Effect of ionizing radiations on metalpolymersilicon structuresBui Ai, H. Carchano, and D. Sanchez Citation: Applied Physics Letters 22, 108 (1973); doi: 10.1063/1.1654569 View online: http://dx.doi.org/10.1063/1.1654569 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/22/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect of ionizing radiation on the optical attenuation in polymerclad silica fiberoptic waveguides Appl. Phys. Lett. 32, 95 (1978); 10.1063/1.89949 Effects of ionizing radiation on tyrosine J. Chem. Phys. 61, 2222 (1974); 10.1063/1.1682295 Carrier injection and trapping phenomena in metalpolymersilicon structures J. Appl. Phys. 43, 3794 (1972); 10.1063/1.1661812 RADIATION EFFECTS AND ELECTRICAL STABILITY OF METALNITRIDEOXIDESILICONSTRUCTURES Appl. Phys. Lett. 12, 385 (1968); 10.1063/1.1651866 Biological Effects of Ionizing Radiations J. Appl. Phys. 12, 279 (1941); 10.1063/1.1712910

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.113.111.210 On: Sun, 21 Dec 2014 12:01:25

Effect of ionizing radiations on metal-polymer-silicon structures

Sui Ai, H. Carchano, and D. Sanchez Universite Paul Sabatier, Laboratoire de Genie Electrique, 2, Rue Camichel, 31071 Toulouse, Cedex, France

(Received 18 October 1972)

The effect of electronic bombardment on metal-polymer-silicon structures is considered. The polymer film was obtained by polymerization of monomer vapor (styrene) in an ac glow discharge. The relative dielectric constant of the films is 3 and the dissipation factor is 0.01. Under 25-keV electronic bombardment at various flux levels and with different polarizing voltages applied, the shifts of the C(Y) curve are always in the opposite direction to that induced by the polarizing voltage. The displacement under irradiation is lower than observed with the metal-silicon dioxide-silicon structure. It is remarkable that the radiation effects are not permanent. The structures do not have any memory of irradiation constraints.

Recently interest has been focused on the effect of ion­izing radiation on the performance of semiconductor devices. The advent of field-effect devices has high­lighted the importance of surface effect.

Indeed the metal-silicon dioxide-silicon structures com­monly used are adversely affected by ionizing radiations as they exhibit a shift of their device characteristics. The extent of the shift and possible deformation of the dynamic capacitance curve depends on the polarity of applied voltage and the oxidation conditions. Permanent effects are poSSible, appearing as a positive charge within the oxide and as the creation of fast surface states at the silicon-oxide interface.

In view of this fact, numerous studies have been de­voted to finding different hardening processes against radiations. We can quote, for example, the aluminum doping of silicon dioxidei and the shielding metallic in­terlayer technique. 2 On the other hand, insulators other than Si~ have been examined. Among these insulators, we can report studies concerning silicon nitride3 (SiaN,) which was later rejected because of its surface in­stabilities. SiON has been developed by Schmidt and Ashner4

; it behaves well under radiation but also pre­sents instabilities. The good behavior under radiation of Al20a 5 obtained by plasma anodization or evaporated aluminum seems promiSing. It is the purpose of this letter to report results obtained with devices that em­ploy polymer film on silicon devices.

The polymer film used in this study is formed by the polymerization of a monomer vapor (styrene) in an ac glow discharge at low pressure (0.2 Torr). 6 One of the discharge electrodes supports a freshly cleaned silicon wafer. The thickness of the polymer film increases from some hundred angstroms up to several microns with discharging time. After the formation of polymer film, gold electrodes 2 mm in diameter were depOSited onto the sample to form the metal-polymer-silicon structure.

Relative dielectric constant determination based on di­electric measurement at 30 kHz, coupled with Tolansky thickness measurement, led to a numerical value of 3 and the films dissipation factor generally remained around 0.01 at 1 kHz. The breakdown of the insulator occurs at a field strength of about 5X 106 V cm-I •

The electric charge distribution at the silicon-polymer interface was determined by the dynamiC capacitance

108 Appl. Phys. Lett., Vol. 22, No.3, 1 February 1973

method at a frequency of 30 kHz. 7 The dynamic capaci­tance curve has shown the presence of hysteresis phe­nomena attributed to carrier injection in the polymer film from electrodes. 8

The units were subjected to x radiations (150 kV voltage, 10 rnA anode current) and to an electron beam provided by an electron gun giving energies between 20 and 50 keY and various flux levels (107 _1011 e cm-2 sec-I).

v(v o 8 16

v(v) _16 _8 o 8 16

(b)

FIG. 1. Capacitance-voltage dependence of an M-P-S structure before and after irradiation (a) with a polarizing voltage of + 36 V and (b) with a polarizing voltage of - 36 V. (1): Structure un­irradiated and unstressed; (2): polarizing voltage having been applied for 1 h; (3): after exposure to electron bombardment (25 keV, 1010 e cm-2 sec-I), polarization maintained for 8 min; (4) after exposure to electron bombardment (25 keY, 1010 e cm-2 sec-I), polarization maintained for 15 min; (5): after ex­posure to electron bombardment (25 keY, 1010 e cm-2 sec-I), polarization maintained for 30 min.

