The application of high energy ion implantation for silicon radiation detectors

Nuclear Instruments and Methods in Physics Research A 377 (1996) 514-520

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A

ELSEVIER

The application of high energy ion implantation for silicon radiation detectors J. von Borany*, B. Schmidt, R. Grrtzschel Forschungszentrum Rossendorf e.ld, lnstitut fiir Ionenstrahlphysik und MateriaIj'brschung, Post/ach 51 O1 19, D-01314 Dresden, Germany

Abstract High energy ion implantation of phosphorous and boron in the MeV-range has been applied to modify the vertical electric field distribution of silicon pn-junction radiation detectors. Low dose phosphorous implantation (10 MeV, 101~-10 ~2cm 2) was used to realize detectors with local high field regions characterized by an internal electric field strength up to 150 kV/cm. A field related decrease of the effective window down to 35 nm and a corresponding reduction of the pulse height defect (PHD) tbr the spectroscopy of low energy heavy ions have been obtained. Additional subjects for the application of the MeV-ion implantation technique are briefly summarized.

1. Introduction Passivated ion implanted planar silicon detectors are widely used for the spectroscopy of charged particles or soft X-rays. This detector technology has been introduced in the early 70"s by the fundamental work of Kemmer [1] and a variety of advanced structures such as strip or pixel detectors, the silicon drift chamber (SDC) or fully depleted junction CCD's has been developed during the last decade [2]. Until now the application of ion implantation is mainly limited to the fabrication of the shallow pn-junction region and the n +-contact using low energy implantation of boron and arsenic/phosphorous (10-30keV, 1014_10]Scm z), respectively. Besides, relatively high energy ion implantation (300-500 keV) has been used to implant deep n ~~' or p ' ~ ' layers into high resistivity silicon for fully depleted CCD's, SDC's or for JFET's integrated on these detectors [3,4]. Nowadays, MeV-implantation facilities with increased ion beam current densities of several i.tA/cm 2 are also frequently applied in the field of materials science. The use for defect engineering, retrograde well formation or soft error reduction in microelectronic devices integrated on low resistivity silicon should be mentioned here

[51. In this paper first applications of the MeV-ion implantation technique in the detector technology based on high resistivity silicon will be discussed. The motivation arises from the fact, that local changes of the substrate doping can be used to modify the vertical electric field distribution of silicon pn-junction detectors. Low dose MeV-implanta*Corresponding author. Tel. +49 351 260 3378, fax +49 351 260 3285, e-mail [email protected].

tion of phosphorous in n-type silicon has been applied to realize pn-junction detectors with high field regions extending from the surface to a depth of several ixm. After the description of the working principle, first experiments will be presented to characterize the properties of the proposed detector structure for the spectroscopy of charged particles. Some further ideas for integrated dE/dx-detectors based on MeV-implanted dopant layers will be summarized including preliminary experimental results.

2. High field region detectors (HFRD) for heavy ion spectroscopy Using silicon pn-junction detectors for heavy ion spectroscopy two particular problems arise, namely the plasma delay and the pulse height defect (PHD). This PHD is usually defined as the energy difference obtained in the spectrum between the peak position of heavy ions and protons (or c~-particles) of the same incidence energy. The PHD is caused by three effects: the nonnegligible energy loss of heavy ions in the detector entrance window, the increasing contribution of the nonelectronic nuclear stopping with increasing ion mass and the deficit due to the plasma recombination of charge carriers along the ionization track of the incident particle [6-8]. It is known that the effective silicon dead layer and the plasma recombination at ionization densities ~>10'" e/h-pairs per cm 3 depends on the electric field strength [9,10]. Therefore, detectors with enhanced internal electric fields of F - > 10 kV/cm are preferred for the spectroscopy of heavy ions owing large electronic stopping power. Such detectors are usually realized using low resistivity silicon (p = 300-

0168 9002/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0168-9002(96)00235-5

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700 ~ cm) as bulk material. For low energies ( < 20 MeV) and ions lighter than silicon the PHD is mainly determined by the detector window and the contribution from nonelectronic energy loss [11]. At higher energies and/or heavier ions the plasma recombination has to be taken into consideration. 2.1. Principle and properties q t MeV-implanted H F R D

