RTP — temperature monitoring by means of oxidation

IOURNA

L OF

NO C SOgDS Journal of Non-Crystalline Solids 187 (1995) 23 28

ELSEVIER

R T P - temperature m o n i t o r i n g by means of oxidation Jens-Peter Z611ner*, Volker Cimalla, J6rg Pezoldt TU llmenau, lnstitut ffir FestkSrperelektronik, PSF 327, 98684 llmenau, Germany

Abstract

In rapid thermal processing (RTP), an accurate temperature distribution adjusting is necessary for manufacturing of homogeneous thin layers and junctions. A common practice is to evaluate RTP systems in this direction by ellipsometric measurements of oxide thickness grown on silicon wafer. The oxide thickness is a function of the temperature distribution during the complete rapid thermal oxidation process including heating and cooling. The goal of this work is to judge the effect of the heating up period on oxide growth. The influence of different heating cycles (different ramp rates and process temperatures) on the oxide thickness distribution is investigated.

1. I n t r o d u c t i o n

Rapid thermal processes (RTP) are characterized by the advantage to carry out high-temperature processes in the range of some seconds up to few minutes. This is the basis for the manufacturing of ultra-thin layers and small structures used in submicron devices El]. For manufacturing of layers with constant thickness homogeneous temperatures across the whole wafer are a fundamental prerequisite. For temperature measurement and adjustment, thermocouple wafer are normally used. This method allows only the temperature determination at few points of the wafer. For temperature mapping, ion implanted test wafers are annealed or bare wafers are oxidized. With modulated optical reflectance and sheet resistance measurements, respec-

* Corresponding author. Tel: +49-367769 1673. Telefax: +49-3677 69 3132.

tively, the modification of the implanted wafer is determined, which corresponds to the annealing temperature. For oxidized wafers the measured oxide thickness distribution yields the two-dimensional temperature profile. This method is suitable for processing temperatures > 900°C [2]. The calculation of the temperature from the oxide thickness is carried out by using a kinetic growth model. The initial oxide growth under dry environment, typical for gate oxide manufacturing with rapid thermal oxidation (RTO), has an enhanced growth rate described by different models [3,4]. Recently, Messaoud et al. [5] compared the experimental growth kinetics of ultra-thin oxide films with the parallel oxidation model [3] for R T O and found an excellent agreement. This parallel oxidation model was applied in this work, too. Because the oxide thickness depends on the temperature-time behaviour of the complete oxidation process, especially during the periods of m a x i m u m temperature, for short processes, the heating up and cooling down phase must be taken into

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J.-P. Ziillner et al. / Journal o f Non-Crystalline Solids 187 (1995) 23 28

account. In [2,6,7] was shown, that temperature inhomogeneities occur in scalar controlled RTPequipments. Temperature overshooting was found to occur near the wafer edge due to steep ramping

up. The aim of this work is to investigate this effect in RTO, and to determine consequences for temperature mapping with oxidation, which are also valid for implant monitoring.

2. Experimental procedure The oxidation processes were carried out with 100 mm-diameter Czochralski silicon wafers that were phosphorus doped (1~-50 fl cm) with (100) surface orientation. Prior to the oxidation, the wafers were cleaned using the RCA process [8] with additional dip in dilute HF. The cleaning sequence consisted of solutions of H 2 0 - H 2 0 2 - N H 4 O H (mixture of 6:1 : 1 by volume, 85°C, 10 min) and H 2 0 - H 2 O z - H C I (6: 1: 1, 85°C, 10 min) with 5 min DI water rinse after each process finally followed by a dip in 1 : 50 by volume HF: HaO and a DI water rinse of 12 min duration. The wafers were dried by a N2 gas flow. The loading of the RTO reactor took place under an inert gas flow (argon, 21/min). After 20s

the chamber was pumped out under a reduced argon flow (10 sccm) for 60 s and then under an oxygen flow (15 sccm) for the same time. After the reactor was filled under oxygen flow of 3 1/min, the 02 gas flow rate was set to 250 ml/min for the normal pressure oxidation process. The complete thermal cycle is illustrated schematically in Fig. 1. All processes were started with a 60 s preheating period of 400°C to obtain a defined initial condition in the chamber because of the memory effect due to slow cooling down of the quartz glass window from the previous process. With different ramp rates (50°C/s, 100°C/s and 200°C/s) the wafers were heated for oxidation at temperatures of 1050°C, ll00°C and 1150°C and times of 10 s, 30 s and 60 s, respectively. 30 s after the heat source was switched off, the gas flow was changed from oxygen to argon. The used R T P system consists of a cylindrical cooled wall chamber with a rotation symmetrical lamp heating made of three concentric lamp rings [7]. The process temperature was monitored with three thermopiles with a time responds of 25 ms and controlled in scalar closed-loop mode with fixed relative lamp power setting (inner lamp ring 100%, middle 64% and peripheric 43%). The oxide thickness distribution was determined with a PLASMOS SD 2300 ellipsometer (light

