Depth-of-focus

The depth of focus (DOF) image quality metric refers to the maximum amount of focus change that can be tolerated before the printed pattern size falls outside the specification. It is defined as the distance along the optical axis that produces an image of some suitable quality. The Rayleigh depth of focus for a feature at 

Resolution Enhancement Techniques in Optical Lithography, p. 61, SPIE Press, Bellingham, WA (2001). 

Field Guide to Optical Lithography, p. 53, SPIE Press, Bellingham, WA (2006) A.K. K. Wong, Resolution Enhancement Techniques in Optical Lithography, p. 60, SPIE Press, Bellingham, WA (2001) C.A. Mack, Fundamental Principles of Optical Lithography The Science of Microfabrication, p. 123, John Wiley Sons, Hoboken, NJ (2007). 

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Traditionally, the first instrument that would come to mind for small scale materials characterization would be the optical microscope. The optical microscope offered the scientist a first look at most samples and could be used to routinely document the progress of an investigation. As the sophistication of investigations increased, the optical microscope often has been replaced by instrumentation having superior spatial resolution or depth of focus. However, its use has continued because of the ubiquitous availability of the tool. 

Run-of-the-mill instruments can achieve a resolution of 5-10 nm, while the best reach 1 nm. The remarkable depth of focus derives from the fact that a very small numerical aperture is used, and yet this feature does not spoil the resolution, which is not limited by dilfraction as it is in an optical microscope but rather by various forms of aberration. Scanning electron microscopes can undertake compositional analysis (but with much less accuracy than the instruments treated in the next section) and there is also a way of arranging image formation that allows atomic-number contrast, so that elements of different atomic number show up in various degrees of brightness on the image of a polished surface. 

Tiefebene,/. low plain, lowland, tiefeingreiiend, a. penetrating thoro(ugb tiefen, v.t. deepen deep-draw. cup. Tiefen-gestein, n. plutonic rock, lehre, /., -messer, m. depth gage. -schSrfe, /. (Pho tog.) depth of focus. 

The depth of focus, or penetrating power, of a lens is the maximum... 

The primary purpose of using CMP in back-end interconnect processes is to planarize the surface. Hence, the depth of focus of existing photolithography tools can be extended into sub-0.35-jum technologies [1]. A question arises, however, as to how much sacrificial thickness is required for polishing away to planarize the surface. Intuitively, the more thickness polished, the better planarity achieved however, at the same time, the across-wafer final thickness nonuniformity becomes worse. This concept of the relationship between the thickness polished and the remaining step height can be understood from Fig. 11. Planarity is a local, microscopic term, which is defined as... 

Fig. 3.3. Acoustic micrographs taken in superfluid helium at 0.2 K. (a) Bipolar transistor on a silicon integrated circuit. The aluminium lines making connections to the base and the emitter are 2 fan wide and 0.5 fan thick. Three images were taken at different heights, and superimposed with colour coding. The lens had a numerical aperture N.A. = 0.625 and a depth of focus less than 150 nm, / = 4.2 GHz. (b) Myxobac-terium, with different planes similarly colour-coded and superimposed, / = 8 GHz...

A good example of the difficulties involved in accessing toxicological data of organo-fluorine compounds can be seen by examining the work of Hodge, Smith and Chen,5 published in 1963. This included results many of which were mainly preliminary toxicity data, reported in the literature until 1961. Much data that was not comparable in depth of focus, meaningfulness, mode of application, animal material, doses, and quality of research was summarized in this article. Information contained within such an article is difficult for experimental chemists to assess. Further data on the toxicity of fluoroalkanes, fluoroalkenes and fluoropolymers has been published.6... 

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