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December 2005
library  >  PAPERS  >  Design

Highly emissive ion beam textured surfaces for improved cooling of electronic devices

continuous improvements in the cooling of electronic systems will be required to keep pace with the rapid development of higher power chips. currently, effective cooling relies (in part) on high surface area heat exchangers. further improvements could be realized by increasing the surface area of these devices. however, increasing the size of heat exchanger components is not a viable option, since system space is extremely limited. therefore, creative methods must be developed to increase effective surface area, without increasing the size of the parts. microscopic texturing is one such method.


microscopically textured surfaces not only increase surface area, they can also substantially improve emissivity (radiative cooling), if surface features are of the correct size and shape. although conventional microscopic texturing processes (e.g. chemical etching) can be used to increase surface area, they do not provide sufficient control over surface feature morphology to permit formation of optimal surface structures.


ion beam bombardment is used to selectively etch materials in applications ranging from electronic substrate patterning to the creation of high-surface-area pacemaker electrode tips. it is based on a phenomenon known as sputtering, which is the process of removing material from a surface at the atomic level due to collisions between energetic (kev) ions and substrate atoms. since it is done on so fine a scale, the process provides a great degree of control over surface features. in principle, it can be used to create almost any type of surface topography that is desired, whether smoother or rougher than the original surface. examples of ion beam textured surface are shown in figure 1.



figure 1. examples of ion beam textured surfaces. a wide range of different surface features can be produced using the ion beam texturing process.



a successful application of ion beam texturing has been the creation of highly textured, light absorbing surfaces for telescope baffles. in this application, a number of different materials, such as titanium and aluminum, have been textured to the degree that they absorb almost all incident light. according to kirchhoff's law, a highly light absorbing surface is also highly emissive (at the same wavelengths that it absorbs). figure 2 shows a total hemispherical reflectance spectrum from an ion beam textured aluminum surface. this material absorbs over 90% of the incident radiation. such a surface would also be a highly effective emitter of radiation across these wavelengths.


the reason that surfaces texture under ion bombardment is that the sputtering process is extremely sensitive to substrate material variations. in general, different materials sputter, or erode, at different rates. even different phases of a given material or different crystal orientations can exhibit different sputtering rates. therefore, any material that is not perfectly homogeneous, either in elemental or phase composition, will form a "natural" texture that is dependent on composition and distribution of substrate atoms. this texture may be enhanced or further controlled by "seeding" the substrate with impurities that sputter at a slower rate than the substrate material.



figure 2. total hemispherical reflectance for an ion beam textured aluminum surface. this particular surface absorbs over 90% of the incident light across a wide range of wavelengths. the spectrum implies that the surface is also highly emissive. a highly textured surface, such as the one that produced this spectrum, would be ideal for electronic cooling due to significantly increased surface area (enhanced convective cooling) and increased emissivity (enhanced radiative cooling).



the size, shape, and distribution of surface features formed by ion beam texturing also depends strongly on the treatment conditions, such as the type of ion that is used to bombard, the surface and its energy, the level of the vacuum in the treatment system, the partial pressures of impurity gasses that are present, etc. these factors, when properly controlled, can be used to tailor surface features to obtain a desired morphology.


for electronic cooling applications, a surface with features that increase emissivity in the infrared should significantly increase the efficiency of radiative cooling. typically, surface features of a given size most effectively absorb radiation with wavelength about the same size as the features. therefore, creating a surface with features ranging from approximately one to ten micrometers should very effectively emit infrared radiation (heat).


at the same time, the increased surface area provided by the treatment should also enhance the efficiency of convective cooling. such a treatment has potential for creating high efficiency, low-profile heat exchangers for use in high-end immersion cooling applications.


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