(Invited) Atmospheric-Pressure Reactive Thermal Plasma Jet Technology for Decarbonization of Semiconductor Manufacturing
Authors: Higashi Seiichiro , Kato Hibiki , Yu Jiawen , Matsumoto Kyohei , Hanafusa Hiroaki 
Published: February 2024

Semiconductor market continues to grow and is expected to hit a trillion dollar in 2030. On the other hand, CO2 emission from semiconductor industry has been anticipated to become one of the serious issues regarding carbon neutral commitments. Organic solvent, which is used for cleaning and wet etching processes, is one of the sources to emit CO2 during device manufacturing. They are recycled and reused, nevertheless, huge amount is disposed. Replacing the wet etching by dry process is a quite effective approach, however, the etching rates by conventional plasma etchings are quite low compared to that of organic solutions (~30 µm/s). In this work, we propose an introduction of atmospheric-pressure reactive thermal plasma jet (R-TPJ) to perform rapid heating and etching reaction simultaneously, which markedly enhances the etching rate of photoresist(PR).
In the experiment, n-type Si(100) wafer was treated by RCA cleaning and HF, HMDS solutions and PR (TOKYO OHKA KOGYO CO., LTD. TSMR iP-3300 17cP) were spin coated at 4400 rpm and baked at 130 °C for 2 min to from ~ 900-nm-thick PR layer. The R-TPJ was generated by DC arc discharge under atmospheric pressure with a supply current (I) of 20 A between a W cathode and a metal anode separated by electrode spacing (ES) of 2.0 mm. Ar flow rate (f
Ar) was varied from 1.0 to 2.0 L/min, and O2 flow rate (f
O2) was varied from 0.3 to 3.0 L/min. The samples were linearly moved by a motion stage in front of the R-TPJ with a scanning speed (v) ranging from 20 to 260 mm/s. The working distance (d) was varied from 0.5 to 3.0 mm. The nozzle diameter is 1 mm.
An emission line from atomic oxygen radicals was observed at 777nm and its intensity increased with increasing O2 flow rate, accordingly, the etching rate of PR increased in a similar manner. An etching rate of 6.3 µm/s was obtained with the f
Ar of 1.0 L/min, f
O2 of 1.0 L/min, and d of 1.0 mm. Moreover, by decreasing the working distance d, etching rate increased with an exponential manner, and an etching rate as high as 61.5 µm/s was obtained when d was set at 0.5 mm, as shown in the figure. It is assumed that millisecond heating of PR surface and simultaneous supply of reactive species have realized an ultra-fast etching of PR. The activation energy for PR etching has been reported to be 0.53 eV [1,2], and according to the experimental fitting on Arrhenius plot, the temperature corresponding to the etching rate of 61.5 µm/s is estimated to be ~820 K. In addition, we conducted a heat diffusion simulation considering PR/Si layer structure annealed by R-TPJ. It was suggested that the annealing temperature reached as high as ~800K with the annealing duration of 4ms. These results suggest that ultra-fast etching of PR by R-TPJ offers a new etching concept in millisecond time regime.
In conclusion, R-TPJ provides millisecond thermal etching and is a candidate to replace conventional wet etching and will contribute to the decarbonization of semiconductor device manufacturing.
[1] K. Yamakawa, M. Hori, T. Goto, S, Den, T. Katagiri, and H. Kano, J. Appl. Phys. 98, 043311 (2005).
[2] A. Granier, D. Chereau, K. Henda, R. Safari, and P. Leprince, J. Appl. Phys. 75, 104 (1994).

Figure 1