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Growth and physical properties of in situ phosphorus-doped RTLPCVD polycrystalline silicon thin films
PERGAMON
Materials Science in Semiconductor Processing 1 (1998) 299±302
Growth and physical properties of in situ phosphorusdoped RTLPCVD polycrystalline silicon thin ®lms S. Kallel a, *, B. Semmache a, M. Lemiti a, Ch. Dubois a, H. Jarezic b, A. Laugier a a
Laboratoire de Physique de la MatieÁre, (CNRS, UMR-5511) BaÃt 502, Institut National des Sciences AppliqueÂes de Lyon, 20 Avenue Albert Einstein, F69621 Villeurbanne Cedex, France b Laboratoire MMP, Ecole Centrale de Lyon, F69131 Ecully Cedex, France
Abstract In situ phosphorus-doped (P-doped) polysilicon (poly-Si) thin ®lms are obtained by rapid thermal low pressure chemical vapor deposition (RTLPCVD) in a single chamber RTP machine by using diluted silane (SiH4/Ar = 10%) and phosphine (PH3=200 ppm). Deposition kinetics of poly-Si ®lms were studied in the 600±8508C temperature range at ®xed total pressure of 2 mbar and gas ¯ow rate (100 sccm). Activation energy of 1.82 eV was calculated in the surface reaction deposition regime. Dopant activation has been obtained sequentially by RTO at 10008C in pure O2 atmosphere. This later process permits to both activate the phosphorus dopant and forms an ultrathin polyoxide which blocks dopant outdiusion. Secondary ion-mass spectrometry (SIMS) analysis showed ¯at P-dopant pro®les throughout the ®lm thickness with a P concentration varying from 5.5 1020 to 2.4 1019 at/cm3 when the deposition temperature increases in the 600±8508C range. Grazing incidence X-ray diraction (XRD) has been used to study the structural properties of the poly-Si layers. It appeared particularly that the amorphous to crystalline temperature transition occurs at around 6508C. Finally, four-point probe measurements showed that sheet resitivities in the mO cm range can be routinely achieved for in situ P-doped RTLPCVD poly-Si ®lms. # 1999 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction LPCVD poly-Si thin ®lms are largely used in integrated circuits technology for various applications. There are used as gate or interconnects in MOS devices and also as both a dopant source and a low resistance contact in high gain bipolar transistors fabrication [1±3]. Other applications include photovoltaic conversion [4, 5] and mechanical sensors [6]. For most, if not all, of these applications tight control of doping levels and the degree of the crystallinity of the layers is needed. Incorporation of dopants into poly-Si layers is usually made by means of a two-stage postdeposition process, which can be either a POCl3 diusion
* Corresponding author.
accompanied by an additional drive-in heat treatment or ion implantation, followed by activation annealing step at a temperature in-between 1000 and 12008C [7]. In situ doping during LPCVD poly-Si deposition is an attractive technique insofar that it insures a ¯at doping depth pro®le of As-deposited layers and permits a process step economy. However, dopant activation is generally implemented ex situ in an appropriate reactor, whilst, RTLPCVD deposition with in situ doping of poly-Si ®lms can be immediately followed by an RTA dopant activation treatment in the same reactor such that cross-contamination associated with wafer handling is avoided. In addition, low thermal budget requirements needed by the continuous scaling down in state of the art VLSI technologies could not be ful®lled by conventional furnace processing. Thereby, RTLPCVD deposition oers undoubtably a valid re-
1369-8001/99/$ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 9 8 ) 0 0 0 3 6 - 5
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S. Kallel et al. / Materials Science in Semiconductor Processing 1 (1998) 299±302
sponse to develop a low thermal budget manufacturing scheme for several components. In the present paper, RTLPCVD in situ P-doped poly-Si ®lm deposition was studied. Structural and electrical properties were investigated as a function of the deposition temperature. P dopant incorporation mechanism during deposition is also discussed with respect to process and structural parameters. 2. Experimental RTLPCVD poly-si layers were deposited on p-type (1±2 Ocm) (100) oriented Cz-grown silicon 2 inches diameter wafers covered with 100 nm thick thermally grown oxide. Deposition were carried out in a FAV4 (Jipelec2) rapid thermal reactor which consists of a water-cooled, stainless-steel cylindrical chamber. Substrates are front-heated through a single quartz window by a bank of 12 tungsten halogen lamps. The temperature is closed-loop controlled by an optical pyrometer viewing the sample backside. A base pressure of about 10ÿ4±10ÿ5 mbar can be attained within 10 min using a combination of mechanical and turbomolecular pumps. After usual degreasing and buered-HF cleaning processes, the wafer is immediately loaded in the processing chamber. In situ P-doped RTLPCVD poly-Si thin ®lms were prepared by pyrolysis of SiH4/ Ar (10%) mixture with adding 200 ppm phosphine gas at deposition temperature in the 600±8508C range and a total process pressure of 2 mbar. Total gas ¯ow rate was kept at a ®xed value of 100 sccm. In these conditions, the PH3/SiH4 mole ratio is ®xed at 2.2 10ÿ3. A preheating step of the reactant gas at 3508C, just below the decomposition temperature of silane, was ®rst applied in order to adjust the total process pressure then the ramp up step (1008C/s) starts toward the deposition temperature plateau at various hold time (30 s to 3 min). 