Extreme Chemical Reduction on Photoresist Stripping Process by Using Sulfuric Acid Ozone Mixture in Liquid Film
Authors: Lin Tsung-Ju , Yamaguchi Yu , Shibata Shuichi , Tamura Keigo , Fujiwara Naozumi , Inaba Masaki 
Published: December 2024

Introduction
Sulfuric peroxide mixture (SPM) has been widely used in the photoresist (PR) stripping process of semiconductor manufacturing. However, the consumption of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) increase environmental burden such as water pollution. To reduce the chemical consumption, one of the ideas is that chemicals are supplied to the wafer until liquid film is formed and stopped during PR stripping process instead of continuous dispensing. It is concerned that chemical species in SPM become inactive while chemicals are stopped. Therefore, in this work, H2SO4 liquid film is formed on the wafer and then exposed to ozone (O3) atmosphere so that chemical species remain active. Peroxydisulfuric acid (H2S2O8) is generated by reacting H2SO4 with O3 and dissociates in peroxydisulfate ion (S2O8
2-) which is one of the chemical species for PR stripping. [1-2] In this study, PR removal efficiency and removal time of SOM in liquid film was evaluated and activation energy of the chemical species generated in SOM was calculated.
Experimental
Sample: 435 nm thickness Boron (B) doped 5E+15 /cm2 5 keV KrF PR film was coated on 300mm Si wafer. And two kinds of 435 nm thickness KrF PR film, which were Arsenic (As) doped 1E+15 /cm2 10 keV and B-doped 1E+16 /cm2 10 keV, were coated on patterned 300mm Si wafer respectively. (Supported by AIST, Japan)
Stripping process: SOM process was performed in a O3 gas chamber on a single coupon/wafer cleaning tool. Coupon size was 25mm x 25mm. H2SO4 was heated to 60°C and then supplied to the wafer. Hot plate was heated to 140~250°C. O3 concentration was 200 g/Nm3 and O2/O3 flow rate was 3 L/min. Experiment was conducted on coupon and 300 mm wafer with consumption of H2SO4 among 0.06ml to 0.5ml and 80ml respectively.
Characterization: Stripping images of PR were observed by optical microscope (OM), and removal efficiency was calculated in image binarization method by using ImageJ software. PR residues were observed by scanning electron microscope (SEM). The activation energy of chemical species in SOM were verified by Ab initio calculation between S2O8
2- and peroxomonosulfate ion (HSO5
-).
Results/discussion
With a little quantity of H2SO4, B-doped 5E+15 PR coupon with 0.3ml H2SO4 liquid film had the best removal efficiency between 0.06ml to 0.5ml. (Fig. 1) It was considered that quantity of H2SO4 should be moderate. With higher hot plate temperature, both As-doped and B-doped PR removal efficiency of patterned coupon were also higher with 0.3 ml H2SO4. At 250°C, the removal efficiency of As-doped 1E+15 was 99% and B-doped 1E+16 was 87% which was comparable to SPM with 170°C-H2SO4 and room temperature (RT) H2O2 mixture in the same process time. (Fig. 2) On 300 mm wafer, As-doped 1E+15 took 120 seconds for PR stripping by SOM in 80ml H2SO4 liquid film at 250°C and it was better than SPM with 90°C-H2SO4 and RT-H2O2 mixture. (Fig. 3) SEM images of the PR residues were observed in the dashed line block of OM images, and it was considered that PR collapse was the reason why PR residues were hardly removed. (Fig.4) By the Ab initio calculation, S2O8
2- had the activation energy of 0.78 eV which was lower than HSO5
-(1.79 eV). It’s implied that the reaction of S2O8
2- generation at higher rate was proved by the Ab initio calculation results. (Fig.5)
Conclusion
A novel PR stripping method of “expose H2SO4 to O3 atmosphere in liquid film” was developed to reduce the chemical consumption. This method was comparable to conventional SPM even though chemical consumption is lower, and it is expected to be next generation cleaning technology with smaller environmental burden.
References
[1] Steven Verhaverbeke, et al. “Method for sulfuric acid resist stripping,” US6032682A, filed June 24, 1997, and issued March 7, 2000.
[2] H. Tomita, M. Sato, S. Nadahara and T. Saitoh, "Photoresist stripping using novel sulfuric/ozone process," 2001 IEEE International Symposium on Semiconductor Manufacturing. ISSM 2001.

Figure 1