A proper surface treatment, such as O2 plasma, helps to improve particle removal efficiency (PRE) because of the
formation of hydrogen bonding between particles, water and the mask surface after treatment. The effectiveness of
surface treatments cannot be determined only by the static wettability after processes. More key indexes should be
considered. In this paper, we report our findings on the relationship between surface treatments on photomasks and the
resulting wettability. In addition, added defects after the treatment and the cleaning process were inspected with a 193-
nm KLA inspector on 193-nm immersion and EUV photomasks, which consist of SiO2, MoSi, Cr, Ta-based absorber
and Ru. Based on our work, three indexes can be built for determining the effectiveness of surface treatments. The first is
to check whether the surface becomes super-hydrophilic after treatment. The second is to determine the efficiency of
surface treatments on enhancing wettability. The last is to quantify the added watermark count after the surface treatment
and the cleaning process. With a proper surface treatment, watermarks can be greatly eased. These three indexes can
quickly determine possible effective methods for treating the surfaces of different materials.
We present a series of baseline techniques for inspection, cleaning, repair, and native defect mitigation of extreme ultraviolet (EUV) masks. Deep-ultraviolet inspectors are capable of inspecting patterns down to about 45 nm in pitch on wafer. Cleaning methods involving both chemical and physical forces have achieved good particle removal efficiency while minimizing absorber shrinkage and have realized 90% PRE in removing particles from the backside of an EUV mask. In addition, our compensation method for native defect repair has achieved partial success.
We investigated methods to extend the damage-free process window for fragile Sub-Resolution Assist Features (SRAF)
in mask cleaning using MegaSonic and binary spray techniques. Particle removal efficiency (PRE) was found to increase
by 8% and damage reduced from 7 ppm to 0 ppm with the optimization of the spray droplet characteristics through liquid
media control. MegaSonic damage was eliminated completely from 10 ppm to 0 ppm by varying physical and chemical
properties of the cleaning media. Since particles in the deep trenches are very difficult to remove using droplet spray
alone, a combination of MegaSonic and Binary Spray processes was tested. The acoustic effects generated through the
MegaSonic combined with optimized droplet impact showed an improvement of 4% in PRE of hard-to-remove trench
particles. Overall, the improved process points to a promising solution for overcoming the roadblock in mask cleaning
for the advanced mask cleaning.
This paper studies the repeatability and the reliability of CDUs from a mask inspector and their correlation with CD
SEM measurements on various pattern attributes such as feature sizes, tones, and orientations. Full-mask image
analysis with a mask inspector is one of potential solutions for overcoming the sampling rate limitation of a mask
CD SEM. By comparing the design database with the inspected dimension, the complete CDU behavior of specific
patterns can be obtained without extra work and tool time. These measurements can be mapped and averaged over
various spatial lengths to determine changes in relative CDU across the mask. Eventually, success of this
methodology relies on the optical system of the inspector being highly stable.
As semiconductor manufacturing advances to sub-20-nm nodes, specification (size < 50 nm) for extremely fine particles on photomasks is getting more and more stringent. Photomask cleanliness, which seriously impacts manufacturing cycle time and productivity, is a serious challenge in the development of sub-20-nm node mask cleaning process. Several cleaning approaches, including the use of chemical and physical forces, are widely used in mask cleaning. In this study, we focus on the chemical force through zeta potential (ZP). ZP indicates the degree of repulsion between the particles and the mask surface (mostly quartz). In the nano-scale, stronger repulsion means easier removal of particles from mask surfaces. By controlling ZP of different chemicals from -10 mV to -150 mV in the cleaning process, the particle removal efficiency (PRE) is further improved by about 10%, especially for extremely fine particles. The ZP measurement methodology for different cleaning chemicals on quartz surface is also carried out. ZP is a helpful index in evaluating the performance of new chemicals for mask cleaning. To enhance photomask cleaning for sub-20-nm nodes, the chemical force needs to be increased because the physical force has been constrained to avoid pattern damages, especially when much smaller assistant features are commonly used to gain a greater lithography process window. How to choose a suitable cleaning approach for the next generation mask cleaning is very critical.
