high-fluence co implantation in si, sio2/si and si3n4/si: part iii: heavy-fluence co bombardment...

High-¯uence Co implantation in Si, SiO2/Si and Si3N4/SiPart III: heavy-¯uence Co bombardment induced surface

topography development

Yanwen Zhang a,*, Thomas Winzell a, Tonghe Zhang b, Ivan A. Maximov c,Eva-Lena Sarwe c, Mariusz Graczyk c, Lars Montelius c, Harry J. Whitlow a

a Department of Nuclear Physics, Lund Institute of Technology, Box 118, S-221 00 Lund, Swedenb Institute of Low Energy Nuclear Physics, Beijing Normal University, Beijing 100875, People's Republic of China

c Department of Solid State Physics, Lund Institute of Technology, Box 118, S-221 00 Lund, Sweden

Received 25 March 1999; received in revised form 15 June 1999

Abstract

The surface topography development of Si(1 0 0), Si(1 1 1), oxide/Si and nitride/Si structures under high normal

¯uence (1 ´ 1016±2.6 ´ 1018 ions cmÿ2) keV Co metal vapour vacuum arc (MEVVA) irradiation has been investigated by

scanning electron microscopy (SEM). The results show that for normal ¯uences up to �1017 ions cmÿ2, the surface

topography remains ¯at. As the ¯uence increases, pores develop and grow to form a columnar structure. At even higher

¯uences the columns are eroded to form an acicular structure. Deposition of a silicon dioxide or nitride layer on the Si

surface leads to a signi®cant suppression of the onset ¯uence for the formation of a rough surface. The porous surface

could not be transformed to the network of acicular structures or a ¯at surface by high temperature annealing. Ó 1999

Elsevier Science B.V. All rights reserved.

PACS: 68.35.B; 68.55.J; 68.35.G; 82.65.D; 82.65.J

Keywords: Surface topography; Sputtering; Scanning electron microscopy (SEM); Silicon; Oxide; Nitride

1. Introduction

All ion±solid interactions during ion implanta-tion result in changes in the surface topography.

These changes can be either on an atomic ormacroscopic scale. The extent is governed by fac-tors such as ion ¯uence and energy, target crystalstructure and orientation, impurity inclusions,defects, etc. In microelectronic technology it is ofprime importance that the development of roughsurface topographies resulting from processing arerigorously controlled.

A key technological problem in the formationof thin silicide layers by ion implantation in Si has

Nuclear Instruments and Methods in Physics Research B 159 (1999) 158±165

www.elsevier.nl/locate/nimb

* Corresponding author. Present address: Division of Ion

Physics, �Angstr�om Laboratory, Uppsala University, Box 534,

S-751 21 Uppsala, Sweden. Tel.: +46-18-4713058; fax: +46-18-

555736; e-mail: [email protected]

0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 5 3 9 - X

been the development of the surface morphology[1±5]. An amorphous surface layer helps to main-tain a ¯at surface topography after ion implanta-tion even for high ¯uence bombardment due to theabsence of crystal anisotropy in sputtering erosion.The formation of thick amorphous Si layers is notstraightforward. From other studies [6,7] it isknown that amorphous oxide and nitride layerscoated on top of Si substrates are uniformly erodedunder high-¯uence Co ion implantation. This leadsto a surface layer with composition close to that ofCoSi2 at normal ¯uences of �5 ´ 1017 Co ions cmÿ2

when the coating layer has been completely erodedaway by sputtering. Moreover, there was minimalincorporation of O and N into the substrate [7].Here, the ``normal ¯uence'' is taken to be the timeintegral of the particle current of ions crossing thesurface plane of the target [6].

