Extended Abstracts of the 20th (1988 International) Conference on Solid State Devices and Materials, Tokyo, 1988, pp. 105-108 A-7-3

T. rzAwA, Pt. KASE*, H.

Advanced Tech. Division and

Lateral Diffusion 0f Dopants beyond Preamorphized Region

1015, Kamikodanaka,

A preamorphization technique with a Si-inplantation was optimizedin terms of both reduction of junction depth and suppression of leakagecument, and was applied to the fabrication of source and drain ofPMOSFETs. However, short-channel effects weren't improved as expected.Estimated 2-D profiles of an amorphized region by a Si-implant andsubsequently inplanted boron suggested there was a non-overlapped regionunder an edge of gate, which allows the dopants to spread laterallythrough channeling. The phenomenon was confirmed by preamorphizationwith a Si-inplantation of tilted incidence.

MORI*, K. KOBAYASHI, ANd M. NAKANO

Process Development Division*, FUJITSU LTD.

Nakahara-ku, Kawasaki zIL, JaPan

1. INTRODUCTION

A preamorphization technique witha Si-implantation 1-l) has been studiedto eliminate an ion-channeling tail ofboron for the fornation of shallow 9'/njuhction. Residual defects caused by

Si-inplantation are hard to be annealed

out completely by subsequent thernalprocesses. Therefore, on applicationof the technique to the formation ofsource and drain of PIIOSFETs, the depthof an amorphized region and implantedboron must be properly adjusted not toIeave the defects remainingr in channel

regrions and depletion layers, because

they act as generation-recombination

centers.In addition, it has been clarified

that only optinization in terns of the"depth" is not enough to achieve quarter-micron devices. In this paper, Iateraldiffusion of boron beyond the preamor-phized region at an edge of gate is dis-cussed.

2. p'/n DIODE FABRICATION

p'/n Diodes were fabricated firstin order to obtain optinized conditionsof the preamorphization technique interms of junction depth and leakage

current due to residual defects. Si-ionswere implanted to zx1-}r5 cm-z at 40 keV,

and then BFz-ions were inplanted to 2x10ls

cm-2 at 5 or 30 keV. Boron was activatedby RTA (Rapid Thermal Annealing) at 800

"C or 1000 oc for 10 sec. Junction depth

was measured by angle-lapping-stainingtechniques and the leakagre curuent was

measured at -2.5 V. The results are

summarized in Table I and the junction

depth was reduced by about 70 nm fora BFz -implant at 30 keV and 1000 "C RTA

by preamorphization with a Si-implant.

3. ''OPTIFIIZED'' FABRICATION OF PIITOSFETS

If lateral spreads of boron-ionsare assumed to be suppressed as much

as the reduction of junction depth, effec-tive channel length of MOSFETs is expectedto be longer by about twice of the reduc-tion of junction depth due to contribu-

105

Table 1 Effects of preamorphization on junction depth x1 and leakage current Jp.

SI-IMPLAI\T . 2E'15 cm-2, 40 keV Vp:2.5V

tions from both sides of source and drain.

Increments of electrical effective channel

Iength were studied for the MOSFETs fabri-

cated with the optimized condition of

a Si-inplantation obtained above. The

I{OSFETs had gate def ined bv EB-direct-

writing and deviation of the length was

about 30 nm, The result is shown in

Figr.1. Though the junction depth reduced

as the energry of BFr-ions hras decreased,

the expected reduction was not observed

at 25 keV. This result was contrary

to that obtained in the diodes previously

mentioned. If BFz -ions are imPlanted

at a lower energy than 25 keV, Ieakage

current is observed as shown in Fig.2 -

Theref ore, suppr.ession of both leakage

current and channeling seems to be a

trade-off.

1E-l4-505

GATE VOLTAGE VG3 (V)

Fig.2 Io-V6 characteristics of PMOSFETswith a Si-implant.

4. ESTIMATION OF 2-D PROFILES

Two-dimensional spreads of theamorphized region and a boron profileunder an edge of gate was estinated.The amorphized region ldas def ined as

a region whose energy density griven by

atomic colLisions exceeded the criticalenergy4) of 6x1023 eV.cm-3 and the boronprofile was assumed to be a Gaussian

distribution. The result suggested therewas a non-overlapped region of the boronprofile to the amorphized region, and

channeling might not be suppressed enough.

It allows the dopants to spread Iaterallybeyond the calculated profile illustratedin Fig.3(a).

5. Si-IIIPLANT hIITH TILTED INCIDENCE

The phenomenon suggested above

was confirned by a Si-inplantationperformed with tilted incidence on bothsides of source and drain of IIOSFETs

1E-3

gF-lrlEGDo

=Go

50

EcFzJc

=!oo-oJI

oJ

RTA: ldXl.c

o

IIl2xaxi I

\\k-==-1-

o102030BF2 ACCELERATION ENERGY (kev)

Fig. l Dependence of increments of effec-tive channel length on BF2 -ionenergy. An open circle indicatestwice of the reduction of junctiondepth.

