wb coursepart 2

AML – Wafer Bonding Machines & Services

Bond Strength Measurement Techniques

Adhesive

Bonded Dies

Tensile Force

Tensile Force Shear Force

Shear Force Wafer 1

Adhesive

AdhesiveThin Film

Tensile Force

Wafer 2

Wafer 1 Adhesive

Blade Adhesive

Pressure

Wafer 1

Wafer 2

Wafer 1

Tensile & Shear Load Test Thin Film Bond – Tensile Load Test

Crack Opening Test Blister Test


Bond Strength and Crack Propogation

• Bond strength can be estimated from crack length measurements

• Inserting a blade of known thickness between wafers will cause bond fracture of a length related to the bond strength

• Very large error on this measurement


Wafer Bond Strength

Note: E = bulk modulus, t = wafer thickness, and y = blade tip radius are all known quantities. The crack length L needs to be measured to determine the bond energy, γ. The 4th power dependence means this is not an accurate technique


Bond Strength and Crack Propogation

Crack Length vs. Bond energy

0.1

1

10

100

1000

10000

100000

0 0.005 0.01 0.015 0.02

crack length m

surf

ace

ener

gy J

m-2

ref 20


Anodic Bonding: Advantages over Direct Bonding

• Anodic Bonding temperature typically 400oC compared with ~1000oC for fusion (Note that this advantage is reversed with the new plasma activated direct bonding)

• Surface roughness requirement for fusion bonding is ~ few Ångströms compared with a few 10’s nm for anodic bonding (4)

• Anodic bonding more tolerant of surface particles and is generally a more robust process


Anodic Bonding: Drawbacks compared with Direct Bonding

• Silicon thermal expansion is not an exact match with glasstherefore some stress in bonded wafers

• Direct bonded wafers have higher temperature capabilitywhereas anodic bonding limited by strain point of the glass

• Direct bonded wafers can be used for subsequent IC processing, whereas the anodic bonding process introduces alkali metal ions:not allowed for CMOS processing (but AML working on process to overcome this)


Comparison of Other Various Wafer Bonding Techniques


Types of Wafer Bonding

Wafer bonding processes to be compared:

• Anodic bonding

• Direct (fusion) bonding

• Glass frit bonding

• Eutectic bonding

• Adhesive bonding

• Solder Bonding


Glass Frit BondingThis process involves the deposition of a layer that contains glass frit on one of the surfaces to be bonded

The frit can be spun-on, screen printed or applied as a pre-form tape

The process typically involves controlled ramp heating and dwells at set temperatures to drive off the bonding material

Wide range of glass frits available with different reflow temperatures and thermal expansion coeff’s.


Glass Frit BondingThe most widely used glass fritfor MEMS is Ferro’s FX11-036. This can be used for bonding silicon, glass and quartz.

Drawbacks of FX11-036 are :

1. relatively high process temperature (450C)

2. material contains lead

New low temperature (320C) glass frit (DM2700P/H848)and also lead-free glass frits (DM2995P/J141) have recently become available


Glass Frit Bonding

One of the main reasons for using glass frit bonding is for achieving hermetic sealing on substrates with high topography or with a multitude of conducting lateral feedthroughs. The only other bonding option in this case is adhesive and this does not exhibit hermeticity

Can achieve vacuums of 1mBar


Glass Frit Bonding

Organic Burn-out and Glazing Profile for DM2995P/J141


Glass Frit Bonding

Sealing Profile for DM2995P/J141


Lead-free frit properties (1 of 3)


Lead-free frit properties (2 of 3)


Lead-free frit properties (3 of 3)Sealing Time and Temperature Options (a) lead-free, (b) low T

Glazing and sealing profiles may vary depending on the size and thermal properties of the components being processed.

Clean, dry air atmosphere is preferred. Nitrogenatmosphere may require higher process temperatures or longer process times.

Vacuum atmosphere should be avoided.

(a)

(b)


Anodic Bonding: Comparison with Glass Frit Bonding

Advantages:

• No glass flow, therefore better dimensional control for micromachined cavities etc.

• Process temperature lower (for Si wafers) compared with 400 - 500oC for frit bonding

• typically shorter cycle times

• Precise contact forces not needed for anodic bonding

Drawbacks:

• Frit bonding can produce better vacuums


Eutectic BondingEutectic bonding can be performed with a wide range of

alloys.

The eutectic composition of an alloy is the composition with the minimum melting point, and the eutectic temperature is the temperature at which this composition melts.

The standard methodology for eutectic bonding is to have one substrate (wafer 1) coated with a thin film of eutectic composition, and the other substrate (wafer 2) to be bonded, coated with a thin film of one of the two constituents of the eutectic material.


Eutectic Bonding

Binary alloy phase diagram

The lowest melting point is at the eutectic concentration


Eutectic BondingThe wafers are brought into contact at a temperature just below

the eutectic temperature and a force, typically ~2kN, applied.

