염기성 수용액에서 전기화학적 수정진동자를 이용한 구리의 전해 용출에 대한 특성조사

최성훈, 탁용석

인하대학교 화학공학과

Electrochemical study of the anodic dissolution of copper combined with EQCM in alkaline solution

Sunghoon Choi, Yongsug Tak

Department of Chemical Engineering, Inha University

INTRODUCTION

The chemical mechanical polishing (CMP) of Cu has emerged as a promising technology for delineating inlaid Cu patterns and planarizing the wafer surface on a global scale[1]. Recently, a number of studies on copper chemical mechanical polishing have achieved multilevel interconnections with low electrical resistance, excellent electromigration resistance, and global planarization[2-4]. However, the local passivation offered by copper oxide during CMP process could result in increased surface roughness. A clear understanding of mechanism on CMP process is important to accomplish Cu metallization. Copper CMP with NH4OH solutions have been studied by many research groups. In electrochemical measurements, they suggested that copper dissolved into Cu(Ⅰ) followed by Cu(Ⅱ) and formed copper oxide. However there were no electrochemical studies combined with Electrochemical Quartz Crystal Microbalance (EQCM) and surface analysis. In this work, in order to describe copper dissolution mechanism, we concentrated on electrochemical study with EQCM and surface analysis. In NH4OH-containing media, there are two anodic current peaks during potentiodynamic measurement and they show different mass change ratio in each peak and analyzed by SEM, XPS and XRD.

EXPERIMENTAL

The electrochemical studies were conducted with a conventional three electrode. The working electrode was Pt electrochemical quartz crystal (Seiko Instruments, Inc., AT-cut 9MHz, area = 0.196cm2). A platinum wire and saturated calomel electrode were used as counter and reference electrode. All potentials are referred to the SCE. Two electrolytic solutions were used. For electroplating of Cu, 0.5M H2SO4 + 0.01M CuSO4 was used, and for anodic dissolution of Cu, 0.5M NH4OH was used.

All measurements were carried out at room temperature using an Potentiostat/Galvanostat (EG&G PAR Model 273A) and EQCM (Seiko EG&G, QCA 917). In the EQCM, the equation of the resonant frequency change for mass change of deposited or dissolved species has been presented by Sauerbrey equation[5-6],

[eqn: TRIANGLE f=-2 { { f}_{o } }^{2 } TRIANGLE m/A{( { mu }_{ q} { rho }_{q })}^{1/2}]

Where f0 is the resonant frequency of the quartz, 9MHz; electrode area (A), 0.196 cm2; □q is the shear modulus of quartz, 2.497 □ 1011 gcm-1; □q is the density of quartz, 2.648gcm-3

[eqn: TRIANGLE f= { -C}_{f } TRIANGLE m]

In which Cf(935.7 Hz/□g) is a constant which is solely determined by he properties of the quartz crystal.

A combination of liner sweep voltammetry, chronoamperometry and chronopotentiometry were used with mass changes.

RESULTS AND DISCUSSION

Figure 1 shows linear sweep voltammetry and mass change of Cu dissolution in 0.5M NH4OH. Cu was electroplated on Pt-Quartz in sulfuric acid electrolyte. There are two anodic current peaks and different mass change ratio. Q. Luo et. al suggested that the first current peak can be ascribed to the formation of Cu2O and the second current peak is due to the formation of CuO film[7]. However, from frequency measurements, there were no evidence of surface passivation. Figure 2 shows galvanostatic experiments in various film thickness. Figure 3 shows potentiostatic experiments in different potential. From figure 2 and 3, it can be seen that there are two reactions which depend on potential. Figure 4 shows concentration effect of NH4OH on Cu dissolution. As increasing NH4OH concentration, anodic peak potential is shifted to more cathodic potential. It could be ascribed to decrease solution resistance. Moreover, as increasing NH4OH concentration, mass change range decrease due to surface passivation.

REFERENCES

[1] F.B. Kaufmann, D.B. Thompson et al, J. Electrochem. Soc., 138, 3460(1991).

[2] J.M. Steigerwald, D.J. Duquette, S.P. Murarka and R.J. Gutmann, J. Electrochem. Soc., 142, 2379(1995).

[3] J.M. Steigerwald, R. Zirpoli, S.P. Murarka, D.Price and R.J. Gutmann, J. Electrochem. Soc., 141, 2842(1994)

[4] Z. Stavreva, D. Zeidler, M. Plotner and K. Drescher,  Appl. Surface Sci., 108,       39(1997).

[5] J. Lee and Y. Tak,  Electrochem. Solid-state Lett., 2, 559(1999).

[6] C.H. Pyun and S.M. Park, J. Electrochem. Soc., 133, 2024(1986).

[7] Q. Luo, R.A. Mackay and S.V. Babu, Chem. Mater., 9, 2101(1997).

Fig. 1. Linear sweep voltammetry and mass changes in NH4OH 0.5M

(Scan rate = 20mV/sec)

(a)                                 (b)

Fig. 2. Chronopotentiometry and mass changes in NH4OH 0.5M

(constant current ; +100□A, 240s)

(a) Voltammetry

(b) Mass change

(a)                                  (b)

Fig. 3. Chronoampeerometry and mass changes in NH4OH 0.5M

(different applied potential ; -0.35V and 0V, 60s)

(a) Voltammetry

(b) Mass change

(a)                                   (b)

Fig. 4. Linear sweep voltammetry and mass change in various NH4OH concentration

(Scan rate = 20mV/sec)

(a) Voltammetry

(b) Mass change