22P015-O

Measurement of spatial distributions of metal atom densities in magnetron
sputtering plasmas employing a CZTS targetCZTS?<2CH^0MHms9QC?js0Wi:^K*1kb0)Y,[W,Shunya Kiku hi1 and Koi hi Sasaki1

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Division of Quantum S ien e and Engineering, Hokkaido University,Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Japan

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We investigated spatial distributions of metal atom densities in magnetron sputtering plasmas employing Cu and Cu2 ZnSnS4 (CZTS) targets by laser-indu ed uores en e (LIF) imaging spe tros opy. LIF pi tures whi h represented distributions of Cu atom densities had di erent shapes
at di erent Ar gas pressures. The omparison between the density distributions observed using Cu
and Cu2 ZnSnS4 targets suggests that sulfuration of Cu atoms in the gas phase is not signi ant.
1. Introdu tion

Cu2 ZnSnS4 (CZTS) thin lm is an attra tive material as a light absorption media of a solar ell. It hasa band gap energy of 1.4-1.5 eV and an absorption oeÆ ient greater than 104 m 1 . The most attra tive
point of CZTS in omparison with Cu(In,Ga)Se2 isits element omposition. CZTS in ludes no toxi elements, and is omposed of abundant elements on theearth. Re ently, a onversion eÆ ien y of 9.6 % has
a hieved in CZTS-based solar ells [1℄. For these reasons, many resear hers has been investigating fabri ation te hnologies of CZTS thin lms for many years.
There are several deposition methods for CZTS thinlms, for example, o-evaporation, sol-gel methods,
pulsed laser deposition and RF magnetron sputtering.
RF magnetron sputtering is the most suitable
method for the mass-produ tion of CZTS-based solar ells, sin e it has advantages of the large-area deposition and the uniformity of lm thi kness. However, magnetron sputtering has diÆ ulty in depositing
CZTS thin lms with ne stoi hiometry. The prin ipal
problem is the less abundant omposition of sulfur andzin atoms in the lm.
A method to understand the deposition me hanismof CZTS thin lms and to understand the reason forthe less abundant omposition of S and Zn is observing density distributions of various atom densities in
the gas phase of magnetron sputtering plasmas. In
this study, we measured two-dimensional distributionsof Cu densities by laser-indu ed uores en e (LIF)
imaging spe tros opy. We employed Cu and CZTS
targets in this experiment. We dis uss the signi an eof gas-phase rea tions of Cu on the basis of the om-parison between the density distributions observed in
Cu and CZTS magnetron sputtering plasmas.
2. Experiment

Figure 1 shows the experimental apparatus. We
used a magnetron sputtering sour e whi h was onne ted to an RF power supply via a ir uit for
impedan e mat hing. The magnetron sputteringsour e onsisted of ylindri al permanent magnets andan indire t water- ooling system. The RF power andthe frequen y were 80 W and 13.56 MHz, respe tively.
A sintered CZTS or Cu target (50 mm in diameter) was
pla ed on the permanent magnets. The hemi al omposition of the CZTS target was perfe tly stoi hiometri , i.e., Cu:Zn:Sn:S = 2:1:1:4. Ar gas was used fordis harges, and the pressure was varied between 3 and30 mTorr.
We used an opti al parametri os illator (OPO)
pumped by a Nd:YAG laser for ex iting Cu atoms inplasmas. The laser beam from the OPO laser was arranged to have a planer shape with a width of 80 mm
and a thi kness of 2 mm using two ylindri al lenses.
Ex itation wavelength for Cu atoms at the ground statewas 324.754 nm (the 4 s2 S 1=2 - 4 p2 Po3=2 transition). On
the other hand, the wavelength of the LIF emission
was 510.554 nm whi h orresponded to the 4 p2 Po3=2 4 s2 D5=2 ℄transition. The image of LIF formed on theplanar laser beam was aptured using a harge oupleddevi e amera with a gated image intensier (ICCDamera) [2℄. An interferen e lter was pla ed in frontof the amera to separate the LIF emission from selfemissions of the plasma. The tone of the LIF image
represented the two-dimensional density distribution    ! "#$%& 

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of ground-state Cu atoms.
3. Results

Figure. 2 shows two-dimensional distributions of
Cu atom densities in CZTS and Cu magnetron sputtering plasmas at pressures of 5 and 20 mTorr. The verti al and horizontal lengths of the pi tures orrespondto 80 mm  80 mm. The target is pla ed at the top of
the images.
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HIJJJJtarget. The spatial distribution of the Cu density wasdependent on the dis harge pressure, but the normalized distributions were roughly the same when we employed Cu and CZTS targets.
4. Dis ussion

As reported previously [3℄, the variation of the axial distribution of the Cu density with the dis hargepressure is a long-time mystery. The distribution observed at 20 mTorr, whi h has the peak at a separated
distan e from the target, annot be explained by thespatial distribution of the speed of sputtered Cu atoms[4℄. Although the fundamental understanding on thespatial distribution is still insuÆ ient, the omparison
between the Cu atom densities in the Cu and CZTS
sputtering plasmas gives us valuable insight into thetransport and hemi al rea tions of Cu atoms.
A point is the ratio of Cu densities in the Cu andCZTS sputtering plasmas. The density ratio of approximately 1/6 is not deviated signi antly from the ratioof the number of Cu atoms ontained in the Cu andCZTS targets. The gas-phase loss of Cu atoms in theCu sputtering plasma is basi ally negligible. Therefore, the unremarkable deviation of the ratio of the Cu
densities from the ratio of the numbers of Cu atoms
ontained in the targets suggests negligible gas-phaseloss of Cu atoms in the CZTS sputtering plasma. Theinsigni ant gas-phase loss of Cu atoms in the CZTS
sputtering plasma is supported by the similar spatial
distributions of the Cu densities in the Cu and CZTS
sputtering plasmas. The similar distributions indi ates
negligible loss of Cu atoms during the transport fromthe target to the downstream region.
The most likely gas-phase rea tion in the CZTS
plasma is sulfuration of metal atoms. The produ tionof suldes in the gas phase means the transport of Satoms to the lm with tight hemi al bonding withother elements, whi h would result in eÆ ient sulfuration of the lm. The present experiment suggests
negligible sulfuration of Cu in the gas phase, and wewill examine sulfurations of Zn and Sn in future work.
Referen es( ) Cu target,(d) Cu target,[1℄ T. K. Todorov, K. B. Reuter and D. B. Mitzi, Adv.
5 mTorr
20 mTorr
Mater. 22, E156 (2010).
Figure 2: Spatial distributions of Cu atom densities in [2℄ K. Fukaya, K. Sasaki, J. Gao, T. Kimura, M.
Cu and CZTS sputtering plasmas. The gas pressure
Watanabe, M. Inoue, A. Fujimaki, and H. Sugai,
were 5 and 20 mTorr, and the RF power was 80 W.
Thin solid lms. 517, 2762 (2009).
[3℄ K. Sasaki, N. Nafarizal, J.-S. Gao, and K. ShibaFigures 2(a) and 2(b) were observed with a CZTS
gaki, Va uum 83, 463 (2008).
target, while we observed Figs. 2( ) and 2(d) with a [4℄ K. Shibagaki, N. Nafarizal, and K. Sasaki, J. Appl.
Cu target. It was found that the Cu atom density with
Phys. 98, 043310 (2005).
a CZTS target is approximately 1/6 of that with a Cu