characterization of shallow implants with sims using electron-beam-assisted oxygen bombardment with...

SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 26, 851È860 (1998)

Characterization of Shallow Implants with SIMSusing Electron-beam-assisted OxygenBombardment with Oxygen BackÐll¤

M. Puga-Lambers1,* and P. H. Holloway1,21 MICROFABRITECH, University of Florida, Gainesville, FL 32611, USA2 Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA

Secondary ion mass spectrometry (SIMS) depth proÐles of shallow implants of boron and arsenic in silicon havebeen measured. It was demonstrated that improved depth resolution was achieved at low (1.5 keV) beam energy forboth and Cs‘ beams. Oxygen backÐll from the base pressure (10—10 Torr) to 10—6 Torr also improved theO

2‘

depth resolution. Simultaneous electron bombardment during oxygen backÐll further improved the depth resolution,as measured both by the surface transients in the Si‘ and SiO‘ substrates as well as the B‘ signal. The mechanismof improvement by electron bombardment during oxygen backÐll was discussed and concluded to be electron-beam-stimulated oxidation of the Si surface. 1998 John Wiley & Sons, Ltd.(

KEYWORDS: SIMS; shallow implants ; oxygen backÐll ; electron beam irradiation ; electron-beam-stimulated oxidation

INTRODUCTION

Characterization of shallow features in integrated cir-cuits is currently one of the major challenges to theSIMS community. For several decades, SIMS has beenone of the most indispensable techniques for quantiÐca-tion of low-level impurities and dopants in semicon-ductor materials due to its high sensitivity and excellentdepth resolution. However, the incessant shrinkage ofsemiconductor device features to well below 1 lm hasplaced new demands on the development of SIMS hard-ware and analytical protocols in order to ensure sub-nanometer depth resolutions with proÐle accuracy inthe near-surface region.1,2

Shallow doping is formed mainly by low-energy ionimplantation. Implant energies of \5 keV and, mostrecently, in the sub-keV range are used for boron.Arsenic is being implanted at energies of O10 keV. Thismeans that most of the implanted dose is located within20È30 nm of the sample surface. For example, fromTRIM calculations, a 11B` implant into Si (100) at 5keV has a projected range of 23.3 nm, and a 10 keV75As` implant into Si (100) yields a projected range of12.1 nm. The depth resolution of SIMS shallowimplants may therefore be limited by the penetrationdepth of the primary ion beam. For example, the depthresolution limit has been reported to be D2.5 times theprojected range of the SIMS primary ions in thesample.3 Cascade mixing and knock-on e†ects causedby the primary ion impact tend to induce proÐlebroadening, which lead to distortion of the measured

* Correspondence to : M. Puga-Lambers, MICROFABRITECH,University of Florida, Gainesville, FL 32611, USA.

¤ Dedicated to Professor Siegfried Hofmann on the occasion of his60th birthday.

dopant depth distribution. Cascade mixing e†ects canbe minimized by using heavier primary ions, by lower-ing the primary ion energy and by increasing the inci-dence angle relative to the sample normal up to 60¡.4,5These conditions signiÐcantly improve depth resolution.

In addition to depth resolution limitations, transientchanges in ion yields and sputter rates occur in thenear-surface region during the initial period of sputter-ing. This transient condition is associated with pertur-bation of the sample by the primary ion beam, and withmatrix e†ects caused by the presence of a native surfaceoxide on the sample. Both changes cause an error inquantiÐcation of shallow depth proÐles.6 Particularlyfor shallow implants, the pre-equilibrium region maycontain a signiÐcant percentage of the implanteddopant species, thus any analysis that excludes thisregion will lead to an incorrect dose calibration. Conse-quently, the use of sufficiently low SIMS primary ionbeam energies (\ 3 keV) is also critical towards estab-lishing a rapid equilibrium condition between theprimary ion and the samples surface. Recent studieshave suggested that a primary ion beam energy of lessthan half the implant energy should be applied forshallow boron proÐling with a primary oxygen beam.7This low energy poses difficulties for some instrumentsin obtaining a well-focused primary ion beam and acurrent density beam high enough to ensure good detec-tion sensitivity and detection limits.

