In¯uence of heavy ion irradiation damage on silicon chargedparticle detector calibration

Yanwen Zhang a,*, Harry J. Whitlow b, Thomas Winzell b

a Division of Ion Physics, �Angstr�om Laboratory, Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Swedenb Department of Nuclear Physics, Lund Institute of Technology, P.O. Box 118, SE-221 00 Lund, Sweden

Abstract

The full analytical potential of heavy ion backscattering and elastic recoil detection analysis (ERDA) depends

critically on establishing a reliable energy calibration. In order to make accurate measurements of thin ®lm samples we

have investigated the changes in the energy calibration of a Si p�-n-n� charged particle detector subjected to heavy ion

irradiation over 24 h in a time of ¯ight-energy elastic recoil detection analysis (ToF-E ERDA) measurement. In this

study a set of similar Al/ZrO2/Zr samples were analysed sequentially with 60 MeV 127I11� ions. The calibration change

for 16O, 27Al and 90±92;95;96Zr were monitored by tagging individual recoils with their energy derived from the ToF. The

calibration parameters for a wider range of elements (Li±Ag) were measured before and after the sequential irradiation

with O, Al, Zr and I atoms.

The results show that the change in the calibration could be characterised by an increase in the energy interval

spanned by one channel and a slight decrease in the channel zero energy. The calibration shift for a given projectile

atomic number depends linearly on the ¯uence of heavy particles impinging on the detector and the consequential

increase in detector leakage current. This indicates that for similar irradiation conditions, a correction to account for the

calibration shift may be simply determined for each sample from the number of heavy recoil counts registered or from

the change in leakage current. Furthermore, the silicon charged particle detector calibration depends on recoil atomic

number both before, and after, the heavy ion irradiation. The ¯uence-induced calibration shift for di�erent recoils can

be described by a linear dependence on recoil atomic number. Ó 2000 Elsevier Science B.V. All rights reserved.

PACS: 29.40.Wk; 29.30.Ep; 82.80.Yc; 61.80.Jh

Keywords: Si charged particle detector; ToF-E ERDA; Energy calibration; Radiation damage

1. Introduction

Si charged particle detectors are widely used forneutral- and charge-state inclusive particle mea-surements of energies in heavy ion elastic recoildetection analysis (ERDA) [1,2] and heavy ionbackscattering spectrometry (BS) [3,4]. For theseapplications, it is of central importance to establish

Nuclear Instruments and Methods in Physics Research B 161±163 (2000) 297±301

www.elsevier.nl/locate/nimb

* Corresponding author. Tel.: +46-18-4713058; fax: +46-18-

555736.

E-mail address: yanwen.zhang@angstrom.uu.se (Y.

Zhang).

0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 9 2 8 - 3

accurate energy calibration. It is well-known thatheavy ion irradiation results in the introduction ofdamage centres into the Si, which may cause cali-bration drift (ref. [5] and references therein). Thiscalibration change arises from reduced charge-carrier collection e�ciency. It can be associatedwith direct recombination and also from the re-duced electric ®eld due to the increased voltagedrop across the bias resistor associated with thegeneration current through the defect centres. Oneapproach to compensate for the latter is to use afeedback system to maintain a constant potentialdi�erence across the detector [6,7]. The calibrationshifts have serious implications for measurementsthat are based on reproducible line shapes andpositions [8,9] and where accurate assignment ofthe depth vs. concentration in ERDA studies oflight elements in heavy substrates is important.(e.g. O in Zr [10]) as well as BS with heavy ionbeams where the substrate mass is heavier than theion mass. In such measurements, progressive cali-bration shifts will manifest themselves as an ap-parent loss of resolving power in individual spectraand a shift of spectral features in cases where anumber of spectra are measured sequentially (e.g.from a batch of samples).

The goal of this study was to determine if thedamage associated calibration shifts could becharacterised in a way that permits a correction tobe established by appropriate interpolation andextrapolation methods.

2. Experimental

The ToF-E ERDA measurements have beencarried out at the Tandem Laboratory in Uppsala,Sweden [11]. 60 MeV 127I11� ions were producedfrom the Uppsala 6 MV EN-tandem Van deGraa� accelerator. The ToF-E detector telescopeconsisted of two carbon-foil time detectors [12]separated by a 0.4375 m ¯ight length (L) and fol-lowed by a silicon p-i-n diode energy detector. Thecarbon foils were 4 lg cmÿ2 thick (� 2� 1017 at-oms cmÿ2) which corresponds to 17.6 nm assum-ing bulk graphite density. The energy detectorplaced 25 mm behind the second time detector wasa 10� 10 mmÿ2 ion implanted p�-n-n� silicon

charged particle detector [13] with an 8 mm di-ameter collimator placed just in front of it. Thedetector was misaligned to inhibit the channellinge�ect [14]. During the measurements, a constantbias voltage of 40 V was applied across the 100MX bias resistor and detector. This was initiallysu�cient to fully deplete the detector into the p�

and n� regions. The angle of incidence h1 of pri-mary ions and the exit angle for recoils h2 wereboth 67.5° to the sample surface normal. The re-coiling target atoms were thus detected at an angle/ � 45° relative to the incoming beam direction.