Our work on high normal ¯uence Co implan-tation into Si, SiO2/Si and Si3N4/Si is presented inthree parts. The ®rst part (Part I) [6] focuses on theformation of thin silicide surface ®lms, Part II [7]discusses sputtering yield transients and the ap-proach to high normal ¯uence equilibrium. Here inPart III, we have studied the development of sur-face topographies of Si3N4/Si(1 0 0), SiO2/Si(1 0 0)and SiO2/Si(1 1 1) structures that have been subjectto high normal ¯uence MEVVA ion source [8±12]Co implantation using scanning electron micros-copy (SEM).

2. Experimental

2.1. Sample preparation and Co bombardment

The details of sample preparation and implan-tation parameters are given in Part I and II of thiswork [6,7]. Pure Si(1 0 0) and Si(1 1 1), 144 nmSiO2/Si(1 0 0), 142 nm SiO2/Si(1 1 1) and nitride/Si(1 0 0) (with thickness: 13, 37, 72 and 145 nm)samples were implanted by Co ions acceleratedusing 40 kV acceleration voltage at an angle of 30°to the surface normal. The implanted Co normal¯uence ranged from 1 ´ 1016 ions cmÿ2 up to2.6 ´ 1018 ions cmÿ2. The beam current was at �51lA cmÿ2 and the vacuum was 2 ´ 10ÿ6 mbar duringthe implantation of the samples.

2.2. Sample annealing

After implantation the Si(1 1 1) samples im-planted with a normal ¯uence of 5 ´ 1017 ions cmÿ2

were cleaved and some parts of the samples wereannealed at di�erent temperatures using a rapidthermal annealing (RTA) system [6,13]. The RTAsystem was equipped with tungsten±iodine lampsoperating in a high purity N2 atmosphere. Heattreatment of samples was carried out at tempera-tures from 700°C to 1260°C for 15 s (plateau du-ration). During annealing the uncapped sampleswere placed in face to face contact with a clean Siwafer.

2.3. Scanning electron microscopy (SEM) mea-surements

In order to have a fresh cross-section andminimise contamination, the sample was cleavedjust before transfer to the SEM chamber. SEMmicrographs were taken with 100 pA, 5 and 10 kVelectron beams.

3. Results and discussions

3.1. Morphology of pure Si(1 1 1) and Si(1 0 0)

The SEM micrographs of the surface ofSi(1 0 0) samples bombarded with di�erent ¯uencesare shown in the Fig. 1. Similar surface morphol-ogies were obtained for Si(1 1 1) samples that wereirradiated with the same ¯uence of Co ions. Fornormal ¯uences up to 1 ´ 1017 ions cmÿ2, the ob-served surface topography remains ¯at. However,atomic scale discontinuities at the surface mayexist.

At a normal ¯uence of 5 ´ 1017 ions cmÿ2, thesurface starts to become eroded. For a normal¯uence of 1018 ions cmÿ2 the pore coverage of thesurface is about 50%. Fig. 2 shows the corre-sponding cross-sectional micrographs at these¯uences. The (dark) left-hand side of the micro-graphs corresponds to the above surface, and theright-hand side is the bulk Si substrate. In Fig. 2the magni®cation was optimised for the di�erent

Y. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 159 (1999) 158±165 159

samples. As shown in Fig. 2(a), the cross-sectionalmicrograph of the 5 ´ 1017 ions cmÿ2 sample is

di�erent compared to Fig. 2(b) in which the sam-ple was bombarded with a normal ¯uence of 1018

ions cmÿ2. Apparently, the pores extend along thedirection of ion incidence. In the case of ourMEVVA implantation where the high beam cur-rent employed may cause considerable self-an-nealing [14], this may lead to growth of silicidecrystallites. However, the amorphisation of thesurface layer as a result of Co bombardment willact to reduce this growth rate. The apparent ori-entation of the pores along the beam directionsuggests that the formation of pores is at leastnucleated by the passage of ions.

