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Enlargement of “location controlled” Si grains by dual-beam excimer-laser with bump structures
Applied Surface Science 154–155 Ž2000. 152–158 www.elsevier.nlrlocaterapsusc
Enlargement of ‘‘location controlled’’ Si grains by dual-beam excimer-laser with bump structures A. Burtsev ) , R. Ishihara Laboratory of Electronic Components, Technology and Materials (ECTM), Delft Institute of Microelectronics and Submicron Technology (DIMES), Delft UniÕersity of Technology, Feldmannweg 17, P.O. Box 5053, 2600 GB Delft, Netherlands Received 1 June 1999; accepted 31 August 1999
Abstract The effect of thickness variation of an intermediate insulator layer on the grain size of a recrystallized large Si grain in an a-SirSiO 2rmetal stack with an array of bumps in the oxide has been investigated. Increased thickness of the intermediate oxide portion and bump height resulted in grain size enlargement of the Si grain. Si crystal grains as large as 5.1 mm were obtained located exactly at the desired position on the oxide. The explanation of the growth-enhanced mechanism by the solidification rate behavior, based on numerical simulation in terms of temperature gradient arguments is given. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Excimer-laser; Single crystal silicon; Location control; Grain size; Thin-film transistors
1. Introduction The extensive application of silicon thin-film transistors ŽTFTs. in integrated active matrix-addressed flat-panel liquid-crystal displays ŽAMLCDs., static random-access memory ŽSRAM., and three dimensional integrated circuits demands an in-depth materials science oriented research activity to improve the performance of these devices w1x. Current TFT devices based on amorphous Si Ža-Si. or polycrystalline Si Žpoly-Si. material have a num-
) Corresponding author. Tel.: q31-15-278-7061; fax: q31-15262-2163. E-mail address: [email protected] ŽA. Burtsev..
ber of limitations such as low electron field-effect mobility and high off-current w2,3x. The fabrication technology of the applications listed above requires the use of transparent substrates and requires the avoidance of exposure to excessive high temperatures during processing. These requirements envisage the excimer-laser crystallization technique as a promising conversion method from an amorphous precursor state to poly-Si thin films w2x. Although the electron mobility of poly-Si is higher than that of a-Si, the deviation of poly-Si TFTs’ characteristics, due to the random location of grain boundaries in the channel region degrade the device-to-device uniformity w4x. One can refer to the crystal Si Žc-Si. TFT which was fabricated on a 10 mm large crystal grain made
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 4 3 9 - 0
A. BurtseÕ, R. Ishiharar Applied Surface Science 154–155 (2000) 152–158
by a double-pulse and dual-beam excimer-laser crystallization method w5x. An outstanding performance of c-Si TFT with e-mobility ) 460 cm2rVs by elimination of the energy barrier in the channel has been reached. At the same time, the off-current of the c-Si TFTs was significantly reduced to less than 10y1 3 A, since the crystal pn junction could be formed at the drain edge. However, it is not necessary to recrystallize the entire poly-Si film but only a desired portion of it while controlling its accurate position. This is the basis for the new concept of ‘‘location controlled’’ Si grain by dual-beam excimer-laser irradiation w6x. Thus, the TFT can be placed on a single grain region. The importance of c-Si grain size, its position and influence on the device characteristics as mentioned above, is a key factor in our current research w6–8x. In this paper, we concentrate on the effect of thickness variation of the intermediate SiO 2 layer on the grain size of the recrystallized large Si grain in the a-SirSiO 2rmetal stack with an array of bumps in the oxide. The ‘‘location control’’ of the grain is examined by its position with respect to the underlying oxide bump. We will also discuss the factors enhancing the growth mechanism in the ‘‘location control’’ of the large c-Si grain. Specifically, we employ a numerical simulation method of two-dimensional transient temperature distribution to give a
153
thermodynamical explanation of growth-enhanced mechanism.
2. Experimental 2.1. Experimental setup Fig. 1 represents schematically the tri-layer structure which has been fabricated in this research. Hoya NA35 glass, 500 mm thick, with a transmittance of 91% at 308 nm wavelength was used as substrate material. A 575-nm thick tungsten layer ŽW. was deposited on the glass substrate by sputtering. The W layer was used as an underlying metal layer instead of Cr to prevent cracking, as reported in Ref. w6x. Secondly, an intermediate SiO 2 layer with thickness F varying from 190 to 400 nm was deposited by chemical vapor deposition ŽCVD. at a temperature of 4508C. Circular bumps ranging from 0.8 to 3.5 mm in diameter and square bumps ranging from 1 to 4 mm in width were formed in the oxide. The height H of the bump structure was chosen as 30, 50 and 70 nm for an F value of 190 nm, thus resulting in a corresponding thickness S of the thin oxide of 160, 140 and 120 nm, respectively. Only 50 nm height bumps were formed in the 400-nm thick oxide. The oxide thickness deviations are summa-
Fig. 1. Schematic view of the fabricated structure for grain size enlargement by dual laser beam.
