ABSTRACT Spacer technology has been developed to fabricate nanostructures for NEMS application. It provides a parallel nanofabrication method with double or quadplex device density at a certain lithography node. By controlling the deposited film thickness, the feature size of the SiO 2 spacer hard mask is reduced down to 35 nm. After the spacer pattern is transferred to Si, a precise thermal oxidation is performed to improve the profile and reduce the plasma damage. Finally, sublimation or HF vapor phase etching is introduced to release the nanostructures according to different structure dimensions. As a result, with better surface morphology, suspended Si nanobeams with a width of 20 nm are obtained. Actuated by mechanical vibration and electrostatic forces, vibrations of the obtained cantilever beams and fixed-fixed beams are observed in SEM. In addition, a metallic nano-nozzle with a diameter of 140 nm is established by electroless plating around the suspended Si nano-beam served as a mold. As a development of the spacer technology, nano-needle array is demonstrated at the cross points of crossed SiO 2 spacers by anisotropic etching. The diameters of the hybridized nano-needles are 300 nm so far and can be further reduced by smaller spacer dimension

Guizhen Yan

Jun Xu

Ling Xia

Wengang Wu

Xiang Han

Yilong Hao

CiteSeerX

DownloFABRICATION OF NANO-ELECTROMECHANICAL STRUCTURES DOWN TO 20 NM 
BY SPACER TECHNOLOGY 
 
 
Xiang Han1 Ling Xia1 Wengang Wu1 Guizhen Yan1 
 
 
Jun Xu2 Yilong Hao1 
 
 
1National Key Laboratory of Micro/Nano Fabrication Technology, Institute of Microelectronics, 
Peking University, Beijing 100871, China 
 
 
2Electron Microscopy Laboratory, Peking University, Beijing 100871, China 
 
Proceedings of MNC2007 
MicroNanoChina07 
January 10-13, 2007, Sanya, Hainan, China 
 
MNC2007-21174 
ABSTRACT 
Spacer technology has been developed to fabricate nano-
structures for NEMS application. It provides a parallel nano-
fabrication method with double or quadplex device density at a 
certain lithography node. By controlling the deposited film 
thickness, the feature size of the SiO2 spacer hard mask is 
reduced down to 35 nm. After the spacer pattern is transferred 
to Si, a precise thermal oxidation is performed to improve the 
profile and reduce the plasma damage. Finally, sublimation or 
HF vapor phase etching is introduced to release the nano-
structures according to different structure dimensions. As a 
result, with better surface morphology, suspended Si nano-
beams with a width of 20 nm are obtained. Actuated by 
mechanical vibration and electrostatic forces, vibrations of the 
obtained cantilever beams and fixed-fixed beams are observed 
in SEM. In addition, a metallic nano-nozzle with a diameter of 
140 nm is established by electroless plating around the 
suspended Si nano-beam served as a mold. 
As a development of the spacer technology, nano-needle 
array is demonstrated at the cross points of crossed SiO2 
spacers by anisotropic etching. The diameters of the 
hybridized nano-needles are 300 nm so far and can be further 
reduced by smaller spacer dimension. 
 
Keywords: NEMS, spacer technology, suspended nano-beam, 
nano-nozzle, nano-needle array  
1
aded From: https://proceedings.asmedigitalcollection.asme.org on 06/28/2019 Terms of U1. INTRODUCTION 
  
Nano-electromechanical systems (NEMS), characterized 
by dimensions in nanometers, will surely take a revolution in 
mass detection, medical diagnostics, data storages and 
mechanical miniaturization [1-4]. It also provides a new 
platform to take further research in fundamental physics [5-7]. 
NEMS devices can be fabricated by either top-down or bottom-
up method. Conventional top-down method is based on 
different kinds of “lithography” technologies such as extreme 
ultraviolet and electron beam lithography. They are always 
involved with either expensive complex commercial apparatus 
or low fabrication efficiency. Bottom-up method, usually in a 
chemical synthesis way [8], however, still has great challenges 
on controllable construction. Both of the two methods are 
promoting the development of NEMS nowadays. 
Spacer technology has been demonstrated in integrated 
circuit industry for decades. Defined by the deposited film 
thickness, the minimum feature size of the spacer has been 
demonstrated to be 20 nm, or even further smaller [9]. In 
addition, it also provides a double or quadplex device density 
for a given lithography node [10]. Based on this spacer 
technology, Choi et al. has obtained Si nanowire array [9] and 
applied it to produce novel electronic devices with sub-40 nm 
gate length recently [10, 11]. 
In this paper a parallel fabrication method for NEMS 
manufacture is proposed by using spacer technology. SiO2 
spacer hard mask of 35 nm is fabricated and Si structure of 20 Copyright © 2007 by ASME
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Donm is obtained combined with a precise thermal oxidization. 
Sublimation release or HF vapor phase etching is introduced to 
guarantee high yield of suspended nano-structures. Vibrations 
of the obtained cantilever beams and fixed-fixed beams are 
then observed in scanning electron microscope (SEM), under 
external actuations. By developing the spacer technology, some 
other nano-electromechanical structures such as nano-nozzle 
and nano-needle array are also demonstrated. 
 
