Showing papers by "Ming Li published in 2017"

Enhanced NO2 sensing performance of reduced graphene oxide by in situ anchoring carbon dots

Abstract: Two-dimensional (2D) reduced graphene oxide–carbon dot (rGO–CD) hybrids were synthesized via a green one-pot method, in which the reduction of GO is accompanied by in situ generation of CDs with ultra-small sizes of 3–8 nm on its surface. The introduction of CDs enhanced the gas sensing properties by ∼3.3 times compared to bare rGO, and the rGO–CD hybrids could detect an extremely low NO2 concentration (10 ppb) at room temperature. Upon exposure to 25 ppm NO2 at room temperature, the as-prepared rGO–CDs show a sensitivity of ∼120% and exhibit excellent selectivity for NO2. After six consecutive gas sensing tests or unencapsulated storage for 90 days, the rGO–CD hybrids still retained their sensing characteristics. The enhanced gas sensing properties of the rGO–CDs can be attributed to the increased hole density on the surfaces of rGO due to the introduction of CDs as well as the formation of all-carbon nanoscale heterojunctions with few residual N atoms, significantly promoting charge transfer. It is believed that this low-cost and environmental strategy provides a facile route to design and fabricate a high sensitivity, stability and repeatability NO2 gas sensor based on 2D materials.

One-step synthesis of 2D C3N4-tin oxide gas sensors for enhanced acetone vapor detection

Abstract: This work proposed a one-step method to fabricate the two-dimensional (2D) heterostructural C 3 N 4 -tin oxide (SnO 2 ) nanocomposites with excellent acetone vapor sensing performance. A facile calcination treatment of melamine and SnCl 2 ·2H 2 O without further processing can obtain the expected C 3 N 4 -SnO 2 sensor. Specially, the SnO 2 nanoparticles directly anchor onto C 3 N 4 layer to construct a heterostructure, giving enhanced sensing properties. Compared with pure SnO 2 the heterostructural C 3 N 4 -SnO 2 Exhibits 22 times enhancement of sensing sensitivity as well as fast response/recovery (7 s/8 s). The limit of detection (LOD) of acetone can be as low as 67 ppb, which is far below the concentration in exhaled breath of a diabetic and predicts a possible for diagnosis of diabetes. Such enhancement can be interpreted as the transformation of electrons from SnO 2 to C 3 N 4 layer to form an asymmetric electronic structure in the electron depletion layer of SnO 2 , for which few electrons can change its resistance significantly. Moreover, the large surface area of C 3 N 4 layer provide vast adsorption sites for target gases. Importantly, SnO 2 and C 3 N 4 are low-cost, easy fabrication and eco-friendly materials, and the synthesis strategy presented here is simple, repeatable and operable, thus can be extended to build other-type metal oxides-based nanocomposites for various applications.

Synthesis of CuInS2 nanowire arrays via solution transformation of Cu2S self-template for enhanced photoelectrochemical performance

Abstract: A novel universal solution method has been proposed to prepare CuInS 2 nanowire arrays (NWAs) using Cu 2 S NWAs as the template. The screw-like CuInS 2 nanowires are constructed by many stacked nanoplates oriented along the direction of [221], and show diameters of 400–700 nm and lengths of several micrometres. Experimental results suggest that relatively higher In 3+ concentration is beneficial to the insertion of In 3+ into Cu 2 S to form CuInS 2 with conserving the nanowire morphology. A novel exchange-erosion formation mechanism is proposed to illustrate the formation of the screw-like CuInS 2 NWAs. The photoelectrochemical (PEC) performance has been investigated using the CuInS 2 NWAs as photocathodes, and they exhibit double-increased photocurrent (0.3 mA cm −2 at −0.1 V vs. RHE) compared to the pristine Cu 2 S NWAs, which can be ascribed to the enhanced light absorption and increased contact area for fast interfacial photocarrier separation and PEC reactions. The photocurrent has been further increased to 0.71 mA cm −2 via decorating CdS quantum dots on the surfaces of CuInS 2 nanowires. It is believed that this facile method can be generalized to prepare other copper chalcopyrite-based NWAs for highly efficient PEC water splitting.

Facile synthesis of amine-functionalized graphene quantum dots with highly pH-sensitive photoluminescence

Abstract: Graphene quantum dots (GQDs) as promising materials have gained increasing attention due to their optical and chemical properties, but the complicated synthetic procedures restrict their large-scal...

