Fracture Toughness of Silicon and Thin Film Micro-structures by Wedge Indentation
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FRACTURE TOUGHNESS OF SILICON AND THIN FILM MICRO-STRUCTURES BY WEDGE INDENTATION M.P. dE BOER*, HE HUANG*, J.C. NELSON*, Z.P. JIANG**, and W.W. GERBERICH* *Dept of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455 **Dept of Electrical Engineering, University of Minnesota, Minneapolis, MN 55455 ABSTRACT Silicon rooftop structures 30 gtm high and of 1 gtm linewidth were fabricated in a microelectronics facility using anistropic KOH etching. Aluminum-1% silicon was sputterdeposited to investigate the effects of a ductile thin film on the fracture toughness of a brittle material. The rooftop structures were indented by a Knoop indenter purposely placed off-center so that the indenter was wedge-shaped. Deformation and cracks were photographed by SEM. Crack threshold load per unit width are reported for normal cracks, running perpendicular to the roofline. Fracture toughness is calculated by first order approximation and finite element modeling using SEM measurements of the length of normal cracks. Without a thin film, values for silicon W , fracture toughness are in reasonable agreement with the literature. With a thin film of only 600 is increased by 45%. Cracks nearly parallel to the roofline also appeared at higher load values. Characteristics of both types of cracks are discussed. INTRODUCTION Indentation studies have been used to obtain information on fracture toughness in ceramics 1 . Also, the effect on fracture toughness of brittle thin films has been studied&,3 . In this study we report the effect of a ductile metal thin film on the fracture toughness of a brittle ceramic. After oxide etch (step 4)
11;0] direction
Photoreslst S102 silicon
Wedge Indenter
S,
Sllcon Rooftop
Silicon Substrite (100) plane
After KOH etch and oxide removal (so 6)
sie30 a (a11)plane2
I
I I 0 (11"0)
41
im
oe(100)
silicon Fig I Idealized schematic of loading situation
Fig 2 Cross-sectional view of process flow
An idealized schematic of the loading situation is shown in figure 1. The reasons for choosing such a structure rather than a flat substrate are as follows. 1) To investigate the indentation fracture effects on small structures and at edges. Most indentation fracture studies have been carried out on flat surfaces. 2) To observe cracks from the side. Cracks which might otherwise
remain subsurface at low loads, such as median cracks4 , may be observed in this configuration. 3) To demonstrate that wedge indentation can be appropriate when the material being probed is small in at least one dimension. A near plane strain condition arises and is easier to analyze than the complicated stress state arising in the normal bulk problem. In this case, the geometry Mat. Res. Soc. Symp. Proc. Vol. 308. ©1993 Materials Research Society
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resembles a simple mode I situation, and the effect on mode I fracture toughness due to a thin film being bonded to the sides of a brittle material is measured. As will be seen, the fracture is perpendicular to the plane of the film-substrate interfa
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