Micro Materials Integration and Evolution in Digital Electronics Manufacturing

Transcription

1 Micro Materials Integration and Evolution in Digital Electronics Manufacturing Ink Jet Deposition of Self-assembled Monolayers David A. Hutt, Changqing Liu, Stephen Ebbens Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University

2 Outline Introduction Motivation Project Overview Background Self-assembled Monolayers (SAMs) Results Conclusion

3 Motivation Traditional high volume manufacture of patterned interconnects for PCBs Photolithography Photoresist, etching / electroplating Conductive paste Stencil printing Requires tooling Long lead time Cost Unsuitable for low volumes / prototypes

4 Data driven (soft tooling) Digital Manufacturing Active response to substrate feature locations Low cost prototyping / low volume products Scalable to high volume Print etch / plating resists Print silver inks - expensive Highly flexible Ink Jet Technology Positioning system Printhead Substrate z y x Ø μm Ink supply CAD images Control variables Ink deposits

5 Project Overview Positioning system Ink supply z y x CAD images Printhead Control variables Substrate Ø μm Self-assembled Monolayer Ink deposits Patterned Electronic Circuit

6 Project Overview Investigate the ink jet deposition of Selfassembled Monolayers (SAMs) for electronics manufacture One year feasibility study Funded by the IeMRC One Research Associate for one year Industrial Support Xaar plc

7 Self-Assembled Monolayers (SAMs) Adsorption of organic molecules (from solution) onto surfaces Densely packed single layer of molecules = organic molecule Self-assembled monolayer Two commonly used types of SAMs Silane molecules on oxide surfaces, glass, silicon Alkyl thiol molecules on metal surfaces

8 Alkyl Thiols on Copper Alkyl thiols on fcc metals (gold, silver, copper, nickel) Changing tail group (X) gives different surface chemical functionality CH 2 CH 2 CH 2 CH 2 X + Copper Surface HS(CH 2 ) n-1 X Thiol Compound H S CH 2 CH 2 CH 2 CH 2 X = CH 3, COOH, CH 2 OH etc. S(CH 2 ) n-1 X S(CH 2 ) n-1 X S X S - (CH 2 ) n -X 2-3 nm

9 SAM Deposition on Copper Surface pre-treatment required to remove oxide layers Oxide layer Copper Chemical Etch Apply Coating Coating Oxide Removed Material can now be stored

10 SAM Deposition on Copper Copper Micro Etch H 2 O Rinse HCl Etch Rapid IPA Rinse 15 mins to 18 hrs Transfer to 5-10mM solution of thiol in solvent IPA Rinse Blow dry

11 Alkanethiol Coating Structure Barrier properties Tight packing of chains prevents diffusion of oxidizing species e.g. O 2, H 2 O to the copper surface tail group CH 2 backbone S head group

12 Fluxless Soldering of Copper Oxide free copper surfaces can be soldered without flux Oxide layer Copper Chemical Etch Solder Heat Oxide Removed Coating Apply Coating Inert Atmosphere Heat Material can now be stored Coating Displaced by Heat and Molten Solder

13 Fluxless Soldering of Copper Fluxless wave soldering of preserved PCBs

14 Patterning of SAMs SAMs can be selectively deposited to create patterns Lithography (photo-oxidation) Micro-contact printing (rubber stamping) Requires tooling Areas with different chemical functionality Areas with / without SAM coverage

15 Patterned SAM Applications Patterned SAM Plating resist Etch resist Localised Fluxless Solder Wetting

16 Feasibility study Project Overview To understand the ink jet deposition of SAMs Investigate the use of patterned SAMs in electronics manufacture Positioning system Printhead z y x Thiol supply CAD images Variables control Substrate Cu, Ni, Au Ø μm SAMs deposits

17 Methodology Ink Jetting Thiol solution ink formulation Ink jet printer waveform Circuit Patterning Pre-treatment of surfaces Patterning strategies

18 Ink Formulation Translation of standard solution chemistry to ink IPA solvent was unsuitable for ink jetting New solvent system identified for ink jetting Low volatility Surface tension Viscosity No strong acids / bases Solvent evaporation Thiol components may concentrate at the edge of the droplet Limited exposure time for SAM formation Printer waveform Close collaboration with partners essential

19 Effect of Solvent on SAM Quality Many alternative solvents trialled Fluxless wetting balance tests used to assess SAM coating performance Wetting Force / mn IPA SAM Butanol SAM 80:20 IPA:Ethylene Glycol SAM 0 Air (18 hours) Freezer (5 days) Air 40 C 1.5 hours

20 Surface Pre-treatment and Thiol Spread Initial trials using micro pipettor to deposit small droplets (3 μl) onto surface Spread of thiol solution very rapid on pre-treated surface Strong dependence on surface microstructure / roughness Pre-treatments assessed to enable more stable droplet formation

21 Localised Fluxless Solder Wetting After localised SAM deposition, entire sample immersed in molten solder without flux Preserved areas wetted by solder Solder Copper Approx 5mm

22 SAMs as Plating Resists SAM deposited using micro pipette Electroless copper deposited from standard bath Plating only builds where Bare Electroless copper copper copper is exposed SAM coated region Plating resist

23 AFM Images of Deposit Boundary Electroless Copper SAM protected Electroless Copper SAM protected 0 40 µm 0 20 µm

24 Analysis Through Boundary (c) nm 0 20 µm Approximately 1.5μm step height

25 Ink Jetting of SAMs Preliminary work at partner site to demonstrate process / establish experimental parameters Ink jet system being commissioned Deposition onto pretreated surfaces Positioning system z Printhead Substrate Cu, Ni, Au y x Thiol supply CAD images Variables control Ø μm SAMs deposits

26 Ink Jetting of SAMs SEM reveals ~50μm diameter SAM preserved areas on copper SAM modified regions

27 Conclusion SAMs offer a potential route forward for the manufacture of patterned interconnects An existing deposition technique has been modified for ink jetting Preliminary experiments have shown localised surface modification Ink jetting has been used to create SAM dots on copper