structural changes in thin sio2 on si after rie-like nitrogen plasma action

a ~ surface science

ELSEVIER Applied Surface Science 120 (1997) 306-316

Structural changes in thin S i t 2 on Si after RIE-like nitrogen plasma action 1

E. Atanassova *, A. Paskaleva Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria

Received 19 February 1997; accepted 26 June 1997

Abstract

Reactive ion etching (RIE) damage effects on thin (13 nm) thermal S i t 2 on Si have been studied using X-ray photoelectron spectroscopy. It is found that 5 min exposure of the oxide to N 2 plasma operating in RIE-mode causes structural modifications which manifest only as a deterioration of the oxide quality but without actual nitridation of the oxide. The presence of a small (< 10%) constant amount of S i t species through the oxide and a broadening of Si-SiO 2 interface transition region are detected as consequences from the RIE process. © 1997 Elsevier Science B.V.

I. Introduction

Reactive ion etching (RIE) is by far the most widely used dry etching technology because it en- ables high degree of anisotropy. The S i t 2 used as a gate insulator is often exposed to RIE in the multi- layer gate processing. Unfortunately, RIE has been identified as one of the processes that can cause radiation damage [1-5] resulting in the generation of interface states at Si-SiO 2 interface and traps in S i t 2. Damage caused by ion bombardment and UV radiation is a real concern especially in plasma strip- ping and ashing processes where the gate is fully exposed to plasma action. Even when the oxide is protected by the gate or by the photoresist there are regions which are exposed to particle flux, photon

* Corresponding author. Fax: +359-2-9753632; e-mail: ele- [email protected].

This work was partly supported by the Bulgarian Ministry of Science and Education in the frame of Project dP537.

flux or both [6]. The use of thinner and thinner gate oxides in modern IC only exacerbates the problem and the RIE damage effect becomes more and more prominent [7,8]. The effect of the RIE irradiation on S i t 2 is of great interest with regard to both the solid state electronic device and the electron spectroscopic studies of S i t 2. It is very important to compare the RIE plasma (as a typical hard-type plasma) contribution in the defect generation in Si-SiO 2 system with the effect of soft plasma [9,10]. Although a considerable amount of work has already been done, it is far from enough, due to the extreme complexity of the RIE process. On the other hand, a number of models have been proposed to describe the local structure of S i t 2 after different types of radiation [11,12]. Very often the X-ray photoelectron spectroscopy (XPS) study of radiation induced changes in the spectrum permits indication of various suboxides that occur in the damaged regions. In this paper XPS is used to investigate a RIE-plasma induced structural disorder in S i t 2. Thin thermal (13 nm) S i t 2 has been sub- jected to the action of nitrogen plasma operating in

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E. Atanassova, A. Paskaleva / Applied Surface Science 120 (1997) 306-316 307

RIE conditions. The reasons to choose nitrogen as a working gas are as follows: (i) to avoid any possibil- ity of actual oxide etching and to study only the effect of radiation damage associated with the RIE process and (ii) the information concerning the nitrogen plasma interaction with the SiO 2 is of value due to the considerable attention at present to the plasma nitrided SiO 2. The investigated layers have been analyzed by means of XPS and have been depth profiled using energetic ion sputtering. In addition, nondestructive angle resolved XPS has been utilized. A special attention has been focused on the near interface region.

2. Experimental procedure

The samples used were prepared on p-type (100) silicon substrates with resistivity 13-17 II cm. The wafers were chemically cleaned by a standard process for MOSLSI ICs and oxidized in dry oxygen at 1273 K (with 60 min post-oxidation annealing in N 2 ambient) to oxide thickness d = 13 nm. The oxide was then exposed to the action of nitrogen plasma in RIE-mode. The RIE system used was a commercial parallel plate diode type and the plasma process parameters were the following: substrate temperature 293 K; pressure 3.3 Pa; frequency 13.56 MHz; rf power density 500 mW/cm2; plasma exposure time 5 min. The working nitrogen gas used was ultra high purity grade (Matheson Gas Product). On the basis of preliminary experiments plasma conditions which do not change stoichiometry of the oxide (i.e. there is no nitridation) were chosen. So, we expect that only structural modifications occur as a result of the plasma treatment. XPS data were obtained in an ESCALAB MklI high vacuum apparatus (V.G. Sci- entific) with a residual gas pressure better than 1 X 10 -8 Pa. A1K, X-rays ( h u = 1486.6 eV) were used as an excitation source. All spectra were taken at 300 K. The photoelectrons were separated by a hemi- spherical analyser with a pass energy of 10 eV. The instrumental resolution measured as the full width at half maximum (FWHM) of the Ag3ds/2 photoelectron peak was 1.0 eV. The positions of the energy peaks were determined with an accuracy of 0.1 eV. Si2p and O ls lines were recorded. In order to obtain some information about the composition of

