Charges in metaloxidesemiconductor samples of various technologies induced by 60Coγ and xray quantaH. Ringel, M. Knoll, D. Bräunig, and W. R. Fahrner Citation: Journal of Applied Physics 57, 393 (1985); doi: 10.1063/1.334763 View online: http://dx.doi.org/10.1063/1.334763 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/57/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Combining and matching optical, electronbeam, and xray lithographies in the fabrication of Si complementarymetal–oxide–semiconductor circuits with 0.1 and sub0.1 μm features J. Vac. Sci. Technol. B 13, 2741 (1995); 10.1116/1.588257 Plasmainduced gateoxide charging issues for sub0.5 μm complementary metal–oxide–semiconductortechnologies J. Vac. Sci. Technol. A 13, 905 (1995); 10.1116/1.579850 Charge trapping and device degradation induced by xray irradiation in metaloxidesemiconductor fieldeffecttransistors Appl. Phys. Lett. 63, 1646 (1993); 10.1063/1.110723 Characterization of the electrical bias induced strain variation in metal–oxide–semiconductor field effecttransistors using xray double crystal topography J. Vac. Sci. Technol. A 10, 1012 (1992); 10.1116/1.578195 Doublecrystal xray topographic determination of local strain in metaloxidesemiconductor device structures Appl. Phys. Lett. 51, 1827 (1987); 10.1063/1.98483

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Charges in meta~-oxijde-semiconductor samples of various technologies induced by 60Co-y- and x-ray quanta

H. Ringel, M. Knoll, D. Brt:i.unig, and W. R. Fahrner Hahn-Meitner-Institute of Nuclear Research, Department of Data Processing and Electronics, Glienicker Strasse 100, D-l000 Berlin 39, Federal Republic of Germany

(Received 8 August 1983; accepted for publication 2 July 1984)

Results of x-ray and 6OCo_y irradiations on differently processed metal-oxide-semiconductor (MOS) capacitors are carried out. From experiments at 80 K, the field-dependent charge yield IN(E) and the ionization coefficient K are found. In addition, a simple method is described for calculating the hole trapping factor A. For this purpose, just the measured values of the midgap voltage shifts after irradiation at 80 K and room temperature (RT) are used. The investigations of oxide charge buildup Q fr and interface state generation D jt by x-ray irradiation at RT indicate that samples subject to high-temperature (HT) annealing exhibit a more sensitive interface and a less sensitive bulk of the oxide than those not subject to this step. This is found to be in contrast to results of 6OCo_y irradiations at R'f. In this case, samples subject to HT annealing show higher sensitivities of the interface and the oxide bulk than corresponding samples without HT annealing. Independent of technology and oxide thickness, the oxide bulk and the interface are more sensitive to x-ray irradiation than to 6OCo-y irradiation. The dependency ofthe rnidgap voltage shift, the interface state density and the hole trapping factor on oxide thickness dox is shown. For both x-ray and 6OCo-y irradiation the same relationship~..1 U MG a: d ~x; ..1Dit a: d ~.- I;

A a: d ~x- 2 (n = 2.6) are found (..1 U MG is the midgap voltage shift). Differences between x-ray and 6OCo-y irradiations occur in the coefficients of the LI U MG , .aD;t, and A vs dox proportionalities.

I. INTRODUCTION

The prediction oflong-term operation and reliability of metal-oxide-semiconductor (MOS) devices exposed to radi­ation in space or laboratory environments is desirable. One approach to this goal consists of investigating the damage behavior ofMOS devices during and after exposure to ioniz­ing irradiation.

The radiation-induced buildup of oxide charge and in­terface states strongly depends on process parameters such as oxidation and annealing temperatures and ambients. Moreover, the oxide thickness, the applied field, and the ab­sorbed dose are important. Due to the temperature-depen­dent mobility of holes, irradiation at different temperatures results in different responses of generation arid charge build­up.

In the present paper, the charge bui1dup and interface state generation within various samples, caused by x-ray and 6OCo-y irradiation will be investigated. The paper presents an attempt to describe the impact of radiation on the damage behavior of MOS capacitors prepared by commonly used device fabrication steps such as dry and wet (with and with­out HCl addition) oxidation with various annealing and oxi­dation temperatures. Furthermore, the impact of the oxide thickness will be investigated. Other parameters are electric field across the oxide, radiation species, and the absorbed dose.

