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Capacitors are everywhere in electronics. They can be used for delaying, filtering, decoupling, converting, storing, etc. Various materials and technologies are used to manufacture components with different characteristics in terms of capacitance, leakage current or breakdown voltage depending on the requirements imposed by the application field. Silicon capacitors jointly developed by Ipdia and CEA-Leti combine high performances and a level of reliability that makes them suitable for high-end applications. They are also particularly appropriate for use under harsh environmental constraints such as those encountered in oil and gas prospection [1] thanks to their very good stability versus temperature [2]. In this paper, we present the developments carried out for stacking 10 chips in a package so as to obtain a 10μF capacitor capable of working continuously at 220°C during 1000h in a down-hole environment. For a first demonstrator, 100μm-thick capacitor dies were alternatively stacked and wire-bonded onto a ceramic substrate using 60μm-thick pieces of silicon as spacers [3]. Thermal tests (thermogravimetry, aging followed by shear tests) were performed in order to qualify the epoxy glue to be spread at the interfaces. A reverse ball bonding process using a fine pitch, straight bottleneck capillary was developed to obtain very low profile loops in order to prevent the wires from touching the dies above and beneath, which would have been critical for leakage. Optical profilometry and in-line electrical tests (C and I_leak measurements) were performed at each level to monitor the device functionality during the assembly. Finally Kovar lids were used to hermetically seal the packages by reflowing a AuSn preform under a N2 atmosphere. Hermeticity was measured by Residual Gas Analysis and a pressure vs. time projection was done, showing no significant pressure increase in the cavity during the lifetime of the device. Motivated by the good electrical results (capacitance within the specification, leakage current lower than 100nA) and to go further in the miniaturization and the robustness of the device, a second demonstrator was performed using a QFN package [4]. The same assembly strategy with “off-the-shelf” dies was applied on a lead frame including innovative patterns and designed with very aggressive dimensions so as to minimize the volume of the module. A new molding compound, evaluated in terms of weight loss and processability, was used to encapsulate the stacked and wire-bonded dies. After sawing, a module with a volume less than 44mm3 was obtained and showed equally good results as the first demonstrator in terms of capacitance and leakage current. A high capacitance, high reliability module was thus realized using capacitor dies and assembly materials compatible with high temperatures up to 220°C. This opens interesting prospects for the oil and gas industry since it allows drilling deeper and reaching currently inaccessible resources. The low volume of the capacitor also makes it suitable for other application fields with more drastic size constraints such as avionics or automotive. |
