olid tate Phenomena Vol. 134 (2008) pp 79-82 Online available since 2007/ov/20 at www.scientific.net (2008) Trans Tech Publications, witzerland doi:10.4028/www.scientific.net/p.134.79 Peracetic acid as active species in mixtures for selective etching of ige/i layer systems Aspects of chemistry and analytics Mathias Guder 1, a, B. O. Kolbesen 1,b, Cécile Delattre 2,c, C. Fischer 3,d, H. chier 3,e, G.Wagner 3,f 1 Johann Wolfgang Goethe-University Frankfurt am Main, Institute of Inorganic and Analytical Chemistry, Max-von-Laue-tr. 7, D-60438 Frankfurt, Germany 2 OITEC, Advanced Technology Devision, Parc technologique des Fontaines, F-38926 Bernin Crolles Cedex France 3 EZ, Draubodenweg 29, A-9500 Villach, Austria a m.guder@chemie.uni-frankfurt.de, b kolbesen@chemie.uni-frankfurt.de, c cecile.delattre@soitec.fr, d C.Fischer@at.sez.com, e H.chier@at.sez.com, f G.Wagner@at.sez.com Keywords: ige, soi, peracetic acid, selective etching, photometry, iodometric titration, kinetics, equilibrium Introduction The ever increasing demand for faster computers has exhausted the capabilities of silicon as the basic material for chip production. This is due to the physical limits of silicon itself and can only be overcome by switching to another material. Until this problem is solved a variant of the normal silicon, called strained silicon (si) is used. In the production of soi-wafers (strained silicon on insulator) an intermediate ige/si layer system is created (Figure 1) from which the ige layer is then removed to expose the strained silicon layer. i 1-X Ge X si HO 3 si si io 2 io 2 io 2 i i i Figure 1 - Intermediate ige/si layer system in the production of soi-wafers and removal of ige Most of the ige is removed by smart cut, a method from OITEC 1. After the smart-cut process the remaining ige is normally removed by polishing or etching with HF/HO 3 /HAc giving the soi-wafer. This mixture has a good removal rate but poor selectivity between ige and si of about 10:1. Wang, Tanner, Carns [1] showed that another mixture containing HF/H 2 O 2 /HAc in the ratio 1:2:3, with peracetic acid () formed as the active species, shows the same positive etching effects with a selectivity of up to 200:1 depending on the amount of Ge and the type and amount of doping. o a good way to reduce the loss of expensive si during polishing or etching with the HO 3 -mixture is to replace it with the -mixture. It is therefore important to have methods with which to determine the concentration even in the presence of a large excess of H 2 O 2 and in very aggressive solutions. The next step is to speed up the formation of and achieve a higher concentration at equilibrium, to improve the etching rates. Therefore methods for monitoring the concentration and additives to increase the concentration were investigated. Further etch rates and selectivity on ige/oi were studied and correlated with the concentration of. Methods This section deals with two methods for the selective determination and quantification of (peracetic acid) even in the presence of a large excess of H 2 O 2 which is a strong oxidizing agent. The two selected methods are the ABT-method [2] based on UV-Vis-spectrophotometry and the classic iodometric titration modified and extended by the use of catalase [3]. 1 Company producing 300mm OI- and soi-wafer (Company home: Bernin near Grenoble in France) All rights reserved. o part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 130.203.136.75, Pennsylvania tate University, University Park, United tates of America-14/06/14,13:42:07)
80 Ultra Clean Processing of emiconductor urfaces VIII ABT-method [2]. The ABT-method is based on the selective formation of the green dye ABT +*, a radical generated from the pale green ABT (2,2 -Azino-bis(3-ethylbenzothiazoline)-6-sulfonate diammonium salt) by but not by H 2 O 2, with a strong absorption at 415 nm. O 3 O 3 H 2 O 2 O 3 O 3 ABT pale green ABT +* green dye (415nm) Figure 2- Chemical reaction of ABT with Because of the high sensitivity of this method and the low concentration range necessary for detection (10-6 - 10-4 [mol/l]) the first step is a dilution sequence of the sample. A sample of 100 µl was diluted with H 2 O to 1 ml, from this 10 