1192 IEEE Transactions on Nuclear Science, Vol. 35, No. 6, December 1988

Annealing of Total Dose Damage: Redistribution of Interface State Density on < loo>, < 110 > and < 111 > Orientation Silicon

R.E. Stahlbush', R.K. Lawrence*, H.L. Hughes3, and N.S. Saks'

'Naval Research Laboratory, Washington, DC 'ARACOR, Sunnyvale, CA

3Defense Nuclear Agency, Washington, DC

Abstract

The annealing of interface states after an x-ray dose of 10 Mrad (Si02) under 1 MV/cm bias is studied on <loo>, <110> and < 11 1 > orientation silicon. During annealing the bias is 1 MV/cm. Annealing times range from under an hour to hundreds of hours and the temperature ranges from 75 to 175°C. Using charge pumping, the energy distribution of interface states within the bandgap is deter- mined. After annealing, the shape of the interface state density curve implies that a defect@) other than PbO and pbl is present. In addition, comparison of the interface state density curves before and after annealing shows that a redistribution of interface state density occurs over a large portion of the time-temperature range studied. The den- sity near 0.4 eV above the valence band decreases and the density near 0.7 eV inmases although the average density does not sign& cantly change. Based upon the time scale and activation energy of the redistribution, a model is proposed. In this model, the rate limiting step is water diffusion within the gate oxide to the interface. This model provides a framework for a transformation among interface defects that accounts for the observed redistribution. Further tests for this water model are discussed.

Introduction

' The elusive nature of interface defects at the silicodsilicon diox- ide interface has been and continues to be an active field of research driven by technological needs and scientific curi~sity.'-~ One aspect of this field if the study of the chemical structure of the electrically active defects and the reactions involved with their formation and pas- sivation. In this paper the effect of annealing on interface states under positive bias after x-ray irradiation is investigated on three crystallo- graphic orientations of silicon: < loo>, < 110> and < 11 1 > . An important feature of these experiments is the use of the spectral charge pumping technique. Using a single MOSFET, either N channel or P channel, the density of interface states spanning 70% of the bandgap can be determined. The accurate tracking of interface state density redistribution presented below relies upon this capability.

The annealing behavior of the interface state density has been studied over a range of times and temperatures. Annealing tempera- tures ranged from 75 to 175°C and the interface state density was followed for times of less than an hour to hundreds of hours. Within a significant portion of this time-temperature range, we observed that there is a redistribution of interface state density during which changes to the total number of interface states were small compared to the redistribution. A similar redistribution has also been observed by other authors at room temperature to 75Oc.l' In our paper, it is argued that the interface state density after annealing shows that inter- face defects other than PbO and Pbl centers are present. Characteris- tics of the additional defect@) are discussed and the similarity of these characteristics to those of a defect that has been modeled by molecular orbital calculations is noted." While the interface state density before and after annealing is different for each orientation, the redistribution is similar for all orientations. In each case, the interface density below midgap decreases and above midgap increases. The redistribution is

shown to be an activated process with an activation energy of 0.85 f 0.15 eV. A model for the annealing chemistry is proposed in which the rate limiting step is the diffusion of water in the gate oxide to the interface. There two reactions occur in sequence. The first reaction passivates a defect. The second reaction involves products of the first reaction and creates a defect. The proposed reaction sequence provides a mechanism to change the mixture of defects at the interface and thereby accounts for the observed interface state redistribution. Further experiments to test the model are also discussed.

Experimental Procedure

The silicon wafers used in this study were all from the same boule and were cut with < loo>, < 110> and < 11 1 > orientations. N-channel MOSFETs with polysilicon gates were fabricated at the NRL processing facility. Channel dimensions of W / L of 150115 pm or 150/10 pm were used and the thickness of the dry oxide grown at 900°C ranged from 550 to 750 A . There was a heavy p + + implant under the field oxide to prevent inversion after irradiation. To simplify processing and eliminate residual implantation damage, no threshold adjust implant was used.

