Heat spreaders

In all cooled appliances, the heat from the device s heat sources must first arrive via thermal conduction at the surfaces exposed to the cooling fluid before it can be transferred to the coolant. For example, as shown in Fig. 2.2, it must be conducted from the chip through the lid to the heat sink before it can be discharged to the ambient air. As can be seen, thermal interface materials (TIMs) may be used to facilitate this process. In many cases a heat spreader in the form of a flat plate with high thermal conductivity may be placed between the chip and the lid. 

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 membranes of the microhotplates were released by anisotropic, wet-chemical etching in KOH. In order to fabricate defined Si-islands that serve as heat spreaders of the microhotplate, an electrochemical etch stop (ECE) technique using a 4-electrode configuration was applied [109]. ECE on fully processed CMOS wafers requires, that aU reticles on the wafers are electrically interconnected to provide distributed biasing to the n-well regions and the substrate from two contact pads [1 lOj. The formation of the contact pads and the reticle interconnection requires a special photolithographic process flow in the CMOS process, but no additional non-standard processes. 

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]. 

C/ tm. With a Si-island underneath, the temperature homogeneity is further improved. For a comparable device with a Si heat spreader a relative deviation of less than 2% equivalent to a temperature gradient of 0.07 °C/pm at 300 °C in the active area was achieved (see Sect 4.4.4 and [81]). 

Another possibihty to improve the temperature homogeneity is to introduce an additional polysiHcon plate in the membrane center. The thermal conductivity of polysilicon is lower than that of crystalline siHcon but much higher than the thermal conductivity of the dielectric layers, so that the heat conduction across the heated area is increased. Such an additional plate constitutes a heat spreader that can be realized without the use of an electrochemical etch stop technique. Although this device was not fabricated, simulations were performed in order to quantify the possible improvement of the temperature homogeneity. The simulation results of such a microhotplate are plotted in Fig. 4.9. The abbreviations Si to S4 denote the simulated temperatures at the characteristic locations of the temperature sensors. At the location T2, the simulated relative temperature difference is 5%, which corresponds to a temperature gradient of 0.15 °C/pm at 300 °C. 

The conclusion from the results of this chapter is, that a sihcon island fabricated by ECE is not absolutely necessary, if a relative temperature difference of 5% within the active area between the electrodes is acceptable. A microhotplate with a dielectric membrane and a polysilicon heat spreader in the center features sufficient temperature homogeneity. Moreover, the tin-oxide droplet serves as additional heat spreader and smoothes out temperature gradients. 

In conclusion, a simple KOH-etching process without ECE is applicable for future microhotplate designs, although the best temperature homogeneity is achieved with the silicon island heat spreader. The island remains an important design feature, especially for the use of thin-film sensitive layers, where the additional heat spreading effect of the sensor materials is small. 

Some of the present industrial uses of diamond coatings include cutting tools, optical windows, heat spreaders, acoustic wave filters, flat-panel displays, photomultiplier and microwave power tubes, night vision devices, and sensors. Because its thermal conductivity and electrical insulation qualities are high, diamond is used for heat sinks in x- ray windows, circuit packaging, and high-power electroific devices. Moreover, the high chemical stability and inertness of diamond make it ideal for use in corrosive environments and in prosthetic devices that require biocompatibility. 

Lefevre E., Revelin R.,. Lallemand M, (2003), Theoretical Analysis of Two - phase Heat Spreaders with Different Cross - section Micro Grooves, The Preprints of the 7 International Heat Pipe Symposium, October 12 - 16, Jeju, Korea, 97 - 102... 

Kang S., Tsai S., Ko M., (2004), Metallic micro heat pipe heat spreader fabrication. Applied Thermal Engineering, 24,299 - 309. 

Hear Sink, Interface Material Heat Spreader. Chip Substrate Epoxy Interface... 

Heat spreader Effective and reliable Advanced MEMS heat spreader needed... 

At the chip level, thermal management solutions need high performance heat spreaders to minimize thermal contact resistance ... 

Diamond has the highest thermal conductivity when compared to other substances. For type la natural diamond the thermal conductivity is about 2000-4000W/m/K. For type Ila it is up to 17500W/m/K. A major problem in heat transfer is an effective heat sink that depends on the effective contact area, forces between both the materials, and the gap interface. Diamond exhibits the best characteristics for a heat spreader, which is the interface material that transfers heat between heat source and heat sink. 

At a microscopic level the contact surface is restricted by peaks and valleys and even highly polished surfaces may exhibit a high peak to valley ratio. This makes it necessary to use an extremely flat contact surface between the heat source and thermal spreader to guarantee an efficient transfer of heat. Chemical vapor deposition diamond with a thickness of 1000 pm has been used for Multi Chip Modules (MCM) for this purpose. Heat spreaders are used in the electronic industry for IC packaging and solid-state lasers. 

Assuming Mi = Ri, M2 = V or H, where R, Fi, and H are entire thermal resistance in the main heat spreader s volume, and its real volume or its height (thickness), the key issue of the method appears as determination... 

P. Hui and H. S. Tan, Temperature Distributions in a Heat Dissipation System Using a Cylindrical Diamond Heat Spreader on a Copper Sink, J. Appl. Phys. (75/2) 748-757,1994. 

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