Current and future lithography techniques require complex imaging improvement strategies. These imaging
improvement strategies require printing of sub-resolution assist-features (SRAF) on photomasks. The size of SRAF’s has
proven to be the main limiting factor in using high power Megasonic cleaning process on photomasks. These features,
due to high aspect ratio are more prone to damage at low Megasonic frequencies and at high Megasonic powers.
Additionally the non-uniformity of energy dissipated during Megasonic cleaning is a concern for exceeding the damage
threshold of the SRAFs. If the cavitation events during Megasonic cleaning are controlled in way to dissipate uniform
energy, better process control can be achieved to clean without damage. The amount and type of gas dissolved in the
cleaning liquid defines the cavitation behavior. Some of the gases possess favourable solubility and adiabatic properties
for stable and controlled cavitation behaviour. This paper particularly discusses the effects of dissolved Ar gas on
Megasonic characteristics. The effect of Ar Gas is characterized by measuring acoustic energy and Sonoluminscense.
The phenomenon is further verified with pattern damage studies.
As the feature size of the mask shrinks, the feature becomes more fragile and the potential for physical force damage during cleaning increases. At the same time, increased feature density of the mask makes it difficult to remove particles from congested trenches without physical force cleaning. Acoustic energy has the ability to suppress the hydro-dynamic boundary layer thereby transferring the physical force impact closer to particles trapped in the deep trenches of the mask. MegaSonic, which employs acoustic energy, is a preferred physical force cleaning technology for advanced masks. However MegaSonic can be extremely aggressive if the energy distribution is not contained within the narrowest process window available. In this paper, liquid media properties and their effect in controlling MegaSonic energy is evaluated. A chemistry is identified which provides favorable gaseous properties for controlling MegaSonic cavitation. The effect of this chemistry is characterized by measuring acoustic energy and Sonoluminscense. The phenomenon is further verified with pattern damage studies.
Megasonic energy transfer to the photomask surface is indirectly controlled by process parameters that provide an
effective handle to physical force distribution on the photomask surface. A better understanding of the influence of these
parameters on the physical force distribution and their effect on pattern damage of fragile mask features can help
optimize megasonic energy transfer as well as assist in extending this cleaning technology beyond the 22nm node. In this
paper we have specifically studied the effect of higher megasonic frequencies (3 & 4MHz) and media gasification on
pattern damage; the effect of cleaning chemistry, media volume flow rate, process time, and nozzle distance to the mask
surface during the dispense is also discussed. Megasonic energy characterization is performed by measuring the acoustic
energy as well as cavitation created by megasonic energy through sonoluminescence measurements.
Extreme ultra-violet (EUV) lithography has become the technique of choice to print the ever-shrinking nanoscale features on the silicon wafer. For successful transfer of patterns on to the wafer, the EUV photomask cannot contain defects greater than 30 nm. Megasonic cleaning is a very successful cleaning technique for removal of particles on photomasks, but also causes a relatively high amount of damage to the fragile EUV photomasks thin film structures. Though it is believed that acoustic cavitation is the primary phenomenon responsible for cleaning as well as pattern damage, a fundamental picture of the acoustic cavitation mechanisms in play during megasonic cleaning has not yet clearly emerged. In this study, we characterize the role of acoustic cavitation in megasonic cleaning by examining the effects of acoustic power densities, cleaning solution properties, and dissolved gas content on cavitation via experiments and molecular dynamics (MD) simulations. MD is an atomistic computation technique capable of modeling atomic-level and nanoscale processes accurately making it well suited to study the effect of cavitation on nano-sized particles and patterns.
Megasonic cleaning has been a traditional approach for the cleaning of photomasks. Its feasibility as a damage free approach to sub 50 nm particulate removal is under investigation for the cleaning of optical and EUV photomasks. Two major mechanisms are active in a megasonic system, namely, acoustic streaming and acoustic cavitation. Acoustic streaming is instrumental in contaminant removal via application of drag force and rolling of particles, while cavitation may dislodge particles by the release of large energy during cavity implosion or by acting as a secondary source of microstreaming. Often times, the structures (substrates with or without patterns) subjected to megasonic cleaning show evidence of damage. This is one of the impediments in the implementation of megasonic technology for 45 nm and future technology nodes. Prior work suggests that acoustic streaming does not lead to sufficiently strong forces to cause damage to the substrates or patterns. However, current knowledge of the effects of cavitation on cleaning and damage can be described, at best, as speculative. Recent experiments suggest existence of a cavity size and energy distributions in megasonic systems that may be responsible for cleaning and damage. In the current work, we develop a two-dimensional atomistic model to study such multibubble cavitation phenomena. The model consists of a Lennard-Jones liquid which is subjected to sinusoidal pressure changes leading to the formation of cavitation bubbles. The current work reports on the effects of pressure amplitude (megasonic power) and frequency on cavity size distributions in vaporous and gaseous cavitation. The findings of the work highlight the role of multibubble cavitation as cleaning and damage mechanism in megasonic cleaning.
Removal of nano-scale contaminant particles from the photomasks is of critical importance to the implementation of EUV lithography for 32nm node. Megasonic cleaning has traditionally been used for photomask cleaning and extensions to sub 50nm particulates removal is being considered as a pattern damage free cleaning approach. Several mechanisms for removal are believed to be active in megasonic cleaning systems, e.g., cavitation, and acoustic streaming (Eckart, Schlichting, and microstreaming). It is often difficult to separate the effects of these individual mechanisms on contamination removal in a conventional experimental setup. Therefore, a theoretical approach is undertaken in this work with a focus on determining the contribution of acoustic streaming in cleaning process. A continuum model is used to describe the interaction between megasonic waves and a substrate (fused silica) immersed in a fluid (water). The model accounts for the viscous nature of the fluid. We calculate the acoustic vibrational modes of the system. These in turn are used to determine the acoustic streaming forces that lead to Schlichting streaming in a narrow acoustic boundary layer at the substrate/fluid interface. These forces are subsequently used to estimate the streaming velocities that may in turn apply a pressure and drag force on the contaminant particles adhering to the substrate. These effects are calculated as a function of angle of incidence, frequency and intensity of the megasonic wave. The relevance of this study is then discussed in the context of the cleaning efficiency and pattern damage in competing megasonic cleaning technologies, such as immersion, and nozzle-based systems.