Nuclear Instruments and Methods in Physics Research B50 (1990) 109-113

North-Holland

109

INFLUENCE OF ELECTRONIC CHARGE COMPENSATION ON RUTHERFORD BACKSCATTERING SPECIXA OF BIASED INSULATORS

J.D. MEYER

Institut ftir Kernphysik, Universitiit Frankfurt, FRG

D.E. ARAFAH

Departmeni of Physics, Universi@ of Jordan, Jordan

On irradiating quartz targets with 2 MeV ‘He+ ions the spectra show a drastic shift of the RBS edge towards higher energies. This shift depends strongly on the biasing voltage applied to the target holder. For positive biasing up to 200 V no significant shift of

the edges compared to the theoretical RBS edges of SiO, is observed. On the other hand, negative biasing up to - 200 V shifts the

edges by 120 keV. This effect can be explained by a negative (electronic) charge compensation of the positive charge on the irradiated

insulator. This charge compensation is affected by the potential of the holder, the positive potential of the target and the potential of

the space charge in front of the target. The experimental results are explained according to this model with realistic parameters, i.e.

mean electron energy and electron density.

1. Introduction

The interest in dielectric materials has grown in recent years due to the impetus applications and prom- ising technological future [l-4]. The modification of the properties of insulating materials using ion beam tech- niques, e.g. by ion implantation, and their analysis by, e.g., RBS or nuclear reaction analysis (NRA) reveals unique problems that are insignificant when the target material is a metal or semiconductor [5,6]. The main problem is the surface charging of the insulator during ion beam irradiation or analysis, due to the injection of positive charges.

The effect of surface charging has been the subject of much literature and many techniques have been pro- posed to avoid such an effect [7,8]. Surface charging results in large energy shifts accompanied by distortions in the RBS or NRA spectra by the slowing down of ingoing charged particles and their acceleration along the outgoing path [9].

This paper investigates the effects on the RBS spec- tra of quartz targets due to the biasing of the holder to voltages of up to &2OO V. A model is proposed to describe the observed energy shifts of backscattered ions in relation to surface charge buildup. The latter is correlated to the conductivity of the insulator and to the compensation by the attracted negative charges flowing towards the target.

2. Experimental

Monocrystalline quartz plates (20 x 20 mm2) of vari- ous thicknesses (0.4, 1.0 and 1.4 mm), cut perpendicular to the optical c-axis, have been used in this experiment. The conduction of quartz mainly along these directions has a strong dependence on the impurities and electrode material. Therefore, the dc electrical conduction along this axis was measured separately with contacts of the same geometry as the beam diameter (0 = 2 mm). No special guard rings were used beside a holder of 60 mm diameter, to which the target was attached by a silver dag, since the main goal of this work was to study the undisturbed charge buildup on the sample’s surface and its dependence on the target holder biasing. The holder was situated in the center of the target chamber which has a radius of 30 cm. The insulation resistance of the holder against ground was larger than 10 GO. The holder could be cooled to liquid-nitrogen temperature.

Spectrum acquisition was made using the conven- tional RBS technique, with 2 MeV helium ions from the University of Jordan Van de Graaff accelerator. Back- scattered particles were detected using a silicon surface-barrier detector with 16 keV FWHM resolution at a scattering angle of 135 “. Samples were biased up to + 200 V, by biasing the target holder (fig. 1). The beam diameter was kept at 2 mm, but various beam currents ranging from 1 up to several nA were employed. When

0168-583X/90/$03.50 0 Elsevier Science Publishers B.V.

(North-Holland) II. ELASTIC SCATTERING

110 J.D. Meyer, D. E. Arafah / RBS spectra of biased insulators

Fig. 1. Schematic diagram of the backscattering geometry and biasing arrangements for irradiation experiments of quartz

plates.

studying the biasing dependence of a sample, the beam current was kept at a fixed value. Also the entrance aperture (0 = 2 mm) to the chamber, 30 cm in front of the target, could be biased with different potentials. All measurements were performed at normal incidence with respect to the beam direction.

3. Results and discussion

One consequence of ion beam irradiation and analy- sis of insulating materials is the persistent problem of surface charging. There are two processes involved when probing with a positive ion beam. The first one is that positive ions are injected into the sample and charge it to positive potentials. The second process is due to secondary electron emission which tends to enhance the positive nature of the surface. However, for surface potentials larger than about +50 eV, the secondary electron emission factor is nearly zero. Hence, very few secondary electrons leave the surface. This implies that the positive potential is established mainly by the injec- tion of the ions. Some of the charge may flow to the metallic target holder depending on the conductivity of the insulator. For the quartz under investigation, the resitivity is 3.5 X 10” B cm.

