Fabrication and characterization of sub-100/10nm planar nanofluidic channels by triple thermal oxidation and silicon-glass anodic bonding Wei Ouyang and Wei Wang Citation: Biomicrofluidics 8, 052106 (2014); doi: 10.1063/1.4894160 View online: http://dx.doi.org/10.1063/1.4894160 View Table of Contents: http://scitation.aip.org/content/aip/journal/bmf/8/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in DNA translocation through short nanofluidic channels under asymmetric pulsed electric field Biomicrofluidics 8, 024114 (2014); 10.1063/1.4871595 Cylindrical glass nanocapillaries patterned via coarse lithography (>1μm) for biomicrofluidic applications Biomicrofluidics 6, 046502 (2012); 10.1063/1.4771691 High throughput fabrication of disposable nanofluidic lab-on-chip devices for single molecule studies Biomicrofluidics 6, 036502 (2012); 10.1063/1.4740231 Translocation of nanoparticles through a polymer brush-modified nanochannel Biomicrofluidics 6, 034101 (2012); 10.1063/1.4732799 Complementary metal oxide semiconductor compatible fabrication and characterization of parylene-C covered nanofluidic channels with integrated nanoelectrodes Biomicrofluidics 3, 031101 (2009); 10.1063/1.3212074
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 152.3.102.242 On: Tue, 11 Nov 2014 09:38:49
BIOMICROFLUIDICS 8, 052106 (2014)
Fabrication and characterization of sub-100/10 nm planar nanofluidic channels by triple thermal oxidation and silicon-glass anodic bonding Wei Ouyang1,a) and Wei Wang1,2,b) 1
Institute of Microelectronics, Peking University, Beijing 100871, People’s Republic of China 2 National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Beijing 100871, People’s Republic of China (Received 13 May 2014; accepted 18 August 2014; published online 25 August 2014)
We reported the fabrication and characterization of nanofluidic channels by Triple Thermal Oxidation and Silicon-Glass Anodic Bonding. Planar nanochannels with depths from sub-100 nm down to sub-10 nm were realized by this method. A theoretical model was developed to precisely predict the depth of nanochannels. The depth and uniformity of nanochannels showed good stability during anodic bonding. This method is promising for various nanofluidic studies, such as nanofluidic C 2014 electrokinetics, biomolecule manipulation, and energy conversion. V AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4894160]
I. INTRODUCTION
Nanofluidics, with its unique properties at dimensions less than 100 nm, has raised tremendous interest in various fields, such as energy conversion,1 biomolecule manipulation,2 and water desalination.3 In particular, increasing efforts have been devoted to the sub-10 nm regime for anomalous fluidic transport phenomena studies,4,5 and novel applications such as DNA analysis6,7 and biosensing.8 Undoubtedly, the advance of nanofluidics relies on the development of nanofabrication technology capable of realizing well-defined and highly reproducible nanopores/nanochannels.9 Although nanolithography provides precise patterning of nanoscale structures, microelectromechanical (MEMS) based techniques have been most widely adopted for nanofluidic device fabrication benefitting from their high reproducibility, high throughput, and low cost.9 Most MEMS based techniques were only able to realize nanostructures with one dimension at nano-scale (vertical nanochannels with nano-width and planar nanochannels with nano-depth). But these methods met the requirements of most nanofluidic studies, which only need one nano-confined dimension to realize electrical double layer overlapping. Xie et al.10 proposed edge lithography as a wafer-scale scheme of fabricating vertical nanochannels from 500 nm down to 50 nm. By controlling the process recipe, several groups adopted Reactive Ion Etching (RIE) to fabricate planar nanofluidic channels with depth down to 20 nm.11–14 Several unconventional methods were developed to realize sub-10 nm nanochannels. Kim et al.15 produced 1 nm deep nanochannels by PDMS (Polydimethylsiloxane) molding and reversible plasma bonding. Song and Wang16 took advantage of silicon native oxide to fabricate sub15 nm planar nanochannels by HF-dips. However, these methods are limited to the fabrication of either sub-10 nm or sub-100 nm to 10 nm nanochannels. Therefore, a more versatile fabrication technique capable of scaling from sub-100 nm to sub-10 nm is highly desirable for the nanofluidics society. In this paper, we proposed triple thermal oxidation and silicon-glass anodic a)
Present address: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA. b) Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel: 86-10-6276-9183. Fax: 86-10-6275-1789. 1932-1058/2014/8(5)/052106/11/$30.00
8, 052106-1
C 2014 AIP Publishing LLC V
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 152.3.102.242 On: Tue, 11 Nov 2014 09:38:49
052106-2
W. Ouyang and W. Wang
Biomicrofluidics 8, 052106 (2014)
bonding (TTO-SGAB) as a versatile method of sub-100/10 nm planar nanofluidic channel fabrication. Although Wu et al.17 and Persson et al.18 used Double Thermal Oxidation (DTO) process to fabricate nanochannels for capillary filling study, we extended this process by an etching step and third oxidation to obtain shallower nanochannels. Moreover, a detailed mathematical model on TTO was developed to precisely predict the depth of nanochannel fabricated by this method. Next, the stability of nanochannel depth and uniformity after SGAB was investigated by SEM. Finally, the collapsing of TTO-fabricated nanochannels during SGAB was scrutinized.
