Contact and Proximity Printing

Conventional proximity maskaligners offer a cost-efficient alternative to expensive projection optics in (sub)micron photolithography. The simpler contact and proximity printing is frequently utilized at universities, research centers, and also small and medium enterprises (SMEs). There is no need of the expensive projection lens system in proximity printing. The wafer with a deposited photoresist layer is brought in a close distance of several microns to the mask. The lithographic image inside the photoresist is created as the shadow of the structured mask.

Proximity printing problems include mask areas up to 200 microns x 200 microns and resist thicknesses up to 100 microns. Moreover, usually partial coherent and broadband illumination is used. Rigorous electromagnetic modeling is generally not suitable for the simulation of such large problems.

The Fraunhofer IISB in-house package Dr.Fresnel offers fast and robust approximate methods for the simulation of mask proximity printing. Comparisons with rigorous electromagnetic simulations have shown that the approach meets the precision requirements of the modeling of mask proximity printing.

Features of Dr.Fresnel are:

  • Different scalar and vector near-field diffraction approximations
  • Handling of 2D (lines/spaces) and 3D (polygons, ellipses) mask geometries
  • Illumination with partial coherent broadband light
  • Simulation of photoresist bleaching
  • Mask optimization, process window analysis

The coupling of Dr.Fresnel with existing modules of Dr.LiTHO enables one to simulate the entire lithographic process including imaging, resist development and analysis of resist profiles.

Diffracted intensity behind a hexagon-shaped aperture
Diffracted intensity behind a hexagon-shaped aperture
The images show diffraction profiles behind a hexagon-shaped aperture.