Wet Benches are used in a wafer FAB to clean debris and/or to etch away unwanted or no longer needed films from wafers. They are also used to prepare a wafers surface for the next step in the overall process. Less sophisticated wet benches are often used in a FAB to clean quartz and other semiconductor equipment parts.
Wet Benches are generally manufactured for compatibility with Acid or Base chemistries. Base chemistry wet benches are often called Solvent Benches. You can recognize a Solvent Bench immediately because it's made of stainless steel. It will generally have integrated fire suppression due to the dangerous flash points of solvents. Acid wet benches are far more plentiful in the FAB and they are constructed of plastic. Most FABs require acid wet benches to be made of Factory Mutual approved plastics like FM4910. This fire-safe plastic is expensive but it is often required to qualify for insurance.
The architecture of a wet bench is one of multiple baths (tanks) sized to accommodate the submersion of a batch or lot of wafers. Typical batch sizes are 50 wafers or 25 wafers. Some common bench layouts are:
· In Line Linear - a linear series of process and rinse baths from left to right or vice versa.
· Rotary Benches - a single arm centered about three or four process baths
· Front to Back usually a process bath in the rear and a rinse bath in front
· Once Through Bath Rather than move wafers from bath to bath, wafers sit in a single bath while dilute chemicals are exchanged within that same bath. These tools are usually configured In Line Linear and are used in sophisticated factories.
FEOL (Front End of Line) refers to the manufacturing processes used to manufacture the transistor portion of an integrated circuit. BEOL (Back End of Line) refers to the manufacturing processes used to interconnect the transistors to one another and to the outside world.
When a wafer reaches the BEOL stage of production, the interconnect is constructed of metal, typically Tungsten, and Copper or Aluminum. Common FEOL chemistries are harmful to these metals making it necessary for Wet Bench engineers to embrace solvents for BEOL cleaning. BEOL will be discussed in detail (see Solvent Wet Benches).
A very important, sophisticated, and usually quite large wet bench used for FEOL cleaning is called a critical clean bench. It is also called the pre-diffusion clean (PDC), pre-furnace clean, RCA, or Standard Clean bench. This clean usually takes place just prior to a thermal, CVD, or ALD processes. Its effectiveness directly impacts the devices switching speed, so its critical.
Other common FEOL cleaning wet benches are:
1. Resist Strip
2. Nitride etch or strip
3. Residue clean, post etch, or post ash clean
4. Oxide etch
5. STI (shallow trench isolation) clean
These benches often incorporate cleaning in addition to their etching or stripping tasks making it difficult to offer up standard configurations. So, maybe its better to offer up some common chemistries instead.
After each chemical bath, it is common to perform a DI water rinse. Sometimes the rinse is performed in the same chemical bath as the process. Other times, the rinse is performed in a separate rinse tank.
There are basically 3 major types of rinses and combinations of same:
1. QDR (Quick Dump Rinse) - Dumps the rinse water down the drain very quickly
2. Overflow Rinse Rinse water is introduced from below the bath, forcing the water to overflow out of the bath and into a weir or drain
3. Spray Rinse DI water spray bars are used to shower the wafers with rinse water
Rinse baths can be equipped with resistivity monitors. The resistance of pure DI water at 25C is some known number. If the rinse water is something other than that known number, we must assume there are contaminants still present, and we continue rinsing. This is a more scientific method of rinsing, in contrast to just a timed rinse.
It is common practice that chemical baths and sometime rinse baths are equipped with megasonics or ultrasonics. Sonic energy creates cavitations that improve the cleaning efficiency of most chemistries. Harsh cavitation causes damage to the wafer. Generally, overly aggressive cavitation comes from ultrasonics frequencies, so megasonics are more popular.
Ozone generators may accompany wet benches. Ozonated DI water is environmentally friendly compared to most legacy chemicals. Ozonated DI is a popular substitute for peroxide, and it is being studied and used to replace other common FEOL chemistries as well.
Critical cleaning baths are often equipped with concentration monitors. These monitors look for the presence or absence of certain chemistries in the bath. When the bath chemistry is spent and not optimal, the concentration monitor complains. Action is taken to refresh the bath chemistry, ideally restoring it to the perfect mix. This bath chemistry restoration process is called spiking.
Process engineers often deploy an HF Last process. This means that just prior to drying, the wafers are etched in HF. The result is a Hydrogen terminated surface. This is desirable because it delays the inevitable growth of native oxide; an undesired oxide that grows on the wafer while waiting in queue for next process.