At 32nm technology node and beyond, the number of defect to be repaired is increased because pattern size is shrunk
and the detecting ability of inspection system higher than before. In repair system, mask surface is exposed to the
various contaminations such as contamination from vacuum chamber wall, reaction gas for repair etc. Although
contaminations deposited on mask surface are removed by followed cleaning process, it makes reflectance change on
scan area detected by high resolution inspection system. This reflectance change on scan area in repair system detected
during inspection is big burden for mask making because the number of scan area requires more time to confirm and
need AIMS simulation if there is any issue on the area. Most of all, it is hard to find where the exact repaired pattern is
and verify whether there are no problems because inspection system does not detect exact repaired pattern but detect all
scan area. Especially, this phenomenon is more observed to MoSi absorber of OMOG mask.
In this paper, we demonstrate the findings of contamination source and the root cause of contamination using surface
analyzing methods, ToF-SIMS(Time of Flight-SIMS) and AFM(Atomic Force Microscope). In addition, preventive
strategy to minimize and remove reflectance change issue in repair system will be discussed.
Scanner expose condition is simulated to reproduce Chrome (Cr) migration on COG mask, known as a new type of mask
degradation, with high energy fluency ArF acceleration tool. Contact hole size is decreased by forming rim inside
sidewall of contact hole. CD change is observed as expose energy accumulates. Bridge defect is found on the unexposed
line end feature adjacent to grounded pattern. Grounded mask showed much worse Cr migration than not grounded one.
It may be analogized that the driving forces of Cr migration is relatively higher ArF energy (6.8eV) than self diffusion
activation energy of Cr (3.5eV). Isolated exposed pattern will be positively charged by photovoltaic effect. Electric field
potential with adjacent ground pattern formed by photovoltaic effect possibly boosts the Cr migration and material loss.
New prevention treatment has been developed to mitigate Cr migration and it worked quite well. Cr migration by ArF
expose wasn't found even grounded condition.
In mask manufacturing process, some soft defects generated through co-interaction of dry etch and PR coating are hard
to be removed in the conventional cleaning or repair process. It is on MoSi layer with smooth surface (lower roughness
than MoSi), very thin and higher transmittance than MoSi film, it looks like half-tone pin-hole. Also, the defects are hard
to detect in the conventional PSM inspection tool because of its thin and higher transmittance. In this paper, the root
cause and control method of dry etch related half-tone pin-hole like soft defect is studied.
High PRE (Particle removal efficiency) and damage free cleaning became main cleaning challenges over haze prevention
in photomask industry, nowadays. SRAF (Sub-resolution assist feature) size became small down below 0.1um as pattern
size become small. Acoustic frequency and power is the main parameter to increase PRE in photomask cleaning. 1 MHz
of acoustic frequency was good enough to remove particles and soft defects until recently. But it has shown pattern
damages for SB (Scattering bar) size of below 0.1um unfortunately.
In this paper, we optimized photomask cleaning process to achieve high PRE and low pattern damage. Its haze
prevention capability and cycle cleaning durability was verified with in-house-built HATB and AIMS, respectively.
Although sulfate free cleaning has reduced number of residual ions on mask surface drastically, the lifetime of
photomask has improved marginally. New haze generation mechanism in sulfate free cleaning has been studied and
evaluated based on surface properties of photomask thin film materials. It was found that haze generation is co-related
with substrate surface properties as well as ionic re-combination under ArF illumination. Based on the haze generation
study, the surface modification treatment has been studied and investigated in the view of surface energy. The surface
modification treatment increases storage lifetime as well as cumulative haze threshold energy in wafer shops.
A lot of research has been carried on sulfate free cleaning process to minimize haze generating residual ions on mask
surface. However sulfate free cleaned mask has been suffered from short life time of haze generation than we expected,
because pellicle outgassing combines with ammonium residuals and formed haze near pellicle frame area and decrease
yield. Therefore physical and PKL developed chemical surface modification treatment was studied and evaluated in term
of near pellicle haze threshold energy, surface energy of mask substrate components (Qz/MoSi/Cr), AFM and AES depth
profile. Dehydration bake treatment (physical surface modification treatment) and PKL developed chemical treatment
increased near pellicle haze threshold energy by 2.5 and 4 times, respectively. Surface modification treatments didn't
show negative effect on phase angle and transmittance losses of ArF EAPSM mask. The effect of illumination sources on
surface modification treatment was also studied.
ArF exposure tool have been implementing as a main work force of lithography. And haze generation by high actinic
wavelength energy is big issue to be resolved. Many studies have been reported to remove or minimized ion residual on
photomask surface and PKL developed haze free process. Even though the surface of photomask is free from ions
generating haze defect by haze free process, but the ions from environment like pellicle and packing box make worsen to
keep cleanness of photomask. The evaluation of environment effect like outgas from pellicle and packing box have been
reported, but it was hard to know pure environmental effect because the surface of photomask was not enough clean to
Several pellicles and boxes with different material from supplies were tested in terms of outgas, contamination of ion and
threshold energy generating haze. Some material of packing box and pellicle showed very sensitive to keep haze free
For high quality products in the semiconductor and photomask industries, exposure wavelength has been shortening
from i-line to ArF to embody the high resolution as critical dimension (CD) shrinkage and the specifications have been
restricted. However, a new defect issue called haze has appeared that is shortening the wavelength. This defect is caused
by the photoreaction of chemical residues exposed to SO<sub>4</sub>
<sup>+</sup> and other chemicals. Accordingly, in this paper we
investigated the generation of haze in thin film materials.