II. PRINCIPLE A. Fabrication process
The fabrication process of the TTO-SGAB method is shown in Figure 1. (1) A SiO2 layer (thickness: d1) was thermally grown on the silicon substrate. (2) A lithography process was conducted to define the pattern of nanochannels. The pattern was transferred to the substrate by wet etching. (3) The second oxidation was conducted, with the thickness of the oxide layer in the nanochannel region being d2. Due to diffusion of oxygen molecules through the oxide layer, the thickness of the oxide layer in the non-nanochannel region increased to d3. (4) All oxide layers on the substrate were completely etched to obtain a shallower nanochannel.19 A nanochannel with depth of h was obtained on the silicon substrate. (5) The third oxidation was conducted to realize electrical insulation. Due to the uniformity of the thermal oxidation process, the nanochannel depth remained h. (6) SGAB was conducted to form enclosed nanofluidic channels. During the thermal oxidation process, silicon was consumed in the oxidation reaction. For a silicon dioxide layer with thickness of d, the depth of silicon consumed was 0.46d. Based on the analysis of the silicon consumption, the depth of the nanochannel could be calculated as h ¼ 0:46d2 0:46ðd3 d1 Þ ¼ 0:46ðd1 þ d2 d3 Þ:
(1)
Because of the growth of oxide layer in the non-nanochannel region during the second oxidation (d3 > d1), the nanochannel depth obtained was smaller than 0.46d2. It can potentially be
FIG. 1. Fabrication process of the TTO-SGAB scheme.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 152.3.102.242 On: Tue, 11 Nov 2014 09:38:49
052106-3
W. Ouyang and W. Wang
Biomicrofluidics 8, 052106 (2014)
extremely small by tuning the thickness of the first oxide layer d1. In Eq. (1), d1 and d2 are design parameters. d3 is determined by d1 and d2, and can be precisely calculated by the DealGrove Model for silicon thermal oxidation. B. Nanochannel depth prediction
The Deal-Grove Model20 is widely used for modeling the silicon thermal oxidation process, according to which the relationship between oxide layer thickness Xox and oxidation time t is rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A2 A þ Bðt þ sÞ ; Xox ¼ 4 2
(2.1)
where A and B are parameters determined by the oxidation conditions (temperature, pressure, oxygen concentration, etc.), s is the effective time corresponding to the initial oxide layer thickness. The parameter s was introduced to offset the effect of oxidation rate enhancement at thin regime (
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 152.3.102.242 On: Tue, 11 Nov 2014 09:38:49
BIOMICROFLUIDICS 8, 052106 (2014)
Fabrication and characterization of sub-100/10 nm planar nanofluidic channels by triple thermal oxidation and silicon-glass anodic bonding Wei Ouyang1,a) and Wei Wang1,2,b) 1
Institute of Microelectronics, Peking University, Beijing 100871, People’s Republic of China 2 National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Beijing 100871, People’s Republic of China (Received 13 May 2014; accepted 18 August 2014; published online 25 August 2014)
We reported the fabrication and characterization of nanofluidic channels by Triple Thermal Oxidation and Silicon-Glass Anodic Bonding. Planar nanochannels with depths from sub-100 nm down to sub-10 nm were realized by this method. A theoretical model was developed to precisely predict the depth of nanochannels. The depth and uniformity of nanochannels showed good stability during anodic bonding. This method is promising for various nanofluidic studies, such as nanofluidic C 2014 electrokinetics, biomolecule manipulation, and energy conversion. V AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4894160]
I. INTRODUCTION
Nanofluidics, with its unique properties at dimensions less than 100 nm, has raised tremendous interest in various fields, such as energy conversion,1 biomolecule manipulation,2 and water desalination.3 In particular, increasing efforts have been devoted to the sub-10 nm regime for anomalous fluidic transport phenomena studies,4,5 and novel applications such as DNA analysis6,7 and biosensing.8 Undoubtedly, the advance of nanofluidics relies on the development of nanofabrication technology capable of realizing well-defined and highly reproducible nanopores/nanochannels.9 Although nanolithography provides precise patterning of nanoscale structures, microelectromechanical (MEMS) based techniques have been most widely adopted for nanofluidic device fabrication benefitting from their high reproducibility, high throughput, and low cost.9 Most MEMS based techniques were only able to realize nanostructures with one dimension at nano-scale (vertical nanochannels with