SURFACE AND INTERFACE ANALYSIS, VOL. 14, 771--780 (1989)

Secondary Ion Yield Matrix Effects in SIMS Depth Profiles of Si/Ge Multilayers

Greg Gillen* and John M. Phelps Center for Analytical Chemistry, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

Randall W. Nelson and Peter Williams Department of Chemistry, Arizona State University, Tempe, AZ 85287, USA

Steven M. Hues Chemistry Division, Code 61 70, Naval Research Laboratory, Washington, DC 20375-5000, USA

Thin multilayer samples of Si/Ge, with individual layer thicknesses of 4-33 om, have been analyzed by secondary ion mass spectrometry (SIMS) using AT', 0,' and Cs' primary ion beams. Bombardment with both Ar' and 0,' produced positive secondary ion depth profiles in which pronounced distortions were observed. Similar effects were found in negative secondary ion depth profiles with Cs* bombardment. In each case, the SIMS depth profiles were characterized by abrupt interfacial secondary ion signal variations and a shift in the secondary ion signal maxima indicating that the layers were superposed, a condition that was not consistent with sample preparation, as verified by Auger electron spectroscopy. Auger electron spectroscopy depth profiling was also used to quantify the level of oxygen in the films. From these data it was concluded that the distortions in the positive secondary ion depth profiles under Ar' bombardment were the result of secondary ion yield variations induced by enhanced incorporation of ambient oxygen, during sample preparation, into the stronger oxide-forming silicon layers. Under 0,' and Cs' bombardment, the profile distortions were introduced by differential incorporation of the implanted primary species into the lower-sputter-yield silicon layers.

INTRODUCTION

Secondary ion mass spectrometry (SIMS) is used exten- sively for the analysis of metal and semiconductor devices. Of particular importance are compositional depth profiles in which the secondary ion signals of one or more elements are monitored as a function of sput- tered depth. Unfortunately, interpretation of SIMS depth profiles is often complicated by distortions resulting from various instrumental, sputtering or sec- ondary ion emission phenomena (for general reviews, see Refs 1 4 ) . A detailed understanding of these effects is necessary to separate secondary ion yield variations from true variations in the elemental composition of the sample.

Thin multilayer structures are currently of consider- able interest because of their unusual mechanical properties and their application as x-ray mirrors,' quantum well and heterojunction lasers,6 potential optical storage media,7 and depth profiling stan- dards. '~~

In this paper, we present studies of secondary ion yield variations in SIMS depth profiles of thin multi- layer structures. Because significant interfacial second- ary ion signal variations have previously been observed in SIMS depth profiles of Si/Ge multilayers,'O~ll we have examined this system with emphasis on identifying the effect of secondary ion yield variations resulting from :

* Author to whom correspondence should be addressed.

01 42- 242 1 /89/11077 1-10 $05.00 (c) 1989 by John WIley & Sons, Ltd.

(1) differential incorporation of reactive species, such as oxygen, into the films during preparation;

(2) variations in the concentration of implanted reactive primary ions resulting from sputter yield changes among the multilayer components.

In addition, we are also interested in characterizing the influence of various experimental parameters, such as primary ion impact energy, on the observed SIMS depth profiles. Auger electron spectroscopy (AES) depth profiling has been used to verify the as-prepared sample composition and to quantify the oxygen levels in the films.

EXPERIMENTAL

A Commonwealth Scientific (Alexandria, VA) Millatron sputter deposition system? was used for preparation of the Si/Ge multilayer samples. Ar + ion bombardment was used to sputter alternating layers of each material onto 2.5 x 7.6 cm slices of a silicon (100) wafer. The germanium (Cerac Inc., Milwaukee, WI; 5.08 cm dia- meter disk) and silicon (7.6 cm diameter silicon wafer) targets had a purity of at least 99.9%.

t Certain commercial equipment, instruments or materials are iden- tified in this paper to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

Receiued 24 April 1989 Accepted 22 June 1989

772 G. GILLEN ET AL.

Table 1. Mulayer structure and thickness of individual layers in samples of Si/Ge

