Nanopantography: A method for parallel writing of etched and deposited nanopatterns

Transcription

1 Nanopantography: A method for parallel writing of etched and deposited nanopatterns Vincent M. Donnelly 1, Lin Xu 1, Azeem Nasrullah 2, Zhiying Chen 1, Sri C. Vemula 2, Manish Jain 1, Demetre J. Economou 1 and Paul Ruchhoeft 2 1 Department of Chemical and Biomolecular Engineering 2 Department of Electrical and Computer Engineering University of Houston, Houston, TX 77204

2 Nanopantography Pantograph is a mechanical device used to copy a figure or plan on a different scale Multiscale and Parallel process

3 Charge-induced Notching during plasma etching Leads to undesired formation of features much smaller than the pattern size. On the origin of the notching effect during etching in uniform high density plasmas, G. S. Hwang and K. P. Giapis, J. Vac. Sci. Technol. B 15, (1997) 70. Charge accumulation effects on profile distortion in ECR plasma etching, N. Fujiwara, S. Ogino, T. Maruyama, M. Yoneda, Plasma Sources Sci. Technol. 5 (1996) 126. Notching as an example of charging in uniform high density plasmas T. Kinoshita, M. Hane, J. P. McVittie, J. Vac. Sci. Technol. B 14, (1996), e

4 Nanopantography Side View: Broad area ion beam V b TopView: + - metal, V m dielectric conducting film or substrate, V s Essential components: Monoenergetic ion beam and electrostatic lens built on the wafer Capabilities and Characteristics: ~10nm patterning Self-aligned, immune to vibration and thermal expansion; Etching and deposition

5 Nanopantography: Write many nanopatterns simultaneously by tilting the substrate Whatever is written once on the imaginary plane here is reproduced at the bottoms of all the lenses here. Side view Top view

6 Nanopantography Side View: Broad area ion beam V b TopView: metal, V m dielectric conducting film or substrate, V s Requires: Monoenergetic ion beam with low divergence

7 Generation of monoenergetic ion beams with selectable energy ON Coil or electrode Pulsed Plasma + V 0 + V OFF Pulsed radio freq. plasma Ring electrode voltage (pulsed or continuous) Ring electrode 0 Strategy: Extracting ions at low RF plasma potential (V RF p ), electron temperature (T e ) and ion temperature (T i ) in afterglow (plasma OFF) decreases the energy spread and increases the directionality of ions. Ring electrode voltage sets the desired ion beam energy.

8 Why pulsed rf plasma for monoenergetic, reduced angular spread ion beam? 2Ti m i 2V m sheath i pre-sheath sheath Higher energy Lower energy grid Small T i means smaller angular spread Ion temperature T i ~ T e / 2 V sheath = V 0 + V RF P Energy spread T i,, V RF P V RF P = 0 and T i is lowered by turning plasma off

9 Time-resolved electron current in response to a 200 V pulse applied in the afterglow of an inductively-coupled pulsed plasma Coil or electrode Pulsed Plasma Ring electrode + V 0 + V ON OFF Pulsed radio freq. plasma Ring electrode voltage (pulsed or continuous) Current to the acceleration ring (A) Electron current spike Voltage on the acceleration ring (V) 0 8 mtorr, 5 KHz modulation frequency, 50% duty cycle, 100 W average RF power on the internal coil, no power on the target electrode Time in the plasma afterglow (μs)

10 Time-resolved Langmuir probe I-V characteristics in the afterglow of a pulsed plasma 0 or +30V DC bias applied on the acceleration ring (15.9 mtorr, 5 KHz modulation frequency, 50% duty cycle, 30 W average RF power on the target electrode). electrode Langmuir probe 0 or +30V Current(mA) Time after plasma power-off 6µs 15µs 31µs 92µs 0 V dc bias 30 V dc bias Probe bias(v)

11 Langmuir probe measurement of evolution of plasma potential (V p ), electron temperature (T e ) and positive ion density (n i+ ) in the afterglow electrode Langmuir probe +30V V p (V), 10xT e (ev) n i V p x T e Decay Time(μs) n i + (cm -3 ) 16 mtorr Ar plasma, 13 MHz, capacitively-coupled plasma (CCP), 5 KHz modulation frequency, 50% duty cycle). +30V continuous bias on the ring electrode.

