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!