Optical Ellipsometric Scatterometry: Instrumentation and Applications

Optical ellipsometric scatterometry  techniques, such as scatterometry based on conventional spectroscopic  ellipsometry (SE), have currently become one of the most important approaches  for in-line metrology of geometric parameters and nanostructures in high-volume-production  nanomanufacturing, due to their attractive advantages of high throughput, low  cost, non-contact, and non-destruction. Optical ellipsometric scatterometry is  essentially model-based metrology, which involves two key issues - how to  collect highly precise ellipsometric scattering data, and how to rapidly and  accurately reconstruct the profiles of nanostructures from the measured data.  The former issue involves the development of specific ellipsometric  scatterometry tools, while the latter involves the theory and method of  computational metrology for nanostructures. In order to collect highly precise  ellipsometric scattering data, we have developed three kinds of ellipsometric  scatterometry instruments for different metrology requirements of practical  nanostructures.

(1) Development of a broadband  Mueller matrix ellipsometer

Mueller matrix  ellipsometry (MME), sometimes also referred to as Mueller matrix polarimetry (MMP)  or generalized ellipsometry, can change three measurement conditions, i.e., the  wavelength, the incidence, and azimuthal angles. It can also provide up to 16  quantities of a 4 by 4 Mueller  matrix in each measurement. Compared with conventional SE-based scatterometry,  which only obtains two ellipsometric parameters, MME-based scatterometry can  thereby achieve much more useful information about the sample. By choosing an  appropriate configuration of the three measurement conditions and by fully  exploring the rich information contained in the collected Mueller matrices, MME-based  scatterometry is expected to achieve a higher measurement accuracy. We  have developed a broadband Mueller matrix ellipsometer under the support of the  National Natural Science Foundation and the National Instrument Development  Specific Project of China.

(2) Development of a Mueller matrix imaging  ellipsometer    

Although optical  ellipsometric scatterometry techniques have achieved wide applications in high-volume-production  nanomanufacturing for nanostructure metrology, they are inherently limited by  the size of the illumination spot, which limits the lateral resolution of the  instrument as well as the efficiency in constructing a map of the sample over a  large area. Aiming at these issues, we have developed a prototype of the Mueller  matrix imaging ellipsometer (MMIE) by introducing conventional imaging  techniques to optical ellipsometric scatterometry. The experimental results  have demonstrated that we can realize Mueller matrix measurement and analysis  for nanostructures with a pixel-sized illumination spot by using MMIE. We can  also directly construct parameter maps of nanostructures over a large area with  pixel-sized lateral resolution by performing parallel ellipsometric analyses  for all the pixels of interest.

(3) Development of a Mueller  matrix ellipsometer with scatterfield tomography

Both the MME and MMIE are used only for the metrology of periodic  nanostructures. They are not suitable for the metrology of much more complex  nanostructures, especially aperiodic nanostructures or nanostructures with  limited periods corresponding to the illumination spot size. In order to deal  with this issue, we are developing a novel instrument based on our previously  developed ellispometric scatterometry techniques. We call this novel instrument  as Mueller matrix ellipsometer with scatterfield tomography (MMEST). By  combining the diffractive tomography techniques, MMEST can collect the scatterfield  information about the sample at different incidence angles over a large range at  a high speed. Therefore, MMEST can acquire much more information in comparison  with the MME and MMIE techniques, which can only obtain the scatterfield  information in a specific direction, namely the direction of the zeroth-order  diffraction. By fully exploring the collected scatterfield information, it is  expected that MMEST can realize the measurement of aperiodic nanostructures.

     (4) Applications to nanostructure metrology

Based on the above  in-house developed ellipsometric scatterometry instruments, we have conducted a  lot of experiments involving some typical nanostructures in cooperation with  several research institutions and corporations at home and abroad. The measured  nanostructures include e-beam patterned structures with small critical  dimensions, photoresist structures with natural line edge roughness, nanopillar  arrays with a high-aspect ratio, and nanoimprinted structures with nonuniform  residual resist layers. As an example, for the nanoimprinted structures, we  first found the depolarization effect induced by the residual layer thickness  variation. After incorporating depolarization effects, not only could improved  measurement accuracy be achieved for the line width, line height, and residual  layer thickness measurement, but the residual layer thickness variation could  also be directly determined over the illumination spot.



