Dependence of Microstructural Evolution of Nanoindented Cu/Si Thin Films
on Annealing Temperature
Woei-Shyan Lee
1;*, Tao-Hsing Chen
2, Chi-Feng Lin
3and Yu-Liang Chuang
11
Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan, R. O. China
2Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, R. O. China 3National Center for High-Performance Computing, Hsin-Shi Tainan County 744, Taiwan, R. O. China
The nano-mechanical properties of as-deposited Cu/Si thin films indented to a depth of 2000 nm are investigated using a nanoindentation technique. The nanoindented specimens are annealed at a temperature of either 160C or 210C, respectively. The microstructures of the
as-deposited and annealed samples are then examined via transmission electron microscopy (TEM). The results show that both the loading and the unloading regions of the load-displacement curve are smooth and continuous, which suggests that no debonding or cracking occurs during nanoindentation. In addition, the hardness and Young’s modulus of the Cu/Si thin films are found to vary with the nanoindentation depth, and have maximum values of 2.8 GPa and 143 GPa, respectively, at the maximum indentation depth of 2000 nm. The TEM observations show that the region of the Cu/Si film beneath the indenter undergoes a phase transformation during the indentation process. In the case of the as-deposited specimens, the indentation pressure induces a completely amorphous phase within the indentation zone. For the specimens annealed at a temperature of 160C, the amorphous nature of the microstructure within the indented zone is maintained. However, for the specimens annealed
at a higher temperature of 210C, the indentation affected zone consists of a mixture of amorphous phase and nanocrystalline phase. Copper
silicide (-Cu3Si) precipitates are observed in all of the annealed specimens. The density of the -Cu3Si precipitates is found to increase with an
increasing annealing temperature. [doi:10.2320/matertrans.M2010212]
(Received June 22, 2010; Accepted August 23, 2010; Published October 14, 2010) Keywords: nanoindentation, silicon, microstructural evolution, annealing temperature
1. Introduction
As micro-electro-mechanical systems (MEMS) techniques continue to mature, the fabrication of thin-film structures has become commonplace in the microelectronics and optoelec-tronics fields.1–3) It is well known that the mechanical properties and microstructural characteristics of thin-film materials differ quite significantly from those of bulk materials, and generally vary in accordance with the fabrication process,4)the substrate effect,5,6) the film thick-ness,7,8) the interface structure,9,10) and so on. Therefore, in optimising the thin-film structures used in MEMS and integrated circuit (IC) applications, it is necessary to study the mechanical properties and microstructure of the thin-film system at an appropriate (i.e. nanometer) scale.
Nanoindentation provides a convenient means of analy-sing the mechanical properties of both bulk materials and thin films.11,12)Previous studies have shown that the indentation process results in a pressure-induced phase transformation of the material directly beneath the indenter, which has a direct effect on the characteristics of the corresponding load-displacement curve.10,13)Furthermore, it has been shown that the indented microstructure of an as-deposited specimen can be modified via the application of a suitable annealing temperature.12,13)Amongst all the face-centered cubic (fcc) materials, copper (Cu) tends to be one of the most commonly used for the coating of silicon substrates in the fabrication of modern electronic devices dues to its high chemical stability, low resistivity, good patterning ability, good reliability, ready availability, and low cost.14,15)Various physical and chemical methods have been proposed for the fabrication of Cu/Si systems with coherent layers.16–19) It is known that the
bonding strength of the Cu/Si system is enhanced via the precipitation of copper silicide particles via a solid state reaction when the interface between the thin Cu film and the Si substrate is heated to a sufficient temperature. Accord-ingly, this study investigates the nano-mechanical properties of as-deposited Cu/Si samples indented to a depth of 2000 nm and then anneals the indented samples at temper-atures of either 160C or 210C, respectively. Thereafter, the microstructures of the as-deposited and annealed samples are examined via transmission electron microscopy (TEM) in order to examine the effects of the annealing temperature on the microstructural evolution of the indented specimens and the degree of copper silicide formation.
