Radio frequency packaging structure of lithium niobate electro-optical modulator

Abstract

The radio frequency packaging structure comprises a ceramic substrate, a bottom metal layer, a coplanar waveguide transmission line, a left side metal layer and a right side metal layer, the bottom metal layer is tightly attached to the bottom of the ceramic substrate, and the coplanar waveguide transmission line is tightly attached to the top of the ceramic substrate; the left side metal layer is tightly attached to the left side part of the ceramic substrate, and the right side metal layer is tightly attached to the right side part of the ceramic substrate; the coplanar waveguide transmission line comprises a first ground electrode, a second ground electrode and a signal electrode which are arranged in parallel; the first end of the signal electrode is of an isosceles trapezoid transition structure, and the size of the transition structure is gradually reduced along with approaching the modulator chip. According to the radio frequency packaging structure of the lithium niobate electro-optical modulator disclosed by the invention, the bandwidth of the packaged lithium niobate modulator can be improved on the premise that the bandwidth of a chip is not improved, and the loss in device packaging is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of electro-optic modulators, and in particular relates to a radio frequency packaging structure of a lithium niobate electro-optic modulator. Background Art

[0002] Electro-optic modulators are important devices that convert radio frequency electrical signals into optical signals carrying effective information. They are widely used in the fields of optical communications and radar military equipment. Lithium niobate is widely used in various optoelectronic devices because of its very high electro-optic coefficient. Electro-optic modulators based on lithium niobate have the characteristics of high bandwidth and low loss.

[0003] The packaging of the electro-optic modulator is an important link on the road to practical application. The RF transition chip is an important part of the electro-optic modulator packaging. The RF signal is transmitted to the RF transition chip through the RF coaxial connector, and then transmitted to the modulator chip, realizing the RF signal conversion from the coaxial RF connector to the modulator chip.

[0004] like Figure 1 As shown, the current RF transition chip uses a ceramic substrate 1, the back of the substrate is metal 2, and a microstrip line 3 is used on the ceramic substrate 1. The microstrip line 3 has the advantages of simple production, convenient installation, and low cost. However, since the conductor is exposed in the air, part of the electromagnetic field of the microstrip line will be radiated into the air, causing transmission loss. Compared with the microstrip line, the grounded coplanar waveguide transition transmission line has a ceramic substrate 1 in the middle, and a coplanar waveguide transmission line 4 is made on the ceramic substrate 1 using thin film lithography technology. By making a metal through hole 5 on the coplanar waveguide line 4, the ground electrode on the surface of the transmission line is short-circuited with the metal electrode on the back. The presence of the metal through hole will destroy the electromagnetic field boundary conditions of the microstrip line mode of the coplanar waveguide transmission line, and to a certain extent, form a suppression of microwave resonance. The metal through hole 5 needs to make a through hole in the ceramic substrate 1 and fill it with metal, which increases the difficulty and cost of production.

[0005] Therefore, further improvements are made to the above problems. Summary of the invention

[0006] The main purpose of the present invention is to provide a radio frequency packaging structure of a lithium niobate electro-optic modulator, which can improve the bandwidth of the lithium niobate modulator after packaging without increasing the chip bandwidth and reduce the loss in the device packaging.

[0007] To achieve the above purpose, the present invention provides a radio frequency packaging structure of a lithium niobate electro-optic modulator, comprising a ceramic substrate, a bottom metal layer, a coplanar waveguide transmission line, a left metal layer and a right metal layer, wherein the bottom metal layer is closely attached to the bottom of the ceramic substrate and the coplanar waveguide transmission line is closely attached to the top of the ceramic substrate, the left metal layer is closely attached to the left side of the ceramic substrate and the right metal layer is closely attached to the right side of the ceramic substrate, wherein:

[0008] The first end of the coplanar waveguide transmission line is connected to the modulator chip through gold wire bonding and the second end of the coplanar waveguide transmission line is connected to the radio frequency coaxial signal connector, the coplanar waveguide transmission line comprises a first ground electrode, a second ground electrode and a signal electrode arranged in parallel, and the signal electrode is located between the first ground electrode and the second ground electrode;

[0009] The first end of the signal electrode is an isosceles trapezoidal transition structure and the transition structure gradually decreases as it approaches the modulator chip. The width of the end of the transition structure close to the modulator chip is the same as the width of the corresponding connection position of the modulator chip (the width of the signal electrode is consistent with the electrode PAD area size design of the modulator chip while ensuring that the characteristic impedance is close to 50 ohms, ensuring continuous matching of the mode), and the width of the second end of the signal electrode is equal to the width of the end of the transition structure away from the modulator chip (that is, the width of the signal electrode close to the RF coaxial connector is widened to obtain better impedance matching with the RF coaxial connector);

[0010] Two ends of the left metal layer are connected to the first ground electrode and the bottom metal layer respectively, and two ends of the right metal layer are connected to the second ground electrode and the bottom metal layer respectively.

[0011] As a further preferred technical solution of the above technical solution, the material of the ceramic substrate is aluminum nitride.

