In the relentless pursuit of device miniaturization and sustainable yield performance, resolution enhancement techniques
(RET) such as optical proximity correction (OPC) and sub-resolution assist feature (SRAF) are identified as enabling
technologies that fuel the industry. The introduction of advanced reticles, however, considerably augments the mask
error enhancement factor (MEEF) where the growth of progressive defects or haze is accelerated by repeated laser
exposure, and continues to be a source of reticle degradation threatening device yield. Previous investigations have
identified ammonium sulfate, cyanuric acid and ammonium oxalate as the primary and most concerning species found in
both mask shop and wafer fabs.
In this work, magnesium sulfate is used to emulate crystal growth due to its identical optical properties to ammonium
sulfate. A technique has been developed to deposit magnesium sulfate of varying concentrations onto chemically cleaned
reticle surfaces. These defects are then inspected with a high resolution reticle inspection system enabled with MEEF
detector Litho3. Upon inspection, defects are classified and analyzed with respect to their location relative to device
geometry, optical transmission loss as well as the residing surface. Ammonium oxalate crystals are also deposited
separately onto reticle surface to comprehend the impact of crystal type and population on defect printability.
Compositional analysis are carried out using Raman spectroscopy and time-of-flight secondary ion mass spectroscopy
(TOF-SIMS) to correlate the amount of magnesium sulfate and ammonium oxalate crystals with transmission loss. Such
emulation study of various crystal formulation mimics progressing stages of crystallization and allows a mechanistic
understanding of crystal congregation, transmission loss and defect printability.
Readiness of new mask defect inspection technology is one of the key enablers for insertion & transition of the next
generation technology from development into production. High volume production in mask shops and wafer fabs
demands a reticle inspection system with superior sensitivity complemented by a low false defect rate to ensure fast
turnaround of reticle repair and defect disposition (W. Chou et al 2007).
Wafer Plane Inspection (WPI) is a novel approach to mask defect inspection, complementing the high resolution
inspection capabilities of the TeraScanHR defect inspection system. WPI is accomplished by using the high resolution
mask images to construct a physical mask model (D. Pettibone et al 1999). This mask model is then used to create the
mask image in the wafer aerial plane. A threshold model is applied to enhance the inspectability of printing defects. WPI
can eliminate the mask restrictions imposed on OPC solutions by inspection tool limitations in the past. Historically,
minimum image restrictions were required to avoid nuisance inspection stops and/or subsequent loss of sensitivity to
defects. WPI has the potential to eliminate these limitations by moving the mask defect inspections to the wafer plane.
This paper outlines Wafer Plane Inspection technology, and explores the application of this technology to advanced
reticle inspection. A total of twelve representative critical layers were inspected using WPI die-to-die mode. The results
from scanning these advanced reticles have shown that applying WPI with a pixel size of 90nm (WPI P90) captures all
the defects of interest (DOI) with low false defect detection rates. In validating CD predictions, the delta CDs from WPI
are compared against Aerial Imaging Measurement System (AIMS), where a good correlation is established between
WPI and AIMSTM.
Ever-tightened design rules and ensuing aggressive OPC features pose significant challenges for wafer fabs in the pursuit
of compelling yield and productivity. The introduction of advanced reticles considerably augments the mask error
enhancement factor (MEEF) where progressive defects or haze, induced by repeated laser exposure, continue to be a
source of reticle degradation threatening device yield. High resolution reticle inspection now emerges as a rescue venue
for wafer fabs to assure their photomask integrity during intensive deep UV exposure. Integrated in the high resolution
reticle inspection, a MEEF-driven lithographic detector "Litho3" can be used run-time to group critical defects into a
single bin. Previous investigations evinced that critical defects identified by such detector were directly correlated with
defects printed on wafer, upon which fab users can make cogent decisions towards reticle disposition and cleaning
therefore reduce cycle time.
One of the challenges of implementing such detector resides in the lengthy set up of user-defined parameters, from
practitioner standpoint, can considerably extend reticle inspection time and inevitably delay production. To overcome
this, an automatic simulation program has been written to optimize Litho3 settings based off a pre-inspection in which
only default Litho3 values are needed. Upon completion of the pre-inspection, the images are then scanned and
processed to extract the optimal Litho3 parameters that are largely dependent upon the feature size characteristics and
local MEEF. Thus optimized Litho3 parameters can then be input into the recipe set up to enable a real-time inspection,
as such fab user can timely access the defect criticality information for subsequent defect disposition. In the interest of
printability validation, such defect information and associated coordinates can be passed onto defect review via XLINK
for further analysis. Corresponding MEEF values are also available for all identified critical defects. Through this
automatic program the set up time for Litho3 can be reduced by up to 90%.
For high capacity production fabs running a pre-inspection is deemed infeasible; this automatic optimization program
can also serve as a direct interpretation of any regular reticle inspection even without invoking Litho3 set up, yet in the
end provide output in the context of defect criticality. Results acquired from this program were found in good accordance
with those from the real-time Litho3 inspection, for both critical and non-critical layers of 90 nm design node. Such
capability allows detailed study of defect criticality in relation to its size, defect optical transmittance, residing surface,
its proximity to a printing pattern as well as lithography parameters such as NA and sigma. Furthermore, coupling this
automatic program with high resolution inspection also assists in determining lithography process window and an indepth
comprehension of defect progression mechanism.
The advent of device miniaturization necessitates sub-half-micron features delineated on reticles where photomask quality, more so than ever, exerts remarkable yield impact on 65 nm node and below. The introduction of advanced reticles considerably augments the mask error enhancement factor (MEEF) in the non-linear regime ensuing aggressive OPC features. The increased MEEF leads to tightened defect capture criteria, in which many of the previously
insignificant defects become of interest and may have substantial yield impact. To provide desired sensitivity, a high resolution inspection is a must; it also effectively monitors mask reliability. However, the productivity of such inspection greatly depends on defect disposition efficacy in sorting out critical defects from the large population detected on contaminated masks [1-3].
Anchoring high resolution reticle inspection, wafer fabs are in a relentless pursuit of optimal defect disposition method to meet the throughput demand. In particular, progressive defects or haze, induced by repeated laser exposure, continue to be a source of reticle degradation threatening device yield. Early detection of these defects to circumvent the printability impact becomes vitally important yet challenging. In addition to its size, the defect criticality also largely depends upon defect optical transmittance, residing surface, its proximity to a printing pattern as well as lithography parameters such as NA and sigma [4-6].
A MEEF-driven lithographic detector named "Litho3" has been designed that can be used run-time during mask inspection to effectively group the critical defects into a single bin based on their potential yield impact. The coordinates of these critical defects, identified by the above Litho3 detector, can then be transferred from reticle to wafer and subsequently subject to printability validation, upon which defective sites can be analyzed thoroughly on reticle or wafer review tools. Such capability reduces inspection cycle time by improving defect disposition efficacy, also assists in
determining lithography process window and a further comprehension of defect progression mechanism.