HOFFMANN, György

NAGY, András

VESZPRÉMI, Tamás

English

Periodica Polytechnica (Budapest University of Technology and Economics)

PROPERTIES OF IR SPECTRA OF PHOSPHORUS DOPED Si02 FILMS By G. HOFFl\IANN, * T. VESZPREl\II, and A. NAGY** Department of Inorganic Chemistry, Technical University, Budapest Receiyed February L 1979 Presented by Doc. dr. J. XAGY 1. Introduction The phosphorus doped SiOz films deposited by pyrolysis are succesfully used as diffusion sources in the semiconductor technology. The characteristics (surface concentration of the dopant, p-n junction depth) of the diffusion layer in the Si substrate obtained with the help of the doped SiOz film can be calcu-lated kno,ving the P-concentration of the film and some parameters of the diffusion. The phosphorus concentration of the SiOz film can be d~termined with the help of infrared (IR) absorption [1, 2]. This procedure is based on the fact that the P-concentration of the SiOz film is directly proportional to the area below the absorption-wave number curve corresponding to the P=O bond. Since the absorption band of the P=O bond is found in the 1200 to 1500 cm- 1 wave number interval - according to NAKAl\IOTO [3] - therefore the absorp-tion spectra of the SiOz films were determined primarily in the 1200 to 1600 cm -1 interval. Several absorption peaks were observed to appear in this wave number interval. Two interdependent problems arise in connection with the foregoing peaks: 1. Which absorption bands are related to the P=O bond? 2. Which atomic groups correspond to the remaining absorption peaks? Considering the use of the P-doped SiOz films as diffusion sources, it is important to answer the above mentioned questions. 2. Experimental For the purpose of IR transmISSIOn measurements, phosphorus doped SiOz films were deposited onto two-side polished Si slices. For deposition the pyrolytical decomposition of tetraethoxysilane (TEOS; Si(OCzH5)4) and dopant '" Research Institute for Technical Physics of the Hungarian Academy of Sciences ** Tungsram Co., Budapest. 176 G. HOFFMAiViV et al. material was used. The dopant materials were: triethylphosphate (TEF, C6H 1S0 4P) and phosphoroxychlorid (POCI3). The mixture ofTEOS and dopants (POCI3 , TEF) was introduced through bubbling system with N2 gas into the reaction space, where oxidation occurred at 520 QC. Table 1 Data of samples used in IR transmission and diffusion measurements Substrate C, Sample [Ohm· cm] [atom/cm') dopant AT 6.3 135 TEF X 1018 BT S.3 139 TEF CT 6.3 153 TEF 9.2 DT 6.3 158 TEF ET 6.3 167 TEF FT 7.85 651 TEF 3.0 GT 7.85 633 TEF HT 7.85 632 TEF IT 7.85 597 TEF JT 7.85 610 TEF 1.0 KT 7.85 602 TEF LT 7.85 537 TEF MT 7.85 889 TEF ::\"T 7.85 910 TEF OT 7.85 926 TEF PT 7.85 868 TEF AP 6.3 i 338 POCI3 BP 6.3 334 POCI3 CP 6.3 306 PO CIa 47.5 DP 6.3 301 POCla EP 6.3 304 POCla FP 4.5 382 POCla 80 GP 4.5 386 POCla 300 HP 4.5 391 POCla IP 8.0 336 POCI3 JP 8.0 344 POCla KP 8.0 367 POCla LP 8.0 398 POCI3 MP 8.0 426 POCI3 NP 8.0 503 POCI3 400 PHOSPHORUS DOPED SW, FILMS 177 In order to find the limits of the spectrum relating to the p=o group in the 1200 to 1600 cm -1 interval, diffused layers of different surface dopant concentrations were prepared making use of Si02 films of different initial P-concentrations. The diffusion processes were carried out at 1100 QC in N2-atmosphere. The surface concentrations were determined by the method of IRVIN [13], making use of the surface resistivity of the diffused layer and the p-n junction depth. The data of the Si02 films and Si substrates are summarized in Table l. The IR transmission and attenuated total reflection (ATR) spectra of