•Achieving a TADF-based SEL-hybrid-WOLED with a high-efficiency, low roll-off, high CRI and superior CIE coordinates.•Demonstration zone of charge recombination is on the host of the white device.•Demonstrating energy transfer pathway between red and green dopants in the white device.Single-emitting-layer hybrid white organic light-emitting diodes (SEL-hybrid-WOLEDs) are promising candidates for large-area lightings, however, ideal hybrid WOLEDs with a simple structure and high-efficiency, low roll-off, high color rendering index (CRI) and superior CIE coordinates have been rarely reported. In this paper, high-performance SEL-hybrid-WOLEDs are demonstrated by utilizing a thermally activated delayed fluorescence (TADF) host emitter combined with green and red phosphors. The optimized WOLED exhibits an external quantum efficiency (EQE) of 20.2%, CIE coordinates of (0.360, 0.390) and a CRI of 85. Remarkably, an extremely low efficiency roll-off is also realized, with an EQE of 19.4% remained even at the practical luminance of 1000 cd/m2, resulting from the wide recombination zone as well as the well-tuned energy transfer in the emitting layer. Moreover, benefited from the stable recombination zone, superior color stability was also achieved. The intriguing results, we believe, greatly manifest the great potential of such a strategy and may pave the way towards real applications.Download high-res image (271KB)Download full-size image

•The delamination of BCB in KOH is attributed to the decomposition of Si-O-Si bonds.•Delamination rates decrease with the increase in the densities of Si-O-Si.•Increase in the density of Si-O-Si improves the bonding strength of BCB.BCB is emerging as an attractive bonding adhesive for wafer bonding in 3-D integration. Although the bonding strength of BCB is satisfactory with the assist of adhesion promoter, it is found that BCB suffers from interface delamination in harsh chemical or thermal conditions. This paper proposes, at chemical bond level, that the mechanism of interface delamination in KOH solution is attributed to the decomposition of SiOSi bonds at the interface between substrates and AP3000 adhesion promoter as a result of hydrolysis. Silicon dioxide and silicon nitride films with various densities of SiH and SiN bonds are prepared, and the bond densities are measured using infrared spectroscopy. The corresponding interface delamination rates of these films and BCB in KOH solution are measured, and the relations between the bond densities and the delamination rates are obtained for silicon dioxide and silicon nitride. It shows that the delamination rates decrease with the increase in the densities of SiOSi. These results demonstrate that the decomposition of SiOSi in KOH is the main reason for BCB delamination, and increase in the density of SiOSi improves the bonding strength.

A void-free bonding interface is critical to yield and reliability for high-quality wafer bonding. Although adhesive bonding using polymers as the bonding interface material is inherently able to restrain void-formation, for the wafers with uneven patterns like metal interconnects and alignment marks, void-free bonding is still challenging. This paper reports the void-formation in uncured and partially-cured benzocyclobutene (BCB) adhesive for bonding wafers with patterns. Experimental results show that uncured and partially-cured BCB has different behaviors in void-formation, and four types of voids, namely center voids, flower voids, micro voids, and floccules voids, are founded and their formation mechanisms are investigated. By optimizing bonding parameters for uncured BCB bonding, the center voids and flower voids are avoided and void-free bonding can be obtained. For partially-cured BCB, the micro voids and floccules voids tend to appear for uneven wafer surfaces. A reflow pretreatment at 140 °C for 2 h before curing process is beneficial to reducing the void areas.

Due to the flowability of benzocyclobutene (BCB) and the unavoidable shear components of the bonding force applied by bonding facilities, it is quite challenging to achieve void-free BCB adhesive bonding with simultaneous high post-bonding alignment accuracy. To solve this problem, this paper reports a compensation method for alignment errors to achieve simultaneous void-free and accurate wafer bonding using soft-baked BCB. By characterizing the wafer shift induced bonding force, it is found that the wafer shift is a systematic error associate with the bonders but independent of the wafers. Upon this investigation, a compensation method presetting a pre-bonding alignment shift opposite to the post-bonding shift is proposed to compensate the bonding induced wafer shift. Using this method, void-free bonding with soft-baked BCB has been achieved, and the alignment errors are improved significantly from around 35–40 μm to around 3 μm. Test results show that the average bonding strength of soft-baked BCB is about 26.4 % higher than that of partially-cured BCB. The preliminary results demonstrate the efficacy of the proposed compensation method, which has potential to improve the alignment accuracy of BCB bonding for three-dimensional integration, MEMS, and microsensors.

