An official website of the United States government
Radiolysis is a complex phenomenon in which molecules subjected to ionizing radiation form new chemical species. Electron-beam irradiation has proven to be a versatile approach for significantly altering materials’ properties and forms the basis for electron-beam lithography using both organic and inorganic resists. Electron-beam exposure is normally carried out under high vacuum conditions to reduce contamination and allow for unhindered interaction between the electrons and the resist material. Exposure under an ambient gas at sub-atmospheric pressures has been found to provide a distinct mechanism which can be exploited to circumvent some of the challenges associated with material processing and significantly alter or enhance material’s properties. This dissertation discusses the modifications in standard electron beam resist characteristics during gas assisted electron beam pattering. We studied the effect of water vapor pressure on positive and negative tone electron-beam patterning of poly methyl methacrylate (PMMA). For both positive and negative-tone patterning, it was found that increasing the water vapor pressure considerably improved the contrast of PMMA. As expected from electron scattering in a gas, the clearing dosage for positive tone patterning gradually increased with vapor pressure. Also, electron scattering in water vapor yielded a substantially larger clear region around the negative-tone patterns. This effect could be useful for increasing the range of the developed region around cross-linked PMMA far beyond the backscattered electron range. As a result, VP-EBL for PMMA offers a new means of tuning clearing/onset dose and contrast while enabling more control over the size of the cleared region around negative-tone patterns. We provide a novel way to simultaneously tune the emission wavelength and enhance the fluorescence intensity of fluorophores formed by irradiating polystyrene with a focused electron beam under various gaseous environments. We studied the effect of electron dose and gas pressure on the emission spectra and photon yield of irradiated polystyrene film on a variety of substrates. Up to 10x enhancement in fluorescence yield was achieved using water vapor and the peak emission wavelength tuned over a wide wavelength range. Thus, localized electron-beam synthesis of fluorophores in polystyrene can be controlled by both dose and by ambient water-vapor pressure. This technique could enable innovative approaches to photonics where fluorophores with tunable emission properties can be locally introduced by electron-beam patterning. We also studied the effect of ambient gases on contrast and resolution of PMMA on conducting and insulating substrates. E-beam exposures were conducted under vacuum conditions and 1 mbar of water vapor, helium, nitrogen and argon to study their effect on contrast and resolution of PMMA on silicon, fused silica and soda lime glass substrates. On silicon, exposure under water vapor yielded contrast values significantly higher than vacuum exposure, consistent with our previous work. However, exposure under helium yielded slightly improved contrast compared to vacuum exposure. On insulating substrates exposure under helium environment yielded contrast values significantly higher compared to vacuum exposure. The clearing dose was found to increase with the gases’ molecular weight and proton number, consistent with the increase in scattering cross-section. The improved contrast and sensitivity (dose to clear) of PMMA under helium motivated us to study the resolution under various gases. Resolution testing indicated that despite the lower clearing dose, helium still exhibited the best resolution with 25-nm half-pitch dense lines and spaces clearly resolved on soda lime glass. Thus, VP-EBL of PMMA under helium yields higher sensitivity and contrast on insulating substrates without sacrificing resolution.