the Si02 films were measured using a Zeiss UR 10 double beam recording spectrophotometer. The absorption coefficients as a function of "wave number were calculated by the method described in [2]. The ATR measurements were performed making use of the ATR attach-ment produced in the Institute for Technical Physics [4]. Samples of paral-lelepiped shape, prepared from high purity Si were applied in the ATR mea-surements. The sample surfaces were coated by the Si02 films to be examined. The data of the ATR samples and the Si02 films are contained in Table 2. Table 2 Data of samples used in ATR measurements 5iO, film Si substrate Sample \'- , ! d' [urn] dopant preparation I Ohm· cm type X I TA 2 321 TEF pyr. I 195 n 38 H3 Si 821 therm. I 300 n 10 pyr:. pyrolytically deposited therm.: thermally grown * The total thickness of the SiOz films being OIl both sides of the ATR sample I 100 freiuJ 50~~------~-------4~-+----~~----r---~V--O~~-------L--------~------~--~--~----~~ ?OOO 1600 1200 800 700 600 500 J} !cm-1j Fig. 1. The transmission spectrum of the TEF doped pyrolytically deposited SiOz film in the 400 to 2100 cm -1 interval 178 The transmission spectrum of TEF doped SiOz film is shown in Fig. 1, while the ATR spectrum of the same film is shown in Fig. 2. For the purpose of comparison, the transmission (Fig. 3) and the ATR spectra (Fig. 4) of the pure Si02 film were recorded. Comparing Figs 1 and 2, the difference hetween the spectra of doped Si02 and pure film is seen to he small: under these conditions considerahle errors can occur in the calculated ahsorption coefficients. The mean yalues of wave numhers v corresponding to the ahsorption peaks, their standard devia-tions and prohahility frequencies of the peak appearance p are contained iD Tahle 3. I lOG [reiuJ ;-1 ;~ -- - ---------L ' ....... ~ .\ i \~,\....,.. . -.,!'~ ,;1 I I, 50 1_+-____ '-___ --+11_ -r-t-n~ I - ~f\ ,,11, I .1 . I • 1--"-----+---------'1- :-1Jj 1 ' ,] . i! o~---------------------------------~ 21,00 2000 1600 1200 v fcm-7j Fig. 2. The ATR spectrum of the TEF doped pyrolytically deposited SiO~ film in the 1000 to 2500 cm -I interval Table 3 The mean values of the wave numbers j:. relating to the absorption peaks, their standard deviations QV and probability frequency p Serial number of the peaks V [cm-1 ] 1. 1305 2. 1340 3. 1363 i 1395 ... 5. 1423 6. 1461 7. 1501 8. 1536 9. 1469 10. 1598 Dopant: TEF QV [cm-1 ] 9 8 6 7 5 9 6 5 5 9 p 1 0.25 1 0.94 0.75 0.90 0.34 Dopant: POCI, v [cm-1] OV [c01-1] P 1303 8 1 1332 10 1 1361 6 1 1403 9 0.9 1419 2 0.47 1456 10 1 1498 7 0.76 1534 5 0.53 1564 9 0.88 1588 3 0.20 I 100 [reI. u.J PHOSPHORUS DOPED SiO, FILMS o~~-----------------------------------------179 2ltOO 2000 1600 1200 BOO 700 600 500 II [cm-07 Fig. 3. The transmission spectrum of the pure thermally grown Si02 film. in the 400 to 2500 cm-1 interval 100 I [ref.u] OL-____________________________ __ 2400 2000 1600 1200 V [cm-I) Fig. 4. The ATR spectrum of the pure thermally grown Si02 film in the 1000-2500 cm-' inten-al 500 J \ \) \ o~~_W ____ ~ ____ ~ ____ ~ __ ___ 1200 1300 1400 1500 1600 )/ [cm-!j Fig. 5. The difference between the absorption coefficients of TEP doped and pure Si02 films as a function of waye number 180 G. HOFFMAj'i"j'i" et -I. o(p [cm-Ij 2000 :tf 1000 \ r x 0 1200 1300 1400 1500 1600 ufcm-1J Fig. 6. The difference between the absorption coefficients of POCIa doped and pure Si02 films as a function of wave number The difference between the absorption coefficients relating to the doped and pure Si02 films as a function of wave number is shown in Fig. 5 for TEF dopant (sample CT); and this function for POCla dopant is shown in Fig. 6 (sample HP). The absorption coefficients were calculated from the recorded curve, using the method described in [2]. 