RTA(l0sec)800 'c 1000 'c

NO SI-IMPLANT Si-IMPLANT NO SI-IMPLANT Si-IMPLANT

BFz +

(2E15

cm-2)

5 keVx 1 (nm)

Jn (A.cm-2)50

1.8E-8

30 keVx i (nm)

J n (A. cm-z)200

9.6E-9100

6.6E-6200

5.0E-9r30

1.8E-9

W/L= 1O/2O fn Vos: -3 V

BF2: r5 koV

106

si*

o'lt

BFz*

I

GATE

1o2ocm-3w_-- l-- 1 d1 5cm-3

AMoRPHous-REGlo*

, ,oon,

,

(a)

(b)

(c)

Si* BFZ*

"\IGATE

1o2ocm-3

-

I r bt u",n-'

AMORPHOUS-REGtON lOOnm

Fig.3 Estinated regrions amorphized bySi-inplants and boron profilesassuned a Gaussian distribution;the ions hrere assumed not to occurchanneling. Si-ions were implantedwith incident anqle of (a) 0o,(b) 15", and (c) 30".

to fully amorphize 'the region under gate.The expected regions amorphized by thetitted incidence of 15" and 30" of Si-ionsIrere illustrated in Fig.3(b) and (c)respectively. The fabrication process

of the FIOSFETs is described below.

Active regions of transistors weredefined by conventional LOCOS process.After 20-nm thick gate oxides were ftrown,a surface inplant was perforned with

BFr-ions to a dose of 1.5x1013 crn-2 at25 keV, and As-ions to 5x1012 cm-2 at25 keV and P-ions to 3x1012 cn-2 at I7O

keV were implanted to form punch-through

stoppers. Gate-electrodes were made

of n*-polysilicon, and defined by EB-

direct-writing. Then the Si-inplantationhras performed with a tilted incidenceof 15", 30", or 45o neasured from normalof wafers toward source or drain, toa dose of 2x101s cn-z at 40 keV on bothsides of grate; a dose hras corrected foreach angrle. Then BFz-ions were implantedto 2xlOls cm-2 at 15 or 25 keV and !{ereactivated by RTA at 10OO"e for 5 sec.

(mA)

-2.O

NO S|-IMPLANT

(a) to

-5.o (v)

(mA)

-2.O

(b) lo

ooo -5.O (v)

a Vo

(mA)

-2.O

(c) to

o o -5.o N)Vo

Fig.4 Io-Vu characteristics of PFIOSFETs(a) without a Si-inplant, (b) withSi-inplants of incident anglesof 0" and (c) 15". Lsare : 0.35unt, V6 : 0 to 5 V, Vsub t 2.5 Y.

107

si*ii

151:T

BFz*

I

GATE

1o2ocm-3

-

1d1scm-3

nrvronprlous-REGtoNlOOnm

l-{

S|-IMPLANT: 15"

Io-VD characteristics of finisheddevices were shown in Fig.4. AII devicesshown here have 0.35-pm long 9ate. The

Si-inplantation with tilted incidencenade nuch improvement in punch-throughvulnerability. Electrical effectivechannel length was neasured for each

implantation condition and Fig.5 shows

increases of the channel length overno Si-inplanted samples. The maximurn

increase of about O.2 lrm was observed

at 15o incidence of Si-ions for 25 keV

of BFr-ions. The reduction of effectivechannel lengrth for larger angles than15" might be explained by enhanced diffu-sion5) due to defects created by theSi - inplantation.

Threshold voltage dependence on

gate length is shown in Fig.6. Remarkable

improvements of short-channel effectswere observed for Si-inplanted samples

with tilted incidence. Inverse short-channel effects5) seen here are due toa punch-through stopper which forms a

non-uniformprofile.

impurity concentration

6. CONCLUSION

It has been confirmed that a profileof dopants spreads laterally throughchanneling beyond because of a non-over-

BFz 25 keV Vsua

NO SHMPLANT

oo

o.5 1.O 1.5GATE LENGTH Lgatc (lm)

Fig.6 Channel length dependence of thresh-old voltage for various incidentangles of Si-inplants.

lapped region of a dopant profile toan amorphized region at a mask edge even

if their depth is optimized. Therefore,preamorphization conditions should be

optimized in terms of two-dimensionalprofiles, and it can also be concludedthe preamorphization technique usingtilted incidence of Si-ions is effectiveto suppress the phenonenon.

ACKNOT.ILEDGETIIENT

Authors would like to thank J.Shige-yoshi and S. Kuroda for arrangement ofan implantation systen of tilted inci-dence. The cooperation of the cleanroom staff for processing the wafersis greatly acknowledged.

REFERENCES1) ll.Y.Tsai and B.G.Streetman; J.AppI.

Phys. ,8( 1979 ) 1S3.2) B. Y. Tsaur and C. H . Anderson, Jr. ; J. Appl .

Phys.,54(1983)63363) C.Carter et al.; AppI.Phys.Lett.,

44(L984)49s4, J.R.Dennis and E.B.Hale; J.Appl.Phys.,

49(1978)1119.5) R.B.Fair; IEDPI87, (19871260.6) K. -Y. Fu; IEEE Trans. on Electron

Devices,ED -29 ( 1982) 181 0 .

2.51.O

c:1oll

E5oIC

ill

35 -o.s

.oLo

-1

Etv= ''-F-4.T n.rai -''ozoJI

!ooJ

BFz : 25 keVt

-t' 'a-

a\ 15keV

o153045TILT-ANGLE OF Si-tMpLANT (deg)

Fig.5 Increnents of effective channelIength as a function of Incidentangles of Si-imPlants.

108