Some analytical strategies have been devised to over-come these problems, particularly for boron shallowproÐle measurements in silicon. A combination of lowprimary ion energy at oblique impact angles (60¡) andoxygen backÐll during depth proÐling of boron insilicon with an oxygen primary beam has been reportedto improve depth resolution and minimize the surfaceion yield transient.2,8 This is due to the continuous for-mation of a thin surface oxide layer during sputtering,which stabilizes the ionization yields and allows for a

CCC 0142È2421/98/110851È10 $17.50 Received 14 May 1998( 1998 John Wiley & Sons, Ltd. Revised 24 July 1998

Accepted 27 July 1998

852 M. PUGA-LAMBERS AND P. H. HOLLOWAY

constant matrix to be reached within a few from theÓsurface.2,9 It was demonstrated that this technique pro-vided uniform sensitivity across the interfaceSiO2/Siand, consequently, should yield reliable quantiÐcationof the implanted dopant.2 However, the accuracy of theoxygen backÐll technique has been challenged, based onthe fact that it does not provide an instantaneous fulloxidation and therefore is not sufficient to eliminatetransient e†ects altogether.6 Indeed, it has been report-ed that before full oxidation is achieved, a fraction ofthe outermost surface material is sputtered away at dif-ferent sputter rates due to oxygen adsorption inducedby exposing the surface to gas.10 The variation insputter rate across this transient region results in signiÐ-cant distortions of the proÐle shape, and subsequentlycauses errors in the depth scale calibration.11,12 Forexample, boron proÐles shifted towards the surface byP4 nm have been observed during oxygen bombard-ment with oxygen backÐll, with the amount of proÐleshift depending not only on the beam conditions butalso on the oxygen pressure.6,13 An alternativeapproach towards eliminating transient e†ects is toencapsulate the surface with a thin layer of amorphoussilicon.14 In this method, the primary ion beam equi-librium is established while bombarding the cap layer,but ion yields and sputter rates still vary through theunderlying native oxide and the substrate. It has beenshown that the enhancement e†ects of the native oxidecan be reduced substantially by using an oxygen backÐllin conjunction with the amorphous cap layer.15 Even so,this method su†ers from some drawbacks, whichinclude the presence of extrinsic impurities trapped atthe interface, difficulty in identifying the original surfaceposition and extra sample preparation steps.16

This paper presents SIMS depth proÐle resultsobtained for shallow arsenic and boron implantswithout using the encapsulation procedure. Thepurpose of this study was twofold : to evaluate thee†ects of primary ion mass and beam energy on depthresolution and peak shape while assessing the lowestpractical energy that we could use with our quadrupoleSIMS system; and to investigate the possibility of elimi-nating the surface transient by applying an electronbeam in conjunction with oxygen backÐll duringprimary oxygen bombardment. A 0.5 keV boronimplant in silicon was used to test this hypothesis. Thisapproach is based on the idea that oxidation is stronglystimulated by electron bombardment.17,18 Severalstudies19h22 have been reported on electron-stimulatedoxidation of Si during oxygen exposure. In one recentreport,22 Auger electron spectroscopy clearly demon-strated the presence of SiÈO bonding at electron-beam-irradiated Si surfaces. Electron bombardment is knownto dissociate oxygen originally adsorbed in molecularform, leading to the formation of An assess-SiO2 .23ment to the e†ectiveness of this method is presented,based on the width of the near-surface transient region.

EXPERIMENTAL

The samples used in this work were an 8 keV/3] 1015cm~2 75As` implant into Si, a 5 keV/5] 1015 cm~211B` implant into Si and a 0.5 keV/1] 1015 cm~2

11B` implant into Si. A silicon sample uniformly dopedwith 1 ] 1019 cm~3 11B` was used for calibration ofthe 0.5 keV 11B` implant with an oxygen backÐll to10~6 Torr. The arsenic and boron depth proÐles in Siwere obtained with a quadrupole Perkin-Elmer 6600PHI SIMS system. A cesium primary ion beam wasused for the analysis of arsenic in silicon, whereasoxygen bombardment was used for the boron analysis.For comparison, boron proÐles were also acquired withthe Cs` beam. The angle of incidence of either of theprimary ion beams with respect to the sample normalwas 60¡. Both cesium and oxygen primary ion beamenergies were varied from 5 keV down to 1 keV.Primary ion beam currents were set within the range200È20 nA and the raster size was varied between 350and 650 lm, with a 70% gating to reduce crater edgee†ects and to maximize sensitivity. The sputter rateswere determined from crater depth measurements per-formed with a Tencor Alpha-Step 500 surface proÐlerafter the SIMS measurements.