The commonly used models 1-3 for oxide charge buildup and interface state generation at 80 K and room temperature (RT = 295 K in our case) are described in Sec. II. Moreover, a simple method is described to calculate the hole trapping factor or the radiation hardness by using only the measura­ble radiation-induced midgap voltage shifts at 80 K and RT.

After outlining the experimenta1 procedure in Sec. IIr, the results ofx-ray and 6OCo_y irradiation experiments, per­formed at 80 K, will be presented in Sec. IV A. They include charge yield and ionization coefficients.

In Sec. IV B, the charge buildup, the interface state gen­eration, and the relation between oxide charge and interface states of differently processed samples, irradiated by x rays at R T and the dependency of the damage behavior on oxide thickness are described. These results will be compared with results of 6OCo_y irradiation experiments, performed with identically processed samples at R T.

II. THEORETICAL CONSIDIERATIONS

Electron-hole pairs are created within the oxide of an MOS capacitor by ionizing irradiation. Some of them un­dergo geminate or columnar recombination. The rest of them are separated by the electric field E applied to the ox­ide.

At 80 K, the electrons are rapidly swept out of the oxide via the positively biased interface while the holes essentially remain immobile at or near their point of creation. The ho­mogeneous distribution of fhese holes causes a midgap vo]­tage shift ~ U MG given by4

d~x LlUMG = - -KDfh(E). (1)

2£

Here dox is the oxide thickness, € is the oxide dielectric con­stant, K is the infinite field ionization coefficient which equals the charge of holes generated per unit volume by the absorbed dose D of one rad (Si). IN is the field-dependent charge yield parameter which describes the ratio ofseparat­ed to generated electron-hole pairs. Its range is between 0

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and 1. It is the usual approach to assume K andfH (E) to be independent of temperature and technological processing parameters. rt might be anticipated from our results, how­ever, that this is not true at least for a singular technological condition. Since the establishment of a model of this special case is beyond the scope of this work and will be dealt with in a forthcoming paper, we restrict ourselves to the above-men­tioned simple theory. Then/H(E) can be deduced from the AU MG vs E curve, measured at 80 K, by normalizing the A U MG (E) curve to the saturation value of AU MG for high fields.

By means ofEq. (1) and a set of A UMG - fH{E) values, the infinite field ionization coefficient K can be calculated to be

(2)

In contrast to the process-independent retention of the holes in Si02 at 80 K,5 the permanent trapping of the radiation­generated holes observed at room temperature is process de­penden 1. 6 The holes move in the direction of the electric field towards the Si02-Si interface (worst case for positively bi­ased gate), and part of them is captured by neutral traps. These trapped holes are distributed around their centroid X. The ratio of captured to separated holes is described by the hole trapping factor A which is a strong function oftechnolo­gical processing parameters.3

Recent research has shown7 that the variation of the midgap voltage is the distinguished point to evaluate the ra­diation-induced change of the oxide charge Q fr alone, as the interface charge becomes neutral for this band bending. This !,eads to

x X AUMG = - ~fr = - -KfH(E}doxAD (3)

€ €

by simple electrostatic considerations. The hole trapping factor A can be extracted from the

.:1 U MG (E) characteristics, measured at RT, ifx is assumed to be approximately do .. and if the values for K and f H (E) are known.

As described in Sec. IV A, the radiation nonequilibri­urn in MOS structures resulting in dose enhancement within the oxide can lead to erroneousl,y calculated values for the absorbed dose D and the ionization coefficient K. For this reason, a simple method to calculate the hole trapping factor A without using the ionization coefficient K [andfH (E), tooJ is described in the last part of this section.