the layers in depth and eventually about the presence of nonstoichiometric oxides, angular dependent measurements were carded out. Photoelectron spectra were acquired under two different angles of observa- tion ( 0 = 15 ° and 90 ° with respect to the surface plane) of the photoemitted electrons. As is known, the signal fades away exponentially with the depth into the layer. Consequently the registered intensity as a function of distance d s from the surface is l = I o e x p ( d J ( A s i n 0)), where A is the mean free path of the electrons, i.e. the intensity depends on 0. So, the method of angle variation gives the possibil- ity to 'look inside' the sample at different depths without sputtering, therefore without any specific artificial effects. Measurements of all XPS spectra including the depth profiles were made just after performing plasma treatment. The sputter profiling of the films was accomplished using an argon ion beam with energy of 1.5 keV and current density of approximately 10 /xA/cm 2. The angle of incidence of the sputtering beam was 50 ° with respect to the surface of the film. On using the ion sputtering technique it is very important to minimize the ad- verse effects of the ion beam on the sample stoichiometry. As is known [13], 1-1.5 keV argon ions remove Si and O at a rate very close to 1 : 2. That is why, we assume that eventual unfavorable effects of the ion beam sputtering (i.e. preferential sputtering) should not influence considerably the experimental results and conclusions. During sputtering the structural characteristics are probably in some way damaged, but most likely this is not reflected in the Si 2p binding energy or its line width. For sputtering times t~ of 1 to 26 min the thickness reduction as measured by XPS [14] was linear with° t S and the sputtering rate was obtained to be 5.26 A/min. The sputtering rate determined b), the oxide thickness and ts was approximately 5 A/min, indicating that both meth- ods are correct.

3. Results and discussion

X-ray photoelectron spectra in the region of the Si2p and O ls peaks were monitored under two different angles. All XPS binding energies were calibrated to the siSip peak position at 98.7 eV [15]. Fig. l(a), (b) and (c) illustrate Si 2p and O ls spectra

308 E. Atanassova, A. Paskaleva / Applied Surface Science 120 (1997) 306 -316

(a) Si2p / k

e=9oo i \

..... 172; .,,_:. . . . . . i _.16_rain

_ _ . _ - - ~ . . . . . . 62~Z /--,,

. . . . / > . . . . . . . . lmun -

. . . . . ~ ".,. . . . . . . . t, = Omin

........ /"~ _~-fow,

Si2p / ~ f 2 15.___°_ . . . . . . ~ ~ m i ~

~ . ~ / ~ \ 21mir~

. . . . . . . . . . . / " ~ _ 1 2 _ m _ _ ! I

.......... ~ / / "~"X . . . . . . . . . l_lmi~

......... / . ~ .~ . 6rain

j ~ " \ . . _ 1 rain . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . 1 ..................... b..=o~i,~

~./ ..~ as-grown

(C) Ols 0 = 90 °, 15 °

tS~=-2~ ..............

[21rain / \

..... ~ / /.., \ ....

S 6 m i n / ~ i

t I I Y ~ _ _ ~ / \,.. ....

lmin / / ' \

538 534 530 109 164 ~9 94 109 164 9'9 94

Binding Energy, eV Binding Energy, eV Binding Energy, eV

Fig. 1. (a, b) Si2p and (c) O ls depth XPS spectra of the RIE treated samples for two values of the take-off angle. The spectra of the as-grown sample are also given.

of the layers obtained at two values of the take-off angle and for different sputtering times. The O l s spectra corresponding to 0 = 90 ° and 0 = 15 ° are identical within the range of the measurement accuracy. It should be mentioned that the spectrum at the surface shows two peaks in the C is region for both the as-grown and the RIE treated samples. The major peak is detected at 286-287 eV (in dependence on 0) indicating the presence of a C - O bond and a smaller one at 284.6 eV due to C - C contamination. The carbon signal is most likely due to the adsorbate which disappears when the surface is sputtered slightly. A nitrogen peak (at 403.0 eV) was observed at the surface of the RIE treated samples. Its intensity, however, is very small even with respect to the carbon signal (about nine times smaller). After 1 min of sputtering neither carbon nor nitrogen was detected in the RIE treated samples. Variation of 0 indicates that N is present only at the oxide surface, i.e. as we assumed, RIE-like treatment in N 2 plasma leads only to N-rich very thin layer at the surface (limited to under 0.5 nm). So, further we shall discuss the effect of RIE only as a source of struc-