An oxygen backÐll in the main chamber from 10~8Torr up to 10~6 Torr was used during analysis of the0.5 keV 11B` implant with the oxygen primary beam. A60¡ incident electron beam was sometimes appliedsimultaneously with the oxygen backÐll during oxygensputtering in order to investigate the e†ects of electron-beam-assisted oxidation on the transient region. Theelectron beam used for stimulated oxidation was set toproduce a 2 keV beam of 105 nA. The width of the areascanned by the electron beam was 1000 lm.

During cesium bombardment, boron and arsenicwere monitored using 39BSi~ and 103AsSi~, respec-tively. Mass 11B` was followed for boron analysisduring oxygen bombardment. Silicon and the primaryoxygen beam were followed with 30Si, 44SiO and58Si2 ,16O. QuantiÐcation of the SIMS proÐles was accom-plished by processing the raw data into concentrationvs. depth using PHI-Matlab data processing software.

RESULTS AND DISCUSSION

The e†ect of primary Cs` beam energies on As depthproÐles is shown in Fig. 1. The As depth proÐles wereacquired for Si implanted with an 8 keV As at a dose of3 ] 1015 cm~2. It can be seen that proÐle broadeningdecreased as the primary beam energy was reducedfrom 5 to 1.5 keV. Better proÐle depth resolution atlower sputtering energies was expected and observed.4,5Surprisingly, the As proÐle at 1 keV showed depthresolution similar to the 1.5 keV proÐle, and at concen-trations below 1 ] 1020 cm~3 it broadened substan-tially. This may be due to either a poor primary ionbeam shape at 1 keV or sputter-induced roughening.Increasing electron gating and raster sizes to excludecrater edge e†ects did not improve depth resolution.The width of the surface transient region before steady-state conditions are established (not shown) decreasedby D8 nm when the Cs` beam energy was reducedfrom 5 to 1.5 keV. It is well known that lowering theprimary ion beam energy decreases not only cascademixing e†ects but also the width of the surface tran-sient.24 The depth of the As peak [in Fig. 1(a)] was 10nm for all primary beam energies, which was beyond

Surf. Interface Anal. 26, 851È860 (1998) ( 1998 John Wiley & Sons, Ltd.

SIMS OF SHALLOW IMPLANTS WITH OXYGEN BACKFILL 853

a

b

Figure 1. The 103AsSiÉ depth profiles from an 8 keV implant in Si for various Cs½ primary SIMS beam energies : (a) complete depth profile ;(b) peak region.

the transient depth measured under these conditions, asdiscussed below. The location of the As peak is in agree-ment with the projected range of 10.7^ 3.7 nm esti-mated from TRIM calculations. In Fig. 1(b), the peak inAs concentration sharpened and increased gradually inconcentration with decreasing energy (except at 5 keV).

The maximum concentration was observed to be2.5] 1021 cm~3 for a 1.5 keV primary beam. Thisimprovement in depth resolution results from adecrease in the surface transition region. Thus, the bestdepth resolution was achieved with 1.5 keV Cs`, whichis well below the implant energy.