154
A. BurtseÕ, R. Ishiharar Applied Surface Science 154–155 (2000) 152–158
rized in Table 1. Subsequently, a 100-nm thick a-Si layer was deposited on the patterned oxide by low pressure chemical vapor deposition ŽLPCVD. using SiH 4 decomposition at 5458C. This tri-layer structure was then patterned to form an array of islands 100 mm wide. Finally, the backside of the structure was irradiated by XeCl excimer-laser with wavelength of 308 nm and pulse duration of 66 ns. The laser transmitted light through the glass substrate was reflected back from an Al mirror, with a reflectance of 72%, and subsequently irradiated the sample from the front, as shown in Fig. 1. The laser energy densities E b are summarized in Table 1. Irradiation energies were adjusted in such a way as to obtain the maximum average grain size for each oxide thickness S. 2.2. Effect of total SiO2 thickness The thickness influence F of SiO 2 on the final grain size D was investigated as follows: two different samples were prepared with F values of 190 and 400 nm, while the H value was kept constant at 50 nm. The maximum grain size obtained for F s 190 nm and F s 400 nm was 3.5 and 5.1 mm, respectively. The laser irradiated energy density E b at the back side of the sample is 459 mJrcm2 . An SEM image of the 5.1 mm large Si grain is shown in Fig. 2. The hillocks of the Si film represent grain boundaries. The diameter of the SiO 2 bump, which can be seen as a slightly dark circle in the center of the grain, is 4 mm. The enlarged c-Si grain is placed on top of the SiO 2 bump.
Fig. 2. Planar-view SEM image of the ‘‘location controlled’’ large Si grain. Hillocks of Si film represent grain boundaries. The grain size D is 5.1 mm. The enlarged crystal-Si grain is placed on top of the SiO 2 bump. The diameter of the circle SiO 2 bump is 3.5 mm. The irradiated laser energy density E b at the backside of the sample is 459 mJrcm2 .
SiO 2 portion was kept at 190 nm, the grain diameter increased with the H value reaching the maximum value of 4.2 mm for an H of 70 nm, as shown in Fig. 3. It should be noted that the ‘‘location control’’ of the resulting large grains was achieved for all these S
2.3. Height effect of the SiO2 bump The thin oxide thickness S varied from 120, 140 and 160 nm, resulting in corresponding H values of 30, 50 and 70 nm. While the thickness of the thick
Table 1 Experimental setup Bump height, H wnmx Thickness of thin SiO 2 , S wnmx Total thickness of SiO 2 , F wnmx Laser irradiated energy density at the back of the sample, E b wmJrcm2 x
50 350 400 459
30 160 190 415
50 140 190 410
70 120 190 405
Fig. 3. Grain size diameter D of ‘‘location controlled’’ large grains as a function of bump height H. F value is kept constant at 190 nm.
A. BurtseÕ, R. Ishiharar Applied Surface Science 154–155 (2000) 152–158
155
knowledge of the two-dimensional temperature profile during the melt and regrowth processes. Especially, when Si is expected to be in the solid-melt phase. The following numerical analysis was performed without implementation of the principles of nucleation and growth, Ži.e., interface-controlled growth regime and supercooling . since they have not been clarified yet. Hence, solidification velocity V is explained here in terms of temperature gradient arguments, which have been frequently utilized in two-dimensional numerical simulations w6x. The relationship between the V and grain size D assumes: the grain size increases with decreasing solidification velocity w5x. We solved the following two-dimensional nonlinear thermal diffusion equation: Fig. 4. Planar-view SEM image of the ‘‘location controlled’’ large Si grain. The grain size D is 4.2 mm. The underlying circle oxide bump has a diameter of 3 mm. The oxide H value is 70 nm. Irradiated laser energy density E b at the backside of the sample is 405 mJrcm2 .
CŽT .
ET Ž x , y,t . Et
E s Ex
ž
K ŽT .
E q Ey
and H values. An SEM image of an obtained grain with 4.2 mm diameter is shown in Fig. 4.
ž
ET Ž x , y,t .
K ŽT .
Ex
/
ET Ž x , y,t . Ey
/
q Q Ž x , y,t .
Ž 1.
where C ŽT . is thermal capacity, T the temperature, t the time, K ŽT . the thermal conductivity, and Q the specific internal energy of the heat conducting medium. The thermal diffusion equation was solved using the finite element method. For greater accuracy, the temperature dependence of the specific material constants for each layer and the latent heat during the phase change were taken into account.
