• Immersion technology offers resolution enhancement and higher numerical aperture compared with today’s conventional advanced projection lithography.
• Using immersion lithography, the space between the projection lens and the wafer in a tool is filled with a liquid, in this case pure water with a refractive index of 1.43. The technology will extend 193nm exposure tools to the 65nm node and below.
• An immersion liquid between the mask and the objective lens in a CD tool has two major advantages in mask metrology.
• First, immersion allows lens designs with numerical apertures up to 1.20, and therefore allowing improved resolution.
• Second, the linearitiy of the CD tool is clearly enhanced down to 230nm and below.

• Under a "Joint Technology Agreement" with MueTec, Leica has developed a water immersion objective lens with NA of 1.20.
• MueTec integrated this lens in its most advanced, and fully automated mask CD metrology system . MueTec developed a "Water Application Unit" and a "Water Removal Unit" which prohibits contamination on the masks, and solves also the temperature control problems of the tool itself, the mask, and the immersion medium.
• Result is an optical system which clearly resolves contacts down to 160nm and below and lines down to 80nm on proper reticles.
• Tool’s linearity is extended down to 230nm and below in a manner that allows accurate CD measurements.
• These data are not only valid for binary masks but also for ArF (193nm) HT Phase Shifting Masks.

• Mask makers are often driven by their customers to use CD-SEM for CD control. The reason is that usually wafer manufacturers use Electron Microscopes for their daily measurement tasks on wafer level. Here optical metrology on single lines cannot be applied due to the lack of the optic’s resolving power.
• A widely unknown fact is, that a CD-SEM doesn’t measure a CD how it matters in a stepper but only the so called “Top-CD” which is obtained on the surface of the masks. Investigations clearly show the difference between Top-CD and CD results measured in stepper like, transmitted illumination. Quite often an explicit “fingerprint” between the Top-CD results and the results in silicon cannot be found.
• Analyses also show the benefit of optical CD metrology on masks regarding the important long term repeatability in comparison with CD-SEM metrology.
• The claims of CD-SEMs to provide an unachievable and unlimited resolution are now substancially restricted by the water immersion technology.
• Each system has individual pros and cons. Based on to day’s knowledge leading edge mask makers may need both systems in parallel to supervise their mask production in different stages (resist measurements and final CD control).

• Whilst a DUV objektive lens with NA=0.90 reaches its linearity limit at about 300nm, a Water Immersion lens with NA=1.20 measures linearly down to 230nm and below.
• The Water Immersion tool with NA=1.20 achieves on qualified masks the following long term repeatability performance:

     Long Term Performance <M5k> WI (3sd)

CoG Mask	Lines / Spaces	0,2 – 1,0 µ	0,47 nm
	Dots / Holes	0,3 – 1,0 µ	0,85 nm
	Dots / Holes	0,2 µ	1,26 nm

ArF (193nm) Mask	Lines / Spaces	0,2 – 1,0 µ	0,85 nm
	Dots / Holes	0,3 – 1,0 µ	1,35 nm
	Dots / Holes	0,2 µ	1,75 nm