半导体材料的飞秒激光微纳加工
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摘要
随着微电子器件的小型化和集成化的发展,半导体材料的微细加工技术显得尤为重要。很多传统的微细加工技术可以满足精细加工缩小尺寸和提高精度的要求,而不能满足复杂的微结构高度集成需求。本论文主要采用飞秒激光微纳加工技术,分别针对常规的半导体材料SnO2:Sb体系和新颖的石墨烯材料进行了可设计微图案化的研究。
对于SnO2:Sb氧化物半导体的加工,首先采用乙酰丙酮对金属无机盐进行化学修饰配置溶胶体系,并利用紫外-可见吸收光谱对溶胶的感光性进行了分析。然后利用飞秒激光直写微细加工技术对SnO2:Sb凝胶膜体系进行逐点扫描,扫描过后的区域溶解度在丙酮里发生明显变化,经显影可留下凝胶微结构,凝胶结构分辨率可达400nm(烧结后减小到150nm)。最后进一步高温煅烧使材料晶化得到SnO2:Sb氧化物半导体微结构,通过扫描电子显微镜对烧结前后,结构的形貌进行了表征,通过X射线衍射分析表征材料晶体结构。此外还制作了微湿敏传感器件,利用keithley4200SCS半导体测试系统对微结构的电学性质和湿敏特性进行了测试。
在对于石墨烯材料的微纳加工中,我们首先采用Hummers’法制备了在水中分散性良好的石墨烯氧化物,该材料可以很方便地在玻璃衬底上旋涂成膜。然后我们利用飞秒激光脉冲对石墨烯氧化物薄膜进行图案化直写还原,得到导电微结构,对表面形貌进行原子力显微镜和扫描电子显微镜表征,图形形貌完好,加工分辨率可达500nm。利用X射线衍射、X射线光电子能谱和拉曼光谱,分析了加工区域与未加工区域化学组成及结构的变化,结果表明加工后还原区域含氧基团大部分被除去。还原后的石墨烯表现出了良好的导电性。最后,我们还讨论了加工功率对还原成度的影响,结果发现随着加工功率的增大,材料电导率增大,最大可达2.56×104S/m。
飞秒激光直写做为一种方便、快速、有效的方法,实现了对以上两种半导体材料体系的任意细微图案化,为微电子器件小型化与集成化提供了一种高效、精密的加工技术。
In recent years, micro-nanofabrication of conductive semiconductors has been paid much attention due to the requirements of miniaturization and integration of microelectronic devices. Conventional micro-nanofabricative methods such as photoresist-wet chemical etching (Wet etching), reactive ion etching (RIE), focused ion beam etching (FIB), e-beam directwritten (EBDW), nano-imprint lithography (NIL) and sol-gel direc patterning have been successfully used for reducing the size of micro-devices. However, it is still very difficult to create complex microstructures arbitrarily for highly integrated micro-nanodevices .
Femtosecond(FS) laser micro-nanofabrication exhibits a series of unique advantages. For example, the focus area of femtosecond laser is very small, which give rise to a much higher resolution compared with the traditional micro-processing technologies. Additionally, the low thermal effects of femtosecond laser is of benefit to good topography. FS laser induced multi-photon absorption could be used for fabrication of three-dimensional microstructures. Therefore, FS laser would be a powerful technique for micro-nanofabrication of desire microstructures towards miniaturization and integration of microelectronic devices.
In this work, as two representative examples, we demonstrate the FS laser micro-nanofabrication of conventional semiconductor of SnO2: Sb and novel graphene-based materials for microelectronic devices use.
In the fabrication of microstructured SnO2: Sb semiconductor, we prepared a photosensitive SnO2: Sb sol as precusor. Typically, Acetylacetone(AcAc) was introduced into metal ions solution to forme chelate rings in the solvent of 2-methoxyethanol. The as-formed sol-like precursor shows long-term stability at room temperature. From UV-vis absorption spectrum, the obvious absorption could be identified at around 302 nm. After UV irradiation for 30 min, the absorption peak decreases, and the gel film behaves insoluble in acetone. In this work, the light source is FS laser with 790nm wavelength, while the gel has no absorption of 790nm wavelength. Therefore, it is believable that two-photon or multi-photon absorption occurs in processing of the fabrication.