3. Results and discussion 3.1. Polysilicon deposition kinetics In Fig. 1 the deposition rate of RTLPCVD P-doped poly-Si vs. the deposition temperature is shown. Results of undoped series extracted from Ref. [8] are reported in order to put in evidence the phosphine species in¯uence on the deposition rate. In the intrinsic series case, the Arrhenius plot identi®es both a surface reaction-limited and a mass transport-limited regime already cited in classical LPCVD processes [9]. Transition between the two regimes occurs at 7508C and the activation energy for the surface-limited reaction is 1.7 eV (40 kcal/mol). Besides, it appears that
Fig. 1. Arrhenius plot of RTLPCVD in situ P-doped poly-Si deposition rates in the 650±8508C temperature range (P is 2 mbar). Eect of in situ P-doping.
the doped series displays only a surface reaction-limited regime with a slightly higher activation energy at around 1.82 eV (43 kcal/mol). This discrepancy is likely due to additional surface adsorption and desorption energies of SiH4 source and H2 byproduct species. Furthermore, it can be noted that despite a twice process pressure the deposition rate is notably reduced for the doped series. Indeed, as usually reported in the literature dedicated to classical LPCVD, phosphine species block the surface reaction sites and then inhibate the silane surface adsorption mechanism which control the poly-Si ®lm deposition process [10, 11]. This feature appears to be less dominant at higher deposition temperatures when SiH4 surface decomposition reactions are much more accelerated. 3.2. Structural analysis Grazing XRD analysis were used in order to assess the degree of crystallinity, crystallite size and texture of RTLPCVD poly-Si ®lms as a function of the deposition temperature. Only contribution of the major diffraction peaks, ie. (111), (220) and (311) was taken into account. Crystallite sizes, Dhkl, were calculated using the classical Sherrer formula [12]. Crystallographic orientation factors of poly-Si ®lms were normalized with respect to an appropriate polycrystalline reference powder. XRD spectra showed that, as previously observed for the intrinsic series, the amorphous to crystalline temperature transition occurs at deposition temperature from about 6508C [13]. We have postulated that this discrepancy with respect to classical LPCVD ®lms (580±6008C) is probably linked to the way by which the deposition is started. Gasswitching in classical furnaces allows previous residual
S. Kallel et al. / Materials Science in Semiconductor Processing 1 (1998) 299±302
water vapor thermal desorption of both substrates and internal wall, whereas temperature-swiching in RTP reactors does not. Moreover, it emerges that the structural amorphous phase contribution disappeared at deposition temperature from 7508C. Beyond this later temperature, the poly-Si ®lms structure seems to be totally crystalline. Balanced (220) and (311) major crystalline orientations are also noted. Furthermore, as seen in Fig. 2, RTLPCVD poly-Si ®lms presents a small-grain structure with an average grain-size of about 20 nm. Even though, grain-size enhancement is observed at deposition temperature from 8008C, when silane species surface mobility is less inhibited by the initial preferential phosphine adsorption mechanism. Note that after a subsequent RTA treatment (11008C/ 20 s) under N2 ambient, grain-size is slightly improved (30±40 nm). It seems that poly-Si grain size growth mainly depends on the initial structural state and is limited by the ®lm thickness. Besides, according to the literature, high P-doping grain size growth mechanism could be eective only when dopant concentration is over 4 1020 at/cm3 [14]. 3.3. Phosphorus SIMS depth pro®les The phosphorus concentration vs. the deposition temperature is shown in Fig. 3 together with the Pdopant incorporation rate which corresponds herein to the polysilicon deposition rate and phosphorus concentration product. It appears that the phosphorus dopant concentration decreases from 5.5 1020 to 2.4 1019 at/cm3 when the deposition temperature increases in the 600±8508C range. One may emphasize that the phosphorus concentration of 1.2 1020 at/cm3 obtained at deposition temperature of 7508C corresponds approximately to the solid P solubility limit in
301
Fig. 3. SIMS phosphorus concentration together with incorporation rate vs. deposition temperature of RTLPCVD in situ P-doped poly-Si ®lms.
