One Stop Micro

In normal powered and non-fast-laser micromachining of metals, the most commonly used wavelength/laser is a YAG, followed by green lasers.

IR lasers do a very good job of micromachining metals. However, feature sizes that can be predictably micromachined on metals are much larger than, say, using UV lasers on polymers.

Although it is common to believe that the IR wavelength (fours times that of excimers: 1064nm to 248nm) is the limiting factor, the wavelength limitation applies only to the feature-sizes that can be made. It does not apply to the straightness/cleanness/roughness of line edges. You might have difficulty in cutting out 200 micron squares from a steel shim- the corners will be rounded. What we are talking about is how clean the edges comes out- irregularities.

There are many factors that affect this outcome. One of them relates to the fact that most metlas are made of crystalline grains (more the alloy, smaller the grain size). Another more subtle factor is the finish of the shim- artifacts introduced during fabrication of the shim, like roller marks etc.
Both during laser processing as well as post-processing, grain size and other factors can play a (small) role in defining edge regularity. Simple acid-cleaning can cause exaggeration of already present irregularities.