  A single-mask Gas Electron Multiplier (GEM) technique overcomes the
cumbersome practice of alignment of two masks and allows the production of
foils with very large area as needed for the CMS muon forward region upgrade.
However, the holes obtained with refinements in the single-mask technique are
asymmetrically bi-conical in shape compared to symmetrically bi-conical holes
of double-mask technology. The hole geometry and their uniformity define the
performance of the detectors which are constructed with such GEM foils. To
evaluate the effect of this asymmetry, the foils have been characterized
experimentally using a special prototype with three single-mask GEM foils. The
structure allowed to change the orientation of foils, testing from above with
foils having a large hole opening, testing from the bottom with all the foils
having small hole opening. The effective gain, energy resolution and the
charging up behavior are compared for the two different hole orientations.
