NBSIR 77-1404

Control of Mobile-Ion

Contamination in Oxidation

Ambients for MOS Device

Processing

Santos Mayo, Richard Y. Koyama, and Thomas F. Leedy

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

January 1978

Final

Prepared for Defense Nuclear Agency

Washington, D.C. 20305

NBSIR 77-1404

CONTROL OF MOBILE-ION

CONTAMINATION IN OXIDATION

AMBIENTS FOR MOS DEVICE

PROCESSING

Santos Mayo, Richard Y. Koyama, and Thomas F. Leedy

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D C. 20234

January 1978

Final

Prepared for Defense Nuclear Agency

Washington, D.C. 20305

U S. DEPARTMENT OF COMMERCE. Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
'

.
TABLE OF CONTENTS

Page

Abstract 1

1. Introduction 1

2. The Experiment 3

3. Experimental Results 6

I

4. Discussion 9

Acknowledgment 10

References 10

Appendix 1 13

Appendix 2 19

Appendix 3 27

Distribution 33

LIST OF FIGURES

Page

Figure 1. Double-wall transparent fused-silica tube used to

grow silicon dioxide films 4

Figure 2. Mobile-ion density in oxide films grown in the

double-wall oxidation tube with different atmospheres in the jacket 7

iii

Control of Mobile-Ion Contamination in Oxidation Ambients

for MOS Processing

Santos Mayo, Richard Y. Koyama, and Thomas F. Leedy

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

Abstract

An alternative method for controlling the mobile-ion

contamination in the oxidation ambients for MOS device

processing is explored. Mobile-ion contamination in silicon

dioxide films thermally grown in dry oxygen at 1000°C on silicon substrates has been studied by use of a double-wall fusedsilica oxidation tube. The space between the tubes was alternatively filled with chlorine, room air, or sodium hydroxide gas to determine if a correlation exists between the

presence of these substances in the jacket and the mobile-ion

density in the oxide films. MOS capacitors were prepared on

these films and mobile-ion densities were measured using con- ventional C-V techniques. The ion densities ranged from 10 13

to 10 10 cm

-2 as a function of the jacket atmosphere. These

preliminary results suggest that there is a correlation between the presence of cleaning or contaminating agents in

the jacket and the mobile-ion density in the oxide films.

Both cleaning and contaminating actions occur through the

tube wall.

Key words : Double-wall oxidation tube; dry oxidation;

mobile-ion contamination; MOS device processing; MOS devices; oxidation ambient control; oxide growth; semiconductor device processing; silicon dioxide; thermal silicon dioxide films.

1. Introduction

Studies of the electrical properties of thin oxide films used in

metal-oxide-semiconductor (MOS) structures have established that mobileion contamination in these oxide films causes instabilities of microelec

tronic devices. 1 Although the properties of the silicon dioxide film

and its interfaces at the silicon and metal are not fully understood, 2 and more systematic experimental observation is required, it is generally agreed that alkali contamination of these films should be avoided in

order to produce high quality MOS devices. 3 Considerable efforts have

been made to detect and neutralize, or eliminate, such contamination dur

ing process steps required for device fabrication. Extreme cleanliness

during process operations has been recommended, and the use of ultrapure low mobile-ion chemicals has been explored. 4

> 5 The use of thin

1
passivating layers of phosphosilicate glass or silicon nitride on silicon dioxide films has been reported, 6 * 7 and the addition of small quantities (a few mole percent) of chlorine or hydrogen chloride into the

oxidation atmosphere 8-10 has been shown to facilitate the growth of high

quality thermal silicon dioxide films.

The initial purpose of this task was to develop a measurement technique which could be used to determine directly the density of sodium in

oxidation atmospheres used for the growth of thermal oxide films on silicon. Ideally, a technique for in situ measurement of trace contamination in furnace atmospheres would meet the following conditions: 1) the

measuring system should be sensitive enough to detect impurity densities

in the range of interest for semiconductor applications and 2) the equilibrium of the furnace atmosphere should be kept unperturbed during the

measurements; for example, introduction of probes into the oxidation

tube or the extraction of atmospheric samples should be avoided. In order to meet these requirements, a laser-induced resonance fluorescence

technique was developed. This technique, which is sensitive to sodium

in an atomic state only, was used to detect sodium in an open fusedsilica oxidation tube operated at 1000°C. The minimum detectable sodium

density was estimated to be approximately 5 x 10 5 atoms/cm 3

. A copy of

the paper which reported the details of this work 11 is included as Appendix 1. The results obtained through this technique led to the conclusion that atomic sodium is not the most abundant species in oxidation

atmospheres contained in f used-silica tubes. Molecular sodium compounds

appear to play an important role in the contamination of such atmospheres .

In view of the above conclusion, a thermodynamic study was made to

evaluate the equilibrium sodium species in oxidation atmospheres con- tained in both fused silica and silicon tubes operated at 1000°C. During thermal oxidation of silicon, impurities contained in the substrate

or in the oxidation atmosphere come in contact with the growing oxide

film and may become incorporated into it. Contamination of the oxidation ambient may result from impurities in the tube bulk incorporated

during its fabrication process,

12-14 and from contaminants which diffuse

through the tube wall into the oxidation chamber. 15 It was assumed that

the source of sodium contamination in oxidation atmospheres was the tube

wall. Commercially available transparent f used-silica tubes contain

about 10 ppm sodium 12 * 13 while silicon tubes contain about 10 ppb sodium. 14 Sodium in such tubes is assumed to be included as a solid solution of sodium metasilicate in the tube matrix (Na20.Si02(c) in Si0 2 (c)).

The thermodynamics of sodium vaporization from sodium silicate-silica

glasses has been previously studied in the temperature range of 1200 to

2000 K where the equilibrium constant of the reaction Na 2 0.Si02 (c) in

Si02(c) -* 2Na(g) + ^02(g) + Si02(c) has been measured. 16 Using these

results, reactions occurring at 1300 K on the tube wall were analyzed

when oxygen, water, chlorine, or hydrogen are present in the tube atmosphere. Copies of the papers which report the results of these calculations 17 * 18 are included as Appendices 2 and 3. These calculations show

that the presence of water in the oxidation ambient produces a rapid in2
crease of the sodium number density from the 10 7 cm

-3

range corresponding to dry oxidation to about two orders of magnitude higher for water

concentration of a few ppm. Also, the relevant reactions taking place

during oxidation tube cleaning were thermodynamically analyzed. Empirically, it is known that the use of chlorine or hydrogen chloride for in

situ cleaning of the oxidation tube in processing facilities is needed

to grow oxide films with low mobile-ion content. Periodically, tubes

are flushed for several hours with 5- to 10-percent hydrogen chloride

diluted in an inert carrier gas . The calculations show that during

cleaning the equilibrium sodium density increases by several orders of

magnitude above the sodium level in the normal oxidation atmosphere.

This cleaning produces a sodium-depleted layer at the tube wall adequate

for growing clean oxide films.

Sodium diffuses easily in the tube bulk [diffusion coefficient in

silicon dioxide: 2 x 10” 6 cm2 /s] 19 to the tube x^all surface and replenishes the layer depleted by cleaning, thus necessitating periodic cleaning. Contaminants from the furnace heating elements, the refractories,

and the room air continually replenish and add to the sodium contamination in the tube wall. If the tube wall were essentially depleted of

sodium and if external sources of sodium were eliminated for replenishing the tube wall, the oxidation ambient should contribute little mobileion contamination during MOS device processing. These arguments suggest

the use of a double-wall oxidation tube containing chlorine gas between

the inner and outer walls to provide a reactive sink and a barrier for

alkali contamination.

This report summarizes preliminary data on this alternative method

for the preparation of electrically stable thermal oxide films grown at

1000°C without specifically introducing any cleaning or passivating

agents into the oxidation atmosphere.

2. The Experiment

Commercially available transparent fused-silica tubes of appropriate diameter were used to build a double-wall oxidation chamber with a 7-

mm wall separation as shown in figure 1. This tube was operated at

1000°C in a three-zone resistance-heated furnace. At the tube ends, the

inner and outer units were fused together to form a jacket around the

oxidation chamber; no furnace liner was used. This configuration is

similar to a single-wall oxidation tube using a fused-silica liner except

for the joining at both ends. The tube was not cleaned prior to installation in the furnace. The oxidation tube temperature was constantly

maintained at 1000 ± 2°C. Except for short periods during in situ nitrogen anneal, the inner oxidation chamber was continuously filled with

electronic grade dry oxygen flowing at a rate of 4 cm 3 /s. Although this

oxidation is nominally referred to as dry oxidation, it cannot strictly

be considered so. The commercial electronic grade oxygen used contains

about 5 ppm water and 20 ppm hydrocarbons measured as methane. 20 At

oxidation temperature these hydrocarbons decompose producing additional

water in the oxidation ambient. Moreover, it has been pointed out that

3
B

r om

C_5 4

Figure

1.

Double-wall

transparent f

used-silica

tube

used

to

grow

silicon

dioxide

films.

