Flip chip soldering packaging structure of differential MZM chip

Abstract

The application provides a flip-chip packaging structure of a differential MZM chip, comprising: a differential MZM chip and a substrate; the electrode side of the differential MZM chip faces downward and is opposite to the substrate; the upper surface of the substrate is paved with metal, and the area of the metal exceeds a preset value; the upper surface of the substrate is interconnected with the ground electrode of the electrode side of the differential MZM chip through a flip-chip solder joint, and the metal on the upper surface of the substrate forms an effective alternating current signal ground structure, so that an equivalent microstrip line structure is formed between the electrode of the differential MZM chip and the metal on the upper surface; when the differential MZM chip works, the positive electrode of the electrode side of the differential MZM chip produces a microstrip line effect, so that the performance of the differential MZM chip is optimized. The flip-chip packaging scheme of the substrate of the application comprehensively considers the parasitic between the flip-chip packaging process and the chip, and utilizes the parasitic to make the electrode of the MZM form a microstrip line effect, so that the effects of reducing microwave loss, improving impedance matching and speed matching are achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical communications, and more particularly, relates to a flip-chip packaging structure of a differential Mach-Zehnder Modulator (MZM) chip. Background Art

[0002] The rapid development of information technology has led to an increasing demand for network capacity. As the next-generation interconnect technology, optical interconnects offer greater bandwidth and stronger interference resistance than electrical interconnects. To address bandwidth, size, and power consumption considerations, optical chips, optical chips, and control chips must be packaged together to form optical modules. This means that packaging has a significant impact on the performance of optical modules.

[0003] Traditional packaging uses gold wire bonding to interconnect chips. This interconnection is low-cost and relatively stable, but it suffers from significant parasitic effects, low interconnect density, and limited bandwidth. To improve bandwidth, flip-chip packaging has been adopted, offering high bandwidth, high interconnect density, and low loss. Compared to single-ended links, differential links offer the advantage of high common-mode rejection, making them widely adopted in optical modules. However, the differential MZM chip in an optical module lacks a ground plane between the electrodes, making it equivalent to a two-wire transmission line, which suffers from high radiation loss and low bandwidth. Summary of the Invention

[0004] In view of the defects of the prior art, the object of the present invention is to provide a differential Mach-Zehnder modulator chip flip-chip packaging structure, aiming to solve the problems of large radiation loss and low bandwidth of the existing differential MZM chip.

[0005] To achieve the above objectives, in a first aspect, the present invention provides a differential MZM chip flip-chip packaging structure, comprising: a differential MZM chip and a substrate;

[0006] The electrode side of the differential MZM chip faces downward, facing the substrate;

[0007] The upper surface of the substrate is paved with metal, and the area of ​​the metal exceeds a preset value. The upper surface of the substrate is interconnected with the ground electrode on the electrode side of the differential MZM chip via flip-chip solder joints. The metal on the upper surface of the substrate forms an effective AC signal ground structure, so that an equivalent microstrip line structure is formed between the differential MZM chip electrode and the upper surface metal. When the differential MZM chip is working, the positive electrode on the electrode side of the differential MZM chip produces a microstrip line effect, thereby optimizing the performance of the differential MZM chip.

[0008] In an optional example, the area of ​​the metal on the upper surface of the substrate may exceed or deviate from the projection range of the MZM chip electrode, so as to enable the positive electrode to produce a microstrip line effect.

[0009] In an optional example, the pattern of the metal on the upper surface of the substrate is a whole piece of metal, a grid, a densely interconnected grid, a metal strip deviating from the projection range of the MZM electrode, or other pattern of an equivalent AC signal ground structure required for the positive electrode to produce a microstrip line effect.

[0010] In an optional example, the substrate is a high-frequency and low-loss substrate.

[0011] In an optional example, the substrate is a microwave PCB board, a ceramic substrate or a silicon substrate.

[0012] In an optional example, when the differential MZM chip is working, the microstrip line effect of the positive electrode causes the microwave refractive index to decrease, and then the microwave refractive index remains consistent with the light refractive index, so that the microwave speed matches the light wave speed; in addition, the microstrip line effect of the positive electrode reduces microwave loss, and at high frequencies, the characteristic impedances of the positive and negative electrodes of the differential MZM chip remain close to the preset target impedance, the high-frequency impedance matching is improved, and the corresponding high-frequency bandwidth is increased.