When the ¯uence increases, more pores appearand their penetration depth becomes greater. At anormal ¯uence of 1 ´ 1018 ions cmÿ2, the surfaceshown in Fig. 1(d) is completely reticulated form-ing a ``columnar structure''. As judged fromFig. 2(b), the penetration of the columnar struc-ture extends much deeper than the range of the Co

Fig. 2. SEM images of cross sections of pure Si(1 0 0) wafers

implanted by Co with di�erent normal ¯uences. (a) 5 ´ 1017 ions

cmÿ2, and (b) 1 ´ 1018 ions cmÿ2.

Fig. 1. SEM images of pure Si(1 0 0) wafer bombarded by Co to di�erent normal ¯uences. (a) 1 ´ 1016 ions cmÿ2, (b) 1 ´ 1017 ions cmÿ2,

(c) 5 ´ 1017 ions cmÿ2, and (d) 1 ´ 1018 ions cmÿ2.

160 Y. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 159 (1999) 158±165

(�65 nm) [15]. Moreover, the top surface of theridges between the columns is ¯at.

3.2. Morphology of 142 nm SiO2/Si(1 1 1) and 144nm SiO2/Si(1 0 0) samples

The SEM images in Fig. 3 show the develop-ment of the surface topography for the 142 nmSiO2/Si(1 1 1) samples. For normal ¯uences up to5 ´ 1017 ions cmÿ2 (shown in Figs. 3(a), (b) and4(a), (b)), the observed surface topography re-mains ¯at. With the increase of bombardmentnormal ¯uence up to 1 ´ 1018 ions cmÿ2 (shown inFigs. 3(c) and 4(c)) the columnar structure startedto form. A further increase in Co ¯uence results inthe appearance of more columns and penetrationto greater depths. Moreover, the column eventu-ally overlapped with others resulting in a roughacicular ``noodle'' structure where the top surfaceof the noodle is pointed (Fig. 3(e)). At higher¯uences, this structure is very similar to the one

that can be seen for the pure Si targets (Figs. 1 and2). Comparing Fig. 3 with the pure Si sample inFig. 1, shows that oxide layers protect the surfaceand the onset of roughening has increased from1017 to 5 ´ 1017 Co ions cmÿ2.

The results for 144 nm SiO2/Si(1 0 0) (Fig. 4) aresimilar to those for 142 nm SiO2/Si(1 1 1). As forthe former, the columnar structure does not de-velop for Co normal ¯uences up to 1017 ions cmÿ2.The needle structure is well developed in the2.6 ´ 1018 ions cmÿ2 irradiation. An electron dif-fraction analysis indicates that the acicular noo-dles can be crystalline silicon [14]. A fewdi�erences between di�erent surface orientationscould be observed.1. Although the column layer had the same thick-

ness, in the case of SiO2/Si(1 0 0) the columnshape was more acicular.

2. The thickness of the acicular noodle layerwas 1.2 lm for SiO2/Si(1 0 0) as compared to�0.8 lm for the SiO2/Si(1 1 1) structures. It is

Fig. 3. Surface SEM micro-graphs (left column) and cross sections (right column) of 142 nm SiO2/Si(1 1 1) samples bombarded with

high normal ¯uence Co. (a) and (b) 5 ´ 1017, (c) and (d) 1 ´ 1018, (e) and (f) 2.6 ´ 1018 ions cmÿ2.


commensurate with our previous observationthat Co penetrated deeper in SiO2/Si(1 0 0)structures than in SiO2/Si(1 1 1) [6]. At the ener-gies and ¯uences used here, we do not expectchannelling to play a major part in in¯uencingthe Co distributions. One possible explanationis thermal e�ects, which may be strongly in¯u-enced by the details of mounting of the sampleon the holder or non-uniformities in the beamcurrent pro®le.