Multilayer Individual layer Sample structure thickness

Si,Ge, Si,Ge, Ge,Si,

7 layers Si/6 layers Ge 5 layers Si/4 layers Ge 4 layers Si/5 layers Ge

Si 4 nm, Ge 12 mm Si 9 nm, Ge 27 nm Si 11 nm, Ge 33 nm

Targets were mounted to the sample carousel with conductive silver paste and were completely masked when not being sputtered. The silicon substrates were mounted on a water-cooled, rotating planetary sample stage. Base pressure of the system (which was cryopumped) was better than 4 x Torr. During operation with argon, the chamber pressure increased to 2 x lo-* Torr. In a typical deposition cycle, the targets were first sputter-cleaned in situ with the samples masked. A 30 nm layer of silicon was deposited onto each silicon substrate to cover the native oxide layer. Alternating layers of each material were then deposited for the same period of time (typically 6-12 min per layer). Because sputter yields differ for the two target materials,I2 the use of the same deposition time resulted in germanium layers that are approximately three times thicker than the silicon layers.

Standard targets for this system are 12.7 cm diameter disks. To accommodate the smaller sputter targets used in this study, the primary beam spot size was reduced by the use of a physical aperture. Typical current den- sities were 500 pA cmP2. Stability of the primary beam during deposition of an individual layer was better than 5%.

The samples used in this study are listed in Table 1. Approximate individual layer thicknesses were deter- mined by measuring the depth of at least three sput- tered craters, produced during analysis, with the assumption that the germanium layer thickness is three times that of silicon.

To study effects resulting from contaminants intro- duced during sample preparation, one Si/Ge multilayer sample (Ge,Si,) was prepared in a dual-capillaritron ion gun sputter deposition system at Arizona State Uni- versity. This glass-walled, oil-diffusion pumped system had a base pressure in the Torr range. A full description of this system and the procedure for making multilayer samples is given elsewhere.'

Secondary ion mass spectrometry depth profiles were performed on a Cameca IMS-3F ion microscope using Ar', 0,' and Cs' primary ion beams. In this instru- ment, the sample potential is maintained at +4.5 kV for positive secondary ions or -4.5 kV for negative sec- ondary ions. The primary ion impact energy for 02+ and Ar' primary ion beams, with positive secondary ion detection, is variable from 2.0 to 10.5 keV. The Cs' primary ion impact energy, with detection of negative secondary ions, is variable from 9.5 to 14.5 keV. The primary ion beam nominally impacts the sample at 30" to the sample normal (fixed by the geometry of the system). However, for detection of positive secondary ions, the primary ion beam will be deflected by the sample p~ ten t i a l ' ~ to give an impact angle that varies from 37" (10.5 keV impact energy) to 64" (2.0 keV impact energy). Similarly, detection of negative second- ary ions results in attraction of the positively charged

ion beam and an impact angle of <3W. Typical primary currents were of the order of 100 nA. For all profiles, the primary beam was rastered over a 250 x 250 pm area. A circular aperture (field aperture) was used to limit the analyzed region to a 60 pm dia- meter area in the center of the sputtered crater. The depth of the sputtered craters was determined with a Tencor Alpha Step 2 surface profilometer.

Auger electron spectroscopy depth profiles were obtained with a PHI 660 Scanning Auger Multiprobe. Sputtering was conducted with a 3.0 keV Ar' primary ion beam rastered over a 4 x 4 mm area. The sputtered area was limited to a 2 x 2 mm square by masking with aluminum foil. The primary electron energy was 5.0 keV at a current of 0.75 pA. Concentration scales were determined using the relative sensitivity factor method."

RESULTS

The disparity in sputter yield between silicon and germanium'2 results in non-linear depth scales for the SIMS depth profiles. Therefore, all profiles will be dis- played as a function of time rather than depth.

Ar + SIMS depth profiling

A series of Ar' SIMS depth profiles of sample Si,Ge,, at impact energies of 2.5, 5.5 and 8.0 keV, is shown in Fig. 1. The positive secondary ion signals for 29Si and 70Ge were monitored as a function of time. As the primary ion impact energy is increased, the Ge' sec- ondary ion signal maxima shift toward the sample surface. At the highest impact energy, the Ge+ and Si' signal maxima overlap, giving the impression that the layers are superposed.

Calculation of the mean projected Ar+ range in both targets (using the transport of ions in matter program (TRIM)',), under the conditions used to generate the profiles in Fig. 1, indicated that the superposition of the Ge+ and Si' secondary ions signals occurs when the primary ion range, corrected for the appropriate inci- dence angle, is greater than the thickness of an individ- ual silicon layer.

