Low-energy Electron Exposure Research Articles

Effect of Oxygen on Thermal and Radiation-Induced Chemistries in a Model Organotin Photoresist.

Organotin photoresists have shown promise for next-generation lithography because of their high extreme ultraviolet (EUV) absorption cross sections, their radiation sensitive chemistries, and their ability to enable high-resolution patterning. To better understand both temperature- and radiation-induced reaction mechanisms, we have studied a model EUV photoresist, which consists of a charge-neutral butyl-tin cluster. Temperature-programmed desorption (TPD) showed very little outgassing of the butyl-tin resist in ultrahigh vacuum and excellent thermal stability of the butyl groups. TPD results indicated that decomposition of the butyl-tin resist was first order with a fairly constant decomposition energy between 2.4 and 3.0 eV, which was determined by butyl group desorption. Electron-stimulated desorption (ESD) showed that butyl groups were the primary decomposition product for electron kinetic energies expected during EUV exposures. X-ray photoelectron spectroscopy was performed before and after low-energy electron exposure to evaluate the compositional and chemical changes in the butyl-tin resists after interaction with radiation. The effect of molecular oxygen during ESD experiments was evaluated, and it was found to enhance butyl group desorption during exposure and resulted in a significant increase in the ESD cross section by over 20%. These results provide mechanistic information that can be applied to organotin EUV photoresists, where a significant increase in photoresist sensitivity may be obtained by varying the ambient conditions during EUV exposures.

Thermal and radiation chemistry of butyltin oxo hydroxo: A model inorganic photoresist

We have investigated the thermal and radiation chemistry of an organotin-based model photoresist to elucidate patterning mechanisms related to extreme ultraviolet (EUV) lithography. Butyltin oxide hydroxide (BuSnOOH) was dissolved in organic solvents and spin-coated to form uniform thin films. The thermal stability and radiation sensitivity of the thin films were characterized by temperature programmed desorption (TPD), electron stimulated desorption (ESD), and X-ray photoelectron spectroscopy (XPS). From the TPD studies, we determined that decomposition of BuSnOOH occurs at ~653 K through cleavage of the butyl-tin bond. Low kinetic energy electron (Ekin = 80 eV) and X-ray (1486.6 eV) exposure also resulted in cleavage of the butyl-tin bond with the resulting desorption of butyl ligands from the film. From the ESD data, we estimated that the butyl ligand desorption cross section was ~4.3 × 10−14 cm2 for electrons with 80 eV kinetic energy. From the XPS results, we found a significant reduction in C 1s intensity for extended X-ray exposures, which corresponds to butyl ligand loss. There were also changes in the O 1s spectra indicating loss of hydroxyl groups and tin oxide network formation during radiation exposure. These results indicate the preferential dissociation and desorption of butyl ligands that can occur through either thermal or radiation induced processes, and that butyl ligand loss leads to the solubility contrast.

Low-energy electron exposure of ultrathin polymer films with scanning probe lithography

Scanning probe lithography (SPL) describes a group of patterning techniques, wherein localized tip-sample interactions are utilized in order to directly or indirectly (via a resist film) generate nanoscale features. In this context, in field-emission SPL (FE-SPL) a beam of low energy electrons (<100eV) is induced by a Fowler-Nordheim type emission from a sharp tip. The emission is a result of a strong, non-uniform electric field applied between a biased sample surface and the tip apex of a proximal scanning probe. Attributed to the application of low energetic electrons, and due to operation in ambient conditions a wide spectrum of electron-related resist modifications is enabled. Here, we demonstrate that different practical lithographic processes can be triggered in polymeric resist films. Typical electron beam lithography resists like PS, PHS and PMMA were applied as references. Within our investigation three patterning modes were identified, in particular: positive tone in self-development manner (1) as well as positive (2) and negative tone (3) registered after a subsequent wet development step. Demonstrated for the first time, all three types of patterning are occurring simultaneously in a superimposed fashion, which gives novel opportunities in nanofabrication.

