Measurement and Control of Trace Moisture in Corrosive Gases

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1 Mesurement nd Control of Trce Moisture in Corrosive Gses Jinlong Yo, Hns H. Funke nd Mrk W. Rynor, Mtheson Tri-Gs, Inc., Advnced Technology Center, Longmont, Colordo, 851, USA Astrct Mesurement nd control of trce moisture in corrosive gses used in etch nd other semiconductor mnufcturing steps is importnt to minimize line corrosion, mintin process consistency nd mximize device yields. A commercil cvity ring-down spectrometer (MTO-1, Tiger Optics) specificlly designed for corrosive gs nlysis hs een investigted for trce moisture detection in nitrogen, chlorine nd hydrogen romide with regrds to ccurcy, sensitivity, linerity, response time nd durility. The instrument response to moisture level etween nd 2 ppm were liner nd the instrument equilirted within 2 minutes to 9% of the finl redings fter concentrtion chnges etween c.1 pp nd 3 ppm. The nlyzer demonstrted good corrosion resistnce over the test period nd ws cple of moisture detection t low pp levels. The cpility of point-of-use purifier (Nnochem Metl-X) to remove moisture from oth HBr nd Cl 2 to <.1 ppm ws verified with the instrument. Introduction The presence of trce impurities such s moisture in process gses for semiconductor mnufcturing is known to negtively ffect device qulity. Moisture not only cuses corrosion in gs delivery systems tht cn led to prticle contmintion nd equipment filure, ut lso prticiptes in wfer surfce chemistry nd thus leds to electricl defects nd other undesirle side effects. Additionlly, moisture concentrtions from liquefied gs sources potentilly re not stle throughout the lifetime of the source (1,2), nd cn cuse process instilities. These fctors hve resulted in growing demnd for more sensitive nd relile nlyticl instrumenttion tht is cple of mesuring trce moisture in gs production nd purifiction processes, s well s for on-line monitoring of gses supplied to process tools. Furthermore, efficient purifier mterils re required for removl of trce moisture from gses t the point-of-use. Vrious instrumentl nd sensor-sed techniques for mesurement of trce moisture in electronic specilty gses hve recently een reviewed (3). Spectroscopic methods re widely ccepted nd lso viewed s the most promising cndidtes for the next genertion of rel-time trce moisture nlyzers for severl resons. Opticl techniques re non-destructive nd only require opticl ccess to the rective or corrosive gs. Further, they usully provide rpid response times, high sensitivity nd selectivity nd re suitle for on-line monitoring pplictions. Among these is cvity ring-down spectroscopy (CRDS), which hs recently een dopted in commercil instrumenttion. Reports of optimized sensitivity studies in the literture show tht the CRDS nlyzer, when used in inert gs service, is cple of su-pp moisture detection (4,5). CRDS studies in corrosive gses such s HCl with singledigit pp level detection limits hve lso een reported (6). In this work, the performnce of commercil CRDS nlyzer (Tiger Optics MTO-1) hs een investigted for the nlysis of moisture in chlorine nd hydrogen romide nlysis. The ccurcy, linerity, response time nd durility of the instrument ws mesured. In ddition, the performnce of point-of-use purifier for moisture removl to <1 pp ws verified using this instrument. Operting Principles CRDS is sed on the sorption of IR rdition nd hs een explored for detection of moisture t su-pp to ppm levels (4-12). Figure 1 shows schemtic digrm of the min instrumentl components nd operting principles of the CRDS system. Some of the components in the commercil instrument tested in this work re shown in the photogrph in Figure 2. The instrument opertes y directing n IR lser em into the ring-down cvity equipped with highly reflective mirrors tht contins the smple gs. As the cvity fills with light, n incresing mount of the light leks through the mirror into the photodiode detector. At predetermined intensity of light reching the detector, the lser em is