tural damage in the oxide. In the Si 2p region a main peak is clearly evident in the spectrum located at about 103.1 eV for the as-grown samples (0 = 90 °, 15 °) and at 103.3 eV ( 0 = 15 °) and 103.7 eV (0 = 900 for the starting (t s = 0 min) RIE treated sam- pies. The peak positions extracted from the depth profile spectra are listed in Table 1. The Si 2p peak for the as-grown sample is typical of thermally grown SiO 2 [16]. After RIE action (t s = 0 min) the peak tends to shift to the higher energies. This shift is very small for 0 = 15 ° and it is very close to the accuracy of the experimental technique. When 0 = 90 °, however,the shift is large enough (0.6 eV, Table 1) and it is out of the measurement error, i.e. for the starting RIE samples (t s = 0 min) the peak positions indicate a slight dependence on the take-off angle. The depth profile spectra show that Si 2p line of the starting sample develops to a double peak structure as the sputtering process proceeds. At oxide depth of 3 nm and more the Si 2p spectra are separated into two

~ ' oxide parts: the high binding energy line referred as al2p and located at 102-104 eV (in dependence on the distance d o from the oxide surface) is associated


Table 1 Si~ xide and Ols peak positions of the RIE treated samples detected after each sputtering step for two values of the take-off angle. The positions at the surface of the as-grown oxide are also given

Samples, distance from 0 Si~p ide the oxide surface/ (deg) (eV) interface d o (nm)/d i (nm)

(eV)

As-grown oxide 90 103.1 532.7 0/13 15 103.1 532.7 After RIE treatment 0/13 90 103.7 533.0

15 103.3 533.1 0.5/12.5 90 104.0 533.2

15 103.9 533.4 3/10 90 103.9 533.4

15 103.8 533.2 5.5/7.5 90 103.6 533.0

15 103.6 533.2 8/5 90 102.4 532.0

15 102.5 532.1 10.5/2.5 90 102.4 531.0

15 102.3 531.3 13/0 90 101.9 530.7

15 101.7 530.9

with an oxidation state of Si and the low binding energy line referred as siSip (located at 98.7 eV) is associated with Si signal from the substrate [15,17]. As can be seen from Table 1 the a"'°xidelzp peak position is not changed (0 = 90 °) up to d o = 5 .5-6 nm, i.e. the oxide is essentially SiO 2 in this case. With increasing sputtering time (at a distance d i of about 7 nm from the interface) the ~"~'°xidel2p peak shows a clear shift to lower energies. At the interface (d i = 0) this shift is 1.8 eV in comparison with the siSip peak position. The analysis of the evolution of the O ls peak with t~ (Fig. l(c)) shows the following. The O ls signal is recorded at 532.7 eV for the as-grown oxide which is the expected position characteristic of SiO 2 [15]. After RIE treatment, a small shift towards higher energies (0.3 eV for 0 = 90 ° and 0.4 eV for 0 = 15 °) is observed for the starting sample. As the sputtering proceeds up to d o = 5 .5 -6 nm (for both angles) a chemical shift of the O ls line cannot be convincingly established. For long sputtering times, however, as the sputtering approaches the S i -S iO 2 interface, the O 1 s peak position shift to lower ener-

gies is more pronounced indicating that the oxidation state of silicon has been reduced. At the interface this O ls position shift is about 2.3 eV and practically it is independent of 0 (the difference is very close to the experimental accuracy, Table 1). The variation of the intensity of both the ~12p"~'°xide and the siSip peaks when thinning the oxide by sputtering depends, however, significantly on 0. Even simple comparison of the spectra in Fig. l(a) and (b) shows that the intensity ratio of these two peaks changes markedly as the sputtering time increases and as the emission angle changes from 90 ° to 15 ° . This behavior of the peak intensity could be traced out in Fig. 2, where the intensity ratio, .oxide .Si I(Sl2p )//l(Sl2p) =los is presented with 0 as a parameter. For 0 = 90 °, los decreases steeply in depth of the oxide. For 0 = 15 ° the behavior of los(d 0 ) curve could be related to the influence of both the carbon and the nitrogen presence at the oxide surface. At first los decreases, then follows a relatively rapid increase of los to values close to those typical of the oxide surface passing through a maximum and subsequently los diminishes until the oxide film is removed completely. If this carbon and nitrogen rich surface layer did not exist, the los(ts) dependence for 0 = 15 ° would probably be very similar to that representing the case 0 = 90 ° (dashed line in Fig. 2). The fact that the C and N effect is well manifested for 0 = 15 ° and is blurred for 0 = 90 ° confirms the suggestion that this layer is

/Jo

60"

50"

40-

do, n m

2.6 5.3 7.9 10.5 13.2 I I I I 1

i//./*'\\

\ "\ , \ \ o = 15"

\9o°

; 1'o 1; io 2;- t,, min

Fig. 2. Intensity ratio 1o~ vs. sputtering time t s for two values of 0. The distance d o from the oxide surface corresponding to each t S is also indicated.

