Phosphorus diffusion into silicon from chemically vapour-deposited phosphosilicate glass

Thin Solid Films, 87 (1982) 373 378

373

GENERAL FILM BEHAVIOUR

P H O S P H O R U S D I F F U S I O N I N T O SILICON F R O M V A P O U R - D E P O S I T E D P H O S P H O S I L I C A T E GLASS

CHEMICALLY

G. F. CEROFOLINI, M. L. POLIGNANO AND P. PICCO

SGS-A TES, 20041 Agrate, Milan (Italy) M. FINETTI AND S. SOLMI

Istituto LAMEL, Consiglio Nazionale delle Ricerche, Vio Castagnoli, 1,40126 Bologna (Italy) M. GALLORINI

Centre of Radiochemistry and Activation Analysis, Consiglio Nazionale delle Ricerche, Pavia (Italy) (Received June 29, 1981 ; accepted August 4, 1981)

The diffusion coefficient of phosphorus in phosphosilicate glass in the temperature range 940-1050 °C was determined on the basis of etch rate measurements. Higher diffusivity values and a lower activation energy than the corresponding values for the thermal oxide are obtained. For diffusion of phosphorus into silicon we have shown that, within experimental error and for concentrations lower than the solubility value, the whole of the dopant is electrically active. This is in disagreement with the hypothesis of the existence of compensated phosphorus in the form of E centres in equilibrium with substitutional phosphorus. Moreover, it is shown that the dopant concentration profile predicted by the model of Fair and Tsai is in disagreement with experimental data.

1. INTRODUCTION Phosphosilicate glass (PSG) is an interesting dopant source in MOS technology. For instance, it can be used in CMOS fabrication to form n + regions and simultaneously to mask them against boron deposition in p+ regions. In addition to this technological advantage, it can be used for the investigation of phosphorus diffusion models. Indeed, when PSG is used as a source, etch rate measurements 1 give an easy and almost direct determination of the total amount QSi of phosphorus diffused in silicon. It is well known 1'2 that simple profiles, such as gaussian or error function profiles, cannot describe phosphorus diffusion in silicon from highly doped PSG. Fair and Tsai 3 have developed a quantitative model of phosphorus diffusion in silicon based on the formation of neutral or negative complexes (E centres) between vacancies and phosphorus atoms. This model takes into account some anomalous effects in the phosphorus diffusion profile, such as the plateau in the surface region, the kink and the high diffusivity tail, and is widely used in simulation programs employed in the electronic industry for integrated circuit design. In this work we studied phosphorus diffusion both in PSG and into silicon; the data were analysed with the model of Fair and Tsai and a disagreement between the experimental results and the theory was obtained. 0040-6090/82/0000-0000/$02.75

(c)ElsevierSequoia/Printedin The Netherlands

374

G.F. CEROFOLINIet al.

2. EXPERIMENTALDETAILS For our experiments we used (100)-oriented p-type doped silicon slices of resistivity 2 f ~ c m PSG was deposited in an open tube reactor by simultaneous oxidation of silane and phosphine. The P205 concentration was about 10 mol.°o and the thickness was about 0.5 ktm. More details of the process are reported in ref. 1. After a moderate annealing (in nitrogen at 800 °C for 30 mini the wafers were treated at three different temperatures (940, 1000 and 1050 °C) for 2.25 h. Etch rate measurements of the oxide were performed and the kinetics profiles were transformed into phosphorus concentration profiles, as explained in ref. 1. to allow us to determine by numerical integration the total amount of phosphorus diffused into the silicon. For the sample treated at 940 °C, this quantity was also determined by performing a neutron activation analysis with subsequent measurements of the 15ray activity. The sample was irradiated in the central thimble facility of the T R I G A M a r k II research reactor of the University of Pavia for t0,h; in this position the nominal neutron flux is 9 x 1 0 1 2 c m - 2 s 1. The consequent 3 ray activity was monitored with a 2rt Geiger counter previously calibrated with silicon samples of the same geometry implanted with 31p at increasing doses 13 x 10 t5 5 x 1015 and 8 x 1015 cm-21. The distribution of electrically active phosphorus in the silicon was determined by measurements of the sheet resistance and the Hall effect, after anodic oxidations and oxide stripping, by using the Van der Pauw geometry. To minimize measurement errors, these values were calculated by averaging eight determinations obtained by inverting the current and the magnetic field and interchanging the current and voltage contacts. 3. RESULTSAND DISCUSSION 3.1. Phosphorus diffusion in phosphositicate glass In order to calculate the phosphorus diffusion profile in PSG, a careful analysis was performed by employing S U P R E M 4 (Stanford University process engineering models), a program widely used in the electronic industry for process simulation in integrated circuit design. The accuracy of the solution with respect to the grid spacing was verified. The amounts of phosphorus diffused into the silicon, calculated by using the default value of the diffusion coefficient in the thermal oxide, were found to differ markedly from our data, as shown in Table I. We can alter the diffusion coefficient in

