We compared a simulator's predictions with the critical dimension (CD) value measured on the
wafer. We used sub resolution assist features (SRAF) in the experiment to keep the focus margin, the
minimum size of the mask was small and comparable with the absorber's thickness. Therefore, it
seems that we need a rigorous model and a variety of parameters for high prediction accuracy.
We investigated the prediction error and found its behavior was not complicated. The dependence
of the prediction errors was related to the space until the next feature, but the relationship was not
linear; rather, it went up and down periodically like a Bessel function. This fact gave us the idea that it
might be possible to improve the simulation accuracy by using a special convolution kernel but not a
We used a complementary kernel and tried to find a suitable shape to match the prediction error.
The convolution kernel consisted of a complex number in order to represent phase change and
amplitude loss. The kernel was applied to the simulator's mask plain. The results showed a significant
improvement in simulation accuracy and a reduction in the route mean square (RMS) of the CD fitting
error for all features with or without SRAFs.
We used this model for optical proximity correction (OPC) and verified its accuracy with a printed
wafer image. The range of the final CD variation of 40 nm line on the wafer was 1.9 nm, and the
model also showed good agreement with the experimental two-dimensional feature shape.
According to the ITRS Roadmap, for 45nm Node (as 65nm Half Pitch), the requirement of Gate CD Control
is defined as 2.6nm. One of the most challenging CD errors is Iso-Dense Bias (IDB). Assuming 40% of CD errors are
dominated by IDB, IDB should be less than 1nm. In general, the majority of IDB is due to: primarily, exposure tool-
related factors such as aberrations, flare, and sigma fluctuation, and secondly, the change in photoresist characteristics.
However, due to the rapidly increasing usage of ArF exposure tools, Band Width (BW) characteristics of the laser
source is an additional factor whose contribution is becoming more critical.
Ideally, BW is monochromatic, thereby not affected by chromatic aberration change. However, in reality, the
BW exhibits a shape of spectral distribution with a finite width.
This study describes experimental and simulation results for E95%, and how performance of both CDs and Laser is
dependent on E95% in order to meet 1nm of IDB towards 45nm Node.
-IDB vs. E95%
-CD at through pitch vs. E95%
-Process Latitude vs. E95%
-Pattern shortening vs. E95%
The double exposure technique using alternating phase shift mask (alt-PSM) has been proposed and it is well
used for the gate layer of the high performance logic devices as strong resolution enhancement technology (RET). This
technique has advantage that the fine resist profile is obtained on wafer with extensive process margin. However, this
double exposure technique is very expensive because of the alt-PSM cost. This time, the new double exposure technique
without alt-PSM is developed for gate layer of 45 nm node logic devices. In this new double exposure method,
attenuated phase shift mask (att-PSM) or binary mask (BIM) is used with dipole illumination. It is thought that this new
double exposure method is effective for random logic devices which have various pattern pitches by the optimization of
dipole illumination condition and pattern placement. Firstly, the optical contrast and depth of focus (DOF) is calculated.
From these results, dipole illumination condition is optimized. It is found that DOF of new double exposure method is
wider than that of conventional method. In addition, mask pattern is optimized to obtain wide process margin. For dense
pattern, mask biasing is effective and optimization of shifter width is effective for isolated pattern. Furthermore, it is
found that assist pattern is very effective for isolated pattern. From experimental results, it is proved that new double
exposure method have wider process margin than that of conventional one. The strong design for manufacturing (DFM)
rule that required the severe line width control is placed at single direction is proposed to realize the new double
exposure method. Finally, it is found that the lithographic performance of new double exposure method has same level as
conventional method with alt-PSM for gate layer of 45 nm logic devices.