The

inner

chamber

contains

the

oxidation

atmosphere

that

is

fully

surrounded

by

the

outer

jacket.

Chlorine,

roo

air,

or

sodium

hydroxide

atmospheres

are

alternatively

established

in

the

jacket

for

different

oxide

film

growth.

The

temperature

profile

in

the

oxidation

chamber

shows a

central

plateau

extending

about

a

third

of

the

tube

length

with ±

1°C

variation.

The

temperature

at

the

furnace

center

is

1000°C.
at 1000°C, water from the room air around the furnace can permeate

through the 1-mm thick f used-silica oxidation tube wall. Assuming

30-percent relative humidity in the room air at 25°C, the number of

water molecules 21 permeating through the tube wall is of the order of

10 111 cm

-2

*s

-1

. It has been shown that small amounts of water (ppm) in

oxygen increase the silicon oxidation rate. 22-24 The chlorine barrier

around the oxidation chamber constitutes a sink for water permeating

through the tube wall, thus reducing the inclusion of water from room

air humidity into the oxidation ambient. The jacket space between the

inner and outer tubes was filled with either chlorine (99.9 percent),

atmospheric room air, or dilute sodium hydroxide in nitrogen carrier gas,

depending upon the desired condition while oxide films were grown in the

excitation chamber. While chlorine was used, a porous quartz-wool plug

was inserted into the exhaust port of the jacket to minimize chlorine

loss to the furnace scavenger. This plug was removed during periods

when room air was allowed to backs tream into the jacket. The change in

jacket atmosphere from air to chlorine could be made in a short interval,

while the change from chlorine to air takes a much longer period until

the residual chlorine in the jacket is exhausted. In the final stages

of the experiment, external contamination was introduced into the jacket

by passing nitrogen through a one-percent sodium hydroxide solution at room temperature; the gas was allowed to flow through the jacket by re- moving the plug from the exhaust.

MOS capacitors were used as test structures to determine the mobile

ion content in 100-nm thick, thermal oxide films grown on 5-cm diameter

(100) n-type silicon substrates with resistivity in the range from 5 to

10 fi*cm. The silicon wafers (with one face polished, the other lapped

and etched) were cleaned with conventional hydrogen peroxide-ammonia and

hydrogen peroxide-acid solutions, rinsed in 18 Mfi*cm deionized water and

spin-dried in nitrogen. 25 After dry oxidation at 1000°C for 212 min,

the wafers were annealed in situ for 30 min in dry nitrogen.

Metallization by electron-gun evaporation of a 0.5-ym aluminum layer on the oxide film immediately followed the annealing step. Photolithographic techniques were used to define the gate area.* The substrate contact was made by evaporation of a 0.2-ym gold (with 0.6-

percent antimony) layer on the entire back side. Finally, the capacitor

was annealed at 500°C for 30 min in dry nitrogen. Processing steps such

as wafer cleaning, oxidation, and metallization were accomplished in a systematic sequence with no delays or storage periods. To monitor accidental contamination introduced during steps other than oxidation, each

lot of four processed wafers was divided into two groups of two wafers

each. Two control wafers were oxidized in a conventional single-wall.

The metallization pattern was taken from the fourth level of the NBS-3

mask set. 26 Four capacitors of 381~ym diameter are available per cell

and each cell is repeated across the wafer at 5.08-mm intervals. Three

of the four capacitors have guard rings to control surface current if

necessary

.

5
f used-silica oxidation tube provided with a liner

;

+ the other two wafers

were divided into halves along a diameter^

4- and oxidized in the doublewall tube. With the exception of the oxidation step, the lot of four wafers was processed as a unit. The data on the control MOS capacitors

were used to gauge the uniformity in the lot processing during the experiment .

The mobile-ion density of the thermal silicon dioxide films was determined from the flat band voltage shift of the MOS capacitors induced

by bias-temperature-stress (BTS) testing using cycles of ±10 V at 300°C

for 5 min. The resulting electric field applied across the oxide film

is about 10 6 V/cm. The capacitance versus voltage (C-V) measurements

were made at room temperature using a commercial capacitance bridge operating at 1 MHz with an applied test signal of 15 mV. Although the en- tire wafer was temperature stressed for each BTS cycle, only specific ca- pacitors (about 8 on each wafer) were bias stressed for a given BTS test.

Assuming that the flat-band voltage shift is due entirely to the change

in position of the mobile ions with respect to the silicon dioxidesilicon interface, the density of mobile ions, Q /q, is given in cm

-2 by

the expression: 27

Q C e e — = AV — = AV ~v~~ q fb fb qX

q o

where AV is the change in the flat-band voltage (V) observed as a re- sult of the BTS test, C is the oxide film capacitance per unit area

(F/cm2 ), e is the relative dielectric constant of the oxide, e is the

permittivity of the free space (F/cm), X is the oxide film thickness

(cm), and q is the electronic charge.

3. Experimental Results

The results reported here were obtained during a time period of

more than one year. Figure 2 shows the mobile-ion density in the MOS

capacitors prepared in the double-wall tube. The dot on each bar is the

average value of the span of data represented by the vertical bar. The

variation of the mobile-ion density for a given data point represents

the spread of the data taken over the complete lot of processed half wafers. This variation shows nonuniformity of the mobile-ion density with

The single-wall oxidation tube used a fused silica liner separated

from the oxidation tube by quartz wool. The space between the liner

and the oxidation tube was not sealed from the room air. The inner

tube was periodically cleaned in situ for several hours with 5-percent

hydrogen chloride diluted in dry nitrogen.

The diameter of the double-wall oxidation tube was such that only halfwafers (divided along a diameter) could be processed.

6
GAS

IN

JACKET

(

HOURS/

DAY )

MOBILE

ION

DENSITY

Q

0

/q

(cm' 2)

0 IOO 200 300 400 500

DAYS

Figure 2. Mobile-ion density in oxide films grown in the double-wall

oxidation tube with different atmospheres in the jacket. The dot on each bar is the average value of the span of data corresponding to a given wafer lot represented by the bars. The two dashed lines represent the range of mobile-ion density measured on the control wafers.

The bottom part shows periods when chlorine, room air, or sodium hydroxide gas is allowed to fill the jacket.

7
position on the wafer and does not reflect the measurement precision.

(The technique is capable of resolving mobile-ion densities in the range

of 4 x 10 9 cm

-2 for 100-nm thick oxides.) Such variation could possibly

be interpreted in terms of localized clustering of the mobile ions. 28

The dashed lines represent the range of mobile-ion density of the control wafers with oxide films prepared in the single-wall oxidation tube.

The bottom part shows the periods with chlorine, room air, and sodium

hydroxide in the jacket. During the first, second, and third chlorine

periods the gas was allowed to flow into the jacket for 5, 18, and 24

hours per day, respectively, while the quartz-wool plug was inserted

into the exhaust port of the jacket. This allowed a gradual treatment

of the tube with increasing chlorine concentration in the jacket.

After 200 days from the end of the last chlorine cleaning period,

external contamination was introduced into the jacket. Nitrogen was passed through a one-percent sodium hydroxide solution at room temperature and the resulting gas was allowed to flow into the jacket for a period of 181 h. Silicon dioxide films were grown during this period

with sodium hydroxide in the jacket. The mobile-ion density in the

oxide films was measured and an order-of-magnitude increase was observed

(see fig. 2). This is due to sodium contamination in the oxidation atmosphere resulting from the passage of sodium through the tube wall.

The initial mobile-ion density in the 10 12 to 10 13 cm

-2

range cor- responds to MOS capacitors prepared on wafers which were processed prior

to the first chlorine period; the double-wall tube initially had been

installed in the furnace with no previous tube clean-up. The data show

that the first two chlorine periods were not effective in achieving com- plete cleaning of the oxidation atmosphere. By the end of the third

chlorine period the double-wall tube data are comparable to mobile-ion

density data corresponding to control wafers processed in the singlewall oxidation tube. After the third chlorine period, contamination levels in oxides grown in both the double-wall and the single-wall oxidation tubes were similar. This result that was consistently observed for

about 200 days is extremely encouraging considering that the tube was initially severely contaminated.

In order to investigate the possibility of chlorine from the jacket

diffusing through the tube wall and becoming part of the oxidation am- bient and thus passivating the mobile-ion content of the oxide, neutron

activation analysis was used to obtain a measure of chlorine in the oxide films prepared in both the double-wall and the single-wall oxidation

tubes. Samples of area about 1 cm2 were packaged in pure polyethylene

bags and irradiated in the NBS reactor for periods of 10 and 20 min to

thermal neutron fluences of 1.5 to 3 x 10 16 cm

-2

. These results indicate that the chlorine content in oxide films prepared in both the

single-wall and the double-wall tubes during chlorine periods is about

the same and equal to 10 ppb. Similar results obtained on plain unoxidized silicon substrates indicate higher chlorine content, presumably

due to residual chlorine left on the wafers after the cleaning procedure. 25

8
4. Discussion

The results presented here suggest that the mobile-ion density in

oxide films prepared in the double-wall fused-silica oxidation tube is

influenced by the atmosphere established in the jacket. The first oxide

film was grown immediately after the tube was installed in the furnace.

