Title: Solder Choice in the FlipChip Bonding of Optoelectronic Integrated Circuits Undergraduate Research S
1Solder Choice in the Flip-Chip Bonding of
Optoelectronic Integrated Circuits(Undergraduate
Research Symposium Presentation)
- Hanching Fuh
- Microelectronics Laboratory
- University of Illinois at Urbana-Champaign
- Urbana, IL
- Professor K.C. Hsieh, Faculty Advisor
- April 21, 2001
2Motivation for ResearchWhat do you want from
technology?
? Higher Integration
3Motivation for Research (Part 2)
- Communications networks of the future require
higher integration too!
- Analog and Digital circuits
- Detectors and other optical devices
4A Vision for the Future
- A Communications System on a Chip
5What enables integration?
What is Flip-Chip Bonding?
- Flip-Chip Bonding attaches one device to a
separate chip with solder
6What does that mean exactly?
?
?
- Enables the Electrical and Mechanical Connections
of Dissimilar Devices
7How do you do Flip-Chip Bonding?
?
?
?
?
8What do we want in a solder for flip-chip bonding?
- A rose is a rose is a rose is a rose
- - Gertrude Stein
- All men are created equal
- - Declaration of Independence
- A solder is not a solder is not a solder is not a
solder! - Not all solders are created equal!
9Differences in solder
- What we want
- ? Low ( lt 450 C)
- ? High
- ? High yield
- ? Cheap
- Avoid Toxic metals
- (Pb, Cd, etc.)
- ? De-wetting/Surface Tension abilities
- Melting temperature
- Conductivity
- Process yield
- Cost
- Environmental Friendliness
- Self-alignment
10Overview of Research
- Devices/chips fabricated and solder is put on
bonding pads
11Overview of Research (pt. 2)
- Chip and device brought into optical alignment
- Samples heated until solder bumps melt and bond
together
12Overview of Research (pt.3)
- Measure resistance through a path
- Extrapolate resistance of wires to find bond
resistance
13Solder Choices
- Three Tin (Sn) based binary solders
- Tin/Indium (Sn/In)
- 48/52
- 117 C
- Tin/Zinc (Sn/Zn)
- 72/28
- 325 C
- Tin/Gold (Sn/Au)
- 60/40
- 310 C
14Processing of Flip-Chip Structure (1)
- Create wires and bonding pads
15Processing of Flip-Chip Structure (2)
- Put solder onto bonding pads
16Final Flip-Chip Structure
17Results
- Nothing
- At least in the beginning!
- Significant Progress made on project
18Results (pt. 2)
- Sn-In
- Low melting temperature (117 C)
- Bonding done at 200 C
- Processing yield high w/ and w/o adhesion layer
- Bonding yield high
- Processing yield high w/ and w/o adhesion layer
- Bonding yield high
19Results (pt. 3)
- Bond Resistance
- (Measured resistance Wire resistance) /
bonds - (Measured R (Pl)/(wt)) / bonds
- P resistivity, l length, w width, t
thickness - Average bond resistance 0.877 ?
- This is low!
20Results (pt. 4)
- High melting temperature
- 325 C
- Bonding done at 400 C
- High melting temperature
- 310 C
- Bonding done at 400 C
- Problems with Solder Sticking to Bonding Pad
- Sn-Zn solved with adhesion layer of titanium
- Sn-Au
- Indirect sample heating
- evaporation rate
- evaporation depth
- Successful in Sn-Au solder sticking to bonding pad
21Picture of Tin-Gold Pad
22Results (pt. 5)
- Most problems with Sn-Zn and Sn-Au solved
- Problems with bonding, not processing
- Alignment
- Heating temperature
- Heating time
- Further trials in progress
- Resistance will be even better than Sn-In for
Sn-Au - Resistance close for Sn-Zn
23How do we predict this?
- Approximation
- Resistivity is weighted average of individual
resistivity
Example 48 Tin and 52 Indium ? .48
1.149E-7 .52 8E-8 9.68E-8 ?-m
Sn-Zn 2.7 higher resistivity Sn-Au 18.6
lower resistivity
24Recommendations
25What to remember about Flip-Chip Bonding
- Low resistance
- Allows High Integration
- Enables what we want in technology
- Low cost
- Helps alleviate problems like EMI, crosstalk
- Increasingly important
26QA
27Thank you!
- To the audience for listening to me
- John Hughes and everyone who keeps the
Microelectronics Lab running - All the members of the MBE group
- Particularly.
- Bruce Flachsbart
- John Epple
- Professor K.C. Hsieh