Jinling Yang

Fracture Properties of LPCVD Silicon Nitride Thin Films from the Load-Deflection of Long Membranes

The bulge test is successfully extended to the determination of the fracture strength of thin films by accurately describing the deflection profile of the loaded long membranes. The model includes the prestress and bending stiffness of the membrane material into the load-deflection response. The feasibility of this method is demonstrated with LPCVD silicon nitride films with maximum strengths between 10.8 GPa and 11.7 GPa.

Fracture Properties of Silicon Carbide Thin Films by Bulge Test of Long Rectangular Membrane

This paper reports the mechanical properties and fracture behavior of silicon carbide (3C-SiC) thin films grown on silicon substrates. Using bulge testing combined with a refined load-deflection model of long rectangular membranes, which takes into account the bending stiffness and prestress of the membrane material, the Youngs modulus, prestress, and fracture strength for the 3C-SiC thin films with thicknesses of 0.40 and 1.42 mum were extracted. The stress distribution in the membranes under a load was calculated analytically. The prestresses for the two films were 322 plusmn 47 and 201 plusmn 34 MPa, respectively. The thinner 3C-SiC film with a strong (111) orientation has a plane-gstrain moduli of 415 plusmn 61 GPa, whereas the thicker film with a mixture of both (111) and (110) orientations exhibited a plane-strain moduli of 329 plusmn 49 GPa. The corresponding fracture strengths for the two kinds of SiC films were 6.49 plusmn 0.88 and 3.16 plusmn 0.38 GPa, respectively. The reference stresses were computed by integrating the local stress of the membrane at the fracture over edge, surface, and volume of the specimens and were fitted with Weibull distribution function. For the 0.40-mum-thick membranes, the surface integration has a better agreement between the data and the model, implying that the surface flaws are the dominant fracture origin. For the 1.42-mum-thick membranes, the surface integration presented only a slightly better fitting quality than the other two, and therefore, it is difficult to rule out unambiguously the effects of the volume and edge flaws. [2007-0191].

Fracture Properties of LPCVD Silicon Nitride and Thermally Grown Silicon Oxide Thin Films From the Load-Deflection of Long

The bulge test is successfully extended to the determination of the fracture properties of silicon nitride and oxide thin films. This is achieved by using long diaphragms made of silicon nitride single layers and oxide/nitride bilayers, and applying a comprehensive mechanical model that describes the mechanical response of the diaphragms under uniform differential pressure. The model is valid for thin films with arbitrary z-dependent plane-strain modulus and prestress, where z denotes the coordinate perpendicular to the diaphragm. It takes into account the bending rigidity and stretching stiffness of the layered materials and the compliance of the supporting edges. This enables the accurate computation of the load-deflection response and stress distribution throughout the composite diaphragm as a function of the load, in particular at the critical pressure leading to the fracture of the diaphragms. The method is applied to diaphragms made of single layers of 300-nm-thick silicon nitride deposited by low- pressure chemical vapor deposition and composite diaphragms of silicon nitride grown on top of thermal silicon oxide films produced by wet thermal oxidation at 950degC and 1050degC with target thicknesses of 500, 750, and 1000 nm. All films characterized have an amorphous structure. Plane-strain moduli Eps and prestress levels sigma0 of 304.8 plusmn 12.2 GPa and 1132.3 plusmn 34.4 MPa, respectively, are extracted for Si3N4, whereas Eps = 49.1 plusmn 7.4 GPa and sigma0 = -258.6 plusmn 23.1 MPa are obtained for SiO2 films. The fracture data are analyzed using the standardized form of the Weibull distribution. The Si3N4 films present relatively high values of maximum stress at fracture and Weibull moduli, i.e., sigmamax = 7.89 plusmn 0.23 GPa and m = 50.0 plusmn 3.6, respectively, when compared to the thermal oxides (sigmamax = 0.890 plusmn 0.07 GPa and m = 12.1 plusmn 0.5 for 507-nm-thick 950degC layers). A marginal decrease of sigmamax with thickness is observed for SiO2, with no significant differences between the films grown at 950degC and 1050degC. Weibull moduli of oxide thin films are found to lie between 4.5 plusmn 1.2 and 19.8 plusmn 4.2, depending on the oxidation temperature and film thickness.

\hbox{Si}_{3}\hbox{N}_{4}

Sn-rich Au?Sn solder bonding has been systematically investigated for low temperature wafer-level hermetic packaging of high-end micro-electro-mechanical systems (MEMS) devices. The AuSn2?phase with the highest Vickers-hardness among the four stable intermetallic compounds of the Au?Sn system makes a major contribution to the high bonding strength. A maximum shear strength of 64 MPa and a leak rate lower than 4.9???10?7?atm-cc?s?1?have been obtained for Au46Sn54 solder bonded at 310??C. In addition, several routines have been used to effectively inhibit the solder overflow and preserve a good bonding strength and water resilience: producing dielectric SiO2?structures which do not wet the melting solder to the surrounding bonding region, reducing the bonding pressure, and prolonging the bonding time. This wafer level bonding technique with good hermeticity can be applied to MEMS devices requiring a low temperature package.

