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.
OPC (Optical Proximity Correction) technique is inevitable and getting more complex to resolve finer features on
wafer with existing optical lithography technology. Some SRAFs generated with special model-based OPC engines
are so sophisticated that we can hardly imagine final patterns on wafer simply by seeing patterns on reticle. These
model-based OPCs consist of many kinds of assist features since they are designed differently according to various
target features on wafer and lithographic conditions. Not only small main features but also even smaller and
aggressive SRAFs (Sub Resolution Assist Features) may cause too many false counts and/or nuisance defects during the
reticle inspection, which makes inspection TAT (Turn Around Time) longer and inspection process more laborconsuming.
To improve the inspectability of this sort of complex OPC patterns, appropriate MRC (Mask Rule Check)
rules should be considered. As far as the inspection methods are concerned, several approaches have been
developed, such as TLD (Thin Line Desense), LPI (Lithographic Plane Inspection), and Aerial Image Based
Inspection to relax MRC rules. In this paper, we've compared and analyzed the functionalities of enhanced
inspection methods for complex OPC features of 4x nodes and beyond.
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 shrinks continuously, a reticle inspection is getting harsh and harsh and is now one of the most
critical issues in the mask fabrication process. The reticle inspection process burdens the entire mask process with the
inspectability and detectability problems. Not only aggressive assist features but also small and dense main features
themselves may cause many false detection alarms or nuisance defects, which makes the inspection TAT (Turn-around
Time) longer. Moreover, small and dense patterns inspections always come with the defect detectability issues.
Detectability of a defect in small and dense patterns is usually inferior to the printability of it because of the high MEEF
(Mask Error Enhancement Factor) resulted by those small and dense patterns.
Double Patterning Technology (DPT)<sup></sup> can relief the pattern pitch effectively, therefore, DPT reticle pattern can
have a larger pitch than normal Single Patterning Technology (SPT) reticle. We investigate the effect of this pitch
relaxation of DPT reticle on the inspection process.
In this paper, we compare and analyze the difference of pattern inspectability and defect detectability between DPT
reticles and SPT reticles when they have same size of patterns on them. In addition to these results, we also
investigate the printability of defects in comparison with the detectability and derive the requirement of the inspection
for 4x nodes DPT reticles from the results.
The importance of mask pattern inspection is increased as design node shrinks below. The major reason is as follows.
Firstly, inspection systems have to enhance sensitivity because the high grade devices are seriously affected from small
defects compared with low grades. The other is SRAFs RET masks. In order to inspect SRAF properly, inspection
systems need severer conditions such as small pixel size, short wavelength and special algorithms. Therefore, it takes
more than 3 hours to inspect a mask and this increasing inspection time is a serious burden in mask making process.
Moreover in spite of mask market and its infrastructure, cost of inspection system is too high.
In this paper, the advantages of using Xe-Hg lamp instead of a DUV laser are presented. Special defect algorithms get
over low sensitivity of lamp optics. We have evaluated performances of the defect inspection system with programmed
defect mask and production mask.
The inspection system is cost-effective because the optic part is configured by DUV lamp and fiber optic delivery
system. The fast scanning speed is enough to charge the inspection capacity in the fabrication line. These features of the
system well match with the flexibility of the facility layout in the mask production.