Elsevier

Microelectronic Engineering

Volume 98, October 2012, Pages 270-274
Microelectronic Engineering

Scatterometry analysis of sequentially imprinted patterns: Influence of thermal parameters

https://doi.org/10.1016/j.mee.2012.05.048Get rights and content

Abstract

Scatterometry technique has been used to characterize Thermal Step and Repeat NIL processes in the framework of NAPANIL project. Two hundred and fifty nanometers dense lines were imprinted in mr-7030 polymer and their profiles have been analyzed and compared to the mold one. It has been demonstrated that scatterometry on silicon exhibits a very high accuracy and that a change of the sidewall verticality of only few degrees can been measured. The results show that some reflow occurs in lines during the imprint of neighboring dies due to heat diffusion. This phenomenon has been studied as a function of printing temperature, demolding temperature, and space between adjacent dies. The conclusion is that a trade off has to be made between imprint temperature and chuck temperature to be able to fill mold cavities with a limitation of the reflow.

Highlights

► Thermal Step and Repeat NIL processes characterized by scatterometry. ► Scatterometry demonstrates high accuracy to quantify imprinted lines on silicon. ► Polymer reflow due to heat diffusion has been studied as a function of printing parameters.

Introduction

Metrology techniques have to be adapted for NIL processes in order to improve the control of printed patterns for industrial applications. It has already been demonstrated that scatterometry is well adapted for this purpose as long as it is used to characterize polymer patterns imprinted on silicon wafers [1], [2]. This very fast and non destructive technique can be used to establish some statistics on patterning processes. Nanoimprint Lithography (NIL) exhibits many advantages such as high resolution, throughput and reasonable cost. NIL techniques are largely developed at sample or wafer scale and it has been already demonstrated that printing uniformity is very good even on 200 mm silicon wafers which are printed directly with a 200 mm mold [3]. But some applications require so high resolution that it will be difficult to produce directly by E-beam lithography a large scale mold fully covered by nanostructures. Depending on the resolution and pattern design, exposure times in E-beam lithography can be as long as several days to pattern a full 200 mm wafer, even with a shaped beam technology. Other technique is needed to produce such a mold, especially for industrial applications. A common solution is to fabricate a small mold of few square centimeters size by E-beam lithography, and to up-scale this master to obtain a larger stamp. NIL Step and Repeat processes, such as step and stamp Nanoimprint Lithography [4], [5] can be used to fabricate large scale stamps. This technique can be implemented using Thermal NIL or UV-NIL. Such processes already exist, but the reproducibility of the process has to be carefully characterized at the wafer scale, with a fast and non destructive technique. Scatterometry is the best candidate for this purpose. This paper presents scatterometry analysis of SSIL patterned grating structures developed for day-lighting applications. Some estimation of scatterometry accuracy will be detailed. Then the influence of printing parameters on the pattern profile will be presented, using a Thermal Step and Repeat process. It will be shown that heat diffusion can induce a reflow of polymer and how this limitation can be avoided.

Section snippets

Experimental

Scatterometry is based on the comparison between the optical signature of a periodic grating and the result of a simulation which uses MMFE method combined with a modeling of the pattern profile. Details of simulation model are described in Ref. [1]. The optical signature is obtained using a Jobin Yvon ellipsometer with wavelength varying from 250 to 800 nm.

The master was fabricated by standard DUV lithography and plasma etching processes. The patterns are dense lines, as shown on Fig. 1. Main

Results and discussion

Four hundred dies have been imprinted on the wafer shown in the photo of Fig. 2. All the dies have been characterized by scatterometry. Fig. 3 presents ellipsometric spectra of lines obtained in die No. 400 (the last one) and in die No. 45 which is representative of all the dies except the last one. Table 1 shows the geometrical parameters measured for these both dies. After this last imprint of die No. 400, the patterns exhibit geometrical parameters in good agreement with the stamp ones,

Conclusion

Step and Repeat Thermal NIL processes have been analyzed in terms of pattern reproducibility using scatterometry technique. It has been shown that the scatterometry can give the geometrical parameters of structures in all dies and at different area of one die with a very high accuracy. We highlight that polymer reflow occurs during the imprint of adjacent dies over a long distance dependent of mold and chuck temperatures. Nevertheless, a trade off has to be made, since enough heating energy is

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