Fabrication of thermal microbridge actuators and characterization of their electrical and mechanical responses

This study presents thermal silicon microbridge actuators which have been made by a novel fabrication process utilizing dry processes for all critical steps. The fabrication process results in microbridges which are fully oxide covered, with excellent surface quality and dimensional control. The microbridges are made in the device layer of a silicon-on-insulator (SOI) wafer which ensures uniform doping profile and accurate thickness control. The electrical and mechanical responses of the bridges were measured upon rapid heating up to near the melting point of silicon. Up to 12 μm mechanical deflection due to thermal expansion was detected by white light interferometry (WLI) which allowed accurate measurement. Mechanical deflection has previously not been measured for silicon microlamps. Thermal conduction in the air gap between the actuator and the neighbouring solid silicon parts was analysed and shown to be more important than convection or radiation, even at very high operation temperatures.     

Effects of bending curvature and text/background color-combinations of e-paper on subjects’ visual performance and subjective preferences under various ambient illuminance conditions

The study used the simulated e-paper to investigate how the bending radius of curvature (−10 cm, plane, and 10 cm) and 12 text/background color-combinations of e-paper affect subjects’ visual performance and subjective preference under various ambient illuminance conditions (200 and 500 lx). Analysis results indicated that the bending curvature and ambient illuminance did not significantly affect subjects’ visual performance. However, subjects visual performance differed significantly for different text/background color combinations of the simulated e-paper. When the background color of the simulated e-paper was set to yellow-like condition and the luminance of the text was low (2.2 and 4.6 cd/m²), subjects’ visual performance was best. Regarding the subjective preferences of subjects, the results of this research also demonstrated that the bending curvature, text/background color combinations and ambient illuminance all significantly affected the subjective preferences of subjects. Subjects exhibited the best preference under the following settings: bending curvature of the simulated e-paper set to plane; background color of the simulated e-paper set to yellow-like condition and low text luminance (2.2 or 4.6 cd/m²); high ambient illuminance (500 lx).     

Reliability of transparent conducting substrates for rollable displays: A cyclic loading investigation

Abstract— Failure mechanisms for flexible conducting substrates are investigated herein in the context of rollable/flexible display applications. Cyclic loading experiments (substrates subjected to multiple cycles of tensile strain) were carried out on both ITO‐coated PET and PEDOT:PSS‐coated PET substrates. The resistance was measured after each bending cycle. The resistance increased with the number of cycles and was not reversible. Even when the tensile strain on the ITO/PET was below the virgin cracking threshold (～2%) previously reported [Appl Phys Lett ⁷⁶, 1425 (2000)], slight increases in resistance were measurable after just a few cycles.     

Bending stress analysis of flexible touch panel

Flexible touch panel is required to undertake extensive bending operations during service; thus, bending testing of flexible touch panel for mechanical behaviors and even reliability is crucial for realizing the technology. This study aims at exploring the bending behaviors of a flexible touch panel under a four-point bending test using finite element analysis. The touch panel is a laminated structure composed of seven thin film layers, namely a polyester layer, an adhesive layer, a polyimide layer, two organic layers and two indium tin oxide conductive layers. Because the touch panel structure is symmetry, only a quarter of the flexible touch panel is established in the analysis model. The mechanical properties of the materials are first obtained using both nanoindentation and uniaxial tensile/compressive testing. Furthermore, a modeling technique based on global and local finite element analysis is applied to evaluate the bending stress at a specified radius of curvature. Special emphasis of the calculation is put on the critical region with respect to stress, which is identified as the most susceptible cracking site.     

Temporary bond—debond technology for high‐performance transistors on flexible substrates

Abstract— A processing technology based upon a temporary bond—debond approach has been developed that enables direct fabrication of high‐performance electronic devices on flexible substrates. This technique facilitates processing of flexible plastic and metal‐foil substrates through automated standard semiconductor and flat‐panel tool sets without tool modification. The key to processing with these tool sets is rigidifying the flexible substrates through temporary bonding to carriers that can be handled in a similar manner as silicon wafers or glass substrates in conventional electronics manufacturing. To demonstrate the power of this processing technology, amorphous‐silicon thin‐film‐transistor (a‐Si:H TFT) backplanes designed for electrophoretic displays (EPDs) were fabricated using a low‐temperature process (180°C) on bonded‐plastic and metal‐foil substrates. The electrical characteristics of the TFTs fabricated on flexible substrates are found to be consistent with those processed with identical conditions on rigid silicon wafers. These TFTs on plastic exhibit a field‐effect mobility of 0.77 cm²/V‐sec, on/off current ratio >10⁹ at Vds = 10 V, sub‐threshold swing of 365 mV/dec, threshold voltage of 0.49 V, and leakage current lower than 2 pA/μm gate width. After full TFT‐array fabrication on the bonded substrate and subsequent debonding, the flexible substrate retains its original flexibility; this enables bending of the EPD display without loss in performance.     

