BackgroundIn extreme ultraviolet (EUV) photoresist exposure, the primary and secondary electrons drive chemistry rather than the EUV photons themselves. These electrons have a wide range of energies below approximately 80 eV, which are capable of complex network of reactions during exposure.AimTo better understand the ability of electrons of different energies within the EUV primary and secondary electron range, we want to characterize and compare the chemistry induced in pure polymer films by direct exposure to electrons.ApproachThin films of poly(tert-butyl methacrylate), poly(methyl methacrylate), and poly(4-hydroxystyrene) were exposed to a 20 to 80 eV electron beam. Outgassing during exposure was characterized in-situ using a quadrupole residual gas analyzer. The thickness changes were measured using ellipsometry and chemical bond structure data were collected using Fourier-transform infrared spectroscopy (FTIR) after exposure to compare different exposure conditions.ResultsPoly(4-hydroxystyrene) demonstrated stability during electron exposures. Exposures of the other two materials led to outgassing of protecting groups, the intensity of which decayed in time. Outgassing, FTIR, and thickness loss data exhibited approximately linear relationships to each other.
EUV photon-generated primary and secondary electrons, produced by a cascade of inelastic scattering events, have a wide range of energies and drive EUV patterning by initiating chemical transformations. To study electron-induced chemistry we exposed photoresist films to an electron beam varied from 20 to 80 eV. Chemical transformations were characterized using a residual gas analyzer, FTIR, and ellipsometry. Total outgassing was determined for key fragments and compared to film thickness changes via ellipsometry and FTIR bond structure changes. Estimates of electron penetration depth and reaction efficiency (bonds broken per electron) will be presented.
Resists are needed to advance extreme ultraviolet (EUV) lithography. In EUV resists, due to the high energy of the incident photons, most of the chemistry arises from the emitted primary and secondary electrons and not the EUV photons themselves. Because the electrons are playing a leading role in EUV patterning, initiating chemical transformations, it is important to characterize their generation, transport, and energy distribution. In this work, we present several experimental techniques to probe model polymer materials to investigate the impact of specific chemical groups on critical resist properties: EUV absorption, electron emission, electron attenuation length (EAL), and energy distribution of emitted electrons. Total electron yield provides information on the conversion of absorbed EUV photons to electrons, and photoelectron spectroscopy provides information on energy distribution of generated electrons. The EAL reveals the distance that the electrons can travel in a resist film, which is related to the electron blur. Correlations between the obtained experimental values are discussed. We explore how different elements or functional groups change the yield, EAL, and energy distribution of emitted electrons, aiming to understand how to control the electron cascade.
In extreme ultraviolet (EUV) resists, due to the high energy of the incident photons, most of the radiation chemistry arises from the photon-generated primary and secondary electrons and not the EUV photons themselves. These electrons are generated by a cascade of inelastic scattering events, have a wide range of energies, and play a leading role in EUV patterning by initiating chemical transformations. To characterize electron-induced chemical transformations we exposed photoresist films to an electron beam 7-9 mm in diameter with energies varied from 20 to 80 eV to study the effect of fast primary electrons as well as slow secondary electrons on photoresist materials. Chemical changes of poly-tert-butyl methacrylate (tBMA), poly-methyl methacrylate (PMMA), and poly-4-hydroxystyrene (PHS) were characterized in-situ during exposure using a quadrupole residual gas analyzer (RGA). Full RGA spectra were used to identify the outgassed compounds from samples, and the time dependence of key chemical fragment masses was studied to observe the depletion of fragments from the sample over time. Transmission FTIR data were collected through exposed spots to quantify the changes in chemical bond structure. The number of molecules lost from each resist was estimated by the integrated partial pressure rise observed in RGA and compared to the total thickness change of exposed areas measured using ellipsometry.