•A unique two-step coating of PPC and PI has been developed to planarize surfaces with high topography.•Substrates with 70 μm deep holes and 100 μm large cavities have been well planarized for photolithography.•Silicon dioxide films are allowed to be deposited on the planarized substrates using PECVD.Pattern transfer on structured surfaces is still a technical challenge in MEMS fabrication due to the accumulation of photoresist at the structure base and the discontinuity at the structure opening. This paper reports a planarization method which employs a two-step polymer spin-coating to cap the structures to facilitate pattern transfer on structured surfaces. The first spin-coating uses heat-depolymerizable poly (propylene carbonate) (PPC) to either partially fill or completely intersect the structures, and the following spin-coating uses polyimide (PI) to form a complete capping film on the structured surfaces. The thermal decomposable feature of PPC allows it to decompose as a sacrificial layer during PI curing, leaving only PI films on the structured surfaces after the two-step coating. High topography surfaces with deep holes with diameter of 25 μm, long trenches with length of 500 μm, and large cavities with width of 100 μm, have been well planarized using the proposed method. Subsequent photolithography verifies that photoresist can be well applied onto the planarized surfaces, and the pattern transfer results are satisfactory. Silicon dioxide deposited using plasma enhanced chemical vapor deposition (PECVD) and aluminum deposited using sputtering are also demonstrated on planarized surfaces. The preliminary results demonstrate the feasibility of planarization of high topography surfaces using the two-step coating method.

•A fabrication method has been developed for thinning individual sensor chips using a reconfiguration method.•Ultra-thin and highly-sensitive stress sensor chips with thickness of 35 μm have been achieved, and the sensitivity is 70 times better than metal strain gauge.•The ultra-thin stress sensor chips have been used to measure pulse on wrist and orthodontic force of invisible aligners.This paper reports the development of a highly-sensitive and ultra-thin silicon stress sensor chip (UTSC) and its applications for wearable sensors. Stress sensor chips are fabricated using CMOS technology, and after dicing the individual chips are reconfigured into a virtual wafer on a carrier wafer using temporary adhesive bonding. The reconfigured wafer is then thinned using mechanical grinding, polishing, and wet etching. After thinning, the sensor chips with thickness of 35 μm are laminated to a thin Kapton PI film, followed by de-bonding to separate the carrier wafer. Measurement results show that the UTSC is able to comply with curved surfaces, and the sensitivity is around 70 times that of metal strain gauge. The specifications of the UTSC are characterized in terms of linearity, repeatability, hysteresis, and zero drift. The UTSCs are demonstrated to measure human pulses on wrist and orthodontic forces of invisible aligners for dental treatment. The preliminary results show that the reconfigured method is applicable to thinning individual chips, and the UTSCs are flexible and sensitive enough for measurement of stress and strain on curved surfaces on human bodies.

Polymer insulation layers (liners) have several potential advantages in terms of capacitance and reliability over conventional silicon dioxide for through-silicon-via (TSV) applications. This paper reports the development and measurement results of a TSV using poly (propylene carbonate) (PPC) polymer as the insulation layers. To address the challenge in coating thin and uniform PPC liners on the sidewalls of high aspect-ratio vias, a spin-on technique using vacuum treatment and solvent refill is developed to prevent formation of air bubbles in the vias, and using this technique circular vias with aspect-ratio as high as 9:1 have been coated with uniform PPC layers. Based on these results, TSVs with thick PPC polymer liners have been fabricated and the capacitance and the current leakage are measured. Thanks to the relatively large thickness and the low dielectric constant of PPC liners, the TSV capacitance density is about 2.45 nF/cm2 and the leakage current is around 40 nA/cm2 at 5 V voltage, indicating that replacing silicon dioxide with PPC as liners considerably reduces TSV capacitance. The preliminary results demonstrate the feasibility of fabrication of high aspect ratio and low capacitance TSVs with polymer liners.Graphical abstractHighlights► TSVs using poly (propylene carbonate) (PPC) polymer as insulators have been developed. ► The TSV capacitance density is as low as 2.45 nF/cm2, about one tenth of TSVs using SiO2 insulators. ► The leakage current at 5 V biased voltage is as low as 40 nA/cm2.