3. Numerical analysis of the transient temperature profile 3.1. Methodology As elaborated in greater detail elsewhere w6x, grain size enlargement needs a detailed analysis and Table 2 Physical values used in numerical simulation Properties
a-Si
SiO 2
W
Heat capacity C wJ cmy3 Ky1 x
C ŽT . s 0.952 q Ž0.171Tr685. Ref. w9x
C ŽT . s 10y19 T 2 q 10y16 T q3 = 10y12 Ref. w10x
Thermal conductivity K wW cmy1 Ky1 x
K ŽT . s 1.3 = 10y1 1 ŽT y 900. 3 q1.3 = 10y9 = ŽT y 900. 2 q10y6 ŽT y 900. q 10y2 Ref. w11x 0.69 Ref. w12x 2.9 = 10 3 Tsolidus s 1268 K; T liquidus s 1278 K Ref. w13x 10 6 Ref. w14x
C ŽT . s Ž aT q Cm .rŽ bT q 1. Cm s Ž aT q 0.1657.rŽ bT q 1. a s 4.63 = 10y4 , b s 1.46 = 10y3 Ref. w9x K ŽT .
Reflectivity Latent heat wJrcm3 x
Absorption coefficient wcmy1 x
Ref. w9x, Fig. 2. – –
K ŽT . s y10y14 T 3 q7 = 10y11 T 2 y10y7 T q 2 = 10y4 Ref. w10x 0.451 Ref. w13x –
–
1.43 = 10 6 Ref. w14x
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A. BurtseÕ, R. Ishiharar Applied Surface Science 154–155 (2000) 152–158
The parameters used in this calculation are shown in Table 2. The square-shaped laser pulses are assumed to irradiate both the front and back of the tri-layer structure with t s 0 to 66 ns. The bump diameter is 1.6 mm.
Table 3 Numerically calculated velocity of molten Sirsolid Si interface propagation Ž H value is fixed at 50 nm. Total oxide thickness F wnmx Average solidification velocity V wmrsx
190 0.99
400 0.45
3.2. SiO2 thickness effect According to our experiment an increase of the total SiO 2 thickness F resulted in grain size enlargement. Two different F values of 190 and 400 nm, were used in the numerical calculation of the transient temperature profile, while H was kept constant at 50 nm. The temperature profiles after 66 ns of laser pulse duration for the molten-SirSiO2 interface at the center of the bump are shown in Fig. 5. Although for different F values, the top Si layer in the middle of the bump achieves different maximum temperatures where Si is completely molten. In both cases the temperature decreases rapidly, reaching the solidification temperature Tm , which is represented in the plot as a plateau. One can see that solidification takes place earlier for the smaller F value. The higher F value provides longer melting duration and therefore a slow average solidification ve-
locity V of the molten Si Žsee Table 3.. The difference between solidification velocities of molten Si can be explained as follows: Ža. the vertical temperature gradient dTrd y at the SirSiO 2 interface, which determines V, is proportional to the temperature difference between the top SirSiO 2 interface and bottom SiO 2rW interface, which becomes gentler for higher F value, as governed by equation : ET s Ey
TOX 2 y TOX 1 F
Ž 2.
where TOX 2 is the temperature at SirSiO 2 interface and TOX 1 is the temperature at SiO 2rW interface w15x and Žb. the larger amount of lateral heat flow occurs for F s 400 nm rather than for F s 190 nm due to the higher maximum temperature of the SirSiO 2 interface at the center of the bump, 1670 K as compared to 1580 K for the smaller F value.
Fig. 5. Temperature–time plots of a molten SirSiO 2 interface at the center of the bump for an F value of 190 and 400 nm after a 66 ns laser pulse. H value was kept at 50 nm. Bump diameter is 1.6 mm. The energy densities E b at the back of the sample are 410 and 459 mJrcm2 , for F thickness of 190 and 400 nm, respectively.
