Evolution of Lithographic Systems

The lithographic projection system is a relatively recent development in the field of optical design. Historically, early attempts at designing lithographic projectors results in a high NA but small FOV systems. The wavelengths used are in the blue region of the visible spectrum, where conventional glass materials can be used.

Since its early stages, improvements in the lithography techniques has been partly driven by the extrapolation of Moore’s observation in 1965, that the number of compo-nents on an integrated circuit chip would double every year reaching roughly 1000 times to what it was by 1975, [9] and indeed for the first decade the semiconductor industry kept up with the trend. However for future decades to come the trend was proven to be too difficult and optimistic for the industry to follow, and Moore himself made adjustments to his observations extrapolated a gentler slopes for future semiconductor manufactur-ers. [10] [11] Of course, simply by common sense would dictate that Moore’s Law cannot continue forever, however as the years advance the market and the industry has seen an ever increasing need for higher lithography resolution.

This need for higher and higher resolution pushed later development towards shorter wavelengths, with increasing NA and FOV, and consequently tighter performance require-ments. Among the designs, some special cases exist which are quite insightful in terms of optical system design. In particular, the designs of a field flattener, and the design of a chromatic aberration corrector with only one available glass material. The technol-ogy and unerstanding of several aspheres within one system, as well as the principles of catadioptric and mirror systems have been possible as a result of these developments.

More physical optical questions occur during the development of the various

gen-Figure 3: Early iterations of lithography projections.

erations of lithographic systems. The understanding of polarization, the correction and control of birefringence, the theory of simulation for high NA and artial coherent illumi-nation are all developed further within this context. Therefore, although the systems of this type are very situational, they will be discussed in more details.

Figure 3 are some examples of early lithographic projectors. [12] [13] Historically, early projectors consists of a retrofocus photographic lens at the front, and a scaled mi-croscopic lens in the rear. [44-6] The mask side will have a larger field due to the reduction ratio (usually between 0.2 to 0.25), and the wafer side has a smaller field but high NA.

Combined together, the two systems forms a complete lithographic system.

Following, in the years of development, the size of the lithography system gradually increased. To enhance the resolution (Equation 1), one possible way is to increase the NA of the projector, as can be seen in Figure4. [14] [15] However, systems with larger NA generally results in more severe geometric aberrations, which results in the need for lithographic systems to house more and more lenses in the hope to correct the aberrations.

To help supressing the aberrations, the angle of refraction at each interface are kept to a minimum, which results in the characteristic smooth bulges and waists in lithographic systems.

Figure 4: Further developments with increasing NA, for higher resolution.

Aspheric Lenses and Immersion

Beginning around year 2000, the need to reduce the size of the lenses and progress in man-ufacturing technology allows the inclusion of aspherical surfaces inside the lithographic projection lenses. This considerably reduces the number of lenses required, and the nec-essary lens diameter of the high NA lenses compared to purely spherical systems of the same NA, as can be seen in Figure5. [16] With aspheres, the NA of dry systems can be enhanced to roughly 0.95, and if immersion fluids are used (e.g. water), increasing the NA beyond 1.0 is possible. [17]

Catadioptric Systems

Since high NA system leads to large lens diameters in the rear lens groups, problems occur in correcting aberrations and obtaining a good uniformity in the material. Immersion sys-tems are particularly different in their behavior. For this reason, the catadioptric syssys-tems, shown in Figure6, have been introduced. [18] [19] [20] The mirrors help in correcting the Petzval curvature, while the Schupman principle can be used to correct axial chromatic aberrations. The size of the systems can be reduced by a considerable factor, and a NA of 1.35 can be achieved. In Figure7, a further development upon the catadioptric systems removes the need of a beam splitter cube (which are used at the cost of a 75% decrease

Figure 5: Introduction of aspheric lenses to reduce the number of lenses, and immersion techniques are employed to further increase NA (even beyond 1.0 NA),

Figure 6: The lens-mirror hybrid catadioptric systems. Intentional back-reflected paths using mirrors and beam-splitters are introduced to reduce chromatic aberration and Petz-val curvature.

Figure 7: Further developments on the catadioptric systems sacrifice portions of the im-aged field to eliminate the need for beam-splitters.

in the aerial image brightness), by trading in half of the usable FOV of the lithographic system. [21] [22] [23] [24]

Reflective Systems

The use of all reflective projection system coupled with a short wavelength source in optical lithography can be dated back over 30 years ago. Figure8shows one such early iteration of an all reflective projector, with 1:1 reduction ratio for printing features with size on the order of ∼1 µm. [1]

Table 1 shows a general trend of source wavelength usage from the early history of lithography. For higher resolution, the natural course of action would be to further reduce the wavelength of the light source, since the optical system NA can only be increased to so much, to a maximum of 1 in vacuum or ∼1.3 by immersion. Following the trend in choosing shorter and shorter wavelengths, the next wavelength after the 193 nm line is the 157 nm line. The problem of using the F₂ laser line at 157 nm wavelength is to be used in projection lithography to obtain a better resolution, only calcium flouride CaF₂can be used as the transparent material due to the absorptions in other materials.

At 157 nm, the corresponding spectral line has a poor efficiency and therefore the line narrowing cannot be achieved as it can for other longer wavelengths. Therefore the need to achromatize the projection lenses has generated some investigation into how to use diffractive elements to correct the system for chromatic aberrations. In principle, this is a possibility, however no current industrial system employs this method. The problem of stray light and the manufacturing of the microstructure tolerances to guarantee the high performance are a severe problem.

Therefore, instead of progressing to the 157 nm lithography, the advent of the immer-sion method using 193 nm lithography became the next milestone. However, this solution is only temporal, as the NA increase provided by the immersion method is limited. As the search continued for shorter wavelengths, the next possible source discovered is the 13.4 nm line in the EUV region. In the past, the decrease in wavlength has been grad-ual, however this time the wavelength is an entire magnitude lower from the previous

Figure 8: A one to one reduction ratio reflective lithographic projector curtesy of Perkin-Elmer, 1986. The intended feature size to be printed are on the order of 1 3 µmm. [1]

Table 1: The general trend of source wavelength reduction from early lithography history.

Wavelength (nm) Source

405 Hg lamp h-line

365 Hg lamp i-line

248 KrF excimer laser (DUV)

193 ArF excimer laser (VUV)

157 F₂excimer laser

(193) In conjunction with immersion technique 13.4 Syncrotron radiation or plasma source (EUV)

Figure 9: Completely reflective system designed to cater to the short wavelength EUV light source. [2]

Figure 10: An early iteration of a lithographic projector.

generation.

Due to the high absorption of light in the EUV region exhibited in most mediums, the optical system is forced to use only reflective elements (i.e. mirrors). Figure9is an example of such an all reflective optical system. [2] Aside from having only reflective ele-ments, the system must also be placed inside a vacuum environment, since the absorption at the EUV wavelengths is high even in air.