We propose a novel silicon detector structure with a high internal field strength up to 200 kV/cm, which is shown schematically in Fig. 1. The conventionally employed pn-junction structure based on high resistivity ntype material (p,, -> 2000 ~ cm) is modified by a high energy, low dose ion implantation of donors (P+, As*), leading to a buried region of enhanced bulk doping. The depth of the HFR and the maximum electric field strength can be established separately by the energy and the dose of the MeV-implantation, respectively• For P+-ions of E = (3-20) MeV the projected range of the implanted ions is between (2-10)I*m. The depletion depth as a function of the bias is shown in Fig. 2 for detectors with additional 10 MeV P+ -implanted layers. Due to the high resistivity of the bulk material the region between the pn-junction and the buried, implanted n ~+Mayer is depleted only from the diffusion voltage. The extension of the depletion depth with increasing detector bias is delayed by the enhanced donor concentration over the MeV-implanted profile up to the full depletion of the implanted region. The critical voltage, characterized by the drop of the curves depth in Fig. 2 is strongly influenced by the implantation dose. The further extension of the depleted depth into the high resistivity substrate requires only small additional voltage. Fig. 3 shows the calculated electric field distribution for a detector modified by a P~-implantation at 10MeV and different doses. The local enhancement of the bulk doping due to the MeV implantation results in an increased and nearly constant electric field up to the depth of the buried layer. Behind the n<+Llayer the electric field linearly decreases analogously to the conventional detector configuration. This field distribution is similar to diodes, which are produced using epitaxial layers on high resistivity material.

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Concerning an application to charged particle spectroscopy, some properties of conventional heavy ion detectors and the novel detector structure are compared in Table 1. The advantage of enhanced electric field strength has been already mentioned. In contrast to conventional heavy ion detectors made from low resistivity silicon, the proposed structure enables a full depletion for a detector of (300-500) I.zm thickness. Therefore a detector is realized,

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VIII. NEW .APPLICATIONS AND DEVICES

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.I. yon Boram' et al. / Nucl. Instr. and Meth. in Phys. Res. A 377 11996)514-520

Table I Comparison o f conventinnal heavy ion detectors and HFRD Parameter

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which offers simultaneously enhanced field strength in an extended near-surface region, large depletion depth and low capacitance ( - l o w electronic noise). These detectorproperties appear to be very attractive for the simultaneous spectroscopy of light charged particles and low energy heavy ions.

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2.2. Detector preparation

Small area detectors (A 24 mm -~) on high resistivity n-type material ( p , ~ l . 5 k ~ c m ) have been prepared including high energy ion implantation of phosphorous with E = 1 0 M e V and D (2.5 10) lOL~cm " into the standard planar process. The MeV-implantation step has been perlormed alter the oxidation process without an 5, implantation mask or alternatively using thick photoresist masks (20 Ia,m thick), which limits the region of MeV implantation to the active detector area of the pn-junction. Concerning the detector properties only slight differences (not discussed here) in the reverse current and the capacitance characteristics have been established for both cases, however the use of an implantation mask should be preferred. The MeV-implantation results in a buried pearson-shaped doping profile with a peak concentration of N D = ( 2 - 1 0 ) I 0 ' S c m ~ in the depth in the depth of (4.7+().2)1,tm, as found by' spreading resistance and capacitance-vohage profiling. The pn-junction was fabricated by a boron implantation (15 keV, 5 x 10 '~ cm -') through a 5 0 n m SiO_~-Iayer. The annealing step for all implantations was performed at 600°C, 30rain in dry nitrogen. For metallization thin layers of AI/Si(3%) with a thickness of 3Ohm and IO0nm were sputtered on the detector front and the rear side, respectively. Measurements of the detector reverse current (IV) characteristics show no significant influence of the additional MeV-implantation step on the detector current even for internal electrical fields of 150kV/cm (U 100V, l<30nA). However, the buried implantation profile strongly changes the capacitance characteristics in comparison with conventional p +- n n - d e t e c t o r structures, as shown in Fig. 4 for different implanted P - d o s e s up to

10': crn :. The sharp transition in the CV-characteristics corresponds to the bias voltage at which the detector begins to deplete behind the buried n~ ~ '-implant (see Fig. 2). To illustrate the influence of different implantation doses on the properties of the high field region, the dependence of the maximum electric field strength on the applied detector voltage is shown in Fig. 5. Implantation doses above D > 2 . 5 X 10 't c m -~ result in high field regions with F > 50 kV/cm, which exceed those of conventional heavy ion detectors and lor an implantation dose of 1 x

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J. yon Boranr et al. t Nut/. hlstr, and Meth. in Phys. Res. A 377 (1996) 514 520

