Electrode transmission line and silicon-based electro-optical modulator including the same

Abstract

The present invention discloses an electrode transmission line, comprising: at least two metal layers, and the two sides of the metal electrodes at corresponding positions in the adjacent metal layers are electrically contacted through through holes to form a rectangular frame-shaped electrode structure. The present invention also discloses a silicon-based electro-optical modulator including the electrode transmission line. Under the condition of being fully compatible with the current CMOS manufacturing process, the structural design of the present invention optimizes the electrical bandwidth of the electrode transmission line, thereby improving the electrical bandwidth of the modulator, thereby increasing the electro-optical bandwidth of the modulator, and thus can adapt to the use scenarios of higher modulation rates; the structural design of the multi-layer metal layer can increase the cross-sectional area of ​​the electrode transmission line, thereby improving the DC current bearing capacity of the electrode transmission line; the rectangular frame-shaped electrode structure formed by the present invention can reduce the influence of the skin effect on high-frequency electrical signals.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon-based electro-optical modulators, and in particular to an electrode transmission line and a silicon-based electro-optical modulator comprising the electrode transmission line. Background Art

[0002] Silicon-based electro-optical modulators have received continuous attention from academia and industry due to their outstanding overall performance. They are manufactured using CMOS-compatible processes and can integrate multiple modulators on a single chip. They can also form more highly integrated silicon photonic integrated chips with silicon photonic passive devices and silicon-based germanium detectors.

[0003] Silicon-based electro-optic modulators in the form of Mach-Zehnder interferometers (MZIs) typically use traveling-wave electrodes as the electrode transmission lines for the modulation signal. Light waves and electrical modulation signals propagate in the same direction within the MZI modulator. To improve the modulator's electro-optical bandwidth, one approach is to reduce microwave loss in the traveling-wave electrode transmission lines. Microwave loss primarily stems from two factors: microwave loss in the electrode transmission lines themselves, primarily due to the skin effect; and microwave loss caused by the PN junction in the modulation region, which is equivalent to the electrical loss added by the RC load attached to the electrode transmission lines.

[0004] Among them, skin effect is a common phenomenon in radio frequency signal transmission. Figure 1 As shown, the DC current on the conductor is almost uniformly distributed inside the conductor, as shown in Figure 2 and Figure 3 As shown in the figure, as the frequency of alternating current increases, an alternating electromagnetic field appears in the conductor, and the current distribution inside the conductor changes. The current is mainly concentrated in the thin layer on the surface of the conductor. The closer to the surface of the conductor, the greater the current. The current inside the conductor is very small or even non-existent. Therefore, the skin effect causes the resistance of the conductor to increase during high-frequency signal transmission, and the conductor loss also increases accordingly.

[0005] like Figure 4As shown in the figure, in high-baud-rate operating scenarios, the driver chip and the modulator chip are connected using direct electrical coupling. The output stage of the driver chip requires a DC current to flow back from the modulator chip to the driver chip. This current typically flows from the load end of the modulator chip along the signal line of the modulator electrode transmission line. This DC current is likely to be large. Some driver chips with large output voltage swings require a current of up to 100mA or even greater to flow back from the modulator chip under normal operating conditions. This places high demands on the current-carrying capacity of the modulator electrode transmission line. However, the modulator electrode transmission line is manufactured using a CMOS-compatible process, and its thickness is often very limited, generally within 2um. Although increasing the electrode width of the signal line can enhance its current-carrying capacity, due to the design requirements of the modulator's characteristic impedance, it is difficult to widen the modulator signal line to a width sufficient to safely carry a large current exceeding 100mA. Summary of the Invention

[0006] To address the above-mentioned technical problems, the present invention provides an electrode transmission line and a silicon-based electro-optical modulator incorporating the same. To provide a basic understanding of some aspects of the disclosed embodiments, a brief summary is provided below. This summary is not intended to be a comprehensive review, identify key or important components, or delineate the scope of protection of these embodiments. Its sole purpose is to present some concepts in a simplified form as a prelude to the detailed description that follows.

[0007] The present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides an electrode transmission line comprising: at least two metal layers, each metal layer being composed of a plurality of metal electrodes, and the two sides of the metal electrodes at corresponding positions in adjacent metal layers being electrically contacted through through holes to form a rectangular frame-shaped electrode structure.

[0009] Furthermore, the distribution, shape and size of the metal electrodes in each metal layer are consistent.

[0010] In a second aspect, the present invention further provides a silicon-based electro-optic modulator, which includes the electrode transmission line.

[0011] Furthermore, the topmost metal layer in the electrode transmission line serves as the electrode PAD layer of the silicon-based electro-optic modulator.

