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Capture and tunnel emission of electrons by deep levels in ultrathin nitrided oxides onsiliconShuoTung Chang , N. M. Johnson , and S. A. Lyon

Citation: Applied Physics Letters 44 , 316 (1984); doi: 10.1063/1.94737 View online: http://dx.doi.org/10.1063/1.94737 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/44/3?ver=pdfcov Published by the AIP Publishing

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apture nd tunnel emission of electrons y deep levels in ultrathin nitridedoxides on silicon

Shuo-Tung Chang 8) and N M Johnson

Xerox Palo lto Research Center. Palo Alto California 94304

S A LyonPrinceton University Princeton New Jersey 08544

(Received24

October 1983; accepted for publication 8 November1983)

Electron injection into ul trathin nitrided oxides on silicon reveals both high densities of electronicdefects, which readily capture electrons, and efficient tunnel emission of trapped charge. Hightemperature nitridation of thermally grown oxides was verified with Auger depth profiling. In11-17-nm-thick nitrided oxides, the electron trap density is;;;. 1 X 10 19 cm- 3 as determined fromsatura ted charge accumulation, the majority of he traps are energetically situated more than 2 e Vbelow the conduction band as determined by post-injection anneals up to 300 ·C, and the capturecross section is of the order of 10- 14 cm 2 as estimated from the trapping kinetics. Completeextraction oftrapped charge is achieved in the thinnest films (e.g., ;;; 11 nm thick), and the tunnelemission mechanism is evidenced by the independence of the discharge time on temperature.Implications ofthe above findings for applications of ultra thin nitrided oxides in very large scaleintegration and for the ir low sensitivity to ionizing radiation are discussed.

PACS numbers: 73.60.Hy, 71.55. - i, 85.40.Ek

Nitridation of thermally grown Si 0 2 films on silicon iscurrently of high technological interest for achieving ultrathin gate insulators suitable for very large scale integration (VLSI). t has been reported that thermal nitridationimproves several critical properties of the dielectric as compared to the pure oxide. l

6 Specifically, it increases the dielectric strength, provides a better barrier against dopantand impurity diffusion, is less reactive with gate electrodes,has a h igher electrical permittivity, is chemically more stable, is more oxidation resistant, and possesses comparable or

superior interface properties. In addition, nitrided oxides areless sensitive to ionizing radiation. 7

The existence of traps in nitrided oxides may be anticipated from defect studies in deposited Si3N 4,8 and evidenceof rapping has been noted in the characteristics of prototypecircuit components (e.g., Ref. 9). A study of ammonia annealed oxides reported an increase in trap density. 10 However, these investigators were unable to demonstate successful nitridation of the oxides, and the traps were consistentwith and ascribed to an increase in the OH content of thefilms. On the other hand, a recent study using high-fieldstress in ultrathin dielectric films reported comparablecharge trapping in nitrided oxides, reoxidized nitrided ox

ides, and pure Si 0 2.

In the present study, we have measured electron trapping 'in nitrided oxides, using avalanche injection to avoidthe complications of trap generation and high-field assisteddetrapping which are generally known to be associated withhigh-field Fowler-Nordheim tunneling experiments. Successful nitridation is verified by Auger depth profiling. Largedensities of electron traps ;;;,10 19 cm -3) are measured, andthe cross section of the traps for capturing electrons differssignificantly from that for the water-related center, 12 13

which was implicated in Ref. 10 as discussed above. Further-

.) Present address: Princeton University, Princeton, New Jersey 08544.

more, this study focuses on ultrathin films in which it isdemonstrated that tunnel emission of trapped charge is anefficient mechanism for charge extraction. This observationprovides a further explanation for the inability of high-fieldmeasurements to accurately assess the presence of traps insuch films.