310 E. Atanassova, A. Paskaleva / Applied Surface Science 120 (1997) 306-316

30- ~ t, y- o0=90°

/×*</i i\ B ,

20 ~ 0 = 15 ° , ', \

{

10 '. ~ \ . . . , .

0 ,5 10 1'5 2'0 25

t~, min

Fig. 3. O ls peak intensity vs. sputtering time for 0 = 90 ° and 0 = 15 °.

really very thin. To check this we shall trace the variation of the O ls peak intensity when thinning the oxide by sputtering. The depth profiles of O Is peak heights are plotted in Fig. 3. As is seen, for both angles the intensity varies with t s in a similar manner. The profiles show three main regions: region A near the surface with thickness of ~ 0.5 nm is characterized with a relatively quick increase of the intensity, region B with thickness of about 8 nm, marks the 'bulk' SiO 2, where the intensity is almost constant and region C is the transition interface region, where the progressive reduction of the oxygen intensity from the value typical of SiO: to its lowest level at the interface is observed. The profile behavior in the region A is caused, as discussed above, by the C and N absorption at the surface and the region C is associated with the existence of intermediate (nonstoichiometric) oxidation states. Concerning the as-grown samples, the depth profiles show that both the ~12p~'°xide and the O ls peaks do not change their position up to t s = 21 min. At the interface (t s = 21-26 min) the peaks shift to lower energies and the intensity ratio los strongly reduces to a negligible value. These data imply a relatively sharp Si-SiO 2 interface for the as-grown sample.

The Si~p ide peak for the as-grown sample is placed as a symmetrical line at 103.1 eV with a full width at half maximum of 1.7 eV. The FWHM of the O ls peak is 1.6 eV (Table 2). A broadening of both the S. oxide and the O 1 s peaks occurs at the oxide surface 12p after the plasma treatment, and the two peaks have a FWHM of 2.3 and 2.7 eV respectively, suggesting the presence of at least two bonding states, i.e. the

plasma action leads to deterioration of the oxide structural parameters. Thinning the oxide by sputtering the FWHM of the Si~p ide remains constant up to d o of about ~ 8 - 1 0 nm, and then for t s = 2 1 min (d o = 10.5 nm, 0 = 90 °) it decreases to the value typical of the as-grown oxide. In general, the alteration of the O 1 s FWHM keeps a similar tendency. It should be mentioned, however, that after the initial plasma-created broadening of the peak, subsequently it slightly narrows with t s reaching the value of the as-grown oxide at the interface. These results suggest that the major modifications in the depth of the oxide caused by the RIE action are structural (not chemical) and implies that the enhanced contribution from intermediate oxidation states is the main result of the plasma action. Note that the FWHM of siSip is not affected neither by the RIE treatment nor by the sputter etching, which could be easily understood if we have in mind that the plasma acts on the oxide surface and it should not influence the substrate signal.

When 0 = 90 ° (the case we are mainly dealing with till now), the signal from the bulk is maximized

Table 2 The values of the FWHM for different distances from the oxide surface for the two take-off angles

d o (nm)/t~ (nm) 0 bl2p~'°xide O 1S

(deg) FWHM FWHM (eV) (eV)