A. BurtseÕ, R. Ishiharar Applied Surface Science 154–155 (2000) 152–158
3.3. SiO2 bump height effect According to our experimental results, for thick oxide value F s 190 nm, the grain size D of the ‘‘location controlled’’ large grains increased with the bump height H value. In order to obtain a thermodynamical explanation of this effect, a numerical analysis of the two-dimensional temperature profile with respect to solidification velocity of molten Si was performed. Two different values of bump heights H s 30 nm and H s 70 nm for F s 190 nm were used. The solidification velocity was numerically calculated for both cases, bearing in mind that: Ža. the top a-Si layer was molten completely after the dual-beam laser irradiation and Žb. The maximum achieved temperature Tmax of the SirSiO 2 interface in the middle of the bump after the laser pulse Žwhich is at the same time the minimum temperature
157
along the SirSiO 2 interface. was the same in both cases. The calculated temperature distribution along the SirSiO 2 interface near the bump immediately after laser irradiation is shown in Fig. 6. It should be noted that Tmax was attained with different laser energy densities for different H values of the samples. The temperature outside the bump for H s 70 nm Ž1418 K. is higher than that for the H s 30 nm Ž1409 K. due to the fact that the heat capacitance outside the bump is smaller for the larger H value. Hence, the energy conservation for an arbitrary small fixed volume outside the oxide for H s 70 nm is higher than that for H s 30 nm, resulting in a larger lateral heat flow from the thin oxide to the thick oxide portion in the vicinity of the SirSiO 2 bump interface. Therefore, heat accumulation occurs more efficiently for the thick oxide portion with H s 70 nm than for H s 30 nm, leading to
Fig. 6. Temperature distribution along the SirSiO 2 interface in the vicinity of the bump immediately after a laser pulse of 66 ns for different H values of oxide. The diameter of the bump is 1.6 mm. The SirSiO 2 interface is traced from the outside Ž5 mm. to the middle of the bump.
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A. BurtseÕ, R. Ishiharar Applied Surface Science 154–155 (2000) 152–158
Table 4 Numerically calculated velocity of molten Sirsolid Si interface propagation Ž F value is fixed at 190 nm. Bump height H wnmx Average solidification velocity V wmrsx
30 1.08
70 0.89
a gentler temperature slope between the molten Si and metal layer. As follows from Eq. 2, since the solidification velocity V is proportional to this temperature difference, V decreases with increasing bump height resulting in larger grains Žsee Table 4.. Hence, the slow solidification velocity for the high H value gave a large grain in the experiment.
4. Conclusion We have investigated the effect of thickness variation F of intermediate SiO 2 layer and bump height H on the grain size of the ‘‘location controlled’’ large Si grains in an a-SirSiO 2rmetal stack with an array of bumps in the oxide. The size of the ‘‘location controlled’’ c-Si increased with F achieving a maximum of 5.1 mm for F s 400 nm. The grain diameter increased with H, reaching a maximum of 4.2 mm for H s 70 nm, while F was fixed at 190 nm. An attempt to explain the growth mechanism enhancement in ‘‘location control’’ of the large crystal grains using the numerical simulation result of two-dimensional transient temperature distribution and solidification velocity has been made. Solidification velocity decreased with F or H value, which is consistent with the experimental result.
Acknowledgements The authors are grateful to Professor Dr. C. Beenakker for his warm encouragement and motivation; and to Dr. J.W. Metselaar, C.C.G. Visser, and Alexei Bereznitski for their useful discussions. This work was supported in part by the Dutch Technology Foundation STW Project No. DEL66.4542 and XMR Santa Clara, CA. References w1x T. Noguchi, Jpn. J. Appl. Phys. 32 Ž1993. L1585. w2x T. Sameshima, S. Usui, M. Sekiya, IEEE Electron Device Lett. 7 Ž1986. 276. w3x K. Sera, F. Okumura, H. Uchida, S. Itoh, S. Kaneko, K. Hotta, IEEE Trans. Electron Devices 36 Ž1989. 2868. w4x N. Yamauchi, R. Reif, J. Appl. Phys. 75 Ž1994. 3235. w5x R. Ishihara, M. Matsumura, Electron. Lett. 22 Ž1995. 1956. w6x R. Ishihara, A. Burtsev, Jpn. J. Appl. Phys. 37 Ž3B. Ž1998. 1071. w7x R. Ishihara, A. Burtsev, Extented abstracts of the 1997 Int. Conf. on Solid State Devices and Materials, Hamamatsu C-8-3 Ž1997. 360. w8x R. Ishihara, A. Burtsev, Digest of Technical Papers AMLCD98 Tokyo Ž1998. 153. w9x H. Kuriyama, S. Kiyama, S. Noguchi, Jpn. J. Appl. Phys. 30 Ž12B. Ž1991. 3701. w10x V.E. Zinov’ev, in: Handbook of Thermophysical Properties of Metals at High Temperatures, New York,1997, p. 288. w11x H. Webber, A. Cullis, N. Chew, Appl. Phys. Lett. 43 Ž1983. 669. w12x L.A. Lompe, J.M. Liu, H. Kurtz, N. Bloembergen, Appl. Phys. Lett. 43 Ž1983. 168. w13x Handbook of Chemistry and Physics, Boca Raton, Florida, 1989, p. B34. w14x D. Bauerle, in: Laser Processing and Chemistry, Berlin, 1996, p. 581. w15x K. Shimizu, O. Sigiura, M. Matsumura, IEEE Trans. Electron Devices 40 Ž1. Ž1993. 112.
