We have calculated radii and lifetime of the molten regions or the regions heated
to the melting point that are formed under irradiation of amorphous SiO2 and Si3N4 with
swift ions. A computer simulation was carried out on the base of thermal spike model. A
comparison of calculated track parameters with ion track etching data have been made
for these materials.
It is shown that an existence of molten region along swift ion trajectory may be a
criterion for a track “etchability” in the case of SiO2. In the same conditions of chemical
etching diameter of etched tracks in SiO2 is proportional to the radius and lifetime of the
molten region. This information is important for a correct choice of irradiation regime
aimed at preparation of nanoporous layers with high pore density ( 10 10 cm-2).
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/2087

Komarov, V.N.

Skuratov, V.A.

Vlasukova, L.A.

Yuvchenko, V.N.

SocioEconomic Challenges Journal (Sumy State University)

Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  393 
 
“ETCHABILITY” OF ION TRACKS IN SIO2/SI AND 
SI3N4/SI THIN LAYERS 
Liudmila A. Vlasukova1*, Fadei F. Komarov1, Vera N. Yuvchenko1, 
Vladimir A. Skuratov2 
 
1 Belarusian State University, Nezaleznasti Ave. 4, 220030, Minsk, Belarus 
2 Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, 141980, Dubna, Russia 
 
ABSTRACT 
We have calculated radii and lifetime of the molten regions or the regions heated 
to the melting point that are formed under irradiation of amorphous SiO2 and Si3N4 with 
swift ions. A computer simulation was carried out on the base of thermal spike model. A 
comparison of calculated track parameters with ion track etching data have been made 
for these materials. 
It is shown that an existence of molten region along swift ion trajectory may be a 
criterion for a track “etchability” in the case of SiO2. In the same conditions of chemical 
etching diameter of etched tracks in SiO2 is proportional to the radius and lifetime of the 
molten region. This information is important for a correct choice of irradiation regime 
aimed at preparation of nanoporous layers with high pore density ( 10 10 cm-2).    
 
Key words: SiO2 ɢ Si3N4, swift ions, ion tracks, chemical etching, nanoporous 
layers. 
 
INTRODUCTION 
The discovery of ion tracks dates back to 1959 when Silk and Barnes pub-
lished transmission electron micrographs of mica with long, straight damage 
trails created by single fragments from the ¿ssion of 235U [1]. Soon after that, 
it has been realized that ion tracks are narrow (<5 nm), stable, chemically reac-
tive centers of strain that are composed mainly of displaced atoms rather than 
of electronic defects [2]. It also became clear that these displaced atoms are not 
due to direct collisions between ions and the target nuclei, but are the result of 
the interaction of the projectile ions with the target electrons. Later it was found 
that tracks were formed in insulators and badly conducting semiconductors, if 
the electronic stopping power Se exceeded a material-dependent threshold 
value Se0. At that time no tracks could be detected in metals [3]. Nanochannels 
can be created in track regions by means of proper chemical etching. Thin die-
lectric layers (SiO2 and  Si3N4) with the etched tracks integrated into silicon 
wafers are of special interest for nanotechnologies and microelectronics [4 – 8]. 
A material-dependent threshold value Se0 is often treated as a criterion for ion 
                                               
* e-mail: vlasukova@bsu.by, tel: (+375)0172095929 
394                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II           
 
