Development of a 3D-AFM for true 3D measurements of nanostructures

Abstract

The development of advanced lithography requires highly accurate 3D metrology methods for small line structures of both wafers and photomasks. Development of a new 3D atomic force microscopy (3D-AFM) with vertical and torsional oscillation modes is introduced in this paper. In its configuration, the AFM probe is oscillated using two piezo actuators driven at vertical and torsional resonance frequencies of the cantilever. In such a way, the AFM tip can probe the surface with a vertical and a lateral oscillation, offering high 3D probing sensitivity. In addition, a so-called vector approach probing (VAP) method has been applied. The sample is measured point-by-point using this method. At each probing point, the tip is approached towards the surface until the desired tip–sample interaction is detected and then immediately withdrawn from the surface. Compared to conventional AFMs, where the tip is kept continuously in interaction with the surface, the tip–sample interaction time using the VAP method is greatly reduced and consequently the tip wear is reduced. Preliminary experimental results show promising performance of the developed system. A measurement of a line structure of 800 nm height employing a super sharp AFM tip could be performed with a repeatability of its 3D profiles of better than 1 nm (p–v). A line structure of a Physikalisch-Technische Bundesanstalt photomask with a nominal width of 300 nm has been measured using a flared tip AFM probe. The repeatability of the middle CD values reaches 0.28 nm (1σ). A long-term stability investigation shows that the 3D-AFM has a high stability of better than 1 nm within 197 measurements taken over 30 h, which also confirms the very low tip wear.

Challenges in nanometrology: high precision measurement of position and size

“…Among them also is a large range metrological scanning probe microscope (LR-SPM) on the basis of a nanopositioning machine of company SIOS [29]. For traceable CD metrology on larger substrates such as photomasks or wafers, we are using a Nanostation 300 as the basic positioning machine, a scanning head based on a 6-axis flexure stage and a self-developed operation mode for probing the surface under test, the so-called vector approach probing [30]. Figure 15 shows the reproducibility of repeated measurements of the surface at the top and the sidewall of a lithographic Si line using a flared or reentrant CD tip, made with e-beam assisted deposition techniques (CDR-EBD tip) [31].…”

Comparison of CD measurements of an EUV photomask by EUV scatterometry and CD-AFM

“…Both techniques are realized in an AFM developed at PTB 28 . It uses the classic optical lever technique to detect the bending and torsion of the cantilever.…”

“…The measurements are mostly performed in the intermittent-contact mode, where the amplitude modulation technique is applied for detecting the tip sample interaction. For achieving better CD measurement performance, new probing and measurement strategies have been developed 28 . For instance, the tip is capable of probing surfaces with a vertical and/or a torsional oscillation to enhance the 3D probing sensitivity.…”

Adaptive tilting angles to achieve high‐precision scanning of a dual probes AFM

“…Cho et al [9] developed a novel 3D-AFM, which can tilt the tip before scanning so that it can access the sidewall and corner of the sample in order to reconstruct a precision 3D topographic image of the sample. Dai et al [11] designed a 3D-AFM head with vertical and torsional oscillation modes and combined it with the proposed vector probing approach to achieve the higher accuracy critical-dimension (CD) measurement. Dai et al [11] designed a 3D-AFM head with vertical and torsional oscillation modes and combined it with the proposed vector probing approach to achieve the higher accuracy critical-dimension (CD) measurement.…”

“…Fouchier et al [10] proposed a method to measure roughness of the line edge by tilting the sample at different angles. Dai et al [11] designed a 3D-AFM head with vertical and torsional oscillation modes and combined it with the proposed vector probing approach to achieve the higher accuracy critical-dimension (CD) measurement. Jayanth et al [12] designed a new type of scanning probe, the tip orientation of which can be changed such that the tip can fully access the sample so that a true 3D image can be acquired.…”