Integrated heat spreader

To bring the die into good thermal contact with an integrated heat spreader, and... 

To bring the integrated heat spreader into contact with the electronic devices. 

High thermal conductivity CVD-diamondfilms deposited on heat spreaders or heat slugs to dissipate the heat of high-density integrated circuits. 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

The electrochemical etch-stop technology that produces the silicon island is rather complex, so that an etch stop directly on the dielectric layer would simplify the sensor fabrication (Sect. 4.1.2). The second device as presented in Fig. 4.6 was derived from the circular microhotplate design and features the same layout parameters of heaters and electrodes. It does, however, not feature any sihcon island. Due to the missing heat spreader, significant temperature gradients across the heated area are to be expected. Therefore, an array of temperature sensors was integrated on the hotplate to assess the temperature distribution. The temperature sensors (nominal resistance of 1 kfl) were placed in characteristic locations on the microhotplate, which were numbered Ti to T4. 

Instead of a silicon island underneath the dielectric layer, a polysilicon plate can be placed in the membrane center. Such a device was not fabricated, but the effect of a heat spreader that is integrated in the dielectric membrane was demonstrated by simulations. The results of the simulations are discussed in Sect. 4.2.2 [115,116]. 

Power modules are integrated in electric and hybrid vehicles, for example, in order to connect the transistors of the power electronics to the heat sinks. The use of power modules is typical for power levels above one kilowatt (Hensler 2012). Power modules have electrical, mechanical, and thermal functions. The silicon chip, the bonds, and the copper layer form the electrical circuit. This electrical circuit is insulated from the rest of the module by means of an electrical insulator (typically ceramic). The insulation is applied to a heat spreader that ensures that heat is distributed uniformly within the module and that the heat is thus dissipated away from the chip. The heat spreader is typically made of copper and also provides mechanical stabilization. It is connected to the heat sink by a so-called thermal grease, and the heat sink in turn is cooled by means of a cooling medium (air or liquid). 

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