# Achieving High Aspect Ratio of Track Length to Width in Molds for Discrete Track Recording Media

Authors: K. O. Aung; C. Shankaran; R. Sbiaa; E. L. Tan; S. K. Wong; S. N. Piramanayagam
Journal: Research Letters in Nanotechnology
 (2008)

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## Abstract

Discrete track media (DTM) fabricated by nanoimprint lithography (NIL) is considered as a potential technology for future hard disk drives (HDD). In the fabrication of a master mold for NIL, patterning the resist tracks with a narrow distribution in the width is the first critical step. This paper reports the challenges involved in the fabrication of high aspect ratio discrete tracks on Polymethylmethacrylate (PMMA) resist by means of electron beam lithography. It was observed that fabrication parameters applied for successful patterning of discrete tracks in nanoscale length were not directly suitable for the patterning of discrete tracks in micron scale. Hence different approaches such as thick layer resist coating, introducing of post exposure baking process, and varying of exposure parameters were used in order to achieve uniform sharp discrete tracks in micron scale length on the resist. The optimal parameters were used to pattern 20μm long tracks with 70 nm track pitch on the resist.


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## Body

## 1. Introduction

In hard disk drives (HDD), achieving a high signal-to-noise ratio (SNR)
without sacrificing thermal stability or writability is one of the major
concerns. Recently, perpendicular recording was introduced in HDDs, because of
its ability to improve SNR, thermal stability and writability as compared to the traditional
longitudinal media which has gradually been phased out due to superparamagnetism. Even
though perpendicular recording technology shifts the onset of superparamagnetic
effect to some extent, the effect still exists and will pose a limitation to
magnetic recording technology in the coming few years. Therefore, alternative
technologies such as heat assisted magnetic recording (HAMR), and bit-patterned
media (BPM) need to be considered. A probable intermediate step towards HAMR or
BPM is discrete track media (DTM) technology [1–7].In the conventional perpendicular
recording scheme, recorded data is stored along circumferential tracks. In this
scheme, the write head determines the track positions and widths. Tolerance
between the write and read head locations can induce noise along the
magnetically defined tracks [6]. In the case of DTM, the tracks are
lithographically defined in such a way that the track widths and locations are
physically fixed. Nonmagnetic region between physically defined magnetic tracks
can reduce the noise during the reading process, which helps in increasing the
track density [6]. To meet areal densities of 1 Tb/in2 or higher, track densities higher than 300 ktpi are sought; however, one of the major
hurdles in the introduction of discrete track recording lies in the media
fabrication. Introducing DTM will require lithographic process with low cost
and high throughput manufacturing capabilities. There are many state-of-the-art
lithographic tools with a capability of fabrication sub 100 nm patterns. Among
them, nanoimprint lithography (NIL) becomes the most promising approach with
low cost and high throughput manufacturing capabilities for DTM fabrication in
a large-scale area.Previous studies presented various
approaches and resist materials used for the fabrication of DTM [8–10]. Hattori et
al. reported the fabrication of discrete tracks with 90 nm pitch by using
ZEP520A electron beam resist and Ni and TiN hard mask layers [10]. In our
study, we have investigated the fabrication of discrete track mold on resist
with electron beam lithography which was designed for the target of 320 ktpi in
20 μm length. It was observed that for such high
track dentist,  making tracks in micron scale length with the track width in
nanometer scale was one of the lithographic challenges. Those narrow width and
long resist tracks require higher mechanical strength and stronger adhesion to
a substrate underneath to withstand an indirect exposure by electron scattering
and mechanical stress from surrounding exposed lines during the process.
Similar issues were reported in simulation and experimental study from Jones
and Paraszczak that narrow width of resist collapsed because of the stresses
introduced by the processing conditions [11]. In our study, the lithographic
challenges involved in the patterning of discrete tracks on resist with an
aspect ratio of multiple of hundreds (track length/track width) and the ways
to solve these problems are reported here. Resist tracks with 120 nm pitch and
20 μm length were patterned to establish the optimal process parameters and
later those parameters were applied to fabricate 70 nm track pitch with 20 μm
track length.


## 2. Experimental Details

A thermally oxidized silicon substrate was spin coated with PMMA resist.
The same substrate type was used throughout experiments, since
patterning results with electron beam lithography can be changed with different
types of substrate or underlayer beneath PMMA resist. More details about
exposure parameters and resist coating are described in the text, as the
conditions of these procedures were varied
for optimization. Scanning electron microscope (SEM)
and atomic force microscopy (AFM) were the main tools used for the evaluation of
the fabricated patterns.  TESP7 AFM probes were used throughout experiments since its maximum radius is 10 nm and tip height of 10μm allows it to scan the bottom of resist trenches which are less than 150 nm in depth.


