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Polystyrene negative resist for high-resolution electron beam lithography.

Ma S, Con C, Yavuz M, Cui B - Nanoscale Res Lett (2011)

Bottom Line: It demonstrated fairly well-defined patterning of a 20-nm period line array and a 15-nm period dot array, which are the densest patterns ever achieved using organic EBL resists.It is also considerably more resistant to dry etching than PMMA.With a low sensitivity, it would find applications where negative resist is desired and throughput is not a major concern.

View Article: PubMed Central - HTML - PubMed

Affiliation: Waterloo Institute for Nanotechnology (WIN), University of Waterloo, 200 University Ave, West, Waterloo, ON N2L 3G1, Canada. bcui@uwaterloo.ca.

ABSTRACT
We studied the exposure behavior of low molecular weight polystyrene as a negative tone electron beam lithography (EBL) resist, with the goal of finding the ultimate achievable resolution. It demonstrated fairly well-defined patterning of a 20-nm period line array and a 15-nm period dot array, which are the densest patterns ever achieved using organic EBL resists. Such dense patterns can be achieved both at 20 and 5 keV beam energies using different developers. In addition to its ultra-high resolution capability, polystyrene is a simple and low-cost resist with easy process control and practically unlimited shelf life. It is also considerably more resistant to dry etching than PMMA. With a low sensitivity, it would find applications where negative resist is desired and throughput is not a major concern.

No MeSH data available.


Related in: MedlinePlus

Contrast curves for polystyrene exposed at 20 and 5 keV, and developed by xylene and cyclohexane for 90 s at room temperature.
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Figure 1: Contrast curves for polystyrene exposed at 20 and 5 keV, and developed by xylene and cyclohexane for 90 s at room temperature.

Mentions: Figure 1 shows the contrast curves for 2000 g/mol polystyrene resist exposed at 20 and 5 keV, using a relatively thick film (125, 135, and 92 nm), which gave more accurate measurement by AFM. Here, in the contrast curves, D0 and D100 are the intersections of the line having the highest slope with the zero and full resist thickness lines, respectively. The contrast for exposure at 20 keV, defined as γ = [log(D100/D0)]-1, is calculated to be 4.4 for both xylene and cyclohexane developers, which is higher than the contrast for ZEP520 resist developed at room temperature [18]. However, the sensitivity for polystyrene resist is rather low with D50 ≈ 4000 μC/cm2, which would limit its application to small scale nano-patterning in R&D. The threshold dose where the contrast curve starts to rise (D0) is the "gel point" that is roughly inversely proportional to the molecular weight for simple negative polymer resists according to the Charlesby theory [19]. This is because the number of crosslinks necessary to make the resist insoluble in the developers decreases with higher molecular weight. We also developed the resist using chlorobenzene but found no apparent difference (the contrast curve is not shown). As for the development temperature, it is well known that generally cold development improves the positive resist contrast and resolution [18,20], whereas hot development increases the contrast for negative resists like HSQ [21]. However, we found no evident improvement for polystyrene (negative) resist development at an elevated temperature of 50°C. One way to alleviate the issue of low resist sensitivity is to carry out exposure at low beam energy such as 5 keV, and the sensitivity was indeed increased to D50 = 1170 μC/cm2. This is in fair agreement with the fact that sensitivity is roughly inversely proportional to the beam energy (E) as predicted by the Bethe equation for electron energy loss (Eloss) in the resist: Eloss ∞ 1/E log(αE) with α being a constant. Sensitivity can be further increased using higher molecular weight polystyrene, but at a cost of reduced resolution. When exposed at 5 keV, the contrast is reduced to 3.4, which is close to the ZEP520 resist developed at room temperature [18]. The sensitivity and contrast for 5 keV exposure is expected to be similar for cyclohexane and chlorobenzene developers, as it is for the case of 20 keV exposure. As seen below and pointed out also by Cord et al. [13], the reduced contrast did not seriously affect the resist resolution.


