Implantation and post-annealing charImplantation and post-annealing characteristics when impinging small Bacteristics when impinging small Bnn cl cl

usters into silicon at low fluenceusters into silicon at low fluence

J.H. Liang, H.M. Han

Department of Engineering and System ScienceNational Tsing Hua University

Hsinchu, Taiwan 30043, R.O.C.

June 28-July 2, 2004

2

IntroductionIntroduction

As the scale of modern devices becomes increasingly smaller, the formation of high-quality shallow junctions using the advanced Bn cluster ion implantation technique becomes more urgent.

In this study, an in-depth investigation of Bn cluster size and post-annealing dependence on the corresponding junction depth (xj) and sheet resistance (Rs) are extensively discussed.

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Specimen preparation:

Specimens of 2 cm x 2 cm squares were well prepared from CZ-grown <100> n-type silicon wafers with a resistivity of 4-7 -cm.

Ion implantation:

Bn- (n=1-4) cluster ions were extracted from a source of negative ion

s by cesium sputtering (SNICS) on a NEC 9SDH-2 3MV tandem accelerator.

Specimens with a tilt of 7o were room-temperature implanted with the Bn

- cluster ions using the same atomic energy level (20 keV/atom) and atomic fluence (5×1013 atoms/cm2 ).

The implanted area on the specimen was 1.5 cm in diameter.

Experimental DetailsExperimental Details

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Post-annealing treatments:

one-step: RTAtwo-step: FA+RTARTA = rapid thermal annealing at 1050 oC 10 s (Heatpulse 610i

system)FA = furnace annealing at 550 oC 1 h (Lindber system)

Characteristic measurements:

SIMS (IMS4F): boron depth profiles => xj (where CB = 1018 atoms/cm3) oxygen depth profiles four-point probe (Napson RT-7): sheet resistance => Rs

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Results and discussionResults and discussion

The mass spectrum of negative ions detected at the Faraday cup located behind the 30o inflection magnet with a slit size of 2 mm.

0 2 4 6 8 10 12

Magnetic fie ld ( kilo-gauss )

0

500

1000

1500

2000

Ion

curr

ent (

nA

) O -

H -

B 4 - / BO 2

-

Ag -

B 2 - B 3

-B 1 -

Cu-

B /Ag ca thodeslit s ize = 2 m m

BO -

O 2 -

Bn- cluster ions can be successfully extracted.

B3- signal does not clearly discriminate from the O2

- signal. BO2

- signal does not clearly discriminate from the B4- signal.

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SRIM-calculated depth profiles of boron atoms, net defects, recoiled silicon atoms, and total defects for the as-implanted 20 keV B1 specimen. The implantation ion fluence is 5×1013 atoms/cm2 .

0 50 100 150 200

D ep th ( nm )

-12

-6

0

6

12B

, N

et

de

fect

s (

10

18

cm

-3 ) 20 keV B 1 in to S i

-4

-2

0

2

4

Si,

To

tal d

efe

cts

( 1

0 2

1 c

m -3

)

B

net defects

Si

to tal defe cts

Average range (Rp), longitudinal range straggling ( R p), skewness (), and kurtosis () are

75 nm, 28 nm, -0.26, and 2.6 for boron atoms; 53 nm, 28 nm, 0.23, and 2.3 for recoiled silicon atoms. Total defects peak at 54 nm. The average ranges of the vacancy-rich, neutral, and interstitial-rich

regions in the depth profile of net defects are 6, 23, and 84 nm.

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SIMS-measured depth profiles of oxygen atoms in the as-implanted B3 and B4 specimens. The implantation ion fluence is 5×1013 atoms/cm2 .

0 50 100 150 200

Depth ( nm )

0E +0

2E +18

4E +18

6E +18

8E +18O

xyge

n co

ncen

trat

ion

( cm

-3 )

B 3 im p lan t

B 4 im p lan t

R p

No indication of O2- contamination in the B3 implant.

BO2- ions do co-implant with B4

- ions in the B4 implant.

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SIMS-measured depth profiles of boron atoms in Bn specimens. The implantation ion fluence is 5×1013 atoms/cm2 .

symbol: SIMS data : as-implanted X: RTA annealed : FA+RTA annealed

line: Pearson distribution dashed: as-implanted dotted: RTA annealed solid: FA+RTA annealed

0 100 200 300 400 500

D epth ( nm )

1E +17

1E +18

1E +19

As-

impl

ante

d bo

ron

conc

entr

atio

n (

cm -3

)

1E +17

1E +18

1E +19

1E +17

1E +18

1E +19

1E +17

1E +18

1E +19

1E +20

1E +16

1E +17

1E +18

As-

anne

aled

bor

on c

once

ntra

tion

( cm

-3 )

1E +16

1E +17

1E +18

1E +16

1E +17

1E +18

1E +16

1E +17

1E +18

1E +19

as-im plan tedF A+RT A

RT A

B 1 im p lan t

B 2 im p lan t

B 3 im p lan t

B 4 im p lan t

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Measured range parameters, retained boron fluence, junction depth, and sheet resistance in the as-implanted and as-annealed Bn specimens.