[0012] Furthermore, the bottom metal layer in the electrode transmission line is connected to the PN junction of the silicon-based electro-optic modulator through a through hole.

[0013] Furthermore, the silicon-based electro-optic modulator further includes: a silicon substrate and a silicon dioxide isolation layer located above the silicon substrate, and the PN junction is prepared on the silicon dioxide isolation layer.

[0014] Furthermore, the silicon-based electro-optic modulator further includes: a silicon dioxide covering layer, wherein the silicon dioxide covering layer is located above the silicon dioxide isolation layer, and the electrode transmission line is located in the silicon dioxide covering layer.

[0015] Furthermore, the silicon-based electro-optic modulator further includes: an optical waveguide, a beam combiner, a beam splitter, and a terminal load.

[0016] The beneficial effects brought by the present invention are:

[0017] 1. The electrode transmission line is formed using at least two metal layers, with adjacent metal layers electrically contacting each other through through-holes. While being fully compatible with existing CMOS manufacturing processes, the structural design of the present invention optimizes the electrical bandwidth of the electrode transmission line, thereby increasing the electrical bandwidth of the modulator and thus the electro-optical bandwidth of the modulator, thus adapting to scenarios with higher modulation rates.

[0018] 2. The multi-layer metal layer structural design can increase the cross-sectional area of ​​the electrode transmission line, thereby improving the DC current carrying capacity of the electrode transmission line;

[0019] 3. The rectangular frame-shaped electrode structure formed by the present invention can reduce the influence of the skin effect on high-frequency electrical signals, increase the electrical bandwidth of the electrode transmission line, and further increase the electro-optical bandwidth of the modulator. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0021] Figure 1 It is a schematic diagram of the DC current distribution on the conductor;

[0022] Figure 2 It is a schematic diagram of the low-frequency current distribution on the conductor;

[0023] Figure 3 It is a schematic diagram of the high-frequency current distribution on the conductor;

[0024] Figure 4 This is a schematic diagram of the connection between the driver chip and the modulator chip;

[0025] Figure 5 It is a structural diagram of the modulator;

[0026] Figure 6It is a partial schematic diagram of the cross section of the modulation area of ​​the modulator of the present invention in which two layers of metal electrodes are combined to form an electrode transmission line;

[0027] Figure 7 It is a partial schematic cross-sectional view of the non-modulation region of a modulator in which two layers of metal electrodes are combined to form an electrode transmission line according to the present invention;

[0028] Figure 8 It is a partial schematic diagram of the cross section of the modulation area of ​​the modulator of the present invention in which three layers of metal electrodes are combined to form an electrode transmission line;

[0029] Figure 9 It is a partial schematic cross-sectional view of the non-modulation region of a modulator in which three layers of metal electrodes are combined to form an electrode transmission line according to the present invention;

[0030] Figure 10 is a schematic cross-sectional view of a rectangular frame-shaped electrode structure of the present invention;

[0031] Figure 11 This is a partial schematic diagram of the cross section of the modulation region of a conventional traveling wave electrode modulator;

[0032] Figure 12 This is a partial schematic diagram of the cross section of the non-modulation region of a conventional traveling wave electrode modulator;

[0033] Figure 13 This is a curve chart comparing the bandwidth response simulation results of the coplanar waveguide transmission line formed by a single-layer metal electrode and a double-layer metal electrode. DETAILED DESCRIPTION

[0034] The following describes embodiments of the present invention in detail with reference to the accompanying drawings. It should be understood that the embodiments described are only some of the embodiments of the present invention, and not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0035] like Figure 5-9 As shown, in order to improve the DC current carrying capacity of the silicon-based electro-optical modulator when used in conjunction with a driver chip, and to improve the electro-optical bandwidth of the modulator to adapt to the use scenario of higher modulation rate, the present invention provides an electrode transmission line, comprising: at least two metal layers, and each metal layer together constructs the electrode transmission line. Each metal layer is composed of a plurality of metal electrodes 1, and the two sides of the metal electrodes at corresponding positions in adjacent metal layers are electrically contacted through through holes, forming an electrode structure in the shape of a rectangular frame, as shown in FIG. Figure 10 The corresponding positions herein refer to metal electrodes on the same vertical line in adjacent metal layers.

[0036] The distribution, shape and size of the metal electrodes in each metal layer are consistent, that is, from a top-down view, the metal electrodes are distributed in the same position, but are in different layers. Since the shapes and sizes are also consistent, the metal electrodes are exactly the same when viewed from above on different layers, so that the rectangular frame-shaped electrode structure can be utilized to the maximum extent when current flows through to cope with the skin effect generated by the conductor.

