Papers by Author: Herbert Struyf

Abstract: 3 formulated etchants were prepared and their etch rates were measured using blanket wafers in order to confirm that the etching reactions on Si_1-XGe_X and Si are controllable. Si_1-XGe_X selective etching with those formulations was also verified using the wafers which had Si_1-XGe_X and Si multi-stacked structures. Cross-sectional transmission electron microscope (TEM) images suggested that the formulations were usable for Si_1-XGe_X selective etching processes.

Abstract: We discuss several mechanistic approaches and experimental data for improving post-CMP cleaning of W plugs with TiN as barrier liner, and dielectric substrates SiO₂ and Si₃N₄ for use at the 10 nm technology node (metal pitch of 40 nm). Particle charge in the low pH, W CMP slurries are usually positive, and the W surface is always negatively charged at pH >3. Therefore, a strong electrostatic attraction is expected to occur between the W surface and the residual particles during post-CMP cleaning. Two main approaches were chosen to break down the strong particles-W surface post-CMP electrostatic interactions, as well as particles dispersion and prevention of redeposition: (1) using cleaning additives able to adsorb at the W surface and reverse the W surface charge; (2) using organic additives to reverse the particle charge. The latter approach results in two strongly negative charged surfaces, which are able to repulse each other, and leads to the best cleaning.

Abstract: The cumulative installed solar power generation has been rising exponentially over the past decade. This has lead to a concomitant rise in production capabilities, leading eventually to excess production capabilities and rapid price declines per unit. In order to compete with the standard electricity generation the cost of solar panel production and installation needs to decrease even further. At the same time the solar panel and cell makers need to be able to keep a healthy margin. A crucial element in this exercise is a close control on the Cost of Ownership (CoO) of a solar cell / panel fabrication site.

Abstract: With the downscaling of devices, due to device geometry shrinkage, the total number of cleaning steps has increased dramatically. As a result, the number of drying cycles after cleaning has increased as well. As the device shrinks with the integration density increase, it is noteworthy that a perfect drying efficiency is mandatory to obtain a high performance device [. Basically, the mechanism of wafer drying in semiconductor industry can be explained as: first reducing the amount of liquid on the wafer surface by mechanical forces. There are some approaches for removing the liquid such as spinning, high pressure gas blowing by nozzle or air-jet, vertical withdrawal from the liquid bath, using surface gradient tension and so on [2]. Second: if the mechanical forces in the liquid removal part are not sufficient for drying and some droplets or a thin liquid layer remain on the wafer surface, complete drying will be achieved by evaporation of the remaining layer on the wafer. After this evaporation step, known as state transformation, the wafers will be completely dried. Evaporation of the remaining liquid layer is the main mechanism for generating drying defects (watermarks, residues, particles, and etc.)[3]. In this study, we propose a new methodology for semiconductor wafer drying based on a high-pressure gas flow. In comparison to conventional drying tools, the new drying set up combines high speed drying (wafer drying time down to 2 sec at 150mm.s^-1) and a low number of added drying defects.

Abstract: In semiconductor fabrication, pattern collapse of high aspect ratio structures after wet processing has been a critical issue and attracted a lot of interest. On the other hand, very little attention is spent on the potential wetting issues as feature dimensions are continuously scaled down and novel materials with different wetting properties are used in new technology nodes. In this work we investigate the wettability of nanopatterned silicon substrates with different surface modifications.

Abstract: Although evaporation as a pure bulk phase transformation is well understood, when one adds solutes to the liquid, or brings the liquid into contact with a substrate, we obtain a new and rich variety of possible behaviors that we can access experimentally to better understand the drying dynamics of residual water droplets. Evaporation of sessile droplets with a small contact angle (below 90°) is studied here extensively on silicon substrates. We focused our work on the origin of the creation of watermarks on silicon wafers. A thorough understanding of droplet evaporation is of vital importance for examining the drying rate, the flow patterns observed inside drying drops, and the residual deposits. The concentration of each potential dissolved species (e.g. silica or silicic acid) can also be predicted and confronted to their solubility. We developed a theoretical model to predict the evaporation rate and the behavior of submillimetric droplets taking into account the characteristics of the ambient and the substrate during the drying process. We discuss also the topology of watermarks on silicon wafers in the case of a predominant evaporation phenomenon.

