobate layer;

[0240] S304: Based on the second metal chromium mask, the lithium niobate layer is dry-etched using an argon-based plasma gas, wherein the argon-based plasma gas includes a mixed gas of trifluoromethane and argon, and the inlet molar ratio is 1:5; the etching rate is between 20 nm / min, and the etching selectivity to Cr is about 1:8;

[0241] S305: using CF4 gas to etch the exposed bottom dielectric layer in the S304 etching area to the back electrode using oxygen plasma technology to obtain an exposed back electrode; the CF4 gas intake volume is 10 sccm; cleaning is performed in a solution of sulfuric acid and hydrogen peroxide in a mass ratio of 2 to 4:1 at 60°C to 80°C for 1 to 10 minutes to further remove residual photoresist.

[0242] Step 4: Spin-coat photoresist on the upper surface of the preformed chip prepared in step 3 and develop the photoresist, form a top dielectric layer on the top of the top waveguide layer and the bottom modulation layer and the etched area in step 3, and further etch the top dielectric layer in the etched area in step 3 to obtain a bare back electrode;

[0243] S401: Spin-coat photoresist using an automatic spin coating machine, SU8 photoresist, thickness 0.5 μm to 1.5 μm, spin coating speed 1200 rpm / s to 1800 rpm / s;

[0244] S402: Expose and develop the photoresist to expose the back electrode.

[0245] Step 5: Form an aluminum primary electrode layer on the etched area of ​​step 4 and on top of the top dielectric layer.

[0246] Step 6: Etching the original electrode layer to a target shape, forming a top electrode on the top of the top dielectric layer, and forming an external electrode connected to the back electrode in the exposed area of ​​the back electrode;

[0247] S601: Spin-coat AR-N 7520.18 negative photoresist on the original electrode layer, use electron beam exposure technology for exposure, and obtain a soft mask of target shape after exposure and development;

[0248] S602: Based on a soft mask, the aluminum original electrode layer is dry-etched for the seventh time, and the etching depth is such that the original electrode layer forms a top electrode on the top dielectric layer, and an external electrode connected to the back electrode is formed in the exposed area of ​​the back electrode; wherein, the original electrode layer is dry-etched for the seventh time, and the etching depth reaches the back electrode layer, and the external electrode is connected to the substrate and the bottom dielectric layer through the back electrode.

[0249] This embodiment discloses an electro-optic modulation chip, which is prepared by the above method, comprising: a thin-film lithium niobate wafer having a lithium niobate layer on top, a silicon nitride top waveguide layer formed on the lithium niobate layer, a SU8 photoresist top dielectric layer, and a top electrode on top of the top dielectric layer;

[0250] The silicon nitride top waveguide layer is provided with waveguide units connected end to end, and the input optical signal propagates in the waveguide region formed by the waveguide unit and the lithium niobate bottom modulation layer;

[0251] SU8 top dielectric layer, formed on the free area of ​​the bottom modulation layer and the top and sides of the waveguide unit;

[0252] A top electrode is formed on top of the top dielectric layer.

[0253] The thickness of the bottom modulation layer is 400nm; the thickness of the top waveguide layer is 200nm; the width of the top waveguide layer is 1um; and the spacing between adjacent waveguide units is 8um.

[0254] This embodiment discloses an electro-optic modulator, such as Fig.14 As shown, it includes an input coupler 01, an output coupler 02, a first modulation arm 03 and a second modulation arm 04; the first modulation arm and the second modulation arm each include an electro-optical modulation chip prepared in this embodiment;

[0255] The input coupler 01 is coupled to one end of the top waveguide layer of the thin film lithium niobate chip in the first modulation arm 03 and the second modulation arm 04, and the output coupler 02 is coupled to the other end of the top waveguide layer of the thin film lithium niobate chip in the first modulation arm 03 and the second modulation arm 04.

[0256] Example 2

[0257] This embodiment discloses a typical commercially available thin film lithium niobate wafer structure, such as Figure 6 As shown, including:

[0258] Substrate layer 1;

[0259] A back electrode 2 formed on the top of the substrate layer 1;

[0260] A bottom dielectric layer 3 formed on the top of the back electrode 2;

[0261] The bottom modulation layer 4 is formed on the top of the bottom dielectric layer 3 .