Sandwich structure based on back-side etching silicon (100) wafers for flexible electronic technology

An SU-8-silicon(100)-SU-8 flexible composite sandwich structure is studied. Besides preventing corrosion to the thin silicon membrane, SU-8 photoresist coated on the silicon membrane improves its flexibility as shown by an ANSYS finite element simulation. Using a plasma enhanced chemical vapor deposited SiO₂/Si₃N₄ composite film as an etch mask on the polished side, a 4″ (100) silicon wafer was thinned to 26 µm without rupture in a 30 wt% KOH solution. The wafer was coated on both sides with 20 µm of SU-8 photoresist and then bent over cylinders with various diameters, to flex the sandwich in a controlled manner. The determined minimal bending radius of the fabricated thin silicon-based sandwich structure is approximately 3.5 mm. The fabrication of this sandwich structure is compatible with conventional microelectronic fabrication processing. Thus, it allows one to thin fully fabricated devices in a post-fabrication process to make high-performance flexible electronics.

     

Manufacture and characterization of sol–gel V1−x−yWxSiyO2 films for uncooled thermal detectors

V_1−x−yW_xSi_yO₂ films for uncooled thermal detectors were coated on sodium-free glass slides with sol–gel process, followed by the calcination under a reducing atmosphere (Ar/H₂ 5%). The V_1−x−yW_xSi_yO₂ films as prepared inherit various phase transition temperatures ranging from 20 to 70 °C depending on the dopant concentrations and the fabrication conditions. Compared to the hysteresis loop of plain VO₂ films, a rather steep loop was obtained with the addition of tungsten components, while a relaxed hysteresis loop with the tight bandwidth was contributed by Si dopants. Furthermore, the films with switching temperature close to room temperature were fabricated to one-element bolometers to characterize their figures of merit. Results showed that the V_0.905W_0.02Si_0.075O₂ film presented a satisfactory responsivity of 2600 V/W and detectivity of 9 × 10⁶ cm  Hz^1/2/W with chopper frequencies ranging from 30 to 60 Hz at room temperature. It was proposed that with appropriate amount of silicon and tungsten dopants mixed in the VO₂, the film would characterize both a relaxed hysteresis loop and a fair TCR value, which effectively reduced the magnitude of noise equivalent power without compromising its performance in detectivity and responsivity.     

Simulation and fabrication of piezoresistive membrane type MEMS strain sensors

Two piezoresistive (n-polysilicon) strain sensors on a thin Si₃N₄/SiO₂ membrane with improved sensitivity were successfully fabricated by using MEMS technology. The primary difference between the two designs was the number of strips of the polysilicon patterns. For each design, a doped n-polysilicon sensing element was patterned over a thin 3 μm Si₃N₄/SiO₂ membrane. A 1000×1000 μm² window in the silicon wafer was etched to free the thin membrane from the silicon wafer. The intent of this design was to fabricate a flexible MEMS strain sensor similar in function to a commercial metal foil strain gage. A finite element model of this geometry indicates that strains in the membrane will be higher than strains in the surrounding silicon. The values of nominal resistance of the single strip sensor and the multi-strip sensor were 4.6 and 8.6 kΩ, respectively. To evaluate thermal stability and sensing characteristics, the temperature coefficient of resistance [TCR=(ΔR/R₀)/ΔT] and the gage factor [GF=(ΔR/R₀)/] for each design were evaluated. The sensors were heated on a hot plate to measure the TCR. The sensors were embedded in a vinyl ester epoxy plate to determine the sensor sensitivity. The TCR was 7.5×10⁻⁴ and 9.5×10⁻⁴/°C for the single strip and the multi-strip pattern sensors. The gage factor was as high as 15 (bending) and 13 (tension) for the single strip sensor, and 4 (bending) and 21 (tension) for the multi-strip sensor. The sensitivity of these MEMS sensors is much higher than the sensitivity of commercial metal foil strain gages and strain gage alloys.     