The test devices were metal-insulator-semiconductor(MIS) capacitors fabricated on (100)-oriented p-type siliconwafers with resistivities of 0.17-0.33 n cm. Chemicallycleaned wafers were oxidized in dry O 2 at 900-1000·C to

grow Si 0 2layers of9-IOO nm in thickness, as determined byellipsometry. The wafers were then annealed in pure ammonia at 1150 C for up to 6 h. As has been generally observed (e.g., Refs. 1,6, and 14), the film thickness increasedby less than 4 nm and saturated after 3-5 h of nitridation.Both thermal oxide controls and nitrided oxides received ahigh-temperature post-oxidation/nitridation anneal in N2for 20 min. Aluminum films were vacuum evaporated overthe gate dielectric and back surface of the silicon wafers.Standard photol ithography was used to define 650-J.lm-diamgate electrodes, and device processing was completed with aforming gas anneal at 450 ·C for 20 min.

The chemical depth profile of a 17-nm-thick nitrided

oxide is shown in Fig. 1. The chemcial composition was determined by Auger depth profile. Standards of pure Si3N4and Si 0 2 were used to determine the relative sensitivities inthe Auger spectra for the constituent elements. In 11- and17-nm-thick films annealed for 6 h, the profiles show a nearly uniform nitrogen concentration in the dielectric layer,with a nitrogen concentration of - 27 at. ; the carbon signal arises from Ubiquitous hydrocarbon contamination onexposed surfaces. In films of ~ 100 nm in thickness and forthe same anneal time the nitrogen concentration detectablydecreases with d istance from the exposed surface, with evidence for a slight peak at the interface. These results demonstrate successful nitridation of he oxide films and verify pre-

316 Appl. Phys. Lett. 44 3), 1 February 1984 0003-6951/84/030316-03 01 .00 © 1984 American Institute of PhYSics 316

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100

Auger Depth Profiles

80Nitrided Oxide 17 nm)NH3 , 1150°C, 6 hr.

c 60

11.uE

s0

c

Sputtering Time min)

FIG. 1. Chemical composition of a 17-nm-thick nitrided oxide on siliconobtained by Auger depth profiling. The thermally grown oxide layer wasannealed in pure NH3 at 1150 C for 6 h.

vious reports on this process. 1.6.14.15 While high-temperaturenitridation may prove to be impractical for VLSI applica

tions, it is anticipated that the electronic properties of thenitrided oxide layer will be indicative of those achievable byalternative nitridation techniques (e.g., plasma enhanced nitridation) which produce films of comparable chemical composition.

Electron injection into the dielectric layers was accomplished by avalanche injection from the p type silicon substrates. Voltage pulses were used to induce avalanche injection, and an electrometer operated in the Coulomb metricmode was used to record the net charge transported in theexternal c ircuit following the application of a selected number of pulses; the measured charge nearly equals in magnitude the charge injected into the dielectric. Charge accumulation in the nitrided oxide layers was detected by paralleltranslation of the high-frequency capacitance-voltage C-V)characteristic along the voltage axis, with shifts to positivegate voltages identifying negative charge accumulation. Theflatband voltage shift .J V FB) versus measured (or injected)electron dose QM) is shown in Fig. 2 for three samples of

16

14

12

~ 10

<II 8;;

Breakdown

j<l 6

4

2

010 11 10 12 10 13 10 14 10 16

OM cm - 2 )

FIG. 2. Shift of the ftatband voltage, .d V FB due to avalanche injection ofelectrons into nitrided oxides of three different thicknesses Ii The variableQM is the accumulated net charge measured in the external circuit due tovoltage-pulse stimulated avalanche injection of electrons; it approximatelyequals the accumulated density of electrons injected into the dielectric.