[0262] As an example, a typical lithium niobate wafer has: the substrate layer is a silicon substrate, the back electrode is a gold electrode, the bottom dielectric layer is silicon dioxide, and the bottom modulation layer is a Z-cut thin film lithium niobate.

[0263] This embodiment discloses a method for forming an electro-optical modulation chip. Figure 6-Figure 13 As shown, the following steps are included:

[0264] Step 1:

[0265] S101: when the chamber pressure is 150 Pa, the temperature is 320° C., and the raw material gas and its intake amount are SiH4 (12 sccm): NH3 (8 sccm): N2 (350 sccm), a silicon nitride layer is deposited on the lithium niobate layer of the lithium niobate wafer by using a reduced pressure chemical vapor deposition technology;

[0266] S102: forming a first metal chromium layer on the silicon nitride layer, the thickness of the metal chromium layer being 100 nm;

[0267] S103: Spin-coat a negative photoresist, and obtain a soft mask of a target shape after exposure and development.

[0268] Step 2:

[0269] S201: based on the soft mask, performing a first dry etching on the first metal chromium layer to obtain a first metal chromium mask corresponding to the developed shape of the soft mask, wherein the first dry etching satisfies the etching depth to the silicon nitride layer, and the etched area forms a bare silicon nitride layer;

[0270] S202: Based on the first metal chromium mask, dry-etching the silicon nitride layer when the gas raw materials and their intake volumes satisfy: SF6 (7 sccm): C4F8 (7 sccc): Ar (110 sccm); the chamber pressure is 2.0 Pa, and the etching rate is 3.2 nm / sec;

[0271] S203: performing a third dry etching on the preformed chip etched in S202 to remove the remaining photoresist; removing the first metal chromium layer by selective wet etching to obtain a waveguide unit connected end to end.

[0272] Step 3: In a blank area where no waveguide unit is set, the lithium niobate layer and the bottom dielectric layer of the thin-film lithium niobate wafer are etched to the back electrode to obtain a bare back electrode in the etched area;

[0273] S301: forming a second metal chromium layer on lithium niobate;

[0274] S302: Spin-coat AR-N 7520.18 negative photoresist, and obtain a soft mask of target shape after exposure and development;

[0275] S303: based on the soft mask, performing a fourth dry etching on the second metal chromium layer to obtain a second metal chromium mask corresponding to the developed shape of the soft mask, wherein the fourth dry etching satisfies the etching depth to the lithium niobate layer, and the etched area forms a bare lithium niobate layer;

[0276] S304: Based on the second metal chromium mask, the lithium niobate layer is dry-etched using an argon-based plasma gas, wherein the argon-based plasma gas includes a mixed gas of trifluoromethane and argon, and the inlet molar ratio is 1:8; the etching rate is between 30 nm / min, and the etching selectivity to Cr is about 1:10.

[0277] S305: Use CF4 gas to etch the exposed bottom dielectric layer in the S304 etching area using oxygen plasma technology to the back electrode to obtain an exposed back electrode. The CF4 gas intake volume is 50 sccm; clean in a solution of sulfuric acid and hydrogen peroxide in a mass ratio of 4:1 at 80°C for 10 minutes to further remove residual photoresist.

[0278] Step 4: Spin-coat photoresist on the upper surface of the preformed chip prepared in step 3 and develop the photoresist, form a top dielectric layer on the top of the top waveguide layer and the bottom modulation layer and the etched area in step 3, and further etch the top dielectric layer in the etched area in step 3 to obtain a bare back electrode;

[0279] S401: Spin-coat photoresist using an automatic spin coating machine, SU8 photoresist, thickness 1.5 μm, spin coating rate 1800 rpm / s;

[0280] S402: Expose and develop the photoresist to expose the back electrode.