Enlargement of ‘‘location controlled’’ Si grains by dual-beam excimer-laser with bump structures A. Burtsev ) , R. Ishihara Laboratory of Electronic Components, Technology and Materials (ECTM), Delft Institute of Microelectronics and Submicron Technology (DIMES), Delft UniÕersity of Technology, Feldmannweg 17, P.O. Box 5053, 2600 GB Delft, Netherlands Received 1 June 1999; accepted 31 August 1999
Abstract The effect of thickness variation of an intermediate insulator layer on the grain size of a recrystallized large Si grain in an a-SirSiO 2rmetal stack with an array of bumps in the oxide has been investigated. Increased thickness of the intermediate oxide portion and bump height resulted in grain size enlargement of the Si grain. Si crystal grains as large as 5.1 mm were obtained located exactly at the desired position on the oxide. The explanation of the growth-enhanced mechanism by the solidification rate behavior, based on numerical simulation in terms of temperature gradient arguments is given. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Excimer-laser; Single crystal silicon; Location control; Grain size; Thin-film transistors
1. Introduction The extensive application of silicon thin-film transistors ŽTFTs. in integrated active matrix-addressed flat-panel liquid-crystal displays ŽAMLCDs., static random-access memory ŽSRAM., and three dimensional integrated circuits demands an in-depth materials science oriented research activity to improve the performance of these devices w1x. Current TFT devices based on amorphous Si Ža-Si. or polycrystalline Si Žpoly-Si. material have a num-
) Corresponding author. Tel.: q31-15-278-7061; fax: q31-15262-2163. E-mail address: [email protected] ŽA. Burtsev..
ber of limitations such as low electron field-effect mobility and high off-current w2,3x. The fabrication technology of the applications listed above requires the use of transparent substrates and requires the avoidance of exposure to excessive high temperatures during processing. These requirements envisage the excimer-laser crystallization technique as a promising conversion method from an amorphous precursor state to poly-Si thin films w2x. Although the electron mobility of poly-Si is higher than that of a-Si, the deviation of poly-Si TFTs’ characteristics, due to the random location of grain boundaries in the channel region degrade the device-to-device uniformity w4x. One can refer to the crystal Si Žc-Si. TFT which was fabricated on a 10 mm large crystal grain made
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 4 3 9 - 0
A. BurtseÕ, R. Ishiharar Applied Surface Science 154–155 (2000) 152–158
by a double-pulse and dual-beam excimer-laser crystallization method w5x. An outstanding performance of c-Si TFT with e-mobility ) 460 cm2rVs by elimination of the energy barrier in the channel has been reached. At the same time, the off-current of the c-Si TFTs was significantly reduced to less than 10y1 3 A, since the crystal pn junction could be formed at the drain edge. However, it is not necessary to recrystallize the entire poly-Si film but only a desired portion of it while controlling its accurate position. This is the basis for the new concept of ‘‘location controlled’’ Si grain by dual-beam excimer-laser irradiation w6x. Thus, the TFT can be placed on a single grain region. The importance of c-Si grain size, its position and influence on the device characteristics as mentioned above, is a key factor in our current research w6–8x. In this paper, we concentrate on the effect of thickness variation of the intermediate SiO 2 layer on the grain size of the recrystallized large Si grain in the a-SirSiO 2rmetal stack with an array of bumps in the oxide. The ‘‘location control’’ of the grain is examined by its position with respect to the underlying oxide bump. We will also discuss the factors enhancing the growth mechanism in the ‘‘location control’’ of the large c-Si grain. Specifically, we employ a numerical simulation method of two-dimensional transient temperature distribution to give a
153
thermodynamical explanation of growth-enhanced mechanism.
2. Experimental 2.1. Experimental setup Fig. 1 represents schematically the tri-layer structure which has been fabricated in this research. Hoya NA35 glass, 500 mm thick, with a transmittance of 91% at 308 nm wavelength was used as substrate material. A 575-nm thick tungsten layer ŽW. was deposited on the glass substrate by sputtering. The W layer was used as an underlying metal layer instead of Cr to prevent cracking, as reported in Ref. w6x. Secondly, an intermediate SiO 2 layer with thickness F varying from 190 to 400 nm was deposited by chemical vapor deposition ŽCVD. at a temperature of 4508C. Circular bumps ranging from 0.8 to 3.5 mm in diameter and square bumps ranging from 1 to 4 mm in width were formed in the oxide. The height H of the bump structure was chosen as 30, 50 and 70 nm for an F value of 190 nm, thus resulting in a corresponding thickness S of the thin oxide of 160, 140 and 120 nm, respectively. Only 50 nm height bumps were formed in the 400-nm thick oxide. The oxide thickness deviations are summa-
Fig. 1. Schematic view of the fabricated structure for grain size enlargement by dual laser beam.
