A current mirror circuit has an input portion including a first transistor, which is adapted to establish a reference current. A polarimetric scatterometer, having optics directing a polarized illumination beam at non-normal incidence onto a periodic structure on a sample, can measure optical properties of the periodic structure. The inner annulus between the shroud and screen provides an alternate flow path for the slurry to bypass the blocked intervals and continue with its placement. Therefore, a display quality of the LCD apparatus is improved.