SURFACE AND INTERFACE ANALYSIS, VOL. 24, 469-475 (1996)

SIMS Depth Profiling of Delta-doped Layers in Silicon

V. I(. Smirnov, S. G. Simakin and E. V. Potapov Institute of Microelectronics of Russian Academy of Sciences, Krasnoborskaya 3, Yaroslavl 150051, Russia

V. V. Makarov Centre for Analysis of Substances, Elektrodnaya 9, Moscow 11 1524, Russia

Boron and germanium 6-doped silicon samples were studied using SIMS depth profiling on a Cameca IMS-4F instrument with O,', Nz+ and Cs' primary beams at various energies and incidence angles. The depth resolution characteristics were compared. We show that, with experimental conditions being the same, the N, + primary beam provides for better decay lengths, while the leading edges of the profiles turn out to be significantly broadened because of the beam-induced roughness which develops at an early stage of silicon bombardment by N, + ions.

The influence of ripplelike roughness on the SIMS profile shape is considered within the present simple analyti- cal model.

The effect of a modified layer on the SIMS profile depth was experimentally investigated. For profiles obtained with an N, + primary beam, it was shown that swelling of the modified layer produced by the implantation and trapping of primary particles shifts the profile as a whole toward the surface.

~~

INTRODUCTION

The development of molecular beam epitaxy in recent years has made it technologically possible to grow semi- conductor layers with atomic plane dopant distribution (&doped layers). In addition to the importance of their application in semiconductor technology, these struc- tures turned out to be very useful for SIMS method- ology. Depth profiling of a b-doped layer gives direct information on the dopant redistribution in the sample during the analysis. The complete form of the depth resolution function obtained in this case may be used further in profile correction procedures. Moreover, the depth profiling of a 6-doped structure allows one to compare advantages and/or disadvantages of various experimental conditions under which the analysis is carried out. In particular the effect of different primary particles may be determined.

According to a number of recent publications,' the use of a N2+ primary beam is of interest for SIMS depth profiling. In the present work we performed depth profiling of silicon &doped with Ge and B to look into the depth resolution characteristics that enables application of an N, + primary beam for SIMS analysis of such structures. We compare these charac-

teristics to those provided by conventional 0, + and Cs+ primary beams. Furthermore, in the present work some specific features of N, ion interaction with Si are revealed and their influence on the depth resolution is shown.

EXPERIMENTAL

The depth profiling measurements were performed on a Cameca IMS-4F instrument. Three types of samples were exposed to SIMS depth profiling: epitaxial Si with a single 6-layer of Ge at 60 nm depth, epitaxial Si with two Ge &layers located at 80 and 100 nm and epitaxial Si with a single 6-layer of B at 140 nm depth. The primary ions were O,', N2+ and Cs+. The types of registered secondary ions, primary impact energies E, and angles of primary beam incidence 8 for each of the analyses are indicated in Table 1.

The incidence angle of the primary beam depends on the primary energy, because of the electric field between the sample surface and the immersion lens in a Cameca configuration., This is reflected in Table 1, where the boundary experimental energies E, and corresponding 8 are given.

Table 1. Experimental conditions of depth profiling

02'

Primary ions

Nz* CS'

Detected €0 Detected E, Type of Detected EP &-layer ions ( k W 6" ions (kev) 8" ions (keV) 8"

Ge/Si 14Ge+ 3-8 47-37 7'Ge+ 3-8 47-37

B/Si "B+ 3-8 47-37 "B+ 3-8 47-37 74Ge- 9.5-17 21-25 74Ge- 9.5-17 21-25 14Ge- 14.5 24.5

27BO- 9.5-17 21-25 26BN- 9.5-17 21-25

CCC 0142-242 1/96/070469-07 0 1996 by John Wiley & Sons, Ltd.