During the laser processing, FS laser directly wrote on the SnO2: Sb gel films. The regions irradiated by laser become insoluble in acetone. So after development in acetone, desired microstructures were successfully obtained. 2D and 3D microstructures could be fabricated according to preprogrammed patterns.
Crystallized microstructures were finally obtained after high temperature calcination at 500℃for two hours, XRD study shows that the micronanostructures was highly crystallized. The whole set of diffraction peaks were attributed to the tetragonal rutile structure. From the SEM images, before sintering, the gel structure showed the highest resolution of 400nm. After high temperature calcinations in air, the structure shrank, but remained intact, and the resolution is as high as 150nm.
Finally, we also fabricated a SnO2: Sb micro-wire between two gold electrodes as a humidity microsensor. Resistance of the wire was calculated to be 5500 ?·cm . When changing the humidity from 11% to 95%, the range of the variational resistance is as wide as 5 orders of magnitude, exhibiting excellent sensing property.
In the second case, we present a direct imprinting of graphene microcircuits on graphene oxide (GO) film by FS laser induced deoxidation. Graphene oxides used for processing were prepared by Hummers' methode. Then graphene oxides aqueous solution was spin-coated on glass substrate at 1000 rpm, giving a transparent film. The thickness of the film was estimated to be about 55nm from AFM characterization. The as-obtained graphene oxide film was used for subsequent processing. After FS laser direct writing on this film according to preprogrammed patterns, various microcircuits were created on the GO film. We carefully characterized the surface height of patterned part and compared it with primary GO film. The microcircuits surface is lower than that of unpatterned part. Possible reason for this phenomenon would be the laser induced deoxidation of GO. XRD, XPS and Raman spectra confirm this explanation. After exposure to FS laser, most of oxygen-containing groups were removed. The reduced GO show conductivity and could be used for electrical devices.
We have also reduced and patterned GO film with different output laser power (0.5-3.0mW). The results show that the conductivity of reduced GO has strong dependence on laser power. With the increase of laser power, the conductivity of reduced and patterned GO also increased. The highest conductivity was about 2.56×104S/m, which was obtained with 3.0mW of FS laser. Further increase of laser power could result in a broken GO film.
In conclusion, by using FS laser micro-nanoprocessing, we have successfully fabricated microstructured semiconductors of SnO2:Sb and graphene microcircuits towards microelectronic devices. It is believable that the FS laser micro-nanofabrication would be a powerful technique for miniaturization and integration of various microdevices.
对于SnO2:Sb氧化物半导体的加工,首先采用乙酰丙酮对金属无机盐进行化学修饰配置溶胶体系,并利用紫外-可见吸收光谱对溶胶的感光性进行了分析。然后利用飞秒激光直写微细加工技术对SnO2:Sb凝胶膜体系进行逐点扫描,扫描过后的区域溶解度在丙酮里发生明显变化,经显影可留下凝胶微结构,凝胶结构分辨率可达400nm(烧结后减小到150nm)。最后进一步高温煅烧使材料晶化得到SnO2:Sb氧化物半导体微结构,通过扫描电子显微镜对烧结前后,结构的形貌进行了表征,通过X射线衍射分析表征材料晶体结构。此外还制作了微湿敏传感器件,利用keithley4200SCS半导体测试系统对微结构的电学性质和湿敏特性进行了测试。
在对于石墨烯材料的微纳加工中,我们首先采用Hummers’法制备了在水中分散性良好的石墨烯氧化物,该材料可以很方便地在玻璃衬底上旋涂成膜。然后我们利用飞秒激光脉冲对石墨烯氧化物薄膜进行图案化直写还原,得到导电微结构,对表面形貌进行原子力显微镜和扫描电子显微镜表征,图形形貌完好,加工分辨率可达500nm。利用X射线衍射、X射线光电子能谱和拉曼光谱,分析了加工区域与未加工区域化学组成及结构的变化,结果表明加工后还原区域含氧基团大部分被除去。还原后的石墨烯表现出了良好的导电性。最后,我们还讨论了加工功率对还原成度的影响,结果发现随着加工功率的增大,材料电导率增大,最大可达2.56×104S/m。
飞秒激光直写做为一种方便、快速、有效的方法,实现了对以上两种半导体材料体系的任意细微图案化,为微电子器件小型化与集成化提供了一种高效、精密的加工技术。
In recent years, micro-nanofabrication of conductive semiconductors has been paid much attention due to the requirements of miniaturization and integration of microelectronic devices. Conventional micro-nanofabricative methods such as photoresist-wet chemical etching (Wet etching), reactive ion etching (RIE), focused ion beam etching (FIB), e-beam directwritten (EBDW), nano-imprint lithography (NIL) and sol-gel direc patterning have been successfully used for reducing the size of micro-devices. However, it is still very difficult to create complex microstructures arbitrarily for highly integrated micro-nanodevices .