Post-CMOS processing for high-aspect-ratio integrated siliconmicrostructures

Presents a new fabrication sequence for integrated-silicon microstructures designed and manufactured in a conventional complementary metal-oxide-semiconductor (CMOS) process. The sequence employs a post-CMOS deep silicon backside etch, which allows fabrication of high aspect ratio (25:1) and flat (greater than 10 mm radius of curvature) MEMS devices with integrated circuitry. A comb-drive resonator, a cantilever beam array and a z-axis accelerometer were fabricated using this process sequence. Electrical isolation of single-crystal silicon was realized by using the undercut of the reactive ion etch (RIE) process. Measured out-of-plane curling across a 120-μm-wide 25-μm-thick silicon released plate was 0.15 μm, which is about ten times smaller than curl of the identical design as a thin-film CMOS microstructure. The z-axis DRIE accelerometer structure is 0.4 mm by 0.5 mm in size and has a 25-μm-thick single-crystal silicon proof mass. The measured noise floor is 1 mG/√Hz, limited by electronic noise. A vertical electrostatic spring "hardening" effect was theoretically predicted and experimentally verified     

High‐temperature thin‐film barriers for foldable AMOLED displays

We present a thin‐film dual‐layer bottom barrier on polyimide that is compatible with 350°C backplane processing for organic light‐emitting diode displays and that can facilitate foldable active‐matrix organic light‐emitting diode devices with a bending radius of <2 mm. We demonstrate organic light‐emitting diodes that survive bending over 0.5 mm radius for 10.000× based on the high‐temperature bottom barrier. Furthermore, we show compatibility of the bottom barrier with the backplane process by fabricating active‐matrix organic light‐emitting diode displays on GEN1‐sized substrates.     

Electrothermally activated paraffin microactuators

A new family of electrothermally activated microactuators that can provide both large displacements and forces, are simple to fabricate, and are easily integrated with a large variety of microelectronic and microfluidic components are presented. The actuators use the high volumetric expansion of a sealed, surface micromachined patch of paraffin heated near its melting point to deform a sealing diaphragm. Two types of actuators have been fabricated using a simple three mask fabrication process. The first device structure consists of a 9 μm thick circularly patterned paraffin layer ranging in diameter from 400 to 800 μm all covered with a 4-μm-thick metallized p-xylylene sealing diaphragm. All fabricated devices produced a 2.7-μm-peak center deflection, consistent with a simple first order theory. The second actuator structure uses a constrained volume reservoir that magnifies the diaphragm deflection producing consistently 3.2 μm center diaphragm deflection with a 3-μm-thick paraffin actuation layer. Microactuators were constructed on both glass and silicon substrates. The actuators fabricated on glass substrates used between 50-200 mW of electrical power with response times ranging between 30-50 ms. The response time for silicon devices was much faster (3-5 ms) at the expense of a larger electrical power (500-2000 mW)     

Flexible AMOLEDs with low‐temperature‐processed TFT backplane technologies

Developments of backplane technologies, which are one of the challenging topics, toward the realization of flexible active matrix organic light‐emitting diodes (AMOLEDs) are discussed in this paper. Plastic substrates including polyimide are considered as a good candidate for substrates of flexible AMOLEDs. The fabrication process flows based on plastic substrates are explained. Limited by the temperature that plastic substrates can sustain, TFT technologies with maximum processing temperature below 400 °C must be developed. Considering the stringent requirements of AMOLEDs, both oxide thin‐film transistors (TFTs) and ultra‐low‐temperature poly‐silicon TFTs (U‐LTPS TFTs) are investigated. First, oxide TFTs with representative indium gallium zinc oxide channel layer are fabricated on polyimide substrates. The threshold voltage shifts under bias stress and under bending test are small. Thus, a 4.0‐in. flexible AMOLED is demonstrated with indium gallium zinc oxide TFTs, showing good panel performance and flexibility. Further, the oxide TFTs based on indium tin zinc oxide channel layer with high mobility and good stability are discussed. The mobility can be higher than 20 cm²/Vs, and threshold voltage shifts under both voltage stress and current stress are almost negligible, proving the potential of oxide TFT technology. On the other hand, the U‐LTPS TFTs are also developed. It is confirmed that dehydrogenation and dopant activation can be effectively performed at a temperature within 400 °C. The performance of U‐LTPS TFTs on polyimide is compatible to those of TFTs on glass. Also, the performance of devices on polyimide can be kept intact after devices de‐bonded from glass carrier. Finally, a 4.3‐in. flexible AMOLED is also demonstrated with U‐LTPS TFTs.     

