Interconnect Applications Research Articles

Meter-Scale Long Connectorized Paper-like Polymer Waveguide Film for 100 Gbps Board-Level Optical Interconnects Application

We design and fabricate meter-scale long connectorized paper-like flexible multimode polymer waveguide film with a large bandwidth-length product (BLP) for board-level optical interconnects application. The measured BLP of the multimode waveguide is greater than 57.3 GHz·m at a wavelength of 850 nm under the strictest overfilled launch condition with a maximum length of 2.1 m and 10-dB insertion loss. The fabricated waveguide films are as flexible as regular printing paper and can be conveniently interfaced with standard mechanically transferable (MT) fiber connectors with low loss. The average insertion loss of the connectorized waveguide is about 0.042 dB/cm with inter-channel crosstalk as low as −46.4 dB, and the bending loss is less than 1 dB at a bending radius of 1 mm under the overfilled launch condition. We also demonstrate a vertical-cavity surface-emitting laser (VCSEL)-based single-lane 100 Gbps PAM4 transmission. Our results show that the meter-scale long paper-like polymer waveguide film has both excellent optical properties and large bandwidth and is ideal for high-speed board-level optical interconnects application with a single-lane data rate of 100 Gbps and beyond, especially those that have a strict requirement on the length of connection and compactness.

Demonstration of a three-active-section DFB laser with photon-photon resonance and detuned loading effects to obtain enhanced bandwidth and maintained high output power

Enhancement of the modulation bandwidth of the directly modulated semiconductor laser has attracted tremendous attention for applications in optical fiber communication and optical interconnects. However, modulation bandwidth is enhanced by some special technology such as integrating the active region or passive region regularly, which lead to low power and complex fabrication. Here we design and demonstrate a monolithic integrated three-active-section distributed feedback (TAS-DFB) laser with enhanced bandwidth theoretically and experimentally. The laser includes three sections: the DFB section, feedback section and active phase section. The three sections share the same multiple quantum-well structure. To enhance the modulation bandwidth beyond the intrinsic modulation bandwidth, both photon-photon resonance (PPR) and detuned loading (DL) effects are utilized to achieve over 55 GHz. The active phase section shows promise in achieving high bandwidth with PPR and relatively high output power of ∼ 10 mW at 25 °C under continuous-wave (CW) operation simultaneously. The fabrication process of the TAS-DFB laser is simplified due to the conventional and identical active layer structure.

Characteristics of metallic coated ultra-short semiconductor Fabry-Pérot lasers

Fabry-Pérot (F–P) lasers have attracted significant attention due to their extensive applications in optical communication and precision measurement fields. However, traditional F–P lasers are relatively large in size and face challenges in directly achieving single-mode lasing, which limits their applications. In this study, we employ a metallic coating layer to significantly enhance the light confinement of the F–P cavity, thereby substantially reducing the cavity length of the F–P laser. We have experimentally developed an F–P laser with a cavity length of approximately 6.8 μm, successfully achieving single-mode lasing at room temperature. The lasing mode exhibits an ultra-narrow linewidth of 0.06 nm and a relative high side-mode suppression ratio exceeding 30 dB, along with good linear polarization. Additionally, we propose a type of on-chip integrated laser based on metallic-coated F–P cavities, characterized by high coupling efficiency and large fabrication tolerances. Simulation results indicate that the coupling efficiency for double-ended and single-ended output waveguides reaches 79% and 55%, respectively. The metallic-coated ultra-short F–P lasers have great potential for applications in on-chip optical interconnects and photonic integrated chip systems.

Sputtered quaternary alloy coating of Fe4CoNiCu for solid oxide fuel cell steel interconnects application

A quaternary Fe4CoNiCu alloy coating is applied on SUS 430 steel substrate for solid oxide fuel cell (SOFC) interconnects application via magnetron sputtering technology. The oxidation behavior of the coated steels is investigated in air at 800 °C. During initial oxidation, Fe and Co in the alloy coating is oxidized preferentially, forming Fe-rich oxide. Ni is oxidized to NiO by inward diffusion of oxygen. Slight Cu diffuses to or near the surface of the oxide scale to form CuO. Some Cu reacts with Fe3O4 to form CuFeO2 inside the oxide scale. The alloy coating is thermally converted into a quaternary spinel coating of (Fe,Co,Ni,Cu)3O4 with a small quantity of CuO existing on the surface and a protective Cr2O3 layer is formed at the steel/coating interface. The (Fe,Co,Ni,Cu)3O4 spinel layer effectively inhibits the growth of Cr2O3 layer and the outward diffusion of Cr. The scale ASR is 13.08 mΩ cm2 at 800 °C after 1680 h oxidation.

