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

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.

Resolution limits

Masks

Nanometric levels

Various lithography sources

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

Deep UV source

E-beam writers Electrons form a beam to hit the surface on the desired area. Thermionic sources, field emission sources, (W or LaB 6 ) Brightness, W: 10 4 A/cm 2, LaB 6 : 10 5, field emission 10 7

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,

E-beam writers

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.

Resolution improvement

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.

Phase contrast microscopy

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 E 0, 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(k x x + k y y + k z z –ωt) + Φ) For zero order, k x =k y =0, k z =2π/λ and E 0 (x,y,0)=Aexp(j(Φ –ωt)) At z=0 plane, E +1 (x)=ε exp(j(k x x–ωt)) and E -1 (x)=ε exp(j(-k x x–ωt)) So, finally, E tot (x) = E 0 + E +1 + E -1 and I(x) = E tot E tot * And I(x)= A ε 2 + E 0 E* +1 + E 0 E +1 * + E 0 E -1 * + E 0 E -1 * + E +1 E -1 * + E +1 *E -1 Eventually!!, I(x)= A ε 2 + 4A ε[cos Φ cos (k x x)] + 2 ε 2 cos(2k x x) By ignoring the last term, I(x)= A ε 2 + 4A ε[cos Φ cos (k x x)] I(x)= A ε 2 + 4Aε 2 [cos Φ cos (k x x)], 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.

Differential interference contrast

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