track “etchability”. Se0  varies from 4 to 1.5 keV/nm for SiO2 according different 
Refs [2, 9 - 11]. There is practically no information about Se0 for silicon nitride 
in literature. We know three papers only that devoted to track investigation in 
this material [12 - 14]. Though, in addition to Se0  it is necessary to know track 
parameters in order to obtain dielectric layers with the definite nanopore densi-
ty by means of swift ion irradiation followed by chemical etching.  
In this paper, we evaluate a possibility to use track formation parameters 
calculated on the base of thermal spike as criterions for ion track “etchability” 
in amorphous   SiO2 and Si3N4 layers. 
METHODS OF SAMPLE MANUFACTURING AND ANALYSIS 
SiO2/Si samples were cut from the thermally oxidized Si wafer. The 
thickness of SiO2 layer was evaluated from RBS measurements and equal to 
600 nm. Si3N4/Si samples were cut from the Si wafer with the amorphous sili-
con nitride layer deposited by gas phase low pressure deposition from ammonia 
and dichlorosilane at 300° C. The Si3N4 layer thickness measured by the Ruth-
erford backscattering method was 600 nm, too. SiO2/Si and Si3N4/Si structures 
were irradiated normally to the surface with Ar (290 MeV), Fe (56 MeV), Kr 
(84 MeV), and W (180 MeV) ions in the Flerov Laboratory of Nuclear Reac-
tions of the Joint Institute for Nuclear Research (Dubna). The ion fluence in all 
cases did not exceed 5×108 cm–2.  The ion flux was kept constant and equal to 
2×108 cm-2s-1. To provide reliable thermal contacts the samples were fixed on a 
massive metallic holder with a heat conducting paste. The irradiated samples 
were treated in hydrofluoric acid (HF) dilute aqueous solutions at room temper-
ature. Then the samples were investigated using the scanning electron micro-
scope Hitachi S-4800.  
THE COMPUTER SIMULATION OF THE TRACK FORMATION IN 
SILICON DIOXIDE AND NITRIDE IN TERMS OF THERMAL SPIKE 
The track formation processes in amorphous SiO2 (Į-SiO2) and Si3N4 were 
simulated in terms of the thermal spike model [15] with the application of a 
software system we designed. The model was successfully used earlier to de-
scribe swift ion travel in silicon dioxide and other dielectrics. The track radii in 
Į-SiO2 we  computed  by  this  model  were  in  good  agreement  with  the  experi-
mental results in [16, 17]. The model involves the thermalization of the elec-
tronic subsystem of a solid within a time not exceeding 10–14 s. A few picose-
conds later, the electron–phonon interaction leads to fast heating of the region 
along the fast ion trajectory. The process of energy transfer from the electronic 
to atomic subsystem of a solid is described by a system of two differential 
equations. The macroscopic parameters of the target material are used to calcu-
late thermal fields. The model involves one free parameter: the electron mean 
free path under electron–phonon interaction Ȝ. If the density of the energy re-
leased in electronic excitations is sufficiently high, we observe the melting of 
Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  395 
 
the material and the formation of a cylindrical domain with a diameter of a few 
nanometers - the future track. A few tens of picoseconds later, the melt cools to 
the temperature of the surrounding matrix.  
The thermophysical parameters for SiO2 were taken from [15], the param-
eter Ȝ was 4 nm. For Si3N4 the parameter Ȝ was 4.8 nm [15]. Thermophysical 
properties of amorphous SiO2 - one of the basic materials of microelectronic 
processing - are thoroughly studied and described in the literature. The situation 
is different for amorphous Si3N4. We failed to find reference data on latent heat 
of melting of this material. In addition, distillation or thermal decomposition of 
the material can take place upon the heating of Si3N4, along with solid phase 
melting. Therefore, as a track region in Si3N4 we took the region heated to the 
melting point instead of the melted region. The latent heat of melting of Si3N4 
was  taken to  be  108 J  kg–1, which is higher by two orders of magnitude than 
average values of semiconductors. 
RESULTS AND DISCUSSION 
Table 1 presents the results of computer simulation of ion track formation, 
namely, the radius and lifetime of molten region for SiO2 and Si3N4.  
 
