It is necessary to develop a nano-bubble detector similar as a conventional particle counter for reducing micro and nano defects caused by nano-bubble (NB) in immersion lithography. In this regard, we discuss adhesion and removal mechanisms of NB adhered on a resist surface for immersion lithography. The micro and nano bubbles are more likely to adhere to the micro defect on the resist surface and lens surface. Keeping cleanness of lens and resist surface is necessary in order to prevent the micro bubble adhesion. We employed the AFM (Atomic Force Microscope) for the observation of NBs on a Si substrate and a resist surface. The diameter and height of NBs observed are approximately 40~100nm and 3~8nm, respectively. By approaching the AFM tip onto the NBs, the repulsive force can be detected but the attractive force on the resist surface. The interaction analysis between the AFM tip and the ArF excimer resist surface is effective in order to identify the NBs and to distinguish from solid particles. These phenomena can be discussed on the basis of Lifshitz theory. The separation procedure of the NB is accomplished with the AFM tip. The applying load at which the NB can be separated into the minute one is approximately 5nN. In addition, by the thermodynamic analysis, it can be considered that the NB adhered on the resist surface tends to be a flat shape and spread on the resist surface. It is difficult to adhere the bubbles on the resist surface.
In immersion lithography technique, some defects such as a watermark and a nanoscale bubble have been focused as the serious problems to be solved. In order to clarify the formation mechanism of the watermark, the in-situ observation of the drying behavior of the water drop containing the particles and without the particles, are conducted on the Si substrates. In the static watermark formation on the flat substrate, we can classify the watermark formation processes based on the watermark shapes. From the surface energy balance analysis, the particles dispersed in the DI-water adhere on the Si substrate. In addition, from the Laplace force balance, the particles adhered on the Si substrate will attract the surrounding particles. Hence, we can clarify the formation mechanism of the static watermark condensed in the ring shape. Meanwhile, in the dynamic watermark formation, we can observe clearly the condensed watermark is formed on the Si substrate and the particles move to lower region in inclined drop. In actual immersion lithography system, we can discuss the particles are more likely to remain in the immersion liquid under the lens system.
Recently, pattern collapse is becoming one of the critical issues in semiconductor manufacturing and many works have been done to solve this issue<sup>1</sup>) <sup>2</sup>). Since pattern collapse occurs when outer force onto the resist pattern such as surface tension, impact of rinse solution, etc. surpasses the resistance of the resist pattern such as mechanical strength, adhesion force between resist and substrate, it is considered effective for improvement of pattern collapse to control resist film properties by track process, i.e., optimization of the mechanical properties of the resist film and enhancement of the adhesion force between resist and substrate<sup>3</sup>) <sup>-5</sup>). In this study, we focused on the mechanical strength of the resist film and examined how post applied bake (PAB) condition affects the pattern collapse behavior. From ellipsometry measurement, it was found that increasing PAB time and temperature resulted in thickness reduction and refractive index increase, which suggested that the density of the resist film became high. Then we analyzed the mechanical strength of the resist film with the tip indentation method using atomic force microscope. It was found that the hardness of the resist film was affected by PAB conditions and regardless of PAB condition, hardened layer existed beneath the film surface. Finally, we carried out the measurements of loads to collapse 180nm resist dot patterns using the direct peeling with atomic force microscope tip (DPAT) method. Loads ranged from 600 to 2000nN overall and essentially increased as seen for indentation measurements when PAB temperature or time was increased, except some critical conditions. Through these evaluations using AFM, we succeeded in quantitatively evaluate the mechanical properties of the resist films processed with various PAB conditions. It was found that PAB condition obviously impacts on the hardness of the resist film and it is closely related to pattern collapse load.
Mechanical strength of resist film processed by various post apply bake (PAB) conditions were measured utilizing the tip indentation method using atomic force microscope (AFM). With the tip indentation method, we could quantify mechanical strength of resist film in terms of “degree of softening.” It was found that PAB at our standard baking temperature tends to lead to softening of the resist film which is considered due to existence of softening point of the resist polymer. Also changing baking time at this temperature showed very complicated softness behavior. By control of baking temperature, we could obtain harder resist film as baking time becomes longer. Further analysis of these resist film properties by ellipsometry suggested that changes in mechanical strength occur by the evaporation of the resist solvent and/or structure changes inside the resist film, depending upon baking conditions.
Various sizes of concave square patterns are used for microscale bubble adhesion and removal investigation in a water/methanol mixture solution. As decreasing the surface energy of the solution, the micro bubbles are more likely to remove from the square patterns. However, the micro bubble is less likely to remove as decreasing the square size of patterns. The threshold concentration of water/methanol solution for bubble removal can be determined experimentally. Based on the surface energy analysis, the adhesion and removal mechanisms of micro bubble can be explained. The nanoscale bubbles adhered on an ArF excimer resist surface can be observed clearly by using atomic force microscope (AFM). The growth of bubbles on the ArF excimer resist surface can be imaged. By the AFM technique, nanoscale bubble can be divided into some minute bubbles on the ArF resist surface under applying certain force about 5nN. The condensation nature of nanoscale bubbles is discussed.