Xiangmeng Jing

Multi-layer microstructure fabrication by combining bulk silicon micromachining and UV-LIGA technology

A novel method for fabrication of multi-layer microstructures of microelectro-mechanical system (MEMS) devices is described. This technique, which combines bulk silicon micromachining technique and UV-LIGA technique can overcome some shape limitations of single technique on complex microstructures. To demonstrate this combination, the SU-8 microstructure fabricated in the etched silicon grooves is presented. In this fabrication process, a SU-8 removal method by fuming sulfuric acid was introduced and a novel type of plastics PETG was tried in microhot embossing process. The proposed fabrication process can be applied to fabricating a high-aspect-ratio microstructure for a large displacement actuator and precision sensors. Moreover, this combined process enables the fabrication of more complex structures, which cannot be fabricated by bulk micromachining or UV-LIGA alone.

Effect of thermal annealing on TSV Cu protrusion and local stress

Through silicon vias (TSVs) are regarded as one of the key enabling component to achieve three-dimensional (3D) integrated circuit (IC) functionality. In this paper, we present the investigation on TSV protrusion and stress at different annealing conditions tested by means of optical profiler and high efficiency micro-Raman microscopy. Finite element method is utilized to model and simulate the thermo-mechanical behavior of the TSV having a diameter of 20 μm and a depth of 120 μm under different annealing temperatures. The measured protrusion increases with annealing temperature below 400°C, and then decreases when being further annealed. The maximum measured silicon stress as a function of annealing temperature has shown similar trend to the protrusion. The pre-annealing has limited effect on protrusion, but is helpful to reduce the silicon stress.

An aerodynamically efficient sphere anemometer with integrated hot-film sensors for 2-D environmental airflow monitoring

This paper reports a novel anemometer configuration for airflow measurement. The flow over a sphere provides an ideal flow geometry. Four micro hot-film sensors are orthogonally mounted on the circumference of a sphere. The airflow speed distribution on the sphere as well as flow separation point will affect heat dissipation of the hot-film sensors, and result in variance of resistance of the sensors. By using the information from four positions, 2-D airflow speed and direction can be reconstructed. The micro hot-film sensor die was first designed and fabricated, and then packaged and assembled into a plastic sphere. The wind tunnel testing of the sensor prototype reveals good sensitivity and directivity, indicating it is feasible for environmental airflow measurement.

Elastic MEMS probe card based on the PDMS substrate

A novel type of MEMS probe card that uses polydimethylsiloxane (PDMS) as an elastic substrate was designed and fabricated by MEMS technology. This proposed probe card structure has the advantages of good planarization, low resistance, higher probe density and a simplified fabrication process. To realize the probe card prototype, polyimide (PI) was employed as the interlayer to avoid micro cracks of the sputtered metal layer. Then oxygen plasma treatment was utilized to enhance the adhesion of chrome to PI. Also, silicon wet chemical etching was used to form the step adopted to compensate the height of the bonding wire. When the thickness of the PDMS and the PI layer was 150 µm and 50 µm respectively, the measured spring constant of the PI/PDMS substrate was 4582 N m−1, and that of the probe card structure was 7317 N m−1. The resistance from the probe tip to the end of the copper conductive line was as low as 1.4 Ω. In the RF range of 0–40 MHz, the characteristic impedance was above 20 kΩ, and the capacitance between the two adjacent probes was between 0.19 pF and 0.28 pF. It indicated that the designed probe card structure was suitable for testing of high-speed signal ICs.

Non-destructive testing of through silicon vias by high-resolution X-ray/CT techniques

Detecting through silicon via (TSV) associated defects non-destructively immediately after the fabrication process, or in failure analysis is of great interest and is a challenge. This paper reports on the inspections of 5 to 30 μm diameter TSVs by the state-of-the-art, commercially available X-ray systems, exemplifying a generally preferred method for non-invasively identifying metallization defects in vias as well as in joining structures. The principle of X-ray imaging for TSV measurement is discussed and illustrated, and three dimensional TSV structures are reconstructed by computed tomography (CT). Methods to achieve high-resolution TSV X-ray imaging are discussed.

Numerical simulation and experimental verification of copper plating with different additives for through silicon vias

The Filling of high aspect ratio through silicon vias (TSVs) using copper plating without any void or seam is one of the technical challenges for 3D integration. This paper presents numerical simulation and experimental verification of copper plating with different additives and a guideline for process optimization is proposed. Theoretical models are derived and a generic calculating approach is developed by employing a variable boundary method using commercial software. The simulation results can predict the behaviors of copper plating with different levels of additives for blind vias. The further experimental results verified the theoretical model and the simulation results. TSVs with diameter of 30μm and depth of 160μm on 8 inch wafers without void or seam have been achieved.