317 App/. Phys. Lett., Vo/. 44. No.3. 1 February 1984

4

Charge Relaxation in NitridedOxides (11 nm Thick) after

3 Saturated Electron Injection

~ 1 5 K<II 20300 K

0 7Time hr)

FIG. 3. Relaxation of the ftatband voltage, .d V PB, with time for II-nmthick nitrided oxides after saturate d avalanche injection of electrons at ISOand 300 K. During the isothermal relaxations the MIS capacitors were biased in accu mulati on (i.e., V = - 5 V). Note the scale change on the lineartime axis.

different insulator thicknesses. The logarithmic scale forcharge was chosen only to illustrate the extreme sensitivityof.J V FB to charge injection and is not intended to suggest afunctional dependence. Saturation of .J V FB was achievedonly in the Il-nm sample. The 17- and 32-nm samples experienced catastrophic dielectric breakdown before reachingsaturation. Even the saturation in the I-nm sample does notrepresent complete filling of traps but rather a balancebetween electron trapping and detrapping,as discussed below. f the traps are assumed to be spatially uniform, theminimum trap density is 1.2X 10 19 cm- 3 for the ll-nm sample. By comparison, the thermal oxide controls and oxidesannealed in pure N 2 at 1150·C for 6 h showed negligibletrapping under comparable electron injection conditions.For the nitrided oxides, a capture cross section of 1 X 10 - 14

cm2

was estimated from a least squares fit of a standard trapping kinetics model 12 to .JVFB QM data obtained fromthicker samples (e.g., ;;;.60 nm) which displayed minimumdetrapping. This cross section is significantly larger thanthat assigned to the water-related center in thermal oxides(_10 - 17 cm 2 from Refs. 12 and 13) and therefore is ascribedto a nitridation-related defect.

The C- V characteristic relaxes with time after termination of avalanche injection, thus revealing detrapping of electrons. Results for an I1-nm sample are shown in Fig. 3. Atroom temperature (i.e., 300 K), the nitrided oxide was firstinjected with electrons to saturation as illustrated in Fig. 2),then the MIS capacitor was biased in accumulationV = - 5 V) and the C- V curve periodically recorded. In

II-nm samples the ftatband voltage relaxes rapidly duringthe first 5-10 min and then more slowly on a time scale ofhours; it had fully recovered to its original value after - 18 h.Biasing the device in inversion VG = 5 V) significantly increased the relaxation time. In thicker samples i.e., ;;; 17 nm)which were injected with charge to just below breakdownsee Fig. 2) only partial relaxation was observed; for example,

in a 6O-nm sample .J V FB recovered to roughly one-third ofits near-breakdown value.

Two mechanisms must be considered to account for theobserved electron detrapping: 1) thermal emission fromshallow traps and 2) tunnel emission. To probe the relative

Chang. Johnson. and Lyon 317

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importance of these mechanisms, the above charge-relaxation experiment was performed at 150 K on an It-nm sample. The decay of 1:1VFB at 150 K is shown in Fig. 3 forcomparison with the room-temperature results. I t can beseen that temperature in this range has negligible effect onelectron detrapping. In addition, annealing an electron injected dielectric at 300 ·C (for 30 min with leads open circuited) produced no significant enhancement of the detrapping

rate. These results argue against the mechanism of thermalemission from shallow traps and permit an estimate of thedepth of the dominant deep traps at > eV below the insulator conduction band. The independence of discharge time ontemperature in the ultrathin insulators and incomplete relaxation in the th icker films support tunnel emission of electrons as a dominant detrapping mechanism. The indicatedbias dependence of detrapping would then reflect the dependence of the tunneling probability on the density of emptyextended states at the same energy in the adjacent electrodes.Another tunneling process may also contribute to bias-dependent discharging: namely, high-field Fowler-Nordheimtunneling of holes from the silicon into the insula tor valenceband which subsequently recombine with trapped electrons.Thus, the saturation of l:1V FB for the I1-nm sample in Fig. 2is due to a balance between trapping of avalanche injectedelectrons and detrapping by tunnel emission.

