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Subthreshold characteristics of submicrometer polysilicon thin film transistor
Thin Solid Films 382 Ž2001. 271᎐274
Subthreshold characteristics of submicrometer polysilicon thin film transistor D.N. Yaung a , Y.K. Fang a,U , K.C. Huang a , Y.J. Wang a , C.C. Hung a , M.S. Liang b , S.G. Wuub a
VLSI Technology Laboratory, Department of Electrical Engineering, National Cheng Kuang Uni¨ ersity, P.O. Box 7-200, Tainan, Taiwan, PR China b Taiwan Semiconductor Manufacturing Company, Hsin-Chu, Taiwan, ROC Received 16 February 2000; received in revised form 4 October 2000; accepted 6 October 2000
Abstract The subthreshold characteristics of both hydrogenated and unhydrogenated sub-micrometer polysilicon thin-film transistors have been investigated in detail. The subthreshold slope of unhydrogenated TFTs becomes steeper at higher drain voltage. This is attributed to the floating-body effect that results from the positive feedback between transistor current and impact ionization current. The floating-body effect of poly-TFTs was found to relate to grain size, drain voltage and hydrogenation. For the TFTs with larger poly-grain, the floating-body effect was more obvious and appeared at lower drain bias. After hydrogenation, the floating-body effect was suppressed. Therefore, the subthreshold swing of hydrogenated TFTs is continuously increasing with drain voltage. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Grain boundary; Hydrogenation; Polysilicon; Thin-film transistor
1. Introduction Polysilicon thin-film transistors ŽTFTs. grown by low-pressure chemical vapor deposition ŽLPCVD. are currently used for liquid crystal display and static random-access memory ŽSRAM. w1,2x. As the TFTs are further scaled into submicron to meet the requirement of higher circuit density, the subthreshold characteristics of the TFTs become critical, thus attracting more attention to study w3,4x. In the past, it has been found that the subthreshold swing is improved at a larger drain voltage w5x. However, the detailed mechanism and factors that affect the swing of the submicrometer TFTs have not been investigated. In this paper, for the first time, we depict the results of a systematical invesU
Corresponding author. Tel.: q886-6-275-7575; fax: q886-6-2345482. E-mail address: [email protected] ŽY.K. Fang..
tigation on the subthreshold characteristics of submicron TFTs. These results reveal that the floating body effect is the major cause for the improvement of subthreshold swing at larger drain voltage. However, the effect is dependent on the grain size of polysilicon and related to the hydrogen-plasma treatment. 2. Device fabrication and measurement A nq polysilicon film was deposited by LPCVD at 620⬚C to be the gate electrode of TFTs. Then, the gate geometry was defined, followed by a 30-nm gate oxide deposited by LPCVD at 780⬚C. Next, another 30 nm amorphous silicon was deposited by thermal decomposition of silane or disilane in a conventional LPCVD reactor. Then, the channel was implanted with phosphorous at a dose of 5 = 10 12 cmy2 and recrystallized at 600⬚C in N2 ambient for 24 h. Source and drain regions are implanted with boron at a dose of 5 = 10 15
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 7 6 6 - 1
272
D.N. Yaung et al. r Thin Solid Films 382 (2001) 271᎐274
Fig. 1. Experiment curves of the drain current vs. gate voltage for unhydrogenated and hydrogenated p-type TFTs ŽW s 0.5 m, L s 0.45 m with 0.2 m offset. deposited by disilane with VDS varying from y1 V to y15 V step-2 V.
cmy2 . Planarization can be achieved by boron phosphorous tetra ethyl ortho silicate ŽBPTEOS. flow at 850⬚C for 30 min. Undoped oxide Ž100 nm. deposited between BPTEOS and poly-TFTs was used to prevent boron and phosphorous diffusing into poly-channel. The channel width is 0.5 m and the channel length is defined at 0.45 m with a 0.2-m offset structure at the drain side to suppress the effect of gate-induced drain leakage ŽGIDL. w6x. Hydrogenation was performed with the plasma-enhanced chemical vapor deposition ŽPECVD. system at 250⬚C in pure H 2 ambient at 1 torr and a rf power of 100 W. 3. Experiment results and discussions The grain size of poly-Si after recrystallization was examined by means of transmission electron microscopy ŽTEM.. The average grain size of polysilicon deposited by pyrolysis of silane and disilane gas is approximately 0.6 m and 1.3 m, respectively. In Fig. 1, the Id ᎐Vg characteristics of unhydrogenated TFT deposited by disilane are shown. The continuously improving swing was observed at drain voltage up to 6 V.