nano-width and planar nanochannels with nano-depth). But these methods met the requirements of most nanofluidic studies, which only need one nano-confined dimension to realize electrical double layer overlapping. Xie et al.10 proposed edge lithography as a wafer-scale scheme of fabricating vertical nanochannels from 500 nm down to 50 nm. By controlling the process recipe, several groups adopted Reactive Ion Etching (RIE) to fabricate planar nanofluidic channels with depth down to 20 nm.11–14 Several unconventional methods were developed to realize sub-10 nm nanochannels. Kim et al.15 produced 1 nm deep nanochannels by PDMS (Polydimethylsiloxane) molding and reversible plasma bonding. Song and Wang16 took advantage of silicon native oxide to fabricate sub15 nm planar nanochannels by HF-dips. However, these methods are limited to the fabrication of either sub-10 nm or sub-100 nm to 10 nm nanochannels. Therefore, a more versatile fabrication technique capable of scaling from sub-100 nm to sub-10 nm is highly desirable for the nanofluidics society. In this paper, we proposed triple thermal oxidation and silicon-glass anodic a)
Present address: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA. b) Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel: 86-10-6276-9183. Fax: 86-10-6275-1789. 1932-1058/2014/8(5)/052106/11/$30.00
8, 052106-1
C 2014 AIP Publishing LLC V
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 152.3.102.242 On: Tue, 11 Nov 2014 09:38:49
052106-2
W. Ouyang and W. Wang
Biomicrofluidics 8, 052106 (2014)
bonding (TTO-SGAB) as a versatile method of sub-100/10 nm planar nanofluidic channel fabrication. Although Wu et al.17 and Persson et al.18 used Double Thermal Oxidation (DTO) process to fabricate nanochannels for capillary filling study, we extended this process by an etching step and third oxidation to obtain shallower nanochannels. Moreover, a detailed mathematical model on TTO was developed to precisely predict the depth of nanochannel fabricated by this method. Next, the stability of nanochannel depth and uniformity after SGAB was investigated by SEM. Finally, the collapsing of TTO-fabricated nanochannels during SGAB was scrutinized.
II. PRINCIPLE A. Fabrication process
The fabrication process of the TTO-SGAB method is shown in Figure 1. (1) A SiO2 layer (thickness: d1) was thermally grown on the silicon substrate. (2) A lithography process was conducted to define the pattern of nanochannels. The pattern was transferred to the substrate by wet etching. (3) The second oxidation was conducted, with the thickness of the oxide layer in the nanochannel region being d2. Due to diffusion of oxygen molecules through the oxide layer, the thickness of the oxide layer in the non-nanochannel region increased to d3. (4) All oxide layers on the substrate were completely etched to obtain a shallower nanochannel.19 A nanochannel with depth of h was obtained on the silicon substrate. (5) The third oxidation was conducted to realize electrical insulation. Due to the uniformity of the thermal oxidation process, the nanochannel depth remained h. (6) SGAB was conducted to form enclosed nanofluidic channels. During the thermal oxidation process, silicon was consumed in the oxidation reaction. For a silicon dioxide layer with thickness of d, the depth of silicon consumed was 0.46d. Based on the analysis of the silicon consumption, the depth of the nanochannel could be calculated as h ¼ 0:46d2 0:46ðd3 d1 Þ ¼ 0:46ðd1 þ d2 d3 Þ:
(1)
Because of the growth of oxide layer in the non-nanochannel region during the second oxidation (d3 > d1), the nanochannel depth obtained was smaller than 0.46d2. It can potentially be
FIG. 1. Fabrication process of the TTO-SGAB scheme.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 152.3.102.242 On: Tue, 11 Nov 2014 09:38:49
052106-3
W. Ouyang and W. Wang
Biomicrofluidics 8, 052106 (2014)
extremely small by tuning the thickness of the first oxide layer d1. In Eq. (1), d1 and d2 are design parameters. d3 is determined by d1 and d2, and can be precisely calculated by the DealGrove Model for silicon thermal oxidation. B. Nanochannel depth prediction
The Deal-Grove Model20 is widely used for modeling the silicon thermal oxidation process, according to which the relationship between oxide layer thickness Xox and oxidation time t is rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A2 A þ Bðt þ sÞ ; Xox ¼ 4 2
(2.1)
where A and B are parameters determined by the oxidation conditions (temperature, pressure, oxygen concentration, etc.), s is the effective time corresponding to the initial oxide layer thickness. The parameter s was introduced to offset the effect of oxidation rate enhancement at thin regime (
