New Single-Wafer Processes Offer Alternative Backside Cleans

According to the International Technology Roadmap for Semiconductors, the backside particle requirement for optical lithography is <94 particles per 200 mm wafer for 0.18 µm technology, and <63 particles for 0.13 µm geometry for particles >0.2 µm. Single-wafer cleaning processes developed by Semitool Inc. (Kalispell, Mont.) have been shown to effectively remove backside particles for copper removal and pre-lithography cleans. Results from independent experiments performed at Semitool and IMEC (Heverlee, Belgium) demonstrated Cu removal to below detection limits(<1E11 atoms/cm² Cu) from the wafer backside, bevel and opposite side edge exclusion zone. In addition, an O₃/hydrofluoric mixture was developed for backside particle removal before photolithography.

Two separate chemical mixtures, sulfuric/peroxide and hydrofluoric peroxide, were used in the company's capsule minienvironment, a single-substrate process chamber. Backside copper contamination removal experiments of 200 mm bare silicon substrates at Semitool showed contamination levels of 1E13 atoms/cm² and <1E10 atoms/cm² on unprocessed wafers. Processing contaminated wafers in a capsule using dilute sulfuric/peroxide mixture (dSPM) showed post-clean contamination levels near the detection limits of TXRF. Following dilute HF:H₂O₂ (dHFPM) processing, post-clean copper contamination levels were ~1E10 atoms/cm².

The low flow(<400 mL/min.) needed for processing allowed the dissolved ozone concentration to remain at sufficient levels during processing. The solution is reclaimed after processing to a chemical tank for reuse. Silicon nitride particle contamination levels prior to processing averaged >5000 particles measuring 0.15 µm and larger. Post-processing particle levels averaged <60 particles 0.15 µm and larger (Fig. 2). •