2.5.1 Contact and Proximity Printing

A simple and straight forward approach is contact printing. In contact printing, the mask is pressed against the resist-coated wafer during exposure, i.e., the optical part shown in Figure 2.3 or Figure 2.4 is missing, but the other components like the illuminator and mask are kept. An important advantage is that feature sizes as small as 0.1 $\mu$m can be made using comparatively inexpensive equipment [18]. The mask is held chrome-side down in intimate contact with the wafer. Ideally, the gap between mask and wafer goes to zero, which minimizes diffraction effects. The resolution is only limited by scattering effects that might occur inside the resist due to its finite thickness. In reality additional limitations of contact printing result from the non-uniformity of mask and wafer. This problem can be reduced by applying pressures ranging from 0.05-0.3 atm. The major disadvantage of such hard contact methods is the extremely high defect generation. Defects are generated both on the wafer and mask during every contact cycle. Mask lifetime is severely reduced and printed defect levels are increased. Therefore, contact printing is only used in device research or other applications that tolerate high defect rates.

Proximity printing avoids defect generation because a small gap ranging from 10-50 $\mu$m is introduced between mask and wafer. The separation is usually controlled by a flow of nitrogen gas. The gas flow keeps the mask away from the wafer surface. The main disadvantage of proximity printing is a severe reduction in resolution due to diffraction spreading. The achievable resolution is governed by the expression

 

$$W = k\sqrt{\lambda d_g},$$ (2.5)

whereby d_g denotes the mask-to-wafer distance, $\lambda$ is the exposure wavelength, and the technology parameter k ranges between 1-2 depending on the resist process. The square root behavior is a consequence of the Fresnel diffraction theory valid in the near field region just below the mask openings. For optical lithography, typical values are k = 1.6, $\lambda$ = 0.4 $\mu$m, and d_g = 25 $\mu$m, yielding a resolution of W = 4 $\mu$m. Resolution can be enhanced by either decreasing the gap at the risk of contact and defect generation or by reducing the wavelength. Using wavelengths in the DUV or even EUV range will not suffice for optical proximity printing to compete with projection printing. However, using X-rays with a wavelength of about 1 nm feature sizes below 0.2 $\mu$m can be produced with proximity methods. This makes 1x proximity X-ray a promising candidate for the 0.13 $\mu$m and 0.10 $\mu$m technology [3].