Ozone chemistry for BEOL resist stripping – A systematic ... - Chemie

Ozone chemistry for BEOL resist stripping – A systematic analyticalattempt to understand the interaction of O 3 with modern DUV-resistsMathias Guder 1a , Maren Pellowska, Maximilian Pohland, Michael Dalmer 2cBernd Kolbesen 1b1 Goethe University Frankfurt/Main, Institute of Inorganic and Analytical Chemistry,Max-von-Laue-Str. 7, D-60438 Frankfurt, Germany2 SEZ AG, SEZ-Straße 1, A-9500 Villach, Austriaa m.guder@chemie.uni-frankfurt.de, b kolbesen@chemie.uni-frankfurt.de, c M.Dalmer@at.sez.comKeywords: ozone, radicals, decomposition, radical trapping, resist characterizationIntroductionThis work deals with the application of ozonated water for the BEOL stripping of DUV-resists. Forthis purpose analytical techniques for the quantification of molecular O 3 and the radicals generatedin the water as well as methods for the non-destructive analysis of resists on wafers have beenstudied. The aim is to be able to determine the concentrations of O 3, its decomposition, and theradicals produced thereby, under the influence of various parameters, and to correlate these datawith the polymer structure of the resist on the wafer and the efficiency of resist removal.Ozone decomposition (UV-Vis)The ozone decomposition was studied via direct UV-Vis measurements at 260 nm. The effects ofrecirculation as opposed to stopped flow, pH-value (1-9) and temperature (25 °C vs. 50 °C) wereinvestigated. Concentration curves were recorded, half lifetimes τ were determined and reactionorders n, reaction constants k and activation energies E A were calculated (Table 1).O 3-decompositionO 3-decomposition[O 3] [mol/l]0,00100,00090,00080,00070,00060,00050,00040,00030,00020,0001[O 3] [mol/l]DIW/O 3: recirculation vs. stopped flow at RT0,00110,00100,00090,0008+H2SO4 RT pH=1 (recirc)+H2SO4 RT pH=1 (stopped flow)pure RT pH=3,5 (recirc)pure RT pH=3,5 (stopped flow)0,00070,00060,00050,00040,00030,00020,00010,00000 600 1200 1800 2400 3000 3600decomposition time t [s]Fig. 1 - Effect of recirculation on decompositionO3-decompositionDIW/O 3: RT vs. 50 °C with recirculation+H2SO4 RT pH=1+H2SO4 50 °C pH=1pure RT pH=3,5pure 50 °C pH=3,5+NH4OH RT pH=9+NH4OH 50 °C pH=90,00000 100 200 300 400 500decomposition time t [s]Fig. 3 - Effect of temperature on decomposition+H 2 SO 4pH=1purepH=3.5+NH 4 OHpH=9DIW/O 3: Effect of pH at RT with recirculation[O 3] [mol/l]0,00100,00090,00080,00070,00060,00050,00040,00030,00020,0001+H2SO4 RT pH=1pure RT pH=3,5+NH4OH RT pH=90,00000 100 200 300 400 500decomposition time t [s]Fig. 2 - Effect of pH-value on decompositionτ [s] / n -k [1/s]nE A[kJ/mol]25 °C 70; 80; 80 6.34x10 -3 17.57n=1.150 °C 75; 60; 55 n=0.825 °C 110; 95; 6.49x10 -3 22.89100 n=1.050 °C 60; 60; 50 n=0.925 °C 3; 4.3; 7.4 6.27x10 -2 22.5350 °C 4; 6.6; 6Table 1 - Ozone decomposition

Of the three parameters studied recirculation unexpectedly produced the largest increase in thespeed of decomposition - up to 80 times faster with H 2 SO 4 and even 35 times in pure ozonatedwater. Altering the pH from pH=3.5 to 9 increased the speed of decomposition by a factor of 22 andthe increase in temperature only produced an increase of 1.5 times.1; 2; 3; 4The DDL-method for radical trappingThe DDL-method is based on the detection of stable formaldehyde (CH 2 O) formed by the oxidationof either DMSO (Fig. 5) or MeOH (Fig. 7) which is then further converted by the Hantzschreaction 2 to the yellow dye DDL (Fig. 4) and quantified via photometry. The calibration plot forboth trapping reactions is obtained by measuring the OH radicals from the solution and the onesgenerated by UV irradiation of known amounts of H 2 O 2 (Fig. 6; Fig. 8) as standardized additives.2OOacetylacetoneRadical trapping with DMSO 12 * OH + 22 *CH 3 2 O 2+HFig. 5 - Trapping with DMSORadical trapping with MeOH 3OSDMSOOHformaldehydeOHAc/Ac -NH 4+O43 5(pH=4.75)N +1H3,5-diacetyl-1,4-dihydrolutidine (DDL)Fig. 4 – Conversion of CH 2 O to DDLSOHmethanesulfinic acid+ 2CH 3OO *O2CH 3OO * + CH 3OH +H H2+O 22 * CH 3O(yellow, absorbs at 412 nm)*OH generation by UV decomposition of H 2O 2at 310 nm3,0 +H2SO4+H2O2 pH=1+H2O2 pH=3.52,5 +NH4OH+H2O2 pH=90,00,0000 0,0005 0,0010 0,0015 0,0020[*OH] [mol/l] in 10mlFig. 