구리 전착에서의 첨가제와 전류파형의 효과

노태근, 김재홍, 탁용석

인하대학교 화학공학과

Effects of Additives and Current Waveform

in Copper Electroplating

Tae-Geun Noh, Jae-Hong Kim, Yongsug Tak

Department of Chemical Engineering, Inha University

INTRODUCTION

Interconnections are increasingly determining the limitations in density, performance, and reliability of ultra large scale integration (ULSI) and require new materials and deposition technologies for more complex structures.

Until now, aluminum(resistivity = 2.67□cm) and its alloys have been used as an interconnect material, but these alloys are susceptible to electromigration failure and their resistivities are still not low enough to operate at ultrahigh speed. Recently, copper(1.67□cm) is suggested as an interconnecting material which replace aluminum[1]. Since the geometry of interconnector has a severely restricted morphology(width < 0.18□, aspect ratio = 1:5), it is very difficult to obtain a compact Cu filling which does not reduce a Cu conductivity in itself. Leading candidate for the copper deposition technology is electroplating because of high productivity and low cost.

To fill copper compactly inside trenches, inhibitor and leveller is required because an ohmic-drop between trench tip and mouth exist. We propose three additives, which are polyethylene glycol(PEG), chloride ions and benzotriazole(BTAH). Generally, the polymer, sometimes referred to as a carrier, along with chloride ions is commonly used with substances known as brighteners and levelers[2]. BTAH is the best corrosion inhibitor[3]. When such additives are present in small concentrations in the bulk electrolyte, their consumption rate at the electrode surface can be affected by mass transfer process, and inhibition of the deposition reaction tends to be more pronounced where diffusion layer thickness is smaller[4]. Furthermore, to fill copper without void and seams in copper electroplating, copper grain size control is necessary and current waveform should be modulated. Current waveform can be classified ; direct current method, on-off pulse waveform, reverse pulse waveform,  and etc. Especially, copper grain size can be controlled by different pulse waveform.

In this work, we investigate the effect of additives on Cu electrodeposition by using rotating disc electrode (RDE). Different current waveforms were applied for controlling the grain size.

EXPERIMENTAL

The basic composition of the electrolyte was 0.01M CuSO4□5H2O + 1M H2SO4 and  various quantities of PEG(MW 3400), chloride ions and BTAH were added. Electroplated Cu rotating disc electrode was connected HR-103A electrode rotator (HOKUTO DENKO). Copper electrode prepared on the glasses by DC magnetron sputtering was used as working electrode. The counter electrode was a platinum wire and a saturated calomel electrode (SCE) served as a reference electrode.

Current waveforms were modulated with potentiostat/galvanostat (EG&G273A). Morphological changes were investigated with scanning electron microscope (SEM, Hitachi S-4200) and atomic force microscopy (AFM, Topo Matrix AccurexⅡ). The growth orientation of copper crystal and copper oxide were investigated with x-ray diffraction (XRD, Phillips DY616).

RESULTS AND DISCUSSION

Investigation of polarization curves in RDE experiments

Figure 1 shows the effect of Cl-, PEG and BTAH on Cu electrodeposition. The addition of Cl- increases the diffusion limited current density(DLC) and favors the Cu deposition. However, DLC was not evidently defined when PEG was added and it  suggests the rate of electron transfer is slow to act as a limiting factor. When BTAH was added, polarization curve becoms totally different from the other curves. DLC was not observed and the Cu deposition is only limited to charge transfer reaction. The addition of both Cl- and PEG retarded the deposition severely but DLC region was defined. The results indicated that Cl- counteract the effect of PEG and BTAH in Cu deposition.

Investigation of copper morphology

In order to elucidate the effect of copper seed layer in the trench on-chip, The copper thin film electrode was prepared by DC magnetron sputtering method.

Figure 2 shows the effect of additives on the grain size when the Cu is deposited at 0.65V. Distribution diagram of grain size shows that most of grain sizes with additives were less than 0.18□ and the predominant range of grains were from 0.02 to 0.08□. However, the grain size of Cu becoms larger when additives are absent. It tells that Cl- and PEG inhibits the growth of nuclei but favors the nucleation.

Since Cu should be deposited into the narrow trench, the precise controlling method of grain size is required and Figure 3 shows an applied current waveform. Cathodic current or zero current was periodically applied. Compared to dc current deposition, pulse deposition brought a compact film which is composed of smaller grains. Zero current pulses caused an epitaxial growth on to the substrate Cu structure. On the other hand, Cu oxide peaks were detected when reverse pulse waveform was used. Since the presence of Cu2O reduces the conductivity of Cu, on-off pulse waveform can be a promising tool for obtaining a defect-free trench filling.

REFERENCES

[1] M. K. Lee, J. J. Wang and H. D. Wang, J. Electrochem. Soc., 144, 1777(1997).

[2] J. J. Kelly and A. C. West, J. Electrochem. Soc., 145, 3472(1998).

[3] Y. Ling, Y. Guan and K. N. Han, Corrosion, 51, 367(1995)

[4] W. U. Schmid and R. C. Alkire,  J. Electrochem. Soc., 143, 3122(1996).

[5] J. J. Kelly, C. Tian, and A. C. West, J. Electrochem. Soc., 146, 2540(1999).

[6] M. Metikos-Hukovic, J. Electrochem. Soc., 145, 4045(1998).

[7] C. Madore, M. Matlosz and Landolt, J. Electrochem. Soc., 145, 3477(1998).

[8] J. J. Kelly, A. C. West, Electrochemiacl Society Proceedings, 98-6

Fig. 1. Effect of PEG , Cl- and BTAH on the polarization behavior

(Scan rate = 20mV/sec)

Fig. 2. Grain size distribution in AFM image

(constant potential ; -0.65V, 120s)

(a) 1M H2SO4 + 0.01M CuSO4 + 40ppm Cl- + 50ppm PEG

(b) 1M H2SO4 + 0.01M CuSO4

Fig. 3. Applied current waveform

(a) On  off pulse waveform

(b) Reverse pulse waveform