Copyright © 1973 American Institute of Physics 108 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.113.111.210 On: Sun, 21 Dec 2014 12:01:25

109 Ai, Carchano, and Sanchez: Effect of radiation on metal-polymer-Si structures 109

LWFB V 5 10 5 100 5 1000 5 10 lOQ --

~ t(s) --... -- I'-.... /

( ) o

-, '- ~ / ......... ~ ~

/ -"" ~-II --~ ~ I --- -

1/ ~ I ~

FIG. 2. Time dependence of flat-band voltage. (1): With polarizing voltage of -16 V; (2): under irradiation and with polarizing voltage of - 16 V; (3): after irradiation and with polarizing voltage of -16 V.

._-;

;1

I /

// .

-'/'

Exposure to x radiations of M-P-S structures with and without applied polarizing voltage did not show any shift of the C(V) curve with the experimental procedure used, Therefore the results mentioned in this letter are re­lated to irradiation effects occurring during electron bombardment.

To show the evolution of radiation effect a rapid record of the C(V) curve is used to obtain the value of the flat­band voltage_ Before irradiation, a polarizing voltage is applied on the M- P-S structures and this v'oltage is held during the exposure to radiations.

Typical results are summarized in Fig. 1. The polymer thickness in this case was 2500 A deposited on the (111) face of an n-type silicon wafer whose resistivity was 4 n cm. For curves (a) the polarizing voltage was + 36 V referring to the silicon electrode and - 36 V for curves (b). Characteristics (1) are obtained with structures in the virgin unirradiated and unstressed state. Curves (2) represent the shift of the C(V) curves provoked by the polarizing voltage applied for 1 h.

Characteristics (3), (4), and (5) represent the evolution of the C(V) curves, the exposure times to electron bom­bardment being 3, 15, and 30 min, respectively. It can

VFB

(v)

15

------ --......

10 ""

///

//

"..-

5 ,..,"" ,..,""

0 ,..,"" ,..""

,..,""

1 5 'KJ 5 100 5 'KJOO

Appl. Phys. Lett., Vol. 22, No.3, 1 February 1973

, '" ~~

~

be noted that the radiation effect induced a shift of the C(V) curve in the opposite sense of that caused by the polarizing voltage. On the other hand, irradiation which is used in the presence of positive polarizing voltage al­lows deformations of the dynamiC capacitance curve to become visible.

Figure 2 shows on the same diagram successive curves representing the variations of the flat-band voltage shift with time. Curve (1) is relative to this evolution before irradiation, a polarizing voltage of - 16 V being applied. Curve (2) is obtained during the electron bombardment process after the preceding polarizing procedure, the polarizing voltage being maintained. The variation of the flat-band voltage in this case tends to bring the struc­ture back to a stage comparable to that existing before the applied polarizing voltage. Curve (3) is plotted after the electron bombardment is stopped, the polarizing voltage remaining applied to the structure. The flat-band voltage shift progresses again as before the irradiation process. Figure 3 reports the same experiments with a positive polarizing voltage of + 16 V. Identical phenom­ena are observed with shifts in an opposite sense. How­ever it can be considered that the determination of the

t (s ) 5 10000

FIG. 3. Time dependence of flat-band voltage. (1): With polarizing voltage of + 16 V; (2): under irradiation and with polarizing voltage of + 16 V. (3): after irradiation and with polarizing voltage of +16 V.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.113.111.210 On: Sun, 21 Dec 2014 12:01:25

110 Ai, Carchano, and Sanchez: Effect of radiation on metal-polymer-Si structures 110

flat-band voltage is not precise on account of deforma­tion of the curve during irradiation.

These preliminary results obviously show that the phe­nomena occurring with M-P-S structures are notably different from those observed with metal-silicon diox­ide-silicon structures. Indeed, when the irradiation pro­cess was stopped, we always noticed the return of the C(V) curve to its initial position in a time comparable to the polarizing process. It seems that the M-P-S struc­ture does not have any memory of the irradiation con­straint. The charges appearing during irradiation are always of the opposite sign of those inj ected by the po­larizing voltage before irradiation.

It can be supposed that radiation induced hole-electron pairs in the polymer film. Under the action of the elec­tric field, these pairs separate and the charges which are drained toward the silicon-polymer interface are of

Appl. Phys. Lett., Vol. 22, No.3, 1 February 1973

opposite sign relative to the charges which are injected by the silicon electrode.

A certain compensation is then obtained which gives a new equilibrium state.

1A.G. Revesz, K.H. Zaininger, and R.J. Evans, J. Electro­chern. Soc. 116, 1146 (1969).

2D. Esteve and J. Buxo, Thin Solid Films 10, 155 (1972). 3p. A. Newman and H. A. R. Wegener, IEEE Annual Conference on Nuclear and Space Radiation Effects, Columbus, Ohio, 1967 (unpublished).

4p.F. Schmidt and F. Ashner, IEEE Trans. Nucl. Sci. NS-17 11 (1970).

5K.H. Zaininger and A.S. Dauxman, IEEE Trans. Elec. Develop. 16, 333 (1969).

GH. Carchano and R. Lacoste, Suppl. Vide 147, 369 (1970). 7D. Sanchez, These de Doctorat de Specialite (Universite Paul Sabatier, 1972) (unpublished).

8Bui Ai, H. Carchano, and D. Sanchez, J. Appl. Phys. 43, 3794 (1972).

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.113.111.210 On: Sun, 21 Dec 2014 12:01:25