IdenticaUy processed samples are irradiated by the same radiation source at 80 K and RT. For these experi­ments, the same absorbed dose D and applied field E are chosen. At RT, x is assumed to be approximately equal to dox • The field-dependent charge yield parameter/ H (E) and the ionization coefficient K are assumed to be independent of temperature and technological processing parameters.2•8•9

(This is true at least for aluminum gate devices as anticipated from Sec. IV A.) By means ofEqs. (2) and (3), the ratio ofthe measured midgap voltage shift AU MG at 80 K and A U MG at RT leads to

AU MG d~xKD/H(E) 1 ---= =-. AUMG 2KDxdox fH(E)A 2A

(4)

394 J. Appl. Phys .• Vol. 57, No.2, 15 January 1985

Since the retention of holes in Si02 at 80 K is process inde­pendent, irradiation experiments performed with the same radiation source at 80 K on differently processed samples will lead to equal values of the midgap voltage shift A U MG for all samples. This is true as long as they have the same oxide thicknesses and the same applied field during irradia­tion (the same absorbed dose). Therefore, the value of A U MG' evaluated for just one sample which is irradiated at 80 K, is valid for all differently processed samples.

Irradiation experiments at R T lead to different values of.a U MG for differently processed samples. This is caused by different radiation sensitivities of the oxides and different values of the hole trapping factor A. If the RT experiments are performed by using the same radiation source and the same values of D and E as used for the 80-K experiment, the hole trapping factor A of differently processed samples with the same oxide thicknesses can be calculated by means ofEq. (4) from the midgap voltage shift A U MG (RT) and the mid­gap voltage shift J U MG' deduced from irradiation experi­ments at 80 K for any fixed reference sample. The calcula­tion of A is now independent of the values of the ionization coefficient K and the absorbed dose D.

III. EXPERIMENTAL PROCEDURE

A. Sample preparation

n-type Si wafers of a resistivity between 4.1 and 7 n cm were used. They were (111) oriented. In Table I, the oxida­tion and annealing parameters for the individual runs are summarized. Different oxide thicknesses were obtained by etching back part of the top oxide. After removing the back oxide, aluminum was evaporated, by e gun on the front and back side. A dot size of 1 X 1 mm2 was obtained by photolith­ography. Finally, the samples were sintered for 30 min at 450 ·C in nitrogen in order to remove the e-gun-induced ra­diation damage. No efforts (e.g., crucible evaporation) have been made to manufacture radiation hardened devices.

B. Measurement setup

For the charge yield experiments, the samples were cooled to 80 K with liquid nitrogen. In the x-ray equipment, the samples were mounted in a sample holder which is evac­uated. For the 6OCo-r irradiation experiments, the samples were submerged in a liquid-nitrogen bath. During irradia­tion, an electric field across the oxide was maintained by applying a dc v01tage to the gate. The measurements of the high-frequency (HF)-C ( V) characteristics before and. after ir­radiation at 80 K were performed by a Boonton l-MHz ca­pacitance meter (model 72B) and by a linear ramp generator with a ramp velocity of 500 m V Is. No differences in the HF­C (V) curves were observed for different ramp velocities rang­ing from 10 to 500 m V Is.

For samples irradiated at RT, the interface state density was determined by a setup which permitted the simulta­neous measurement of the high- and low-frequency (HF-LF) MOS capacitance. 10 The radiation-induced midgap voltage shift was extracted from the HF-C(V) data.

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TABLE I. Oxidation/annealing conditions, symbol identifications, and hole trapping factors A. The A values have been normalized to 15ao-A. oxides.

Hole trapping factor HT annealing Oxide

Sample Symbol Oxidation (Forming gas) thickness y-quanta x-ray

SA I + Dry l000'C 1000 ·C, 30 min 1550 A. 0.12 0.13 SA2 X Dry l000'C IS50A. 0.09 O.IS SA3 • Wet l000'C 1000 'C, 30 min 1630A 0.27 0.37 SA4 0 Wet l000'C 1700 A 0.17 SA5 1m Wet l000'C 1000 'c, 30 min 1500 A 0.16 0.20

1490 A O.IS 0.21 (HCl)

SA6 0 Wet l000'C (HCI)

SA 7-15 .,\7 Wet l000'C 1000 ·C, 30 min 6S0-14SoA see Fig. 6

C. irradiation sources

For the irradiation experiments, two different sources were used. The 6OCo-r source emits r quanta of energies of 1.17 and 1.33 MeV. The dose rate was 1.5 X lOS rad (Si)/h.

The x-ray source emits quanta in the energy range between 20 and 150 ke V. The total dose was measured by integrating the radiation-induced photocurrent of a calibrat­ed solar cell which was placed near the MOS capacitance under irradiation. The typcial dose rate was 1.44 X 10" rad (Si)/h.

