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Hydrogenation in laser annealed polysilicon thin-film transistors (TFTs)
Thin Solid Films 383 Ž2001. 151᎐153
Hydrogenation in laser annealed polysilicon thin-film transistors Ž TFTs. F.V. Farmakis a,U , D.M. Tsamadosa , J. Brini a , G. Kamarinos a , C.A. Dimitriadis b , M. Miyasakac a
LPCS, ENSERG, 23 rue des Martyrs, BP 257, 38016 Grenoble Cedex 1, France Department of Physics, Uni¨ ersity of Thessaloniki, 54006 Thessaloniki, Greece c Seiko Epson Corporation, Base Technology Research Center, Owa 3-3-5, Suwa, Nagano 392, Japan b
Abstract Hydrogenation effects in excimer laser annealed polysilicon thin-film transistors ŽTFTs. were studied. Hydrogen plasma formed from hydrogen diluted with Ar or He was used in order to passivate defects at the polysiliconrsilicon oxide interface, as well as in the polysilicon material. It was found that, after hydrogenation, no more than a 10% increase in the carrier mobility is attained, accompanied by a threshold-voltage decrease, due to passivation of deep states at the polysiliconrsilicon oxide interface and at the grain boundaries. However, the most important feature of hydrogenated devices is the improvement in the dispersion of their transfer characteristics. In addition, hot-carrier stress experiments showed that optimization of the type of dilution gas ŽAr or He. and the relative concentration of hydrogen can be carried out in order to improve the device reliability. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Polycrystalline; Thin-film transistor; Plasma hydrogenation
1. Introduction Polycrystalline silicon thin-film transistors Žpolysilicon TFTs. are widely investigated, mainly due to their application in active-matrix liquid crystal displays ŽAMLCDs.. The performance of polysilicon TFTs depends strongly on defects in the polysilicon and at the polysiliconrgate oxide interface w1x. Recently, laserannealing techniques were applied to improve the polysilicon TFT performance through grain-size enlargement and reduction of the grain boundary and in-grain defect density w2x. A traditional technique to reduce defects in TFTs is hydrogenation. Various hydrogenation techniques are currently applied, such as plasma hydrogenation w3x, hydrogen implantation w4x
U
Corresponding author. Tel.: q33-476856046; fax: q33-476856070. E-mail address: [email protected] ŽF.V. Farmakis..
and SiN x :H encapsulation w5x. An important issue of the hydrogenation process is its duration, resulting in improvement of the turn-on voltage, carrier mobility and sub-threshold swing, and in the dispersion of the static device characteristics. In this work, we used plasma hydrogenation, with the hydrogen plasma formed by a low % hydrogen diluted with a rare gas, namely helium or argon. The aim of this work was to investigate the effect of various hydrogen concentrations, diluted with helium or argon, on the device performance, the dispersion of the device parameters and the device reliability. 2. Experimental The studied polysilicon TFTs were fabricated on glass substrates by a low-temperature process described elsewhere w2x. Amorphous silicon Ž ␣-Si. films 51.8 nm thick were deposited by low-pressure chemical vapor
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 5 8 8 - 1
F.V. Farmakis et al. r Thin Solid Films 383 (2001) 151᎐153
152
Table 1 Mixtures of H 2 , He and Ar during plasma hydrogenation H2 gas flow Žsccm.
He or Ar gas flow Žsccm.
H2 concentration Ž%.
0 1400 100 150 200
0 0 4900 4850 4800
0a 100 2 3 4
a
Unhydrogenated, reference.
