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Microelectronic Engineering 85 (2008) 1311–1313
A new way to integrate solid state nanopores fortranslocation experiments
E.M. Huisman a, A.-L. Biance a,b,c,*, A. Madouri a, G. Patriarche a, E. Bourhis a,G. Oukhaled b, L. Auvray b, J. Gierak a
a LPN/CNRS, Route de Nozay, F-91460 Marcoussis, Franceb MPI, Universite d’Evry Val d’Essone, Bd. Franc�ois Mitterrand, F-91025 Evry, France
c LPMDI, UMR8108, Universite deMarne-la-Vallee, 5 Bd. Descartes, F-77450 Champs sur Marne, France
Received 1 October 2007; received in revised form 17 January 2008; accepted 21 January 2008Available online 26 February 2008
Abstract
The integration of the nanopore drilled in a nanometric thin and therefore fragile membrane in a macroscopic set-up necessitates amultiscale approach. An integration technique to select and support such a nanopore is described, using a Pyrex device with a micropore.Therefore, we developed a method to fabricate micrometer sized apertures in Pyrex wafers relying on a specific control of HF etching.The integration of FIB nanodrilled membrane in this Pyrex set-up is here discussed.� 2008 Elsevier B.V. All rights reserved.
Keywords: Nanopores; HF etching; Pyrex; Micro-nanofluidic; Bioapplications; FIB
1. Introduction
The detection of the translocation of biological mole-cules like proteins or DNA in a pore of a few nanometresby an electrical method (as drawn in Fig. 4) has beenproved to be an efficient way to understand more deeplysome biological mechanisms [1], such as DNA dezipping[2] or protein denaturation [3]. The nanopore fabricationcan be achieved by the insertion of a protein channel in alipid bilayer or by a nanopatterning technique [4,5]. Inthe latter case, the integration of the nanopore drilled ina thin (down to 9 nm) and therefore fragile membranenecessitates a multiscale approach from a macrometricdevice to the nanopore. We will discuss here this integra-tion process.
0167-9317/$ - see front matter � 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.mee.2008.01.106
* Corresponding author. Address: LPN/CNRS, Route de Nozay, F-91460 Marcoussis, France.
E-mail address: [email protected] (A.-L. Biance).
1.1. Nanopore fabrication and integration requirements
Our nanopore fabrication technique consists in drillingby FIB of an array of 5�5 pores separated by 25 lm withan increasing dose on a SiC membrane by FIB, asdescribed in details in Ref. [6]. Surface marks (achievedby a low dose FIB scanning on the membrane surfacenear the pore) allow us to trace the pores for character-ization in TEM, SEM and optical microscopy. On thecentre of each mark, the membrane is drilled with a porewith variable size (Fig. 1). We can thus choose the opti-mal pore. Then, we can select this pore by aligning andsticking a substrate of Pyrex with a 50 lm aperture atthe place of the selected hole, thus closing the others(Fig. 1).
Several requirements are motivating the fabrication of aspecific integration set-up: the fragility of the membrane inrespect to manipulation, the re-usability and the bio-com-patibility of the set-up, the variety of the pore we want todefine on a specific membrane and the possibility of choos-ing and isolating the optimal one.
Fig. 1. Optical photography of the alignment marks on membrane. Thecrosses are separated by 25 nm. Insert: TEM image of a 3 nm pore.
1312 E.M. Huisman et al. / Microelectronic Engineering 85 (2008) 1311–1313
The choice of Pyrex appears to be the best, for its elec-tronic, mechanical and chemical properties. Indeed, Pyrexis a good electrical insulator, and it has a large young mod-ulus (especially compared to PDMS mostly used for micro-fluidic integration), which mechanically isolate andtherefore preserve the membrane. A number of techniqueshave been developed in recent years to create micropores inPyrex, for example, the drilling of micropores by a mechan-ical tip [7], etching a trace of a heavy ion through Pyrex [8]or LASER sewing [9]. We have investigated a standardclean room available method, the use of defining lm-sizedhole in a 250 lm thick Pyrex substrate using selective chem-ical etching, by coating the Pyrex wafer with a protectionlayer resistant to HF, drilled by a micrometric hole.
2. HF etching of micropores
The Pyrex is immersed in a solution of 40% HF in a tef-lon container. To avoid the formation of AlF3 � Al3+ whichwill diminish the etching productivity and cause roughetched surfaces, HCl is added to the HF solution at a ratioof 1:10 [10].