[0037] Since the current silicon photonics process platform usually uses two metal layers, the present invention is described below using a typical structure of two metal layers.

[0038] like Figure 6-7 As shown, the electrode transmission line includes two metal layers. In the present invention, the upper metal layer of the two metal layers is defined as the first metal layer 2, and the lower metal layer is defined as the second metal layer 3. The first metal layer 2 and the second metal layer 3 together constitute the electrode transmission line.

[0039] The two sides of the metal electrodes at corresponding positions in the first metal layer 2 and the second metal layer 3 are electrically contacted through the first through-hole 4 to form a rectangular frame-shaped electrode structure, which brings two benefits: on the one hand, compared with a single-layer electrode, the cross-sectional area of ​​the conductor is increased, and the maximum current that the electrode can withstand can be effectively increased, which has obvious advantages when used in conjunction with an open-collector driving amplifier. This is because when used in conjunction with an open-collector driving amplifier, a larger DC current needs to be transmitted on the signal line of the modulator electrode transmission line; on the other hand, the influence of the skin effect on the electrode transmission line is weakened, and the corresponding electrode for DC transmission is a rectangular frame, and the corresponding electrode for high-frequency signal transmission is also a rectangular frame. Therefore, the additional high-frequency loss is reduced, and the electrical bandwidth is effectively increased, which in turn has a certain positive effect on improving the electro-optical bandwidth of the modulator.

[0040] As a comparison, Figure 11-12 As shown in the figure, a partial schematic diagram of the cross section of a conventional traveling wave electrode modulator in the modulation area and the non-modulation area is shown, which uses a single-layer metal electrode as a high-frequency transmission line. The single-layer electrode transmission line has a dilemma: if the electrode thickness of the metal layer is selected to be very thin, such as below 1μm, the DC resistance of the electrode transmission line is large, which will increase the electrical signal loss of the electrode transmission line, and at the same time, the DC current bearing capacity of the electrode signal is weak; if the electrode thickness of the metal layer is selected to be thicker, such as above 3μm, although the DC resistance and low-frequency resistance are significantly reduced, due to the influence of the skin effect, its effective resistance is significantly increased during high-frequency microwave transmission, so the electrical bandwidth of the electrode transmission line is still affected. In addition, according to the actual situation of the current CMOS process, it is also very difficult to make a single thicker metal layer. Changing the metal thickness of the single-layer electrode can balance the above problems to a certain extent, but the effect is still slightly worse than that of the combined electrode provided by the present invention.

[0041] Although current approaches to manufacturing silicon optical modulators already utilize two or even multiple electrode layers, the high-frequency electrode transmission line for the modulator is often implemented in only one layer, with the remaining layers simply serving to form good electrode contact. Either the lower metal layer 102 is used as the high-frequency transmission line to reduce high-frequency signal transmission loss and achieve impedance matching, while the upper metal layer 101 is used as the electrode PAD layer for gold wire bonding or flip-chip soldering to establish an electrical connection with the outside world; or the upper metal layer 101 is used as the high-frequency transmission line to reduce high-frequency signal transmission loss and achieve impedance matching, while the lower metal layer 102 serves as the structural component for achieving electrical connection to the modulator's PN junction, working together with the lower via 103 to achieve the electrical connection between the electrode transmission line and the PN junction.

[0042] When large DC currents need to be carried on the electrode transmission lines, existing silicon optical modulators require that the majority of this current flow through the upper metal layer 101 or the lower metal layer 102. Compared to the two-layer metal electrode combination of the present invention, this results in greater resistance and relatively weaker current-carrying capacity. Furthermore, using only one metal layer as the electrode transmission line allows DC and low-frequency electrical signals to be uniformly distributed across the metal's cross-section. However, high-frequency signals are limited by the skin effect, resulting in a reduced equivalent cross-section and increased equivalent resistance, leading to increased losses in high-frequency signal transmission.

[0043] The present invention utilizes the same dimensions for the first metal layer 2 and the second metal layer 3, connecting the metal electrodes within these two layers via a first through-hole 4. This allows both the first metal layer 2 and the second metal layer 3 to effectively carry current. Existing technologies utilize only one metal layer as the actual modulator's high-frequency electrode transmission line, while the other metal layer serves only as electrical contact. The two metal layers are not designed to form a rectangular frame to function as an electrode transmission line.

[0044] like Figure 8-9 As shown, in some cases, if the DC current carrying capacity of the modulator's electrode transmission line is very high, and considering that the characteristic impedance design of the modulator cannot be effectively alleviated by increasing the electrode width, it is also possible to consider using more than two layers of multi-layer metal electrodes to form an electrode transmission line.

