S10-4
Beam study of plasma-surface interactions for etch resistant materials
ビーム実験による難エッチング材料のプラズマ表面相互作用
Kazuhiro Karahashi, and Satoshi Hamaguchi
唐橋一浩，浜口智志
Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University
2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
大阪大学大学院工学研究科アトミックデザイン研究センター，〒565-0871 大阪府吹田市山田丘2-1
In an attempt to develop low-damage, highly selective etching techniques for difficult etching materials
such as magnetic thin films, surface reactions of thin films exposed to reactive ions have been studied withthe use of a mass selected ion beam system. Especially focused is etching of Pt, Ni, Co and Ta films by
CO+, C+ and O+ irradiation. It is found that, with CO+ ion irradiation, etching of Pt, Ni, Co proceeds
whereas etching of Ta hardly occurs due to the formation of an oxidation layer. The results suggest etchingmechanisms of carbon monoxide or methanol based plasmas and also possibilities of developing a
selective etching technique by reactive plasmas.

1. Introduction
Recently magnetic thin films have becomematerials of choice for some specificmicroelectronics applications such as magnetic
random access memory (MRAM). However, it iswell known that magnetic films are difficult to etchbecause they hardly react with gaseous species thatare present in plasmas. Therefore Ar ion millingseems to be almost the only etching technique in thecurrent manufacturing processes. However,capabilities of Ar ion milling for anisotropic andselective etching of magnetic films are severely
limited and therefore new technologies of reactive
ion etching for magnetic films are now seriously
sought [1][2][3].
Reactive ion etching based on noncorrosive CO
based gas (CO/NH3 or CH3OH etc.) are candidatesof magnetic film etching. However, the mechanism
for etching reactions has not yet been clearly
understood. In this study, we have focused on
etching processes of Pt, Ni, Co and Ta (typicalmaterial of had mask) thin films and examined theirsurface reactions caused by energetic CO+, C+, andO+ ions, which are generated in CO based gas
plasmas. Specifically we evaluated etching yields ofthe samples and examined chemical states of ion
irradiated sample surfaces, using a mass-selectedion beam system equipped with multiple in-situ
diagnostic systems.
2. Experimental
The ion beam system can inject mono-energetic
single-species ions (e.g., CO+, C+ or O+ ions) to asample (e.g., Pt, Ni, Co or Ta) film surface inultra-high vacuum conditions[4][5][6]. The reaction
chamber, where the sample is placed, is equippedwith a quadrupole mass spectrometer (QMS), andan X-ray photoelectron spectroscopy (XPS) (Fig.1).
The QMS is used for the detection of desorbed
products during ion beam injection and XPS is used
for in-situ chemical analyses of the irradiatedsample surface. The ion beam energy used in thisstudy was in the range of 300-1000eV. The etchingyield was determined from the depth profile of abeam-irradiated surface, which was measured bystylus profilometry. For each sample, a 200nm thickfilm of the sample material was deposited on Si
substrates by sputtering.
3. Results
Etching yields of Pt, Co and Ta films by CO+ and+O irradiation are shown in Fig.2, Fig.3 and Fig.4.
The etching yields of Pt by CO+ ions are inagreement with the yields of equivalent physicalsputtering (indicated by the blue straight line inFig.2), which are estimated from interpolation ofsputtering yield data of inert atom ions (He+, Ne+,Ar+, Kr+ etc.)[7][8]. The etching yields of Co by
CO+ ions are higher than that by O+ ions but
slightly smaller than the yields of equivalent
physical sputtering. On the other hand, the etchingyields of Ta by CO+ ions are far smaller than theyields of equivalent physical sputtering and almost
the same as that by O+ ions. When C+ ions irradiate
Ni and Co surfaces, no etching took place and a:Clayer grew.
From XPS measurements of Ni2p, Co2p andTa2p for Ni, Co and Ta surfaces after exposure tovarious ions, it is found that Ta is oxidized, whichsuggests that the oxide layer on a Ta film prevents
etching by ions listed here. It is also found thatoxidation occurs on a Ni and Co surface under O+

ion irradiation. On the other hand, the oxide layeron Ni or Co is shown to be removed by CO+ ionirradiation.
4. Conclusions
It has been found that CO+ ions etch Pt, Ni andCo without oxidation whereas O+ ions hardly etch
Ni and Co due to oxidation of the surface. On theother hand, CO+ ions hardly etch Ta, forming aoxidation layer on the Ta surface. The etchingyields of Ta by CO+ and O+ are far smaller than theyields of equivalent physical sputtering. Thus it has
been found that high selectivity of Pt, Ni and Co
etching against Ta by CO+ irradiation arises fromthe prevention of etching by Ta oxidation.
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Okada, and T. Tsukada: Vacuum 66 (2002) 479[3] K. Kinoshita, H. Utsumi, K. Suemitsu, H. Hada, andT.Sugibayashi: Jpn. J. Appl. Phys. 49 (2010)
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[4] K. Karahashi, K. Yanai, K. Ishikawa, H. Tsuboi, K.
Kurihara, M. Nakamura: J.Vac. Sci.Technol. A22(2004) 1166.
[5] K. Karahashi, and S. Hamaguchi: J.Phys. D: Appl.
Phys, 47 (2014) 224008.
[6] K. Ishikawa, K. Karahashi, H. Tsuboi, K. Yanai, andM. Nakamura: J. Vac. Sci. Technol. A21 (2003) L1.
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[8] D. Rosenberg, and G. K. Wehner: J. Appl. Phys. 33(1962) 1842.

Fig.2. Etching yield of Pt by CO+

Fig.3. Etching yield of Co by CO+ and O+

Fig.1.(a) A schematic diagram of Mass selectedion beam system
Fig.4. Etching yield of TA by CO+ and O+

Fig.1.(b) UHV scattering chamber