( 1998 John Wiley & Sons, Ltd. Surf. Interface Anal. 26, 851È860 (1998)


Figure 2(a) shows the SIMS boron depth proÐlesobtained from Si implanted with 5 keV boron to a doseof 5 ] 1015 cm~2 using an primary ion beam. ForO2`the purpose of comparison, the proÐles were alsoacquired with a Cs` primary ion beam [Fig. 2(b)]. Theboron peak was located at a depth of 22 nm for all

energy conditions (consistent with the projected rangeof 23.7 ^ 1.7 nm estimated from TRIM) and showed anapproximate concentration of 1.5 ] 1021 cm~3. Forboth Cs` and bombardment, reducing the primaryO2`ion beam energy slightly improved the depth resolution.This indicates that the beam conditions provided suffi-

a

b

Figure 2. Boron depth profiles of a 5 keV implant in Si at various energies using: (a) to detect 11B½ ; (b) Cs½ to detect 39BSiÉ.O2½



a

b

Figure 3. Secondary ion distribution of 30Si½ from the sample in Fig. 2 using (a) and Cs½ (b) at different energies.O2½

cient depth resolution for the proÐles. Boron proÐlesacquired at 4, 3 and 2 keV show similar depthresolution for both Cs` and whereas the 1 keVO2`,proÐle showed both poorer depth resolution and alower peak concentration than at higher energies. WithCs` bombardment, the 1 keV proÐle depth resolution

deteriorated at concentrations below 1020 cm~3. Usingan primary beam at 1 keV resulted in a poorerO2`depth resolution at concentrations \7 ] 1018 cm~3.This result is probably due to a poor primary ion beamshape at 1 keV or increased surface-induced roughnessby oxygen sputtering, which degrades depth



a

b

Figure 4. Comparison of boron depth profiles (a) and 30Si½ surface transient (b) obtained from a 0.5 keV boron implant in Si using O2½

and Cs½ at 1.5 and 1 keV.

resolution.25 The proÐles in Fig. 2 also show a changein the slope of the boron distribution at concentrationsof D1019 cm~3. This is due to channeling e†ects of theimplant into silicon, which increase the penetrationdepth of the implanted ions.

The location of the arsenic and boron maximumpeaks [Figs 1 and 2] is independent of sputteringenergy over all the primary ion beam energies used.This result was observed because the penetration depthof Cs` and primary ions was smaller than theO2`



Figure 5. Boron depth profiles of a 0.5 keV boron implant using electron-assisted bombardment in combination with oxygen backfill.O2½

depth of the implant proÐles. The penetration depth ofeither of the primary ion beams was estimated from theprimary energy and angle of incidence by using anequation derived in Ref. 24.

Figure 3 illustrates the surface transient of the siliconmatrix signal 30Si` at various energies obtained with

and Cs`, respectively, from the sample implantedO2`with boron at 5 keV 5] 1015 cm~2. The surfaceenhancement observed with sputtering is largerO2`than with Cs` sputtering, and increases with decreasingprimary ion energy because the native oxide is mixed toa deeper point with increased energy. The width of thetransition region decreased by D15 nm (from 25 nm to10 nm) as the energy was reduced from 5 keV to 1O2`keV [Fig. 3(a)]. For the same Cs` energy range, thewidth of the transition region only decreased by D3 nm(from 6 nm at 5 keV to 3 nm at 1 keV) [Fig. 3(b)]. Thisagrees with previous Ðndings26 in which a 2 keV Cs`beam at 60¡ produced a 2 nm deep transition regionsufficient to resolve a 5 keV boron implant in Si. Thesame energy resulted in a much deeper transitionO2`region width due to the lower mass of oxygen vs.cesium.

From the results presented in Figs 1È3, it is feasible touse beam energies down to 1 keV with our quadrupoleSIMS system, although this inherently resulted in a lossin maximum current and problems in focusing theprimary ion beam. Optimizing the beam shape at verylow beam energies required a substantial increase inraster size in order to minimize crater edge e†ects andto compensate for reduced dynamic range. Obviously,this resulted in longer analysis times. In order to investi-gate the possibility of enhanced oxidation by electronbombardment, a 0.5 keV boron implant in Si wasanalyzed with and Cs` using 1.5 and 1 keV beamO2`

energies. Although using sub-keV primary energieswould have been more appropriate to characterize a 0.5keV boron implant, our choice of conditions was dic-tated by instrumental constraints and sample avail-ability.

