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Production sequence of Si-spheres and interferometrical determination of the sphere volume

AFM Linewidth Metrology

Working Group 5.23

Development of new 3D AFM probing techniques

 

It is desired that the new 3D-AFM can perform measurements in both static and dynamic modes.  In dynamic mode of conventional AFMs, however, the cantilever is usually driven by one shaking piezo and oscillates in the vertical direction only. This mode is principally not ideal for sensing vertical sidewalls since the tip oscillates almost parallel to the surface being measured, i.e. the tip-sample distance does not change within the oscillation cycles. Consequently, the tip sample interaction force such as van der Waals force changes little during the oscillation cycles, which leads to poor probing sensitivity.  To overcome such disadvantages, we developed a new 3D-AFM with vertical and torsional oscillation modes.

 

Combined tapping and torsional probing mode

The configuration of the developed probing head is shown in figure 14(a). The cantilever with tip is mounted to an alignment chip, and can be oscillated by two shaking piezos. The driving signals of the two piezos are shown in figure 14(c).  By applying a pair of driving signals Sd1,v and Sd2,v in phase at a frequency near the cantilever’s vertical (tapping) resonance frequency, a vertical oscillation of the tip can be generated. By applying  a pair of driving signals Sd1,t and Sd2,t with a phase shift of 180° at a frequency near the cantilever’s torsional resonance frequency, a lateral oscillation of the tip can be generated.  If the driving signal Sd1 and Sd2 include both components of the Sd1,v  Sd1,t  and Sd2,v  Sd2,t respectively, the tip can be oscillated in both vertical and lateral directions simultaneously. An optical beam deflection method is applied as detection technique. The beam from a laser diode is focused on the backside of the cantilever and is received by a quadrant photodiode (QPD) after being reflected. The vertical (bending) and torsion signals of the cantilever Sv , St can be obtained by processing the signals output from QPD as

Sv = (SI+SII-SIII-SIV)/(SI+SII+SIII+SIV)

St = (SI+SIV-SII-SIII)/(SI+SII+SIII+SIV)

where the signals SI, SII , SIII, and SIV  are the output signals of the QPD segments I, II, III, and IV respectively.

The photography of shaking piezos of the realized 3D-AFM head is shown in figure 14(b).

 

Fig.14 Principle of 3D-AFM head with vertical and torsional oscillation modes.

In order to illustrate the function of the 3D-AFM head, a picture of an oscilloscope panel showing  the driving and measurement signals of the 3D-AFM head is given in figure 15. It can be seen that the driving signals Sd1, Sd2 include two frequency components. The component with a low frequency of about 250 kHz is used for the vertical oscillation while the component with a high frequency of about 1.8 MHz is used for the torsional oscillation.  From the measurement signals, it can be seen that the vertical signal has only the low frequency component while the torsional signal has only the high frequency component. The measurement signals confirm that the cantilever has been oscillated in both directions, the vertical and torsional, simultaneously under its own resonance frequencies.  The cross talk between the vertical and torsional oscillations is very small.

 

Fig.15 Photography of an oscilloscopic panel showing the driving and measured signals of the 3D-AFM head.

Different demodulation techniques such as amplitude demodulation (AM) and frequency demodulation (FM) can be applied for processing the vertical and torsional measurement signals. In this study, we have used the AM demodulation technique, where Sv, St are processed by two commercial lock-in amplifiers.

The 3D-AFM is capable of measuring in four different modes by configuring the driving signals using designed software in different ways. They are:

  • Static mode, where both Sdt and Sdv are switched off.
  • Vertical oscillation mode , where Sdt is switched off while Sdv is on.
  • Torsional  oscillation mode, where Sdt is switched on while Sdv is off.
  • Combined vertical and torsional oscillation mode, where both Sdt and Sdv are switched on.

 

The 3D-AFM combines the design features of the CD-AFM, contact and tapping mode AFM and torsion mode AFM. The torsion mode AFM is initially developed for mapping in-plane mechanical properties on hard surfaces, such as friction, shear stiffness and other tribologically relevant properties [8].

The cantilever is oscillated near its vertical and torsional resonance frequencies in related dynamic modes. The resonance frequencies are determined by frequency tuning of the cantilever. To demonstrate the frequency tuning properties of the 3D-AFM, measured frequency tuning curves (amplitude vs. frequency and phase vs. frequency) with an applied flared tip AFM probe type “CDR120” are shown in figure 9. It can be seen that the cantilever has a torsional resonance frequency of 1,822 MHz with a Q factor of about 800, and a vertical resonance frequency of 260,49 kHz with a Q factor of about 400.

 

Fig.16 Frequency tuning curves measured on a AFM probe type “CDR-120”

The 3D-AFM combines the design features of a CD-AFM, a contact mode AFM, a tapping mode AFM, and a torsion mode AFM. The torsion mode AFM has initially been developed for mapping in-plane mechanical properties on hard surfaces, such as friction, shear stiffness and other tribologically relevant properties.

 

Probe-sample interaction curves

To compare the measurement performance of the tapping and torsion measurement modes, a vertical sidewall of a sample is measured at the same position using both the vertical oscillation and torsional oscillation modes with a flared AFM tip – type CDR130. The probing curves recorded for the vertical and torsional oscillation modes are shown in figures 17 (a) and (b), respectively. It can be seen that the curve recorded in the torsional oscillation mode (b) has a much smoother interaction region than that of (a). This phenomenon can be understood from their measurement principles. In the vertical oscillation mode, the tip sample interaction force such as van der Waals force is (almost) orthogonal to the oscillation direction of the tip during probing. It is thus predominantly the surface frictional forces which lead to the (abrupt) change of tip oscillation. In the torsional oscillation mode, however, the tip sample interaction force lies along the tip oscillation direction. Consequently, the oscillation of cantilever is more readily modeled and interpreted. On the other hand, it should be noted that the interaction region of the curve (b) is wider than that of the curve (a). The reason for this is the relative lower lateral stiffness of the flared tip compared to that of the cantilever.

 

Fig. 17   Recorded probing curves in measuring a vertical sidewall source by using the vertical oscillation mode (a) and the torsional oscillation mode (b).

 

Probing Repeatablity

The same sidewall point was repeatedly probed 50 times using two different oscillation modes. The measurement results are shown in figure 18. The performance of the torsional oscillation mode (standard deviations of 0.11 nm, 0.12 nm and 0.10 nm in three repeated measurements) was slightly better than that of the vertical oscillation mode (standard deviations of 0.13 nm, 0.15 nm and 0.12 nm in three repeated measurements). However, this result doesn’t fully represent the potential measurement capability of the torsional oscillation mode, since the comparison is limited by the position noise of the piezo stage (about 2.2 nm peak-peak). In addition, the flared AFM probe could be further optimised for sidewall probing performance. Specifically, the torsional stiffness of the cantilever could be reduced or the lateral stiffness of the flared tip increased.

 

Fig. 18   Measurement repeatability on a same sidewall point using the vertical oscillation mode (a) and the torsional oscillation mode (b).

 

Novel kind of true 3D AFM probe

(This part is under construction)