Guy Vereecke

Evaluation of megasonic cleaning for sub-90-nm technologies

Cleaning of nanoparticles (< 50nm ) is becoming a major challenge in semiconductor manufacturing and the future use of traditional methods, such as megasonic cleaning, is questioned. In this paper the capability of megasonic cleaning to remove nanoparticles without inflicting damage to fragile structures is investigated. The role of dissolved gas in cleaning efficiency indicates that cavitation is the main cleaning mechanism. Consequently gas mass-balance analyses are needed to optimize the performance of cleaning tools. When gas is dissolved in the cleaning present tools can remove nanoparticles down to about 30 nm using dilute chemistries at low temperature. Ultimate performance is limited by cleaning uniformity, which depends on tool design and operation. However no tool reached the target of high particle removal efficiency andlow damage. Significantly lower damage could only be obtained by decreasing the power, at the cost of a lower cleaning efficiency for nanoparticles. The development of damage-free megasonic is discussed.

Particle adhesion and removal mechanisms during brush scrubber cleaning

Brush scrubbers are among the most commonly used instruments for wafer-cleaning applications nowadays. However, the removal mechanisms of nanosized particles are far from clear, especially because no direct experimental data are available to backup theoretical models in the literature. This study combines a theoretical approach based on a force analysis with an experimental study of the removal of nanosized slurry particles. In the theoretical part, all forces affecting the adhesion and the removal of particles are evaluated to determine which are dominant in two extreme removal mechanisms: lifting and rolling. In the experimental part, the removal efficiency of 34nm SiO2 particles is investigated by using the haze approach. Based on a study of the aging of contaminated wafers, conditions are selected where no chemical bonds are formed between a particle and a substrate. Force analysis and experimental observations both show that nanosized particles cannot be lifted directly by a brush. Instead, rolling s...

Capturing Wetting States in Nanopatterned Silicon

Spectacular progress in developing advanced Si circuits with reduced size, along the track of Moores law, has been relying on necessary developments in wet cleaning of nanopatterned Si wafers to provide contaminant free surfaces. The most efficient cleaning is achieved when complete wetting can be realized. In this work, ordered arrays of silicon nanopillars on a hitherto unexplored small scale have been used to study the wetting behavior on nanomodulated surfaces in a substantial range of surface treatments and geometrical parameters. With the use of optical reflectance measurements, the nanoscale water imbibition depths have been measured and the transition to the superhydrophobic Cassie-Baxter state has been accurately determined. For pillars of high aspect ratio (about 15), the transition occurs even when the surface is grafted with a hydrophilic functional group. We have found a striking consistent deviation between the contact angle measurements and the straightforward application of the classical wetting models. Molecular dynamics simulations show that these deviations can be attributed to the long overlooked atomic-scale surface perturbations that are introduced during the nanofabrication process. When the transition condition is approached, transient states of partial imbibition that characterize intermediate states between the Wenzel and Cassie-Baxter states are revealed in our experiments.

Fundamental study of the removal mechanisms of nano-sized particles using brush scrubber cleaning

To ensure high device yields, wafer surface contamination and defects must be monitored and controlled during the entire process of semiconductor manufacturing. Particle surface concentrations on the wafers, mostly related to chemical mechanical polishing (CMP) processes, must be kept at the lowest possible levels. Brush scrubber cleaning has the potential to achieve this goal. However, the particle removal mechanisms are still under discussion especially the removal of nano-sized particles. This paper investigates the interactions between the particle, the brush and the wafer surface and explores the potential and limitations of the brush scrubbing technique. Furthermore the effect of the various brush/wafer parameters on the particle removal efficiency (PRE) is studied. From a mechanistic viewpoint it is shown that brush scrubbing acts in a mixed lubrication regime. From an extensive analysis of the relevant forces and moments it can be concluded that in the hydrodynamic lubrication regime, particles ar...

Megasonics : a cavitation driven process

In this paper attention is given to different parameters playing a role in megasonic cleaning, which is based on gaseous cavitation such as gas type and concentration, acoustic drive pressure and their influence on sonoluminescence, the motion of a void, particle removal efficiency, uniformity and damaging. The cavitation process is a non-uniform process in itself, but also larger scaled non-uniformities can be observed for SL and PRE. Single bubble simulations show that only a limited bubble size distribution will be active in a megasonic field and that the main parameters to consider are the power and the bubble size affected by the saturation level of gas. Depending on the megasonic system and process conditions, a positive relation between SL and PRE is not always found but damage always increases along with PRE. In fact not the type of gas but the level of saturation is determining the success of the particle removal in a megasonic process.