Fig. 6 shows the measured ion energies corresponding to the (x-particle calibration as a function of the electric field strength. The bias voltage for all detectors was changed from 5 V up to 200 V to obtain an electric field overlap between detectors with different high field regions. Three regions can be clearly distinguished. The measured energy continuously increases with the electric field up to a constant value, which is obtained at around 35 kV/cm. The following nearly constant part of the curves can be interpreted as the region of maximum carrier collection from the window and the ionized track in the depletion zone. At electric fields above 100kV/cm a significant enhancement of the pulse height was measured caused by charge carrier multiplication. The beginning avalanche multiplication at moderate field strength of about 80 kV/ cm is surprising and gives rise to the conclusion, that the electric field is strongly enhanced near the end of the "'conducting needle" of the plasma track. The enhanced PHD at low ion energies is caused by an increasing influence of the detector window. The effective silicon dead layer thickness, measured for =4'Am e~-particles using the well known tilting technique [12], is found to be a strong function of the electric field strength and decreases from 130 nm to 35 nm with increasing field strength of 12 and 85 kV/cm, respectively (Fig. 7). The effective dead layer thickness of (35_+ 1 0 ) n m corresponds to the lowest values ever obtained for pn-junction detectors. Further investigations concerning the spectroscopy of heavier ions and the timing properties of high field region detectors are being in progress. During our experiments the decrease of the silicon dead layer is referred to a reterence detector with relatively thick dead layer of 130 nm. It is known that for optimized

10'= cm -~even an electric field strength above I00 kV/cm can be easily realized. 2.3. H F R D test .fi)r o x y g e n s p e c t r o s c o p y

First experiments have been performed at the Rossendoff Tandetron accelerator to characterize the behaviour of detectors with different high field regions for oxygens ions with energies between 0.9 and 8.5 MeV. The oxygen ions were backscattered from a 150nm thick Au layer on vitreous carbon. The test detector was installed at an angle of 164 ° concerning the beam direction and the kinematic broadening due to the detector acceptance angle of <0.5 ° is less than 20 keV in the worst case. Although we did not observe long term shift of the peak position in the spectrum due to carbon buildup in the beam spot, the beam position at the target was changed regularly. The determination of the PHD requires a very accurate knowledge of the incidence ion energy. Therefore the absolute energy of the system was calibrated using the narrow resonance at 3.045 MeV in the cross section of the elastic 4He scattering at oxygen ( '"O(e~, a ' )~'O, P - 10 keV). The setting of the various incidence energy values was performed with a precision GVM with a linearity better than 2 × 10 3. This procedure results in a total uncertainty of the incidence oxygen oxygen energy of less than 3 × 10 3. The detectors were calibrated with e~-particles of -~4]Am(5485.7keV) and '42Gd-(3182.SkeV) sources and the stability of the electronics was controlled during the whole measurement using a peak from a precision pulser (ORTEC-444). The energy of oxygen ions in the spectrum was evaluated from the high energy edge of the flat-topped gold peak.

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implantation and annealing conditions ( T - 9 0 0 ° C ) the effective silicon dead layer of conventional detector structures can be reduced to (40-50) rim [13]. Therefore, further experiments are necessary to investigate the influence of the field strength for F > 20kV/cm to the effective dead layer of detectors, prepared at optimized technology. The advantages of HFRD are expected particularly for heavy ions at elevated energies (e.g. for fission fragments with E--~ 100MeV), where the energy deficit from plasma recombination should dominate. For this application the high field region has to be extended up to 15 Ixm, which requires phosphorous ion implantation energies of about 50 MeV.

obtained for dE-transmission detectors made from thin silicon membranes. This behaviour is determined by the following properties of the proposed structure: - The variations in the "thickness" of the dE-detector are very low ( < 0.1 ~m) and comparable with dE-detectors made from epitaxial layers. - Despite the low detector bias the electric field strength is above 20kV/cm necessary tor sufficient charge carrier collection. The field strength is nearly constant across the full depth of the dE-detector. - The contribution of hole diffusion from the undepleted bulk into the depletion region to the signal amplitude is reduced due to the increased electron concentration in the buried implanted n (-)-layer. These effects result in a measured energy of the dEsignal (dE = 725 keV), which is not far away from the expected value (x = 4.7 I~m ~ dE = 670 keV). The residual influence of the diffusion can be further reduced by an optimized electronic signal processing based on small shaping times. Although this detector principle cannot replace a real telescope, where the dE- and the E-signal are produced from the two (different) detectors, this structure enables to distinguish between different charged particles in a fixed radiation field. The structure can be also advantageously used for particle discrimination between light charged particles like protons or alphas and heavy ions. In conventional silicon detectors, the discrimination is common realized using a low detector bias. However, the properties concerning heavy ion spectroscopy deteriorate due to the low electric field related with insufficient charge carrier collection. This problem can be reduced by the proposed, implantation modified structure, where small depletion depth and high

3.

