10 nm. The large spring constant prevents the cantilever being pulled down by the
electrostatic force between the tip and the substrate which have opposite polarity.
Due to noncontact operation the conducting AFM tip gives out an electron field
emission. Typically, the current was in the range of a few tens of pA and it is the
emission current that was kept constant by the feedback loop which moves the Zpiezo positioner. The line width written by this method depends on the voltage
applied. For a fixed emission current a higher bias will ensure a larger line width
due to the geometric effect.
Combination of a pulsed bias and noncontact AFM has been found to improve
the control of the writing process [78]. This method reduces the tip–substrate interaction time and thus improves the reliability and lithographic resolution. The
frequency of oscillation and the field pulsing frequencies need to be adjusted to
create a definite phase relation between the two and it was found that the minimum line width is obtained when the applied field is on during the time the cantilever tip is furthest from the substrate. The process also needs adjustment of the
duty cycle.
Recent applications of AFM for oxidation include lithography of amorphous
chalcogenides such as GeSb 2 Te 4 films [79]. The particular choice of the material is
due to its application potential as an electronic phase memory material. Selective
oxide growth of Si using ultrathin SiO 2 or Si 3 N 4 as a mask for AFM-based lithography has been done [80]. An ultrathin PtaSi bilayer patterned by SPM oxidation
has been used as a lithography mask [81]. An H-passivated Si surface was patterned by SPM-enabled oxidation. This local oxide prevents reaction of a deposited
Pt film with the underlying Si, thus inhibiting silicide formation. The unreacted Pt
on the oxide is removed by selective etching and the pattern is then annealed to
transfer the pattern though silicide formation.
Judging by the present trend it is apparent that more innovative use of SPMbased oxidation of Si and other semiconductor materials will be applied in nanoelectronic device fabrication.
21.5
Use of Scanning Probe Microscopy in Dip Pen Nanolithography
One of the innovative applications of scanning probe microscopy for nanolithography is dip pen nanolithography (DPN). In this special technique the water meniscus formed between the tip and the substrate acts as a medium for molecular
transport. The technique depends on the key phenomenon that the molecule to be
deposited on the substrate (which is referred as the ‘‘ink’’) can be transported in a
controlled way from the tip (which is initially coated with the ink) to the substrate.
The molecule (the ink) to be deposited on the substrate should interact with the
substrate to form a chemical bond, leading to a stable structure [82].
The core concept that the water meniscus at the tip–substrate contact can indeed
be controlled and can be used as the molecular transport medium came from basic
investigations of water meniscus on lateral force microscopy (LFM) [83]. It was
21 Nanolithography and Nanomanipulation
712
electrostatic force between the tip and the substrate which have opposite polarity.
Due to noncontact operation the conducting AFM tip gives out an electron field
emission. Typically, the current was in the range of a few tens of pA and it is the
emission current that was kept constant by the feedback loop which moves the Zpiezo positioner. The line width written by this method depends on the voltage
applied. For a fixed emission current a higher bias will ensure a larger line width
due to the geometric effect.
Combination of a pulsed bias and noncontact AFM has been found to improve
the control of the writing process [78]. This method reduces the tip–substrate interaction time and thus improves the reliability and lithographic resolution. The
frequency of oscillation and the field pulsing frequencies need to be adjusted to
create a definite phase relation between the two and it was found that the minimum line width is obtained when the applied field is on during the time the cantilever tip is furthest from the substrate. The process also needs adjustment of the
duty cycle.
Recent applications of AFM for oxidation include lithography of amorphous
chalcogenides such as GeSb 2 Te 4 films [79]. The particular choice of the material is
due to its application potential as an electronic phase memory material. Selective
oxide growth of Si using ultrathin SiO 2 or Si 3 N 4 as a mask for AFM-based lithography has been done [80]. An ultrathin PtaSi bilayer patterned by SPM oxidation
has been used as a lithography mask [81]. An H-passivated Si surface was patterned by SPM-enabled oxidation. This local oxide prevents reaction of a deposited
Pt film with the underlying Si, thus inhibiting silicide formation. The unreacted Pt
on the oxide is removed by selective etching and the pattern is then annealed to
transfer the pattern though silicide formation.
Judging by the present trend it is apparent that more innovative use of SPMbased oxidation of Si and other semiconductor materials will be applied in nanoelectronic device fabrication.
21.5
Use of Scanning Probe Microscopy in Dip Pen Nanolithography
One of the innovative applications of scanning probe microscopy for nanolithography is dip pen nanolithography (DPN). In this special technique the water meniscus formed between the tip and the substrate acts as a medium for molecular
transport. The technique depends on the key phenomenon that the molecule to be
deposited on the substrate (which is referred as the ‘‘ink’’) can be transported in a
controlled way from the tip (which is initially coated with the ink) to the substrate.
The molecule (the ink) to be deposited on the substrate should interact with the
substrate to form a chemical bond, leading to a stable structure [82].
The core concept that the water meniscus at the tip–substrate contact can indeed
be controlled and can be used as the molecular transport medium came from basic
investigations of water meniscus on lateral force microscopy (LFM) [83]. It was
21 Nanolithography and Nanomanipulation
712