[0013] In an optional example, the differential MZM chip flip-chip packaging structure is suitable for high-speed optical communication packaging links with a bandwidth of 100 GHz and above.

[0014] In a second aspect, the present invention provides a flip-chip packaging method for a differential MZM chip, comprising the following steps:

[0015] Laying a layer of metal on the substrate, wherein the area of ​​the metal exceeds a preset value;

[0016] The differential MZM chip is flip-chip bonded to the upper surface of a substrate; wherein the upper surface of the substrate is interconnected with the ground electrode on the electrode side of the differential MZM chip via the flip-chip solder joints, and the metal on the upper surface of the substrate forms an effective AC signal ground structure, so that an equivalent microstrip line structure is formed between the differential MZM chip electrode and the upper surface metal; when the differential MZM chip is in operation, the positive electrode on the electrode side of the differential MZM chip produces a microstrip line effect, thereby optimizing the performance of the differential MZM chip.

[0017] In an optional example, the substrate is a high-frequency and low-loss substrate.

[0018] In an optional example, the substrate is a microwave PCB board, a ceramic substrate or a silicon substrate.

[0019] In general, the above technical solutions conceived by the present invention have the following beneficial effects compared with the prior art:

[0020] The application provides a flip-chip packaging structure of a differential MZM chip, compared with a traditional differential MZM scheme, the substrate flip-chip scheme of the application comprehensively considers the parasitic between the flip-chip process and the chip, and utilizes the parasitic to make the electrode of the MZM form a microstrip line effect, so as to achieve the effects of reducing microwave loss, improving high-frequency impedance matching and speed matching, and further, when the high-frequency impedance matching is improved, the loss of high-frequency transmission is reduced, and the bandwidth of high frequency is improved, therefore, the microstrip line effect of the electrode in the application is still applicable in the high-frequency band, and can be used for high-speed optical communication packaging links with a bandwidth of 100GHz and above in the future. The differential MZM flip-chip packaging scheme based on the substrate paving has no additional requirements for the substrate processing and flip-chip process precision, and has high practicability and stability. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is a flip-chip packaging structure of a differential MZM chip based on substrate paving provided by the application;

[0022] Figure 2 is a typical equivalent ground plane pattern of a substrate surface metal provided by the application;

[0023] Figure 3 is a comparison chart of insertion loss of the application and other schemes provided by the application;

[0024] Figure 4 is a return loss chart of the application and other schemes provided by the application;

[0025] Figure 5 is a comparison chart of electrode characteristic impedance of the application and other schemes provided by the application;

[0026] Figure 6 is a comparison chart of microwave refractive index of the application and a traditional scheme provided by the application. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical scheme and advantages of the embodiments of the application clearer, the technical scheme in the embodiments of the application will be described clearly and completely below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. The description of the at least one exemplary embodiment is actually only illustrative, and is by no means any limitation on the application and use or application of the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the application.

[0028] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0029] Unless otherwise specifically stated, the relative arrangement of the parts and steps, numerical expressions and numerical values ​​set forth in these embodiments do not limit the scope of the present invention. At the same time, it should be clear that, for ease of description, the sizes of the various parts shown in the drawings are not drawn according to the actual proportional relationship. The technology, methods and equipment known to those of ordinary skill in the relevant art may not be discussed in detail, but in appropriate cases, the technology, methods and equipment should be considered as part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary and not as a limitation. Therefore, other examples of the exemplary embodiments may have different values. It should be noted that similar numbers and letters represent similar items in the following figures, and therefore, once an item is defined in one figure, it does not need to be further discussed in subsequent figures.

[0030] In the description of the present invention, it should be understood that the directions or positional relationships indicated by directional words such as "front, back, up, down, left, right", "horizontal, vertical, vertical, horizontal" and "top, bottom" are usually based on the directions or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description. Unless otherwise specified, these directional words do not indicate or imply that the device or element referred to must have a specific direction or be constructed and operated in a specific direction. Therefore, they cannot be understood as limiting the scope of protection of the present invention: the directional words "inside and outside" refer to the inside and outside relative to the outline of each component itself.

[0031] In the description of the present invention, reference to terms such as "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples" means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the exemplary expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0032] In order to solve the above-mentioned problems of differential MZM chips, the present invention proposes a differential MZM flip-chip packaging solution based on substrate paving. During flip-chip bonding, the substrate adopts surface metal paving to make the electrodes form a microstrip line effect, which can simultaneously optimize microwave loss, impedance matching, and speed matching performance.