interrupted nd the ring-down decy lifetime of the cvity is recorded. The moisture concentrtion in smple gs is clculted from the following eqution(4, 8): N = (1/τ (ν) 1/τ empty ) /(c σ(ν)) c is the speed of light, N is the moleculr density of moisture, σ is the sorption cross section of wter molecule in cm 2, τ ( ν) is the smple gs ring-down time, τ empty is the ring-down time without moisture present, nd ν is the lser light frequency. The effective pth-length is of the order of kilometers, which results in high sensitivity. The Tiger Optics CRDS nlyzer is cple of scnning the spectrum etween nm y regulting the temperture of the lser diode. The two moisture lines tht cn e used for quntifiction in this rnge re t nd nm, corresponding to lser diode temperture of nd C, respectively. These lines rise from rottionl trnsitions (6,6, 6,6,1) nd (2,,2 3,,3) in the ν1 + ν3 (1,,1,,) virtionl SEMICON West 24 ISBN # SEMI 24 Semiconductor Equipment nd Mterils Interntionl

2 1/τ (1/µs) 1/τ (1/µs) nd of wter molecules (13, 14). As spectr tke severl hours to scn, they re not routinely mesured. However, these dt cn e used to ccurtely confirm the moisture concentrtion nd ccount for ny mtrix gs sornce tht my ffect the moisture redings in norml opertion. Smple in Cvity Ring -down Asorption Cell Smple out Mirror Mirror Highly Reflective Mirrors Intensity Acousto -Opticl Modultor Photodiode Detector Cvity Ring -down Decy Time Opticl Fier Trigger Isoltor Computer CW Diode Lser Figure 1. Schemtic digrm of CRDS instrument nd nm t concentrtion of 9 pp nd 23 pp respectively. The stright seline of the Cl 2 spectrum indictes miniml interference from the mtrix gs nd single-point seline correction is justified. The HBr spectrum showed curved seline in the H 2 O spectrl region tht initilly resulted in n offset of the redings. The dt processing routines were modified to ccount for the seline fetures Wvelength (nm) Wvelength (nm) detector cell mnifold Figure 2. Internl view of the Tiger Optics MTO-1 CRDS instrument. For routine moisture nlysis, the instrument monitors the ring-down time t nm t predetermined intervl. Prior to inititing run, the ring-down time t nm corresponding to lser temperture of 17.8 o C (where wter does not sor) is mesured. This is used to clculte the seline t nm. The seline is employed s reference ginst which totl moisture mesurements in the cvity re susequently mde. Becuse the ring-down is only dependnt on the moisture present inside the gs cell, the mesurements re unffected y externl humidity nd purge of the opticl ench s often used in FTIR is not necessry. Figure 3. CRDS spectr showing moisture nds in () Cl 2 nd () HBr mtrices. Experimentl The instrument used in this work (MTO 1, Tiger Optics, Wrrington, PA) hd een in prior corrosive gs service ut ws certified to e in good working order y the mnufcturer. A schemtic of the mnifold used for verifying the clirtion of the CRDS instrument, gs smple nlysis nd purifier evlution is shown in Figure 4. Clirtions were performed y dding nitrogen tht ws humidified with moisture genertor (Spn Pc 261, Kin- Tek, LMrque, TX ) to the HBr or Cl 2 smple gs. The output from the moisture genertor ws verified using chilled mirror hygrometer (CR-3, Buck Reserch, Boulder, CO). The ddition of N 2 to the smple gses cused seline shift, nd ws compensted y seline-tune. Ring-down mesurements were performed t flow rte of 1 slpm nd cvity pressure of 4 psig. Figure 3 shows two spectr tht were collected in Cl 2 nd HBr mtrix nd tht cover the two moisture lines t SEMICON West 24 ISBN # SEMI 24 Semiconductor Equipment nd Mterils Interntionl

