C. Gui

The effect of surface roughness on direct wafer bonding

A theory is presented which describes the initial direct wafer bonding process. The effect of surface microroughness on the bondability is studied on the basis of the theory of contact and adhesion of elastic solids. An effective bonding energy, the maximum of which is the specific surface energy of adhesion, is proposed to describe the real binding energy of the bonding interface, including the influence of the wafer surface microroughness. Both the effective bonding energy and the real area of contact between rough surfaces depend on a dimensionless surface adhesion parameter, theta. Using the adhesion parameter as a measure, three kinds of wafer contact interfaces can be identified with respect to their bondability; viz. the nonbonding regime (theta > 12), the bonding regime (theta < 1), and the adherence regime (1 < theta < 12). Experimental data are in reasonable agreement with this theory

Nonlinearity and hysteresis of resonant strain gauges

The nonlinearity and hysteresis effects of the electrostatically activated voltage-driven resonant microbridges have been studied theoretically and experimentally. It is shown that in order to avoid vibration instability and hysteresis to occur, the choices of the ac and dc driving voltages and of the quality factor of a resonator, with a given geometry and choice of materials, are limited by a hysteresis criterion. The limiting conditions are also formulated as the hysteresis-free design rules. Expressions for the maximum allowable quality factor and maximum attainable figure of merit are given. Experimental results, as obtained from electrostatically driven vacuum-encapsulated low-pressure chemical-vapor deposition (LPCVD) polysilicon microbridges, are presented and show good agreement with the theory.

Spontaneous direct bonding of thick silicon nitride

Wafers with LPCVD silicon-rich nitride layers have been successfully direct bonded to silicon-rich nitride and boron-doped silicon surfaces. A chemical - mechanical polishing treatment was necessary to reduce the surface roughness of the nitride before bonding. The measured surface energies of the room-temperature bond were comparable to values found for Si - Si hydrophilic bonding. A mechanism similar to this bonding is suggested for silicon nitride bonding.

Present and future role of chemical mechanical polishing in wafer bonding

Almost all direct wafer bonding has been conducted between chemical-mechanically polished substrates or between thin films that were present on top of the polished substrates. Introducing chemical mechanical polishing in the wafer bonding will make a large range of materials suitable for direct wafer bonding, which has found and will find more, applications in integrated circuits, integrated optics, sensors and actuators, and microelectromechanical systems.

Fabrication of multi-layer substrates for high aspect ratio single crystalline microstructures

This paper reports a new method for making multi-layer substrates (MLS) for high aspect ratio single crystalline movable microstructures using a group of technologies, such as direct wafer bonding (DWB), chemical mechanical polishing (CMP), and reactive ion etching (RIE). As a first example, Si-SiO2-polySi-SiO2-Si sandwich wafers were fabricated using CMP and DWB. Subsequently, free-standing micro cantilever beams and double side clamped bridges were fabricated on these sandwich wafers using a one-run self-aligned RIE process, where polysilicon was used as the sacrificial layer. Polishing and bonding of low pressure chemical vapour deposition (LPCVD) polysilicon were studied. An LPCVD Si3+xN4 polishing stop layer technique was presented to accurately control the final thickness of the device layer. The uniformity of the device layer was improved as well.

Static friction in elastic adhesive MEMS contacts, models and experiment

Static friction in shearing mode can be expressed as the product of the shear strength of the interface and the real contact area. The influence of roughness on friction in elastic adhesive contact is analyzed. Special attention is paid to low loading conditions, in which the number of contact points is small. The models are used to analyze a friction experiment in a MEMS friction meter.

A high-T/sub c/ superconductor bolometer on a silicon nitride membrane

In this paper, we describe the design, fabrication, and performance of a high-T/sub c/ GdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// superconductor bolometer positioned on a 2/spl times/ 2-mm/sup 2/ 1-/spl mu/m-thick silicon nitride membrane. The bolometer structure has an effective area of 0.64 mm/sup 2/ and was grown on a specially developed silicon-on-nitride (SON) layer. This layer was made by direct bonding of silicon nitride to silicon after chemical mechanical polishing. The operation temperature of the bolometer is 85 K. A thermal conductance G=3.3/spl middot/10/sup -5/ W/K with a time constant of 27 ms has been achieved. The electrical noise equivalent power (NEP) at 5 Hz is 3.7/spl middot/10/sup -2/ WHz/sup -1/2/, which is very close to the theoretical phonon noise limit of 3.6/spl middot/10/sup -12/ WHz/sup -1/2/, meaning that the excess noise of the superconducting film is very low. This bolometer is comparable to other bolometers with respect to high electrical performance. Our investigations are now aimed at decreasing the NEP for 84-/spl mu/m radiation by further reduction of G and adding an absorption layer to the detector. This bolometer is intended to be used as a detector in a Fabry-Perot (FP)-based satellite instrument designed for remote sensing of atmospheric hydroxyl.

Q-factor dependence of one-port encapsulated polysilicon resonator on reactive sealing pressure

Micromachined encapsulated polysilicon resonators have been fabricated in different reactive sealing pressure, 200, 50 and 20 mTorr, in order to investigate the dependence of the Q-factors on the sealing pressure. Q-factors as high as 2700 have been measured. The experimental results show that the q-factors of one-port encapsulated resonators are proportional to 1/p and the resonant frequency is independent of the sealing pressure. However, the measured Q-factors are more than two orders of magnitude lower than theoretical prediction.

Electrostatically actuated mechanooptical waveguide ON-OFF switch showing high extinction at a low actuation-voltage

This paper reports on an integrated mechanooptical waveguide ON-OFF switch, where an absorbing element is moved into and out of the evanescent field of the guided mode in order to achieve switching. For the electrostatically driven devices, an extinction ratio of 65 dB at an actuation voltage of 2.5 V has been achieved in a 9.5-mm-long device for a wavelength of 632.8 nm. The calculated mechanical fundamental resonance frequency is 1.95 kHz. The device design enables future vacuum sealing that will annihilate the squeeze damping which, in the present device, reduces the response time to 10 s. Full-wafer scale fabrication is enabled by using standard silicon technology, chemical mechanical polishing and aligned wafer bonding. The devices can he used for channel selection purposes in integrated optical sensor arrays.

Etching pits and dislocations in Si{111}

The nature of etch pits that arise during anisotropic etching in KOH on Si{111} surfaces was investigated. It was verified that bulk stacking faults in the crystal lattice give rise to deep etching pits. Other types of dislocations, of which the nature is still unclear, were also found to be present, but these do not give rise to etching pits.