As-grown 90 1.7 1.6 oxide 0 / 0 15 1.7 1.6 After RIE treatment 0 / 0 90 2.3 2.7

15 2.2 2.7 0 .5/1 90 2.2 2.6

15 2.2 2.7 3 / 6 90 2.4 2.5

15 2.5 2.6 5.5/11 90 2.4 2.4

15 2.5 2.5 8 /16 90 2.3 2.2

15 2.2 2.2 10.5/21 90 1.6 1.8

15 1.7 2.0 13/26 90 1.7 1.6

15 1.7 1.7


with respect to that from the surface layer. If h is the photoelectron mean free path in the SiO2 film (ex- perimentally determined to be about 3 nm [18] for A1K~ line), the signal intensity is mostly derived from a distance of 3 h within the solid. (The vertical depth investigated is given by d~ = 3A sin 0 and it has a maximum when 0 = 90°). As 0 decreases from 90 ° to 15 ° the vertical depth was reduced to 2.3 nm and the signal from the oxide surface dominated the signal from the bulk. As is seen (Table 1) we do not observe a noticeable difference in the positions of the two peaks ~12p"~'°xide and 0 Is with 0 changing from 90 ° to 15 °, excepting the case t~ = 0 rain. This means that for the RIE treated samples the oxide is homogeneous in depth (i.e. for all t s) within the limits of the information depth defined by the two angles used. In general, the FWHMs of the peaks remain also unchanged as compared to the case 0 = 90 °. On the contrary, at the surface t h e S i ~ ; ide peak position of the plasma treated samples depends on 0. The behavior of the intensity ratio Ios in the oxide depth also depends on the emission angle (Fig. 2). As we mentioned above, the surface sensitivity may be improved by tilting the specimen to small angles. So,

since the escape depth at 15 ° is smaller than that at 90 °, the curve Io~(ts) at 15 ° clearly detected carbon and nitrogen and demonstrated that they are indeed situated in a very thin layer at the oxide surface. After removing C and N by sputtering, the curve for 15 ° behaves similarly to that for 90 ° and this similar- ity implies that the effect of the RIE treatment is practically equal within the take-off angle variation of the escape depth. This suggestion is clearly con- firmed by Fig. 3, where the curves for the two take-off angles show similar behavior. The observed different values of the O ls intensity for equal t~ are related with the well known dependence of the intensity on the take-off angle.

In order to obtain more information about the RIE related changes in the oxide, the depth spectra have been deconvoluted to Gaussian curves. Peaks fitting has been made with a full width at half maximum of standard components and Gaussian-Lorentzian func- tions. The final fitting was made iteratively. Some results are given in Fig. 4 where the Si 2p spectra of the RIE treated samples for several selected sputter times are compared. The experimental curves are as-recorded data after Shirley background subtrac-

I

e

y a,.tl.

- - ezperimenl/"\/z" BE, eV A, eV caleulalio¢ \ f i 4 + 98.7 0.00

101.1 2.37 ~\ 103.7 5.01

/ .. Si ~+ / ) ~"-~. Si o

1;8 1;6 1;4 102 I;0 9'8 Binding Energy, eV

BE, eV A, eV

/' '\ Si 4+ 100.9 2.18

s / ~ 103.6 4.87

/ \ Si °

l~S 1;6 1;4 1;2 1;0 9~ Binding Energy, eV

I

Z .

.4+ ~ S~ BE, eV A, O

I ~\ ~ 0.00 [ ~\ Si° 100.3 1.60

1;8 1;6 1;4 1;2 1;0 9'8 9'6 Binding Energy, eV

L

Sio A BE, eVA, eV ][ / 98.7 O.00

[! /100 .1 1.40 /; /101.4 272

/ , ~t1°2-4 371

106 104 102 100 98 96 Binding Energy, eV

I n t e n s 1 t y

&.u.

~)

SIOABE , eV A, eV ~ / 98.7 O.OO

/ 100.0 1.28 f /101:2 2:485

i i i 106 104 1;2 100 9'8 9'6 94

Binding Energy, eV

Fig. 4. Si~p ide XPS spectra of the RIE treated samples for different t s (0 = 90°). The spectra are deconvoluted to Gaussian curves (see text). The corresponding intermediate oxidation states are seen in the figures: (a) t~ = 0 min; (b) t~ = 11 min; (c) t S = 16 rain; (d) t s = 21 min; (e) t~ = 26 min.


tion. The binding energies of the different peaks as well as their shifts A with respect to the siSip peak are presented (tabular inset) in the figure.

The analysis of the data providing the best fit shows the following. For the starting RIE treated sample, the siSip peak associated with the elemental silicon is fitted with a symmetrical line centered at 98.7 eV with a FWHM of 1.24 eV. The O12p<'°xide peak is composed of a high binding energy line located at 103.7 eV which is associated with SiO 2 (100% Gaussian, FWHM = 2.3 eV) and a second one located at 101.1 eV (100% Gaussian, FWHM = 2.3 eV). This means that in fact the ~12p~'°xide peak at the oxide surface is actually due to photoelectrons from the SiO 2 and from another phase, namely SiO (or Si2 +) according to the classification of Grunthaner et al. [19] and Himpsel et al. [20]. As is seen the peak associated with the silicon substrate is very weak. (In fact, the sum of all peaks gives a spectrum equal to the experimental one, Fig. 4.) The three peaks (elemental Si (Si°), stoichiometric SiO 2 (Si 4+ ) and intermediate Si 2+ oxidation state) present in the spectra until t S = 11 min (5.5-5.8 nm from the oxide surface) but their intensities are different for the different ts: the intensities of both the Si and the SiO 2 increase, while the intensity of SiO feature is nearly constant. A qualitative change in the spectra is observed after 16 min of sputtering where obviously the transition interface region starts. The most pronounced effect of this change is the strong increase of the elemental Si intensity and the intensity drop of SiO 2 signal. So, the deconvolution of the ~12p~'°xide

