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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 18 — Aug. 29, 2011
  • pp: 17790–17798
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Increased process latitude in absorbance-modulated lithography via a plasmonic reflector

Charles W. Holzwarth, John E. Foulkes, and Richard J. Blaikie  »View Author Affiliations


Optics Express, Vol. 19, Issue 18, pp. 17790-17798 (2011)
http://dx.doi.org/10.1364/OE.19.017790


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Abstract

Absorbance-modulated lithography is a relatively new optical patterning method where a thin layer of photochromic molecules is placed between the far-field optics and photoresist. These molecules can be made transparent or opaque by illuminating with wavelengths λ1 or λ2, respectively. By simultaneously illuminating this layer with patterns of both wavelengths it is possible to create an absorption mask capable of subwavelength resolution. This resolution comes at the price of limited contrast and depth-of-focus resulting in poor process latitude. Here it is shown that by using TM polarization for λ1 and integrating a plasmonic reflector process latitude is increased by up to 66%.

© 2011 OSA

1. Introduction

The resolution of far-field optical lithography is typically limited by diffraction, requiring the use of shorter wavelengths and higher numerical apertures to improve performance [1

1. S. Okazaki, “Resolution limits of optical lithography,” J. Vac. Sci. Technol. B 9(6), 2829–2833 (1991). [CrossRef]

]. This has resulted in the exponential increase in complexity and cost of state-of-the-art optical lithography systems [2

2. E. Muzio, “Optical lithography cost of ownership (COO) – final report for LITG501,” SEMATECH, (2000) http://www.sematech.org/docubase/document/4014atr.pdf.

]. In contrast, near-field optical techniques such as evanescent-field contact lithography, can achieve sub-diffraction limited resolution by utilizing the evanescent high spatial frequencies to form the image. Although impressive results have been achieved [3

3. T. Ito, T. Yamada, Y. Inao, T. Yamaguchi, N. Mizutani, and R. Kuroda, “Fabrication of half-pitch 32 nm resist pattern using near-field lithography with a-Si mask,” Appl. Phys. Lett. 89(3), 033113 (2006). [CrossRef]

], fundamental problems such as expensive mask fabrication, the need for intimate contact between a rigid photomask and photoresist, and mask wear have prevented this method from being widely adopted.

To overcome these issues while maintaining sub-diffraction limited resolution a relatively new lithography technique, absorbance-modulated optical lithography, has been developed that uses far-field optics to control near-field waves [4

4. R. Menon and H. I. Smith, “Absorbance-modulation optical lithography,” J. Opt. Soc. Am. A 23(9), 2290–2294 (2006). [CrossRef] [PubMed]

]. This technique uses a spin-on absorbance-modulation layer (AML), located directly above the photoresist, as an optically-activated photomask [5

5. T. L. Andrew, H. Y. Tsai, and R. Menon, “Confining light to deep subwavelength dimensions to enable optical nanopatterning,” Science 324(5929), 917–921 (2009). [CrossRef] [PubMed]

,6

6. R. Menon, H. Y. Tsai, and S. W. Thomas 3rd, “Far-field generation of localized light fields using absorbance modulation,” Phys. Rev. Lett. 98(4), 043905 (2007). [CrossRef] [PubMed]

]. The AML is made up of photochromic molecules that can be optically switched from opaque to transparent via exposure to short-wavelength ultraviolet light (λ1) and switched back, transparent to opaque, via exposure to longer-wavelength visible light (λ2). It is possible to optically control the localized level of transparency of the AML by locally controlling the intensity ratio, λ12. One simple way to do this is absorbance-modulated interference lithography (AMIL) where the AML is simultaneously exposed to a standing-wave interference pattern of λ2 and uniform illumination of λ1 (Fig. 1
Fig. 1 In absorbance modulation interference lithography (AMIL) the AML is illumined uniformly by λ1 and by a standing wave interference pattern of λ2 resulting in large power ratios at the optical nulls. This creates transparent regions in the AML through which λ1 can expose the underlying photoresist.
) [6

6. R. Menon, H. Y. Tsai, and S. W. Thomas 3rd, “Far-field generation of localized light fields using absorbance modulation,” Phys. Rev. Lett. 98(4), 043905 (2007). [CrossRef] [PubMed]

].