Process windows have become narrower as nano-processing technology has advanced. The semiconductor industry, faced with this situation, has had to impose extremely severe tool controls. Above all, with the advent of 90-nm device production, demand has arisen for strict levels of control that exceed the machine specifications of ArF exposure systems. Consequently, high-accuracy focus control and focus monitoring techniques for production wafers will be necessary in order for this to be achieved for practical use. Focus monitoring techniques that measure pattern placement errors and resist features using special reticle and mark have recently been proposed. Unfortunately, these techniques have several disadvantages. They are unable to identify the direction of a focus error, and there are limits on the illumination conditions. Furthermore, they require the use of a reticle that is more expensive than normal and they suffer from a low level of measurement accuracy. To solve these problems, the authors examined methods of focus control and focus error measurement for production wafers that utilize the lens aberration of the exposure tool system. The authors call this method FMLA (focus monitoring using lens aberration).
In general, astigmatism causes a difference in the optimum focal point between the horizontal and vertical patterns in the same image plane. If a focus error occurs, regardless of the reason, a critical dimension (CD) difference arises between the sparse horizontal and vertical lines. In addition, this CD difference decreases or increases monotonously with the defocus value. That is to say, it is possible to estimate the focus errors to measure the vertical and horizontal line CD formed by exposure tool with astigmatism.
In this paper, the authors examined the FMLA technique using astigmatism. First, focus monitoring accuracy was investigated. Using normal scholar type simulation, FMLA was able to detect a 32.3-nm focus error when 10-mλ astigmatism was present. Furthermore, we verified that it was possible to experimentally detect a 20-nm focus error for gate layer of 90-nm logic devices. In tilt error evaluation, the estimated tilt error value was separated by 0.3-ppm from the input value into exposure tool parameters. Finally, when FMLA was applied to gate layer of 90-nm logic devices, inter lot distribution was decreased from 6.8-nm to 2.8-nm, and it was proved that FMLA using astigmatism was an effective method in device manufacturing.
An alternating phase shift mask (alt. PSM) must be fabricated in such a way that imbalances in optical intensities are minimized. The mask structure must be optimized to obtain a balanced distribution of optical intensities and this means that the shifter thickness/quartz depth that corresponds to a phase angle of 180 degrees and the correct amount of undercutting should be estimated. There are two key points in the optimization of an alt. PSM. One is to find the optimum structure in terms of reducing the amount of undercutting. Narrower chrome (Cr) line widths are required for ArF laser lithography than for KrF laser lithography, so the undercutting must be restricted to prevent peeling of the Cr patterns, degradation of cleaning durability, and so on. Another key point is to investigate the effect of Cr line widths and pattern pitches on imbalances in the optical intensities. A variety of pattern pitches and Cr line widths are available from actual devices. All patterns, however, have same shifter thickness and amount of undercutting on each mask produced by a given mask fabrication process. It is thus necessary to study the effect on optical intensities of changes in Cr line widths and pattern pitches so that it is possible to optimize mask structures for a variety of patterns. From our simulation and experimental results, we found that an alt. PSM with vertical sidewalls has advantages in terms of reducing the amount of undercutting and is effective in the fabrication of sub 100-nm devices. We also discovered that imbalances in optical intensities vary periodically with Cr line widths. It was found that a structure for an alt. PSM should be optimized for each Cr line widths on these bases.
A dual exposure method with an alternating phase shift mask has been proposed for using KrF laser lithography to fabricate 100 nm gate patterns for logic devices. Fine and uniform patterns can be formed and so this process is considered very advantageous in terms of the formation of gate for logic devices. Several factors determine the lithographic performance of the alternating phase shift mask: phase accuracy, amount of undercutting, quartz and chromium defects, and so on. It is thought that these factors need to be strictly controlled. We thus investigated the impact of errors in the fabrication of alternating phase shift masks to determine the quality required for the dual exposure method, focusing on three factors: phase accuracy, amount of undercutting, and defects. A phase error causes CD variation and lateral shift in the defocused condition. Unsuitable undercutting causes lateral shift at the best focus. Shifter and chromium defects cause CD variation and distortion of the gate patterns. Our experimental results showed that these factors do not need to be strictly controlled. We thus propose a fabrication process for alternating phase shift masks to be used in the dual exposure method. Keywords: Alternating phase shift mask, dual exposure, phase accuracy, undercutting, defect