Femtosecond(FS) laser micro-nanofabrication exhibits a series of unique advantages. For example, the focus area of femtosecond laser is very small, which give rise to a much higher resolution compared with the traditional micro-processing technologies. Additionally, the low thermal effects of femtosecond laser is of benefit to good topography. FS laser induced multi-photon absorption could be used for fabrication of three-dimensional microstructures. Therefore, FS laser would be a powerful technique for micro-nanofabrication of desire microstructures towards miniaturization and integration of microelectronic devices.
In this work, as two representative examples, we demonstrate the FS laser micro-nanofabrication of conventional semiconductor of SnO2: Sb and novel graphene-based materials for microelectronic devices use.
In the fabrication of microstructured SnO2: Sb semiconductor, we prepared a photosensitive SnO2: Sb sol as precusor. Typically, Acetylacetone(AcAc) was introduced into metal ions solution to forme chelate rings in the solvent of 2-methoxyethanol. The as-formed sol-like precursor shows long-term stability at room temperature. From UV-vis absorption spectrum, the obvious absorption could be identified at around 302 nm. After UV irradiation for 30 min, the absorption peak decreases, and the gel film behaves insoluble in acetone. In this work, the light source is FS laser with 790nm wavelength, while the gel has no absorption of 790nm wavelength. Therefore, it is believable that two-photon or multi-photon absorption occurs in processing of the fabrication.
During the laser processing, FS laser directly wrote on the SnO2: Sb gel films. The regions irradiated by laser become insoluble in acetone. So after development in acetone, desired microstructures were successfully obtained. 2D and 3D microstructures could be fabricated according to preprogrammed patterns.
Crystallized microstructures were finally obtained after high temperature calcination at 500℃for two hours, XRD study shows that the micronanostructures was highly crystallized. The whole set of diffraction peaks were attributed to the tetragonal rutile structure. From the SEM images, before sintering, the gel structure showed the highest resolution of 400nm. After high temperature calcinations in air, the structure shrank, but remained intact, and the resolution is as high as 150nm.
Finally, we also fabricated a SnO2: Sb micro-wire between two gold electrodes as a humidity microsensor. Resistance of the wire was calculated to be 5500 ?·cm . When changing the humidity from 11% to 95%, the range of the variational resistance is as wide as 5 orders of magnitude, exhibiting excellent sensing property.
In the second case, we present a direct imprinting of graphene microcircuits on graphene oxide (GO) film by FS laser induced deoxidation. Graphene oxides used for processing were prepared by Hummers' methode. Then graphene oxides aqueous solution was spin-coated on glass substrate at 1000 rpm, giving a transparent film. The thickness of the film was estimated to be about 55nm from AFM characterization. The as-obtained graphene oxide film was used for subsequent processing. After FS laser direct writing on this film according to preprogrammed patterns, various microcircuits were created on the GO film. We carefully characterized the surface height of patterned part and compared it with primary GO film. The microcircuits surface is lower than that of unpatterned part. Possible reason for this phenomenon would be the laser induced deoxidation of GO. XRD, XPS and Raman spectra confirm this explanation. After exposure to FS laser, most of oxygen-containing groups were removed. The reduced GO show conductivity and could be used for electrical devices.
