a single mask 34 may be sufficient to shield the substrates 20 and or that a single concavity 46 may be effective for shielding the three dimensional features 28 from the plasma 26 for example a single mask 34 having a single concavity 46 extending about the periphery of the mask 34 may be effective for shielding the lower surface 20 a of the substrates 20 from the reactive species in the plasma 26 with continued reference to fig2 4 the upper frame 36 is positioned on the substrates 20 held by the masks 34 the mass of the upper frame 36 applies a downward force that secures the substrates 20 and masks 34 with the lower plate 32 the upper frame 36 includes equidistant parallel ribs 48 extending between two opposite sides of the generally rectangular opening defined inside the annular peripheral portion 44 which divide this space inside the peripheral portion 44 into a plurality of windows 50 when the shielding assembly 30 is assembled the ribs 48 are generally positioned between adjacent substrates 20 cross members 52 function to strengthen the upper frame 36 and only cover portions of the substrates 20 for which plasma treatment is not required or desired the specific location of the cross members 52 will depend upon the arrangement of the three dimensional features 28 on substrate 20 and will operate to divide the windows 50 into even smaller windows the present invention contemplates that the upper frame 36 may be constructed to deliberately shield areas of the upper surface 20 b of the substrate 20 from the plasma key pins 54 in the diagonal corners of the upper frame 36 are registered with corresponding key bores 56 in the lower plate 32.