154
A. BurtseÕ, R. Ishiharar Applied Surface Science 154–155 (2000) 152–158
rized in Table 1. Subsequently, a 100-nm thick a-Si layer was deposited on the patterned oxide by low pressure chemical vapor deposition ŽLPCVD. using SiH 4 decomposition at 5458C. This tri-layer structure was then patterned to form an array of islands 100 mm wide. Finally, the backside of the structure was irradiated by XeCl excimer-laser with wavelength of 308 nm and pulse duration of 66 ns. The laser transmitted light through the glass substrate was reflected back from an Al mirror, with a reflectance of 72%, and subsequently irradiated the sample from the front, as shown in Fig. 1. The laser energy densities E b are summarized in Table 1. Irradiation energies were adjusted in such a way as to obtain the maximum average grain size for each oxide thickness S. 2.2. Effect of total SiO2 thickness The thickness influence F of SiO 2 on the final grain size D was investigated as follows: two different samples were prepared with F values of 190 and 400 nm, while the H value was kept constant at 50 nm. The maximum grain size obtained for F s 190 nm and F s 400 nm was 3.5 and 5.1 mm, respectively. The laser irradiated energy density E b at the back side of the sample is 459 mJrcm2 . An SEM image of the 5.1 mm large Si grain is shown in Fig. 2. The hillocks of the Si film represent grain boundaries. The diameter of the SiO 2 bump, which can be seen as a slightly dark circle in the center of the grain, is 4 mm. The enlarged c-Si grain is placed on top of the SiO 2 bump.
Fig. 2. Planar-view SEM image of the ‘‘location controlled’’ large Si grain. Hillocks of Si film represent grain boundaries. The grain size D is 5.1 mm. The enlarged crystal-Si grain is placed on top of the SiO 2 bump. The diameter of the circle SiO 2 bump is 3.5 mm. The irradiated laser energy density E b at the backside of the sample is 459 mJrcm2 .
SiO 2 portion was kept at 190 nm, the grain diameter increased with the H value reaching the maximum value of 4.2 mm for an H of 70 nm, as shown in Fig. 3. It should be noted that the ‘‘location control’’ of the resulting large grains was achieved for all these S
2.3. Height effect of the SiO2 bump The thin oxide thickness S varied from 120, 140 and 160 nm, resulting in corresponding H values of 30, 50 and 70 nm. While the thickness of the thick
Table 1 Experimental setup Bump height, H wnmx Thickness of thin SiO 2 , S wnmx Total thickness of SiO 2 , F wnmx Laser irradiated energy density at the back of the sample, E b wmJrcm2 x
50 350 400 459
30 160 190 415
50 140 190 410
70 120 190 405
Fig. 3. Grain size diameter D of ‘‘location controlled’’ large grains as a function of bump height H. F value is kept constant at 190 nm.
A. BurtseÕ, R. Ishiharar Applied Surface Science 154–155 (2000) 152–158
155
knowledge of the two-dimensional temperature profile during the melt and regrowth processes. Especially, when Si is expected to be in the solid-melt phase. The following numerical analysis was performed without implementation of the principles of nucleation and growth, Ži.e., interface-controlled growth regime and supercooling . since they have not been clarified yet. Hence, solidification velocity V is explained here in terms of temperature gradient arguments, which have been frequently utilized in two-dimensional numerical simulations w6x. The relationship between the V and grain size D assumes: the grain size increases with decreasing solidification velocity w5x. We solved the following two-dimensional nonlinear thermal diffusion equation: Fig. 4. Planar-view SEM image of the ‘‘location controlled’’ large Si grain. The grain size D is 4.2 mm. The underlying circle oxide bump has a diameter of 3 mm. The oxide H value is 70 nm. Irradiated laser energy density E b at the backside of the sample is 405 mJrcm2 .
CŽT .
ET Ž x , y,t . Et
E s Ex
ž
K ŽT .
E q Ey
and H values. An SEM image of an obtained grain with 4.2 mm diameter is shown in Fig. 4.
ž
ET Ž x , y,t .
K ŽT .
Ex
/
ET Ž x , y,t . Ey
/
q Q Ž x , y,t .
Ž 1.
where C ŽT . is thermal capacity, T the temperature, t the time, K ŽT . the thermal conductivity, and Q the specific internal energy of the heat conducting medium. The thermal diffusion equation was solved using the finite element method. For greater accuracy, the temperature dependence of the specific material constants for each layer and the latent heat during the phase change were taken into account.