TABLE I T O T A L A M O U N T OF P H O S P H O R U S D I F F U S E D F R O M PSG I N T O S I L I C O N AS D E T E R M I N E D F R O M E T C H RATE M E A S U R E M E N T S A N D AS C O M P U T E D W I T H S U P R E M

T (C)

940 1000 1050

OSi ( x 10 l 5 c m - z)

Measured by etch r a t e

Calculated with SU PREM

5.1 7.9 10.0

1.84 4.05 7.43

P DIFFUSION INTO Si FROM CVD PSG

375

the oxide such that the calculated values just fit the experimental results. The diffusion coefficient so obtained (referred to as the diffusion coefficient in PSG) in the experimental range is greater than that in the thermal oxide (Fig. 1) and depends upon the temperature as follows: DpsG = 12.88 exp

1.71 eV']

ka T / gm 2 min-

1

(1)

where ka is the Boltzmann constant and T the absolute temperature. Since both the pre-exponential factor and the activation energy differ markedly from the corresponding quantities in the thermal oxide, for which

/ 3.5

Dox = 4.56 x l0 v e x p ~ )

I.tm 2 min "1

we can deduce that high temperature diffusion in highly doped PSG occurs by a different mechanism from that of diffusion in thermal oxide. In particular, the much lower activation energy (1.7 eV compared with 3.5 eV) and pre-exponential factor suggest that the motion of phosphorus in PSG is energetically facilitated with respect to its motion in the thermal oxide, but that it requires a more complex configuration which lowers the pre-exponential factor because of the associated entropy of activation 5. This suggests that diffusion in PSG resembles diffusion in a liquid phase. In Fig. 2 the profile calculated by SUPREM (using the diffusion coefficient given by eqn. (1)) is compared with the experimental profile. Although these profiles are in fair agreement, the nature of the deviation of the experimental points from the calculated profile seems to indicate that the diffusion coefficient in the highly doped zones (c/? >~ 2, i.e. c ~> 7 mol.~o) is higher than the value given by expression (1) while in the zones with low doping (c/? <~ 2) it is lower than that value. This suggests that lOOO

T/°C

0oo

±

/~¢

, I

10

\

PSG

\

\

\ \

o18

o10

5~o

io'oo ~ PSG thickness

4soo

sooo

5500

(~ )

Fig. L Arrhenius plot of phosphorus diffusion coefficients in thermal oxide (Dox) and PSG (Desk). The unusual unit (square microns per minute) used for D is that required by SUPREM. Fig. 2. Experimental and computed profiles for a diffusion of 2.25 h in nitrogen at 940 °C (~ = 1.34 × 102 t c m - 3, defined in ref. 1).

376

G.F. CEROFOLINIet al.

the mechanism of phosphorus diffusion is enhanced by the presence of P 2 0 5 (possibly due to softening) rather than because the chemically vapour-deposited oxide is loosely packed compared with the thermal oxide. 3.2. Phosphorus diffusion into silicon In order to investigate the models proposed for phosphorus diffusion into silicon, the total amount of phosphorus diffused from PSG, as computed from etch rate measurements, was compared with the amount of electrically active phosphorus deduced from the experimental carrier concentration profile for samples treated at 940 °C for 2.25 h in an atmosphere of nitrogen. The electrical measurements were carried out on three samples in order to obtain an accurate determination of the active phosphorus concentration. The resulting concentration distribution is reported in Fig. 3. By integration of the profile we obtained QSiac t = (5.2+0.5) x 1015 c m - 2

compared with QSi = (5.1 + 0 . 5 ) x 1015 c m - 2

determined from etch rate measurements. This comparison shows that, within experimental error, all the phosphorus atoms are electrically active, i.e. QSi = QSiact. This is well predicted by the model which assumes that precipitation is the phenomenon that is responsible for electrically inactive phosphorus because the solid solubility of phosphorus at 940 °C is 3.3 x 1020 cm 3 (ref. 6) compared with the surface concentration of 2.4 x 1020 cm 3. In contrast, the model of Fair and Tsai 3' v predicts for any concentration an equilibrium between electrically active and electrically inactive phosphorus. The total amount of phosphorus, computed by the formulae C~ = ns(1 + 2.04 x 10 4x ns2)

(2)

Xo = 2 Ol =

(3)

QSi ~ ½Csx °

\n~/

2ht

)

