Mechanical time-of-flight filter based on slotted disks and helical rotor for measurement of velocities of nanoparticles

Abstract

A mechanical time-of-flight filter intended for measurement of velocities of nanoparticles exiting a gas aggregation source has been developed. Several configurations maximizing simplicity, throughput or resolution are suggested and investigated both theoretically and experimentally. It is shown that the data measured using such filters may be easily converted to the real velocity distribution with high precision. Furthermore, it is shown that properly designed filters allow for the monitoring of the velocity of nanoparticles even at the conditions with extremely low intensity of the nanoparticle beam.

Figure 1

(a) Schematics of the nanoparticle source; (b) copper NPs deposited at pressure 20 Pa (mean size 18 nm, the standard deviation of mean 2.5 nm); (c) copper NPs deposited at pressure 100 Pa (mean size 26 nm, the standard deviation of mean 7 nm); The images have been obtained using a scanning electron microscope (SEM, JSM-7200F, JEOL). The samples have been deposited on single side polished silicon wafers and observed in secondary electron mode. The sizes have been determined by fitting NPs circumference by a circle.

Figure 2

Schematics of the nanoparticle time-of-flight filter rotors: (a) slotted disks with 8 slits of angular width 5° on each disk. The relative angle between the disks is 9°, disks-to-disk distance is 19 mm, disk thickness is 1 mm; (b) helical rotor with 90 slits of angular width 1°, the relative angle between entrance and exit 9°, total thickness 20 mm and printing resolution 0.2 mm.

Figure 3

Schematic illustration of the model: (a) schematic of the whole system; (b) geometric quantities for calculation of δ_max or v_min; (c) geometric illustration of NPs collected on the inner wall of the entrance slit/transmitted NPs in the zoomed area around the upper part of the slit in disk 1.

Figure 4

Calculated transmission signal maps for input nanoparticle velocities in the form of delta-functions, relative angle of the disks 9°, the distance of the disks 19 mm, disk thickness 1 mm and angular width of the slit (a) 5°; (b) 2°; (c) 1°. The last map under letter (d) shows the transmission of a helical rotor with a relative angle of the slit between the inlet and outlet of the slit again 9°, slit angular width 1° and thickness of the rotor 20 mm. The print resolution for the helical rotor (thickness of each subsequent incremental disk) is 0.2 mm.

Figure 5

(a) Gaussian velocity distributions; (b) Simulation of the signals obtained from 5° disks for Gaussian velocity distributions; (c) log-normal velocity distributions. (d) simulation of the signals obtained from 5° disks for log-normal velocity distributions.

Figure 6

Comparison of fits using Gaussian (v_peak = 35.05 m/s; w_G = 2.624; FWHM = 6.18 m/s; RSS = 0.002813) and lognormal (v_peak = 35.08 m/s; w_L = 0.074; FWHM = 6.09 m/s; RSS = 0.002871) distributions of nanoparticle velocities. Nanoparticles have been deposited at aggregation chamber pressure 20 Pa, deposition chamber pressure 0.015 Pa, DC magnetron current 200 mA and QCM averaging over 20 values.

Figure 7

Comparison of the model with real measured data. (a–d) nanoparticles deposited at aggregation chamber pressure 20 Pa, deposition chamber pressure 0.015 Pa, DC magnetron current 200 mA and QCM averaging over 5 values; (e–h) nanoparticles deposited at aggregation chamber pressure 100 Pa, deposition chamber pressure 0.15 Pa, DC magnetron current 200 mA and QCM averaging over 20 values. The relative angle of the inlet and exit slit was 9°, disk distance 19 mm, disk thickness 1 mm, thickness of helical rotor was 20 mm printed with layer thickness 0.2 mm and the angular width of the slits was (a,e) disks 5°; (b,f) disks 2°; (c,g) disks 1° and (d,h) helical rotor 1°. The lower black axis is the directly measured frequency of the rotor, the upper blue axis is formally converted to velocity using Eq. (15). The fits of the measured data in red are accompanied by the derived NPs velocity distributions in blue.

Figure 8

Fit parameters for measurements shown in Fig. 7. The bars show the mean velocity, while the error bars show the FWHM.