M. Le Berre

Fabrication and experimental investigation of silicon micro heat pipes for cooling electronics

An experimental investigation was conducted to determine the thermal behaviour of micro heat pipe (MHP) arrays micromachined in silicon wafers. Two types of MHP arrays were tested, one with triangular channels, 230 μm wide, 170 μm deep, and the other with triangular channels, 500 μm wide, 340 μm deep, coupled with arteries. Both types of arrays were fabricated using an anisotropic etching process. Once fabricated, a plain Si wafer was used to seal the pipe array hermetically. Two working fluids were tested, ethanol and methanol. A polysilicon heater was used to supply the heat input, and cooling water flowing through the condenser was used as a heat sink. Fill charges from 0% up to 66% were tested. The axial temperature variation along the length of the pipe was measured using T-type thermocouples connected to a data acquisition system. The effective thermal conductivity was evaluated using the experimental temperature profiles and 3D thermal modelling. The results show a maximum improvement of 300% in effective thermal conductivity at high heat flux, which demonstrates enhanced heat transfer in a prototype with liquid arteries.

Magnetic properties of sputtered barium ferrite thick films

The development of devices that combine a magnetic material with a semiconductor chip is a major focus of current research. Barium hexaferrite (BaFe12O19 or BaM) thick films are deposited here using a rf sputtering system. The films are amorphous and nonmagnetic after deposition. Postdeposition thermal annealings are employed to obtain magnetic properties. The effects of the substrate, thermal annealing and thickness of BaM on the magnetic properties are studied using a vibrating sample magnetometer. The initial results show good magnetic properties for the two subtrates studied and after thermal annealing above 800 °C. The magnetic properties of the thick films are close to the bulk (BaM) ones.

Feasibility of an Integrated Self Biased Coplanar Isolator With Barium Ferrite Films

The development of passive devices using a ferrite is a major focus of current research for electronic applications in the microwave range (circulators and isolators). Hexagonal ferrite, such as barium ferrite (BaFe12O19 or BaM), which has a large resistivity and high permeability at high frequencies are indeed of great interest for microwave device applications. In this work we developed an integrated and self-biased coplanar isolator using BaM sputtered films. BaM films, 1-36 mum thick, were deposited under optimized conditions by radio frequency magnetron sputtering on alumina substrates. The films were crystallized using a 800degC thermal annealing. Isolators were then realized using patterning of coplanar waveguides (CPW) with standard lift-off technique. The slots and the central width were 300 mum wide and gold was used for the conductor lines. We evaluated the influence of various parameters on the device performances: the magnetic films thickness, the positioning of the magnetic film (CPW deposited onto the magnetic film or directly on the substrate), the CPW metallic thickness and the polarizing field. As standard design, the CPW were deposited on the top of the magnetic film. At the remanent magnetization (no polarizing field applied), the transmission coefficients then showed a non reciprocal effect, which reached 5.4 dB per cm of line length at 50 GHz for a 26.5 mum thick BaM film. Both the insertion losses and the non-reciprocal effect measured increased with the magnetic film thickness with a saturation effect. In the second design where the CPW is deposited directly on the substrate after a selective etching of the BaM film, we measured that the non reciprocal effect reached higher values for lower BaM thicknesses than for the first design and that the insertion losses also decreased. The interaction between the field lines created by the conductors and the magnetic film was indeed favored in the second case. Finally, we show the tunability of the isolator with the polarizing field.

Stress analysis at singular points of micromachined silicon membranes

From 3D Finite Element simulations of square silicon micromachined membrane, we know that stress singularities are located close to the membrane median lines. As 3D simulations do not reach a sufficient level of accuracy to describe singularity behaviour, we use an iterative method to calculate the stress field components in the immediate vicinity of the singularity with more precision. However, this implies a reliable knowledge of the stress field in the surrounding of the singularity as a limiting condition. This was achieved through the development of a 2D axisymmetric Finite Element model (FEM) with a refined meshing around the singularity. The consistency of the results of the 3D and 2D model is discussed with regards to the description of the membrane behavior in both cases. Finally, the singular stress field is described numerically and its ability to raise up the local stress field is discussed in terms of crack initiation mechanisms.

Effect of clamping conditions and built-in stresses on the thermopneumatic deflection of SiO2/Si membranes with various geometries

Abstract Built-in stresses are known to induce deflections in microstructures such as composite membranes. The thermoelastic behaviour of large square SiO 2 /Si membranes with a side length from 3 to 7.5 mm, a Si thickness in the [7.7–50] μm range and a SiO 2 thickness up to 1.5 μm, was studied by optical profilometry. A very satisfactory agreement within 10% has been found between the experimental and the simulated deflections of prestressed SiO 2 /Si membranes. However, clamping conditions have been found to play a major role in built-in stress relaxation. This tendency was confirmed by Finite Element Modelling. Oxidized (up to 1.5 μm) and bare Si membranes with thicknesses from 7.7 to 50 μm were studied under pressure in the [0–1] bar range. We observed that the membrane stiffness is affected by the oxide depending on SiO 2 /Si thickness ratio. Finally, F.E.M. results are in good agreement for various membrane geometries, and the interest of this tool for the design of packaging structures is shown.