12 Nearly monoenergetic Ar + beam extracted from a pulsed Ar CCP, continuous DC ring electrode voltage electrode Pulsed Plasma Retarding field ion energy analyzer resolution 3% gate Normalized Ion Energy Distributions Ions extracted while plasma OFF 30.5 ev Ions extracted while plasma ON Gate always open 51.0 ev 71.5 ev ev Gated 3.4 ev FWHM Ion energy (ev) L. Xu, D. J. Economou, V. M. Donnelly, P. Ruchhoeft, Appl. Phys. Lett. 87, (2005).

13 Time-resolved ion current downstream from the plasma Ion current on a 3 -dia. collector, 60 cm downstream of the plasma source. 100 V acceleration ring pulsed between 14 μs and 61 μs into the afterglow. 8 mtorr, 5 KHz modulation frequency, 50% duty cycle, 100 W average coil power Current to the collector (μa) Flight time Voltage on the coil (V) -4 Plasma ON Plasma OFF Time (μs)

14 Nearly monoenergetic Ar + beam extracted from a pulsed Ar Inductively coupled plasma (ICP) coil Pulsed Plasma V peak = 99 ev 1.0 gate IED (A.U.) FWHM = 2.2 ev Retarding field ion energy analyzer resolution 2% Ion energy (ev)

15 Si nano-etching Ar ion beamlets and Cl 2 gas

16 Nanopantography for Ar + -assisted etching of Si by Cl 2 Matching Network Ar pulsed plasma Ar + -assisted etching by Cl 2 Cl 2(g) + 2Si 2Si-Cl (ads) monoenergetic Ar ion beam xsi-cl (ads) SiCl x(ads), x=1-3 Ar + + SiCl x(ads) SiCl y(g), y=1-4 + Ar Lens array voltage Cl 2 effusive beam Sample with lens array

17 Proof of Principle (PoP) Experiment: Focusing depends on aspect ratio of lens structure and potential across the dielectric material Nano version: Nanopantography Micron version: PoP Experiment metal, V m dielectric 100μm thick transparency Metal mesh with 75 μm holes conducting film or substrate, V s Highly doped Si wafer PoP Experiment: Prove the principle and characterize the quality of ion beam Easy to analyze the sample

18 SEM picture of PoP experiment after Ar+ etching a) c) b) 1.3 μm 75 μm The etched hole size is around 1.3 μm, resulting in a reduction factor of ~60x From AFM measurement, the hole is around 1 μm deep.

19 Micro-Einzel Lenses: Simulations 50 nm 500 nm Si Substrate Ion beam Metal Insulator Solve Laplace eq. in cyl. Coord. 2 1 V V ( r ) + 2 r r r z Find electric field = 0. Integrate eqs. of motion for du dt r E = V e = Er, m du dt z = e m E z Relation Ion flux at bottom Simulation for 260 nm dia. lens 1 0 ~3nm FWHM ( i.e. ~100:1 reduction) Distance along bottom (nm) Fine focusing requires: 1. Narrow energy spread, ΔE ~2eV 2. Narrow angular spread, Δθ 1 O

20 Frame Sep 2005 Monte Carlo Computations Nanopantography Simulations Lens structure Conductor / dielectric / conductor 950nm l m = 50 nm a) b) l d = 1000 nm At normal incidence Vm=197.2V, Vs=100V Relative Ion Flux c) FWHM = 10 nm d) 5E-07 4E-07 3E-07 2E-07 1E-07 Y 0-1E-07-2E-07-3E-07-4E-07-5E-07-5E E-07 X Tilted by 30 o Vm=196.7V, Vs=100V Relative Ion Flux e) FWHM = 32 nm f) Distance from Center (nm)

21 Nanopantography results: Ar + -assisted etching of Si by Cl 2 SEM Top view of lens array L. Xu et al. Nano Letters, 5, 2563 (2005). Cr layer (top electrode) Lens bottom surface TEM HAADF-STEM, dots 5 o tilt b) ~110 nm normal incidence 950 nm 20 nm 10 nm 50 nm Size reduction factor of ~95X Focused point can be displaced along the substrate surface

