Copper damascene process-based high-performance thin-film lithium tantalate modulators

Domain: chemistry
Published: 2026-02-26
Editorial outcome: minor_revision

Abstract
The conversion between electrical and optical signals underpins modern optical communication systems and increasingly requires tight co-integration with electronics at short length scales. Thin-film lithium tantalate has emerged as a promising electro-optic platform due to its large Pockels coefficient, low bias drift, and high power handling, yet its integration with standardized microelectronic processes remains limited. Here we show that incorporating the copper Damascene process into thin-film lithium tantalate modulators enables a scalable, electronics-compatible fabrication approach. The resulting devices exhibit approximately 10% lower microwave loss than conventional gold-electrode designs, while simultaneously supporting watt-level on-chip optical power handling and maintaining a stable quasi-static half-wave voltage from 1 Hz to 1 MHz, with a bias point drift of only 0.4 dB over a 15-hour period when operated at 1.75 mW on-chip optical power. High-speed transmission experiments demonstrate line rates of 416 Gbit/s (PAM4) and 540 Gbit/s (PAM8) below the 25% soft-decision forward-error-correction threshold, together with watt-level on-chip optical power handling. These results establish a practical route toward scalable chip-on-wafer integration of electro-optic modulators with microelectronic circuits.

Introduction
Integrated electro-optic (EO) modulators provide a scalable platform for converting information between electrical and optical domains, serving as key building blocks for high-capacity optical communications 1–3 and emerging chip-to-chip interconnects 4,5. Recent progress in thin-film lithium niobate and thin-film lithium tantalate photonics has transformed traditional bulk devices 6,7. The adoption of a layered “on-insulator” structure, combined with improved etching techniques, enables strong optical confinement and allows electrodes to be placed closer to the waveguide, thereby enhancing modulation efficiency. This structure also facilitates velocity matching between optical and microwave signals, resulting in higher modulation bandwidths. However, as the electrode gap is reduced to achieve tighter optical confinement and stronger modulation, the currents in the signal line and ground plane become increasingly concentrated near the gap due to the electromagnetic proximity effect 8. This enhanced current concentration increases the per-unit-length resistance of the transmission line, leading to higher microwave loss. The resulting loss limits further improvements in voltage–bandwidth performance. Employing electrodes with periodic microstructured “T-rails” mitigates the proximity effect by introducing a larger effective gap while preserving the electric-field distribution required for efficient modulation 9–12. An alternative approach to reducing microwave loss replaces conventional gold electrodes with low-resistivity metals, such as copper which has a resistivity approximately 24% lower than gold 13.

While continuous efforts have been made to extend the voltage–bandwidth performance of thin-film lithium niobate and lithium tantalate modulators 14,15, most reported devices remain standalone prototypes, requiring bulky electrical probes and external control electronics. Meanwhile, AI-based applications are driving an ever-growing demand for higher data throughput, lower latency, and improved energy efficiency in modern computing systems 16. To meet these requirements, co-packaged optics (CPO) has emerged as a transformative technology that tightly integrates photonics and electronics 17. Such integration reduces high-frequency signal degradation, reduces energy consumption, and enables large-scale system scalability. Among various integration approaches, three-dimensional (3D) integration, as illustrated in Figure 1(a), offers the shortest metal interconnects by vertically stacking photonic and electronic chips, thereby minimizing signal loss and improving overall efficiency 18. While this stacking is typically achieved through flip-chip bonding using microbumps 19, the industry is transitioning toward hybrid copper-copper bonding 20. In this approach, copper surfaces are directly joined through thermally activated diffusion, eliminating solder interfaces and thereby reducing parasitic effects. A key enabler of this transition is planarized copper metallization achieved via the copper Damascene process, already widely adopted in both microelectronics 21 and silicon photonics 22. However, ferroelectric photonic platforms still rely on conventional gold electrodes, highlighting the need for further innovation to achieve fully integrated, high-performance photonic–electronics systems.

Here, we address this challenge by incorporating the copper Damascene process into the fabrication of lithium tantalate photonic integrated circuits (PICs). This approach leverages a mature, low-cost, and widely adopted method used for metallization in microelectronics and silicon photonics. Using this process, we demonstrate integrated electro-optic modulators with microwave losses approximately 10% lower than those employing conventional gold electrodes, while preserving the intrinsic benefits of thin-film lithium tantalate, including low optical loss, high modulation efficiency, and broad modulation bandwidth. The resulting planar electrode surface also supports direct chip-on-chip and chip-on-wafer bonding with driver electronics.

Results
Fabrication. The devices were fabricated from a commercially available X-cut thin-film lithium tantalate (TFLT) wafer (NANOLN), which consists of 600nm of lithium tantalate (LiTaO3, LT), 4.7µm of SiO2, and a 525µm-thick high-resistivity silicon substrate. The TFLT PICs were manufactured using an etching technique based on a diamond-like carbon hard mask, which can reliably produce both lithium niobate and lithium tantalate PICs 23,24. The etch depth of the LT was 320nm, leaving a 280nm-thick slab for efficient electro-optic modulation and proper phase matching between microwave and optical signals.

Next, the copper Damascene process, outlined in Figure 1(b), was introduced to fabricate traveling-wave electrodes. This Damascene process differs from silicon nitride Damascene methods 25, which reduce optical propagation losses in waveguides whereas we apply it to copper electrodes to minimize microwave loss. In this process, dielectric trenches are patterned and filled by electroplated copper followed by chemical–mechanical polishing (CMP). This process produces well-defined electrodes with a smooth surface potentially suitable for chip-on-wafer bonding. Further information on the fabrication is available in the Methods. Figure 1 (c) shows two fabricated Cu-TFLT MZMs extending over an effective modulation length of 6mm. Figure 1 (d) illustrates the fabricated 4-inch wafer with hundreds of Cu-TFLT MZMs, highlighting its potential for scalable integration of next-generation photonic systems. To improve the wafer’s uniformity following the CMP process, inactive regions feature a filler copper pattern, which is visible in Figure 1(c-e). As respectively shown in Figure 1(e-g), each modulator consists of 50:50 1×2 multimode interference (MMI) couplers, EO phase shifters operating in a push-pull configuration, and double-layer tapers for enhanced PIC-to-fiber coupling efficiency 26 (Figure S1). As illustrated in Figure 1(f), the tight confinement of the modulator’s microwave and optical modes ensures strong overlap between the two fields and thus enhanced modulation efficiency.