[0281] Step 5: forming an aluminum electrode layer on the etched area in step 4 and on top of the top dielectric layer;

[0282] Step 6: Etching the original electrode layer to a target shape, forming a top electrode on the top of the top dielectric layer, and forming an external electrode connected to the back electrode in the exposed area of ​​the back electrode;

[0283] S601: Spin-coat AR-N 7520.18 negative photoresist on the original electrode layer, use electron beam exposure technology for exposure, and obtain a soft mask of target shape after exposure and development;

[0284] S602: Based on a soft mask, dry-etch the aluminum original electrode layer to a depth of the original electrode layer to form a top electrode on the top dielectric layer, and form an external electrode connected to the back electrode in the exposed area of ​​the back electrode; wherein, the original electrode layer is dry-etched to a depth of the back electrode layer, and the external electrode is connected to the substrate and the bottom dielectric layer through the back electrode.

[0285] This embodiment discloses an electro-optic modulation chip, which is prepared by the above method, comprising: a thin-film lithium niobate wafer having a lithium niobate layer on top, a silicon nitride top waveguide layer formed on the lithium niobate layer, a SU8 photoresist top dielectric layer, and a top electrode on top of the top dielectric layer;

[0286] The silicon nitride top waveguide layer is provided with waveguide units connected end to end, and the input optical signal propagates in the waveguide region formed by the waveguide unit and the lithium niobate bottom modulation layer;

[0287] SU8 top dielectric layer, formed on the free area of ​​the bottom modulation layer and the top and sides of the waveguide unit;

[0288] A top electrode is formed on top of the top dielectric layer.

[0289] The thickness of the bottom modulation layer is 600nm; the thickness of the top waveguide layer is 300nm; the width of the top waveguide layer is 1.5um; and the spacing between adjacent waveguide units is 10um.

[0290] This embodiment discloses an electro-optic modulator, such as Fig.14 As shown, it includes an input coupler 01, an output coupler 02, a first modulation arm 03 and a second modulation arm 04; the first modulation arm and the second modulation arm each include an electro-optical modulation chip prepared in this embodiment;

[0291] The input coupler 01 is coupled to one end of the top waveguide layer of the thin film lithium niobate chip in the first modulation arm 03 and the second modulation arm 04, and the output coupler 02 is coupled to the other end of the top waveguide layer of the thin film lithium niobate chip in the first modulation arm 03 and the second modulation arm 04.

[0292] Example 3

[0293] This embodiment discloses a typical commercially available thin film lithium niobate wafer structure, such as Figure 6 As shown, including:

[0294] Substrate layer 1;

[0295] A back electrode 2 formed on the top of the substrate layer 1;

[0296] A bottom dielectric layer 3 formed on the top of the back electrode 2;

[0297] The bottom modulation layer 4 is formed on the top of the bottom dielectric layer 3 .

[0298] As an example, a typical lithium niobate wafer has: the substrate layer is a silicon substrate, the back electrode is a gold electrode, the bottom dielectric layer is silicon dioxide, and the bottom modulation layer is a Z-cut thin film lithium niobate.

[0299] This embodiment discloses a method for forming an electro-optical modulation chip. Figure 6-Figure 13 As shown, the following steps are included:

[0300] Step 1:

[0301] S101: when the chamber pressure is 100 Pa, the temperature is 300° C., and the raw material gas and its intake amount are SiH4 (10 sccm): NH3 (6 sccm): N2 (300 sccm), a silicon nitride layer is deposited on the lithium niobate layer of the lithium niobate wafer by using a reduced pressure chemical vapor deposition technology;

[0302] S102: forming a first metal chromium layer on the silicon nitride layer, the thickness of the metal chromium layer is 80 nm;

[0303] S103: Spin-coat a negative photoresist, and obtain a soft mask of a target shape after exposure and development.

