Volume 3. number 3 MATERIALS LETTERS January 1985

ATOMICSTRUCTUREOFCOLLISIONCASCADESINION-IMPLANTEDSILICON

ANDCHANNELINGEFFECTS

J. NARAYAN Microelectronics Center of North Carolina, Research Triangle Park, NC 2 7709, USA and Materials Engineering Department, North Carolina State University, Raleigh, NC 27695.7907, USA

O.S. OEN

Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

and

D. FATHY and O.W. HOLLAND Microelectronics Center of North Carolina, Research Triangle Park, NC 27709, USA

Received 29 October 1984

We have investigated the atomic structures of collision cascades in Bi+-implanted silicon. The formation of subcascades or bunching of the primary cascades is clearly observed. The central regions of the cascades were found to be amorphous with a high density of disorder in the surrounding regions. The experimental results are discussed in terms of the computer simulation of the deposited damage energy profiles. The effect of channeling on the deposited damage energy profile is examined. The peak and integrated damage energies are considerably lower in channeling directions compared to random directions.

Ion implantation represents a crucial step in the

fabrication of microelectronic devices. The implanta- tion by monoenergetic ions into a random amorphous

solid produces an approximate gaussian profile of

dopant, and displacement damage. The dopant as well as displacement damage profiles are very sensitive to the incident directions with respect to axes and planes in crystalline solids. The critical angle for channeling is particularly sensitive to the nuclear charge of the in-

cident ion (Zl) and substrate atoms (Z2), and energy of the incident ions (E) [ 1,2 1. There is a critical angle for channeling [JI a (Z1, Z2/E)li2] below which signif- icant deviations from a gaussian profile corresponding to a random solid are observed. Knowledge of the do- pant and damage profiles for a given set of variables is necessary to determine the characteristics of the p-n junctions formed by ion implantation. In the previous studies, TEM techniques were used to investigate the structure and amorphous nature of collision cascades. However, detailed studies of atomic structure of the

cascades were lacking [3].

In this paper, we have used high-resolution trans-

mission electron microscopy [4] to study the atomic

structure of individual cascades. Clear evidence of

bunching or subcascade formation is observed. The effect of ion implantation conditions and channeling are examined theoretically and experimentally. Pene- trating damage profiles with significant reduction in

total damage production in the channeling directions are emphasized.

Single crystals of (100) and ( 110) orientations were implanted with 100 keV, 208Bi+, 50 keV, 3oSi+, and 35 keV, l1 B+ ions to doses ranging from 1 .O X 1 012 to 1 .O X 1015 cmM2 while the substrate temperature was kept either at liquid-nitrogen (LN2) or at liquid-helium (LHe) temperature. The (011) cross sections from (100) specimens were prepared by an ion thinning pro- cedure, whereas (110) plan-view specimens were pre- pared by a chemical polishing technique. The (110) cross-section and plan-view specimens were studied by

a JEOL 2OOcx electron microscope operating at 200

0 167-577x/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

67

Volume 3, number 3 MATERIALS LETTERS January 1985

Fig. 1. Bright-field electr_on micrograph of damaged regions in (110) silicon produced by implanting 100 keV “*Bi+ions to a

dose of 5.0 x lOI cmeL.

kV with a spherical aberration coefficient (C,) of 1.2 Fig. 1 shows cascades after implantation with 100

mm. The high-resolution images were taken under axial keV Bi+ ions to a dose of 1 .O X 1012 cmv2. The im- illumination with optimum defocus of -650 a, which plantations were done at 7” from [ 1 lo] surface nor-

is about 1.2 times the Scherzer defocus. The experi- mal along the [ 1 IO] axis. The cascades appear as black

mental results on defect clusters were compared with spots with average size of 50 8. The formation of sub-

theoretical damage production calculations. cascades or bunching of the primary cascade is clearly

Fig. 2. High-resolution electron micrograph (plan-view) showing (I IO) chains of atoms around damaged regions. The amorphous structure in the central regions of the cascades is clearly shown.

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Volume 3. number 3 MATERIALS LETTERS January 1985

observed. Some of the cascades containing subcascades are encircled in fig. 1. The number density of cascades was determined to be (5.0 + 1 .O) X 1011 cm-2. For

specimens that were implanted in the channeling [ 1 lo]

direction, the number density of black spots was con- siderably lower, about a factor of 10.

Fig. 2a shows a high-resolution micrograph of a cas-

cade, in which the central region contains amorphous structure with the absence of (110) chains of atoms. The outer regions contain significant amounts of dis- order in the form of disrupted (110) chains of atoms. Fig. 2b shows a cascade in which amorphous region is slightly larger than that shown in fig. 2a, presumably reflecting higher recoil energy. The structureless region in the middle is presumably amorphous. This amor- phous region is surrounded by a region containing con- siderable disorder, exhibited by broken (110) chains. It is interesting to note that the boundary between

the amorphous and crystalline structure is atomically sharp, indicating a first-order phase transition.