IV. EXPERiMENTAL RESULTS AND DISCUSSION

A. Irradiation at 80 K

According to Eq. (1), the field-dependent charge yield parameter/H (E ) can be extracted from the.::1 U MG (E ) curves. Figure 1 presentsfH(E) after x-ray and 6OCo-r irradiation. For comparison, results from the literature 1.9 are added. The values of the electric field are obtained by dividing the gate voltage by the oxide thickness. Charging effects in the oxide are neglected. (A field of 1 MV em - I corresponds to a bias of 15.50 V for a sample with an oxide thickness of 1550 A.)

The.1 V MG values obtained from x-ray irradiation un­der positive and negative biases differ slightly. Boesch and McGarrityl explained this effect by initial transport of holes through extended states in the valence band prior to capture

1.0

0.5

• 6OCO _}'

'" x-ray Us"' >0 .. x - ray USi .. < 0 o e- Boesch 111 • x - ray Sraur 191

~~

0.5 to

A

E. MVcm-1

1.5 2.0

FIG. 1. Fie1d-dependent charge yield/HIE) deduced from irradiation ex­periments at 80 K in comparison with results from literature. 1.9 (A common field axis instead of individual voltage axes has been chosen to render com­parable the data from various sources. This assumes that the change in ox­ide field due to charging effects is negligible.)

395 J. Appl. Phys., Vol. 57, No.2, 15 January 1985

in traps. For the initial hole displacement, values of about 85 A are obtained from their model. 1 Using this model, we found for our "j U MG data displacement values of about 65 A. Since this is a small distance compared to an oxide thick­ness of about 1550 A, we neglect this effect and consider the holes to be immobile near their point of creation.

The saturation region at a high electric field was used to eva1uate theK va1ue because here the/H(E) values are unity. We found K = 1.26x 10-6 C/(rad (Si) cm3

] (Co-r irradia­tion) and K = 1.57 X 10- 6 C/(rad (Si) cm3

] (x-ray irradia­tion). The commonly used value for K is K = 1.22 X 10 -6 C/ [rad (Si) cm3

]. 2 The comparison of these data shows that the K value is nearly independent of the processing parameters at least for aluminum gate devices. The large value for the x­ray irradiation was possibly caused by dose enhancement within the oxide resulting from radiation nonequilibrium. It is found in the literature I that interface states are not gener­ated by ionizing irradiation at 80 K. This is in contrast to Fig. 2 where C (V) curves for different x-ray dose values are presented. Obviously, not only a parallel shift but also a dis­tortion of the curves occurs. This effect has already been observed by Boesch 12 who postulated a new kind of surface states. On the other hand, these states would exhibit proper­ties not familiar to the "normal" ones. For instance, we ob­served no change in the HF-C (V) curves when the ramp speed was varied in the range of 10 to 500 mV Is. Thus a comment on this phenomenon will be postponed until further investigations have been carried out.

Cle Q,

0.75

0.50

0.25

-8 -6 -4 -2 o

FIG. 2. HF-C! V) characteristics before and after x-ray irradiation at SO K, sample SA I in Table I, ..:1 U 1..:11 = 500 m Vis, E = 1.2 MV lern.

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lB. Irradiatloln at room temperature

1. Influence of technology

a. Oxide charge. To study the influence of technology, a number of wafers was differently processed, with regard to their oxidizing ambient and their oxidation and annealing temperature. This is summarized in Table I. The samples were irradiated by x rays under applied fields across the ox­ide in the range from 0 to 1.2 MV fcm at RT. [The electric field was calculated from bias and oxide thickness (Table I) by neglecting charging effects in the oxide.]