T (°C) process

temperature

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

/

linear heating up / period ~

400

..p.reh_e.atl~ mperature /

14sl

60S

i

process time

~,~

30

s ~-,:

oxygen flow

Ar flow v

Fig. 1. Schemeof the entirethermalcycleof the RTO process.

J.-P. Z61lner et al. / Journal of Non-Crystalline Solids 187 (1995) 23-28

wavelength 632.6 nm, incident angle 70°, fixed refractive index mode with n = 1.465 for the thermal oxide).

B1 A1

The oxidation model developed by Han and Helms [3] was used for theoretical thickness determination of oxides produced by RTO. The model based on the assumption of two parallel non-interacting processes, which respond to the fundamental linear-parabolic law [9]. The total oxidation rate can be expressed as

B2 +

2dox+

(1) A2'

where dox is the oxide thickness, t the time and A and B growth constants for the mechanisms 1 and 2, respectively. For calculation of oxide growth on (100) silicon the expressions for the parabolic rate constants are B l = l . 0 8 3 3 x 1 0 8 e x p ( 2 " 2 enVm)2 k T s

(2a)

and

I

(2c)

0o

Be 4.333-104 exp A-S =

(

1.9 eV'] nm /

4. Results

The measured radial oxide thickness distributions in relative units, to the average thickness in the wafer centre, are shown in Fig. 2 for a oxidation temperature 1150°C. With increasing process time,

I

I

1 1 5 0 °C proc,=ssing time = 10 s

.._.. 115 v

e"-'

processing time = 30 s

A7

._o

o~e"---~^

105

x o 1 O0

95

,

0

I

1

=

I

2

(2d)

were applied. Because of the short processing time during RTO, the complete heating cycle must be taken into account in oxide growth calculations. According to recently published results of Messaoud et al. [5], the initial oxide film thickness is necessary to simulate the kinetic growth in RTO. The initial oxide thickness measured before loading the chamber was 0.5 nm.

heating up rate = lO0°C/s

ffl ~ 110 ¢-

(2b)

and

I

processingtemperature=

120

1.6eV']nm 2 kT J s

and for the linear rate constants --=

3. O x i d e thickness calculation

ddox B1 -dt 2dox + A1

B 2 = 4.333 x 106 exp

25

=

I

3

=

I

4

,

5

radial position (cm) Fig. 2. R a d i a l relative oxide thickness d i s t r i b u t i o n for different o x i d a t i o n times. Lines are d r a w n as guides for the eye.

J.-P. Z611ner et al.

26

/ Journal o f Non-Crystalline Solids 187 (1995) 23 28

the relative oxide thickness near the wafer edge (mainly at r > 3 cm) is reduced. The corresponding temperature behaviours in situ measured at the radial position r = 0 cm and r = 4 cm show the temperature overshooting near the wafer edge. The

occurring temperature difference is represented in Fig. 3. It is clearly seen, that the temperature overshooting appears during heating up and at the beginning of the essential oxidation period (plateau region). Because the overshooting has relaxed in

100

I

p r o c e s s i n g t e m p e r a t u r e = 1150 *C. h e a t i n g up rate = 1 O0 *CIs

80 60

L)

J

40

In

•

measured

values

smoothed

behaviour

20 0

i--

'==\_~ ==1= u=~..~ •

.Z.

nin

==

lB._ I

i i

i i

.

-

i

_=.=~lmm _

_

•

• i I

i

• e

~

-20 •

•;,at;au region I

-40

.

=

I

80

6O

1 O0

time (s) Fig. 3. Measured temperature difference between the wafer edge (r = 4 cm) and the centre (r = 0 cm) for a oxidation process of 30 s and 1150 °C.