[0033] It should be noted that the metal grounding on the substrate surface means that the upper surface of the substrate is interconnected with the ground electrode on the electrode side of the differential MZM chip through flip-chip solder joints, and the metal on the upper surface of the substrate forms an effective AC signal ground structure.

[0034] In view of the defects of the prior art, the purpose of the present invention is to provide a differential Mach-Zehnder modulator flip-chip packaging solution, aiming to solve the problem of microwave loss, impedance matching and speed matching of differential MZM chips.

[0035] The present invention provides a flip-chip packaging solution for a differential Mach-Zehnder modulator (MZM) based on substrate paving. The metal paving on the substrate surface can improve the performance of the MZM, including reducing microwave loss and improving impedance matching and speed matching.

[0036] Furthermore, the differential MZM chip is flip-chip bonded to a substrate with a metal ground plane. This allows the MZM electrodes to be transformed from a two-wire transmission line structure to a microstrip transmission line structure, thereby reducing losses caused by electromagnetic wave dissipation. Because the two-wire transmission line structure lacks a ground plane, the electromagnetic field radiated by the conductors extends to infinity. In this case, the electrodes act like antennas, resulting in significant radiation losses that increase with frequency. The microstrip transmission line structure prevents excessive electromagnetic field leakage, thereby reducing radiation losses.

[0037] Furthermore, the metal-paved substrate can be considered the ground plane of the microstrip line. The flip-chip solder balls are used to connect the ground between the MZM chip and the substrate. By controlling the height of the flip-chip solder balls, the characteristic impedance of the MZM electrode can be determined. The dielectric layer of the equivalent microstrip line structure serves as the medium between the chip and the substrate. This dielectric layer can be made of air, resin, or other filler materials.

[0038] Furthermore, the microstrip effect significantly improves impedance matching, especially for the middle electrode. When the substrate is not grounded, the middle electrode acts as a two-wire transmission line structure, and the impedance response is similar to that of an antenna, with periodic peaks, which makes impedance matching difficult. When the substrate is grounded, the middle electrode acts as a microstrip transmission line, and the impedance peak is significantly reduced, resulting in better impedance matching.

[0039] Further, the packaging scheme improves the speed matching of microwave speed and light speed, the microwave refractive index of the MZM is usually large, and the microstrip line effect can reduce the microwave refractive index, so that the microwave refractive index is consistent with the optical refractive index, that is, the speed matching; the microwave refractive index is the ratio of the speed of light in vacuum c and the phase velocity v of microwave in the MZM electrode, and the microwave refractive index can be compared with the optical refractive index to determine the speed matching; the speed matching is a key indicator of the MZM, when the microwave speed is completely consistent with the light speed, the signal modulation effect is best, and the mismatch of the speed will reduce the bandwidth of the electro-optical modulation. p

[0040] Further, the flip-chip process flips the MZM chip on the substrate to form a short-distance connection, which is suitable for high-speed packaging interconnection.

[0041] Further, the substrate is not limited to a certain specific material, and can be a microwave PCB board, a ceramic substrate or a silicon substrate, etc., and selecting a high-frequency and low-loss substrate is beneficial to expand the bandwidth of the packaging link, and since a large-area paving form is adopted, the processing precision requirement of the microwave board is low, so that the substrate has high practicability and high reliability.

[0042] Further, the flip-chip packaging scheme of the substrate paving can be applied to the high-speed optical communication packaging link with a bandwidth of 100GHz and above in the future.

[0043] The application can solve the problem of poor differential MZM electrode matching in existing optical communication, and proposes a differential MZM flip-chip packaging scheme based on substrate paving, which significantly improves the performance of the MZM.

[0044] The MZM flip-chip packaging scheme based on substrate paving provided by the application has three improvements: reducing microwave loss, improving impedance matching and speed matching. The differential MZM chip is flip-chip packaged on the substrate, and the substrate adopts a surface metal paving form, so that the electrode of the MZM has a microstrip line effect, thereby realizing the performance optimization of the MZM.