The radiation-induced midgap voltage shift versus ap­plied field and voltage, respectively, is shown in Fig. 3. The absorbed dose acts as parameter. At fields above 0.5 MV f cm, all curves saturate except for the steam-oxidized sample not subject to high-temperature (HT) annealing. All other samples virtually show similar ..:i U MG (E) characteristics, namely a linear range of.::1 U MG for small doses and satura­tion for E> 0.5 MY fcm (typically 7.5 V for an oxide thick­ness of 1500 A) in agreement with Eq. (3) [.::1 U MG ex: D, satu­ration behavior offH (E)]. Differences occur in the slopes and final values. As to the hole trapping factor A [calculated by means ofEq. (4) and normalized to the same oxide thickness 1500 A, Table 11, the best radiation hardness was obtained on dry oxidized samples subject to HT annealing ("with an­nealing"), the worst one on wet oxidized samples where the HT annealing step was omitted ("without annealing"). The

dry. wilh HT

·6U.G, Y 1.5

dry, withoul HT

r--------><-­----+--+ 0.5 ~ .... :-x-w-x--.-

~ ..... -« ___ :.c _______ x_

k"II I ! • I 0.5 1.0 E.MY/: I ! I

0.5 to [.MV/em

10 15 U. Y 5 10 15 U. Y

-6U.G,V

wei, with HI 1.5

0.5 1.0. f.MV/em 0.5 1.0 E.MV/em

10 15 U,V 10 15 U,V

wet(HCII,wllh HT wet IHW, without HT

o

a

0.5 1.0 E.MV/em 0.5 1.0 [,MY/em

10 15 U.V 10 15 U.Y

FIG. 3. Midgap voltage shift ~ U MG vs applied field E. - : x-ray irradiation at RT. - - -:MlCo-r irradiation at RT. D = 0.9; 2.7; 4,5 krad (Sil.

396 J. AppL Phys., Vol. 57, No.2, 15 January 1965

A value of sample SA 4 cannot be calculated due to the anomalous behavior described above. For the wet oxidized sample without annealing, a calculation of the hole trapping factor A was not possible, caused by the nonsaturating be­havior of the .L1 U MG (E ) characteristics.

Generally, samples with anneating appear less sensitive to x-ray irradiation than the corresponding oxidized samples without annealing. This is in contrast to results for the hole trapping factor A, deduced from 6OCo-r irradiation experi­ments 7 which were performed by using identically processed. samples (cf. Table I). Furthermore, the sensitivity of the bulk ofthe oxide and of the hole trapping factor A to x-ray irradia­tion was higher than to 6OCo-r irradiation.

To investigate thenonsaturating..::i U MG(E)characteris­tic observed for wet oxidized samples without annealing in Fig. 3, identically processed samples were irradiated by 6OCo-r quanta. The resulting L1 U MG VS E plot is shown in Fig. 3 (dashed lines)_ For x-ray and 6OCo-r irradiation, the same doses were chosen. The J. U MG curves, deduced from 6OCo-r irradiation experiments, show the wen-known satu­ration of L1 U MG' The value of the hole trapping factor can be evaluated as described above.

For the unexpected results of the sample labeled "wet, without HT", the field range was extended up to 4 MV fcm (not shown in the figures). As to x rays, r::; change .of.the positive slope was observed. In the case of Co-r radlatlOn, however, a decrease of the apparent oxide charge was found [i.e., negative slope of the..:1 U MG (E) curve]. These effects are not yet understood and are still under investigation.

h. Interface state density. Up to now, we have onlly con­sidered the radiation-induced oxide charge or .::1 U MG . Ioniz­ing irradiation, however, leads to interface state generation, too. 13-15 Samples with different process parameters were ir­radiated by x rays at room temperature (R T). During irradia­tion [dose of 2.7 krad (Si)], an electric field E = 1 MV fcm was applied to the oxide. Figure 4(a) shows the resulting interface state densities. For comparison, the results of60Co­r irradiation experiments l6 (dose of 3 krad (Si)] performed with identically processed samples and the same ~onditions (E = 1 MV fcm, i.e., 16 V for an oxide of 1600 A, R!) are shown in Fig, 4(b). It can be observed that samples With an annealing treatment have a more pronounced maximum at about W - Wi = 0.15 eV than samples without annealing but otherwise identical oxidation parameters. The highest value of Djt is shown by wet oxidized samples with anneal­ing the smallest value by wet (with HC!.) oxidized samples

, 60 . without annealing. Differences between x::!ay and Co-r Ir-radiation occur in the absolute values of D jt • Generally, the interface state density of x-ray-irradiated samples is higher compared with .the i~terface ~ate density of identi~an.y ~~o­cessed samples Irradiated by Co-r quanta. The S102-S1 tn­

terface of MOS capacitors appears to be more sensitive to x rays than to r quanta.

