Rapid thermal annealing of arsenic-implanted poly-Si layers on insulator

352

Nuclear

RAPID THERMAL ON INSULATOR M. TAKAI,

ANNEALING

and Methods

OF ARSENIC-IMPLANTED

M. IZUMI, T. YAMAMOTO

Faculty of Engineering

Instruments

in Physics

Research B39 (1989) 352-356 North-Holland, Amsterdam

POLY-Si LAYERS

and S. NAMBA

Science and Research Center for Extreme

Materials,

Osaka Uniwrtrity,

Toyonaka,

Osaka 560, Japan

T. MINAMISONO Faculty of Science, Osaka University, Toyonnka, Osaka 560, Japan

Atomic and carrier concentration profiles in poly-Si on insulator structures implanted with arsenic ions after furnace and rapid thermal annealing (RTA) have been investigated by Rutherford backscattering (RBS) and Hall effect measurements. Arsenic atoms at and near the implanted peak region show slow diffusion. while those at the tail region show fast diffusion. The diffusivity for As in cm*,/s for the tail region and D =1.7x103 exp(-3.8/kT) poly-Si on insulator is represented by D = 8.4 x IO4 exp(-3.8/kT) cm2/s for the peak re8ion. Poly-Si layers after implantation and annealing were found to have tensile stresses of 1.7-3.7 kbar.

1. Introduction Poly-silicon-on-insulator

(SOI)

structures

have

re-

attention because of the potential for application in thin-film transistors (TFT) such as drivers for flat panel display devices or liquid crystal shutter devices for printers. Improvement in electrical properties of poly-Si TFTs is possible by optimizing processes to fabricate TFT structures, such as short channel devices, or by increasing grain-size and reducing defects in poly-Si layers. Implanted-impurity behavior in SO1 structures such as redistribution of impurities in poly-Si layers during various heat treatments has not been studied in detail though it is a very important factor for device applications [1,2]. Arsenic (As) implantation followed by annealing is one of the possible doping processes for TFT in poly-Si layers with SOI structures, where precise controls of impurity profiles, i.e. junction depth, are necessary. Diffusions of implanted As in poly-Si layers on crystalline Si [3,4] or on SiO, after furnace annealing [l] or RTA [2] have been measured. The obtained profiles or diffusivity of As depend on the preparation process of the interface between poly-Si and underlying crystalline Si layers [4] or on the grain size of poly-Si, where fast diffusion of As through grain boundaries occurs [1,2]. In our previous study [2], As was implanted at 100 keV with a dose of 10” crne2 in poly-Si on insulator structures to obtain the diffusivity of As in poly-Si during RTA or furnace annealing. D.iffus~~ties of L3 = 0.28 exp( -2.84/kT) cm2/s for RTA and D = 1.81 exp( - 3.14/kT) cm2/s for furnace annealing were obtained at and near the peak region of the implanted As profiles. cently

attracted

great

0168-583X/89/$03.50 Q Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

In this study, As profiles in poly-Si layers on insulator (SiO,) structures after RTA and conventional furnace annealing have been measured by Rutherford backscattering (RBS) to investigate parameters suitable for shallow As profiles. Implantation doses ranged from 5 X 10” to 1 X lOI cm-*, where faster diffusion took place in the tail region of the As profiles than in the peak region. Hall effect measurements in combination with successive layer removal techniques were used to obtain activated carrier and mobility distributions. We obtained the As diffusivity in poly-Si on insulator structures after two different annealing processes, including the activation energy of diffusion. Raman scattering spectroscopy was used to measure the residual stress in the poly-Si film and the crystallinity of poly-Si layers, i.e., grain boundary defects before and after implantation, followed by annealing. Preliminary results were published elsewhere IS].

2. Experimental procedures Poly-Si layers with a thickness of 540 nm were deposited by a low-pressure CVD method on a 50 nm thick thermal oxide of a (100) Si wafer. The pressure and substrate temperature during deposition were 1.5 Torr and 61OOC. The deposition rate was 7.5-7.8 nm/min. 100 keV As ions were implanted in poly-Si layers with a dose of S-10 x 1Or5 cme2. Conventional furnace annealing (FA) in a flowing nitrogen atmosphere was performed for 15 min at temperatures from 800 to 1000° C. RTA with a halogen flash lamp was performed for 15 s in an Ar atmosphere at temperatures from 900 to 1050 o C, as described elsewhere [2].