The MOSFETs were irradiated at room temperature under a positive bias of 1 MVlcm with x-rays generated from a tungsten target bombarded by 60 keV electrons. Each device received a total dose of 10 Mrad (SO2), and the dose rate was 1.8 krad/sec. Annealing tem- peratures were 75, 125 and 175°C; during annealing the bias was 1 MVlcm. The whole annealing evolution of the three orientations was checked by measuring two or more MOSFETs for the 125°C data and there was excellent agreement of the results. At 75 and 175°C spot checks at a couple of annealing times were made. Additional con- sistency checks were provided by the fact that the pre-annealing data of the three MOSFETs for the three temperatures showed the same interface state density and by the fact that the same evolution of inter- face state density was observed at each temperature. Temperature altered the rate, but not the form, of the evolution.

Before annealing and at times ranging from under an hour to hundreds of hours at logarithmically spaced intervds the MOSFETs were removed from the oven and charge pumping was performed at room temperature. The annealing that occurred during the measure- ment time (under half an hour) was insignificant. Based upon observa- tions of the annealing rate versus temperature, half an hour at room temperature was equivalent to annealing for less than one minute even at the lowest annealing temperature. The energy distribution of inter- face state density was determined by the charge pumping method described in reference 12. By varying the rise and fall times of the pulse train applied to the gate, the portion of the bandgap sampled is altered. The lower energy limit of interface states measured is below midgap and is set by the rise time. Conversely, the upper limit is above midgap and is determined by the fall time. All the interface states between these limits are measured. In practice, the lower and upper limits can be adjusted to within about 0.1 eV of midgap and to within about 0.15 eV of the respective band edge. Measurements at multiple rise and fall times allows one to determine the energy distri- bution of interface state density as illustrated in figure 1. In the

0018-9499/88/1200-1192$01.00 0 1988 IEEE

1193

3.0

2.0

1 .o

h

3.0 r

t N ' 2.0 E N c

$2 1.0 v

.e 0

3.0

2.0

1 .o

0 1

(b) (110)

. . . . . . , - .

0.0 0.2 0.4 0.6 0.8 1 .o Energy ( e V )

Figure 1. Interface state density before (0) and after (A) annealing at 125°C for the <loo>, <110> and <111> orientations. The annealing times were 138, 167 and 164 hours, respectively. The x- ray dose was 10 Mrad (SiO,). The dots in (a) for the < loo> orienta- tion show the annealed data offset by the decrease of the average interface state density from 0.20 to 0.90 eV (see text).

region within about 0.1 eV of midgap, the average interface state den- sity is determined. The point plotted at midgap in the figures shows the midgap average.

Results

The effect of annealing upon the density of radiation induced interface states is shown in figure 1 for the < loo>, < 110> and < 111 > orientations. Data before annealing and after annealiig about one hundred hours at 125°C is shown. Pronounced changes in the shape of the interface state density curves are apparent after annealiig. At 75 and 175OC, the annealing rate is very different but the changes to the shape of the interface state density curves are the same. These changes include a large redistribution of interface state density as well as smaller changes in the total number of interface states. In figure 2, the average interface state density from 0.20 to 0.90 eV above the valence band is plotted versus time for the three orientations. Each datum point is determined from a single charge pumping measurement for which the lower limit is set to 0.20 eV and the upper limit is set to 0.90 eV. Data at the three temperatures and for times ranging from less than an hour to hundreds of hours are plotted. The average is constant within a 10% range for the < 110> and < 11 1 > orienta- tions. In contrast, the <loo> Orientation exhibits a steady decline whose rate increases with temperature. However, only after annealing several hours at 175°C does the average drop more than 10%. In comparison to the change in the average, the redistributions shown in figure 1 are large.

