EUVL (Extreme Ultraviolet Lithography) for semiconductor processing

1. Introduction: Resolution

Resolution

▶ The achievable, repeatable minimum feature size (Critical Dimension)
▶ Determined by the wavelength of the light and the numerical aperture of the system.
▶ The resolution can be expressed as:
(1)
where k1 = the system constant, l is the wavelength of the light, NA = the numerical aperture of the lens
▶ Increase NA
→ Larger lens, could be too expensive and unpractical
→ Reduce DOF (Depth of Focus) and cause fabrication difficulties
▶ Reduce wavelength
→ UV to DUV, to EUV, and to X-Ray
→ Need develop light source, photoresist, exposure system, and equipment
▶ Reduce k1
→ Phase shift mask

To improve resolution!

1. Introduction: Depth of Focus

Depth of Focus
▶ The range that light is in focus and can achieve good resolution of projected image
▶ Depth of focus can be expressed as:
(2)
where k2 = the system constant, l is the wavelength of the light, NA = the numerical aperture of the lens

Fig. 4. Illustration of DOS (depth of focus) and COS (center of focus)

2. Current status: ITRS of Lithography 2010 edition