[0045] ​In an embodiment of the present invention, the MZM adopts a differential solution of lithium niobate, that is, there are three electrodes, S-, S+, and S-. The characteristic impedance requirements of the S+ and S- electrodes are 25 ohms and 50 ohms, respectively. The potential difference between the electrodes is used to modulate the differential optical path. In order to ensure the modulation efficiency, the spacing between the electrodes is strictly limited. In addition, the lithium niobate MZM chip itself does not have a ground plane, so the microwave loss, characteristic impedance, and refractive index of the electrode are relatively large, especially for the middle S+ branch. In order to solve this problem, the present invention flip-chip solders the differential MZM chip onto a substrate with a metal ground surface to form a microstrip line effect, which significantly improves the performance of the MZM. The metal ground surface of the substrate has low requirements for the substrate processing technology and is highly practical and stable.

[0046] In an embodiment of the present invention, a substrate-to-ground flip-chip differential MZM solution is compared with traditional and air-bridge differential MZM solutions, demonstrating improvements in microwave loss, impedance matching, and velocity matching. As a preferred embodiment of the present invention, a flip-chip differential lithium niobate MZM circuit architecture is frequently used in fiber optic transmitters.

[0047] In order to further illustrate the differential MZM flip-chip packaging method based on substrate paving provided by the embodiment of the present invention, the following is described in detail with reference to the accompanying drawings and in combination with specific embodiments:

[0048] like Figure 1 The left two sections illustrate the circuit architecture used in an embodiment of the present invention, namely a flip-chip differential lithium niobate MZM. In addition to the three sets of electrodes (S-, S+, and S-) on the MZM chip, a large area of ​​ground metal is located on the top and bottom sides. This design optimizes the impedance matching of the electrodes. The ground plane on the substrate is interconnected with the ground metal of the MZM chip via flip-chip balls, achieving an effective common ground effect. The front view shows the parasitic effects of the electrodes, namely the series inductance and the parallel capacitance to the ground. This creates a microstrip effect, which reduces microwave loss, improves impedance matching, and enhances speed matching.

[0049] like Figure 2 As shown, the pattern of the ground metal on the substrate surface includes but is not limited to the following forms: a whole piece of metal, a mesh metal, a grid metal, and a metal strip. These metal patterns can provide an equivalent ground plane for the MZM electrode, thereby forming a microstrip line effect. In addition, the pattern of the above metal can also be any other pattern that can produce any equivalent AC signal ground structure required for the positive electrode to produce a microstrip line effect.

[0050] like Figure 3As shown, the traditional differential MZM chip is directly exposed to the air, and the radiation loss is large due to the absence of a ground plane for the middle S+ electrode. The air bridge scheme can provide a ground plane for the middle S+ electrode to some extent, but the optimization effect is limited. Compared with the above two schemes, the present application adopts a surface metal paving flip-chip mode, Figure 2 The S21 response of the traditional and air bridge schemes and the present application scheme is shown on the right, and for both the S+ branch and the S- branch, the present application scheme can significantly improve the insertion loss of the electrode and the fluctuation of the response curve, and overall reduce the microwave loss.

[0051] As shown in the figure, Figure 4 For the traditional scheme and the air bridge scheme, the return loss of the S+ and S- electrodes is at the level of -5dB and -10dB, while the return loss of the S+ and S- electrodes under the present application scheme can reach below -20dB, which indicates that the impedance matching is significantly improved.

[0052] As shown in the figure, Figure 5 Whether in the traditional scheme or the air bridge scheme, the characteristic impedance of the S+ and S- electrodes is significantly larger than the required 25 ohms and 50 ohms. After the improvement of the present application, the characteristic impedance of the S+ and S- electrodes is about 26 ohms and 54 ohms, which is close to the target impedance, and the fluctuation of the impedance curve is significantly smaller, which significantly improves the matching of the electrode characteristic impedance, thereby reducing the ripple of the bandwidth response and improving the electro-optical eye diagram quality.

[0053] As shown in the figure, Figure 6 The optical refractive index in lithium niobate MZM is fixed at 2.2, while in the traditional and air bridge schemes, the refractive index of the S+ and S- electrodes is significantly larger, which will cause the mismatch between the optical speed and the microwave speed, thereby deteriorating the eye diagram quality. By using the substrate paving scheme of the present application, the microwave refractive index of the S+ and S- electrodes is significantly reduced and is close to the optical refractive index, reaching a level close to 2.2, which improves the matching between the optical speed and the microwave speed and is beneficial to improving the bandwidth and the eye diagram quality.