The tube was not cleaned prior to installation and the jacket was open

to the room air. The corresponding mobile-ion density resulted in the

high 10 12 cm

-2

range. During the next 100 days, chlorine was used for

two short periods separated by an interval with room air in the jacket.

Oxide films grown during these 100 days show a one-order-of-magnitude re- duction in the mobile-ion density. After the third chlorine period, room air was allowed into the jacket and several oxides were grown at

various times during an interval of about 180 days. These oxides consistently exhibited mobile-ion densities within the range of control oxides

prepared in the single-wall tube. This represents a reduction of two

orders of magnitude in mobile-ion density with respect to the initial result. By using sodium hydroxide contamination in the jacket, the mobileion density was increased again by one order of magnitude. Both the

cleaning action of chlorine and the contaminating action of the sodium

hydroxide occur through the tube wall. A model describing the interactions in the tube with the jacket atmosphere would be valuable to interpret the present results. More theoretical and experimental work is re- quired to understand the permeability of fused silica at 1000°C to various gases.

Neutron activation analysis results indicate that the chlorine content in oxides grown in the double-wall oxidation tube is only about 10

ppb (about 10 14 cm

-3

). These results are consistent with results from

ellipsometric determination of the index of refraction of these oxides

at wavelength 632.8 nm where no significant departures from the nominal

value n = 1.462 are observed. Changes in the index of refraction of An = 0.0018 to - 0.0079 have been reported in silicon dioxide films grown in

oxygen with the addition of 0.5 to 2 volume percent chlorine at 1000°C. 29

Experimental evidence shows that thermal silicon dioxide films grown at

1100°C in one-percent hydrogen chloride in oxygen on (111) phosphorusdoped 10 fi*cm silicon wafers contain large amounts of chlorine. The re- sidual chlorine profile measured by the Rutherford backscattering technique using a 2 MeV- 4He beam indicates a chlorine content in these oxides higher than 1020 cm

-3

. 30

Oxide films grown in the double-wall oxidation tube may also be of

interest for fabricating radiation hardened devices. Although it has

been established that there is little correlation between mobile-ion con- tamination in silicon dioxide films and radiation sensitivity of MOS devices, it has been recognized that the use of hydrogen chloride during

oxidation to reduce the mobile—ion content in these films increases the

device radiation sensitivity. 31 Although the radiation sensitivity of

the oxides produced in the double-wall tube has not been investigated,

the technique provides an alternative to present radiation-hard oxide

preparation methods requiring extensive hydrogen chloride tube cleaning

9
periods prior to the oxide growth. 31 The double-wall oxidation tube

with the chlorine barrier requires no specific cleaning of the oxidation

chamber. Hence, in principle, oxidation can occur on a continuous basis

with no interruptions for routine clean-ups as required by the conventional hydrogen chloride oxidation chamber cleaning. However, additional experimental work is required to completely characterize these oxides.

Acknowledgment

The authors are grateful to R. M. Lindstrom and the Activation Analysis Section for measurements of chlorine content in silicon and thermal

silicon dioxide films prepared for this work. Thanks are due to Drs. W.

Murray Bullis and Kenneth F. Galloway for their constant stimulus for

and criticism of this work.

References

1. Hickmott, T. W. , Thermally Stimulated Conductivity of Sodium in

Thermal Si0 2 , J. Appl. Phys. 46_, 2583-2598 (1975).

2. Deal, B. E.., The Current Understanding of Charges in the Thermally

Oxidized Silicon Structure, J. Electroehem. Soc. 121 , 198C-205C

(1974) and Charge Effects and Other Properties of the Si0 2 -Si Interface: The Current Understanding, Semiconductor Silicon 1977, H. R.

Huff and E. Sirtl, Eds,, pp. 276-296. Proceedings of the Third International Symposium on Silicon Materials Science and Technology,

The Electrochemical Soc., Inc. (1977).

3. Derbenwick, G. F., Mobile Ions in Si02

: Potassium, J. Appl. Phys.

48, 1127-1130 (1977).

4. Yurash, B., and Deal, B. E. , A Method of Determining Sodium Content

of Semiconductor Processing Materials, J. Electroehem. Soc. 115 , 1191-1196 (1968).

5. Knolle, W. R. , and Retajczyk, T. F., Jr., Monitoring Sodium Contamination in Silicon Devices and Processing Materials by Flame Emission Spectrometry, J. Electroehem. Soc. 120 , 1106-1111 (1973).

6. Kerr, D. R. , Logan, J. S., Burkhardt, P. J. , and Pliskin, W. A.,

Stabilization of Si0 2 Passivation Layers with P 2 05 , IBM J. Res. Develop. 8, 376-384 (1964).

7. Eldridge, J. M. , Sodium Drift Through Phosphosilicate Glass-Si0 2 Films, J. Electroehem. Soc. 118 , 986-991 (1971), Balk, P., and Eldridge, J. M. , Phosphosilicate Glass Stabilization of FET Devices,

Proc. IEEE 57_, 1558-1563 (1969).

8. Kriegler, R. J. , Cheng, Y. C., and Colton, D. R., The Effect of HC1

and Cl 2 on the Thermal Oxidation of Silicon, J. Electroehem. Soc.

119, 388-392 (1972).

10
9 . Kriegler , R. J. , Neutralization of Na+ Ions in "HCl-Grown" Si0 2 , Appl. Phys. Lett. 20, 499-451 (1972).

10. Kriegler, R. J. , The Role of HC1 in the Passivation of MOS Structures, Thin Solid Films 13 , 11-14 (1972).

11. Mayo, S., Keller, R. A., Travis, J. C. , and Green, R. B., Detection

of Sodium Trace Contamination in Furnace Atmospheres at 1000°C, J. Appl. Phys. 4_7 , 4012-4106 (1976).

12. Hetherington, G. , Stephenson, G. W. , and Winterburn, J. A., Vitreous Silica in Electronics, Electronic Engineering 41 , No. 495, 5256,

and No. 496, 4447 (1969).

13. Owen, A. E. , and Douglas, R. W. , The Electrical Properties of Vitreous Silica, J. Soo. Glass Technology 4_3, 159T-178T (1969).

14. Dietze, W. , Hunt, L. P., and Sawyer, D. H. , The Preparation of CVDSilicon Tubes and Boats for Semiconductor Device Technology, J. Electrochem. Soc. 121, 1112-1115 (1974).

15. Fewer, D. R. , and Gill, W. L. , Investigation of Reliability Testing

and Prediction Techniques for Integrated Circuits, Technical Report

No. RADC-TR-66-345 , pp. 33-46 (1966). Also available as NTIS Document AD-489-969.

16. Vidale, G. L. , Measurement of the Absorption of Resonance Lines III,

Vaporization of Sodium from Sodium Silicate Glasses, General Electric Co., Technical Information Series, R60SD390 (1960).

17. Mayo, S., and Evans, W. H. , Development of Sodium Contamination in

Semiconductor Oxidation Atmospheres at 1000°C, J. Electrochem. Soc.

124, 780-785 (1977).

18.

Mayo, S., and Evans, W. H. , Thermodynamic Considerations in the Use

of Polysilicon Oxidation Tubes for Clean Si02-Film Preparation, J.

Electrochem. Soc. 125, 106-110 (1978).

19. Wolf, H. F., Silicon Semiconductor Data , pp. 526-527 (Pergamon

Press, Oxford, 1969).

20. Matheson Gas Products, Oxygen Data Sheet.

21. Revesz, A. G. , Noncrystalline Silicon Dioxide Film on Silicon: A

Review, J. Eoncrystalline Solids 11^, 309-330 (1970).

22. Irene, E. A., The Effects of Trace Amounts of Water on the Thermal

Oxidation of Silicon in Oxygen, J. Electrochem. Soc. 121 , 1613-

1616 (1974).

11
23. Irene, E. A., and van der Meulen, Y. J. , Silicon Oxidation Studies:

Analysis of Si02 Film Growth Data, J. Electrochem. Soo. 123, 1380-

1384 (1976).

24. Irene, E. A., and Ghez, R. , Silicon Oxidation Studies: The Role of

H2 0, Semiconductor Silicon 1977 , H. R. Huff and E. Sirtl, Eds.,

pp. 313-323 (The Electrochemical Society, Princeton, 1977).

25. Kern, W. , and Puotinen, D. A., Cleaning Solutions Based on Hydrogen

Peroxide for Use in Silicon Semiconductor Technology, RCA Review

31, 187-206 (1970).

26. Buehler, M. G., Microelectronic Test Pattern NBS-3 for Evaluating

the Resistivity Dopant Density Relationship of Silicon, NBS Special

Publication 400-22 (June 1976).

27. Grove, A. S., Physics of Semiconductor devices , p. 339 (J. Wiley

and Sons, Inc., New York, 1967).