spectra results in a set of three peaks for t s = 21 min (and more) which are attributed to the different intermediate oxidation states. In Fig. 4 one can clearly identify the peaks between the Si and the SiO 2 lines corresponding to SiO for t s = 16 min and to Si20 (Si J+) and Si203 (Si 3+) [19-21] for t s = 21 min, i.e. the near interface region of the RIE treated samples, which extends to about 4-4.5 nm from the Si-SiO 2 interface, consists of all the three possible intermediate oxidation states Si I +, Si 2+ and Si 3+. At the interface (Fig. 4(e)) the Si 2p is fitted with a main peak from the elemental Si and a small peak corresponding to Si I +. Fig. 5(a), in which the relative area under the peaks of the four oxidation states is plotted as a function of the sputtering time, exhibits clearly the distribution of the observed oxidation states. A

:) ~--~/'-\ / . / /~

o = 9 0 7 / ", f s

t \ t t Y

&.tl.

' ; 1~0 ll5 20 2'5 t,, min

q b ~ = i S ° / ' N ' ~ S i 4 +

.

;t/ Z a.u. Si 2+ /~ ~

6 5 1'0 1~5 2'0 25 6, min

Fig. 5. In-depth distribution (peak areas) of the four silicon oxidation states and unoxidized silicon (Si °) for RIE samples: (a) 0 = 90°; (b) 0 = 15 °.

close examination of Figs. 4 and 5(a) shows that at the interface region the actual oxide signal falls below the detection limit of the measurement technique; the SiO and Si203 peaks also disappear. On the contrary, the absolute intensity of intermediate oxidation states goes down with increasing d i due to attenuation by the SiO 2 overlayer and at the oxide surface there exists in fact only SiO in a small amount. In fact, SiO species in a small but a constant amount exist along the whole oxide thickness. The coexistence of all the three intermediate oxidation states in the transition interface region of our RIE treated samples clearly indicates deviation from the ideal atomically abrupt interface. Obviously, the structure of the layer in this region is substantially different from that of the true SiO 2. As is seen, the relative amounts of the Si 1÷, Si 2÷, Si 3÷ oxidation states are different in the depth of the transition region. At the beginning of this transition region (t s = 16 min) only the SiO state is present. At a distance of about 2.5 nm from the Si-SiO 2 interface

E. A tanassova, A. Paskaleca / Applied Surface Science 120 (1997) 306-316 313

the prevalent oxidation state is Si20 and the quantity of Si20 3 is about threefold smaller; SiO disappears. At the very interface a small amount of Si l+ (Si20) oxidation state is only observed in addition to the strong signal from the Si crystal. The rest of the layer, approximately 8 nm thick, according to the spectra, behaves like a typical SiO 2, so it must be considered as a bulk SiO v (The peak at 103.6-104.0 eV, in dependence on t s, corresponds to S i - O - S i bond angle very close to 144 ° which is typical of a bulk amorphous oxide [19].) Additional support for the peak identification as intermediate oxidation states of Si is their energy position (for ts = 21; 26 min) with respect to the unoxidized line at 98.7 eV. The Si t+ peak is shifted by 1.3-1.4 eV from that of the elemental Si and the Si 3+ peak is shifted to 2.7 eV respectively. This is consistent with the observa- tion by other authors [19-21] of oxidation state peak shifts of about 1 eV, (with respect to each other) starting from the line of the elemental Si. The depth profiles of the as-grown samples do not indicate the presence of suboxides up to d o ~ 10 nm, where obviously the transition region begins. This region also contains Si atoms in intermediate oxidation states (Si I+, Si 2+ and Si 3+) and extends to about 3 nm into the SiO 2 layer (i.e. the width of the interface in this case is smaller than that of the RIE treated samples). Therefore, R/E treatment leads to a broadening of the interface region and to a relatively small deterioration of the quality of the 'bulk' oxide (giv- ing rise to a small constant amount of SiO through the whole oxide). Concerning the actual width of the interface for both the as-grown and the RIE treated sample, the following should be taken into account. As mentioned in [20] many studies of the Si-SiO 2 interface generally agree in identifying two distinct regions: the near interface region (a few atomic layers region that 'covers' the interface) and a second one which extends at about 3 nm in the SiO 2 overlayer. Our results support this concept of the interface rather than the earlier studies [21-23] which indicated that the true interface was < 2 nm (even < 1 nm) thick. In addition, all the same, it is very likely that some unavoidable effects of the ion sputtering take place in our experiments. As commented by Barr [15] due to the change of the sputtering rate in the transition region, most of the oxide species detected are 'knock-on implants' driven into the

