Use of Malonic Acid in Chemical-Mechanical Polishing (CMP) of Tungsten
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ABSTRACT The use of malonic acid as an additive in alumina slurries used for the chemical mechanical polishing ( CMP ) of tungsten has been explored for the reduction of particulate contamination. The principal objective of this work was to delineate conditions under which alumina contamination on polished surfaces could be reduced. The interaction between malonic acid and alumina particles has been investigated through electrokinetic and adsorption measurements. At suitable malonic acid concentrations and pH values, tungsten and alumina surfaces develop a negative zeta potential resulting in conditions conducive to reduced particulate contamination. Small scale polishing experiments have been carried out to relate electrokinetic results to the level of particulate contamination after polishing. INTRODUCTION Chemical mechanical polishing ( CMP ) is being increasingly considered as the only viable method to achieve global planarity in the fabrication of ultralarge-scale integrated ( ULSI ) circuit devices. It has been applied to planarization of interlevel dielectric ( ILD), polysilicon and metal films. As the feature size of ULSI devices scales down to sub-0.25 .im, contamination of
sub-micron particles obviously becomes a critical issue. Particles in CMIP slurries are typically in the size range of 0.1-0.2 u.m. Particulate contamination during CMv~iP arises mostly from the interaction between abrasive particles and the film being polished. For a tungsten structure subjected to CMP, the final exposed surface is composed of silicon dioxide and tungsten. Therefore, in order to reduce or minimize the particulate contamination during tungsten CMP, the interaction between the abrasive alumina particles in polishing slurries with oxide and tungsten films must be understood and controlled. In Fig. 1, zeta potentials of some materials of interest to tungsten CMP are plotted as a function of solution pH [ 1, 2 ]. Alumina based slurries at acidic pH values are most commonly used for tungsten CMP. From Fig. 1, it may be seen that, in the acidic pH range, alumina and tungsten have opposite signs of zeta potential. Electrostatic attraction should prevail under these conditions and alumina contamination of tungsten may be expected. If the slurry pH is higher than 4, contamination will also occur on the silicon dioxide surface. One strategy to minimize alumina contamination during tungsten CMNis to make alumina surface negatively charged at polishing pH values. Previous studies [ 1, 2 ] have shown that this can be accomplished by adding certain chemicals to the polishing slurry. In
115 Mat. Res. Soc. Symp. Proc. Vol. 477 ©1997 Materials Research Society
this paper, the effect of malonic acid in alumina slurries on contamination during tungsten CMP is reported. 80
TOX
CVD W ýýalumina --*'ýaýý luminV
> 40
A
0 CL ,w• -40N
-------- ---_x_ XX
-80 1
3
5
pH
7
11
9
Figure 1: Electrokinetic characteristics of high purity alumina powder, thermal oxide ( TOX ) film and chemical vapor deposited ( CVD ) W film in
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