Two typical RBS spectra of 2 MeV helium back- scattered from a 1 mm thick SiO, target are shown in fig. 2. The well separated steps represent the surface scattering energies from oxygen and silicon. These mea- surements were done at room temperature (RT) for zero/positive (0 symbol) and negative (solid line) bias- ing. The spectrum for zero/positive biasing shows a normal RBS behaviour with relative sharp Si and 0 edges nearly at the theoretically expected energy values (k,,,E,), whereas for the negative biasing lower than about -60 V one observes that drastic changes in the spectrum have occurred. Clearly the upper edges of the signal, corresponding to the step in the spectrum, do

not vary as much as the lower parts (onset). This distortion, which is the same for the 0 and Si edges, shifts the spectrum onset to higher energies by some 100 keV when a negative biasing voltage of about - 200 V is applied to the holder.

This observation may be explained in the following way. Due to surface charging, the surface energy of backscattered particles (i.e. Et = k,E,) is modified according to

E,=k,,,(&,-@)+nq+> (1)

where q+ is the potential energy which slows down the He+ on its way to the target, n is the charge state of the accelerated ion on its way to the detector. E, is the accelerator energy and k, the kinematic factor for element m. Since about 98% of the backscattered helium ions with incident energy of 2 MeV are doubly charged, n is equal to 2 [lo].

From eq. (1) the surface potential of the irradiated quartz target can be calculated. For doing this, the energy shift is taken as the difference between surface scattering, measured at half the height of fig. 2 for the negative- and zero-biased (E, = k, E,) curves. Fig. 3 shows the surface potential qcp for both RT and 77 K measurements as a function of the biasing voltage ob- tained in this way. As stated above, this potential vanishes for a zero or positive biasing voltage. For negative biasing lower than about -60 V, the surface potential suddenly appears and tends to saturate below - 200 V. This behaviour is the same for both RT and 77 K measurements. The latter fact is unexpected, since due to the increase in electrical resistance and decrease in the mobility of ions for dielectric materials at 77 K a larger surface potential should be found. The line drawn

ME , I I I I I

! 1a 21 310 488 m

CHANNEL Fig. 2. RBS energy spectrum of 2 MeV 4HeC backscattered from SiO, and measured at RT. Data are for zero (0) and negative (solid line) biasing voltage on the holder. Beam cur-

rent *as 10 n.4.

J.D. Meyer, D.E. Arafah / RBS spectra of biased insulators 111

L J

BlASlffi VOLTAGE I V

Fig. 3. Surface potentials of quartz plate between zero and negative biasing as a function of applied biasing voltage for RT (0) and 77 K (0) measurements. Tire line is according to

the compensation model.

through the data points is calculated according to the

proposed model, which will be discussed later.

In order to increase the spread of accumulated charge

on the surface, 200 run thick electrically floating Pt

films of different areas were evaporated on quartz targets. Fig. 4 shows the surface potential, again derived from RBS spectra, as a function of film area. Higher beam currents as well as lower areas for fixed beam current increase this potential. For a given beam current the potential should be inversely proportional to the film area. Since the potential on the metal film surface has to be constant, the data suggest that the current flow to the target holder is uniformly distributed over the whole deposited Pt spot. The solid lines stem from the model calculations.

e 0 18 20 38 40 50

Pt AREA / d

Fig. 4. Surface potentials for floating Pt film (200 nm thick) on quartz versus area of the film. Data are for different currents and - 200 V applied biasing voltage. The lines are according to

the compensation model.

A parallel example with LiNbO, [ll] confirms that a floating metal coating over insulating surfaces does not necessarily reduce surface fields to zero. In another experiment the entrance aperture of the beam to the chamber (30 cm in front of the target) was biased by +200 V, whereas the holder was kept at zero voltage. The resulting RBS spectrum showed the same distorted behaviour as in the case of biasing the holder by -200 V (see fig. 2).

These measurements demonstrate that the potential on the surface is markedly influenced by negative bias- ing. On the other hand, insignificant energy shifts are observed for positive biasing and negative biasing larger than about -60 V. It should be emphasized that rela- tively small negative voltages (up to - 200 V) influence the energy of the backscattered ions by some 100 keV. These results may be accounted for by the following suggested model. Four distinct facts govern the accu- mulated charge on insulator surfaces: (i) the conductiv- ity of the material, which allows the charge to flow to the target holder; (ii) the biasing voltage of the holder, which establishes a potential in front of the target; (iii) the positive potential of the beam spot, which is super- imposed by the potential of the holder and, in case of a negative biasing voltage, gives rise to a potential well in front of the target; (iv) the negative (electronic) charge flow in the beam and from chamber regions, which is attracted by the positive beam spot modifying by its presence the depth and position of the potential well in front of the target.

The solution of the mathematicaly complicated prob- lem of solving the Poisson equation, with the boundary conditions given by the biasing voltage on the holder, the surface potential of the target and the ionic and electronic space charges in front of the target, is ap- proximately found in a two-step procedure. First, the superposition of the holder and target potentials is calculated without taking into account the space charge.