For fabrication of various thin films, the materials which were metal, compound material without nitrogen, and
compound material with nitrogen, were deposited on a quartz substrate using sputtering. Then, we chemically treated the
thin film materials using various conditions including sulfuric peroxide mixture (SPM) and standard cleaning (SC-1).
First, the concentration of ions on the thin film materials was measured using ion chromatography (IC) analysis. Second,
haze defects were inspected after exposure in order to evaluate the difference in haze generation on the thin film
materials. Also, we investigated the numbers and shape of the occurrences of haze.
Haze generation has been serious issue on wafer lithography process, as illumination wavelength become shorter with
248nm and 193nm. Several published papers have been reported that ammonium and sulfate residual ion on mask
surface is major source of haze generation. These ions are come from conventional photomask cleaning process. PKL
have been studied new cleaning process to minimize haze generation and found cleaning process condition. Also, PKL
found that residual ammonium ion is major source of haze generation than residual sulfate ion. New cleaning process
improved residual ammonium ion concentration to less than 45 ppb from 900 ppb with conventional RCA cleaning. And
illumination doses generating haze have been tested on five residual ammonium ion, 1500 ppb, 900 ppb, 160 ppb, 70 ppb,
45 ppb, respectively. In house designed Haze Acceleration Test Bench (HATB) was used to expose masks. Haze were not
generated until from 25 kJ to 100 kJ, on 160 ppb to 45 ppb of ammonium ion concentration, respectively. And the
residual of sulfate ion and its haze generation dose did not correspond. Residual ammonium ions need to be controlled
tightly than sulfate ion. PKL concentrated on minimizing ammonium residual with new cleaning process and found the
optimized cleaning process for preventing 100kJ of cumulative energy on ArF embedded attenuated PSM (EAPSM).
We choose thermal treatment as part of a methodology to remove chemical residue on the surface of a mask. This new step of thermal treatment is inserted into our standard cleaning process for embedded attenuate phase shift masks (EAPSMs). The treatment is carried out in a modified hot plate system at various temperatures and times. After thermal treatment, ion chromatography measures the residual ions on a given surface. The thermal treatment is found to considerably reduce residual sulfate ions on the mask surface. The remaining sulfate ions on the mask are <0.18 ng/cm2 using thermal treatment.
In recent years, organic compounds have been clarified as one of major root causes of the haze, and carbon and amine components are major of them to organize. Therefore, both two should be controlled simultaneously for preventing haze defects on photomask. It is well known that UV/O<sub>3</sub> treatment has a strong efficiency of removing organic matters<sup>1</sup>. For that reason, we have inserted it into our cleaning process, especially for EA.PSMS. And the surface variation of mask, after UV/O<sub>3</sub> treatment, has been confirmed by the change of surface free energy on it. And organic matters remaining on mask surface have been identified by Gas Chromatography Mass Spectrometry (GC MS) with two different sample preparation methods: Thermal Desorption (TD) and direct extraction. As a result of UV/O<sub>3</sub> treatment, we confirmed that it has an excellent removing efficiency of aromatic compounds and semi-volatile organics on mask surface. Finally, through the haze accelerating tests, we have known that conventional SPM/SC-1 cleaning with UV/ O<sub>3</sub> treatment has been having a much higher threshold energy value in terms of causing haze defects on masks about 20 times higher than that of the cleaning with just SPM/SC-1.
We have chosen a combination of thermal treatment and hot D.I water rinsing as a part of methodologies to remove chemical residuals on mask surface. A new step of thermal treatment has been inserted in our standard manufacturing procedure for EAPSM. After thermal treatment, Ion Chromatography (IC) methods are used to confirm the surface cleanliness. As a result of our study, thermal treatment can considerably reduce residuals (e.g. ammonium, sulfuric and others) on mask surface. So, it could be suggested that the thermal treatment is an effective way to minimize residual ions. Also, in order to understand on haze source and mechanism, we investigated on artificial acceleration method for haze formation. We preceded haze acceleration test by using modified haze acceleration system (UV 172nm light). From the acceleration test, we found that humidity, irradiation energy, concentration of chemical residuals are dominant factors of haze formation.
We investigated the control of residual ions on the mask surface and the phase/transmission change rate by using thermal treatment after a conventional cleaning process. We hypothesized that the remaining sulfuric ions on the mask surface could combine with other ions and produce compounds during the thermal treatment. These compounds are easily removed by a hot D.I water rinse. Our study shows that the amount of remaining sulfuric ions is 250ng/mask when the mask has been thermally treated. The amount of sulfuric ions is substantially reduced compared to the results of other cleaning processes. Additionally we have found that the thermal treatment can be reduced varying the phase/trans value according to the cleaning cycle and the variation was stable even with a higher concentration of SC-1 solution.