3.3. Morphology of nitride/Si(1 0 0) samples

Fig. 5 presents the topologies for the Si(1 0 0)samples with a nitride layer on the surface. It canbe clearly seen that the Co ion ¯uence requiredfor the onset of the columnar structure increaseswith increasing nitride layer thickness. The cor-responding cross sections are presented in Fig. 6.One surprising result is that the columnar struc-ture is better developed for the 13 nm thick ®lm

than for the bare Si(1 0 0) sample (Figs. 1 and 2).We note that such thin nitride layers are di�cultto deposit by plasma CVD. Elastic recoil detec-tion (ERD) with a DE ÿ E detector telescope re-vealed the presence of considerable H in thesethin ®lms. In this case the ®lm may be inhomo-geneous which would lead to non-uniform erosionand in turn enhanced development of a roughtopography.

3.4. Morphology of annealed Si(1 1 1) samples

It is interesting to consider the developmentof the columnar structure and its transformationto the acicular structure. The general trend is thatthe initially ¯at surface develops pores with in-creasing Co ion ¯uence. When the ¯uence in-creases further, the pores grow wider and deeperto form a columnar structure with ¯at tops. Asthe ¯uence increases even further the columns areeroded, transforming the ¯at top columns into

Fig. 4. Surface SEM micro-graphs (left column) and cross sections (right column) of 144 nm SiO2/Si(1 0 0) samples bombarded with

high normal ¯uence Co. (a) and (b) 5 ´ 1017, (c) and (d) 1 ´ 1018, (e) and (f) 2.6 ´ 1018 ions cmÿ2.


sharp acicular noodles. The orientation of theacicular noodles is not perpendicular to the sur-face but inclined (Fig. 6(C3)). Bearing in mindthat the o�-normal ion bombardment contributesan anisotropy in the sputtered ¯ux, this maysimply re¯ect this anisotropy. In view of thenarrow geometrical cross section of the acicularnoodles, the noodle coverage is small. One mighttherefore attribute them to growth of crystallitesof cobalt silicide(s). If this was so, one wouldexpect grain growth under high temperature an-nealing. Fig. 7 shows SEM images for Si(1 1 1)samples that have been implanted to 5 ´ 1017 ionscmÿ2 and subjected to RTA at 940°C and 1260°Cfor 15 s.

Clearly no transformation to a ¯at or aciculartopographic structure is observed in Fig. 7. In thecase of the 1260°C annealing, which is close to themaximum temperature where CoSi2 can be inequilibrium with Si [16], the growth of a discon-tinuous network of grains is apparent. It followsthat the development of the columnar and noodlestructure must be associated with the implantationand not merely the result of grain growth.

4. Conclusions

High ¯uence bombardment of Si surfaces withCo ions from a MEVVA ion source leads to a

Fig. 5. Topography of nitride/Si(1 0 0) samples with di�erent thicknesses (top to bottom: A 13, B 37, C 72, and D 145 nm) bombarded

with high normal ¯uence Co (from left to right): (1) 5 ´ 1017, (2) 1 ´ 1018, and (3) 2.6 ´ 1018 ions cmÿ2.


transformation of the originally smooth surface.Initially pores are formed that extend throughoutand deeper than the implanted layer to form anetwork of columns. Further bombardment erodesthe ends of the columns to form a network ofsharp acicular noodle-like structures. Coating thesample with ®lms of Si3N4 and SiO2 can increasethe Co ¯uence required for the onset of roughtopography development. Coating the surface of aSi(1 0 0) wafer with a nitride ®lm increase the onset¯uence for the formation of a rough topographyfrom less than 5 ´ 1017 ions cmÿ2 to greater than1 ´ 1018 ions cmÿ2. The transformation from poresto acicular noodle structures or ¯at surfaces couldnot be achieved by high temperature rapid thermalannealing.

Acknowledgements

We are grateful to the sta� at the Tandem Ac-celerator Laboratory in Uppsala and the Instituteof Low Energy Nuclear Physics, Beijing NormalUniversity for help and assistance. This investiga-tion has been carried out under the auspices of theNFR/NUTEK Nanometer Structure Consortium.

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high-fluence co implantation in si, sio2/si and si3n4/si: part iii: heavy-fluence co bombardment...

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