c. Relation between oxide charge and interface states. Results of tunnel. injection and 6OCo-r irradiation experi­ments performed. at RT7 indicate that the generation of in­terface states is proportional to the amount of trapped. holes. In the case of negJjgible electron trapping by positively filled traps, the radiation-induced midgap voltage shift A U MG will

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D= 2.7 krod (Si)

a !W-Wd.eV

0~7----7----~----~--~~~ 0.1 0.2 0.3

2 D.3 krod (Si)

b (W-WI l,eV O~~----~----~----~----~~

D 0.1 D.Z 0.3 MG

FIG. 4. Interface state density D,t vs energy of the forbidden band after x­ray irradiation (a) and 6OCo-r irradiation (b). (Wi - Wi is the energy of the forbidden band above midgap.) The electric field during irradiation was I MV em-I. This corresponds to a bias of 15.50 V for a sample with an oxide thickness of 1550 A, for example.

be caused only by the trapped hole charge Q fr and a linear relationship betwen .aDit and.a U MG will result. 7

In Fig. 5, we see linear ADit (..1 U MG ) characteristics for differently processed samples, irradiated by x rays at RT. (The correlation coefficient of this linearization is 0.86. This leads to a ADit = SA U MG law with S ranging from 0.9X 1011 to 2.9X 1011 cm- 2 ey-1/V.) Parameters are dose values [D = 0.9; 2.7; 4.5 krad (Si)) and electric field (0.1 MY I em <E < 1.0 MY Icm). For an oxide thickness of 1480 A, these field limits correspond to bias values of 1.48 and 14.8 V. Charging effects in the voltage-to-field conversion have been neglected. We can assume in this field range that elec­tron trapping by positively filled traps is also negligible for x­ray irradiation. The scattering of the ..1Dit (A U MG) charac­teristics is larger than it is found for 6OCo-y irradiation (full line). This is caused by differences in the response of the

397 J. Appl. Phys., Vol. 57, No.2, 15 January 1985

Dit

.l0ncm,leV"

• dry, with HT x- roy-irradiation • dry, without Hl • wet. with HT o wet. without Hl • wet (with HCIL with HI c wet (with HCIL without HT

.+

c

• Knoll e\ a\. [7]

c

FIG. 5. Peak values .JDit vs midgap voltage shift.J U MG. evaluated from x­ray irradiation experiments and 6OCo-r irradiation experiments 1 (fullline). The irradiations have been performed with different bias values ranging between 1.5 and 15 V (i.e., with different fields nominally ranging between 0.1 and I MVcm- I

).

interface and the bulk of the oxide to x-ray irradiation. These differences have already been described above. The ratio of radiation sensitivity of the interface (.aDit I D) to radiation sensitivity of the oxide (..::1 U MG I D) differs for x-ray irradiated samples with different process parameters. The ADi! (A U ~ ) characteristics can be regarded as a combination of theADi ! (D ) and A U MG (D ) curves, so we cannot get the same .aDi! (..1 U MG ) characteristics for all samples.

Let us now compare the results for the hole trapping factorsA and the interface state densities [Fig. 4(a) and Table I] deduced from x-ray irradiation experiments. Recall that samples with annealing have a higher value of Di! and a lower value of A than the corresponding oxidized samples without annealing. Thus we can assume that HT annealing causes a more sensitive interface and a less sensitive oxide than no HT annealing. For dry oxidized samples, this is at least true if we consider the interface state density in the induced peak at about (W - Wj ):::::0.15 eY. This is in con­trast to the results of 6OCo_y experiments, Table I and Fig. 4(b). There samples with HT annealing ex.hibit higher values of A and D jt than corresponding samples without HT an­nealing. Furthermore. the SiOrSi interface and the bulk of the oxide appear to be more sensitive to x rays than to y­quanta when identically processed samples under x-ray and 6OCo_y irradiation, respectively, are compared (Fig. 4 and Table I).