Si at this temperature [15]. One may also remark that the PH3/SiH4 mole ratio is, by a mere chance, strictly respected at this deposition temperature. While, outside this deposition temperature, phosphorus concentration is either higher or lower than the solid solubility limit depending on the structural nature of the deposited layers. On one hand, the dopant concentration is relatively higher when the deposited ®lms are amorphous owing to the disordered structure (dangling bonds). On the other hand, Si atom incorporation in the growing layer is more accelerated because the PH3 decomposition rate is lower than one of SiH4 at higher deposition temperature [11]. This behavior is well supported by the incorporation rate variation against deposition temperature (see Fig. 3) which clearly indicates that from deposition temperature of 7008C, P atoms incorporation is somewhat slown down at the expense of a better Si atoms incorporation. 3.4. Sheet resistivity
Fig. 2. XRD grain size, Dhkl, vs. deposition temperature of RTLPCVD in situ P-doped poly-Si ®lms.
In Fig. 4 are represented RTLPCVD poly-Si ®lms sheet resistivity variations as a function of deposition temperature before and after an additional RTO treatment (10008C/20 s) under pure O2 atmosphere. This later process permits to both activate the phosphorus dopant and forms an ultrathin polyoxide which blocks dopant outdiusion [16]. It can be observed that resistivity starts to strongly decrease (1 order of magnitude) and thereafter saturates at deposition temperature from about 7008C. This trend is probably linked to the dopant segregation at grain boundaries which reduces electrically active dopant atoms in poly-Si grains and especially at lower deposition temperature [17]. In contrast, after the RTO treatment, the resistivity increases with the deposition temperature. A signi®cant variation (two orders of magnitude) is observed for in-
302
S. Kallel et al. / Materials Science in Semiconductor Processing 1 (1998) 299±302
addition, poly-Si thin ®lms displayed a small-grain crystalline structure without a marked preferential grain orientation. Furthermore, dopant activation process performed sequentially by RTO at 10008C in pure O2 atmosphere seems to be operational in order to both activates the phosphorus dopant and forms an ultrathin polyoxide which blocks dopant outdiusion. Finally, in situ P-doped RTLPCVD poly-Si ®lms with sheet resistivity in the mO cm range were routinely achieved at deposition temperature of 7508C followed by optional RTO heat treatment.
Fig. 4. Sheet resistivity variations as a function of deposition temperature of RTLPCVD poly-Si ®lms before and after subsequent RTO treatment (10008C/20 s).