Influence of environmental temperature on the dynamic properties of a die attached MEMS device

Die attach is one of the major processes that may induce unwanted stresses and deformations into micro-electro-mechanical systems (MEMS). The thermo-elastic coupling between the die and package may affect the performance of MEMS under various temperature loads, causing unreasonable effects of the output signal, such as zero offset, temperature coefficient of offset (TCO), nonlinearity, ununiformity and hysteresis, etc. A complete characterization of these effects is critical for a more reliable design. This work presents experimental studies of the temperature effects on the dynamic properties of MEMS. Microbridges and strain gauges with different dimensions were used as test structures. They were surface-micromachined on test chips and the chips were die attached on organic laminate substrates using epoxy bonding as well as tape adhering. The material and dimension of the substrate were specially defined to amplify the magnitude of the coupled deformation for the convenience of investigation. Modal frequencies of the microbridges under a set of controlled environmental temperature before and after die attach were measured using a laser Doppler vibrometer system. The average initial residual strain was also measured from the strain gauges to help analyze the dynamic behavior. Nonlinear TCO of the frequencies were observed to be as large as 2,500–5,000 ppm for the epoxy-bonded samples, in contrast with much smaller values for the tape-adhered and unpackaged ones. The frequencies recovered to their original values beyond the curing temperature of the epoxy. A distributed feature was also observed in frequencies of the microbridges with the same length but at different locations of the chip with a maximum relative difference of 20%. The process of thermal cycling and wire bonding was also applied to the samples and caused tender shifts of the frequencies. The experiments reveal major factors that are related to the temperature effects of die attached MEMS and the results are useful for improving the reliability of a package–device co-design.     

Modeling of the intrinsic stress effect on the resonant frequency of NEMS resonators integrated by beams with variable cross-section

Nano-electro-mechanical systems (NEMS) resonators integrated by a double clamped beam with variable cross-section are used in several applications such as chemical and biological detectors, high-frequency filters, and signal processing. The structure of these resonators can experience intrinsic stresses produced during their fabrication process. We present an analytical model to estimate the first bending resonant frequency of NEMS resonators based on a double clamped beam with three cross-sections, which considers the intrinsic stress effect on the resonant structure. This model is obtained using the Rayleigh and Macaulay methods, as well as the Euler–Bernoulli beam theory. We applied the analytical model to a silicon carbide (SiC) resonator of 186 nm thickness reported in the literature. This resonator has a total length ranking from 80 to 258 μm and is subjected to a tensile intrinsic stress close to 110 MPa. Results from this model show good agreement with experimental results. The analytic frequencies have a maximum relative difference less than 6.3% respect to the measured frequencies. The tensile intrinsic stress on the resonant structure causes a significantly increase on its bending resonant frequency. The proposed model provides an insight into the study of the intrinsic stress influence on the resonant frequency of this nanostructure. In addition, this model can estimate the frequency shift due to the variations of the resonator geometrical parameters.     

Silicon condenser microphones with corrugated silicon oxide/nitride electret membranes

Corrugated electret membranes are used in a micromachined silicon microphone. The membranes consist of a permanently corona-charged double layer of silicon dioxide and silicon nitride, known to have excellent charge-storing properties. This electret can replace the external bias needed for condenser microphones. The well-known LOCOS technique—also combined with dry etching—is used for the first time to fabricate membranes with corrugation depths of several microns. The membrane thickness amounts to 600 nm.

The interferometrically measured center deflection is up to 40 nm/Pa for diaphragms with four corrugations of up to 3.3 μm depth and a size of A_M=1 mm². This high mechanical sensitivity limits the dynamic range to sound pressures below 50 Pa. The obtained mechanical sensitivities are in excellent agreement with the theory.

The most compliant corrugated diaphragms result in a microphone sensitivity of 2.9 mV/Pa, an equivalent noise level of 39 dB(A) and a total harmonic distortion below 1.7% at 28 Pa (123 dB SPL). The corrugation depth in the sensors has been only 1.3 μm. All sensors cover the whole audio and low ultrasonic range. The temperature coefficient is between 0.05 and 0.1 dB/K.     