Electrical Transmission. To assess the potential of copper as an electrode material for high-performance modulators, we characterized the electrical properties of fabricated copper Damascene CPWs, including resistivity and microwave performance, and compared them with their conventional gold-based counterparts. Figure 2 (a) shows a photograph of the test structures. For context, Figure 2 (b) presents the bulk resistivity of the seven most conductive elemental metals commonly used in microelectronics and photonics. Although thin films are typically employed in these applications, their resistivity depends on thickness and fabrication process; thus, bulk values are used here for a general comparison. Silver (1.59µΩcm) exhibits the lowest resistivity, followed by copper (1.68µΩcm), which is only 5.6% higher, and gold (2.21µΩcm), which is 39% higher 27. We focus on copper and gold due to the impracticality of using silver (see Discussion).

Figure 2 (c) compares the measured resistivity of thin-film gold and copper and its evolution with time. The gold thin film (796nm) was deposited by electron beam evaporation, and the copper thin film (1170nm) by electroplating. Both were formed on silicon carrier wafers with a 2µm silicon dioxide layer and characterized using a four-point probing and mechanical profilometry. The electroplated copper exhibited a 24% decrease in resistivity within 48 hours at room temperature due to self-annealing 28, stabilizing at 2.04µΩcm - approximately 20% lower than that of gold. Studies suggest that the resistivity of electroplated copper could be further reduced to 1.77µΩcm 29 via process optimization. Using these measured values, we modeled the microwave behavior of the fabricated CPWs through full-wave electromagnetic simulations (Figure S2). Figure 2 (d) shows the simulated microwave loss of a 16mm-long copper Damascene CPW with a 6µm gap and a 27µm signal line, alongside simulated CPWs with varying resistivities but identical geometries. The measured microwave loss agrees well with the simulation, validating the model. The data indicate that copper electrodes yield approximately 10% lower microwave loss than gold, while the propagation index and characteristic impedance change by less than 0.3%. The dependence of the microwave effective index on design parameters is provided in Figure S3.

Electro-optic Modulation. A direct approach to assess the modulators’ performance is to measure their EO response, including the quasi-static half-wave voltage (Vπ) and the 3 dB EO bandwidth. Finite-element simulations (COMSOL Multiphysics) were performed to analyze the VπL product and optical loss for various modulator geometries. A design with 6µm electrode gap and a 4µm-wide waveguide was selected. This relatively wide waveguide maintains a similar VπL and optical loss while allowing a larger electrode gap that reduces microwave loss (Figure S4) 30. A 100µm-long adiabatic linear taper connects the modulation region to the single-mode routing waveguide, ensuring single-mode operation. The quasi-static Vπ was measured by applying a low-frequency triangular voltage signal to the CPW electrode and recording the optical output simultaneously. Figure 3 (a) shows the normalized transmission of a modulator as a function of the applied voltage, linearly swept at 100Hz, exhibiting a near-sinusoidal response with a Vπ of 1.7V. For a device length of 16mm, this corresponds to a VπL of 2.7V·cm, closely matching the simulated value of 2.64V·cm. The inset dB-scale plot shows an extinction ratio of 20dB, suitable for advanced modulation formats such as PAM4 and PAM8. The dependence of the Vπ on the modulation frequency was also examined. As shown in Figure 3 (b), Vπ remains stable across a wide frequency range from 1Hz to 1MHz. More detailed results are shown in Figure S5. Similar measurements were performed over a range of on-chip optical powers up to 1.17W, and the results are shown in Figure S6. This flat frequency response contrasts with the pronounced low-frequency instability previously reported for lithium niobate modulators with gold CPWs 31. A small fluctuation (< 0.1V) in Vπ between 1kHz and 100kHz is attributed to dielectric relaxation intrinsic to the Cu–LiTaO3 stack. Such effects arise from slow polarization dynamics or charge trapping in the ferroelectric layer when the modulation frequency approaches the characteristic relaxation rate. The magnitude and frequency of this fluctuation remain unchanged as the on-chip optical power increases from 1mW to 1.17W, confirming that is not due to optical intensity-dependent photorefractive effects. As indicated by the stable quasi-static Vπ, the modulator exhibits excellent long-term stability, showing only a 0.4 dB bias drift over 15 hours at 1.75mW on-chip power (Figure 3(c)). This stability removes the need for active thermal biasing and its associated power-hungry heaters 30. More detailed bias-drift measurement results are provided in Figure S7. Figure 3 (d) presents the measured EO response up to 110GHz. The 3 dB EO bandwidth reaches 40GHz for a 16mm-long device and 100GHz for a 6mm-long device, both with a 6µm electrode gap. The simulated EO response is derived from the measured electrical response and agrees well with the experimental data, validating the measurement. Simulations further indicate that the 3-dB EO bandwidth is approximately 10% higher than that of comparable gold-electrode devices (Figure S8). These results demonstrate that the copper-electrode-based modulators combine high EO bandwidth with low drive voltage, enabling high-speed optical communication applications.

Optical Communications. To demonstrate the exceptional performance of our Cu-TFLT MZM, we performed a high-speed intensity-modulation and direct-detection (IMDD) signaling experiment. The experimental apparatus for this task is illustrated in Figure 4(a) and further described in the Methods. We generated and received PAM4 and PAM8 data signals with symbol rates between 144GBd and 208GBd. Figure 4 (b) displays the measured bit error ratios (BER) of the various PAM signals as a function of symbol rate. The horizontal dashed lines indicate the thresholds for typical soft-decision (SD) forward-error correction (FEC) with 25% and 15% overhead, and for hard-decision (HD) FEC with 7% overhead. The results show that we can transmit 180GBd PAM8 signals and 208GBd PAM4 signals while the respective BER of 3.76×10−2 and 3.56×10−2 are still below the 25% SD-FEC limit. In Figure 4 (c), we further calculate the generalized mutual information (GMI) of our measurements based on log-likelihood ratios by using an additive white Gaussian noise channel model 32. The dashed curves depict the achievable information rate (AIR), which is the product of the GMI and the symbol rate. The solid curves in Figure 4 (c) show the net data rates for measurements with BER below the 25% SD-FEC limit, where the net data rates are determined by multiplying the transmitted line rates with the code rate that is associated with the normalized GMI threshold, extracted from 33. The results indicate that the highest AIR of 449Gbit/s is achieved by using PAM8 signals at a symbol rate of 176GBd. This corresponds to a net data rate of 423Gbit/s, which is on par with results demonstrated for high-bandwidth thin-film lithium niobate MZMs 34. Comparison of these data rates and other metrics achieved in this work along with those from other state-of-the-art lithium niobate and lithium tantalate modulators are available in Table S1. It should be noted that some of the results such as the PAM4 symbol rate were limited by the bandwidth of our driver electronics, not by the MZM itself.