In the 2.5 keV impact energy profile, an increase in the Ge+ secondary ion signal is observed at each inter- face. The magnitude of this Ge+ signal enhancement is greater at the Si/Ge interface than at the Ge/Si inter- face.

As the impact energy, and therefore the depth of the atomically-mixed zone, was increased, we also observed the expected decrease in depth resolution. This is mani- fested in the profiles as a reduction in the peak-to-valley ratio of the Si+ secondary ion signal and the loss of the lower intensity Ge' peak found at the Ge/Si interface.

As shown in Fig, 2, the Ar' SIMS depth profiles of sample Si,Ge, did not exhibit an observable shift in the apparent layer positions with increasing primary impact energy. However, at the lowest impact energy, two interfacial Ge+ signal spikes of approximately the same magnitude are observed at both the Si/Ge and the Ge/Si interfaces. The 5.5 and 8.0 keV profiles of the

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same sample do not exhibit these enhancements and suggest only a slight decrease in resolution as the impact energy is increased. The Sif secondary ion signal does not display interfacial variations, but there is a gradual increase in the maximum Si' signal within a silicon layer as we sputter from the surface to the last silicon layer (which would correspond to the first depos- ited silicon layer of the film). The Ge+ secondary ion signal spike observed in the first deposited germanium layer may be an artifact (contamination) introduced during preparation of the sample.

In comparison, Fig. 3 shows the Ar+ SIMS depth profiles of sample Ge,Si, as a function of impact energy. These profiles each exhibit significant interfacial enhancement of the Ge+ secondary ion signals, even at the highest impact energies. There is also a general increase in the Si+ secondary ion signal intensity across each silicon layer. However, as in the Ar+ profiles of sample Si,Ge, (Fig. 2), no observable shift in the appar- ent layer positions as a function of increasing primary ion energy is seen. It is interesting to note that, in both

samples, the projected primary ion range is significantly less than the thickness of an individual silicon layer.

Figure 4 shows a comparison of the 2.5 keV Ar' SIMS depth profiles of all three samples in which we have monitored the secondary ion signals of Si', Ge+ and 0'. The data are plotted on a logarithmic scale. Each of these SIMS profiles demonstrates high 0' sec- ondary ion signals, with the highest oxygen localiza- tions occurring in the silicon layers. It is not clear from these data whether the oxygen concentration in the silicon is sufficient to influence the Si+ or Ge' second- ary ion yields.

AES depth profiling

Auger electron spectroscopy sputter depth profiles (obtained with 3.0 keV Arf bombardment) showing the atomic concentration of germanium, silicon and oxygen as a function of sputter time are shown in Fig. 5. The AES profile for sample Si,Ge, (Fig. 5(a)) shows the expected sample periodicity (i.e. there is no shift in the AES signal maxima) and a peak oxygen concentration in the silicon of 22 at.%. Similarly, the profiles of sample Si,Ge, (Fig. 5(b)) and Ge,Si, (Fig. 5(c)) also show high localizations of oxygen in the silicon, with peak values of 4 and 20 at.%, respectively. The AES profiles from both Si,Ge, and Ge,Si, also demonstrate a gradual decrease in oxygen concentration from the first deposited layer to the last (surface layer). This indi- cates that the available oxygen in the deposition

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116 G. GILLEN ET AL.

chamber is reduced over time. Also, in the profile of sample Ge,Si, , there is an obvious variation in oxygen concentration across an individual silicon layer.

Because enhancement of positive secondary ion yields depend on a high power of the surface oxygen concen- tration," it is reasonable to suggest that the observed interfacial increases in the Ge + secondary ion signal (in the SIMS depth profiles) result from changes in the Ge+ secondary ion yield as the oxygen concentration varies between each of the silicon and germanium layers. The apparent shift in layer positions observed in the higher- impact-energy Ar + SIMS depth profiles of sample Si,Ge, also appears to be related to these variations in oxygen incorporation. This will be explored in more detail in the discussion section.

rate for silicon. Also, oxygen forms a much stronger chemical bond with silicon than it does with germa- nium." This would act to increase the sticking coefi- cient for oxygen on silicon relative to germanium.

The gradual decrease in oxygen concentration from the first to the last deposited silicon layers suggests a decrease in the desorption rate of oxygen and oxygen- containing species induced by heating of the targets, target holders and primary beam aperture during the deposition of the first layers. Alternatively, the reduction in oxygen concentration across the sample may result from gettering of the residual oxygen by silicon, which is typically sputtered throughout the sample chamber during sample preparation.

