We report inspection results of early 22-nm logic reticles designed with both conventional and computational
lithography methods. Inspection is performed using a state-of-the-art 193-nm reticle inspection system in the reticleplane
inspection mode (RPI) where both rule-based sensitivity control (RSC) and a newer modelbased
sensitivity control (MSC) method are tested.
The evaluation includes defect detection performance using several special test reticles designed with both conventional
and computational lithography methods; the reticles contain a variety of programmed critical defects which are
measured based on wafer print impact. Also included are inspection results from several full-field product reticles
designed with both conventional and computational lithography methods to determine if low nuisance-defect counts can
be achieved. These early reticles are largely single-die and all inspections are performed in the die-to-database
inspection mode only.
A new methodology - Aerial Plane Inspection (API) - has been developed to inspect advanced photomasks
used for the 45 nm node and beyond. Utilizing images from a high resolution mask inspection system, a
mask image is recovered by combining the transmitted and reflected images. A software transformation is
then performed to replicate the aerial image planes produced in a photolithography exposure system. These
aerial images are used to compare adjacent die in a Die-Die inspection mode in order to find critical defects
on the photomask. The mask recovery process and modeling of the aerial plane image allows flexibility to
simulate a wide range of lithographic exposure systems, including immersion lithography. Any source
shape, Sigma, and numerical aperture (NA) can be used at all common lithographic wavelengths.
Sensitivity of the inspection can be fully adjusted to match photomask specifications for CD control, lineend
shortening, OPC features, and for small and large defective areas. An additional adaptive sensitivity
option can be utilized to automatically adjust sensitivity as a function of MEEF.
Using the Aerial Plane Inspection to compare pattern images has the benefit of filtering out non-printing
defects, while detecting very small printing defects. In addition, defects that are not printing at ideal
exposure condition, but may be reducing the lithographic process window, can also be detected.
Performing defect detection at the aerial image plane is more tolerant to small Optical Proximity Correction
(OPC) sub-resolution assist features (SRAFs) that are difficult to inspect at the reticle image plane.
Inspection of aggressive Optical Proximity Correction (OPC) designs, improvement of usable sensitivity,
and reduction of cost of ownership are the three major challenges for today's mask inspection
methodologies. In this paper we will discuss using aerial-plane inspection and wafer-plane inspection as
novel approaches to address these challenges for advanced reticles.
Wafer-plane inspection (WPI) and aerial-plane inspection (API) are two lithographic inspection modes.
This suite of new inspection modes is based on high resolution reflected and transmitted light images in the
reticle plane. These images together with scanner parameters are used to generate the aerial plane image
using either vector or scalar models. Then information about the resist is applied to complete construction
of the wafer plane image. API reports defects based on intensity differences between test and reference
images at the aerial plane, whereas WPI applies a resist model to the aerial image to enhance discrimination
between printable and non-printable defects at the wafer plane.
The combination of WPI and API along with the industry standard Reticle Plane Inspection (RPI) is
designed to handle complex OPC features, improve usable sensitivity and reduce the cost of ownership.
This paper will explore the application of aerial-plane and wafer-plane die-to-die inspections on advanced
reticles. Inspection sensitivity, inspectability, and comparison with Aerial Imaging Measurement System
(AIMSTM) or wafer-print-line will be analyzed. Most importantly, the implementation strategy of a
combination of WPI and API along with RPI leading-edge mask manufacturing will be discussed.
As the design rule continues to shrink towards 3x nm and below, lithographers are searching for new and
advanced methods of mask lithography such as immersion, double patterning and extreme ultraviolet
lithography (EUVL). EUV lithography is one of the leading candidates for the next generation lithography
technologies after 193 nm immersion and many mask makers and equipment makers have focused on
stabilizing the process. With EUV lithography just around the corner, it is crucial for advanced mask makers
to develop and stabilize EUV mask processes. As a result, an inspection tool is required to monitor and
provide quick feedback to each process step.