Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, Italyµm3210-1230 µm300 µm -5Figure 6. Optical profilometer 3D image of a deformed pressure sensorthin membrane.The approach consisting of integrating the temperaturesensors closest to the interior of the vapor chamber targetedtwo main objectives: to have fast in-situ measurement in orderto determine the temperature distribution along the length ofthe heat spreader (between the evaporator and condenser),with precise values taken at specific positions along the heatspreader. Two solutions represent the best choice: dopedpolycrystalline silicon thermistor and the diode (pn junction).Both devices are easily integrable, the technological stepsneeded being CMOS process technology compatible. Thedevices are simple and easy to produce and model, withreduced production costs and good reproducibility.Ud (V)1.601.501.401.301.201.101.00d275 300 325 350 375 400T (K)Figure 7. Diode temperature sensor response as a function of thetemperature.The temperature sensors consisting of a pn junction offerthe possibility of measuring the temperature locally, with anactive area of only 20x40 µm². The average measuredsensitivity for a large number of sensors is 1.91mV/K (for aconstant current of 0.5mA). The maximum value of thesensitivity (2.33mV/K) shows a good agreement with thesimulation results as well as with the values in the literature(2.5mV/K [10]), the standard deviation from linearity being inthe order of 0.8%.III. IN-SITU TEMPERATURE DISTRIBUTION MEASUREMENTSIn order to make in-situ pressure and temperaturemeasurements during the heat spreader operation, the cavitywas sealed with a glass plate provided with two openingsconceived for the air evacuation and liquid filling (Figure 8).The purpose of this type of closure is to have the most realisticconditions possible while keeping the possibility of modifyingimportant parameters for the functioning of the heat spreadersuch as the filling ratio of the cavity and the temperature ofthe hot spot. This solution also permits to have a goodvisibility of the liquid flow into the channels.-2-3-4Cooling device Heat sourceFigure 8. Experimental set-up that allows the in-situ testing of theprototypes.On Figure 9 one can observe an example of thetemperature distribution along the cavity for a hot spottemperature of 70°C, a temperature reduction of about 24%being observed for a 10% ethanol filling of the cavity.Temperature (°C)80706050403020Empty cavityEthanol 10% cavity filling0 10 20 30 40 50Position along cavity length (mm)Figure 9. Temperature distribution measured along the heatspreader –comparison between empty and 10% filled cavity [11].The heat extraction capability differences have beenevidenced using different filling ratio and different coolingliquids. In figure 10 one can observe the temperature reductionat the evaporator side for three filling ratios using ethanol andDI water and for 70°C evaporator temperature. While the 10%ethanol filing provides a reduction of 23.4% from theevaporator – condenser temperature, while the 10% H 2 Ofilling removes only about 2%.Temperature (°C)7060504030Heat source20AirEthanol 10%Ethanol 50%Ethanol 90%H 2 O 10%H 2 O 50%H 2 O 80%0 5 10 15 20 25 30 35 40 45 50Position along heat spreader (mm)Figure 10. The temperature variation as a function of the liquid type andfilling ratio.IV. CONCLUSIONSIn this contribution we present the results concerning themicromachining of a silicon integrated heat spreader equipped©EDA Publishing/THERMINIC 2008 175ISBN: 978-2-35500-008-9