O2 + SIMS depth profiling Origin of oxygen incorporation

It is of interest to determine the source of the oxygen contamination in the silicon layers. The nominal purity of the silicon wafer target is sufficiently high that it is unlikely that the oxygen found in the multilayer films was sputtered from the target wafer. Surface oxide con- tamination on the target was removed by in situ sputter-cleaning before the layers were deposited. The most likely explanation for these observations is that the oxygen is adsorbed from the residual gas onto the silicon layers during sample preparation. The high localization of oxygen in the silicon layers is not unex- pected, considering the factor of three slower deposition

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Figure 9. (a) 5.5 keV and (b) 8.0 keV "O,+ SIMS depth profiles of sample Si,Ge,. The "0-containing primary beam is used to evaluate the effect of primary ion implantation on the observed profiles. Si+ (- - -), Ge+ (-) and "0' (. . .) secondary ions were monitored as a function of time. The "O+ curve in (a) was multiplied by a factor of 200, and the "O+ curve in (b) by a factor of 100.

0, i. primary ion bombardment of multilayer sample Si,Ge, produced SIMS depth profiles that are very similar to those produced under Ar + bombardment. Figure 6 shows SIMS depth profiles of this sample, using 02+ primary ions at impact energies of 2.5, 5.5 and 8.0 keV.

The 2.5 keV 02+ profile shows interfacial Ge+ peaks at both the Si/Ge and Ge/Si interfaces. These peaks are of approximately the same magnitude as observed in the Ar+ profiles, with the Si/Ge peak again being larger. As the primary impact energy is increased, the SIMS profiles exhibit an energy-dependent shift in the Ge+ secondary ion signal maxima, similar to that seen under

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Figure 10. (a) 5.5 keV and (b) 8.0 keV "Oz+ SIMS depth pro- files of sample Si,Ge, . The "0-containing primary beam is used to evaluate the effect of primary ion implantation on the observed profiles. Si+ (---), Ge+ (-) and '*O+ (. . .) secondary ions were monitored as a function of time. The Si+ and l80+ curves in both (a) and (b) were multiplied by a factor of 5.

SECONDARY ION YIELD MATRIX EFFECTS 717

Arf bombardment. Also, the smaller Ge' peak at the Ge/Si interface is no longer present. Slightly greater impact energies (10.5 k e y were required under 0,' bombardment to achieve superposition of the Si' and Ge' secondary ion signal maxima. This difference may reflect the decrease in the straggle of the oxygen project- ed range distribution.

In contrast to sample Si,Ge, , 0,' bombardment of both Si,Ge, and Ge,Si, generated SIMS depth profiles that are markedly different from the Ar' SIMS depth profiles of the same samples. Figure 7 shows 0,' depth profiles of sample Si,Ge, at impact energies of 2.5, 5.5 and 8.0 keV. In general, the peak-to-valley ratios of both the Ge' and Si' secondary ion signals are greater than in the Ar' profiles, reflecting the improvement in depth resolution that is expected under oxygen bom- bardment (resulting from a reduction in ion bombardment-induced surface roughness). Each of the profiles in Fig. 7 also exhibits an increase in the Ge' secondary ion signal at each interface, with a higher transient at the Si/Ge interface. The magnitude of the Ge' signal spike at the Si/Ge interface is significantly higher than in the Ar' profile data for the same sample. Additional structure is also observed in the Ge+ sec- ondary ion signals at the Si/Ge interface as the primary energy is increased. Even more apparent is the increase in the Si+ secondary ion signal as each individual silicon layer is sputtered. This variation is most pro- nounced at the 5.5 and 8.0 keV primary ion energies.

Similar effects were observed in sample Ge,Si,, as shown in Fig. 8. However, in the first few germanium layers of the 2.5 keV profile, we also observed a down- ward transient in the Ge' secondary ion signal before the Ge/Si interface was reached.

'*02+ SIMS depth profiling

Use of 02+ primary ions would not be expected to produce distortions in positive secondary ion depth profiles unless there are significant variations in the concentration of primary beam oxygen implanted into the different materials, or the ratio of the primary ion range to the layer thickness is such that the implanted oxygen concentration could not build up to a steady- state level within the depth of an individual layer.

