1. Separation Techniques, Valencia
High Compressive Strength Au film Fabricated by Advanced Electrochemical Technique in Supercritical CO2 Emulsified Electrolyte for MEMS Accelerometers
Chun-Yi Chen*, Masaharu Yoshiba , Hau-Chun Tang, Tso-Fu Mark Chang, Daisuke Yamane,
Katsuyuki Machida, Kazuya Masu, Masato Sone
Institute of Innovative Research
Tokyo Institute of Technology
September 27th , 2016

2. Outline Results and discussion Experimental section Introduction
Current uses and demand for gold
MEMS accelerometer
Strategy for Strengthening
Size Effect of Mechanical Property
Experimental section
Pulse electroplating
Electrolyte bath
Electroplating with scCO2
Micro mechanical compression test
Results and discussion
Au film characterization
Micro mechanical property of Au micro-pillars

3. Current Uses and Demand for Au
Au fabrication Demand
Advantage of Au
High chemical stability
High electrical conductivity
High Corrosion Resistance
Au in Electronics Industry
Circuit boards
Electrical connectors
Relays
Catalyst
MEMS sensors
Unit: Ton
References: GFMS Gold Survey and Morgan Stanley Research

4. MEMS Motion Sensor-Accelerometer
Concern: Typical yield strength of bulk Au is MPa  Too soft   Reliability of MEMS sensor
Thermal-mechanical noise of the proof mass
Higher density than Si
Au (19.3×103kg/m3 )
>> Si (2.33×103kg/m3 )
Brownian noise ↓
Size of Sensor↓
Sensitivity ↑
𝑩 𝑵 : Boltzmann constant
( J/K)
𝑩 𝑵 = 𝟒 𝒌 𝑩 𝐓𝒃 𝒎
𝐓: the absolute temperature
𝒌 𝑩 : the viscous damping coefficient
𝒎: mass of proof mass
D. Yamane et al., APL 104 (2014)

5. Strategy for Strengthening
Grain boundaries are barriers to slip.
Barrier "strength“ increases with Increasing angle of mis-orientation.
Smaller grain size: more barriers to slip.
Hall-Petch Equation:
Reference: A Textbook of Materials Technology, by Van Vlack, Pearson Education, Inc., Upper Saddle River, NJ.
Reduce Grain Size
Strengthening mechanisms like work hardening, precipitate, and grain boundary strengthening

6. Size Effect: Mechanical Properties
Strengthening mechanisms like work hardening, precipitate, and grain boundary strengthening
Uchic et al., Science, 305 (2004) 986.

7. Outline Results and discussion Experimental section Introduction
Current uses and demand for gold
MEMS accelerometer
Strategy for Strengthening
Size Effect of Mechanical Property
Experimental section
Pulse electroplating
Electrolyte bath
Electroplating with scCO2
Micro mechanical compression test
Results and discussion
Au film characterization
Micro mechanical property of Au micro-pillars

8. Pulse Peak Current (Ip)
Experimental Section-Pulse Electroplating
Pulse Electroplating (PE):
High nucleation rate and inhibition of grain growth
Electrochemical
parameters
Temperature
Electrolyte
Constant Current
Electroplating (CE)
The periodically shift of current between two different values.
Pulse Peak Current (Ip)
Pulse Off Current (Io)
Average Current (I A)
Pulse
ON-time
Off-time
Duty
Cycle
10 mAcm-2
0 mAcm-2
5 mAcm-2
10 ms
0.5

9. Sulfite-based electrolyte Cyanide-based electrolyte
Experimental Section-Electrolyte Bath
Sulfite-based electrolyte
Na2SO3
50g/L
(NH4)2SO3
Na3[Au(SO3)2]
21.63g/L
Sodium gluconate
5g/L pH 8
Temperature
40°C
Cyanide-based electrolyte:
Constant current electroplating (CE-1)
as comparison reference
Sulfite-based electrolyte:
Pulse electroplating (PE) and
Constant current electroplating (CE-2)
Cyanide-based electrolyte
KCN
85g/L
K [Au(CN)2]
14.63g/L
Di-ammonium hydrogen citrate
230g/L pH 5
Temperature
60°C
圖有誤

10. Application of sc-CO2 in electroplating
T. Nagoshi et al., Appl. Mech. Mater. 284 (2013) 163
T.F.M. Chang et al., Thin Solid Film 529 (2013) 25
Smoother morphology and smaller grain size
Higher mechanical strength in Ni film

11. Au Electroplated with Sc-CO2

12. Displacement rate: 0.1 μms-1 Load resolution: 10 mN
Micro Mechanical Compression Test
Fabrication of Au micro-pillar
FIB image
Displacement rate: 0.1 μms-1 Load resolution: 10 mN
圖有誤

13. Outline Results and discussion Experimental section Introduction
Current uses and demand for gold
MEMS accelerometer
Strategy for Strengthening
Size Effect of Mechanical Property
Experimental section
Electrolyte bath
Pulse electroplating
Electroplating with scCO2
Micro mechanical compression test
Results and discussion
Au film characterization
Micro mechanical property of Au micro-pillars

