New Con­cept for Point-of-Use Aba­te­ment of Harm­ful Gases in Wet Bench Semi­con­duc­tor Processes

In the field of was­te gas treat­ment it is well esta­blished prac­ti­ce to treat exhaust air from wet-che­mi­cal pro­ces­ses in semi­con­duc­tor manu­fac­tu­ring with cen­tral wet-scrub­bers or, if loa­ded with VOCs, with cen­tral ther­mal oxidizers.

Swit­ching Boxes repla­ced by Point-of-Use Systems

Neverthe­less, with sin­gle-wafer wet-clea­ning tools coming into mass pro­duc­tion local wet-scrub­bers have advan­ta­ges. They repla­ce swit­ching boxes, which direct the exhaust air — depen­ding on actual process

con­di­tions — to the dif­fe­rent cen­tral sys­tems. The cha­llen­ge in the design of a local scrub­ber for this appli­ca­tion was a tra­de-off to meet the spe­ci­fic res­tric­tions con­cer­ning size, effi­ciency and pres­su­re drop.

Point-of-Use Con­cepts offer clear advantages

The point-of-use con­cept by DAS Envi­ron­men­tal Experts was suc­cess­fully eva­lua­ted for this appli­ca­tion. The advan­ta­ges com­pa­red to the swit­ching-box con­cept are sma­ller and less com­plex exhaust-piping, sma­ller load to cen­tral treat­ment sys­tems, redu­ced loss of clean room air, sma­ller foot­print and higher fle­xi­bi­lity for pro­cess chan­ges. Low emis­sion con­cen­tra­tions are achie­ved and salt par­ti­cle for­ma­tion from acids and bases is eliminated.

Sin­gle Wafer Clea­ning in Semi­con­duc­tor Manufacturing

Many pro­cess steps in the semi­con­duc­tor industry emit harm­ful was­te gases. For che­mi­cal vapor depo­si­tion or dry etching whe­re very reac­ti­ve gases are used, so-called point-of-use treat­ment of was­te gases clo­se to the sour­ce is com­mon prac­ti­ce. In con­trast, for wet che­mi­cal pro­ces­ses it is a well-esta­blished prac­ti­ce to treat the exhaust air with cen­tral wet-scrub­bers or, if con­tai­ning many VOCs, with cen­tral ther­mal oxi­di­zers, both typi­cally loca­ted in or on top of the buil­ding. Neverthe­less, with sin­gle wafer clea­ning beco­ming more and more com­mon in mass pro­duc­tion, local wet scrub­bers have advan­ta­ges. Tech­ni­cal and com­mer­cial reasons have led to the request for an adap­ted local scrub­ber solu­tion with limi­ted foot­print, which is able to treat in one sys­tem all the exhaust air with the clea­ning pro­cess che­mi­cals. Such a con­cept is des­cri­bed in this article.

In wet ben­ches, which have been in use for a long time, a carrier with mul­ti­ple wafers is sub­mer­ged into a sequen­ce of liquid baths, whe­re each com­part­ment of the wet bench always con­tains the same type of liquid. The ven­ti­la­ted air from each com­part­ment is direc­ted into one of seve­ral exhaust sys­tems, typi­cally clas­si­fied as aci­dic, alka­li­ne, VOC and gene­ral exhaust.

In con­trast, in a sin­gle wafer wet-clea­ning sys­tem sin­gle wafers are loa­ded in a pro­cess cham­ber. A wet-clean tool has mul­ti­ple pro­cess cham­bers to pro­cess seve­ral wafers at one time or to per­form dif­fe­rent pro­cess steps in dif­fe­rent cham­bers. At the time of clea­ning, the wafers are spra­yed with dif­fe­rent liquid che­mi­cals in a sequen­ce, which are then remo­ved by spin­ning the wafer. Besi­des pure water, typi­cal ingre­dients are ammo­nia, sulphu­ric acid, hydro­gen pero­xi­de, ozo­ni­zed water, hydro­fluo­ric acid or iso­propyl alcohol. During the spra­ying and spin­ning some liquid eva­po­ra­tes and small liquid dro­plets can be suc­ked into the ven­ti­la­tion ducts.

The liquids con­tai­ning ammo­nia and liquids con­tai­ning hydro­fluo­ric acid or sulphu­ric acid can cau­se pro­blems due to the for­ma­tion of salt crys­tals in the exhaust, whe­re vapors or dro­plets get into con­tact with each other. The reac­tion of ammo­nia and hydro­gen fluo­ri­de is rever­si­ble (see Equa­tion 1); the for­ma­tion of ammo­nium fluo­ri­de salt can the­re­fo­re be sup­pres­sed, if the con­cen­tra­tion of both gases is kept below an equi­li­brium value, which can be esti­ma­ted ther­mody­na­mi­cally as

[NH3][HF]< 100 (ppm)² at room temperature.

