Role of the dense amorphous carbon layer in photoresist etching

Abstract

The development of new photoresists for semiconductor manufacturing applications requires an understanding of the material properties that control the material's plasma etching behavior. Ion bombardment at ion energies of the order 100 s of eV is typical of plasma-based pattern-transfer processes and results in the formation of a dense amorphous carbon (DAC) layer on the surface of a photoresist, such as the PR193-type of photoresist that currently dominates the semiconductor industry. Prior studies have examined the physical properties of the DAC layer, but the correlation between these properties and the photoresist etching behavior had not been established. In this work, the authors studied the real-time evolution of a steady-state DAC layer as it is selectively depleted using an admixture of oxygen into an argon plasma. Observations of the depletion behavior for various DAC layer thicknesses motivate a new model of DAC layer depletion. This model also correlates the impact of the DAC layer thickness with the etch rate of the bulk photoresist. The authors find that up to a 40% depletion of the DAC layer thickness does not have a significant impact on the bulk photoresist etch rate. However, further depletion results in an exponential increase in the etch rate, which can be up to ten times greater at full depletion than for the fully formed DAC layer. Thus, with these trends the authors show that the photoresist etch rate is controlled by the thickness of the DAC layer. Furthermore, thickness loss of the DAC layer in an O2-containing plasma coincides with a chemical modification of the layer into an oxygen-rich surface overlayer with properties that are intermediate between those of the DAC layer and the bulk photoresist. Support for this interpretation was provided via x-ray photoelectron spectroscopy characterization. Atomic force microscopy was used to gauge the impact on surface roughness as the DAC layer is formed and depleted. The trends established in this work will provide a benchmark in our development of new photoresists, which will be suitable for pattern transfer processes that will ultimately be a part of enabling smaller semiconductor device feature sizes and pitches.

Evolution of photoresist layer structure and surface morphology under fluorocarbon‐based plasma exposure

“…Once the DAC layer has been thinned by a certain amount, there is a corresponding decrease in the bulk layer thickness. This reactive etching of the DAC layer is consistent with prior works, which observed selective depletion of the DAC layer under either a reactive oxygen or FC plasma environment . However, based on the directionality of the ellipsometric data, the interaction of the C 4 F 8 with the DAC layer does not purely reduce the thickness of the DAC layer, but also induces another surface modification that has a directionality opposite to that of bulk layer etching.…”

“…The fundamental details of our baseline ellipsometric model have been discussed extensively in our previous work . The components of the model, which correspond to the directionality of the raw data in psi‐delta space, are depicted in Figure a.…”

“…The samples used in this study were 200 nm thick blanket films of 193 nm photoresist material (PR193) on a Si substrate produced by the Japan Synthetic Rubber Corporation (JSR); these films have been characterized extensively in previous works . The 193 nm photoresist consists of a terpolymer structure with a polymethylmethacrylate (PMMA)‐based backbone with leaving, lactone, and polar groups serving as the functionalized side groups…”

“…Sufficiently energetic ion bombardment forms a dense amorphous carbon (DAC) layer on the sample surface, as a result of the depletion of oxygen, hydrogen, and volatile‐product‐forming species . This depletion results in the surface layer becoming significantly denser than the underlying bulk material, resulting in an enhancement of the etch resistance . However, as the DAC layer grows from the underlying bulk layer, the increase in density introduces compressive stresses within the surface layers, causing a buckling instability that can lead to the enhancement of surface roughness .…”

“…However, maintaining a controlled and consistent etching selectivity depends on the stability of the etch resistance of the photoresist material. For a 193 nm photoresist exposed to energetic plasmas, the etch resistance is proportional to the developed thickness of the surface DAC layer . However, the DAC layer is susceptible to chemical sputtering from reactive plasmas, which depletes the layer and decreases the etch resistance .…”