itially amorphous layers deposited at 6008C resulting both on the higher dopant concentration and an eventual grain size growth. Furthermore, it might be speci®ed that sheet resistivities of poly-Si ®lms deposited on virgin Si substrates are comparatively lower. In addition, the subsequent RTO treatment (10008C/20 s) is relatively more bene®cial owing to structural changes essentially at the poly-Si/Si interface. Finally, in situ Pdoped RTLPCVD poly-Si ®lms with sheet resistivity in the mO cm range are routinely obtained when deposition is performed at 7508C. 4. Conclusions In situ P-doped RTLPCVD poly-Si ®lms were prepared using decomposition reactions of diluted silane (SiH4/Ar = 10%) with PH3 (200 ppm) mixture at a ®xed total process pressure of 2 mbar. It has been shown that, like for classical LPCVD deposition, adjunction of PH3 in the Si gas source induced a polySi deposition rate reduction. It appeared also that P concentration is principally controlled by deposition temperature and the structural nature of the deposited ®lms. XRD analysis showed that amorphous to crystalline temperature transition was around 6508C. In
References [1] Wolstenholme GR, Jorgensen N, Ashburn P, Booker GR. J Appl Phys 1987;61(1):225. [2] Castener LM, Ashburn P, Wolstenholme GR. IEEE Electron Device Lett 1991;12(1):10. [3] Keyes EP, Tarr NG. Can J Phys 1989;67:179. [4] Tarr NG. IEEE Electron Device Lett 1985;6(12):655. [5] Tarr NG, Thomas RE, Wong SK. In: 11th E.C.P.S.E.C Proc., 1992. p. 434. [6] French PJ, Van DrieeÈnhuizen BP, Poenar D, Goosen JFL, MalleÂe R, Saro PM, Wollenbuttel RF. J Microelectromech Syst 1996;5(3):187. [7] Wu CP, Schnable GL, Lee BW, Stricker R. J Electrochem. Soc 1983;131(1):216. [8] Lemiti M, Semmache B, Leà QN, Barbier D, Laugier A. In: 1st WCPEC Proc., II, 1994. p. 1375. [9] Harbeke G, Krausbauer L, Steigmeier EF, Widmer AE, Kappert HF, Neugebauer G. J Electrochem Soc 1984;131(3):675. [10] Mulder JGM, Eppenga P, Hendriks M, Tong JE. J Electrochem Soc 1990;137(1):273. [11] Learn AJ, Foster DW. J Appl Phys 1987;61(5):1898. [12] Joubert P, Loisel B, Chouan Y, Haji L. J Electrochem Soc 1987;134(10):2541. [13] Semmache B, Kleimann P, Le Berre M, Lemiti M, Barbier D, Pinard P. Sensors Actuators A 1995;46-47:75. [14] Wada Y, Nishamatsu S. J Electrochem Soc 1978;125:1500. [15] Mackintosh JM. J Electrochem Soc 1961;109:392. [16] Kallel S, et al., Paper presented in this conference. [17] Mandurah MM, Krishna, Saraswat C, Robert Helms C, Kamins TI. J Appl Phys 1980;51:5755.
Materials Science in Semiconductor Processing 1 (1998) 299±302
Growth and physical properties of in situ phosphorusdoped RTLPCVD polycrystalline silicon thin ®lms S. Kallel a, *, B. Semmache a, M. Lemiti a, Ch. Dubois a, H. Jarezic b, A. Laugier a a
Laboratoire de Physique de la MatieÁre, (CNRS, UMR-5511) BaÃt 502, Institut National des Sciences AppliqueÂes de Lyon, 20 Avenue Albert Einstein, F69621 Villeurbanne Cedex, France b Laboratoire MMP, Ecole Centrale de Lyon, F69131 Ecully Cedex, France
Abstract In situ phosphorus-doped (P-doped) polysilicon (poly-Si) thin ®lms are obtained by rapid thermal low pressure chemical vapor deposition (RTLPCVD) in a single chamber RTP machine by using diluted silane (SiH4/Ar = 10%) and phosphine (PH3=200 ppm). Deposition kinetics of poly-Si ®lms were studied in the 600±8508C temperature range at ®xed total pressure of 2 mbar and gas ¯ow rate (100 sccm). Activation energy of 1.82 eV was calculated in the surface reaction deposition regime. Dopant activation has been obtained sequentially by RTO at 10008C in pure O2 atmosphere. This later process permits to both activate the phosphorus dopant and forms an ultrathin polyoxide which blocks dopant outdiusion. Secondary ion-mass spectrometry (SIMS) analysis showed ¯at P-dopant pro®les throughout the ®lm thickness with a P concentration varying from 5.5 1020 to 2.4 1019 at/cm3 when the deposition temperature increases in the 600±8508C range. Grazing incidence X-ray diraction (XRD) has been used to study the structural properties of the poly-Si layers. It appeared particularly that the amorphous to crystalline temperature transition occurs at around 6508C. Finally, four-point probe measurements showed that sheet resitivities in the mO cm range can be routinely achieved for in situ P-doped RTLPCVD poly-Si ®lms. # 1999 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction LPCVD poly-Si thin ®lms are largely used in integrated circuits technology for various applications. There are used as gate or interconnects in MOS devices and also as both a dopant source and a low resistance contact in high gain bipolar transistors fabrication [1±3]. Other applications include photovoltaic conversion [4, 5] and mechanical sensors [6]. For most, if not all, of these applications tight control of doping levels and the degree of the crystallinity of the layers is needed. Incorporation of dopants into poly-Si layers is usually made by means of a two-stage postdeposition process, which can be either a POCl3 diusion
* Corresponding author.