Wettability evolution of different nanostructured cobalt films based on electrodeposition

Abstract: The wettability evolution of eight different types of nanostructured cobalt films is investigated using galvanostatic electrodeposition. The morphology, which evolves from a plump, pea-like structure to cone, pyramid, shell, fluffy cone, fluffy shell and flower structures can be controlled by changing the anion type in the electrolyte or the deposition current density. A growth mechanism is proposed based on cathodic anomalous absorption theory, which explains both the influence of the anion type on the surface structure and the current-dependent morphology evolution. The contact angle and sliding angle of all films are measured, and fractions of water drop contact area with the surrounding air are calculated to illustrate the roughness induced wettability. The hierarchical (nano and micro) structures exhibit eminent superhydrophobicity compared with single dimensional surfaces, indicating this as an effective way to prevent transition from Cassie–Baxter mode to Wenzel mode.

Grafting and properties of a porous poly(methyl methacrylate) film on a silicon surface by a one-step dipping method

Abstract: In this study, we prepared a porous poly(methyl methacrylate) (PMMA) film on an Si surface with a novel dipping method We conducted the process by directly dipping the Si substrate into acidic aqueous media in a simple flask at 10 °C First, 4-nitrobenzene diazonium tetrafluoroborate (NBD) was spontaneously reduced at the Si surface Then, the aryl radicals during the reduction of NBD were directly grafted onto the Si surface Meanwhile, the aryl radicals initiated the polymerization of methyl methacrylate (MMA) monomers, and the radical-terminated PMMA chains formed in the solution were grafted onto the Si surface Because water was a poor solvent for MMA, the grafted PMMA chains easily aggregated together, and this resulted in a porous polymer film The porosity of the film depended on the grafting time and the MMA concentration Furthermore, the permittivity of the porous PMMA film was relatively low, and its dielectric dissipation factor was extremely small Therefore, its excellent dielectric properties should allow the porous film to have many applications in industry © 2017 Wiley Periodicals, Inc J Appl Polym Sci 2017, 134, 44930

Impact of substrate materials on packages warpage

Abstract: Unit warpage in Ball Grid Array (BGA) packages with different substrates due to coefficient of thermal expansion (CTE) and Young's Modulus mismatch was investigated. The effect of substrate materials on BGA packages warpage was analyzed. Thermal Mechanical Analyzer (TMA) and Dynamic Mechanical Analyzer (DMA) were used to measure Glass Transition Temperature (Tg) and CTE and Young's Modulus of the substrates and the properties of these substrates were analyzed. Shadow Moire was used to measure the unit warpage at room temperature (25°C) and during reflow. A Finite Element Analysis (FEA) method was used to simulate the warpage results to compare with results from the experiment and to verify the results. From this work, Cu, core materials, and solder mask (SM) have a combined effect on Tg, CTE and Young's Modulus of the substrate. Higher CTE of substrate than that of moldcap results in units presenting Cry, and compared with CTE, the modulus plays a dominate role in the strip compress and decreasing modulus of substrate helps to lower unit warpage. The Shadow moire test results and the simulation results state that the two results are credible.

Impact of tensile strength on thermal fatigue properties and failure modes of Sn-Ag-Cu-Ni solder joints

Abstract: With the requirements of higher board level reliability, researchers gradually turned attention to complex composition solders like Sn-Ag-Cu-Ni-Bi solders. This quinary system solder has lower melting point and higher tensile strength than traditional Sn-Ag-Cu solder, which may good for the reliability. In this research, the effects of tensile strength on thermal fatigue properties of Sn-2Ag-0.5Cu-0.05Ni and Sn-4Ag-0.5Cu-0.05Ni-3Bi solders were studied. After 1000 thermal cycles, SEM observation and nanoindentation test were used to characterize the failure mechanism and mechanical properties. The results demonstrated that Sn-4Ag-0.5Cu-0.05Ni-3Bi solder joints had better thermal fatigue properties than Sn-2Ag-0.5Cu-0.05Ni solder joints. The tensile strength of Sn-4Ag-0.5Cu-0.05Ni-3Bi solder was higher, so it can endure more stress and have less deformation in thermal cycling. And with the diffusion of Cu/Sn couples, the modulus and hardness of Sn-2Ag-0.5Cu-0.05Ni solder near the IMC interface decreased a lot, which promoted the crack formation and finally caused the fracture at the bulk solder. While for Sn-4Ag-0.5Cu-0.05Ni-3Bi solder, it didn't decrease too much due to the Bi concentration effect at the interface, which led to the crack formation in IMC. Hence, solders with higher tensile strength, like Sn-4Ag-0.5Cu-0.05Ni-3Bi solders, may have better thermal fatigue properties.

