INDUSTRY PHOTORESISTS
Figure 2: The working principle of the Bubbles & Beyond phase fluid lisoPUR. The layer (orange) is penetrated, fragmented and finally lifted off from the substrate (red)
Figure 3: Spectra reveal surface recovery after processing. The upper trace shows the silicon sample prepared with a phase fluid (without rinse), while the middle and lower plots reveal surface recovery after rinse off with deionised water and the spectra for the bare silicon reference, respectively
Scrutinising surfaces with Fourier transform infrared spectroscopy provides evidence of full surface recovery after the application of a phase fluid (see Figure 3). It is clear from these spectra that the fluid leaves no residues after a rinse off with water. Regular cleaning rinses, such IPA, can also be used. Another characteristic of the phase fluids is revealed by these results: Adding water (or alcohol) to the phase fluid immediately stops its activity.
Figure 4: A KLA Tencor tool enables particle measurement on a 300 mm silicon wafer subjected to a single spin process. The scans compare before and after phase fluid application (removal is followed by a deionised water rinse and SC1 short cleaning)
In addition to this study of molecular surface recovery, we have undertaken a ‘high scale’ assessment of remaining residues, searching for particles with a KLA-Tencor SP2 tool. This effort commenced by applying a fluid, via a single spin process, to a 300 mm silicon wafer.
Post-cleaning followed, using deionised water, followed by a shorter step with diluted, cold SC1 – a mixture of ammonium hydroxide and hydrogen peroxide. The KLA-Tencor tool, set to detect particles with sizes from 0.12 µm to 1 µm,
determined that just 38 more defects were found after processing (see Figure 4).
Although these first results are only on the laboratory scale, they are very encouraging, showcasing the promise that phase fluids have throughout the entire semiconductor industry.
By selecting the optimum conditions – such as the right temperature, ultrasonic treatment and drying procedure – it is possible to realise fast stripping of various photoresists, including positive and negative varnishes. For example, these harmless process fluids can remove a photoresist greater than 10 µm-thick in less than 5 minutes to leave a pristine surface.
We can alter the formulation of our phase fluids so that they can be tailored to a particular task. Through our work with the Fruanhofer Center Nanoelectronic Technologies, we have found that cleaning efficiency increases by heating the fluid, and also using steps such as agitation and ultrasonic treatments.
The opportunity to work with a water-based formulation, rather than harmful fluids, has caused quite a stir within the market. Trials have revealed that if our phase fluid is heated to 80°C, metal lift off takes just 10 minutes. To realise full compatibility with a semiconductor environment, the approach involves a deionised rinse process, followed by SC1 cleaning when necessary. Note that phase fluids can be used in either bath or single-wafer processing.
These results highlight the very optimistic outlook for phase fluids. Initially they’ll be used for photoresist removal and metal lift-off, but as time goes by, they will start to also make an impact in the cleaning of various pieces of equipment, such as catch cups and photomasks.
Further reading
www.intelligent-fluids.de
© 2013 Angel Business Communications. Permission required.
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www.compoundsemiconductor.net August / September 2013
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