novel wafer bonding technology survey

Department of Electrical Engineering, National Taiwan University

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NOVEL WAFER BONDING NOVEL WAFER BONDING TECHNOLOGY SURVEYTECHNOLOGY SURVEY

Po-Wen Chen

Department of Electrical Engineering and Graduate Institute of Electronics Engineering

National Taiwan University, Taipei, Taiwan, R.O.C.

Department of Electrical Engineering, National Taiwan University

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Outline

•Introduction•Wafer bonding•Wafer bonding application•TEM inspection of our bonding achievement

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Introduction

• Clean(hydrophilic)

• Alignment• Pre-bonding• Anneal

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Wafer Bonding

• Anodic Bonding

•Silicon Direct Bonding/Fusion Bonding

• Intermediate-Layer Bonding

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Anodic Bonding

•Rely on charge migration

•Silicon and glass with alkali metal

–Glass with 3.5% Na2O

•Negative voltage to glass to attract and neutralize Na+

•Due to electric field , O2- transported to glass-silicon interface form SiO2

•Electrostatic attraction between glass-silicon interface

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•Enhance positive ion mobility at 500℃•Produce uniform bonds,but charged carriers make it incompatible with active device•Useful for pressure sensors, solar cells and piezoresistive and package applications

Anodic Bonding

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Silicon Direct Bonding/Fusion Bonding

• Join silicon wafer by – Create and contact hydrophobic or

hydrophilic surfaces– Anneal at high temperature

• Hydrophobic case – HF dip before contact– More challenging than hydrophilic

wafer ,but ultimately better?• Hydrophilic case

– SC1(standard cleaning) before bonding

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Silicon Direct Bonding/Fusion Bonding

• AfterSC1 ,the mirror polished silicon wafer filled with hydroxyl radicals(OH- )

• OH- on polished silicon face permit a good initial contact bond

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Silicon Direct Bonding/Fusion Bonding

• Subsequent heating dehydrates• OH- cause oxidation of the bondi

ng surface resulting in a Si-O-Si bond• As annealing temperature are inc

reased beyond 1000℃ ,the strength of the bond approaches that of silicon itself

• Viscosity and pressure of ambient gas ,wafer contact energy influence speed

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• Press in the middle of wafer to create a preliminary point of contact

• While mechanical spacer maintain wafer physically separated

• Retract spacer to form a single bonding wave from center to wafer

• Spacer integrity is important– Multiple bonding waves promote warpage

and voids– Gas trapped in pocket form by multiple wa

ves

Silicon Direct Bonding/Fusion Bonding

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Silicon Direct Bonding/Fusion Bonding

• From room temperature 110C– Slow fracture effect and interface

water rearrangement• From 110 C to 150 C

– Polymerization of silanol groups across the interface

• From 150 C to 800 C– Bonding energy limited by conta

cted area• From 800 C and above

– Complete bonding via oxide flow

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Influence of particle

Schematic of particle leading to an unbonded areaEx :a particle of about 1um diameter leads to an unbonded area with a diameter of about 0.5cm of typical 4-in diamet

er silicon wafers with a thickness of 525um

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Influence of surface

R＞ 2tw

R＜ 2tw

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Intermediate-Layer Bonding

Options for intermediate layer bonds

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Intermediate-Layer Bonding

Eutectic and glass-frit bonding techniques– Deposition of intermediate metallic and glass films

• Eutectic bond– Examine a two component phase diagram

• Little solubility between the component , reveal eutectic point located at the lowest melting temperature

• Alloy– Formed by solid-liquid inter-diffusion at contact interface – Followed by solidification upon cooling

• Gold and silicon– 363℃,2.85%Si,97.1%Au by weight

• Good eutectic bond– Remove silicon oxide films that hamper gold diffuse into

silicon

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Intermediate-Layer Bonding

• Before bonding– Exposure to ultraviolet light to remove organic contaminants

• Low temperature – Reach eutectic point make this technique attractive for

active device processes

• Glass frit bond– Create hermetic seals using relatively low temperature– Thin glass layer is deposited and pre-glazed– Wafer brought into contact at rated melting temperature of

glass ,less than 600℃– Pressure is applied to maintain contact– Lead borate with significant lead oxide content is often used

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Checking for wafer-to-wafer

bonding integrity• Three dominant methods for

imaging a bonded pair of silicon wafer– Infrared transmission– Ultrasonic– X-ray topography

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Visualization of the bonded wafer pair

X-ray topography Ultrasonic IR transmission

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Bonding Strength Measurement

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Wafer bonding application

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BESOI(ELTRAN)

Smart-cut

Wafer bonding application

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TEM inspection of our bonding achievements

Department of Electrical Engineering, National Taiwan University

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TEM inspection of our bonding achievements