M. Takai et al. / Rapid thermal annealing ofpoly-Si

RBS measurements with 2 MeV helium ions were performed to investigate As redistribution in poly-Si layers on insulators. Samples were tilted 60 o off-axis to obtain higher depth resolution. The system energy resolution of 18.4 keV corresponds to 18.9 nm in As RBS spectra. The concentration profile of As before and after annealing was obtained by converting the energy scale of RBS spectra to a depth scale. Diffusion coefficients (D) of As were obtained by Gaussian fitting, as described elsewhere [2], at the peak (O-150 nm) and tail (150-400 nm) regions of the profiles, respectively, since, in this study, the diffusion behaviour near the peak region differed from that at the tail region, which was not observed for 5 X 1Ol5cm-2 implantation in our recent study [2]. Gaussian-fitted curves were illustrated in the concentration profiles in each of the figures. Raman scattering spectroscopy was used before and after implantation, followed by annealing, to measure residual stress and crystallinity, i.e. grain boundary defects in poly-Si layers. The relative peak position can be determined with an accuracy of about 0.1 cm-‘. A 488 nm line of an Ar laser with a penetration depth of about 500 nm was used to excite the sample, in which Raman signals only come from poly-Si layers. The experimental arrangement was described in detail elsewhere [6].

“w [L 2

353

layers on insulator

% O

4000

0

5 8 2000

0

8OO’C.

.

95O’C

15min

F A

D

. 15mm F A O

0

0

I

I

100

200 CHANNEL

00

1

0

CT.

300

400

500

NUMBER

Fig. 1. RBS spectra for As-implanted poly-Si on SiO, /Si after furnace annealing at 800 and 950 o C for 15 min.

keeps a Gaussian profile, while that after annealing at 950 o C shows a flat-top profile with a slight pileup at the interface between the poly-Si layer and the underlying insulating layer. Further, the As profiles were measured by RBS with samples tilted off-axis by 60” to investigate the diffusion behavior with better depth resolution [2]. Fig. 2a shows the As concentration profiles before and after furnace annealing at 800 and 850 o C. The As profile after annealing at 800 o C shows slight diffusion with a Gaussian shape, while that after annealing at 850°C shows fast diffusion at the tail region. The diffusion behavior at and near the peak region (O-150 nm) of the profile differs from that at the tail region (150-400 nm).

3. Results and discussion Fig. 1 shows the RBS spectra for As-implanted polySi layers on SiO,/Si after furnace annealing at 800 and 950” C. The As spectrum after annealing at 800” C 22

‘022y-yiigj

10

Furnace

Anneal

Id'

20

10

Id” 0

0.1

0.2 DEPTH

0.3

0.1

0.4

0.2 DEPTH

(pm)

0.3

0.4

(/HII1

Fig. 2. As concentration as a function of depth before and after furnace annealing (a) and RTA (b). Solid curves stand for Gaussian-fitted profiles. Arrows indicate the interface between the implanted amorphous layer and the underlying poly crystalline layer. IV. SEMICONDUCTORS:

Si

M. Takai et al. / Rapid thermal annealing ofpoly-Si

354

Furnace

layers on insulator

Anneal 10” z t

1

lo'71

0

0.2

0.1 DEPTH

I

’

1o”L

0.3

0

I

I

0.1

(pm)

I

’

0.2

DEPTH

11

0.3

(pm)

Fig. 3. Carrier concentration and Hall mobility as a function of depth for furnace annealing (a) and RTA (b).

Fig. RTA

2b

shows

the As concentration

slight

diffusion

900°C.

at

Further

hanced

the

tail

region

the diffusion

by annealing

at the peak

after

of the profile These

is applicable

95O’C

results

Fig. 3 shows the comparison at 900°C. nealing

profiles The

after

carrier

the profile

concentration is almost

profile

results

suggest

former

mechanism.

that

-1, 1100 10

’

by furnace

in diffusion

this difference

at

anneal[2]. These

may be due to the

’

TEMPERATURE

( ‘C )

1000

000

‘.\I

900

’

\\

and RTA

for furnace

has

followed

ing, did not show such difference

concentration

to the atomic

RTA

5 x 1015 cm-2,

that RTA

annealing

similar

as shown in fig. 3. The low dose implantation

and below

’

’

700

Ij

’

1 X10’6As’ cm+ OFA peak A RTA

an-

-

profile

a shallow

peak

of 3 x 102’ cmm3 at 60 nm. The mobility

constant

for both

also indicate

furnace

of carrier

region,

from

layers.

furnace

after

behavior

slightly

indicate

at

is en-

for shallow

concentrations

show flat profiles

[5], while

results

differs

in 540 nm thick poly-Si

and mobility

annealing

at 950 o C. The diffusion

region

at and below formation

after

shows

at the tail region

that at the tail region.

to

profiles

for 15 s at 900 and 950 o C. The As profile

annealing

that the RTA

annealing

processes.

