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Nano-Electronics. S. Mohajerzadeh University of Tehran. Lithography, nano-technology. Lithography is transferring a desired pattern from a “mask” onto a processed substrate. Lithography remains essentially the same for micro and nano-electronics. Standard Photolithography.
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Nano-Electronics S. Mohajerzadeh University of Tehran
Lithography, nano-technology • Lithography is transferring a desired pattern from a “mask” onto a processed substrate. • Lithography remains essentially the same for micro and nano-electronics.
Lithography approaches • Contact mode, mask sits on the resist-coated sample, best resolution is achieved. • “d” is the resist thickness and “g” is the gap between the sample and mask (proximity mode). • In projection mode (most used for nano-lithography), numerical aperture of the lens plays a crucial role.
UV sources • Mercury high pressure lamps, strong peaks at 436, 405 and 365nm • Nothing below 300nm, • Plasma torch, Extreme UVs, at 10nm • Transparency of the various glasses drops at lower wavelengths. • Quartz or fused silica can be used for deep UV illuminations
Extended UV lithography • No transmission lens • Reflection condensing, mirrors • Reticle, reflecting metals • High resolution
E-beam writers • Electrons form a beam to hit the surface on the desired area. • Thermionic sources, field emission sources, (W or LaB6) • Brightness, W: 104 A/cm2, LaB6: 105, field emission 107
Electron Trajectories • Resolution is limited by the spot size, and the exposed area • Backscattered electrons can expose unwanted regions • Proximity effect, the shape of pattern affects the resolution
E-beam lithography • Underetching results in the reduction of the pattern resolution. • Sharp vertical patterns are obtained by high energy electron beam writing.
Possible parallel processing • Scattering limited projection e-beam lithography (SCALPEL) • Use of scattering layer (Au, W) to stop the electrons in the undesired regions,
X-ray lithography • X-ray beam has a wavelength of the order of 1Ǻ, suitable for high resolution lithography. • Adsorption is a problem with the mask. Shadow masks or thin membranes are used. • No lens is available for X-ray. Long pipes are used to form a coherent beam.
AFM lithography • The AFM tip is used to deliver liquid (resist) onto the surface of sample. • Nano-metric resolution is achieved by this “true writing” approach.
Phase shift masks • Phase shift leads to diffraction on the image side. • Phase shift is suitable only when two windows are placed close together.
Dark field microscopy • Illuminating the object at glancing incidence, to ensure the main reflecting beam does not enter the microscope • Surface irregularities are highlighted and features as small as 10nm high are detected. • Dust particles of the order of 100nm are observed.
Phase Objects • A pure phase object has no contrast and we cannot see it. • A glass with a step on it will be seen as a flat surface. • Phase object changes the phase of the light and our eyes can see the variations in the intensity and not the phase. • Zernike proposed the phase contrast microscopy and received a Nobel prize for this invention.
Phase contrast scheme • The idea of phase retarding plate causes contrast on the image plane. • Light passes the phase object and after passing through the lens, the zero-order diffraction faces the phase retarding plate and on the image plane we see contrast.
Phase shift • Instead of plate, a phase shifting ring is used.
Only first and second order diffracted beams are considered. • Other directions cannot reach the lens and are not important. • The parallel diffraction beams forms new beams in diverging directions. • On the transform (focal) plane, small dots are formed corresponding to various diffraction beams. • Without a phase shift object, contrast is not formed and no clear image is formed on the image plane. • A phase retarding/advancing plate yields a constructive/destructive interference and hence a true image is formed.
Simple explanation! • If E0, E+1 and E-1 are amplitudes of zero, +1 and -1 diffracted beams and assuming a plane wave nature for these light beams, • E(x,y,z,t)=Aexp(j(kxx + kyy + kzz –ωt) + Φ) • For zero order, kx=ky=0, kz=2π/λ and E0(x,y,0)=Aexp(j(Φ –ωt)) • At z=0 plane, E+1(x)=ε exp(j(kxx–ωt)) and E-1(x)=ε exp(j(-kxx–ωt)) • So, finally, Etot(x) = E0 + E+1 + E-1 and I(x) = Etot Etot* • And I(x)= A2 + 2 ε2 + E0 E*+1 + E0E+1* + E0E-1* + E0E-1* + E+1E-1* + E+1*E-1 • Eventually!!, I(x)= A2 + 2 ε2 + 4A ε[cos Φ cos (kxx)] + 2 ε2 cos(2kxx) • By ignoring the last term, I(x)= A2 + 2 ε2 + 4A ε[cos Φ cos (kxx)] • I(x)= A2 + 2 ε2 + 4Aε2[cos Φ cos (kxx)], • Playing with (cos Φ) can cause a contrast image to occur on the image plane. Say Φ=0, π,… in normal case, Φ =π/2 or ..
Wollaston prism, made of a material with two anisotropic charactersitics such as Calcite. • The light which is polarized, when traveling in the direction parallel with the optical axis, has a speed different with the perpendicular case, so phases are different. (higher index of refraction in one direction) • Normaski prism produces two images of an object, one for each polarization with a small relative displacement. • For the case of DIC resolution is less than optical resolution.
