Fabrication of small Contact using practical pupil filtering Hoyoung Kang, Bruce W. Smith Rochester Institute of Technology Microelectronic Engineering Department 82 Lomb Memorial Dr. Rochester, NY 14545 hxk6337 @rit.edu, bwsemc@rit.edu Abstract Fabrication of small contacts is one of most important challenges of semiconductor lithography. The use of Bessel contact, pupil filtering or phase shift mask has been suggested by various groups, however those were not easily applicable. Currently, only an attenuated phase shift mask has been demonstrated as a useful enhancement approach for contact level lithography. As a practical pupil filtering method, a pellicle plane angular transmission filter is suggested. An angular transmission filter can be realized using multiple dielectric organic or inorganic coatings on glass or membrane substrates. Filter have been designed and fabricated for high NA DUV lithography and initial filter fabrication and exposure evaluation have been carried out. Lithographic results shown up to 20% improvement of minimum contact size for dense contacts, and some improvement for isolated and semi-dense contacts. With further optimization of filter, it is expected to have better results with matched lithography set with filter. Key words : lithography, pupil filter, contact 1. Introduction As the semiconductor feature sizes are reduced, there are increased demands placed on small contact fabrication, which is generally more challenging than other feature types. Dense line and spaces can be printed with strong phase shift or off axis illumination [1]. One method that can be used to enhance contact lithography is the attenuated phase shift mask. [2]. A possible enhancement method for contact level lithography is pupil-filtering [3][4][5][6]. This can be done by inserting transmission-phase filter in the pupil of projection lens. Using pupil filtering, resolution and image profile can be modified. Resolution limit of the optical lithography system is mainly governed by the wavelength and numerical aperture of projection lens. As well known theory, the image of a contact can be described using Fourier transformation methods[7]. A mask with contact size a imaged with coherent illumination of wavelength λ, the diffraction field is Besinc function shape. Near the resolution limit, the pupil width is much smaller than the width of this Besinc function and the resulting low-pass filtered diffraction pattern resembles a cylinder function. The field intensity at the center of pupil remains higher than edge of pupil, shown in Figure 1 and summarized in Table 1. The final image resembles the inverse Fourier transform of pupil and the reduced intensity at the pupil edge will produce a wider final contact image. If the intensity at the center of pupil is reduced, the width of the image can be reduced. This is the goal of both the Bessel contact approach and the attenuated phase shift mask. The Bessel contact approach is useful for large enough contacts and when contacts are sparse to have sufficient room for additional features. Pupil filtering is an obvious alternative to achieve the desired pupil modification. Inserting a transmission filter in pupil, however is very difficult because of phase errors and flatness requirements over the pupil. Any absorption by such a filter can also generate heat and can change optical properties, introducing lens aberration.