- Email: [email protected]
Dynamic characteristics of MICC polycrystalline thin film transistors
Journal of Non-Crystalline Solids 352 (2006) 1745–1748 www.elsevier.com/locate/jnoncrysol
Dynamic characteristics of MICC polycrystalline thin film transistors Yong-Duck Son, Kyung-Dong Yang, Byung-Seong Bae, Kyu-Chang Park, Jin Jang
*
Advanced Display Research Center, Department of Information Display, Kyung Hee University, Hoegi-dong 1, Dongdaemun-gu, Seoul 130-701, Republic of Korea Available online 30 March 2006
Abstract We studied the DC bias-induced changes in the performance of the PMOS ring oscillator made of poly-Si TFT. The poly-Si was prepared by metal-induced crystallization of amorphous silicon through a cap layer (MICC). The p-channel MICC poly-Si TFT exhibited the field-effect mobility of 31 cm2/V s, on/off current ratio of 107 and threshold voltage of 2.8 V. A 41-stage PMOS inverter chain exhibited an operation frequency of 1.2 MHz at an applied voltage of VDD = 18 V. The degradation of the ring oscillator made of MICC poly-Si TFT is much less than that of ELA TFT by DC bias stress. The difference appears to be due to the smooth surface of MICC poly-Si as compared with a typical excimer laser annealing (ELA) poly-Si. 2006 Elsevier B.V. All rights reserved. PACS: 68.47.Fg; 85.25.Am; 85.30.De; 85.30.Hi Keywords: Thin film transistors; Crystallization
1. Introduction Low-temperature polycrystalline silicon (LTPS) is of increasing interest recently to realize a system-on-glass (SOG). The concept of SOG is to build displays, memory, sensors, controllers and even microprocessors on glass as well as drivers. The poly-Si can be used in high-resolution, integrated active-matrix liquid-crystal displays (AMLCDs) and active-matrix organic light-emitting diodes (AMOLEDs) because of its high field-effect mobility and better stability during operation of thin-film transistor (TFT) as compared with hydrogenated amorphous silicon TFTs (a-Si:H TFTs). A number of techniques have been proposed during the last two decades to achieve large-area poly-Si on glass [1,2]. Among various techniques, laser annealing (LA) is the most frequently used. Excimer LA (ELA) of a-Si gives a high-quality poly-Si and is suitable for low-temperature process using a buffer layer on glass. But, there are still sev*
Corresponding author. Tel.: +82 2 961 0270; fax: +82 2 961 9154. E-mail address: [email protected] (J. Jang).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.11.131
eral issues such as high manufacturing cost, nonuniformity over a large area, high surface roughness, and degradation in TFT performance under bias stress. In this paper, we have investigated the stability of the ring oscillator made of p-channel LTPS poly-Si TFT. The poly-Si was made using metal-induced crystallization of a-Si through a cap layer (MICC) [3,4]. 2. Experimental procedure A self-aligned poly-Si TFT was used for a ring oscillator. The process for the poly-Si TFT is as follows. To crystallize a-Si:H at low temperature by MICC, hydrogenated amorphous silicon (a-Si:H) films were deposited on SiO2 (100 nm)/glass. After dehydrogenation of a-Si:H, the SiNx was deposited by PECVD using a SiH4/NH3 mixture at the substrate temperature of 270 C, and then small amount of Ni particles were sputtered onto the SiNx/a-Si. The sample was crystallized in a furnace at 570 C or rapid thermal annealing system (RTA). After forming of poly-Si islands, 70 nm-thick SiO2 and 100 nm metal layers were deposited on it. The substrate temperature was fixed at 400 C during
1746
Y.-D. Son et al. / Journal of Non-Crystalline Solids 352 (2006) 1745–1748
Fig. 1. The grain size of MICC poly-Si as a function of crystallization temperature. The Ni density was 4.6 · 1012 atoms/cm2.
SiO2 deposition. After patterning both gate metal and insulator on a channel, the source/drain regions were doped with 1% B2H6 diluted in He plasma using an ion doping system, which consisted of ion generation and ion acceleration parts. The ion acceleration voltage, gas pressure and substrate temperature were fixed at 12 kV, 0.3 mTorr and 280 C, respectively. To isolate source/drain from gate, 200 nm thick SiNx was deposited and contact holes were formed. Then, a 400 nm thick AlNd was deposited to form the source/drain electrodes. Finally, TFT was annealed at 400 C for 2 h under H2/N2. A high nickel density on a-Si leads to small grains as a result of vertical grain growth. However, with decreasing the Ni density, the distance between NiSi2 crystallites increases and leads to large grains as a result of lateral grain growth. In case of MICC, the amount of Ni diffused into the a-Si is much less than that on the SiNx surface because most of the Ni atoms are inside of the nitride. The average grain size seen by optical microscopy was found to be 40 lm. Note that the grain size can be controlled by the amount of Ni on the surface. The a-Si was crystallized by MICC in RTA system. Fig. 1 shows the grain size as a function of crystallization temperature by RTA system. The Ni area density on the a-Si was fixed at 4.6 · 1012 atoms/cm2. The grain size was measured by an optical microscopy. The grain size decreases from 51 lm to 26 lm when the crystallization temperature increases from 650 C to 750 C even though the same amount of Ni density is used to crystallize the a-Si.