[0012] As a further preferred technical solution of the above technical solution, a first distance between the signal electrode and the first ground electrode is equal to a second distance between the signal electrode and the second ground electrode (same position section).

[0013] As a further preferred technical solution of the above technical solution, the first ground electrode and the second ground electrode are connected into a whole through a left metal layer and a right metal layer.

[0014] As a further preferred technical solution of the above technical solution, the RF coaxial connector is connected to the RF packaging structure by gold-plated tape / gold-tin welding.

[0015] The beneficial effects of the present invention are:

[0016] 1. The RF signal emitted by the RF coaxial signal connector is loaded onto the lithium niobate electro-optic modulator chip through the RF transition transmission chip. The RF transition transmission chip adopts a coplanar waveguide transmission line, and a metal layer is applied under and on both sides of the substrate to connect all ground electrodes into a whole. This transmission line has a stronger ability to bind the electric field and greatly improves the bandwidth.

[0017] 2. The electrode width of the coplanar waveguide transmission line close to the chip area is designed to be as consistent as possible with the chip electrode PAD area size while ensuring the characteristic impedance is close to 50 ohms, ensuring continuous matching of the mode. The isosceles trapezoidal transition structure is adopted to widen the electrode width close to the RF coaxial connector to obtain better impedance matching effect.

[0018] 3. The RF transition transmission chip is RF-connected to the lithium niobate modulator chip through gold wire bonding. The number and relative positions of the gold wires are designed to improve the transmission performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a packaging structure diagram of an existing RF transition chip.

[0020] Figure 2 It is a schematic diagram of the structure of the radio frequency transition chip of the present invention.

[0021] Figure 3 It is a schematic diagram of the structure of the radio frequency transition chip of the present invention.

[0022] Figure 4 It is a schematic diagram of the structure of the radio frequency transition chip of the present invention.

[0023] Figure 5 It is a schematic diagram of the structure of an existing modulator chip.

[0024] Figure 6 Schematic diagram of return loss of the present invention.

[0025] Figure 7 Schematic diagram of transmission loss of the present invention.

[0026] Figure 8 It is a schematic diagram of the transmission characteristics of the present invention.

[0027] Fig. 9 It is a schematic diagram of the transmission characteristics of the present invention.

[0028] The reference numerals include: 1, ceramic substrate; 2, bottom metal layer; 4, coplanar waveguide transmission line; 4-1, first ground electrode; 4-2, signal electrode; 4-3, second ground electrode; 6, left metal layer; 7, right metal layer. DETAILED DESCRIPTION

[0029] The following description is used to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments described below are only examples, and those skilled in the art can think of other obvious variations. The basic principles of the present invention defined in the following description can be applied to other embodiments, variations, improvements, equivalents, and other technical solutions that do not deviate from the spirit and scope of the present invention.

[0030] In the preferred embodiment of the present invention, those skilled in the art should note that the modulator chip and the RF coaxial signal connector involved in the present invention can be regarded as prior art.

[0031] Preferred embodiments.

[0032] like Figure 2-9 As shown, the present invention discloses a radio frequency packaging structure of a lithium niobate electro-optic modulator, comprising a ceramic substrate 1, a bottom metal layer 2, a coplanar waveguide transmission line 4, a left metal layer 6 and a right metal layer 7, wherein the bottom metal layer 2 is closely attached to the bottom of the ceramic substrate 1 and the coplanar waveguide transmission line 4 is closely attached to the top of the ceramic substrate 1, the left metal layer 6 is closely attached to the left side of the ceramic substrate 1 and the right metal layer 7 is closely attached to the right side of the ceramic substrate 1, wherein:

[0033] The first end of the coplanar waveguide transmission line 4 is connected to the modulator chip through gold wire bonding and the second end of the coplanar waveguide transmission line 4 is connected to the RF coaxial signal connector 14. The coplanar waveguide transmission line 4 includes a first ground electrode 4-1, a second ground electrode 4-3 and a signal electrode 4-2 arranged in parallel, and the signal electrode 4-2 is located between the first ground electrode 4-1 and the second ground electrode 4-3;

[0034] The first end of the signal electrode 4-2 is an isosceles trapezoidal transition structure and the transition structure gradually decreases as it approaches the modulator chip. The width of the end of the transition structure close to the modulator chip is the same as the width of the corresponding connection position of the modulator chip (the width of the signal electrode is consistent with the electrode PAD area size design of the modulator chip while ensuring the characteristic impedance is close to 50 ohms, ensuring continuous matching of the mode). The width of the second end of the signal electrode 4-2 is equal to the width of the end of the transition structure away from the modulator chip (that is, the width of the signal electrode close to the RF coaxial connector is widened to obtain better impedance matching with the RF coaxial connector). The isosceles trapezoid is gradually widened or narrowed to ensure both impedance matching with the RF coaxial connector and mode matching when connected to the lithium niobate modulator chip.

[0035] Two ends of the left metal layer 6 are connected to the first ground electrode 4 - 1 and the bottom metal layer 2 respectively, and two ends of the right metal layer 7 are connected to the second ground electrode 4 - 3 and the bottom metal layer 2 respectively.

