How can free-standing ALD alumina membranes be made?

1. S-69.4114 Post-graduate course on Electron Physics IIThin film technology How can free-standing ALD alumina membranes be made? Ville Vähänissi7.5.2010

2. Outline • Introduction • Why alumina? • ALD alumina process • Fabrication & Application examples • Conclusions

3. Introduction (1) • Impermeable, continuous, suspended thin films • Membrane applications: pressure sensors, thermopiles, bolometers, microheaters, etc. • Thermal and electrical insulation: reduced power consumption, enhanced performance & speed

4. Introduction (2) • Membrane material traditionally silicon nitride (LPCVD or PECVD) • Insulator • Good mechanical properties • Tailorable tensile stresses • Drawbacks • Mechanical properties and stresses strongly depend on deposition parameters • High temperature processing (LPCVD) • Selectivity in etching (silicon-rich PECVD) • Poor conformality Stress tailoring troublesome

5. Why alumina? • Thermal and electrical insulator • Can be deposited using ALD • Low temperature process • Fully conformal coating • Superior film thickness control • Good mechanical properties and low stresses • Flexible post-deposition processing • Patternability • Selectivity (66 000:1)

6. ALD alumina process • One of the most widely investigated and developed ALD processes TMA H2O

7. Membrane fabrication Through-etching from the wafer backside Sacrificial silicon etching from the wafer frontside

8. Through-wafer etching (example 1) • Pressure sensor based on carbon nanotubes • SWNTs act as piezoresistive strain gauges • High sensitivity and high scalability • Thickness control, stable & good mechanical properties

9. 1. alumina deposition, alignment markers2. alumina backside openings by ICP, DRIE-ICP of silicon (Bosch process) 3. SWNT deposition 4. AFM imaging, e-beam lithography5. contact forming (metallization & lift-off) 6. isotropic release by RIE (crucial step, optimized to prevent damage) • Fabrication steps:

10. Through-wafer etching (example 2) • Nanocorrugated membranes • Applications as extremely small liquid containers • Good mechanical strength, low stress, stability, conformal deposition, low deposition temperature

11. 1. creation of nanostructures 2. alumina deposition3. backside mask deposition & patterning4. cryogenic DRIE of silicon • Fabrication steps:

12. Sacrificial silicon etching (example 1) • Free-standing bridges • Fabrication steps: • Suspended metallic devices limited to small sizes due to stress 1. bridge material deposition and patterning 2. isotropic inductively coupled SF6 plasma release (lateral etch rate 3 μm/min) help from alumina

13. Sacrificial silicon etching (example 2) • Perforated membranes as supports for metallic devices • robustness of alumina enables the fabrication of very large suspended metallic structures • fabrication easy due to the isotropic frontside release • structures thermally and electrically isolated 1. alumina deposition & patterning (200 nm thick, hexagonally packed perforations) 2. metal deposition and patterning (for example Al, wet etching rate ratio 1:20)3. membrane release by isotropic SF6 plasma etching

15. Conclusions • Many applications which could utilize the beneficial properties of ALD alumina • Two fabrication styles • through-wafer etching • sacrificial etching • Through-wafer etching • Sacrificial etching + impermeable large membranes+ shape control - alignment- membrane protection - release last step + straightforward and easy+ small devices possible + processing after release - large impermeable devices problematic

16. Thank you!

17. References • L. Sainiemi, Doctoral Dissertation, Cryogenic deep reactive ion etching of silicon micro and nanostructures, Helsinki University of Technology (2009) • L. Sainiemi, K. Grigoras and S. Franssila, Nanotechnology, vol. 20, 075306 (2009) • M. K. Tripp, et. al., Sensors and Actuators A, vol. 130-131, p. 419-429 (2006) • C. Stampfer et. al., Nanoletters, vol. 6, no. 2, p. 233-237 (2006) • T-Y. Zhang et. al., Acta Mater, vol. 48, p. 2843 (2000)