Danqi Zhao

Inter-CMOS process for monolithic integrated MEMS resonator

In this work, novel processes for the simultaneous fabrication of the MEMS (micro electro mechanical system) and CMOS (complementary metal oxide semiconductor) components on a monolithic integrated micro-cantilever resonator are proposed. A universal cavity shared by the entire MEMS unit is designed, which decreases the altitude difference between the MEMS and CMOS parts, thus benefiting the subsequent lithography for metal interconnection. The substrate for the CMOS unit is independent on the MEMS unit, assuring electrical isolation. Furthermore, an additional step of selectively removing silicon nitride (Si3N4) capping layer and polysilicon (poly-Si) layer upon the CMOS area is added to lower the stress in the MOSFET channel regions. The fabrication of this resonator with on-chip circuits according to the presented inter-CMOS process is investigated first by theoretical analysis, and then moving to experimental verification.

Humidity sensing characteristics of tin oxide thin film gas sensors varying with the operating voltage

In this paper, the tin oxide thin film gas sensors were successfully manufactured under a certain process condition and the humidity sensing properties of the devices varying with the operating voltage were tested. To begin with, a model of the gas sensor was built and simulated to determine the steady-state temperature distributions of the whole device and the gas sensing film using ANSYSTM. Then X-ray photoelectron spectroscopy (XPS) was used to analyze the composition of the tin oxide film fabricated under the specific process condition, showing that it has a high oxygen content. At last the humidity sensing characteristics of the devices at different operating voltages were tested. A conclusion can be drawn from the experimental results that the humidity sensitivity of the gas sensors can be enhanced from 1.429E-2 to 4.651E-2 when the operating voltage is increased from 3.000 V to 4.000 V and can be reduced from 4.651E-2 to 4.077E-3 when the operating voltage is increased from 4.000 V to 5.000 V.

Mechanical strength measurement of micro anchor-beam combined structure fabricated by silicon-on-glass process

Two series of devices, bonding quality testing devices and torsional strength testing devices, were designed to fully investigate the mechanical strength of the micro anchor-beam combined structure, which is fabricated by silicon-on-glass process. Its proved that the bonding quality of the anchor degenerates severely when the anchor size becomes very small. And the results of anodic bonding quality testing device demonstrated that array-shaped anchor design helped to improve the anodic bonding yield. According to the bending fracture test of the torsional strength testing devices, the array-shaped anchor design has an almost equal, even bigger torsional strength compared with single anchor.

Study on compatible CMOS-MEMS process with surface micromachining for the application of monolithic integration

In this work, compatible CMOS-MEMS process with surface micromachining is investigated. Surface micromachining method for cantilever fabrication has been merged with conventional CMOS process, and release of MEMS structure is conducted after CMOS process. We designed polysilicon MEMS structures as well as CMOS devices and circuits on a monolithic sensor chip for the investigation of the influence of stress induced by non-adequate post-CMOS annealing. The impact of step coverage and the releasing process on both the MEMS and CMOS components are also discussed.

Device for measuring the flexural fracture strength of etched surface

To evaluate the quality of deep-reactive-ion-etching process quantitatively, a novel device was designed and fabricated to obtain the flexural fracture strength of etched surface. The device is composed of beam of uniform strength and on-chip multifunctional micro needle. The testing method only requires probe station in the measurement. Devices designers can utilize the method to research the mechanical reliability of the devices easily, and as for foundry, the testing result can be used as a parameter to reflect the etching capacity. From our bending test, it can be demonstrated that the fracture strength of the etched surface in our etching conditions is about 2.1 GPa.

A novel method to fabricate the black silicon for the solar cell

In this paper, an one-step method to fabricate the black silicon with varying scale was introduced. This novel method was conducted at room temperature and completely compatible with the traditional process. This method was based on a standard Bosch deep reactive ions etching with a phase-delay producer. The relationship between the process parameter and the key physical factor was investigated and the practical reflectance of the black silicon was measured. The results showed the black silicon, produced by this method, had a very low reflectance and was easy to change the scale.

Effect of phosphorus doping on the performance of Au/Si inter-diffusion

In this work, both phosphorus doped and undoped Au/Si contact structures were investigated by scanning electron microscope (SEM) and Rutherford backscattering spectrometry (RBS) analysis after annealing at 350°C for 30 min. The effect of phosphorus doping on the performance of Au/Si inter-diffusion is discussed in this paper. The SEM image of the undoped Au/Si contact structure revealed that inverted pyramid-shaped diffusion outline formed at the contact interface after annealing due to the non-uniformity and anisotropy of Au/Si inter-diffusion. However, when the crystal Si substrate was heavily phosphorus doped by ion implantation, the inverted pyramid-shaped outline was eliminated and a smooth contact interface was obtained. In addition, RBS analysis showed that the average diffusion depths in both cases were nearly the same, which indicates the phosphorus doping can alter the anisotropy of Au/Si interdiffusion but has no significant influence on the Au/Si interdiffusion rate.

Electrical interconnect in MEMS/NEMS devices by Au/a-Si eutectic reaction

The quality and the reliability of the electrical interconnection have a direct impact on the performance of the MEMS/NEMS devices. In this work, reliable electric interconnection for MEMS/NEMS devices was realized by Au/a-Si (amorphous Si) eutectic reaction in the anodic wafer bonding process. In order to evaluate the qualities of the anodic bonded contact, the electrical property of the Au/a-Si contact was characterized by a modified vertical Kelvin method. The resistor network model of the anodic bonded modified Kelvin structure indicates that the relationship between the contact resistance and the measured resistance can be easily established. The contact resistance was precisely measured by minimizing the interferences from parasitic resistances. The test results indicated that the anodic bonded Au/a-Si contact is Ohmic contact and the qualities of small-size bonded contact is greatly improved compared to that of traditional Au/Si contact. In addition, the fabrication process was also simplified by eliminating the high temperature annealing process after ion implantation.

Fabrication of monolithic integrated MEMS resonator with wet-release-monitoring array

In this work, a monolithic integrated MEMS resonator was fabricated and tested. Surface micromachining method was employed to fabricate the cantilever MEMS resonator after a standard 3 μm CMOS process. The wet release method with dilute HF solution was chosen and compared to the anhydrous HF vapor release process. A release-monitoring structure with polysilicon/Au cantilever array was used to determine the corrosion time of the sacrificial material. Results showed that the MOSFETs function well after proposed release process.

A novel design of super-capcitor based on black silicon with atomic layer deposition

In this paper, a novel super-capacitor design based on the black silicon was introduced. The atomic layer deposition was designed to fabricate the dielectric and electrode layer. And the black silicon was chosen for the substrate. A specific MEMS process was also designed to achieve the contact between the metal electrode and electrode layer. The parallel-plate capacitor theory was employed for the capacitance calculation. And the simulation has showed that this novel super-capacitor design has big capacitance and quick charge/discharge speed.