The strongest feature of the redistribution observed for all orien- tations is a decrease of interface state density centered near 0.4 eV and a corresponding increase centered near 0.65 eV for the < 100>

A A A A A A

0 0

1.61 A 1 A

le4 t 0

lm4 I (c) (111)

Preannwl 1 10 100

Time (hours)

Figure 2. The average interface state density from 0.20 to 0.90 eV above the valence band versus annealing time for the <loo>, < 110> and < 1 1 1 > orientations. For each orientation there are data at 75°C (O), 125" (A), and 175°C (0).

and < 110> orientations and near 0.75 eV for the < 1 1 1 > orienta- tion. All energies are with respect to the valence band. The redistribu- tion is most pronounced for the < 110> orientation. Before anneal- ing, the density at 0.4 eV and 0.65 eV is nearly equal and after annealing 167 hours at 125°C the average density between 0.20 eV and 0.90 eV increases less than 3%. In contrast, there is a 25% decrease at 0.4 eV and a 40% increase at 0.65 eV. While there is a small amount of redistribution near the band edges, the primary redis- tribution occurs between the depression at 0.40 eV and the peak at 0.65 eV. The < 11 1 > orientation shows a U shaped continuum that is stronger above midgap and that skews the peak to higher energy. This continuum continues to grow for longer times and is responsible for the upturn in figure 2(c) of the 175°C datum point at 330 hours. The <loo> orientation exhibits a peak near 0.65 eV before annealing. After annealing, the peak grows and the interface state density at 0.4 eV decreases to the lowest level observed for any of the orientations. Other work, in which the interface state density only above midgap was measured, has suggested that the important redistribution for the < loo> orientation is between the peak at 0.65 eV and the tail near 0.85 eV.I3 The interface state density above midgap, such as shown in figure l(a), is complicated by ,the small change in the average density during annealing. After annealing 138 hours at 125"C, the average decreases by 0.20 x 10" cm-'eV-'. The dotted curve shows the annealed data after adding this offset. Comparing the dotted curve to the pre-annealing curve, the redistribution looks very similar to the redistribution seen on the < 110 > orientation. Thus, all three orienta- tions exhibit a similar redistribution.

In figure 3, the time evolution of the interface state redistribution is shown for the three orientations by plotting the difference between the interface state density at 0.40 eV and the density at the peak above

1194

1.6

1.2

n .- I

@l 3 ' 0.8 E 2

0.4 v Y .- n a

1.6

0.8

i -0-

(b) (110)

I (c) (111)

Pmanmol 1 10 100

Time (hours) Figure 3. The redistribution of interface state density versus time tracked by plotting the interface state densities difference. In (a) and (b) the difference between the density at 0.68 and 0.40 eV is plotted for the < 100> and < 110> orientations, respectively, and in (c) the difference between the densities at 0.75 and 0.40 is plotted for the <111> orientation. Data at 75°C (0) , 125" (A), and 175°C (0) are shown. Times along the dashed line are used in the Arrhenius plot.

midgap. The linear sections of the curves in this figure are fits to the data points and the curved sections of the curves are guides to the eye. The density near 0.68 eV is used for the < 100> and < 110> orien- tations whereas a higher value near 0.75 is used for the < 11 1 > orientation because this peak is skewed to higher energy by the U shaped background. The density is calculated by averaging the five data points centered around the chosen energy. By plotting the differ- ence, errors introduced by the changes in the average interface state density are minimized. The figure for the < 11 1 > orientation, figure 3(c), only includes data at 125 and 175°C. There was a low yield of MOSFETs fabricated on <111> wafers that had sufficiently low leakage currents to obtain complete interface state density curves. Many devices that were adequate for measuring total interface state density gave inaccurate results for the interface density near 0.4 eV and 0.7 eV. However, even on the poorly working devices the evolu- tion of the density of states is consistent with the observations at the higher temperatures.