Table 1 – The results of computer simulation for SiO2 and Si3N4  
Material 
 
Ion type  
and energy, 
MeV 
The electronic 
stopping power  
Se *, keV/nm 
The radius of 
the molten 
region r**, nm 
The lifetime of 
the molten 
region t**, ps 
 
  SiO2 
Ar (290) 
Kr (253) 
Kr  (84) 
W (180) 
Fe (56) 
2.8 
9.8 
9.4 
15.8 
6.9 
- 
3.4 
3.8 
6.3 
2.7 
- 
7.0 
8.9 
24.4 
3.9 
 
Si3N4 *** 
W (180) 
Kr  (84) 
Fe (56) 
20.4 
11.9 
9.0 
5.3 
3.0 
1.7 
4.7 
1.3 
0.5 
*  Se were calculated by SRIM’ 2003 code 
** for  Si3N4 the parameters r and t are the radius of the region heated to the melting point and the 
lifetime of this region, accordingly 
*** the density of Si3N4 was taken to be 2.85 g cm–3 [18] for  Se calculation  
 
In the case of Ar (290 MeV) Se is equal to 2.8 keV/nm. It is higher than 
the threshold value for the preferential track etching reported in [10, 11]. One 
can see that in the case of SiO2 irradiation with Ar ions (290 MeV) there is no 
formation of molten region along the ion trajectory. Accordingly, we did not 
found the etched tracks in the SiO2/Si samples irradiated with Ar even after the 
treatment in 6% HF for 20 minutes. Evidently, in order to forecast the “etcha-
bility” of ion tracks it is not enough to know Se only. We have to take into ac-
count “velocity effect” [19]. In our calculation this effect is demonstrated for 
396                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II           
 
SiO2 irradiated with Kr ions with the energies of 253 and 84 MeV. For Kr (253 
MeV) the electronic stopping power is 9.8 keV/nm. This is higher than a value 
9.4 keV/nm for Kr (84 MeV). However, the radius and the lifetime of the mol-
ten region are larger in the case of slower ion. In the conditions of similar Se the 
most part of released energy is localized in narrow regions along ion trajecto-
ries during slow ions passing. During swift ion passing a substantial part of 
energy goes away from the ion trajectory for significant distances with high-
energy G-electrons. Thus, a substance temperature in the region of future track 
ought to be higher in the case of slow ion passing. Evidently, the molten region 
formation along ion trajectory is more reliable criterion for track “etchability” 
in comparison with Se. Figure 1 shows images of etched tracks in SiO2/Si sam-
ples irradiared with Fe, Kr and W ions and chemically treated in the same con-
ditions.  
One can see from Fig.1 that tracks were etched in all samples. It should to 
note a correlation between etched pore diameters and calculated radii and life-
times of the molten regions. Pore diameter in SiO2 irradiated with Fe ions was § 
15 nm after the etching in 1.5% HF solution during 9 min. For the samples 
irradiated with Kr and W ions pore diameters were 30 and 45 nm, accordingly. 
Thus, the smallest pore diameter was observed for silicon dioxide irradiated 
with ions that formed molten regions with small radii and lifetime. The biggest 
one was observed in the case of W irradiation. According to our calculation W 
ions created the biggest molten regions with the longest lifetime from three ion 
species used in our experiment. Earlier for Į-SiO2 irradiated with Bi (710 MeV) 
it was shown a good correlation between minimum etched tracks diameters and 
molten region radii calculated by thermal spike [17]. In this way, one can fore-
cast minimum etched tracks diameters on the base of the molten region radius 
knowledge. It is important for a choice of proper irradiation regime aimed at 
preparation of nanoporous layers with high pore density ( 10 10 cm-2).   
 
 
Irradiated samples were treated in 1.5% HF for 9 min. 
 
Fig. 1 – Etched tracks in SiO2 . Ⱥ – irradiation with Fe (56 MeV, 2-4u108 cm2),  
B –Kr (84 MeV, 2-4u108 cm-2), C – W (180 MeV, 3u108 cm-2) 
Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  397 
 