To pattern Pyrex, a protective mask is applied to coverthe parts of the Pyrex that must be preserved. After testingseveral materials (Cr/Au mono and multilayers [11]) wehave selected a bilayer of PECVD deposited amorphousSi (400 nm + 700 nm) [12]. The Si mask is patterned byUV lithography of AZ512 resist and the pattern is trans-ferred in the Si layer by RIE. Next, the wafer is etched inthe HF/HCl solution. Due to isotropic etching characteris-tics of HF-etching, it is practically impossible to etch holesof 60 lm in Pyrex with a thickness of 250 lm directly.
To achieve the desired aspect ratio, we first etch a pla-teau at one side of the Pyrex, thus creating an area wherethe thickness of the Pyrex is only 40 lm (Fig. 2). By manual
Fig. 2. Scheme of a membrane (top-red) sticked and aligned on the Pyrexdevice (bottom) drilled by the micropore. The yellow regions below themembrane and on the side of the membrane correspond to the waximpregnation region necessary for an efficient sealing. (For interpretationof the references to color in this figure legend, the reader is referred to theweb version of this article.)
alignment, an aperture of 50 lm can be patterned in theprotection layer at the backside of the Plateau region. Thenthe backside HF etching of the Plateau region is performedtill the hole is created. In the end, the Si protection layer isremoved in a TMAH solution at 80 �C.
3. Integration and results
After the fabrication of narrow holes in ultra thin SiCmembranes [6], several routes were explored to allow theirreliable and efficient integration in an electrophoresis setup.The key step remains the sealing of the Pyrex hole with theSiC patterned membrane, as described in Fig. 2. Two mainrequirements must be taken into account: the sealing mustbe very efficient but still smooth to avoid membrane cracks;the alignment must be done on the selected nanopore(Fig. 1).
Several methods were tested:
– A resist layer was spin coated on the Pyrex template inorder to act as a sealing media. The main problemencountered here occurred with the resist filling theholes.
– An anodic bonding process was tested due to the intrin-sic qualities such as robustness and ease of use. Unfortu-nately even if the positioning of the membrane withrespect to the Pyrex aperture is relatively easy, the bond-ing process was found to be not compatible with our SiCmembranes of thicknesses ranging between 9 and100 nm. A possible reason may be found in the differ-ence between the thermal expansion coefficients of SiCand Pyrex, leading to membrane rupture after the bond-ing (Fig. 3).
– A wax glue was tested on a sample heated at 50 �C. ThePyrex set-up coated by the wax is smoothly heated, andthe SiC auto-supported membrane is manually alignedunder a microscope (Fig. 3). By a fastidious reheatingand washing, the membrane and the Pyrex set-up canbe used again separately.
We used then the integrated solid-state nanopore to sep-arate two tanks filled with a conductive solution (Fig. 4). Acurrent of good order of magnitude was measured but itwas drifting with time for unresolved reasons (3 GX resis-
Fig. 3. Right: Pyrex hole bonded to a SiC membranes: many cracksappeared resulting in a bad sealing. Left: Image of the Pyrex hole gluedwith the wax on a SiC membrane. The alignment cross is visible inside thePyrex pore and the membrane intact.
Fig. 4. Macroscopic electrical set-up (Ag/AgCl electrodes, Axon patch-clamp amplifier) to detect molecule translocation (schematic representa-tion and picture).
E.M. Huisman et al. / Microelectronic Engineering 85 (2008) 1311–1313 1313
tance for an 8 nm pore drilled in a 50 nm thick membraneplunged in a 1 M KCl solution, measured with an AXONpatch-clamp amplifier in a Faraday cage).
4. Conclusions
The use of micropores is a promising way to select andintegrate nanopores in SiC membranes for translocationexperiments. The choice of Pyrex smoothly glued withwax on the fragile SiC membrane appeared efficient.
Moreover, HF etching is proved to be a good alternativeto expensive experimental techniques to etch micropores inPyrex with a high aspect-ratio.
Besides, these micropores in Pyrex appear to be astraightforward and efficient alternative as a supporting
material for lipid bilayers which could integrate biologicalnanopore, another tool for translocation experiments.
Acknowledgement
The authors would like to thanks the ANR, the CNRS,the region Ile-de-France, and the reseau franco-neerlandaisfor financial support.
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