Koji Kikuchi

New optimization method of exposure with alternative phase-shifting masks

A new method to optimize the structure of an alternative phase shifting mask (alt-PSM) with dual trench type has been developed form the viewpoint of pattern placement error as well as conventional factors of light intensity contrast and exposure-defocus window (ED-window). By using this, the pattern placement error caused by either the phase error or defocus has been reduced. By the simulation, the optimum structure for reducing the pattern placement error is with a phase difference of about 170 degree(s) between phase shifted and non-shifted area. The pattern placement error for 0.13 micrometers lines and spaces can be reduced to less than 2.5 nm from -0.3 to 0.3 micrometers defocus by this optimum. A high contrast and a large ED-window can be obtained as well. However, high accuracy of mask fabrication technique is required. In the experiment, we have evaluated line and space patterns for a 0.13 micrometers node logic gate design rule. It showed less than 7.5 nm errors in the required DOF for the mask with the optimized mask structure.

Simulation for estimating smear noise and sensitivity of CCD image sensors having square pixels less than 2.0 μm

As a consequence of the reduction in the pixel sizes of charge coupled device (CCD) image sensors, the sensitivity of these sensors has decreased, which means that their signal-to-noise ratio (SNR) has also decreased even though the amount of noise is kept constant. In order to maintain and even increase the SNR, we evaluated a simulation method for estimating the sensitivity and smear noise. Smear noise and sensitivity are defined by the number of electrons in vertical registers and photodiodes, respectively. We used a finite-difference time-domain (FDTD) method to simulate the light energy which is proportional to electrons generated in a Si substrate. Using this simulation, we were able to estimate sensitivity and smear noise accurately and optimize the structure of on-chip lenses (OCLs) with respect to these parameters. When we optimized an OCL structure for an interline transfer (IT)-CCD having 1.86-&mgr;m-square pixels, we found that the optimal thickness of the OCL in regards to the smear noise was 0.25 &mgr;m thinner than the optimal thickness for the sensitivity. This result demonstrates that when designing the structures of image sensors, including the OCL shape, it is not only necessary to consider the sensitivity, but it is also important to take the smear noise into consideration.

High accuracy simulation method for CCD image sensors below 2.5 um square cell size

We found that accurate estimation of the actual resist patterns and impurity profiles is the key point in the case of image sensors below 2.5 um square cell size. We apply a resist patterning process model to our process/device simulation. In the photolithography process simulation, each patterned resist layer exhibits own resist corner rounding regarding as differences such as resist thickness and wavelength of stepper. For the ion implant processes and thermal processes, channeling and doped impurity diffusion models are newly applied. We introduced two dimensional Monte Carlo simulation in order to estimate channelings affected by impurity species, accelerating voltage of implanter and crystallographic orientation. This enables to get impurity profiles of implant processes with mega order accelerating energy. Three dimensional impurity diffusion profiles can be obtained by using the optimized ratio of lateral diffusion to perpendicular diffusion. We have confirmed the advantage of the new simulation method by evaluation of device characteristics in small size CCDs.

Defect printability of alternating phase-shift mask: a critical comparison of simulation and experiment

An alternative phase shift mask (alt-PSM) is a promising device for extending optical lithography to finer design rules. There have been few reports, however, on the masks ability to identify phase defects. We report here an alt-PSM of a dual-trench type for KrF exposure, with programmed quartz defects used to evaluate defect printability by measuring aerial images with a Zeiss MSM100 measuring system. The experimental results are simulated using the TEMPEST program. First, a critical comparison of the simulation and the experiment is conducted. The actual measured topography of quartz defects are used in the simulation. Moreover, a general simulation study on defect printability using an alt-PSM for ArF exposure is conducted. The defect dimensions, which produce critical CD errors are determined by simulation that takes into account the full 3-dimensional structure of phase defects as well as a simplified structure. The critical dimensions of an isolated defect identified by the alt-PSM of a single-trench type for ArF exposure are 240 nm in bottom diameter and 50 degrees in height (phase) for the cylindrical shape and 240 nm in bottom diameter and 90 degrees in height (phase) for the rotating trapezoidal shape, where the CD error limit is +/- 5%.

New method for reducing across chip poly-CD variation with statistical OPC/gauge capability analysis

In this paper, we propose a new analysis method to quantify the performance of optical proximity effect correction (OPC) as well as the gauge capability of in-process quality control (IPQC) monitors. The method consists of two stages. The first stage is to quantify the gauge capability of IPQC monitor patters for representing across chip line width variation (ACLV). The second one is to verify OPC by using the IPQC monitor patterns as verified in the first stage. Our new analysis method has bene found to be remarkably effective and quite useful to optimize OPC parameters and other methods in attempting to reduce ACLV, the most critical factor in producing high-end logic devices with a design rule of 0.15 micrometers and below.

Optimization method of the double exposure technique with alt-PSMs for below a 0.13 μm node

A new method to optimize the structure of alternative phase shifting masks (alt-PSMs) with dual trench type and optical proximity effect correction (OPC) has been developed from the viewpoint of pattern placement error and gate width fluctuation. The pattern placement error and gate width fluctuation have been reduced significantly to meet the requirements of overlay margin and critical dimension (CD) accuracy for the 0.13 μm node devices by this method. The optimum mask structure for reducing a pattern placement error was determined by the simulation that takes account of the mask topography effect. The alt-PSM with a deep trench depth of 570 nm and a phase difference of about 170° between the phase shifted and nonshifted area was optimum to reduce pattern placement error. At the optimum condition, a high contrast was also obtained. For the experiment, we used a set of masks to study dependence of mask cross sectional structure. While the dependence of pattern placement error on focus shift is much large...