µl were taken and diluted up to 1 ml, the second step in the dilution was repeated to give a final dilution of 1:1000. To this diluted sample 333 µl of a 0.0012M KI solution (0.02 g/l) were added followed by 267 µl of a 0.001M ABT solution (0.5 g/l ABT). A ten minute interval before the UV measurement was necessary to ensure that the reaction was complete with full development of the dye. The absorption of the sample was then measured at 405 nm (the absorption could not be determined at 415 nm on the instrument available) with a UV-Vis-photometer (Merck Q118 V1.21) against water with KI and ABT added serving as a blank sample. By using this procedure a calibration curve (Figure 4) was generated using different amounts of a commercially available 32% solution. The procedure was repeated three times with the following variations prior to dilution of the sample: - to rule out the influence of H 2 O 2 on the detection 333 µl of 30% H 2 O 2 were added to the same amount of - to rule out the influence of the ph value 500 µl of 100% HAc were added - to rule out the influence of F - 167 µl 50% HF were added. All concentrations chosen were the maximum possible concentrations in the mixtures to be tested later. Iodometric titration with catalase [3]. The iodometric titration with catalase is a variation of the classical iodometric titration and allows the determination of the sum of both oxidizing compounds (+H 2 O 2 ) or that of alone. 500 µl of the sample were made up to 250ml with water and an aliquot of 25ml taken for every titration. Figure 3 shows the 2 different titrations conducted. To determine the concentration of both and H 2 O 2 the aliquot taken was acidified with H 2 O 4 to ph < 2, 10ml of 0,1M KI, a drop of a saturated solution of the catalyst (H 4 ) 6 Mo 7 O 24 *4H 2 O added and the resulting dark brown solution titrated with 0,1 a 2 2 O 3. To determine the concentration of alone an aliquot of 25ml of the sample was buffered with a phosphate buffer to ph 7. 10 µl of catalase were added and the solution left to react for 5 minutes for the complete decomposition of H 2 O 2. 1ml sample dilute with H 2O acidic + H 2O 2 acidify; add I - titrate with 2O 3 2- [] + [H 2O 2] take aliquot calculate 250ml diluted sample less acidic 25ml aliquot of diluted sample [H 2O 2] calculate buffer ph=7; add 10µl catalase acidify; add I - titrate with 2O 3 2- [] Figure 3 - Modified titration sequence for the separate determination of as well as H 2 O 2
olid tate Phenomena Vol. 134 81 It was then acidified with H 2 O 4 to ph < 2 and the mixture titrated according to the procedure used for +H 2 O 2. The concentration of H 2 O 2 is taken to be the difference in concentration obtained from the two titrations. To verify the results both types of titration were performed on the commercially available 32%. Kinetics & Equilibrium. To study the kinetics of the formation the more efficient ABTmethod was used. The standard mixture was HF/H 2 O 2 /HAc 1:2:3 which produced the best etching results [1]. amples of 1ml were taken at intervals and analyzed and the concentration read off the calibration curve. Factors which could possibly influence the kinetics were varied (amount of H 2 O addition of water or removal by H 2 O 4 ). To study the influence of the addition of water three mixtures were analyzed (HF/H 2 O 2 /HAc 1:2:3; H 2 O/HF/H 2 O 2 /HAc 0.5:0.5:2:3; H 2 O/H 2 O 2 /HAc 1:2:3). To study the influence of the removal of water HF was replaced by 95-97% H 2 O 4. For the equilibrium iodometric titration was used as an additional method to verify the results of the ABTmethod. Etch rates and selectivity. The practical application of and its influence on etch rates and selectivity with ige and soi were also investigated using 2 mixtures, each with a different amount of H 2 O 4. Wafers were etched at different bath ages and the removal determined by ellipsometry. Parallel to this the concentration of in the mixtures was determined with the ABT method to correlate concentration and etch rates. These experiments were carried out on a spin etch tool at EZ. Results and Discussion ABT-method. The two plots in Figure 4 obtained with the ABT tests show very good calibration curves with very low standard deviations. Moreover all the plots are nearly identical, indicating that the detection and quantification of is not affected by the presence of HAc, F - or even high concentrations of H 2 O 2. Hence the only obvious source of inaccuracy is the process of dilution. This is easily overcome by a skilled operator or by automation. The time required for the ABT test is moderate, taking 15 min. to complete from sample extraction to measurement. 