Dependence of photoresist surface modifications during plasma-based pattern transfer on choice of feedgas composition: Comparison of C4F8- and CF4-based discharges

Abstract

The authors report a study of the impact of surface chemical factors on etch rates along with surface and line edge roughness introduction for prototypical photoresist (PR) materials and structures during plasma-based pattern transfer employing fluorocarbon (FC) discharges. For selected photoresist materials and model polymers (193nm PR, 248nm, PR, and polymethyladamantyl methacrylate), the influence of bulk polymer properties on plasma durability was clarified by comparing etch rates, surface roughness introduction, and profile evolution of nanostructures. They studied the effects of both fluorocarbon fragment deposition and polymer surface fluorination by gas phase fluorine atoms on plasma etching resistance and surface roughness evolution of the organic materials by comparing discharges fed with C4F8∕Ar or CF4∕Ar∕H2 gas mixtures. The spatial frequency distribution of surface roughness was obtained using fast Fourier transformation of atomic force microscopy data. A graphitic layer was formed for Ar containing discharges on the polymer surfaces. Fluorocarbon deposition on the damaged photoresist affected roughening in two opposing ways: ion-induced mixing with the damaged polymer layer increased surface roughening, whereas for simple FC precursor deposition a reduction in surface roughness was seen. The latter effect was especially important during profile evolution of three-dimensional structures. Fluorination of the photoresist surfaces by fluorine-rich plasma increased polymer etching yields, and for highly fluorinated surfaces inhibited the formation of the graphitic surface layer. The destruction of the adamantyl structure is usually found in fluorocarbon/argon discharges and is a major origin of roughness evolution for 193nm PR materials. Process conditions having high etch yields were found to improve the roughness results of 193nm PR after etch. The fluorination of the photoresist materials prevented the formation of characteristic small scale roughness features at the cost of large scale roughness introduction. Use of low energy density process conditions prevented the introduction of large scale roughness and can be explained by suppression of surface roughness growth by an ion-induced transfer mechanism. Alternatively, CF4∕H2 processing also showed improved roughness results due to a separate layer deposition of the fluorocarbon film on top of the photoresist material. The etch results for all process conditions can be combined in a surface roughening model where the roughening behavior of the film scales linearly with the energy density delivered to the polymer surface during processing. Even for a range of feedgas chemistries, adamantyl-containing polymers show enhanced roughening rates, suggesting that the instability of the adamantyl structure used in 193nm PR polymers is the performance limiting factor for processing these PR materials.

Benefits of plasma treatments on critical dimension control and line width roughness transfer during gate patterning

“…But, as reference resist is etched faster, it develops higher roughness for a given processing time. This is consistent with other studies 13,14,35,36 that show that polymers with greater plasma induced material loss present increased surface roughness. The reason is that during steady state etching, the existing surface roughness can be amplified by an ion induced transfer mechanism where locally nonuniform removal rates lead to enhanced surface roughness since low and high surface features will not be etched at the same rate.…”

“…This is consistent with previous works. 3,5,13,14,36 Hua et al 13 observed that rough surfaces develop within few seconds of exposure of 193 nm PR to FC plasma, and that the transfer of surface roughness produces rough sidewalls that exhibit vertical aligned features. Similar observations were made on 248 nm photoresist.…”

“…Those chemical changes convey the cleavage of pending groups and the removal of lactone group caused by photoetching mechanisms and that are typically observed when 193 nm PR are exposed to wavelength below 200 nm. 14,15,19,20,[25][26][27][28] In the case of HBr or VUV cured resists, those modifications have already been performed during the cure treatment and those polymers become less sensitive to Si-ARC plasma light.…”

“…[3][4][5][6][7][8][9][10] Moreover, 193 nm photoresist polymers show poor etch resistance and develop important surface roughness when exposed to fluorocarboned plasma processes typically used to open hard mask layers. [11][12][13][14] The consequence is that the photoresist LWR tends to be degraded during hard mask opening steps and the degradation is transferred into the final gate. Some authors have shown the benefit of introducing plasma treatment prior plasma transfer in order to decrease the resist LWR and also to limit the resist degradation during the plasma process.…”

Real-time measurements of plasma photoresist modifications: The role of plasma vacuum ultraviolet radiation and ions

“…2 This change in the material platform introduced new challenges during plasma processing such as increased roughness development, reduced etch resistance, and pattern distortion. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Oehrlein et al have recently reviewed progress on this topic. However, improved CD control by reducing roughness development during plasma etching has become increasingly important.…”