The Characterization of Resins in Lithographic Processes

Introduction

The Laboratoire d’Electronique de Technologie et d’Instrumentation (LETI) specializes in the application of photosensitive resins for lithographic processes in the semiconductors industry (transistors for integrated circuits). The ever-increasing miniaturization of transistors is the driving force behind numerous technological advances. Currently, the best possible resolution that can be achieved for isolated lines is 40 nm, and 60 nm for lines in dense patterns. These limits are most probably due to the different resin components presently used. The current challenge is therefore to develop new resin formulations (chemically amplified resists or so-called CARs) that will allow a resolution of 20 nm to be obtained

CARs are multi-component mixtures consisting of a polymer matrix (the combination of two polymers), a photo acid g;enerator, PAG, and other additives, depending on the properties desired. In order to improve the properties of CARs, the production process has to be optimized. This in turn requires an understanding of the physicochemical behavior of each component of the CAR and its effect on the process.

The LETI-LTM (Laboratoire de Technologie Microélectronique) and the “Laboratoire de Thermodynamique des Solutions et des Polymères” cooperate in this area of research. Temperature-modulated DSC (ADSC), FTIR and thickness measurement techniques are used to investigate the influence of the different resin components on resolution in the lithographic process. The main advantage of ADSC is that the glass transition temperature and the thermal effects associated with the lithographic process (vaporization and cross-linking) can be determined in one single measurement [1].

Experimental Details 

Various resins manufactured by Sumitomo Incorporated (Japan) differing in at least one component were investigated in order to understand the physicochemical behavior of the mixtures. To do this, the influence of several parameters such as the polymer matrix, the molecular weight of the individual polymers, the PAG and the solvent was measured. First of all, ADSC curves of all the resins were measured to determine the glass transition region and cross-linking temperature. This is illustrated in Figure 1 with the NEB22 resin. 

These investigations are necessary to optimize the processing conditions, to control diffusion processes and to minimize the sensitivity to contamination. The type of CAR and the processing temperatures influence the film thickness. If the bake temperature is increased, the films become thinner whereby so-called stable temperature domains can also occur. This is illustrated in Figure 2 with the NEB22 resin. The right processing temperature helps to optimize the compactness of the resin layer. The lithographic process is initiated at about 100 °C. It is therefore important to investigate what happens in this temperature range.

The decrease in thickness of the resin with increasing processing temperature (see Fig. 2) indicates that the film is not completely stable. This is due to internal processes such as the vaporization of different components. Free volume is thereby generated, which is then eliminated during further baking. In order to identify the components responsible for this instability, partial CAR formulations were prepared in which one of the components was missing each time. The thermal behavior of each formulation was measured. This led to the following results [2]:

Conclusions 

DSC, ADSC and FTIR measurement techniques enable valuable information for the improvement of CAR formulations to be obtained. The study shows that thermal analysis can help one to understand the behavior of CAR components and to optimize the lithographic process. ADSC is a useful technique to measure the glass transition and vaporization/curing effects that take place almost simultaneously.

^{The Characterization of Resins in Lithographic Processes | Thermal Analysis Application No. UC 153 | Application published in METTLER TOLEDO Thermal Analysis UserCom 15}