deposition ŽLPCVD. at 425⬚C and 1.1 torr, using Si 2 H 6 as the reactant gas. Then, the ␣-Si films were crystallized by XeCl excimer laser Ž s 308 nm, 14 shots. with energy density 370 mJ cmy2 . After forming a 121-nm thick SiO 2 gate-insulator by electron cyclotron resonance ŽECR.-PECVD at 100⬚C, some wafers were loaded into the RF Ž13.56 MHz. PECVD chamber for hydrogenation processing. The hydrogenation was performed with RF power density 0.038 W cmy2 , pressure 1 torr and temperature 340⬚C for 180 s. The mixtures of H 2 and He or Ar introduced into the chamber are shown in Table 1. After hydrogenation, a standard self-aligned NMOS low-temperature Ž300⬚C. process was used to fabricate TFTs with gate width Ws 10 and length L s 10 m. For each hydrogenation regime, a batch of approximately 20 transistors has been studied. The device parameters Žturn-on voltage VON , effective mobility eff and sub-threshold swing S . were extracted from the transfer characteristics in the linear regime. The ratio Ion rIoff ŽVGS s 0 and 10 V, respectively. was determined for VDS s 5 V. With the aid of photo-emission measurements, hot-carrier stress conditions Ž VGs tress - VDstress . were defined for maximum light emission at the drain edge w6x.
3. Results and discussion 3.1. Static de¨ ice parameters Fig. 1 shows the device parameters as a function of the H 2 concentration diluted in He or Ar. It is apparent that VON is improved when a plasma of 2᎐4% H 2rAr is used for hydrogenation, while a high concentration of H 2 is required for the improvement of VON when a plasma of H 2rHe is used. After hydrogenation in a plasma of 4% H 2rHe, the devices exhibit a VON value approximately 1 V lower than the unhydrogenated ones, and even lower than those hydrogenated with 100% H 2 . Concerning the effective mobility, a slight improvement Žapprox. 10%. is noted after hydrogenation. In contrast, the Ion rIoff ratio and the subthreshold swing are improved, especially after hydrogenation with 4% H 2rAr or He. The sub-threshold swing decreases by approximately 20% from its value in the unhydrogenated devices. However, some differences are observed between devices hydrogenated in H 2rAr or He plasma, particularly for a hydrogen concentration of 2%. In addition, in hydrogenated TFTs, a shrinkage in the dispersion of the device parameters is observed, a result presenting the most important feature of the hydrogenation process. As it is generally known w3x, interface and grain boundary passivation is acquired when atomic hydrogen H andror Hq ions are introduced to the device. In contrast, molecular hydrogen H 2 presents no significant passivation effect. According to a previous study w7x, the hydrogen plasma formed from pure hydrogen gas Ž100% H. consists mainly of hydrogen molecular radicals. These hydrogen molecular radicals must be thermally decomposed to two hydrogen atoms in order to terminate silicon dangling-bonds. This is why pure
Fig. 1. Ža. Turn-on voltage; Žb. effective mobility; Žc. Ion rIoff ratio; and Žd. sub-threshold swing as a function of hydrogen concentration diluted with helium or argon.
F.V. Farmakis et al. r Thin Solid Films 383 (2001) 151᎐153
hydrogen plasma works inefficiently for silicon hydrogenation. In comparison, the hydrogen plasma formed from a low % of hydrogen diluted with rare gas, such as helium and argon, can generate hydrogen atomic radicals more efficiently w7x. The differences observed between the use of argon or helium as the dilution medium can possibly be attributed to the different cross-sections that they present for hydrogen ionization. 3.2. Hot-carrier experiments As the reliability of polysilicon TFTs represents an important issue for device integration into circuits, hot-carrier stress experiments were performed. The applied stress conditions were determined by photoemission experiments, which demonstrated that
153
VGs tress y VON remains constant, where VGstress is the gate voltage for maximum device degradation. Fig. 2 shows turn-on voltage variation and g mm ax degradation as a function of stress duration applied to the devices. First of all, we observe that unhydrogenated devices present less degradation than hydrogenated ones, indicating that hydrogenation deteriorates device reliability w8x. Devices hydrogenated by the hydrogen plasma formed from 100 and 4% H 2 diluted with He exhibit less degradation than those hydrogenated with a plasma of 4% H 2 diluted with Ar. This is generally attributed to breaking of weak Si᎐H bonds, generating traps at the grain boundaries and at the polysiliconrSiO 2 interface. In addition, released hydrogen atoms can be injected into the gate oxide, enhancing the device degradation. The role of helium and argon in the degradation mechanism is not yet clear and, hence, it merits more study. 4. Conclusions The effects of various hydrogenation processes were investigated in polysilicon TFTs. The device parameters are improved after hydrogenation in a hydrogen plasma formed by H 2 diluted with He or Ar. In addition, the dispersion of device characteristics is improved after hydrogenation. However, hot-carrier stress experiments demonstrate that hydrogenation severely affects the device reliability. Furthermore, it was found that a hydrogenrhelium mixture gives more reliable transistors than hydrogenrargon mixtures, in terms of hotcarrier effects. References
Fig. 2. Ža. VON variation; and Žb. g mmax degradation during hot carrier stress. Stress conditions: VDs tress s 14 and VGstress y VON s 2 V.