In the following, we present the damage profile

calculations, using the computer code MARLOWE

[51 *l where we examine the effect of incident-ion direction with the lattice direction on deposited damage

energy. The computer code MARLOWE has been used to calculate damage energy deposition as a function of depth. The results show a large reduction in the damage peak and integrated damage energy, and a big increase in the average penetration depth. Presently, we are

examining the effects of various ion implantation (in- cident-ion energy and mass) and substrate parameters on deposited damage energy profile and ion ranges. The channeling effects become particularly important as the incident-ion energy decreases, because accep- tance angles for channeling increase with decreasing

energy. Since low-energy implants are needed to make shallow junctions in VLSI devices, the effects of ion channeling on damage energy and ion ranges become increasingly important.

The computer code MARLOWE, which is a collision cascade simulation program that uses the binary-colli-

sion approximation to construct the trajectories of

” Ref. [6] describes a version of the program which is avail-

able from the National Energy Software Center, Argonne

National Laboratory, Argonne, IL 60439, and from the

Radiation Shielding Information Center, Oak Ridge Na-

tional Laboratory, P.O. Box X, Oak Ridge, TN 37831.

energetic particles moving in crystalline material, has been used to make some illustrative damage produc-

tion calculations in Si by Bi ions. In these calculations the Moliere [7] approximation to the Thomas-Fermi

interatomic potential with a Firsov [8] screening dis-

tance is used to describe the binary elastic collisions.

The electronic slowing down theory of Lindhard et al.

[9] is used to account for the inelastic energy losses. In silicon, only the elastic collisions lead to displaced

atoms and thereby stable Frenkel pair production. In

the simulation program the incident Bi ion is followed collision by collision in the silicon target until it slows down to rest. In addition, any Si target atom that is set into motion when it receives an energy greater than the displacement threshold energy Ed (15 eV) is followed until its energy drops below Ed. The spatial position of each and every recoil target atom is recorded

when its energy drops below the displacement energy and this directly gives the number of displaced atoms as a function of depth. The net effect of incorporating this atom transport is to produce a damage profile at

deeper target depths than that deduced from the energy loss profde of the incident ion alone. In calcu-

lating ion irradiation damage in crystalline media it is

important to realize that ion channeling may affect the results. In channeling the motion of the ion is con-

strained to move in the more open avenues of the crys- tal thus greatly reducing the probability of violent col-

lisions that produce cascade damage. Effects of chan-

neling on damage production have been known for a

long time since they were first predicted [ l] and ex-

perimentally confirmed [ 10 J . MARLOWE can also model random or structureless targets by rotating the

target crystal randomly in three dimensions about a lattice site before each collision. This preserves target density, but destroys directional correlations. Thermal vibrations of the target atoms are also included in the computational model.

Fig. 3 gives the calculated number of displaced atoms as a function of target depth for a 100 keV Bi ion incident onto a random target and a crystalline target. To minimize possible channeling effects in the crystal calculation the incident-beam direction was 7” from the normal of the (110) crystal surface. (A criti- cal axial channeling angle of 4.5’ is estimated from Lindhard’s [ 1 l] channeling theory.) The overall agree- ment of the two curves in fig. 3 is quite good. For in- stance, the total number of displacements agree to

69

Volume 3, number 3 MATERIALS LETTERS January 1985

DISPLACED ATOMS

NUMBER OF DISPLACED ATOMS (PER HISTOGRAM) vs PENETRATION DEPTH IN Si PRODUCED BY A 100 keV Bi ION

RANDOM

I ‘-rY--l I 1

100 200 300 400

DEPTH (UNITS OF a0 = 5.43 8)

Fig. 3. Number of displaced Si atoms as a function of penetration depth in a Si target produced by a 100 keV Bi ion. The curve labeled random is for a structureless target and that labeled crystal is for a crystalline target. For the crystalline target the polar angle of the beam direction is 7’ from the [ 1 lo] crystal normal and the azimuthal angle is 0” from the [ liO] . The integrated average number of displacements per incident ion is 1954 for the crystal case and 1916 for the random case.

within ~2% of each other (see table 1). There is a slight overall damage. The effects are twofold on the damage penetration tail in the crystal case indicating that a few profile: First, there is a large reduction of the central

ions are channeled which produce damage at greater damage peak; second, there is a component of the

than normal depths. Fig. 4 shows how the number of damage profile that extends more deeply into the tar-

displaced Si atoms depends on the incident Bi beam get. There are several reasons why damage is produced

direction in a (110) single crystal. Curve 1 is the same when the beam is incident within the critical channel-

as the crystal case discussed above (fig. 3) in which the ing angle. First, some ions never become channeled

polar angle 8 = 7” was chosen to be greater than the since they undergo large deflection angles in collisions

critical angle for axial channeling. In each of the three with surface atoms. Secondly, dechanneling mecha-

curves the azimuthal angle Q, (measured from [ liO]) nisms, such as thermal vibrations, cause some chan-

was chosen to minimize channeling from any major neled ions to dechannel before they slow down to rest.