Further improvements are seen in the FEM simulations by integrating a plasmonic reflector beneath the photoresist to improve contrast and depth of focus. This is counterintuitive because typically in optical lithography bottom-layer antireflection coatings (ARC) are used to minimize substrate reflections and suppress the unwanted formation of a standing wave in the photoresist. However, plasmonic reflectors have the ability to regenerate evanescent waves in the near field with the forward and reflected fields interfering to create a symmetric intensity profile through the depth of the photoresist [8

8. M. D. Arnold and R. J. Blaikie, “Subwavelength optical imaging of evanescent fields using reflections from plasmonic slabs,” Opt. Express 15(18), 11542–11552 (2007). [CrossRef] [PubMed]

]. FEM simulations show that this effect increases the contrast and depth of focus of the evanescent fields of λ1 in the photoresist with minimal effects on resolution.

To confirm the FEM results, various AMIL exposure are performed using TE or TM polarization for λ1 and with or without an integrated plasmonic reflector. The process latitude, the percent increase in dose required to increase the linewidth by 50%, is measured for each exposure condition. An increase in process latitude signals an improvement in contrast and depth of focus for the exposing wavelength. Similar to the FEM results the experiments showed that the process latitude increases when switching from TE to TM polarized light for λ1. A further increase is seen for the samples exposed with TM polarized light when a plasmonic reflector is placed below the photoresist.

For these experiments the AML layer is comprised of a polymerized azobenzene-based molecule chosen for its optical and thermal characteristics. However, this class of photochromic molecules exhibits the formation of photoinduced surface-relief gratings (SRG) in the presence of a standing wave interference pattern. In the last section of this paper, 4.2, we will analyze the formation of SRG in the AML and discuss its consequences on the contrast of AMIL.

2. Finite element method modeling

For modeling AMIL a two-dimensional (2D) full FEM modeling system using Comsol Multiphysics software suite [9

9. COMSOL Inc, 744 Cowper Steet, Palo Alto, CA 94301, www.comsol.com.

], has been developed. The accuracy and validity of this model had been tested previously [7

7. J. Foulkes and R. J. Blaikie, “Influence of polarization on absorbance modulated subwavelength grating structures,” J. Vac. Sci. Technol. B 27(6), 2941–2946 (2009). [CrossRef]

], by comparing the results for idealized 2D absorption gratings to an analytical calculation method developed by Rytov [10

10. S. M. Rytov, “Electromagnetic properties of a finely stratified medium,” Sov. Phys. JETP 2, 466–475 (1956).

]. For AMIL, full FEM modeling is needed to model the fields beneath the AML (i.e. in the photoresist and at the plasmonic reflector) since analytical methods such as Rytov analysis, assume that the gratings are infinite.

In the model the inputs are defined as a TE-polarized standing wave (800nm period) at λ2 with an imperfect null of 0.25% of the peak intensity and uniform illumination of λ1 with either TE or TM polarization. The material stack defined in the model was given parameters to mimic the recording stack used in the experiments: a 200-nm thick AML, a 10-nm thick polyvinyl alcohol (PVA) layer, a 40-nm thick photoresist layer above either an anti-reflective coating (ARC) or plasmonic reflector (see Fig. 6
Fig. 6 The recording stacks used for (a) the polarization and (b) plasmonic reflector experiments.
). The results for λ1 intensity in the photoresist for the TE, TM and TM with a plasmonic reflector case are shown in Fig. 2
Fig. 2 Intensity profile for λ1 in the photoresist when using (a) TE, (b) TM, and (c)TM polarization with a plasmonic reflector. Line-spread function at (i) 0 nm, (ii) 20nm, and (iii) 40nm into the resist layer.
. Also plotted in Fig. 2 are the line-spread function for each case at depths of 0 nm, 20 nm, and 40 nm into the resist layer. From these plots it is possible to see that contrast and depth of focus is increased by using TM polarization and adding a plasmonic reflector

