Nanoimprint lithography (NIL) technology is in the spotlight as a next-generation semiconductor manufacturing technique for integrated circuits at 22 nm and beyond. NIL is the unmagnified lithography technique using template which is replicated from master templates. On the other hand, master templates are currently fabricated by electron-beam (EB) lithography. In near future, finer patterns less than 15nm will be required on master template and EB data volume increases exponentially. So, we confront with a difficult challenge. A higher resolution EB mask writer and a high performance fabrication process will be required. In our previous study, we investigated a potential of photomask fabrication process for finer patterning and achieved 15.5nm line and space (L/S) pattern on template by using VSB (Variable Shaped Beam) type EB mask writer and chemically amplified resist. In contrast, we found that a contrast loss by backscattering decreases the performance of finer patterning. For semiconductor devices manufacturing, we must fabricate complicated patterns which includes high and low density simultaneously except for consecutive L/S pattern. Then it’s quite important to develop a technique to make various size or coverage patterns all at once. In this study, a small feature pattern was experimentally formed on master template with dose modulation technique. This technique makes it possible to apply the appropriate exposure dose for each pattern size. As a result, we succeed to improve the performance of finer patterning in bright field area. These results show that the performance of current EB lithography process have a potential to fabricate NIL template.
ArF immersion lithography combined with double patterning has been used for fabricating below half pitch 40nm devices. However, when pattern size shrinks below 20nm, we must use new technology like quadruple patterning process or next generation lithography (NGL) solutions. Moreover, with change in lithography tool, next generation mask production will be needed. According to ITRS 2013, fabrication of finer patterns less than 15nm will be required on mask plate in NGL mask production 5 years later . In order to fabricate finer patterns on mask, higher resolution EB mask writer and high performance fabrication process will be required. In a previous study, we investigated a potential of mask fabrication process for finer patterning and achieved 17nm dense line pattern on mask plate by using VSB (Variable Shaped Beam) type EB mask writer and chemically amplified resist . After a further investigation, we constructed higher performance mask process by using new EB mask writer EBM9000. EBM9000 is the equipment supporting hp16nm generation's photomask production and has high accuracy and high throughput. As a result, we achieved 15.5nm pattern on mask with high productivity. Moreover, from evaluation of isolated pattern, we proved that current mask process has the capability for sub-10nm pattern. These results show that the performance of current mask fabrication process have the potential to fabricate the next-generation mask.
Photomask used for optical lithography has been developed for purpose of fabrication a pattern along with finer
designed rules and increase the productivity. With regard to pattern fabrication on mask, EB (Electron beam) mask
writer has been used because it has high resolution beam. But in producing photomask, minimum pattern size on mask is
hits a peak around 40nm by the resolution limit of ArF immersion systems. This value is easy to achieve by current EB
writer. So, photomask process with EB writer has gotten attached to increase turnaround time.
In next generation lithography such as EUV (Extreme ultraviolet) lithography and Nano-imprint lithography, it is
enable to fabricate finer pattern beyond the resolution limit of ArF immersion systems. Thereby the pattern on a mask
becomes finer rapidly. According to ITRS 2012, fabrication of finer patterns less than 20nm will be required on EUV
mask and on NIL template. Especially in NIL template, less than 15nm pattern will be required half a decade later. But
today’s development of EB writer is aiming to increase photomask’s productivity, so we will face a difficulty to fabricate
finer pattern in near future.
In this paper, we examined a potential of mask production process with EB writer from the view of finer pattern
fabrication performances. We succeeded to fabricate hp (half-pitch) 17nm pattern on mask plate by using VSB (Variable
Shaped Beam) type EB mask writer with CAR (Chemically Amplified Resist). This result suggests that the photomask
fabrication process has the potential for sub-20nm generation mask production.
A new photomask technology with the Advanced Binary Film (ABF) by HOYA has been established. The film of
relatively low thickness is expected to show the best lithography performance. The simple film structure of thin film of
chemically amplified resist, as a mask layer for etching, on the thin ABF film enables us to obtain sub-50nm small
features in a photomask. The thinness of the film also helps to avoid pattern collapse in cleaning steps. The photomask
with ABF expecting the best currently available lithography performance shows the best achievable durability for use in
ArF lithography process steps and the best attainable feasibility in the fabrication process steps for leading edge
The requirement for image placement accuracy on photomask has been rising. The ITRS road map says that we need
to achieve 4.3nm accuracy in 2012 for HP 36nm device with single exposure process, further more we must achieve
3.0nm accuracy if double patterning process is selected.
Fig.1 shows the today's performance of image placement accuracy. Some sample photomasks which have same pattern
shape are produced during 3 months, and the mask to mask overlay accuracy of them was measured. The average of
them was 3.6nm. This data shows the possibility to achieve the accuracy of photomask for HP 36nm devices. The image
placement accuracy of actual device pattern during same period is showed in Fig.2. The image placement accuracy of
actual device pattern is worse than that showed before. We categorized these data according to the pattern density, low
density pattern and the high density pattern. The density of the test pattern is categorized into very low density pattern.
The results are showed in Fig.3. We can see the degrading of image place accuracy according to the pattern density.
This degrading of image placement accuracy is caused by resist charging effect. In photomask production process,
electron beam writer is mainly used as lithography tool. Each pattern on photomask is formed by step by step exposure
of electron beam. The surface of resist film will be charged with exposed electron beam, and electric field will be
generated around that charged area. So the orbit of electron beam for next exposure will be bended by the electric field
which generated by previous beam shot, and image placement accuracy will degrade. To achieve the demanded image
placement accuracy, we need to remove the error caused by this phenomenon.
We researched in resist charging effect for correcting it, and we studied that this phenomenon have so complex feature.
After that we tried to research in it on EBM-7000, newly developed electron beam writer, and we found out the reduction
of the image placement error caused by the resist charging effect on EBM-7000.
We achieved highly accurate Local CD in the vicinity of 1nm with the newly developed low sensitivity
chemically amplified resist (CAR) for the e-beam reticle writer, EBM-6000. We applied shot noise model to
estimate Line Edge Roughness (LER). According to the estimation result, LER is improved by increasing the
threshold dosage. We evaluated the performance of newly developed low sensitivity CAR. Local CD
accuracy, LER, pattern resolution and drawing time are evaluated. We concluded that the performance with
the low sensitivity CAR was good enough to produce photomasks for 45nm half pitch (HP) devices.
The performance of electron beam reticle writer EBM-5000(NFT) was examined with higher
current density. The current density was raised up to 70A/cm<sup>2</sup> against to its standard current density
50A/cm<sup>2</sup>, and sufficiently good results were obtained with that operating condition. We concluded
that the performance with that operating condition was good enough to produce photomasks for
65nm node devices.