Strip/Cleaning Process Uses Near-Visible Light

A new resist strip/wafer cleaning technology has been developed that is "fortunately not intuitively obvious," according to developer David J. Elliott of UVTech Systems Inc., Wayland, Mass. The technology employs visible light (λ=380-780 nm) with simple gas chemistries to remove contaminants from the wafer surface. What's not intuitively obvious is that most wafer cleaning techniques based on photoablation have so far been based on light with much shorter wavelengths, requiring, for example, sophisticated excimer lasers. "The UVTech process uses a different photochemical reaction," said Elliott. "The new process uses only 'green' reactive gases, such as oxygen and ozone, to remove resists and polymer-based residues. The process produces no waste treatable by-products."

The process was developed primarily for all-dry photoresist removal, including ion implanted resist. Secondary applications under investigation include photo-stabilization of thin dielectrics, metals removal and photopolymerization reactions.

By working in the near-visible range, the UVTech system, dubbed XLC-100, uses a simple solid-state commercial laser that is much less expensive than excimer lasers, and requires less maintenance. Wavelengths in the 350-550 nm range are used to create a low-level plasma in conjunction with a custom gas recipe. "The low energy per photon of wavelengths in the near-visible and visible spectrums allows for wide process latitude without damage, even to the most sensitive films," said Elliott. For example, the eV/photon with the XLC-100 process is in the range of 2.3 to 3.5, while prior ultraviolet cleaning processes with excimer lasers have 6.42 eV/photon. "The advantage of longer, low eV wavelengths is evident in AFM (atomic force microscopy) analysis photos of cleaned samples," Elliott said.

Elliott said the gas recipes (patents pending) permit very high concentrations of atomic oxygen and ozone, as well as hydroxyl radicals. This highly oxidative mixture enables a total removal of carbon in resist strip applications. "Previous UV/reactive gas organic layer removal processes always resulted in a carbon residue of >20 Å, requiring additional wet cleaning and drying steps. The new gas recipe developed by UVTech will replace processes using large volumes of toxic industrial chemicals and even larger volumes of highly purified rinse water."

A typical process cycle takes about 3 minutes, yielding a throughput of 20 wafers per hour. Conventional resist strip applications require several minutes, as they involve a wet bench process, ashing, and alcohol drying. The new process is one single step, which is all dry.

XPS analysis for residual carbon and metals (Figs. 1 and 2 ) was performed by outside analytical services, and the results show that carbon is removed to the limit of detectability, or about 2 Å. Samples cleaned over a wide range of system parameters were also virtually free of carbon, evidencing a robust cleaning process suitable for manufacturing. Some samples were intentionally delayed for 24 to 48 hours, then processed. Results showed the accumulation of ~3 Å (~1 atomic diameter) of atmospheric carbon, a predictable result, Elliott said. Several samples cleaned with the XLC-100 actually had less carbon that the control samples, showing that the system removed the atmospheric carbon as well.

1. XPS analysis for carbon shows that carbon is removed to the limit of detectability, or about 2 Å.

2. XPS analysis for metals was performed by an outside analytical service.

Some samples were first implanted with low, medium and high ion doses. These samples were stripped in the same amount of time as the standard hard-baked samples, with no damage or residual carbon. "Surprisingly, the highly oxidative plasma of the XLC-100 showed little to no differentiation between highly cross-linked or carbonized materials and standard hard-baked resist layers," Elliott said.

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