We consider that the steepening of the subthreshold slope is correlated to floating-body effect w5x. In a submicrometer device, the applied drain bias will produce an electric field of the order of 10 5 Vrcm, which is high enough to cause impact ionization. The body of submicron TFTs is floating, so the impact-generated electrons will accumulate above the channel to lower the body potential. This causes positive shift of the threshold voltage of p-type poly-TFT, and increases Id s and impact ionization rate, thus resulting in a positive feedback and leading steep subthreshold slopes. It has been observed that the swing of single transistor for submicrometer TFT or SOI can be smaller than 100 mVrdec w7x. However, the grain boundaries existing in poly-Si channel will trap the carriers and decrease the energy of carriers, then suppress the floating-body effect. So most TFTs have a larger swing Ž) 600 mVrdec.. In order to minimize the device variation of poly-TFTs caused by grain boundaries, our samples have 400 TFTs connected in parallel. For parallel TFTs with channel dimension smaller than the grain size, grain boundaries are existing in some TFTs but are not in others, so the combined subthreshold swing of the parallel TFTs is smaller than TFTs with grain boun-
Fig. 2. Experiment curves of the drain current vs. gate voltage for unhydrogenated and hydrogenated p-type TFTs ŽW s 0.5 m, L s 0.45 m with 0.2 m offset. deposited by silane with VDS varying from y1 V to y15 V step-2 V.
D.N. Yaung et al. r Thin Solid Films 382 (2001) 271᎐274
daries, but larger than TFTs without grain boundaries. However, when TFTs have a larger distance between source and grain boundaries, carriers can store up enough energy to generate impact ionization even if grain boundaries are existing in the channel. As drain voltage increases, the distance for carriers to store enough impact energy is decreased and more TFTs can generate the floating-body effect to steepen the subthreshold slope. So the combined swing of parallel TFTs is improved with drain voltage. Above 8 V, sourcerdrain punchthrough occurs and swing begins to degrade w8x. Fig. 2 shows the Id ᎐Vg curves of TFTs deposited by silane. The continuously decreasing swing is only observed for 3 V- VD - 8 V. That differs from TFTs deposited by disilane whose swing is continuously decreasing for drain voltage up to 6 V. The average grain size of poly deposited by silane is half of disilane, so it needs higher electric field to store up enough energy to generate impact ionization before carriers reach to grain boundaries; thus floating-body effect is insignificant at low drain voltage. So the improving swing wasn’t observed until VD ) 3 V. Fig. 1 and Fig. 2 also show the Id ᎐Vg curves of 8-h hydrogenated TFTs deposited by disilane and silane. The characteristics of hydrogenated TFTs are obviously improved due to the passivation of trap states. Differing with unhydrogenated TFTs, the subthreshold swing of hydrogenated TFTs is continuously increasing with drain voltage. The defect bonds are much weaker than normal bonds; less energy is required to generate impact ionization in poly-channel w9x. After hydrogenation, the impact ionization is significantly suppressed because the number of defect states is reduced w10x. As a result, the floating-body effect is not apparent for hydrogenated TFTs. The dependency of subthreshold swing on drain voltage is summarized in Fig. 3 for a clearer illustration. For unhydrogenated TFTs, the phenomena of the floating-body effect is the most significant at the valley of curves i.e. approximately Vd s 8 V for silane TFTs. Because the grain size is smaller for TFTs deposited by silane, it needs higher electric field to generate impact ionization. Hence the drain voltage of the valley for silane TFTs is larger than disilane. For hydrogenated TFTs, the hydrogen passivation made the swing smaller at low drain voltage and suppressed the floating body effect. But as the drain voltage is larger than 12 V, the punchthrough causes the increasing of swing rapidly. The difference of threshold voltage Ž ⌬VTH . caused by drain voltage is another method to describe the subthreshold characteristics. Here the threshold voltage is defined as the gate voltage at which the drain current normalized by WrL is fixed to 10y8 A w11x. Fig. 4 shows ⌬VTH vs. drain voltage. As drain bias increases, increasing ⌬VTH shows the floating body effect
273
Fig. 3. Subthreshold swing vs. drain voltage for different type TFTs.