To study the implantation of oxygen into the multi- layer samples during analysis, SIMS depth profiles were performed using a 1 8 0 2 + primary beam while monitor- ing the secondary ion signals of "0, ,'Si and ',Ge. Profiles of sample Si,Ge, are shown in Fig. 9 at impact energies of 5.5 and 8.0 keV. The Ge' and Si' secondary ion signals exhibit the same signal variations observed in the previous oxygen profiles. However, in this case the effects of primary beam implantation can be seen clearly. The "0' signal shows an increase across each silicon layer, reaching a maximum at the Si/Ge inter- face. As the sputtering front penetrates into a germa- nium layer, the "O+ signal shows a rapid decrease. A steady-state level of implanted is not reached within an individual silicon layer. In these profiles, we have also observed a slight increase in the l60' second- ary ion signal (which for clarity was not plotted in Fig. 9). The increase in l60+ signal is believed to result from implantation of ',0,', which is incompletely resolved from the 1 8 0 , ' beam in the primary mass filter of the

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Figure 11. (a) Cs+ SIMS depth profile of sample Si,Ge, at an impact energy of 14.5 keV. Si- (- - -) and Ge- (-) secondary ions were monitored as a function of time. (b) Cs+ SIMS depth profile of sample Si,Ge, at an impact energy of 5.5 keV. Si+ (- - -) and Ge+ (-) secondary ions were monitored as d func- tion of time. The Ge- curve in (a) was multiplied by a factor of 3, and the Si+ curve in (b) by a factor of 4.

Cameca IMS-3F. The large spike in the Ge' secondary ion signal at the Si/Ge interface tracks'the increase in implanted oxygen concentration. As the primary energy is increased, the "0' secondary ion signal increases and the profile distortions (especially in the Ge+ signal) become more pronounced, as observed in the previous l60,+ profiles.

primary ion depth profiles were also con- ducted on sample Si,Ge,, as shown in Fig. 10 for impact energies of 5.5 and 8.0 keV. These profiles indi- cate that the concentration of implanted "0, when compared to the level of oxygen previously introduced into the sample during preparation, is not sufficient to influence the secondary ion yields of the matrix species.

Cs+ SIMS depth profiling

Analogous to the effect of oxygen on positive secondary ion yields, it has been shown that negative secondary ion yields depend on a high power of the surface cesium concentration.'' No significant cesium incorporation was observed in the as-prepared multilayer films. Similar to oxygen bombardment, the disparity in sputter yield between silicon and germanium should allow higher levels of implanted cesium to build up in the lower-sputter-yield si l i~on. '~ The negative second- ary ion depth profiles would, therefore, be predicted to exhibit the same types of secondary ion yield variations in the Ge- secondary ion signals as observed in the positive secondary ion depth profiles under oxygen bombardment.

778 G. GILLEN ET AL.

In the Cameca IMS-3F, the use of a Cs+ primary ion beam with detection of negative secondary ions requires the use of a negative sample potential. The attraction of the positive ion beam to the sample results in a decrease in primary beam impact angle and increases the minimum primary ion impact energy that can be achieved to 9.5 keV. The combination of the increased impact energy and closer-to-normal impact angle results in an increase in the atomically mixed depth, making resolution of individual layers in a sample such as Si,Ge, difficult. Profiles of this sample are poorly resolved and the Ge- and Si- secondary ion signals were completely superposed.

Better SIMS depth profiles were obtained from sample Si,Ge,. Figure ll(a) shows a 14.5 keV Cs+ profile of Si,Ge, in which the negative secondary ions of "Si and 70Ge were monitored as a function of time. As predicted, we observed the same types of profile dis- tortions seen in the oxygen profile data, with a similar increase in the Ge' secondary ion signal at each inter- face. It appears that the implanted Cs concentration is building up preferentially in the silicon layers, although we cannot directly confirm this because cesium does not form a negative secondary ion of sufficient intensity.

If the polarity of the sample is changed, and positive secondary ions are monitored, the profile in Fig. 1 l(b) is obtained. This profile exhibits the correct periodicity because positive secondary ion yields are insensitive to the local concentration of implanted cesium. However, we would expect positive secondary ion yields under Cs+ bombardment to be enhanced by the intrinsic oxygen level in the multilayer sample. In Fig. ll(b), the Si+ secondary ion signal is clearly enhanced at the surface. Also, there is an increase in the Si' signal at the end of the profile. Similar results are obtained for Ar' bombardment of the same sample, as shown in Fig. 2(b). In both cases, the Si' secondary ion signal follows the variations in the intrinsic oxygen level, as indicated in Fig. 4(b).