3. Numerical analysis of the transient temperature profile 3.1. Methodology As elaborated in greater detail elsewhere w6x, grain size enlargement needs a detailed analysis and Table 2 Physical values used in numerical simulation Properties
a-Si
SiO 2
W
Heat capacity C wJ cmy3 Ky1 x
C ŽT . s 0.952 q Ž0.171Tr685. Ref. w9x
C ŽT . s 10y19 T 2 q 10y16 T q3 = 10y12 Ref. w10x
Thermal conductivity K wW cmy1 Ky1 x
K ŽT . s 1.3 = 10y1 1 ŽT y 900. 3 q1.3 = 10y9 = ŽT y 900. 2 q10y6 ŽT y 900. q 10y2 Ref. w11x 0.69 Ref. w12x 2.9 = 10 3 Tsolidus s 1268 K; T liquidus s 1278 K Ref. w13x 10 6 Ref. w14x
C ŽT . s Ž aT q Cm .rŽ bT q 1. Cm s Ž aT q 0.1657.rŽ bT q 1. a s 4.63 = 10y4 , b s 1.46 = 10y3 Ref. w9x K ŽT .
Reflectivity Latent heat wJrcm3 x
Absorption coefficient wcmy1 x
Ref. w9x, Fig. 2. – –
K ŽT . s y10y14 T 3 q7 = 10y11 T 2 y10y7 T q 2 = 10y4 Ref. w10x 0.451 Ref. w13x –
–
1.43 = 10 6 Ref. w14x
156
A. BurtseÕ, R. Ishiharar Applied Surface Science 154–155 (2000) 152–158
The parameters used in this calculation are shown in Table 2. The square-shaped laser pulses are assumed to irradiate both the front and back of the tri-layer structure with t s 0 to 66 ns. The bump diameter is 1.6 mm.
Table 3 Numerically calculated velocity of molten Sirsolid Si interface propagation Ž H value is fixed at 50 nm. Total oxide thickness F wnmx Average solidification velocity V wmrsx
190 0.99
400 0.45
3.2. SiO2 thickness effect According to our experiment an increase of the total SiO 2 thickness F resulted in grain size enlargement. Two different F values of 190 and 400 nm, were used in the numerical calculation of the transient temperature profile, while H was kept constant at 50 nm. The temperature profiles after 66 ns of laser pulse duration for the molten-SirSiO2 interface at the center of the bump are shown in Fig. 5. Although for different F values, the top Si layer in the middle of the bump achieves different maximum temperatures where Si is completely molten. In both cases the temperature decreases rapidly, reaching the solidification temperature Tm , which is represented in the plot as a plateau. One can see that solidification takes place earlier for the smaller F value. The higher F value provides longer melting duration and therefore a slow average solidification ve-
locity V of the molten Si Žsee Table 3.. The difference between solidification velocities of molten Si can be explained as follows: Ža. the vertical temperature gradient dTrd y at the SirSiO 2 interface, which determines V, is proportional to the temperature difference between the top SirSiO 2 interface and bottom SiO 2rW interface, which becomes gentler for higher F value, as governed by equation : ET s Ey
TOX 2 y TOX 1 F
Ž 2.
where TOX 2 is the temperature at SirSiO 2 interface and TOX 1 is the temperature at SiO 2rW interface w15x and Žb. the larger amount of lateral heat flow occurs for F s 400 nm rather than for F s 190 nm due to the higher maximum temperature of the SirSiO 2 interface at the center of the bump, 1670 K as compared to 1580 K for the smaller F value.
Fig. 5. Temperature–time plots of a molten SirSiO 2 interface at the center of the bump for an F value of 190 and 400 nm after a 66 ns laser pulse. H value was kept at 50 nm. Bump diameter is 1.6 mm. The energy densities E b at the back of the sample are 410 and 459 mJrcm2 , for F thickness of 190 and 400 nm, respectively.