Measured parameters

as-implanted / RTA annealed / FA+RTA annealed

20 keV B1 40 keV B2 60 keV B3 77 keV B4

Rp (nm) 81 / 115 / 96 88 / 126 / 91 97 / 97 / 95 98 / 93 / 99

Rp (nm) 37 / 63 / 48 42 / 68 / 47 46 / 53 / 48 47 / 49 / 47

0.86 /1.10/1.05 0.95/1.10/1.10 0.99/1.15/1.10 1.00/1.10/1.05

4.4 / 5.1 / 4.9 4.6 / 5.1 / 5.0 4.7 / 5.2 / 5.1 4.8 / 5.1 / 5.1

Φr (1013

atoms/cm2 )6.3 / 7.2 / 7.5 6.4 / 6.1 / 5.9 7.8 / 6.5 / 7.1 2.6 / 2.7 / 3.0

xj (nm) --- / 220 / 182 --- / 222 / 171 --- / 184 / 180 --- / 146 / 152

Rs (Ω /sq.) --- / 1.74 / 1.93 --- /1.64 / 1.76 --- / 2.35 / 1.96 --- / 3.78 / 3.52

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as-implanted boron depth profile: The clear-the-way and acceleration effects outweigh the nonline

ar damage effect and cause the boron depth profile becomes broader and extends deeper into the specimen bulk as cluster size increases. That is, Rp , Rp , , and increase as cluster size increases.

The increase in Rp , Rp , , and inclines to reach a saturation point in larger cluster ions.

The variation of versus for all the implants of concern closely correlates to the parabolic fitting formula of

suggested by Gibbons.

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RTA annealed boron depth profile: In the B1 and B2 implants, the boron depth profile in the RTA anneale

d B2 specimen becomes significantly broader and extends much deeper into the specimen bulk than does that of the B1 implant.

Rp , Rp , , and increase as cluster size increases when compared to those of the as-implanted specimens.

TED is stronger in the RTA annealed B1 and B2 specimens, thus leading to larger junction depths but smaller sheet resistance.

In the B3 and B4 implants, the boron depth profile in the RTA annealed B4 specimen becomes slightly broader but shifts a little more toward the specimen surface than does that the B3 implant.

Rp decreases but Rp , , and increase as cluster size increases when compared to those of the as-implanted specimens. .

TED is smaller in the RTA annealed B3 and B4 specimens, thus results in smaller junction depths but larger sheet resistance.

Oxygen contamination and greater residual defects in the near-surface region causes an anomalous increase in sheet resistance in the RTA annealed B4 specimen.

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FA+RTA annealed boron depth profile: In the B1 and B2 implants, Rp , Rp , , and increase only slightl

y as cluster size increases when compared to those of the as-implanted specimens.

In the B3 and B4 implants, Rp and Rp are practically unchanged while and increase slightly as cluster size increases when compared to those of the as-implanted specimens.

FA+RTA annealed boron depth profiles indeed show less TED when compared to those of RTA.

Less TED is obtained among the boron depth profiles in the FA+RTA annealed B1, B2, and B3 specimens and all the boron depth profiles are similar to each other. Therefore, the corresponding junction depth and sheet resistance varies insensitively to cluster size.

Oxygen contamination and greater residual defects in the near-surface region causes an anomalous increase in sheet resistance in the FA+RTA annealed B4 specimen.

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ConclusionsConclusions

The characteristics of the shallow junction produced by low-fluence small Bn cluster ion implantation are strongly dependent on cluster size as well as post-annealing method.

TED of boron atoms favors one-step RTA for smaller Bn cluster ions, but a noticeable reduction of TED is obtained for larger Bn cluster ions.

The TED of two-step FA+RTA is smaller than that of one-step RTA.

Sheet resistance of FA+RTA is larger than that of RTA in the B1 and B2 implants, indicating that boron activation favors RTA.

Sheet resistance of FA+RTA is smaller than that of RTA in the B3 and B4 implants, implying that crystalline recovery favors FA+RTA.

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It is possible to use larger Bn cluster ions together with the subsequent FA+RTA method to obtain an optimum shallow junction without sacrificing extremely large sheet resistance.

The results of this study confirm the availability and feasibility of Bn cluster ion implantation as a novel technique in fabricating high-quality shallow junctions for modern devices.