Metrology of thin resist for high NA EUVL

Abstract:

One of the many constrains of High Numerical Aperture Extreme Ultraviolet Lithography (High NA EUVL) is related
to resist thickness. In fact, one of the consequences of moving from current 0.33NA to 0.55NA (high NA) is the Depth
of Focus (DOF) reduction. In addition, as the resist feature lines shrink down to 8nm half pitch, it is essential to limit
the aspect ratio to avoid pattern collapse. The direct consequence of such a situation is that a resist thickness of 30nm,
usually used for 32nm pitch dense line/space (LS), will not be suitable for 16nm pitch, where the target thickness is
expected to be 15nm thickness or less to ensure a similar aspect ratio.
The question we need to answer is how the resist thickness reduction will impact the various metrology techniques
needed to properly set up a process. To address this question, a set of wafers using both Chemical Amplified Resist
(CAR) and Metal Oxide Resist (MOR) at different thicknesses and with different types of underlayer have been
generated for LS patterns at 32nm pitch. We first investigated the impact of film thickness by scanning electron
microscope (SEM) on the imaging of CAR resist lines. To start with, our current Best-Known Methods (BKM’s) were
used to acquire the SEM images. As resist thickness decreases, noise level and image contrast are observed to degrade
dramatically. Such an image quality degradation may directly impact the quality of the CD measurements both in
terms of accuracy and precision.
In this paper we investigated the thin resist wafer set described above using various techniques, such as Critical
Dimension Scanning Electron Microscope (CD SEM), Atomic Force Microscopy (AFM), Low Voltage SEM (LV
SEM), scatterometry, Pattern Shift Response (PSR), and optical defect inspection. The impact of the resist thickness
is estimated for each approach, and optimal settings were investigated to minimize the relative impact on metrology.
Our results indicated that, in most cases, alternative operation conditions and BKM settings, sometimes drastically
different from the usual operation condition, must be used to guarantee the metrology requirements. Our results show
that, despite the impact of thinning resist materials, it is possible to find appropriate settings to strengthen the
metrology quality output.
Keywords: Thin Resist, High NA EUVL, e-beam, CD SEM, Scatterometry, AFM, inspection, PSR