relatively conductive Si ° matrix during the sputter etching. This effect is related with the fact that some of the SiO x (x < 2) and Si ° detected in the transition region are not present before sputter etching but are produced by the sputtering itself. As a result part of the silicon substrate appears to mimic the oxide which manifests in an artificial broadening of the transition region. This effect applies to the as-grown as well as to the RIE-treated sample. Having all this in mind we believe that our real interface appears at about t s = 21 min. The effects of the ion sputtering require detail investigation which is out of the scope of this study. However, they do not change the major conclusions concerning the influence of the RIE treatment on the samples and affect mainly the actual width of the transition region.

The chemical shift A of the SiO 2 peak referred to the energy of the pure Si peak is 5.0 eV for the starting RIE samples. This value is larger than that for the as-grown sample as usually reported (3.8 eV) for thin SiO 2 films [21]. The relatively large value of the al2p"'°xide chemical shift for the RIE samples is attributed to the observed structural modifications as a result of the RIE treatment as well as to the effect of C and N present on the surface (t s = 0; 1 rain). Similar values of A have been obtained previously [24] and have been related to structural imperfections and nonhomogeneities in the layers. The alteration of A with t s (as obtained by fitting of the spectra) is given in Fig. 6. Generally, the behaviour of A as a function of d o reflects in fact the degree of structural homogeneity of the layer. In this sense, the relatively small alteration of A in depth of the oxide (up to thickness of ~ 8 nm, 0 = 90 °) could be assigned to a relatively uniform distribution of eventual plasma induced structural disorder as well as to a lack of clearly pronounced plasma strongly damaged layer

A~ el/

* " 0 = 1"~o] 5.0- o o - 0 = 90

o

4.5-

4.0 2

3.5 ~ :'o I; 20

t,, min

Fig. 6. The chemical shift of SiO 2 peak vs. t s for two values of 0.

314 E. Atanassova, A. Paskaleva /Applied Surface Science 120 (1997) 306-316

in this part of the oxide. In the interface transition region, A decreases to the value of the as-grown sample and does not change any more. On the other hand, the shape of the curves in Fig. 6 could be explained with the existence of a positive charge in the oxide traps, which we have detected with electrical measurements of the same samples. The capaci- tance-voltage curves show that these traps are dis- tributed throughout the whole oxide including also the near interface region. The existence of this positive charge retards the photoemission of electrons from the SiO 2 layer and hence their kinetic energy is smaller. This results in a higher binding energy of the ,12pq'°xide peak. With increasing t s (i.e. with reduc- ing the oxide and the positive charge respectively) A has to decrease which is indeed observed (Fig. 6). The prime candidate for these RIE induced electrical active traps are the structural damage and modifications as evidenced by our XPS spectra. It is well known that broken and strained bonds (i.e. Si atoms in intermediate oxidation states) electrically manifest themselves as traps. The analysis of the deconvoluted spectra shows that the shift in binding energy relative to the line of elemental Si for each of the intermediate oxidation states does not change through the transition region (for SiO A ~ 2-2.4 eV; for Si203 A ~ 3-3.5 eV; for Si20 A ~ 1.2 eV, i.e. there is no noticeable difference).

Fig. 7 shows the deconvolution of the ol2pq'°xide

spectra for a take-off angle of 15 ° and for ts = 0; 21 and 26 rain respectively. Comparing the data from the fitting procedure and referring again to Fig. 4, we note that the major differences observed for 0 = 90 ° and 15 ° are related with the composition of the transition region and especially with the relative contribution of the lower oxidation states with respect to the peak of elemental Si. Upon deconvolution the spectra for both the angles produced the same three component peaks corresponding to Si 1 +,

Si 2+ and Si 3+ states (Fig. 5(a) and (b)). In fact, the angle change does not have a significant effect on the distribution behaviour of the oxidation states; the depth profile only shows different content of these states in the transition region in dependence on 0, i.e. the interfacial region is slightly inhomogeneous in its depth. The part of the oxide assigned as a bulk SiO 2 (d o ~ 8 nm) is homogeneous in terms of the take-off angle values (a similar behaviour of Si and

(a) I

L

14+ BE, eV A, eV 99.2 0.00 10l.O 1.83 103.3 4.11

a) 1;8 1;6 164 1;2 160 9'8

Binding Energy, eV

(b) I

,,.Yu.