[0036] Specifically, the material of the ceramic substrate 1 is aluminum nitride.

[0037] More specifically, a first distance between the signal electrode 4 - 2 and the first ground electrode 4 - 1 and a second distance (same position segment) between the signal electrode 4 - 2 and the second ground electrode 4 - 3 are equal.

[0038] Furthermore, the first ground electrode 4-1 and the second ground electrode 4-3 are connected as a whole through the left metal layer 6 and the right metal layer 7, and all the ground electrodes are connected as a whole by side metallization. This transmission line has a stronger ability to bind the electric field and a higher transmission frequency.

[0039] Furthermore, the RF coaxial connector is connected to the RF packaging structure by gold-plated tape / gold-tin welding.

[0040] For the structure of the modulator chip: Figure 5 As shown, it includes a substrate 8, which is silicon; a silicon oxide layer 9 is formed on the substrate 8; a lithium niobate thin film layer 10 and a ridge waveguide structure 11 are formed on the silicon oxide layer 9; a silicon oxide upper cover layer 12 is disposed above the ridge waveguide structure 11; and a GSG traveling wave electrode 13 is disposed on the silicon oxide upper cover layer 12.

[0041] Through 3D electromagnetic software HFSS simulation calculation Figure 2 The transmission performance of the structure of the present invention, such as Figure 6 , 7 As shown, it can be seen that the structure of the present invention has high return loss and low transmission loss.

[0042] like Figure 4 As shown, the RF coaxial connector 14 loads the RF signal into the lithium niobate electro-optic modulator (i.e., modulator chip) through the RF packaging structure (i.e., RF transition transmission chip). The RF coaxial connector 14 is connected to the RF transition chip through gold-plated tape / gold-tin welding. The RF transition chip and the lithium niobate modulator chip are connected through gold wire 15. The gold wire bonding interconnection technology is used to realize the connection between solid-state devices or monolithic integrated circuits and passive circuits, the interconnection of passive circuits, and the interconnection of multiple chips. A model of signal transmission between the RF transition transmission chip and the lithium niobate electro-optic modulator chip was established through HFSS, and the dielectric constant of the chip medium, impedance matching, and gold wire bonding parameters were adjusted to obtain the ideal transmission model of the chip so that S11≤-12.5dB within the working bandwidth, and the transmission characteristics are obtained as shown below. Figure 8 , 9As shown, where S11max = -12.5dB@60GHz.

[0043] It is worth mentioning that the technical features such as the modulator chip and the RF coaxial signal connector involved in the patent application of the present invention should be regarded as the prior art. The specific structure, working principle and possible control method and spatial layout method of these technical features can be selected by conventional methods in the field, and should not be regarded as the inventive point of the patent of the present invention. The patent of the present invention will not be further elaborated.

[0044] For those skilled in the art, it is still possible to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A radio frequency packaging structure of a lithium niobate electro-optic modulator, characterized in that: The invention comprises a ceramic substrate, a bottom metal layer, a coplanar waveguide transmission line, a left metal layer and a right metal layer, wherein the bottom metal layer is closely attached to the bottom of the ceramic substrate and the coplanar waveguide transmission line is closely attached to the top of the ceramic substrate, the left metal layer is closely attached to the left side of the ceramic substrate and the right metal layer is closely attached to the right side of the ceramic substrate, wherein: The first end of the coplanar waveguide transmission line is connected to the modulator chip through gold wire bonding and the second end of the coplanar waveguide transmission line is connected to the radio frequency coaxial signal connector, the coplanar waveguide transmission line comprises a first ground electrode, a second ground electrode and a signal electrode arranged in parallel, and the signal electrode is located between the first ground electrode and the second ground electrode; The first end of the signal electrode is an isosceles trapezoidal transition structure, and the size of the transition structure gradually decreases as it approaches the modulator chip. The width of the end of the transition structure close to the modulator chip is the same as the width of the corresponding connection position of the modulator chip. The width of the second end of the signal electrode is equal to the width of the end of the transition structure away from the modulator chip. Two ends of the left metal layer are connected to the first ground electrode and the bottom metal layer respectively, and two ends of the right metal layer are connected to the second ground electrode and the bottom metal layer respectively.

2. The radio frequency packaging structure of a lithium niobate electro-optic modulator according to claim 1, characterized in that: The material of the ceramic substrate is aluminum nitride.

3. The radio frequency packaging structure of a lithium niobate electro-optic modulator according to claim 1, characterized in that: A first distance between the signal electrode and the first ground electrode is equal to a second distance between the signal electrode and the second ground electrode.

4. The radio frequency packaging structure of a lithium niobate electro-optic modulator according to claim 1, characterized in that: The first ground electrode and the second ground electrode are connected as a whole through the left metal layer and the right metal layer.

5. The radio frequency packaging structure of a lithium niobate electro-optic modulator according to claim 1, characterized in that: The RF coaxial connector is connected to the RF packaging structure by gold-plated tape / gold-tin welding.

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