DISCUSSION

Differential incorporation of residual oxygen

One of the more interesting observations from both the Auger and Ar+ SIMS depth profile data presented in the previous sections is the periodic incorporation of high levels of oxygen in the silicon layers of the films. This periodic variation in oxygen concentration results in Ar+ SIMS depth profiles of Si/Ge multilayer samples in which the Ge' secondary ion signals are significantly enhanced at each interface. For thinner Si/Ge multi- layer samples and higher primary impact energies (i.e. when the primary ion range is greater than or equal to the thickness of one silicon layer), Ar' SIMS depth pro- files are obtained in which the secondary ion signals of both Si' and Ge' are superposed.

Preferential incorporation of oxygen into the silicon layers was explained earlier on the basis of the strong oxide-forming ability of the silicon.18 Other samples that we have examined, including Cu/Ni and Cr/Ni multilayers," in which one species forms a stronger

oxide bond, have also exhibited significant incorpo- ration of oxygen in the stronger oxide-forming species. For Cu/Ni multilayer samples, the weaker oxide- forming Cu (analogous to the germanium in the Si/Ge samples) is the species that exhibits significant inter- facial secondary ion signal transients. Also, the Cu/Ni samples exhibit similar superposition of the Cu' and Ni ' secondary ion signals at higher impact energies, when the primary ion range was approximately equal to the thickness of an individual Ni layer. This behavior may be typical of Ar' SIMS depth profiles of multilayer systems, in which the components have significantly dif- ferent oxygen affinities.

Oxygen-concentrationdependent secondary ion yield enhancement

For a matrix such as silicon or germanium, the magni- tude of oxygen-induced secondary ion yield enhance- ment varies with the local oxygen concentration. Wittmaack has demonstrated that, for oxygen implanted into silicon at concentrations < 3.0 at.%, the secondary ion yield enhancement of Si' was directly proportional to the oxygen concentration.' He further demonstrated that at local oxygen concentrations between 3.0 and 35.0 at.%, the yield enhancement of Si' was proportional to a power law of the oxygen con- centration (power as high as 3.7).17 This suggests that the higher the oxygen incorporation into our films, the greater the magnitude of the secondary ion signal varia- tions.

Comparing the 2.5 keV Ar' SIMS depth profiles of all three multilayer samples (Fig. 4), it is observed that for the two samples in which the local oxygen concen- trations were highest (Ge,Si,, 20 at.% and Si7Ge6, 22 at.%), the interfacial signal variations are significantly higher than those found in the SIMS depth profile of Si,Ge, , which contained < 4.0 at.% oxygen. Addi- tionally, in contrast to the other profiles, the profile of Si,Ge, exhibits interfacial enhancements of the Ge +

secondary ion signal only at the 2.5 keV impact energy. The loss in signal enhancement at the higher energies could be due to a loss in depth resolution, but may also result from cascade dilution of the oxygen in the inter- facial region, thus reducing the local concentration to a level that does not influence sputtered secondary ion yields.

Primary-energydependent shift in Ge' secondary ion signal

The apparent shift in the Ge+ secondary ion signal as a function of primary ion impact energy is shown in Figs 1 and 6. This shift is observed for both Ar+ and 02+ primary bombardment of sample Si7Ge6 and is attrib- uted to an increase in primary ion range, coupled with the oxygen-induced secondary ion yield enhancement. As stated earlier, the apparent shift only occurs when the primary ion range is approximately equal to or greater than the thickness of an individual silicon layer. This shift may develop in the following fashion. At a primary ion energy of 2.5 keV, we begin sampling silicon and high levels of oxygen in the first silicon