The wafers are then heated to above the eutectic temperature and the eutectic composition on wafer 1 will melt and material from the coating on wafer 2 will begin to dissolve into the melt.

This changes the composition to a non-eutectic state and the material solidifies to form a bonding layer with a higher reflowtemperature than the eutectic temperature.

This results in a controllable, reproducible process and resulting bond that has higher temperature performance than the original M.P.


AuSn Eutectic Bonding

The most commonly used material combination for eutectic bonding of this type is Au:Sn.

The phase diagram shows that the desired 280 ° C AuSneutectic point is at 20% wt tin, and has steep walls.

A 1% shift in the AuSn solder composition to the Au rich area can raise the melting point of the solder 30 ° C, making the solder unusable. [ref ]


AuSn Eutectic Bonding

Au:Sn Phase Diagram


Depositing the Eutectic Layer (1/4)Stamped AuSn Solder Preforms

The minimum preform size is 25 microns thickness and 1mm x 1mm areas - due to difficulties in preform stamping and handling.

Vacuum-Deposited AuSn PreformsDeposit alternate layers of gold and tin until the target deposit

thickness and metal stoichiometry are reached.

Thin deposits are common (less than 1µm) and are usually made up of gold and tin layers from 0.1-0.5µm thick.

Gold is used for the top layer to protect the tin from oxidationwhile ensuring solderability during reflow.


Depositing the Eutectic Layer (2/4)

Paste Screening AuSn PreformsSolder pastes are comprised of 80:20 alloy spheres with a

thixotropic carrier material to support the spheres and fluxing agents.

Metal content is controlled to ±1%

The solder feature size, volume, and placement accuracy are limited to the capabilities of the selected screen printing process.

Drawback of paste is the need for flux. When using flux there is always the possibility of out-gassing volatile compounds that may condense on the device surfaces or be trapped inside the package.



Electroplated AuSn Preformsphotoresist defines the shape, thickness and location

of the desired solder deposit.

Gold and tin are then sequentially plated in steps alternating between gold and tin.

Alternating Au and Sn plating layers, deposited in a 1.5:1 thickness ratio of Au to Sn, are necessary to achieve the proper 80Au:20Sn stoichiometric ratio.

The outer plated surface is always gold to insure proper wetting and reflow of the solder and substrate metalization during assembly.



With the proper selection of plating tools and chemistry it is possible to deposit AuSn solder features with a liquidus onset consistency of ±1°C. Solder features as small as 20 microns, with pitches as close as 5 microns, can be deposited at wafer scale with dimensional variations of better than ±5.0 %.


AuSn Eutectic BondingBenefits & Drawbacks

Benefits

Can be performed at temperatures as low as 300oC therefore compatible with wide range of materials

Metallic bond has high strength & good hermeticity

Drawbacks

Not compatible with lateral feedthroughs

Exact (<1%) compositional control required


AuSn Eutectic BondingExample of Bond Quality

Obtained using Bonder with Good platen parallelism and good force uniformity


AuSn Eutectic BondingExample of Bond Quality

Obtained using Bonder with poor platen parallelism and poor force uniformity


Anodic Bonding: Comparison with AuSn Eutectic Bonding

Advantages:

•No pre-forms or deposited interlayers required

Compatibility with conducting leadthroughs

Drawback:

AuSn bonding can be performed at ~300C – but very important to have exact eutectic composition


Types of Eutectic Bonding

(Ref 42)


Only need to gold coat one of the two wafers (assuming that we are bonding silicon wafers).

The silicon substrate itself provides the material for forming the AuSi eutectic.

For this reason it is important that the silicon is not covered by an oxide.

Standard practice is to dip the silicon in dilute or buffered HF prior to bonding in order remove any native oxide.

AuSi Eutectic Bonding


Quality of the thin film deposition is important

The deposited layer should be low stress and include a diffusion barrier to prevent the movement of unwanted material to the surface.

If the process is performed correctly then high quality, strong bonds can be realised.

Vacuum cavities in 10-4 mBar range have been achieved [Ref 42)].



Example of AuSi eutectic bonds (in-situ pre-treatment of wafer surfaces to remove native oxide prior to bonding



AuSi Eutectic Bonding (Ref 42)



μ-probe analysis of TiAu-Si bond

(Ref 42)



SEM cross section of CrAu-Si bond

(Ref 42)



SEM cross section of NiAu-Si bond

(Ref 42)



SEM cross section of TiPtAu-Si bond

(Ref 42)



VABOND Conclusion

(Ref 42)


Anodic Bonding: Comparison with AuSi Eutectic Bonding

Advantages:

• Anodic bonding can be used for Si that has surface coatings whereas eutectic bonding requires bare silicon

• No pre-forms or deposited interlayers required

• Compatibility with conducting leadthroughs

Note: process temperatures are comparable for the two processes


Anodic Bonding: Comparison with AuSi Eutectic Bonding

•Au-Si eutectic temperature 370ºC at 31% Au in Si Bonding temperature is well below Al interconnect melting point

wb coursepart 2

Documents