1. Lithography Topics: Wafer exposure systems Photoresists
Manufacturing Methods & Equipment
Measurement Methods

2. Projected Lithography Requirements

4. Projection systems Proximity Printing: Contact Printing:
Mask/wafer separated 5-25 mm
Separation results in poor resolution
Limited to mm-sized features
Both systems require 1X masks
Contact Printing:
Limited diffraction effects
Inexpensive
Contact between mask & resist results in damage – low yield
Oldest & simplest

5. Projection systems Produce high resolution w/o defects
Resolution limited by diffraction effects
Mask is 4X – 5X image size
Sub-micron features
50 wafer/hour throughput

6. Smaller openings mean greater diffraction
Diffraction effects
Light passing through aperture, with an opening ≈l, diffracts, creating a larger image than that on the mask
Light travels as a plane wave in free space. The spherical wavelets combine to for a uniform front. When it passes through an aperture, the waves from the limited number of wavelets superimpose, spreading out in all directions.
Smaller openings mean greater diffraction

7. The light that diffracts at the highest angels carries detailed information about the shape of the aperture, is not collected passing through the focusing lens and is lost.
Projected intensity of light through a circular aperture. Rings around center bright spot result from diffraction
D = diameter of central disk
f = focal length
d = objective lens diameter

8. A Fourier series is used to describe the path of light:
The intensity is given by: I(fx,fy) = |e(fx,fy)|2 = |F[t(x1,y1)]|2

9. Resolution

10. Depth of Field
The Rayleigh criteria for depth of focus states that the path difference for a ray on the center line and one coming from the edge of the aperture should not differ by more than l/4.

11. Example
Estimate the resolution and depth of focus of an excimer laser stepper using KrF light source (l = 248 nm) and NA=0.6 Assume k1 = 0.75 and k2 = 0.5.
Solution:
R = k1*l/NA = 0.75(0.248/0.6) = 0.31 nm
DOF = ± k2*l/NA2 = ±0.5(0.248/(0.6)2) = ±0.34 mm

12. Modulation Transfer Function (MTF)
The MTF is a measure of the quality of contrast between features. As features move closer together, diffraction affects cause their Airy disks to begin to overlap, changing the degree of intensity between the two features.
Generally, a MTF>0.5 is needed. Smaller values limit the minimum feature size

13. Aerial images produced by contact printing (dashed line) proximity, and projection systems. g=0 in contact system and ~25 microns in proximity system.
The quality of the image decreases as the mask is removed from the surface, with gap size g. The image can be calculated when the gap falls between l < g < W2/2 where W = mask opening width
The minimum resolvable feature size is: Wmin ≈ √(lg)

14. Simulated aerial image
Simulated aerial image. Shading corresponds to optical intensity in the aerial image. The black borders correspond to the mask image that is being printed. The exposure system simulated has an NA = 0.43, partially coherent g-line (l = 436nm). Min feature size is 1 mm.
Below: Same example, but wavelength has been reduced to nm and NA has been increased to Note improvement in image quality
Left: same example as above except feature size has been reduced to 0.5 mm. Note the much poorer image quality

15. Light Coherency
A perfect point source creates parallel beams after passing through the condensor lens. A light source with finite size causes light to strike mask at a variety of angles
Coherent
Partially Coherent
A definition of spatial coherence of light sources is:
S = light source diameter/condenser lens diameter = s/d
Or S = NAcondenser optics/NA projection optics
As s becomes more incoherent, MTF degrades for larger features but improves for smaller ones.

