Exhausted Solvent Trash Cans

Roger Robbins                                                                                                 10/7/2011

Purpose

In the University of Texas at Dallas Natural Science and Engineering Research Lab Clean Room, Lithography Bay, solvents are used extensively in the lithography process flow, and considerable solvent solid waste, such as cleanup wipes is generated.  We have solvent cans to store the waste, but experienced excessive vapor levels in the air recirculating through the lab.  In order to reduce this we attached flexible exhaust ducts to the solvent trash cans.  This paper reports on the solvent containment performance of the system.

Introduction

It is well chronicled that breathing solvents such as Acetone and the like for long periods can result in serious health damage, including such targets as:

  • Brain –
  • Skin – dermatitis
  • Liver – liver damage
  • Blood – blood forming functions
  • Kidneys
  • Fertility of both men and women
  • Foetus in a pregnant woman
  • Cancer – example Benzene

Some solvents will even have magnified synergistic effects when combined with other hazards and drugs.  That is, the solvent exposure will have greater health effects when a person is exposed to a combination of other hazards such as smoking or alcohol use.

Since the Cleanroom re-circulates the majority of the air in each bay, any evaporation of solvent into the bay spreads to all of the bay air volume and slowly builds up concentration as long as solvent vapor is evaporating.  The subtlety in this evaporation and recirculation results in a dilution and a loss of smell sensitivity by the users – this lulls the active users into thinking that the air is ok, even though the solvent smell might overcome a user coming into the bay from a solvent-free office.  Therefore the Cleanroom staff has put considerable effort into reducing the amount of solvent vapor released into the air.  We have installed hoods over the resist spinners, exhausted the hotplates and ovens with local hoods, and hooked up building solvent vent lines to the solvent trash cans.

The two solid waste cans next to the solvent hood and the photoresist spinner hood have been connected to the building solvent exhaust to reduce the level of solvent vapor in the bay.  Figure 1 shows the simple four inch flex line hooked up to a port at the bottom of the trash can.  The trash cans are designed with an internal bucket to catch the trash but can be lifted out of the trash can to more easily remove the trash.  We line the removable bucket with a solvent compatible plastic bag used to transport the waste to the solvent bunker where it is repackaged for delivery to the UTD EHS department for disposal.  Thus the exhaust pulls air from the lid region, around the bucket, out the lower back and up the building exhaust line.

 

Figure 1.  Two exhausted solid waste solvent cans in the lithography bay of the NSERL Clean Room.

Figure 2 shows the inner bucket removed for trash pickup.  The inner bag (black plastic) is immediately removed, tied at its top for partial sealing, and immediately transported to the solvent bunker where it is transferred to a container for later routine EHS solvent pickup.

Figure 2.  Inner trash can bucket removed for trash pickup.

Exhaust Air Flow Data

In order to insure that the exhaust airflow would sufficiently capture solvent vapor from the material in the inner can when the lid was opened or remains slightly open because of misfit or excess contents, we installed a 4 inch diameter flex line of corrugated thin Aluminum with an inner coiled wire support to prevent collapse under vacuum.  The airflow in the can is through the annulus between the inner contents can and the outer can and out the lower exhaust port.  In Table 1, the measured airflow rate is listed.  The airflow measurements were taken with a linear hot-wire anemometer.

Table 1

Airflow Rates at various points in the Exhausted Solvent Can

Location ft/min
4” dia exhaust Port at bottom 1100
Annulus at back with lid open 304
Annulus at front with lid open 300
Lid open 1” at front 132

Discussion

Airflow measurements show that the annulus exhaust rate and the inflow rate with the lid slightly open meets standard hood flow specs.  However when the lid is opened with the foot pedal or lifted with the hand there is still some solvent vapor that escapes into the room.  However, by the olfactory comparison test method, the amount of solvent entering the room during a quick lid opening is far less than that with no exhaust.  Users are encouraged to quickly open the lid, dispose of the solid waste and just as quickly close the lid.  With the negative air pressure inside the waste can, the lid is held shut fairly firmly and no solvent vapor can be smelled at the lid seams when it is closed.  Again by the olfactory comparison method, the amount of solvent vapor in the recirculating air in the bay is reduced to an un-noticeable level compared to that with no exhaust.

Conclusion

By exhausting the solvent solid waste cans, we have reduced the amount of solvent vapor in the lithography bay significantly.  However, the amount of reduction still depends on users managing the release of solvent vapors into the room air.  Transport of the solvent laden material from the hood to the waste can must be quick and if the waste can be “wrapped’ inside un-laden material in the hood, the transport release of solvent vapor can be further reduced.