Received 26 June 1995 Accepted 28 March 1996

470 V. K. SMIRNOV ET AL.

In all the experiments the primary beam was rastered to a 250 pm x 250 pm square on the sample surface. A 10 pm analysed area was used. After the analysis the depths of craters were measured with a Talystep profi- lometer. The transfer from sputtering time scale to the depth scale was performed using the routine procedure implying a constant sputtering rate during the analysis.

The special investigation of silicon sputtering by O2 +

and N, + ions was undertaken using the following pro- ~ e d u r e . ~ The Cameca IMS-4F instrument allows the rastered primary beam to be moved along the sample surface parallel to one of the raster sides at a well- controlled constant speed. Consider a situation where the raster motion begins simultaneously with the switching on of the primary beam. In the top of Fig. 1, the initial and final raster positions are shown by solid squares. Points A, B, C and C' are fixed at the surface within the initial raster square. After the raster moves far from its initial position the total fluences of primary ions in points A, B, C and C' become different. If the primary beam current is stable, raster motion speed is constant, primary ion distribution in the raster square is uniform and primary beam size is much less than the size of the raster, then one can write for the total fluence @(C) in point C

@(C) = @(B)AC/AB AC < AB (1) where @(B) is the total fluence at point B. (This expres- sion is valid only if the lateral speed of raster displace- ment is much less than the raster scan speed. In our experiments this condition was always fulfilled because the raster scan speed in a Cameca instrument is 100 s- ' while a typical time of raster displacement for the dis- tance equal to the raster size was -30 s). An expected dependence of sputtered depth Z(x) along the AB line is schematically shown in the bottom of Fig. 1 for the case where the sputtering rate is constant and the primary beam is very small (point-like). In our experiments the beam size was - 30 pm.

Consider some point C close to C. If one writes for this point an equation similar to Eqn (l), it is easy to

Raster motion

Initial position Final position

A1 C C' B X

Figure 1. Measurements of sputtering rate on the basis of ras- tered beam motion across the surface.

obtain

A@ = @(C') - @(C) = @(B)CC'/AB (2) If the primary particle flow is stable, A@ may be expressed through the time interval At necessary for the raster to come the distance C C as follows

A@ = @'At (3) where @' = const. is a corresponding time derivative. Let u be the raster motion speed. Note that

@(B) = @'AB/u (4)

CC/v = A@/@' (5)

and hence Eqn (2) may be rewritten as

The difference in fluence A@ leads to a difference in depth AZ between points C and C . Introducing a local sputtering rate 2' one may express A 2 through the time interval At

A 2 = Z ( C ) - Z(C) = 2 At (6) Substituting Eqn (3) into Eqn (5) and then Eqn (5) into Eqn (6) one obtains

A Z / C C = Z'/V (7) which means that the instantaneous sputtering rate at any depth Z when A < x < B (see Fig. 1) is proportion- al to the slope of the Z(x) curve at the corresponding point. Therefore, by measuring the Z(x) dependence with a profilometer one can investigate the time evolu- tion of the crater depth.

As shown below, this method may be successfully used to explore the initial stages of silicon sputtering by N, + ions.

RESULTS AND DISCUSSION

Depth resolution

To characterize the depth resolution in our experiments and to compare our results with those reported in the literature we used the exponential decay length A, which is defined as the inverse of the exponential slope of the trailing edge of an abrupt structure depth p r ~ f i l e . ~ In addition, in the present work we use the parameter characterizing a broadening of the profile leading edge. This parameter is denoted as t and defined similarly to Ref. 5 as the length where the signal rises from 0.01 to 0.5 of its maximum. To order the data presentation, A is considered as a function of primary ion penetration depth R, as was proposed earlier by Vandervorst and Shepherd.6 The penetration depth for each experimental configuration was calculated with the TRIM p r ~ g r a m . ~ The leading edge broadening 5 is presented as a func- tion of the energy of the normal component of primary ion motion E, cos2 8.

