Wafer bonding sami.franssila@aalto.fi

Introduction Bonding means attaching two objects together with a seal (usually permanent) Microtechnology involves various types of bonding processes Wafer bonding Wire bonding Flip chip bonding Used in IC packaging and printed circuit board assembly. Will not be covered in this lecture.

Accelerometer example

Accelerometer, Part 2

Microturbine Silicon-to-silicon bonding 5 wafers stacked together alignment ±1 µm D. Epstein, MIT C.-C. Lin and M. Schmidt of MIT.

Bonding requirements smooth surfaces (on nm-scale) flat wafers (on cm-scale) no particles (voids larger than particles) matching CTEs (otherwise stresses) suitable surface chemistry (hydrophilic) intimate contact

Wafer bonding procedure -particle removal -surface chemistry modification -(optional) vacuum pumping -(optional) wafer alignment -room temperature joining -application of force/heat/voltage -(optional) wafer thinning

Bonding methods direct bonding = fusion bonding (Si/Si; glass/glass; SiO2/Si )  anodic bonding (AB) Si/glass, glass/Si/glass   thermo-compression bonding (TCB) Eutectic/TLP bonding Si/glass frit bonding adhesive bonding limited by “glue” properties No intermediate layer Metallic intermediate layer Insulating intermediate layer (or conducting glue)

Bonding in microfabrication 1: as a way to make advanced starting wafers (SOI) 2: as a way to create complex 3D structures and cavities which create device functionality (reference pressure cavity, µ-channel, nozzles... 3: as a packaging method to create closed environments (vacuum packages for resonators, and for optical and IR devices) 4. as a integration technology for joining 2 chips fabricated separately (e.g. CMOS + MEMS; or MEMS + power; or CMOS+optics) 5. as a way to join systems by wires

Thermo-Compression Bonding apply enough heat & pressure Double Sided Heating: Contact Force: Bonding Atmosphere: up to 550°C 100 - 40000N 1E-5 - 3000mbar

Adhesive bonding surface cleaning spin coating of polymer initial curing (solvent bake) evacuate vacuum (optional) join the wafers final curing pressure and/or heat

Adhesive bonding benefits temperatures around 100oC (>Tglass) tolerant to (some) particle contamination structured wafers can be bonded low cost, simple process

Bonding considerations Bond properties What are the chemical bonds that will bond ? Exist naturally or formed by treatment? Bond strength ? Permanent vs. temporary ?

Bonding: materials Chemical compatibility Temperature tolerance Bond formation temperature Device operation temperature CTE (coefficient of thermal expansion) mismatch between materials Surface quality (roughness; waviness) Surface particles

Bonding: productivity considerations Availability of equipment (production tools have automated, cassette to cassette operation) Process compatibility (IC compatibility: temperature & contamination) Process yield Throughput Cost Maturity

Anodic bonding Relies on mobile Na+ and oxygen ions in glass glass Na+ Na+ Na+ O2- O2- O2- <Si> anode heater block Relies on mobile Na+ and oxygen ions in glass Temperature raises ion mobility in glass Voltage attracts oxygen ions to interface Si + 2 O2- ==> SiO2 + 4 e- reaction at glass-silicon interface

Anodic bonding Anodic bonding (AB) = Field-assisted thermal bonding Glass and metals bonded Various glass types Corning 7740 Pyrex glass most common for anodic bonding CTE close to that of Si Annealing the glass before of after bonding can reduce stresses due to CTE mismatch

Thermal matching Coefficients of thermal expansion of silicon and glass must be matching, otherwise cracking upon cooling  only certain glasses suitable for anodic bonding: Pyrex, Borofloat

Anodic bonding Double Sided Heating: Contact Force: High Voltage Supply: Bonding Atmosphere: up to 550°C 1 - 1000N 0 - 2000V / 0 - 50mA 1E-5 - 3000mbar

