Radio frequency package structure of a lithium niobate electro-optic modulator

Abstract

The application discloses a radio frequency packaging structure of a lithium niobate electro-optic modulator, which comprises a ceramic substrate, a bottom metal layer, a coplanar waveguide transmission line, a left metal layer and a right metal layer, the bottom metal layer is close to the bottom of the ceramic substrate, the coplanar waveguide transmission line is close to the top of the ceramic substrate, the left metal layer is close to the left side of the ceramic substrate, and the right metal layer is close to the right side of the ceramic substrate; the coplanar waveguide transmission line comprises first ground electrodes, second ground electrodes and signal electrodes which are arranged in parallel; the first end of the signal electrode is a transition structure in the shape of an isosceles trapezoid, and the transition structure gradually decreases in size as it approaches the modulator chip. The radio frequency packaging structure of the lithium niobate electro-optic modulator can improve the bandwidth of the lithium niobate modulator after packaging, and reduce the loss in the device packaging without improving the chip bandwidth.

Description

Technical Field

[0001] This invention belongs to the field of electro-optic modulator technology, specifically relating to an RF packaging structure for a lithium niobate electro-optic modulator. Background Technology

[0002] Electro-optic modulators are crucial devices that convert radio frequency electrical signals into optical signals that carry effective information, finding wide applications in optical communication and military radar equipment. Lithium niobate, due to its extremely high electro-optic coefficient, is widely used in various optoelectronic devices, and lithium niobate-based electro-optic modulators feature high bandwidth and low loss.

[0003] Packaging of electro-optic modulators is a crucial step towards their practical application. The radio frequency transition chip is an important part of the electro-optic modulator packaging. Radio frequency signals are transmitted to the radio frequency transition chip through the radio frequency coaxial connector, and then to the modulator chip, realizing the radio frequency signal conversion from the coaxial radio frequency connector to the modulator chip.

[0004] like Figure 1 As shown, current RF transition chips use a ceramic substrate 1 with a metal backing 2. A microstrip line 3 is fabricated on the ceramic substrate 1. The microstrip line 3 has advantages such as simple fabrication, convenient installation, and low cost. However, because the conductor is exposed to air, a portion of the electromagnetic field of the microstrip line radiates into the air, causing transmission loss. In contrast to the microstrip line, the grounded coplanar waveguide transition transmission line is located on the ceramic substrate 1. A coplanar waveguide transmission line 4 is fabricated on the ceramic substrate 1 using thin-film lithography. Metal vias 5 are fabricated on the coplanar waveguide line 4, short-circuiting the surface ground electrode with the back metal electrode. The presence of these metal vias disrupts the electromagnetic field boundary conditions of the coplanar waveguide transmission line-like microstrip line mode, thus suppressing microwave resonance to some extent. The metal vias 5 require fabrication and metal filling within the ceramic substrate 1, increasing fabrication difficulty and cost.

[0005] Therefore, further improvements will be made to address the aforementioned issues. Summary of the Invention

[0006] The main objective of this invention is to provide a radio frequency packaging structure for a lithium niobate electro-optic modulator, which can increase the bandwidth of the packaged lithium niobate modulator without increasing the chip bandwidth and reduce the loss in the device packaging.

[0007] To achieve the above objectives, the present invention provides an RF packaging structure for 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. The bottom metal layer is in close contact with the bottom of the ceramic substrate, and the coplanar waveguide transmission line is in close contact with the top of the ceramic substrate. The left metal layer is in close contact with the left side of the ceramic substrate, and the right metal layer is in close contact with the right side of the ceramic substrate.

[0008] The first end of the coplanar waveguide transmission line is connected to the modulator chip via 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 includes a first ground electrode, a second ground electrode, and a signal electrode arranged in parallel, with the signal electrode 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 size of the transition structure gradually decreases as it approaches the modulator chip. The width of the end of the transition structure near 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 designed to be consistent with the size of the electrode PAD area of ​​the modulator chip while ensuring that the characteristic impedance is close to 50 ohms, thus ensuring continuous mode matching). 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 near the RF coaxial connector is widened to obtain better impedance matching with the RF coaxial connector).

[0010] The two ends of the left metal layer are connected to the first ground electrode and the bottom metal layer, respectively, and the 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 embodiment of the above technical solution, the material of the ceramic substrate is aluminum nitride.

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

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

[0014] As a further preferred technical solution to the above technical solution, the RF coaxial connector is connected to the RF package structure by gold-plated strip / gold-tin soldering.