6 – Calibration of DMSO-trapped ·OHHOH OOH2 OCalibration of MeOH-trapped *OH+ * OHC**OH generation by UV decomposition of H 2O 2at 310 nmHH HH-H 2O - * 4,0+H2SO4+H2O2 pH=1OOH H H+H2O2 pH=3.53,5MeOHformaldehyde+NH4OH+H2O2 pH=93,0Fig. 7 - Trapping with MeOH2,5+++ H 2 SO 4 pH=1 DMSO: y = 584−25 + 0 .06−0 . 032,0++MeOH: y = 922−189 + 0 .4−0 . 21,5++pure pH=3.5 DMSO: y = 1093−39 + 0 .06−0 . 041,0MeOH:++y = 1052−90 + 1 .35−0 . 080,5+++NH 4 OH DMSO: y = 1356−32 + 0 .23−0 . 030,0++0,0000 0,0005 0,0010 0,0015 0,0020pH=9MeOH: y = 3990−652 + 0 .8−0 . 2[*OH] [mol/l] in 10 mlTable 2 – Calibration curves for the DDL-methodFig. 8 - Calibration of MeOH-trapped ·OHBoth calibrations show good linearity over the selected region. In both cases the gradient increaseswith the pH-value of the sample. With faster O 3 -decomposition at high pH-values the radicalconcentration increases as well. Both trapping variations show the same tendency but unfortunatelythe difference between the ·OH concentrations obtained which is only slight at pH 1 increases up toa factor of 10 at pH 9. A reason for this might be known advanced oxidation processes (AOP’s) 4 .absorbanceabsorbance2,01,51,00,5Calibration of DMSO-trapped *OH

Iodometric titration for radical trappingWhile the UV-Vis method allows a selective determination of molecular O 3 , with iodometrictitration the sum of all oxidizing components (O 3 and ·OH in this case) can be measured. Theconcentration of radicals is then obtained as the difference between the two values.2·OH + 2S 2 O 2- 3 → 2OH - + S 4 O 2- 6 (5)O 3 + 2H + + 2S 2 O 2- 2-3 → O 2 + S 4 O 6 + H 2 O (6) mol ·OH = mol S 2 O 2- 3 - 2x mol O 3Two variations of the iodometric titration for pure ozonated water were tried out: one with(NH 4 ) 6 Mo 7 O 24 as a catalyst for fast and complete oxidation of 2I - to I 2 and the other without.Methodlower limit [·OH]DIW/O 3 pureupper limit [·OH]DIW/O 3 pureDMSO trapping direct calibration / /standard addition 1.07x10 -3 1.43x10 -3MeOH trapping direct calibration 0.39x10 -3 0.401x10 -3standard addition 0.69x10 -3 1.07x10 -3iodometric titration without catalyst 0.61x10 -3with catalyst 1.02x10 -3Table 3 – Comparison of radical concentration calculated from the DDL-method (Table 1) and iodometric titrationResist stripping efficiency for a polyhydroxystyrene (PHS)-type DUV-resistThe resist stripping was studied with different additives to vary the pH-value from 1 to 8 for each ofthe four applied process steps. The measurements were carried out in a beaker with 150 mLstripping solution under constant stirring. The experiments were conducted in 2 sets, one at 25 °Cand the other at 50 °C. The ozone concentration was continuously monitored by UV/Vis and thestripping solutions were renewed when it dropped to half its initial value (0.85 mM/41 ppm → 0.42mM/21 ppm in 5-7 min.). For pH=7 & 8 one minute was chosen as the shortest manageableinterval.Fig. 9 - Results of resist strippingA= +H 2 SO 4 ; B= +HNO 3 ; C= +HAc; D= +HAc/Ac - ; E= +PO 4 3- ; F= +PO 43-The results show that the minimum resist stripping time was obtained for pH=4.8 with an HAc/Ac - -buffer. Higher and lower pH-values decreased the stripping efficiency. Surprisingly an additionaloxidant like HNO 3 produced a strong negative effect on the stripping efficiency making it the worststudied additive.