From the line-spread function it is possible to qualitatively determine the relative process latitude for the three cases. This is done by looking at the slope of the line-spread function at the bottom of the photoresist, where the higher the slope results in higher process latitude. Using this metric it is clear the TM polarization should result in higher process latitude than TE polarization. The highest process latitude is achieved by using TM polarization with a planar plasmonic reflector below the photoresist. Also noticed in these results is that when using a plasmonic reflector the intensity is higher due to the reflected power traveling back through the photoresist. This should result in a lower exposure dose when using a plasmonic reflector.

3. Absorbance-modulated interference lithography

A schematic of the Lloyd’s mirror based AMIL setup used is shown in Fig. 3
Fig. 3 Diagram of the Lloyd’s mirror based AMIL system used.
. The source used for λ1 was a mercury lamp fiber light. The broadband light from the lamp travels through uniform illumination optics, a 405 nm filter, and a linear polarizer before reaching the sample. At the surface of the sample the 405 nm light is either TE or TM polarized with an intensity of 1.5 mW/cm2 uniform over a 25 mm × 25 mm area.

For λ2 a 50 mW, 532 nm wavelength solid-state diode pumped laser with a coherence length >1 m was used. This laser travels through a spatial filter with a non-collimated output, resulting in a Gaussian shaped intensity profile, centered at the intersection of the mirror and the sample, with peak intensity of 9 mW/cm2. A silver mirror is used for the Lloyd’s mirror, instead of the conventional aluminum mirror, due to its higher reflectivity at 532 nm. Also the sample chuck has an integrated Peltier cooler for controlling the sample’s temperature. For the process latitude experiments the Lloyds mirror is set to θ=19.4° for a period of 800 nm. Using the combination of uniform illumination for λ1 and a Gaussian shaped intensity profile for λ2 allows for the testing of many different intensity ratios on a single sample for a given λ1 exposure dose (Fig. 4
Fig. 4 Plot of how the intensity and intensity ratio varies with distance from the mirror along the samples surface.
).

3.1 Absorbance- modulation layer

The AML used in these experiments was comprised of polymerized 4’-[[(2-methacryloyoxy)ethyl]ethylamino]azobenzene (pMAEA). The synthesis of this photochromic material is described in detail in [11

11. H. S. Ho, A. Natansohn, C. Barrett, and P. Rochon, “Azo polymers for reversible optical storage. 8. The effect of polarity of the azobenzene groups,” Can. J. Chem. 72(11), 1773–1778 (1995). [CrossRef]

,12

12. C. Barrett, A. Natansohn, and P. Rochon, “Cis-trans thermal isomerization rates of bound and doped azobenzenes in a series of polymers,” Chem. Mater. 7(5), 899–903 (1995). [CrossRef]

]. pMAEA is a polymerized azobenzene based molecule that has two distinct absorption spectrums for its trans and cis isomers. The thermally stable trans isomer undergoes a photoisomerization reaction when exposed to λ1 = 405 nm, forming the cis isomer. The reverse reaction can take place quickly by exposing the cis isomer to λ2 = 532 nm or slowly at room temperature via thermal decay. The absorbance spectrum of a 200 nm thick film of pMAEA is shown in Fig. 5
Fig. 5 UV-Vis spectrum of 200 nm thick film of pMAEA.
after exposure to 405 nm light, to maximize the cis concentration, and 532 nm light, to maximize the trans concentration. The important parameters for this material are the absorption contrast at the exposing wavelength and the thermal decay constant, measured to be 2.3 and 2 × 10−3 at 15°C, respectively.