3 N 2 Cl 2 or HBr N 2 N 2 Cl 2 or HBr SPAN PAC MFC MFC Purifier MFC Inlet 2 Inlet 1 CRDS to scruer Figure 4. Schemtic digrm of the CRDS smpling mnifold. Using the instrument, the moisture removl efficiency of 5 ml Nnochem Metl-X purifier (Mtheson Tri- Gs) ws evluted with moisture chllenges of ~2.3 ppm in HBr nd ~.9 ppm in Cl 2 mtrix t 4 psig nd flow rte of 125 sccm. The purifier ws lso used to generte dry gs for the instrument testing. Results Instrument Response Figure 5 shows plot of instrument moisture reding versus ctul moisture concentrtion in N 2 tht ws used to verify tht the nlyzer ws clirted correctly. The instrument moisture reding greed well with the ctul moisture concentrtion in the -2 ppm rnge investigted. The correltion coefficient ws A 5. pp regression sed (weighted lest squres) detection limit ws clculted from the clirtion dt (15), which is pproximtely two to three times higher thn the prcticl detection limit previously reported y Hrdimn nd coworkers (1). This is likely ecuse the instrument hd een exposed to corrosive environments prior to the tests nd ws not fully optimized for highest sensitivity in inert gses. Figure 6 shows the moisture response of the CRDS instrument to different moisture levels with time in chlorine. The response curve generted from these dt in Figure 6 ws lso liner with correltion coefficient of However, in contrst to mesurements in N 2, the instrument overestimted the ctul moisture concentrtion y ~16% in Cl 2. This discrepncy is likely cused y spectrl prmeters such s rodening coefficients in the instrument softwre tht were not optimized for moisture detection in Cl 2. A detection limit of 11.3 pp using weighted lest squres method ws clculted from the dt [H 2 O] ( pp) Figure 6. () Moisture response of the CRDS instrument with time during clirtion of moisture in Cl 2, nd () Plot of instrument moisture reding versus ctul moisture concentrtion in Cl [H 2 O] (pp) Cylinder Anlysis The MTO 1 hs een used to nlyze moisture in severl HBr nd Cl 2 cylinder sources. Figure 7 shows n exmple of such nlysis for Cl 2 cylinder tht contined ~14 pp moisture in the gs phse. The smple gs ws pssed through Nnochem Metl-X purifier nd fter the system ws dried elow ~5 pp, the purifier ws ypssed to otin the gs phse moisture concentrtion in the cylinder. Figure 5. Plot of instrument moisture reding versus ctul moisture concentrtion in N 2. SEMICON West 24 ISBN # SEMI 24 Semiconductor Equipment nd Mterils Interntionl

4 [H2O] (pp) [H2O] (pp) Wet-up: Reches 9% of finl reding in 2 minutes strem strem Dry-down: Reches 1% of initil reding in 25 minutes Figure 7. CRDS mesurement of moisture in gs phse Cl 2 from cylinder source. Response Time The nlysis of nd humidified smple strems tht were introduced through two instrument inlets ws used to chrcterize the response time of the instrument to chnges in moisture level. Figure 8 shows the response to ppm chnges of moisture in nitrogen. The redings reched 9% of the finl vlue in <5 minutes, confirming erlier work y Yn nd coworkers(4). [H2O] (pp) Wet-up: Reches 9% of finl reding in 2 minutes strem strem Dry-down: Reches 1% of initil reding in 25 minutes [H2O] (pp) Figure 8. Response of the CRDS instrument to chnges in moisture concentrtion when switching from dry (Nnochem OMX ) nitrogen strem to strem contining 1 ppm moisture. For oth Cl 2 nd HBr, the instrument response to moisture chnges ws slower nd equilirtion to 9% of the finl redings were otined within ~2-25 minutes. Dry-down times typiclly were few minutes longer then the time required for wetting the system. Two exmples in Cl 2 nd HBr re shown in Figure 9. A humidified HBr strem with concentrtion of 2.3 ppm ws used to chllenge 5 ml Nnochem Metl-X purifier. The moisture level ws reduced to 83 pp within 3 hours. The Metl-X purifier mteril ws lso effective in removing moisture in Cl 2 where 9 pp chllenge resulted in outlet concentrtions of 72 pp fter out 3 hours of gs flow. Reducing the moisture concentrtion in process gses to these levels will significntly reduce line corrosion nd improve process stilities. Figure 9. Moisture in Metl-X nd humidified gs strems from two inlets during purifier testing: () Cl 2 nd () HBr. Resistnce to Corrosive Gses The CRDS instrument ws used in HBr nd Cl 2 service for out 6 months. The ring-down time slowly decresed during this time period likely due to decresing mirror reflectivity. This ehvior is shown in Figure 1 t lser diode temperture of 17.8 C ( nm) in oth nitrogen