X-ray Radiation Damage in P-N Junction Diode

Transcription

1 X-ray Radiation Damage in P-N Junction Diode Itsara Srithanachai 1, Surada Ueamanapong 1, Yuwadee Sundarasaradula 1, Amporn Poyai 2, Surasak Niemcharoen 1 1 Department of Electronics, Faculty of Engineering, King Mongkut s Institute of Technology Ladkrabang 2 Thai Microelectronics Center (TMEC), Chachoengsao Abstract The effect of X-ray radiation damage in P-N junction diode is discussed. Electrical characteristics of P-N junction diode can be analyzed by current-voltage (I-V) measurement. This paper investigates X-ray irradiation by its electrical characteristics of difference X-ray exposure time. The X-ray energy use to expose 40 kev various time in the range second of exposure. Leakage current after irradiation at 5 and 15 second are increase, while increase irradiation time the leakage current are reduced back to close before irradiation. X-rays are induced defect at 5 and 15 second, while defects reduced after irradiation at 125 second. The result shows that the time of X-ray exposure at 75 and 125 second can reduced defect in P-N junction diode. Keywords: X-ray radiation, P-N junction diode, Radiation damage

2 1. Introduction Silicon sensors are widely used in high energy and nuclear physics experiments, however they suffer from severe radiation damage that leads to degradations of the sensor s performances. These degradations include significant increase in leakage current, bulk resistivity and free carrier trapping. Trapping center is the main factor for analysis the effect of radiation on device [1]. Highenergy particles may produce at least two different types of effects in semiconductor devices, i.e., ionization damage and displacement damage. Initial ionization damage creates free electron-hole pairs in the SiO 2 layer by disrupting electronic bonds, which can cause either transient or long-term ionization damage. On important transient ionization degradation mechanism is the Single Event Upset (SEU). It corresponds to a single high-energy particle striking a critical node of the device, leaving behind an ionized track passing through the well area or storage capacitor. For sufficiently high particle energies, the energy transferred during an elastic or inelastic nuclear collision may be large enough to knock an atom from its lattice site. This creates a vacancy (V) and the atom in an interstitial (I) position. Such one-atom disorder in a crystalline lattice is called appoint defect [2-3]. The radiation induced lattice defects of Si diodes by irradiation and their effect on device performance are investigated [4]. This lattice damage consists of bounce out a Si atom from the normal lattice site and atomic hydrogen or another impurity atom substituts on Si site. Consequently, the defects induced by irradiation are composed of complexes of the vacancy with impurity atoms such as phosphorus or oxygen and of two adjacent sites in the lattice. The aim of this paper is to investigate a detailed characterization of radiation defects in P-N junction diode by the irradiation with lowenergy (40 kev) X-ray. The relationship between the defect or trapping center and leakage current of the low-energy X-ray irradiation in silicon electronics are also discussed [5-8]. 2. Experimental The P-N junction diode were fabricated using n-type CZ silicon wafers, with (100) orientation, Ω cm resistivity and 625 µm of thickness. Silicon wafers were thoroughly cleaned with an ultra-sonic washer to remove organic contaminants. The surface of the samples were chemically cleaned using a mixture of acids (H 2 SO 4 : H 2 O 2 ) and HF [6]. The wafers were sent into photolithography and etch processes to open 2 mm 2 for active area and 10 μm for guard ring of 1 μm thickness silicon dioxide window. Then wafers were boron implanted with dose of 1x10 16 cm -2 at energy of 120 kev. Then phosphorous implanted with the same condition on the backside wafer for ohmic contact and followed by an 1050 o C, 60 min thermal annealing. After that wafers were sent into metallization process to create 1 μm thickness of aluminum layer at both sides. The second photolithography step and etch processes were conducted to create aluminum patterns then anneal at 400 o C for 30 min. Wafers were sawn and assembled on PCB before finishing with wire bond process. Finally, the chip is ready for testing with external circuit. The structure of a P-N junction diode after fabrication was shows in Fig. 1. After its fabrication process, the diodes were irradiated by X-ray for energy 40 kev with exposure time of 4, 15,75 and 125 second. The semiconductor parameter analysis of model HP4156B was used to measure electrical properties of diode, before and after irradiation. The current-voltage (I-V) characteristics of the P-N diode were measured at room temperature to examine the change of the dark current (I D ) by X-ray irradiation. The current-voltage (I-V) characteristics were measured on wafer with

3 bias step of 25 mv for both reverse (V R ) and forward (V F ) voltage, sweeping in the range of - 10 to +1 V. Capacitance-voltage (C-V) measurements were performed on the same diode at a frequency of 100 khz [7]. Fig. 2 The forward and reverse bias semilogarithmic I-V characteristics of P-N diode under various time of exposure at room temperature. Fig.1 P-N junction diode structure. 3. Results and discussions The electrical properties of the irradiated P-N diode are examined. Fig. 2 shows the I-V characteristics of 40 kev X-ray irradiated P-N diode. Forward current increases by irradiation, which is caused by decreasing the recombination lifetime. Note that this should be considered for a forward voltage larger than 0.5 V, since the resistivity of Si substrate is decreasing [8]. Fig. 3 shows the reverse current of P-N diode after 40 kev X-ray irradiation. From this figure, it is found that leakage current increases after irradiation. As can be seen from the figure, 5 and 15 second exposure time caused higher leakage current. However, at longer exposure times (75 and 125 second) leakage current decreases and approaches the original unexposed characteristic. This implies the benefits of the longer X-ray exposures which assist damage curing effect on the silicon bulk lattices. This could point to a different origin of the degradation, i.e., bulk versus surface damage. In the first case, radiation-induced defects will affect the bulk generation/recombination lifetime, while in the second case, the creation of interface traps can increase the surface generation/recombination velocity. Therefore, a further study of the microscopic damage in necessary for a better understanding of the device degradation.

