Microelectronic Engineering 87 (2010) 927–930

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Microelectronic Engineering

journal homepage: www.elsevier .com/locate /mee

22 nm silicon nanowire gas sensor fabricated by trilayer nanoimprintand wet etching

Chen Gao a, Shao-Ren Deng a, Jing Wan a, Bing-Rui Lu a,b, Ran Liu a, Ejaz Huq b, Xin-Ping Qu a,*, Yifang Chen b,*

a State Key Lab of ASIC and System, Department of Microelectronics, Fudan University, Shanghai 200433, Chinab Micro and Nanotechnology Centre, Rutherford Appleton Laboratory, STFC, Didcot, Oxon OX11 0QX, UK

a r t i c l e i n f o

Article history:Received 15 September 2009Received in revised form 27 November 2009Accepted 30 November 2009Available online 5 December 2009

Keywords:Trilayer nanoimprintWet etchingSilicon nanowire sensorGas detection

0167-9317/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.mee.2009.11.173

* Corresponding authors. Tel.: +86 21 65643561;Qu), tel.: +44(0)12355159; fax: +44(0)12356283 (Y. C

E-mail addresses: xpqu@fudan.edu.cn (X.-P. Qu), y

a b s t r a c t

In this work, we demonstrate the fabrication of silicon nanowires down to 22 nm wide using trilayernanoimprint lithography and wet etching. Using the same template prepared by E-beam lithography(EBL), nanowires with top width of 22 nm and 75 nm are fabricated on boron-doped top silicon layerof SOI substrate. The two samples are tested in 250 ppm NO2 ambient for gas detection. The 22 nm wideone shows a much higher relative sensitivity than the 75 nm wide one. The simulation which calculatesthe carrier density by solving Poisson equation was carried out and the results well explain the sensitivitydisparity between the two samples.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Semiconductor gas sensors were intensively investigated in re-cent decades. There are two major classes of them: semiconductornanowire/nanotube/nanorod sensors and porous silicon sensors.Their mechanisms of oxidizing/reducing-gas-detection are similar:the oxidizing gases (like NO2) adsorbed on the semiconductor sur-face can withdraw electrons from the semiconductor, make thesurface negatively charged, and cause hole accumulation or elec-tron depletion, while reducing gases (like NH3) can act oppositely.Thus the detection of such gases can be realized by measuring theelectrical characteristics of the semiconductor structures.

Despite their high relative sensitivities, most of the porous sili-con sensors [1] and metal oxide nanowire/nanotube/nanorod sen-sors [2] have various drawbacks such as lack of reliability anduniformity, high measuring temperature, and poor compatibilitywith VLSI silicon technology.

In recent years, semiconductor nanowire fabricated with top-down techniques has been applied as a promising candidate toconstruct devices for not only gas detection [3], but also chemicalsensing [4] and biological sensing [5]. The top-down process has itsunique advantages in fabricating the nanowire FET sensors [6,7]due to its compatibility with the VLSI silicon technology, highercontrollability, and better reproducibility.

ll rights reserved.

fax: +86 21 65643768 (X.-P.hen)..chen@r1.ac.uk (Y. Chen).

Nanoimprint lithography, as one of the top-down methods, at-tracts much research interest, because of its high resolution, highthroughput, and low cost. However, its application to nanowireFET sensor fabrication was still rarely reported.

Traditional top-down fabrication process usually includes areactive ion etching (RIE) step to transfer the nanowire patternfrom the resist to the substrate [5,6]. However, the surface chargeinduced by RIE may deteriorate the transport properties and causeunexpected and sometimes uncontrollable influence on the de-vices performance. The application of wet etching for the substitu-tion of RIE process can effectively avoid the above problem [7].

In our previous work [8], by using nanoimprint lithography andRIE process, the nanowire sensors with linewidth of 135 nm and75 nm were fabricated. In this work, we elaborate the silicon nano-wire arrays fabrication with trilayer nanoimprint and wet etching.Samples with top widths of 22 nm and 75 nm were fabricated onthe SOI substrate. The samples were electrically measured in NO2

for their properties in gas detection.

2. Experiment details

The silicon templates used in our nanoimprint process werefabricated by EBL and a following nickel deposition and lift-off pro-cess, which details were given in our previous work [8]. The nano-wire array was 2 lm in period and 70 nm in line width.

