To understand the effect of line width on textural and microstructural evolution of Cu damascene interconnect, three Cu interconnects samples with different line widths are investigated. According to x-ray diffraction (XRD) results, the (111) texture is developed in all investigated lines. Scattered {111} and {111} texture components are present in 0.18-μm-width interconnect lines, and the {111} texture was developed in 2-μm-width interconnect lines. The directional changes of the (111) plane orientation with increased line width were investigated by XRD. In addition, microstructure and grain-boundary character distribution (GBCD) of Cu interconnect were measured using electron backscattered diffraction (EBSD) techniques. This measurement demonstrated that a bamboo-like microstructure is developed in the narrow line, and a polygranular structure is developed in the wider line. The fraction of Σ3 boundaries is increased as the line width increases but is decreased in the blanket film. A new interpretation of textural evolution in damascene interconnect lines after annealing is suggested, based on the state of stress and growth mechanisms of Cu deposits.

INTRODUCTION

Recently, Cu interconnect processing technology, the so-called damascene process, became an important issue in the integrated circuitry chips industry because it decreases resistance and capacitance delay losses and the number of processing operations. One of the most important stages in this technology is the Cu electroplating process, which is characterized by excellent gap filling, high deposition rate, low-temperature processing, system simplicity, and good process controllability.1

Because Cu has been introduced as an interconnect material to replace Al, significant research on the relationship between texture and reliability of copper interconnects has been undertaken.2-7 It is well known that a strong (111) texture is beneficial for the improvement of electromigration failure in Al interconnects.8,9 In addition, it was demonstrated10 that the electromigration failure in aluminum thin films can be correlated with the frequency of coincidence site lattice (CSL) boundaries, low or high diffusivity boundaries, and the strength of the {111{ texture.10 In copper damascene lines, however, such correlations have not been firmly established, and the driving force that can affect the evolution of texture and microstructure as line width increases were not clearly identified until now.

In this study, a fully quantitative description of texture in the interconnect lines will be presented and a possible explanation of texture evolution with an increase of the line width will be proposed.

EXPERIMENTAL PROCEDURE

Three copper damascene lines in tetraethylorthosiliate (TEOS) oxide having different line widths from 0.18-2 μm, all having a trench depth of 0.5 μm, and one copper blanket film were investigated. These are listed in Table I. A 400-Å-thick TaN layer was deposited on the surface of a single crystalline Si (100) wafer as the barrier layer; a copper seed layer and then copper electrodeposits are deposited on the top of the barrier layer. To encapsulate Cu interconnects, overlayers of 7,000-Å SiN and TEOS oxide were deposited. The Cu damascene lines and blanket film were fabricated with the same process steps and conditions.

To obtain direct, quantitative information of the sample surface texture, diffracted intensity measurements were performed at low grazing-incidence angle geometry using a Rigaku x-ray diffractometer (Tokyo, Japan). The crystallographic texture of the copper interconnects was measured using a Siemens D500 x-ray goniometer with copper tube (Munich, Germany). Pole figures were obtained using the reflection technique, up to a maximum tilt of the specimen of 80° in 5° intervals. The recalculated pole figures, orientation distribution function (ODF), and the fraction of different texture components were calculated by TexTools v. 3.0, commercial software for texture analysis (Resmat, Montreal, PQ, Canada).11

An orientation imaging microscope (OIM) mounted on a Philips XL30 field-emission scanning electron microscope (Eindhoven, the Netherlands) was used to identify the orientation of each grain and the types of grain boundaries in the copper interconnects and blanket film. The CSL grain boundaries were identified from the electron backscattered pattern. The frequency of occurrence of these boundaries, up to Σ29, was calculated.

The passivation layer had to be removed to reveal the underlying interconnect lines for the OIM texture measurements. It was found that etching in 15% HF for 5-10 min allows removal of the top passivation layer without causing damage to the copper interconnect lines.