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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.
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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