gradually significant Žmore and more TFTs show the floating body effect.. Then, ⌬VTH will be decreased because the floating-body effect began to saturate Žmost TFTs have shown the floating body effect.. Finally, ⌬VTH is increasing due to punchthrough. For unhydrogenated TFTs, the floating body effect of disilane TFTs is more significant, so the difference of ⌬VTH between the maximum and the minimum of the curve is larger and the maximum and the minimum are located at smaller drain voltage. In addition, the influence of the floating-body effect on ⌬VTH also was observed in hydrogenated disilane TFTs. But it is not obvious for silane TFTs because the hydrogenation of smaller grain size samples shows better passivation effect w12x than suppress the floating-body effect. 4. Conclusion A detailed investigation of the subthreshold characteristics of submicrometer hydrogenatedrunhydrogenated TFTs has been presented. For unhydrogenated TFTs, The subthreshold slope of unhydrogenated TFTs becomes steeper at higher drain voltage due to the
Fig. 4. The difference of threshold voltage per step of drain voltage, ⌬VTH s VTH Ž VDS s N . y VTH Ž VDS s N y 1., vs. drain voltage. Here VTH is defined as the gate voltage when I DSrŽWrL. s 10y8 A.
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floating-body effect which depends on grain size, drain voltage and hydrogenation. After 8 h of hydrogenation, the impact ionization is suppressed because the number of defect states is reduced. Therefore, the subthreshold swing of hydrogenated TFTs is continuously increasing with drain bias. References w1x T.P. Brody, IEEE Trans. Electron Devices 31 Ž1984. 1614. w2x S.D.S. Malhi, H. Shichijo, S.K. Banerjee, R. Sundareson, M. Elahy, G.P. Pollack, W. Richaedson, A.H. Sha, L.R. Hite, R.H. Womack, P. Chatterjee, H. William, IEEE Trans. Electron Devices 32 Ž1985. 258. w3x R.K. Watts, J.T.C. Lee, IEEE Electron Device Lett. 14 Ž1993. 515. w4x D.N. Yaung, Y.K. Fang, K.C. Hwang, K.Y. Lee, K.H. Wu, J.J. Ho, C.Y. Chen, Y.J. Wang, M.S. Liang, J.Y. Lee, S.G. Wuu, IEEE Electron Device Lett. 19 Ž1998. 429.
w5x N. Yamauchi, J.J. Hajjar, R. Reif, IEEE Trans. Electron Devices 38 Ž1991. 55. w6x S. Ikeda, S. Hasiba, I. Kuramoto, H. Katoh, S. Ariga, T. Yamanaka, T. Hashimoto, N. Hashimoto, S. Meguro, IEDM Ž1990. 469. w7x J.R. Davis, A.E. Glaccum, K. Reeson, P.L.F. Hemment, IEEE Electron Device Letters 7 Ž1986. 570. w8x K. Yu, Y.L. Tsang, IEEE Trans. Electron Devices 44 Ž1997. 847. w9x M. Hack, A.G. Lewis, IEEE Electron Device Lett. 12 Ž1991. 303. w10x M. Cao, T. Zhao, K.C. Sarawat, J.D. Plummer, IEEE Trans. Electron Devices 42 Ž1995. 1134. w11x M.J. Deen, Z.X. Yan, IEEE Trans. Electron Devices 37 Ž1990. 1707. w12x I.W. Wu, T.Y. Huang, W.B. Jackson, A.G. Lewis, A. Chiang, IEEE Electron Device Lett. 12 Ž1991. 181.
