Department of Electrical Engineering, National Taiwan University NOVEL WAFER BONDING TECHNOLOGY 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 Outline Introduction Wafer bonding Wafer bonding application TEM inspection of our bonding achievement

Department of Electrical Engineering, National Taiwan University Introduction Clean(hydrophilic) Alignment Pre-bonding Anneal

Department of Electrical Engineering, National Taiwan University Wafer Bonding Anodic Bonding Silicon Direct Bonding/Fusion Bonding Intermediate-Layer Bonding

Department of Electrical Engineering, National Taiwan University Anodic Bonding Rely on charge migration Silicon and glass with alkali metal –Glass with 3.5% Na 2 O Negative voltage to glass to attract and neutralize Na + Due to electric field, O 2- transported to glass-silicon interface form SiO 2 Electrostatic attraction between glass-silicon interface

Department of Electrical Engineering, National Taiwan University 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

Department of Electrical Engineering, National Taiwan University 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

Department of Electrical Engineering, National Taiwan University 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

Department of Electrical Engineering, National Taiwan University Silicon Direct Bonding/Fusion Bonding Subsequent heating dehydrates OH - cause oxidation of the bonding surface resulting in a Si-O-Si bond As annealing temperature are increased beyond 1000 ℃,the strength of the bond approaches that of silicon itself Viscosity and pressure of ambient gas,wafer contact energy influence speed

Department of Electrical Engineering, National Taiwan University 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 waves Silicon Direct Bonding/Fusion Bonding

Department of Electrical Engineering, National Taiwan University Silicon Direct Bonding/Fusion Bonding From room temperature 110  C –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 contacted area From 800  C and above –Complete bonding via oxide flow

Department of Electrical Engineering, National Taiwan University Influence of particle Schematic of particle leading to an unbonded area Ex :a particle of about 1um diameter leads to an unbonded area with a diameter of about 0.5cm of typical 4-in diameter silicon wafers with a thickness of 525um

Department of Electrical Engineering, National Taiwan University Influence of surface R ＞ 2t w R ＜ 2t w

Department of Electrical Engineering, National Taiwan University Intermediate-Layer Bonding Options for intermediate layer bonds

Department of Electrical Engineering, National Taiwan University 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

Department of Electrical Engineering, National Taiwan University 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

Department of Electrical Engineering, National Taiwan University Checking for wafer-to-wafer bonding integrity Three dominant methods for imaging a bonded pair of silicon wafer –Infrared transmission –Ultrasonic –X-ray topography

Department of Electrical Engineering, National Taiwan University Visualization of the bonded wafer pair X-ray topographyUltrasonicIR transmission

Department of Electrical Engineering, National Taiwan University Bonding Strength Measurement

Department of Electrical Engineering, National Taiwan University Wafer bonding application

Department of Electrical Engineering, National Taiwan University BESOI (ELTRAN) Smart-cut Wafer bonding application

Department of Electrical Engineering, National Taiwan University TEM inspection of our bonding achievements

Department of Electrical Engineering, National Taiwan University TEM inspection of our bonding achievements