Jiankui Chen

Highly sensitive, temperature-dependent gas sensor based on hierarchical ZnO nanorod arrays

The low-cost growth of patterned zinc oxide (ZnO) nanorod arrays (NAs) has attracted much attention with the rapid development of electronics and nanotechnology. A mechanoelectrospinning-assisted continuous hydrothermal synthesis method (MES-CHSM) is proposed to direct-write the precursor patterns for the growth of the ZnO-NAs, in a digital, low-cost, and mask-free manner. The morphology and distribution of the hierarchical ZnO nanorods, having a tremendous impact on the gas response, are determined by the process parameters of the MES-CHSM. It is highly desirable that the diameter, interval, orientation and distribution of the ZnO nanorods can be tuned proactively by changing the growth time, the solution concentration, the nature of the precursor layer, and the pattern by MES. The ZnO-NAs exert excellent Ohmic contact with interdigital electrodes when exposed to dry air, NO2 gas and then dry air again. The gas response of the ZnO sample is surface-reaction-determining. The gas sensing results show highly sensitive and repeatable response–recovery cycles following NO2 gas exposure and air purging, respectively. The dynamic response of the gas sensor shows a temperature-dependent response to NO2, even at low concentrations (1–50 ppm). The best gas response is located between 200 °C and 225 °C. Gas sensors, prepared by different process parameters, show two laws regarding the corresponding responses and the NO2 concentrations: approximately linear and saturation regions. The optimal process parameters are presented to postpone the occurrence of the saturation region, to enlarge the measuring range.

Roll-to-Roll Processing of Flexible Heterogeneous Electronics With Low Interfacial Residual Stress

The roll-to-roll (R2R) processing plays an increasingly important role in the high-throughput fabrication of flexible electronics. This paper highlights the dramatic influence of material properties, geometrical dimensions of film-on-substrate structure, process temperatures, and web tension of the R2R processing. A co-optimization approach has been presented for the R2R processing, and all the design and process parameters are to be simultaneously optimized to reduce the interfacial residual stress. First, the misfit strain between the thin film and the substrate is established based on the process parameters. The temperature-dependent properties are involved in polymer/rubber substrate. Nonlinear phenomena are discovered when the process temperatures and the web tension are involved in substrate together. Then, the interfacial shear and peeling stresses resulted from R2R processing are derived in analytical model based on the generalized plane strain theory. The interfacial stresses are related with process temperatures, material properties, and structural dimensions. Finally, a multivariable optimization model is established to compensate the misfit strain. The results imply that device structures and process parameters are correlated phenomena and therefore should be simultaneously optimized in the R2R processing of flexible electronics.

Tunable Peeling Technique and Mechanism of Thin Chip From Compliant Adhesive Tapes

The efficient pick-up of thin/ultrathin integrated circuit chip from the wafer is one of the key techniques in advanced microelectronic packaging, whose success rates are determined by the peeling-off of chip from the adhesive tapes. Analytical models for estimating the effects on process parameters represent important design tools. Here, solutions for adhesive stresses and mixed-mode peeling are presented in chip-adhesive-substrate structure based on mixed boundary conditions, in which the geometrical dimensions and material properties of this three-layer system are included. Analytical expressions are defined in terms of structural parameters and several integration constants, which can be obtained by a given system of linear equations. These models agree well with finite element models with virtual crack-closure technique. In particular, the mechanism of tunable mode peeling is uncovered, and the technological limit of normal single ejecting needle is discussed.

A hybrid control method of tension and position for a discontinuous web transport system

Tension control and precise conveyance in web-handling machines are critical to ensure product quality. This paper is focused on actualizing a hybrid control method of tension and position for a discontinuous web transport system. First, the simplified algebraic dynamical models of the unwind/rewind section and feed section are presented, which plays an important role for the moving control of flexible web. Second, the hybrid control method, the PID tension control, the synchronous position control and the hybrid control, are introduced, respectively. Finally, based on the mathematical model and control strategies, an experiment platform is established. Several experiments have been performed to show the efficiency and feasibility of this hybrid control method for discontinuous web transport system.

Reliable Peeling of Ultrathin Die With Multineedle Ejector

Flip chip packaging as a mainstream packaging interconnect technology has proliferated rapidly within the last decade or so. With the applications of high-performance chip, its thickness and size have been much thinner and bigger, which is challenging the current assembly technique, especially for the reliable peeling of ultrathin die from the wafer due to its vulnerability and flexibility. Here, we present some significant analytical formulas to estimate die cracking stress and peeling energy in die peeling process. The effects of two factors, including peeling cracking propagation and ejecting needle configuration, are investigated using a fracture mechanics framework. Meanwhile, all analytical predictions have been verified via finite element modeling with virtual crack technique. Theoretical results have shown that die cracking stress could be effectively reduced, but it rarely works to improve peeling energy when more needles are embedded below the adhesive tape. In particular, the essence of the technique with the multi-needle is discussed, compared with the normal single-needle technique, which can be used to guide the design of ultrathin die peeling process.

