Self organization and properties of Black Silicon

Transcription

1 TECHNISCHE UNIVERSITÄT ILMENAU 51st IWK Internationales Wissenschaftliches Kolloquium September 11-15, 2006 Self organization and properties of Black Silicon M. Fischer, M. Stubenrauch, Th. Kups, H. Romanus, F.M. Morales*, G. Ecke, M. Hoffmann, C. Knedlik, O. Ambacher and J. Pezoldt Institute for Micro- and Nanotechnologies TU Ilmenau *Universidad de Cádiz, España

2 1 Introduction RIE: reactive ion etching of silicon a widely used process for MEMS Black Silicon: needle like structures with micro- and nanometer dimensions Only an unwanted side effect of anisotropic silicon etching? 2 Common Applications: - optical absorber elements - antireflective coatings - field emitter arrays RIE-textured solar cells, Ruby et al Sandia Labs USA AlN coated field emitter array V. Lebedev et al

3 1 Introduction New Applications: - new bonding technologies based on the Velcro effect - superhydrophobic surfaces, contact angle >170 deg - catalytic enhanced microreactors bonding interface between two bl. silicon coated chips black silicon microreactor coated with a catalysator water droplet on a hydrophobic black silicon surface Fabrication: lithography-free, self-organized etching process on standard silicon etching machines (RIE, ICP) 3

4 2 Technologies Standard RIE System STS 320 RIE System 4 anisotropic structuring process is a sensitive combination of etching e and deposition of a passivation layer (generated material or pass. gases) etching and passivation take place simultaneously physical and chemical etching parts are not separately controllable

5 2 Technologies ICP System STS Multiplex ICP System 5 Deep RIE (ASE or Bosch ) => Etching and Passivation separated in time HD-Plasma (ICP) for high etch rates and precise control of the process A second HF source for the platen electrode enables separate control of physical and chemical etching parts

6 2 Technologies Comparison of RIE and ASE needles processes RIE etching/passivation Mixture Parameter C4F8 gas flow [sccm] SF6 gas flow [sccm] O2 gas flow [sccm] cycle time [s] coil power [W] platen power [W] chamber pressure [mtorr] passivation cycle ASE etching cycle chamber temperature [ C] substrate temperature [ C] Process time [min]

7 3 Characterization Scanning Electron Microscope 7 RIE up to Mio needle properties legth [μm] diameter on top [nm] diameter on bottom [nm] density [/mm²] ASE up to > 1 Mio

8 3 Characterization Transmission Electron Microscope I RIE ASE Side view of the silicon bulk with needles at the surface Side view of ASE -needles 8

9 3 Characterization Transmission Electron Microscope II RIE up to Mio smooth corrected properties length [μm] diameter on top [nm] diameter on bottom [nm] density [/mm²] surface ASE up to > 1 Mio ribbed 9

10 3 Characterization Auger Electron Spectroscopy RIE ASE only strong silicon and oxygen peaks as well as a verry low fluor peak (close to detection limit) => only SiO x passivation and no SiO x F y? carbon peak => carbon contaminations due to transportation strong carbon and fluor peaks and nearly no silicon and oxygen peak => fluor carbon (teflon like) coverage of the silicon needles oxygen peak => oxygen contaminations due to transportation 10

11 4 Black Silicon Formation Model Reactive Ion Etching Loading Model Chemistry for needle growing with RIE 11 Local excessive deposition of reaction products (masking spots) Electric loading on the surface of these dielectric areas Influence around a needle spot -> > more needles Vertical growth of the needles (full wafer or unmasked areas)

12 4 Black Silicon Formation Model Advanced Silicon Etching AFM scan of a 9 sec pass. layer Needle growing with ASE 12 non uniform passivation deposition non uniform ion attack on wafer surface, supported by loading effectse multi-spot formation, influencing each other growth by standard etching phenomena

13 5 Further Investigations Spectroscopic Ellipsometry ellipsometric data for standard silicon substrates (crystalline, amorphous) ellipsometric data for nanostructured silicon surfaces (RIE, ASE-ICP) 13 totally different ellipsometer spectra for nanostuctured surfaces standard volume models for silicon do not apply a mixture of air, crystalline and amorphous silicon does not fit t the parameters further research will focus on the optical properties of black silicon

14 6 Conclusions - Different types of homogenous Black Silicon were formed by using two dry etching methods - Density and geometry of the needles depend strongly on the formation conditions - The needle types show no significant difference in their tip radii (app. 5 nm) - For RIE needles the sidewall passivation is mainly a oxide based layer - ASE structures have a teflon like coating with excellent surface properties (wetting and tribology) - The self organization is based on local differences in etch rate, electric loading and the inhomogeneous passivation layers > single spots - Every spot has an influence zone around its tip and induces the creation of more spots -> spreading of black silicon on large areas 14

15 Acknowledgement Thanks to all the colleagues who contributed to this work: Mrs. Elvira Remdt Dr. J. MüllerM Dr. A. Schober Dr. G. Ecke Dr. Iannev Dr. Krischok Dipl.-Ing. S. Stoebenau Dipl.-Ing. Ch. Kremin 15