Subthreshold characteristics of submicrometer polysilicon thin film transistor D.N. Yaung a , Y.K. Fang a,U , K.C. Huang a , Y.J. Wang a , C.C. Hung a , M.S. Liang b , S.G. Wuub a
VLSI Technology Laboratory, Department of Electrical Engineering, National Cheng Kuang Uni¨ ersity, P.O. Box 7-200, Tainan, Taiwan, PR China b Taiwan Semiconductor Manufacturing Company, Hsin-Chu, Taiwan, ROC Received 16 February 2000; received in revised form 4 October 2000; accepted 6 October 2000
Abstract The subthreshold characteristics of both hydrogenated and unhydrogenated sub-micrometer polysilicon thin-film transistors have been investigated in detail. The subthreshold slope of unhydrogenated TFTs becomes steeper at higher drain voltage. This is attributed to the floating-body effect that results from the positive feedback between transistor current and impact ionization current. The floating-body effect of poly-TFTs was found to relate to grain size, drain voltage and hydrogenation. For the TFTs with larger poly-grain, the floating-body effect was more obvious and appeared at lower drain bias. After hydrogenation, the floating-body effect was suppressed. Therefore, the subthreshold swing of hydrogenated TFTs is continuously increasing with drain voltage. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Grain boundary; Hydrogenation; Polysilicon; Thin-film transistor
1. Introduction Polysilicon thin-film transistors ŽTFTs. grown by low-pressure chemical vapor deposition ŽLPCVD. are currently used for liquid crystal display and static random-access memory ŽSRAM. w1,2x. As the TFTs are further scaled into submicron to meet the requirement of higher circuit density, the subthreshold characteristics of the TFTs become critical, thus attracting more attention to study w3,4x. In the past, it has been found that the subthreshold swing is improved at a larger drain voltage w5x. However, the detailed mechanism and factors that affect the swing of the submicrometer TFTs have not been investigated. In this paper, for the first time, we depict the results of a systematical invesU
Corresponding author. Tel.: q886-6-275-7575; fax: q886-6-2345482. E-mail address: [email protected] ŽY.K. Fang..
tigation on the subthreshold characteristics of submicron TFTs. These results reveal that the floating body effect is the major cause for the improvement of subthreshold swing at larger drain voltage. However, the effect is dependent on the grain size of polysilicon and related to the hydrogen-plasma treatment. 2. Device fabrication and measurement A nq polysilicon film was deposited by LPCVD at 620⬚C to be the gate electrode of TFTs. Then, the gate geometry was defined, followed by a 30-nm gate oxide deposited by LPCVD at 780⬚C. Next, another 30 nm amorphous silicon was deposited by thermal decomposition of silane or disilane in a conventional LPCVD reactor. Then, the channel was implanted with phosphorous at a dose of 5 = 10 12 cmy2 and recrystallized at 600⬚C in N2 ambient for 24 h. Source and drain regions are implanted with boron at a dose of 5 = 10 15
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 7 6 6 - 1
272
D.N. Yaung et al. r Thin Solid Films 382 (2001) 271᎐274
Fig. 1. Experiment curves of the drain current vs. gate voltage for unhydrogenated and hydrogenated p-type TFTs ŽW s 0.5 m, L s 0.45 m with 0.2 m offset. deposited by disilane with VDS varying from y1 V to y15 V step-2 V.
cmy2 . Planarization can be achieved by boron phosphorous tetra ethyl ortho silicate ŽBPTEOS. flow at 850⬚C for 30 min. Undoped oxide Ž100 nm. deposited between BPTEOS and poly-TFTs was used to prevent boron and phosphorous diffusing into poly-channel. The channel width is 0.5 m and the channel length is defined at 0.45 m with a 0.2-m offset structure at the drain side to suppress the effect of gate-induced drain leakage ŽGIDL. w6x. Hydrogenation was performed with the plasma-enhanced chemical vapor deposition ŽPECVD. system at 250⬚C in pure H 2 ambient at 1 torr and a rf power of 100 W. 3. Experiment results and discussions The grain size of poly-Si after recrystallization was examined by means of transmission electron microscopy ŽTEM.. The average grain size of polysilicon deposited by pyrolysis of silane and disilane gas is approximately 0.6 m and 1.3 m, respectively. In Fig. 1, the Id ᎐Vg characteristics of unhydrogenated TFT deposited by disilane are shown. The continuously improving swing was observed at drain voltage up to 6 V.