The large temperature dependence of the redistribution is evident in figure 3. A saturation of the redistribution is also seen at 175°C. The onset of saturation starts after several hours for the < 100> and < 110 > orientations and after tens of hours for the < 11 1 > orienta- tion. There is also an indication that the <loo> data at 125°C is saturating after hundreds of hours. The drop of the < 100> redistri- bution curve at 175°C after saturation is associated with the decrease of the average density shown in figure 2(a). Another feature in com- mon between the < 100 > and < 110 > orientations is the magnitude of the redistribution. Before annealing, the difference between the

interface state density near 0.7 eV and the density near 0.4 eV is about 1 x 1OI2 cm-2eV-' for the <100> orientation and close to zero for the < 110> orientation. However, figures 3(a) and (b) show that an any point during the annealing process the difference between the interface state density near 0.7 eV and 0.4 eV increases a similar amount for both the <100> and <110> orientations. The magni- tude of the redistribution for the < 111 > orientation is roughly twice as large. One explanation for larger magnitude is than a portion of it is due to the asymmetric U shaped background previously mentioned.

The data over the time and temperature studied provide a win- dow through which annealing processes are viewed. During the initial annealing, the dominant process for all orientations is the redistribu- tion of interface state density from below midgap near 0.4 eV to above midgap near 0.65 eV to 0.75 eV. The redistribution is a tem- perature activated process. However, it is not the only process and after sufficient annealing time it is not even the dominant process. While the form of the redistribution is very similar for the three orien- tations, other effects of the annealing exhibit more variation among the orientations. For example, after long term annealing the interface state density drops very rapidly for the < 100> orientation compared to the other two, and the growth of an asymmetric U shaped continuum is observed for the < 11 1 > orientation but not for the other two orienta- tions.

An activation energy for the redistribution of interface state den- sity is obtained by fitting to the expression

+ = [+3 C-E.*T (1)

where 7 is the time for the difference of interface densities plotted in figure 3 to increase to an arbitrarily chosen value, Ea is the activation energy, T is the temperature in Kelvin and T~ is a normalizing con- stant. This temperature dependence for the redistribution rate follows from the assumption that diffusion in the oxide is the rate limiting step. There is strong evidence that the rate limiting step of interface state creation after a short radiation burst is governed by the diffusion of H+ through the gate ~ x i d e . ~ . ' ~ The form of equation (1) is derived from diffusion equations. The time, t , to diffusion a distance d , is given by

1 40 t d2 _ = _

where D is the diffusion rate and Do is a constant.

An Arrhenius plot of the redistribution is shown in figure 4. The dashed horizontal l i e s in figure 3 show the level of interface state density difference chosen to extract the time, 7. This level is half the interface state density difference at which the 175°C data saturates. A linear extrapolation of the 175°C data is used to deter- mine 7 for the <100> and <110> orientations. For annealing curves that are parallel, the choice of the level has no effect upon the derived.activation energy. However, there is a strong effect upon 70.

Thus, the activation energy, E,, is derived from the Arrhenius plot but the constant 7o is not determined, i.e., only the slope of the dashed line fit is used. The vertical bars at 75°C and 175°C show the devia- tion from parallel annealing curves. As the level for determining 7 is raised or lowered, the ratio of the annealing rate at 125°C and the rate at 175°C changes because the annealing curves at 125°C and at 175°C are not exactly parallel. The spread of ratios is shown by plot- ting a vertical bar at 175°C. Similarly, the bars at 75°C show the spread of ratios between the 75°C annealing curve and the 125°C curve. The activation energy is 0.85 f 0.15 eV and within experi- mental error is the same for all three orientations.

Discussions

In this section, a model is proposed that provides the framework for explaining the observed redistribution in the density of interface

1195

75°C

2 x 0.6 0.03 40

1.5 x lo6 (400 hrs)

100

n r ' 10 e 3 0 -c v

1 l- \ r

2 0.1 (3 IY

0.01

125°C 175°C

1 x lo-' 6 x 0.02 0.002

0.005 0.001 2 0.3

6 x lo4 4000 (17 hrs) (1.2 hrs)