We have also reduced and patterned GO film with different output laser power (0.5-3.0mW). The results show that the conductivity of reduced GO has strong dependence on laser power. With the increase of laser power, the conductivity of reduced and patterned GO also increased. The highest conductivity was about 2.56×104S/m, which was obtained with 3.0mW of FS laser. Further increase of laser power could result in a broken GO film.
In conclusion, by using FS laser micro-nanoprocessing, we have successfully fabricated microstructured semiconductors of SnO2:Sb and graphene microcircuits towards microelectronic devices. It is believable that the FS laser micro-nanofabrication would be a powerful technique for miniaturization and integration of various microdevices.
引文
[1]朱江峰,魏志义.飞秒激光精密微纳加工的研究进展[J].前沿进展, 2006, 35(8): 679-683.
[2]杨建军.飞秒激光超精细“冷”加工技术及其应用[J].激光与光电子学进展, 2004, 41(3).
[3]黄剑锋.溶胶-凝胶原理与技术[M].北京:化学工业出版社,2005.
[4] TOHGE N, SHINMOU K, MINAMI I. Effects of UV-irradiation on the formation of oxide thin films from chemically modified metal-alkoxides[J]. Journal of Sol-Gel Science and Technology, 1994, 2: 581-585.
[5] TOHGE N, ZHAO G, CHIBA F. Photosensitive gel flms prepared by the chemical modification and their application to surface-relief gratings[J]. Thin Solid Films, 1999, 351: 85-90.
[6] TADANAGA K, OWAN T, MORINAGA J, et al. Fine patterning of transparent, conductive SnO2 thin films by UV-irradiation[J]. Journal of Sol-Gel Science and Technology, 2000, 19: 791-794.
[7] ZHANG W H, ZHAO G Y, CHEN Z M. Photosensitive PZT gel films and their preparation for fine patterning[J]. Materials Science and Engineering, 2003, B99: 168-172.
[8] ZHAO G Y, Zhang W H, Du Z H. Preparation of Pb0.98La0.02(Zr0.65Ti0.35)O3 films and their fine patterning[J]. Materials Research Bulletin, 2004, 39: 449-456.
[9] LI Y, ZHAO G Y, ZHANG W H, et al. Fine pattern fabrication on SnO2 : Sb thin films formed by the sol-gel process[J]. Surface and Interface Analysis, 2006, 38: 1291-1295.
[10] PASINGER S, SAIFULLAH M S M, REINHARDT C, et al. Direct 3D patterning of TiO2 using femtosecond laser pulses[J]. Advance Materials, 2007, 19: 1218-1221.
[11] NOVOSELOV K S, GEIM A K, MOROZOV S V, et al. Electric field effect inatomically thin carbon films[J]. Science, 2004, 306: 666-669.
[12] ALLEN M J, TUNG V C, KANER R B, et al. Honeycomb carbon: a review of graphene[J]. Chemical Reviews, 2010, 110: 132-145.
[13] GEIM A K, NOVOSELOV K S. The rise of graphene[J]. Nature Materials, 2007, 6: 183-19.
[14] LIANG X G, FU Z L, CHOU S Y. Graphene transistors fabricated via transfer-printing in device active-areas on large wafer[J]. Nano Letters, 2007, 7: 3840-3844.
[15] DI C A, WEI D C, YU G, Liu Y Q, et al. Patterned graphene as source/drain electrodes for bottom-contact organic field-effect transistors[J]. Advance Materials,. 2008, 20: 3289-3293.
[16] ROBINSON J T, PERkins F K, Snow E S, et al. Reduced graphene oxide molecular sensors[J]. Nano Letters, 2008, 8: 3137-3140.