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This paper presents an effective method to restrain galvanic corrosion of polycrystalline silicon (polysilicon) that is electrically coupled with noble metals of microelectromechanical systems (MEMS) devices by hydrofluoric-acid (HF)-based solutions. A titanium (Ti) redox sacrificial layer is added on the noble-metal layer and then covered by photoresist. Benefiting from the lower electrochemical potential of Ti than that of polysilicon in HF-based solutions, Ti is preferentially corroded in HF-based solutions, and the polysilicon is well protected. The thickness of the Ti layer should be optimized for effectively suppressing galvanic corrosion of polysilicon; a 50-nm-thick Ti film is able to preserve the resistivities of polysilicon unchanged in concentrated HF (49% HF by weight percent) solution for 1 h. This approach is simple and compatible with MEMS batch-fabrication technology and provides a solution for the longstanding issue in microfabrication technology, i.e., galvanic attack to the polysilicon structural layer by HF-based solutions.

\hbox{SiO}_{2}/\hbox{Si}_{3}\hbox{N}_{4}

A novel microcantilever biosensor was batch-fabricated with IC compatible MEMS technology for joint detection of liver cancer biomarkers with high sensitivity, high throughput, high specification, and good precision. A micro-cavity was designed in the free end of the cantilever for local antibody-immobilization using micro printing system, which can dramatically reduce the effect of adsorption-induced k variation. A linear relationship between the resonance frequency shift and the antigen concentration was observed for three liver cancer biomarkers, AFP, GGT-2, and HGF. In addition, the presented immunosensing method has little cross-reactivity to different antigen, paving the way to a highly specific technique. These approaches will promote clinical application of the cantilever sensors in cancer early diagnosis.

Diaphragms

This paper systematically investigates the frequency stability of a radial-contour-mode micromechanical disk resonators with high quality factor (Q-factor) and high resonance frequency. Microelectromechanical system (MEMS)-based oscillator prototype consisting of the resonator and off-chip circuit is realized for frequency reference. The sustaining circuit is designed using impedance matching networks and two op-amp stages with automatic gain control circuit. The oscillator reaches a short-term frequency stability of ±1 ppm and a medium-term frequency stability of ±5 ppm over industrial temperature range (-40 °C-85 °C), which outperforms some of the best MEMS oscillators. Meanwhile, the phase noise is -95 dBc/Hz at 10 kHz offset and 149-MHz carrier. This paper also presents a simple and effective compensation scheme by combining built-in microoven and bias voltage tuning, which can achieve a frequency stability range of 2 ppm at temperature ranges from 20 °C to 100 °C.

A wafer-level Sn-rich Au–Sn intermediate bonding technique with high strength

This paper introduces two novel approaches to effectively eliminate the influence of the scattering light from the wafer chuck and enhance the lithography precision of the SU-8 photoresist on a glass substrate. The first method is based on the complete reflection of light from Si substrate, and the second one employs materials which has low optical transparency and can achieve complete absorption of the near ultraviolet light transmitted from the SU-8 photoresist and the glass substrate. The SU-8 structures produced by these two methods have much better profiles than those made by the conventional process, and the line width deviation is smaller than 1 μm. These two routines have advantages of simplicity, low cost, therefore are applicable to batch fabrication and can significantly enhance the performance of MEMS devices.

An Effective Approach for Restraining Galvanic Corrosion of Polycrystalline Silicon by Hydrofluoric-Acid-Based Solutions

This paper presents a novel method to effectively protect the structural material polycrystalline silicon (polysilicon) from electrochemical corrosion, which often occurs when the MEMS device is released in HF-based solutions, especially when the device contains a noble metal. This corrosion seriously degrades the electrical and mechanical performance as well as the reliability of MEMS devices. In this method, a photoresist (PR) is employed to cover the noble metal, which is electrically coupled with the underlying polysilicon layer. This PR cover can effectually prevent an HF-based solution from diffusing through and arriving at the surface of the noble metal, thus cutting off the electrical current of the electrochemical corrosion reaction. The polysilicon is well protected for longer than 80 min in 49% concentrated HF solutions by a 3 µm-thick AZ 6130 PR film. This fabrication process is simple, reliable and suitable for mass production of high-end micromechanical disk resonators. Benefiting from the technology breakthrough mentioned above, a novel low-cost microfabrication method for disk resonators with high performance has been developed, and the VHF polysilicon disk resonators with resonance frequencies around 282 MHz and Q values larger than 2000 at atmosphere have been produced at wafer level.

Cantilever array sensor for multiple liver cancer biomarkers detection

The mechanical properties and fracture behavior of silicon carbide (3C-SiC) thin films grown on silicon substrates were characterized using bulge testing combined with a refined load-deflection model for long rectangular membranes. Plane-strain modulus Eps, prestress s0, and fracture strength smax for 3C-SiC thin films with thickness of 0.40 mum and 1.42 mum were extracted. The Eps values of SiC are strongly dependent on grain orientation. The thicker SiC film presents lower s0 than the thinner film due to stress relaxation. The smax values decrease with increasing film thickness. The statistical analysis of the fracture strength data were achieved by Weibull distribution function and the fracture origins were predicted.