Figure 4(a) shows a comparison of the boron depthproÐles obtained from Si implanted with 0.5 keV boronto a dose of 1 ] 1015 cm~2 using or Cs` primaryO2`ion beams at 1.5 and 1 keV and at an incidence angle of60¡. It is quite clear that reducing the energy from 1.5 to1.0 keV substantially improved the depth resolution.Furthermore, proÐling provided a much betterO2`depth resolution than Cs`. This is true because underCs` bombardment there is a linear dependence betweenthe decay length and the penetration depth of primaryions, which exclusively determines the amount ofmixing.27 Therefore, stronger mixing (i.e. larger decaylengths) occurs when the penetration depth of theprimary ion exceeds the depth of the implant proÐle.24In the case of bombardment, the additionalO2`incorporation of oxygen into the Si matrix (swellingmodel) causes a change in the matrix density of thesurface region, leading to smaller decay lengths and,consequently, to improved depth resolution.28

The width of the transition region before the matrixsignal 30Si` reaches steady state is shown in Fig. 4(b).The transition region obtained with 1 and 1.5 keV O2`beams is D 5 nm deep. The peak of the boron implan-tation distribution was measured to be at a depth of 3nm [Fig. 4(a)], but this value may not be correct for tworeasons : the calculation of the sputter rate in Figs. 4(a)and 4(b) did not take into account the increased sputterrate in the surface oxide, which is about 2.2 times thatin Si ; and, as reported previously, when the penetrationdepth of the primary ion exceeds the projected range of



a

b

Figure 6. Comparison of 30Si½ (a) and 44SiO½ (b) surface transients from a 0.5 keV boron implant in Si for only bombardment or forO2½

simultaneous electron bombardment at different oxygen backfill pressures.

the implant, the peak position tends to shift towards thesurface due to continuous oxygen incorporation and,consequently, a variation in sputter rate within thesurface oxide region itself.6 Therefore, quantiÐcation ofthe boron depth proÐles illustrated in Fig. 4(a) is signiÐ-cantly a†ected by the ionization yield changes due to

oxygen incorporation and by the native surface oxide.Furthermore, wider transition regions result in largero†sets on the depth scale due to sputter rate di†erencesbetween (with x O 2) and Si. A 1 keV beamSiO

xO2`was used to analyze this sample under oxygen backÐll

conditions (5 ] 10~6 Torr) vs. base pressure



(5.5] 10~10 Torr). The SIMS proÐles [Fig. 5] alsoshow the e†ect of an electron beam under the sameoxygen backÐll conditions. Oxygen backÐll has beenused extensively to reduce surface transient e†ects, but asimple backÐll is not sufficient to instantaneously forma stoichiometric oxide.2,6,11,12 By applying electronirradiation simultaneous with ion sputtering, we stimu-lated the dissociation of molecular oxygen and theimmediate formation of SiO2 .

In Fig. 5, depth resolution improved as the pressurewas raised from 1.4 ] 10~7 to 5.4] 10~6 Torr. It wasimproved even further by an electron beam at backÐllpressures of 3.5 ] 10~7 and 2.8 ] 10~6 Torr. Thedynamic range of the boron proÐles measured with highoxygen pressure was reduced as the oxygen pressurewas raised above the base pressure. This was observedboth with and without an electron beam; however, theelectron beam tended to reduce this undesirable e†ect.Increased raster sizes to exclude crater edge e†ects didnot improve the dynamic range during oxygen backÐll,in agreement with previous reports.24 In addition, pro-Ðles measured shortly after the system recovered backto a base pressure of 10~10 Torr still exhibited lowdynamic ranges, even after 1 day of pumping. Thisreduced dynamic range has been attributed to a highboron background signal caused by memorye†ects.24,25 Memory e†ects have been attributed toincreased scattering of primary and secondary ionsunder oxygen backÐll conditions, a high boron stickingcoefficient on oxidized surfaces and higher ionizationyields.24 The boron background level was consistentlylower at all pressures when an electron beam wasapplied. Presumably, the electron beam reduces thememory e†ect by stimulating the conversion of boronto oxide with a reduced SIMS signal. In addition, theimproved depth resolution from simultaneous electronand ion bombardment may be inÑuenced by reducedion-beam-induced roughness. Previous studies29 haveshown that electron beam irradiation simultaneous withion beam bombardment suppresses ripple growth in Si.Wittmaack has reported30 that low-energy oxygenbombardment (0.5È2 keV) of Si at incidence anglesabove 40¡ during vacuum and oxygen backÐll condi-tions caused rapid surface roughening with ripple for-mation, which may be suppressed by the electron-beam-stimulated oxidation.