Understanding the deformation creep and role of intermetallic compound-microstructure in Sn-Ag-Cu solders

Tin-based alloys are commonly used in lead-free solder joints in electronic interconnection applications, where creep can limit joint reliability. In this work, directionally solidified bulk solder alloys of different compositions (pure tin, SAC105 and SAC305) are mechanically tested under a constant load for each composition at room temperature to understand mechanical creep performance and quantify the role of different content of Ag3Sn and Cu6Sn5 intermetallic compounds (IMCs) in terms of secondary creep strain rate, microstructural evolution, and total amount of accumulated strain. In this work, microstructures are fabricated using directional solidification to promote a systematic variation in IMC sizes, shapes, and distributions within similar crystal orientations of the primary β-Sn matrix. Analysis of creep data indicates that secondary creep is dominated by obstacle-controlled dislocation motion. This is further confirmed by electron backscatter diffraction (EBSD) analysis, which reveals that the formation of subgrains and internal structure, correlating to the initial microstructure of the sample, i.e. changes in secondary dendrite arm spacing (λ2) and eutectic intermetallic spacing (λe). The experimental observations are supported further by crystal plasticity simulations that explicitly model the presence of IMCs within the Sn-based samples and show the dependence of the secondary creep rate upon the IMC content, and therefore be beneficial for the possible future composition design in solders.

Low-temperature Cu-Cu direct bonding with ultra-large grains using highly (110)-oriented nanotwinned copper

With the application of Cu-Cu direct bonding technology in high-performance electronic devices, improving bonding reliability is essential for achieving high-density 3D IC integration. In this study, we innovatively applied highly (110)-oriented perpendicular nanotwinned Cu (p-ntCu) to achieve Cu-Cu direct bonding at low temperature and pressure. The anisotropic growth of p-ntCu during annealing at 200 °C resulted in the formation of ultra-large grains, which could improve the conductivity of the electrodeposited Cu film. Two Cu films were bonded at 200–250 °C with 2 MPa pressure for 1 h, and there was almost no void at the bonding interface. Anisotropic grain growth occurred within the bonding joint and grain boundary migration was observed at the bonding interface. The shear strength after bonding at 250 °C was measured as 58.3 MPa on average, indicating a relatively high quality of such Cu-Cu bonding. The p-ntCu can achieve Cu-Cu bonding and improve the conductivity by anisotropic grain growth at low temperature and pressure, which has great potential for Cu interconnect applications.

Single-fundamental-mode cryogenic (3.6 K) 850-nm oxide-confined VCSEL

Cryogenic oxide-confined vertical-cavity surface-emitting laser (VCSEL) has promising application in cryogenic optical interconnect for cryogenic computing. In this paper, we demonstrate a cryogenic 850-nm oxide-confined VCSEL at around 4 K. The cryogenic VCSEL with an optical oxide aperture of 6.5 μm in diameter can operate in single fundamental mode with a side-mode suppression-ratio of 36 dB at 3.6 K, and the fiber-coupled output power reaches 1 mW at 5 mA. The small signal modulation measurements at 298 and 292 K show the fabricated VCSEL has the potential to achieve a high modulation bandwidth at cryogenic temperature.

Unconventional magnetoresistance and resistivity scaling in amorphous CoSi thin films

The resistivity scaling of Cu electrical interconnects represents a critical challenge in Si CMOS technology. As interconnect dimensions reach below 10 nm, Cu resistivity increases significantly due to surface scattering. Topological materials have been considered for application in ultra-scaled interconnects (below 5 nm), due to their topologically protected surface states that have reduced electron scattering. Recent theoretical work on the topological chiral semimetal CoSi suggests that this material could offer lower resistivity than Cu at dimensions smaller than 10 nm. Here we investigate the scaling trend of textured and amorphous CoSi thin films, deposited by molecular beam epitaxy in a thickness range between 2 and 82.5 nm. Contrary to predictions of standard resistivity models, we report here a reduction in resistivity for thin amorphous CoSi films, which is instead consistent with surface-dominated transport. Moreover, magnetotransport measurements reveal significant enhancement of the magnetoresistance in scaled films, highlighting the complex transport mechanisms present in these highly disordered films at thicknesses of a few nanometers.