[0304] Step 2:

[0305] S201: based on the soft mask, performing a first dry etching on the first metal chromium layer to obtain a first metal chromium mask corresponding to the developed shape of the soft mask, wherein the first dry etching satisfies the etching depth to the silicon nitride layer, and the etched area forms a bare silicon nitride layer;

[0306] S202: Based on the first metal chromium mask, dry-etching the silicon nitride layer when the gas raw materials and their intake volumes satisfy: SF6 (5 sccm): C4F8 (5 sccc): Ar (90 sccm); the chamber pressure is 1.0 Pa, and the etching rate is 2.6 nm / sec;

[0307] S203: performing a third dry etching on the preformed chip etched in S202 to remove the remaining photoresist; removing the first metal chromium layer by selective wet etching to obtain a waveguide unit connected end to end.

[0308] Step 3: In a blank area where no waveguide unit is set, the lithium niobate layer and the bottom dielectric layer of the thin-film lithium niobate wafer are etched to the back electrode to obtain a bare back electrode in the etched area;

[0309] S301: forming a second metal chromium layer on lithium niobate;

[0310] S302: Spin-coat AR-N 7520.18 negative photoresist, and obtain a soft mask of target shape after exposure and development;

[0311] S303: based on the soft mask, performing a fourth dry etching on the second metal chromium layer to obtain a second metal chromium mask corresponding to the developed shape of the soft mask, wherein the fourth dry etching satisfies the etching depth to the lithium niobate layer, and the etched area forms a bare lithium niobate layer;

[0312] S304: Based on the second metal chromium mask, the lithium niobate layer is dry-etched using an argon-based plasma gas, wherein the argon-based plasma gas includes a mixed gas of trifluoromethane and argon, and the inlet molar ratio is 1:10; the etching rate is between 20 nm / min and 30 nm / min, and the etching selectivity to Cr is about 1:12.

[0313] S305: Use CF4 gas to etch the exposed bottom dielectric layer in the S304 etching area using oxygen plasma technology to the back electrode to obtain an exposed back electrode, and the CF4 gas intake volume is 100 sccm; clean in a solution of sulfuric acid and hydrogen peroxide in a mass ratio of 3:1 at 70°C for 5 minutes to further remove residual photoresist.

[0314] Step 4: Spin-coat photoresist on the upper surface of the preformed chip prepared in step 3 and develop the photoresist, form a top dielectric layer on the top of the top waveguide layer and the bottom modulation layer and the etched area in step 3, and further etch the top dielectric layer in the etched area in step 3 to obtain a bare back electrode;

[0315] S401: Spin-coat photoresist using an automatic spin coating machine, SU8 photoresist, thickness 1.0 μm, spin coating rate 1500 rpm / s;

[0316] S402: Expose and develop the photoresist to expose the back electrode.

[0317] Step 5: forming an aluminum electrode layer on the etched area in step 4 and on top of the top dielectric layer;

[0318] Step 6: Etching the original electrode layer to a target shape, forming a top electrode on the top of the top dielectric layer, and forming an external electrode connected to the back electrode in the exposed area of ​​the back electrode;

[0319] S601: Spin-coat AR-N 7520.18 negative photoresist on the original electrode layer, use electron beam exposure technology for exposure, and obtain a soft mask of target shape after exposure and development;

[0320] S602: Based on a soft mask, dry-etch the aluminum original electrode layer to a depth of the original electrode layer to form a top electrode on the top dielectric layer, and form an external electrode connected to the back electrode in the exposed area of ​​the back electrode; wherein, the original electrode layer is dry-etched to a depth of the back electrode layer, and the external electrode is connected to the substrate and the bottom dielectric layer through the back electrode.

[0321] This embodiment discloses a Z-cut thin-film lithium niobate chip, which is prepared by the above method, including: a thin-film lithium niobate wafer having a lithium niobate layer on the top, a silicon nitride top waveguide layer formed on the lithium niobate layer, a SU8 photoresist top dielectric layer, and a top electrode on the top dielectric layer;

[0322] The silicon nitride top waveguide layer is provided with waveguide units connected end to end, and the input optical signal propagates in the waveguide region formed by the waveguide unit and the lithium niobate bottom modulation layer;

[0323] SU8 top dielectric layer, formed on the free area of ​​the bottom modulation layer and the top and sides of the waveguide unit;

[0324] A top electrode is formed on top of the top dielectric layer.

