As future patterning processes reach the limit of lithographic printability, continuous innovation in mandrel trim or shrink strategies are required to reach sub-20 nm line-space patterning. Growing concerns of lithography defectivity, mask selectivity, line edge roughness (LER), line width roughness (LWR), and critical dimension uniformity (CDU) present significant challenges towards this goal. The authors compare various alternative mandrel trim strategies to highlight potential solutions and drawbacks towards enabling successful printing of mandrels used in extreme ultraviolet (EUV) multi-patterning schemes. Through this comparison, the authors demonstrate the challenges of maintaining adequate pattern transferability while keeping aspect ratio-driven line roughness and material selectivity under control. By process partitioning, the limitations of traditional lithography and etch trimming strategies are highlighted, suggesting the need for new methods of CD reduction after the pattern has been transferred. These new trimming methods offer flexibility in CD control without negatively impacting the mandrel profile and demonstrates better tunability across different material sets, allowing for evaluation of different mask and mandrel material combinations for downstream process optimization.
The photomechanism of extreme ultraviolet (EUV) exposures in chemically amplified photoresists is much different than that of previous lithographic wavelengths. Electrons generated during EUV exposure are demonstrated to be a source of acid production through a process referred to as electron trapping. Density functional theory modeling indicates that it is energetically favorable for the photoacid generator (PAG) molecule to decompose if an electron is trapped. Low-energy electrons (<10 eV) that are unlikely to produce holes and secondary electrons generate acid-indicating electron–PAG interactions that are capable of inducing decomposition. Additionally, solution phase reduction in PAGs via electrolysis is shown to produce acid. Furthermore, a more easily reduced PAG (i.e., higher likelihood of trapping an electron) produces a higher acid yield, further supporting electron trapping as a process of acid production regardless of the polymer matrix. An acid indicator, Coumarin 6, was used to determine the number of acids generated per absorbed EUV photon. The results of these measurements indicate that electron–PAG interactions are a source of acid production through electron trapping; thus, an increase in the number of electron-hole pairs available to induce chemical reactions would improve sensitivity.
During the photolithographic process, a photoresist is exposed to EUV photons; it is believed that the secondary low energy electrons generated during this exposure decompose the PAG molecule, producing acid. Regardless of how these secondary electrons are produced, whether by incident electrons or photons, the number of acids produced will lead to a solubility change within the photoresist. The goal of this study is to observe the solubility changing reactions due to low energy electron exposures (approximately 5-80 eV). The reactions occurring in the photoresist are monitored through outgassing measurements during EUV photon exposures, and low energy electron exposures. Outgassing results indicate that PAG decomposition occurs with electrons as low as 4.5 eV, and subsequent deprotection reactions are observed due to the acid generated from the PAG. Without being in the presence of PAG decomposition, deprotection reactions are caused by electron exposures with energies down to at least 15 eV. These deprotections that occur in the absence of PAG decomposition are referred to as direct deprotection reactions. Sentaurus Lithography simulations show that these direct deprotection reactions can affect the resist modeling.
The photo-mechanism of EUV exposures in chemically amplified photoresists are much different than that of previous lithographic wavelengths. Electrons generated during EUV exposure are demonstrated to be a source of acid production through a process referred to as electron trapping. Density functional theory modeling indicates that it is energetically favorable for the PAG molecule to decompose if an electron is trapped. Low-energy electrons unlikely to produce holes and secondary electrons generate acid indicating electron-PAG interactions are capable to induce decomposition. Additionally, a more easily reduced PAG (i.e. higher likelihood of trapping an electron) produces a higher acid yield supporting electron trapping as a process of acid production. An acid indicator, Coumarin 6, was used to determine the number of acids generated per absorbed EUV photon. The results of these measurements indicate that electron-PAG interactions are a source of acid production through electron trapping; thus, increasing the number of electron-hole pairs available to induce chemical reactions would improve sensitivity. It is expected that lower band gap materials produce more electron-hole pairs after an absorption event. Subsequently, these measurements show that lower band gap polymers generate higher acid yields.
Extreme ultraviolet (EUV, ~13.5 nm) lithography is the prospective technology for high volume manufacturing by the microelectronics industry. Significant strides towards achieving adequate EUV source power and availability have been made recently, but a limited rate of improvement in photoresist performance still delays the implementation of EUV. Many fundamental questions remain to be answered about the exposure mechanisms of even the relatively well understood chemically amplified EUV photoresists. Moreover, several groups around the world are developing revolutionary metal-based resists whose EUV exposure mechanisms are even less understood. Here, we describe several evaluation techniques to help elucidate mechanistic details of EUV exposure mechanisms of chemically amplified and metal-based resists. EUV absorption coefficients are determined experimentally by measuring the transmission through a resist coated on a silicon nitride membrane. Photochemistry can be evaluated by monitoring small outgassing reaction products to provide insight into photoacid generator or metal-based resist reactivity. Spectroscopic techniques such as thin-film Fourier transform infrared (FTIR) spectroscopy can measure the chemical state of a photoresist system pre- and post-EUV exposure. Additionally, electrolysis can be used to study the interaction between photoresist components and low energy electrons. Collectively, these techniques improve our current understanding of photomechanisms for several EUV photoresist systems, which is needed to develop new, better performing materials needed for high volume manufacturing.