[17] FOWLER J D, ALLEN M J, Tung V C, et al. Practical chemical sensors from chemically derived graphene[J]. ACS Nano, 2009, 3: 301-306.
[18] STANKOVICH S, DIKIN D A, DOMMETT G H. B, et al. Graphene-based composite materials[J]. Nature, 2006, 442: 282-286.
[19] STANKOVICH S, DIKIN D A, PINER R D, et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide[J]. Carbon, 2007, 45: 1558-1565.
[20] FAN X B, PENG W C, LI Y, et al. Deoxygenation of exfoliated graphite oxide under alkaline conditions: A green route to graphene preparation[J]. Advance Materials, 2008, 20: 4490-4493.
[21] BECERRIL H A, MAO J, LIU Z F, et al. Evaluation of solution-processed reduced graphene oxide films as transparent conductors[J]. ACS Nano, 2008, 2: 463-470.
[22] EDA G, FANCHINI G, CHHOWALLA M. Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material[J]. Nature, 2008, 3: 270-274.
[23] WU Z S, REN W C, GAO L B. Synthesis of graphene sheets with high electricalconductivity and good thermal stability by hydrogen arc discharge exfoliation[J]. ACS Nano, 2009, 3: 411-417.
[24] ZHOU M, WANG Y L, ZHAI Y M. Controlled synthesis of large-area and patterned electrochemically reduced graphene oxide films[J]. Chem. Eur. J, 2009, 15: 6116-6120.
[25] ALLEN M J, TUNG V C, LEWIS G. Soft transfer printing of chemically converted graphene[J]. Advance Materials, 2009, 21: 2098-2102.
[26] BELL D C, LEMME M C, STERN L A, Precision cutting and patterning of graphene with helium ions[J]. Nanotechnology, 2009, 20: 455301.
[27] PANG S P, TSAO H N, FENG X L. Patterned graphene electrodes from solution-processed graphite oxide films for organic field-effect transistors[J]. Advance Materials, 2009, 21: 1-4.
[28] COTE L J, SILVA R C, HUANG J X. Flash reduction and patterning of graphite oxide and its polymer composite[J]. J. Am. Chem. Soc, 2009, 131: 11027-11032.
[29] HEILAND G, KOHL D. Chemical Sensor Technology[M]. Kodansha, Tokyo, 1988.
[30] HENRICH V A, COX P A. The Surface Science of Metal Oxides[M]. Cambridge University Press, Cambridge, 1994.
[31]全宝富,邱法斌.电子功能材料及元器件[M].长春:吉林大学出版社,2001.
[2]杨建军.飞秒激光超精细“冷”加工技术及其应用[J].激光与光电子学进展, 2004, 41(3).
[3]黄剑锋.溶胶-凝胶原理与技术[M].北京:化学工业出版社,2005.
[4] TOHGE N, SHINMOU K, MINAMI I. Effects of UV-irradiation on the formation of oxide thin films from chemically modified metal-alkoxides[J]. Journal of Sol-Gel Science and Technology, 1994, 2: 581-585.
[5] TOHGE N, ZHAO G, CHIBA F. Photosensitive gel flms prepared by the chemical modification and their application to surface-relief gratings[J]. Thin Solid Films, 1999, 351: 85-90.
[6] TADANAGA K, OWAN T, MORINAGA J, et al. Fine patterning of transparent, conductive SnO2 thin films by UV-irradiation[J]. Journal of Sol-Gel Science and Technology, 2000, 19: 791-794.
[7] ZHANG W H, ZHAO G Y, CHEN Z M. Photosensitive PZT gel films and their preparation for fine patterning[J]. Materials Science and Engineering, 2003, B99: 168-172.
[8] ZHAO G Y, Zhang W H, Du Z H. Preparation of Pb0.98La0.02(Zr0.65Ti0.35)O3 films and their fine patterning[J]. Materials Research Bulletin, 2004, 39: 449-456.
