Introduction
Immersion lithography has its roots in the proven technology of immersion microscopy. Already in the 19th century scientists from Carl Zeiss made use of a principle first found and described by the Italian physicist Giovanni Battista Amici (1786 – 1863). A thin liquid film between the final lens element from the microscope objective and the speciman itself improves depth of focus and allows the construction of objectives with a Numerical aperture higher than 1 – thus increasing resolution.
Principles of Technology
Using an immersion fluid between the wafer and the lens substantially changes the light path, resulting in two benefits. First, it enhances depth of focus (DOF) for a given Numerical Aperture. Second, immersion allows lens designs with Numerical Apertures significantly larger than 1.0, therefore allowing improved resolution.
NA<1: Gain in DoF due to the smaller angles in water.
NA>1: Only feasible for immersion. Dry lenses would show total reflection at last surface. With immersion lenses smaller CDs can be resolved. Resolution enhancement !
Starlith 1150i – the lens that changed the roadmap of optical lithography
The principal of immersion has been adopted by a group of scientists at Carl Zeiss SMT AG for the development of a lithography lens, capable of immersion lithography. For several reasons the illumination wavelength of 193 nm and purified water as immersion liquid have been chosen. Starting point for the development was the already proven Starlith® 1150 lithography lens with a Numerical aperture of 0.75 and a resolution of 90 nm. Starting the development in Summer 2002 it only took a few months until the Starlith® 1150i, the very first fullfield immersion lens was presented, displaying identical technical data compared to to the "dry" version of the lens. Already in Summer 2003 the Starlith® 1150i lens was installed at an ASML immersion pilot tool the AT:1150i. Feasibility of immersion lithography was demonstrated during the following months. The encouraging results led to an unforeseen change in the roadmap of optical lithography. While only two years ago, 157nm lithography (F2) was the technology of choice to close the resolution gap between 193nm "dry" lithography and EUV-Lithography, 193nm-immersion-lithography has completely replaced F2 – technology and is targeted to at least 45 nm resolution.
In an exposure series, the wafer was deliberately moved along the optical axis away from the ideal position. While in "dry lithography" (above) the features can no longer be clearly imaged with defocusing of only 0.3µm , defocusing of +/- 0,5 µm is uncritical with the immersion version of the same type of objective lens (below). This clearly shows the increase in Depth of Focus (DoF) with an immersion liquid.
Starlith 1250i – capable of volume production
At Semicon West Show 2004, Carl Zeiss SMT has then presented its newly developed Starlith® 1250i objective. It is the first lens for immersion lithography at 193 nanometer exposure wavelength capable of volume production. The lens will be incorporated into ASML's TWINSCAN™ XT:1250i , a 193 nm pre-production lithography scanner. The TWINSCAN™ platform combines the improved depth of focus of immersion tools with the precision of "dry" lithography systems. With the Starlith® 1250i lens, 193 nm immersion-lithography takes an important step towards volume production. The lens offers an NA of 0.85 and is specified with a resolution down to 70nm. The main advantage of this lens is an increase in depth of focus of about 50% compared to the "dry" version, resulting in significantly enlarged process latitudes during exposure. That lens will enable chip manufacturers to develop their production processes for immersion lithography, thus giving them substantial competitive advantage.
Starlith® 1400i
Starlith® 1400i was the first step in the further development of immersion lithography for the exposure wavelength 193 nm. With a NA of 0.93, it achieves a resolution of 65 nm. It offers an even higher depth of field than its counterpart, the "dry" system Starlith® 1400, of which over 100 systems have already been delivered to ASML.
Starlith® 1700i
Starlith® 1700i crosses the NA 1.0 threshold for the first time. With a NA of 1.2 – with a full image field - it offers a 30% higher NA and therefore a 30% higher resolution than its predecessor. As a result, it enables the economical volume production of microchips featuring 45 nm structures.
 Read More
With the Starlith® 1700i, Carl Zeiss SMTnumbers among the five finalists in the “Large Companies“ category of the "Innovation Award of German Industry" in 2006.
 Bild Download (JPG, 857 kB)
 Top |  Introduction
Principles of Technology
Starlith 1150i
Starlith 1250i
Starlith 1400i
Starlith 1700i
Cooperation agreement
International SEMATECH teams with Carl Zeiss SMT |