Methods
Copper Damascene Fabrication Flow. To begin with, the TFLT PICs were cladded with a 2.5µm-thick silicon dioxide layer by plasma-enhanced chemical vapor deposition (PECVD). This cladding layer will embed the subsequently formed copper electrodes. The layout for these electrodes was defined with a DUV stepper lithography (ASML PAS 5500/350C) and transferred as preforms into the cladding layer through fluorine-based dry etching in a reactive ion etcher. These preforms were coated with 10nm of titanium (Ti) and 100nm of copper (Cu) through sputtering (Alliance-Concept DP 650). The titanium layer serves as a barrier to prevent copper from diffusing into the surrounding dielectric 41. It also provides adhesion to a subsequently sputtered copper layer acting as a seed to facilitate copper electroplating. The electroplating was performed in a Silicet Electroplating unit and contributed to a 2.8µm thick copper layer with low resistivity (2.04µΩcm). Chemical mechanical planarization (CMP) was employed to remove the excess copper, leaving copper only in the etched preforms with a final thickness of 1.8µm. This relatively thick metal layer minimizes microwave losses in the CPW electrodes across frequencies ranging from tens of GHz to 110GHz (Figure S14). Since the electrodes are embedded within a SiO2 cladding layer and share a common top surface, their thickness can be indirectly monitored by measuring the thickness of nearby SiO2 windows using spectroscopic reflectometry, as shown in Figure 1 (c). Subsequently, a 100nm-thick silicon nitride passivation layer was deposited with PECVD to prevent copper oxidation. The electrode pads were then opened and capped with gold (Au) for efficient and stable electrical probing. Finally, chip singulation was achieved through a combination of dry etching - standard reactive ion etching for LT and silicon dioxide, deep reactive ion etching for silicon - and backside grinding. This process ensures smooth facets for edge coupling to single-mode fibers.

Data Transmission Apparatus. An external-cavity laser (ECL, 17.8dBm at 1550nm) provides the optical carrier. A high-speed arbitrary-waveform generator (AWG, M8199B, Keysight) is used to generate the electrical drive signal, which is fed to the CPW of the MZM via a 20cm-long RF cable, a broadband RF amplifier, and a 110GHz RF probe. We synthesize various pulse-amplitude modulation (PAM) signals based on pseudo-random bit sequences and apply root-raised cosine pulse-shaping filters with a roll-off of β = 0.05. We account for the frequency-dependent RF loss up to the input of the feeding probe by applying a linear minimum-mean-square-error (MMSE) predistortion. The CPW is terminated by a 50Ω resistor via a second 110GHz RF probe. As previously demonstrated in Figure 3 (c), the EO stability of the Cu-TFLT modulator allows reliable DC biasing at the quadrature point for intensity modulation with a bias-T attached to the second probe. The optical power at the output fiber of the MZM operated at the quadrature point is 5.6dBm, which exceeds the power requirements in typical specifications for high-speed optical ethernet transceivers 42. Still, an additional erbium-doped fiber amplifier (EDFA) was needed in the experiment to reach sufficient power levels for the high-speed photodiode (8.5dBm) at the receiver. Note that a practical transceiver implementation could rely on a sufficiently broadband amplifier after the photodiode, thereby rendering the EDFA unnecessary. In our experiments, the out-of-band amplified spontaneous-emission (ASE) noise of the EDFA is suppressed by a tunable bandpass filter (BPF), and a variable optical attenuator (VOA) is used to adjust a power level of 8.5dBm at the input of the photodiode. The electrical signal at the photodiode output is digitized by a real-time oscilloscope (RTO, UXR 1004A, Keysight) with an analogue bandwidth of 105GHz and a sampling rate of 256 GSa/s. The data is finally extracted by offline receiver DSP (Rx-DSP), which contains resampling to 2 Sa/symbol, timing recovery, linear Sato equalization, and an additional decision-directed least-mean-square (DD-LMS) equalizer.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript presents an advancement in the implementation of electro-optic modulator using a copper Damascene process with ~10% lower microwave loss compared to conventional gold electrodes. The demonstration of high data rates (416 and 540 Gbit/s) and compatibility with industry-standard processes are useful for next-generation photonic-electronic co-integration. This work can be considered for publication in Nature Communications after addressing the following issues.\n1. The copper Damascene process is well-described, and the use of CMP for planarization is a key technique for hybrid bonding. However, more details on the uniformity and yield of the process across the 4-inch wafer would strengthen the scalability arguement. Statistical data on device performance variation would be beneficial.\n2. While comparisons are made with silver-based modulators, a more detailed table comparing key metrics (e.g., VpL, bandwidth, loss, data rate) with recent LN and LT modulators would help contextualize the advancements.\n3. The manuscript emphasizes the potential for chip-on-wafer bonding, but no actual bonding or co-integration results are shown. A preliminary demonstration or cross-section image of a bonded structure would greatly strengthen the claim.\n4. It would be beneficial if the authors could provide more detailed information on how the electrode structure (e.g., gap width, signal line width, thickness) was specifically designed to achieve phase velocity matching between the microwave and optical modes.\n5. While Supplementary Figure S8 shows microwave loss stability over 7 months, more information on environmental stability (e.g., temperature cycling, humidity tests) would be valuable for assessing suitability in commercial applications. This is especially relevant given copper’s susceptibility to oxidation and diffusion.\n6. A critical metric for the practical deployment of Mach-Zehnder modulators (MZMs) in communication systems is the long-term stability of the operating bias point. The authors comprehensively demonstrate the remarkable stability of the half-wave voltage across frequency (Fig. 3c, Suppl. Fig. S2). However, they do not present data on the dynamics of the relative output intensity with the bias set to quadrature over extended operating time.", "reviewer_2": "The authors present a thin-film lithium tantalate (TFLT) modulator utilizing a copper damascene process. I am not able to recommend the manuscript for publication in a high-profile journal such as Nature Communications due to the following major weaknesses:\n1. The Damascene process is claimed to be a key innovation of this work, but its practical advantages for optical transmission applications are insufficiently demonstrated, particularly in achieving notably lower propagation loss [Optica 5, 884-892 (2018)].\n2. The observed improvement in electro-optic modulation performance stems primarily from the lower electrical resistivity of copper, rather than structural or process-related innovations.\n3. The work employs a sophisticated Damascene fabrication technique, but the overall device performance shows only marginal improvement compared with their previous work [Optica 11, 1614-1620 (2024)].\nIn addition, below are my technical comments/questions:\n1. Figure 2b shows that the resistivity of Ag is lower than that of Au and Cu. Should Ag be a better material than Cu for metal electrodes in electro-optic (EO) modulators?