## 3. Results and Discussion

### 3.1. Initial Studies on Sub-μm Tracks

An array of tracks with 150 nm length, 40 nm width, and 40 nm spacing were patterned using PMMA resist dissolved in 2%
anisole. The resist was spun at 4000 RPM for 60 seconds on the substrate and baked at 180°C for 2 minutes. Under these
conditions, 55 to 60 nm thick resist was obtained after 30 seconds development time.
Figure 1 shows SEM images of the
developed resist patterns, showing that 40 nm line widths and spacings can be produced
at exposure doses between 300 and  450 μC/cm2   with a constant electron beam
current of 100 pA. We selected the range of exposure doses between 300 and  350 μC/cm2 for
further investigation since measured line widths and spacings produced at those
doses are closer to the targeted value of 40 nm compared to the doses between
400 and 450 μC/cm2.SEM images of developed resist patterns exposed at (a) 300μC/cm2 and (b) 450 μC/cm2.
(a)(b)


### 3.2. Longer Track Investigation

The fabrication of a
master mold for DTM requires high aspect ratio tracks with good uniformity, where
the uniformity is defined as the percentage of the difference between the
widest line and the narrowest line over the sum of those values. For this
purpose, an array of 20μm long tracks with 120 nm track pitch (70 nm
width and 50 nm spacing) was patterned on the PMMA resist. Parameters for
resist coating, baking, and developing for 20 μm long tracks were kept the same
as those described above for the fabrication of an array of tracks with 150 nm length.
Figure 2 shows that the tracks were removed at the centre of the
exposed area, while broken tracks of resist remained only at the edges of the array patterns.
It is anticipated that etching rates are different for the edge area and the
center area, since the developer solution encountered more unexposed resist
bulk at four edges of the exposed area of tracks. Reducing the resist
development time resulted in under developed patterns at the edges, while still
giving a partial removal of the exposed resist in the centre area. These
results suggest that the higher aspect ratio of the tracks leads to more
difficult process control in order to achieve good intact and adhesion of those
tracks to the substrate. Hence further process improvements are necessary to
overcome this issue.Figure 2
AFM images of (a) 20μm long-tracks with 70 nm track width and 50 nm spacing. The
center of the exposed area is washed away after development. Image (b) is an enlarged area which shows that the exposed lines are broken and
nonuniform. Right side figure shows a curled-shape track which is due to scanning artifact.As a first attempt to overcome this problem, post exposure baking (PEB) process was introduced just after exposure and
before resist development. PEB process was expected to harden the resist and
reduce the resist sensitivity during its development. A baking temperature of
80°C was chosen which was below the glass transition temperature of PMMA.
Reducing the sensitivity and hardening the resist can minimize its mechanical
stress and electron scattering into unexposed resist lines from adjacent
exposed lines, resulting in minimal resist deformation during its development. From
AFM images, the whole exposed area is now filled with an array of 20 μm long tracks, but there is nonuniformity of
resist tracks although the tracks remain present for the process with PEB. It
can be concluded that the PEB process can help to improve the fabrication of long tracks but not good enough
to be used for making a mold for DTM.