[0325] The thickness of the bottom modulation layer is 500nm; the thickness of the top waveguide layer is 250nm; the width of the top waveguide layer is 1.2um; and the spacing between adjacent waveguide units is 9um.

[0326] This embodiment discloses an electro-optic modulator, such as Fig.14 As shown, it includes an input coupler 01, an output coupler 02, a first modulation arm 03 and a second modulation arm 04; the first modulation arm and the second modulation arm each include an electro-optical modulation chip prepared in this embodiment;

[0327] The input coupler 01 is coupled to one end of the top waveguide layer of the thin film lithium niobate chip in the first modulation arm 03 and the second modulation arm 04, and the output coupler 02 is coupled to the other end of the top waveguide layer of the thin film lithium niobate chip in the first modulation arm 03 and the second modulation arm 04.

[0328] This embodiment discloses an electro-optic modulator, such as Fig.14 As shown, it includes an input coupler 01, an output coupler 02, a first modulation arm 03 and a second modulation arm 04; the first modulation arm and the second modulation arm each include an electro-optical modulation chip prepared in this embodiment;

[0329] The input coupler 01 is coupled to one end of the top waveguide layer of the thin film lithium niobate chip in the first modulation arm 03 and the second modulation arm 04, and the output coupler 02 is coupled to the other end of the top waveguide layer of the thin film lithium niobate chip in the first modulation arm 03 and the second modulation arm 04.

[0330] Example 4

[0331] The only difference compared with Example 3 is that the waveguide layers on the top of the thin-film lithium niobate chip in the first modulation arm 03 and the second modulation arm 04 output the modulated optical signals to obtain optical signals with different frequencies and phases from the input optical signals.

[0332] Comparative Example 1

[0333] The only difference compared with Example 3 is that the thickness of the top waveguide layer in the electro-optic modulation chip is set to 150 nm.

[0334] Comparative Example 2

[0335] The only difference compared with Example 3 is that the spacing between adjacent waveguide units in the electro-optic modulation chip is set to 6 um.

[0336] Comparative Example 3

[0337] The only difference compared with Example 3 is that the width of the top waveguide layer in the electro-optic modulation chip is set to 0.5 um.

[0338] Comparative Example 4

[0339] The only difference compared with Example 3 is that the width of the top waveguide layer in the electro-optic modulation chip is set to 3.0 um.

[0340] Comparative Example 5

[0341] The only difference compared with Example 3 is that when preparing the electro-optic modulation chip, the first metal chromium layer is not provided in step S102, and the second metal chromium layer is not provided in step S301.

[0342] Comparative Example 6

[0343] The only difference compared with Example 3 is that when preparing the electro-optic modulation chip, the argon-based plasma gas in step S304 does not contain trifluoromethane.

[0344] Experimental example

[0345] The electro-optical modulators designed by the embodiment and the comparative example were subjected to a photoelectric modulation performance simulation test, and the test results are as follows:

[0346]

[0347]

[0348] Experimental conclusion:

[0349] It can be seen from the above table:

[0350] It can be seen from Examples 1-3 that the half-wave voltage of the electro-optic modulator prepared with the Z-cut thin film lithium niobate chip is 4V to 7V, the modulation bandwidth is 19GHz to 28GHz, and the insertion loss is 8dB to 11dB.

[0351] By comparing Example 3 and Comparative Example 1, it can be seen that when the thickness of the top waveguide layer is less than 200 nm, the modulation bandwidth decreases, the insertion loss increases, and the performance of the electro-optic modulator decreases significantly.

[0352] By comparing Example 3 and Comparative Example 2, it can be seen that the spacing between adjacent waveguide units is set too small, the modulation bandwidth is reduced, the insertion loss is increased, and the performance of the electro-optic modulator is significantly reduced.

[0353] By comparing Example 3 with Comparative Examples 3 and 4, it can be seen that if the width of the top waveguide layer is set too small, the modulation bandwidth is reduced, the insertion loss is increased, and the performance of the electro-optic modulator is significantly reduced; if the width of the top waveguide layer is set too large, the modulation bandwidth remains basically unchanged, the insertion loss is slightly reduced, and the volume of the modulator is increased.