22 Ni nano-dot deposition Low energy Ni ion beamlets

23 Nanopantography results: Ni nanodot deposition Ni target Ar pulsed plasma Ni coil Pulsed Ni lamp Optical diagnostics (OAS and OES) Pumping Pumping Lens array voltage Sample with lens (ion landing energy: 20eV) NOTE: Low landing energy prevents sputtering and diffusion

24 Plasma Neutral Gas Temperature Measured by Trace N 2 (C 3 П u - B 3 П g ) Emission Spectroscopy W, 20 mtorr, 2% N 2 in Ar Intensity (A. U.) N band T rot =350 K N band T rot =300 K Wavelength (Å)

25 Total Ni Atom Number Densities points: measurements; lines: model predictions Total Ni density (10 9 cm -3 ) mtorr 12 mtorr 15 mtorr 20 mtorr 20 mtorr 15 mtorr 12 mtorr 8 mtorr Power on the coil (W)

26 Total Ni Ion Number Densities lines: model predictions Total Ni + density (10 7 cm -3 ) mtorr 15 mtorr 12 mtorr 8 mtorr Power on the coil (W)

27 SEM images (top view) of sample after Ni deposition 0 degree tilting Bright: Metal top electrode 5 degree tilting(~30 nm nanodots) Dark: bottom surface of lens 700 nm 900 nm 10nm The focusing ration is 70X for the Ni nanodots!

28 AFM images of deposited Ni nanodots The heights of the deposited nanodots are 5~10 nm.

29 Nanopantography apparatus for continuous substrate tilting and writing Pulsed RF power ~ External gear In-vacuum gear Ar pulsed plasma Lens sample Grounded mesh Vacuum divider To pump To pump Electric pins Lens sample Cl 2 nozzle Loadlock External motor To pump In-vacuum motor Motorized stage To pump Etching of nanopatterns in silicon by nanopantography, L. Xu, A. Nasrullah, Z. Chen, P. Ruchhoeft, D. J. Economou and V, M. Donnelly, Appl. Phys. Letters, 92, (2008).

30 Influence of the top electrode potential of a microlens assembly on the ion current to the substrate without grounded top mesh Ion current to the collector (A.U.) Ion energy: 200 ev 100 ev Sweeping voltage (V) (a) with grounded top mesh 14 Ion energy: 200 ev ev : Figs. (b) and (d) are schematic illustrations of ion trajectories) Ion beam Si substrate (collector) (b) Sweeping voltage Ion current to collector (A.U.) Ground mesh Ion beam Si substrate (collector) Sweeping voltage Sweeping voltage (V) (C) (d)

31 Nanopantography results: continuous writing of etched Si nano-trenches with Ar + / Cl 2 15 nm FWHM Etching of nanopatterns in silicon by nanopantography, L. Xu, A. Nasrullah, Z. Chen, P. Ruchhoeft, D. J. Economou and V, M. Donnelly, Appl. Phys. Letters, 92, (2008).

32 Nanopantography results: continuous writing of etched Si nano-ts with Ar + / Cl 2 Etching of nanopatterns in silicon by nanopantography, L. Xu, A. Nasrullah, Z. Chen, P. Ruchhoeft, D. J. Economou and V, M. Donnelly, Appl. Phys. Letters, 92, (2008).

33 Our Vision for Nanopantography ion-focusing lens Upper layer circuitry Upper-to-lower layer via metal Nano-devices formed by nanopantography Lower layer circuitry

34 Conclusions We have demonstrated Nanopantography, a new approach for fabrication of nanometer scale selected patterns over large areas. A nearly mono-energetic and directional Ar ion beam has been achieved for the realization of nanopantography. 10 nm holes were etched ~100 nm deep into Si by Ar + - assisted etching in Cl 2 with a reduction of ~95 x. 10 nm diam. Ni nanodots with heights of 5~10 nm were deposited with a Ni + beam By tilting the sample, the focal points were displaced, making possible the writing of etched nano-patterns (trenches and Ts) into Si

35 Acknowledgement NSF (NIRT, Nanoscale Interdisciplinary Research Team Projects ) and the Texas Advanced Research Program for funding. Dr. Alain Diebold and Dr. Hugo Celio, Sematech for TEM measurements.

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37 Outline Overview of nanofabrication techniques What is Nanopantography? A proof-of-principle experiment Experimental results from Nanopantography Summary and future work Acknowledgements