The development of nanotechnologies requires patterning techniques capable of producing nanometric features. A standard approach is represented by electron beam lithography (EBL), where a thin film of resist is exposed prior to the pattern transfer process, such as metal evaporation and lift-off or reactive ion etching, among the others. However, both forward and backward electron scattering in the resist layers hinders the ultimate lateral resolution and the minimum pitch achievable. The latter in particular can suffer from the so-called proximity effects, where backscattered electrons end up exposing regions between closely irradiated features. An alternative approach is given by the exposure of the organic resist by low energy (10 keV) ion beam irradiation using a Focused Ion Beam (FIB) instrument [1]. Ion beam exposure allows the patterning with high lateral resolution (close to 10 nm or below), and a mask-less approach over large areas with minimal stitching issues. Moreover, the ion-based lithography would lead to a drastically reduced ion fluence or dose to expose the resist layer, given the denser energy release, resulting in a faster exposure compared to electrons as long as the current is comparable. Finally, since ion backscattering is negligible, the lateral resolution would be mainly defined by the beam focusing and the forward scattering. As a main disadvantage, ions accelerated at 10 keV energies can only penetrate 100-200 nm of most of organic films, limiting the thickness of the resist films that can be exposed. Recent FIB tools equipped with different multi-species ion sources [2] enlarge the palette of ion species that can be tested in order to optimize lateral resolution, feature density and resist thickness [3, 4]. In this work, we have developed the processes of ion beam lithography on resist using the ion species available in a liquid metal alloy ion source (LMAIS) installed on a Raith Velion FIB system: Si++, Ge++ and Au ions. We tested two acceleration voltages, 35 (optimal focus) and 17.5 kV resulting in kinetic energies of 70 and 35 keV, respectively. Poly(methyl methacrylate) (PMMA) resist was used in two different dilutions in anisole, resulting in 2 thicknesses, 105 and 200 nm, respectively. At first, dose matrix was carried out to identify the process window for the different combinations of ion species and resist thicknesses, Figure 1 shows optical images of the resist exposed with the different ions. Measuring the irradiated areas after development with a mechanical stylus profilometer, it resulted clear that the 70 keV Si ions were able to expose both 105 and 220 nm resists, whereas the 70 keV Ge and 35 keV Si only exposed the thinner one. On the other end, Au++ ions never fully exposed the tested resist thicknesses due to the reduced penetration depth. Contrast curves versus ion fluence were acquired with a different denser dose matrix for the relevant ion species, in where depths were measured by atomic force microscopy as reported in Figure 2. From this it was possible to identify the dose windows to be used for a resolution test. The latter was carried out by exposing ‘L structures’ as depicted in Figure 3, where the minimum linewidth and pitches for different ion irradiation/resist combination could be identified. Finally, pattern transfer processes like reactive ion etching were carried out in order to test the achievable typical sizes of line-width. [1] L. Bruchhaus et al., Microelectron. Eng. 97 (2012) 48. [2] L. Bishoff et al., Appl. Phys. Rev. 3 (2016) 021101. [3] N. Ravi Kiran et al., Appl. Electron. Mater. 2 (2020) 12. [4] L. Zhang et al., Nanotechnology 31 (2020) 325301-13.
Enhancing nanolithography: ion beam lithography with multi-species ion sources for high-resolution patterning
Elia Scattolo;Alessandro Cian;Damiano Giubertoni
;Lorenza Ferrario
2024-01-01
Abstract
The development of nanotechnologies requires patterning techniques capable of producing nanometric features. A standard approach is represented by electron beam lithography (EBL), where a thin film of resist is exposed prior to the pattern transfer process, such as metal evaporation and lift-off or reactive ion etching, among the others. However, both forward and backward electron scattering in the resist layers hinders the ultimate lateral resolution and the minimum pitch achievable. The latter in particular can suffer from the so-called proximity effects, where backscattered electrons end up exposing regions between closely irradiated features. An alternative approach is given by the exposure of the organic resist by low energy (10 keV) ion beam irradiation using a Focused Ion Beam (FIB) instrument [1]. Ion beam exposure allows the patterning with high lateral resolution (close to 10 nm or below), and a mask-less approach over large areas with minimal stitching issues. Moreover, the ion-based lithography would lead to a drastically reduced ion fluence or dose to expose the resist layer, given the denser energy release, resulting in a faster exposure compared to electrons as long as the current is comparable. Finally, since ion backscattering is negligible, the lateral resolution would be mainly defined by the beam focusing and the forward scattering. As a main disadvantage, ions accelerated at 10 keV energies can only penetrate 100-200 nm of most of organic films, limiting the thickness of the resist films that can be exposed. Recent FIB tools equipped with different multi-species ion sources [2] enlarge the palette of ion species that can be tested in order to optimize lateral resolution, feature density and resist thickness [3, 4]. In this work, we have developed the processes of ion beam lithography on resist using the ion species available in a liquid metal alloy ion source (LMAIS) installed on a Raith Velion FIB system: Si++, Ge++ and Au ions. We tested two acceleration voltages, 35 (optimal focus) and 17.5 kV resulting in kinetic energies of 70 and 35 keV, respectively. Poly(methyl methacrylate) (PMMA) resist was used in two different dilutions in anisole, resulting in 2 thicknesses, 105 and 200 nm, respectively. At first, dose matrix was carried out to identify the process window for the different combinations of ion species and resist thicknesses, Figure 1 shows optical images of the resist exposed with the different ions. Measuring the irradiated areas after development with a mechanical stylus profilometer, it resulted clear that the 70 keV Si ions were able to expose both 105 and 220 nm resists, whereas the 70 keV Ge and 35 keV Si only exposed the thinner one. On the other end, Au++ ions never fully exposed the tested resist thicknesses due to the reduced penetration depth. Contrast curves versus ion fluence were acquired with a different denser dose matrix for the relevant ion species, in where depths were measured by atomic force microscopy as reported in Figure 2. From this it was possible to identify the dose windows to be used for a resolution test. The latter was carried out by exposing ‘L structures’ as depicted in Figure 3, where the minimum linewidth and pitches for different ion irradiation/resist combination could be identified. Finally, pattern transfer processes like reactive ion etching were carried out in order to test the achievable typical sizes of line-width. [1] L. Bruchhaus et al., Microelectron. Eng. 97 (2012) 48. [2] L. Bishoff et al., Appl. Phys. Rev. 3 (2016) 021101. [3] N. Ravi Kiran et al., Appl. Electron. Mater. 2 (2020) 12. [4] L. Zhang et al., Nanotechnology 31 (2020) 325301-13.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