Effect of annealing after copper plating on the pumping behavior of through silicon vias

Though Silicon Vias(TSVs) are regarded as a key technology to achieve three dimensional(3D) integrated circuit(IC) functionality. Annealing a silicon device with TSVs may cause high stress and cause TSV protrusion because of high Coefficient of Thermal Expansion(CTE) between silicon substrate and TSVs. The TSV wafers could be annealed right after copper plating process, or after chemical mechanical polish (CMP) process, or both. In this paper, we report our research progress on the effect of annealing right after copper plating on the pumping behavior at different temperatures. Then the copper overburden is removed by CMP. The TSV wafers are tested at different temperatures for 30 minutes, 250°C, 300°C, 350°C, 400°C, 450°C, respectively. The pumping is measured by optical profiler, BRUKER Contour GT-X3. The finite element analysis method, ANSYS, is used to model and simulate the copper pumping at different temperatures. The pumping results with annealing at different temperatures are compared with those without annealing. It reveals that the pumping with annealing is larger than that without annealing. This is possibly due to higher level of stress release and microstructure evolution.

Study of equivalent thermal modeling and simulation of 2.5D/3D stacked dies module

In the paper, an equivalent modeling method is proposed to simplify thermal simulation model of 2.5D stacked dies modules. A TSV and its surrounding silicon substrate or a micro bump and its surrounding underfill will be equivalent to a single body of material. Through this method, we will not only be able to obtain thermal characteristics of each part of the stacked dies module, but also can greatly simplify the calculation amount of numerical simulation. According to this method, we obtained thermal distribution map of 2.5D/3D stacked dies module; in addition, to guide and optimize thermal management design, we analyzed the influence of several parameters on maximum junction temperature of 2.5D stacked dies as well, such as spacing among dies, thermal conductivity of TIM2 (Thermal Interface Material), ambient temperature, wind speed and so on. It was found that with the increase in spacing among dies, the maximum junction temperature of dies decreases and the maximum decreasing amplitude is 4.4°C. Secondly, impact on the maximum junction temperature of die, the ambient temperature of the cabinet is the most serious. Finally, the wind speed of the cabinet and the thermal conductivity of TIM2 (Thermal Interface Material) also have a great effect on the maximum junction temperature of die.

Design and fabrication of a micromachined bilayer cantilever probe card

We present a bilayer cantilever microelectromechanical systems probe card configuration aiming to achieve an optimization of the mechanical and electrical properties of the probes. This bilayer cantilever structure is analyzed by an analytical method, and then further validated by finite element analysis. A prototype probe card structure is designed for the parallel I/O pads layout with a pitch of 100 µm, and developed via combining Si micromachining and ultraviolet Lithographie, Galvanoformung, Abformung (lithography, electroplating, and molding) (UV-LIGA) technique. The measured spring constant of the cantilever is 0.6362 Nm-1, close to the theoretical prediction. The resistance from the probe tip to the end of the Cu conductive line is as low as 0.035 , indicating a very small electrical loss on the probe structure. In the radio frequency (rf) range of 0 to 40 MHz, the characteristic impedance is higher than 20 k, while the capacitance between two adjacent probes is around 0.13 pF. These measurement data indicate that the designed cantilever probe card structure has a good rf isolation property that makes it suitable for the testing of high-speed signal ICs.

Hair-like airflow sensing with piezoelectric vibrating diaphragm

Inspired by filiform hairs, we propose a three dimensional (3D) microelectromechanical systems (MEMS) airflow sensing structure for wind velocity measurement. The piezoelectric Pb(Zr,Ti)O3 (PZT) diaphragm vibrating at its resonant frequency is used as the sensing element to mimic the neuron. The standing beam on the diaphragm behaves like the hair to collect the airflow drag force. The bending of the standing beam can induce the mechanical strain, resulting the changes in the diaphragm stiffness and subsequently the resonant frequency. Then the airflow velocity can be characterized by the resonant frequency shift of the piezoelectric diaphragm. The microstructure design, fabrication and packaging sequence are described. A specific electronic printed circuit, which converts the resonant frequency shift to a voltage (conversion rate of 0.7 mV/Hz) and enables the data communication to a PC, has been developed. Testing result shows that a 25Hz resonant frequency shift is found when the packaged device is subjected to a wind speed of 10 m/s.