selective material as a hard mask reference layer 16 by highly selective it is meant that hard mask reference layer 16 does not etch appreciably in the etch procedure used which in this instance is a plasma etch relative to the material of device feature layer 14 a suitable substance for hard mask reference layer 16 where feature layer 14 is aluminum and the etch is a plasma etch would be silicon dioxide also seen in fig1 a is photoresist layer 18 which has already been patterned fig1 b illustrates a cross section of a device 10 or critical dimension feature under construction after the hard mask reference layer 16 has been patterned it is entirely reasonable using current technology to expect that edges of the remaining photoresist pattern 18 and the edges of hard mask reference layer 16 to exactly correspond as seen in fig1 b note that no measuring step is yet required shown in fig2 a is semiconductor device or critical dimension feature 20 under construction where substrate 22 supports device feature layer 24 and photoresist layer 28 note the absence of a hard mask reference layer fig2 a shows device 20 in the stage after photoresist layer 28 has been exposed and the unwanted areas of photoresist have been stripped away in prior art methods of monitoring bias the selected critical dimension for example the width of the pattern shown as d m must be measured during the step illustrated in fig2 a this is the first discrete measuring step of at least two required in the prior art method it should also be emphasized that the measurement of the critical dimension at the photomask level d m is done by optical methods from above the feature however since.