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Introduction

In photolithography, immersion lithography is a resolution enhancement technique that interposes a liquid medium between the optics and the wafer surface, replacing the usual air gap. This liquid has a refractive index greater than one. With the 193 nm wavelength, the typical liquid used is ultra-pure, degassed water. Immersion lithography increases the effective depth-of-focus for a given numerical aperture and permits the use of optics with numerical apertures above 1.0, thus raising the maximum resolution potential of extant wavelength technologies. The depth of focus for an immersion tool is given by: DOF = (immersion index)*wavelength/(N.A.)^2, where N.A. is the numerical aperture. For a deep ultraviolet (DUV) wavelength of 193 nm, an immersion index of 1.44 for water, and a N.A. of 1.2, the depth of focus is 193 nm. By comparison, for an extreme ultraviolet (EUV) wavelength of 13.5 nm, an immersion index of 1 (i.e., vacuum), and an N.A. of 0.3, the depth of focus is 150 nm. Both systems are capable of imaging 100 nm features, but the 193 nm immersion system has the better depth of focus by virtue of longer wavelength and higher immersion index in this case.

Using an immersion fluid also confers the advantage of reducing reflections by virtue of reducing refractive index differences.

Implementation

As of 2005, it is expected that immersion lithography at the 193 nm wavelength will be used in 2009 to print 45 nm lines and spaces . Following its aggressive introduction, it is speculated that enhancements will be used to prolong the use of the technology to smaller features. Such enhancements include the use of higher refractive index materials in the final lens, immersion fluid, and photoresist. Each of these materials puts a limit on the largest angle that the light makes with the optical axis normal to the image plane. For example, by using LaF3 with a refractive index n=1.67 as the final lens material, along with a recently demonstrated immersion fluid with refractive index n=1.64, or doped water, a numerical aperture of 1.5 can be reached. This is the minimum value needed for 32 nm line-space resolution.

Constraints

As numerical apertures increase, the degree of polarization of the light becomes critical to the image quality. This is because the interference of light inside the photoresist becomes polarization-dependent. Specifically, the imaging of straight lines near the resolution limit is best done with light polarized parallel to the lines. This requires special illumination preparation which is available on the most advanced lithography systems. Imaging of holes or islands is more problematic unless the holes or islands are closely spaced near the resolution limit in one direction and widely spaced in the orthogonal direction; otherwise, polarization effects will be a hindrance rather than a benefit. The highest hole or island density is achieved by effectively superimposing two orthogonal line-space images. Preferably, these images will be polarized parallel to the lines. Due to these imaging constraints, integrated circuit (IC) layouts utilizing dimensions near the resolution limit will be required to be 'lithography-friendly'.

Photomask Impact

For immersion lithography, as the minimum resolvable half-pitch linewidth decreases, features on the photomask will eventually approach subwavelength sizes. Subwavelength features no longer obey the laws of classical imaging optics but need to be rigorously analyzed using electromagnetic theory (see for example, ). One way to delay this outcome would be to increase the magnification of the photomask image relative to the wafer image.

Resolution Extension at 193 nm

The upper limit of the numerical aperture should asymptotatically approach the refractive index of the photoresist. At this theoretical point, there would be light traveling parallel to the photoresist surface on the wafer. For a wavelength of 193 nm and a refractive index of 1.7, this would correspond to 28 nm lines and spaces. The practical upper limit for undoped water at 193 nm is around 1.3. The practical upper limit for current lens materials is around 1.4. Once the maximum numerical aperture is reached, the only way immersion lithography can print denser features at the same wavelength would be to split a dense layer into two looser layers .

Immersion at Shorter Wavelengths

The other choice is to use shorter wavelengths, e.g., 157 nm. However, it would be necessary to consider the mechanisms by which these wavelengths interact with the materials used in photolithography. Absorption is greatly enhanced, and the ionization potential is exceeded. The success of 157 nm immersion, for example, would depend on the use of materials with minimal absorption. Water itself is not transparent enough to serve as an immersion fluid for this wavelength. It also is important to make sure that when the wavelength is divided by the refractive index, it gives a sufficiently smaller value than 118 nm, which is the 193 nm wavelength divided by the refractive index of the next-generation immersion fluid at that wavelength (n=1.64). This is the measure of the degree of resolution enhancement.

Remaining Technical Concerns

Other considerations which are important to immersion lithography systems are the elimination of bubbles in the immersion fluid, temperature and pressure variations in the immersion fluid, and immersion fluid absorption by the photoresist. Degassing the fluid, carefully constraining the fluid thermodynamics and carefully treating the top layer of photoresist are key to the implementation of immersion lithography. Defects intrinsic to immersion lithography have been identified. Reducing particle generation due to the water dispensing unit should help reduce the incidence of defects. Water also has been shown to extract acid from photoresist. Specifically, photoacid generators (PAGs) are extracted into the water, which produce acid upon radiation exposure. This must be managed to ensure the lens is not corroded by the acid and the photoresist is not chemically altered to the point of being defective.

Impact on Lithography Industry

In addition to the technical concerns, there are also significant business concerns. Immersion lithography tools are naturally expected to cost more than dry lithography tools. However, it is not unexpected that lithography tool costs will go up with time. As soon as the current technology approaches limits, the demand for and the cost of finding, developing, and continually improving an alternative that fits the infrastructure will increase further. A simpler tool that disrupts the infrastructure, such as one based on nanoimprint lithography, can still incur costs due to change of infrastructure. Simply tightening the quality specs in increments also drives up costs. Hence, a series of tools based on extreme ultraviolet lithography will only continue the same trend that DUV tools have followed, by virtue of tighter overlay requirements and contamination control.

As of 2006, orders have been placed for 193 nm immersion tools, yet the question remains: how long will this technology be used? How far will numerical aperture increase? Will mask-to-wafer image demagnification need to increase as well, and if so, how much? A significant increase in demagnification also leads to a significant reduction in field size, and hence throughput. The willingness of chipmakers to continue to invest in this technology will depend on their willingness to deal with these changes as well as preserve the current 193 nm-based lithography infrastructure.

References

1. M. LaPedus, "Litho race," EE Times, October 21, 2005.
2. D. Ristau et. al., Appl. Opt. 41, pp. 3196-3204 (2002).
3. A. Hand, "High-Index Fluids Look to 2nd-Generation Immersion," Semiconductor International, April 1, 2005.
4. B. W. Smith et. al., Proc. SPIE 5377, pp. 273-284 (2004).
5. C-W. Chang et. al., Laser Physics Letters 2, pp. 351-355 (2005).
6. A. Grenville, "Prospects for Immersion Lithography at the 45nm Half-Pitch and Beyond," Future Fab International, Issue 20, pp. 76-79 (2006).
7. G. Vandenberghe, "How Optical Lithography Prints a 32 nm Node 6T-SRAM Cell," Semiconductor International, June 1, 2005.
8. M. Switkes et. al., J. Vac. Sci. & Tech. B 21, pp. 2794-2799 (2003).
9. U. Okoroanyanwu et. al., "Defectivity in water immersion lithography," Microlithography World, Nov. 2005.
10. J. C. Taylor et. al., SPIE 5376, pp. 34-43 (2004).

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