The original SOI substrate had an 80 nm-thick (1 0 0) top siliconlayer, with a boron doping concentration of 8 � 1017 cm�3. For the

Fig. 1. Schematic diagram of the trilayer nanoimprint and wet etching.

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nanowire sample with 22 nm top-width, the top silicon was chem-ically etched to 38 nm-thick.

Trilayer nanoimprint lithography was applied to transfer thenanowire patterns from the template onto the resist on the SOIsubstrate, as illustrated in Fig. 1. The advantages of the novel trilay-er technique include low imprint temperature and pressure, highcontrollability of critical dimensions, a wide choice of resists, andeasy lift-off of the relatively thick metal layer. The details of the tri-layer nanoimprint process were illustrated in our previous work[9]. First, the trilayer of SU8/SiO2/PMMA was deposited onto theSOI substrate. The imprint was carried out at 100 �C with an im-print pressure of 20 MPa. Then a subsequent RIE process was ap-plied to form patterns and create undercut in the PMMA layer.After that, a 20 nm thick Cr layer was deposited onto the structureby ion beam sputtering, following a lift-off process in acetone.

To form silicon nanowire, tetramethylammonium hydroxide(TMAH) solution was used to carry out the anisotropic wet etchingon the Si layer. Since TMAH etches Si (1 1 1) planes about 100times more slowly than it etches the other planes, such anisotropicwet etching forms silicon nanowire with trapezoidal cross-section,with the patterned Cr layer as the etching mask.

The wet etching was carried out in 60 �C with TMAH solution of8%. The sample with 80 nm-thick top silicon layer was etched forabout 1 min, and then the 75 nm-top-wide silicon nanowire wasformed. The sample with 38 nm thick top silicon layer was etched

Fig. 2. SEM images of the silicon nanowires wi

for about 8 min, and the nanowire top width was reduced to 22 nm(Fig. 2). Aluminum contact pads were made on the sample throughphotolithography, and an annealing process helped the aluminumand p-type silicon form reliable ohmic contact. For both samples,the nanowire arrays consist of about 50 wires, which are about20 lm long.

3. Results and discussion

3.1. Experiment result

The testing gas was 250 ppm NO2 (diluted by N2). As NO2 canadsorb onto the p-type Si nanowire surface, it will strongly with-draw electrons from silicon, thus causing holes accumulation,and finally the nanowire conductance will be increased.

In the electrical measurement, a constant DC voltage of 2 V wasapplied across the aluminum electrodes. After flowing N2 into thetesting chamber for enough time, the current through the nano-wires became stable. Then 250 ppm NO2 was filled into the cham-ber, and it caused dramatic increase in the current, as shown inFig. 3.

The relative sensitivity (S) of a gas sensor is defined as:S = (Ig � I0)/I0, where I0 and Ig are the currents of the samples mea-sured in pure N2 and 250 ppm NO2 respectively. The conductance

th top widths of (a) 22 nm and (b) 75 nm.

Fig. 3. The current responses of (a) 22 nm wide and (b) 75 nm wide nanowires to 250 ppm NO2.

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of the nanowires in our cases can be roughly considered as propor-tional to the average hole density within the whole cross-sectionarea of the nanowires. Then the relative sensitivity S = (Ig � I0)/I0

can be considered as (pg � p0)/p0, as p0 and pg are the average holedensity before and after exposing the sample to NO2.

The initial carrier density (p0) which is equal to the doping con-centration for the sample made with wet etching is same for both22 nm and 75 nm nanowire sensors. Therefore, approximately, I0

(or p0) is proportional to the area of the cross-section of the nano-wire, and (Ig � I0) (or pg � p0) is proportional to the area of the ex-posed surface of the nanowire. So an increased surface-to-volumeratio will cause a corresponding increment in relative sensitivity.Table 1 lists the measured resistance and relative sensitivity forthe two devices extracted from Fig. 3. The 22 nm sample shows amuch larger relative sensitivity (2.1 times of that of the 75 nmone), as its surface-to-volume ratio is 2.4 times of that of the75 nm one. The measurement results are in accordance with theprediction very well.

The relative sensitivity in this work is much lower than that inour previous work [8] and others’ work in which the sensors weremade with RIE process. In our previous work, a 75 nm-wide nano-

Table 1The resistance and relative sensitivity for NO2 for nanowire sensors with 75 nm and22 nm width.

