The properties of thermal hillocks as a function of linewidth and process parameter in Al-on-chemical-vapor-deposited W
- PDF / 2,127,588 Bytes
- 9 Pages / 612 x 828 pts Page_size
- 38 Downloads / 214 Views
The formation of hillocks has been studied as a function of process parameter in patterned and unpattemed Al98.5wt.%Sii.owi.%Cuo.5Wt.% films deposited on chemical-vapordeposited W-coated substrates. The effects of linewidth, substrate temperature during film deposition, and sintering time and temperature on hillock size were investigated. Three types of hillocks are found: the "surface hillocks", the "side hillock", and the "line hillock". These are further classified by their shapes. The surface hillock and side hillock, which have been seen previously, form on patterned metal lines having linewidths greater than the larger Al alloy grain sizes (~3 /im). None is seen on linewidths between 0.9 and 2 ^ m where long-range grain boundary diffusion cannot occur. A new type of hillock, the line hillock, is seen to occur on metal structures having linewidths of 0.6 /nm. The line hillock is inconsistent with the current understanding of hillock formation and may present severe restrictions on the downsizing of ultra-large-scale integrated devices.
I. INTRODUCTION Hillocks, i.e., small protrusions, formed during thermal or electrical stressing of thin ( < 1 /JLVO) Al films on Si substrates cause problems in the manufacturing and reliability of integrated circuits. The impact of electrical stressing on hillock formation can be controlled by circuit design. However, thermal stressing, typically in the form of 350 °C to 450 °C sinters, is required in order to heal Si substrate damage incurred during processing. An added benefit of the sinter process is that it increases the Al film's grain size and size uniformity, resulting in an increased resistance to electromigration.12 Thermal hillock formation has been explained3'4 to be a direct result of compressive film stress relief. In the case of Al on Si, during the sinter process, a high compressive stress occurs in the Al film caused by the high Al thermal expansion coefficient with respect to that of Si. This compressive film stress, in turn, causes Al atoms to preferentially migrate from the stressed bulk of the film to its surface. 34 Instead of redistributing itself evenly across the surface, the diffusing Al atoms grew into small surface protrusions known as hillocks and, in some cases, whiskers. The net result is a smooth Al film with micron-sized Al hillocks on its surface. These hillocks interfere with the polymer resist coating process used in a subsequent patterning process, making the fabrication
^Present address: Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.
1010
http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 5, May 1993
Downloaded: 06 Apr 2015
of metal leads in integrated circuits unreliable. To work around this difficulty, the patterning step is done prior to the sintering process. In this case, hillocks form both on the top surface (defined here as "surface hillock") of the Al film and on the sides (defined as a "side hillock") of the patterned metal structures during the
Data Loading...
