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MICROLITHOGRAPHY

MICROLITHOGRAPHY 9.25

∆CD—reduce the magnitude of the individual process errors (∆v ), or reduce the response of CD to
i
that error (∂CD/∂v ). The separation of CD errors into these two source components identifies the
i
two important tasks that face the photo engineer. Reducing the magnitude of process errors is gen-
erally considered a process control activity and involves picking the right material and equipment for
the job and ensuring that all equipment is working in proper order and all materials are meeting their
specifications. Reducing the process response is a process optimization activity and involves picking
the right process settings, as well as the right equipment and materials. Often, these two activities are
reasonably independent of each other.
A note of caution—the derivation of Eq. (9.25) assumed that the process errors were small
enough to be linear and independent in their influence on CD. This will not always be the case in a
real lithographic process. One needs only to consider the two variables of focus and exposure to see
that the response of CD is certainly nonlinear (it is approximately quadratic with focus) and the two
variables are highly dependent on each other. Usually, a linear expansion such as that of Eq. (9.25)
is most useful as a guide to understanding rather than as a computational tool.

9.5 THE LIMITS OF OPTICAL MICROLITHOGRAPHY

To date, optical lithography has been the technology of choice of IC manufacturing. What are the
limits of this lithographic approach? The resolution limit as described in Eq. (9.1) outlines obvious
challenges. Lower wavelengths require expensive, unproven materials (superpure fused silica and
fluoride salts, for example, are required if the wavelength is reduced to 157 nm). Higher numerical
apertures result in increasing aberrations, which can only be reduced by more complicated designs
and more exacting lens manufacturing processes.
To make the situation worse, any improvement in resolution is always accompanied by a decrease
in depth of focus (DOF). According to the Rayleigh criterion, the DOF for small features should
decrease as the feature size squares. In reality, empirical results have shown the DOF to decrease, as
about the feature size, to the first power (due to improvements in photoresists and other factors).
Today’s 100 nm features typically have a DOF of a few tenths of a micron. An important require-
ment of any improvement in the “practical” resolution of an imaging system is the ability to live
within the confines of this reduced DOF. Improvements in wafer and mask flatness, autofocusing and
autoleveling systems, and wafer planarization by chemical-mechanical polishing of the wafer are
examples of how the industry is coping with reduced DOF.
Attempts to simultaneously improve the resolution and DOF by optical means—sometimes
called optical “tricks”—include (Fig. 9.16)

• Optimization of the mask pattern shape, called optical proximity correction (OPC)
• Optimization of the angles of light illuminating the mask, called off-axis illumination (OAI)
• Adding phase information to the mask in addition to intensity information, called phase shifting
masks (PSM)
• Control of the polarization of the illumination

Collectively, these optical approaches are known as resolution enhancement technologies
(RETs).
The limits of optical lithography may be further pushed with immersion lithography. By replac-
ing the air between the lens and the substrate with a higher index fluid, it is possible to design and
build a lens with a numerical aperture greater than 1.0. At 193 nm, water has very low absorption
and an index of refraction of 1.44, thus increasing the maximum theoretical NA from 1.0 to 1.44. It
seems likely that numerical apertures up to 1.3 will one day be available. These hyper NAs in con-
junction with aggressive RETs should enable 193 nm lithography to extend to resolutions of 45 nm
and below.

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