Polystyrene negative resist for high-resolution electron beam lithography.

Ma S, Con C, Yavuz M, Cui B - Nanoscale Res Lett (2011)

Contrast curves for polystyrene exposed at 20 and 5 keV, and developed by xylene and cyclohexane for 90 s at room temperature.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211865&req=5

Figure 1: Contrast curves for polystyrene exposed at 20 and 5 keV, and developed by xylene and cyclohexane for 90 s at room temperature.
Mentions: Figure 1 shows the contrast curves for 2000 g/mol polystyrene resist exposed at 20 and 5 keV, using a relatively thick film (125, 135, and 92 nm), which gave more accurate measurement by AFM. Here, in the contrast curves, D0 and D100 are the intersections of the line having the highest slope with the zero and full resist thickness lines, respectively. The contrast for exposure at 20 keV, defined as γ = [log(D100/D0)]-1, is calculated to be 4.4 for both xylene and cyclohexane developers, which is higher than the contrast for ZEP520 resist developed at room temperature [18]. However, the sensitivity for polystyrene resist is rather low with D50 ≈ 4000 μC/cm2, which would limit its application to small scale nano-patterning in R&D. The threshold dose where the contrast curve starts to rise (D0) is the "gel point" that is roughly inversely proportional to the molecular weight for simple negative polymer resists according to the Charlesby theory [19]. This is because the number of crosslinks necessary to make the resist insoluble in the developers decreases with higher molecular weight. We also developed the resist using chlorobenzene but found no apparent difference (the contrast curve is not shown). As for the development temperature, it is well known that generally cold development improves the positive resist contrast and resolution [18,20], whereas hot development increases the contrast for negative resists like HSQ [21]. However, we found no evident improvement for polystyrene (negative) resist development at an elevated temperature of 50°C. One way to alleviate the issue of low resist sensitivity is to carry out exposure at low beam energy such as 5 keV, and the sensitivity was indeed increased to D50 = 1170 μC/cm2. This is in fair agreement with the fact that sensitivity is roughly inversely proportional to the beam energy (E) as predicted by the Bethe equation for electron energy loss (Eloss) in the resist: Eloss ∞ 1/E log(αE) with α being a constant. Sensitivity can be further increased using higher molecular weight polystyrene, but at a cost of reduced resolution. When exposed at 5 keV, the contrast is reduced to 3.4, which is close to the ZEP520 resist developed at room temperature [18]. The sensitivity and contrast for 5 keV exposure is expected to be similar for cyclohexane and chlorobenzene developers, as it is for the case of 20 keV exposure. As seen below and pointed out also by Cord et al. [13], the reduced contrast did not seriously affect the resist resolution.

Bottom Line: It demonstrated fairly well-defined patterning of a 20-nm period line array and a 15-nm period dot array, which are the densest patterns ever achieved using organic EBL resists.It is also considerably more resistant to dry etching than PMMA.With a low sensitivity, it would find applications where negative resist is desired and throughput is not a major concern.

View Article: PubMed Central - HTML - PubMed

Affiliation: Waterloo Institute for Nanotechnology (WIN), University of Waterloo, 200 University Ave, West, Waterloo, ON N2L 3G1, Canada. bcui@uwaterloo.ca.

ABSTRACT
We studied the exposure behavior of low molecular weight polystyrene as a negative tone electron beam lithography (EBL) resist, with the goal of finding the ultimate achievable resolution. It demonstrated fairly well-defined patterning of a 20-nm period line array and a 15-nm period dot array, which are the densest patterns ever achieved using organic EBL resists. Such dense patterns can be achieved both at 20 and 5 keV beam energies using different developers. In addition to its ultra-high resolution capability, polystyrene is a simple and low-cost resist with easy process control and practically unlimited shelf life. It is also considerably more resistant to dry etching than PMMA. With a low sensitivity, it would find applications where negative resist is desired and throughput is not a major concern.

No MeSH data available.


Related in: MedlinePlus