Uncertainty analysis for phase measurement on PSM with a 193nm common-path shearing interferometer

Abstract

Scaling down the critical dimension (CD) also requires a reduction of systematic and stochastic errors in the measurement of phase shifts introduced by phase shift masks (PSM). Furthermore, in current mask design all degrees of freedom are used to satisfy the requirements and consequently different types like attenuated or alternate PSM are in use. A direct measurement of the introduced phase shift - the wave field propagating from the mask - is mandatory to further reduce the CD error budget. Although different interferometer set-ups might be used, a common path design enables a high repeatability of the measured phase values by being less sensitive to vibrations and air turbulence. Furthermore systematic errors due to different optical aberrations present in different arms of an interferometer are eliminated. A lateral shearing interferometer was realized by adding two phase gratings and one amplitude grating to a microscope working in transmission. Thus a very stable tool for the measurement of phase shifting structures at λ = 193nm was obtained. The repeatability of the phase measurement is less than 2π/1300. A simple evaluation can be used if the lateral restricted phase shifting region is doubled, i.e. the phase ridge is compared with a neutral surrounding area. This situation of a totally sheared object is obtained if the amount of the shear s is larger than the object's lateral distance d. Moreover, this method provides a larger number of measured phase values compared to the case of differential phase contrast. To reduce the contribution of the visibility to the uncertainty budget the spatial degree of coherence was tailored to obtain a visibility V > 0,8 even at large shear distances. The 5-phase algorithm was used to introduce the reference phase steps Δφ i . A model based description of the measurement was made. The uncertainty of the measured phase value of the introduced interferometer is |Δφ max | = 2π/330 if an optimal alignment of the system is achieved. This value can be reduced by a factor of 2 if the dependence of the phase shifting algorithm on the measured phase value is taken into account. A further reduction can be obtained if an intensity monitoring is implemented. Measurements performed at different adjustment states are given to discuss e.g. the influence of the adjustment of the two phase gratings on the uncertainty budget. The estimation of the phase values and the dependence of this values on the layout of the object, respectively the surrounding area will be discussed. Although the results obtained - due to the prototype status - are not yet completely satisfactory, the described analysis proves the potential of the newly developed interferometer for further reduction of uncertainty and possible use as a reference phase measurement instrumentation.