The electronics system consisted of standardNIM electronics [1,11]. Data analysis was carriedout o�-line using CERN physics analysis work-station (PAW) [15] and TASS macro routines forERDA analysis [16]. The stopping powers for therecoils in the carbon foil were obtained usingZieglers STOP code [17].

The thick calibration standard samples of Li,Be, BNx, C, Si3N4, SiO2/Si,Al, Si, Mn, Nb Mo andAg speci®ed in [11] were chosen to give isolatedelement signals with little or no overlapping signalsfrom scattered projectiles or other elements. Themetallic Li sample was freshly prepared [11]. Thecalibration constants determined from calibrationsamples were measured before and after analysisof a set of similar Al/ZrO2/Zr samples [10]. Duringthis analysis, the evolution of the calibration con-stants for 16O, 27Al and 90±92;95;96Zr was monitoredsample by sample. During this sequence of mea-surements the detector was bombarded with heavyelements, (mainly Zr and 127I) for 25 h with similarprojectile beam currents. The Si detector exhibiteda signi®cant increase in leakage current as a resultof this bombardment (Fig. 1(a)). After the masshad been assigned to each recoil, the recoil energyE, in keV, was obtained from:

E � k2

MLT

� �2

ÿ DEfoil; �1�

where M is the tabulated recoil mass, T the ToF,DEfoil the energy lost in traversing the secondcarbon foil and k is a scaling constant. The ToFwas calibrated from the edge positions in the timespectra that correspond to each recoil isotope anda correction was applied for the energy loss in

298 Y. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 297±301

traversing the ®rst carbon foil [11,18]. The energycalibration for each recoil element

E �keV� � aE �ch:� � b �2�between the energy signal in terms of channelnumber E (ch.) and in keV E (keV) (from Eq. (1))was obtained by regression, where a is the slope(keV/ch.) and b is the intercept (keV).

3. Results and discussions

Fig. 1 shows the detector leakage current andcalibration parameters for 16O, 27Al and Zr recoils

as a function of the number of heavy particle in-cident on the detector. It is clearly seen that thekeV per channel a and leakage current increaseswith increasing heavy particle ¯uence. However,the intercept b (i.e. the particle energy corre-sponding to channel zero) is largely independent ofthe ion ¯uence. The linear increase in leakagecurrent with ion ¯uence (Fig. 1(a)) indicates that asfor a-particles [5], the increase in generation cur-rent can be attributed to the linear increase instable electrically active defect centres in the activeregion. The observation that the energy span perchannel a (Fig. 1(b)) increases with ¯uence,whereas the intercept b does not, is consistent withthe concept of a detector structure where the ma-jority of collected charge carriers are the result ofenergy deposition in the depleted region. It followsthat b, which is associated with electronic o�sets,energy deposition in the p� contact and metallicoverlayer as well as energy transport into and outof these layers by secondary particles, will belargely insensitive to an increase in electricallyactive defect centres in the entrance window. The¯uence dependent increase in a (Fig. 1(b)) can thenbe attributed to reduced collection e�ciency. Thisis con®rmed by the data in Fig. 2, which show thecalibration constants a and b for a wide range ofrecoil elements measured before and after the ir-radiation. Clearly, the same behaviour is observedfor all recoils from 7Li to 107;109Ag. The results inFig. 2(a), which are in contrast to those reportedin [19], imply that the energy deposition requiredto collect an electron±hole pair from the activelayer increases with increasing recoil atomicnumber. (i.e. an increasing pulse height defect [20±28].) In our measurements the higher atomicnumber recoils will have shorter ranges and hencethe plasma density will be higher. This would leadto a greater charge carrier recombination in theplasma which is consistent with the increase in awith increasing recoil atomic number. It is alsonoteworthy, that in common with previous re-ports [19] we observe a change of slope of a vs.atomic number at Z � 8 (Fig. 2(a)). Moreover, thedi�erence in a (Fig. 2(a)) before and after irradi-ation also changes slope at this point. The impli-cation of this is that, for measurement of particleswith a wide range of recoil atomic numbers, it

Fig. 1. The measured leakage current (a) and calibration con-

stants (Eq. 2) (b and c) vs. total number of the heavy particles

registered in the energy detector. In (b) and (c), the circles,

triangles and squares correspond to 16O, 27Al and Zr recoils,

respectively. The dotted lines are straight-line ®ts.

Y. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 297±301 299

may be inadequate to automatically correct forcalibration shift by correction of the detectorvoltage [6,7].

4. Conclusions

1. For analysis of closely similar samples, the cal-ibration coe�cients for a Si detector exhibit alinear dependence on the heavy ion ¯uencecharacterised by the number of heavy particlesregistered.

2. For similar irradiation conditions the increasein leakage current is linearly correlated withthe change in energy interval spanned by achannel.

3. For similar irradiation conditions, the increasein leakage current and the number detected par-ticles may be used to correct by interpolationthe change in calibration parameters associatedwith heavy particle bombardment.

Acknowledgements

The work has been carried out under the aus-pices of the NFR-NUTEK Nanometer StructureConsortium.

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