3.2 Experimental procedure

The recording stack for the polarization experiments consisted of four spin-coated layers; 200 nm of pMAEA, 10 nm PVA, 40 nm of AZ 1518 photoresist, and 230 nm of ARC on a silicon substrate (Fig. 6a). For the plasmonic reflector experiment the ARC was replaced with 60 nm of Ag deposited via sputtering (Fig. 6b). The PVA interlayer was necessary to prevent the solvent, chlorobenzene, used for spin-coating the pMAEA from attacking the underlying photoresist.

4. Results and discussion

AFM images of the gratings are taken at various distances from the sample’s edge, corresponding to different λ21 intensity ratios. Figure 7
Fig. 7 AFM images of samples exposed to various λ1 doses using TE polarization, TM polarization, and TM polarization with a plasmonic reflector.
shows AFM images of samples exposed with TE, TM and TM polarization with a plasmonic reflector for various λ1 exposure doses at an incident intensity ratio of 4. The full-width half-maximum (FWHM) of the exposed regions (trenches) were measured and averaged over 36 μm of grating length.

4.1 Process latitude

By plotting the FWHM versus exposure dose (Fig. 8
Fig. 8 Process latitude plots for intensity ratios of 4 and 5.
) it is possible to determine the minimum linewidth and process latitude for the various exposure conditions, as is summarized in Table 1

Table 1. Minimum Linewidth and Process Latitude

table-icon
View This Table
. The minimum linewidth is approximately the same for samples exposed with TE and TM polarization without a plasmonic reflector. However, for TM polarization the process latitude is improved by 32% and 22%, for intensity ratios of 4 and 5, respectively. It is also noticed that the process latitude decreases as the power ratio increases. This is consistent with previously reported FEM simulation results where both the contrast and depth of focus are reduced as the intensity ratio increases past a relatively low threshold value, with TM polarization out performing TE [7

7. J. Foulkes and R. J. Blaikie, “Influence of polarization on absorbance modulated subwavelength grating structures,” J. Vac. Sci. Technol. B 27(6), 2941–2946 (2009). [CrossRef]

].

By replacing the ARC with a plasmonic reflector it is seen that the process latitude when using TM polarization is increased even further, 34% and 82% for power ratios of 4 and 5 respectively. This increase is due to the refocusing upon reflection of high spatial frequencies resulting in an increased depth of focus and the creation of a symmetric intensity profile of the evanescent waves throughout the depth of the photoresist. As expected, when we go to higher power ratios where high spatial frequencies begin to dominate the exposure, the benefit of the plasmonic reflector becomes greater. However, this increase process latitude does not come with out a price. As is evident in the experimental data the minimum linewidth achievable at a given power ratio increases when the plasmonic reflector is used. This increase can be explained by a broadening of the transparent aperture in the AML by the reflected λ1 intensity.

4.2 Surface-relief gratings

In the FEM simulations the AML is modeled as a static layer where only its absorption characteristics can be changed. In reality this is not true for pMAEA, which, like most polymerized azobenzene molecules, exhibits the formation of photoinduced SRG when illuminated with a standing-wave interference pattern. This well known phenomenon is not trivial and can have undesirable effects on the performance of AMIL.

Since the material flows from areas of high intensity light to low intensity the thickness of the AML at the optical null grows during the exposure. This increases the absorption in the transparent regions. Likewise, the thickness of the AML is decreased at the optical peaks resulting in decreases absorption in the opaque regions. This effectively reduces the absorption contrast, calculated as the optical density ratio between the opaque regions at the optical peaks and the transparent apertures at the nulls as seen in Fig. 10
Fig. 10 Change in absorption contrast during exposure due to the formation of SRGs. The contrast of this material without the SRG present is 2.3 as shown in Fig. 5.
. An absorption contrast reduction of up to 25% is observed due to this effect, which may be responsible for some of the linewidth broadening observed in our experiments, and points to the need for AML materials with reduced susceptibility for forming SRGs in future.

