Mengwei Li

Controllable shrinking of inverted-pyramid silicon nanopore arrays by dry-oxygen oxidation

A novel and simple technique for the controllable shrinkage of inverted-pyramid silicon (Si) nanopore arrays is reported. The Si nanopore arrays with sizes from 60 to 150 nm, made using a combination of dry and wet etching, were shrunk to sub 10 nm, or even closed, using direct dry-oxygen oxidation at 900 ° C. The shrinkage process of the pyramidal nanopore induced by oxidation was carefully modeled and simulated. The simulation was found to be in good agreement with the experimental data within most of the oxidation time range. Using this method, square nanopore arrays with an average size of 30 nm, and rectangular nanopores and nanoslits with feature sizes as small as 8 nm, have been obtained. Furthermore, focused ion beam cutting experiments revealed that the inner structure of the nanopore after the shrinkage kept its typical inverted-pyramid shape, which is of importance in many fields such as biomolecular sensors and ionic analogs of electronic devices, as well as nanostencils for surface nano-patterning.

Direct surface nanopatterning using pyramidal silicon nanopore arrays as templates

Abstract. Template-based surface nanopatterning techniques are highly efficient methods in realizing different surface nanopatterns, which are the fundamental structures of various nanodevices. We demonstrate a template of pyramidal silicon nanopore array (PSNA) for direct surface nanopatterning. Using the PSNA templates, deposition experiments of platinum (Pt) and gold-palladium (Au-Pd) particles on chromium-coated silicon substrates were performed. Individual Pt nanocubes and nanocube arrays with different shapes and feature sizes as small as 82 nm were obtained. By tuning the gap between the PSNA template and the substrate, Au-Pd microdot arrays with an average diameter of 7.6 μm were also fabricated. As the nanopore size and shape of the PSNA template can be easily controlled, the corresponding size and shape of the transferred surface nanopatterns are tunable. These results indicate the potential of the PSNA templates for large-scale production of nanopatterns with desired sizes and shapes.

Pressure sensing element based on the BN–graphene–BN heterostructure

In this letter, we report a pressure sensing element based on the graphene–boron nitride (BN) heterostructure. The heterostructure consists of monolayer graphene sandwiched between two layers of vertically stacked dielectric BN nanofilms. The BN layers were used to protect the graphene layer from oxidation and pollution. Pressure tests were performed to investigate the characteristics of the BN–graphene–BN pressure sensing element. A sensitivity of 24.85 μV/V/mmHg is achieved in the pressure range of 130–180 kPa. After exposing the BN–graphene–BN pressure sensing element to the ambient environment for 7 days, the relative resistance change in the pressure sensing element is only 3.1%, while that of the reference open-faced graphene device without the BN protection layers is 15.7%. Thus, this strategy is promising for fabricating practical graphene pressure sensors with improved performance and stability.In this letter, we report a pressure sensing element based on the graphene–boron nitride (BN) heterostructure. The heterostructure consists of monolayer graphene sandwiched between two layers of vertically stacked dielectric BN nanofilms. The BN layers were used to protect the graphene layer from oxidation and pollution. Pressure tests were performed to investigate the characteristics of the BN–graphene–BN pressure sensing element. A sensitivity of 24.85 μV/V/mmHg is achieved in the pressure range of 130–180 kPa. After exposing the BN–graphene–BN pressure sensing element to the ambient environment for 7 days, the relative resistance change in the pressure sensing element is only 3.1%, while that of the reference open-faced graphene device without the BN protection layers is 15.7%. Thus, this strategy is promising for fabricating practical graphene pressure sensors with improved performance and stability.

Micro-Raman spectroscopy analysis of residual stress in polysilicon MEMS resonators

This paper presents the recent results of utilization of micro-Raman spectroscopy to measure and characterize residual stress in polysilicon doubly-clamped MEMS resonators with small lateral size. Due to imprecise prediction of the magnitude of intrinsic residual stress, detrimental effect of the residual stress severely shifts the resonant frequency of MEMS resonator from the analytical pre-designed value. The stress is not only determined by the fabrication process but also related to the structural dimensions of resonators. In this work, microRaman spectroscopy was used to measure the residual stress of resonators with widths down to 2μm. The results show that the optimized resonator with length shorter than 50μm and width between 3.2μm and 4.1μm exhibits minimum residual stress.

Symmetric toggle structured MEMS linear variable capacitor with large tunning ratio

A microelectromechanical-system (MEMS) variable capacitor with symmetric toggle structure is proposed to achieve an excellent linearity of the C-V response and a large capacitance tuning ratio. Based on lever principle, flexible top plate of the capacitor moves upwards when applying the voltage on the control electrodes increases, which results in a highly linear decrease of the capacitance with increasing control voltage. The proposed MEMS variable capacitor was modeled and simulated using ANSYS software and fabricated using surface micromachining process. The results show a high linearity factor (LF) of 96.3% in C-V response and a large tuning ratio of 160% in a low actuation voltage range from 0 V to 30 V. The LF even reaches 98.9% from 10 V to 30 V.

Shrinking of silicon nanopore arrays by direct dry-oxygen oxidation

Solid-state nanopores have emerged as useful single-molecule sensors for DNA and proteins. Dry-oxygen oxidation was proposed to directly shrink pyramidal silicon (Si) pore arrays. With this method, inverted-pyramid Si nanopore arrays with feature sizes of over 60 nm, prepared using a combination of dry and wet etching, were shrunk to be less than 20 nm with nanometer precision. The shrinkage was found to be dominated by the deposition of the SiO2 layer on the nanopore surface and its surface-tension-driven mass flow. The inner structure of the nanopore after the shrinkage kept its typical inverted-pyramid shape, which theoretically permits high-resolution DNA sequencing. Furthermore, this method can process many nanopore samples at one time and reduce the inbuilt stress in the Si nanopores during the annealing.

Study on polysilicon resistivity control with nano-scale grain size

Resistivity of boron-doped polycrystalline-silicon (polysilicon) film was investigated experimentally with wide range of doping concentration (5×10¹⁸~1.4×10²⁰cm^-3) and various annealing conditions (950~1050°C, 10-30mins), which results in sheet resistance of 89.5~149000Ω/°C and resistivity of 2.6×10^-3~4.3Ω·cm. Mathematical fitting curve shows perfect consistency of resistivity as function of average dopant concentration lower than 8.5×10¹⁹cm^-3. While at higher concentration where sheet resistance is lower than 170Ω/°C, it is verified that dependence of resistivity of annealing temperature can be represented by the average nano-scale grain size. The average diameter of heavily doped (1.4×10²⁰cm^-3) polysilicon is 85nm, 99nm and 111nm at 950°C, 1000°C, 1050°C, respectively. Combining curve-fit and annealing temperature, resistivity of polysilicon thin film can be precisely controlled with an error less than 10%. The resistivity changes with nano-scale grain size are discussed.