16. Typical process for g-line and i-line resists
Photoresists
Resist changes chemical properties when exposed to light
Resist may be positive or negative. In positive resists, the exposed areas dissolve when developed
Resist is poured on to wafer, then wafer is spun. Viscosity is controlled by solvents, which evaporate
Following patterning and developing, resist is baked to increase hardness
Typical process for g-line and i-line resists

17. Photoresists
Sensitivity is a measure of the light required to expose a resist. In Deep UV resists (DUV), this is mJ/cm-2. Lower sensitivity gives higher contrast and increased processing latitude.
The above diagram illustrates how diazoquinone changes when exposed to light. The final molecule (bottom left) is carboxylic acid, which is soluble in common developers (e.g., KOH, NaOH, TMAH (tetramethyl ammonium hydroxide))
Basic structure of diazoquinone, a photo active compound (PAC) in positive resists

18. Sensitivity of these resists is ~ 20-40 mJ/cm-2
As wavelengths get shorter, photoresist has to be reformulated to have the desired reactivity and sensitivity
In DUV resists, incoming photons react with a photo-acid generator (PAG) molecule, creating an acid molecule that acts as a catalyst to make resist molecule soluble. This process can repeat tens or hundreds of times during the post-exposure bake (PEB).
Sensitivity of these resists is ~ mJ/cm-2
This scheme is being exploited in a new generation of resists designed to work at short wavelengths.

19. Properties and Characterization of Resists
Resist is characterized by its contrast and critical modulation transfer function (CMTF)
Contrast is its ability to distinguish light from dark areas in the aerial image
Resists with high contrast can sharpen a poor aerial image
Idealized contrast curves for positive and negative resists
Qo = dose at which exposure begins to have an effect
Qf = dose at which exposure is complete

20. The MTF was used to measure the “dark” versus “light” intensities in the aerial image. A similar quantity is used for the resist, called the Critical MTF. It is the minimum optical transfer function needed to resolve a pattern in the resist.
Example of how the quality of the aerial image and the resist contrast combine to produce the resist edge profile. The left side shows a sharp aerial image and steep resist edges. The example on the right shows a poorer aerial image and the resulting gradual edges on the resist profile.

21. Resist Exposure Issues
Resist coatings are:
Non-uniform thickness
Not exposed simultaneously
Light absorption falls off exponentially with increasing depth: I = Ioexp(-az)
Highly reflective substrates create standing waves that systematically over/underexpose resist, resulting in “wavy” lines:

22. Mask Engineering: Optical Proximity Correction
Because high frequency detail is lost due to diffraction defects and apertures and lenses are round, square details on mask are lost on aerial image (left). OPC adjusts the image on the right to provide greater detail in the projected pattern

23. Mask Engineering: Phase Shifting
In the image at left, diffraction effects result in a loss of contrast where the waves overlap.
On the right, a material is added to the mask to cause the light to shift phase 180o, thereby canceling the effect of overlap.
Phase shifting can be used to improve the quality of the aerial image or to improve the depth of focus of the exposure system at constant resolution by using a lower NA system

24. Wafer Exposure Systems
The schematic at left illustrates a scanning system for transferring information to the wafer. This requires a 1X mask.
Modern systems use a 5X (or larger mask) and combine scanning with a stepper to systematically cover small portions of the wafer.

25. Techniques to improve image quality
In the Kohler illumination system (left), light is focused at the entrance pupil of the projection lens, which captures the diffracted light from any features on the mask equally well
Off-axis illumination captures some of the higher-order diffracted light which was lost in the normal illumination process

26. Typical mask flaws include:
Measurement Methods
Mask inspection measurements read the mask and compare it to another mask or a mask in a design database
Typical mask flaws include:
Opaque defects: Cr where it should not be
Clear defects: Cr isn’t where it should be
The actual size of the feature on the mask, which is influenced by the size of the e-beam spot used to write the mask (typ. ~0.125 mm)
Proximity effects from electron backscattering in the resist resulting in distortion

27. Test Structures
This is an example of a typical test structure built into the edges of a chip/sheet.
It is used to extract the sheet resistance
The geometry is chosen to define one square of the material (labeled rs)
A current, I5-6 is applied, and voltage, V3-4, is read at terminals 3 & 4. The sheet resistance is then:
Given the sheet resistance, the line width can be calculated from: