Effects of Electrostatic Discharge Stress on Electrical Propertiesof Bidirectional TVS Zener Diode with Abrupt Junctions

Daoheung Bouangeune1, Yeon-Ho Kil1, Sang-Sik Choi2, Deok-Ho Cho2,Kyu-Hwan Shim1,2,+ and Chel-Jong Choi1,3,+

1School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University,Jeonju 561-756, Republic of Korea2R&D Division, Sigetronics, Inc., Jeonju 561-756, Republic of Korea3Department of BIN Fusion Technology, Chonbuk National University, Jeonju 561-756, Republic of Korea

A bidirectional transient voltage suppressor (TVS) Zener diode was fabricated with abrupt junctions using the low-temperature epitaxyprocess. The effects of various electrostatic discharge (ESD) stresses on the electrical properties are demonstrated, such as the current­voltage(I­V) and 1/f noise power spectral density (PSD). Very sharp and uniform bidirectional multi-junctions result in good symmetric I­V behaviorover a wide range of operating temperatures of 300­450K. The differential resistance in the breakdown region is only 0.2³, and the reverseleakage current density is completely suppressed to 1.5 © 10¹4 A/m2. The thermal activation energy obtained from the Arrhenius plot is nearlyequal to half the band gap of Si, indicating that the reverse leakage current is dominated by thermal generation at the depletion edges for theentire reverse bias regions. The manufacture bidirectional TVS devices exhibit excellent ESD robustness, regardless of the stress conditions ofthe human body model and electrical fast transient. However, a «4.5 kV machine model and «13 kV IEC61000-4-2 stresses led to severedamage of the epitaxially grown junction, resulting in rapid increases in both the reverse leakage current and 1/f noise PSD.[doi:10.2320/matertrans.M2013144]

(Received April 12, 2013; Accepted August 21, 2013; Published October 11, 2013)

Keywords: electrostatic discharge (ESD), 1/f noise, transient voltage suppressor (TVS), zener, diode, human body model (HBM), machinemodel (MM), international electrotechnical commission (IEC), electrical fast transient (EFT)

1. Introduction

Electrostatic discharge (ESD) is a well-known transientthreat to sensitive integrated circuit (IC) components.1­3)

Common ESD events lead to permanent device damageassociated with the breakdowns of junctions, metal inter-connects and dielectrics, caused by high current transientsand high voltage overstress, resulting in the malfunction ofIC chips. IC dimensions have been continuously scaled downto realize higher package density, faster operation speed andlower power dissipation. As such, ICs are more vulnerablethan before to the damaging effects associated with ESDevents.1,4) Moreover, ESD continues to be the primary causeof poor reliability in IC components. It was reported thatapproximately 35% of total IC failures are related to ESD,incurring tremendous annual costs to the IC industry.5,6)

Therefore, dedicated ESD protection devices are required toprotect IC chips against ESD damage. Among the varioustransient voltage suppressor (TVS) technologies as summa-rized in Table 1, the non-snapback TVS device like the TVSdiode has received much attention close to an ideal TVSdevice, due to the simple structure and good performancecharacteristics, such as lowest leakage current, clampingvoltage and dynamic resistance. Generally, the conventionalTVS devices are constructed with reverse-biased p­njunctions designed to breakdown at a well-controlled voltage,and operate in the avalanche condition.12) However, the largeleakage current and capacitance make them unsatisfactory forhigh-performance technology applications.13,14) Furthermore,the conventional TVS devices are focused only unidirectionaldevices, despite the fact that ESD can have either a positive

or negative current/voltage waveform. Therefore, the bidirec-tional TVS device has emerged to ensure the reliability of theprotective component.

We have fabricated a bidirectional TVS Zener diode withepitaxial Si layers consisting of very sharp planar junctions,and investigated the effects of ESD on its electricalproperties. In particular, the current­voltage (I­V) and 1/fnoise properties were analyzed in conjunction with ESDstresses to understand the failure process and correlate themto noise protection. It is shown that the manufactured TVSZener diode is very promising, with a low differentialresistance, good thermal stability and strong ESD protectioncapability.

2. Experimental Procedures

The proposed bidirectional TVS diode consisted of thep­n­p structure as the main part to avoid the snapbackphenomenon compare the n­p­n structure, as shown inFig. 1(a).15) Although the snapback phenomenon can achievelow clamping voltage, the low holding voltage below powersupply voltage is still main problem.8) In addition, a very thinand heavily doped n++ epilayer enabled I­V curves steep riseafter breakdown, resulting in small dynamic resistance valueand the improvement of ESD performance. Therefore, thep-type (100) Si wafer with a resistivity of 0.003³·cm wasused as a starting material. In addition, the bidirectionalbreakdown voltage was control by doping and thicknessin multi-junction. To form an abrupt planar junction witha doping gradient of >5 © 1025 cm¹4, thin Si films withvarious doping conditions were epitaxially grown usingreduced-pressure chemical vapor deposition (RPCVD). Thep- and n-type epitaxial Si layers in the junction were formed+Corresponding author, E-mail: khshim@jbnu.ac.kr, cjchoi@jbnu.ac.kr

Materials Transactions, Vol. 54, No. 11 (2013) pp. 2125 to 2130©2013 The Japan Institute of Metals and Materials

in-situ by the addition of diborane (B2H6) and phosphine(PH3) gases during the deposition, respectively. To minimizethe degradation of device performance caused by the non-uniform local electric field concentrated in the SiO2/Siinterfaces associated with a linearly-graded junction formedby the conventional high-temperature drive-in process, theRPCVD process was performed at low temperatures of 600­800°C. For the same reason, based on the well-establishedcomplementary metal­oxide­semiconductor (CMOS) tech-nology, the process temperature for the fabrication of abidirectional TVS Zener diode was maintained below 800°C.In order to form a top contact electrode, 100-nm-thick Aufilm was deposited by means of an e-beam evaporator,followed by patterning with a 140 © 140-µm2 square shapeusing lift-off lithography. The details of the device structureand fabrication process for a bidirectional TVS Zener diodecan be found elsewhere.16) The I­V characteristics wereanalyzed using a parameter analyzer (Agilent 4156C). TheESD properties and reliably of the manufactured TVS diodewere analyzed using an ESD simulator (NoiseKenESS-6008), which can supply an output voltage of up to «8 kVin a human body model (HBM) and machine model (MM).An ESS-2000 with a discharge gun TC-815R and FNS-AXIIwere used for the IEC61000-4-2 and IEC61000-4-4 standardanalyses, respectively. For convenience, IEC61000-4-2 andIEC61000-4-4 are referred to here as IEC and electrical fasttransient (EFT), respectively. The 1/f noise power spectraldensity (PSD) was measured using the experimental setupshown in Fig. 1(b), which consists of a low-noise currentamplifier (SR570) and a spectral analyzer (Agilent 35670A).

The HBM and MM tests referred as ESD simulator modelsrepresent the effect of statistic charges created by humanbody and machine discharge on electronic components.Generally, human bodies and machine components arecharged by walking across a carpeted floor or removinga sweater, and by rubbing insulative materials during theiroperation, respectively.17) Meanwhile, IEC61000-4-2 and

IEC61000-4-4 are part of a larger family of IEC 61000-4(IEC: International Electrotechnical Commission) defined asthe performance required of all electronic devices in a varietyof electromagnetic interference (EMI) conditions. The IECwas developed from HBM, which is referred as the minimumESD level of acceptable performance required on allelectronic devices sold into the European Union.18) TheEFT test simulates the effect of the burst of very fast pulseson electronic components, which is created by switchingcircuits, such as relay contact bounce and some air contactorswhen interrupting inductive-capacitive loads.19) To under-stand the difference of each ESD models, all ESD waveformsused here were shown in Fig. 1(c).20,21)

3. Results and Discussion

Figure 2 represents the I­V characteristics of a bidirec-tional TVS Zener diode measured at temperatures in therange of 300­450K. Generally, a bidirectional TVS Zenerdiode can be described as two unidirectional TVS diodes inback to back contact, i.e., one works in forward bias andanother is reverse bias. When applied voltage or transientvoltage like ESD is larger than breakdown voltage of TVSdiode, the reverse biased one suddenly shows breakdownin avalanche breakdown mode, resulting in passing extratransient current to ground. For all temperatures, the deviceshows good symmetric I­V behavior without degradation ofthe device performance. The increase in temperature led toan insignificant change of the breakdown voltage, the valueof which is ³«19V at 102A/m2. Since the carriers in thesemiconductor follow Boltzmann statistics, the reverseleakage current density increased with increasing temper-ature. However, regardless of temperature, the reverseleakage current density was kept at less than 10A/m2 at«15V. This small reverse leakage current is essential toassure the improvement in reliability, noise performanceand ESD robustness of the TVS device. The temperature

Table 1 Comparison of electrical and ESD performance of various TVS technologies.

Device characteristicsand

electrical parametersIdeal TVS device

TVS technologies based on Si

Diode7) SCR8) Punch-thoughdiode9)

Gate groundedMOSFET10)

Structure, polarity Vertical or Lateral, Uni- or Bi- Vertical, Uni- Lateral, Bi- Vertical, Uni- Lateral, Uni-

Device area, A/mm2 Small size 0.0196 N/A 0.0786 N/A

Snapback device N/A Non Yes N/A Yes

Leakage current, I/A Smaller 1 © 10¹9 6 © 10¹8 ³1 © 10¹9 1 © 10¹8

Breakdown voltage, BV/Vor fist triggering voltage, Vt1/V

Depend on application 6 ³15 5.5V ³6.5

Holding voltage, Vh/V Vh µ Vt1 > VCC*1 Non 7.5 Non 4.5

Clamping voltage, VC/V VC µ BV or Vh 7.7*2 ³18*2 ³14*2 6*3

Maximum ESD robustness(IEC 61000-4-2), IEC/kV

>8 (level 4) 19 >6*4 7.2*4 ³1*4

Dynamic resistance orturn on resistance, RD/³

Smaller 0.12 ³1.16 0.8 ³1.5

Capacitance, C/pF Smaller 60 N/A ³22 N/A

*1VCC = Normal operator voltage of system or circuits.*2Clamping voltage measured by transmission line pulse (TLP) at 10A.*3Clamping voltage at 1A TLP.*4Converted from TLP to IEC 61000-4-2 (1A TLP µ 600V IEC).11)

D. Bouangeune et al.2126

coefficients of the breakdown voltage and the reverseleakage current were measured to be ³10¹3 V/K and³3.42 © 10¹3 A/m2·K (at ¹10V). These values are reason-able, because Zener breakdown occurs correspondingly in theswitching regime from the tunneling mode to the avalanchemultiplication mode.22)

The temperature dependence of the reverse leakage currentcan provide useful insight into the leakage mechanism. Thetemperature dependence of reverse current Ir can be describedby:

Ir / T 3 exp � Ea

kT

� �ð1Þ

where k is the Boltzmann constant, Ea is the activate energyof the junction and T is the absolute temperature. The slope ofan Arrhenius plot of log10 Ir(T)/T3 versus 103/T yields theactivation energy Ea.23) The activation energy of the reverseleakage current should in principle be close to the Si band

gap Eg or Eg/2 in diffusion or recombination-dominatedregimes, respectively.24) Figure 3 shows the Arrhenius plot ofthe reverse leakage current of a bidirectional TVS Zenerdiode measured at various reverse biases in a temperaturerange of 300­450K. Irrespective of the reverse biases, thevalues of activation energy determined from the slope ofthe Arrhenius plots ranged from 0.56 to 0.57 eV, which arenearly equal to Eg/2. This result clearly indicates that thereverse leakage current of the bidirectional TVS Zener diodewas dominated by thermal generation at the depletion edgesfor the entire reverse bias regions. The generation andrecombination of carriers from generation-recombinationcenters in the space charge region of a p­n junction in thepresent device could be a main contribution to carrierconduction in reverse bias.

The differential resistance (Rz = dV/dI) in the breakdownregion is one of the most important parameters of the TVSdevice. A low differential resistance guarantees low clampingvoltages and minimizes the resistive Joule heating withstrong and rapid ESD surges. Moreover, the high differentialresistance leads to unstable current drivability. Figure 4presents the plots of differential resistance as a function ofreverse current obtained from the I­V characteristics in the

Fig. 2 I­V characteristics measured from the bidirectional TVS Zenerdiode at various temperatures of 300, 350, 400 and 450K.

Fig. 3 Arrhenius plot of the leakage current of a bidirectional TVS Zenerdiode measured at various reverse biases in temperature range of 300­450K.

(a)

(b)

(c)

Fig. 1 (a) Schematic cross-sectional view of the bidirectional TVS Zenerdiode with abrupt junctions, (b) the experimental setup for low-frequencynoise measurements and (c) ESD and EFT waveform used in proposedwork.

Effects of Electrostatic Discharge Stress on Electrical Properties of Bidirectional TVS Zener Diode with Abrupt Junctions 2127

temperature range of 300­450K (Fig. 2). With increasingtemperature, the differential resistances gradually increased.For instance, the differential resistances near the breakdownvoltage were less than 1 and 30³ at 300 and 450K,respectively. These values are quite low compared to those ofa conventional Zener structure with linearly graded junctionsformed by high-temperature diffusion process.16) The verylow differential resistance obtained from the present devicecould be attributed to abrupt junctions with a doping gradientof >5 © 1025 cm¹4 formed by the epitaxial growth.