\n2. Compared with the Ag-electrode-based TFLT EO modulator reported in [Optica 11, 1614-1620 (2024)], the measured bit error ratios (BER) for both PAM8 and PAM4 signals as a function of symbol rate demonstrate only marginal improvement. What are the reasons for this limited performance?\n3. How did the authors determine the thickness and geometry of the Cu electrodes to achieve lower microwave loss? How was the thickness controlled precisely through Cu planarization method?\n4. In Figure 3c, why does V_π fluctuate between 10^3 and 10^5 Hz?\n5. Figure S7 shows that the EO modulation performance depends on the structure of the Cu electrodes. It is crucial to evaluate how the electrode structure impacts optical transmission, as this will consequently influence the overall modulation performance."}, "author_response": "We thank the reviewers for their thoughtful and constructive comments on our manuscript\n“Copper Damascene Process-Based High-Performance Thin Film Lithium Tantalate\nModulators.” We have carefully considered all suggestions and have revised the\nmanuscript and supplementary information accordingly. Below, we provide a detailed\npoint-by-point response, with the reviewers’ comments copied verbatim in blue.\nThe manuscript presents an advancement in the implementation of electro-optic\nmodulator using a copper Damascene process with ~10% lower microwave loss\ncompared to conventional gold electrodes. The demonstration of high data rates (416 and\ngeneration photonic-electronic co-integration. This work can be considered for publication\nin Nature Communications after addressing the following issues.\nWe are grateful to the Referee for their time and effort in reviewing our work.\n1. The copper Damascene process is well-described, and the use of CMP for planarization\nis a key technique for hybrid bonding. However, more details on the uniformity and yield\nof the process across the 4-inch wafer would strengthen the scalability argument.\nStatistical data on device performance variation would be beneficial.\nResponse: We thank the reviewer for this valuable suggestion. We have now included\nstatistical data from 9 fields across the 4-inch wafer (new Supplementary Figure S11). The\nresults show that the device-to-device variation of the electrical S parameter is below\n21\n0.1 dB at 60 GHz, corresponding to a variation of less than 3% in linear scale, the\nmicrowave group indices are all well matched to the optical group index of 2.22 within a\ntolerance of ± 0.03, and the quasi-static V varies by less than 5 %.\nπ\nThese results confirm\nhigh process reproducibility and support the scalability of our approach. We have added\nthe following passage in the Discussion accordingly:\nWe further characterized the microwave metrics and quasi-static Vpi values on\ndevices from nine fields across a 4-inch wafer (Supplementary Figure S11). The\nresults show that the device-to-device variation of the electrical S parameter is\n21\nbelow 0.1 dB at 60 GHz (less than 3% in linear scale), the microwave group indices\nare all well matched to the optical group index of 2.22 within a tolerance of ± 0.03,\nand the quasi-static Vpi varies by less than 5%. These results confirm high process\nreproducibility and support the scalability of our approach.\n2. While comparisons are made with silver-based modulators, a more detailed table\ncomparing key metrics (e.g., VpiL, bandwidth, loss, data rate) with recent LN and LT\nmodulators would help contextualize the advancements.\nResponse: We agree and now include a comparison table (Supplementary Table I) that\nsummarizes the key metrics of our devices alongside recent state-of-the-art LN and LT\nmodulators. This contextualizes our achievements and clarifies the advantages of the Cu–\nTFLT platform.\n2\n3. The manuscript emphasizes the potential for chip-on-wafer bonding, but no actual\nbonding or co-integration results are shown. A preliminary demonstration or cross-section\nimage of a bonded structure would greatly strengthen the claim.\nResponse: We appreciate this comment. While a full bonding demonstration is beyond\nthe scope of the present study, we have provided detailed morphological characterization\nof the CMP planarized copper electrodes (new Supplementary Figure S13). The AFM\nscans show an RMS roughness below 11 nm and a maximum step height of ~37 nm at\nthe Cu–SiO interface, which is within accepted limits for chip-on-wafer bonding\n2\n[Microelectron. Reliab. 59, 134–139 (2016)]. We have emphasized this evidence in the\nrevised Discussion as strong support for the feasibility of chip-on-wafer\nintegration with\nthe following passage:\nWe also envision a full co-packaging of Pockels modulators with their driving\ncircuits using emerging copper-copper hybrid bonding techniques. Although a full\nbonding demonstration is beyond the scope of this study, we have shown\nmorphological results that strongly support the feasibility of chip-on-wafer\nintegration using our process. Specifically, the CMP-treated copper electrodes\nexhibit an RMS roughness below 11 nm and a maximum step height of\napproximately 37 nm at the Cu–SiO interface (Supplementary Figure S13), values\n2\nthat fall within accepted limits for hybrid bonding.\nWe also restructured our manuscript’s introduction and discussion sections to better set\nexpectations for the scope of our work. We now primarily motivate our choice of copper\nas the CPW material to limit microwave losses while relying on standardized\nmanufacturing methods in-line with industry standards and potentially compatible with\nemerging co-packaging methods.\n4. It would be beneficial if the authors could provide more detailed information on how the\nelectrode structure (e.g., gap width, signal line width, thickness) was specifically designed\nto achieve phase velocity matching between the microwave and optical modes.\nResponse: We thank the reviewer for the suggestion. We use FEM simulations (Ansys\nQ2D) to optimize the electrode gap, signal width, and layer stack to minimize velocity\nmismatch. The optimized structure yields a microwave effective index of ~2.24, well\nmatched to the optical group index of 2.22, ensuring efficient electro-optic modulation. The\nparameter dependence of the microwave group index is shown in new Supplementary\nFigure S3.\n5. While Supplementary Figure S9 shows microwave loss stability over 7 months, more\ninformation on environmental stability (e.g., temperature cycling, humidity tests) would be\n3\nvaluable for assessing suitability in commercial applications. This is especially relevant\ngiven copper’s susceptibility to oxidation and diffusion.\nResponse: We thank the reviewer for the valuable suggestion. To evaluate the\nenvironmental stability of the copper electrodes, we performed a temperature cycling\nexperiment, as shown in new Supplementary Figure S10. The device was repeatedly\nheated to 80 °C for 45 minutes and then cooled to room temperature before each re-\nmeasurement. The electrical S and the corresponding microwave group index remained\n21\nnearly unchanged across all four measurements, indicating no observable degradation or\nchange in performance. These results demonstrated the excellent robustness of our\ncopper electrodes under repeated temperature stress and confirmed their suitability for\nlong-term and practical microwave applications. We have added the following passage in\nthe Discussion accordingly:\nTo assess the stability of our copper Damascene electrodes, we measured their\nmicrowave performance over an extended period and conducted a thermal cycling\ntest. The results show that the electrodes maintained their loss and propagation\ngroup index over six months of ambient storage (Supplementary Figure S9). These\nproperties also remained unchanged under a 45-miniute-per-cycle thermal cycling\ntest between room temperature and 80 °C (Supplementary Figure S10).