When transitioning to a liquid immersion process using ultra-pure water (UPW), water clarity must be addressed

By Bipin Parekh, Entegris

In response to consumer demand for smaller, faster, and more capable electronics, semiconductor process designers and chip fabricators have adopted optical lithography processes to increase chip capacity by decreasing circuit linewidths. To stay competitive, fabs are gearing up for a transition from dry lithography to a liquid immersion lithography (LIL) processing technique, using ultra-pure water (UPW). Using LIL, manufacturers can create higher resolution images than a “dry” lens system will allow. UPW immersion presents several unique challenges relating to water clarity, which can be addressed through liquid purification and flow control techniques.

Ultimate LW (resolution) in optical lithography is given by:

LW = k₁λ/NA

where λ is the illumination wavelength; k₁ is an optics/process parameter; and NA is numerical aperture, given by NA = n*sinθ, n being fluid refractive index.

In immersion lithography a higher refractive index liquid (e.g., UPW, index n = 1.44) is placed between the final lens and the wafer (replacing the lower index air, index n = 1). The higher refractive index of the DI water delivers two benefits: improved resolution and increased depth of focus of up to 50 percent for printing the finer circuit lines onto wafers.^1,2

Despite the potential benefits, UPW immersion presents several unique challenges surrounding maintaining water clarity (low light absorbance) and purity (ppt levels of contaminants). To achieve high yields, manufacturers must be able to protect the wafer from dissolved materials or particles that contaminate the water and may cause staining. They also must ensure that the UPW is free of bubbles formed during the scanning and exposure processes, or in the fluid delivery, recovery, and recirculation system. Since immersion lithography requires that all water impurities be removed to trace levels, manufacturers must comply with the stringent water quality guidelines, shown in Table 1 (ITRS Ultra Pure Water Guidelines 2006, http://www.itrs.net/), and strictly control the DI water temperature to eliminate patterning defects related to the refractive index.^3-6

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The information provided here describes the effects of various water contaminants and attributes on the immersion process and illustrates how to prevent patterning defects by removing these contaminants from DI by purification and conditioning techniques.

UPW parameters and impact on immersion process

Dissolved gases and bubbles, particles, TOC, and extractables (ionic and organic) added from process materials and piping components all have the potential to contaminate the wafer and influence the refractive index of the DI water. UPW with a higher TOC value than specified in Table 1 could absorb DUV energy from the scanner, causing defects. It also could lead to build-up of adsorbed species on the lens and cause transmission loss.

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Figure 1 demonstrates lens clouding by adsorption of fluoro-organics in water. TOC monitoring instruments did not detect the fluoro-organics, but non-volatile residue testing did indicate the presence of these harmful materials. Selecting cleaner materials of construction (such as Teflon^®) for the system components is key to alleviate the contamination. A UV oxidation-ion exchange process can reduce TOC to ppt levels by breaking down most organic molecules into CO₂ and H₂O. Other organic molecules might only be ionized and removed by ion exchange.

Dissolved gases

Dissolved gases in UPW can create bubbles, which can have a negative effect when found moving within the fluid. In Fig. 2, a 10 μm bubble disrupts an unpolarized plane wave and casts a strong shadow. Since the water is moving and carrying the bubble, a longer exposure time can lower the impact of a bubble on any one spot. The distance of the bubble from the resist makes a difference in the impact upon feature resolution and accuracy.⁷

Vacuum degassing reduces dissolved gas concentration from UPW feed (and gases/bubbles generated by the UV oxidation source). To remove dissolved gases to ppb levels, it is critical to use clean devices with low TOC extractables and particle shedding. Conventional degassers are efficient at typical flow rates (>75 percent efficient) but high TOC shedding from these units limits their use upstream of the UV oxidation source, as roughing degassers. Teflon^® degassers are more expensive due to lower efficiency (>40 percent efficient), but their superior cleanliness makes them ideal for use after the UV source.⁸

Particles in UPW

Since particles in UPW can deposit on the wafer or cast a shadow during wafer exposure and cause defects, it is imperative to remove particles down to 0.03 μm size using point-of-use (POU) filtration. One solution is to use a 0.03 μm rated “all” Teflon^® filter with particle retention efficiency of >99.7 percent removal for 0.03 μm. An out-of-the-box filter test extracted metals at the necessary detection limit (an ICP-MS profile of HCl extract), while maintaining the integrity of the UPW.⁹

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Metal ions, silica, and boron purification. POU mixed bed purifiers installed in series with silica/boron purifiers can help remove ions from the immersion system. Strong resin enables the removal of most metals and weakly dissociated silica and boron.

Temperature control. A stable water temperature prevents imaging defects by eliminating refractive index changes. Inlet fab water temperature swings are not well known, nor controlled to the degree required. To be successful, manufacturers must accurately manage of the point-of-dispense temperature within 0.001°C or 1 mK.

Figure 3 demonstrates the temperature stability results for a POU UPW system that achieves the target temperature of 20.5°C. The system is stable to <0.01°C for target temperature at 20.5°C. Temperature measurement details show accuracy is ±0.0013°C and resolution is ±0.0001°C. The cooling water is degassed to prevent any gas transfer through the PFA tubes in the heat exchanger to the process UPW.

Liquid flow control

It is also important to deliver a stable DI flow precisely/repeatedly to the illuminated area to prevent bubbles from attaching to the wafer or lens during filling. The water filling rate, over the wafer topography, should remove resist reaction byproducts, water soluble resist components, and the heat generated during exposure to prevent refractive index changes. The current flow-rate control required ranges from 0.4 to 1 LPM at steady state. In some newer designs, a flow rate up to 3 LPM is required to maintain the temperature stability. A slower initial fill flow rate, to ensure complete filling under the lens, followed by a faster rate during scanning, is required to ensure byproduct removal and also meniscus integrity during stage movement. We employed an Entegris flow control module in our system to maintain a highly repeatable and stable flow rate through the illuminated area.

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Figure 4 shows the Computational Fluid Dynamics (CFD) simulation of the liquid flow processes in the lens area.¹⁰ An optimum flow velocity is key to achieving the stable water meniscus free of entrapped bubbles.

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Summary
Research shows that liquid immersion lithography using UPW at the 193 nm laser infrastructure will enable photolithography capabilities beyond 45 nm. However, eliminating liquid contamination and reducing defects through process controls are required at this lower resolution.

Immersion lithography applications must be designed to deliver purified DI water with low TOC, particles, and dissolved oxygen levels. Contaminants must be removed to trace levels to eliminate interference for the laser transmission through UPW. The purification system should be able to maintain a stable thermal control within 0.001°C of the target temperature to ensure stable refractive index. When designing a UPW purification system, manufacturers must select clean construction materials, control ions and organics extractables from the components, control dissolved gas content, and maintain precise water temperature and flow control.

Bipin Parekh is a senior consulting engineer at Entegris (www.entegris.com). Co-authors who contributed to this piece include Michael Clarke, Annie Xia, and Joseph Smith.

References

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