Temperature dependence of the transmission of keV negative ions through tapered glass capillary

Hanyu Song Daijie Zhou Dianwei Zhou Jianxiong Shao

Abstract：We investigated the transmission of 15-keV ?ions through tapered borosilicate glass capillary with various temperatures at 0° tilt angle. The properties of transmitted particles were analyzed by the 2-D images and corresponding angular distributions. The results showed that the intensity of transmitted particles was decreased with the increasing temperature and the negative ion fraction of scattering particles was increased at the same time， which was interpreted by the influence of changing deposited charge and the blocking effect.

1.Introduction

With the development of micro-technology， the micro tapered glass capillary was widely applied in physics， chemistry and material science and technology. It was used to create focused ion beam （FIB） which can modified the surface of nano-structured ?materials [1]. In Y. Yamazakis work， FIB with the taper glass capillary scheme was first applied in ion irradiation in μm3 region for cell surgery [2].

In 2002， the experiment of slow highly charged ions （3-keV） transmitted through the insulating polyethylene terephthalate （PET） nano-capillaries was reported by N. Stolterfort et al [3]. The guiding effect was observed and interpreted by the self-organized charge patches deposited on the inner wall of insulting capillaries [4]. These patches established the electrostatic field which led to the dynamic equilibrium in the transmission process [5-7]. Then tapered glass capillaries were developed as ideal tools instead of insulating nano-capillaries. For the positive highly-charged ions， the dominant physical process was distinct with different initial energy range. The experiment of 2 MeV/ufocusing through single tapered glass capillary was performed which was attributed to the small angle scattering[8]. But Ikeda and his group used 8-keVions through borosilicate glass capillary to produce a microbeam which based on guiding effect [9].

In recent year， The investigation of negative ions transmission through tapered glass capillaries is still lacking， and the mechanism of this process is unknown. So in this work， we performed experiments to study the transmission of 15-keV ?ions through tapered glass capillaries at different temperatures and the properties of the transmitted particles were discussed. The intensity， FWHM and peak position of profiles were also analyzed. These results enable us to understand the mechanisms that should exist in this transmission process.

2.Experiment

Fig. 1. A schematic picture of experimental setup

The experiment was performed at the tandem accelerator facility at the School of Nuclear Science and Technology， Lanzhou University. As the Fig. 1. shown， our main experimental setup consists of slits， a sample holder， a tapered glass capillary， a pair of parallel-plate electrostatic deflectors and a 2D-PSD （two-dimensional position sensitive detector）. The whole system installed in a sealed tubular vacuum chamber and the heating coils twined on the outer wall of the vacuum chamber which connected to a temperature control system. The ion beam was induced by ion source and collimated by two pairs of 0.5×0.5 mm2 slits which were 75cm apart from each other. The beam bombarded into a tapered glass capillary mounted on the sample holder which was placed in the high vacuum chamber. The angles were defined as detection angle θ and Ф which the primary beam direction respect to the tapered glass capillary axes in horizontal and vertical， respectively. The transmitted ions were detected by 2D-PSD， so we can analyze the 2D images and corresponding angular distributions on θ and Ф. For analyzing charge state of the transmitted particles， a pair of parallel-plate electrostatic deflectors was installed between the sample holder and the detector. During the experiment， the vacuum was better than .

The material of tapered capillary is borosilicate， and it composition is 81% SiO2， 13% B2O3， 4% Na2O， 2% Al2O3 and some others. The inlet diameter and outlet diameter are 580 μm and 68 μm， respectively.

In the experiment， the 15-keV ?ions transmitted through tapered capillary with different temperatures. The accuracy of temperature control system was about 0.1℃. The temperature was increased by setting the heating value. When the temperature reach predetermined value， it would remained and fluctuated in a small range about2℃. The deflectors were used to analyse the charge state of transmitted particles， so different charge states of transmitted particles and corresponding intensity in the angular distribution can be easily observed.

3.Result and discussion

Fig. 1. The two dimensional images and angular distributions of 15-keV ?ions through tapered capillary with different temperatures. The left column is 2-D image of transmitted particles without charge state analysis and the right column is the corresponding projection onto X axis.

The 2-D images and angular distributions at 0° tilt angle are shown in Fig. 5. An intense core at the center and some scattering particles appear in the 2-D image with the room temperature. Here， we use ‘core to denote the former. In the corresponding angular distribution， the peak position of the core locates at 0° and the angular range of scattering particles is from -2° to 2.4°. The intensity of the scattering particles is much lower than that of the core. When the temperature raises form 30℃ to 115℃， the intensity of the core is gradually decreased from 1366 cps to 109 cps， which is about one tenth of that in the room temperature. But in this transmission process， the peak position of the core is almost unchanged. The scattering particles are gradually disappeared with the increasing temperature. We can image that the core component is dominant by the deflections of incident particles occur at the capillary inner wall： Incident ?ions transmit through tapered capillary via specular scattering at the inner surface of tapered shape and lose electrons with a certain probability in the collisions because of the geometric shape limitation. Electrons are captured by inner surface atoms of the tapered shape and the deposited charge fields should influence the exitance of subsequent ?ions. So the core and the scattering pattern are formed in the 2-D images.

Table 1. The intensity of the scattering particles with different temperatures.

Temperature （℃） 30 45 75 115

Intensity （cps） 924 487 348 90

The properties of the scattering particles with different temperatures by analyzing the angular distribution spectrum are given in the Table 1. The intensity of the halo is decreased from 924 cps to 90 cps with the increasing temperature.

Fig. 2. The contributions of the negative ions and the neutral particles to total transmitted particles. The black square dot and red dot represent the former and latter， respectively.

The contributions of different charge particles to the scattering particles are shown in the Fig. 2. It shows that with the change of temperature， the particle composition of the scattering ring which composed of negative ions and neutral particles will produce a certain trend. The specific performance is as follows： the proportion of neutral particles that account for 60% at low temperatures （30 ℃） decreases with increasing temperature， while the proportion of relatively small negative ions （about 40%） rises with increasing temperature， the number of neutral particles and negative ions tends to approach， and eventually there will be a trend of a stable proportion of both， at the same time， the proportion of both tends to be equal （50%）.

According to the Ref. [10]， the increasing temperature influences the dielectric constant of the inner wall of the tapered capillary. When the temperature increases， the dielectric constant shows a down trend and the conductivity （directly proportional to the derivative of the dielectric constant） increases. The deposited charges will be lost more quickly， which makes the following charged particles repelled from the axis of the tapered capillary in the transmission process， which cause a blocking effects. The transport charge is difficult to deposit， which also makes the contribution of negative ions in the scattering particles larger.

4.Conclusion

In this work， the transmission experiments of 15-keVions through tapered capillary with various temperatures are performed. The results show that when the temperature raises， the intensities of the core and scattering particles are declined and the peak position is unchanged. The contribution of the negative ions to the scattering particles is increased. It is indicated that the high temperature influences the dielectric constant of the inner wall of the tapered capillary and causes the blocking effects.

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