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Process optimisation of BCB-polymer for use in CMUTs

Process optimisation of BCB-polymer for use in CMUTs. Martin Lind Ommen , Andreas Spandet Havreland, Rikke Thiesson, Rune Sixten Grass and Erik Vilain Thomsen. Outline. Introduction What is BCB? Motivation Process Development Contrast curve Exposure mode Curing Study Breakdown and FTIR

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Process optimisation of BCB-polymer for use in CMUTs

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  1. Process optimisation of BCB-polymer for use in CMUTs Martin Lind Ommen, Andreas Spandet Havreland, Rikke Thiesson, Rune Sixten Grass and Erik Vilain Thomsen

  2. Outline • Introduction • What is BCB? • Motivation • Process Development • Contrast curve • Exposure mode • Curing Study • Breakdown and FTIR • Conclusion • What is next?

  3. Outline • Introduction • What is BCB? • Motivation • Process Development • Contrast curve • Exposure mode • Curing Study • Breakdown and FTIR • Conclusion • What is next?

  4. IntroductionWhat is BCB? • Photosensitive polymer • Spin coat • Can be used as insulating material in CMUT

  5. IntroductionWhat is BCB? BCB Divinylsiloxane-bisbenzocyclobutene BAC-M – photo sensitive component Cured Polymer R1 R2 R3 Zuo, Journal of polymer Science, V47-I22, pp 6246-6258 2009.

  6. IntroductionBCB for CMUTs • Process overview

  7. IntroductionComparison to state of the art LOCOS RC CMUT BCB CMUT

  8. IntroductionWhat is the intended use? • Rapid array prototyping • Potentially inexpensive and fast production

  9. IntroductionBCB state of the art Dry Etch BCB Photo sensitive BCB Wire phantoms Pulse-echo Manwar, Microsystems Tech, p1-13, 2016. Li, Journal of micromech. and microeng., V26-I11, pp 115019, 2016.

  10. IntroductionCritical points for BCB CMUTs • Need good control of film thickness and structuring • Critical regarding pull-in voltage • Need to know breakdown field • Non-destructive characterisation of electrical parameters for final CMUTs

  11. IntroductionFTIR • Transmission/Reflection

  12. IntroductionFTIR of BCB Divinylsiloxane-bisbenzocyclobutene Cured Polymer BCB R1 R2 R3 b – Optical path length in film How does this correlate to breakdown? 1475 cm-1 1500 cm-1

  13. IntroductionOverview of BCB processing • Reduced Process flow in this study

  14. Outline • Introduction • What is BCB? • Motivation • Process Development • Contrast curve • Exposure mode • Curing Study • Breakdown and FTIR • Conclusion • What is next?

  15. Process developmentContrast curve • Submersion development in beaker • High dose and long development stabilises resulting thickness 40

  16. Initial ChallengesFilm uniformity Contact mode Mean thickness subtracted After BCB Spin After Development Mean: 894 nm sd: 1.7 nm SUP: < 0.2% Mean: 652 nm sd: 67 nm SUP: 10%

  17. Initial ChallengesExposure mode • Contact exposure mode • Embossing and Newton rings ~5 cm

  18. Initial ChallengesOptimised Film uniformity Mean thickness subtracted Hard contact After development Proximity mode After development Mean: 652 nm sd: 67 nm SUP: 10% Mean: 330 nm sd: 3.9 nm SUP: 1.2%

  19. Outline • Introduction • What is BCB? • Motivation • Process Development • Contrast curve • Exposure mode • Curing Study • FTIR and breakdown • Conclusion • What is next?

  20. Curing StudyExperimental concept • Used curing process similar to Li et al. – 2 step ramp

  21. Curing StudyFTIR Spectra – before development R2 Cured Polymer BCB R3

  22. Curing StudyFTIR Spectra – before development Hard Cure Soft Cure

  23. Curing StudyFTIR Spectra – after development R2 Cured Polymer BCB R3

  24. Curing StudyFTIR Spectra – after development

  25. Curing StudyBreakdown fields – General behaviour • IV Characterisation

  26. Curing StudyBreakdown fields – General behaviour • Large variation across wafer

  27. Curing StudyBreakdown fields – before development

  28. Curing StudyBreakdown fields – after development

  29. Outline • Introduction • What is BCB? • Motivation • Process Development • Contrast curve • Exposure mode • Curing Study • Breakdown and FTIR • Conclusion • What is next?

  30. Conclusion • Larger doses and longer development times improves replicability • BCB is a soft polymer – Proximity exposure mode is necessary • Film thickness of 330 nm ± 4 nm possible • FTIR and breakdown still needs optimisation for ideal use • No clear correlation optainable yet • Design devices for 0.1 V/nm

  31. What is next? • Creative designs to minimise the electric field within the BCB • Next talk – BCB implementation

  32. Thank you for your attention!

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