Abstract: In semiconductor fabrication, mixtures of isopropyl alcohol (IPA) and deionized water (DIW) are commonly used in wafer cleaning processes due to their superior wetting and drying performance on many types of substrates. To achieve a maximum cleaning performance with reduced IPA consumption, it is of great interest to understand the wetting mechanism of such mixtures. In this work, we investigate the spreading and evaporation process of IPA-DIW drops with different concentrations.

Abstract: The continuous miniaturization of electronic building blocks in the semiconductor industry imposes more stringent requirements on the different cleaning processes. Purely chemical particle cleaning is based on weak etching of the substrate. This etching reduces the attractive van de Waals interaction between particle and substrate and at the same time, electrostatic repulsion ensures the removal of particles. This technique is not applicable anymore for future technology nodes, since an unacceptable large substrate loss (up to 3 nm) is necessary to obtain high particle removal efficiencies with pure chemical cleaning alone [. As a result, an additional physical force has to be considered to overcome this limitation. Several physical cleaning techniques exist, but all of them suffer from too much damage creation when fragile structures are cleaned. Currently, the industry is more focusing on spray cleaning and state-of-the-art spray tools show a high control over droplet size and droplet velocity [2]. Despite all of the advancements in spray cleaning, damage creation of fragile elements remains an issue, which could partially be attributed to the chaotic behavior of the water layer on the wafer surface [3]. Therefore, megasonic cleaning is still considered as a possible alternative to reduce damage formation during a physical cleaning process. Recently, it has been shown that the acoustic pressure amplitude can be reduced while maintaining the same particle removal efficiency level. This is achieved by (1) using pulsed acoustic fields which makes it possible to control the average bubble size and maximize the number of resonant bubbles, by (2) increasing the dissolved gas concentration which facilitates bubble nucleation and, finally, by (3) introducing traveling waves to transport bubbles to the wafer surface which needs to be cleaned. These conditions are briefly discussed and are applied during the investigation of the influence of dissolved CO₂ on bubble activity. Dissolved CO₂ is particularly interesting since it has been reported that sonoluminescence (i.e. strong bubble collapse) as well as damage formation is reduced when CO₂ is added to the cleaning liquid [4,5]. Here, it is shown that also particle removal efficiencies (PREs) diminish with increasing CO₂ concentrations.

Abstract: Removal of particulate residues represents a very challenging step in current CMOS-technology nodes. The continued miniaturization and the introduction of novel materials in the semiconductor industry have resulted in very stringent requirements for device fabrication steps such as cleaning processes [. Physical forces, acting directly on the surface to be cleaned, are currently employed for delicate particle removal as an alternative to more aggressive chemistries [2]. High frequency ultrasounds (500 kHz 4 MHz), or megasonics, rely on the action of oscillating bubbles created during the ultrasonic agitation of the cleaning liquid. Strongly oscillating gas bubbles are able to generate shear forces, which are considered to be responsible for cleaning [3]. However, collapsing bubbles close to a surface can also produce water jets and shockwaves which lead to damage of fragile structures. Fundamental research is needed in order to overcome these issues by improving the understanding of the physical parameters playing a role in the acoustic cavitation of bubbles. This study reports the effects of lowering the surface tension of the liquid bulk on the bubble activity in the MHz range. A lower surface tension (45 mN/m) with respect to water (72 mN/m) is obtained by adding a non-ionic surface-active agent (TritonX-100). After fully characterizing its wettability, a cleaning solution containing surfactant is investigated under pulsed and continuous acoustic fields, for different acoustic amplitudes and gas concentrations. The aim is to increase bubble activity while reducing the strength of the bubble collapse. The results obtained can be useful in tuning megasonic cleaning systems towards more efficient processes.

Abstract: Cleaning photoresist from semiconductor wafers during the transistor formation in the front end of the line (FEOL) becomes more challenging with ever smaller nodes. First of all the resists do become more difficult to clean with decreasing node size, because implantation energy increase and the resist becomes more complex (to comply with the reduced wave length of the laser light for the lithography) at future node sizes. This results in more cross linked / polymerized photoresist, which is harder to (wet) strip. Additionally, the requirements on material compatibility of the cleaning solution increase, as more elements are used to build the transistor less than a monolayer of these materials can be removed during a single cleaning step.