There has been a growing demand for more precise Mask CD MTT (Critical Dimension Mean to Target) control by
shrinking the semiconductor device. Generally, The CD MTT is determined by patterning process such as writing,
develop, and etch. But, additional CD MTT variation often occurs by cleaning process after patterning process. As a
result, it is important to preserve the CD MTT for minimizing CD variation by cleaning process. The cleaning process of
photomask is becoming more critical for 32nm node and below because the size of defect and SRAF pattern is in the
same range. In order to achieve high first cleaning pass yields, intensive cleaning method depending on media not
physical force is still essential to photomask manufacturing and these cleaning processes bring about considerable CD
MTT change. Therefore, it is necessary to increase the durability of MoSi material of attenuated HTPSM (Half Tone
Phase Shift Mask) by the new surface treatment method.
In this study, we presented the plasma etching technique for Cr strip etch in the 2nd process of the attenuated HTPSM for
minimizing CD variation by cleaning process. Diverse dry etching processes are investigated to improve the durability of
the MoSi patterns. In order to evaluate the surface modification of the MoSi film, surface compositions are analyzed by
XPS (X-ray Photoelectron Spectroscopy), TOF-SIMS (Time of Flight Secondary Ion Mass Spectrometry), and EELS
(Electron Energy Loss Spectroscopy). The variation of CD MTT and optical properties are also evaluated by CD SEM
and AIMS (Aerial Image Measurement System), respectively. The XPS analysis shows that sidewall passivation films
are formed during the main etch process and then modified at the over etch step and additional in-situ O2 plasma
treatment. The concentration of the MoO3 is increased when over etch step and in-situ O2 plasma treatment are added.
The difference of CD shift between initial measurement and 2nd measurement after cleaning process depends on plasma
etching conditions. Consequently, the increase in the peak intensity of MoO3 that is less soluble than that of MoO2 leads
to preserve CD by cleaning chemicals.
Backside defects a few micrometers in size are serious concern in lithography because they can degrade the image
quality on a wafer. It was known that defects attached on the backside affected the printing images on a wafer by locally
altering the partial coherence (σ) and the transmitted intensity of the illumination. The ability to detect and to simulate
their impact of defects on the backside is one of the key components in ensuring quality of photomask.
The purpose of this study is to determine the minimum size of defects on the backside which would be affected
printability in 193nm photolithography. It was investigated to the influence of wafer critical dimension (CD) variation
according to illumination and NA, that of refraction according to defect size.
For this study, a reticle was designed to include line and space patterns, contact patterns and isolated patterns on the front
side. And the type of defects attached on the backside was made of chrome to investigate the relation between
transmittance of backside defects and its printability.
The correlation of measurements made with UV and DUV-based inspection system; simulation performed with a
193nm aerial image measurement system. Besides the allowable size of backside defects was determined using the
criterion of a maximum intensity variation of 10%.
The Aerial Image Measurement Tool (AIMS) can estimate the wafer printability without exposure to wafer by using
scanner. Since measured aerial images are similar with wafer prints, using AIMS becomes normal for verifying issue
points of a mask. Also because mask design rule continues to shrink, defects and CD uniformity are at issues as factors
decreasing mask yield. Occurred defects on a mask are removed by existing mask repair techniques such as
nanomachining, electron beam and focused ion beam. But damages and contaminants by chemical and physical action
are found on the mask surface and contaminants above special size lead to defects on a wafer. So cleaning has been
necessary after repair process and detergency has been important. Before AIMS measurement, cleaning is done to make
same condition with shipped mask, which method brings repeated process - repair and cleaning - if aerial image was
So cleaning effect after the FIB repair is tested by using the AIMS to find the optimized process minimizing the
repeated process and to get similar scanner results. First, programmed defect mask that includes various defect size and
type is manufactured on some kinds of patterns in DRAM device and sub-80nm tech. Next the defects on the
programmed mask are repaired by FIB repair machine. And aerial images are compared after the chemical cleaning,
non-chemical cleaning and without cleaning.
Finally, approximate aerial images to scanner results are taken regardless of cleaning process. It means that residue
originated from repair process doesn't affect aerial images and flexible process is possible between AIMS, repair and
cleaning process. But as the effect of minute particles and contaminations will be increased if pattern size is much
smaller, it needs to reconfirm the effect below the sub-60nm in DRAM device.
In sub-60nm technology node, cleaning process becomes specialized to clear the defects without pattern damage as
decreasing critical particle size to control. While cleaning process has to meet the primary requisite, removal of particle
including organic residue and prevention of particle re-deposition, it should enable to suppress haze phenomena for a
long life of photomask. However, to solve the problem of haze, the chemical materials caused haze seed should be
hardly used and physical force becomes strengthen as the compensation for cleaning efficiency. Unfortunately it brings
about another problem, pattern damage seriously.
In this paper, adequate cleaning conditions which are applicable in sub-60nm technology node are evaluated to meet the
dilemma among three requirements, high cleaning efficiency, and prevention of pattern collapse, and prevention of haze
phenomenon. All cleaning steps in photomask process were set up using only 172nm UV irradiation for degradation of
organic contaminants and deionized water (DI) with acoustic power for particle lift-off. The effect of UV and DI
cleaning on cleaning efficiency and haze phenomena was derived from carrying out chemical and physical analysis
simultaneously. Also, we could quantify the statistical probability of pattern collapse in each of technology node and
layer shape as different condition of megasonic frequency and its power. As a result, it was known that this cleaning
process have various merits to make out dilemma mentioned above, if it satisfies optimized conditions.