28. DiStefano, T. H. , Dielectric Breakdown Induced by Sodium in MOS

Structures, J. Appl. Phys. 44^, 527-528 (1973). DiStefano, T. H. , and Viggiano, J. M. , Interface Imaging by Scanning Internal Photoemission, IBM J. Res. Develop. JL8, 94-99 (1974).

29. van der Meulen, Y. J. , Osburn, C. M. , and Ziegler, J. F., Properties of Si0 2 Grown in the Presence of HC1 on Cl 2 , J. Electroahem.

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Meek, R. L. , Residual Chlorine in 0 2

: HC1 Grown Si0 2 , J. Electrochem. Soc. 120 , 308-310 (1973).

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Gregory, B. L. , Process Controls for Radiation-Hardened Aluminum

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12
Appendix 1

Detection of sodium trace contamination in furnace

atmospheres at 1000 C*

Santos Mayo

Electronic Technology Division, Institute for Applied Technology. National Bureau of Standards, Washington. D.C. 20234

Richard A. Keller

Physical Chemistry Division, Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

John C. Travis and Robert B. Green

Analytical Chemistry Division. Institute for Materials Research, National Bureau of Standards, Washington, D C. 20234

(Received 12 May 1976)

Free sodium atoms were detected by resonance fluorescence in an open contaminated quartz tube heated

to 1000‘C. The quartz tube and furnace were similar to those used in semiconductor device processing. Fluorescence was excited by a cw dye laser tuned to the sodium D, or D2 transition and directed along the

axis of the furnace. Fluorescence from the sodium £>; line emitted in the axial direction was collected by a

telescopic system and focused onto a photomultiplier tube. The

density in the furnace is 5 x10s atoms/cm3

. No free sodium was

not been intentionally contaminated.

PACS numbers: 07.20.Ka, 32.10.Hg, 82.80.Hn

I. INTRODUCTION

Sodium contamination in microelectronic devices has been correlated with erratic electrical behavior of these devices. 1 The electrical properties of silicon di- oxide films in metal-oxide-semiconductor (MOS) devices have been the subject of many investigations in which

sodium was shown to be one of the principal causes of

instabilities. 2 Among other sources of sodium conta- mination such as those due to materials associated with

the processing of semiconductor devices, 3 the oxidation furnace atmosphere itself was suggested as a pro- minent one because of the high diffusion coefficient of sodium in silicon and silicon dioxide at temperatures used for oxidation. 4 It has been postulated that sodium

is transferred from the furnace material to the silicon wafer through the furnace atmosphere as free sodium

atoms. Contamination control during device processing

has been highly recommended to produce radiation hardened stable MOS devices. The mechanisms by which

alkali contamination affects hardness are not well established5 although ion microprobe studies of MOS devices show a correlation of sodium content in the oxide

with radiation sensitivity. 6 The purpose of this work

is to develop a measurement technique for measuring

sodium impurities in furnace atmospheres used in the growth of MOS oxides and in their subsequent annealing.

It is difficult to detect free sodium atoms at atmospheric concentration levels concomitant with the observed device contamination. Atomic absorption techniques are limited to concentration ranges above 10s atoms/cm3 by the difficulty of measuring small changes in transmitted light. 7,8 Sodium concentrations as small as 100 atoms/cm3 were recently detected in an evacuated tube at - 28 °C by a resonance fluorescence technique. 9-11 This paper

reports on an extension of this method to an open quartz

tube at 1000 °C. Problems associated with Doppler and

4012 Journal of Applied Physics, Vol. 47, No. 9, September 1976

estimated minimum detectable sodium

detectable in a processing tube that had

collisional cross-section reduction, reaction of the sodium with oxygen and water, quenching of the emission by molecular collisions, and scattering from furn- ace gases, thermal gradients, suspended dust particles, and geometrical restrictions related to the structure of the furnace result in a poorer detection limit.

II. EXPERIMENTAL METHOD

A diagram of the apparatus used for the detection of resonance fluorescence from sodium vapor is shown in

FIG. 1. Experimental arrangement for sodium detection in a semiconductor processing furnace. E, 0. 5-mm etalon mounted

on torsion motor; A, power amplifier; W, sinusoidal wave generator (0. 5 Hz); S, sweep counter; MS, multiscaler; V/F,

voltage to frequency converter; PSD, phase sensitive detector; C, chopper (2 kHz); P, power meter; L, lens; M, mirror; T,

telescope; D, diaphragm; F, interference filter Cl.25-nm FWHM, 588. 5 nm); PM, photomultiplier tube; l, length of observation region.

American Institute of Physics 4012

13
FT

: 1

IJII Mil 1 1 1 1 1 1 1 IMP

:"ii i i 1 i : 1 llll M M Mil

1 TIT II I I 1 1 1 1 1 1 1 1 t 1 l_ M 1 Ml till llll llll

. -J ...I

0 01 NM

FIG. 2. Typical laser beam mode structure. The repetition of the pattern is due to the two spectral ranges covered by the spectrum analyzer.

Fig. 1. Amplitude -modulated radiation from a'tunable dye laser passes down the furnace slightly off axis (2°) and through a hole in the laboratory wall. The fluores- cence emission traveling out of the tube in the direction opposite to the laser beam is collected by a telescopic

system, passed through an interference filter (centered on the D2 line), and focused onto a photomultiplier tube. The ac signal corresponding to the fluorescence is syn- chronously detected, digitized, and recorded on a multi- channel analyzer as a function of the wavelengh of the excitation laser.

The excitation source was a commerical cw dye

laser which was longitudinally pumped with powers up

to 5 W by an all-line argon -ion cw laser. An intracavity

birefringent filter was used to coarse tune the dye laser

with a resultant bandwidth of 0. 03 nm. Insertion of a 0.5-mm etalon into the cavity further narrowed the output to 0. 003 nm. The mode structure of the dye laser was monitored by observing a small fraction of the beam

with an interferometric spectrum analyzer (8-GHz free spectral range). A typical mode structure is shown in Fig. 2. These laser modes fall within the broadened

absorption line of sodium (~ 0. 01 nm). The laser emission wavelength could be repetitively and reproducibly scanned over the absorption line by rotating the etalon through an angle of about 1° with an oscillatory torsion motor driven by a low-frequency (0.5 Hz) low-amplitude

sinusoidal wave. The laser was tuned to center the so- dium Z), resonance on the approximately linear portion

of the sinusoidal wavelength scan. The signal-to-noise

ratio was increased by repetitive scanning of the laser wavelength and summing the data from approximately

100 sweeps in the multichannel analyzer. The wavelength

scale was calibrated by passing a portion of the output

of the laser through the spectrum analyzer.

A major limitation to the minimum observable sodium

signal was scattered laser light. Extreme care was

necessary in the placement of optical components, apertures, and the dumping of the laser beam to minimize

back reflection. Contributions from scattered light were

further reduced by excitation of the Dx line (589.6 nm)

and observation of the D2 emission (589. 0 nm). At atmospheric pressure, collisions with molecular nitrogen and oxygen in the furnace completely equilibrate the population of these two levels according to their statistical weights. 12,13

The calibration necessary to relate the density of sodium atoms to the observed fluorescence signal was

done by two methods. The first method involved a comparison of the fluorescence intensity from the sodium

with the fluorescence intensity from a dilute solution

of rhodamine 6G placed in a glass cell in the cooled

furnace. The dimensions of the glass cell were the same

as those of the quartz tube. The second method was

based upon a calculation of the geometric factors in- volved in the detection optics and a measurement of the

ratio of the intensity of the fluorescence emission to the excitation intensity. The methods yielded results that agreed within a factor of 3. Sodium fluorescence signals were not observable in clean quartz tubes. Nonquantitative sodium contamination was intentionally introduced by aspirating dilute sodium chloride solutions into the hot tube for short periods of time and then waiting for several hours until a relatively constant level of contamination was ob- served.

III. FLUORESCENCE SIGNAL RESULTS

The fluorescence signal from sodium vapor inside of the quartz tube is shown in Fig. 3. These data are the accumulation of 100 2 -sec sweeps. The average excitation power was 70 mW. The observed linewidth

of the excitation spectrum (7. 5 GHz or ~ 0.01 nm) agrees

well with the calculated linewidth taking into account Doppler (2. 7 GHz) and pressure (6.2 GHz) broadenings under the conditions of the experiment. This good agree- ment is indicative of the frequency stability of the laser system. Background blackbody emission from the fur- nace was discriminated against by the sodium interfer- ence filter and synchronous detection. The nonzero

signal observed off resonance is caused by scattered

laser light. The scattered light signal is reduced ap- proximately an order of magnitude when the L>, tran2500

X

2

= 588 997nm >-

t= 2000 - GOzUJ

F—35 1500 - UJO

z^ 1000 1 . GO • V* • ••

• .w* • * UJ •*• •• CE * •**••• .** *•. .. O

ZD bUO - Un-O-' 1 1 1 1 1 1 -35 -25 -15 -5 5 15 25 35 FREQUENCY SHIFT (GHZ)

FIG. 3. Sodium fluorescence signal detected in contaminated

furnace atmoshpere pressure, 1000 °C. This signal corresponds to 2.5xl07 sodium atoms/cm3

.

