H. Majjad

Dynamic determination of Young's modulus of electroplated nickel used in LIGA technique

Mechanical properties of materials involved in the fabrication of new microactuators have to be well characterized in order to be used in CAD and for the simulation of Microsystems. To achieve that goal, we present a study of the Youngs modulus E of electroplated nickel used in the LIGA technique. This mechanical parameter was obtained by the analysis of vibration frequencies of free-clamped microcantilevers. The resonant frequencies of in-plane and out-of-plane flexural modes were measured with an optical bench. The experimental results are compared to the frequencies derived from a pure elastic finite element model. The variation of the boundary conditions, in particular the description of the clamped part of the devices, leads to a good agreement between the experimental and the simulation. The correlation between these two methods leads to the determination of the Youngs modulus of the device. First results lead to an average value of 195 GPa, which is lower than the data reported for the bulk material. These results are in good agreement with our previous values obtained by steady-state bending tests and other works reported in the literature. E is insensitive to the direction of the excited mode which is characteristic of an isotropic behaviour of the electroplated metal.

Vibration maps of capacitive micromachined ultrasonic transducers by laser interferometry

A 1.8-mm /spl times/ 1.8-mm capacitive micromachined ultrasonic transducer (CMUT) element is experimentally characterized by means of optical measurements. Optical displacement measurements provide information on the resonant behavior of the single membranes and also allow us to investigate the dispersion in the frequency spectrum of adjacent membranes. In addition, higher order mode shapes are observed, showing that either symmetrical or asymmetrical modes are excited in CMUT membranes. Laser interferometry vibration maps, combined with quantitative displacement measurements, provide information about the quality and repeatability of the fabrication process, which is a basic requirement for 2D array fabrication for ultrasound imaging.

Modeling and Characterization of Lamé-mode Microresonators Realized by UV-LIGA

We report in this paper the study of a new metallic microresonator realized by UV-LIGA technique. This kind of device is excited electrostatically and takes advantage of the contour modes or Lame-modes of the structure. Design methods of such device are presented and simulated with a Finite Element Program. Details on the microfabrication process are also presented. The vibration modes are detected with an optical bench setup and preliminary electrical results are presented. A comparison between experiments and numerical predictions are finally discussed.

Low temperature bonding of interface acoustic waves resonators on silicon wafers

Surface acoustic wave (SAW) devices are widely used for high frequency wireless telecommunications. The use of interface acoustic waves (IAW) (1), also termed boundary waves, is promising to push back the limitations set by the sensitivity of the surface to disturbances. The IAW principle consists in guiding elastic waves at the interface between two materials, one of which at least is piezoelectric to allow for excitation and detection. Interface waves vanish in the depth of the substrates from both sides of the interface with the acousto-electric energy confined at the interface. From this perspective, the realisation of high frequency passive devices based on the coupling of piezoelectric transducers with semiconductor components is attractive. We developed a process which enables the bonding of a 3 inches processed lithium niobate wafer with a silicon substrate. In order to prevent thermal strain due to thermal expansion coefficients mismatch in the stack, a low temperature wafer bonding (125°C) is performed by enhancing energy surfaces thanks to chemical surface activation. Aluminium interdigital transducers (IDT) are buried inside the lithium niobate wafer. The problems inherent to surface pollution are sought to be solved by the self-encapsulation of the component. IDT resonators with 10 µm, 20 µm and 30 µm pitch were manufactured. Back side collective wet etching of vias through the Si wafer in KOH gives access to the pad contacts for measurements. We use a finite element analysis/boundary integral method (FEA/BIM) model to discuss the response of devices in Y-cut LiNbO3 wafers bonded on (100) silicon.

