Tightening requirements on resolution, CD uniformity and positional accuracy push the development of improved
photomask blanks. One such blank for 45nm node attenuated phase shift masks (att-PSM) provides a thinner chrome
film, TF11, with a higher etch rate compared to previous generation NTAR5 att-PSM blanks from the same supplier.
FEP-171, a positive chemically amplified resist, is commonly used in mask manufacturing for both e-beam and DUV
laser pattern generators. TF11 chrome allows the FEP-171 resist thickness to be decreased at least down to 2000 Å while
maintaining sufficient etch resistance, thereby improving photomask CD performance. The lower stress level in TF11
chrome films also reduces the image placement error induced by the material.
In this study, TF11 chrome and FEP-171 resist are evaluated with exposures on a 248 nm DUV laser pattern generator,
the Sigma7500. Patterning is first characterized for resist thicknesses of 2000 Å to 2600 Å in steps of 100 Å, assessing
the minimum feature resolution, CD linearity, isolated-dense CD bias and dose sensitivity. Swing curve analysis shows a
minimum near 2200 Å and a maximum near 2500 Å, corresponding closely to the reflectivity measurements provided by
the blank supplier. The best overall patterning performance is obtained when operating near the swing maximum. The
patterning performance is then studied in more detail with a resist thickness of 2550 Å that corresponds to the reflectivity
maximum. This is compared to the results with 2000 Å resist, a standard thickness for e-beam exposures on TF11. The
lithographic performance on NTAR5 att-PSM blanks with 3200 Å resist is also included for reference. This evaluation
indicates that TF11 blanks with 2550 Å resist provide the best overall mask patterning performance obtained with the
Sigma7500, showing a global CD uniformity below 4 nm (3s) and minimum feature resolution below 100 nm.