We consider that the steepening of the subthreshold slope is correlated to floating-body effect w5x. In a submicrometer device, the applied drain bias will produce an electric field of the order of 10 5 Vrcm, which is high enough to cause impact ionization. The body of submicron TFTs is floating, so the impact-generated electrons will accumulate above the channel to lower the body potential. This causes positive shift of the threshold voltage of p-type poly-TFT, and increases Id s and impact ionization rate, thus resulting in a positive feedback and leading steep subthreshold slopes. It has been observed that the swing of single transistor for submicrometer TFT or SOI can be smaller than 100 mVrdec w7x. However, the grain boundaries existing in poly-Si channel will trap the carriers and decrease the energy of carriers, then suppress the floating-body effect. So most TFTs have a larger swing Ž) 600 mVrdec.. In order to minimize the device variation of poly-TFTs caused by grain boundaries, our samples have 400 TFTs connected in parallel. For parallel TFTs with channel dimension smaller than the grain size, grain boundaries are existing in some TFTs but are not in others, so the combined subthreshold swing of the parallel TFTs is smaller than TFTs with grain boun-
Fig. 2. Experiment curves of the drain current vs. gate voltage for unhydrogenated and hydrogenated p-type TFTs ŽW s 0.5 m, L s 0.45 m with 0.2 m offset. deposited by silane with VDS varying from y1 V to y15 V step-2 V.
D.N. Yaung et al. r Thin Solid Films 382 (2001) 271᎐274
daries, but larger than TFTs without grain boundaries. However, when TFTs have a larger distance between source and grain boundaries, carriers can store up enough energy to generate impact ionization even if grain boundaries are existing in the channel. As drain voltage increases, the distance for carriers to store enough impact energy is decreased and more TFTs can generate the floating-body effect to steepen the subthreshold slope. So the combined swing of parallel TFTs is improved with drain voltage. Above 8 V, sourcerdrain punchthrough occurs and swing begins to degrade w8x. Fig. 2 shows the Id ᎐Vg curves of TFTs deposited by silane. The continuously decreasing swing is only observed for 3 V- VD - 8 V. That differs from TFTs deposited by disilane whose swing is continuously decreasing for drain voltage up to 6 V. The average grain size of poly deposited by silane is half of disilane, so it needs higher electric field to store up enough energy to generate impact ionization before carriers reach to grain boundaries; thus floating-body effect is insignificant at low drain voltage. So the improving swing wasn’t observed until VD ) 3 V. Fig. 1 and Fig. 2 also show the Id ᎐Vg curves of 8-h hydrogenated TFTs deposited by disilane and silane. The characteristics of hydrogenated TFTs are obviously improved due to the passivation of trap states. Differing with unhydrogenated TFTs, the subthreshold swing of hydrogenated TFTs is continuously increasing with drain voltage. The defect bonds are much weaker than normal bonds; less energy is required to generate impact ionization in poly-channel w9x. After hydrogenation, the impact ionization is significantly suppressed because the number of defect states is reduced w10x. As a result, the floating-body effect is not apparent for hydrogenated TFTs. The dependency of subthreshold swing on drain voltage is summarized in Fig. 3 for a clearer illustration. For unhydrogenated TFTs, the phenomena of the floating-body effect is the most significant at the valley of curves i.e. approximately Vd s 8 V for silane TFTs. Because the grain size is smaller for TFTs deposited by silane, it needs higher electric field to generate impact ionization. Hence the drain voltage of the valley for silane TFTs is larger than disilane. For hydrogenated TFTs, the hydrogen passivation made the swing smaller at low drain voltage and suppressed the floating body effect. But as the drain voltage is larger than 12 V, the punchthrough causes the increasing of swing rapidly. The difference of threshold voltage Ž ⌬VTH . caused by drain voltage is another method to describe the subthreshold characteristics. Here the threshold voltage is defined as the gate voltage at which the drain current normalized by WrL is fixed to 10y8 A w11x. Fig. 4 shows ⌬VTH vs. drain voltage. As drain bias increases, increasing ⌬VTH shows the floating body effect
273
Fig. 3. Subthreshold swing vs. drain voltage for different type TFTs.