### 3.3. Effect of Resist Thickness and Beam Current

In order to see if the resist thickness
plays a vital role to achieve the uniform resist tracks with high aspect ratio,
two different resist thicknesses were evaluated. The first sample was spin coated
with a speed of 2500 rpm and the second sample was coated twice with a speed of
4000 rpm for each resist layer. A baking at 180°C for 2 minutes was conducted
for each layer to improve interfacial adhesion between them and surface
uniformity. The measured resist thicknesses for the first and second samples
were about 90 nm and 120 nm, respectively. In addition to the resist thickness
variation, the electron beam current was reduced to 30 pA to minimize proximity
effect (electron scattering on adjacent unexposed areas). The exposure dose for
the two samples was increased to 600 μC/cm2 to accommodate the
increase in exposure energy requirement for thicker resists. In this study, no PEB was applied to investigate only the effect of
beam current and resist thickness on tracks uniformity. As expected, both
samples had uniform resist tracks intact over the whole exposed area, improved
by the lower beam current (30 pA) which minimizes the proximity effect compared
to previous processes in which 100 pA of beam current was applied.Figure3 shows AFM images of the samples
scanned over 5 μm range in the centre of the processing area.
The track pitch was 120 nm with 60 nm width and spacing, respectively. It can
be seen from AFM images that more uniform resist tracks can be obtained on 120
nm thick resist compared with tracks on 90 nm thick resist. In order to
evaluate the resist thickness effect on the uniformity of resist tracks, the widths
of tracks and grooves were tabulated and the width distributions for both
resist thichnesses were compared. Since the tracks and grooves have different intensity
levels, simple thresholding was used to distinguish between them. Figure 4 shows
groove width distributions for different resist thickness. For the sample
coated with 120 nm thick resist, average widths of grooves and tracks were 22
+/− 3 nm and 122 +/− 3 nm, respectively, whilst for the sample with 90 nm of
resist thickness, 35 +/− 5 nm and 109 +/− 4 nm, respectively. Although the
sample with the thicker resist gave smaller variation in line widths, it was
still not sufficient to achieve good signal to noise ratio as there was a ∼20 nm
difference between the widest and narrowest tracks. Hence further study on the
optimization of electron beam energy to achieve even lower track pitch and
narrower distribution of line widths was carried out for 120 nm thick resist.
As a result, it was found that 600 μC/cm2 is the
optimal exposure dose for achieving long tracks of more than 20 μm with sharper edges
and good uniformity.AFM images of resist patterns for (a) 90 nm and (b) 120 nm of resist thickness.
(a)(b)Figure 4
Groove width distributions for two different resist thicknesses.In DTM, to be able to optimize the
readback signal while reducing the noise, sharper tracks edges are needed; and from SEM images, it can be
seen clearly from Figure5 that with thicker resist, 30 pA beam current and
optimal dose, it is possible to achieve sharper and uniform tracks with large
aspect ratio (length/width).Figure 5
An SEM image of 70 nm pitch tracks.


## 3.1. Initial Studies on Sub-μm Tracks

An array of tracks with 150 nm length, 40 nm width, and 40 nm spacing were patterned using PMMA resist dissolved in 2%
anisole. The resist was spun at 4000 RPM for 60 seconds on the substrate and baked at 180°C for 2 minutes. Under these
conditions, 55 to 60 nm thick resist was obtained after 30 seconds development time.
Figure 1 shows SEM images of the
developed resist patterns, showing that 40 nm line widths and spacings can be produced
at exposure doses between 300 and  450 μC/cm2   with a constant electron beam
current of 100 pA. We selected the range of exposure doses between 300 and  350 μC/cm2 for
further investigation since measured line widths and spacings produced at those
doses are closer to the targeted value of 40 nm compared to the doses between
400 and 450 μC/cm2.SEM images of developed resist patterns exposed at (a) 300μC/cm2 and (b) 450 μC/cm2.
(a)(b)


## 3.2. Longer Track Investigation

The fabrication of a
master mold for DTM requires high aspect ratio tracks with good uniformity, where
the uniformity is defined as the percentage of the difference between the
widest line and the narrowest line over the sum of those values. For this
purpose, an array of 20μm long tracks with 120 nm track pitch (70 nm
width and 50 nm spacing) was patterned on the PMMA resist. Parameters for
resist coating, baking, and developing for 20 μm long tracks were kept the same
as those described above for the fabrication of an array of tracks with 150 nm length.
Figure 2 shows that the tracks were removed at the centre of the
exposed area, while broken tracks of resist remained only at the edges of the array patterns.
It is anticipated that etching rates are different for the edge area and the
center area, since the developer solution encountered more unexposed resist
bulk at four edges of the exposed area of tracks. Reducing the resist
development time resulted in under developed patterns at the edges, while still
giving a partial removal of the exposed resist in the centre area. These
results suggest that the higher aspect ratio of the tracks leads to more
difficult process control in order to achieve good intact and adhesion of those
tracks to the substrate. Hence further process improvements are necessary to
overcome this issue.Figure 2
AFM images of (a) 20μm long-tracks with 70 nm track width and 50 nm spacing. The
center of the exposed area is washed away after development. Image (b) is an enlarged area which shows that the exposed lines are broken and
nonuniform. Right side figure shows a curled-shape track which is due to scanning artifact.As a first attempt to overcome this problem, post exposure baking (PEB) process was introduced just after exposure and
before resist development. PEB process was expected to harden the resist and
reduce the resist sensitivity during its development. A baking temperature of
80°C was chosen which was below the glass transition temperature of PMMA.
Reducing the sensitivity and hardening the resist can minimize its mechanical
stress and electron scattering into unexposed resist lines from adjacent
exposed lines, resulting in minimal resist deformation during its development. From
AFM images, the whole exposed area is now filled with an array of 20 μm long tracks, but there is nonuniformity of
resist tracks although the tracks remain present for the process with PEB. It
can be concluded that the PEB process can help to improve the fabrication of long tracks but not good enough
to be used for making a mold for DTM.


