**1. Introduction**

50 Recent Advances in Nanofabrication Techniques and Applications

Yasaka, A. et al., 2008. Application of vector scanning in focused ion beam photomask repair

Ziegler, J.F., 2004. SRIM-2003. *Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms*, 219-220(1-4), pp.1027-1036.

*Structures*, 26(6), pp.2127-2130.

system. *Journal of Vacuum Science and Technology B: Microelectronics and Nanometer* 

Lithography is a top-down technology to produce sub-micrometer feature sizes for the fundamental research and widespread applications. The conventional optical lithography has been developed to not only make the tiny electronic devices to form the basic circuits in today's computer chips, but also fabricate micro/nano electromechanical systems (MEMS/NEMS). However, the conventional photolithographic techniques have now reached their ultimate spatial resolution because of both the light diffraction and the photoresist restrictions [1]. Alternative techniques for traditional photolithography have been proposed and developed to meet the requirements of a high spatial resolution of sub-100 nm as well as the capability to fabricate an arbitrary structure and achieve mass production [1-10]. Research teams around the word have been investigating alternative techniques including extreme ultraviolet lithography, electron and ion beam lithography, Xray lithography and atom lithography. Each technique has its own advantages and disadvantages, and no one yet knows which one will be the method of choice for the next generation lithography (NGL).

The arrays of micro- and nanostructures fabricated on silicon substrates have now attracted more attention for its interesting characteristics in applications such as photonics, electronics, optoelectronics and sensing [11-15]. The array properties can be tuned further by varying the geometry, the doping, the periodicity, and the size of the micro- and nanostructures [11,12]. These controllable and elaborate arrays are commonly fabricated by conventional optical lithography, X-ray lithography, electron-beam or ion-beam lithography [1,16-18]. Unfortunately, there are inherent limitations to pattern over large areas at nanoscale using these lithographical techniques due to the light diffraction, the long-range inter-particle interactions and the proximity effects. To overcome these limitations, it's necessary to develop new alternative techniques with a shorter wavelength and a thinner resist for traditional optical lithography.

Among the alternative techniques for traditional photolithography, atom lithography based on metastable atoms beam (MAB) and self-assembled monolayers (SAMs) has shown significant potential in fabricating arrays of micro- and nanostructures, which is a major goal in nanoscience and nanotechnology [10,19-29]. Metastable atom is one of the atom's electron is

Atom Lithography: Fabricating Arrays of Silicon Microstructures

nanostructures of silicon.

**2. Principle and procedure** 

**2.1 The general principle and procedure** 

in brief.

helium atom: He).

Using Self-Assembled Monolayer Resist and Metastable Helium Beam 53

passing through a stencil is directly transferred into the underlaying Si substrate by KOH etching, which is a single step etching process. Undoubtedly, to realize and understand this direct transfer process has opened a novel way in the practical application of the atom lithography in micro- and nanofabrication of silicon, especially in arrays of micro- and

In this context, we summarized our experimental results obtained during the near past years in detail in the field of atom lithography, where organosilane SAMs and He\*-MAB were used to pattern the surface of Si(111)/Si(110)/Si(100) wafer substrates without coating intermediate layer. The principle and procedure of atom lithography used in our experiments were introduced firstly. Then the experimental process and achievements were given. Finally, the problems and perspectives about this new technique were also discussed

Sheme 1. Shematic illustration of the principle and procedure for the frabrication of periodic arrays of Si(111)/Si(100) by He\*-MAB lithography(excited helium atom:He\*; ground-state