accompanied by an additional drive-in heat treatment or ion implantation, followed by activation annealing step at a temperature in-between 1000 and 12008C [7]. In situ doping during LPCVD poly-Si deposition is an attractive technique insofar that it insures a ¯at doping depth pro®le of As-deposited layers and permits a process step economy. However, dopant activation is generally implemented ex situ in an appropriate reactor, whilst, RTLPCVD deposition with in situ doping of poly-Si ®lms can be immediately followed by an RTA dopant activation treatment in the same reactor such that cross-contamination associated with wafer handling is avoided. In addition, low thermal budget requirements needed by the continuous scaling down in state of the art VLSI technologies could not be ful®lled by conventional furnace processing. Thereby, RTLPCVD deposition oers undoubtably a valid re-
1369-8001/99/$ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 9 8 ) 0 0 0 3 6 - 5
300
S. Kallel et al. / Materials Science in Semiconductor Processing 1 (1998) 299±302
sponse to develop a low thermal budget manufacturing scheme for several components. In the present paper, RTLPCVD in situ P-doped poly-Si ®lm deposition was studied. Structural and electrical properties were investigated as a function of the deposition temperature. P dopant incorporation mechanism during deposition is also discussed with respect to process and structural parameters. 2. Experimental RTLPCVD poly-si layers were deposited on p-type (1±2 Ocm) (100) oriented Cz-grown silicon 2 inches diameter wafers covered with 100 nm thick thermally grown oxide. Deposition were carried out in a FAV4 (Jipelec2) rapid thermal reactor which consists of a water-cooled, stainless-steel cylindrical chamber. Substrates are front-heated through a single quartz window by a bank of 12 tungsten halogen lamps. The temperature is closed-loop controlled by an optical pyrometer viewing the sample backside. A base pressure of about 10ÿ4±10ÿ5 mbar can be attained within 10 min using a combination of mechanical and turbomolecular pumps. After usual degreasing and buered-HF cleaning processes, the wafer is immediately loaded in the processing chamber. In situ P-doped RTLPCVD poly-Si thin ®lms were prepared by pyrolysis of SiH4/ Ar (10%) mixture with adding 200 ppm phosphine gas at deposition temperature in the 600±8508C range and a total process pressure of 2 mbar. Total gas ¯ow rate was kept at a ®xed value of 100 sccm. In these conditions, the PH3/SiH4 mole ratio is ®xed at 2.2 10ÿ3. A preheating step of the reactant gas at 3508C, just below the decomposition temperature of silane, was ®rst applied in order to adjust the total process pressure then the ramp up step (1008C/s) starts toward the deposition temperature plateau at various hold time (30 s to 3 min). 3. Results and discussion 3.1. Polysilicon deposition kinetics In Fig. 1 the deposition rate of RTLPCVD P-doped poly-Si vs. the deposition temperature is shown. Results of undoped series extracted from Ref. [8] are reported in order to put in evidence the phosphine species in¯uence on the deposition rate. In the intrinsic series case, the Arrhenius plot identi®es both a surface reaction-limited and a mass transport-limited regime already cited in classical LPCVD processes [9]. Transition between the two regimes occurs at 7508C and the activation energy for the surface-limited reaction is 1.7 eV (40 kcal/mol). Besides, it appears that
Fig. 1. Arrhenius plot of RTLPCVD in situ P-doped poly-Si deposition rates in the 650±8508C temperature range (P is 2 mbar). Eect of in situ P-doping.