Vacuum-based picking-up of thin chip from adhesive tape

The reliable picking-up of thin chip using the vacuum sorption determines the success rate of flip chip assemblies from donor tape to receptor substrate. An analytical solution to model the chip–adhesive–tape structure with vacuum loads is introduced to understand the fracture mechanism of chip picking-up. The critical process parameters (the length of bonded region, vacuum strength, and pick-up displacement, etc.) are investigated. Theoretical predictions are used in combination with virtual crack-closure-based finite-element technique to reveal the detaching behavior between the chip and the adhesive tape. The results show that the length of the bonded region should be controlled less than 40% of chip length to eliminate the effects of chip thickness, and the higher vacuum strength acting on the adhesive tape is able to accelerate the detachment of the chip from the adhesive tape. In particular, a process window is proposed to enhance the reliability and efficiency of picking-up for a thin chip.

Analytical Evaluation of Interfacial Crack Propagation in Vacuum-Based Picking-up Process

Chip picking-up with vacuum sorption is a critical technology in flip-chip packaging for separating chip intactly from donor adhesive tape. With increasing length/thickness ratio of chips, the incomplete separation affects the success rate of this process, even leads to the failure of chips. Here, we present a theoretical model with a bisection algorithm to calculate the effective size of tape that contributes to the crack growth, and introduce an energy release rate and pick-up force to evaluate the ability of thin chip picking-up. The finite-element method with a virtual crack-closure technique is constructed to validate the established process model. The effects of the type of pick-up head, length of chip, thickness, and material of tape on chip picking-up process are also revealed to improve the ability to propagate a crack in the adhesive layer. Furthermore, the crack propagation in the adhesive layer with increasing pick-up displacement is predicted to determine the critical value of the thin chip complete separation from compliant tape. These results suggest that the thinner and more compliant tape should be adopted in the chip picking-up process.

Competing Fracture of Thin-Chip Transferring From/Onto Prestrained Compliant Substrate

The transferring of thin chip from donor to receptor plays a critical role in advanced electronic package, and the productivity is determined by the interfacial behavior between chip and substrate during chip transferring. The paper investigates analytical competing fracture model of chip–adhesive–substrate structure in thin-chip transferring (peeling-off and placing-on), to discover the critical process condition for distinguishing the interfacial delamination and chip crack. The structure is continuously subjected to ejecting needle, vacuum pick-up head, and wafer fixture, which leads to concentrated and distributed loads and dynamic boundary conditions. Additionally, two criterions based on competing fracture model are presented to determine the extreme chip dimension for peeling-off and the elimination of residual stress for placing-on. The theoretical results are validated by the finite-element simulation with virtual crack-closure technique (VCCT). This paper provides an insight for process optimization, to improve the success ratio and productivity of chip transferring.

Out-of-Plane Vibration Frequency Estimation for Flexible Substrate in Roll-to-Roll Processing.

Out-of-plane displacements of a moving web is well known to be a main limiting factor of roll-to-roll (R2R) manufacturing for flexible electronics. To tackle this problem, in this paper, a new contactless approach for the measurement of membrane vibration is addressed. This technique, which allows catching out-of-plane membrane vibration involves a vision system composed of a CCD camera and a laser stripe device. With gray centroid method and principal component analysis, out-of-plane vibration distribution is estimated in real time. Experiments are conducted on the RFID inlay in R2R experimental platform to confirm the effectiveness and accuracy of the proposed method.

Stretchable Tactile and Bio-potential Sensors for Human-Machine Interaction: A Review.

Human machine interaction (HMI) technologies have been widely applied to the fields of the complicated task assignment, biological health monitoring, prosthesis techniques, and clinical medicine. In this paper, different kinds of HMI modes are reviewed, such as tactile sensors, biological sensors, and multi-sensory data. Stretchable electronics integrated with multi-function sensors on the polydimethylsiloxane (PDMS) substrate are laminated onto the skin surface for collecting temperature, strain, pressure, biological signals simultaneously. More conformable and natural human-machine interaction methods would be realized, which will provide effective ways for human-robot interaction similar to human-to-human interaction, and finally drive the development of the coexisting-cooperative-cognitive robot (Tri-Co Robot) technology.