Figure 5 presents the I­V characteristics of a bidirectionalTVS Zener diode measured at room temperature after theapplication of various ESD stresses. The manufactured TVS

device was subjected to 10 discharges of both positive andnegative polarities with 1 s time intervals for MM, HBM andIEC tests. For the evaluation of EFT immunity, a number ofunidirectional EFT pulses with a constant voltage of +4.5 kVwere applied to the TVS device. It is clear that the TVSdevice exhibits excellent strength against HBM stress(Fig. 5(a)). Almost identical I­V behaviors were observedeven after the HBM pulses with both positive and negativepolarity at «8 kV (maximum supply voltage). As for the EFTtest (Fig. 5(b)), the increase in the numbers of EFT pulses ledto a gradual increase in the reverse leakage current density.For instance, the reverse leakage current densities measuredat ¹10V were respectively found to be 1.32 © 10¹4 and2 © 10¹3 A/m2 before and after applying 106 EFT pulses.Although the EFT stress led to the degradation of deviceperformance to some extent, the reverse leakage currentdensity level was still maintained below 10¹1 A/m2. Thisimplies that the TVS device demonstrated here has sufficientreverse leakage current margins for device application.Similarly, the TVS device was capable of withstanding«4.0 kV MM and «12 kV of IEC without degradation ofthe I­V characteristics, as shown in Figs. 5(c) and 5(d),respectively. However, both «4.5 kV MM and «13 kV IECcaused a rapid increase in the reverse leakage current density,indicating a typical electrical failure signature. The scanningelectron microscope (SEM) results (insets of Figs. 5(c) and5(d)) showed that the application of either «4.5 kV MM or«13 kV IEC incurred the creation of pinholes surrounded byrefrozen features on the metal electrode. This indicates thatthe epitaxially grown junction was severely damaged by«4.5 kV MM or «13 kV IEC, which could be responsible forthe device failure.

Fig. 5 I­V characteristics measured before and after various ESD stresses: (a) HBM, (b) EFT, (c) MM and (d) IEC.

Fig. 4 The corresponding differential resistance curves measured from thebidirectional TVS Zener diode at various temperatures of 300, 350, 400and 450K.

D. Bouangeune et al.2128

The ESD immunity of the bidirectional TVS Zener diodewas also confirmed by low-frequency noise measurement,which is a valuable diagnostic tool to assess the reliabilityof semiconductor active devices.25­28) Figure 6 exhibits theplots of 1/f noise PSD as a function of the frequency of thebidirectional TVS Zener diode measured before and aftervarious ESD stresses. Similar to the ESD stress-dependentI­V characteristics (Fig. 5), the manufactured TVS deviceshowed excellent 1/f noise properties for HBM and EFTtests, as shown in Figs. 6(a) and 6(b), respectively. Forinstance, the 1/f noise PSD remained unchangeable(³10¹19 A2/Hz at a frequency of 100Hz) even aftermaximum HBM and EFT stresses. On the other hand, asshown in Figs. 6(c) and 6(d), the 1/f noise PSD changedinsignificantly up to «4.0 kV MM and «12 kV IEC, and thenincreased rapidly. Such a large increase in the 1/f noisePSD could be associated with the defect-induced noise at thetrap-and-detrap centers, when considering damaged featureson of the present device produced by «4.5 kV MM and«13 kV IEC (insets of Figs. 5(c) and 5(d)).

4. Conclusion

A bidirectional TVS Zener diode was fabricated with anepitaxially grown abrupt junction, and its ESD performancewas demonstrated using I­V characteristics and 1/f noisePSD. Due to the planar junctions achieved using low-temperature epitaxial growth technology, the reverse leakagecurrent density and differential resistance in the breakdownregion can be suppressed to 1.5 © 10¹4 A/m2 and 0.2³,respectively. The manufactured TVS device presented anexcellent ESD robustness against «8 kV HBM and 106 EFT

pulses of 4.5 kV without degradation of the I­V and 1/f noisecharacteristics. On the other hand, «4.5 kV MM and «13 kVIEC resulted in critical damage to the epitaxially grownjunction, which could be responsible for the large increases inreverse leakage current and 1/f PSD.

Acknowledgements

This work was supported by the Priority Research CenterProgram (2011-0031400), and the Converging ResearchCenter Program (2012K001428) through the NationalResearch Foundation of Korea (NRF), funded by theMinistry of Education. It was also supported by the ITR&D program of the MKE (KI002083, Next-GenerationSubstrate Technology for High-Performance SemiconductorDevices).

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