Other

Applications

3.1. Application o f H F R D f o r particle discrimination

In Fig. 2 it is clearly shown, that a critical voltage is necessary to extend the depletion depth of the detector behind the implanted buried profile. For a detector bias below this value, the depletion is limited to the high field region and the depleted depth changes only slightly with increasing voltage. For this condition the detector generates a signal, which corresponds to the energy loss dE in the depletion region. Therefore, the detector can be operated alternatively in the dE- and the E-mode switching only the detector bias between two specified values. This principle was tested with 24'Am-~-particles and the results are shown in Fig. 8 where the dE- and E-signals can be clearly distinguished. The peak width of the dE-signal is quite good and corresponds to the energy resolution

J. yon Borany et al. / NucL Instr. and Meth. in Phys. Res. A 377 (1996) 514-520

electric field can be simultaneously realized in the dEmode described above. An example for the advantageous use of particle discrimination is the reduction of the aparticle background in ERD-experiments (Elastic Recoil Detection) applied for the elemental analysis of solid state surfaces using alphas as incidence particles. 3.2. Monolithically integrated d E / d x - E - t e l e s c o p e s

High energy boron implantation can be used to realize n +np +nn +-detector structures, which exhibit the combination of a transmission dE/dx-detector with a E-stop-detector. The schematic structure of the proposed integrated detector telescope is shown in Fig. 9. The common pnjunction for both the d E / d x - and the E-detector is realized by the buried boron implant. The contact areas for the p+-layer as well as the n+-contact of the dE/dx-detector are located on the front side of the detector and allows simplified detector housing. Earlier published detector telescopes [14] have been fabricated as single detector samples using special tapering etching and electrical leads (indium pellets) to make contact to the deep implanted layer. Two main problems have to be solved to enable the application of the planar batch detector fabrication process: a suitable implantation mask for ions in the MeV energy range and the electrical contacting of the buried p +-layer. Structurable photoresist mask up to 50 l~m has been developed based on modified AZ1350H photoresist with a high specific weight of 1.1022 g / m l and 1.1125 g/ml. The resist was spun on the wafers using low rotation velocity of 1500rpm to achieve a homogeneous layer thickness. Applying double resist exposition and development an edge slope of the pattern in the range from 0.5 up to 0.6 has been observed for layer thicknesses of 15 t-tm and 50 txm, respectively. The masks can be applied for ion energies up to E ~ 10MeV. For higher energies masks made from (100)-oriented silicon wafers should be preferred. The structure on the masking wafer will be realized by a standard photolithography process and subsequent wet anisotropic chemical etching through the whole wafer. The alignment of the wafers to each other can be realized using infrared transmission microscopy.

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The electrical contact to the buried p'-layers were established using wet chemical anisotropic etching (KOH, 80°C) of deep contact holes into the (100)-oriented silicon up to the depth of the buried layer. The connection between the deep implanted p+-layer to the planar p+contact area on the frontside of the detector is realized by an additional boron implantation (50 keV, 5 x 10 ~4 cm 2) into the bottom and the edges of the etched grooves. The properties of such dE/dx-detectors and first spectroscopic results will be published in a separate paper.

4. Conclusions The application of MeV ion implantation to the technology of silicon pn-junction detectors has been discussed. The increase of the local substrate doping or the formation of buried pn-junctions leads to the modification of the vertical electric field distribution, which can be used to realize high field region detectors or integrated detector telescopes. First detector structures have been tested. The preliminary results show the influence of enhanced electric fields to the reduction of the silicon dead layer and the possibility to use such detectors as dE/dx-detectors. Avalanche multiplication effects have been observed for oxygen ions at electric field strength above 80kV/cm. Further technological effort is necessary to fabricate detectors with a larger depth of the high field region using increased implantation energies.

Acknowledgements The basic idea of the proposed high field region detector structure was developed by M. Deutscher to improve the signal linearity of electron detectors for high current densities ( > 2 0 A / c m 2 ) . The authors would like to thank W. Skorupa for stimulating discussions on the application of MeV-ion implantation, I. Beatus for her technological support concerning the preparation of thick photoresist masks and the colleagues from the accelerator laboratory in Rossendorf for their encouragement during the experiments.

A ~ 2

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519

References

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[11 J. Kemmer, Nucl. Instr. and Meth. 169 (1980) 499. [21 J. Kemmer, Nucl, Instr. and Meth. B 45 (1990) 247. [31 V. Radeka et al., IEEE Trans. Electron Devices Lett. ED10(21 (1989) 91. 141 L. Striider, Nucl. Instr. and Meth. A 283 (1989) 387. [5] Handbook of Ion Implantation Technology, ed. J.F. Ziegler (Elsevier Science, 1992) ISBN 0 444 89735 6, 346. I61 B.D. Wilkins et al., Nucl. Instr. and Meth. 92 (1971) 381.

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