Mask Critical Dimension Error Research Articles

Flare Impact and Correction for Critical Dimension Control with Full-Field Exposure Tool

Extreme ultraviolet lithography (EUVL) requires flare variation compensation technology and highly accurate critical dimension (CD) control for the fabrication of devices with feature sizes of 32 nm and beyond. To deal with these issues, a mask pattern suitable for evaluating flare and CD variation was designed based on the power spectral density (PSD) of the projection optics of the EUV1. The CD of the replicated patterns clearly varied with the local density of mask patterns, and the variation was affected by mask CD error and flare level. The impact of flare on CD variation was estimated very precisely (<±2 nm) from aerial simulations that took into account the flare level based on the PSD and resist blur, which was modeled as a simple Gaussian function. Mask resizing was found to be useful in compensating for flare variations and was used in the fabrication of the metal-layer patterns for a 22-nm-node static random access memory.

Flare-variation compensation for 32nm line and space pattern for device manufacturing on extreme-ultraviolet lithography

One of the critical issues in extreme-ultraviolet lithography (EUVL) is flare, which degrades the contrast of aerial images and control of the critical dimension (CD) across the exposure field and is related to the density of the absorber layout. It is necessary, therefore, to determine a process window under flare-variation compensation (FVC), taking into account residual FVC error and estimated resist properties. In this article, the authors specify a process window for 32nm line/space patterns under a FVC framework based on rigorous aerial-image simulations. FVC, by means of proper mask resizing, can provide an exposure latitude of up to 23% or more. A 0.5nm grid can be used for mask-data preparation due to the low mask-enhancement error factor of EUVL. In addition, resist blur, estimated by convolving a Gaussian function to aerial images, has a significant impact and the standard deviation of the blur needs to the kept below 7nm to obtain a usable process window, considering mask CD error and the process margin for flare levels of 0%–10%.

Critical Dimension Controllability Evaluation Based on Process Error Distribution for 150 nm Devices

In this study, the critical dimension (CD) controllability of 150 nm patterns was evaluated by aerial image simulation. We considered three types of process errors (dose errors, focus errors, and mask CD errors) and evaluated the CD error distribution by calculating both the CD and the probability for each of the various combinations of process errors. The effects of high numerical-aperture (NA) KrF exposure on CD controllability were investigated in detail. In contact hole patterns and line patterns with various pitches, the CD controllability was significantly improved by increasing the NA, up to about 0.72. However, NA of more than 0.72 does not seem to have further significant effects on the CD controllability. We also investigated the CD controllability in ArF exposure. ArF exposure is expected to exhibit better CD controllability for contact hole patterns and line patterns with various pitches, even under lower NA conditions than KrF exposure.

Optically induced mask critical dimension error magnification in 248 nm lithography

One form of optical proximity effect that further complicates lithography is the unexpected response of the printed image to small perturbations [critical dimension (CD) errors] in the reticle. In this way mask CD errors are actually magnified (they are reduced by less than the reduction factor of the optics) during the optical transfer to the wafer. This effect will require even tighter specifications for mask CD control when the error magnification factor is significantly above unity. The effect is particularly pronounced for tight pitches of small features, but can also impact the printing of small isolated lines. Both resist and optical nonideal responses are involved in this mask error factor (MEF). This article discussed the optical effects that produce the MEF. This article will show where the MEF due to optical effects can be ignored and where they cannot when using 248 nm lithography with a high numerical aperture (NA) tool. We will demonstrate how the NA, partial coherence, and variations in focus can effect the mask error magnification factor. We will also show that resolution enhancement techniques can be used to reduce the mask error magnification factor. In particular, Levenson phase shift masks show particularly low mask error magnification factors for small lines. For some applications it should be possible to design the mask so that the mask error magnification factor of the smallest features is significantly below unity. This would allow loosened reticle CD specifications and/or better CD control of the lithography process.