[0045] like Figure 5-10 As shown, the present invention also provides a silicon-based electro-optical modulator, comprising: a PN junction 5, a silicon substrate 7, a silicon dioxide isolation layer 8, a silicon dioxide cover layer 9, an optical waveguide 10, a beam combiner 11, a beam splitter 12, and a terminal load 13.

[0046] After passing through the beam splitter 12, the input light is divided into two equal parts and enters the upper arm and lower arm of the MZI structure respectively. A modulation area 14 and a non-modulation area 15 are designed in the upper arm and the lower arm of the MZI. The voltage change of the electrical signal brings about a change in the effective refractive index of the optical waveguide in the modulation area 14, resulting in a change in the phase of the two beams when they are combined, thereby achieving a corresponding change in the light intensity of the output light.

[0047] The silicon-based electro-optic modulator of the present invention includes the aforementioned electrode transmission line. The topmost metal layer in the electrode transmission line serves as the electrode PAD layer of the silicon-based electro-optic modulator. The bottommost metal layer in the electrode transmission line is connected to the PN junction 5 of the silicon-based electro-optic modulator via a second through hole 6.

[0048] A silicon dioxide isolation layer 8 is located above the silicon substrate 7, and a PN junction 5 is formed on the silicon dioxide isolation layer 8. A silicon dioxide capping layer 9 is located above the silicon dioxide isolation layer 8, and an electrode transmission line is located within the silicon dioxide capping layer 9.

[0049] The structural design of the silicon-based electro-optic modulator proposed in the present invention is fully compatible with the process system of existing commonly used solutions, and the electrode transmission line is optimized to reduce the high-frequency microwave loss of the electrode transmission line, thereby improving the electro-optical bandwidth of the modulator and enhancing the high current carrying capacity of the electrode transmission line.

[0050] like Figure 13 As shown in the figure, the bandwidth response results of the coplanar waveguide transmission line formed by a single-layer metal electrode and a double-layer metal electrode are simulated and compared. It can be seen from the figure that after the double-layer metal electrode is used to form the electrode transmission line, the electrical bandwidth characteristics of the transmission line are significantly improved.

[0051] The electrode material in the simulation model is aluminum, and the model length is 1 mm. Figure 13 In the figure, H=0.5um, H=1.0um, and H=2.0um refer to the simulation result curves when the thickness of the single-layer metal electrode is 0.5um, 1.0um, and 2.0um, respectively. H=0.5um+0.5um refers to the simulation result curve when the thickness of one layer of the metal electrode in the double-layer metal electrode is 0.5um and the thickness of the other layer of the metal electrode is 0.5um. H=1.0um+0.5um refers to the simulation result curve when the thickness of one layer of the metal electrode in the double-layer metal electrode is 1.0um and the thickness of the other layer of the metal electrode is 0.5um. H=1.5um+0.5um refers to the simulation result curve when the thickness of one layer of the metal electrode in the double-layer metal electrode is 1.5um and the thickness of the other layer of the metal electrode is 0.5um.

[0052] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. An electrode transmission line, characterized in that: include: At least two metal layers, and each metal layer is composed of a plurality of metal electrodes, and the metal electrodes at corresponding positions in adjacent metal layers are electrically contacted through through holes on both sides along the width direction to form an electrode structure, and the transverse cross-section of the electrode structure in the width direction is a rectangular frame.

2. The electrode transmission line according to claim 1, characterized in that: The distribution, shape and size of the metal electrodes in each metal layer are consistent.

3. A silicon-based electro-optical modulator, characterized in that: The silicon-based electro-optic modulator comprises the electrode transmission line according to claim 1 or 2.

4. The silicon-based electro-optic modulator according to claim 3, characterized in that: The topmost metal layer in the electrode transmission line serves as the electrode PAD layer of the silicon-based electro-optic modulator.

5. The silicon-based electro-optic modulator according to claim 4, characterized in that: The bottom metal layer in the electrode transmission line is connected to the PN junction of the silicon-based electro-optical modulator through a through hole.

6. The silicon-based electro-optic modulator according to claim 5, characterized in that: Also includes: A silicon substrate and a silicon dioxide isolation layer located above the silicon substrate, wherein the PN junction is prepared on the silicon dioxide isolation layer.

7. The silicon-based electro-optic modulator according to claim 6, characterized in that: It also includes: a silicon dioxide covering layer, the silicon dioxide covering layer is located above the silicon dioxide isolation layer, and the electrode transmission line is located in the silicon dioxide covering layer.

8. The silicon-based electro-optic modulator according to claim 7, characterized in that: Also includes: Optical waveguides, beam combiners, beam splitters, and terminal loads.

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