4013 J. Appl. Phys., Vol. 47, No. 9, September 1976 14

Mayo etal. 4013
TABLE I. Constants for calibration using rhodamine 6G.

/B (589.0)//Nl(589. 0) 0. 19“ /„(589.6) 70 mW“

oB

(571. 8) 2.4 xio- 18 cm2 “ 0.85 (Ref. 14)

crNa(589.6) 7. 8 xlO" 13 cm2 b 0.01-0.04 (Refs. *R 3.5 xl013/cm3 “ 6.1 xl0-9b

4(571.8) 9.0 mW“ ^Na 0.31 b

“Experimentally determined. b Calculated.

sition is excited and the 02 emission observed, in agreement with the transmission characteristics of the interference filter. Any reduction in the absolute magnitude of the scattered light results in a reduction in the associated shot noise and in noise resulting from

fluctuations in the scattering medium. After careful a- lignment of the optical system, a major contribution to the scattered light was reflections from suspended dust

particles in the tube. This contribution could be reduced

by flushing the tube with clean nitrogen. A statistical analysis of the data in Fig. 3 shows that the ratio of the fluorescence signal at the peak to the noise in the scat- tered light is> 50. The long-term precision is better than 10%.

IV. CALIBRATION I The quartz tube was replaced by a room-temperature

cell containing a dilute solution of rhodamine 6G dis- solved in ethyl alcohol. Absorption cross sections and

fluorescence quantum yields for this material are well known. 14 For the measurement of the fluorescence intensity at the wavelength of the sodium D2 line, it was expedient to irradiate at a shorter wavelength to provide

a reasonable cross section and to reduce scattered light from the windows of the cell and dust particles in the

liquid. A convenient wavelength was found to be around

570 nm. The laser beam was deflected by the solution and windows on the cell, requiring a slight realignment

of the optics to maximize the fluorescence signal and minimize the scattered light. The calibration process can be understood by considering the following equations:

/R( 589. 0) = <tr ( 571 . 8)iVRf/0(571. 8)(? RAR G, (1)

/Na(589. 0) = eNa(589. 6)NNaW0(589. 6>QNaANaG . (2)

IR and /Na are the measured fluorescence intensities from rhodamine and sodium, the cr’s are the absorption cross sections at the indicated wavelengths, the

Af’s are atomic or molecular densities, l is the length

from which fluorescence is being detected, the Q’s are

fluorescence quantum yields, the A’s are the spectral

fraction of the emitted light passed by the interference

filter, and G is a geometric collection factor assumed

similar for both samples. Taking the ratio of Eq. (1) and Eq. (2) and solving for NNa gives Eq. (3),

N =N /„,( 589,0) oR (571. 8) 4(571.8) <? B A R {3 ) Na R

7r ( 589. 0) oNa(589.6) /0 (589.6) (? Na ANa

'

All quantities in Eq. (3) are known or can be measured. Ar

is found by folding the transmission curve of the

4014 J. Appl. Phys., Vol. 47, No. 9, September 1976

interference filter [T(A)] into the spectral emission

curve of the dye [F (A)]. Ar = / T(VF(\)d\/J FMd\. (4) ANa is calculated by assuming equal population among

the substates of and D2 levels and including the

effects of the interference filter,

4».= H7J) r(589 ' 6) + (TT2Tr<589 0) '

The statistical weights of theD, and U2 levels are 2 and

4 respectively. crNa can be calculated from the oscillator strength and the calculated linewidth contributions. 13 The quantities for Eq. (3) are given in Table l.

15 * 16

V. CALIBRATION 2 Equation (2) relates the observed fluorescence intensity to the density of sodium atoms. Rearrangement of

this equation for the case where both excitation and

observation are at 589.0 nm gives

^'

ifsSlLO) Ka(589.0)^NaANaG]-, (6)

All quantities to the right of the equality sign are known

or can be measured. The values of o, l, Q, and A are

listed in Tables I and II. The intensities of the sodium

fluorescence and laser excitation were measured by

comparison with an electrically calibrated pyroelectric

detector. 17 Neutral-density filters were used with PIN

diodes to change intensities in known ratio, into the range of linearity for these devices. The geometric

collection factor was evaluated by consideration of the optical design shown in Fig. 1. The results of both calibrations are listed in Table

III. The data for calibration 2 were preliminary and are recorded here only as an independent check on later results.

VI. OXIDE IMPURITY RESULTS

A clean (the quartz tube was rinsed in hydrofluoric and nitric acid solutions and washed in deionized water)

TABLE II. Additional constants for calibration with calculated geometric factors.

<rNa (589.0) 1.5X10-12 cm2 “ G S.SXlO-4 *

/Na (589.0) 360 pW b

l 60 cm b

/0 (58 9.0) 140 mWb

“Calculated.

b Experimentally determined.

Mayoef al. 4014

15
TABLE III. Observed sodium densities and estimated minimum

detectable quantities. a

Calibration Na density S/N Minimum detectable (atoms/cm3

) quantity 11 (atoms/cm3

)

1 (rhodamine) 3—12X107 56 5-20 X105

2 (geometric) 4-16 X106 22 2—8 X105

“Range reflects uncertainty in QNa. b Signal to noise equal to two.

quartz furnace tube with end caps was used to grow

oxide films on silicon wafers. After the oxide film was prepared, the end caps were removed (water from

ambient humidity may have diffused into the tube during

the measurement; see Sec. VII) and it was found that no sodium fluorescence was observable within the detection limits reported in Table III. The oxide films grown on silicon in this clean tube showed an order of magnitude higher sodium contamination level than is normally present on similar films prepared in production-type semiconductor processing oxidation tubes operated in a controlled environment. These results were obtained by capacitance -vs -voltage measurements

performed on MOS capacitors after application of simultaneous electric bias and temperature stress cycles. Electric fields of about 10

6 V/cm were established a- cross the oxide film by use of positive or negative 10-V

gate polarization while the device temperature is maintained at 300 °C for 10 min (MOS-CV-BT stress

technique). 18

Similar oxide layers were grown on silicon in a sodi- um-contaminated oxidation tube where sodium fluores- cence from approximately 101 atoms/cm3 was measureable. MOS capacitors built on this oxide exhibited two orders of magnitude more sodium content than similar

control capacitors prepared in a standard processing

facility as measured by the MOS-CV-BT stress

technique.

VII. DISCUSSION

It is important to note that the technique described above is a measurement of free sodium atoms only. Sodium present in compounds (e.g. ,Na20 and NaOH)

would not be detected. These results are important for understanding the apparent discrepancy between the oxide impurity measurements and the resonance fluo- rescence results. It is a difficult thermodynamic pro- blem to estimate the concentration of oxides and the hydroxide from a knowledge of the free sodium concentration because the concentrations of other species

(e. g. , H2 and OH) are uncertain by many orders of magnitude. It should be possible to measure the hydroxyl

radical concentration in the furnace by a similar reso- nance fluorescence technique. 19 Knowledge of both the free sodium and the hydroxyl radical concentration could be used to determine the sodium hydroxide concentration.

In a production semiconductor processing furnace,

where the oxidation atmosphere is completely enclosed

in the quartz tube and carefully controlled dry oxygen

is used, a more favorable ratio of free sodium to total

4015 J. Appl. Phys., Vol. 47, No. 9, September 1976

sodium might be expected. Such conditions would improve the atomic measurement except for additional sources of light scattering at the capped tube ends. However, these sources of interference could be practically eliminated by an alternative geometry discussed below.

There are several improvements which should de- crease the detection limits reported above. The first of these involves frequency modulation of the excitation beam and synchronous detection of the fluorescence to reduce contributions from frequency -independent scattering. Frequency modulation of the cw dye laser is relatively straightforward.

Scattered light could be significantly reduced by using

a monochromator in place of the interference filter to completely isolate the D1 excitation from the D2 emission. The insertion of the monochromator may reduce

the total number of photons reaching the photomultiplier

tube to the extent that photon counting might be necessary.

A reduction of scattered light and an increase in light gathering power could be accomplished by mounting the detecting optics in ports distributed down the tube and

viewing the fluorescence at right angles to the excitation. This has two additional advantages; (1) information on the spatial distribution of the contamination would now be available by viewing through different ports; (2) windows on the quartz tube would not inter- fere with the measurement because laser scattering

from them would be far from the field of view subtended by the detector. ACKNOWLEDGMENTS

The completion of this project was facilitated by many

fruitful discussions with Gabriel Luther of the Quantum

Metrology Section. Thanks are also due to John Geist and Edward Zalewski of the Radiometric Physics Section for help in the absolute measurement of light intensities.

•This research was conducted under the joint auspices of the Semiconductor Technology Program and the Laser Chemistry

Program of the National Bureau of Standards. Partial support was provided by the Defense Nuclear Agency.