Thermo-fluid simulation of the advanced IGBT module in a power stack

Abstract: Insulated gate bipolar transistor (IGBT) modules are essential in power electronic application. Due to the improvement in chips and packaging, the maximum operation junction temperature T vj,op of the latest IGBT module has been increased up to 175°C, comparing to 150°C of the previous generation. With the enhancement of power density, its application at high temperature gives rise to more concerns of the reliability. Nevertheless, few literatures exist concerning the thermal design of the advanced IGBT module. External cooling system such as forced air cooling and liquid cooling is commonly utilized to make sure that the junction temperature T j is not larger than 175°C. But the employment of such cooling components consequently increases the variance in T j among chips within one single module during its usage. The heat dissipated by chips, transferred by packaging and carried by coolant leads to higher T j of chips in the downstream area and lower T j in the upstream. Nonetheless, this phenomenon has not been adequately evaluated. The present paper describes a thermo-fluid simulation model of the advanced IGBT modules in a power stack, by employing Computational Fluid Dynamics (CFD) and Finite Volume Method (FVM). Through grid independent study, the numerical error of model in T j is found less than 1°C. The ambient temperature is set at 55°C for simulating the extreme environment condition in the cabinet. Furthermore, the variance in T j among chips of IGBT components within one single module is inspected. Its correlations with power loss and air flow velocity are examined.

Power distribution safety protection monitoring system

Abstract: The invention discloses a power distribution safety protection monitoring system, which comprises a power distribution master station and a plurality of substations. The power distribution master station is provided with a master processing module, a master check module and a master communication module. Each substation is provided with a slave processing module, a slave check module and a slave communication module. The master communication module is in communication connection with the slave communication modules; the master communication module and the slave communication modules are used for communication data two-way transmission; the master check module is connected with the master processing module and the master communication module, and is used for checking substation identity information in the communication data received by the master communication module, and after the check passes, the master processing module carries out processing on substation control information in the communication data; and the slave check modules are connected with the slave processing modules and the slave communication modules. The new power distribution safety protection monitoring system can improve power distribution safety protection in both-way communication.

A new approach to minimize package warpage in reflow process

Abstract: Package warpage is often a problem in surface mount reflow process especially for thin package. Large warpage prevents package solder balls to be connected to PCB pads and results in low process yield. In order to effectively minimize warpage in reflow process, this paper proposed a new approach to reduce package warpage by temporally bonding the back of the packaged component to a rigid plate. In the reflow process, the bonded rigid plate reduces the bending of package and makes sure that its warpage is in process limit at all temperatures. The component is soldered on PCB after reflow process. The last step is to remove the plate that is bonded to the packaged component. We employed a thin Flip Chip Ball Grid Array (FCBGA) package with flexible substrate as an example to prove the effectiveness of the new warpage minimization approach.

Research on the reliability of Cu/Sn copper pillar bump

Abstract: As the electronic packaging density continues to increase, flip chip or stacked packaging via bump bonding is gradually replacing traditional wire bonding and will become the mainstream packaging form in the future. For copper bumps, this new type of electronic interconnection has not yet been fixed by industry standards. Therefore, this paper has made a preliminary study on the reliability of this new type of package. We find that the growth rate of IMCs was faster at the initial stage of aging and decreased with the aging time increasing. In the early stage of IMCs growth, the form of the IMCs was mainly Cu 6 Sn 5 . As the thickness of Cu 6 Sn 5 layer increased, the brittle phase Cu 3 Sn appeared. The growth of IMCs increased the interfacial adhesion. But after aging for 750h, the interfacial adhesion had a certain degree of decline.

Copper pillar bump surface smoothness simulation studies in through-silicon via technology

Abstract: With the strength of fine pitch, high electrical conductivity and excellent reliability, copper pillar interconnect becomes a promising alternative to traditional solder bump. However, bad surface smoothness is a severe problem and significantly affects the reliability of the bump connection. In this paper, numerical simulations on the shape evolution of copper pillar bump and the effect of bump dimension are investigated. Simulated result shows that the shape evolution of copper pillar bump is specified into three stages regardless of the excessive deposition, including uniform growth stage, convex growth stage, and the dished growth stage. Consequently, the final bowl-shaped surface brings about the unevenness of the bump. Under the same aspect ratio of copper pillar bump, the smoothness is improved with the bump diameter decreases. Further study finds that the polarization phenomenon of electric field on growth front is relieved when the bump diameter is reduced.