The influence of the biasing voltage V,, on the poten- tial in front of the target qV’ is given by

qV’ = qV, [ 1 - arctan( z/rs)/arctan( R,/r,)] , (2) where ra (= 30 mm) indicates the holder radius and R, (= 300 mm) is the chamber radius; z is the distance in front of the holder on the beam axis (one-dimensional formula). The potential becomes zero at the entrance aperture to the chamber.

The influence of the target surface potential qV,,

measured by the RBS energy spectra, on the potential qV” in front of the target can be approximated by

qV” = qV, arctan(2/z2)/arctan(2). (3) This equation is the solution of the Laplace equation for a beam spot of 1 mm radius with a constant surface potential on a 1 mm thick quartz plate. The z2-depen- dence reflects the mirror charge of the beam spot in- duced in the metallic target holder surface.

II. ELASTIC SCATTERING

112 J.D. Meyer, D. E. Arafah / RBS spectra of biased insulators

The total potential, without space charge, results from the addition of qV’ and qV” (fig. 5). For nega- tive biasing voltages V,, a minimum of the potential is developed in front of the target. The depth of this potential well governs the number of negative charges (electrons) which may contribute to the charge com- pensation on the target surface and, on the other hand, modify the potential itself by varying the space-charge distribution. For positive biasing the superposition of qV’ and qV” gives rise to a positive, monotonically decreasing potential in front of the target.

Hence, the second step of the problem of solving the Poisson equation will be the calculation of the space charge density of the electrons. A one-dimensional Boltzmann distribution is used as the model velocity distribution for the negative charges. According to the experiment in which the entrance aperture was biased, showing strong distortions of the RBS spectrum for positive biasing, this aperture is regarded as the source of the flow of electrons. With the help of fig. 5 the electronic charge density in a potential eU can be calculated. On the source (entrance aperture) side of the potential minimum, eU,, the charge density is given by

Pe G =n,e{l +erf[lieb((i-]} exp(eUb). (4)

On the target side the charge density is given by

p,, =n,e{l -erf[ileb((i-]} exp(eUb). (5)

In these equations ne is the electron density; n, is about 6 times larger than the ion density ni for elec- trons from secondary-electron emission sources [12]. The factor b = 1/(2(E)) is of the order of 0.05 eV_‘, which corresponds to a mean energy (E) of secondary electrons of 10 eV [12]. The symbol erf( ) represents the error function.

280 240 200 160 120 80 40 0

POSITION IN FRONT OF TARGET , mm

Fig. 5. Potential in front of the target resulting from superposi- tion of the biasing voltage on the holder and the measured surface potential of the target. No space charge is taken into

account.

-30 3: Vh=-184 V:.U,=-Il.59 eV 4: Vh=-138 V: slJ, =-19.59 eV

t

5: Vh=-170 V: eU, z-25.72 eV 6: Vh=-202 V: eU, =-30.21 eV

-40

280 240 200 168 120 80 40 E

POSITION IN FRONT OF TARGET / mm

Fig. 6. One-dimensional solution of the Poisson equation for

experimental biasing voltages on the holder and measured surface potentials of the target.

The Poisson equation has to be solved in the ap- propriate regions with the above charge densities:

A+= -l/ce(pi-p~). (6)

In principle, this equation should be solved in full three-dimensional form. However, for mathematical simplicity and for the reason of understanding, it is solved here only for one dimension in the beam direc- tion. Furthermore, the ion charge density pi is kept constant in the region in front of the beam spot.

Fig. 6 shows the complete solution of eq. (6) as a function of experimental parameters for the biasing experiments of fig. 3. If Ohm’s law is valid for the quartz target, the surface voltage at the beam spot can be written as

V, = Vh + RZi [ 1 - n,/ni/m exp( e&$)1,

(7)

which for ion energy E = 2 MeV, density ratio n,/ni

= 6.54, the known ion-electron mass ratio m/m, and b = 0.05 eV_’ simplifies to

K=P,+RZi[l-exp(eU,b)]. (8)

The calculated saturation value RZi (for Z, = 10 nA) of the surface voltage V, amounts to about 100 kV, whereas fig. 3 extrapolates to 96 kV. It was observed experimen- tally that for constant beam diameter and biasing volt- ages the breakthrough voltage (destruction of the quartz) could be reached by increasing the beam current Z,. The solid lines in figs. 3 and 4 are calculated from eq. (8) with the theoretically obtained (from eq. (6)) depth of the potential well eU, in front of the target.

The comparison of these calculations with our ex- periments on biasing insulating materials shows that the results fit nicely into the model of negative-charge com- pensation of the injected positive charge. Only in the

J. D. Meyer, D. E. Arafah / RBS specira of biased insulators 113

case of applying a negative voltage lower than about -60 V to the holder, the space charge in front of the holder can be balanced in such a way that it can be

experimentally observed.

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