Fig. 2. Transfer characteristics of a MICC poly-Si TFT.
of 2.8 V, and a gate voltage swing of 0.4 V/dec. The poly-Si TFT was designed to have the channel length of 6 lm. The ring oscillators consist of 41-stages inverters and two stage buffer amplifiers. The buffer amplifier is used to see the output signal of the ring oscillator that is not affected by the impedance of the measurement equipment. Fig. 3 shows the schematic circuit and photograph of a ring oscillator with buffer amplifier. Fig. 4 shows the output frequency of the ring oscillator measured as a function of the applied voltage (VDD). The output characteristics of the ring oscillator at VDD = 18 V are shown in the inset. The shape of a waveform is determined by the poly-Si TFT performance. The average propagation delay time was found to be 0.16 ns when the average propagation delay time is defined as 1/(2Nfosc), where fosc is the frequency
3. Results and discussion Fig. 2 shows the transfer characteristics of a MICC poly-Si TFT. The p-channel TFT exhibited a field-effect mobility of 31 cm2/V s, the minimum leakage current of about 2 · 10 12 A/lm at Vds = 10 V, a threshold voltage
Fig. 3. Schematic for the MICC poly-Si TFT ring oscillator circuit (a) and photograph of the completed 41-stage ring oscillator (b).
Y.-D. Son et al. / Journal of Non-Crystalline Solids 352 (2006) 1745–1748
1747
Fig. 4. The frequency characteristics of MICC poly-Si ring oscillator.
of ring oscillator in output waveform and N is the inverter chain number [5]. At the applied voltage of VDD = 18 V the ring oscillator oscillates at 1.2 MHz. The oscillation frequency increases from 0.01 to 1.6 MHz with increasing applied voltage from 6 to 25 V, and the oscillation amplitude saturates at 24 V. Note that the field-effect mobility of the MICC poly-Si TFTs used for the ring oscillator in this work is not high, as compared with other MICC TFTs manufactured in our lab. Fig. 5 shows the comparison of the degradation as a function of DC bias-stress time at VDD = 18 V for MICC and ELA poly-Si TFT ring oscillators. The performance of a poly-Si TFT degrades by hot carrier-induced defect creation within the channel. According to our experimental results, there is a little change in the frequency for the MICC poly-Si TFT ring oscillator. However, the oscillator
Fig. 5. DC bias-stress (at VDD = 18 V) effect on the oscillator frequency for the MICC and ELA poly-Si TFT ring oscillators.
Fig. 6. The atomic force microscope (AFM) images of a MICC and typical ELA poly-Si films. The root-mean-square surface roughness of a MICC poly-Si and an ELA poly-Si were found to be 1.06 and 12 nm, respectively.
