Research Papers

Microstructural Characterization of Thermal Damage on Silicon Wafers Sliced Using Wire-Electrical Discharge Machining

[+] Author and Article Information
Kamlesh Joshi, Upendra Bhandarkar

Department of Mechanical Engineering,
Indian Institute of Technology Bombay,
Mumbai 400076, India

Indradev Samajdar

Department of Metallurgical Engineering and
Materials Science,
Indian Institute of Technology Bombay,
Mumbai 400076, India

Suhas S. Joshi

Department of Mechanical Engineering,
Indian Institute of Technology Bombay,
Mumbai 400076, India
e-mail: ssjoshi@iitb.ac.in

1Corresponding author.

Manuscript received December 2, 2017; final manuscript received March 7, 2018; published online June 4, 2018. Editor: Y. Lawrence Yao.

J. Manuf. Sci. Eng 140(9), 091001 (Jun 04, 2018) (14 pages) Paper No: MANU-17-1751; doi: 10.1115/1.4039647 History: Received December 02, 2017; Revised March 07, 2018

Slicing of Si wafers through abrasive processes generates various surface defects on wafers such as cracks and surface contaminations. Also, the processes cause a significant material loss during slicing and subsequent polishing. Recently, efforts are being made to slice very thin wafers, and at the same time understand the thermal and microstructural damage caused due to sparking during wire-electrical discharge machining (wire-EDM). Wire-EDM has shown potential for slicing ultra-thin Si wafers of thickness < 200 μm. This work, therefore, presents an extensive experimental work on characterization of the thermal damage due to sparking during wire-EDM on ultra-thin wafers. The experiments were performed using Response surface methodology (RSM)-based central composite design (CCD). The damage was mainly characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. The average thickness of thermal damage on the wafers was observed to be ∼16 μm. The damage was highly influenced by exposure time of wafer surface with EDM plasma spark. Also, with an increase in diameter of plasma spark, the surface roughness was found to increase. TEM micrographs have confirmed the formation of amorphous Si along with a region of fine grained Si entrapped inside the amorphous matrix. However, there were no signs of other defects like microcracks, twin boundaries, or fracture on the surfaces. Micro-Raman spectroscopy revealed that in order to slice a wafer with minimum residual stresses and very low presence of amorphous phases, it should be sliced at the lowest value of pulse on-time and at the highest value of open voltage (OV).

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Fig. 1

Feasible ultra-thin wafer slicing regions as obtained from entire experiment

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Fig. 2

Theme of experiments

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Fig. 3

Central composite design for RSM analysis

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Fig. 4

(a) Wire-EDM machine and (b) setup for Si wafer slicing

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Fig. 5

Schematic showing the slicing process and assessment of cross-sectional thermal damage on wafer surfaces

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Fig. 6

(a) Effect of OV on thermal damage and (b)–(f) SEM micrographs at different OV: (b) OV:62, Exp:04, (c) OV:65, Exp:01, (d) OV:68, Exp:10, (e) OV:71, Exp:05, and (f) OV:74, Exp:26

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Fig. 7

Effect of OV and SV for SR, when all other variables are kept at middle level [22]

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Fig. 8

(a) Effect of SV on thermal damage and (b)–(f) SEM micrographs at different SV: (b) SV:37, Exp:11, (c) SV:39, Exp:08, (d) SV:41, Exp:20, (e) SV:43, Exp:06, and (f) SV:45, Exp:03

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Fig. 9

(a) Effect of Ton on thermal damage and (b)–(f) SEM micrographs at different Ton: (b) Ton: 0.3 μs, (c) Ton: 0.4 μs, (d) Ton: 0.5 μs, (e) Ton: 0.6 μs, and (f) Ton: 0.7 μs

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Fig. 10

(a) Effect of Toff on thermal damage and ((b)–(f)) SEM micrographs at different Toff: (b) Toff: 7 μs, (c) Toff: 9 μs, (d) Toff: 11 μs, (e) Toff: 13 μs, and (f) Toff: 15 μs

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Fig. 11

Interaction plots of OV with other parameters for thermal damage

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Fig. 12

Interaction plots of SV and Ton with Toff for thermal damage

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Fig. 13

Variation of damage along the height

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Fig. 14

SEM micrographs for different processing conditions

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Fig. 16

(a) Bright field TEM image of resolidified layer of Si wafer, high-resolution images of (b) region A, (c) region B, (d) region C and SAD pattern for (e) region A, (f) region B, and (g) region C

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Fig. 15

Energy-dispersive X-ray spectroscopy maps for deposited material on the sliced surface: (c) oxygen Ka1, (d) carbon Ka1, (e) copper Ka1, and (f) zinc Ka1

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Fig. 18

Effect of varying (a) OV on amorphization of Si and (b) Ton on residual stresses

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Fig. 17

Comparison of Raman shift between uncut colloidal silica polished sample and wire-EDM cut sample at OV 77 V




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