A. BurtseÕ, R. Ishiharar Applied Surface Science 154–155 (2000) 152–158
3.3. SiO2 bump height effect According to our experimental results, for thick oxide value F s 190 nm, the grain size D of the ‘‘location controlled’’ large grains increased with the bump height H value. In order to obtain a thermodynamical explanation of this effect, a numerical analysis of the two-dimensional temperature profile with respect to solidification velocity of molten Si was performed. Two different values of bump heights H s 30 nm and H s 70 nm for F s 190 nm were used. The solidification velocity was numerically calculated for both cases, bearing in mind that: Ža. the top a-Si layer was molten completely after the dual-beam laser irradiation and Žb. The maximum achieved temperature Tmax of the SirSiO 2 interface in the middle of the bump after the laser pulse Žwhich is at the same time the minimum temperature
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along the SirSiO 2 interface. was the same in both cases. The calculated temperature distribution along the SirSiO 2 interface near the bump immediately after laser irradiation is shown in Fig. 6. It should be noted that Tmax was attained with different laser energy densities for different H values of the samples. The temperature outside the bump for H s 70 nm Ž1418 K. is higher than that for the H s 30 nm Ž1409 K. due to the fact that the heat capacitance outside the bump is smaller for the larger H value. Hence, the energy conservation for an arbitrary small fixed volume outside the oxide for H s 70 nm is higher than that for H s 30 nm, resulting in a larger lateral heat flow from the thin oxide to the thick oxide portion in the vicinity of the SirSiO 2 bump interface. Therefore, heat accumulation occurs more efficiently for the thick oxide portion with H s 70 nm than for H s 30 nm, leading to
Fig. 6. Temperature distribution along the SirSiO 2 interface in the vicinity of the bump immediately after a laser pulse of 66 ns for different H values of oxide. The diameter of the bump is 1.6 mm. The SirSiO 2 interface is traced from the outside Ž5 mm. to the middle of the bump.
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Table 4 Numerically calculated velocity of molten Sirsolid Si interface propagation Ž F value is fixed at 190 nm. Bump height H wnmx Average solidification velocity V wmrsx
30 1.08
70 0.89
a gentler temperature slope between the molten Si and metal layer. As follows from Eq. 2, since the solidification velocity V is proportional to this temperature difference, V decreases with increasing bump height resulting in larger grains Žsee Table 4.. Hence, the slow solidification velocity for the high H value gave a large grain in the experiment.
4. Conclusion We have investigated the effect of thickness variation F of intermediate SiO 2 layer and bump height H on the grain size of the ‘‘location controlled’’ large Si grains in an a-SirSiO 2rmetal stack with an array of bumps in the oxide. The size of the ‘‘location controlled’’ c-Si increased with F achieving a maximum of 5.1 mm for F s 400 nm. The grain diameter increased with H, reaching a maximum of 4.2 mm for H s 70 nm, while F was fixed at 190 nm. An attempt to explain the growth mechanism enhancement in ‘‘location control’’ of the large crystal grains using the numerical simulation result of two-dimensional transient temperature distribution and solidification velocity has been made. Solidification velocity decreased with F or H value, which is consistent with the experimental result.
Acknowledgements The authors are grateful to Professor Dr. C. Beenakker for his warm encouragement and motivation; and to Dr. J.W. Metselaar, C.C.G. Visser, and Alexei Bereznitski for their useful discussions. This work was supported in part by the Dutch Technology Foundation STW Project No. DEL66.4542 and XMR Santa Clara, CA. References w1x T. Noguchi, Jpn. J. Appl. Phys. 32 Ž1993. L1585. w2x T. Sameshima, S. Usui, M. Sekiya, IEEE Electron Device Lett. 7 Ž1986. 276. w3x K. Sera, F. Okumura, H. Uchida, S. Itoh, S. Kaneko, K. Hotta, IEEE Trans. Electron Devices 36 Ž1989. 2868. w4x N. Yamauchi, R. Reif, J. Appl. Phys. 75 Ž1994. 3235. w5x R. Ishihara, M. Matsumura, Electron. Lett. 22 Ž1995. 1956. w6x R. Ishihara, A. Burtsev, Jpn. J. Appl. Phys. 37 Ž3B. Ž1998. 1071. w7x R. Ishihara, A. Burtsev, Extented abstracts of the 1997 Int. Conf. on Solid State Devices and Materials, Hamamatsu C-8-3 Ž1997. 360. w8x R. Ishihara, A. Burtsev, Digest of Technical Papers AMLCD98 Tokyo Ž1998. 153. w9x H. Kuriyama, S. Kiyama, S. Noguchi, Jpn. J. Appl. Phys. 30 Ž12B. Ž1991. 3701. w10x V.E. Zinov’ev, in: Handbook of Thermophysical Properties of Metals at High Temperatures, New York,1997, p. 288. w11x H. Webber, A. Cullis, N. Chew, Appl. Phys. Lett. 43 Ž1983. 669. w12x L.A. Lompe, J.M. Liu, H. Kurtz, N. Bloembergen, Appl. Phys. Lett. 43 Ž1983. 168. w13x Handbook of Chemistry and Physics, Boca Raton, Florida, 1989, p. B34. w14x D. Bauerle, in: Laser Processing and Chemistry, Berlin, 1996, p. 581. w15x K. Shimizu, O. Sigiura, M. Matsumura, IEEE Trans. Electron Devices 40 Ž1. Ž1993. 112.