made of ELA poly-Si TFTs degrades significantly after 28 h. This is to compare the similar initial performances of ring oscillator made by MICC and ELA poly-Si films. Fig. 6 shows the atomic force microscope (AFM) images of MICC poly-Si and a typical ELA poly-Si. The rootmean-square surface roughness of an MICC and an ELA poly-Si films were found to be 1.06 and 12 nm, respectively. The ELA poly-Si was achieved by 20 times exposure to the a-Si at the optimum energy condition (300 mW/cm). This is the typical condition used for the manufacturing of ELA poly-Si TFT-LCD. The grain size of this ELA poly-Si was 200 nm. The roughness comes mainly from the bumps at the grain boundaries, which are due to the collision between the two grains when they are cooled from the molten state. Note that the mass density of molten Si is higher than that of solid Si [6,7]. The bias-stress effect of a poly-Si TFT depends strongly on the surface roughness because the local electric field increases at the rough interface of ELA poly-Si. To remove the bumps, the chemical–mechanical polishing can be done. It is reported that the hot-carrier reliability in a MOSFET is significantly improved by smoothing the Si surface [8]. The smooth surface of MICC poly-Si is due to the solid-phase crystallization, via the Ni-mediated lateral crystallization. This could lead to the formation of strong Si–O bond on the LTPS. Therefore, the stable operation of the TFT under hot-carrier bias is probably due to
1748
Y.-D. Son et al. / Journal of Non-Crystalline Solids 352 (2006) 1745–1748
the smooth surface and a good interface between SiO2 and the poly-Si. Therefore, the poly-Si TFT developed in this experiment can be applied to the ring oscillator and other circuits requiring long lifetime. The Si–O bonds at the interface between MICC poly-Si and SiO2 seem to be quite strong compared to that of ELA poly-Si. 4. Conclusion We studied the DC bias-stress effect on the performance of MICC poly-Si TFT ring oscillator. The TFT exhibited a p-channel field effect mobility of 31 cm2/V s. A 41-stage PMOS inverter chain exhibited an operation frequency of 1.2 MHz at an applied voltage of 18 V. The ring oscillator made of MICC poly-Si TFTs has stable performance against DC bias-stress. This may be due to the less surface roughness of MICC poly-Si and strong Si–O bonds at the
interface in comparison to ELA poly-Si TFT. Therefore, the MICC poly-Si TFT can be applied to the devices and circuits requiring stable operation for long time. References [1] R.B. Iverson, R. Reif, J. Appl. Phys. 62 (1987) 1675. [2] K.H. Kim, S.J. Park, K.S. Cho, W.S. Sohn, J. Jang, SID Tech. Dig. (2002) 150. [3] W.S. Shon, J.H. Choi, J.H. Oh, S.S. Kim, J. Jang, J. Appl. Phys. 94 (2003) 4326. [4] J.H. Choi, D.Y. Kim, B.K. Choo, W.S. Sohn, J. Jang, Electrochem. Solid-St. Lett. (2003) G16. [5] S. Docking, M. Sachdev, IEEE J. Solid-State Circ. 39 (2004) 533. [6] M. Miyasaka, J. Stoemenos, J. Appl. Phys. 86 (1999) 5556. [7] C.T. Angelis, C.A. Dimitriadis, M. Miyasaka, F.V. Farmakis, G. Kamarinos, J. Brini, J. Stoemenos, J. Appl. Phys. 86 (1999) 7083. [8] J.H. Chen, T.F. Lei, C.L. Chen, T.S. Chao, W.Y. Wen, K.T. Chen, J. Electrochem. Soc. 149 (2002) G63.
Dynamic characteristics of MICC polycrystalline thin film transistors Yong-Duck Son, Kyung-Dong Yang, Byung-Seong Bae, Kyu-Chang Park, Jin Jang
*
Advanced Display Research Center, Department of Information Display, Kyung Hee University, Hoegi-dong 1, Dongdaemun-gu, Seoul 130-701, Republic of Korea Available online 30 March 2006
Abstract We studied the DC bias-induced changes in the performance of the PMOS ring oscillator made of poly-Si TFT. The poly-Si was prepared by metal-induced crystallization of amorphous silicon through a cap layer (MICC). The p-channel MICC poly-Si TFT exhibited the field-effect mobility of 31 cm2/V s, on/off current ratio of 107 and threshold voltage of 2.8 V. A 41-stage PMOS inverter chain exhibited an operation frequency of 1.2 MHz at an applied voltage of VDD = 18 V. The degradation of the ring oscillator made of MICC poly-Si TFT is much less than that of ELA TFT by DC bias stress. The difference appears to be due to the smooth surface of MICC poly-Si as compared with a typical excimer laser annealing (ELA) poly-Si. 2006 Elsevier B.V. All rights reserved. PACS: 68.47.Fg; 85.25.Am; 85.30.De; 85.30.Hi Keywords: Thin film transistors; Crystallization