w1x S.D. Brotherton, Semicond. Sci. Technol. 10 Ž1995. 721. w2x C.T. Angelis, C.A. Dimitriadis, M. Miyasaka, F.V. Farmakis, G. Kamarinos, J. Brini, J. Stoemenos, J. Appl. Phys. 86 Ž1999. 4600. w3x T.I. Kamins, P.J. Marcoux, IEEE Electr. Dev. Lett. 1 Ž1980. 159. w4x M. Rodder, D.A. Antoniadis, IEEE Electr. Dev. Lett. 8 Ž1987. 27. w5x G.P. Pollack, W.F. Richardson, S.D.S. Malhi, T. Bonfield, H. Shichijo, S. Banerjee, M. Elahy, A.H. Shah, IEEE Electr. Dev. Lett. 5 Ž1984. 468. w6x F.V. Farmakis, J. Brini, G. Kamarinos, C.T. Angelis, C.A. Dimitriadis, M. Miyasaka, Proceedings of ICNF ’99 ŽHongKong., Ž1999. 457. w7x C.-F. Yeh, T.-J. Chen, C. Liu, J.T. Gudmundsson, M.A. Lieberman, IEEE Electr. Dev. Lett. 20 Ž1999. 223. w8x I.-W. Wu, W.B. Jackson, T.-Y. Huang, A.G. Lewis, A. Chiang, IEEE Electr. Dev. Lett. 11 Ž1990. 167.
Hydrogenation in laser annealed polysilicon thin-film transistors Ž TFTs. F.V. Farmakis a,U , D.M. Tsamadosa , J. Brini a , G. Kamarinos a , C.A. Dimitriadis b , M. Miyasakac a
LPCS, ENSERG, 23 rue des Martyrs, BP 257, 38016 Grenoble Cedex 1, France Department of Physics, Uni¨ ersity of Thessaloniki, 54006 Thessaloniki, Greece c Seiko Epson Corporation, Base Technology Research Center, Owa 3-3-5, Suwa, Nagano 392, Japan b
Abstract Hydrogenation effects in excimer laser annealed polysilicon thin-film transistors ŽTFTs. were studied. Hydrogen plasma formed from hydrogen diluted with Ar or He was used in order to passivate defects at the polysiliconrsilicon oxide interface, as well as in the polysilicon material. It was found that, after hydrogenation, no more than a 10% increase in the carrier mobility is attained, accompanied by a threshold-voltage decrease, due to passivation of deep states at the polysiliconrsilicon oxide interface and at the grain boundaries. However, the most important feature of hydrogenated devices is the improvement in the dispersion of their transfer characteristics. In addition, hot-carrier stress experiments showed that optimization of the type of dilution gas ŽAr or He. and the relative concentration of hydrogen can be carried out in order to improve the device reliability. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Polycrystalline; Thin-film transistor; Plasma hydrogenation
1. Introduction Polycrystalline silicon thin-film transistors Žpolysilicon TFTs. are widely investigated, mainly due to their application in active-matrix liquid crystal displays ŽAMLCDs.. The performance of polysilicon TFTs depends strongly on defects in the polysilicon and at the polysiliconrgate oxide interface w1x. Recently, laserannealing techniques were applied to improve the polysilicon TFT performance through grain-size enlargement and reduction of the grain boundary and in-grain defect density w2x. A traditional technique to reduce defects in TFTs is hydrogenation. Various hydrogenation techniques are currently applied, such as plasma hydrogenation w3x, hydrogen implantation w4x
U
Corresponding author. Tel.: q33-476856046; fax: q33-476856070. E-mail address: [email protected] ŽF.V. Farmakis..
