Adhesive bonding Ville Liimatainen

Contents Introduction – Adhesive bonding – Process overview – Main features Polymer adhesives Adhesive bonding technology Bond characterization Applications Conclusion

Adhesive bonding An intermediate adhesive layer is used to bond two surfaces Successfully used in many industries – Cars – Airplanes – Space shuttles Initially not significant in semiconductor wafer bonding research – Small bonding areas (chip-level bonding) An established wafer bonding method nowadays

Process overview 1.Polymer adhesive applied to one or both surfaces 2.Pressure applied to force the surfaces into close contact 3.Adhesive cured from liquid or viscoelastic state into solid state – UV, heat, pressure...

Main features Bonding conditions – From RT to ~ 400 ° C – Low to moderate bonding pressure Pros and cons + Low bonding temperature + Works with practically any materials + Comparably simple, robust, low-cost - No hermetic bonds - Limited temperature stability - Long-term stability in harsh environments? Applications – MEMS – Sensor packaging – 3D IC – Temporary bonds

Polymer adhesives Polymers are large molecules consisting of linked small molecules (monomers) Hardening – Solvent evaporation (drying adhesives) – Solidification upon cooling (hot melts) – Polymerization by chemical reactions (polymer precursors) Mixing of two components, heat, light, moisture... Deposition on wafer surfaces – Spin coating, spray coating, electrodeposition, stamping, screen printing, brushing, dispensing – CVD (thin films), lamination of films or sheets Selection for wafer bonding – Compatibility – Physical properties: mechanical and thermal stability, creep strength – Chemical resistance, chemical stability Epoxies UV epoxies (e.g. SU8) Positive and negative photoresists Benzocyclobutene (BCB) Flare Polymethylmethacrylate (PMMA) Fluoropolymers Polyimides Methylsilsesquioxane (MSSQ) Polyetheretherketone (PEEK) Thermosetting copolyesters (ATSP) Thermoplastic copolymers (PVDC) Parylene Liquid-crystal polymers (LCP) Waxes Epoxies UV epoxies (e.g. SU8) Positive and negative photoresists Benzocyclobutene (BCB) Flare Polymethylmethacrylate (PMMA) Fluoropolymers Polyimides Methylsilsesquioxane (MSSQ) Polyetheretherketone (PEEK) Thermosetting copolyesters (ATSP) Thermoplastic copolymers (PVDC) Parylene Liquid-crystal polymers (LCP) Waxes

Adhesive bonding technology Tools – Wafer bonders – Substrate lamination tools For temporary bonding, CMP, grinding – Die bonders For packaging Alignment techniques – Back-side alignment, SmartView, IR, transparent wafer, through-wafer holes etc. Wafer bonder schematic Substrate lamination tool schematic Schematic of the ITEC die bonder principle (NXP Semiconductors)

Adhesive bonding processes SU-8 and BCB widely established adhesives for MEMS and electronic component production Example process: Benzocyclobutene (BCB) 1.Clean the wafers (e.g. H 2 O 2 + H 2 SO 4 or oxygen plasma) 2.Apply adhesion promoter (spin-coat, spray-coat) 3.Deposit BCB by spin or spray coating (usually 1 – 50µm) 4.Softbake, pattern and dry etch the BCB or Expose and develop 5.Soft curing to remove solvents and volatile substances (< 300 ° C, ambient air) 6.Bonding in a vacuum chamber a.Establish vacuum b.Bring the surfaces in contact c.Apply bonding pressure and heat ( ° C, 30 to 240 minutes) d.Chamber purge, cool down, release pressure

Bond characterization Wafer bond quality – Defect rate – Bond strength – Stresses – Hermeticity – Stability Common techniques – Bond interface imaging – Bond strength measurement – Bond hermeticity measurement acoustic imaging IR imaging SEM imaging

Applications Fabrication of 3D ICs Integration of ICs with MEMS Fabrication of microcavities for packaging Thin film solar cells RF components BioMEMS 3D integrated circuits Thin film and device transfer for MEMS/IC integration Thin film solar cells Microcavities for packaging Hydrophobic valves

Conclusion Adhesive bonding is an established method for MEMS, ICs, their integration and packaging Advantages – Low bonding temperature < 200 ° C – Applicable to various wafer materials and structures – Wide selection of adhesives adjusted for MEMS and electronic components available – Compensation of surface non-uniformities and contamination – No electric voltage or current involved – Chemical resistance – Simple and low-cost process Drawbacks – No hermetic sealing (possible with a diffusion barrier) – Limited temperature stability – Limited long-term stability in harsh environments – Variance of the gap between the bonded surfaces – (Relatively) low bond strengths