Development and characterisation of a new silicon single crystal linewidth reference material

Progressive miniaturisation of advanced nanomanufacturing techniques currently delivers nanodevices with a feature size of below 20 nm. The accurate measurement of the dimensional parameters, particularly the feature width of the nanostructures both on silicon wafers and photomasks, is a crucial task for process development and process control. Addressing the challenging issues of industry such as tool-to-tool matching, hybrid metrology and AFM tip characterisation, a kind of new linewidth reference material with improved vertical parallel sidewalls (referred as IVPS-100-PTB) has been developed at PTB in collaboration with the company Team-Nanotec.
The new IVPS-100-PTB sample is based on the already commercially available sample IVPS-100. Its layout is shown in figure 1(a). The sample has a size of 6 mm x 6 mm. It consists of four separated groups of 5 x 5 feature patterns, which offers the possibility to further cut the sample into smaller pieces so that, for instance, parts of it can be applied for destructive characterization methods, like e.g. transmission electron microscopy (TEM). The layout of a feature pattern is shown in figure 1(b). It consists of a group of five line features to be used as the reference line features. These line features have improved vertical parallel sidewalls, similar to the IVPS-100 and different nominal CD values of 50 nm, 70 nm, 90 nm, 110 nm and 130 nm, respectively. Thus the sample can be applied for calibrating e.g. both the scaling factor and the tip width of CD atomic force microscopes (CD-AFMs). The sample has been fabricated by Team nanotec GmbH using electron beam lithography to define the structures in a masking material for subsequent lattice plane selective wet etching utilising an alkaline solution.
A number of measurements have been carried out to thoroughly characterise the quality of the developed line features using both high-resolution TEM (HR-TEM) and CD-AFM.
The sample preparation is a critical step for achieving high quality (S)TEM measurements. It has been performed using a dual-beam FIB instrument (FEI helios nanolab 400s) at the target measurement location defined by the apexes of the alignment marks. At first a protective carbon layer is applied to the target structures using the electron beam followed by a platinum layer deposited by the ion beam. Lamellas were produced utilizing the FIB lift out technique. The lamellas were further thinned to have a thickness of approx. 50 nm using 2 kV Ga+ ions before being measured in TEM. Great care was taken to ensure that the feature surfaces are not damaged during the preparation.
Figure 2 illustrates an overview STEM image taken on a feature pattern measured with a magnification of 7100x. The cross-sectional view of two groups of alignment marks (each with 9 grooves) is clearly visible in the figure. The depth of the nine grooves is unequal due to the correlation between the length and depth of the alignment grooves of the marker. It indicates that all grooves of the alignment marks have been cross-sectioned in the sample preparation, confirming the deviation between the actual and desired measurement location being less than +/-0.2 μm. Five line features L5 … L1 are visible in the middle of two alignment marks. As being discussed in a previous study [1], the crystal lattice constant of silicon can be applied as an internal scale for calibrating the scaling factor of (S)TEM measurements, thus offering high accuracy.
CD-AFMs are widely used particularly in semiconductor industry for non-destructive measurements of 3D geometry of nanostructures. Unlike conventional AFMs which usually apply pyramidal shaped tips, the CD-AFM uses flared tips. Such tips have an extended geometry near their free end which enables the probing of steep and even undercut sidewalls. We applied a CD-AFM developed at PTB which features a special probing and measurement strategy referred to as vector approaching probing (VAP) [2] for the measurements.
The HR-TEM and CD-AFM results indicate the high quality of the line features: top corner radius of < 7 nm, vertical sidewall (slope mostly within 90 ̊ ± 0.5 ̊), and very small line width variation (LWR down to 0.36 nm). The sample can be applied to calibrate probe geometry, test or benchmark CD tools, perform comparison measurements between different methods/tools as well as to verify theoretical modelling with experimental results.
The developed reference material samples are commercially available [3] and the calibration service for the linewitdh values of the sample is also available at PTB with an expanded uncertainty (k = 2) of < 2 nm.


Figure 1. (a) Designed Layout of the sample consisting of 4 groups of feature patterns. Each pattern has 5 by 5 feature groups; (b) designed layout of a feature group with two alignment marks and a group of five line features; (c) SEM image of a fabricated feature group and its labeling numbers; (d) SEM image of the alignment marks and line features at the marked area in (c).


Figure 2. (a) An overview image of the alignment marks and five line features measured by STEM (magnification: 7.1k); (b) The line L1 with a nominal feature width of 50 nm measured by STEM (magnification of 1.3M); (c) the averaged intensity profile of the STEM image at the marked area in (b) with an inset figure showing the periodic crystal lattice planes of the silicon.

Reference:

1.  Dai G et al 2013 Reference nano-dimensional metrology by scanning transmission electron microscopy, Meas. Sci. Technol. 24 085001
2. Dai G, Häßler-Grohne W and Hueser D et al. 2012 New developments at Physikalisch Technische Bundesanstalt in three-dimensional atomic force microscopy with tapping and torsion atomic force microscopy mode and vector approach probing strategy. J. Micro/Nanolith. MEMS MOEMS, 11(1), 011004
3. http://www.team-nanotec.de/index.cfm?contentid=10&shopAction=showProductDetails&id=422