S./̂ \;°A BE, eV A, eV

/ ;" '1o-6~-~ ~ _ / / 1ioo.o l to

• ÷,,~.%~÷ , ~ ' s i ~ ~ 191,2 2 .45

,~)

11)6 1;4 1;2 1;0 9'8 9'6 9'4 Binding Energy, eV

(c) S.//\BE,~o~ eV A, eV

n I; 1999 12i

t

4 1;6 1;4 1;2 1;0 98 9'6 9'4

Binding Energy, eV

Fig. 7. Si~p ide XPS spectra of the R1E treated samples for different t~ (0 = 15°). The spectra are deconvoluted to Gaussian curves (see text). The corresponding intermediate oxidation states are seen in the figures: (a) t~ = 0 min; (b) t~ = 21 rain; (c) t~ = 26 min.

SiO 2 signals is observed, Fig. 5; the dependence A(t s) is practically the same, Fig. 6). For both angles this part of the oxide exhibits a dominant SiO 2 state

and a constant level of SiO chemical state not exceeding ~ 10%. For both angles the deconvoluted O 1 s spectra are also identical within the error of the measurement technique. The O ls line located at 533.0 eV at the oxide surface (Fig. 8) can be approx- imated by a Gaussian function with a FWHM of 2.6 eV (92% G). This peak position is practically not affected by the sputter etching up to t s = 11 min and is very close to the O l s signal observed in SiO 2 [15,16]. A significant shift (1 eV) of the O ls line to a lower energy is observed after t~ = 16 min, i.e. at the beginning of the transition region, the oxygen signal also indicates the presence of a lower oxida-


/\ °is/ 0 = 90]

I [ ! / n

e / / \

i min t Y

&.U. ]

z 6min ..,l-

2 r m - Z . . . . . . . . . .

538 ~6 5~4 5~2 5~0 5~s Binding Energy, eV

Fig. 8. O ls XPS spectra of the RIE samples for different distances from the oxide surface (d o = 0 nm, t~ = 0 min; d o = 8 nm, t~ = 16 min; d o = 10.5 nm, t S = 21 min).

by the plasma. (ii) The plasma action in R/E-mode leads to a broadening of the interface transition region. The near interface region extends to 4-4.5 nm from the Si-SiO 2 interface (for the as-grown samples this region proceeds to 3 nm from the interface). The alteration in the local atomic structure of the transition region as a result of the RIE process is not established - - the transition interface region of both the as-grown and the plasma treated samples contains all the three possible intermediate oxidation states (Si I +, Si 2+, Si3+). Therefore, thin oxide films after 5 min action of nitrogen plasma at RIE conditions experience broadening of the Si-SiO 2 interface region and a relatively small deterioration of the quality of the 'bulk' oxide (the presence of a small constant amount of SiO through the whole oxide).

tion state. Near to the interface (t s = 21 min) the O ls peak can be deconvoluted into two peaks (Fig. 8): the main peak at 531.0 eV (FWHM = 1.8 eV) is assigned to the elemental oxygen [15] and the small peak at 532.3 eV associated with a poor oxidation state. The FWHM of the O ls line gradually decreases from 2.6 eV at the oxide surface to 2.0 eV at the interface. Just at the interface (t S = 26 min), the peak at 532.3 eV is with a negligible intensity and the O ls line is practically fitted only with a small peak of the elemental oxygen (FWHM = 1.6 eV).

4. Conclusion

The main conclusion of this study is that only structural modifications in thin SiO2-Si structures occur as a result of RIE treatment in N 2 atmosphere and the process does not lead to actual nitridation of the SiO 2. The plasma action for 5 min forms only a very thin ( < 0.5 nm) N-rich layer on the oxide surface. The primary features of the structural modifications are: (i) RIE process favors the generation of SiO species through the whole oxide. The part of the RIE treated oxide assigned as a bulk oxide exhibits a dominant SiO 2 state and a constant level of SiO not exceeding 10%, i.e. there is no evidence for the formation of a layer in the oxide strongly damaged

Acknowledgements

We are indebted to Dr. G. Tyuliev and Dr. K. Kostov for their help in XPS measurements.

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structural changes in thin sio2 on si after rie-like nitrogen plasma action

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