\n6. A critical metric for the practical deployment of Mach-Zehnder modulators (MZMs) in\ncommunication systems is the long-term stability of the operating bias point. The authors\ncomprehensively demonstrate the remarkable stability of the half-wave voltage across\nfrequency (Fig. 3c, Suppl. Fig. S2). However, they do not present data on the dynamics\nof the relative output intensity with the bias set to quadrature over extended operating time.\nResponse: We thank the reviewer for highlighting this important point. We have now\nperformed long-term measurements of the quadrature bias point stability. As shown in\nnew Supplementary Figure S7, the modulator exhibits a bias drift of only 0.4 dB over 15\nhours without active feedback. This confirms that Cu–TFLT modulators can reliably\nmaintain biasing in practical communication systems.\nThe authors present a thin-film lithium tantalate (TFLT) modulator utilizing a copper\ndamascene process. I am not able to recommend the manuscript for publication in a high-\nprofile journal such as Nature Communications due to the following major weaknesses:\nWe are grateful to the Referee for their time and effort in reviewing our work.\n1. The Damascene process is claimed to be a key innovation of this work, but its practical\nadvantages for optical transmission applications are insufficiently demonstrated,\nparticularly in achieving notably lower propagation loss [Optica 5, 884-892 (2018)].\nResponse: We thank the reviewer for raising this point, which made us realize that there\nmight be confusion between the Damascene process explored in this work and the one\nreported in [Optica 5, 884-892 (2018)], which also came out of our group. When we refer\nto the Damascene process, we generally refer to the forming microstructures by patterning\nthe encapsulating medium and depositing material in the resulting preform. This type of\nprocessing is typically employed when patterning cannot be employed by depositing a film\nof material and etching it due to complications in the etching step. The process reported\nin [Optica 5, 884-892 (2018)] involved producing silicon dioxide cladded silicon nitride\nwaveguides. Specifically, it relies on etching smooth preforms followed by LPCVD\ndeposition of silicon nitride and planarization of the resulting material stack. This resulted\nin the formation of silicon nitride waveguides with ultrasmooth sidewalls which were\ninstrumental in reducing optical propagation losses in silicon nitride photonic integrated\ncircuits.\nUnlike this Damascene process for silicon nitride photonic waveguides, the process\nreported in this work is for copper microwave electrodes. Direct etching of copper is\nparticularly challenging because copper does not form volatile byproducts, making it\ndifficult to produce clean, well-defined features. Although alternative methods such as lift-\noff can avoid direct etching, its resolution becomes limited when the copper layer is thick\n(e.g., >0.5 µm) or when the pattern has a high aspect ratio. Lift-off also leaves copper\nsidewalls exposed, which complicates the subsequent deposition of diffusion barrier\nlayers (e.g., Ti, Ta, TaN, etc.) that are essential in modern interconnect technologies to\nprevent copper diffusion. In addition, metal deposition for lift-off is typically performed by\nevaporation due to its superior directionality compared to other physical vapor deposition\n(PVD) methods such as sputtering. However, evaporation is inherently slow (~20 nm/min)\nand costly, making it impractical for depositing thick copper films or for large-scale\nmanufacturing.\ndielectric to form trenches and vias, then depositing a conformal barrier/seed layer\nfollowed by bottom-up electroplating of copper, and finally performing chemical–\nmechanical polishing (CMP) to remove excess metal. This approach eliminates the need\nfor anisotropic copper etching, achieves void-free features with excellent line-edge and\nwidth control, and allows electroplating to deposit thick copper layers far more efficiently\n(~280 nm/min/A) and economically than physical vapor deposition methods. The CMP\nstep also planarizes each layer, enabling clean stacking of multiple metal levels. For these\nreasons, the copper Damascene process is the dominant method used for fabricating\ninterconnects in integrated circuits.\nOn that note, the process itself is not meant to reduce optical losses as in the case of the\nsilicon nitride Damascene process, but rather allow the deposition of thicker low resistivity\nlayers of copper that can yield lower microwave losses in the modulator and hence better\nmodulation bandwidths. Furthermore, as emphasized in various parts of the manuscript,\nthis process is an industry standard used for multilayer electronic circuits. It also allows\nfor smooth metal surfaces instrumental for chip-on-wafer bonding, thereby making our\nmodulators potentially suitable for co-integration with other chip-scale systems.\nTo clarify the distinction between our work and [Optica 5, 884-892 (2018)], we have\nincluded the following passage in our main text pointing out the relevance of the\nDamascene process for fabricating other types of structures besides electronic ones:\nThis Damascene process differs from silicon nitride Damascene methods, which\nreduce optical propagation losses in waveguides whereas we apply it to copper\nelectrodes to minimize microwave loss.\n2. The observed improvement in electro-optic modulation performance stems primarily\nfrom the lower electrical resistivity of copper, rather than structural or process-related\ninnovations.\nResponse: We agree with the reviewer that the lower electrical resistivity of copper plays\na crucial role in improving electro-optic modulation performance. However, that does not\nnecessarily preclude structural or process-related effects to contribute to this feature. As\nmentioned in our work, one of the key structural effects contributing to our lower resistivity\nelectrodes is copper’s self-annealing effect as quantified in Figure 2(c). As discussed in\nthe Electrical Transmission section of our work, this effect is well studied in the\nmicroelectronics community [J. Appl. Phys. 86, 2516-2525 (1999)]. During this process,\nthe resistivity of electroplated copper decreases after deposition as a result of grain growth\nwithin its crystallographic structure. Given electroplated copper’s dominance in\nmicroelectronics, studies examining this effect in copper films deposited by other means,\nsuch as by evaporation or sputtering, are a little scarcer. However, early work relying on\nthinner evaporated or sputtered copper films seem to rely on an additional annealing step\n6\nto bring down the resistance of their copper films to bulk values [J. Electrochem. Soc. 139,\n992 (1992); Electrochem. Solid-State Lett. 2, 534 (1999); Appl. Phys. Lett. 81, 3774-3776\n(2002); Phys. Rev. B 102, 165102 (2020)]. Such effects could relate to our observed lower\nmicrowave losses compared to prior work involving modulators relying on lithium niobate\nand evaporated copper [Photonics Res. 10, 2380-2387 (2022)]. Though these\nobservations do not definitely conclude the relevance of the structural benefits of using\nelectroplated copper over evaporated copper, they do justify an exhaustive comparison of\nthe effects of both types of copper on electro-optic modulation, which we believe would\nbe more appropriate for a more specialized study.