HF(g) + NH3(g) ↔ NH4F(s)

New Approach for Was­te Gas Treat­ment in Wet Bench Processes

The pro­blem was pre­viously sol­ved by sepa­ra­ting the exhaust of each tool cham­ber into pipes for alka­li­ne, aci­dic or orga­nic exhaust, as shown in Figu­re 1. The sepa­ra­tion into the­se three types of exhaust was reali­zed by con­nec­ting the exhaust pipes from seve­ral cham­bers of a wet-clean tool to a so-called swit­ching box, whe­re lar­ge dimen­sio­ned val­ves direc­ted each exhaust flow from a sin­gle cham­ber into one of the three spe­ci­fic cen­tral exhaust pipes, depen­ding on the actual pro­cess step in each cham­ber. Each exhaust pipe was then trea­ted by a spe­ci­fic cen­tral scrubber.

Although the swit­ching boxes per­for­med well, seve­ral draw­backs for­ced chip manu­fac­tu­rers to search for a modi­fied solu­tion: to avoid pres­su­re modu­la­tions in the exhaust pipes the swit­ching boxes feed air into the dead ends that are tem­po­ra­rily not con­nec­ted with a pro­cess cham­ber. Each exhaust pipe the­re­fo­re has to con­duct the maxi­mum pos­si­ble flow for all pro­cess cham­bers at all times. Sin­ce the boxes have to be loca­ted clo­se to the tool, insi­de the clean envi­ron­ment, a high volu­me of costly clean air is lost and has to be repla­ced. Downs­tream of each cen­tral exhaust pipe, the cen­tral scrub­ber sys­tems are always loa­ded with the maxi­mum flow of air, requi­ring a scrub­ber design capa­city that is a mul­ti­ple of the actual exhaust from the pro­cess tools. The area whe­re the auxi­liary equip­ment is ins­ta­lled usually corres­ponds to the clean room squa­re foo­ta­ge and is the­re­fo­re limi­ted and den­sely pac­ked with equip­ment. Due to the lar­ge dia­me­ter of pipes and val­ves, the swit­ching boxes occupy a lar­ge foot­print. Sig­nals have to be gene­ra­ted to direct the flow from each pro­cess exhaust into a sui­ta­ble cen­tral exhaust pipe. Howe­ver, with a point-of-use wet-scrub­ber the­se disad­van­ta­ges can be overcome.

Gene­rally, wet-scrub­bers are well sui­ted to remo­ve solu­ble gases from a gas flow. Aci­dic and basic gases (here HF and NH3) can be redu­ced to low levels by che­mi­cal absor­ption with alka­li­ne or aci­dic scrub­bing liquid (see Equa­tions 2 and 3). For sol­vents that follow Henr­y’s Law the lowest pos­si­ble exhaust con­cen­tra­tion post scrub­ber is limi­ted by the effec­ti­ve con­cen­tra­tion in the scrub­bing liquid of the final scrub­ber sta­ge (see Equa­tion 4). But the­se theo­re­ti­cal limits requi­re that the scrub­ber is lar­ge enough, spe­ci­fi­cally, the resi­den­ce time of the gas is long enough to get the gas and liquid pha­se into equi­li­brium, and the flow of liquid is high enough so that con­cen­tra­tions in the liquid remain rela­ti­vely sta­ble along the scrub­ber column. The cha­llen­ge in the design of the SALIX was a tra­de-off to meet the spe­ci­fic res­tric­tions con­cer­ning size, effi­ciency and pres­su­re drop.

HF + OH– ↔ F– + H2O (2)

NH3 + H3O+ ↔ NH4+ + H2O (3)

kH = p / caq (4)

whe­re caq is the con­cen­tra­tion in the liquid phase

p is the par­tial pres­su­re in the gas phase

Figu­re 2 shows the sche­ma­tic dia­gram of a new point-of-use wet-scrub­ber solu­tion called SALIX. All pro­cess cham­bers from one pro­cess tool are con­nec­ted into one wet-scrub­ber, which is con­nec­ted to only one cen­tral exhaust pipe. An addi­tio­nal fea­tu­re, reques­ted by cus­to­mers is a bypass line around the scrub­ber to keep the tool ven­ti­la­ted during scrub­ber main­te­nan­ce or in case of fai­lu­re. Loca­ted directly at the entran­ce of the exhaust pipes into the wet-scrub­ber are spray nozz­les that initially redu­ce the con­cen­tra­tion of solu­ble gases befo­re the dif­fe­rent exhaust flows get mixed. The gas then flows through two pac­ked scrub­bing sta­ges, which can be ope­ra­ted with dif­fe­rent scrub­bing liquids. At the end of each scrub­ber sta­ge a demis­ter remo­ves dro­plets from the gas flow. The pres­su­re in the sys­tem is con­tro­lled and kept cons­tant by a fan with an elec­tro­nic fre­quency con­ver­ter. The gas can bypass the two scrub­bing sta­ges in case of fai­lu­re or maintenance.