[0054] The present application discloses a differential MZM flip-chip packaging scheme based on substrate paving, and the surface metal paving of the flip-chip substrate improves the performance of the MZM, including reducing the microwave loss, improving the impedance matching and the speed matching. The present application utilizes the parasitic between the MZM chip and the ground plane during flip-chip to make the differential MZM electrode produce microstrip line effect, especially for the middle electrode branch, the middle electrode is changed from a double-line transmission line structure to a microstrip transmission line structure, which significantly improves the bandwidth, reduces the fluctuation of the response curve, and makes the characteristic impedance closer to the ideal value. The present application proposes a flip-chip architecture based on substrate paving, which can reduce the microwave refractive index and is beneficial to the speed matching between the microwave speed and the optical speed.

[0055] Those skilled in the art can easily understand that the above description is only the preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A flip-chip packaging structure of a differential MZM chip, characterized in that: include: Differential Mach-Zehnder modulator (MZM) chips and substrates; The electrode side of the differential MZM chip faces downward, facing the substrate; The upper surface of the substrate is paved with metal, and the area of ​​the metal exceeds a preset value. The upper surface of the substrate is interconnected with the ground electrode on the electrode side of the differential MZM chip via flip-chip solder joints. The metal on the upper surface of the substrate forms an effective AC signal ground structure, so that an equivalent microstrip line structure is formed between the differential MZM chip electrode and the upper surface metal. When the differential MZM chip is operating, the positive electrode on the electrode side of the differential MZM chip produces a microstrip line effect, thereby optimizing the performance of the differential MZM chip. The area of ​​the metal on the upper surface of the substrate exceeds or deviates from the projection range of the MZM chip electrode, so as to enable the positive electrode to produce a microstrip line effect; The metal pattern on the upper surface of the substrate is a whole piece of metal, a grid, a densely interconnected grid, a metal strip deviating from the projection range of the MZM electrode, or other patterns of equivalent AC signal ground structures required for the positive electrode to produce a microstrip line effect.

2. The structure according to claim 1, characterized in that The substrate is a high-frequency and low-loss substrate.

3. The structure according to claim 1 or 2, characterized in that The substrate is a microwave PCB board, a ceramic substrate or a silicon substrate.

4. The structure according to claim 1 or 2, characterized in that When the differential MZM chip is working, the microstrip line effect of the positive electrode causes the microwave refractive index to decrease, and the microwave refractive index is then kept consistent with the light refractive index, so that the microwave speed matches the light wave speed; in addition, the microstrip line effect of the positive electrode reduces microwave loss, and at high frequencies, the characteristic impedances of the positive and negative electrodes of the differential MZM chip remain close to the preset target impedance, thereby improving high-frequency impedance matching and correspondingly increasing the high-frequency bandwidth.

5. The structure according to claim 1, characterized in that The differential MZM chip flip-chip packaging structure is suitable for high-speed optical communication packaging links with a bandwidth of 100 GHz and above.

6. A flip-chip packaging method for a differential MZM chip, characterized in that: The steps include: Laying a layer of metal on the substrate, wherein the area of ​​the metal exceeds a preset value; A differential Mach-Zehnder modulator (MZM) chip is flip-chip bonded to the upper surface of a substrate; wherein the upper surface of the substrate is interconnected with a ground electrode on the electrode side of the differential MZM chip via a flip-chip solder joint, and the metal on the upper surface of the substrate forms an effective AC signal ground structure, thereby forming an equivalent microstrip line structure between the differential MZM chip electrode and the metal on the upper surface; when the differential MZM chip is operating, the positive electrode on the electrode side of the differential MZM chip generates a microstrip line effect, thereby optimizing the performance of the differential MZM chip; The area of ​​the metal on the upper surface of the substrate exceeds or deviates from the projection range of the MZM chip electrode, so as to enable the positive electrode to produce a microstrip line effect; The metal pattern on the upper surface of the substrate is a whole piece of metal, a grid, a densely interconnected grid, a metal strip deviating from the projection range of the MZM electrode, or other patterns of equivalent AC signal ground structures required for the positive electrode to produce a microstrip line effect.

7. The method according to claim 6, characterized in that The substrate is a high-frequency and low-loss substrate.

8. The method according to claim 6 or 7, characterized in that The substrate is a microwave PCB board, a ceramic substrate or a silicon substrate.

Images