14. Au Film Characterization Fabrication by Pulse Plating
XRD Pattern
AFM Image
Grain size 17.6 nm
Surface Roughness
Ra= 117.1nm (CE-1)
FIB image
Grain size 22.8 nm
Surface Roughness
Ra= 32.5 nm (CE-2)
Grain size 10.5 nm
Surface Roughness
Ra= 10.7 nm (PE)

15. Results of Micro Compression Test-SIM Image
Irregular strip-patterns
Brittle fracture was observed
Cracks along texture boundary
Camouflage patterns
Broad shear banding crossing through top to bottom
No obvious grain/texture
boundary on the Au pillar
Cyanide electrolyte
Sulfite-based electrolyte
Grain-boundary strengthening mechanism
Electrochemistry Communications 67 (2016) 51–54

16. Engineering Stress-Strain Curves of Au
Elastic
region
Plastic
region
Grain-boundary strengthening mechanism
Work hardening is the strengthening of a metal by plastic deformation. This strengthening occurs because of dislocation movements and dislocation generation within the crystal structure of the material.
Yield drop observed from CE1 may correspond to cracks. The cracks are suggested to be caused by the impurities derived from the cyanide-based electrolyte, which lead to a decrease in the adhesion between the texture boundary. 
The micro-pillars prepared from the PE and the CE2 shows parabolic work hardening generally observed in polycrystalline samples.

17. Hall–Petch Plot Hall–Petch relationship PE 𝝈 𝒚 ↑ = 𝝈 𝒐 + 𝒌 𝒚 𝒅↓ CE-1nm
Hall–Petch relationship
𝝈 𝒚 ↑ = 𝝈 𝒐 + 𝒌 𝒚 𝒅↓
𝝈 𝒚 :𝑺𝐭𝐫𝐞𝐬𝐬
𝝈 𝒐 :Materials constant for the
starting stress for dislocation
𝒌 𝒚 : Strengthening coefficient
𝒅:Average grain diameter
Grain-boundary strengthening mechanism
Au micro-pillar with compressive strength of 800MPa by PE is the highest value reported for pure Au.
The high strength is suggested to be due to the grain-boundary strengthening mechanism known as the Hall-Petch effect.
The results demonstrated that PE method and the sulfite-based electrolyte are promising for applications in miniaturization of MEMS devices.

18. Au Film Characterization Fabrication with scCO2CE
EP-SCE
Similar morphology and roughness
Ra ~13 nm (Cu substrate Ra ~19 nm)
Peak broadening in EP-SCE film
⇒ finer grain ~13 nm
Surface of plated Au films
Crystalline structure (XRD)
Grain-boundary strengthening mechanism
Work hardening is the strengthening of a metal by plastic deformation. This strengthening occurs because of dislocation movements and dislocation generation within the crystal structure of the material.

19. Engineering Stress-Strain Curves of Au
Deformation behavior (SIM)
Strain-stress (SS) curve
CONV-EP: 450 MPa (flow stress)
EP-SCE: ~800 Mpa (flow stress)
Grain-boundary strengthening mechanism
Work hardening is the strengthening of a metal by plastic deformation. This strengthening occurs because of dislocation movements and dislocation generation within the crystal structure of the material.
Higher yield stress (520 MPa) and flow stress (~800 MPa) was also achieved by EP-SCE Au.
Hall-Petch effect ⇒ strength enhancement by finer grain

20. Summary
The Au film prepared by PE possessed less defect, lower roughness, smaller grain size, and denser texture when compared with the Au film prepared by the CE.
Grain size of Au micro-pillar was estimated to be 10.5 nm, and the compressive flow stress was 800 MPa (yield stress 670 MPa), which was the highest value reported for pure Au.
High yield stress of 520 MPa was also achieved by electroplating with scCO2.
The results demonstrated that PE method and CE method with scCO2 are both effective for the enhancement of mechanical properties of Au.
The high strength is suggested to be due to the grain-boundary strengthening mechanism known as the Hall-Petch effect.
Future Work
Combining the PE technique with scCO2 is promising to achieve Au film with finer grain and higher strength.

21. Acknowledgement People Founding
Prof. Masato Sone in Tokyo Institute of Technology
Prof. Tso-Fu Mark Chang in Tokyo Institute of Technology
Dr. Takashi Nagoshi in National Institute of Advanced Industrial Science and Technology
Mr. Haochun Tang (PhD Candidate) of Tokyo Institute of Technology
(Poster No. ST-008 )
Founding
CREST Project operated by the Japan Science and Technology Agency (JST)

22. Thank You for Your Attention

23. Engineering Stress-Strain Curves of Au-Cu
5μm
Compression
Flow stress reached 1400 MPa
High mechanical strength was
achieved, and was attributed
to the Hall-Petch relationship
Grain size = 6.7 nm
Comparison reference: Au-Cu-Sn Alloy
Grain-boundary strengthening mechanism
Work hardening is the strengthening of a metal by plastic deformation. This strengthening occurs because of dislocation movements and dislocation generation within the crystal structure of the material.
Scripta Materialia 57 (2007) 301–304

24. Strength Dependence on PE Parameters
Optimization of PE
On-time (Ton)= 10 ms
Off-time (Toff)=10 ms
FIB image
Grain-boundary strengthening mechanism