Prediction and measurement of boundary waves at the interface between LiNbO 3 and silicon

Interface acoustic waves (IAWs) propagate along the boundary between two perfectly bonded solids. For a leakage- free IAW, all displacement fields must be evanescent along the normal to the boundary inside both solids, but leaky IAWs may also exist depending on the selected combination of materials. When at least one of the bonded solids is a piezoelectric material, the IAW can be excited by an interdigital transducer (IDT) located at the interface, provided one can fabricate the transducer and access the electrical contacts. We discuss here the fabrication and characterization of IAW resonators made by indirect bonding of lithium niobate onto silicon via an organic layer. In our fabrication process, IDTs are first patterned over the surface of a Y-cut lithium niobate wafer. A thin layer of SU-8 photo-resist is then spun over the IDTs and lithium niobate to a thickness below one micrometer. The SU-8-covered lithium niobate wafer then is bonded to a silicon wafer. The stack is subsequently cured and baked to enhance the acoustic properties of the interfacial resist. Measurements of resonators are presented, emphasizing the dependence of propagation losses on the resist properties. Comparison with theoretical computations based on periodic finite element/boundary element analysis allows for explanation of the actual operation of the device.

6K-2 Interface Acoustic Wave Devices Made By Indirect Bonding of Lithium Niobate on Silicon

Interface acoustic waves (IAW) propagate along the boundary between two perfectly bonded solids. For a lossless IAW, all displacement fields are evanescent along the normal to the boundary inside both solids, but a variety of leaky IAWs also exist depending on the selected combination of materials. When at least one of the bonded solids is a piezoelectric, the IAW can in principle be excited by an interdigital transducer (IDT) located at the interface. However, the IDT has a finite and non vanishing thickness that must be properly taken into account in actual devices. This difficulty is probably the reason why most studies have remained theoretical and why few experiments have been reported thus far. One possibility which we have discussed at last years symposium is to bury the IDT inside one of the solids and to subsequently achieve a direct bonding onto the other solid. Another possibility is to deposit a thick layer, for instance silicon oxide, atop an IDT patterned above a piezoelectric plate. This layer must be thick enough so that interface waves are excited rather than Sezawa modes. We discuss the fabrication and characterization of IAW resonators made by indirect bonding of lithium niobate onto silicon. In our fabrication process, IDTs are first patterned over the surface of a Y-cut lithium niobate wafer. A thin layer of SU8 photoresist is then spun over the IDTs and lithium niobate to a final thickness below one micron. The viscosity of the SU8 layer is such that a uniform and flat deposition is achieved. The SU8 covered lithium niobate wafer is then bonded to a silicon wafer using a wafer bonding machine. During the bonding process, we heat the material stack at a temperature of 60degC and we apply a pressure of 10 kgmiddotcm-2 to the whole contact surface. The stack is subsequently cured and baked to enhance the acoustic properties of the interfacial resist. Measurements of resonators are presented with an emphasis on the dependence of propagation losses with the resist properties. We find in particular that the resist viscosity is about ten times larger than that of crystalline silicon. This is nevertheless quite a smaller value than could be expected for such a polymer

Design and test of new high-Q microresonators fabricated by UV-LIGA

We report in this paper the study of a new metallic microresonator realized by UV-LIGA technique. This kind of device is excited electrostatically and takes advantage of the contour modes or Lame-modes of the structure. Design methods of such device are presented and simulated with a Finite Element Program. Details on the microfabrication process are also presented. The vibration modes are detected with an optical bench set-up and preliminary electrical results are presented. A comparison between experiments and numerical predictions are finally discussed.

Excitation of acoustic waves at the interface between lithium niobate and silicon plates

We discuss the fabrication and characterization of IAW resonators made by indirect bonding of lithium niobate onto silicon. In our fabrication process, IDTs are first patterned over the surface of a Y-cut lithium niobate wafer. A thin layer of SU-8trade photo-resist is then spun over the IDTs and lithium niobate to a final thickness below one micron. The SU-8trade covered lithium niobate wafer is then bonded to a silicon wafer using a wafer bonding machine. Measurements of resonators are presented and compared with theoretical computations based on our periodic finite element/boundary element code allows for explaining the actual operation of the device.

Characteristics of a torsional resonator with two degrees of freedom fabricated by UV-LIGA

One of the authors has proposed an electrostatically driven torsional resonator with two degrees of freedom (TDF). The main characteristic of the TDF resonator, in which the electrode gap does not directly affect to inconsistencies between low voltage driving and a large range of motion, is reported. The TDF structure is also beneficial for achieving high Q values. However, size reduction was difficult because of the limitations of the fabrication process. In this study, the TDF resonator is miniaturized by the UV-LIGA (UV exposed lithography and electroplated structure) process. The process and the frequency characteristics of the resonator are reported.