T ("C) 175 125 75

I I I

I - \ \ \

\ El '\

\

2.0 2.2 2.4 2.6 2.8 3.0

lOOO/T (K- l ) Figure 4. Arrhenius plot of the characteristic rate, 1 / ~ , of the inter- face state redistribution during annealing for < 100> (0), < 110> (A), and < 1 1 1 > (0). The vertical bars at 75°C and 175°C are explained in the text and show the range of rates between that tem- perature and 125°C. The dashed lines shows the slope corresponding to an activation energy of 0.85 eV.

states. The rate limiting step is the diffusion of water within the gate oxide to the interface. Subsequent reactions are presented that pas- sivate one interface defect and create one interface defect, thus provid- ing a mechanism for changing the mixture of defects present while preserving the total number. To account for the shape of the interface state density curves after annealing, a defect that has two closely spaced levels at the peak observed near 0.7 eV is introduced. This defect is consistent with defects studied by molecular orbital calcula- tions and its failure to be observed by ESR is explained by its closely spaced levels."

Hydrogen species are known to be present in both wet and dry oxide^.'^.'^ There is strong evidence that diffusion of Hf and I-f through the gate oxide to the interface is responsible for the produc- tion of interface states after low temperature, 80K, irradiation and subsequent warming. 17,L* It is certainly plausible that other species, such as water, are also involved in interface reactions. In table I, the activation energies of various hydrogen species are shown. l9 Also included in the table is the time for each of the species to diffuse through the gate oxide at 75, 125 and 175°C. The length of 500 A is chosen since it is approximately the gate thickness. The exact length chosen is not critical since the diffusion times vary by orders or mag- nitude. As is evident from the table, the times for water diffusion agree with the redistribution times whereas the times for all the other hydrogen species are orders of magnitude too rapid. Care must be exercised when using diffusion data for charged species since the drift times are field dependent. With a positive bias, the drift time foi positive species decrease and the time for negative species increases. Thus, proton diffusion can still be eliminated as the rate limiting step. While it is unlikely that OH- diffusion times would be increased several orders of magnitude, this possibility cannot be eliminated. Further experiments observing the bias dependance of the annealing time scale will resolve this question.

Another diffusing species that must be considered is Na' since in its slow diffusing form it has an activation energy and diffusion

Table I. Diffusion constants for hydrogen species in amorphous silicon dioxide. The time, in seconds (except where indicated), to diffuse 500 A at 75"C, 125°C and 175°C is also shown."

5.7 x 1 0 - ~

10-6

- Ea

(eV) - - 0.18 0.76 0.45 0.68 0.79

time similar to water." It has been ruled out for two reasons. Sodium or other mobile ion contamination is routinely checked at the processing facility using bias temperature stressing. Assuming that each sodium ion causes one interface defect to transform, the bias temperature tests show that the contamination is at least an order of magnitude too small to account for the size of the observed transfor- mation. Secondly, during annealing the amount of trapped positive charge is observed to decrease. If Na+ were drifting toward the inter- face, a large increase in the positive charge would be observed.

In the model, once water diffuses to the interface, reactions there first passivate one defect and then create another defect.

Si, =Si. + H20 - Si, =Si-OH + Ho (3)

Si, =Si-H + Ho - Si, =Si. + H, (44

=Si--siE + HO - s i= (4b) I I . H

In the first reaction, equation (3), an interface defect reacts with a water molecule. The notation Si,=Si. indicates an interface defect consisting of a silicon atom with a dangling bond and back-bonded to three silicon atoms. This defect has been identified on the < 1 1 1 > and <100> orientations by electron spin resonance, ESR, as a pbo center." The defect becomes electrically inactive once the dangling bond reacts with the water to form a silanol group and a hydrogen atom. As can be seen in table I, the hydrogen diffuses very rapidly. Within microseconds it moves to a reaction site and forms a new electrically active defect. Two possible reactions are shown. In equa- tion (4a), a hydride is converted to a defect similar to the defect pas- sivated in equation (3) and a hydrogen molecule is released. An alter- native reaction is shown in (4b). A strained silicon-silicon bond, indicated by =Si--Si=, combines with the hydrogen atom. The bond is broken leaving one silicon atom bonded to the hydrogen atom and a dangling bond on the other. The atoms to which the two silicon atoms in equation (4b) are back-bonded are omitted since a variety of configurations are possible. On the < 100> orientation, molecular orbital calculations on this defect have been reported." The results indicate that this defect, labeled p b l ( 0 ) , produces two closely spaced interface states above midgap. While the existence of this defect has not yet been experimentally verified, evidence for this defect or another similar defect is presented below. The essence of the model is thus: (1) water diffuses to the interface, (2) an electrically active inter- face defect is converted to a stable silanol group, and (3) hydrogen produced by that reaction creates a new electrically active defect.

