Magazine fall01 investigation193nm

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Investigation of 193 nm Resist Shrinkage During CD-SEM Measurements Thomas Hoffmann, Greet Storms, Monique Ercken, Mireille Maenhoudt, Ivan Pollentier, Kurt Ronse, IMEC, Belgium Franck Felten, Evelyn Wong, Jonathan England, KLA-Tencor, Europe

193 nm resists are known to shrink during CD-SEM measurements. The large size and non-linear behavior of this shrinkage must be characterized and understood if CD-SEM metrology is to be correctly applied in advanced lithography processing. This paper describes a study in which recommendations for the best measurement conditions were developed and speculations on possible models for the observed shrinkage mechanisms could be made.

Introduction

It is well known that 193 nm resist features change size permanently during CD-SEM measurements.1-5 The size of the shrinkage, often up to 40 nm, should be compared to the CD metrology budget of 1 nm for features in the 100 nm design rule node, when 193 nm lithography is expected to enter production for critical layers. 1 The several classes of 193 nm resist chemistry (COMA, acrylate, cyclo-olefins, VEMA) and layer schemes (single, thin imaging layer and hybrid) all exhibit shrinkage to varying degrees depending on their formulations, process history and measurement conditions. Shrinkage is observed to progress in a nonlinear way with applied e-beam dose and understanding the mechanisms that contribute to this shrinkage is complex. Several studies2, 3 have been reported the attempt to improve this understanding as a basis to improve the resist materials. As yet, complete elimination of e-beam-initiated shrinkage has yet to be achieved. This effect has largely been overcome in 248 nm resist metrology, but we may expect similar or worse effects in some 157 nm materials. It is, therefore, important to understand and minimize resist shrinkage in order to be able to meet the challenges for production worthy CD-SEM metrology of advanced materials. This 32

Fall 2001

Yield Management Solutions

paper discusses investigations of shrinkage effects carried out in joint work between IMEC and KLA-Tencor in a study to develop recommendations for CD-SEM conditions that can minimize shrinkage. Experiments

This study investigates a 193 nm resist exhibiting above-average e-beam shrinkage. Wafers were uniformly exposed several days prior to CD-SEM measurements using an ASML 5500/900 argon fluoride scanner at IMEC. Trenches with a nominal CD of 150 nm were measured using five to ten fresh sites for each experiment. It should be noted that resist shrinkage cause the reported trench CD measurement values to increase. A first set of CD-SEM measurements were carried out at KLA-Tencor, San Jose on an 8200-R CD-SEM and these were repeated and extended in IMEC on 8100XP and 8100-ER systems. It has already been widely reported that e-beam exposure of the measurement position must be minimized. Therefore, a standard 193 nm resist measurement recipe was created with a low-magnification pattern recognition step (magnification of 6.25kX, 24 Âľm field of view (FOV)) to identify the region to be measured. The e-beam spot size was then automatically focused in a region away from the measurement site. A highermagnification pattern recognition step (magnification 12.5kX, 12 Âľm FOV) was then used to identify the exact area of the trench to be measured.


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