2. Variation of oxide thickness

Wet oxidized samples with annealing and of various oxide thicknesses. ranging from 740 to 1480 A. were irra­diated by x rays at room temperature. An electric field of E = 1.0 MY I cm was applied to the oxide during irradiation (D = 0.9 krad). In Table II, the corresponding bias values are given for an electric field of 1 MY em-I. (For this constant field we expect a constant charge yield CEq. (3)] when neg­lecting charging effects in the oxide.) The midgap volta,,$e shift ..1 U MG and the change in interface state density ADit were obtained from HF-LF-C (V) curves taken on the sam­ples before and after irradiation.

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TABLE Il Field-to-voltage conversion for the samples of Fig. 6. Charging effects in the oxide are neglected.

Oxide thickness Voltage

Sample (A) (V)

SAl 680 6.80 SA2 780 7.80 SA3 850 8.50 SA4 870 8.70 SA5 1010 10.10 SA6 1040 10.40 SA 7 1210 12.10 SA8 1310 13.10 SA9 1480 14.80

In Fig. 6, a logarithmic plot of L1 U MG VS dox (solid curve) reveals a relationship Li U MG cc d~., n = 2.6 for sam­ples irradiated with x rays. The results of60Co-r-irradiation experiments (E = 1.0 MV /cm, D = 5 krad) performed with identically processed samples 16 are added. (The bias values corresponding to 1 MV cm- I are given in Table n.) To com­pare the results of 60Co-r and x-ray irradiation experiments, .J U MG and ..::iDit are normalized to an absorbed dose of D = 0.9 krad (.JU MG ccD, LlDit ccD). The value n = 2.6, found for x-ray irradiation, is also valid for 6OCo-r-irradia­tion. From commonly used theory [Eq. (3), x cc dox 1, we ex­pect.J U MG cc d ~x' [It should be pointed out that ad~. law is, at first, true only for low temperatures, thick oxides, and low doses where x = do.!2. The application of this law, how­ever, can be extended to room temperature by assuming A 1=f(dox ) and x:::::dox .] This disagreement between theory and experimental results, also obtained by other authors 17.18

is possibly caused by a smaller number of trapping centers near the Si02-Si interface with decreasing oxide thickness, causing a decrease of the hole trapping factor A. The depen­dency of A on the oxide thickness dox can be computed by meansofEq. (3) with theassumptionx:::::dox ' TheseA (dox ) (x rays) and A * (dox ) (60Co r) dependencies are demonstrated in

0.1

0.05 -

o.OZ -

... AU., /"v \".,,,,/

./"v ..-..,.:If.v","" dO rt ,.,'"

~ ~ .... \.... ........ v

............ _",.................. x-ray-Irradiation ~.... ,,-...., y-irrodiction _ ........

"'!,..o..... datIl ~01=-------~~ ____ ~~ __ ~L-__ ~~~~~

500 700 900 1100 1300 1500

FIG. 6. Hole Ulpping factor A, rnidgap voltage shift A U MO, and interface state density AD,. vs oxide thickness do. after x-ray irradiation (solid lines from least-square fit) and 6OCo-y irradiation 7 (dashed lines from least-square fit), RT.

398 J. Appl. Phys., Vol. 57, No.2, 15 January 1985

Fig. 6. The larger values forA and.JUMG compared with A • and .J U ~G for comparable oxide thicknesses are possibly caused by different sensitivities of the bulk of the oxide to x rays and r quanta, as observed for several samples in Sec. IVB L

For an electric field E = 1 MV /cm (the bias values cor­responding to 1 MV cm- I are given in Table H), electron trapping by positively filled traps is negligible (Sec. IV B 1) and the oxide charge Q fr is only caused by trapped holes. The linear growth of Dit is proportional to the number of trapped holes and thus we get

LlDit cc Qfr cc A (dox )dox cc d ~x- I, n = 2.6. (5)

The expectation ADit cc d ~~6 according to Eq. (5) is fulfilled as shown in Fig. 6. The AD ~ (dox ) data points deduced from 6OCo-r irradiation experiments (AD ~ normalized to D = 0.9 krad) are described by a AD ~ cc d~; 1 (n = 2.6) dependence, too. The differences between the values of ADit and LiD ~ for comparable oxide thicknesses may be caused by different sensitivities of the interface for x rays and y quanta accord­ing to the differences in the values of the hole trapping factor and the midgap volta~ shift;." Note that for the same oxide thickness the ratios .JDit : AD ~ = 1.8 and.J U MG :.J U~G = 2.3 are different. Therefore, the differences between x-ray

and 6OCo-r irradiation cannot be caused by erroneously cal­culated values of K,fH' or D, as these would lead to the same ratios.