\nFurthermore, as emphasized in our reply to the reviewer’s previous comment, the\nstructural benefits that come with the Damascene process provide additional advantages\nbeyond modest propagation-loss reduction: (i) CMP planarization that enables chip-on-\nwafer hybrid bonding, and (ii) compatibility with multilayer metallization flows well-\nestablished in commercial microfabrication. These two factors greatly contribute to the\nindustrial scalability of our platform. We have clarified this point in the revised Discussion\nand supported it with AFM planarity data (new Supplementary Figure S13).\n3. The work employs a sophisticated Damascene fabrication technique, but the overall\ndevice performance shows only marginal improvement compared with their previous work\n[Optica 11, 1614-1620 (2024)].\nResponse: We thank the reviewer for this comment. While the overall communication\nperformance shows only modest improvement compared with our previous Ag-electrode-\nbased TFLT modulator, the main contribution of this work lies in establishing a scalable\nand standardized copper Damascene process for Pockels modulators.\nFrom a device perspective, the achievable data rate and BER depend on both microwave\nand optical factors. The microwave and microwave–optical performance of our Cu-based\ndevice are comparable to those of the Ag-based modulator, as expected from the similar\nresistivity of the two metals. However, our device offers improved optical characteristics,\nincluding lower coupling and transmission losses and higher extinction ratio, which\ntogether enable slightly better transmission performance.\nThe limited BER improvement is therefore not a reflection of material or design limitations,\nbut rather of the fact that the Ag and Cu devices are already near the performance ceiling\ndefined by the experimental setup. The key advance of the present work is the\ndemonstration of a scalable, low-loss, and planarized copper platform that preserves\nthe high-speed capability of TFLT modulators while enabling large-scale electronic–\nphotonic co-integration, which is an essential step toward practical and manufacturable\nhigh-bandwidth EO systems.\n1. Figure 2b shows that the resistivity of Ag is lower than that of Au and Cu. Should Ag be\na better material than Cu for metal electrodes in electrooptic (EO) modulators?\nResponse: We appreciate the reviewer’s comment. While silver has the lowest bulk\nresistivity among elemental metals used in microelectronics (1.59 μΩ·cm vs. 1.68 μΩ·cm\nfor copper, only 5.6% higher), this small difference becomes negligible in thin films, where\nresistivity is dominated by fabrication conditions. In practice, thin-film silver does not\nnecessarily outperform copper and suffers from diffusion and sulfidation issues that are\ndifficult to control. In addition, electroplating and CMP processes are not well optimized\nfor silver. In contrast, the copper Damascene process we use is mature, cost-effective,\nand widely adopted in microelectronics and silicon photonics. It provides planarized\nsurfaces, multilayer routing, and direct compatibility with chip-on-wafer bonding - key for\nminimizing parasitic and enabling dense 3D integration. Such manufacturing maturity and\nintegration capabilities are generally lacking in silver-based processes. Therefore, we\nrespectively don’t think silver is a better material for scalable and reliable applications of\nEO modulators. We have added the following passages in the main text to clarify these\npoints.\nFor context, Figure 2(b) presents the bulk resistivity of the seven most conductive\nelemental metals commonly used in microelectronics and photonics. Although thin\nfilms are typically employed in these applications, their resistivity depends on\nthickness and fabrication process; thus, bulk values are used here for a general\ncomparison. Silver (1.59 μΩ·cm) exhibits the lowest resistivity, followed by copper\n(1.59 μΩ·cm), which is only 5.6% higher, and gold (2.21 μΩ·cm) which is 39%\nhigher. We focus on copper and gold due to the impracticality of using silver (see\nDiscussion).\nFrom a materials perspective, metals with lower resistivity reduce microwave loss.\nGold electrodes are conventionally used in bulk lithium niobate modulators due to\ntheir relatively low resistivity, chemical stability, and ease of fabrication by\nevaporation and lift-off or electroplating followed by etching. With the emergence\nof thin-film platforms and growing demands for high-speed data connectivity,\nreplacing conventional gold electrodes with copper Damascene electrodes has\nbecome strategically important. Copper offers lower resistivity, improved voltage-\nbandwidth performance, and compatibility with scalable electronic integration.\nAlthough silver has slightly lower resistivity than copper, it suffers from diffusion\nand sulfidation issues that cannot be effectively addressed as copper. Moreover,\nsilver's electroplating and CMP processes are less mature. Consequently, despite\nits marginal electrical advantage, silver is less reliable and less cost-effective for\nlarge-scale integration. on.\n8\n2. Compared with the Ag-electrode-based TFLT EO modulator reported in [Optica 11,\n1614-1620 (2024)], the measured bit error ratios (BER) for both PAM8 and PAM4 signals\nas a function of symbol rate demonstrate only marginal improvement. What are the\nreasons for this limited performance?\nResponse: We thank the reviewer for this comment. The achievable data rate and\ncorresponding BER are influenced by (i) microwave properties such as impedance\nmatching and propagation loss, (ii) microwave–optical interactions such as phase-velocity\nmatching, and (iii) purely optical factors including coupling loss, on-chip transmission loss,\nand extinction ratio. In terms of the microwave and microwave-optical performance, our\nCu-electrode-based modulators are similar to those of the Ag-electrode-based TFLT\ndevices. However, our device demonstrates distinct optical advantages - namely, a fiber-\nto-chip coupling loss as low as 3 dB per facet, on-chip transmission loss of only 1 dB, and\nan extinction ratio exceeding 20 dB (Supplementary Figure S1). These improvements\ncontribute to the slightly higher data transmission rates we observe relative to the Ag-\nelectrode-based counterpart.\nWhile the improvement in data rate is modest, we emphasize that the achieved high\nsymbol rates primarily validate the intrinsic high-speed capability of our modulator, which\narises from the Pockels effect and the optimized co-design of the traveling-wave\nelectrodes and optical waveguides. High data rates alone, however, do not directly\ntranslate to energy-efficient operation. As discussed in [OFC 2025, M3C.1], the ultimate\ngoal of reducing drive voltage and increasing bandwidth is to minimize RF attenuation and\nenergy consumption. Nonetheless, all the hard work in achieving the above two goals can\nbe wiped off if the device suffers from high optical insertion loss, as the resulting low optical\noutput power undermines overall system efficiency.\nOf course, a comprehensive assessment of energy efficiency in EO modulators must\nconsider not only the intrinsic device performance but also the energy consumption of\nbroadband RF amplifiers and digital signal processing (DSP) modules required for signal\nrecovery [Nat. Electron. 