4 trap level in Si bulk. A general relationship between a physical variable and its activation energy can be applied to the leakage current of a P N junction, according to I R (T) exp(-e a /kt) (1) Fig. 3 Reverse bias of current-voltage characteristics before and after X-ray irradiation at 40 kev. With I R is a reverse current. The slope of an Arrhenius plot, I R (T) vs 1/kT yields the activation energy E a [9-10]. Fig. 5 shows the effect of different temperature corrections from Arrhenius plot of generation currents versus the temperature of the different biases. The slope of the plot yields the activation energy E T. Fig. 4 Capacitance-voltage of P-N diode at 40 kev, various exposure times. Fig.4 shows capacitance-voltage (C-V) characteristics before and after irradiated at 40 kev on 5, 15, 75 and 125 second. In reverse bias, a capacitance of non-irradiated silicon diode is generally decreasing while increasing voltage until the saturated value equals to the capacitance between the metal contacts. From this figure, capacitance before and after irradiated of various energy and time were obviously not different. The defect in P-N diode after irradiation by X-ray can be explained by trapping center or Fig. 5 Arrhenius plot of generation current versus the temperature of the different biases. From figure 6, the activation energy of samples before and after exposed to the X-ray irradiation is shown. Activation energy or trapping center before irradiation is approximated to be ev. After X-ray irradiations, the result of leakage currents and calculated activation energies has significant changes. In case of 5, 15 second exposure, the leakage currents increase from the original unexposed leakage current while activation energy curves show major deviation from the original curve as well. However when we increase the exposure time to 75 and 125 second, the leakage currents

5 decrease to the original unexposed leakage current. Fig. 6 Activation energy of P-N junction diode before and after 5, 15, 75 and 125 second of X- ray irradiation at 40 kev. This rebound effect is also shown by activation energy curves. It is observed that the longer exposure times give insignificant changes from the original activation energy curve. From the results, the longer X-ray exposure times (75 and 125 second) may help curing the disturbing effects caused by the shorter exposure times (5 and 15 second). 4. Conclusion The main conclusions from this work are that the device degradation observed after 40 kev X-ray irradiation or subsequent isochronal annealing scales well with the observed radiation induces electron trapping. The degradation of the device s performance and the introduction rate of the lattice defect firstly increase by X-ray irradiation at a short period. However, defect reduced after irradiation by optimize X-ray. In the experimental after irradiation time of 75 and 125 second, the leakage currents decrease. When the device has been exposed with a suitable X-ray dose long enough, it is possible to cure the degradation effects caused by the early X-ray exposure. 5. Acknowledgments The authors would like to thank King Mongkut s University of Technology North Bangkok for providing the X-ray exposure equipment for this experiment, Thai Microelectronics Center (TMEC) for fabrication P-N junction diode, National Electronics and Computer Technology Center, Thailand and Thailand Graduate Institute of Science and Technology (TGIST) under scholarship number TG D). Finally, we would like to give our appreciation for the manuscript writing improvement to Mr. Putapon pengpad, Thai Microelectronics Center (TMEC), Chachoengsao 24000, Thailand. References Journal Papers [1] A. Poyai, E. Simoen, C. Claeys, Appl. Phys. Lett. 78 (7) (2001) 949. [2] P.P. Allport, P.S.L. Booth, C. Green, A. Greenall, J.N. Jackson, T.J. Jones, J.D. Richardson, S. Marti i Garcia, N.A. Smith, P.R. Turner, M.P. Wormald, Nucl. Instr. and Meth. A 420 (1999) 473. [3] K. Takakuraa, K. Hayama, D. Watanabe, H. Ohyama, T. Kudou, K. Shigaki, S. Matsuda, S. Kuboyama, T. Kishikawa, J. Uemura, E. Simoen, C. Claeys, Physica B 376 (2006) 403. [4] H. Ohyama, T. Hirao, E. Simoen, C. Claeys, S. Onoda, Y. Takami, H. Itoh, Physica B 308 (2001) [5] P. Hazdra, H. Dorschner, Nucl. Instr. and Meth. B 201 (2003) 513. [6] M. A. Krivov, S. V. Malyanov, L. S. Smirnov, Izv. Vuzov. Fizika. 8 (1968) [7] M. A. Krivov, S. V. Malyanov, V. I. Gaman, Izv. Vuzov. Fizika. 1 (1967) 99. [8] Z. Ya. Kleiman, T. A. Stefanova, Izv. Vuzov. Fizika. 2 (1964) 160.

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