Model-Based Infrared Reflectrometry for Deep Trench Structures

Fig. 1. The in-house developed broadband Mueller matrix  ellipsometer.

Model-Based Infrared Reflectrometry for Deep Trench Structures

Fig. 2. Data  analysis software of the broadband Mueller matrix ellipsometer.

Model-Based Infrared Reflectrometry for Deep Trench Structures

Fig. 3. The prototype of the in-house developed Mueller matrix  imaging ellipsometer.

Model-Based Infrared Reflectrometry for Deep Trench Structures

Fig. 4. Some  typical nanostructures measured by the in-house developed ellipsometric scatterometry  instruments.

Model-Based Infrared Reflectrometry for Deep Trench Structures

Fig. 5. Measurement  of nanoimprinted structures with non-uniform residual resist layer and the  comparison between measured and model-calculated depolarization index spectra.

Selected papers

  1. S.   Y. Liu, W. C. Du, X. G. Chen, H. Jiang, and C. W. Zhang, "Mueller matrix imaging   ellipsometry for nanostructure metrology," Opt. Express 23(13),   17316-17329 (2015). (URL, PDF)

  2. S. Y. Liu, X. G. Chen, and C. W. Zhang, "Development of a broadband Mueller matrix ellipsometer as a powerful tool for nanostructure metrology," Thin Solid Films 584, 176-185 (2015). (URL, PDF)

  3. J. L. Zhu, S. Y. Liu, H. Jiang, C. W. Zhang, and X. G. Chen, "Improved deep-etched multilayer grating reconstruction by considering etching anisotropy and abnormal errors in optical scatterometry," Opt. Lett. 40(4), 471-474 (2015). (URL, PDF)

  4. H. G. Gu, S. Y. Liu, X. G Chen, and C. W. Zhang, "Calibration of misalignment errors in composite waveplates using Mueller matrix ellipsometry," Appl. Opt. 54(4), 684-693 (2015). (URL, PDF)

  5. X. G. Chen, C. W. Zhang, S. Y. Liu, H. Jiang, Z. C. Ma, and Z. M. Xu, "Mueller matrix ellipsometric detection of profile asymmetry in nanoimprinted grating structures," J. Appl. Phys. 116(19), 194305 (2014). (URL, PDF)

  6. X. G. Chen, S. Y. Liu, C. W. Zhang, H. Jiang, Z.  C. Ma, T. Y. Sun, and Z. M. Xu, "Accurate characterization of nanoimprinted  resist patterns using Mueller matrix ellipsometry," Opt.  Express 22(12), 15165-15177 (2014). (URL, PDF)

  7. S. Y. Liu, X. G. Chen, and C. W. Zhang, "Mueller  matrix polarimetry: A powerful tool for nanostructure metrology," Presented at  China Semiconductor Technology International Conference (CSTIC), Shanghai,  China, March 16-17, 2014,  in ECS Trans. 60(1), 237-242 (2014).  (Invited speech) (URL, PDF)

  8. X. G. Chen, S. Y. Liu, C. W. Zhang, Y. P. Wu, Z.  C. Ma, Y. T. Sun, and Z. M. Xu, "Accurate measurement of templates and  imprinted grating structures using Mueller matrix ellipsometry," Acta. Phys. Sin. 63(18), 180701 (2014). (URL, PDF)

  9. X. G. Chen, C. W. Zhang, and S. Y. Liu, "Depolarization  effects from nanoimprinted grating structures as measured by Mueller matrix  polarimetry," Appl. Phys. Lett. 103(15), 151605 (2013). (URL, PDF)