(4)

given by Fair v, is much greater than our measured value. F r o m ns = 2.4 x 102o cm 3 the values C~ = 5.2 x 1020 cm 3, Xo = 0.69 Jam and QSl = 1.8 x 10 t6 cm 2 are calculated. In addition, the width x0 of the fiat region, computed according to the method of Fair and Tsai, is more than twice the value determined from the profile shape (0.69 tam compared with about 0.3 gm) and the diffusion coefficient in the tail is not correctly predicted. The chemical amount of phosphorus per unit area can be evaluated in a more precise way by numerical integration of the chemical concentration profile, which is computed by the application of expression (2) to the observed carrier concentration profile (broken curve in Fig. 3); the integration gives QS~ = 8.3 x 1015 cm-2. As this value has to be compared with our experimental value, to be more certain of it we measured the amount of phosphorus diffused into the silicon by an independent method, namely neutron activation analysis with subsequent measure-

377

P DIFFUSION INTO Si FROM CVD PSG

ment of the [3 ray activity. This technique gives QSi = (6.4_+0.4) × 1015 cm -2, in substantial agreement with the above value. It is worth noting that the error in the value obtained from neutron activation analysis is due only to statistical fluctuations in the number of counts and that the small discrepancy between the two experimental values can be partly attributed to an overestimate with neutron activation analysis, owing to phosphorus doping on the back of the slice which is caused by the evaporation of P 2 0 5 from the PSG 8. In any case, we observe that the QSi value predicted by the model of Fair and Tsai disagrees with the experimental data. Moreover, the model of Fair and Tsai is employed in SUPREM. Figure 4 shows the carrier concentration profile in silicon as obtained by S U P R E M using the previously determined diffusion coefficient in PSG. Although the surface concentration shows a large deviation and the profile is inconsistent with the experimental data, the junction depth xj and sheet resistivity Ps (xj = 1.28 tam; Ps = 18.5 ~ / D ) are in fair agreement with S U P R E M predictions (xj = 1.31 ~tm; Ps = 17.0 ~ / D ) . i

l v1 ~

i

,

\~xc -

***~ r* ..~ e~

.

1(~ ~

i

"2~

E u

oz

Z 0

z o

°1

0

x

J

",

Z w

\

m

U

•

4~8

,

l~

•

x

\xx i j ,

o

, ..

~'.,.,

~1~ s =

1~"[

"..;'~,

,

,

'o'.s

1 ' ' X/~m

10'7 X//lu m

Fig. 3. Carrier concentration profile in silicon after phosphorus diffusion at 940 :C for 2.25 h in nitrogen from PSG : - - -, the total concentration profile computed according to the model of Fair and Tsai. Fig. 4. Electrically active phosphorus concentration profile as measured (O) and as determined ( - - - ) from S U P R E M analysis (model of Fair and Tsai). The large discrepancy between the surface concentration and the profile should be noted.

In conclusion, the reported data seem to preclude the presence of a significant number of E centres in equilibrium with substitutional phosphorus. In fact, for concentrations lower than the solubility values, we find that within the experimental error the whole of the phosphorus is electrically active. The same conclusion has recently been drawn from different experiments 9. The phosphorus concentration profile predicted by the model of Fair and Tsai on the basis of the experimental carrier surface concentration ns differs markedly from the experimental data. However, the simulation with S U P R E M of phosphorus diffusion from PSG gives

378

G.F. CEROFOLINI et al.

satisfactory results as far as the junction depth and the sheet resistivity are concerned, in spite of the large difference in the concentration profile shape. ACKNOWLEDGMENT

We wish to thank Dr. M. Vanzi (SGS-ATES) for having introduced us in S U P R E M calculations. REFERENCES 1 2 3 4 5 6 7 8 9

M . L . Polignano, P. Picco and G. F. Cerofolini, J. Electrochem. Soc., 127 (1980) 2734. M. Yoshida, E. Arai, H. N a k a m u r a and Y. Terunuma, J. Appl. Phys., 45 (1974) 1458. R.B. FairandJ. C.C. Tsai, J. Electrochem. Soc.,124(1977) l107. D.A. Antoniadis and R. W. Dutton, IEEE J. Solid-State Circuits, 14 (1979) 412. S. Glasstone, K. J. Laidler and H. Eyring, The Theory of Rate Processes, McGraw-Hill, New York, 1941. G. Masetti, D. Nobili and S. Solmi, in H. R. Huff and E. Sirtl (eds.), Semiconductor Silicon 1977, Electrochemical Society, Princeton, N J, 1977, p. 642. R.B. Fair, J. Electrochem. Soc., 125 (1978) 323. M . L . Polignano, P. Picco and G. F. Cerofolini, J. Electrochem. Sot., 128 (1981) 2037. D. Nobili, A. Armigliato, M. Finetti and S. Solmi, J. Appl. Phys., to be published.