2. Experimental Procedure
The Cu/Si thin-film specimens were fabricated by depos-iting a Cu film with a thickness of approximately 800 nm on a Si (100) substrate using an evaporation deposition technique. The thickness of the Cu film was monitored continuously during the deposition process using a quartz-crystal micro-balance, and was confirmed following fabrication using an X-ray reflectometry technique. The nanoindentation tests were performed using an MTS Nanoindenter XP system fitted with a Berkovich diamond pyramid tip. The loading-unloading procedure involved the following steps: (1) impressing the indenter into the Cu/Si system until the position of maximum indentation was achieved (2000 nm); (2) holding the indenter in this position for 10 s; and (3) smoothly withdrawing the indenter from the specimen over a period of 30 s. The corresponding load-displacement data were then used to determine the hardness and Young’s modulus of the Cu/Si thin film in accordance with the method proposed by Oliver and Pharr.20)
*Corresponding author, E-mail: [email protected]
Materials Transactions, Vol. 51, No. 11 (2010) pp. 2013 to 2018
Following the nanoindentation tests, the specimens were annealed at a temperature of either 160C or 210C for 2 min in a rapid thermal annealing (RTA) system. During the annealing process, purified nitrogen gas (99.999%) was passed through the furnace at a flow rate of 3 L/min. Thin foil specimens of both the as-deposited samples and the annealed samples were prepared using an FEI Nova 200 focused ion beam (FIB) milling system with a Gaþ ion beam and an operating voltage of 30 keV. The cross-sectional micro-structures of the various specimens were then observed using a Philips Tecnai F30 Field Emission Gun Transmission Microscope operated at 300 keV.
3. Results and Discussion
Figure 1 presents a TEM micrograph of the as-deposited Cu/Si thin-film system. Note that the Pt layer on the sur-face of the Cu thin film serves to protect the indentation region from accidental damage by the ion beam while preparing the TEM samples. As shown in the selected area diffraction (SAD) patterns presented in the upper-left and upper-right corners of Fig. 1, respectively, the substrate has a single-crystal structure, while the Cu film has a poly-crystalline structure. The as-deposited Cu thin film has a tensile residual stress with a magnitude of approximately 280 MPa, as measured using a polarisation phase shift technique.21)
Figure 2(a) presents the loading-unloading curve for the as-deposited Cu/Si thin film when indented to a depth of 2000 nm. It can be seen that both the loading and the unloading regions of the curve are smooth and continuous, which suggests that no debonding or cracking occurs during the indentation process. Moreover, the virtually vertical slope of the unloading curve indicates that the plastic deformation of the Cu/Si thin film during the loading process is followed
by a very weak elastic return in the unloading step. Finally, the slight elbow feature in the final portion of the unloading curve suggests that the silicon substrate transforms from a diamond cubic like structure to an amorphous structure in the indentation affected zone. Note that this inference is consistent with the finding in Refs. 22, 23) that the formation of amorphous phase upon unloading is accompanied by a
Fig. 1 Bright field TEM micrograph of as-deposited thin-film structure comprising Cu film and Si substrate.
0 Depth, D/nm 0 50 100 150 200 250 Load, F /mN (a) 2500 2000 1500 1000 500 0 Depth, D/nm 0 1 2 3 4 5 Hardness, H /GP a (b) 2500 2000 1500 1000 500 (c) 0 Depth, D/nm 0 40 80 160 Modulus, E /GP a 2500 2000 1500 1000 500 120
Fig. 2 (a) Typical load-displacement curve obtained in nanoindentation test performed to depth of 2000 nm; (b) Variation of microhardness with indentation depth; (c) Variation of Young’s modulus with indentation depth.
Acknowledgements
The authors gratefully acknowledge the financial support provided to this study by the National Science Council (NSC) of Taiwan under contract no. NSC 97-2221-E-006-047.
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