gradually significant Žmore and more TFTs show the floating body effect.. Then, ⌬VTH will be decreased because the floating-body effect began to saturate Žmost TFTs have shown the floating body effect.. Finally, ⌬VTH is increasing due to punchthrough. For unhydrogenated TFTs, the floating body effect of disilane TFTs is more significant, so the difference of ⌬VTH between the maximum and the minimum of the curve is larger and the maximum and the minimum are located at smaller drain voltage. In addition, the influence of the floating-body effect on ⌬VTH also was observed in hydrogenated disilane TFTs. But it is not obvious for silane TFTs because the hydrogenation of smaller grain size samples shows better passivation effect w12x than suppress the floating-body effect. 4. Conclusion A detailed investigation of the subthreshold characteristics of submicrometer hydrogenatedrunhydrogenated TFTs has been presented. For unhydrogenated TFTs, The subthreshold slope of unhydrogenated TFTs becomes steeper at higher drain voltage due to the
Fig. 4. The difference of threshold voltage per step of drain voltage, ⌬VTH s VTH Ž VDS s N . y VTH Ž VDS s N y 1., vs. drain voltage. Here VTH is defined as the gate voltage when I DSrŽWrL. s 10y8 A.
274
D.N. Yaung et al. r Thin Solid Films 382 (2001) 271᎐274
floating-body effect which depends on grain size, drain voltage and hydrogenation. After 8 h of hydrogenation, the impact ionization is suppressed because the number of defect states is reduced. Therefore, the subthreshold swing of hydrogenated TFTs is continuously increasing with drain bias. References w1x T.P. Brody, IEEE Trans. Electron Devices 31 Ž1984. 1614. w2x S.D.S. Malhi, H. Shichijo, S.K. Banerjee, R. Sundareson, M. Elahy, G.P. Pollack, W. Richaedson, A.H. Sha, L.R. Hite, R.H. Womack, P. Chatterjee, H. William, IEEE Trans. Electron Devices 32 Ž1985. 258. w3x R.K. Watts, J.T.C. Lee, IEEE Electron Device Lett. 14 Ž1993. 515. w4x D.N. Yaung, Y.K. Fang, K.C. Hwang, K.Y. Lee, K.H. Wu, J.J. Ho, C.Y. Chen, Y.J. Wang, M.S. Liang, J.Y. Lee, S.G. Wuu, IEEE Electron Device Lett. 19 Ž1998. 429.
w5x N. Yamauchi, J.J. Hajjar, R. Reif, IEEE Trans. Electron Devices 38 Ž1991. 55. w6x S. Ikeda, S. Hasiba, I. Kuramoto, H. Katoh, S. Ariga, T. Yamanaka, T. Hashimoto, N. Hashimoto, S. Meguro, IEDM Ž1990. 469. w7x J.R. Davis, A.E. Glaccum, K. Reeson, P.L.F. Hemment, IEEE Electron Device Letters 7 Ž1986. 570. w8x K. Yu, Y.L. Tsang, IEEE Trans. Electron Devices 44 Ž1997. 847. w9x M. Hack, A.G. Lewis, IEEE Electron Device Lett. 12 Ž1991. 303. w10x M. Cao, T. Zhao, K.C. Sarawat, J.D. Plummer, IEEE Trans. Electron Devices 42 Ž1995. 1134. w11x M.J. Deen, Z.X. Yan, IEEE Trans. Electron Devices 37 Ž1990. 1707. w12x I.W. Wu, T.Y. Huang, W.B. Jackson, A.G. Lewis, A. Chiang, IEEE Electron Device Lett. 12 Ž1991. 181.