the doped series displays only a surface reaction-limited regime with a slightly higher activation energy at around 1.82 eV (43 kcal/mol). This discrepancy is likely due to additional surface adsorption and desorption energies of SiH4 source and H2 byproduct species. Furthermore, it can be noted that despite a twice process pressure the deposition rate is notably reduced for the doped series. Indeed, as usually reported in the literature dedicated to classical LPCVD, phosphine species block the surface reaction sites and then inhibate the silane surface adsorption mechanism which control the poly-Si ®lm deposition process [10, 11]. This feature appears to be less dominant at higher deposition temperatures when SiH4 surface decomposition reactions are much more accelerated. 3.2. Structural analysis Grazing XRD analysis were used in order to assess the degree of crystallinity, crystallite size and texture of RTLPCVD poly-Si ®lms as a function of the deposition temperature. Only contribution of the major diffraction peaks, ie. (111), (220) and (311) was taken into account. Crystallite sizes, Dhkl, were calculated using the classical Sherrer formula [12]. Crystallographic orientation factors of poly-Si ®lms were normalized with respect to an appropriate polycrystalline reference powder. XRD spectra showed that, as previously observed for the intrinsic series, the amorphous to crystalline temperature transition occurs at deposition temperature from about 6508C [13]. We have postulated that this discrepancy with respect to classical LPCVD ®lms (580±6008C) is probably linked to the way by which the deposition is started. Gasswitching in classical furnaces allows previous residual
S. Kallel et al. / Materials Science in Semiconductor Processing 1 (1998) 299±302
water vapor thermal desorption of both substrates and internal wall, whereas temperature-swiching in RTP reactors does not. Moreover, it emerges that the structural amorphous phase contribution disappeared at deposition temperature from 7508C. Beyond this later temperature, the poly-Si ®lms structure seems to be totally crystalline. Balanced (220) and (311) major crystalline orientations are also noted. Furthermore, as seen in Fig. 2, RTLPCVD poly-Si ®lms presents a small-grain structure with an average grain-size of about 20 nm. Even though, grain-size enhancement is observed at deposition temperature from 8008C, when silane species surface mobility is less inhibited by the initial preferential phosphine adsorption mechanism. Note that after a subsequent RTA treatment (11008C/ 20 s) under N2 ambient, grain-size is slightly improved (30±40 nm). It seems that poly-Si grain size growth mainly depends on the initial structural state and is limited by the ®lm thickness. Besides, according to the literature, high P-doping grain size growth mechanism could be eective only when dopant concentration is over 4 1020 at/cm3 [14]. 3.3. Phosphorus SIMS depth pro®les The phosphorus concentration vs. the deposition temperature is shown in Fig. 3 together with the Pdopant incorporation rate which corresponds herein to the polysilicon deposition rate and phosphorus concentration product. It appears that the phosphorus dopant concentration decreases from 5.5 1020 to 2.4 1019 at/cm3 when the deposition temperature increases in the 600±8508C range. One may emphasize that the phosphorus concentration of 1.2 1020 at/cm3 obtained at deposition temperature of 7508C corresponds approximately to the solid P solubility limit in
301
Fig. 3. SIMS phosphorus concentration together with incorporation rate vs. deposition temperature of RTLPCVD in situ P-doped poly-Si ®lms.
Si at this temperature [15]. One may also remark that the PH3/SiH4 mole ratio is, by a mere chance, strictly respected at this deposition temperature. While, outside this deposition temperature, phosphorus concentration is either higher or lower than the solid solubility limit depending on the structural nature of the deposited layers. On one hand, the dopant concentration is relatively higher when the deposited ®lms are amorphous owing to the disordered structure (dangling bonds). On the other hand, Si atom incorporation in the growing layer is more accelerated because the PH3 decomposition rate is lower than one of SiH4 at higher deposition temperature [11]. This behavior is well supported by the incorporation rate variation against deposition temperature (see Fig. 3) which clearly indicates that from deposition temperature of 7008C, P atoms incorporation is somewhat slown down at the expense of a better Si atoms incorporation. 3.4. Sheet resistivity
Fig. 2. XRD grain size, Dhkl, vs. deposition temperature of RTLPCVD in situ P-doped poly-Si ®lms.