6, 910–921 (2023)]. Such an analysis lies beyond the scope of\nthe present study, whose primary objective is to demonstrate the integration of the Cu\nDamascene process with Pockels-based platforms (lithium tantalate and lithium niobate).\nThis approach preserves the intrinsic advantages of these materials (i.e., low loss and\nhigh-speed modulation), while paving the way for large-scale electronic–photonic co-\nintegration.\nLastly, it is important to note that the measured data transmission rate is also limited by\nthe bandwidth of the RF components used in the experimental setup rather than the\nmodulator itself. As shown in [IEEE Photon. Technol. Lett. 35, 850–853 (2023)], the\nachievable modulation rate scales with the available RF bandwidth. With the ongoing\ndevelopment of higher-bandwidth RF electronics, the Cu-TFLT modulators introduced\n9\nhere are expected to support substantially higher data rates, as discussed in [J. Lightwave\nTechnol. 41, 3674–3679 (2023)] and [OFC 2025, Th4B.7].\n3. How did the authors determine the thickness and geometry of the Cu electrodes to\nachieve lower microwave loss? How was the thickness controlled precisely through Cu\nplanarization method?\nResponse: We thank the reviewer for raising this important point concerning the\nmethodology behind our modulator design. As described in Supplementary Information\nSection II, we used Ansys Q2D to model the microwave transmission properties of the\nelectrodes. This includes microwave losses. To determine the thickness that achieves\nlower losses, we performed simulations of CPW electrodes with varying metal thicknesses.\nThe results show that microwave losses approach an asymptotic minimum beyond a\ncertain thickness. For example, at 20 GHz, the metal thickness should exceed 1.6 µm to\nachieve low loss; at 40 GHz, this requirement decreases to 1.2 µm, and at frequencies up\nto 100 GHz, a thickness greater than 0.6 µm is sufficient. This trend arises from the skin\neffect in metals. We have included these simulation results in new Supplementary Figure\nS14.\nIt is worth noting that our copper Damascene process readily supports the fabrication of\nelectrodes several micrometers thick. The maximum thickness is mainly limited by the\nability to uniformly coat the trench sidewalls with the diffusion barrier and copper seed\nlayers during sputtering. This capability not only ensures a “safe” electrode thickness for\nachieving low microwave loss, but also enables the realization of deep vias for signal\nrouting, which is crucial for the seamless integration of driving circuits with photonic\nintegrated circuits [IEEE J. Sel. Top. Quantum Electron. 25, 1 (2019)].\nSince the electrodes are fully embedded within the oxide cladding and share a common\ntop surface, their thickness can be monitored by measuring the oxide window thickness\nvia spectroscopic reflectometry, as illustrated in main text Figure 1(c).\nWe have added the following passage in the main text Methods to address these points:\nThis relatively thick metal layer minimizes microwave losses of the CPW electrodes\nacross frequencies ranging from tens of GHz to 110 GHz (Supplementary\nInformation). Since the electrodes are embedded within a SiO cladding layer and\n2\nshare a common top surface, their thickness can be indirectly monitored by\nmeasuring the thickness of nearby SiO windows using spectroscopic reflectometry,\n2\nas shown in Figure 1(c).\n4. In Figure 3c, why does V_π fluctuate between 10^3 and 10^5 Hz?\nPockels modulator, the Vπ should remain constant across frequency, as the electro-optic\neffect is inherently linear and instantaneous. In practice, however, real materials and\nelectrode properties introduce secondary effects that lead to frequency-dependent\nvariations. At high modulation frequencies (e.g., in the tens of gigahertz range), the\npropagating electrical signal experiences attenuation due to factors such as skin effect\nand dielectric losses along the traveling-wave electrodes. This attenuation reduces the\neffective voltage applied to the modulation region, resulting in an apparent increase in Vπ.\nAt lower frequencies (e.g., below 1 MHz, as explored in this work), slow polarization\ndynamics, charge trapping, or ionic relaxation within the ferroelectric lithium tantalate can\ninduce small variations in the effective refractive index. These slow processes are unable\nto follow high-frequency modulation and therefore only manifest when the modulation\nfrequency approaches or falls below their characteristic relaxation frequencies. The mild\nfluctuation of Vπ (<0.1 V) observed between 10^3 Hz and 10^5 Hz can thus be attributed\nto such dielectric relaxation effects intrinsic to the Cu–LiTaO stack. Similar low-frequency\n3\nanomalies have been reported in thin-film lithium niobate modulators, where slow\npolarization and interface charging give rise to enhanced or oscillatory electro-optic\nresponse in the 1 kHz to 100 kHz range [Holzgrafe et al., Opt. Express 32, 3619 (2024)].\nInterestingly, the magnitude and position of the observed fluctuation do not change when\nthe on-chip optical power is increased from 1 mW to 1.17 W (Supplementary Figure S3,\nindicating that it is not caused by photorefractive effects, but rather by intrinsic dielectric\nrelaxation.\nHowever, we agree with the reviewer on the importance of acknowledging its presence\nand pointing to its potential origin. To address this point, we added the following passage\nin the Electro-optic Modulation section in the main text:\nA small fluctuation (< 0.1 V) in Vpi between 1 kHz and 100 kHz is attributed to\ndielectric relaxation intrinsic to the Cu–LiTaO stack. Such effects arise from slow\n3\npolarization dynamics or charge trapping in the ferroelectric layer when the\nmodulation frequency approaches the characteristic relaxation rate. The magnitude\nand frequency of this fluctuation remain unchanged as the on-chip optical power\nincreases from 1 mW to 1.17 W, confirming that it is not due to photorefractive\neffects.\n5. Figure S7 shows that the EO modulation performance depends on the structure of the\nCu electrodes. It is crucial to evaluate how the electrode structure impacts optical\ntransmission, as this will consequently influence the overall modulation performance.\nResponse: We thank the reviewer for raising the potential issue of the copper electrode’s\nstructure affecting the optical transmission of the modulator. The slow-wave (capacitively\nloaded) CPW electrodes used here are well-established designs that have been\n11\nexperimentally demonstrated in prior works [IEEE Trans. Microw. Theory Techn., 42, 1918\n(1994); Optica 8, 357 (2021); APL Photonics 7, 020802 (2022)]. These microstructured\n“T-rail” geometries reduce current crowding near the electrode gaps, thereby lowering\nmicrowave loss, while maintaining a similar electric field distribution across the optical\nwaveguide to preserve modulation efficiency.\nAs a result, the optical transmission loss remains primarily governed by the electrode gap,\nas in the case of conventional CPW electrodes. The impact of the electrode gap on optical\ntransmission loss has been depicted in main text Figure 3(a). The slow-wave geometry\nitself does not introduce additional optical absorption or scattering beyond this\ndependence.