Fig. 1. Schematics of Si nano-structure fabrication process 
based on spacer technology 
2. SPACER TECHNOLOGY FOR SUSPENDED NANO-
BEAM 
2-1. Spacer Technology 
Fig. 1 shows the fabrication process of Si nano-structures 
by spacer technology. After the film deposition (Fig. 1(a)), 
poly-Si is patterned by photolithography (Fig. 1(b)). Then a 
LPCVD SiO2 is conformally deposited at the wafer surface 
(Fig. 1(c)) and anisotropically etched by RIE (Fig. 1(d)). We 
remove the exposed poly-Si (Fig. 1(e)) and etch the SiO2, 
leaving the spacers only (Fig. 1(f)). Then the spacer hard mask 
pattern is transferred to Si combined with traditional photo-
resist masks by anisotropic ICP etching (Fig. 1(g)). Finally 
release the structures by etching SiO2 (Fig. 1(h)). 
Si SiO2 Poly-Si 
(a) 
(b) 
(c) 
(d) 
(e) 
(f) 
(g) 
(h)  
wnloaded From: https://proceedings.asmedigitalcollection.asme.org on 06/28/2019 Terms o 
Fig. 2. SEM image of spacer corresponding to Fig. 1 (d) 
 
Fig. 3. SEM images of the silicon dioxide spacer, with the 
width of 35 nm. 
 
 
Fig. 4. Different nano-structures after release. (a) released 
by sublimation method; (b) released by HF vapor phase 
etching method. 
(a)
35nm 
Poly-Si
Spacer 
(b)2 Copyright © 2007 by ASME
f Use: http://www.asme.org/about-asme/terms-of-use
D
As illustrated above, the feature size of the spacer hard 
mask is defined by the deposited film thickness in Fig. 1 (c). It 
can be precisely controlled at nanometer scale by traditional 
microelectronics process. So it is convenient to obtain nano-
structures at the whole wafer. Meanwhile, for a given 
lithography node, it can provide a double or even quadplex 
device density, with feature size beyond the lithography limit 
simultaneously. 
Fig. 2 shows the SEM image of the SiO2 spacer before 
remove the poly-Si, corresponding to the Fig. 1 (d). By 
controlling the deposited film thickness, a spacer with 
dimension of 35 nm is obtained in Fig. 3, which is 
corresponding to Fig. 1 (f). Combined with these hard masks 
and photo-resist masks, Si structures can be patterned and 
etched then (see Fig. 4). 
Usually there are some variations in the lateral surfaces of 
Si nano-structures fabricated by spacer technology. From Fig. 4 
(b) we can see variations about 50 nm for the Si nano-beam. 
These variations will significantly limit the applications and the 
extent to reduce the structure width. Thus a precise thermal 
oxidization is performed before final release. This will not only 
improve the surface profile, but also reduce the dimension and 
plasma damage. As a result, with better surface morphology, 
the feature size is finally reduced down to 20 nm (see Fig. 5). 
 
 
Fig. 5. 20 nm Si nano-beam after precise thermal oxidation 
and HF vapor phase release. 
2-2. Sacrificial Layer Release 
Two sacrificial release methods for different structure 
dimensions are introduced to obtain suspended nano-structures 
in the last step. For structures with width above 40 nm, we use 
HF solution to wetly etch the surrounding SiO2. Then 
cyclohexane is used to replace the HF solution for sublimation 
release. For structures with width below 40 nm, HF vapor 
phase etching is proposed to prevent the fluid-induced fracture 
as well as surface-tension-induced collapse during the final 
release step. 
The results of the two methods mentioned above are 
shown in Fig. 4 (corresponding to Fig. 1(h)). The sublimation 
method is well established by different kinds of structures up to 
16 µm-long and above 40 nm-wide. By using the dry release  
3
ownloaded From: https://proceedings.asmedigitalcollection.asme.org on 06/28/2019 Terms of Umethod, 20 nm-wide Si suspended beam is released 
successfully (see Fig. 5). As a proof, vibrations of the 
cantilever beams and fixed-fixed beams are observed in SEM, 
actuated by mechanical vibration and electrostatic forces, 
respectively (see Fig. 6). The two release methods applied 
according to the structure dimensions guarantee high yield of 
suspended nano-structures in the release step. 
 