3. Discussion The course of reaction during the deposition of the doped Si02 film can be considered as the pyroly-tic decomposition of alcoxysilanes [5]. Namely, the decomposition of the alcoxysilanes is realized through formation of complex radicals. The radicals of type RaSi; R 2Si-O, O=P(OR), 0=P(OR2) and O=P-O formed in the reaction space are chemisorbed on the surface of the Si substrate, where their further decomposition occurs [6]. Finally a Si02 film containing phosphorus is formed. Considering the above mentioned conception and the conditions of the pyrolysis, the reaction equations of principle can be written as follows: for TEF dopant: TEOS + TEF for POCla dopant: TEOS + POCl3 + O2 --- [Xz] --- Si02 + phosphorus oxyde + H 20 + HCl where [Xl] = Al --+ BI --+ [Xz] = A2 --+ B2 --+ are consecutive reactions. PHOSPHORUS DOPED SiO, FILMS 181 In priuciple several oxides and acids can form with phosphorus (e.g. P 40 6, P 40" PiOS' P 40 9, P 40 l0 and H 3P02 , HP02 , H 4P20 5, H 3P03, H 4P 20 6, HP0 3, H 4P Z0 7, H 3P04) [7]. Since the oxidation value of both TEF and POCl3 is 5, and as the pyroly-sis is basically an oxidizing process, therefore it is probable that the oxide formed consists mostly of P 40 10• It is to be noted that differences hetween the IR spectra of the different phosphorus oxides are too small to be distin-guished even in the case of pure phosphorus oxides. It can be concluded from the foregoing train of thought that the thin film deposited by pyrolysis is composed primarily of Si02 and phosphorus oxide (probably P 40 10). The film would contain H 20, OH groups, C, CO2, hydrocarbon groups and in the case of POC13 dopant HCl as contaminations. Therefore, besides peaks relating to the P=O, P-O and P-O-P groups. peaks corrcsponding to the impurities may appear in the IR absorption spectrum. Examining the absorption peaks contained in Table 3 it can be stated that peaks 1 to 5 relate to the P=O bond. This can be proved in the following way: For the absorption coefficient corresponding to the P=O group the relation holds, where er is the absorption cross section of the P=O group, l' the wave number, (1) Cc the P concentration of the SiOz film. On the other hand, according to [2]: (2) where Cs is the surface P-concentration of the diffused layer, and f is a function of the segregation coefficient, and of diffusion coefficients of P in SiOz and Si, resp. The coefficient f is constant at invariant temperatures, therefore, accord-ing to formulas (1) and (2), IX is in direct proportion to Cs' Consequently the rate of absorption coefficients equals the rate of surface concentrations. The ratio of surface concentrations (CsI/Cd for different diffused layers and the mean values of a(I)/a(2) rates for peaks 1-2, 1-3, ... 1-10 (see Table 3) are summarized in Table 4. CS1/Cs2 is seen to closely approximate et.(I)/a(2) if the peaks 1-5, 1 6 are involved into the mean value. The deviation between CSl/Cs2 and a(l)ja(2) is greater in the ease of peaks 1-7 ... 