\nOur implementation of the slow-wave structures in new Supplementary Figure S12\nprimarily serves as a proof of concept to validate the Cu Damascene process for defining\nfine “T-rail” features with high fidelity and thus electrode with low-loss. As shown in new\nSupplementary Figure S13(a), the process faithfully reproduces these structures with\nexcellent edge definition, confirming that the Damascene platform can support future\niterations employing optimized slow-wave electrodes for further bandwidth and loss\nimprovements.\n12"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have fully addressed all my comments. The manuscript contains sufficient technical details and is now in good shape toward publication. Overall this is a nice work for potential photonics-electronics co-integration using copper electrodes.", "reviewer_2": "I appreciate the authors’ efforts in addressing my comments. Unfortunately, I stick with my previous opinion that this manuscript lacks the key novelty for publishing in a high profile journal such as Nature Communications. In what follows, I provide my detailed argument:\nThe authors argue that “unlike the Damascene process for silicon nitride photonic waveguides, the process reported here is for copper microwave electrodes.” However, the ultimate goal of any process innovation should be to enhance device performance. Both the academic and industrial communities favor processes that are sustainable, simpler, more efficient, and more reliable. In this connection, compared with the same group’s earlier work using Ag electrodes [Optica 11, 1614 1620 (2024)], the considerably more complicated “copper Damascene process” introduced here yields only mild improvement in performance. This substantially diminishes the claimed advantage of a copper based platform relying on Damascene processing. Therefore, only if this complicated process delivers a substantial performance enhancement would I be able to support its publication."}, "author_response": "We thank the reviewers for their thoughtful feedback on our revised manuscript entitled\n“Copper Damascene Process-Based High-Performance Thin Film Lithium\nTantalate Modulators.” We have carefully revised our manuscript according to the editorial\nrequests. Besides, we provide a detailed point-by-point response, with the reviewers’\ncomments reproduced in blue.\nThe authors have fully addressed all my comments. The manuscript contains sufficient\ntechnical details and is now in good shape toward publication. Overall this is a nice work\nfor potential photonics-electronics co-integration using copper electrodes.\nWe are grateful to the reviewer for their time and effort in reviewing our revised manuscript.\nI appreciate the authors’ efforts in addressing my comments. Unfortunately, I stick with my\nprevious opinion that this manuscript lacks the key novelty for publishing in a high profile\njournal such as Nature Communications. In what follows, I provide my detailed argument:\nThe authors argue that “unlike the Damascene process for silicon nitride photonic\nwaveguides, the process reported here is for copper microwave electrodes.” However, the\nultimate goal of any process innovation should be to enhance device performance. Both\nthe academic and industrial communities favor processes that are sustainable, simpler,\nmore efficient, and more reliable. In this connection, compared with the same group’s\nearlier work using Ag electrodes [Optica 11, 1614 1620 (2024)], the considerably more\ncomplicated “copper Damascene process” introduced here yields only mild improvement\nin performance. This substantially diminishes the claimed advantage of a copper based\nplatform relying on Damascene processing. Therefore, only if this complicated process\ndelivers a substantial performance enhancement would I be able to support its publication.\nWe thank the reviewer for their time and effort in evaluating our revised manuscript. We\nrespectfully disagree with the conclusion that the present work lacks sufficient novelty for\npublication in Nature Communications. The key novelty of this work does not lie in\npursuing incremental improvements in a single performance metric, but rather in\nestablishing an industry-standard copper Damascene process as a scalable and\nreliable platform for thin-film lithium tantalate (TFLT) electro-optic modulators,\nthereby enabling large-scale interfacing with microelectronic circuits. The copper\nDamascene process is the dominant metallization technology in modern microelectronics\nand advanced packaging, underpinning multilayer interconnects, chip-on-wafer\nintegration, and emerging copper–copper hybrid bonding. To the best of our knowledge,\nthis work represents the first systematic demonstration of integrating such a process\nwith Pockels-based thin-film ferroelectric modulators, bridging a long-standing gap\nbetween high-performance electro-optic devices and industrial microelectronic\nmanufacturing.\nWhile it is true that the communication performance metrics are comparable to those of\nour earlier Ag-electrode-based TFLT modulators [Optica 11, 1614–1620 (2024)], this\ncomparison overlooks several essential advances introduced here. Beyond high-speed\ndata transmission, we present a comprehensive reliability and scalability assessment\nof copper-Damascene-based TFLT modulators, including wafer-scale uniformity across\na 4-inch wafer, long-term microwave stability over several months, thermal cycling\nrobustness, and exceptional bias-point stability without active feedback. These\naspects are critical for practical deployment and industrial adoption, yet were not\naddressed in prior Ag-electrode-based demonstrations.\nmature and scalable Damascene and CMP infrastructure, suffer from diffusion and\nsulfidation challenges, and are not compatible with established multilayer metallization\nflows. In contrast, the copper Damascene process enables planarized surfaces, thick low-\nloss electrodes, multilayer routing, and direct compatibility with chip-on-wafer bonding—\ncapabilities that extend well beyond modest improvements in microwave loss and are\nindispensable for dense electronic–photonic co-integration.\nTo summarize, we agree with the reviewer that manufacturing processes that are more\nsustainable, simpler, more efficient, and more reliable provide a more compelling case for\ntechnology readiness in both the academic and industrial community. However, we believe\nthat these criteria are not simply met by diminishing the amount of steps in the\ncorresponding process flow, but also an overall verification of the process stability and\nreliability. These factors have been extremely well-studied for the case of Copper\nDamascene metallization as supported by work from two decades ago and its application\nto CMOS manufacturing. Therefore, compliance with standardized metallization used in\nindustrial wafer-scale manufacturing of integrated photonics and electronics provides a\nmore realistic outlook on the simplicity of the process. Satisfying this requirement allows\nthe results of our work to directly translate into manufacturing within industry settings.\nFurthermore, reaping the benefits of accessible copper-copper bonding techniques opens\nthe door to future academic work studying hybrid devices relying on copper metallization.\nTherefore, the significance of this work should be evaluated not solely on incremental\nperformance gains, but on its demonstration of a manufacturable, industry-compatible\nelectro-optic modulator platform that preserves state-of-the-art modulation performance\nwhile enabling large-scale integration with microelectronics. We believe this system-level\nand platform-level advance aligns well with the scope and readership of Nature\nCommunications.\n4"}]

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