1

1. Introduction and application.
2. Light source and photomask, alignment.
3. Photolithography systems.
4. Resolution, depth of focus, modulation transfer function.
5. Other lithography issues: none-flat wafer, standing wave...
6. Photoresist.
7. Resist sensitivity, contrast and gray-scale photolithography.
8. Step-by-step process of photolithography.

Chapter 5 Lithography

NE 343: Microfabrication and thin film technology

Instructor: Bo Cui, ECE, University of Waterloo; http://ece.uwaterloo.ca/~bcui/

Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin

2

Light diffraction through an aperture on mask

3

Three basic methods of wafer exposure

High resolution. But mask wear, defect generation.

Less mask wear /contamination, less resolution (depend on gap).

Fast, simple and inexpensive, choice for R&D.

No mask wear/contamination, mask de-magnified 4× (resist features 4× smaller than mask). Very expensive, mainly used for IC industry.

4

For g=10μm, λ=365nm

W_min ≈ 2 μm

Near field/Fresnel diffraction for contact/proximity exposure

• Interference effects and diffraction result in “ringing” and spreading outside the aperture.
• Edges of image rise gradually (not abrupt) from zero.
• Intensity of image oscillates about the expected intensity.
• Oscillations decay as one approaches the center of the image.
• The oscillations are due to constructive and destructive interference of Huygen’s wavelets from the aperture in the mask.
• When aperture width is small, the oscillations are large
• When aperture width is large, the oscillations rapidly die out, and one approaches simple ray tracing when aperture >> λ.

(t is resist thickness)

Near field:

(g is gap)

Figure 5.14

5

Far field: W² << λ(g²+r²)^1/2, r is position on the wafer.

Sharp maximum intensity at x=0, and intensity goes through 0 at integer multiples of one-half number.

Far field/Fraunhofer diffraction for projection exposure

Far field

Near field

Figure 5.15

6

Lens capturing diffracted light

Large lens captures more diffracted light, and those higher order diffracted light carries high frequency (detail of fine features on mask) information.

7

Numerical aperture of a lens

Numerical aperture (NA) of an optical system is a measure of the ability of the lens to collect light.

NA ≡ nsinα, n is refractive index for the medium at the resist surface (air, oil, water).

For air, refractive index n=1, NA = sinα ≈ (d/2)/f ∝ d for small α.

8

Effect of numerical aperture on imaging

Large lens

Small lens

9

Light diffraction through a small circular aperture

http://en.wikipedia.org/wiki/Airy_disk

Figure 5.7 Image intensity of a circular aperture in the image plane.

Figure 5.6 Qualitative example of a small aperture being imaged.

10

Resolved images

Unresolved images

Lord Rayleigh

Rayleigh criteria for resolution

Rayleigh suggested that a reasonable criterion for resolution is that the central maximum of each point source lie at the first minimum of the Airy pattern.

Strictly speaking, this and next slides make sense only for infinitely far (>>f) objects, like eye. Fortunately, 4x reduction means far object, and near (near focal plane) image.

Figure 5.8

11

Rayleigh criteria for resolution R

To increase resolution, one can:

Increase NA by using large lens and/or immersion in a liquid (n>1).

Decrease k₁ factor (many tricks to do so).

Decrease λ (not easy, industry still insists on 193nm).

K₁ factor has no well-defined physical meaning.

It is an experimental parameter, depends on the lithography system and resist properties.

12

Effect of imaging/printing conditions

Annular means an “off-axis illumination” method, which is one trick to decrease k₁.

EUV: extreme UV, here wavelength 13.5nm. Immersion means exposure in water.

A small aperture was used to ensure the foreground

stones were as sharp as the ones in the distance.

What one need here is a telephoto lens at its widest aperture.

Depth of focus (DOF)

DOF for photography

Small DOF

(background blurred)

Large DOF

DOF is the range in which the image is in focus and clearly resolved.

14

Rayleigh criteria for depth of focus (DOF)

Rayleigh criteria: the length of two optical paths, one on-axis, one from lens edge or limiting aperture, not differ by more than λ/4.

For small θ

On axis, optical path increased by OC-OB=δ.

From edge, increased by AC-AB=DC=δcosθ.

At point B (focal point), two branches have equal path.

D

Again, like the case of resolution, we used k₂ factor as an experimental parameter. It has no well-defined physical meaning.

