SAMCO : Aqua Plasma Supports Reduction of Oxidized Copper@AEI November 2022

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Aqua Plasma Supports Reduction of Oxidized Copper

amco Inc. focuses on product		pressure.(1) As a result of the investiga-
development with emphasis on		tion, the company has found that water
environment, health and safety		vapor can be used for the reduction of
(EHS). The company has been		oxidized metal films as well. This ar-
advancing research on water vapor plas-		ticle introduces the results of the latest
Sma at low pressure , Aqua Plasma®, and		research wherein the company studied
product development using the treatment		the reduction mechanism of the oxidized
technique. Thus far, the company has in-		copper using Aqua Plasma® and com-
troduced reduction effects of oxidized		pared with hydrogen plasma.(2)
copper and silver using Aqua Plasma®.		Experiment
Conventional methods to reduce the
oxidized copper include hydrogen re-		In this study, copper thin film which
duction and hydrogen plasma reduction.		was deposited on a silicon wafer by
It has been reported that these methods		electron beam deposition of copper with
use atomic hydrogen (H) and proton (H+)		99.99 percent purity was used as a sam-
for reduction. However, hydrogen has a		ple. Then, the oxidized copper layer was
wide range of explosion limits of 4 to 75		formed by thermally processing on a hot
vol% in the air. Therefore, it needs to be		plate at 250℃ for 30 minutes in the ambi-
handled with care and safety equipment.		ent air. As a result of X-ray diffraction and
Water vapor excels in terms of EHS		X-ray photoelectron spectroscopy (XPS)
with low inflammable property. Howev-		analyses, the oxidized copper sample was
er, thus far, it has been used as an oxidiz-		composed mainly of Cu2O with CuO ex-
ing agent, such as thermal oxidation of		isting only in the outermost surface (19	Photo 1: AQ-2000 Aqua Plasma® cleaner
silicon wafers. Meanwhile, Samco pre-		nm in the surface layer at the deepest).	the XPS and length measurement of the
sumed that water vapor would work as		The depth of oxidized copper layer was
a reducing agent because it emits strong		identified approximately 600 nm from the	cross section of the cleaved sample using
atomic H emission in plasma at low		results of the depth direction analysis by	a scanning electron microscope (SEM).
			The surface treatment was
			performed using Aqua Plas-
			ma® Cleaner (Photo 1). The
			sample was set to room-tem-
			perature	ground	electrodes
			and treated without applying
			heat. It was processed by fix-
			ing the water vapor flow rate,
			vacuum pressure and power,
			and changing treatment time.
			Hydrogen plasma was pro-
			cessed under the same con-
			ditions except for the gas
			(hydrogen	with	purity of
			99.999%).

Fig. 1. Cross-section SEM images of oxidized copper film treated with (a) water vapor plasma and

1. hydrogen plasma.Thickness of reduced Cu and CuxO (x = 1 or 2) film which were measured from (a) and (b) is shown in (c) and (d). Adapted from (2).

Results and Discussion Fig. 1 shows the thickness

of the reduced copper layer measured by SEM observation of the cross section and plotted by treatment time. In addition, the thickness of oxidized and total copper layer

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AEI November 2022 Special Issue

was also measured and compared them with the results of hydrogen plasma.

An induction period was observed in which the oxidized copper layer was not reduced until 40 or 60 seconds after the plasma ignition. The depth of the reduced copper layer increased not linearly but along an S-shape curve with respect to the treatment time. This tendency were consistent with characteristics reported in previous studies(3, 4) on hydrogen plasma. The hydrogen plasma in this study also showed the similar tendency of that. These results indicate the oxidized copper in Aqua Plasma® can be reduced by H and H+ as in the case of hydrogen plasma. For further details of the results and discus- sion, please refer to reference (2).

In contrast, the depth of the reduction by Aqua Plasma® was 257 nm after 600

seconds, which was more than twice that of 105 nm by hydrogen plasma. There- fore, Aqua Plasma® enables to shorten the reduction-process time. Samco considers that adsorption of polar molecules with OH to the copper surface contributes to the difference in the reduction rate. The company intends to continue further investigation in this area.

References:

(1) H. Terai, R. Funahashi, T. Hashi- moto, and M. Kakuta, "Heterogeneous bonding between cyclo-olefin polymer (COP) and glass-like substrate by newly developed water vapor-assisted plasma, Aqua Plasma Cleaner," Electr. Eng. Ja- pan, vol. 205, no. 4, pp. 48-56, 2018.

(2) H. Terai, K. Okafuji, T. Tanaka, T. Hashimoto, H. Nakano, and O. Tsuji,

"Reduction of Copper Oxide by Water Vapor Plasma at Low Pressure," IEEJ Trans. Sensors Micromachines, vol. 139, no. 7, pp. 157-162, 2019. (in Japanese)

(3) J. Y. Kim, J. A. Rodriguez, J. C. Hanson, A. I. Frenkel, and P. L. Lee, "Reduction of CuO and Cu2O with H2: H embedding and kinetic effects in the formation of suboxides," J. Am. Chem. Soc., vol. 125, no. 35, pp. 10684-10692, 2003.

(4) K. C. Sabat, R. K. Paramguru, and B. K. Mishra, "Reduction of Copper Oxide by Low-Temperature Hydrogen Plas- ma," Plasma Chem. Plasma Process., vol. 36, no. 4, pp. 1111-1124, 2016.

About This Article:

Samco, Inc. provided the contents of the article.

AEI November 2022 Special Issue

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