Co-Electrodeposition of Ni/Co–W and Cu/Co–W Alloys on Type 445 Stainless Steel for Application in SOFC Interconnects

Co-Electrodeposition of Ni/Co–W and Cu/Co–W Alloys on Type 445 Stainless Steel for Application in SOFC Interconnects

Modeling and analysis of crosstalk induced effects in graphene-carbon nanotube composite interconnects

This paper proposes an equivalent circuit model for two-line coupled multilayer graphene nanoribbon - single-wall carbon nanotube (MLGNR-SWCNT) composite interconnects (MSCs), incorporating the effects of coupling capacitance and mutual inductance. We also examine the temperature-dependent crosstalk effect on the victim line of MSCs in the time domain using decoupling techniques and the ABCD parameter matrix approach. This analysis is conducted at the global level of 7 nm, 14 nm, and 22 nm technology nodes, comparing the performance of MSCs with MLGNR, SWCNT, and copper (Cu) interconnects, validated through HSPICE simulations. Our results reveal that the crosstalk delay of all interconnects induced by dynamic crosstalk exhibits superior performance in the in-phase crosstalk mode compared to the out-of-phase mode at room temperature. In particular, MSCs demonstrate less crosstalk delay in both crosstalk modes compared to SWCNT and Cu interconnects. In addition, we analyze the crosstalk delay of the victim line for two-line coupled MSCs at varying temperatures in out-of-phase crosstalk mode, comparing them with MLGNR, SWCNT, and Cu interconnects. Simulation results indicate that the crosstalk delay is temperature-dependent, increasing with rising temperatures, and the crosstalk delay of MSCs is the least of all interconnects. Furthermore, we investigate the crosstalk noise of MSCs induced by functional crosstalk at different temperatures, comparing it with MLGNR, SWCNT, and Cu interconnects. It is observed that the crosstalk noise peak remains constant with temperature changes across all interconnects; however, the holding time and width of crosstalk noise increases with rising temperatures and MSCs have the least crosstalk noise peak and crosstalk noise width of all interconnects. Also, numerical results exhibit that reducing interconnect temperature, SWCNT diameter, and edge roughness of MLGNR are effective strategies to diminish the crosstalk delay of MSCs. In addition, increasing line spacing is identified as an effective method to reduce crosstalk noise peak of MSCs of different lengths. The proposed model results show excellent agreement with HSPICE simulation data. Therefore, our analysis of crosstalk effect manifests that MLGNR-SWCNT composite can be a promising material to replace SWCNT and copper as an ideal material for global interconnect applications in thermally variable environments.

Terminated polymer waveguide with low coupling loss to single-mode fiber for 100 Gbps optical interconnects and beyond.

We designed and fabricated terminated polymer waveguides with low coupling loss to a standard single-mode fiber (SSMF) for 100 Gbps optical interconnects application and beyond. The mode field diameter of the polymer waveguide is designed to perfectly match that of the SSMF with a theoretical coupling loss as small as 0.07 dB. The fabricated polymer waveguides on FR-4 substrates exhibit both a low propagation loss of 0.32 dB/cm and a low coupling loss of less than 0.17 dB to SSMF at a wavelength of 1310 nm. These low loss values indicate good compatibilities with both printed circuit boards and fiber optics. Both single-lane 56 Gbaud (112 Gbps) and 60 Gbaud (120 Gbps) 4-level pulse amplitude modulation (PAM4) optical transmission using terminated polymer waveguides have been successfully demonstrated. The bit error ratio (BER) for PAM4 transmission at both data rates reached a level of 10-4 which is well below the forward error correction (FEC) limitation with a power penalty of less than 1 dB. The bandwidth of the test equipment rather than the polymer waveguide itself limits the high-speed optical interconnect performances in our experiments. The results imply that the polymer waveguide is a promising media for single-lane 100 Gbps optical interconnects application and beyond.

Effect of Mixed Morphology (Simple Cubic, Face-Centered Cubic, and Body-Centered Cubic)-Based Electrodes on the Electric Double Layer Capacitance of Supercapacitors.