These

at 900 o C is superior

in

obtaining

in As

diffusion

a shallow

carrier

profile. The region 2,

difference

is considered

induced peak

by ion

region

to

be

tail region. difference diffusion

due

are trapped

cal activity

residual

the As atoms

diffuse

slower

during

diffusion

peak in fig.

damage near

the

anneal-

than those at the

et al., however,

as a two-stream in the interior

the

the

observed

by the damage

Swaminathan

grain-boundary

to

implantation:

ing so that these atoms

about

between

and the tail region of the profile,

interpret

process

[l]:

this the

of grains near the peak and the

diffusion

at the tail region.

of As at the tail region

30% and was almost

The electri-

in this study

was

the same as that in the peak

,615 ‘-

0.7

1.0

0.9

0.8 103/T

( k-’

)

Fig. 4. Diffusion coefficient of As in poly-Si as a function of inverse temperature.

M. Takai et af. / Rapid thermal annealing ojpofy-Si I

I

A= 48Pnm

1

I

I

I

I

t: 510

520 WAVENUMBER

530 Cd)

Fig. 5. Raman scattering spectra for single-crystalline and poly-Si before and after implantation and annealing.

Fig. 4 shows the Arrhenius plot of the diffusivity for As in poly-Si on insulators after RTA and furnace annealing. Data for As in poly-Si on single-c~stalline Si, reported by Tsukamoto et al. [3] and Ryssel et al. [4], and for As in poly-Si on SiOJSi, reported by Swaminathan et al. [l], are also shown for comparison, The diffusivity for As in poly-Si on insulator in this study is represented by D = 8.4 X lo4 exp( - 3.8/W) cm2/s for the tail region and D = 1.7 X lo3 exp( - 3.8/W) cm2/s for the peak region, where k and T are Boltzman constant and temperature in K, respectively. Though the activation energy in this study, with an uncert~~ty of i-O.2 eV, is very close to that for As in poly-Si on SiO, by Swaminath~ et al. or that for As in poly-Si on crystalline Si by Ryssel et al. and Tsukamoto et al., the diffusivity is higher by a factor of lo-50 than that for As in poly-Si on crystalline Si [3,4]. This difference in diffusivity is considered to be due to the grain size of poly-Si layers [l]: poly-Si layers deposited on single-crystalline Si would have a larger grain size than those on insulators, and fast diffusion of As through grain boundaries occurs. Ryssel et al. [4], for example, observed higher diffusivity for As in poly-Si layers on crystalline Si with native oxide at the interface between the poly-Si and the underlying crystalline Si and attributed it to the structural difference in poly-Si layers, presumably gram size difference. High values of diffusivity for As in poly-Si on insulators by Swaminathan et al. [l] are due to their shallow

layers on insulator

355

As implantation, in which they measured As diffusion profiles far beyond the implanted area. As diffusion in such deep areas suffers less influence of implantationinduced defects. Grain sizes far beyond the implanted layer are smaller than those in the implanted layer. Fig. 5 shows the comparison of the Raman spectra obtained before and after implantation, followed by annealing at 9oo°C. A spectrum for crystalline Si is also shown for comparison. The Raman spectrum for crystalline Si has a peak at 520.2 cm-’ with a half width of 3.5 cm-‘, while the peak for poly-Si before implantation is located at 519.2 cm-’ with a half width of 9.0 cm- ‘. Further, the peak shift is enhanced by 1.5 cm-’ after implantation and RTA. The shift in peak position toward lower wave number is due to the residual tensile stress in poly-Si layers on SiO,/Si, induced by the difference in thermal expansion coefficient between the poly-Si and the underlying layers [S]. The stress in deposited poly-Si layers is estimated by the peak shift of 1.0 cm-’ to be 2.49 kbar, since the shift of 1 cm-’ turns out to be a residual stress of 2.49 kbar (or 2.49 X lo9 dyne/cm*) [6]. Such a stress is enhanced to 3.7 kbar by RTA. The half width of the Raman peak closely correlates with crystalline imperfections such as impurities and defects. The broadening in half width of poly-Si peaks, therefore, indicates grain boundary defects. The half width decreases from 9.0 to 6.1 cm-’ for RTA and to 5.3 cm-’ for furnace annealing, which indicates that the grain in poly-Si layers becomes larger

Table 1 A comparison corresponding

of the Raman peak intensity, width, residual stress for various processes

Sample c-Si poly-Si: as-deposited RTA 15s 900°C 5 x 10” As+/cm* RTA 15 s 900°C 950°c looO”c 1050°c FA 15 min 900°C 1000°C 1 X 10’6As+/cmz RTA 15 s 900°C 950°C 1ooo”c 1050 o c