The general principle and process of atom lithography using He\*-MAB and organosilane SAMs to create the arrays of silicon micro- and nanostructures on a Si substrate were illustrated in Scheme 1. By introducing a hydrogenation process into the pretreatment of silicon substrates (as described in (I) and (II) in Scheme 1), fine organosilanes SAMs was successfully formed as a resist on silicon surface under a controlled argon gas atmosphere at elevated temperature. The chemical modification of non-oxidized silicon surfaces utilizing monolayers could be achieved by neutralizing the silicon surfaces with alkyl chains through

excited from ground state to metastable state. In metastable state, instead of immediately decay to ground state, the electron will stay for some time (long compare to usuall short lifetime-excited state) before it decays to its ground state. For example, the metastable helium atoms have natural lifetimes ≥ 20ms, which is much longer than its typical flight times. The energy stored in metastable noble gas atom can be used to create pattern in a SAM that act as resit. When a metastable noble gas atom strikes the SAM resist, the atom will release energy and it will go to ground state. The energy released will change the characteristic of the SAM resist in radius few angstroms from place where the atom strikes. The atom in ground state will not have any effect toward SAM resist because noble gas atoms are not chemically reactive. The locally change of characteristic of the SAM resist will enable us to do wet chemical etching. This new fabrication technique, which bridges the gap between the bottomup chemical self-assembly techniques and the modern top-down lithography, can overcome the intrinsic resolution limitations of traditional photolithographic techniques [10,19,20]. In principle, the atom lithography based on neutral MAB and SAM ideally meets the required conditions for sub-100nm fabrication [10]. MAB with a de Broglie wavelength of less than 0.1nm eliminates diffractive resolution limitations, and an ultrathin organic SAM resist with a thickness of 1~3nm ensures the sharpness of the edge profile within the resist. Moreover, the neutral metastable atoms are insensitive to the electric and magnetic fields, and the long-range inter-particle interactions are weak. Therefore, this novel method permits the direct and large area manufacturing of micro- and nanostructures on a silicon wafer, avoiding some inherent complications of electron-beam, ion-beam or photolithography. With these unique advantages, the atom lithography with neutral MAB and high-resolution SAM resist makes it possible to achieve nanolithography and provides a potential way in manufacturing structures at a largescale based on micro- and nanoscale features.

During the past fifteen years, considerable attention has also been given to the investigation of the mechanism of metastable atom lithography and microfabrication with different resolutions on various substrates, such as gold, silicon, silicon oxide, and mica [10,19-29]. Direct evidence of the emission of H+ and CHx+ from the SAM on the Au(111) surface under the irradiation of a metastable helium atom beam (He\*-MAB) at thermal energy supports the interpretation that the SAM is damaged through the interaction between the outermost surface of the SAM and the metastable atoms. This damage can cause a characteristic surface change from hydrophobic to hydrophilic or a molecular structural change from condensed to cross-linked, which consequentially affect the etching process in the fabrication of the micro- or nanostructure, resulting in the positive and negative pattern transfers, respectively. The typical micropatterning with sub-100 nm resolution on a silicon substrate covered with a SiO2 or Au layer has been achieved successfully [10,21,22,28,29]. Most of these studies are conducted through multistep etching processes using the cover layers as intermediate masks. For example, in the He\*-MAB lithography with dodecanethiol-SAM [10,21,22], a Au/Ti coating layer is used to create an intermediate mask on silicon substrate by the first etching process, and then, the intermediate mask pattern is transferred to silicon substrate by the second etching process. It is very interesting to directly transfer negative and positive patterns of SAM induced by He\*-MAB into a silicon substrate without metal/oxide coating layer to fabricate arrays of micro- and nanostructures.

Recently, the He\*-MAB lithography of Si with SAM, which formed directly on silicon substrate by direct covalent linkage instead of metal/oxide coating layers, has been accomplished by our research team [19,20]. The latent image formed in SAM by He\*-MAB passing through a stencil is directly transferred into the underlaying Si substrate by KOH etching, which is a single step etching process. Undoubtedly, to realize and understand this direct transfer process has opened a novel way in the practical application of the atom lithography in micro- and nanofabrication of silicon, especially in arrays of micro- and nanostructures of silicon.

In this context, we summarized our experimental results obtained during the near past years in detail in the field of atom lithography, where organosilane SAMs and He\*-MAB were used to pattern the surface of Si(111)/Si(110)/Si(100) wafer substrates without coating intermediate layer. The principle and procedure of atom lithography used in our experiments were introduced firstly. Then the experimental process and achievements were given. Finally, the problems and perspectives about this new technique were also discussed in brief.