nd HBr. Becuse mesurements were not continuous nd were performed with different corrosive nd inert gses, the decresing ring-down time only illustrtes generl trend in loss of performnce. The dt cnnot e used for quntittive predictions. As the ring-down time decresed the sensitivity decresed ccordingly. The instrument ccurcy ws not ffected y this chnge nd the slope of the clirtion for the different gses remined constnt. The loss in mirror reflectivity might e due to deposition products from the smple gses. Etching of the mirror surfce is less likely since exposure to liquid HBr for 6 month y the mnufcturer did not show ny degrdtion. The time-frme of mirror degrdtion is significntly lengthened in comprison to long pth-length FTIR cells where uncceptle reflectivity loss in HBr ws oserved within few dys to weeks. SEMICON West 24 ISBN # SEMI 24 Semiconductor Equipment nd Mterils Interntionl

5 τ empty (µs) τ empty (µs) time (dy) Figure 1. Decy of ring-down time in nitrogen ( ) nd HBr ( ) over test period of 6 months. Conclusions Mesurement of moisture in Cl 2 nd HBr t pp to ppm level ws explored using Tiger Optics CRDS MTO-1 nlyzer. The CRDS instrument operted relily nd ws found to e suitle for cylinder nlysis, on-line monitoring pplictions nd purifier evlutions. For exmple, Nnochem Metl-X purifer ws shown to remove moisture t the ppm level in Cl 2 nd HBr to less thn 1 pp. The instrument is sensitive to low pp moisture with liner response in the tested rnge of -2 ppm. The instrument hs een shown to withstnd the corrosive effects of gses such s HBr nd Cl 2 over test period of 6 months. The response in Cl 2 nd HBr is pproximtely 4 times slower thn in N 2 mtrix nd therefore sufficient equilirtion time is required to otin relile results in the corrosive gses. Liner response to ctul moisture concentrtion ws otined in ll gses, ut the presence of ckground interference nd other spectroscopic rtifcts such s line rodening due to dditionl components in the gs strem resulted in different slopes depending on the smple gs. This illustrtes the need for pproprite dt nlysis nd clirtion procedures if ccurte concentrtion redings re required. 4 W.B. Yn nd C. Yu, SEMI Workshop Proceedings: New dvnces in detection of trce (<1 pp) impurities in ulk inert gses, SEMICON West 23, Sn Frncisco, CA, (22) 5 W.B. Yn nd C. Yu, SEMI Workshop Proceedings: Advnces in detection of trce moisture in specilty gses, SEMICON West 23, Sn Frncisco, CA, (23) 6 V. Vors, S. Dhendhnoo, D. Yesenofski, nd M. Wgner, SEMI Workshop Proceedings: Advnces in detection of trce moisture in specilty gses, SEMICON West 23, Sn Frncisco, CA, (23) 7 W.B. Yn, Proceedings of SEMICON West 23, Sn Frncisco, CA, 89 (23) 8 G. Berden, R. Peeters nd G. Meijer, Interntionl Reviews in Physicl Chemistry, 19, 565 (2) 9 K. W. Busch nd M. A. Busch, Ed., Cvity Ringdown Spectoscopy: An Ultrtrce-Asorption Mesurement Technique, ACS Symposium Series 72, Oxford University Press, Wshington, DC (1999) 1 R. Hrdimn, D. McKee nd K. Kimmerle, Proceedings of SEMICON West 23, Sn Frncisco, CA, 11 (23) 11 D. C. Hovde, J.T. Hodges, G.E. Scre, nd J. A. Silver, Applied Optics, 4, 829 (21) 12 S.Y. Lehmn, K.A. Bertness, nd J.T. Hoges, Journl of Crystl Growth, 25, 262 (23) L. S. Rothmn, et l., Journl of Quntittive Spectroscopy nd Rditive Trnsfer, 6, 5 (23) 15 J. D. Hogn, Ed., Specilty Gs Anlysis, A Prcticl Guide Book, Wiley-VCH, New York (1997) Acknowledgements The uthors would like to thnk Clvin Krusen nd Wen-Bin Yn of Tiger Optics for helpful discussions, nd for their ssistnce with spectrl simultions nd softwre modifictions. References 1 H. H. Funke, M. W. Rynor, B. Yucelen, nd V. H. Houlding, Journl of Electronic Mterils, 3, 1438 (21) 2 E. Flherty, C. Herold, J. Wojcik, D. Murry, A. Amto, nd S. Thompson, Solid Stte Technology, 3, 69 (1987) 3 H.H. Funke, B.L. Grissom, C.E. McGrew nd M.W. Rynor, Review of Scientific Instruments, 74, 399 (23) SEMICON West 24 ISBN # SEMI 24 Semiconductor Equipment nd Mterils Interntionl

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