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WET CLEANING

18.12 WAFER PROCESSING

process a large number of wafers at a time, to ensure consistent process chemicals for all wafers in a
lot, and flexibility in the number of process steps and diversity of chemicals.

18.3.2 Batch Spray

A batch spray typically exposes one to four cassettes of wafers to a specific chemistry at a time. The
cassettes are rotated with fixed nozzles applying the chemicals. The chemicals can be recirculated or
used once. These systems have the benefit of high throughput, consistent wafer-to-wafer results, and
flexibility in mixing chemistries at the nozzle.

18.3.3 Single Wafer
Single wafer is a wide classification that includes immersion, spray, scrub, sonic, and dispense clean-
ing systems. The continuing improvement to smaller and smaller line widths creates the need for bet-
ter process uniformity across the wafer surface and lower defect density. Single wafer offers faster
cycle times than traditional batch processes and incorporates process monitoring that is recorded on
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a per wafer level (a vast improvement over per batch). Single-wafer immersion systems are based
on the concept of larger wet benches. Single-wafer spray systems utilize a moving or static spray
nozzle(s) and a rotating or static wafer. The scrubbing system can be implemented into a spray
machine or can utilize a chemical dispense on the cleaning brushes. Sonic devices are used with
rotating wafers and a chemical dispense system. Chemical dispense systems are used on rotating
wafers. A novel dispense system utilizes a “Bernoulli chuck” that ensures that only one side of the
wafer is treated with a chemical at a time.

18.4 FUTURE TRENDS AND CONCLUSIONS

In addition to the standard processes and technologies discussed previously, there are some emerg-
ing technologies that are being more accepted in wafer manufacturing. Supercritical fluids are being
evaluated for such diverse areas as FEOL critical cleans and BEOL photoresist removal. Pre-lithog-
raphy backside cleans along with silicon stress relief and silicon thinning are being added to the man-
ufacturing flow to address specific technological issues that have arisen.

18.4.1 Supercritical Fluids
Supercritical fluids are compounds at a temperature and pressure above their critical point. They
have long been used as solvents in commercial applications such as the extraction of caffeine from
coffee and essential oils from plants. Their unique properties resemble both gas and liquid phases
specifically in the area of viscosity (gas) and density (liquid). A promising supercritical fluid is
propylene carbonate (referred to as PCO ) that is mixed at 5 vol. percent with CO . This has shown
3
2
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to have no negative effect on the metallization present on wafers. A hypothesis to describe the
mechanism of removal is a combination of photoresist swelling by diffusion of the supercritical fluid
into the polymer matrix followed by a debonding of the film caused by rapid depressurization then
followed by a fluid flow that clears away the removed film. This method of removal offers an envi-
ronmentally benign process for the complete removal of a photoresist. 26
18.4.2 Pre-lithography Backside Clean

Pre-lithography backside cleans are becoming important as linewidths are decreased. The smaller
linewidth creates a smaller focus window that can be impacted by wafer bow due to film stresses on
the backside of the wafer, also known as “litho hot spots” (see Fig. 18.4).
This clean also reduces the likelihood of cross contamination from the lithography chuck by remov-
ing the contamination from the backside of the wafer prior to placement on the chuck. The process uses

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