‘R.J. Krtegler, 12th Annual Proceedings Reliability Physics

1974 (IEEE, New York, 1974), p. 250.

2 T.W. Hlckmott, J. Appl. Phys. 46, 2583 (1975).

3 B. Yurash and B. E. Deal, J. Electrochem. Soc. 115, 1191

(1 968).

4 D.R. Fewer and W. L. Gell, Texas Instruments Inc. , Technical Report No. RADC-TR-66-345, 1966 (unpublished).

S B. L. Gregory, IEEE Trans. Nucl. Set. NS-22, 2295 (1975). ®H. Hughes, R.D. Baxter, and B. Phillips, IEEE Trans.

Nucl. Set. NS-19, 256 (1 972).

7 T.E. Burgess andH.M. Donega, J. Electrochem. Soc. 116, 1313 (1969).

8N. loll, F. Strumla, and A. Morettt, J. Opt. Soc. Am. 61, 1251 (1971).

Mayo era/. 4015

16
9D. A. Jennings and R. A. Keller, J. Am. Chem. Soc. 94, 9249 0 972).

,0W.M. Fatrbank, Jr., T.W. Hansch, andA.L. Schawlow, J. Opt. Soc. Am. 65, 199 0975). "F.C.M. Coolen and H. L. Hagedoorn, J. Opt. Soc. Am. 05, 952 0 975).

1J M. Stupavsky and L. Krause, Can. J. Phys. 46, 2127 0968).

,3 A.C.G. Mitchell and M.W. Zemansky, Resonance Radiation and Excited Atoms (Cambridge U. P. , Cambridge, England,

1971), p. 100.

,4 J.P. Webb, W.C. McColgin, O.G. Peterson, D. L. Stock- man, and J. H. Eberly, J. Chem. Phys. 53, 4227 (1970).

15 D.R. Jenkins, Proc. R. Soc. London A 293, 493 (1966).

,6 H.P. Hooymayers and C.Th.J. Alkemade, J. Quant. Spectrosc. Radlat. Transfer 6, 847 (1966). nJ. Geist, B. Steiner, R. Schaefer, E. Zalewski, and A. Corrons, Appl. Phys. Lett. 26, 309 (1975).

,8R.Y. Koyama and M. G. Buehler, Semiconductor Measure - ment Technology , edited by W.M. Bullis, NBS Special Publication No. 400-4 (U.S. GPO, Washington, D.C., 1974),

pp. 37—39.

I9C. C. Wang and L. I. Davis, Jr. , Phys. Rev. Lett. 32, 349

(1 974).

4016 J. Appl. Phys., Vol. 47, No. 9, September 1976 Mayo ef a! 4016 17
,
Appendix 2

Development of Sodium Contamination in Semiconductor

Oxidation Atmospheres at 1000°C

Santos Mayo

Electronic Technology Division, Institute jor Applied Technology,

National Bureau of Standards, Washington, D.C. 20234

and William H. Evans

National Bureau of Standards, Physical Chemistry Division,

Institute for Materials Research, Washington, D.C. 20234

Reprinted from Journal of the Electrochemical Society

Vol . 124, No. 5, May 1977

Printed in the U.S.A.

19
Development of Sodium Contamination in Semiconductor

Oxidation Atmospheres at 1000°C

Santos Mayo

Electronic Technology Division, Institute for Applied Technology,

National Bureau of Standards, Washington, D.C. 20234

and William H. Evans

National Bureau of Standards, Physical Chemistry Division,

Institute for Materials Research, Washington, D.C. 20234

ABSTRACT

The thermodynamic equilibria established in fused silica oxidation tubes operated at 1000°C are analyzed. Transparent fused silica tubes used for ther- mal oxidation of silicon contain about 10 ppm sodium impurity. At oxidation temperatures sodium diffuses in fused silica, evaporates into the oxidation ambient, and reacts with residual water contaminating the oxidation atmosphere. During the oxidation cycle enough sodium is incorporated into the growing oxide film to be detected later by capacitance measurements in metal

oxide semiconductor (MOS) structures. Reactions taking place during currently used in situ furnace cleaning procedures are analyzed. Calculations in- dicate that the amount of sodium removed from the fused silica tube wall through chlorine or hydrogen chloride cleaning is substantial. The reaction

rate is regulated by diffusion of sodium in the fused silica. The use of iodine and hydrogen iodide as cleaning agents is discussed.

Oxidation of silicon has been the subject of many

investigations aimed at understanding metal oxide semiconductor (MOS) device instabilities caused by

the action of mobile ion impurities localized in the oxide film (1). Test vehicles in the form of simple

MOS capacitors built on silicon dioxide films 50-100 nm thick are frequently used to study ionic con- tamination in thermal silicon dioxide. The mobile

ion density in the oxide film can be calculated from

capacitance measurements after thermal cycles have

been applied to the device (2). Several contamination sources may contribute impurities to the oxide film. Due to their high natural abundance, alkali species are the most common impurities likely to contaminate silicon dioxide. Sodium,

potassium, and lithium may cause ionic-type instabilities in the oxide. Sources of alkali contamination can be classified according to their origin as: (i) bulk and surface impurities already contained in the

silicon wafer before oxidation, (ii) impurities intro- duced into the oxide film during silicon high temperature oxidation and annealing, and (iii) impurities introduced after oxidation through metallization (3). Considerable research has been done to determine

the presence of impurities in microelectronic process

materials. Analytical techniques such as flame emission, atomic absorption, resonance fluorescence, neutron activation, and others have been used for trace contamination detection (4-6). Metallization of microelectronic devices has also received a great deal of attention regarding the introduction of ionic contamination in the oxide film from both the evaporation

process used and contamination in the metal (7). These results show that impurities may be introduced through several processing steps and play a significant role in determining the electrical properties of the

device. During oxidation of silicon, sodium impurity contained in fused silica oxidation tubes evaporates from

the tube wall into the oxidation ambient. In typical oxidation facilities, furnaces employing fused silica tubes are operated at about 1000°C (or 1300°K for Key words: fused silica, oxidation furnace, semiconductor processing, silicon dioxide, thermal oxidation.

the purpose of this paper). The oxidation atmosphere

generally consists of pure dry oxygen flowing along

the tube with laminar flow at about 4 cmVsec (equivalent-to 0.5 std ft3/hr). Transparent commercial fused

silica tubes currently used for this purpose generally have an average sodium content of 10 ppm (8-9). Sodium in the tube bulk diffuses easily [diffusion

coefficient in SiC>2 : 2 x 10 -6 cm2/sec (10)] to the tube wall surface. This in turn causes the appearance

of contamination in silicon dioxide films grown in the oxidation tube (11). The purpose of this work is to study the oxidation atmosphere used to grow thermal silicon dioxide films

as a source of sodium contamination introduced into the film during the growing cycle. The thermodynamic

equilibria established in fused silica oxidation tube atmospheres at 1300‘K were considered.

Reactions Occurring in Fused Silica Oxidation Tubes The thermodynamics of sodium vaporization from sodium silicate-silica glasses has been previously studied in the temperature range of 1200°-2(MFK where

the equilibrium constant of the reaction NajO • Si 02

(in SiC>2 ) -* 2Na(g) + Vi 02 (g) + Si02 (c or glass) has been measured (12). Values of K = p

2 (Na) • p*( 02 )[a(Si02 )/a(Na20 Si02 )] were reported for molar ratios of Si02 to Na20 varying from 1.69:1 to

3.88: 1, where the a’s (activities) were expressed in terms of mole fractions. The results indicated Raoult law behavior was approximated in the higher ratio

solutions. On this basis, extrapolation to larger ratios,

i.e., 105 (about 10 ppm sodium in the silica) leads

to the value K = 2.5 X 10 -19 at T — 1300‘K. Using

this value and thermodynamic data from Table VII we obtain aH°f298 (Na20 • SiO2 L 10 5 mole fraction in Si02 gls]) = —376.8 kcal/mole This value is in rea- sonably good agreement with the corresponding JANAF

(13) value for NajO • SiCMgls) of 373.2 kcal/mole. The difference between these values can be considered

a measure of the solution effects, such as formation

of Na^O • x Si 02 species, and the difference between

the properties of the simple compound as a glass and

the same species in solution. For this reason the use

20
Vol. 124, No. 5 SODIUM CONTAMINATION 781

Table 1. Equilibrium atmosphere in fused silica dry oxidation tube at 1300°K. Sodium content in fused

silica bulk is 10 ppm, oxygen at 1 atm.

Reaction

111 V4 CMg) -» O(g)

121 NaX) • SIOj (in SlOr) -* NasO(g) + SiCMc)

[31 NaaO SIO* (inSiCM 2Na(g) + '/a CMg) t SKMcj

(41 NaX> • SiOs (in SiOj) - Na(g) + NaO(g) + SiCMc)

(51 NaX)(g) + ‘A Oa(g) -» 2NaO(g)

16) NaX) SlOa (InSiOa) + >A CMg) -> 2NaO(g) + SiCMc)

of the former value (—376.8 kcal/mole) derived from measurements (12) was preferred, assuming that any

systematic errors in the calculations would be partially compensated.