5. Conclusion

In this paper we have confirmed experimentally the results from 2D FEM simulations that show increase contrast and depth of focus in AMIL when using TM polarization for λ1 and integrating plasmonic reflector below the photoresist. This was done by measuring the process latitude for various exposure conditions. At an intensity ratio of 4, a 32% increase in process latitude was gained by using TM polarization for λ1 and a further 34% increase was achieved by integrating a plasmonic reflector into the resist stack, for a total increase in process latitude of 66%. We also discussed the formation of photoinduced SRG in the AML during exposure and its detrimental effects on the contrast of AMIL.

Acknowledgments

The authors would like to thank Rajesh Menon, Hsin-Yu Tsai, and Samuel W. Thomas III for synthesizing and donating the pMAEA material. This research is supported by the Marsden Fund Council contract No. UOC0806.

References and links

1.

S. Okazaki, “Resolution limits of optical lithography,” J. Vac. Sci. Technol. B 9(6), 2829–2833 (1991). [CrossRef]

2.

E. Muzio, “Optical lithography cost of ownership (COO) – final report for LITG501,” SEMATECH, (2000) http://www.sematech.org/docubase/document/4014atr.pdf.

3.

T. Ito, T. Yamada, Y. Inao, T. Yamaguchi, N. Mizutani, and R. Kuroda, “Fabrication of half-pitch 32 nm resist pattern using near-field lithography with a-Si mask,” Appl. Phys. Lett. 89(3), 033113 (2006). [CrossRef]

4.

R. Menon and H. I. Smith, “Absorbance-modulation optical lithography,” J. Opt. Soc. Am. A 23(9), 2290–2294 (2006). [CrossRef] [PubMed]

5.

T. L. Andrew, H. Y. Tsai, and R. Menon, “Confining light to deep subwavelength dimensions to enable optical nanopatterning,” Science 324(5929), 917–921 (2009). [CrossRef] [PubMed]

6.

R. Menon, H. Y. Tsai, and S. W. Thomas 3rd, “Far-field generation of localized light fields using absorbance modulation,” Phys. Rev. Lett. 98(4), 043905 (2007). [CrossRef] [PubMed]

7.

J. Foulkes and R. J. Blaikie, “Influence of polarization on absorbance modulated subwavelength grating structures,” J. Vac. Sci. Technol. B 27(6), 2941–2946 (2009). [CrossRef]

8.

M. D. Arnold and R. J. Blaikie, “Subwavelength optical imaging of evanescent fields using reflections from plasmonic slabs,” Opt. Express 15(18), 11542–11552 (2007). [CrossRef] [PubMed]

9.

COMSOL Inc, 744 Cowper Steet, Palo Alto, CA 94301, www.comsol.com.

10.

S. M. Rytov, “Electromagnetic properties of a finely stratified medium,” Sov. Phys. JETP 2, 466–475 (1956).

11.

H. S. Ho, A. Natansohn, C. Barrett, and P. Rochon, “Azo polymers for reversible optical storage. 8. The effect of polarity of the azobenzene groups,” Can. J. Chem. 72(11), 1773–1778 (1995). [CrossRef]

12.

C. Barrett, A. Natansohn, and P. Rochon, “Cis-trans thermal isomerization rates of bound and doped azobenzenes in a series of polymers,” Chem. Mater. 7(5), 899–903 (1995). [CrossRef]

13.

N. K. Viswanathan, D. Y. Kim, S. Bian, J. Williams, W. Liu, L. Li, L. Samuelson, J. Kumar, and S. K. Tripathy, “Surface relief structures on azo polymer films,” J. Mater. Chem. 9(9), 1941–1955 (1999). [CrossRef]

14.

