Quick Drying Enables Single-Wafer Cleans
Maria A. Lester, Associate Editor -- Semiconductor International, 10/1/2000
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The Rotagoni method was developed in collaboration with Verteq (Santa Ana, Calif.) as part of the Goldfinger platform, a single-wafer cleaning tool with a megasonic transducer, where the wafer remains at its position during the drying process. The drying method based on the interaction of Marangoni forces and rotational forces removes the liquid rather than evaporating it. A liquid is dispensed through a narrow tube, while the wafer rotates 300 to 1000 rpm. The tube moves slowly from the center of the substrate toward the edge. A second nozzle, mounted on the trailing side of the liquid dispense tube, dispenses a tensioactive vapor, which reduces the surface tension of the liquid and creates a strong Marangoni force.
Wafers covered with 1100 nm thick PECVD oxide were subjected to an oxide CMP process that thins the oxide to 700 nm. The wafers immediately were stored in a UPW tank, then subjected to a standard spin dry, a spin dry in an IPA vapor environment, and the Rotagoni process. For the two first drying methods, some wafers also received a spin rinse prior to the drying cycle. Adding a spin-rinse cycle helped reduce the residues; however, it was still less effective than the Rotagoni process. The drying method also has been tested for drying marks, which were observed only on spin-dried wafers.
The cleaning was evaluated with respect to the removal of platinum (Pt) particles. The wafers were cleaned using three different cleaning chemistries on two hardware configurations: a spin clean followed by a spin dry, and a clean with megasonic agitation followed by the Rotagoni dry. The spin clean and dry process showed no significant removal of Pt particles, contrary to the combination of megasonic agitation with Rotagoni. A second process qualification was a post-copper-CMP cleaning application. For that, a single-damascene deposition of Cu in an oxide layer was done. The Cu electroplating was followed by Cu-CMP using silica-based slurry. The wafers were cleaned using a proprietary chemistry in combination with megasonic agitation followed by Rotagoni. The wafers were spin-dried right after the CMP process. The spin-dried wafers showed silica particles that were preferentially deposited on Cu during post-CMP rinse. However, the megasonic process terminated with Rotagoni resulted in a good particle removal (Figure). No Cu corrosion or roughening was observed. Resistance measurements showed the Rotagoni drying resulted in 100% yield, whereas the yield after spin-dry was ~94%. The drying time for 200 mm blanket wafers was <20 sec, enabling single-wafer cleaning in 1 min.
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| Post Cu-CMP cleaning results of silica particles after spin-dry cleaning (a) and after Rotagoni process (b). (Source: IMEC) |
The Lineagoni drying method was developed in collaboration with STEAG (Essen, Germany) as part of the DamasClean platform, which consists of a scrubber clean followed by Lineagoni. The DamasClean platform processes wafers horizontally to allow independent top-side and back-side wafer treatment and accelerated wafer handling. The drying method uses a track setup and tensioactive vapor for high-performance drying. The wafer is pushed slowly through a narrow box-shaped reactor via narrow entrance and exit slits. The reactor contains the process fluid (UPW or chemicals). The ambient atmosphere above the liquid is kept at a lower pressure with respect to the pressure adjacent to the chamber close to the slits. At the exit slit, a tensioactive vapor creates a Marangoni effect, resulting in the drying of the wafer.
The Lineagoni module was tested on metal neutrality by measuring the concentration of metallic surface concentration on clean wafers. Results showed good metal neutrality with most metals below or close to the detection limit. Particle removal efficiency also was tested using the DamasClean platform. Each wafer was contaminated with ~5000 Si3N4 particles (0.1 and 0.3 µm diameter). Initial tests showed a drying time of 50 sec for 200 mm wafers with high particle removal efficiency. Further experiments are being done to reduce the drying time.
