Rigorous simulation of alignment for microlithography

Abstract

As the dimensions of semiconductor devices continue to shrink, mask alignment becomes increasingly important and difficult. The required accuracy of alignment is only a few percent of the used wavelengths. In optical projection lithography, the position of a wafer with respect to a mask is determined by the analysis of the light that is diffracted from an alignment mark on the wafer. The profile shape, depth, and asymmetry of the alignment mark significantly affect the observed alignment signal. In order to predict the influence of these factors, the simulation of the alignment system becomes necessary. We compare rigorous methods for the simulation of light diffraction from the alignment mark, such as the finite-difference time-domain method, rigorous coupled wave analysis, and the waveguide method (WG) to the Fresnel method that has been used in the past. It is shown that the three rigorous methods used in this study demonstrate a good convergence. For the specific geometry and material parameters of the alignment mark used in our investigations, WG is most appropriate for fast work. The difference between Fresnel and rigorous methods becomes important when the height of the alignment mark exceeds 30% of the wavelength of the alignment system

Alignment mark signal simulation system for the optimum mark feature selection

“…[1][2][3][4][5] However, the mark selection procedure using real process wafers was time consuming because of numerous experimental evaluations of the mark signal and overlay accuracy. Though mark selection is done with great care, issues concerning the mark frequently arise in production.…”

Simple tilt and height location monitoring of wafers

“…Good wafer alignment is invaluable in lithography, dicing, or bonding, as it permits the consistent application of process parameters. Endeavors in wafer alignment can be broadly classified as efforts to ͑a͒ improve wafer placement and holding, 1-3 ͑b͒ study alignment error sources, [4][5][6] and ͑c͒ monitor the wafer position. [7][8][9][10] Techniques reported thus far for wafer position monitoring include the use of ͑i͒ imaging of marks with a CCD camera, 7 ͑ii͒ imaging of marks with a series of photosensors, 8 ͑iii͒ sensing of diffracted light from marks, 5,9 and ͑iv͒ observation of moiré patterns.…”