SECONDARY ION YIELD MATRIX EFFECTS 119

layer. As the sputtered front penetrates into the first ger- manium layer, germanium secondary ions will begin to be detected. The germanium concentration at this point may be quite low; however, the Ge' secondary ion signal appears artificially high because of the ion yield enhancement from the 10-20 at.% oxygen in the mixed zone. As the sputtering front penetrates into the middle of the germanium layer, the atomic concentration of germanium will be highest and the silicon and oxygen will be rapidly decreasing. However, the germanium ion yield, which was artificially enhanced, will also begin to fall off, tracking the variation in oxygen. As the sputter- ing front penetrates the next silicon layer, the germa- nium concentration rapidly decreases, but is nevertheless enhanced again by the locally high oxygen concentration being added to the mixed zone. As the sputtering front penetrates fully into the silicon layer, the oxygen concentration is highest, but the atomic con- centration of germanium is very low and the Gef signal falls off again. This process then continues throughout the subsequent layers. As the primary ion impact energy is increased, a similar process occurs, except that as soon as the first silicon layer begins to be sputtered, the germanium is also sampled, because of the greater depth of the mixed zone, and the signal maxima appear superposed. This is strictly an ion yield variation, with the germanium secondary ion yield tracking the varia- tion in oxygen as modified by the primary impact energy (nature of the mixed zone). As demonstrated by the AES profiles, the elemental composition of the samples is consistent with sample preparation.

an extent that secondary ion yields were significantly affected.

In the thicker samples (Si,Ge4 and Ge,?,), the implanted oxygen concentration exceeded the intrinsic level of oxygen previously introduced into the film during preparation, and also introduced oxygen con- centration gradients across individual silicon layers. As seen from the "0 experiments, the additional peaks and the variation in the Si' signal appear to track the variations in implanted oxygen across a layer.

In addition to the effects of sputtering yield on the build-up of implanted oxygen, additional variations in the concentration of the primary ion species may be introduced by stopping power transients at interfaces? As is seen in Fig. 8, and to a lesser extent in Fig. 7, there is a downward transient in the Ge+ secondary ion signal before the interface is reached. Such a process has been identified by Williams and Baker4 as being due to stopping power variations at interfaces of dissimilar masses, which result in a transient decrease in back- scattered primary ions and a decrease in the local con- centration of the primary species.

The Si/Ge negative secondary ion depth profiles for samples under Cs + bombardment also exhibited effects similar to the oxygen profiles, which we attribute to the build-up of implanted primary beam cesium into the lower-sputter-yield silicon in a fashion analogous to oxygen bombardment with detection of positive second- ary ions.

CONCLUSION

Implantation of reactive primary beam species

Oxygen SIMS depth profiles exhibited distortions similar to the Ar' depth profiles, including the energy- dependent shift in the Ge' signal observed in sample Si,Ge, . However, additional distortions are observed which we attribute to differential implantation of oxygen into the silicon layers of the sample.

It has been shown for a number of matrices21922 that the steady-state concentration of implanted primary beam species is proportional to 1/S, where S is the sputter yield of the matrix. Variations in sputter yield between multilayer component species, such as the factor of three found in the Si/Ge system, would be expected to result in higher incorporation of primary beam oxygen into the lower sputter yield species (silicon).

It has also been demonstrated that the implanted beam concentration continues to build up, in most matrices, as the sputtering front moves into the sample, reaching a steady state only after erosion of a depth approximately equal to the primary ion range.21 For the thicker samples, at higher impact energies, a steady- state concentration of implanted oxygen is not achieved over the depth of a single silicon layer.

The experimental profiles of Si,Ge, under 02' bom- bardment and the l80 experiment are shown in Figs 6 and 10, respectively. These profiles indicate that the implantation of primary beam oxygen did not increase the total oxygen concentration in the multilayer to such

One of the major difficulties in the interpretation of SIMS depth profiles is the susceptibility of sputtered secondary ion yields to variations in the chemistry of the sputtered surface. These chemical variations can result in depth profiles that are difficult to rationalize in terms of the original sample composition, assuming that it is known. Any depth profile is, therefore, suspect unless any potential distorting effects in a given system are characterized as completely as possible.

In this study we have examined such effects in Si/Ge multilayer systems. This system is particularly prone to secondary ion yield transients for two reasons :

because the slower sputtering rate and strong oxide- forming ability of silicon, relative to germanium, results in significant adsorption of residual oxygen during preparation (even at low deposition system base pressures); because the mass difference between silicon and ger- manium produces large variations in stopping power and sputter yield across the sample, resulting in a varying concentration of reactive primary species implanted into the sample during analysis.

If the multaayers being analyzed are su6ciently thin, such that the interfacial signal enhancements are not resolved from the bulk layers, these effects will result in secondary ion signal intensities that are primarily modulated not by concentration changes but by sec- ondary ion yield variations. This process may result in SIMS depth profiles that are inconsistent with the true elemental composition of the sample.

I80 G. GILLEN ET AL.

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