additive impact on cu properties for fan-out wafer level packaging...
TRANSCRIPT
Additive Impact on Cu Properties for Fan-out Wafer Level Packaging Applications
Dirk RohdeR&D Manager Electronics, Atotech
FO WLP
FO WLP Introduction
Source: Yole
FO WLP Introduction – Copper Plating Process Requirement
Decreasing RDL pitch down to 2x2µm 1. Smaller pitch enables more RDLs in slim packages2. More RDLs enable higher I/O density
Microvia filling capability for Multilayer RDL1. Multilayer architectures enable higher functionality and integration
High Speed Plating capability1. Decrease process time for copper pillar plating
FO WLP Introduction – Required Copper Properties
Good Mechanical Properties1. Fine lines in high stress area (avoid cracks)
High Pure Copper Deposits1. Conductivity
Low Stress1. Warpage of mold wafer
Mechanical Properties of Copper
Mechanical Properties of Copper – How to measure?
0 5 10 15 200
50
100
150
200
250
300
350
Rp0.2 [MPa]
Totalelongation/extensionat fracture
Elongation/extensionafter fracture [%]
Slope of the tangentElastic modulus E [GPa]
Integrated area is deformation energy densityW [mJ/mm3]
In Ductensiomat II units: W = 7.8 Ncm
Tensile strength Rm [kN/cm2]
t = D [%]
Tens
ile s
tres
s (M
Pa)
Elongation / extension (%)
Mechanical Properties of Copper – How to influence?
• Strength-Ductility Dilemma
R. Valiev et al., J. Mat. Res.17/1, 5-8(2002)
• Alloy deposition e.g. Brass (CuZn alloy)– Better mechanical properties than copper – Tensile strength of 450 MPa– Ductility of 27% – Conductivity is lower than pure copper (30 – 50%)
Mechanical Properties of Copper – How to influence?
S. K. Jha et al., IJAME 11, 2317-2331 (2015)
Mechanical Properties of Copper – How to influence?
• Engineering tensile test curves for different grain sizes
– Grain size can be controlled by electro deposition conditions– Smaller grain show higherstrength values – Ductility is decreasing– Toughness is decreasing– Conductivity is decreasing
• Nano-twined Cu via pulsed electrodeposition (20 µm thickness)
Mechanical Properties of Copper – How to influence?
X.H. Chen, Scripta Materiialia 64, 311-314 (2011)
Nano-twin Cu
GNG Cu
Mechanical Properties of Copper – How to influence?
Brass
Gradient Nano Grained Copper
Z. Yin; Materials and Design 105, 89-95 (2016); K. Lu, Science, 345(6203), 1455 - 1456 (2014)
Mechanical Properties of Copper – How to influence?
• Copper in FO WLP – Advanced RDL L/S is in the range of the common copper grain dimension – Temperature load > 250°C due to multiple solder reflow process steps– Copper recrystallization take place at this temperatures, and clean copper
will generate grains as large as the target structures Bamboo vs. Brick Stone Structure
Bamboo Brick stone
Additive Impact to Mechanical Properties of Copper
• Cu deposition requires organic additives to modify properties of the deposit
• Controlled deposition– Smoothness & uniformity of the deposit, metal throw, feature filling, ...
• Physical properties of the deposit– Morphology, elongation/ductility, tensile strength, conductivity, purity, ...
• Usually, three different types of additives are used• Suppressor• Accelerator• Leveler
Additive Impact to Mechanical Properties of Copper: Composition of Copper Electrolyte
Copper Plating Process designed for FO WLP
Copper Plating Process designed for FO WLP
• Atotech Spherolyte Cu UF3 process – Single process for fine line, Microvia filling and high speed
plating– High pure copper deposition – Good surface distribution even @ High speed plating
conditions – Capable for tall pillar plating– Low stress – 20 – 30 MPa– Demonstrated reduction of Cu fine line breakages after
thermal cycle testing.
CD [ASD] WIW [%] WID [%] WIP [%]
15 2.2 1.5 3.6
SIMS < 15ppm
Ductility ≥ 30%
TS > 280MPa
Thank You!