A number of reactions take place in fused silica oxidation tubes operated at 1300°K. Table I shows

the equilibrium1 atmosphere resulting from 1 atm

dry oxygen reacting with the tube wall. The partial pressure corresponding to atomic oxygen resulting

from equilibrium dissociation of the oxygen molecule

is shown through reaction [l] 2 as p(O) = 1.76 x 10 -7 atm indicating that reactions with atomic oxygen are not significant. The evaporation reactions [2], [3], and [4] show that sodium in the oxidation atmosphere is present as both atomic (Na) and molecular

(NaiO, NaO) species. Reaction L5] shows the conversion of Na2 0 into NaO through interaction with

oxygen. These results indicate that in a dry oxidation atmosphere atomic sodium is about three times more

abundant than molecular sodium compounds. The

total number of sodium atoms in the equilibrium

oxidation atmosphere is 1.2 X 107 cm" 3 (both atomic and molecular species included). Table II shows the equilibrium atmosphere re- sulting in fused silica oxidation tubes at 1300°K when

water is added.3 The reaction system [7] shows that water is more stable than OH by factors ranging from

20 to more than 100 times as humidity in the oxidation ambient increases. For increasing water content

the most significant changes are seen in p(Na2 (OH) 2 ) and p(NaOH) as resulting from reactions [8] through

[11]. The sodium number density for 1 ppm water

impurity goes to 3.3 x 10 s cm -3 or a factor 27 higher than the dry oxidation condition. If 30 ppm water

is present in the oxidation ambient the sodium number density at equilibrium raises to 1.7 x 109 cm -3

or a factor 142 higher than the dry oxidation condition. Table III and Fig. 1 summarize these results.'* The above calculations show that sodium contamination developed in oxidation atmospheres could be explained in terms of contamination in the bulk fused

silica even if the furnace tube wall surface were perfectly clean. To reduce sodium density in oxidation atmospheres, it is necessary that the bulk tube material itself be sodium free. Two different approaches have been tried: (i) use of a sodium-free tube wall,

1 All calculated values presented in this work refer to equilib. rium conditions and represent limiting values for the actual sys- tems because of kinetic and diffusion factors

- Data used to calculate reaction equilibrium constants are

listed in Table VII. The actual calculation scheme is illustrated in the Appendix.

1 Experimental evidence shows that in a typical “dry" oxidation atmosphere the water content is in the 20-30 ppm ranee 114) Moreover at oxidation temperatures in the range 800°-1300"C it was reported that water and/or hydrogen from the external ambient can diffuse through the silica tube wall into the oxidation chamber (15).

1 Because of errors and approximations involved in thermody- namic data used to evaluate the equilibrium results, an absolute error of ±50% could be expected in these calculations. The rela- tive error is much smaller. Resonance fluorescence measurements showed that in open fused silica oxidation tubes no free so-hum was detected (16). This experimental result can he understood

in terms of calculations shown in Fig. 1. For sufficiently high water content in the oxidation atmosphere the amount of free so- dium detectable through resonance fluorescence technioue could be substantially reduced due to sodium reactivity with excess water Thus, free sodium is converted into compound sodium and

consequently becomes not detectable by the above-mentioned

technique

K p(atm) N(cm*l

1.76 x 10-7 P(O) = 1.76 x 10-7 7.9 x 10-*’ P ( NazO ) = 7.9 x 10-“

2.5 x 10-10 9.1 x 10-^ 'i P(Na) = 1.6 x 10-^ 9 x 10 l

0.42 } 3.3 x 10--” J p(NaO) = 5.7 x 10-* 3.2 x 10*

Total sodium density: 1.2 x 101

or (it) use of sodium-leaching agents to deplete a region just under the inner wall surface prior to oxidations. The first approach is based on the well-known

fact that polycrystalline silicon tubes can be produced

with 1000 times less sodium content than found in

silica tubes (17). Empirical results indicate that oxide

films grown on silicon using recently installed silicon oxidation tubes produce cleaner films (lower sodium

content) than similar oxides prepared in fused silica tubes. Polycrystalline silicon tubes develop, however, observable sodium contamination after being in operation for several months, when the use of sodium-leaching agents is then required. This contamination, de- veloped under strictly controlled production-oriented

environments, could be explained by assuming that during tube operation at oxidation temperature sodium

arising from external sources reaches the oxidation chamber by diffusion throueh the tube wall. The contamination source could be located in furnace refractories surrounding the oxidation tube (11). Once the tube wall becomes loaded with sodium above a critical level, evaporation into the oxidation atmosphere starts

to predominate. This is a slow process that may take

a long time (several months) in continuous operation

depending on environmental conditions (18).

Reactions Occurring When Cleaning Fused

Silica Tubes

Cleaning of oxidation tubes is a frequent operation

in processing facilities. At room temperature, the cleanWATER CONTENT IN OXIDATION ATMOSPHERE (ppm)

Fig. 1. Total sodium density [cm -3

] at equilibrium in fused

silica oxidation tube atmosphere as a function of temperature and

water impurity. The sodium concentration in silica is assumed 10 ppm, p(Os) is 1 atm.

21
782 J. Eleetrochem. Soe.: SOLID-STATE SCIENCE AND TECHNOLOGY May iy/7

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in situ by using 5-10% chlorine or hydrogen chloride

diluted in an inert gas carrier This treatment re- moves sodium contained in the tube inner “skin" down through a convenient depth. A typical cleaning

action of this kind requires up to 20 hr of continuous treatment flushing the cleaning gas at about 4 cm3 / sec (0.5 std ft3 /hr). The several reactions taking

place during this process are summarized in Table IV

Reactions [16], [17], and [18] are solved as a simultaneous system. The resulting sodium number density

at equilibrium in the tube atmosphere goes to 8.8 x

10' 6 cm -3 or a factor 7.3 x 109 higher than the equilibrium sodium number density in a dry oxidation atmosphere. Reactions [16], [19], and [20] show

similar results due to hydrogen chloride cleaning.

The increase in sodium number density with respect

to the dry oxidation condition is also in the 109 range. Microdroplets of liquid sodium chloride, if formed

on the tube wall hot zone, would be removed by the

gas stream and transported to a cooler tube zone where sodium chloride precipitates on the wall (so- dium chloride melts at 801°C). This “transpiration

mechanism" continues while sodium in the hot zone

is available for reaction. It should be mentioned that sodium chloride formed in the gas phase is carried down the flushing gas stream and condenses also at the cooler zone where the temperature is appropriate

The cleaning action can be interpreted in terms of relocating sodium in the tube, depleting the hot zone, and enriching the exhaust tube end where the wall temperature is appropriate for condensation. Stoichiometric equilibria in reactions [17] through

[20] require the use of about 1.5g of chlorine or hydrogen chloride per gram of extracted sodium. As- suming that the oxidation tube wall surface is about

40C0 cm2 and the reaction depth in the wall is 10~ 3 cm, the total reacting volume is 4 cm3 or about lOg fused silica. Assuming 10 ppm sodium content in the fused silica, the amount of sodium reacting is 10 •*g which requires the use of 1.5 x 10 ‘ 4 g of chlorine or hydrogen chloride for complete reaction equilibrium

The density of 10% chlorine at 1300*K is 6.74 x 10 8 g/cm3 . At 4 cm3 /sec flow, the chlorine mass transported

through the oxidation tube is 2.7 x 10 g/sec This shows that the reaction rate is regulated by diffusion of sodium in the fused silica. When sodium in the wall bulk diffuses to the wall suiface, the reaction with chlorine takes place immediately, in practice, a cleaning period of about 20 hr is used to deplete

the tube wall. The longer the cleaning period, the longer the oxidation tube can latei be operated under

acceptable oxide growing conditions. These calculations indicate that the amount of sodium removed

from the fused silica tube wall through chlotine or hydrogen chloride cleaning is substantial and that such cleaning certainly reduces the sodium content

in the inner tube wall surface Iodine or hydrogen iodide can also be used for in situ oxidation tube cleaning The lelevant reactions taking

place are summarized in Table V. Reactions [25] through 1 28] show the effectiveness of this type of cleaning indicating that iodine or iiydiogen iodide could be used instead of chlorine oi hydrogen chloride. However, due to economic reasons, the latter are more extensively used in practice. Since thermodynamic data are not available to the desired extent, similar calculations are not presented here for bro- mine and hydrogen bromide However bromine should have an intermediate behavior between chlorine and

iodine according to the pattern of the members of the halogen family.

22
Vol. 124, No. 5 SODIUM CONTAMINATION 783

Table III. Equilibrium sodium densities in fused silico oxidation ambient at 1300‘K. Oxygen at 1 atm.