and SiN x :H encapsulation w5x. An important issue of the hydrogenation process is its duration, resulting in improvement of the turn-on voltage, carrier mobility and sub-threshold swing, and in the dispersion of the static device characteristics. In this work, we used plasma hydrogenation, with the hydrogen plasma formed by a low % hydrogen diluted with a rare gas, namely helium or argon. The aim of this work was to investigate the effect of various hydrogen concentrations, diluted with helium or argon, on the device performance, the dispersion of the device parameters and the device reliability. 2. Experimental The studied polysilicon TFTs were fabricated on glass substrates by a low-temperature process described elsewhere w2x. Amorphous silicon Ž ␣-Si. films 51.8 nm thick were deposited by low-pressure chemical vapor
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 5 8 8 - 1
F.V. Farmakis et al. r Thin Solid Films 383 (2001) 151᎐153
152
Table 1 Mixtures of H 2 , He and Ar during plasma hydrogenation H2 gas flow Žsccm.
He or Ar gas flow Žsccm.
H2 concentration Ž%.
0 1400 100 150 200
0 0 4900 4850 4800
0a 100 2 3 4
a
Unhydrogenated, reference.
deposition ŽLPCVD. at 425⬚C and 1.1 torr, using Si 2 H 6 as the reactant gas. Then, the ␣-Si films were crystallized by XeCl excimer laser Ž s 308 nm, 14 shots. with energy density 370 mJ cmy2 . After forming a 121-nm thick SiO 2 gate-insulator by electron cyclotron resonance ŽECR.-PECVD at 100⬚C, some wafers were loaded into the RF Ž13.56 MHz. PECVD chamber for hydrogenation processing. The hydrogenation was performed with RF power density 0.038 W cmy2 , pressure 1 torr and temperature 340⬚C for 180 s. The mixtures of H 2 and He or Ar introduced into the chamber are shown in Table 1. After hydrogenation, a standard self-aligned NMOS low-temperature Ž300⬚C. process was used to fabricate TFTs with gate width Ws 10 and length L s 10 m. For each hydrogenation regime, a batch of approximately 20 transistors has been studied. The device parameters Žturn-on voltage VON , effective mobility eff and sub-threshold swing S . were extracted from the transfer characteristics in the linear regime. The ratio Ion rIoff ŽVGS s 0 and 10 V, respectively. was determined for VDS s 5 V. With the aid of photo-emission measurements, hot-carrier stress conditions Ž VGs tress - VDstress . were defined for maximum light emission at the drain edge w6x.
3. Results and discussion 3.1. Static de¨ ice parameters Fig. 1 shows the device parameters as a function of the H 2 concentration diluted in He or Ar. It is apparent that VON is improved when a plasma of 2᎐4% H 2rAr is used for hydrogenation, while a high concentration of H 2 is required for the improvement of VON when a plasma of H 2rHe is used. After hydrogenation in a plasma of 4% H 2rHe, the devices exhibit a VON value approximately 1 V lower than the unhydrogenated ones, and even lower than those hydrogenated with 100% H 2 . Concerning the effective mobility, a slight improvement Žapprox. 10%. is noted after hydrogenation. In contrast, the Ion rIoff ratio and the subthreshold swing are improved, especially after hydrogenation with 4% H 2rAr or He. The sub-threshold swing decreases by approximately 20% from its value in the unhydrogenated devices. However, some differences are observed between devices hydrogenated in H 2rAr or He plasma, particularly for a hydrogen concentration of 2%. In addition, in hydrogenated TFTs, a shrinkage in the dispersion of the device parameters is observed, a result presenting the most important feature of the hydrogenation process. As it is generally known w3x, interface and grain boundary passivation is acquired when atomic hydrogen H andror Hq ions are introduced to the device. In contrast, molecular hydrogen H 2 presents no significant passivation effect. According to a previous study w7x, the hydrogen plasma formed from pure hydrogen gas Ž100% H. consists mainly of hydrogen molecular radicals. These hydrogen molecular radicals must be thermally decomposed to two hydrogen atoms in order to terminate silicon dangling-bonds. This is why pure
Fig. 1. Ža. Turn-on voltage; Žb. effective mobility; Žc. Ion rIoff ratio; and Žd. sub-threshold swing as a function of hydrogen concentration diluted with helium or argon.