Bulk Properties of MOCVD-Deposited HfO2 Layers for High k Dielectric Applications

The physical bulk properties of metalorganic chemical vapor deposited (MOCVD) deposited HfO 2 layers were characterized as a function of deposition temperature. thickness, and starting surface. It is shown that depositing HfO 2 layers at 300°C results in a lower density film compared to films deposited at higher temperature (e.g., 485 and 600°C). In addition, it is shown that layers deposited at 300°C contain significant amounts of carbon originating from the organic precursor (tetrakis-diethylamidohafnium). As a result of the low density and/or carbon contamination, the dielectric properties of these layers are very poor. It is observed that the density of the film is heavily dependent on the thickness, where very thin layers have a density that is only a fraction of the bulk density regardless of the deposition temperature. For thicker layers, a higher deposition temperature is seen to result in a higher density, although still lower than bulk density, as observed by ellipsometric porosimetry. Finally, the crystalline state of the material is found to be dependent on the deposition temperature, thickness, and post-deposition anneal. Based on our results, MOCVD deposited HfO 2 layers are expected to be polycrystalline and present in its cubic and/or monoclinic phase.

Alternative Photoresist Removal Process to Minimize Damage of Low-k Material Induced by Ash Plasma

Dry ashing of photoresist (PR) using oxygen-containing plasma applied subsequently to an etch plasma leads to degradation of porous low-k material. The surface region is substantially depleted in carbon. The low-k film becomes more hydrophilic after being subjected to plasma etch and especially ash process as evidenced by water absorption results. The amount of absorbed water into a 30% porosity film at moisture saturation is estimated to be about 15% of the film volume, which corresponds to 50% of the total pores in the low-k film. A wet, alternative means for PR removal based on dissolution of PR in organic solvents combined with physical forces is presented. Under certain conditions, megasonic cleaning resulted in complete removal of the PR layer without damaging of the dielectric lines. These results suggest that the PR crust is permeable to these solvents and that out-diffusion of dissolved bulk PR also occurred through the crust. Dissolution of bulk PR in organic solvents first makes the PR structure more fragile, then the physical energy helps to remove the remaining crust mechanically without dissolving it. Compared to plasma ashing, solvent strip shows no carbon depletion and no significant increase in k-value.

All-Wet Strip Approaches for Post-Etch Photoresist Layers After Low-K Patterning

Plasma chemistries, applied during low-k patterning processes in back-end of line (BEOL) applications, modify the photoresist (PR) layer present on top of the etched structures. The remaining resist layer after plasma etch is resilient towards most organic solvents and aqueous solutions. Conventionally, the layer is removed before copper deposition using an oxidizing plasma process. This approach is not acceptable anymore due to damage of the dielectric, i.e. k-value degradation and chemical modifications [1,2]. In order to obtain less damaging photoresist removal processes for post low-k etch, the use of wet-only methods is under investigation.

Measurements of trace gaseous ambient impurities on an atmospheric pressure rapid thermal processor

Gaseous impurities in nitrogen ambient in the chamber of an atmospheric pressure rapid thermal processor were quantitatively measured. We employed atmospheric ionization mass spectrometry (APIMS) for this purpose. APIMS is the most sensitive technique to detect trace impurities in a gas at atmospheric pressure. A wide dynamic range (0.1 ppb→10 ppm) measurement was successfully performed, which allowed real-time monitoring of impurities during rapid thermal annealing. This work reports fundamental behavior of the ambient impurities originating from different sources. The sources discussed in this article are fourfold: source gas, system background, air (wafer loading), and wafer itself. The contribution of the source gas was found to be negligible, whereas the air and the wafer were found to be most crucial. Ambient management requires a better understanding of the independent contribution of each source to processing. It is shown how in situ measurements help to define process recipe for different types o...

Superhydrophobic Breakdown of Nanostructured Surfaces Characterized in Situ Using ATR–FTIR

In situ characterization of the underwater stability of superhydrophobic micro- and nanostructured surfaces is important for the development of self-cleaning and antifouling materials. In this work, we demonstrate a novel attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy-based method for large-area wetting characterization of silicon nanopillars. When air is present in between the structures, as is characteristic of the Cassie-Baxter state, the relative intensities of the water bands in the absorption spectrum change because of the wavelength-dependent attenuation of the evanescent wave. This phenomenon enables unambiguous identification of the wetting state and assessment of liquid impalement. Using mixtures of isopropanol and water with different concentrations, the breakdown of superhydrophobic states and the wetting hysteresis effects are systematically studied on uniform arrays of silicon nanopillars. A transition from the Cassie-Baxter to Wenzel state is observed when the isopropanol concentration exceeds 2.8 mol %, corresponding to a critical surface tension of 39 mN/m. Spontaneous dewetting does not occur upon decreasing the isopropanol concentration, and pure water can be obtained in a stable Wenzel state on the originally superhydrophobic substrates. The developed ATR-FTIR method can be promising for real-time monitoring of the wetting kinetics on nanostructured surfaces.