Anodic bonding parameters Typical conditions for anodic bonding 200 °C … 450 °C (350 °C) 300 V … 2000 V (1000 V) 0 … 1000 N (200 N) 10 minutes with voltage on Cycle time > 30 min (unloading at 200 °C) Current decreases when bonding proceeds It is considered that when current is below 10% of its maximum, the bonding is complete Alternatively, it is possible to limit the max current to a reasonably low level (e.g. 4 mA for 4” wafers) Different electrode configurations Needle (sharp/blunt) Plate (metal/graphite) Dummy glass wafer between For 7740 glass 300 °C might be optimal in terms of thermal expansion but better bond quality is usually obtained with higher temperature Softer glass → increased flux → better conformation to topography mismatch Smaller resistance → smaller voltage and less sparking at electrode Increased diffusion helps to remove contaminants (Dziuban 2006)

Bond alignment Various approaches for aligning wafers to be bonded Silicon wafers are transparent to infrared light → IR alignment Many applications will work even with very coarse alignment by wafer edges Many bonding tools utilize separate alignment apparatus where the wafers are fixed to a bond chuck that is then moved to a separate bonding chamber Risk of misalignment during transport and heating! (Niklaus 2006)

Cavities in anodic bonding Anodic bonding generates outgassing of oxygen in particular Pressure level is not dependent on pattern density as in silicon fusion bonding Glass is permeable to He With Pyrex He partial pressure increases from 10−14 to 10−4 Pa in 30 days Getters are applicable, but better initial vacuum can be obtained with other bonding methods (Dziuban 2006)

Multi-wafer anodic bonding Multi-wafer anodic bonding is possible Different alternatives for Si-glass-Si stack Two electrode process: bonding of one wafer pair at a time Can be done also by inverting voltage polarity after first bond with all wafers stacked to begin with Damage to the first bonded glass during second bond Special clamp can be used to make electrical contact directly to the middle wafer Three electrode process Simultaneous bonding (Dziuban 2006)

Anodic bonding with thin films Anodic bonding can be done for thin-film covered silicon wafers Suitable dielectrics: SiO2 (up to 0.6 µm), Si3N4, Al2O3, SiC Suitable metals: Al, Ti, Ta A glass thin film can be deposited and used for anodic bonding Various glass types: borosilicate, borate, sodium glass Different deposition methods: sputtering, spin-on coating Glass thickness from sub-µm to a few µm Si3N4 film can be used as Na+ diffusion barrier to protect semiconductor devices from contamination (Dziuban 2006)

Anodic bonding: valve (P.W.Barth, Stanford)

Fusion/direct/thermal bonding Identical materials bonded No CTE problems Bonds naturally available Apply heat/pressure to enhance bonding Si-Si Glass-glass Polymer-polymer

Polymer thermal bonding Raise temperature above Tg  softening  intimate contact (hold long enough) cool down below Tg Bond interface indistinguishable from bulk materials (because same bonds !)

Polymer bonding (2) Softening by solvent surface treatment  Intimate contact (hold long enough) Bonding of different polymers, too! In theory a room temperature process In practise difficult to control the thickness of the softened layer

(Ville Saarela, unpublished) Glass fusion bonding Heat above glass softening temperature, e.g. 650 °C works for Pyrex 7740 Intimate contact Danger: the stack of wafers can be distorted if temperature too high (Ville Saarela, unpublished) Isotropically wet etched channel in glass with fusion bonded glass channel.

Bonded SOI (Silicon On Insulator) Thermally oxidized wafer is joined to another silicon wafer. After bond improvement anneal and thinning, the resulting 3-layer structure can be processed as any silicon wafer. Oxide layer thickness 100 nm – 1 µm (4 µm available as an expensive option) Top silicon thickness anything, but 5-50 µm typical in MEMS RCA-1 clean RT joining HT anneal thinning edge conditioning

Chemistry of Si-Si bonding (1) Water removal and siloxane bond formation at 110-150oC; wafers drawn close together Bonding of hydrophilic silicon surfaces via weak hydrogen bonds; large distance between wafers Tong, Q.-Y. & U. Gösele: Semiconductor Wafer Bonding, John Wiley & Sons (1999)