Ge/Si. The profile evolution of the single &layer of Ge in Si measured using an N, + primary beam with differ- ent E , (and consequently different 6) is shown in Fig. 2(a). A similar family of profiles was measured using an 0,' primary beam. A Cs+ primary beam was used

SIMS DEPTH PROFILING OF &DOPED Si 471

m

',E 102'

; 10'9

i 0 d

:: 0

Depth, nm

40 80 120 Depth, nrn

Figure 2. Depth profiling of the Ge delta layer in silicon: (a) sputtering with NZ+ beam at (1) €,=3 keV, 8=47", (2) €,=8 keV, B = 37" and (3) €, = 17 keV, 6 = 25"; (b) sputtering with (3) N2+, €,=I7 keV, 8=25", (4) Cs+, €,=14.5 keV, 8=24.5" and (5) 02+, €, = 17 keV, 8 = 25".

with a constant E , and only in the regime of negative secondary ion registration. The Ge profiles in the same sample measured using all three kinds of primary ions with the highest E , are shown in Fig. 2(b). At this point we pay no attention to the different positioning of the presented profiles on the depth scale. This problem will be discussed below. Consider parameter A, which char- acterizes the depth resolution.

Figure 3 shows 1 as a function of penetration depth. For comparison, similar data obtained by Meuris et al.' and Petravic et al.' are also shown in Fig. 3. It should be noted that in Ref. 8 as well as in our experiments a Cameca IMS-4F instrument was used for the depth

Polarity ( Ge', Ge-)

1

I 0 5 10 15 20 R , nrn

Figure 3. Germanium decay length A as a function of primary ions projected range. 'The curve labelled 1 was measured with charging compensation.

profiling. A thick (57.5 nm) Geo,17Sio,s3 layer on Si was exposed to the analysis. In Ref. 9 a Riber MIQ-256 instrument was used in the regime of positive secondary ion registration, and the data were obtained with inde- pendent variation of E , (4-12 keV) and 8 (16-65'). The sample in Ref. 9 was silicon containing a single &layer of Ge at 35 nm depth. These authors also used Arf and Cs + primary beams and corresponding data are shown in Fig. 3 by the upper straight dashed line.

It can be seen from Fig. 3 that for an 0,' primary beam 1 strongly depends upon the sign of registered secondary ions. In the case of Ge' our data are in good agreement with those of Ref. 8 and differ from the data of Ref. 9 only slightly (a particular difference is difficult to discuss because in Ref. 9 only values of penetration depth R are indicated and E , and 8 are not). The depen- dence 1(R) for the 0,' primary beam lies lower than those for Cs' and Ar+ primary beams, and this fact may be explained (as in Refs 8 and 9) within the model of matrix swellings910 during the analysis. However, when Ge- ions are registered, 1(R) lies - 3 nm higher and this requires an explanation.

It is known* that there is a critical angle of incidence 8, x 30" for 0, + ions when silicon is bombarded. For 8 < 8, the competition between sputtering and primary oxygen implantation leads to the formation of a stoi- chiometric oxide layer on the surface of the sample, while for 8 > BC a solid solution SiO,(x < 2) is formed. Therefore, the modified layer for 8 < BC should differ qualitatively from that for 8 > 8,. Note that in our experiments the regime of Ge' registration corresponds to 8 > 8,, while Ge- ions are registered at B < 6,. This difference may be associated with the difference in A(R) behaviour.

The probable influence of the stoichiometric oxide layer on the measured profile of Ge may be caused by two mechanisms: Ge segregation at the SiO,/Si bound- ary and electromigration of Ge to SiO,/Si boundary caused by SiO, charging under ion bombardment. (The latter mechanism was reported'' to be the main one responsible for the broadening of SIMS depth profiles of Na, Ag, Cs and Cu in Si when 0, + primary ions are used.) To distinguish between these two possible broadening mechanisms a depth profile with charge compensation was carried out. We found that the charge compensation had no effect on the shape of the profiles when Ge- ions were registered, which means that Ge segregation at the SiO,/Si boundary is prob- ably the main reason why 1 increases in our experi- ments with Ge- registration and 0, + primary ions.