IR-studies on a polyhydroxystyrene (PHS)-type DUV-resistWith direct IR-measurements on the wafer it is possible even in transmission mode to followstructural changes at successive stages of the process. Knowledge of the resist composition enablesthe assignment of peaks to functional groups of the components of the resist and allows thedetermination of chemical structures and the changes they undergo. With IR-spectroscopy,structural changes that occur during the process steps (here from the soft bake stage to the softbake+litho stage - Fig. 10) can be followed as well as changes during the resist strip (Fig. 11). Thedifferential spectra in Fig. 11 show the sharp decrease of a peak at 1513 cm -1 corresponding to thedecomposition of an aromatic C=C-bond from the PHS side group as well as an increase inintensity at 1745 cm -1 corresponding to the formation of a carboxylic C=O-bond. These resultssuggest a classic ozonolysis reaction under oxidizing conditions (Fig. 12).IR-study of polyhydroxystyrene type DUV-resist(tracking of structural changes after different process steps)absorbance*PGOpolyhydroxystyrene/PHSwith protecting group PG*resist+softbake+lithoresist+softbake4000 3500 3000 2500 2000 1500 1000wavenumber (1/cm)absorbancesample 1.2 - coated+softbake in DIW/O 3for 5 min.aromatic C=C=C-H0,120,100,080,06PHS-type DUV-resistaromatic C=C=C-H-CH 3C=Oaldehydes,ketonesO0,04R0,020,00-0,02sample 1.2 before O3-0,04 sample 1.2 differential-0,06-0,084000 3500 3000 2500 2000 1500 1000wavenumber (1/cm)-C-Oester * *PGOOO 3*OHOOshould be stable under oxidativeconditions - but conversionOto Acyloin under reductiveconditionsFig. 10 - IR-spectra at successive Fig. 11 – Differential spectra for removal Fig. 12 – Possible ozonolysis ofprocess stages process with DIW/O 3 after softbake the aromatic side groupSummaryThe UV-Vis detection of molecular O 3 is a technically mature method. The decomposition rate ofO 3 increases with recirculation, temperature and pH-value. The faster decomposition corresponds tohigher radical concentrations. ·OH values determined for the two DDL trapping variations differ athigher pH-values up to a factor of 10 whereas that of DMSO (without any additive = 2-3x10 -3[mol/l]) is close to that obtained by iodometric titration (Table 3). IR-spectroscopy can be used tofollow resist changes on the wafer but a precise knowledge of the resist composition is needed.Differential spectra taken during the stripping process support the assumption of classicalozonolysis as a possible decomposition reaction. For a more sophisticated analysis the knowledgeof the exact resist composition is essential. This is neither available from the producers nor couldwe obtain it by GC-MS or HPLC-MS. Resist stripping at the pH of 4.8 showed the best results forall process steps; stripping times increased above and below this value. Increasing the temperatureto 50 °C generally results in increased resist stripping. However, the interval between renewing thesolutions varies and can be too short for a fresh solution to attain this temperature.References[1] Chao Tai et al.; Analytica Chimica Acta 527 (2004) 73-80[2] T. Nash; Biochem J. 1953 Oct;55(3):416-421[3] Dissertation Marion Lackhoff: „Photokatalytische Aktivität ambienter Partikelsysteme“Institut für Wasserchemie und chemische BalneologieLehrstuhl für Hydrogeologie, Hydrochemie und UmweltanalytikDer Technischen Universität MünchenChapter 2.2.2 Page 11[4] Piotr Ulanski, Clemens von Sonntag; J. Chem. Soc., Perkin Trans. 2, 1999, 165-168*