Top-down approach for formation of nanostructures: Lithography with light, electrons and ions Seminar Nanostrukturierte Festkörper, Martin Hulman

Seminar Nanostrukturierte Festkörper, Top-down approach for formation of nanostructures: Lithography with light, electrons and ions Seminar Nanostrukturierte Festkörper, Outline History Physical foundations of lithography Overview of lithographic techniques Resists Future and perspectives Lithography in our lab

Seminar Nanostrukturierte Festkörper, LITHOGRAPHY = „STONE DRAWING“

Seminar Nanostrukturierte Festkörper, A piece of history invented in 1798 first technique for colorprinting pictures made by impressing flat embossed slabs (of limestone), each covered with greasy ink of a particular color, onto a piece of stout paper

Seminar Nanostrukturierte Festkörper, SEMICONDUCTOR MANUFACTURING PROCESS

Seminar Nanostrukturierte Festkörper, Lithographic techniques with electromagnetic waves: optical ultra-violet deep UV X-ray with charged particles: electrons ions

Seminar Nanostrukturierte Festkörper, Physical basis of lithography finite resolution of the image- forming system results in the light distribution which does not have clearly defined edges Diffraction!

Seminar Nanostrukturierte Festkörper, Physical basis of lithography two ingredients of image formation: optics photo-resist The quality of image is determined by: resolution power of the optics focusing accuracy contrast of the resist process

Seminar Nanostrukturierte Festkörper, Physical basis of lithography: Diffraction a circular aperture illuminated by a point source of light the light intensity distribution from a point source projected through a circular aperture Airy function x=r d  z

Seminar Nanostrukturierte Festkörper, Physical basis of lithography: the Rayleigh criterion for resolution two point sources of light separated by a small angle the total light intensity is a sum of individual intensities The Rayleigh criterion: maximum of the Airy pattern from one source falls on the first zero of the Airy pattern from the other source the minimum resolved distance d between the peak and the first minimum of the Airy function d = 0.61  n sin  n sin  is a numerical aperture

Seminar Nanostrukturierte Festkörper, Physical basis of lithography: typical parameters for optics

Seminar Nanostrukturierte Festkörper, Optical printing lithography techniques Contact printing: a photomask is in direct or intimate contact with a resist-covered wafer; the photomask is pressed against the wafer with a pressure of atm; exposed to light with wavelength of about 400nm; a high resolution of less than 0.5 µm m is possible but the resolution varies across the wafer the mask used in contact printing is frequently replaced after short period of use Proximity printing: there is a typical separation between the mask and the wafer in a range of  m; the defects resulting from proximity printing are not as bad as contact printing ; its resolution is not as good as compared to that of contact printing ; the mask used has a longer lifetime Projection printing: larger separation between mask and wafer; higher resolution than proximity printing; the system cost is approximately five times that of contact printing

Seminar Nanostrukturierte Festkörper, Drawbacks of optical systems: Aberrations chromatic aberration: inability to focus light over a range of wavelength distortions: higher resolutions in the center of the fields astigmatism: points to appear as lines

Seminar Nanostrukturierte Festkörper, Optical Lithography: the smallest working device -- with 80 nm features (1999) a flash memory cell made of silicon

Seminar Nanostrukturierte Festkörper, X - ray lithography X – ray wavelength  6 – 14 nm diffraction effects can be ignored because of a small wavelength masks consists of an absorber (Au) on a transmissive membrane substrate (Si, SiC, Si 3 N 4 ) ability to define very high resolution images

Seminar Nanostrukturierte Festkörper, Electron beam lithography no masks required ! the diameter of the electron beam as small as 50 nm electrons with energy 10 – 50 keV(150 eV => 1 A) resolution not limited by diffraction but by scattering in the resist masks for optical lithography aberrations still present slow compared to optical lithography expensive and complicated

Seminar Nanostrukturierte Festkörper, Electron beam lithography

Seminar Nanostrukturierte Festkörper, Ion beam lithography lithography with charged ions (He + and Ar + ) at energies up to 200keV very small particle wavelength ~10 -5 nm electrostatic ion optics with a small numerical aperture ~ resolution down to 50 nm diffraction limit 3 nm

Seminar Nanostrukturierte Festkörper, Resists positive resist – more soluble after exposing to light, chemical bonds are destroyed in a photoactive component negative resist – less soluble after exposing to light, crosslinks between molecules are created PMMA for UV, deep-UV, X-ray and e-beam lithography higher resolution is possible with positive resists in OL factors limiting resist resolution: - swelling of the resist in the developer - index of refraction > 1 (for OL) - electron scattering (neglible for X-ray)

Seminar Nanostrukturierte Festkörper, Comparison of various lithographic techniques

Seminar Nanostrukturierte Festkörper, Future and perspectives: Moore´ s Law Year of introduction Transistors (per IC) , , , , , ™ processor , ™ DX processor ,180,000 Pentium® processor ,100,000 Pentium II processor ,500,000 Pentium III processor ,000,000 Pentium 4 processor ,000,000 Violation of the Moore´s law ? Current technology: 0.13 µm, down to µm in 2007 physical limitations

Seminar Nanostrukturierte Festkörper, Future and perspectives trends for technology for the scaling into deep nanometer regime

Seminar Nanostrukturierte Festkörper, Future and perspectives: Direct imprint S. Chu et al., Nature 2002 Resolution down to 10 nm no masks required !

Seminar Nanostrukturierte Festkörper, Lithography in our lab: Raman microspectroscopy on individual carbon nanotubes carbon nanotubes on a silicon surface position of a nanotube with respect to a predefined marker system AFM images, scale bars 1µm

Seminar Nanostrukturierte Festkörper, Lithography in our lab: Raman spectra

Seminar Nanostrukturierte Festkörper, Lithography in our lab: marker system masks made by e-beam lithography size of letters 1.2 µm

Seminar Nanostrukturierte Festkörper, Lithography in our lab: Suspended carbon nanotubes G.T. Kim et al., Appl. Phys. Lett. 80 (2002)