 
Fig. 6. Vibrations of (a) cantilever beams and (b) fixed-fixed 
beams are observed in SEM under external actuations. 
 
Fig. 7. Metal nano-nozzle fabricated by spacer technology 
and electroless copper plating. 
2-3. Other Nano-electromechanical Structures 
Based on the spacer technology, some other nano-
electromechanical structures can also be parallel fabricated. 
Used as a mold, the Si nano-beams can be conformally 
encapsulated by other materials and then selectively etched to 
form nano-nozzle and nano-channel structures. 
(a) 
(b)Copyright © 2007 by ASME
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DoHere we use electroless copper plating to encapsulate the 
Si after the structures are suspended. Copper will only be 
plated on Si nano-beams while the SiO2 is immune [12]. Then 
we use FIB to cut the fixed-fixed beam to expose the Si and 
selective remove the Si core by KOH etching. Finally a nano-
nozzle with a diameter of 140 nm is successfully obtained (see 
Fig. 7). 
 
 
Fig. 8. (a) SiO2 nano-needle array by crossed spacers, with 
quadplex density for the given lithography node. (b) 
Hybridized nano-needle with a diameter of 300 nm. 
3. DEVELOPMENT OF THE SPACER TECHNOLOGY 
By developing the spacer technology, two layers of SiO2 
spacers are used to fabricate nano-needle array. At the cross 
points, the thickness of the spacer is doubled. After anisotropic 
dry etching, only SiO2 nano-needles are left at the cross points, 
with the height of a single spacer. The SEM image of SiO2 
nano-needle array is shown in Fig. 8 (a), with quadplex density 
compared with lithography density. In Fig. 8 (a), the second 
spacer hasn’t finished etching while the first spacer has been 
completely removed. The needles are generally with a tip 
radius of 37 nm. This top-down method also enables good 
controllability of the needle dimensions and locations beyond 
the photolithography limit. 
Then we take the SiO2 needle as a mask to etch the Si 
below by ICP. Hybridized nano-needles with higher aspect 
ratio are then obtained, with a diameter of 300 nm, as shown in 
Fig. 8 (b). As SiO2 is hydrophilic and Si is hydrophobic, this 
Feature 
size 
Spacer1
Spacer2 
SiO2 
Si 
(a) 
(b)  
4
wnloaded From: https://proceedings.asmedigitalcollection.asme.org on 06/28/2019 Terms of Unano-needle array is sure to have great prospect in bio-medical 
field. 
4. CONCLUSIONS 
The application of spacer technology for NEMS structure 
fabrication has been successfully demonstrated. Combined with 
traditional microelectronics process, the feature size of the 
spacer is reduced to 35 nm, which is far beyond the limitation 
of the lithography we used. The two release methods we 
proposed greatly improve the yield of suspended nano-beams. 
By using a precise thermal oxidization, Si nano-structures with 
good profile and smaller dimension are fabricated. As a result, 
we obtain suspended nano-beam as small as 20 nm 
successfully. We also observe the vibrations of the cantilever 
beam and fixed-fixed beam under SEM, which can be used in 
some device applications. 
Other nano-electromechanical structures are also 
demonstrated based on the spacer technology. Both a nano-
nozzle of 140 nm diameter and nano-needle of 37 nm tip radius 
are established, which have potential applications in fluidic and 
bio-medical field. 
ACKNOWLEDGMENTS 
This work is partially supported by the National Natural 
Science Foundation of China under Grant No. 90307007. The 
authors would like to thank the entire staff of the National Key 
Laboratory of Micro/Nano Fabrication Technology, Peking 
University, for their enthusiastic support. 
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FABRICATION OF NANO-ELECTROMECHANICAL STRUCTURES DOWN TO 20 NM BY SPACER TECHNOLOGY

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FABRICATION OF NANO-ELECTROMECHANICAL STRUCTURES DOWN TO 20 NM BY SPACER TECHNOLOGY

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