Experimental evaluation of MEMS strain sensors embedded incomposites

Micromechanical in-plane strain sensors were fabricated and embedded in fiber-reinforced laminated composite plates. Three different strain sensor designs were evaluated: a piezoresistive filament fabricated directly on the wafer; a rectangular cantilever beam; and a curved cantilever beam. The cantilever beam designs were off surface structures, attached to the wafer at the root of the beam. The composite plate with embedded sensor was loaded in uniaxial tension and bending. Sensor designs were compared for repeatability, sensitivity and reliability. The effects of wafer geometry and composite plate stiffness were also studied. Typical sensor sensitivity to a uniaxial tensile strain of 0.001 (1000 με) ranged from 1.2 to 1.5% of the nominal resistance (dR/R). All sensors responded repeatably to uniaxial tension loading. However, for compressive bending loads imposed on a 2-3-mm-thick composite plate, sensor response varied significantly for all sensor designs. This additional sensitivity can be attributed to local buckling and subsequent out of plane motion in compressive loading. The curved cantilever design, constructed with a hoop geometry, showed the least variation in response to compressive bending loads. All devices survived and yielded repeatable responses to uniaxial tension loads applied over 10 000 cycles     

硅纳米梁的静态特性分析

提出了用于分析硅纳米梁静态弯曲的半连续体模型.与传统的连续体模型相比,该方法考虑了厚度方向进入纳米尺度所带来的物理特性的离散化现象.基于Sun-Zhang提出的应变能量计算模型,运用变分原理,推导出半连续体模型.从计算结果可以看出,几何尺寸和表面原子的重构、弛豫效应对梁的弯曲有一定影响.该模型分析梁的弯曲得到的结果与连续体模型相比偏小,随着尺寸的增大误差逐渐减小,在宏观尺寸下两种模型最终趋于一致.     

Vertically aligned ZnO nanorods and graphene hybrid architectures for high-sensitive flexible gas sensors

We present the fabrication and characterization of new type of flexible gas sensors, composed mainly of a bottom ZnO conductive layer on metal foil, vertically aligned ZnO nanorod channel, and graphene-based top conductive electrode. Multiple cycling tests demonstrated the ZnO nanorods (NRs) and graphene (Gr) hybrid architectures accommodated the flexural deformation without mechanical or electrical failure for bending radius below 0.8 cm under the repeated bending and releasing up to 100 times. In addition, the hybrid architectures fabricated on glass substrate showed good optical transmittance larger than ∼70% for visible light, indicating potential application in transparent devices. Furthermore, our gas sensors demonstrated the ppm level detection of ethanol gas vapor with the sensitivity (resistance in air/resistance in target gas) as high as ∼9 for 10 ppm ethanol.     

Self-Assembly Based on Chromium/Copper Bilayers

In this paper, we detail a strategy to self-assemble microstructures using chromium/copper (Cr/Cu) bilayers. Self-assembly was primarily driven by the intrinsic residual stresses of Cr within these films; in addition, the degree of bending could be controlled by changing the Cu film thickness and by introducing a third layer with either a flexible polymer or a rigid metal. We correlate the observed curvature of patterned self-assembled microstructures with those predicted by a published multilayer model. In the model, measured stress values (measured on the unpatterned films using a substrate curvature method) were utilized. We also investigated the role of two different sacrificial layers: 1) silicon and 2) water-soluble polyvinyl alcohol. Finally, a Taguchi design of experiments was performed to investigate the importance of the different layers in contributing to the stress–thickness product (the critical parameter that controls the curvature of the self-assembled microstructures) of the multilayers. This paper facilitates a deeper understanding of multilayer thin-film-based self-assembly and provides a framework to assemble complex microstructures, including tetherless self-actuating devices. $hfill$[2008-0308]      

Polymer micro valve with a hydraulic piezo-drive fabricated by the AMANDA process

A normally-closed piezo-driven micro valve for gases and liquids has been developed. Due to the small deflection of piezo-actuated bending disks made of PZT a hydraulic transmission was constructed, which increases the displacement of the piezo disk up to the 25-fold. The switching times of the valve are shorter than 1.8 ms.

Silicone gel serves as a transition medium in the hydraulic. It simplifies the manufacturing process and allows a flexible adaptation to different pressure requirements up to a differential pressure of 150 kPa switched with 300 V. Additionally, this silicone gel supports the sealing of the valve so it achieves an open-to-close ratio of 1:150,000 or higher.

The overall dimensions of the valve are 13 mm×13 mm×3 mm, the dead volume is 0.33 μl, and the inner diameter of the valve seat is 200 μm. A transformer as small as the micro valve is able to produce driving voltages of up to 300 V with a response time of 2 ms. Therefore, the valve could be integrated into hand-held systems which are battery powered with 5–15 V.