In extreme ultraviolet (EUV) resists, due to the high energy of the incident photons, most of the radiation chemistry arises from the emitted electrons and not the EUV photons themselves. The absorption of an EUV photon by a resist film leads to the emission of primary electrons, which, through a cascade of inelastic scattering events, cause excitation of molecules and emission of secondary electrons. Because the electrons are playing a leading role in EUV patterning by initiating the majority of chemical transformations, it is important to characterize their generation, transport, and energy distribution. In this work we investigate the emission of electrons in model EUV photoresists using photoelectron spectroscopy. The photoelectron spectroscopy reveals variations in the energy distributions and abundances of emitted electrons for different resist compositions (bare polymer, PAG, and quencher concentrations). EUV exposure of resists induces a change of the electron emission spectra, mostly affecting emission of slow secondary electrons. By applying this approach to each resist component, we begin to decipher the role of resist components in electron generation, capture, and transmission.
In EUV resists, due to the high energy of the incident photons, most of the radiation chemistry arises from the emitted electrons and not the EUV photons themselves. The absorption of an EUV photon by a resist film leads to the emission of a primary electron, which, through a cascade of inelastic scattering events, causes the excitation of molecules and emission of secondary electrons. Those electrons play a leading role in EUV patterning, initiating chemical transformations. To characterize electron induced chemical transformations in photoresist materials we exposed thin films of model resists to an electron beam and quantified electron induced changes employing FTIR spectroscopy. Combination of polymers with PAG served as model resists. Employed electron energies varied from 20 to 80 eV to study the effect of fast primary electrons as well as slow secondary electrons on photoresists.
In EUV resists, due to the high energy of the incident photons, most of the radiation chemistry arises from the emitted electrons and not the EUV photons themselves. The absorption of an EUV photon by a resist film leads to the emission of primary electrons, which, through a cascade of inelastic scattering events, cause excitation of molecules and emission of secondary electrons. Those electrons are playing a leading role in EUV patterning, initiating chemical transformations. The photoelectron spectroscopy of the resist materials provides information on the abundance and kinetic energies of primary and secondary electrons generated by EUV photons, as well as the ratio of slow vs. fast electrons. The photoelectron spectra demonstrate that the primary electrons have narrow energy distributions, whereas all spectra have broad distributions of slow, mostly secondary electrons, dominating the electron yield. The emitted primary electrons were assigned using quantum mechanical calculations.
New photoresists are needed to advance Extreme Ultraviolet (EUV) lithography. Tailored design of efficient resists is impossible without a fundamental understanding of EUV induced chemistry. In EUV resists, due to the high energy of the incident photons, most of the chemistry arises from the emitted electrons and not the EUV photons themselves. The absorption of an EUV photon by a resist film leads to the emission of primary electrons, which, through a cascade of inelastic scattering events, cause excitation of molecules and subsequent emission of secondary electrons. The electrons may travel up to tens of nanometers before losing their kinetic energy via collisions which initiate chemical reactions. Because the electrons are playing a leading role in EUV patterning, initiating chemical transformations, it is important to characterize their generation, transport, and energy distribution. Previously, we demonstrated a suite of experimental techniques capable of characterizing EUV absorption, electron yield, and electron attenuation length of resist materials. In this work we demonstrate how to experimentally investigate the energy spectrum of those electrons via EUV photoelectron spectroscopy of thin films. We explore how different elements or functional groups may change the energy distribution of emitted electrons, aiming to understand how to control the electron cascade.
New resists are needed to advance EUV lithography. Tailored design of efficient photoresist is impossible without fundamental understanding of EUV induced chemistry. The absorption of an EUV photon by a thin film resist leads to emission of primary and secondary electrons. The electrons may travel up to tens of nanometers before losing their kinetic energy via collisions which initiate chemical reactions. The “blur” of an aerial image is directly related to the distance that electrons are able to travel and initiate chemistry in the resist. Thus, identifying how to measure and influence the absorption of EUV photons, emission of electrons, and distance traveled by the secondary electrons is extremely beneficial to the resist community.