more »
« less
Effect of water vapor pressure on positive and negative tone electron-beam patterning of poly(methyl methacrylate)
Variable-pressure electron-beam lithography (VP-EBL) employs an ambient gas at subatmospheric pressures to reduce charging during electron-beam lithography. VP-EBL has been previously shown to eliminate pattern distortion and provide improved resolution when patterning poly(methyl methacrylate) (PMMA) on insulating substrates. However, it remains unknown how water vapor affects the contrast and clearing dose nor has the effect of water vapor on the negative-tone behavior of PMMA been studied. In addition, water vapor has recently been shown to alter the radiation chemistry of the VP-EBL process for Teflon AF. Such changes in radiation chemistry have not been explored for PMMA. In this work, VP-EBL was conducted on conductive substrates to study the effect of water vapor on PMMA patterning separately from the effects of charge dissipation. In addition, both positive and negative-tone processes were studied to determine the effect of water vapor on both chain scission and cross-linking. The contrast of PMMA was found to improve significantly with increasing water vapor pressure for both positive and negative-tone patterning. The clearing dose for positive-tone patterning increases moderately with vapor pressure as would be expected for electron scattering in a gas. However, the onset set dose for negative-tone patterning increased dramatically with pressure revealing a more significant change in the exposure mechanism. X-ray photoelectron spectra and infrared transmission spectra indicate that water vapor only slightly alters the composition of exposed PMMA. Also, electron scattering in water vapor yielded a much larger clear region around negative-tone patterns. This effect could be useful for increasing the range of the developed region around cross-linked PMMA beyond the backscattered electron range. Thus, VP-EBL for PMMA introduces a new means of tuning clearing/onset dose and contrast, while allowing additional control over the size of the cleared region around negative-tone patterns.
more »
« less
- Award ID(s):
- 2135666
- PAR ID:
- 10481208
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Journal of Vacuum Science & Technology B
- Volume:
- 41
- Issue:
- 1
- ISSN:
- 2166-2746
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Variable Pressure - Electron Beam Lithography was conducted to study the effect of water vapor pressure on patterning PMMA. Water vapor does not appear to alter the radiation chemistry of positive-tone exposure. However, water vapor does alter the negative tone behavior in a manner consistent with simultaneous cross-linking and etching.more » « less
-
Ice Lithography (IL) is a new technique for electron beam lithography that employs conformal ices (i.e., solid phase condensed gasses) as sacrificial resist layers for lithographic processes on a variety of sample types. The completely dry in-situ processing inherent in IL allows for high precision patterning of delicate and organic surfaces such as atomic force microscope (AFM) tips and proteins. In this talk I will present an overview of the capabilities of the MU IL instrument and describe our recent work, which is centered on pushing ice species beyond water. Amorphous H2O ice is a material that has been well characterized in the astrophysics community but has received little attention from the nanotechnology field. It is a positive resist that requires large critical doses (about 2C/cm^2 for 200nm thick ice). The alcohols (ethanol, methanol and isopropanol) are attractive candidates to complement water ice. They are negative phase resists which exhibit lower critical dose (0.2C/cm^2 for 200nm thick ice) requirements than water ice. We are exploring the use of alcohol resists in conjunction with Reactive Ion Etching (RIE) for accurate modification of small, delicate, structures such as AFM tips. We are also interested in characterizing the residual material from alcohol ices after exposure to the electron beam which can be achieved through several methods including Electron Energy Loss Spectroscopy (EELS) and simulations such as the high energy chemistry program GEANT-4.more » « less
-
The implementation of aberration-corrected electron beam lithography (AC-EBL) in a 200 keV scanning transmission electron microscope (STEM) is a novel technique that could be used for the fabrication of quantum devices based on 2D atomic crystals with single nanometer critical dimensions, allowing to observe more robust quantum effects. In this work we study electron beam sculpturing of nanostructures on suspended graphene field effect transistors using AC-EBL, focusing on the in situ characterization of the impact of electron beam exposure on device electronic transport quality. When AC-EBL is performed on a graphene channel (local exposure) or on the outside vicinity of a graphene channel (non-local exposure), the charge transport characteristics of graphene can be significantly affected due to charge doping and scattering. While the detrimental effect of non-local exposure can be largely removed by vigorous annealing, local-exposure induced damage is irreversible and cannot be fixed by annealing. We discuss the possible causes of the observed exposure effects. Our results provide guidance to the future development of high-energy electron beam lithography for nanomaterial device fabrication.more » « less
-
To our knowledge, these are the first studies of molecules other than water for EBL in gaseous environments. Exposure of PMMA under helium yields higher sensitivity, contrast (12.5) and the highest resolution (25-nm half-pitch dense lines and spaces) demonstrated to date for EBL on insulating substrates in a gaseous environment.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1116/6.0002118
