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Fabrication of an Electrostatically Actuated Impingement Cooling Device
Keywords: Active cooling device, Fabrication, 3D die stack
This paper reports on the fabrication and characterization of an electrostatically actuated cooling device. The continuous increase of IC power density and the widespread use of 3D integration call for advanced cooling techniques. These are necessary to keep the device temperature low and to improve the reliability. In the electrostatic actuated cooling device, liquid droplets move through channels by applying a voltage. A planar version of this approach, where droplets move in microchannel parallel to the IC die, has been reported earlier. However, this technique has the disadvantage that the liquid heat up resulting in large temperature difference across the device. To overcome this problem, we propose a 3D device in which droplets are electrostatically actuated through vertical channels, perpendicular to the IC surface. The channels are formed by etching through Silicon via; and final assembly is done by stacking four dice. The electrodes on individual channels can, in principle be controlled and activated independently, thus allowing to enhance cooling efficiency in proximity of the hot spots. The main focus of the paper will be the process flow development and the fabrication of a demonstrator for an electrostatically actuated impingement cooling device. The paper will explain the main challenges encountered and novel strategies implemented during the fabrication process. The fabrication is divided into three main components: a) the base holder b) the device fabrication c) the stacking of the dice. The base acts as a reservoir to collect the coolant and provide the support for the final assembly. Coolant is in large cavities and these cavities are formed by etching through full thickness Si wafer that is bonded to carrier wafer. The first step in device fabrication is formation of various channels, with diameters ranging from 100 μm to 300 μm, by means of DRIE in a low resistivity 200mm Si wafer. Next, an oxide is deposited on the surface and the channel walls. This is followed by depositing and patterning of the metal contacts. The wafer is then temporarily bonded to a carrier wafer to allow grinding to reduce the thickness to 150 μm and to open the cavities from the backside. A low temperature oxide is then deposited on the backside of the device wafer to isolate the channels. Lastly, a polymer is deposited and patterned on the backside and this polymer is used as bonding material. Stacking of the dice starts with debonding the individual die from carrier wafer and bonding the first die to the base wafer. A second die is then bonded on top of the first one, and this is repeating for a third die to create a “tower of Hanoi” structure. To characterize the assembly, electrodes are connected to different layers to accelerate and eject the droplets. Dependence of droplet flow as a function of applied voltage value and wave form is being investigated.
Bivragh Majeed, Researcher
Leuven, VB 3001,

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