Pattern-generation and pattern-transfer for single-digit nano devices

Single-electron devices operating at room temperature require sub-5 nm quantum dots having tunnel junctions of comparable dimensions. Further development in nanoelectronics depends on the capability to generate mesoscopic structures and interfacing these with complementary metal–oxide–semiconductor devices in a single system. The authors employ a combination of two novel methods of fabricating room temperature silicon single-electron transistors (SETs), Fowler–Nordheim scanning probe lithography (F-N SPL) with active cantilevers and cryogenic reactive ion etching followed by pattern-dependent oxidation. The F-N SPL employs a low energy electron exposure of 5–10 nm thick high-resolution molecular resist (Calixarene) resulting in single nanodigit lithographic performance [Rangelow et al., Proc. SPIE 7637, 76370V (2010)]. The followed step of pattern transfer into silicon becomes very challenging because of the extremely low resist thickness, which limits the etching depth. The authors developed a computer simulation code to simulate the reactive ion etching at cryogenic temperatures (−120 °C). In this article, the authors present the alliance of all these technologies used for the manufacturing of SETs capable to operate at room temperatures.

Acid Generation Efficiency of EUV PAGs via Low Energy Electron Exposure

Acid Generation Efficiency of EUV PAGs via Low Energy Electron Exposure

Reactions and anion desorption induced by low-energy electron exposure of condensed acetonitrile

Thermal desorption spectrometry (TDS) and electron stimulated desorption (ESD) are employed to investigate mechanisms responsible for the formation of C2H6 in electron irradiated multilayer films of acetonitrile (CH3CN) at 30 K. Using a high sensitivity time-of-flight mass spectrometer, we observe the ESD of anionic fragments H−, CH2 −, CH3 − and CN−. Desorption occurs following dissociative electron attachment (DEA) via several negative ion resonances in the 6 to 14 eV energy range and correlates well with a “resonant” structure seen in the TDS yield of C2H6 (i.e., at mass 30 amu). It is proposed that C2H6 is formed by the reactions of CH3 radicals generated following DEA to CH3CN which also yields CN−. Between 2 and 5 eV, a second resonant feature is seen in the C2H6 signal. While DEA is observed in the gas phase at these energies, no anion desorption occurs since anionic fragments likely have insufficient kinetic energy to desorb. Since the CH2 − ion has not been observed in gas-phase measurements, we propose that it is formed, along with HCN (that is detected in TDS) when dissociation into CH3 − and CN is hindered by adjacent molecules.

Functionalization of a Self-Assembled Monolayer Driven by Low-Energy Electron Exposure

Self-assembled monolayers (SAMs) of 10-undecene-1-thiol on Au were functionalized with nitrogen-containing groups using an approach in which multilayer ammonia (NH(3)) films were deposited at low temperature onto the SAMs and subsequently exposed to 15 eV electrons. The result of this process was investigated after removal of the remaining NH(3) by annealing to room temperature using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). HREELS shows that the CC double bonds disappear during electron exposure, while XPS gives evidence that about 25% of the terminal double bonds of the SAM were functionalized. Also, XPS shows that a sufficiently thick NH(3) layer protects the underlying SAM from electron-induced damage. The process suggested here thus represents a particularly gentle approach to the functionalization of ultrathin molecular layers. Thermal desorption spectrometry (TDS) and electron-stimulated desorption (ESD) experiments on condensed layers of NH(3) reveal production of N(2) but show that significant amounts of the initial NH(3) as well as N(2) produced during electron exposure desorb. Hydrogen released upon formation of N(2) is held responsible for the reduction of double bonds and protection of the SAMs from damage.