As one can see from Fig. 3, the transfer to a negative charge of registered ions in the case of a N, ' primary beam does not lead to a significant change in 1 as in the case of 0, + primary ions. The corresponding points of 1(R) dependence for both Ge+ and Ge- registration lie rather close to a straight line. Note that this line lies lower than the data for Cs+ and Ar+ primary beams. Moreover, this line is even lower than the one for the 0,' primary beam, which means that the process of matrix swelling improving 1 when the 0, + primary beam is used also takes place in the case of an N 2 + primary beam. Apparently in this case a modified layer forms, which is more or less close to a stoichiometric Si,N,, depending on the experimental conditions. In the present work the formation of Si,N, is confirmed

472 V. K. SMIRNOV ET AL.

(though indirectly) for the case where Si is bombarded with N, + primary ions with E , = 17 keV and 8 = 25" (see below). However, the segregaton of Ge at the Si,N,/Si boundary is apparently not so intensive as at the SiO,/Si boundary. We may try to explain it in terms of Ref. 10 where it is proposed that the segregation rate should be predicted by the difference between the enth- alpies AHf of oxide formation (0, + primary beam) of matrix atoms and impurity. Generalizing this approach to the case of nitride formation (N,+ primary beam) one can see'' that [AHfo(Ge3N,) - AHfo(Si3N,)] < [AHfo(GeO,) - AHfo(SiO,)], which should mean (in accordance with Ref. 10) that the segregation of Ge at SiO,/Si boundary is stronger than at the Si,N,/Si boundary.

Anyway, for relatively large E , (and consequently high penetration depths), the 1 values for N, + primary ions turn out to be lower than those for 0,' primary ions. This means there is an advantage of the N 2 + primary beam over the 0,' primary beam from the viewpoint of the decay length characteristic of SIMS depth profiling of Ge in Si with E , 3 8 keV. However, at low energies (and therefore large angles of incidence) the decay lengths are rather similar, reflecting the similar masses of the projectiles.

B/SF The sample containing a single 8-layer of B was exposed to SIMS depth profiling using 0,' and N2+ primary ions with 3 keV d E , < 17 keV. Corresponding values of Iz us. primary particle penetration depth R are shown in Fig. 4. Similar data from Ref. 12 are also pre- sented in Fig. 4 for comparison. One can see a good agreement between our results and those from Ref. 12 in the case of the 0, + primary beam.

h nr

10

8

4

4

5 10 15 20 R,nm Figure 4. Boron decay length A as a function of primary ion pro- jected range.

Two points should be made here. Firstly, the depen- dences R(R) for B/Si depth profiles are not sensitive to the sign of the registered ions. As was mentioned above, the change of the registration regime in our experiments both in the case of N, + and 0, + primary ions leads to a significant change in the quality of the modified layer formed under the interaction of primary ions with the surface of silicon. Nevertheless it does not affect 1(R) behaviour. We assume that it is caused by the relatively low mobility of B atoms in silicon and the correspond- ing SiO, and Si,N, modified layers (i.e. boron does not tend to segregate, whereas in SiO, germanium does). The second point is that from Fig. 4 one can see again the advantage of the N, ' primary beam over the 0, +

primary beam.