120

I

I

l

I

p r o c e s s i n g t e m p e r a t u r e = 1100 °C p r o c e s s time = 30 s 115

v 110 ¢/) u) t-. -'~ 105 t"1o " ~ 100 o

~m"-

-~o--- ramp = lO0*C/s _ A _ ramp = 50 *C/s

95 ,

0

I

1

J

I

2

~

I

3

=

I

4

5

radial position (cm) Fig. 4. Radial relative oxide thi'ckness distribution for a oxidation process of 1100°C with several heating velocities. Lines are drawn as guides for the eye.

J.-P. Z6llner et aL / Journal of Non-Crystalline Solids 187 (1995) 23 28 I

I

I

I

I

27

I

I

12

10

03

.......

radial position: radial position:

r = 4 cm

/

r = 0 cm

~

.............

8

""""

(1) ¢-

J

-"'=

.e,..)

6

."~ ~ X I~

4

. ....... --" .....

--''-"

..-"

~

. .-" ...." ,.-" ." ." .&

I 0

,

o

...--'-

.-'° . . - - ' "

•

. .p--'-.wP-.....

2 0

I 10

*

I 20

=

I 30

*

I 40

*

I 50

*

I 60

process time (s) Fig. 5. Calculated oxide thickness as function of the oxidation time for ideal temperature behaviour with a ramp of 100°C/s at the wafer centre (dotted line, without overshooting) and near the wafer edge (full line, with overshooting) in comparison with experimental results. For the wafer center a 10°C lower process temperature was assumed.

approximately 5 s after reaching the plateau region, its influence on the oxide thickness is reduced with increasing process time. Steeper heating ramp rates lead to greater overshooting and consequently to larger thickness inhomogeneities (Fig. 4). The oxide thickness as function of time was calculated for 3 different process temperatures with and without including of the overshooting by means of the parallel oxidation model (Fig. 5). For the wafer centre the growth was calculated using the ideal temperature behaviour according to Fig. 1. Due to the irradiation deficiency of the applied lamp heater in the wafer centre (see Fig. 3) a 10°C lower process temperature was utilized. The oxide growth near the edge was simulated for the ideal temperature profile superimposed with the overshooting difference. The results are in good agreement with the experimental datas. The reverse procedure is used when monitoring the oxidation temperature from the measured oxide thickness, the corresponding temperature must be determined by iteratively solving the oxidation equation. The accuracy depends on the quality of the oxidation model and for short processes from the local temperature behaviour including the overshoot effect. Disregarding this dynamic temper-

ature effect, overestimates the temperature at the wafer periphery. The deviations are for instance for a process temperature of T = 1150°C:11°C at a 10s-process, 2.4°C at a 60s-process, for a 1050°C-process: 8.3°C at a 10s-process, and 1.4°C at a 60 s-process. The influence of overshooting is quite intensive for short process times.

5. Conclusion Because the oxide thickness is a function of the whole temperature cycle, dynamic temperature inhomogeneities must be taken into account in oxidation temperature monitoring for RTP. The temperature overshooting leads to an overestimation of the oxidation temperature near the wafer edge for short processes. For process times > 60 s the influence of dynamic effects can be neglected.

References [1] R. Singh, J. Appl. Phys. 63 (1988) R59. [2] R. Kakoschke, E. BuBmann and H. F611, Appl. Phys. A50 (1990) 141.

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J.-P. Zbllner et al. / Journal of Non-Crystalline Solids 187 (1995) 23-28

[-3] C.J. Han and C.R. Helms, J. Electrochem. Soc.134 (1987) 1297. [4] H.Z. Massoud and J.D. Plummer, J. Appl. Phys. 62 (1987) 3416. [5] A.Y. Messaoud, E. Scheid, G. Sarrabayrouse, A. Claverie and A. Martinez, Jpn. J. Appl. Phys. 32 (1993) 5805.

[6] R. Kakoschke, E. Bussmann and H. F611, Appl. Phys. A52 (1990) 52. [-7] G. Leitz, J. Pezoldt, I. Patzschke, J.-P. Z611ner and G. Eichhorn, Mater. Res. Soc. Symp. Proc. 303 (1993) 171. [8] W. Kern and D.A. Puotinen, RCA Rev. 31 (1970) 187. [9] B.E. Deal and A.S. Grove, J. Appl. Phys. 36 (1965) 3770.