Low-energy electron-induced reactions in thin films of glucose and N-acetyl-glucosamine

Reactions induced in thin films of α- d-glucose and N-acetylglucosamine by low-energy electron exposure at incident electron energies ( E 0) between 5 eV and 15 eV have been investigated by high-resolution electron energy loss spectroscopy (HREELS). The reactions of α- d-glucose upon electron exposure were also studied in the presence of molecular oxygen. Electron exposure leads to characteristic changes of the vibrational spectra indicating that OH groups are lost with the formation of CC double bonds taking place preferentially above the ionisation threshold of the investigated molecules. At lower E 0, OH groups are equally decomposed suggesting that dissociative electron attachment contributes to the reactions but formation of double bonds is not observed. The results show that different reaction channels are effective depending on E 0 and that the outcome of electron-driven chemistry in saccharides may be controlled by changing from the subionisation regime to E 0 above the ionisation threshold. Generally, low-energy electron exposure in the absence of O 2 produces a material with lower oxygen content, i.e. leads to a reduction of the saccharide. In the case of N-acetylglucosamine, removal of the amide group from the sugar is also important at subionisation energies. In contrast, as shown for α- d-glucose, low-energy electron exposure in the presence of O 2 leads to oxidation of the sugar even at cryogenic temperature.

Modification of surface properties of thin polysaccharide films by low-energy electron exposure

The effect of low-energy electron irradiation on thin layers of two different polysaccharide materials, namely amylose and chitosan, has been investigated. Modification of the samples occurs at electron energies as low as 5 eV. In the case of amylose, a change in the wetting properties is observed upon exposure. Loss of oxygen from the hydroxyl groups and consequent formation of a hydrocarbon material is responsible for this effect as revealed by analysing the samples before and after electron exposure by means of reflection absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS). It is shown that the amide side groups in chitosan are more reactive than the hydroxyl groups and consequently dominate the modifications in this case. Additional experiments by high-resolution electron energy loss spectroscopy (HREELS) on thin condensed films of the model compound α- d-glucose show that facile oxidation of the saccharide occurs in the presence of O 2. Traces of O 2 as a residual gas can therefore be held responsible for the formation of carboxyl groups that are observed by RAIRS in samples of amylose after electron exposure.

Electrical circuit modeling of surface structures for X-ray photoelectron spectroscopic measurements

We model the X-ray photoelectron spectrometer and the sample with lumped electrical circuit elements, and simulate various types of conditions using a widely used computer program (PSpice) and compare the results with experimental measurements. By using the electrical model simulations, the surface voltage and the spectrum can be estimated under various types of external voltage stimuli, and the zero potential condition can be predicted accurately for obtaining a truly uncharged spectrum. Additionally, effects of several charging mechanisms (taking place during XPS measurements) on the surface potential could easily be assessed. Finally, the model enables us to find electrical properties, like resistance and capacitance of surface structures, under X-ray and low-energy electron exposure.

Low-energy Electron Exposure Effects on the Optical Properties of ZnO/K2SiO3 Thermal Control Coating

An experimental investigation was undertaken to study low-energy electron exposure effects upon ZnO/K2SiO3 thermal control coating. The specimens were exposed to 10-, 30-, 50-, and 70-keV electrons, respectively. The spectral reflectance of each specimen was measured in situ before and after electron exposures. The solar absorptance was calculated by assuming a Johnson solar spectral irradiance distribution. Electron paramagnetic resonance and photoluminescence measurements were made before and after electron exposures to explain physical changes induced by the electron exposures. All the specimens exhibited a reflectance decrease throughout the visible and the near-infrared regions, and also the induced optical degradation was found to be electron energy dependent, with increased damage for increasing electron energy. It is believed that the optical degradation of ZnO/K2SiO3 thermal control coating, induced by the low-energy electron exposures, is mainly due to the ionization process of the ZnO pigment.