1-10. Moreover, SIMS (secondary ion mass spectroscopy) measurements on pyrolytically depos-182 G. HOFF.UA-''' et a!. ited SiOz films have revealed the presence of CHz and CH3 groups in the film [12]. The peak 6 can be assigned to these hydrocarbon groups [8]. This means that the peaks 1-5 follow the variations of the P-concentration, therefore the spectrum between 1284 : 12 cm -1 and 1433 : 6 cm -1 relate to the P=O groups. Table 4 The ratio of surface concentrations (CsdCd for different diffused layers prepared with P-doped SiOz films and the mean values of peak absorption coefficient rates (cc(1)/cc(2» for the oxide films in the 1300 to 1600 cm -1 interval i :(1)/~(2) Sample Dopant CdC" 1-3 i 1-4 1-6 I 1-7 , 1-2 1-5 1-8 1-9 1-10 , CT: 1, FT: 2 TEF 3.1 5.1 4.0 3.3 3.0 i 2.8 2.5 2.3 2.3 2.3 .NO: 1, CP: 2 POCla 1.3 1.6 1.5 1.6 1.5 1.5 2.1 2.0 2.0 1.9 CP: I, FP: 2 POCla 0.6 1.4 1.3 l.l 0.9 0.8 1.0 1.0 1.2 1.4 A further evidence of the foregoing statement is provided hy the ATR speetrum of the P-doped SiOz film (Fig. 2). In this spectrum, absorption peaks have been found at 1305 cm 1330 cm -1, 1354 cm -1, 1372 cm -I, 1404 cm-1, 1416 cm -1 and 1424 cm -1 wave numbers; the end of this band is at 1442 cm -1. Remark that the peaks 1-10 are hardly visible in the transmission recorded curve, they are revealed by the calculated absorption coefficient - 'wave number function. Another evidence of the above mentioned statement is that probability frequencies of the peaks 1-4 are equal to about 1. that is in general they appear together in the spectra. Therefore they are caused by intentionally introduced dopant rather than by the contaminants. The absorption band of the P-O hond is in the 900 to 1200 cm-1 inter-val [3, 8]. Though in the transmission recorded curve (Fig. 1) "steps" are found in this interval (at 1130 cm-I, 1160 em 1200 cm-I). ho"wever. at most the peak at 1130 cm -1 ean he assigned to P - ° hond. That is. the steps at 1160 cm -1 and 1200 em -1 are also found in the spectrum of the pure SiOz film (Fig. 3). The recognition of the absorption peak relating to the P-O bond in the SiOz film is still made more difficult by the overlapping absorption bands. An intensive ahsorption hand appears at 1215 cm -1 in the ATR spectrum (Fig. 2) of the P-doped SiOz film. Beside this hand another intensive band (JlSiOz) is seen at 1048 cm-I. In the transmission curve only a small step is seen at 1215 cm- 1 (Fig. 1). WOe are of the opinion that the absorption peak at 1215 cm-1 does not relate neither to P=O nor to P-O bonds. Namely. a similar absorption PHOSPHORUS DOPED SiO, FIL3IS 183 peak is seen at 1200 cm -1 in the ATR spectrum of the pure SiOz film (Fig. 4). (The v(SiOz) band is at 1062 cm-I in this spectrum.) In a previous paper [9] an ATR absorption peak at 1189 9 cm -1 has been described, and assigned to a SiOz modification of denser (perhaps crystal-line) structure with higher oscillator force than the average of the amorphous SiOz film. Comparing the spectra in Figs 2 and 4, the conclusion can be drawn that the above mentioned SiOz modification is present both in pure and in P doped SiOz films. However, there are differences bet'ween the ATR spectra of the two kinds of SiOz films, namely: a) The wave number of the peak in the case of a P-doped film is higher (1215 cm -1) than for a pure thermally grown SiOz film (1200 cm -1). b) In the spectrum of the P-doped film the absorption band (at 1215 cm-I, Fig. 2) is more separated from the v(SiOz) band than the corresponding band (at 1200 cm -1) in the spectrum of the pure film (Fig. 4). The difference between the peak's wave numbers of the v(SiOz) and the other bands in question are: for P-doped SiOz: Llv = 147 cm -1 and for pure SiOz: .:1v = 138 cm-I. From the results described above it can be concluded that the oscillator force is higher in the mentioned SiOz modification of P-doped SiOz and it is more separated from the amorphous SiOz than in the thermally grown SiOz films. Thus the formation of this denser SiOz phase may be enhanced hy the P-dopants. In the case of boron-doped SiOz films, V. G. Litovchenko came to the same conclusion [10]. The