The charge trapping properties of ultrathin nitrided oxides described in this study provide insight into two key technological applications of this gate insulator. First, for VLSIthe improvements over pure Si0 2 , that were enumerated atthe beginning of this letter, must now be weighed against thelikelihood of significant electron trapping and consequentialthreshold voltage shifts from hot-electron injection in shortchannel metal-oxide-semiconductor field-effect transistors(MOSFET's). Only in dielectric layers of ;; 11 nm in thickness should it be anticipated that the deleterious effects of

charge trapping will be alleviated by efficient charge extraction via tunnel emisison. Thus, combined with a fundamental lower limit for dielectric thickness of - 3 nm (before onsetof gross gate leakage from direct tunneling between gate andsubstrate), the results presented here introduce the conceptof an optimum range of insulator thicknesses (e.g., roughly3-11 nm) for stable device operation with highly defectivedielectrics; the upper bound of optimum thickness will likelydepend on gate bias. The second potential application ofni

trided oxides is for radiation-hardened MOSFET's.7 The

318 Appl. Phys. Lett. Vol. 44 No.3 1 February 1984

electron-hole pairs generated in pure oxides on silicon byexposure to ionizing radiation result in a bias dependentbuildup of positive space charge in the oxide layer due toefficient hole trapping and rapid sweep out of he electrons. 16

Our evidence of strong electron trapping in nitrided oxidesestablishes that the improved radiation tolerance can be dueto efficient electron-hole recombination through deep levelsand trapped charge compensation in the insulating layer.

Work is in progress to more fully define in terms of he defectproperties the limits of applicability of nitrided oxides forVLSI and radiation hardening.

The authors are pleased to acknowledge helpful discussions with G. Hu, W. Meuli, and M. D. Moyer and the support of the Integrated Circuit Laboratory at the Xerox PaloAlto Research Center. Auger depth profiling was performedat Charles Evans and Associates (San Mateo, California).The work at the Xerox Corporation was supported by theU.S. Army ERADCOM.

'T. Ito, T. Nozaki, and H. Ishikawa, J. Electrochem. Soc. 127,20 53 (1980).T. Ito, H. Arakawa, T. Nozaki, and H. Ishikawa, J. Electrochem. Soc.127,2248 (1980).

31 Kato, I. Ito, S. Inoue, T. Nakamura, and H. Ishikawa, Jpn. J. App .Phys. 21, 153 (1981).T. Ito, T. Nakamura, and H. Ishikawa, IEEE Trans. Electron DevicesED-29, 498 (982).T. Ito, T. Nakamura, and H. Ishikawa, J. Electrochem. Soc. 129, 184(1982).

6y. Hayafuji and K . Kajiwar a, J. Electrochem. Soc. 129, 2102 (1982).7F. L. Terry, M. L. Naiman, R. J. Aucoin, IEEE Trans. Nuc . Sci. NS-28,4389 (1981).

8B. H. Yun, App . Phys. Lett. 25, 340 (1974).T. T. L. Chang, H. S. Jones, C. S. Jenq, W. S. Johnson, J. Lee, S. K. Lai,and V. Dham, IEDM Tech. Digest 810 (abstract) (1982).

lOS. Lai, D. W. Dong, and A. Hartstein, J. Electrochem. Soc. 129, 2042(1982).

"s. S. Wong, S. H. Kwan, and H. R. Grinolds, Proceeding o he Symposiumon Silicon Nitride Thin Insulati ng Films edited by V. J. Kapoor and H. J.Stein (The Electrochemical Society, Pennington, NJ, 1983), Vol. 83-8, p.346.

I2E. H. Nicollian, C. N. Berglund, P. F. Schmidt, and J. M. Andrews, J.Appl. Phys. 42, 5654 (1971).

I3F. J. Feigl, D. R. Young, D. J. Maria, S. Lai, and 1. Calise, J. App . Phys.52,5665 (1981).

14M. M. Moslehi and K. C. Saraswat, Proceedings o the Symposium o

Silicon Nitride Thin Insulating Films edited by V. J Kapoor and H. J.Stein (The Electrochemical Society, Pennington, NJ, 1983), Vol. 83-8, p.324.

15F. H. P. M. Habraken, A. E. T. Kuiper, and Y. Tamminga, Phillips J. Res.38,19 (1983).

'6K. H. Zaininger, App . Phys. Lett. 8,140 (1966).

Chang Johnson and Lyon 318

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