Effect of La Doping of Ceria Abrasives for STI CMP

Shallow trench isolation (STI) is used to isolate the transistors in an integrated chip and chemical mechanical planarization (CMP) is a key step in the STI integration scheme [1]. Ideally a high silicon dioxide removal rate and a low silicon nitride removal rate (i.e. high selectivity) is needed.  High selectivity can be achieved when amino acids such as L-proline are used as additives along with ceria abrasives [2-3] but they do not work well with other abrasives [4]. Even when ceria is used as abrasive high selectivity is not observed in some cases [5].

In the present study, ceria is synthesized using sol gel and liquid phase synthesis (LPS). Polishing experiments are done on different ceria abrasives with same additives. To study the effect of impurity on polishing, lanthanum is added to ceria during the synthesis stage.  The polishing results show that ceria without any impurity give high selectivity with L-proline while ceria with lanthanum as impurity does not give high selectivity.

To compare the results of polishing, commercial ceria with high and low purity obtained from sigma aldrich (ceria-SA) are considered for polishing experiments. Polishing with commercial ceria show similar results obtained with synthesized ceria. The ceria particles both commercial and synthesized were characterized by x-ray diffraction, energy dispersive spectra and scanning electron microscopy techniques. Polishing experiments were also conducted with milling the commercial ceria to evaluate if the size of ceria abrasive plays a role in modifying the selectivity.

References:

[1] J.M. Steigerwald, S.P. Murarka, R.J. Gutmann, Chemical Mechanical Planarization of Microelectronic Materials, John Wiley & Sons, New York, 1997.

[2] W.G. America, S.V. Babu, Electrochem. Solid-State Lett. 7 (2004) G327 – G330.

[3] R. Srinivasan, S.V. Babu, W.G. America, Y.-S. Her, US Patent 6,468,910 B1 (2002).

[4] R. Manivannan, S. Ramanathan, Microelectron. Eng. 85 (2008) 1748 - 1753.

[5] P.W. Carter, T.P. Johns, Electrochem. Solid-State Lett. 8 (2005) G218 – G221.