The only interface defects that have been shown to produce inter- face states and whose chemical structure has been experimentally veri- fied are the PbO and Pb] centers.'' ~ 0 t h are amphoteric centers. pbo produces interface states near 0.3 eV and 0.85 eV and pbl produces states near 0.45 eV and 0.8 eV. All energies are with respect to the valence band. It is difficult to explain the results of this study using

1196

only those two defects. Before annealing the interface state density below and above midgap is comparable for the < 1 10 > and < 11 1 > orientations and it is plausible that only Pb centers are present. It is likely that other defects are present before annealing for the < 100> orientation because the interface state density for this orientation peaks near 0.65 eV. The strongest evidence showing that other defects are present is provided by the redistribution of interface state density dur- ing annealing. It is difficult to rearrange the concentrations of only PbO and Pbl centers to produce the simultaneous decrease of interface state density near 0.4 eV and increase near 0.7 eV. The most striking example is provided by the < 100> orientation. As shown in figure 1, after annealing, the average interface state density from 0.25 eV to 0.45 eV is 0.8 x 10” cm-’ eV-’ compared to 2.1 x 10’’ cm-’ eV-’ in the span from 0.65 eV to 0.85 eV.

There are other reasons to believe that a defect such as the defect in equation (4b) is present. The important feature of this defect is that the interface states it produces are close together within the bandgap, i.e., the +lo and the ol- transitions have nearly the same energy. Such a defect is not visible by ESR because ESR measurements are only sensitive to the neutral state in which the dangling bond has a single unpaired electron. An ESR measurement on a sample contain- ing both Pb defects and this defect will only be sensitive to the Pb defects. Also, molecular orbital calculations have suggested that it is natural to have such a defect on the c 100> orientation. Finally, it provides a relatively simple explanation for the interface state density redistribution. During the first reaction at the interface, equation (3), primarily Pb defects are passivated. For each defect passivated two interface states are eliminated, one below midgap and one above. Dur- ing the second reaction, primarily defects of the type in equation (4b) are produced and two interface states above midgap are produced. The net result is a redistribution of interface state density from below midgap to above midgap while the number of interface states stays the same. It is reasonable to assume that the same interface defect that is responsible for the growth of the peak above midgap for all orienta- tions is also present along with Pb defects before annealing for the < 100> orientation.

There are additional experiments that need to be undertaken to test the model. The most important is to investigate the bias depen- dence of the annealing. Since the rate limiting step is the diffusion of neutral water molecules, the time scale of the annealing rate should not depend on the bias. Others have reported that at room temperature to 75°C there is a bias dependence to the magnitude of the redistribu- tion.” Work is currently in progress to determine if there is a bias dependence to the interface reactions proposed in equations (3) and (4) in which interface defects are passivated and created. Measurements at a variety of biases are also being made. There may be several reactions, each passivating or creating different electrically active defects and the bias may affect the balance among these reactions. Other experiments include varying the gate thickness and using dif- ferent annealing ambients. The diffusion time varies as the square of the distance traversed. By measuring MOSFETs that are the same except for the gate oxide thickness; changes in the annealing time scale should be observed. It is possible that the source of water or other diffusing species is not from within the device but from the ambient. Varying the ambient would test this hypothesis.