1. Introduction Low-temperature polycrystalline silicon (LTPS) is of increasing interest recently to realize a system-on-glass (SOG). The concept of SOG is to build displays, memory, sensors, controllers and even microprocessors on glass as well as drivers. The poly-Si can be used in high-resolution, integrated active-matrix liquid-crystal displays (AMLCDs) and active-matrix organic light-emitting diodes (AMOLEDs) because of its high field-effect mobility and better stability during operation of thin-film transistor (TFT) as compared with hydrogenated amorphous silicon TFTs (a-Si:H TFTs). A number of techniques have been proposed during the last two decades to achieve large-area poly-Si on glass [1,2]. Among various techniques, laser annealing (LA) is the most frequently used. Excimer LA (ELA) of a-Si gives a high-quality poly-Si and is suitable for low-temperature process using a buffer layer on glass. But, there are still sev*
Corresponding author. Tel.: +82 2 961 0270; fax: +82 2 961 9154. E-mail address: [email protected] (J. Jang).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.11.131
eral issues such as high manufacturing cost, nonuniformity over a large area, high surface roughness, and degradation in TFT performance under bias stress. In this paper, we have investigated the stability of the ring oscillator made of p-channel LTPS poly-Si TFT. The poly-Si was made using metal-induced crystallization of a-Si through a cap layer (MICC) [3,4]. 2. Experimental procedure A self-aligned poly-Si TFT was used for a ring oscillator. The process for the poly-Si TFT is as follows. To crystallize a-Si:H at low temperature by MICC, hydrogenated amorphous silicon (a-Si:H) films were deposited on SiO2 (100 nm)/glass. After dehydrogenation of a-Si:H, the SiNx was deposited by PECVD using a SiH4/NH3 mixture at the substrate temperature of 270 C, and then small amount of Ni particles were sputtered onto the SiNx/a-Si. The sample was crystallized in a furnace at 570 C or rapid thermal annealing system (RTA). After forming of poly-Si islands, 70 nm-thick SiO2 and 100 nm metal layers were deposited on it. The substrate temperature was fixed at 400 C during
1746
Y.-D. Son et al. / Journal of Non-Crystalline Solids 352 (2006) 1745–1748
Fig. 1. The grain size of MICC poly-Si as a function of crystallization temperature. The Ni density was 4.6 · 1012 atoms/cm2.
SiO2 deposition. After patterning both gate metal and insulator on a channel, the source/drain regions were doped with 1% B2H6 diluted in He plasma using an ion doping system, which consisted of ion generation and ion acceleration parts. The ion acceleration voltage, gas pressure and substrate temperature were fixed at 12 kV, 0.3 mTorr and 280 C, respectively. To isolate source/drain from gate, 200 nm thick SiNx was deposited and contact holes were formed. Then, a 400 nm thick AlNd was deposited to form the source/drain electrodes. Finally, TFT was annealed at 400 C for 2 h under H2/N2. A high nickel density on a-Si leads to small grains as a result of vertical grain growth. However, with decreasing the Ni density, the distance between NiSi2 crystallites increases and leads to large grains as a result of lateral grain growth. In case of MICC, the amount of Ni diffused into the a-Si is much less than that on the SiNx surface because most of the Ni atoms are inside of the nitride. The average grain size seen by optical microscopy was found to be 40 lm. Note that the grain size can be controlled by the amount of Ni on the surface. The a-Si was crystallized by MICC in RTA system. Fig. 1 shows the grain size as a function of crystallization temperature by RTA system. The Ni area density on the a-Si was fixed at 4.6 · 1012 atoms/cm2. The grain size was measured by an optical microscopy. The grain size decreases from 51 lm to 26 lm when the crystallization temperature increases from 650 C to 750 C even though the same amount of Ni density is used to crystallize the a-Si.
Fig. 2. Transfer characteristics of a MICC poly-Si TFT.
of 2.8 V, and a gate voltage swing of 0.4 V/dec. The poly-Si TFT was designed to have the channel length of 6 lm. The ring oscillators consist of 41-stages inverters and two stage buffer amplifiers. The buffer amplifier is used to see the output signal of the ring oscillator that is not affected by the impedance of the measurement equipment. Fig. 3 shows the schematic circuit and photograph of a ring oscillator with buffer amplifier. Fig. 4 shows the output frequency of the ring oscillator measured as a function of the applied voltage (VDD). The output characteristics of the ring oscillator at VDD = 18 V are shown in the inset. The shape of a waveform is determined by the poly-Si TFT performance. The average propagation delay time was found to be 0.16 ns when the average propagation delay time is defined as 1/(2Nfosc), where fosc is the frequency
3. Results and discussion Fig. 2 shows the transfer characteristics of a MICC poly-Si TFT. The p-channel TFT exhibited a field-effect mobility of 31 cm2/V s, the minimum leakage current of about 2 · 10 12 A/lm at Vds = 10 V, a threshold voltage
Fig. 3. Schematic for the MICC poly-Si TFT ring oscillator circuit (a) and photograph of the completed 41-stage ring oscillator (b).
