Top-down approach for formation of nanostructures: Lithography with light, electrons and ions Seminar Nanostrukturierte Festkörper, 30.10.2002 Martin Hulman.

Top-down approach for formation of nanostructures: Lithography with light, electrons and ions

Seminar Nanostrukturierte Festkörper, 30.10.2002

Martin Hulman

Seminar Nanostrukturierte Festkörper, 30.10.2002Seminar Nanostrukturierte Festkörper, 30.10.2002

Top-down approach for formation of nanostructures: Lithography with light, electrons and ions


Outline

• History• Physical foundations of lithography • Overview of lithographic techniques• Resists • Future and perspectives• Lithography in our lab


LITHOGRAPHY = „STONE DRAWING“


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


SEMICONDUCTOR MANUFACTURING PROCESS


Lithographic techniques

with electromagnetic waves:

• optical

• ultra-violet

• deep UV

• X-ray

with charged particles:

• electrons

• ions


Physical basis of lithography

finite resolution of the image-forming system results in the light distribution which does not have clearly defined edges

Diffraction!


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


Physical basis of lithography:

Diffraction

a circular aperture illuminated by a point source of light

the light intensity distribution from a point sourceprojected through a circular aperture

Airy function

x=r d z



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 betweenthe peak and the first minimum of the Airy function

d = 0.61n sin

n sinis a numerical aperture



typical parameters for optics


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 0.05 - 0.3 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 20 - 50 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


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


Optical Lithography:

the smallest working device -- with 80 nm features

(1999)

a flash memory cell made of silicon


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, Si3N4)

• ability to define very high resolution images


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


Electron beam lithography


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 ~ 10-5

• resolution down to 50 nm

• diffraction limit 3 nm


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)


Comparison of various lithographic techniques


Future and perspectives: Moore´ s Law

Year of introduction Transistors (per IC)

4004 1971 2,250

8008 1972 2,500 8080 1974 5,000 8086 1978 29,000 286 1982 120,000 386™ processor 1985 275,000 486™ DX processor 1989 1,180,000 Pentium® processor 1993 3,100,000 Pentium II processor 1997 7,500,000 Pentium III processor 1999 24,000,000 Pentium 4 processor 2000 42,000,000

Violation of theMoore´s law ?

Current technology: 0.13 µm,down to 0.065 µm in 2007

physical limitations


Future and perspectives

trends for technology for the scaling into deep nanometer regime


Future and perspectives: Direct imprint

S. Chu et al., Nature 2002

Resolution down to 10 nm

no masks required !


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



Raman spectra

150 200 250

0

1

2

3

4

2.41

2.50

2.60

Eexc

(eV)

174.1

214.4

230.7178.0

174.8

Inte

nsi

ty (

a.u

.)

Raman shift (cm-1)

150 200 250

0

2

4

6

Eexc

(eV)

2.60

2.50

2.41

2.18

1.92

181.7

206.9

231.9212.2180.6

180.7

Inte

nsi

ty (

a.u

.)

Raman shift (cm-1)



marker system

masks made bye-beam lithography

size of letters 1.2 µm



Suspended carbon nanotubes

G.T. Kim et al., Appl. Phys. Lett. 80 (2002)

Top-down approach for formation of nanostructures: Lithography with light, electrons and ions Seminar Nanostrukturierte Festkörper, 30.10.2002 Martin Hulman.

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