Supercapacitors store energy due to the formation of an electric double layer (EDL) at the interface of the electrodes and electrolyte. The present article deals with the finite element study of equilibrium electric double layer capacitance (EDLC) in the mixed morphology electrodes comprising all three fundamental crystal structures, simple cubic (SC), body-centered cubic (BCC), and face-centered cubic morphologies (FCC). Mesoporous-activated carbon forms the electrode in the supercapacitor with (C2H5)4NBF4/propylene carbonate organic electrolyte. Electrochemical interference is clearly demonstrated in the supercapacitors with the formation of the potential bands, as in the case of interference theory due to the increasing packing factor. The effects of electrode thickness varying from a wide range of 50 nm to 0.04 mm on specific EDLC have been discussed in detail. The interfacial geometry of the unit cell in contact with the electrolyte is the most important parameter determining the properties of the EDL. The critical thickness of the electrodes is 1.71 μm in all the morphologies. Polarization increases the interfacial potential and leads to EDL formation. The Stern layer specific capacitance is 167.6 μF cm-2 in all the morphologies. The maximum capacitance is in the decreasing order of interfacial geometry, as FCC > BCC > SC, dependent on the packing factor. The minimum transmittance in all the morphologies is 98.35%, with the constant figure of merit at higher electrode thickness having applications in the chip interconnects. The transient analysis shows that the interfacial current decreases with increasing polarization in the EDL. The capacitance also decreases with the increase of the scan rate.

Optimization of novel two-step curing method for die stack epoxy bonding to reduce voids in Ball Grid Array packages for high-density interconnect applications

This research explores the optimization of epoxy curing parameters to minimize void formation in 3-IC-Chip-MAPBGA packages, a subset of BGA packages, crucial components in high-density interconnect applications. The study utilizes a systematic approach involving design of experiments (DOE) assisted by statistical JMP tool to manipulate curing profiles, aiming to achieve void reduction while preserving adhesion properties. Various analytical techniques, including X-ray imaging, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), die shear strength tests, and C-Sam analysis for delamination, are employed to analyze void formation, material characteristics, mechanical properties, and structural integrity. The findings demonstrate that the sample with a 2nd step curing profile, identified as sample#3, which includes a ramp time of 15 min, a 1st step curing temperature of 90 °C with a soak time of 20 min, and a 2nd step ramp time of 20 min, exhibits the most favourable outcome in void reduction. This sample shows a notably lower void presence of 3.66 % and the highest die shear strength of 126 MPa. In contrast, the control sample, serving as a reference, displays a void percentage of 7.28 %, nearly twice as high as that of sample#3, and much lower die shear strength of 80 MPa at 25 °C. Adopting the curing profile of sample#3 also leads to a substantial 18.75 % reduction in cycle time compared to the control sample. The study highlights the importance of balancing curing parameters to mitigate void formation and maintain optimal mechanical properties, offering valuable insights for improving the reliability of high-density interconnect applications.

Optimization of electrophoretic deposition and thermal treatment of Cu–Mn spinel matrix for spinel/perovskite composite coating used as SOC metallic interconnect

The electrophoretic deposition of a mixture of copper-manganese spinel oxide matrix with perovskite oxide dispersion phase and subsequent two-step sintering was investigated as a means of obtaining a new spinel/perovskite composite coating for application in metallic interconnects for solid oxide cell (SOC) technologies. The effect of a number of factors was examined, including electrophoretic deposition parameters such as solvent type, time, voltage, and dispersant content as well as different heat treatment conditions on the quality of the matrix of the coating material. A solution of ethanol-acetone (25/75 v/v) and 2.0 g L−1 of iodine dispersant with a deposition time of 60 s and a voltage of 60 V provided the thickest, most uniform coating as deposited. SEM observations showed that a dense coating of Cu–Mn spinel was obtained after 8 h of reduction at 1000 °C followed by 6 h of oxidation at 850 °C. Selected preparation parameters were applied to deposit composite coatings with a Cu–Mn spinel matrix and the addition of LNF, LSC and LSCF perovskites.

Fabrication and Oxidation Behavior of the Cu–Fe Spinel Coating for SOFC Steel Interconnect Applications

Fabrication and Oxidation Behavior of the Cu–Fe Spinel Coating for SOFC Steel Interconnect Applications

Development of Ag@Si composite sinter joining with ultra-high resistance to thermal shock test for SiC power device: Experiment validation and numerical simulation

Owing to its cost-effectiveness and low coefficient of thermal expansion (CTE), micron Si is incorporated into sintered Ag matrix to develop an Ag@Si composite sintering strategy. The Si integration helps reduce cost and alleviate CTE-mismatch of Ag-sintered die attachments. An innovative Ag@Si composite sintering structure is achieved by low-temperature and pressure-less sintering process, where a possibly slight diffusion of native SiO2 on the Si surface toward Ag is indicated by TEM observation, enabling the integration of Ag and Si into one continuous network. Owing to the robust well-bonded composite sintering, the Ag@Si joining strategy provides mechanical/microstructural reliability far beyond the pure Ag sinter joining, demonstrating significant prospects in high-temperature interconnection applications. During the harsh thermal cycling (−50–250 °C), the mitigating effect of the Si addition on CTE mismatch is manifested through the substantial suppression of microstructure deterioration in Ag@Si joint that occurs in pure-Ag-sintered joint, while the shear strength retention rate is doubled. Detailed investigation into the mechanism for elevated performance was conducted by material property and Missies stress analysis. The incorporation of Si helps modify the CTE, elastic properties, and stress distribution of Ag@Si-sintered material, synergistically contributing to the satisfactory joining reliability.

