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An original three-step etch process is used in order to beable to seal the vapor chamber either with a transparent plate(pyrex or glass) for microfluidics flow observation andmeasurements, or with a second Si wafer, previously wickpatterned for producing the heat spreader.Figure 2 schematically presents the working principle ofthe heat spreader: the vapors produced in the proximity of theheat source are flowing towards the cooled walls where theycondense, insuring the heat transport. The capillary wickinsures the pumping of the condensed liquid back to theheated zone by the pressure drop between the hot and the coldends of the channels (difference of the radius of the liquid tovapor meniscus inside the channel between the evaporator andthe condenser).Capillary wick pumpingHeat sourceEvaporation24-26 September 2008, Rome, Italy100µmTransversalgaugeLongitudinalgaugeFigure 3 Top view of the sensors, showing the resistors (the stressgauges) and the contact pads.As we can see from Figure 9, a good pressure sensitivityis obtained when using an SOI substrate [8] to micromachine apressure sensor with an ultra-thin membrane of 1.1µm and asize of 150x150µm². The linearity in sensor response is verygood whit an average sensor response of 0.55mV/kPa [9].30Vapor chamberCooling plateCondensationChamber wallsFigure 2 Working principle of the vapor chamber heat spreader.As shown in Figure 1, the pressure sensor membrane ismicromachined in the heat spreader wall in order to be able tomeasure accurately the pressure inside the cavity. Thereforethe membrane size is dependent on the channel width, whichin return has to be optimized in order to provide an efficientliquid pumping [4]. For the specific heat spreader dimensions(Figure 1 – vapor cavity of 5cm in length, 1cm in width and250µm in height) a value of 160µm has been found as channelwidth optimum for a maximal heat flux transfer (using wateras cooling fluid) [5]. This small value of the membrane arearequires an ultra-thin membrane to get a significant deflection,comparable to standard pressure sensors using larger andthicker membranes.The sensitive elements are piezo-resistors consisting ofdoped regions in crystalline Si (Figure 3), connected in aWheatstone bridge configuration. When a pressure is appliedon the membrane, the in-plane stress modifies the carriermobility, changing the electrical resistance of the gauges andtherefore producing a nonzero output voltage at the bridgeoutput [6].The resulting relative resistance variation of eachindividual stress gauge as a function of the in-planemechanical stress is given by [7]:ΔR= πlσl+ πtσtRwhere π l et π t are the longitudinal and transversalpiezoresistive coefficients and σ l and σ t are the longitudinaland transversal components of the mechanical stress in themembrane, relative to the current flow direction.Output voltage (V)25201510500 10 20 30 40 50Pressure (kPa)T=27°CT=80°CFigure 4. Temperature dependent sensor response (C=9x10 18 at/cm 3 ).The silicon integrated heat spreader was achieved using athree-step deep reactive ion eching process. In Figure 5 we canobserve a top view of the backside of the wafer, showing themicrochannels and the holes respective to the pressure sensors.LongitudinalchannelAPressure sensoropeningRefillingchannelSchematic A-A'transversal cut200µmFigure 5. Optical image of the integrated pressure sensors into themicrochannels.Due to the small size of the pressure sensors membranes,their low thickness and the small deformation under appliedpressure, the optical profilometry is the most appropriatetechnique for their characterization (Figure 6), their thicknesscan be also measurable using the visualization of theirvibrational modes in addition to their static deformation. Thepressure sensors membrane deflection was thus measured as afunction of the differential pressure applied, a good linearitybeing observed in the sensor response.A'©EDA Publishing/THERMINIC 2008 174ISBN: 978-2-35500-008-9