Ope­ra­ting Prin­ci­ple of the Point-of-Use-Solu­tion SALIX

In order to accom­mo­da­te limi­ta­tions in foot­print and height the scrub­ber sta­ges have to be as com­pact as pos­si­ble. A typi­cal column cons­truc­tion with seve­ral scrub­bing units stac­ked on top of each other is the­re­fo­re not sui­ta­ble. Two rec­tan­gu­lar pac­ked coun­ter-flow sta­ges are arran­ged side-by-side insi­de of one fra­me. Becau­se the gas trans­fer into the liquid pha­se depends on the con­tac­ted liquid sur­fa­ce, both sta­ges are desig­ned as pac­ked columns. The selec­tion of the tight pac­king mate­rial was a cri­ti­cal point in the design, as sma­ller pac­king mate­rial gene­rally increa­ses the con­tact sur­fa­ce, but also leads to a higher pres­su­re drop across the pac­king. Sin­ce the scrub­ber height is neces­sa­rily fixed, pac­king mate­rial must have a spe­ci­fic sur­fa­ce area as lar­ge as pos­si­ble; yet enough open volu­me bet­ween the indi­vi­dual units of pac­king mate­rial is requi­red so that the exhaust fan can still cons­tantly com­pen­sa­te for the pres­su­re dif­fe­ren­ce across the pac­king. Narrow areas and bends within the gas flow path and the demis­ters con­tri­bu­te to the pres­su­re drop along the sys­tem. Becau­se both sta­ges are coun­ter-flow columns, the gas has to flow a com­plex path from the entran­ce on the side of the scrub­ber to the bot­tom of the first sta­ge and again from the top of the first sta­ge to the bot­tom of the second sta­ge and out of the side of the scrub­ber. The poten­tial for pres­su­re drop in the sys­tem nee­ded to be miti­ga­ted for the com­pact design of the SALIX.

Nasswäscher SALIX reinigt Abwässer aus Nassbank-Prozessen in der Halbleiterindustrie

New Scrub­ber SALIX for Was­te Gas Treat­ment in Wet Bench Processes

SALIX

The design of the SALIX allows dif­fe­rent inlet con­fi­gu­ra­tions with up to 12 sepa­ra­te 6″ inlets with a total flow of 1.000 — 4.000 m³/h. The foot­print of the scrub­ber is only 4 m² (less than 40 % of the older ins­ta­lla­tions) at a height of 2.04 m without the bypass; it mea­su­res 2.57 m inclu­ding the bypass option. With the fre­quency-con­tro­lled fan a pres­su­re sta­bi­lity of +/-10Pa during pro­cess is achie­ved. Total exhaust flow and con­sum­ption of clean room air is about one third of the older ins­ta­lla­tions. For pro­cess tools with fewer cham­bers or less total exhaust flow a sma­ller ver­sion was desig­ned, with only a 1‑stage scrub­ber. Futu­re enhan­ce­ments will con­cen­tra­te on further impro­ve­ment of pres­su­re sta­bi­lity and the opti­mi­za­tion of media consumption.

The SALIX sys­tem was desig­ned, built, and cer­ti­fi­ca­tion obtai­ned accor­ding to the SEMI S2 safety stan­dard within 6 months. A bypass fun­ction was added by cus­to­mer request. The wet-scrub­ber was eva­lua­ted at a cus­to­mer site, whe­re it was con­nec­ted to 12 cham­bers from a sin­gle-wafer pro­cess tool. The­re pres­su­re sta­bi­lity at the scrub­ber inlet during nor­mal ope­ra­tion and in case of swit­ching to bypass was opti­mi­zed and qua­li­fied. Inlet and outlet con­cen­tra­tions were mea­su­red at the pro­cess line using FTIR tech­no­logy. As a result, the sys­tem was qua­li­fied by the cus­to­mer for use at this com­pan­y’s sin­gle wafer wet-clean pro­ces­ses without any need for further modification.

Con­clu­sion

The point-of-use wet-scrub­ber con­cept for the treat­ment of the exhaust from sin­gle-wafer wet-clean tools was reali­zed with the design of the SALIX sys­tem and suc­cess­fully eva­lua­ted at a fab. Low emis­sion con­cen­tra­tions were achie­ved and salt par­ti­cle for­ma­tion from acids and bases eli­mi­na­ted. The advan­ta­ges over the older sys­tem are: sma­ller and less com­plex exhaust-piping; sma­ller load to cen­tral scrub­ber sys­tems; higher fle­xi­bi­lity for pro­cess chan­ges; redu­ced loss of clean room air, and sma­ller footprint.

DAS Headquarter in Dresden

Your Con­tact for Was­te Gas Treat­ment in the Semi­con­duc­tor Industry:

Mr. Ray Morgan
Sr. Sales Manager