Important Elements of Spin-Exciton and Magnon-Exciton Coupling.

The recent discovery of spin-exciton and magnon-exciton coupling in a layered antiferromagnetic semiconductor, CrSBr, is both fundamentally intriguing and technologically significant. This discovery unveils a unique capability to optically access and manipulate spin information using excitons, opening doors to applications in quantum interconnects, quantum photonics, and opto-spintronics. Despite their remarkable potential, materials exhibiting spin-exciton and magnon-exciton coupling remain limited. To broaden the library of such materials, we explore key parameters for achieving and tuning spin-exciton and magnon-exciton couplings. We begin by examining the mechanisms of couplings in CrSBr and drawing comparisons with other recently identified two-dimensional magnetic semiconductors. Furthermore, we propose various promising scenarios for spin-exciton coupling, laying the groundwork for future research endeavors.

The performance of NiO/NiFe2O4 coated pre-oxidized SUS 430 interconnect with different NiO thickness for solid oxide fuel cell

Ni/NiFe2 coatings with different thickness of Ni were deposited on 50-h pre-oxidized SUS 430 steel by magnetron sputtering for solid oxide fuel cell (SOFC) interconnects application, which were subsequently exposed to 800 °C air for durations of up to 5040 h. Ni/NiFe2 coating was thermally converted to an oxide structure comprising inner NiO layer and outer NiFe2O4 spinel layer. The effects of different NiO diffusion barrier thickness on the oxidation behavior, electrical performance and long-term working stability of NiO/NiFe2O4 coated steel were investigated. The chromium vaporization from bare and coated steels was also evaluated at 800 °C in air-12 % H2O atmosphere. Results showed that scale delamination and the spike of area specific resistance (ASR) were achieved for the sample with thicker NiO layer after 2520 h thermal exposure. Moreover, NiO gradually migrated outward through the solid-state reaction between NiO and NiFe2O4 spinel and eventually aggregated near the outer surface. Breakaway oxidation of the bare steel occurred during the chromium vaporization test and undulating Fe oxide nodules were strewn on the steel surface. The chromium vaporization rate was effectively suppressed by the application of NiO/NiFe2O4 coating.

Overcoming oxygen inhibition in UV photolithography for the fabrication of low-loss polymer waveguides.

Perfluorinated acrylate polymer materials exhibit low absorption loss at 1310 and 1550 nm, but molecular oxygen inhibits their photocuring. We propose a novel, to our knowledge, UV photolithography method incorporating a pre-exposure process for fabricating low-loss perfluorinated acrylate polymer waveguides. During the pre-exposure process, a partially cured thin layer forms on the core layer, effectively overcoming oxygen inhibition in subsequent lithography. Furthermore, the functional group contents of the polymerized materials were characterized by a Raman spectrometer to analyze the development reaction under the pre-exposure layer. Utilizing this improved method, we fabricated a straight waveguide with a length of 21 cm. The experiments showed that the propagation losses are 0.14 dB/cm at 1310 nm and 0.51 dB/cm at 1550 nm. The inter-channel cross talk for a core pitch of 250 µm was measured as low as -49 dB at 1310 nm. Error-free NRZ data transmission over this waveguide at 25 Gb/s was achieved, showcasing the potential in optical interconnect and communication applications.

Multi-Objective Optimization for Rapid Identification of Novel Compound Metals for Interconnect Applications.

Interconnect materials play the critical role of routing energy and information in integrated circuits. However, established bulk conductors, such as copper, perform poorly when scaled down beyond 10 nm, limiting the scalability of logic devices. Here, a multi-objective search is developed, combined with first-principles calculations, to rapidly screen over 15,000 materials and discover new interconnect candidates. This approach simultaneously optimizes the bulk electronic conductivity, surface scattering time, and chemical stability using physically motivated surrogate properties accessible from materials databases. Promising local interconnects are identified that have the potential to outperform ruthenium, the current state-of-the-art post-Cu material, and also semi-global interconnects with potentially large skin depths at the GHz operation frequency. The approach is validated on one of the identified candidates, CoPt, using both ab initio and experimental transport studies, showcasing its potential to supplant Ru and Cu for future localinterconnects.