[0015] The beneficial effects of this invention are as follows:

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

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

[0018] 3. The radio frequency transition transmission chip is connected to the lithium niobate modulator chip via gold wire bonding. The number and relative positions of the gold wires have been designed to improve transmission performance. Attached Figure Description

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

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

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

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

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

[0024] Figure 6 This is a schematic diagram of the return loss of the present invention.

[0025] Figure 7 This is a schematic diagram of the transmission loss of the present invention.

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

[0027] Figure 9 This is a schematic diagram of the transmission characteristics of the present invention.

[0028] The reference numerals in the attached figures 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 Implementation

[0029] The following description is intended to disclose the present invention and enable those skilled in the art to implement it. The preferred embodiments described below are merely examples, and other obvious variations will occur to those skilled in the art. The basic principles of the invention defined in the following description can be applied to other embodiments, modifications, improvements, equivalents, and other technical solutions that do not depart from the spirit and scope of the invention.

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

[0031] Preferred embodiment.

[0032] like Figure 2-9 As shown, this invention discloses an RF packaging structure for a lithium niobate electro-optic modulator, including 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. The bottom metal layer 2 is in close contact with the bottom of the ceramic substrate 1, and the coplanar waveguide transmission line 4 is in close contact with the top of the ceramic substrate 1. The left metal layer 6 is in close contact with the left side of the ceramic substrate 1, and the right metal layer 7 is in close contact with the right side of the ceramic substrate 1.

[0033] The first end of the coplanar waveguide transmission line 4 is connected to the modulator chip by gold wire bonding and the second end of the coplanar waveguide transmission line 4 is connected to the radio frequency 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. 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 size of the transition structure gradually decreases as it approaches the modulator chip. The width of the end of the transition structure near 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 designed to be consistent with the size of the electrode PAD area of ​​the modulator chip while ensuring that the characteristic impedance is close to 50 ohms, thus ensuring continuous mode matching). 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 near the RF coaxial connector is widened to obtain better impedance matching with the RF coaxial connector). The isosceles trapezoidal shape 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] The 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 the 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 ceramic substrate 1 is made of aluminum nitride.

[0037] More specifically, the first distance between the signal electrode 4-2 and the first ground electrode 4-1 and the 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 into a whole by the left metal layer 6 and the right metal layer 7. The side metallization connects all the ground electrodes into a whole. This transmission line has a stronger ability to confine the electric field and can transmit at a higher frequency.

[0039] Furthermore, the RF coaxial connector is connected to the RF package structure via gold-plated ribbon / gold-solder soldering.

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

[0041] Simulation calculations were performed using the 3D electromagnetic software HFSS. Figure 2 The transmission performance of the structure of the present invention, such as Figure 6 , 7 As shown, 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., the modulator chip) through the RF package structure (i.e., the RF transition transmission chip). The RF coaxial connector 14 is connected to the RF transition chip via gold-plated ribbon / gold-tin soldering. The RF transition chip and the lithium niobate modulator chip are connected by gold wire 15. Bonding wire 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 signal transmission model between the RF transition transmission chip and the lithium niobate electro-optic modulator chip was established using HFSS. 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, ensuring that S11 ≤ -12.5dB within the operating bandwidth. The transmission characteristics are as follows: Figure 8 , 9As shown, S11max = -12.5dB@60GHz.

[0043] It is worth mentioning that the technical features such as the modulator chip and the radio frequency coaxial signal connector involved in this patent application should be regarded as prior art. The specific structure, working principle and possible control method and spatial arrangement of these technical features can be adopted by conventional choices in the field and should not be regarded as the inventive point of this patent. This patent will not be further elaborated in detail.

[0044] For those skilled in the art, modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the protection scope of this invention.

Claims

1. A radio frequency packaging structure for a lithium niobate electro-optic modulator, characterized in that, The system includes a ceramic substrate, a bottom metal layer, a coplanar waveguide transmission line, a left metal layer, and a right metal layer. The bottom metal layer is in close contact with the bottom of the ceramic substrate, and the coplanar waveguide transmission line is in close contact with the top of the ceramic substrate. The left metal layer is in close contact with the left side of the ceramic substrate, and the right metal layer is in close contact with the right side of the ceramic substrate. The first end of the coplanar waveguide transmission line is connected to the modulator chip via 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 includes a first ground electrode, a second ground electrode, and a signal electrode arranged in parallel, with the signal electrode 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 near 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. The two ends of the left metal layer are respectively connected to the first ground electrode and the bottom metal layer, and the two ends of the right metal layer are respectively connected to the second ground electrode and the bottom metal layer; The first distance between the signal electrode and the first ground electrode and the second distance between the signal electrode and the second ground electrode are equal.

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

3. 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 into a whole by the left and right metal layers.

4. 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 package structure via gold-plated ribbon / gold soldering.

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