Talk:X-ray lithography
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[edit] Secondary electrons and Auger electrons
Secondary electrons have energies of 25 eV or less, and can be generated by any ionizing radiation (VUV, EUV, X-ray, ions and other electrons). Auger electrons have energies of hundreds of eV's. The secondaries (generated by and outnumbering the Auger and primary photoelectrons) are the main agents for resist exposure.
The higher energy Auger electrons tend to have shorter ranges on the order of nanometers due to the larger number of inelastic scattering states available to them. On the other hand, the secondaries follow a different trend below ~100 eV: the lower the energy, the longer the mean free path.
As they decay, primary photo-electrons and Auger electrons eventually become physically indistinguishable (as in Fermi statistics) from secondary electrons. The range of secondary electrons is always smaller than the range of primary photo-electrons or of Auger electrons. What matters for X-ray lithography is the effective range of electrons that have sufficent energy to make or break chemical bonds in negative or positive resists.
It is supposed among some electron-beam lithographers that due to a practical mean free path of ~20 nm for secondary electrons, resolution control begins to become tricky around this dimension.
However, electron-beam lithography has peculair practical difficulties, including charging.
The secondary electron mean free path results in an image blur, which is usually modeled as a Gaussian function (sigma=blur) that is convolved with the expected image. Obviously, as the desired resolution approaches the blur, the image becomes smoothed out and less well-defined.211.72.108.18 14:50, 18 May 2006 (UTC)
The blur that matters is the making or breaking of bonds during the exposure of resist. This governs the effective mean free path. It is much shorter than 20 nm as experiments show, including reference (3). Electrons with energy less than plasmon energies are not able to break bonds and they may wander as far as they can, elastically, without affecting print resolution. Most secondaries are produced by losses in the plasma energy range (~25 eV) and quickly collide with valence electrons having similar energies, so the effective secondary mean free path is short. Plasmon energies are needed for the required chemical changes. In X-ray lithography, the effective mean free path of secondaries in PMMA resist is about a nanometer and is not resolution limiting.
