In recent years, lithographic printability of overlay metrology targets for memory applications has emerged as a significant issue. Lithographic illumination conditions such as extreme dipole, required to achieve the tightest possible pitches in DRAM pose a significant process window challenge to the metrology target design. Furthermore, the design is also required to track scanner aberration induced pattern placement errors of the device structure. Previous workiii, has shown that the above requirements have driven a design optimization methodology which needs to be tailored for every lithographic and integration scheme, in particular self-aligned double and quadruple patterning methods. In this publication we will report on the results of a new target design technique and show some example target structures which, while achieving the requirements specified above, address a further critical design criterion – that of process resilience.
We present a metrology target design (MTD) framework based on co-optimizing lithography and metrology performance. The overlay metrology performance is strongly related to the target design and optimizing the target under different process variations in a high NA optical lithography tool and measurement conditions in a metrology tool becomes critical for sub-20nm nodes. The lithography performance can be quantified by device matching and printability metrics, while accuracy and precision metrics are used to quantify the metrology performance. Based on using these metrics, we demonstrate how the optimized target can improve target printability while maintaining the good metrology performance for rotated dipole illumination used for printing a sub-100nm diagonal feature in a memory active layer. The remaining challenges and the existing tradeoff between metrology and lithography performance are explored with the metrology target designer’s perspective. The proposed target design framework is completely general and can be used to optimize targets for different lithography conditions. The results from our analysis are both physically sensible and in good agreement with experimental results.
In order to handle the upcoming 1x DRAM overlay and yield requirements, metrology needs to evolve to more accurately represent product device patterns while being robust to process effects. One way to address this is to optimize the metrology target design. A viable solution needs to address multiple challenges. The target needs to be resistant to process damage. A single target needs to measure overlay between two or more layers. Targets need to meet design rule and depth of focus requirements under extreme illumination conditions. These must be achieved while maintaining good precision and throughput with an ultra-small target. In this publication, a holistic approach is used to address these challenges, using computationally optimized metrology targets with an advanced overlay control loop.
Resist reflow is a simple and cost effective technique by which the resist is baked above the glass transition temperature (Tg) after the typical contact hole pattern has been exposed, baked and developed. Resist reflow method can obtain very high resolution without the loss of process margin than any other resolution enhancement techniques that can make the same linewidth. But it is difficult to predict the results of the thermal flow and the process optimization. If the results of reflow process can be exactly predicted, we can save great time and cost. In order to optimize the layout design and process parameters, we develop the resist flow model which can predict the resist reflow tendency as a function of the contact hole size, initial shape and reflow temperature for the normal and elongated contact hole. The basic fluid equation is used to express the flow of resist and the variation of viscosity and density as a function of reflow temperature and time are considered. Moreover surface tension and gravity effects are also considered. In order to build a basic algorism, we assume that the fluid is incompressible, irrotational and Newtonian. First, we consider the boundary movement of side wall and we think the basic equations for free surface flow of fluid as 2-dimensional time-dependent Navier-Stokes equations with the mass conservation equation. Surface tension acting on the interface pressure difference and gravity force that enable the resist flow are also included.
The desired minimum feature size is decreasing for the future technology nodes. Immersion lithography has been actively pursued as a method of extending the resolution of optical lithography beyond 65 nm mode. Immersion lithography and hyper NA impact the selection and optimization of the various resolution enhancement techniques (RET). These can be selected as appropriate for each mask pattern. As the line width on target is narrower, the fine-line structure will no longer be discernible. Then this is the resolution limit of the system. Until recent times, the traditional means of determining the quality of an optical element or system of elements was to evaluate its limit of resolution. A useful parameter in evaluating the performance of a system is the modulation transfer function and this is analyzed for the hyper NA immersion lithography.
The minimum feature size of the semiconductor device will be smaller and smaller because of the increasing demand for the high integration of the device. According to recently proposed roadmap, ArF immersion lithography will be used for 65 nm to 45 nm technology nodes. Polarization effect becomes a more important factor due to the increasing demand for high NA optical system and the use of immersion lithography. It is important to know that the polarization effect is induced by mask in small size patterning. The unpolarized plane waves leaving the illumination system are diffracted by the mask. So the light beam going through the mask will experience induced polarization by the mask. In this paper, we considered the change of polarization state as a function of mask properties. We calculated vector diffraction of 193 nm incident light. The masks considered are the chromeless mask, a binary chrome mask and 6 % attenuated phase shifting mask. We use the finite-difference time-domain method to solve the Maxwell equation. The aerial image depends on the polarization states induced by the mask properties such as materials, thickness, and pitch.