## 3.3. Effect of Resist Thickness and Beam Current

In order to see if the resist thickness
plays a vital role to achieve the uniform resist tracks with high aspect ratio,
two different resist thicknesses were evaluated. The first sample was spin coated
with a speed of 2500 rpm and the second sample was coated twice with a speed of
4000 rpm for each resist layer. A baking at 180°C for 2 minutes was conducted
for each layer to improve interfacial adhesion between them and surface
uniformity. The measured resist thicknesses for the first and second samples
were about 90 nm and 120 nm, respectively. In addition to the resist thickness
variation, the electron beam current was reduced to 30 pA to minimize proximity
effect (electron scattering on adjacent unexposed areas). The exposure dose for
the two samples was increased to 600 μC/cm2 to accommodate the
increase in exposure energy requirement for thicker resists. In this study, no PEB was applied to investigate only the effect of
beam current and resist thickness on tracks uniformity. As expected, both
samples had uniform resist tracks intact over the whole exposed area, improved
by the lower beam current (30 pA) which minimizes the proximity effect compared
to previous processes in which 100 pA of beam current was applied.Figure3 shows AFM images of the samples
scanned over 5 μm range in the centre of the processing area.
The track pitch was 120 nm with 60 nm width and spacing, respectively. It can
be seen from AFM images that more uniform resist tracks can be obtained on 120
nm thick resist compared with tracks on 90 nm thick resist. In order to
evaluate the resist thickness effect on the uniformity of resist tracks, the widths
of tracks and grooves were tabulated and the width distributions for both
resist thichnesses were compared. Since the tracks and grooves have different intensity
levels, simple thresholding was used to distinguish between them. Figure 4 shows
groove width distributions for different resist thickness. For the sample
coated with 120 nm thick resist, average widths of grooves and tracks were 22
+/− 3 nm and 122 +/− 3 nm, respectively, whilst for the sample with 90 nm of
resist thickness, 35 +/− 5 nm and 109 +/− 4 nm, respectively. Although the
sample with the thicker resist gave smaller variation in line widths, it was
still not sufficient to achieve good signal to noise ratio as there was a ∼20 nm
difference between the widest and narrowest tracks. Hence further study on the
optimization of electron beam energy to achieve even lower track pitch and
narrower distribution of line widths was carried out for 120 nm thick resist.
As a result, it was found that 600 μC/cm2 is the
optimal exposure dose for achieving long tracks of more than 20 μm with sharper edges
and good uniformity.AFM images of resist patterns for (a) 90 nm and (b) 120 nm of resist thickness.
(a)(b)Figure 4
Groove width distributions for two different resist thicknesses.In DTM, to be able to optimize the
readback signal while reducing the noise, sharper tracks edges are needed; and from SEM images, it can be
seen clearly from Figure5 that with thicker resist, 30 pA beam current and
optimal dose, it is possible to achieve sharper and uniform tracks with large
aspect ratio (length/width).Figure 5
An SEM image of 70 nm pitch tracks.


## 4. Conclusion

Electron beam lithography process for
making a mold for DTM was optimized by various procedures. For 55 nm thick
resist, the exposure doses between 300μC and 350 μC/cm2 give the
best result for resist tracks with 150 nm length. For the tracks in micron
scale length, however, the uniformity in resist thickness was not possible
using the same process parameters suggesting the weak adhesion of the resist to
the sample surface. PEB was found to improve the overall resist performance to
some extent that the resist tracks still remain after development. However, there
is still a thickness variation in the resist by about 15 nm which is about 30%
of the coated resist thickness of 55 nm. In order to overcome this issue, the
electron beam current was reduced in parallel with the application of thicker
resist. Resist tracks with uniform thickness and lateral dimensions were
confirmed by AFM and SEM images.
It is concluded that resist thickness, the optimized e-beam energy corresponding
to the thicker resist, and e-beam current play a major role in obtaining long
tracks with high aspect ratio.



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*Source: 765398-2008-06-08.xml*