Conclusion

Charge pumping has been used to study the annealing of x-ray induced interface state density on < 100>, < 110> and < 111 > orientation silicon. Over a significant portion of the time and tem-

perature range examined the most prominent annealing feature is a redistribution of interface state density from near 0.4 eV above the valence band to near 0.7 eV. Based upon the time scale and activation energy of this process, a model is proposed in which diffusion of water in the gate oxide to the interface is the rate limiting step. At the interface, reactions occur in which one defect is passivated and another defect is created. The reactions offer a mechanism to change the mixture of defects present while preserving the number. Finally, the observed interface state densities after annealing suggest that defects other than the PbO and Pbl centers are present. A possibility for the additional defect is one that has been studied by molecular orbital calculations. An important characteristic of this additional defect is that it produces two closely spaced interface states above midgap. The mixture of interface defects present depends upon the orientation and upon annealing.

Acknowledgements

We thank D.L. Griscom and A.H. Edwards for stimulating conversations during the course of this work.

References

1. C.T. Sah, IEEE Trans. Nucl. Sic. NS-23, 1563 (1976). 2. A.G. Revesz, IEEE Trans. Nucl. Sci. NS-24, 2102 (1977). 3. C.M. Svensson, Proc. of Intl. Con$ on Phys. of Si02 and Its

lnferfaces, ed. S.T. Pantelides (Pergamon, New York, 1978), p. 328.

4. P.S. Winukur, H.E. Boesch, J.M. McGarity and F.B. McLean, J. Appl, Phys. 50, 3492 (1979).

5. F.B. McLean, IEEETrans. Nucl. Sci. NS-27, 1651 (1980). 6. P.M. Lanahan and P.V. Dressendorfer, J. Appl. Phys. 55, 3495

(1984). 7. D.L. Griscom, J. Appl. Phys. 3, 2524, (1985). 8. A.H. Edwards, Phys. Rev. B part 2 vol. 2, %38 (1987). 9. M. Cook, Phys. Rev. Lett. 59, 1741 (1987).

10. Y. Nishioka, E.F. da Silva Jr. and T.P. Ma, IEEE Trans. NUC. Sci. NS-34, 1166 (1987); Y. Nishioka, E.F. da Silva Jr. and T.P. Ma, IEEE Trans. Nuc. Sci. NS-35, (1988);

11. A.H. Edwards, Proc. of the Electrochem. Soc., Atlanta, 1988. 12. G. Groeseneken, H.E. Maes, N. Beltran and R.F. de

Keersmaecker, IEEE, Trans. Elm. Dev. ED-31,42 (1984). 13. C. Barnes, T. Zietlow and K. Nakamura, IEEE Trans. Nucl.

Sci. NS-35 (1988). N.S. Saks, C.M. Dozier, D.B. Brown and D.L. Griscom, IEEE 14. Trans. Nucl. Sci. NS-35, (1988).

15. F.J. Feigl, D.R. Young, D.J. DiMaria, Lai and J. Calise, J. Appl. Phys. 52, 5665 (1981).

16. K.L. Brower, P.M. Lenahan and P.V. Dressindorfer, Appl. Phys. Lett. 41, 251 (1982).

17. D.L. Griscom, D.B. Brown and N.S. Saks, Proc. of the Electro- chem. Soc., Atlanta, 1988.

18. N.S. Saks, R.B. Klein and D.L. Griscom, IEEE Trans. Nucl. Sci. NS-35 (1988).

19. S. Wolf and R.N. Tauber, SSilicon Processing for the VLSl Era, (Lattice Press, Sunset Beach, Ca., 1987), p. 262.

21. E.H. Poindexter, P.J. Caplan, B.E. Deal and R.R. h u k , J. Appl. Phys. 52, 879 (1981).

22. G.J. Gerardi, E.H. Poindexter, P.J. Caplan and N.M. Johnson, Appl. Phys. Lett. 2, 348 (1986).

20. S.R. Hofstein, IEEE Trans. Elm. Dev. ED- 14, 749 (1967).