planes. In curve 2,8 = 3O and @ = 15’ ; and in curve 3, Thirdly, for systems of large projectile-target mass

0 = 0” and 4 = 0”. A beam divergence of 0.25” was ratios it is possible for the projectile to produce dis-

used in curve 2 and one of 0.50” in curve 3. It is seen placements and still remain channeled. For instance,

that channeling of the incident ions greatly reduces the a 10 keV Bi ion undergoes a laboratory deflection of

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300

200

DISPLACER ATOMS

0

MAThRIALS LETTERS January 1985

NUMBER OF DISPLACED ATOMS (PER HISTOGRAM) vs PENETRATION DEPTH IN CRYSTALLINE Si PRODUCED BY A 100 keV Bi ION

I I 1 I

-1

1

0 100 200 300

DEPTH (UNITS OF ao = 5.43 8)

400 500

Fig. 4. Number of displaced Si atoms as a function of penetration depth in a crystalline Si target produced by a 100 keV Bi ion. The three cases represent three different incident directions of the Bi beam onto the (110) Si target. For curve 1 the polar angie B = 7” and the azimuthal angle (measured from [ 1701) @ = 0”; for curve 2, B = 3”) @ = 15”; for curve 3,e = 0” and Q, = 0”. For the fatter two cases channehng reduces the damage near the surface and increases the damage at greater depths. The average num- ber of total displaced atoms per incident ion in the three cases is (I) 1954, (2) 1129, and (3) 491.

less than 1” in transferring an energy of 15 eV (dis- Table 1 summarizes the number of displaced atoms placement threshold) to a Si atom. Both the second and average penetration depth as a function of inci- and third reasons enable the creation of damage at dent ion beam direction. The total number of displaced much greater target depths than in a random target atoms decreases to ~25%. while the average penetra- assemblage because of the greater range of channeled tion depth increases almost sixfold as the beam direc- ions. tion varies from 7” to 0” (channeling direction). In the

Table 1 Displacement damage and average penetration depth as a function of beam direction

Energy OteV)

Beam direction

8 Meg) @ (deg)

Divergence of beam (deg)

Total number Average of displaced penetration depth of atoms incident ion (A)

__.____._~ _ ~________ 100 7 0 0 1954 955

3 15 0.25 1129 3878 0 0 0.50 491 5665 random run - 1916 453

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Volume 3, number 3 MATERIALS LETTERS January 1985

channeling direction, the peak of the damage profile reduced to about a factor of 10, which is consistent with the experimental observations of number density

damaged regions.

ing the MARLOWE computer program used in the energy deposition calculations.

In conclusion, we have shown that high-resolution TEM techniques can be used to image displacement

cascades and to obtain atomic structures of the damage regions. The amorphous nature of cascades produced by 100 keV Bi+ ions has been directly demonstrated. The cascades for these implantation conditions tend

to branch out into subcascades that also contain amor- phous regions. The damage energy calculations based

upon the computer code MARLOWE show that the

damage profile is gaussian, similar to that of a random solid, above a critical angle of channeling. Below these

angles deep penetrating profiles with significant reduc- tion in damage production are obtained. The present results emphasize the control of beam direction with

respect to the crystalline target in obtaining desired

dopant profiles for device fabrication.

References

[l] OS. Oen and M.T. Robinson, Appl. Phys. Letters 2

(1963) 83.

[2] M.T. Robinson and O.S. Oen, Phys. Rev. 132 (1963)

2385.

[3] L.M. Howe and M.H. Rainville, Nucl. Instr. Methods

182/183 (1981) 143.

[4] .I. Narayan, D. Fathy, O.W. Holland and O.S. Oen,

Mat. Letters 2 (1984) 211.

[S] M.T. Robinson, Phys. Rev. B27 (1983) 5347.

[6] M.T. Robinson, User’s Guide to MARLOWE (Version

12), 1984, unpublished.

[7] G. Moliere, 2. Naturforsch. 2a (1947) 133.

[8] O.B. Firsov, Zh. Eksperim. Teor. Fiz. 33 (1957) 696

[English Transl. Soviet Phys. JETP 6 (1958) 5341.

[9] J. Lindhard, M. Scharff and H.E. Schiott, Kgl. Danske

Videnskab. Selskab Mat. Fys. Medd. 33, No. 14 (1963).

[lo] T.S. Noggle and O.S. Oen, Phys. Rev. Letters 16 (1966)

395.

One of the authors (OSO) would like to thank [ 1 l] J. Lindhard, Kgl. Danske Videnskab. Selskab Mat. Fys.

Mark Robinson for many fruitful discussions concern- Medd. 34, No. 14 (1965).

72