[9] LI Y, ZHAO G Y, ZHANG W H, et al. Fine pattern fabrication on SnO2 : Sb thin films formed by the sol-gel process[J]. Surface and Interface Analysis, 2006, 38: 1291-1295.
[10] PASINGER S, SAIFULLAH M S M, REINHARDT C, et al. Direct 3D patterning of TiO2 using femtosecond laser pulses[J]. Advance Materials, 2007, 19: 1218-1221.
[11] NOVOSELOV K S, GEIM A K, MOROZOV S V, et al. Electric field effect inatomically thin carbon films[J]. Science, 2004, 306: 666-669.
[12] ALLEN M J, TUNG V C, KANER R B, et al. Honeycomb carbon: a review of graphene[J]. Chemical Reviews, 2010, 110: 132-145.
[13] GEIM A K, NOVOSELOV K S. The rise of graphene[J]. Nature Materials, 2007, 6: 183-19.
[14] LIANG X G, FU Z L, CHOU S Y. Graphene transistors fabricated via transfer-printing in device active-areas on large wafer[J]. Nano Letters, 2007, 7: 3840-3844.
[15] DI C A, WEI D C, YU G, Liu Y Q, et al. Patterned graphene as source/drain electrodes for bottom-contact organic field-effect transistors[J]. Advance Materials,. 2008, 20: 3289-3293.
[16] ROBINSON J T, PERkins F K, Snow E S, et al. Reduced graphene oxide molecular sensors[J]. Nano Letters, 2008, 8: 3137-3140.
[17] FOWLER J D, ALLEN M J, Tung V C, et al. Practical chemical sensors from chemically derived graphene[J]. ACS Nano, 2009, 3: 301-306.
[18] STANKOVICH S, DIKIN D A, DOMMETT G H. B, et al. Graphene-based composite materials[J]. Nature, 2006, 442: 282-286.
[19] STANKOVICH S, DIKIN D A, PINER R D, et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide[J]. Carbon, 2007, 45: 1558-1565.
[20] FAN X B, PENG W C, LI Y, et al. Deoxygenation of exfoliated graphite oxide under alkaline conditions: A green route to graphene preparation[J]. Advance Materials, 2008, 20: 4490-4493.
[21] BECERRIL H A, MAO J, LIU Z F, et al. Evaluation of solution-processed reduced graphene oxide films as transparent conductors[J]. ACS Nano, 2008, 2: 463-470.
[22] EDA G, FANCHINI G, CHHOWALLA M. Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material[J]. Nature, 2008, 3: 270-274.
[23] WU Z S, REN W C, GAO L B. Synthesis of graphene sheets with high electricalconductivity and good thermal stability by hydrogen arc discharge exfoliation[J]. ACS Nano, 2009, 3: 411-417.
[24] ZHOU M, WANG Y L, ZHAI Y M. Controlled synthesis of large-area and patterned electrochemically reduced graphene oxide films[J]. Chem. Eur. J, 2009, 15: 6116-6120.
[25] ALLEN M J, TUNG V C, LEWIS G. Soft transfer printing of chemically converted graphene[J]. Advance Materials, 2009, 21: 2098-2102.
[26] BELL D C, LEMME M C, STERN L A, Precision cutting and patterning of graphene with helium ions[J]. Nanotechnology, 2009, 20: 455301.
[27] PANG S P, TSAO H N, FENG X L. Patterned graphene electrodes from solution-processed graphite oxide films for organic field-effect transistors[J]. Advance Materials, 2009, 21: 1-4.
[28] COTE L J, SILVA R C, HUANG J X. Flash reduction and patterning of graphite oxide and its polymer composite[J]. J. Am. Chem. Soc, 2009, 131: 11027-11032.
[29] HEILAND G, KOHL D. Chemical Sensor Technology[M]. Kodansha, Tokyo, 1988.
[30] HENRICH V A, COX P A. The Surface Science of Metal Oxides[M]. Cambridge University Press, Cambridge, 1994.
[31]全宝富,邱法斌.电子功能材料及元器件[M].长春:吉林大学出版社,2001.