As analysis of calculated track parameters shows, a region heated to the 
melting point exists during the irradiation with all three ion species in the case 
of Si3N4. 
Though, track formation process needs more energy spend in comparison 
with SiO2. At more high level of the electronic stopping power in Si3N4 for Fe, 
Kr and W ions the calculated lifetime of the region heated to the melting point 
were substantially shorter in comparison with the lifetime of the molten region 
for SiO2.  
In the Si3N4/Si samples irradiated with Fe and Kr, no tracks were revealed 
even after 60 min treatment in an HF solution. Tracks were etched only upon 
exposure to W ions. Figure 2 depicts the scanning electron microscope image 
of the W irradiated sample after etching. Ten pores were recorded on a scan 
area of 7.8u5.5 µm2. The track etching efficiency, defined as ȟ = Npĭ, where 
Np is the number of pores per unit area and ĭ is the fluence, did not exceed 
12%. The low track etching efficiency and the considerable difference in the 
diameters of the etched pores allow us to assume that discontinuous tracks are 
formed in silicon nitride upon irradiation with tungsten (180 MeV). This means 
that the electronic stopping power of W ions are sufficient for order–disorder 
transformation; however, they are lower than the threshold electron losses nec-
essary for the formation of extended etched tracks in this material. This conclu-
sion  on  the  formation  of  discontinuous  tracks  in  amorphous  Si3N4 irradiated 
with lead ions with Se = 19.3 keV/nm was also drawn by Canut et al. [13]. They 
reported about track etching efficiency ȟ  = 60% with an appreciable size 
spread of pores. 
To obtain nanoporous 
Si3N4 with the pore density 
controlled by ion fluence, the 
formation of extended contin-
uous tracks is required. This 
condition can be fulfilled with 
the  use  of  ions  with  S e> 20 
keV/nm. 
According to computer 
simulation data the regions 
heated to the melting point 
exist in Si3N4 during the irradi-
ation with all three ion species 
(Fe, Kr and W). Though, 
chemical treatment results show the preferential etching of discontinuous tracks 
only for the samples irradiated with tungsten. This ion type is characterized 
with the maximal energy losses in electron subsystem for our experiment. 
Probably, we have done incorrect assumptions in computer simulation of track 
 
Fig. 2 - Scanning electron microscope image of 
Si3N4  surface irradiated with W ions (180 MeV, 
2u108 cm–2) and etched in 4% HF for 50 min 
398                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II           
 
formation in Si3N4 because the absence of reliable data on thermophysical prop-
erties of this material. A fitting of thermal spike model in application to amor-
phous Si3N4 is a subject of further investigation. 
CONCLUSIONS 
The radii and lifetime of the molten regions or the regions heated to the 
melting point formed in amorphous SiO2 and  Si3N4 irradiated with swift ions 
have been calculated on the base of thermal spike model. A comparison of 
calculated track parameters and etching of ion track data have been made for 
these materials. 
It is shown that an existence of molten region along swift ion trajectory 
may be used as a criterion for track “etchability” in the case of SiO2.   In  the  
same conditions of chemical etching diameter of etched tracks in SiO2 is pro-
portional to the radius and lifetime of the molten region. This information is 
important for a correct choice of irradiation regime aimed at preparation of 
nanoporous layers with high pore density ( 10 10 cm-2).    
As  a  computer  simulation  shows,  in  the  case  of  irradiation  with  Fe  (56  
MeV), Kr (84 MeV), and W (180 MeV) at more high level of the electronic 
stopping power in Si3N4 the calculated lifetime of the region heated to the melt-
ing point were substantially shorter in comparison with the calculated lifetime 
of the molten region for SiO2.  
In silicon nitride irradiated with swift ions, tracks were etched only after 
exposure to W ions (180 MeV) with S e= 20 keV/nm being maximal for our 
experiment. The low track etching efficiency and the considerable size spread 
of pores allow assuming the formation of discontinuous tracks.  
An existence of the regions heated to the melting point along swift ion tra-
jectory can not be used as a criterion for track “etchability” in the case of Si3N4. 
It needs an additional investigation for a fitting of thermal spike model in appli-
cation to amorphous Si3N4. 
 
Acknowledgements 
This  work  was  supported  by  the  Belarusian  Foundation  for  Basic  Re-
search, grant T10R–142. 
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"Etchability" of ion tracks in Si02/Si and Si3N4/Si thin layers

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"Etchability" of ion tracks in Si02/Si and Si3N4/Si thin layers

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