A novel method for the development of standard EUV photoresists in CO2 using CO2 compatible salts (CCS) is described and examined using a quartz crystal microbalance (QCM) technique in CO2. The fundamental steps of this development process are proposed to be 1) photoresist modification via CCS interaction with Bronsted acid groups in the resist; and 2) dissolution of the modified resist into CO2. Removal rates of non-exposed photoresist from the quartz crystal can be studied in real time under a variety of conditions to help elucidate the kinetics and mechanism of the CCS development process. A series of runs from 35 to 50 C at the same CO2 density showed a strong effect of temperature on the development rate. In the presence of a large excess of CCS, the kinetics of dissolution appear to be zero order. An Arrhenius plot generated by this data gave an activation energy of 79.0 kJ/mol. The zero order kinetics of dissolution was confirmed with a series of runs at various CCS concentration that showed insignificant rate effects. Dynamic flow of the CCS solution in the QCM cell gave ~50% increase in rate of resist removal relative to the standard conditions using static CCS solution. The indication from the three sets of QCM experiments described here is that dissolution of the CCS modified polymer is the slow step in CCS development.
Direct development of EUV resists in homogeneous carbon dioxide (CO2) solutions containing CO2 compatible salts
(CCS) has been demonstrated. These CCS complexes have been designed and prepared such that the anion and/or the
cation of the salt contains at least one CO2-soluble portion. In the described method, standard positive tone EUV resists
are processed in supercritical CO2 containing less than 20 mM CCS, at pressures ranging from 3500 to 5500 psi, 35 to
65 C with cycle times as short as 1 minute to give reverse image development. Substantial reduction in image collapse
and LER/LWR has been observed; large aspect ratios approaching 10 have been measured in dense line/space features,
and dense lines with 3 sigma LWR values that are 30% smaller than comparable TMAH developed samples have been
observed.
This paper will describe results from a two level full factorial design of experiments (DOE) using four factors. Two
experimental samples were processed at each point, along with four center point runs to give a total of 36 experimental
samples. An additional seven supplemental runs were also prepared. Responses measured for the DOE include half
height resist thickness, resist height, contrast, iso/dense bias, aspect ratio of collapse, line edge roughness, and line
profile. Details of how the responses were measured will be presented in the paper. Effects of individual factors on each
response, along with the significant interactions between factors will be reported. An optimized parameter space will be
selected and additional follow-up experiments will be described.
Pattern collapse and line width roughness (LWR) are two issues expected to become increasingly important as feature
sizes approach those expected from EUV lithography. The unique physical properties of supercritical CO2 (low
viscosity, zero surface tension) may provide attractive solutions to these problems, but only if the limitations of CO2
(low polarity, low pH) can be overcome to give high contrast development of standard resists. This paper describes the
use of novel CO2 compatible salts (CCS) in homogenous supercritical CO2 solutions to give highly effective reverse
image development of a standard EUV resist. Results from a factorial DOE verify the robustness of this new process
and indicate that temperature is the most important factor in determining development rate. The absence of surface
tension in the homogenous CO2 solutions used for this development lead to substantially reduced pattern collapse and
standing dense L/S features with aspect ratios >12. Development using CCS chemistry in CO2 is an anisotropic
process, where CD of developed features can be controlled without affecting the resist film height. Plasticization of the
amorphous resist polymer by CO2 during development contributes to the ~30% reduction in LWR for CCS samples
versus standard TMAH development. The feasibility of scaling up CCS based development in CO2 from the small scale
view cell into a 200 mm full wafer prototype tool has been demonstrated. A two-step process for resist development is
proposed, along with a likely mechanism to account for the high level of contrast observed.
Line Width Roughness (LWR) of resists constitutes one of the main obstacles in the race of further shrinking the feature
dimensions of fabricated devices. Thus, the reduction and control of LWR is one of the biggest challenges of next
generation lithographies. In this paper, the LWR output of a new development process of EUV resists which uses
homogeneous carbon dioxide (CO2) solutions containing CO2 compatible salts (CCS) has been examined. The
measurement and characterization of LWR has been made through the analysis of CD-SEM images and the application
of a three-parameter model. The three parameters involved in this model (sigma value σLWR, correlation length ξ,
roughness exponent α) determine both the spatial aspects (spectrum) of LWR as well as the interplay between LWR and
local CD variations. It is found that wafers developed with CCS process gives substantially lower LWR parameters
(σLWR,ξ) than comparable TMAH developed samples. Also, the impact of the preparation of resist wafer (exposure time,
PAG and quencher level) and the development conditions (temperature, CCS concentration) on LWR parameters is
examined so that we are able to identify trends to lead toward optimized LWR performance.
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