Depth resolution and beam-induced roughness

The difference in the interaction of N 2 + and 0,' primary ions with silicon manifests itself in the develop- ment of beam-induced roughness. There are indications' that if N, + primary ions are used, then for 35" < 8 < 65" there exists some depth d, at which the roughness in the form of a wave-like ripple develops very rapidly; d , depends on E , and 8, and d, decreases with a decrease in E, and 8. When d, is reached during SIMS analysis the intensity of a matrix secondary ion current rapidly changes (it may increase or decrease, depending on experimental conditions13) and after a while becomes stabilized again at a new level. Such a behaviour of the matrix signal is apparently caused by the changes in the local chemical conditions of sputter- ing during ripple formation. (For the case of silicon bombardment with oxygen, the development of these changes was discussed by Elst and Vandervo~t.'~) One of the curves in Fig. 5 represents a typical dependence of 30Si+ current on depth in depth profiling of Si with a N, + primary beam. Its increase at the depth of inten- sive ripple development is clearly seen. Using this behaviour of the matrix ion current the measurements of d, for E, between 3 and 8 keV were carried out for N, + primary ions.13 The results are presented in Fig. 6 as d, VS. E, COS' 8.

A similar ripple formation is observed when 0,' primary ions are ~ s e d . ' ~ , ' ~ However, d, values are greater than those for N, + primary ions by one order of magnitude. This circumstance allows one to separate the influence of beam-induced ripple formation on depth resolution. In Fig. 5 the superimposing of two profiles of a single b-layer of B in Si is shown. Similarly

1 d' 4 c, cm-3, Ic""si+),

countsis

Id

lo"

lo"

depth, nm Figure 5. Depth profiling of boron delta-doped layer: (1) N,+ and (2) 02+; E , = 3 keV and 0 = 47.

473 SIMS DEPTH PROFILING OF &DOPED Si

d, nm ,

Figure 6. Ripple-like roughness depth d, as a function of E , cosz 8 for silicon bombarded with N,+.

in Fig. 7 two profiles of a double &layer structure of Ge in Si are compared. In both cases the profile obtained with the 02+ primary beam is not broadened by the beam-induced ripple because d, is not reached during the analysis. In contrast, the profile obtained with the N, + primary ions is significantly broadened because the corresponding depth of ripple formation is less than the depth of the profile location. It is interesting to note that the broadening manifests almost only on the leading edges of the profiles. The ripple does not affect 1 values. Also, the broadening of the leading edge (of the profile obtained with N, + primary beam) manifests in its almost parallel shift towards the surface if one compare the profile with the corresponding unbroadened profile (obtained with an O2 + primary beam). This could be explained as follows. Consider the situation where the beam-induced roughness has a wave-like form, its amplitude 5 being much less than the wavelength L (both for 02+ and N,+ primary beam). Consider some point (point A) on a roughened surface of sputtering as is schematically shown in Fig. 8. If the condition [ $ L is fulfilled, then the instantaneous inter- nal profile at point A should be the same as if a flat sputtered surface was at the corresponding position (this position in Fig. 8 is shown as dashes), because any significant depth variations of the real sputtered surface are too far from the point A. This means that the con- tribution of the roughness to a depth profile may be reduced to an averaging of the unbroadened profile over some interval of depth that is determined by the roughness amplitude. Namely, if the average depth of

-3 2, cm

1

0 40 80 120

Depth, nm

Figure 7. Depth profiling of double delta-doped layer structure of Ge in Si: (1) N,+, €,=3 keV, @=47" ; (2) 02+, E , = 3 keV, 8 = 47".

n

~~

Lateral position

Figure 8. Illustration of SIMS signal averaging by ripple-like roughness.

the sputtered surface at some moment is 2 then, assuming that local depths are equally probable in interval (2 - 5, 2 + l), one can write for the SIMS signal D(Z)

D(2) = 1/25 g(z') dz' (8) zI Z-5

where g(z) is the profile in the absence of roughness. The ripples are assumed to have a wave-like form but we do not use a sinusoidal weighting in Eqn (8) to simplify the consideration.

