Wafer Stepper Lithography

In light projection lithography [49-51] an optical lens system is used to project the pattern of the mask onto the photo-lacquer layer (contact-free technique). The blurring of the pattern is now only restricted to the size of the particles present on the photolacquer layer. The projection system often reduces the pattern of the mask between 1 and 10 times with respect to the projected pattern in the lacquer layer. The typical lacquer area that can be projected in one step is about 1-2 cm2. More steps (of the wafer stepper) are therefore needed to project the full wafer (3-12 inch diameter). A typical single step time is 1-2 seconds. This time cannot easily be reduced because of the mechanical repetition step time and the minimum exposure time per light shot. Although wafer steppers are very expensive (new generation >5 M euro) they are very economical in mass production applications. Also it is relatively more easy to get smaller pattern dimension (e.g., 0.12 micrometer linewidths) than contact mask applications due to the use of high resolution lenses with a large numerical aperture. See Figure 22.

Additional advantages are that the mask pattern is relatively large and can be made with relatively cheap laser writers.

Lithographic reduction has historically been accomplished by optimizing the parameters in the Rayleigh model for image resolution: In this model [53, 54], image resolution = kj/NA, and depth of focus = k2A/NA2, where A = exposure wavelength and NA = numerical aperture (k1, k2 are constants for a specific lithographic process). To pattern devices with decreasing feature sizes, photolacquer exposure wavelengths were reduced and numerical apertures were increased [55, 56]. See Table 3.

Laser Interference Lithography

Figure 22. Wafer stepper. (1) Optical adjustment system, (2) reticule, (3) telecentric lens, (4) alignment optic, (5) laser interference meter, (6) mirror, (7) wafer, (8) wafer chuck, x-y table, (9, 10) projected and unprojected pattern fields, (12) objective. Reprinted with permission from [52], D. Widmann et al., "Technologie hochintegrierter Schaltungen (2. Auflage)." Springer-Verlag, Berlin/Heidelberg/New York, 1996. © 1996, Springer-Verlag.

Figure 22. Wafer stepper. (1) Optical adjustment system, (2) reticule, (3) telecentric lens, (4) alignment optic, (5) laser interference meter, (6) mirror, (7) wafer, (8) wafer chuck, x-y table, (9, 10) projected and unprojected pattern fields, (12) objective. Reprinted with permission from [52], D. Widmann et al., "Technologie hochintegrierter Schaltungen (2. Auflage)." Springer-Verlag, Berlin/Heidelberg/New York, 1996. © 1996, Springer-Verlag.

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