Effects of reduced graphene oxide film on bonding interfaces between Cu microcones and 25 μm Sn/Cu bumps

Abstract: Reduced graphene oxide (RGO) thin film was used to improve the performance of a low-temperature bonding process based on Cu microcones and Sn/Cu bumps (diameter: 25 μm), which has potential applications in high-density packaging. Under bonding conditions of a weight of 1500 g applied to each wafer for 10 min at a temperature of 120 °C, and incorporating a thin RGO layer, a compact bonding structure was obtained without interface voids. The RGO interlayer served as a barrier against interatomic diffusion of metals under zero applied pressure, and the formation of intermetallic compounds at the interface was thereby effectively reduced after bonding. Probable mechanisms for this bonding process are discussed. The investigation used standard 25 μm-diameter Sn/Cu bumps to simulate realistic industrial production.

Aging effect on wettability of negative photoresist with fine-pitch micro-holes after O 2 plasma treatment

Abstract: O 2 plasma treatment was performed on the JSR THB-430N negative UV photoresist to modify the surface wettability for micro-bumps electrodepositing. Wettability of photoresist was enhanced after O 2 plasma treatment. The samples were placed for different time at room temperature. Micro-FTIR indicated that the hydroxyl groups were introduced on the surface of photoresist after O 2 plasma surface treatment, meanwhile, the change of surface roughness and morphology was measured by atomic force microscopy. The results of contact angle between photoresist and test liquid showed that the wettability reduced with the increase of time after O 2 plasma treatment. The best wettability which had the minimum contact angle existed within a short period after O 2 plasma treatment. The combination of proper surface morphology and more hydroxyl groups are the reasons for photoresist to have better wettability. The mechanism of the aging effect on wettability was also discussed.

Preparation of pine-like Cu-Ni-P coating and its application in 3D integration

Abstract: A hierarchically structured Cu-Ni-P film fabricated via one-step route of electroless plating is reported. The morphology of deposits can be controlled by adjusting the additive in electrolyte. Due to the synergistic crystalline modification of polyethylene glycol (PEG), Janus Green B (JGB) and Cl−, the as-prepared Cu-Ni-P film exhibits pine-like architecture, about 2 µm in height and 1.5µm in root diameter. EDX result reveals that the pine-like coating is composed of about 94% Cu, 5%Ni and 1% P (weight %). Combined with the results of Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM), it can be seen that the pine-like Cu-Ni-P structure is ploy-crystalline. Bonding test shows that the adhesion strength between Sn and pine-like Cu-Ni-P substrate is about 8.4MPa. The reason for the excellent bonding performance could be ascribed to the mechanical interlocking effect caused by the roughened surface, which is proved to be an effective way to achieve low temperature bonding of Sn and Cu.

The effect of Sb content in InAs 1−x Sb x /GaSb type-II superlattices

Abstract: InAs/GaSb type-II superlattices materials have large quantum efficiency and responsivity, and smaller tunneling current and Auger recombination rate. However, the lattice constants of two components of the SLs are not exactly the same, which would introduce strain between each layer. InSb is introduced into InAs component, forming InAs 1−x Sb x /GaSb to free the strain. Band structure of SLs is computed with the use of K.P theory under the envelop-function approach. Electron effective mass and relative absorption are also calculated with Sb content varying from 0 to 0.2. The strain is almost negligible when the Sb content is 0.09, electron effective mass and relative absorption increase as Sb content increases. Dark current is analyzed under different temperature.

A low-temperature solid-state bonding method using Ag-modified Cu microcones and Ag buffer

Abstract: A novel low-temperature solid-state bonding method that Cu microcones coated with Ag and Ag buffer has been proposed. Thin Ag layer was used to prevent the oxidation of Cu microcones and Ag layer of several micrometers was used as a buffer layer between Cu microcones and Cu bumps. No brittle IMCs formed in the interfaces.

Effect of reflow time on shear property of Sn-9Zn solder bumps

Abstract: In this paper, the effect of reflow time on shear property of Sn-9Zn solder bumps was studied. The Sn-9Zn solder bumps were prepared by two step electroplating method. With the increase of reflow time, the shear strength decreased first and then increased. The change of shear strength was closely related to the formation of intermetallic compounds (IMCs). Scanning electron microscope (SEM) and Energy dispersive spectrometer (EDS) were used to analyze the composition and morphology of IMCs. The results indicated that IMCs were thickening with increasing reflow time and the morphology was flat, while IMCs thickness decreased and the morphology became fluctuant and unconsolidated when the reflow time extended to 20 min. The change trend of the shear strength was consistent with that of IMCs thickness. In addition, bumps fracture occurred inside the solders and dimples were observed on the fracture surface, which indicated that the solder bumps still had good ductility after reflow.

