After decades of binary mask manufacturing using Cr absorber the material spectrum was extended by phase shift
material in late 90's during introduction of Half Tone Phase Shift Masks (HT-PSM). This change had strong impact on
manufacturing flow as well as several unit processes.
A consequences of phase shifter introduction was the necessity of introducing a second level litho process, as well as
introducing of dry etch processes due to poor etch properties of MoSi using wet chemistry. Less obvious and rather
unremarkable was the impact of this change to clean processes, except the impact of the clean process on the phase
In recent years we've seen several new materials based on varying chemical composition as well as thickness of the
absorber developed by various mask blank vendors namely Hoya and ShinEtsu. These materials are improving
resolution, pattern fidelity and to some degree also mask lifetime. Adding the EUV mask blank materials increases
further the spectrum of materials, taking into account all the absorber stacks available today on market.
Thorough investigation of the clean process performance as a function of surface material shows significant variation in
the critical parameters as defectivity, susceptibility to recontamination and relative cleaning efficiency.
Goal of this work is to
1) Compare the already mentioned clean related properties together with feature damage and impact on the critical
dimension (CD) shift for different materials.
2) Find a compromise between the technology requirements and process limitations resulting from the combination of
available processes with material properties.
Some aspects of the new materials such as stack height and interface between absorber and substrate are making this
task easier, especially with respect to feature damage. On the other hand the most critical parameter - the cleaning
efficiency, dropped due to the introduction of the new materials, mainly due to unfavorable sticking coefficients of
The cleaning processes used today for photomasks were developed over decades and optimized to fulfill customer specifications. Some mask procedures were adapted from wafer cleaning technology. A principal technique, megasonic (MS) cleaning, yields high particle removal efficiencies (PRE). However, MS can frequently cause feature damage, and so damage becomes the principle limitation to MS power levels applied to small feature sizes. The use of lower MS power levels can benefit from a better understanding of removal mechanisms. In several publications the effects influencing the mechanisms of particle cleaning were discussed <sup></sup>. Particle transfer was investigated experimentally on wafer surfaces using bath tools and was tracked using fluorescent optical microscopy <sup></sup>. The goal of our investigation is to test the validity of the aforementioned models for mask cleaning using a spinning mask and a megasonic head mounted on a arm swinging over the mask surface, which is the most common hardware setup used for mask cleaning tools. While this equipment setup provides a useful variability, it also introduces disadvantages e.g. non-equal distribution of the megasonic power across the cleaned surface as will be shown. We will focus on some of the main parameters e.g. chuck speed, arm swing speed and media flow, which are strongly coupled by the fluid dynamics and cannot be treated separately. All three parameters influence particle-mask decoupling and reattachment during particle transport by the media stream across the mask surface. The approach to estimate the particle removal and reattachment rate is illustrated. The experiments performed allow the conclusion that the reattachment rate on a flat spinning mask surface is lower than previously assumed and the most critical part of the cleaning process is the detachment of the particle from the surface.
Scatter bar (SB) breakage presents a mounting confrontation for final cleaning of masks While the industry is strongly dependent on megasonic (MS) energy, MS is hazardous to scatter bars which approach the size of particles which must be removed. The difference in energy needed to remove small particles and the energy needed to remove small features represents a subtle and shrinking domain. Here we observe cleaning effects when the plate is inverted. This gives us a
look that at affects which might otherwise remain hidden. We provide evidence of plate resonance effects and constructive interference from internal reflection We assess the ability to clean a plate without direct exposure to the MS beam We adapt a MS bath qualification method for use on spinning plates and use it to assay cavitation activity and uniformity for Upright and Inverted spinning plates. Cavitation activity is recorded in the spalling on a metallic film, which allows quantification by optical reflectance measurements. Value to both cleaning and SB breakage are assessed.
Researchers have linked the occurrence of reticle haze to many parameters which include cumulative irradiation, the greater use of 193nm in low-k1 lithography, humidity and the presence of ammonium with other process contaminants. Published methods of contaminant reduction include 1) volatilization by thermal treatment 2) induced preemptive crystallization with 172nm eximer energy followed by subsequent ozone-water and megasonic treatments and 3) hot water treatments. In this paper we explore the process characteristics necessary to achieve a new method of continuous ion removal which includes sustained plate temperature during UV treatment and the sublimation of ammonium crystals. The application of these principles are consistent with room temperature (RT) fluid flows which allow us to work within a regime of negligible phase angle, negligible transmissivity change and silicon nitride removal efficiencies above 99% for particles as small as 80nm.
Cleaning becomes increasing important and challenging as feature sizes continue to shrink. Many methods and strategies have been explored to reduce particle defects and ion haze that destroy yield on pelliclized reticles. A successful cleaning method must balance reductions of particles and haze while imposing minimal changes to the transmissivity of the chrome stack, to exposed quartz and to the phase shift of molybdenum silicide surfaces. This paper focuses on the inclusion of many previously explored cleaning methods working in concert within a single reticle cleaning tool. We present our findings on elimination of particles with minimum impact on reflectivity and phase angle. We test the collective effects of Ozonated Water (O3W) and final cleaning methods that employ
ammonia hydroxide and hydrogen water. These methods are presented within the context of spin cleaning applications.
A great deal of work has been done on identifying the sources of reticle haze. Researchers have sited haze contributions from the atmosphere, from pellicle, pellicle adhesive and from sulfate residuals left by mask cleaning. Residual sulfates from otherwise high performance cleaning processes can range from 30 ppb and up. This paper focuses on final clean methods within a single track tool that leave concentrations of ion residues approaching 1 ppb. We compare different spin processes which use ozonated water and ultra dilute ammonia and hydrogen water through a megasonic head. Other sources of haze producing ions may remain but eliminating contributions from the final cleaning process opens a productive path to higher yield with 65 and 45 nm design rules.