V.SUMMARY

The dependencies of x-ray irradiation-induced oxide charge buildup and interface state generation of samples with different technological process parameters on oxide thickness, electric field applied to the oxide and absorbed dose have been described. Comparative investigations of x­ray and 6OCo-y irradiation experiments performed at 80 K and room temperature have shown differences in the values of the hole trapping factor, the ionization coefficient, the midgap voltage shift, and the interface state density. It was found that HT annealing causes a higher x-ray sensitivity of the Si02-Si interface and a lower one of the oxide bulk than no HT annealing, when corresponding samples with and without HT annealing are compared. This is in contrast to results of 6OCo-r irradiation experiments where samples with HT annealing have more radiation-sensitive oxides and interface than corresponding samples without high-tem­perature annealing.

A nonsaturating relationship between the midgap vol­tage shift and the electric field applied to the oxide was ob­served for wet oxidized samples with HT annealing, which were irradiated with x rays. This result was not observed for other samples.

For x-ray irradiated samples with different oxide thick­nesses, the same dependencies of the midgap voltage shift, the interface state density, and the hole trapping factor on oxide thickness as deduced. from 6OCo-y irradiation experi­ments were observed.

ACKNOWLEDGMIENTS

The authors are indebted to Dipt-Ing. B. Mwler and S. Peuser (Rober Bosch GmbH Berl.in) for supplying control

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wafers from their MOS line. Furthermore, they thank Dr. W. Gaebler, W. Jesionek, S. Koschitzky, J. Siegel, and K. Tiedt for sample preparation and technical assistance.

'H. E. Boesch and J. M. McGarrity, IEEE Trans. Nucl. Sci. NS·23, 1520 (1976).

2R. Freeman and A. Holmes-Siedle, IEEE Trans. Nuel. Sci. NS·25, 1216 (1978).

3F. B. McLean, IEEE Trans. Nuc1. Sci. NS-27, 1651 (1980). 4J. M. Aitken, D. J. DiMaria, and D. R. Young, IEEE Trans. Nuc!. Sci. NS·23, 1526 (1976).

sH. H. Sander and B. L. Gregory, IEEE Trans. Nuel. Sci. NS·22. 2157 (1975).

6K. G. Aubuchon, IEEE Trans. Nuc!. Sci. NS·18, 117 (1971). 1M. Knoll, D. Briiunig, and W. R. Fahrner, IEEE Trans. Nuel Sci. NS-29, 1471 (1982).

399 J. Appl. Phys., Vol. 57, No.2, 15 January 1985

"0. L. Curtis, J. R. Srour, and K. Y. Chiu, J. App!. Phys. 45, 4506 (1974). 9]. R. Srour, O. L. Curtis, and K. Y. Chiu, IEEE Trans. Nucl. Sci. NS-21, 73 (1974).

]()R. Ferretti, Diplomarbeit 042, TV Berlin (1976). IIF. B. McLean, G. A. Ausman, H. E. Boesch, and J. M. McGarrity, J.

App!. Phys. 47, 1529 (1976). 12H. E. Boesch, IEEE Trans. Nuc!. Sci. NS·29, 1446 (1982). 13J. M. McGarrity, P. S. Winokur, H. E. Boesch, and F. B. McLean, Pr0-

ceedings of the International Conferences on the Physics of Si02 and Its Interface (Pergamon. New York, 1978), p. 428.

I.p. S. Winokur, H. E. Boesch, J. M. McGarrity, and F. B. McLean, J. App!. Phys. SO, 3492 (1979).

ISp. S. Winokur and M. M. Sokoloski, App!. Phys. Lett. 28, 627 (1976). 16M. Knoll, Dissertation D84, TV Berlin (1983). I1G. W. Hughes, R.I. Powell, and M. H. Woods, App!. Phys. Lett. 29, 377

(1976). I"G. F. Derbenwickand B. L. Gregory, IEEE Trans. Nuel. Sci. NS-22, 2151

(1975).

Ringel et al. 399 [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:

130.209.6.50 On: Mon, 22 Dec 2014 04:13:28