C. J. Barrett, A. L. Natansohn, and P. L. Rochon, “Mechanism of optically inscribed high-efficiency diffraction gratings in azo polymer films,” J. Phys. Chem. 100(21), 8836–8842 (1996). [CrossRef]

OCIS Codes
(110.5220) Imaging systems : Photolithography
(160.4330) Materials : Nonlinear optical materials
(240.6680) Optics at surfaces : Surface plasmons

ToC Category:
Imaging Systems

History
Original Manuscript: July 11, 2011
Revised Manuscript: August 7, 2011
Manuscript Accepted: August 8, 2011
Published: August 25, 2011

Citation
Charles W. Holzwarth, John E. Foulkes, and Richard J. Blaikie, "Increased process latitude in absorbance-modulated lithography via a plasmonic reflector," Opt. Express 19, 17790-17798 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-18-17790


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References

  1. S. Okazaki, “Resolution limits of optical lithography,” J. Vac. Sci. Technol. B9(6), 2829–2833 (1991). [CrossRef]
  2. E. Muzio, “Optical lithography cost of ownership (COO) – final report for LITG501,” SEMATECH, (2000) http://www.sematech.org/docubase/document/4014atr.pdf .
  3. T. Ito, T. Yamada, Y. Inao, T. Yamaguchi, N. Mizutani, and R. Kuroda, “Fabrication of half-pitch 32 nm resist pattern using near-field lithography with a-Si mask,” Appl. Phys. Lett.89(3), 033113 (2006). [CrossRef]
  4. R. Menon and H. I. Smith, “Absorbance-modulation optical lithography,” J. Opt. Soc. Am. A23(9), 2290–2294 (2006). [CrossRef] [PubMed]
  5. T. L. Andrew, H. Y. Tsai, and R. Menon, “Confining light to deep subwavelength dimensions to enable optical nanopatterning,” Science324(5929), 917–921 (2009). [CrossRef] [PubMed]
  6. R. Menon, H. Y. Tsai, and S. W. Thomas, “Far-field generation of localized light fields using absorbance modulation,” Phys. Rev. Lett.98(4), 043905 (2007). [CrossRef] [PubMed]
  7. J. Foulkes and R. J. Blaikie, “Influence of polarization on absorbance modulated subwavelength grating structures,” J. Vac. Sci. Technol. B27(6), 2941–2946 (2009). [CrossRef]
  8. M. D. Arnold and R. J. Blaikie, “Subwavelength optical imaging of evanescent fields using reflections from plasmonic slabs,” Opt. Express15(18), 11542–11552 (2007). [CrossRef] [PubMed]
  9. COMSOL Inc, 744 Cowper Steet, Palo Alto, CA 94301, www.comsol.com .
  10. S. M. Rytov, “Electromagnetic properties of a finely stratified medium,” Sov. Phys. JETP2, 466–475 (1956).
  11. H. S. Ho, A. Natansohn, C. Barrett, and P. Rochon, “Azo polymers for reversible optical storage. 8. The effect of polarity of the azobenzene groups,” Can. J. Chem.72(11), 1773–1778 (1995). [CrossRef]
  12. C. Barrett, A. Natansohn, and P. Rochon, “Cis-trans thermal isomerization rates of bound and doped azobenzenes in a series of polymers,” Chem. Mater.7(5), 899–903 (1995). [CrossRef]
  13. N. K. Viswanathan, D. Y. Kim, S. Bian, J. Williams, W. Liu, L. Li, L. Samuelson, J. Kumar, and S. K. Tripathy, “Surface relief structures on azo polymer films,” J. Mater. Chem.9(9), 1941–1955 (1999). [CrossRef]
  14. C. J. Barrett, A. L. Natansohn, and P. L. Rochon, “Mechanism of optically inscribed high-efficiency diffraction gratings in azo polymer films,” J. Phys. Chem.100(21), 8836–8842 (1996). [CrossRef]

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