Y.-D. Son et al. / Journal of Non-Crystalline Solids 352 (2006) 1745–1748
1747
Fig. 4. The frequency characteristics of MICC poly-Si ring oscillator.
of ring oscillator in output waveform and N is the inverter chain number [5]. At the applied voltage of VDD = 18 V the ring oscillator oscillates at 1.2 MHz. The oscillation frequency increases from 0.01 to 1.6 MHz with increasing applied voltage from 6 to 25 V, and the oscillation amplitude saturates at 24 V. Note that the field-effect mobility of the MICC poly-Si TFTs used for the ring oscillator in this work is not high, as compared with other MICC TFTs manufactured in our lab. Fig. 5 shows the comparison of the degradation as a function of DC bias-stress time at VDD = 18 V for MICC and ELA poly-Si TFT ring oscillators. The performance of a poly-Si TFT degrades by hot carrier-induced defect creation within the channel. According to our experimental results, there is a little change in the frequency for the MICC poly-Si TFT ring oscillator. However, the oscillator
Fig. 5. DC bias-stress (at VDD = 18 V) effect on the oscillator frequency for the MICC and ELA poly-Si TFT ring oscillators.
Fig. 6. The atomic force microscope (AFM) images of a MICC and typical ELA poly-Si films. The root-mean-square surface roughness of a MICC poly-Si and an ELA poly-Si were found to be 1.06 and 12 nm, respectively.
made of ELA poly-Si TFTs degrades significantly after 28 h. This is to compare the similar initial performances of ring oscillator made by MICC and ELA poly-Si films. Fig. 6 shows the atomic force microscope (AFM) images of MICC poly-Si and a typical ELA poly-Si. The rootmean-square surface roughness of an MICC and an ELA poly-Si films were found to be 1.06 and 12 nm, respectively. The ELA poly-Si was achieved by 20 times exposure to the a-Si at the optimum energy condition (300 mW/cm). This is the typical condition used for the manufacturing of ELA poly-Si TFT-LCD. The grain size of this ELA poly-Si was 200 nm. The roughness comes mainly from the bumps at the grain boundaries, which are due to the collision between the two grains when they are cooled from the molten state. Note that the mass density of molten Si is higher than that of solid Si [6,7]. The bias-stress effect of a poly-Si TFT depends strongly on the surface roughness because the local electric field increases at the rough interface of ELA poly-Si. To remove the bumps, the chemical–mechanical polishing can be done. It is reported that the hot-carrier reliability in a MOSFET is significantly improved by smoothing the Si surface [8]. The smooth surface of MICC poly-Si is due to the solid-phase crystallization, via the Ni-mediated lateral crystallization. This could lead to the formation of strong Si–O bond on the LTPS. Therefore, the stable operation of the TFT under hot-carrier bias is probably due to
1748
Y.-D. Son et al. / Journal of Non-Crystalline Solids 352 (2006) 1745–1748
the smooth surface and a good interface between SiO2 and the poly-Si. Therefore, the poly-Si TFT developed in this experiment can be applied to the ring oscillator and other circuits requiring long lifetime. The Si–O bonds at the interface between MICC poly-Si and SiO2 seem to be quite strong compared to that of ELA poly-Si. 4. Conclusion We studied the DC bias-stress effect on the performance of MICC poly-Si TFT ring oscillator. The TFT exhibited a p-channel field effect mobility of 31 cm2/V s. A 41-stage PMOS inverter chain exhibited an operation frequency of 1.2 MHz at an applied voltage of 18 V. The ring oscillator made of MICC poly-Si TFTs has stable performance against DC bias-stress. This may be due to the less surface roughness of MICC poly-Si and strong Si–O bonds at the
interface in comparison to ELA poly-Si TFT. Therefore, the MICC poly-Si TFT can be applied to the devices and circuits requiring stable operation for long time. References [1] R.B. Iverson, R. Reif, J. Appl. Phys. 62 (1987) 1675. [2] K.H. Kim, S.J. Park, K.S. Cho, W.S. Sohn, J. Jang, SID Tech. Dig. (2002) 150. [3] W.S. Shon, J.H. Choi, J.H. Oh, S.S. Kim, J. Jang, J. Appl. Phys. 94 (2003) 4326. [4] J.H. Choi, D.Y. Kim, B.K. Choo, W.S. Sohn, J. Jang, Electrochem. Solid-St. Lett. (2003) G16. [5] S. Docking, M. Sachdev, IEEE J. Solid-State Circ. 39 (2004) 533. [6] M. Miyasaka, J. Stoemenos, J. Appl. Phys. 86 (1999) 5556. [7] C.T. Angelis, C.A. Dimitriadis, M. Miyasaka, F.V. Farmakis, G. Kamarinos, J. Brini, J. Stoemenos, J. Appl. Phys. 86 (1999) 7083. [8] J.H. Chen, T.F. Lei, C.L. Chen, T.S. Chao, W.Y. Wen, K.T. Chen, J. Electrochem. Soc. 149 (2002) G63.