Chemistry of Si-Si bonding (2) Si-OH + HO-Si ==> Si – O – Si + H2O annealing improves bond strength Bond strength of Si-O-Si is so high that the bonded wafer breaks inside silicon; not at the bond interface Tong, Q.-Y. & U. Gösele: Semiconductor Wafer Bonding, John Wiley & Sons (1999)

HRTEM showing thin amorphous oxide between two single crystal silicon layers

Bond energy vs. temperature

Hydrophobic Si-Si bonding (Christiansen et al. 2006) Silicon native oxide is removed in HF or ammonium fluoride Surface will be hydrogen terminated Bonding and annealing will result in direct Si-Si covalent bonds Outgassing of H2 via interface No absorbing oxide – increased void probability compared to hydrophilic bonding Initially, hydrophilic bond energy is small compared to hydrophobic → more sensitive to defects at surface

Silicon fusion bonding: valve (P.W. Barth, Stanford)

Fusion bonding equipment In direct wafer bonding, pressure is applied at the middle of one wafer to create a single point of contact between the pieces. Retracting the mechanical spacers creates a bonding wave that propagates from the middle of the samples toward the periphery. Surface cleanliness and roughness have a strong influence on the quality of the bond.

Low temperature Si-Si direct bonding Ultra high vacuum (UHV) bonding Hydrophilic wafers are put in UHV chamber Heating (e.g. 450 °C) to remove hydrogen from Si surface Very Si-Si strong bonds even at room temperature HF & NaOH bonding Bonding in liquid solution Plasma treatment just prior to contacting wafers can also provide strong bond without high temperature annealing Different explanations: Surface cleaning Chemical activation Increased roughness/porosity increases absorption of gasses at interface

Problems with particles Voids created by particles are Silicon can conform only to very large compared to particles very small particles, < 100 nm themselves because silicon is rigid material

Flatness & microstructures 2h 2R t Only flat wafers can be bonded (=no long wavelength undulation) Shallow channels will be bonded if they are too wide Note: flatness is macroscopic measure; smoothness is local/atomic measure

Cavities and fusion bonding Outgassing of H2 at the bonding interface during annealing If there are cavities at the interface, H2 will be trapped The effect of cavity density to pressure inside cavity in hydrophilic silicon fusion bonding is shown in picture Sparse cavities will take in H2 from larger area → higher residual pressure Getter materials can be added in cavities to improve vacuum level and stability (Christiansen et al. 2006)

Hermeticity and gettering Sparks:

Hermeticity and gettering (2) Sparks:

Visual/IR inspection Particle between anodically bonded silicon and glass wafers under optical microscope (5x) IR transmission of direct bonded silicon wafers. Voids are revealed by interference patterns. Surface damage to glass wafer during AB

Razor blade test Sharp blade is pushed to the bonding interface Debonding takes place with a specific crack length that is proportional to the surface energy Surface energy for two identical wafers: Different substrates have more complex equations. See e.g. Vallin et al. 2005 (Christiansen et al. 2006)

Tensile test method Tensile test method is widely used to quantify bond strengh A test chip is glued to pull studs which are pulled apart and the force at which the chip breaks determines the bond strength Destructive method Several configurations (Vallin 2005)

Chevron test method Chevron test is a variation of tensile test which provides good accuracy Gradually increasing pulling force is applied perpendicular to the bond Fracture begins at the chevron notch tip (Vallin 2005)

Blister test method In blister test method a pressure is applied to a cavity No voids (leaks) allowed in the studies cavity (Vallin 2005) (Christiansen et al. 2006)

Scanning acoustic microscopy (Christiansen et al. 2006) Scanning acoustic microscopy provides information about voids with better accuracy compared to visual/IR inspection Voids change the way the probe resonates

RF switch example 4 wafers, 3 bonding processes: Fusion bonding to make SOI Anodic silicon to glass Glass frit to attach cover oxide gold glass frit glass <Si>