A programmed-defect mask consisting of both bump- and pit-type defects on the LTEM mask substrate has been
successfully fabricated. It is seen that pit-type defects are less printable because they are more smoothed out by the
employed MLM deposition process. Specifically, all bump-type defects print even at the smallest height split of 1.7 nm
whereas pit-type defects print only at the largest depth split of 5.7 nm. At this depth, the largest nonprintable 1D and 2D
defect widths are about 23 nm and 64 nm, respectively.
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.
The fundamentals of droplet-based cleaning of photomasks are investigated and performance regimes that enable the use
of binary spray technologies in advanced mask cleaning are identified. Using phase Doppler anemometry techniques, the
effect of key performance parameters such as liquid and gas flow rates and temperature, nozzle design, and surface
distance on droplet size, velocity, and distributions were studied. The data are correlated to particle removal efficiency
(PRE) and feature damage results obtained on advanced photomasks for 193-nm immersion lithography.
The control of global critical dimension uniformity (GCDU) across the entire mask becomes an important factor for the high-end masks quality. Three major proceses induce GCDU error before after-developing inspection (ADI) including the E-Beam writing, baking, and developing processes. Due to the charging effect, the fogging effect, the vacuum effect and other not-well-known effects, the E-Beam writing process suffers from some consistent GCDU errors. Specifically, the chemical amplified resist (CAR) induces the GCDU error from improper baking. This phenomenon becomes worse with negative CARs. The developing process is also a source of the GCDU error usually appears radially. This paper reports the results of the study of the impact of the global CD uniformity on mask to wafer images. It also proposes solutions to achieve better masks.
In this paper, a quantitative evaluation of mask quality in the domain of 2D pattern fidelity and a method of assessing the OPC model effectiveness are investigated. The spirit of our algorithm is to characterize the wafer lithographic performances of both the real physical mask and the ideal OPCed layout mask that the physical mask is based on. To acquire these performances, we adopted a CD-SEM image process technique for transforming an actual SEM mask image into a simulation-friendly format like GDSII together with the methods to correctly handle the image transformation and interpret the simulation results. Finally, the images, such as the simulated aerial images, the simulated or observed resist top views, are superposed for comparison using logic operation.
In the IC industry the mask cost and cycle time have increased dramatically since the chip design has become more complex and the required mask specification, tighter. The lithography technology has been driven to 65-nm node and 90-nm product will be manufacturing in 2004, according to ITRS's roadmap. However, the optical exposure tools do not extend to a shorter wavelength as the critical dimension (CD) shrinks. In such sub-wavelength technology generation,
the mask error factor (MEF) is normally higher. Higher MEF means that tighter mask specification is required to sustain the lithography performance. The tighter mask specification will impact both mask processing complexity and cost. The mask is no longer a low-cost process. In addition, the number of wafers printed from each mask set is trending down, resulting in a huge investment to
tape out a new circuit. Higher cost discourages circuit shrinking, thus, prohibits commercialization of new technology nodes.
Nowadays, the CD (Critical Dimension) control on masks manufacturing plays an important role in photolithography process for 90-nm node technology and below. The process performance of photolithography
will degrade severely even when the mask CD error is small. One of the most important process-induced mask CD errors comes from the dry etching process. With the loading effect due to environment pattern variations, isolated and dense patterns have different etching biases. Furthermore, the loading effect can induce an overall
CD variation called global loading effect contributed from the pattern density change in large areas and a CD variation on individual monitor pattern called micro-loading effect contributed from various feature dimensions in the near region. The micro-loading effect can also be classified as the “nearest spacing” effect which is dependent upon the space between the nearest neighbor pattern and the monitor pattern, and the “nearest
neighbor” effect which is dependent upon the size of the nearest neighbor feature around the monitor pattern. All of these effects enlarge the total range of mask CD linearity and proximity errors.
In this paper we report the result of the global loading effect and micro-loading effect by varying pattern densities and feature dimensions nearby. With the design of test pattern, the global loading effect and the micro-loading effect can be separated. The CD variation dominated by the micro-loading effect in the dry etching process is observed. This large etching bias change resulted from the micro-loading effect is consistent with the depletion of radical species in the narrow space during the etching process.