TOP -DOWN NANOFABRICATION : OPTICAL LITHOGRAPHY

Question 1

Optical lithography steps

Answer 1

-Deposition(thin film to be etched)
-Photoresist coating
-Photoresist exposure
-Photoresist development
-Thin film etching (using photoresist as mask)
-Photoresist stripping

Question 2

Types of OL

Answer 2

–Contact and proximity lithography – 1:1 reproduction of the mask pattern
–Projection lithography – 1:1 or reduction projection Eliminates mask damage and contamination associated with contact OL.

Question 3

Projection lithography

Answer 3

–Quality of projected image depends on imaging optics
–1:1 projection lithography prevented mask damage while preserving good resolution
–Reduction projection required to further extend the resolution limits
–Equipment cost considerably higher than proximity/contact OL.
–Pattern transfer limited by diffraction

Question 4

Projection lithography – light diffraction from a slit

Answer 4

Difraction involves bending of waves around obstacles. Described by Huygens’ principle that every point on a wave front acts as a source of tiny wavelets (brief oscillations) that move forward with the same speed of the wave. Wavelets interfere with each other, causing light intensity variations

Question 5

Projection lithography – focusing light

Answer 5

–Ligth coming out of mask opening has to be focused by a lens system onto substrate surface.
–Diameter of lens aperture (NA) determines how much or how wide the diffracted light from the mask can be accepted into the lens system.
–For improved resolution we should aim for ligth sources with shorter λ, lens with high NA, and processes with low k1

Question 6

Efeitos focusing light

Answer 6

–Shorter λ and lens with high NA for improved OL resolution
–Lower λ - narrower distribution of light diffraction, more light can be focused by the lens of fixed diameter (NA).
–Larger NA - more diffracted light can be admitted into the lens and be focused onto a substrate
–Both result in improved resolution in imaging, because higher ordered diffractions (light of large diffraction angle) passing the lens results in more details of the mask feature to be imaged onto the substrate.

Question 7

OL at shorter λ – DUV

Answer 7

–Mercury lamps were the illumination source of the early days of OL.
–But these are unsuitable for nanoscale patterning – very low photon energy at shorter λ to meet requirement of volume production of ICs.
–Excimer (pulsed gas) lasers meet the requirements of both high photon energy and shorter λ.

Question 8

OL at shorter λ – EUV

Answer 8

–Efforts/costs to surpass technological issues for 157 nm light sources would only allow “small” resolution improvements.
–More durable new technology required: Extreme UV (EUV)
–Reflective optics (rather than refractive, due to
strong EUV absorption of materials)
–EUV imaging carried out in near-vacuum to
reduce atmospheric absorption of EUV light

Question 9

Critical EUV components:

Answer 9

–EUV source
–EUV projection optics
–EUV mask
–EUV resist

Question 10

EUV source

Answer 10

EUV can be generated by two methods:
* Synchrotron radiation source (SRS), typically only owned by a national government and mainly for
fundamental scientific research
* Laser-produced plasma (LPP) and discharge-produced plasma (DPP)

Main issues:
* Achieve high power for high throughput (>120 wafers/hour).
* Lifetime of collector optics due to contamination caused by debris generated in the source

Question 11

EUV optics

Answer 11

• EUV reflectivity of any single material at near-normal incidence is very low.
• Thin-film multilayer reflective mirror is required – aka Bragg reflectors
• Alternating layers of materials (e.g., Mo and Si) having dissimilar EUV optical constants, providing resonant
  reflectivity when period of the layers≈λ/2

Question 12

EUV mask

Answer 12

Works the same way as EUV mirror (has to be reflective!)
–Substrate should be low-thermal expansion material with flatness better than 50 nm
–Involves fabrication of mask blank and absorber layer (Al, Cr, Ta, W…)
–Patterning of EUV absorver not very demanding doable by e-beam lithography and RIE.
–Main issue is defect inspection and repair:
* any bumps can cause image errors bump in the multilayer surface can cause 20%
deviation of dimension to a 25 nm line)
* for repair e-beam is prefered over FIB (Ga can easily cause damage and staining to the substrate)

Question 13

EUV resists

Answer 13

• Similar to DUV resists, but required to have:
• low line edge roughness (LER), e.g. @ 32-nm node required to be <2.5 nm
• higher sensitivity, because of low power of EUV source. Enhanced with chemically amplified (CA) resists. But
  higher sensitivity typically leads to higher LER.
• higher resolution

Question 14

OL at high NA

Answer 14

–In terms of Fourier optics, smaller spatial detail can be referred to as being of higher “spatial frequency”
–Accurate image requires higher orders of the sinusoidal wave function – requires higher NA
–High NA also means more overall light coming through the lens system → brighter image
–But resolution is only one
requirement for high-quality OL →Depth of focus
–For higher NA, DOF drops even more steeply with NA

Question 15

Increase in NA (>0.8)

Answer 15

• Improved spatial resolution, but…
• Lower DOF
• Complex/heavy/expensive lens design/fabrication
• Polarization effect at high oblique incidence angle → image contrast degradation

Question 16

OL at high NA – immersion OL

Answer 16

Liquid between objective lens and imaging plane reduces the angle of light coming out of the lens, i.e., same NA of optical lens for wet and dry imaging, but effective NA seen from the imaging plane is higher
–Replacing air by a liquid → >n → >NA

Question 17

Main issues immersion OL:

Answer 17

• Fast movement of wafer stage relatively to objective lens, without creating bubbles
• Submerge whole chuck, wafer and lens in pool of water
• Limit pool of water to the top of chuck
• Dispense water between lens and wafer with nozzle (and rely on surface tension…)
• Defects created on PR surface due to water marks and drying stains (diffusion of PR into water
  droplets, topcoats used)

Question 18

OL at low k1
factor

Answer 18

k1 comes from Rayleigh Criterion: minimum resolvable detail of 2 images through an optical system is diffraction-limited.
* 2 closely adjacent light spots are resolvable when the 1st diffraction minimum of the light spot image coincides with the maximum of another light spot image

Question 19

what is the minimum separation between 2 features in a photomask?

Answer 19

Highest density achieved with equal line and space
(or array of holes with space between holes equal
to the diameter of hole). Below this separation the
lines are not resolved