Intensity

shift and

[a, u.1

Width [cm-‘]

Shift [cm-i]

3050

3.5

_

5.50

9.0

1.0

2.5

751

7.4

0.7

1.7

890 950 1140 1200

6.1 5.9 5.1 5.3

1.5 1.3 1.3 1.0

3.1 3.2 3.2 2.5

1135 1850

5.3 4.9

1.2 0.7

3.0 1.7

640 770 910 1200

6.1 5.6 5.6 4.9

1.4 1.4 1.4 1.0

3.5 3.5 3.5 2.5

Stress [kbar]

IV. SEMICONDUCTORS:

Si

M. Takai et al. / Rapid thermal annealing ojpoly-Si layers on insulator

356 and the grain

boundary

defects

decrease

after

anneal-

ing. Table

1 compares

Raman

peak intensity,

peak shift and corresponding samples.

The peak intensity

samples

followed

poly-Si for

before

the

These

as-deposited

results

samples sample

indicate

for various

RTA,

and the peak-

is narrower

before

and

that the implanted

than after

that RTA.

and annealed

have larger grain sizes and less grain boundary

defects

than

grain

unimplanted

size enhancement

is also found by TEM to the residual enhanced

stresses

samples.

Such

and annealing

stress

in this

case,

followed

processes

after

in poly-Si

layers.

however,

is

by annealing.

implantation

In

enhance

4. Conclusions 100

keV

SiO,/Si. surements RTA

As

RBS, on

ions

were

Hall effect,

layers

annealing.

of implanted

As in poly-Si

conventional

furnace

impurity The

profiles

diffusivity

sented

implanted

in

and Raman

implanted

and furnace

implantation

and annealing.

result

in higher furnace

RTA

stresses

processes in poly-Si

annealing.

The

authors

would

Miyauchi

(Matsushita

preparing

the samples.

Nojiri,

K. Matsuta

like The

T. Hirao

Industrial

authors

Co.,

and M. Ltd.)

are indebted

and Y. Takahashi

ation at the Van de Graaff and K. Kawasaki

to thank

Electric

accelerator

for their help during

for

to Y.

for their cooperand to K. Mino the experiment.

[8]. The peak shift, corresponding

tensile

RTA

residual

poly-Si

by implantation

by implantation

particular,

after

layer than conventional

for the implanted

samples

the

layers after implantation

than for the as-depostied

and after

for the implanted

peak width,

stress

is higher

by annealing

samples

width

residual

grain boundary defects than the unimplanted poly-Si layers. Tensile stresses of 1.7-3.7 kbar reside in poly-Si

were

Rapid in

performed

after

thermal

Poly-Si

is superior concentration.

after

is repre-

cm*/s

exp( - 3.8/kT)

layers

to

shallow

on insulator

by D = 8.4 x lo4 exp( - 3.8/kT)

tail region and D = 1.7 x lo3

annealing

obtaining

with a high peak carrier for As in poly-Si

on mea-

on insulators

annealing

poly-Si

scattering

for the

cm2/s for

the peak

region.

annealing

were found to have a larger grain size and less

implantation

and

References [l] B. Swaminathan, K.C. Saraswat, R.W. Dutton and T.I. Kamins, Appl. Phys. Lett. 40 (1982) 795. [2] M. Takai, M. Izumi, K. Matsunaga, K. Gamo, S. Namba, T. Minamisono, M. Miyauchi and T. Hirao, Nucl. Instr. and Meth. B19/20 (1987) 603. [3] K. Tsukamoto, Y. Akasaka and K. Hone, J. Appl. Phys. 48 (1977) 1815. [4] H. Ryssel, H. Iberl, M. Bleier, G. Prinke, K. Haberger and H. Kranz, Appl. Phys. 24 (1981) 197. [5] M. Takai, M. Izumi, T. Yamamoto, A. Kinomura, K. Gamo, T. Minamisono and S. Namba, Polysilicon Films and Interfaces, eds. C.Y. Wong, C.V. Thompson and K.N. Tu (Mater. Res. Sot. Pittsburgh) p. 341. [6] M. Takai, T. Tanigawa, M. Miyauchi, S. Nakashima, K. Gamo and S. Namba, Jpn. J. Appl. Phys. 23 (1984) L363. [7] T. Englert, G. Abstreiter, and J. Pontcharra, Solid State Electron. 23 (1980) 31. [8] L.R. Zheng, L.S. Hung and J.W. Mayer, Appl. Phys. Lett. 51 (1987) 2139.