Consider Eqn (8) when g(z') = const. exp( - .?/A) (the trailing exponential edge of our profile). Calculation in this case gives

D(Z) = const. 1/25 exp( - Z/A)[exp(5/1) - exp( - 5/11)]

If the roughness amplitude 5 is small in comparison with A, then by expanding the two exponential functions in square brackets up to a linear term one can see that

which means that the roughness does not manifest itself on a sufficiently gradual edge of a profile. If c is greater than 1 then the second exponential function in square brackets in Eqn (9) may be neglected and

(9)

D ( z ) - g(Z) (10)

D(Z) - const. 1/25 exp[ - ( Z - [)/A] (1 1)

W ) scz - 5 + 1 W/25)1 (12)

or

which means that an abrupt trailing edge is seen as shifted to the right, the shift increasing as (/A increases, while the exponential slope remains the same until the ripples amplitude changes with depth. A similar result may be obtained for an exponential leading edge (but an abrupt leading edge turns out to be shifted to the left, of course).

As the leading edges of B and Ge profiles are sharper than corresponding trailing edges, the influence of the beam-induced roughness manifests itself mainly in the leading edge shift (see Figs 5 and 7) with almost no changes in the exponential slope. Indeed, the leading edge shift of the Ge(N,+) profile at 80 nm depth is almost the same as that of the B(N2 +) profile at 140 nm depth, and this confirms the validity of the present con- sideration and proves that the ripple amplitude hardly changes with depth in our experiments. In accordance

474 V. K. SMIRNOV ET AL.

with Eqn (12) the distance between two leading edges in Fig. 7 may be treated as an estimation of the roughness amplitude and its value (8-10 nm) seems to be of a correct order of magnit~de.’~’~’

The presented model of the influence of the ripple for- mation on SIMS depth resolution is based on the con- dition that the roughness amplitude is small in comparison with the lateral characteristic dimension of the roughness (c 4 L). Unfortunately, because of the lack of data on the roughness dimensions, we cannot say whether this condition was fulfilled in our experi- ments or not. A more detailed study of the roughness development should be undertaken to find out how strictly the condition 5 G L should be fulfilled to provide model validity. Nevertheless, the presented experimental data may be considered as an indirect confirmation of the model validity.

The leading edge of a profile of a 6-layer is almost always sharper than the trailing edge and hence the leading edge is more sensitive to a roughness. But to characterize the roughness it is not enough to take only an exponential slope of a leading edge. This is why the second parameter characterizing depth resolution 5 in this work is defined as a length where the leading edge of a profile changes from 0.01 to 0.5 of its maximum. Defined in this way, characterizes simultaneously both the leading edge exponential slope and the leading edge shift caused by ripple formation.

In Fig. 9(a) the evolution of 5 is presented as a func- tion of the energy of the normal component of primary ion motion E , cos2 t? for the depth profiles of the single &layer of Ge in Si obtained with O,+ and N z + primary ions. In Fig. 9(b) similar data are presented for the depth profiling of B in Si. At this point the following comments should be made. The behaviour of 5 in the case of analysis of boron using an N, + beam (curve 1, Fig. 9(b)) illustrates well the effect of the roughness on the depth resolution. The first point of the curve ( E , cos2 t? - 1.5 keV) corresponds to the situation where the roughness is developed much earlier than the &layer is reached. For the next point ( E , cos’ t? - 3 keV) the roughness develops at depths much deeper than the depth of the &layer (see Fig. 6), and 5 decreases drastically. If a Ge &layer is analysed (curve 1, Fig. 9(a)), the situation is less clear: 5 degrades with E, cos2 8 even when the roughness does not develop. In our opinion, the only explanation for this phenomenon is that the relative contribution of roughness to 5 in comparison with other mechanisms of broadening in the case of analysis of Ge is smaller than for the analysis of B. In both cases 0,’ primary ions provide better resolution.