2,0 absorbance y=0,50267 + 0783*x (%) - (solid line) + H 2 O 2 y=8033 + 0504*x (1%) - (dashed line) 0,5 1,5 2,0 2,5 3,0 3,5 Figure 4 - Calibration curves for the ABT-method, influence of H2O2. Iodometric titration with catalase. The values determined by iodometric titration were reproducible for both +H 2 O 2 without and alone with catalase. Measurements of the equilibria of real mixtures showed a good match between the results of the ABT-method and the iodometric titration. Iodometric titrations may hence be regarded in this case as suitable for the study of equilibria. Kinetics & Equilibrium. The effect of water - its addition (Figure 5) as well as its removal by H 2 O 4 (Figure 6) - on both the kinetics and the equilibrium is significant. It is easy to see that increasing the amount of water reduced the reaction rate to 1/4 of the original
82 Ultra Clean Processing of emiconductor urfaces VIII ABT-method - Comparison of kinetics at RT Influence of water HF/H2O2/HAc 1/2/3 RT H2O/HF/H2O2/HAc 0,5/0,5/2/3 RT H2O/H2O2/HAc 1/2/3 RT 0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 reaction time t[h] Figure 5 Addition of H 2 O [mol/l] 1,3 [mol/l] 0,5 [mol/l] value and equilibrium was attained at 1/3 of the original concentration of. The replacement of 50% HF with an equal volume of the water withdrawing agent 95-97% H 2 O 4 showed an almost threefold increase in the reaction rate and a higher equilibrium concentration of 2 [mol/l] representing an improvement of 25%. Etch rates and selectivity. The mixture with less H 2 O 4 produced a higher difference in the etch rates of ige and soi (Figure 7). On both substrates the etch rates increase over a relatively long period with the bath age corresponding to the increase in the concentration until all three factors reached a stable level. 2,2 2,0 Kinetics & equilibrium of in-situ formation water withdrawal with H 2 O 4 HF/H2O2/HAc 1/2/3 RT H2O4/H2O2/HAc 1/2/3 RT 0 6 12 18 24 30 36 42 48 reaction time t[h] 2 [mol/l] (dashed) Figure 6 - Water withdrawal with H 2 O 4 [mol/l] (solid) 50 0 45 8 40 1,36 etchrate [A/min] 35 30 25 20 ige OI [] 4 1,12 0 8 concentration [mol/l] 15 0,76 10 4 5 0,52 0 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 Figure 7 - Correlation of etchrates on ige & soi with conc. From the etchrates a selectivity in favour of ige over soi in the proportion 18:1 was calculated. ummary Of the many analytical techniques considered, the UV-Vis method using ABT and the variation of the classical iodometric titration with catalase have proven their reliability and accuracy. Water withdrawal is effective for both kinetics and equilibrium, temperature (reported in [4]) only affects the reaction rate. An ideal combination of water withdrawal and temperature leads to fast and efficient formation. The etchrates correlate well with conc. over the entire time of formation, with an 18:1 preference for ige over soi and high etchrates on ige. References [1] T.K. Carns, M.O. Tanner, and K.L. Wang; Chemical Etching of i 1-X Ge X in HF:H 2 O 2 :CH 3 COOH (J. Electrochem. oc., Vol 142, o. 4, April 1995) bath age [min] [2] Uwe Karst: elective photometric determination of peroxycarboxylic acids in the presence of hydrogen peroxide (Analyst, June 1997, Vol. 122 (567-571)) [3] Fred A. Heitfeld; Method of quantitating organic oxidants using catalase (EP 0 452 120 A1) [4] Mathias Guder: Diploma thesis - elektives Ätzen von i/ge und Ge (2005) http://www.uni-frankfurt.de/fb/fb14/chemie/aac/ak_kolbesen/mitarbeiter/guder/index.html
Ultra Clean Processing of emiconductor urfaces VIII 10.4028/www.scientific.net/P.134 Peracetic Acid as Active pecies in Mixtures for elective Etching of ige/i Layer ystems Aspects of Chemistry and Analytics 10.4028/www.scientific.net/P.134.79