Figure 5.9

15

Depth of focus for projection photolithography

• It can be seen that larger NA gives smaller depth of focus!
• This is also true for camera. A cheap camera takes photos that are always in focus no matter where the subject is, this is because it has small lenses.
• This of course works against resolution where larger NA improves this property.
• In order to improve resolution without impacting DOF too much, λ has been reduced and “optical tricks” have been employed.

Large lens (large NA), small DOF

Small lens (small NA), large DOF

16

Optimal focal plane in photolithography

• Light should be focused on the middle point of the resist layer.
• In IC, DOF is << 1μm, hard to focus if wafer is not super flat.
• People talks more of resolution, but actually DOF can often be a bigger problem than resolution.
• For example, a 248nm (KrF) exposure system with a NA = 0.6 would have a resolution of ≈0.3μm (k₁ = 0.75) and a DOF of only ≈ ±0.35μm (k₂ = 0.5).

17

Modulation transfer function is another useful concept.

It is a measure of image contrast on resist.

Modulation transfer function (MTF)

Figure 5.10

18

MTF and spatial coherence

Usually MTF > 0.5 is preferred.

It depends on λ, light source size (coherency), and optical system.

It certainly also depends on feature size (or period for a grating pattern).

Spatial coherence of light source

Point source is coherent

Partially

coherent

• Coherent light will have a phase to space relationship.
• Incoherent light or light with only partial coherence will have wave-fronts that are only partially correlated.
• Spatial coherence S is an indication of the angular range of light waves incident on mask, or degree to which light from source are in phase.
• Small S is not always good (see next slide).

Figure 5.12

Plane wave

19

MTF and spatial coherence

For a source with perfect spatial coherence S=0, MTF drops abruptly at Rayleigh criterion W=half pitch=R=k₁λ/NA.

Large S is good for smaller features, but bad for larger ones.

Trade-off is made, and industry chooses S=0.5-0.7 as optimal.

MTF vs. diffraction grating period on mask.

W = line width = space width of the grating.

X-axis of the plot: spatial frequency ν=1/(2W), normalized to Rayleigh criterion cutoff frequency ν₀=1/R=NA/(0.61λ).

Grating photomask

Large features Smaller features

(similar to Figure 5.13)

20

1. Introduction and application.
2. Light source and photomask, alignment.
3. Photolithography systems.
4. Resolution, depth of focus, modulation transfer function.
5. Other lithography issues: none-flat wafer, standing wave...
6. Photoresist.
7. Resist sensitivity, contrast and gray-scale photolithography.
8. Step-by-step process of photolithography.

Chapter 5 Lithography

NE 343 Microfabrication and thin film technology

Instructor: Bo Cui, ECE, University of Waterloo

Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin

21

Exposure on patterned none-flat surface

This leads to random reflection/proximity scattering, and over or under-exposure.

Proximity scattering

Both problems would disappear if there is no reflection from substrate.

22

Exposure on patterned none-flat surface

To reduce the problem, one can:

• Use absorption dyes in photoresist, thus little light reaches substrate for reflection.
• Use anti-reflection coating (ARC) below resist.
• Use multi-layer resist process (see figure below)
1. thin planar layer for high-resolution imaging (imaging layer).
2. thin develop-stop layer, used for pattern transfer to 3 (etch stop).
3. thick layer of hardened resist (planarization layer).

23

Surface reflection and standing wave

• Resist is partially reflective, so some light reaches resist bottom and is reflected.
• Constructive and destructive interference between incident and reflected light results in a periodic intensity distribution across the resist thickness.
• With change in exposure (light intensity) comes change in resist dissolution rate, leading to zigzag resist profile after development.
• Use of anti-reflecting coating (ARC) eliminates such standing wave patterns.
• Post exposure bake also helps by smoothing out the zigzag due to resist thermal reflow.
• (Also due to reflection, a metal layer on the surface will require a shorter exposure than exposure over less reflective film.)

Figure 5.24

24

Standing wave effect on photoresist

Is this a positive or negative resist?

n_PR is refractive index of photoresist

25

Position of minimum and maximum intensity

Maximum when optical path difference between incident and reflected beams is mλ.

There may be a 180^o phase shift when light is reflected at the resist/substrate interface, thus it is minimum (rather than maximum) when x=d.

Positive resist