F.V. Farmakis et al. r Thin Solid Films 383 (2001) 151᎐153
hydrogen plasma works inefficiently for silicon hydrogenation. In comparison, the hydrogen plasma formed from a low % of hydrogen diluted with rare gas, such as helium and argon, can generate hydrogen atomic radicals more efficiently w7x. The differences observed between the use of argon or helium as the dilution medium can possibly be attributed to the different cross-sections that they present for hydrogen ionization. 3.2. Hot-carrier experiments As the reliability of polysilicon TFTs represents an important issue for device integration into circuits, hot-carrier stress experiments were performed. The applied stress conditions were determined by photoemission experiments, which demonstrated that
153
VGs tress y VON remains constant, where VGstress is the gate voltage for maximum device degradation. Fig. 2 shows turn-on voltage variation and g mm ax degradation as a function of stress duration applied to the devices. First of all, we observe that unhydrogenated devices present less degradation than hydrogenated ones, indicating that hydrogenation deteriorates device reliability w8x. Devices hydrogenated by the hydrogen plasma formed from 100 and 4% H 2 diluted with He exhibit less degradation than those hydrogenated with a plasma of 4% H 2 diluted with Ar. This is generally attributed to breaking of weak Si᎐H bonds, generating traps at the grain boundaries and at the polysiliconrSiO 2 interface. In addition, released hydrogen atoms can be injected into the gate oxide, enhancing the device degradation. The role of helium and argon in the degradation mechanism is not yet clear and, hence, it merits more study. 4. Conclusions The effects of various hydrogenation processes were investigated in polysilicon TFTs. The device parameters are improved after hydrogenation in a hydrogen plasma formed by H 2 diluted with He or Ar. In addition, the dispersion of device characteristics is improved after hydrogenation. However, hot-carrier stress experiments demonstrate that hydrogenation severely affects the device reliability. Furthermore, it was found that a hydrogenrhelium mixture gives more reliable transistors than hydrogenrargon mixtures, in terms of hotcarrier effects. References
Fig. 2. Ža. VON variation; and Žb. g mmax degradation during hot carrier stress. Stress conditions: VDs tress s 14 and VGstress y VON s 2 V.
w1x S.D. Brotherton, Semicond. Sci. Technol. 10 Ž1995. 721. w2x C.T. Angelis, C.A. Dimitriadis, M. Miyasaka, F.V. Farmakis, G. Kamarinos, J. Brini, J. Stoemenos, J. Appl. Phys. 86 Ž1999. 4600. w3x T.I. Kamins, P.J. Marcoux, IEEE Electr. Dev. Lett. 1 Ž1980. 159. w4x M. Rodder, D.A. Antoniadis, IEEE Electr. Dev. Lett. 8 Ž1987. 27. w5x G.P. Pollack, W.F. Richardson, S.D.S. Malhi, T. Bonfield, H. Shichijo, S. Banerjee, M. Elahy, A.H. Shah, IEEE Electr. Dev. Lett. 5 Ž1984. 468. w6x F.V. Farmakis, J. Brini, G. Kamarinos, C.T. Angelis, C.A. Dimitriadis, M. Miyasaka, Proceedings of ICNF ’99 ŽHongKong., Ž1999. 457. w7x C.-F. Yeh, T.-J. Chen, C. Liu, J.T. Gudmundsson, M.A. Lieberman, IEEE Electr. Dev. Lett. 20 Ž1999. 223. w8x I.-W. Wu, W.B. Jackson, T.-Y. Huang, A.G. Lewis, A. Chiang, IEEE Electr. Dev. Lett. 11 Ž1990. 167.