Depth resolution is frequently quantiÐed in terms ofthe decay length.2,7,24,31 Decay lengths were derivedover an order of magnitude drop along the trailing edgeof the boron proÐles in Fig. 5. They were larger thanthose reported previously.2,31 This discrepancy is prob-ably caused by ion-beam-induced roughening andmemory e†ects as discussed above, and to some extentby ion-beam-induced mixing and sample di†erences. Inaddition, it has been suggested14 that enhanced di†u-sion caused by primary ion-beam-induced damageduring boron analysis can cause the depth resolution todeteriorate.

The sputter rates determined from the proÐles shownin Fig. 5 decreased by a factor of three when oxygenwas backÐlled into the chamber up to a pressure of10~7 Torr. Between 10~7 and 10~6 Torr, the sputterrate remained constant, consistent with previousreports.6 Under identical backÐll conditions, no changesin sputter rate were detected when the sample was also

electron beam irradiated. A lower sputter rate duringoxygen backÐll results from lower sputter yields for Si,which have been reported to decrease by a factor of twoto three25,32 as the oxygen pressure was increased. Thetotal sputter yields of Si and were reported to levelSiO2o† at pressures above 10~6 Torr, which indicatessimilar compositions for the Si surface region andSiO2 .32

Figure 6 shows the surface transient signals of 30Si`and 44SiO`, respectively, as a function of depth undervarious oxygen backÐll conditions. The depth valueshave not been adjusted for sputter rate changes. At basepressure (not included in Fig. 6), the signals from 30Si`and 44SiO` reach their equilibrium levels after a tran-sient depth of 3È4 nm. As the oxygen pressure wasincreased, the matrix-speciÐc 30Si` transient width wassubstantially reduced, as expected, reaching D0.3 nm at10~7 Torr and 0.15 nm at 10~6 Torr. No signiÐcantreduction was observed in the transient depth for elec-tron beam irradiation. It was previously presumed thatfull oxidation was achieved almost instantaneouslyunder oxygen backÐll conditions, based upon the factthat the Si` signal reached equilibrium more rapidlywith an oxygen backÐll than at base pressure.32 Underoxygen backÐll conditions, the sample was saturatedwith oxygen, thus maintaining a constant ionizationyield.2 But the 44SiO` transient [Fig. 6(b)] onlyreached a stable level at much larger depths (2.5 nm at10~7 Torr and 2.0 nm at 10~6 Torr). This indicates thatfull oxidation was achieved at depths greater than thoseobserved with the 30Si` transient. By expanding thenear-surface area, a reduction of D 0.3È0.5 nm in thewidth of the transition region can be detected for elec-tron beam irradiation. This supports the fact that simul-taneous electron beam bombardment accelerates theoxidation process, in agreement with reports by Kirbyand Litchman.23

CONCLUSIONS

The SIMS depth proÐles of 8 keV arsenic and 5 or 0.5keV boron implants into Si (100) wafers were measured.It was shown that the depth resolution of the implantproÐle improved as the beam energy was reduced from5 keV to 1.5 keV for both and Cs` primary beams.O2`The depth resolution was not as good for 1.0 keVbeams, presumably because of trouble in focusing theion beam at this low energy. Oxygen backÐllingimproved the depth resolution but degraded thedynamic range of boron implants. Simultaneous elec-tron bombardment with depth proÐling improved thedepth resolution and maintained the dynamic range ofthe SIMS analysis. By analysis of both the 30Si` and44SiO` transients, it was demonstrated that the extentof surface oxidation was accelerated by simultaneouselectron bombardment.

Acknowledgements

Extensive discussions with Kevin Jones, Erik Kuryliw and Fred Stevieare gratefully acknowledged.



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