semiconductor substrates. A conducting tip AFM (Si 3 N 4 tip metallized by 30 nm
Ti) was used to make 1–2 nm high and 10–30 nm wide oxide structures on Hpassivated n-type Si (100) substrates [70]. The writing was done by contact mode
AFM using a contact force of around 100 nN with a cantilever with a force constant
of around 0.4 N m
À1 . The voltage bias used for the writing was in the range 1.5–
2.5 eV. The threshold time for oxide formation was found to be around 10
À4 to
10
À5 s. The pattern was transferred to the Si substrate below using wet etching
with KOH. The oxide grown by the SPM method is stoichiometric SiO 2 , as has
been shown by recent spectroscopic studies on such oxides [71].
UHV-STM has been used to form oxidation-based nanopatterns on an Hpassivated Si (100) surface showing (2 Â 1) structure [72]. Line widths as small as
1.5 nm could be produced. Although UHV-STM offers a clean way of fabricating
much smaller nanostructures of a few nm dimension as opposed to ten or few tens
of nanometer sized features in SPM operating in the air most applications of SPM
for nanofabrication were carried out in ambient air. This is presumably due to the
ease of operation as well as the cost factor.
A nanometer scale pattern on n-type Si (100) was generated by field induced oxidation using Si cantilever tips which are made metallic by heavy p-type (10
26 m
À3 )
doping [73]. It was found that the oxidation needs a threshold bias (@10 V) before
oxidation begins. The structures formed were oxides (1 nm high) which could be
etched by leaving grooves on the substrate. (Growth of the oxide layer in Si consumes a layer of around 0.5 h inside the Si where h is the height of the oxide layer
grown. As a result it leaves a groove on removal of the protruding oxide formed by
the tip-induced oxidation.)
In the models proposed for oxidation in the above experiments there is no role of
the absorbed moisture layer that can act as the ‘‘electrolyte’’. However, subsequent
models show that the oxide formation can be explained using the moisture layer
[74]. The process is similar to liquid electrolyte based anodization. The oxidation
process is sensitive to the humidity. The diffusion limited electric field E at the
surface determines the terminating thickness of the oxide grown. The model is
similar to what has been proposed for the field-induced oxidation process proposed
by Cabrera and Mott more than 50 years ago [61]. An electron tunnels from the
conducting AFM tip (which is negatively biased with respect to the substrate) and
creates negatively charged oxygen ions. The oxide growth stops when the electric
field is no longer sufficient for the ionic diffusion.
Amorphous Si (a-Si) has been oxidized like crystalline Si in AFM-based lithography and has been used as a resist [75]. a-Si can be H passivated like its single
crystalline counterpart. This makes SPM-based lithography possible in a-Si.
Application of non-contact mode AFM in anodic oxidation based lithography is
an important step forward in this field [76]. Noncontact AFM has been used to
create feature sizes <30 nm on a 65 nm resist (polymer resist) and was transferred through reactive ion etching (RIE) to the Si substrate (P-doped (100)) on
which the resist is coated [77]. A scan speed of 10 mm s
À1 has been achieved using
a heavily B-doped Si cantilever with a large spring constant (@340 N m
À1 ) and
mechanical resonance frequency ( f r A 1 MHz) and with a tip of radius of curvature
21.4 Scanning Probe Based Anodic Oxidation as a Tool for the Fabrication of Nanostructures 711
Ti) was used to make 1–2 nm high and 10–30 nm wide oxide structures on Hpassivated n-type Si (100) substrates [70]. The writing was done by contact mode
AFM using a contact force of around 100 nN with a cantilever with a force constant
of around 0.4 N m
À1 . The voltage bias used for the writing was in the range 1.5–
2.5 eV. The threshold time for oxide formation was found to be around 10
À4 to
10
À5 s. The pattern was transferred to the Si substrate below using wet etching
with KOH. The oxide grown by the SPM method is stoichiometric SiO 2 , as has
been shown by recent spectroscopic studies on such oxides [71].
UHV-STM has been used to form oxidation-based nanopatterns on an Hpassivated Si (100) surface showing (2 Â 1) structure [72]. Line widths as small as
1.5 nm could be produced. Although UHV-STM offers a clean way of fabricating
much smaller nanostructures of a few nm dimension as opposed to ten or few tens
of nanometer sized features in SPM operating in the air most applications of SPM
for nanofabrication were carried out in ambient air. This is presumably due to the
ease of operation as well as the cost factor.
A nanometer scale pattern on n-type Si (100) was generated by field induced oxidation using Si cantilever tips which are made metallic by heavy p-type (10
26 m
À3 )
doping [73]. It was found that the oxidation needs a threshold bias (@10 V) before
oxidation begins. The structures formed were oxides (1 nm high) which could be
etched by leaving grooves on the substrate. (Growth of the oxide layer in Si consumes a layer of around 0.5 h inside the Si where h is the height of the oxide layer
grown. As a result it leaves a groove on removal of the protruding oxide formed by
the tip-induced oxidation.)
In the models proposed for oxidation in the above experiments there is no role of
the absorbed moisture layer that can act as the ‘‘electrolyte’’. However, subsequent
models show that the oxide formation can be explained using the moisture layer
[74]. The process is similar to liquid electrolyte based anodization. The oxidation
process is sensitive to the humidity. The diffusion limited electric field E at the
surface determines the terminating thickness of the oxide grown. The model is
similar to what has been proposed for the field-induced oxidation process proposed
by Cabrera and Mott more than 50 years ago [61]. An electron tunnels from the
conducting AFM tip (which is negatively biased with respect to the substrate) and
creates negatively charged oxygen ions. The oxide growth stops when the electric
field is no longer sufficient for the ionic diffusion.
Amorphous Si (a-Si) has been oxidized like crystalline Si in AFM-based lithography and has been used as a resist [75]. a-Si can be H passivated like its single
crystalline counterpart. This makes SPM-based lithography possible in a-Si.
Application of non-contact mode AFM in anodic oxidation based lithography is
an important step forward in this field [76]. Noncontact AFM has been used to
create feature sizes <30 nm on a 65 nm resist (polymer resist) and was transferred through reactive ion etching (RIE) to the Si substrate (P-doped (100)) on
which the resist is coated [77]. A scan speed of 10 mm s
À1 has been achieved using
a heavily B-doped Si cantilever with a large spring constant (@340 N m
À1 ) and
mechanical resonance frequency ( f r A 1 MHz) and with a tip of radius of curvature
21.4 Scanning Probe Based Anodic Oxidation as a Tool for the Fabrication of Nanostructures 711