[0354] By comparing Example 3 and Comparative Example 5, it can be seen that when preparing the Z-cut thin-film lithium niobate chip, no metal chromium layer is provided, the damage caused by dry etching is increased, resulting in a decrease in the modulation bandwidth, an increase in the insertion loss, and a significant decrease in the performance of the electro-optic modulator.

[0355] By comparing Example 3 and Comparative Example 6, it can be seen that when preparing the Z-cut thin film lithium niobate chip, the argon-based plasma gas in step S304 does not contain trifluoromethane, the modulation bandwidth is reduced, the insertion loss is increased, and the performance of the electro-optic modulator is significantly reduced.

[0356] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. An electro-optic modulation chip, characterized in that: include: substrate layer; a back electrode formed on top of the substrate layer; a bottom dielectric layer formed on top of the back electrode; a bottom modulation layer formed on top of the bottom dielectric layer; Top waveguide layer: formed on the top of the bottom modulation layer; the top waveguide layer is provided with a plurality of waveguide units connected end to end, and the input optical signal propagates in the waveguide area formed by the waveguide unit and the bottom modulation layer; the waveguide unit and the bottom modulation layer below form a continuous and folded modulation area connected end to end; A top dielectric layer formed on the free area of ​​the bottom modulation layer and the top and side of the waveguide unit; A top electrode is formed on top of the top dielectric layer.

2. The electro-optic modulation chip according to claim 1, characterized in that: The bottom modulation layer is Z-cut thin film lithium niobate, and the top waveguide layer is silicon nitride.

3. The electro-optic modulation chip according to claim 1, characterized in that: The back electrode and the top electrode are arranged horizontally to provide a vertical modulation electric field.

4. The electro-optic modulation chip according to claim 1, characterized in that: The top dielectric layer is photoresist.

5. The electro-optic modulation chip according to claim 4, characterized in that: The photoresist is SU8 photoresist.

6. The electro-optic modulation chip according to claim 1, characterized in that: The thickness of the top waveguide layer is 200nm to 300nm; the width of the top waveguide layer is 1um to 1.5um; and the distance between adjacent waveguide units is 8um to 10um.

7. A method for forming an electro-optical modulation chip, characterized in that: The method for preparing the electro-optical modulation chip according to any one of claims 1 to 6 comprises the following steps: Step 1: Grow the top waveguide layer on the bottom modulation layer of the thin-film lithium niobate wafer; Step 2: Etch the top waveguide layer to the target shape to prepare a waveguide unit connected end to end; Step 3: In a blank area where no waveguide unit is set, etching the bottom modulation layer and the bottom dielectric layer of the thin-film lithium niobate wafer to the back electrode, and obtaining a bare back electrode in the etched area; Step 4: Spin-coat photoresist on the upper surface of the preformed chip prepared in step 3 and develop the photoresist, form a top dielectric layer on the top of the bottom modulation layer and the top waveguide layer and the etched area in step 3, and further etch the top dielectric layer in the etched area in step 3 to obtain a bare back electrode; Step 5: forming an original electrode layer on the etched area of ​​step 4 and on top of the top dielectric layer; Step 6: Etch the original electrode layer to a target shape, form a top electrode on the top of the top dielectric layer, and form an external electrode connected to the back electrode in the exposed area of ​​the back electrode.

8. The method for forming an electro-optic modulation chip according to claim 7, characterized in that: After growing the top waveguide in step 1, a metal chromium layer is grown as a protective layer.

9. An electro-optic modulator, characterized in that: It comprises an input coupler, an output coupler, a first modulation arm and a second modulation arm; the first modulation arm and the second modulation arm each comprise an electro-optical modulation chip as claimed in any one of claims 1-6.

10. An electro-optic modulator according to claim 9, characterized in that: The half-wave voltage of the electro-optic modulator is 4V to 7V, the modulation bandwidth is 19GHz to 28GHz, and the insertion loss is 8dB to 11dB.

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