Elimination the CMP defects for TSV process by optimizing the copper electrodeposition

Abstract: Chemical mechanical polishing (CMP) performance influence the reliability of TSV process, because the wafer chip-to-chip vertical interconnections for the next process request the TSV wafer a flat surface without defects after CMP. The influence of ECD process to the CMP defects, further to the via-filling effect, was investigated. Thickness uniformity, top surface morphology and the grain orientation of the copper-filled via prove to be the major factors that lead to the CMP defects: Cu residue, delamination and pits. Optimizing the additives of ECD bath is proved to be a solution to eliminate the CMP defects.

Silicon@conductive porous copper layer anode for rechargeable lithium-ion batteries

Abstract: Silicon anodes, with an extreme high theoretical specific capacity of 4200mAh g−1 and proper stable plateau potential of 0.4V, are considered one of the most promising anode materials in rechargeable Lithium-ion batteries. However, the great structural and volumetric changes during charge/discharge cycles relating to poor cycling performance are still the most critical challenges limiting the breakthrough of silicon anodes. Here we present a silicon@conductive porous copper layer fabricated by one-step electrodeposition with subsequent a magnetron sputtering deposition process. Cyclic voltammetry and galvanostatic charge/discharge tests were carried out to understand the cycling performance. According to the results, this unique structure can relieve volume changes during repeated charge/discharging effectively and achieved 1000 mAh g−1 after 50 cycles at the current density of 0.05mA cm−2 and maintained intact structure after the cycling test. The electrode designed opens up the opportunity to improve the capacity of Li metal anodes by a facile method.

Influence of solder cap thickness on the interfacial reaction in Cu/Sn/Ni copper pillar bump

Abstract: For a limited solder volume interconnect structure, bump interconnect reliability is more sensitive to the growth behavior of the interfacial intermetallic compounds(IMCs). The study of the effect of solder cap thickness on the interfacial diffusion reaction is of great importance to the application of copper pillar bump. Here, we investigated the effect of different solder cap thicknesses on IMCs and void growth. Cu/Sn/Ni copper pillar bump sandwich structure was prepared by multilayer electroplating method with 5µm, 10µm, 20µm Sn layer, then aged at 150°C for 24, 48, 84 and 108 hours without reflow. We find that the morphology, thickness and volume ratio of IMCs are significantly affected by the solder cap thickness. These results are instructive for the reliability of micro-bump interconnects.

Competitive adsorption between suppressor and accelerator in copper methanesulfonic acid bath for electrodeposition

Abstract: The rapid development of three dimensional packaging makes it necessary to develop smaller and more reliable microbumps. In the electrodeposition process of bump cylinder, the filling quality is largely determined by the combination of additives. In this work, the effect and competitive adsorption between suppressor polyethylene glycol (PEG) and accelerator Bis-(3-sodiumsulfopropyl disulfide) (SPS) were investigated in copper methanesulfonic acid (MSA) plating bath. The results indicated that suppressor inhibited the Cu+/Cu reduction process in copper deposition by forming PEG-Cl−-Cue passivation film on the electrode surface. And the inhibiting effect was found to reach saturation when suppressor concentration reached above 9 mg/L, due to the coverage limitation of suppressor absorption. When accelerator was added into plating bath with PEG, SPS was supposed to disrupt or displace the adsorbed PEG and to form a bridge (RS-Cu2+-Cl−) with the electrode metal, which speed up the charge-transfer process. Thus the electrochemical process was converted to be diffusion controlled.

Grafting of perfluorinated organic film on the Si surface in aqueous solution

Abstract: In the past few years, organic film prepared on the silicon (Si) surface has become attractive owing to the wide range of potential application, low cost and good performance. In this paper, perfluorinated organic film was prepared on the Si surface in aqueous solution using one-step dipping method. On the Si surface, 4-nitrobenzene diazonium (NBD) tetrafluoroborate was firstly reduced with the presence of hydrofluoric acid in the aqueous media. The aryl radicals can then directly graft onto the Si substrate surface. In the meantime, the 2-(Perfluorooctyl)ethyl methacrylate can be initiated by aryl radicals and chains formed by the perfluorinated organic monomer in the aqueous solution can grafted onto Si surface. After grafting, the substrate surfaces and cross-section were characterized by SEM (FEI SIRION 200) and AFM which was uses a Digital Instrument Nanoscope IIIa operated in tapping mode in the air. The result showed that perfluorinated organic film was successfully grafted on the Si surface. Furthermore, the polymer film was composed of polyvinyl and polynitrophenylene which was comprised in the polyvinyl chain throughout the entire grafting process. Under this circumstance, the polymer film had uniform chemical composition from inner layer to external layer. In the experiment, we find that the monomer and NBD was grafted on the surface and the properties of the organic film are suitable for through-silicon-via(TSV) which is of good application potential.