1. Wafer bonding

2. 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.

3. Accelerometer example

4. Accelerometer, Part 2

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

6. 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

7. 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

8. 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)

9. 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

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

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

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

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

14. 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

15. 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

16. Anodic bonding Relies on mobile Na+ and oxygen ions in glass
glass Na+ Na+ Na+
O 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

17. 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

18. 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

19. Anodic bonding Double Sided Heating: Contact Force:
High Voltage Supply:
Bonding Atmosphere:
up to 550°C N
V / mA
1E mbar

20. 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)

21. 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)

22. 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)

23. 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)

24. 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)

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

26. 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

27. 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 !)

28. 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

29. (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.

30. 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

31. Chemistry of Si-Si bonding (1)
Water removal and siloxane bond formation at oC;
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)

32. 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)

33. HRTEM showing thin amorphous oxide between two single crystal silicon layers

34. Bond energy vs. temperature

35. 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

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

37. 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.

38. 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

39. 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

40. Flatness & microstructures2h 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

41. 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)

42. Hermeticity and gettering
Sparks:

43. Hermeticity and gettering (2)
Sparks:

44. 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

45. 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)

46. 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)

47. 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)

48. 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)

49. 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

50. 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>