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TECHNICAL PAPERS

# Response of Thin Films and Substrate to Micro-Scale Laser Shock Peening

[+] Author and Article Information
Youneng Wang

Department of Mechanical Engineering, Columbia University, New York, NY 10027yw2119@columbia.edu

Hongqiang Chen, Jeffrey W. Kysar, Y. Lawrence Yao

Department of Mechanical Engineering, Columbia University, New York, NY 10027

J. Manuf. Sci. Eng 129(3), 485-496 (Nov 10, 2006) (12 pages) doi:10.1115/1.2714568 History: Received November 22, 2004; Revised November 10, 2006

## Abstract

Micro-scale laser shock peening $(μLSP)$ can potentially be applied to metallic structures in microdevices to improve fatigue and reliability performance. Copper thin films on a single-crystal silicon substrate are treated by using $μLSP$ and characterized using techniques of X-ray microdiffraction and electron backscatter diffraction (EBSD). Strain field, dislocation density, and microstructure changes including crystallographic texture, grain size and subgrain structure are determined and analyzed. Further, shock peened single crystal silicon was experimentally characterized to better understand its effects on thin films response to $μLSP$. The experimental result is favorably compared with finite element method simulation based on single-crystal plasticity.

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## Figures

Figure 1

Laser shock peening process

Figure 2

Characterization of testing materials (1μm and 3μm Cu polycrystalline films on [004] single crystal Si substrate) by conventional X-ray diffraction

Figure 3

Divergence of X-ray beam incident and θ, and χ scans of sample (distance from sample to the capillary tip is about 3mm, the focused spot size is about 2μm, the divergence angle is 0.6deg, and the spot size on the sample surface is about 5μm)

Figure 4

Diffraction intensity contrast measurements across the shock line of 3μm thin film under different energy levels

Figure 5

3D X-ray profile spatial distribution across the shock line of 3μm thin film (laser energy of 6.30GW∕cm2, spatial resolution is 5μm close to the line and 10μm far away from the line)

Figure 6

ln(An) versus n2 and ln(An) versus n lines at different positions from the center of shocked line (An: the real part of corrected Fourier coefficient; and n: Fourier series number)

Figure 7

Standard strain deviation in depth direction and dislocation density by Fourier transformation for the 3μm copper thin film

Figure 8

Spatial distribution of average mosaic size based on FFT analysis of the initial slopes of the An versus n curves (as shown in the small figure) for 3μm copper thin film

Figure 9

Spatial distribution of strain normal to the irradiated surface of (004) single-crystal silicon based on X-ray diffraction measurements

Figure 10

Diffraction intensity contrast measurements across the shock line of (004) single-crystal silicon

Figure 11

In-plane and out-of-plane lattice rotation on shock peened surface of single-crystal silicon for 4.03GW∕cm2 laser energy (obtained from the θ and χ scans, respectively, shown in Fig. 3)

Figure 12

Texture of 1μm thin film by inverse pole figure: (a) raw sample; and (b) shocked area

Figure 13

Misorientation angle distribution of {001} lattice direction before and after LSP for 1μm thin film and slip systems (111)⟨110⟩ for (111) film texture

Figure 14

Grain size map and subgrain structure changes through LSP of 1μm thin film: (a) before LSP; (b) after LSP: red color: highly deformed region with the highest density of substructure, gray color: grains with medium density of substructures; white color: stress free grains that have less defects and substructures (circles shown in (b) are to be used with Fig. 1)

Figure 15

Distribution of grain size for 1μm thin film

Figure 16

Topography of 1μm copper thin film by AFM: (a) raw thin film; (b) shocked film (scanning area 8μm×8μm, data scale 2μm)

Figure 17

(001) texture component corresponding to Fig. 1, the darker, the closer to the ⟨001⟩ direction; white regions are greater than 20deg

Figure 18

Strain normal to the irradiated top surface for single crystal silicon: (a) strain distribution by FEM; and (b) comparison between FEM and X-ray results

Figure 19

In-plane lattice rotation for single-crystal silicon: (a) lattice rotation distribution by FEM; and (b) comparison of FEM and X-ray results

Figure 20

Plane strain slip systems for (001) single crystal sample

Figure 21

(a) Plastic shear strain for slip system i; (b) plastic shear strain for slip system ii; (c) plastic shear strain for slip system iii; and (d) total plastic shear strain (total simulation region is 800μm×400μm)

Figure 22

Resolved shear stress contour in: (a) slip system i; (b) slip system ii (corresponding to Figs.  2121, respectively, unit is MPa)

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