Thermal management is one of the critical challenges in three-dimensional (3D) integration. As the polymer bonding adhesive is the major source of the internal thermal resistance in 3D integration, this paper reports an approach to increase the thermal conductivity of the bonding adhesive benzocyclobutene (BCB) by loading carbon nanotubes (CNTs). By exploiting the aromatic property of BCB, an ultrasonication and mechanical stirring assisted non-covalent dispersion method is developed to disseminate CNTs with concentrations ranging from 0.25% to 2% to form BCB–CNT composites. The effects of ultrasonication and mechanical stirring on CNT dispersion are experimentally investigated, and the electrical properties, the thermal diffusivity, and the bonding strength of BCB–CNT composites are characterized. The results show that ultrasonication in conjunction with mechanical stirring can significantly improves the CNT dispersion results, and an 80% improvement in the thermal conductivity is achieved for BCB by loading CNTs. The bonding strength of BCB–CNT composites, though varies with CNT concentrations, are improved compared with pure BCB for CNT concentrations less than 1.75%.Graphical abstractHighlights► Ultrasonication with mechanical stirring greatly improves CNT dispersion in BCB. ► Stable BCB–CNT composites with different CNT concentrations are obtained. ► An 80% improvement in the thermal conductivity is achieved for BCB–CNT composites. ► The composite bonding strength is improved for CNT concentrations less than 1.75%.

Through-Silicon-Vias (TSVs) with polymer liners have potential improved electrical and mechanical reliability for three-dimensional (3D) packaging/integration applications. To address the challenge in fabrication of polymer liners in high aspect-ratio TSVs, we have developed a vacuum-assisted spin-coating technology for filling benzocyclobutene (BCB) polymer in high aspect-ratio annular trenches. The newly developed spin-coating technology employs spin-coating twice and vacuum treatment after each coating. The vacuum treatment removes the bubbles at the trench bottom trapped during spin-coating and curing, and facilities the formation of void-free BCB filling. The influences of the BCB viscosities and the spin-coating parameters on the filling ability are evaluated, and upon optimization annular trenches with aspect-ratio as high as 22 have been successfully filled with no void formation, thin overburden, and slight dishing. This method is also applicable to trench filling using polymers with similar properties.

This paper presents the fabrication and characterization of a curved-up piezoresistive microcantilever flow sensor. The microcantilever sensor, fabricated with silicon-on-insulator (SOI) wafers, consists of two layers of silicon dioxide and a silicon piezoresistor in-between. The difference in the residual stresses between silicon and silicon dioxide layers curves the microcantilever upwards and the free-end bends out of plane. The curved-up microcantilever transfers fluidic momentum that acts on it to drag force, which bends the curved-up microcantilever and changes the resistance of the piezoresistor. This configuration has the advantage of high sensitivity for low flow rate measurement and allows the microcantilever to be integrated in microchannels to measure steady flow. The flow sensor is calibrated with respect to low flow rates at range of 0–20 cm/s. The sensitivity, repeatability, and zero drift are characterized in detail.

This paper presents a new method for detecting trimethylamine (TMA) in gas and liquid phases using a piezoresistive microcantilever sensor functionalized with a self-assembled monolayer (SAM). The microcantilever consists of two silicon dioxide layers and a single crystalline silicon piezoresistor in-between. This configuration allows microcantilevers to achieve high sensitivity because of the low stiffness of silicon dioxide and the high piezoresistive coefficients of single crystalline silicon. The surface of the microcantilever is chemically functionalized with a self-assembled monolayer of 11-mercaptoundecanoic acid (11-MUA) via Au–SH covalent bonding on a gold film deposited on the microcantilever. The 11-MUA SAM adsorbs TMA through hydrogen bonding between the SAM and TMA molecules, and the microcantilever is deflected by the changes in the surface stress as a result of the hydrogen bonding based chemisorption. Experimental results show that the deflection of the microcantilever depends on the concentration of TMA, and the minimum detection limits of 10 mg/L for liquid TMA and 1.65 μg/L for gas TMA are achieved. The selectivity of TMA with respect to ethanol, acetone, and butanone is also investigated.

This paper reports two soft lithographic methods, micromolding and hot embossing, to produce biodegradable poly (3-hydroxybutyrate-co-3-ftydroxyhexanoate) (PHBHHx) arrays of microstructures for hosting and culturing cells in a local microenvironment by controlled shape. Silicon masters with high-aspect-ratio microfeatures were fabricated using KOH and DRIE anisotropic etching. These silicon masters were used as molds to construct PHBHHx microstructures using micromolding and hot embossing. Using silicon rather than conventional PDMS as molds allowed microstructures with feature size of 20 μm and height of 100 μm to be realized. PHBHHx microstructures with different configurations including circles, rectangles, and octagons were fabricated to investigate the effects of topography on cell culture. Mouse fibroblast cell lines L929 were cultured on PHBHHx microstructures in vitro to investigate the biocompatibility. This study demonstrates the feasibility of microfabrication of PHBHHx structures with micro-scale feature size using soft lithography, and the results show that PHBHHx microstructures can be created to mimic cellular microenvironment for cell culture, providing a convenient means to investigate relationships of microstructures and cell functions.