In this work, we present several experimental techniques to probe model polymer materials to investigate the impact of specific chemical groups on three critical resist properties: EUV absorption, electron emission, and the electron attenuation length (EAL). EUV absorption dictates the efficiency of the film to absorb photons. Total electron yield (TEY) provides information on the conversion of absorbed EUV photons to electrons, whereas photoelectron spectroscopy (PES) provides information on energies and abundance of generated electrons. The EAL corresponds to the thickness of a material required to reduce the number of emitted electrons to 1/e of the initial value. The EAL reveals the distance the electrons can travel in a resist film, which is directly related to the electron blur. Correlations between the obtained experimental values is discussed.
The absorption of an EUV photon by a thin film resist leads to the emission of a photoelectron as well as several secondary electrons with low kinetic energy. The “universal curve”, used in X-ray photoelectron spectroscopy, indicates that the low kinetic energy electrons may travel tens to hundreds of nanometers before losing their kinetic energy via initiation of chemical reactions. The distance that the electrons are able to travel in the resist is directly related to the resultant “blur” of the aerial image. Thus, identifying how to measure and influence the distance traveled by the secondary electrons is extremely beneficial to the resist community.
In this work, we utilize several model polymer materials to investigate the impact of specific chemistry groups on the secondary electron attenuation length (EAL) – the thickness of resist material required to reduce number of emitted secondary electrons to 1/e of initial. The EAL measures the distance the secondary electrons can travel in a resist film, which is directly related to the electron blur. Possibilities to gain additional information on electron penetration depth in resist films will also be discussed.
Extreme ultraviolet (EUV) lithography is one of the most promising printing techniques for high-volume semiconductor manufacturing at the 14-nm half-pitch device node and beyond. However, key challenges around EUV photoresist materials, such as the exposure-dose sensitivity or the line-width roughness, continue to impede its full adoption into industrial nanofab facilities. Metrology tools are required to address these challenges by helping to assess the impact of the EUV materials’ properties and processing conditions along different steps of the nanofabrication process. We apply the resonant soft x-ray scattering (RSoXS) technique to gain insights into the structure of patterned EUV resists before the development step takes place. By using energies around the carbon K-edge to take advantage of small differences in chemistry, the electronic density contrast between the exposed and unexposed regions of the resists could be enhanced in order to image the patterns with subnanometer precision. Critical-dimension grazing-incidence small-angle x-ray scattering is then performed at energies where the contrast is maximized, enabling the reconstruction of the three-dimensional shape of the latent image. We demonstrate the potential of RSoXS to provide a high-resolution height-sensitive profile of patterned EUV resists, which will help in quantifying the evolution of critical features, such as the line-edge roughness, at a key step of the nanofabrication process.
Extreme ultraviolet lithography is one of the most promising printing techniques for high volume semiconductor manufacturing at the 14 nm half-pitch device node and beyond. However, key challenges around EUV photoresist materials such as the exposure-dose sensitivity or the line-width roughness continue to impede the full adoption into industrial nanofab facilities. New metrology tools are required to address these challenges by helping to determine the impact of the EUV materials’ properties and processing conditions on the roughness through the different step of the process. Here, we apply the resonant soft x-ray scattering (RSOXS) technique to gain insights into the structure of patterned EUV resists before the development step takes place. By using energies around the carbon absorption edge to take advantage of small differences in chemistry, the electronic density contrast between the exposed and unexposed regions of the resists could be enhanced in order to image the patterns with sub-nm precision. Critical-dimension grazing incidence small-angle X-ray scattering (CDGISAXS) was then performed at energies where the contrast was maximized, enabling the reconstruction of the 3D shape of the latent image. This demonstrates the potential of RSOXS to provide a high-resolution heightsensitive profile of patterned EUV resists, which will help to quantify the evolution of critical features, such as the line edge roughness, at each step of the nanofabrication process.
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