Low-energy electron-beam patterning of amine-functionalized self-assembled monolayers

Patterned amine-functionalized self-assembled monolayers have potential as a template for the deposition and patterning of a wide variety of materials on silicon surfaces, including biomolecules. Results are presented here for low-energy electron-beam patterning of 2-aminopropyltriethoxysilane and (aminoethylaminomethyl)phenethyltrimethoxysilane self-assembled monolayers on silicon substrates. On these ultrathin (1–2 nm) monolayers, lower electron beam energies (&amp;lt;5 keV) produce higher resolution patterns than high-energy beams. Auger electron spectroscopy indicates that low-energy electron exposure primarily damages the amine groups. At 1 keV, a dose of 40 μC/cm2 is required to make the patterns observable by lateral force microscopy. Features as small as 80 nm were exposed at 2 keV on these monolayers. After exposure, palladium colloids and aldehyde- and protein-coated polystyrene fluorescent spheres adhered only to unexposed areas of the monolayers.

Charging and discharging of dielectric films irradiated by high-/medium-/low-energy electrons

Exposure of Mylar and Teflon films to combined high-/medium-/low-energy electrons has been performed, its results differing from those of exposure to medium-energy electron environment alone, combined high-/medium-energy electron environment, and combined medium-/low-energy electron environment. The experimental results have shown that dielectric discharge can be initiated by high-energy electron exposure. The results of exposure of Teflon films to combined high-/medium-/low-energy electron environment agree with those of exposure to 1–100 keV dN dE ∝ E −2 spectral energy electron environment and those of NASCAP computer simulation. It is concluded that combined high-/medium-/low-energy electron exposure can perfectly simulate charging and subsequent discharging effects of dielectrics aboard geosynchronous satellites. Necessity and sufficiency of using electrons with the three different energies have been demonstrated. It is inferred that the surface potential of dielectrics on spacecraft surface will be raised and discharges will be initiated if high- and/or medium-energy electron fluxes increase to an adequate value in geosynchronous orbit during geomagnetic storms or substorms.

Low energy electron diffraction studies of the surfaces of molecular crystals (ice, ammonia, naphthalene, benzene)

Low Energy Electron Diffraction (LEED) has been used to study the surface structures of thin films of molecular crystals. The samples were grown epitaxially on metal single crystal substrates at low temperatures. Both Pt(111) and Ag(111) surfaces were used as substrates in order to identify the influence of the substrate on molecular film structure. Previous observations of ice (0001) and naphthalene (001) surfaces on films grown on Pt(111) substrates [Surface Sci. 55 (1976) 413], were confirmed using the Ag(111) substrate. The NH 3(111) and benzene (111) surfaces were also studied on films grown on either substrate. All observed molecular crystal surfaces showed no evidence of surface reconstruction. To minimize sample charging and electron beam induced damage, LEED experiments were performed on samples of thickness less than 10−10 2 nm, with low energy electron exposures less than 1 C cm −2. The maximum thickness and exposure values were characteristic of the particular molecular crystal. The relationship between the structure of the initial adsorbed monolayer and the molecular crystal orientation is discussed.

The Effects of High-Energy Electrons on the Charging of Spacecraft Dielectrics

Results are presented of a laboratory investigation of discharges in spacecraft dielectrics using both a low-energy electron and a combined low- and high-energy electron environment. Comparison of the charging and discharge characteristics indicates that the presence of high-energy electrons in space will result in significantly different behavior from that predicted for low-energy electron simulations alone. For Kapton, fiberglass, and second-surface mirrors, the effect of the high-energy electrons is primarily to increase the conductivity, thus often limiting surface potentials to below discharge threshold values. Variation of the relative fluxes of low- and high-energy electrons can lead to discharge; however, the magnitude of the charge blowoff is significantly smaller (~1/20) than that observed upon exposure to low-energy electrons alone. Astroquartz, which for low-energy electron exposures exhibited no measurable charge blowoff, exhibited significant charge blowoff (Ip = 2 amp) upon exposure to a combined high- and low-energy electron environment.