| Revision as of 05:55, 5 January 2006 edit69.181.80.136 (talk) →Remaining Concerns← Previous edit | Revision as of 06:26, 5 January 2006 edit undoGuiding light (talk | contribs)Extended confirmed users5,905 edits Removed cost figures (let market decide).Next edit → | ||
| Line 17: | Line 17: | ||
| 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 ] 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. | 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 ] 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 Concerns== | ==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 ] 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. | 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 ] 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. | ||
| ==Increasing Lithography Costs== | |||
| In addition, to the technical concerns, there are also significant business concerns. Immersion lithography tools with 1.3NA are expected to cost between $40M and $50M. Few companies can afford this level of investment. Immersion systems with >1.3NA will likely cost as much or more than EUV systems. It is questionable whether companies will be willing to pay such a high price for a single generation tool. | |||
| In addition, to the technical concerns, there are also significant business concerns. Immersion lithography tools are expected to cost more than dry lithography tools. It is unlikely that these tools can compete against ] in the area of cost, or against ] in terms of optical simplicity. The willingness of chipmakers to invest in this technology will depend on their willingness to preserve the current 193 nm-based lithography infrastructure. | |||
| ==References== | ==References== | ||
Revision as of 06:26, 5 January 2006
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. For all numerical apertures, the immersion fluid 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, 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, should enable 32 nm lines and spaces. Alternatively, by doping the water, a numerical aperture of 1.5 can be easily reached. This is the minimum value needed for the same 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
Ultimately, the upper limit of the numerical aperture will be 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. 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.
Increasing Lithography Costs
In addition, to the technical concerns, there are also significant business concerns. Immersion lithography tools are expected to cost more than dry lithography tools. It is unlikely that these tools can compete against nanoimprint lithography in the area of cost, or against extreme ultraviolet lithography in terms of optical simplicity. The willingness of chipmakers to invest in this technology will depend on their willingness to preserve the current 193 nm-based lithography infrastructure.
References
- M. LaPedus, "Litho race," EE Times, October 21, 2005.
- D. Ristau et. al., Appl. Opt. 41, pp. 3196-3204 (2002).
- A. Hand, "High-Index Fluids Look to 2nd-Generation Immersion," Semiconductor International, April 1, 2005.
- B. W. Smith et. al., Proc. SPIE 5377, pp. 273-284 (2004).
- C-W. Chang et. al., Laser Physics Letters 2, pp. 351-355 (2005).
- G. Vandenberghe, "How Optical Lithography Prints a 32 nm Node 6T-SRAM Cell," Semiconductor International, June 1, 2005.
- M. Switkes et. al., J. Vac. Sci. & Tech. B 21, pp. 2794-2799 (2003).
- U. Okoroanyanwu et. al., "Defectivity in water immersion lithography," Microlithography World, Nov. 2005
 | This technology-related article is a stub. You can help Misplaced Pages by expanding it. |