Phase-shift mask
Encyclopedia
Photomask
A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.-Overview:...
s that take advantage of the interference generated by phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
differences to improve image resolution
Image resolution
Image resolution is an umbrella term that describes the detail an image holds. The term applies to raster digital images, film images, and other types of images. Higher resolution means more image detail....
in photolithography
Photolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
. There exist alternating and attenuated phase shift masks.
A conventional photomask
Photomask
A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.-Overview:...
is a transparent plate with the same thickness everywhere, parts of which are covered with non-transmitting material in order to create a pattern on the semiconductor wafer
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...
when illuminated.
In alternating phase-shift masks, certain transmitting regions are made thinner or thicker. That induces a phase-shift in the light traveling through those regions of the mask (see the illustration on the left). When the thickness is suitably chosen, the interference of the phase-shifted light with the light coming from unmodified regions of the mask has the effect of improving the contrast
Contrast ratio
The contrast ratio is a property of a display system, defined as the ratio of the luminance of the brightest color to that of the darkest color that the system is capable of producing...
on some parts of the wafer, which may ultimately increase the resolution on the wafer. The ideal case is a phase shift of 180 degrees, which results in all the incident light being scattered. However, even for smaller phase shifts, the amount of scattering is not negligible. It can be shown that only for phase shifts of 37 degrees or less will a phase edge scatter 10% or less of the incident light.
Attenuated phase-shift masks are already extensively used, due to their simpler construction and operation, particularly in combination with optimized illumination for memory patterns. On the other hand, alternating phase-shift masks are more difficult to manufacture and this has slowed their adoption, but their use is becoming more widespread. For example, the alternating phase-shift mask technique is being used by Intel to print gates for their 65 nm and subsequent node transistors.
While alternating phase-shift masks are a stronger form of resolution enhancement than attenuated phase-shift masks, their use has more complex consequences. For example, a 180 degree phase edge or boundary will generally print. This printed edge is usually an unwanted feature and is usually removed by a second exposure.
A benefit of using phase-shift masks in lithography is the reduced sensitivity to variations of feature sizes on the mask itself. This is most commonly used in alternating phase-shift masks, where the linewidth becomes less and less sensitive to the chrome width on the mask, as the chrome width decreases. In fact, even with no chrome the phase edge can still print, as noted above. Some cases of attenuated phase-shifting masks also demonstrate the same benefit (see left).
As phase-shift masks are applied to printing smaller and smaller features, it becomes more and more important to model them accurately using rigorous simulation software, such as Panoramic Technology or Sigma-C. It becomes especially important as the mask topography starts to play an important role in scattering the light, and the light itself starts to propagate at larger angles. The performance of phase-shift masks can also be previewed with the use of aerial image microscopes. Defect inspection remains a critical aspect of phase-shift mask technology, as the set of printable mask defects has expanded to include those with phase effects in addition to conventional transmission effects.