In Fig. 4 are represented RTLPCVD poly-Si ®lms sheet resistivity variations as a function of deposition temperature before and after an additional RTO treatment (10008C/20 s) under pure O2 atmosphere. This later process permits to both activate the phosphorus dopant and forms an ultrathin polyoxide which blocks dopant outdiusion [16]. It can be observed that resistivity starts to strongly decrease (1 order of magnitude) and thereafter saturates at deposition temperature from about 7008C. This trend is probably linked to the dopant segregation at grain boundaries which reduces electrically active dopant atoms in poly-Si grains and especially at lower deposition temperature [17]. In contrast, after the RTO treatment, the resistivity increases with the deposition temperature. A signi®cant variation (two orders of magnitude) is observed for in-
302
S. Kallel et al. / Materials Science in Semiconductor Processing 1 (1998) 299±302
addition, poly-Si thin ®lms displayed a small-grain crystalline structure without a marked preferential grain orientation. Furthermore, dopant activation process performed sequentially by RTO at 10008C in pure O2 atmosphere seems to be operational in order to both activates the phosphorus dopant and forms an ultrathin polyoxide which blocks dopant outdiusion. Finally, in situ P-doped RTLPCVD poly-Si ®lms with sheet resistivity in the mO cm range were routinely achieved at deposition temperature of 7508C followed by optional RTO heat treatment.
Fig. 4. Sheet resistivity variations as a function of deposition temperature of RTLPCVD poly-Si ®lms before and after subsequent RTO treatment (10008C/20 s).
itially amorphous layers deposited at 6008C resulting both on the higher dopant concentration and an eventual grain size growth. Furthermore, it might be speci®ed that sheet resistivities of poly-Si ®lms deposited on virgin Si substrates are comparatively lower. In addition, the subsequent RTO treatment (10008C/20 s) is relatively more bene®cial owing to structural changes essentially at the poly-Si/Si interface. Finally, in situ Pdoped RTLPCVD poly-Si ®lms with sheet resistivity in the mO cm range are routinely obtained when deposition is performed at 7508C. 4. Conclusions In situ P-doped RTLPCVD poly-Si ®lms were prepared using decomposition reactions of diluted silane (SiH4/Ar = 10%) with PH3 (200 ppm) mixture at a ®xed total process pressure of 2 mbar. It has been shown that, like for classical LPCVD deposition, adjunction of PH3 in the Si gas source induced a polySi deposition rate reduction. It appeared also that P concentration is principally controlled by deposition temperature and the structural nature of the deposited ®lms. XRD analysis showed that amorphous to crystalline temperature transition was around 6508C. In
References [1] Wolstenholme GR, Jorgensen N, Ashburn P, Booker GR. J Appl Phys 1987;61(1):225. [2] Castener LM, Ashburn P, Wolstenholme GR. IEEE Electron Device Lett 1991;12(1):10. [3] Keyes EP, Tarr NG. Can J Phys 1989;67:179. [4] Tarr NG. IEEE Electron Device Lett 1985;6(12):655. [5] Tarr NG, Thomas RE, Wong SK. In: 11th E.C.P.S.E.C Proc., 1992. p. 434. [6] French PJ, Van DrieeÈnhuizen BP, Poenar D, Goosen JFL, MalleÂe R, Saro PM, Wollenbuttel RF. J Microelectromech Syst 1996;5(3):187. [7] Wu CP, Schnable GL, Lee BW, Stricker R. J Electrochem. Soc 1983;131(1):216. [8] Lemiti M, Semmache B, Leà QN, Barbier D, Laugier A. In: 1st WCPEC Proc., II, 1994. p. 1375. [9] Harbeke G, Krausbauer L, Steigmeier EF, Widmer AE, Kappert HF, Neugebauer G. J Electrochem Soc 1984;131(3):675. [10] Mulder JGM, Eppenga P, Hendriks M, Tong JE. J Electrochem Soc 1990;137(1):273. [11] Learn AJ, Foster DW. J Appl Phys 1987;61(5):1898. [12] Joubert P, Loisel B, Chouan Y, Haji L. J Electrochem Soc 1987;134(10):2541. [13] Semmache B, Kleimann P, Le Berre M, Lemiti M, Barbier D, Pinard P. Sensors Actuators A 1995;46-47:75. [14] Wada Y, Nishamatsu S. J Electrochem Soc 1978;125:1500. [15] Mackintosh JM. J Electrochem Soc 1961;109:392. [16] Kallel S, et al., Paper presented in this conference. [17] Mandurah MM, Krishna, Saraswat C, Robert Helms C, Kamins TI. J Appl Phys 1980;51:5755.