Micro-Raman study of thermoelastic stress distribution in oxidized silicon membranes and correlation with finite element modeling

Abstract Silicon on insulator (SOI) pressure sensors show potential applications in high temperature environment. However, as a result of thermal mismatch, large stresses usually exist in the composite SiO 2 /Si membranes which would provide an otherwise promising substrate for such sensors. These stresses can significantly influence the long term reliability of such membranes. In this work a high spatial resolution (10 μm 2 ) Raman spectroscopy method has been used to measure the localized stresses over an oxidized membrane, thus yielding stress maps. The method is based on the frequency shift of the Raman line at 520 cm −1 . Shift between 0.05 and 1 cm −1 are observed. A three-dimensional commercially available finite element modeling (FEM) software (ANSYS) has been used to modelize the thermal stress distribution over the complete micromachined bilayer membrane. Its validity has been checked out through optical profilometer deflection measurements. The experimental Raman shifts were compared with those calculated using stresses from FEM and a biaxial stress hypothesis. Finally, the sensitivity of Raman stress mapping method for high temperature SOI pressure sensors is discussed.

Thermal drift of piezoresistive properties of LPCVD polysilicon thin films between room temperature and 200 °C

Abstract Investigations of the thermal drifts of the resistivity and gauge factors of polysilicon are needed to extend its use as a transducing material for high temperature piezoresistive microsensor applications. Longitudinal gauge factors of LPCVD B-doped and rapid thermal-annealed polysilicon films, patterned onto oxidized silicon using a clamped beam technique, have been measured in the 20–200 °C temperature range as a function of the doping concentration between 8 × 10 18 and 10 20 cm −3 . The gauge factors have been simulated as a function of the B-concentration and the temperature, starting from experimental data for the polysilicon resistivity and literature data for the hole resistivity and the piezoresistive effect in monocrystalline silicon. Only a model involving a significant piezoresistive effect at grain boundaries can adequately fit the temperature dependent gauge factors of polysilicon.

Effect of Thermoelastic Stress on the Pressure Response of a Composite SiO 2 /Si Membrane

The layout design of membrane-based sensors such as Silicon On Insulator micromachined pressure sensors requires the knowledge of the mechanical behavior of the complete structure as a function of pressure and temperature. Unfortunately, as a result of thermal mismatch, large stresses usually exist in composite SiO 2 /Si membranes that significantly affect the pressure response of such sensors, We present here an original study on the 3D-FEM modeling (ANSYS) of pre-stressed SiO 2 /Si membranes submitted to pressure. The model of thermoelastic stress (without any applied pressure) has been first validated by optical profilometry deflection measurements. 2.94 mm width membranes with Si thickness varying from 10.7 to 19.3 μm, as measured by FT-IR, and covered with 1.46 μm thick thermal oxide grown at 1130°C, have been studied. Thermoelastic deflections from I to 25 μm have been measured for decreasing Si thickness in agreement with the simulation. The simulated longitudinal stress relaxation over the oxide varies from 5 to 40 MPa between the membrane center and the frame when decreasing membrane thickness. Under pressure (10 and 100 mbar) and for a 19.3 μm thick composite membrane (0.5 )μm thick oxide), the calculated strains can exceed by 20% that of a bare Si one, due to a lower overall stiffness produced by the oxide compressive stress.

Influence of the Structural Parameters of Polysilicon Films on the Piezoresistive Properties at High Temperature

Polycrystalline silicon, used as a transducing material in high temperature Silicon On Insulator piezoresistive pressure sensor, is well known to be strongly dependent on the process parameters. Indeed depending on the deposition, doping and annealing conditions, the structural parameters of polysilicon namely grain size and texture may be very different. We use here a complete model of the piezoresistivity in polysilicon including a significant effect at grain boundaries to study the influence of structural parameters on the resistivities, gauge factors and their thermal drifts between room temperature and 200°C. The model validity has been verified experimentally for two types of polysilicon materials (LPCVD at 620°C and LPCVD at 580°C), subsequently B-doped (from 9×10 18 to 9×10 19 cm −3 ) and annealed by Rapid Thermal Annealing at 1100°C/20s and 1100°C/40s respectively. A good agreement between theoretical and experimentally determined resistivities and gauge factors at a variable temperature is observed for the two types of polysilicon materials considering different grain sizes with a dominant texture for LPCVD 620°C series and no preferential texture for LPCVD 580°C series.

Towards the integration of barium ferrite sputtered films for coplanar isolators and circulators in the millimeter wave range

Signal processing in communication, instrumentation and radar detection requires low-cost microwave and millimeter wave devices. The integration of millimeter wave passive components like isolators and circulators is thus a major issue. In these components, the nonreciprocal nature of wave propagation in ferrites plays an essential role. Hexagonal ferrites,such as barium ferrite (BaFe12O19 or BaM), which has a large resistivity and high permeability at high frequency, are of great interest for such applications. The present work deals with the characterization of barium hexaferrite sputtered films and with the device integration for the development of coplanar isolators and circulators working at frequencies above 40 GHz. The BaM films (in the thickness range of 10μm) were deposited on alumina substrate by RF magnetron sputtering at room temperature and were then crystallized using a 800°C thermal annealing. Structural properties were evaluated by X-ray diffraction and the magnetostatic properties of the film were determined using a Vibrating Sample Magnetometer (VSM) leading to assess a range of deposition conditions appropriate for device integration. Both coplanar isolators and circulators were implemented using standard lift-off technique. Coplanar circulators were designed by analytical calculation and 3D electromagnetic simulation (HFSS) supposing a saturated material and no losses. The microwave range characterization was performed using a network analyzer and a probing system with a prior OSTL calibration. The coplanar isolators were characterized both at the remanence and under a polarizing field. At the remanence, the gyroresonance occurred at 50 GHz and the nonreciprocal effect - difference between the transmission coefficient in the forward direction and in the reverse direction - was evaluated. When applying a polarizing field, the tunability of the isolator was verified experimentally. Finally first measurements on integrated coplanar circulators are currently under progress.