Effect of profile shift

At this point we return to Fig. 2 to discuss the depth positioning of the profiles. As was mentioned above, to transfer from sputtering time scale to depth scale the routine procedure was used, implying a constant sput- tering rate during the analysis. This procedure ignores the fact that when a chemically active primary beam is used for the depth profiling, a modified layer is formed beneath the surface of the sample, and this layer usually has a partial atomic density of matrix atoms signifi-

lo 1 a

0

0 5 10

Epcos2B, keV

2 4 6 E,cos20 Figure 9. (a) Broadening of germanium profile leading edge (0 as a function of E, cos2 6 : (1) N,+ and (2) 02+; (b) similar dependence for boron.

cantly lower than in the initial matrix because an implantation of primary particles and their trapping beneath the sputtered surface leads to swelling of the modified layer. Because the modified layer is repro- duced permanently during depth profiling, the actual depth of the sputtered surface at each moment turns out to be lower than the true one by some value determined by swelling of the altered layer. (Fox et ~ 1 . ’ ~ pointed out that this value may be important for determination of the sputtering rate by measurement of the sputtering crater depth.) As a result, when the swelling effect is sig- nificant (high E , and 0) the profiles are erroneously found to be shifted toward the surface (see Fig. 2).

The initial modified layer formation obviously happens at the first stages of sputtering. Hence, at the first stage of sputtering there should be variations in the sputtering rate. We investigated the evolution of crater formation in silicon in the case of N, + primary ions at E , = 17 keV and t? = 21”; the method is described in the Experimental section. In Fig. 10 the sputtered depth 2 is plotted as a function of the lateral position x of the N, + primary beam raster on the silicon surface. As was shown above, the slope of this dependence at each point serves as a measure of the relative sputtering rate at the corresponding depth. Three stages of sputtering are clearly seen in Fig. 10. Stages 1 and 3 correspond to sputtering with a constant rate. At stage 2 the sputter- ing rate is almost equal to zero, and this stage obviously

SIMS DEPTH PROFILING OF &DOPED Si 415

Z(X1,

3 0 7 1 10 ,!A

140 180 220 X, pm Figure 10. Profilogram of crater wall sputtered with N,+ ions (moving raster). Labels 1, 2 and 3 show different sputtering stages.

corresponds to modified layer (apparently Si,N,) for- mation, when the swelling almost exactly compensates for the sputtering. It should be noted that in the present case the initial modified layer formation is localized in time and happens only after the fluence of primary ions reaches some critical value. In the case of 0, primary ions the modified layer formation happens more grad- ually and cannot be revealed so evidently with the method that we use. Note that the curve in Fig. 10 allows one to estimate successfully the correction value A - 9 nm, which is the distance of the shift of profile 3 in Fig. 2(a) outward to the surface necessary to return it to the true position.

CONCLUSIONS

The experimental results on the depth resolution char- acteristics of SIMS depth profiling of B and Ge 8-layers in silicon with 0, +, N, + and Cs+ primary ions lead to the conclusion that a N,+ primary beam has a definite advantage over Cs+ and even 0,' primary beams,

especially in the range of high primary ion energies. This advantage is manifested in better values of I that are characteristic of the depth resolution function. The analyses show that the smaller decay lengths of the pro- files obtained with an N, + primary beam are caused by the formation of a nitrogen-containing modified layer at the surface of the analysed sample, the swelling of this layer and the relatively small mobility of boron and ger- manium inside it. Therefore, a N, + primary beam may serve as an alternative to Csf and 0, + primary beams for SIMS analysis of impurity distributions with abrupt trailing edges. On the other hand, the relatively early development of beam-induced roughness at the surface of silicon when a N,' primary beam is used signifi- cantly broadens the leading edge of the depth resolution function and limits the application of an N2 + primary beam.

The influence of a ripple-like roughness on the form of a SIMS profile was considered. An analytical model was presented to explain the preferential influence of roughness on broadening of the abrupt profile edge.

The influence of modified layer formation on SIMS profile depth positioning was investigated experimen- tally. It was shown on the example of profiles obtained with an N,' primary beam that the swelling of the modified layer created by the implantation of primary particles into the sample causes a shift of the whole profile as such towards the surface.

Acknowledgements

The authors would like to thank D. J. Eaglesham for kindly providing the samples.

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