1. Field of the Invention
This invention relates to a heat sink, an electronic device, a method of manufacturing a heat sink, and a method of manufacturing an electronic device. More particular, this invention relates to a heat sink for discharging heat generated by the operation of an electronic device, a method of manufacturing such a heat sink, an electronic device including such a heat sink, and a method of manufacturing such an electronic device.
2. Description of the Related Art
With the progress in the field of semiconductor manufacturing technology, higher integration of semiconductor elements and a higher speed operation of the circuits have been achieved, leading to rapid improvement in performance of semiconductor elements. In contrast to such technical developments, on the other hand, the heat density (i.e. the amount of heat generated per unit area) of a semiconductor substrate on which a semiconductor element is disposed has tended to increase, which has caused a demand for a method of cooling a semiconductor substrate with high efficiency.
As a method for cooling a semiconductor substrate described above, technology in which a material with high heat conductivity (such as carbon, graphite, metal, ceramic, diamond, or the like) is disposed on a semiconductor substrate is generally known (see Japanese Published Unexamined PCT Patent Application (Kohyo) No. 2002-519846, for example). In particular, diamond has very high heat conductivity, and single crystal diamond has about 5 times, and polycrystalline diamond has about 2.5 times as high heat conductivity as that of copper whose heat conductivity is the highest among general industrial materials. Consequently, diamond has been a focus of great attention as a material for forming a heat sink in the field of semiconductor lasers and microwave oscillators or the like which generate a high amount of heat.
Further, diamond is insulating when no impurities are doped therein. Accordingly, with a diamond film being formed on a top surface of a semiconductor substrate, rather than on a rear surface thereof, it is possible, in principle, to cool the substrate using this diamond substrate.
A heat generation portion of a semiconductor substrate is an active element portion where an active element is mainly formed, and this active element portion locally exists on the surface of the semiconductor substrate. Accordingly, by forming a diamond film having an insulating property and high heat conductivity on the surface of a semiconductor substrate, heat generated from the heat generation portions can be transferred to non-heat generation portions through the diamond film without affecting the operation of the semiconductor element. As a result, the temperature of the semiconductor substrate can be made uniform over the entire surface. As described above, it is important to reduce the temperature of the active element portions (heat generation portions) in order to secure the normal operation and the electrical reliability of the semiconductor element. In this regard, the temperature of the active element portions can be reduced by making the temperature of the semiconductor substrate uniform over the entire surface.
However, the method of cooling a semiconductor substrate by a diamond layer formed on the semiconductor substrate described above has the following disadvantages. Specifically, in the semiconductor manufacturing process, the process of forming a diamond thin film is often introduced after forming semiconductor elements on the semiconductor substrate and then forming a wiring for electrically connecting the semiconductor elements. In general, a diamond thin film can be formed by using a CVD (Chemical Vapor Deposition) method, with the temperature of the substrate being approximately 800° C. However, when an aluminum alloy, for example, is used as the material of the wiring, the wiring, and therefore the semiconductor element, would be damaged at the time of forming the diamond thin film, because the melting point of an aluminum alloy is about 600° C. In addition, due to the fact that the coefficient of thermal expansion of a diamond thin film is smaller than that of silicon, a stress resulting from a temperature cycle is generated between the diamond thin film and the semiconductor substrate, which causes damage on the semiconductor substrate having a lower mechanical strength than the diamond thin film.
On the other hand, nanodiamond thin films have attracted an attention in recent years. While the diamond thin film described above is a polycrystalline film having crystal grains with a grain size in units of micrometerss (μm), a nanodiamond thin film, although it is also a polycrystalline film, contains crystal grains with a grain size in units of nanometers (nm). In addition, nanodiamond thin films can be formed with the temperature of a substrate being 400° C. or lower. Namely, use of a nanodiamond thin film as a heat discharging route would provide an advantage that, in the semiconductor manufacturing process, a diamond thin film can be formed after forming a semiconductor element on the semiconductor substrate and further forming a wiring, with the temperature of the substrate at the time of film formation being set to a temperature equal to or below the melting point of the wiring material. However, while the nanodiamond thin film can be formed at a low temperature, the heat conductivity of nanodiamond, which is the most significant, is low. This is because a nanodiamond thin film contains crystal grains with a small grain size and therefore has a large number of grain boundaries in the heat transmitting route, where propagation of lattice vibration is prohibited. It is therefore difficult to use a nanodiamond thin film to cool the semiconductor device.