Nanowire width (nm) Resistance (ohm) Relative sensitivity for NO2 (%)

75 69.9 � 103 7.022 366 � 103 14.7

Fig. 4. Simulation result of the hole density distributions over the nanowire cross-sectheight. The surface charge density was set at �2.3 � 1010 cm�2 in both (a) and (b).

wires sensor (also 8 � 1017 cm�3 boron-doped, 80 nm high, about20 lm long) made with RIE showed a relative sensitivity of61.8%, which is 8.8 times of that of our 75 nm-wide sample madewith wet etching. In Ref. [10], silicon nanowires which were18.5 nm-wide and 32 nm-thick (about 1 � 1018 cm�3 boron-doped) were patterned using a special dry-transfer technique andformed with RIE. Their sensor demonstrated a sensitivity of around3000% after 1.25 min of exposure to 20 ppm NO2.

The relative sensitivity disparity between this work using wetetching and our previous work using RIE is analyzed and explainedas follow. Since RIE process usually induces positive charge withinthe surface region of the silicon nanowire, hole depletion can becaused in such region, and then the initial carrier density p0 is re-duced by orders of magnitude. Meanwhile, to achieve charge-neu-trality, a certain density of negative surface charge induced by NO2

tends to attract the same amount of positive charge in the Si nano-wire, so the numerator (pg � p0) is not likely to change as much asp0 does, Therefore, the relative sensitivity (pg � p0)/p0 can be in-creased because of the small p0 resulted from the RIE process. An-other possible reason for the high sensitivity of RIE sample is that,the surface roughness unintentionally caused by the RIE processmay increase the surface area of the Si nanowire and provide addi-tional binding sites for NO2, while the anisotropic wet etching al-ways results in smooth surface of the Si nanowire.

However, since the RIE process is lack of controllability of sur-face charge density and surface roughness, we found that the ini-tial carrier density and the sensitivity can be very different fromits design and can vary from device to device. So the applicationof wet etching effectively avoids the above problems. In future re-search, to increase the sensitivity of gas detection, we still need to

ion of (a) 22 nm top-width and 38 nm height and (b) 75 nm top-width and 80 nm

Fig. 5. Simulation result of conductance of (a) 22 nm wide and (b) 75 nm wide nanowires. Without surface charge, the conductance of the 75 nm wide nanowire is 5.66 timesof that of 22 nm one. With a surface charge density of �2.3 � 1010 cm�2, the conductance increases by 15% and 6.5% for the 22 nm wide and 75 nm wide nanowires,respectively.

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control the geometry, surface morphology and doping concentra-tion of the Si nanowire.

3.2. Investigation of the sensitivity disparity by simulation

The hole density distribution over the nanowire cross-sectionwas calculated by solving Poisson equation using successiveover-relaxation method. The effect of NO2 adsorption is consideredas an increase of negative surface charge, and will cause hole accu-mulation accordingly. As depicted in Fig. 4, the narrower nanowirecan be more strongly affected by the same surface charge density,for its surface-to-volume ratio is higher. The nanowire conduc-tance can be roughly considered as proportional to the integral ofthe hole density over the entire nanowire cross-section area. It isassumed that the 22 nm wire and the 75 nm one obtain the samesurface charge density in NO2. The original surface charge of thenanowire is not taken into consideration because it would affectthe conductance of the nanowire whether exposed to NO2 or notin the same extent.

From the simulation result in Fig. 5, it is clear that, with a cer-tain surface charge density, which increases the conductance of22 nm-wide nanowire by 15%, the conductance of the 75 nm onewill increase by about 6.5% accordingly. So the simulation resultconforms to the experimental result well, and explains the relativesensitivity disparity between the two samples.

4. Conclusion

In this work, we demonstrate the fabrication of silicon nano-wires down to 22 nm wide on top and 38 nm high by a combina-tion of trilayer nanoimprint and wet etching. With the sametemplate made by EBL, different dimensions of the nanowires are

obtained using different etching time and different SOI substrates.The nanowire samples were tested in NO2 for gas detection. Thesample of 22 nm wide nanowires show a significant sensitivityenhancement over the 75 nm one. The simulation based on succes-sive over-relaxation method is carried out and the results well ex-plain the sensitivity disparity between the two samples.

Acknowledgements

The authors would like to thank financial support from Shang-hai Municipal of Science and Technology (08QH14002), NationalBasic Research Program of China (2006CB302703), and the SeedFunds for Key Science and Technology Innovation Projects ofMOE(708020). This work is also supported by the ‘‘985” Micro/Nanoelectronics Science and Technology Platform.

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