Nanoindentation tests on diamond-machined silicon wafers

Jiwang Yan, Hirokazu Takahashi, Jun'ichi Tamaki, Xiaohui Gai, Hirofumi Harada, John Patten
2005 Applied Physics Letters  
Nanoindentation tests were performed on ultraprecision diamond-turned silicon wafers and the results were compared with those of pristine silicon wafers. Remarkable differences were found between the two kinds of test results in terms of load-displacement characteristics and indent topologies. The machining-induced amorphous layer was found to have significantly higher microplasticity and lower hardness than pristine silicon. When machining silicon in the ductile mode, we are in essence always
more » ... in essence always machining amorphous silicon left behind by the preceding tool pass; thus, it is the amorphous phase that dominates the machining performance. This work indicated the feasibility of detecting the presence and the mechanical properties of the machining-induced amorphous layers by nanoindentation. Silicon is not only a dominant substrate material for the fabrication of microelectronic and micromechanical components but also an important infrared optical material. 1,2 The manufacturing of large-diameter silicon wafers by ultraprecision ductile machining technology has become a subject of concentrated research interests. 3-6 A bottleneck for ductile machining processes is the machining-induced subsurface damages to silicon substrates, which involve dislocations and phase transformations. A number of previous studies have confirmed the presence of amorphous phase within the nearsurface layer of ductile-machined silicon wafers. [7] [8] [9] [10] [11] The subsurface damages, especially the amorphous layer, will significantly influence the mechanical, optical, and electrical functions of silicon parts. Virtually all studies of machined silicon surfaces involve the evaluation of this amorphous layer by default, even if the investigators did not realize it or explain this phenomenon. For example, when considering micromechanical applications where surface contacts and/or frictions exist, the mechanical properties of the amorphous layer, such as hardness, elasticity, and plasticity, become very important. The subsurface damages also influence the subsequent wafer manufacturing processes. That is, a machining operation always involves multiple tool passes due to the cross feed; thus, with the exception of the first cut, all subsequent cuts are made upon an amorphous material and not the starting crystalline material. From this point of view, it is essential to clarify the mechanical properties of this amorphous layer. However, to date, most silicon machining processes are based on the properties of the diamond-cubic single-crystalline phase, and little effort has been placed on the machining-induced amorphous phase. During the past decades, response of single-crystal silicon to micro/nanolevel indentation has received extensive attention. 12-23 Unlike other materials, silicon often shows characteristic features in indentation depth in the unloading part of the load-displacement curve, namely, pop out or elbow, depending on the unloading rate, angle of the indenter, maximum load, or indentation depth. 19-21 These interesting phenomena are believed to be related to high-pressure phase transformations occurring beneath the indenter, which are accompanied by a significant volume change, and the extrusion ͑elbow͒ or containment ͑pop out͒ of the high pressure phase. In this article, we report the results of nanoindentation tests performed on ultraprecision ductile-machined silicon wafers. When performing nanoindentation on machined silicon wafers, the indentation behavior will be dominated by the machining-induced amorphous layer for low loads and shallow depths. This situation can be simply considered as a thin film of amorphous silicon formed on a pristine crystalline substrate. Therefore, if a suitable indentation load is used, it may be possible to detect the presence and the mechanical property of this amorphous layer by nanoindentation. In our experiments, electric device grade p-type singlecrystal silicon ͑100͒ wafers having a doping level of 1.33 ϫ 10 14 atoms/ cm 3 , were used as specimens. These wafers are 25.4 mm in diameter, 0.725 mm in thickness, and obtained with chemomechanical polished finishes. The wafers were fixed to a hydrostatic spindle by heat-softened wax and were machined by fly-cutting using a straight-nosed diamond cutting tool 24 on an ultraprecision diamond lathe, Toyoda AHP 20-25N. We used a cutting tool with a −60°rake angle and a 6°relief angle for experiment. The tool rake angle was set to more negative than the practically used tools ͑−20-−40°͒, for the purpose of generating a thicker subsurface damage layer. 25 Other machining conditions were undeformed chip thickness 100 nm, tool feed 14 m / rev, cutting edge angle 0.4°, depth of cut 2 m, and cutting speed a͒ Present address:
doi:10.1063/1.1924895 fatcat:l7fumxvgbrdmhfosmy4idrbtya