absorption band relating to the b(O-P-O) vihration is in the 300 to 650 cm -1 interval [3]. In the transmission spectrum (Fig. 1) only the peak (at 453 cm -1) corresponding to b(SiO) vihration has appeared in the 400 to 650 cm -1 interval. The spectra revealed the presence of some contaminants in the SiOz films. Primarily peaks of OH groups and H 20 appeared in the spectra, they are shown in Tahle 4 (peak number 10), Fig. 2 (peak at 16] 0 cm -1) and the ATR ahsorption hand ohtained in the 2700 to 3660 cm -1 interval. Accordingly, P=O and OH groups were detected in the doped SiOz film, therefore presence of O=P-OH groups in the film is realizable. This seems to he proved by the hand in the 1580 to 1700 cm-I interval (Fig. 2) containing a step at 1670 cm -1 [8]. According to [11], COz can also he built into the SiOz film. The small peak in the ATR spectrum at 2340 cm -1 indicates the presence of CO2 in the oxide film. 184 G. HOFF.lIA.YN ct al. Acknowledgements The authors express their thanks to Dr. P. SVISZT and Dr. M. G.tL for their valuable comments, to P. VIG, M. D.tNYI, I. UNGV.tRI, G. OR..-I.VECZ, A DERczE, M. HUBAI for their useful contribution to this work. Summary Information has been obtained on the composition of P-doped pyrolytically deposited Si02 films from their IR transmission and ATR spectra. The absorption band (calculated by substracting the pure Si02 absorption from the total absorption) in the interval 1284 and 1433 cm-1 related to the P=O group. This conclusion can be drawn from comparing results of IR transmission, ATR, diffusion and SBIS measurements. The intensive absorption band at 1215 cm -1 in the ATR spectrum of P doped Si02 films have been assigned to inhomogeneities in the film of denser structure, having higher oscillator force than the average amorphous Si02• These inhomogeneities are similar to those previously revealed by the ATR spectra of thermally grown Si02 films [9]. The presence of some contaminants can be inferred from the IR transmission and ATR spectra of pyrolytically deposited Si02 films. Primarily the absorption bands of OH groups and H 20 appear in the spectra, but a minor quantity of hydrocarbon groups, CO2 and O=P--OH groups are also contained by the films. References 1. TEi'ii'iEY, A. S.-GHEZZO. M.: J. Electrochem. Soc. 120, 1276 (1973) 2. HOFBL-I.NN, G.-XAGY, A.-PUSK.tS. L.: J. Phys. D: Appl. Phys. 8, 1044 (1975) 3. NAKAl\lOTO, K.: Infrared Spectra of Inorganic and Coordination Compounds. J. Wiley, New York (1963) 4. SCH.Ai'iDA, J.-SASS, G.: MUFI Kozl. 03, 33 (1971) 5. KHLERER, J.: J. Electrochem. Soc. 112, 503 (1965) 6. VESZPRlbu, T.: HIKI Kozl. 11, 55 (1972) 7. NAGY. J.: Theoretical and Inorganic Chemistry. Ill. Tankonyvkiad6, Budapest, 1976 (in Hungarian) 8. HOLLY, S.-SOH.tR, P.: Infrared Spectroscopy. }Iiiszaki Konyvkiad6, Budapest, 1968 (in Hungarian) 9. HOFDIANN. G.-NElIETH-SALLAY, l\I.-SCHANDA, J.: Acta Phys. Acad. Sci. Hungaricae 36, 349 (1974) 10. LITOVCHENKO, V. G.: (Semiconductor Research Institute, Kiev) (private communication) 11. PLISKIN, W. A.-SnulONs, R. C.-ESCH. R. P.: Infrared Spectra of CO 2 and CO in RF-Sputtered SiO z Films. Thin Film Dielectrics Ed. by F. Vratny The Electrochem. Soc. Inc .. New York )1969) 12. ROlIL-\.NOVA, G. F.: (Semiconductor Research Institute, Kiev) (private communication) 13. IRVIN, J. C.: Bell Syst. Tech. J. 41, 387 (1962) Gyorgy HOFFMANN I Tama~ V~SZPREMI H-1521 Budapest Andras NAGY 

PROPERTIES OF IR SPECTRA OF PHOSPHORUS DOPED SiO₂ FILMS

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PROPERTIES OF IR SPECTRA OF PHOSPHORUS DOPED SiO₂ FILMS

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