Water content In oxygen: 0 Water content In oxygen: 1 ppm Water content In oxygen: 30 ppm

Compound P(atm) N< cm-2

) p(atm) N( cm-*) P(atm) N ( cm-3

)

Na 1.6 x 10-TM 9 x 10° 1.6 x 10-TM 9 x 10* 1.6 x 10-u 9 x 10a NaO 5.7 x 10-“ 3.2 x 10* 5.7 x 10 “ 3.2 x 10* 5.7 x 10-TM 3 2 x 10s NaaO 7.9 x 10-“ — 7.9 x 10-" —

NaOH — — 5.6 x 10-11 3.2 x 10» 3 x 10-TM 1.7 x 10* Nai(OH). — — 1.5 x 10-“ 4.5 x 10-'-“ Sodium number density 1.2 x 10* 3.3 X 10* 1.7 x 10*

Table IV. Equilibrium reactions in fused silica tubes at )3O0°K when cleaning with 10%

CI 2 or HCI in N2 carrier at 1 atm.

Reaction K p(atm) N(cm-*)

112) Cls(g) -» 2Cl(g) 1.6 x 10-4 P(C1) = 4 x 10-3 p(Cla) = 98 x 10-2 (13) HCl(g) -. H(g) + Cl(g) 1 x 10-TM p(HCl) as 0.1

(14) HCl(g) V4 Hal g ) + fca Cla(g) 7.5 x 10-« P(C1)= 3.2 x 10-7 p(Cla) = 7 5 x 10-« p(NaCl) = 1.25 x 10-2 (15) NaCl(l) -. NaCI(g) 1.25 x 10-a (vapor pressure)

(16) 2NaCl(g) -. NaaCla(g) 27.1 > p(NaCl) =1.01 x 10-2 5.7 x 10TM

(171 NaaO • SlOs (In SlOa) + Cla(g) -. 2NaCl(g) + VaOa(g) + SlOa(c) 6.4 i P(NaaCla) = 2.76 x 10-2 3.1 x 10TM

(18) NaaO • SlOs (In SlOa) + CMg) - NaaCb(g) + Vi Oa(g) + SlOa(c) 174 J p(Oa) = 3 9 x 10-2 Sodium number density 8.8 x 10TM

116) 2NaCl(g) NaaCb(g) 27.1 'v p(NaCl) = 4.1 x 10-s 2.3 x 10TM

(19) NaaO • SlOa (In SlOa) + 2HCl(g) - 2NaCl(g) + HaO(g) + SlOa(c) 0.42 l p(NaaCla) = 4.6 x 10-* 5.2 x 10TM

120) NaaO - SlOa (InSlOa) 4- 2HCl(g) NaaCla(g) + HaO(g) + SlOa(c) 11.4 J p(HaO) = 2.5 x 10-* Sodium number density 2.8 x 10’°

Table V. Equilibrium reactions in fused silica tubes at 1300°K when cleaning with 10%

Is or HI in Ns carrier at 1 atm.

Reaction K p(atm) N(cm-*)

(21) % Ia(g> -* 1(g)

122) 2Hl(g) -* a,(g) + big)

1231 2HI(g) -» Hj(g) + 21(g)

124] Nal(l) Nal(g)

(251 NaaO • SlOa (InSlOa) + Ia(g) -» 2Nal(g) + '/a Oa(g) + SlOa(c)

(281 NaaO • SlOa (InSlOa) + 21(g) -* 2Nal(g) + ti Oa(g) + S10a(c)

127) NaaO • SlOa (InSlOa) + 21(g) + Ha(g) - 2NaJ(g) + H.O(g) + SlOa(c)

(281 NaaO • SlOa (In SlOa) + 2HI(g) -* 2NaI(g) + HaO(g) + S)Oa(c)

0.467

0.0501 1

0.0109

| 7.06 X 10-2 9.3 x 10-» 1 4.3 x 10-‘ /

4.9 x 10s

I 54.2 )

p(l) = 1.0 X 10-1 Pda) = 4.5 x 10-2 p(I) = 3.57 x 10-- Pda) = 5.8 x 10-> P(Ha) = 2 36 x 10-2 P(HI) = 5.22 x 10-2 p(Nal) = 7.06 x 10-2

p(Nal) =9.3 x 10-5 Sodium number density p(Nal) = 1.4 x 10-2 Sodium number density

(vapor pressure)

5.3 x 10’*

7.9 x 10“

• p(Naala) can be estimated about 0.5 p(Nal) from similar results obtained with the chlorine system. No thermodynamic data are avail- able for Naala.

The Oxidation Atmosphere as a Sodium

Contamination Source An experimental test was made to check the validity of assuming that sodium becomes trapped in an oxi- dation tube due to the evaporation-transpiration-con- densation cycle mentioned above. In a fused silica tube operated at 1000°C “dry” oxygen5 was allowed to flow

at a rate of 4 cm3 /sec for 120 hr. The exhaust gas was passed through a 5% hydrochloric acid solution. After this period the solution was analyzed by flame emission spectrometry. The detection limit for sodium

in the solution was 10~'° g/cm3

. No evidence of sodium was detected in the solution after correcting for evaporation of the liquid. This test was repeated twice with consistently negative results. Consequently, it was concluded that sodium is not carried out of the oxidation tube. There is no apparent way of extracting from a fur- nace tube a representative sample of the oxidation atmosphere. Other analytical techniques operating on

gas samples removed from the oxidation atmosphere, such as mass spectrometry, are not appropriate to detect sodium due to condensation as the temperature

decreases. On the other hand, any analytical instru- mentation if introduced into the oxidation tube

could change the oxidation atmosphere equilibrium

adding extra contamination, thus perturbing the measurements.

2 Electronic grade oxygen typically contains about 5 ppm water

and 20 ppm hydrocarbons measured as methane.

Contamination of Thermally Grown Silicon Dioxide Films The results of Tables I and II indicate that in typical fused silica oxidation tubes the equilibrium atmosphere

contains atomic and molecular sodium species result- ing in a sodium number density in the 107 -109 cm-3

range, depending on the amount of water present. Due to the low density, sodium atoms and molecules

contained in the oxidation ambient can be considered

as ideal gases. The number of collisions per unit time sodium executes per unit area on the wafer surface

is expressed as (19)

v = 0.25 Nv

where N is the sodium number density in the oxidation atmosphere, v = (8 kT/nm)& is the average

molecular velocity at temperature T, k is the Boltz- mann constant, and m is the weight of the sodium

species. The resulting value of v for T = 1300°K is 1012 cm-2 • sec -1 for N — 5 x 107 cm'3 . The presence

of oxygen (1 atm) in the tube slightly modifies this number due to collisions with sodium atoms and

molecules. A number of reactions can take place between the growing silicon dioxide film and the impinging sodium

molecules as shown in Table VI. Sodium silicate is formed by reactions [291 throueh [35] which have

high equilibrium constants. As sodium from the oxidation atmosphere becomes incorporated into the oxide

23
784 J Electrochem. Soc.: SOLID-STATE SCIENCE AND TECHNOLOGY May 1977

Table VI. Impurity reactions with S1 O2 films in fused silica oxidation tube atmospheres at 1300'K

Reaction K

111 Vi Oa(g) - O(g) 1.76 x 10-7 r Hs(g) -+ 2H(g) 1.2 X 10-u

1 HaO<g) - H ( g) + OH( g) 1.54 x 10-l » 17) 2HaO(g) - Ha(g> + 2 OH(g) 2 x 10- l « l HaO ( g 1 + Vi Oi(g) - 2 OH(g) 2.5 x 10-‘

1291 NasO(g) + Si02(c) -* NaaO SiOa (In SiOa) 13 x 10“

130) 2Na(g) + Vi Oi(g) + SiOa(c) -» NaaO SlOa On SlOa) 4 > 1016 1311 Na(g) + NaO(g) + SiOa(C) -* NasO SiOa (In SiOa) 1.1 x 10‘* 1321 2NaO(g) t S)Oa(c) Na.O SlOa ( in S10.> 1 V 14 0:(g| 3 » 10“

(33] 2NaO(g) + Ha(g) + SlOa(c) -. NaaO SiOa (in SlOa) 1- HaO(g) 3.6 x 10-* (341 Naa(OH) > + SIO.(c) -. Na.-O SlOa (In SlOa) + HaO(g) 6.7 X 10* 135) 2NaOH(g) + SlOa(c) -* NaaO SlOa (In SlOa) + HaO(g) 3 2 x 10>

film, more sodium from the tube wall evaporates into the atmosphere, maintaining the equilibrium. There are not enough kinetic data available to estimate precisely the actual amount of sodium incor- porated into the silicon dioxide film during the oxidation cycle. The typical dry oxidation period used to grow a 100 nm thick oxide film on a (100) sample

of silicon at lOOO'C is about 200 min. During this period the oxide film is subjected to about 10 16 sodium

collisions per square centimeter. Parameters such as the wafer temperature, oxygen flow, and the water

content in the tube certainly influence the oxide film contamination level At equilibrium, the sodium concentration in the oxide film grown on silicon should be similar to that in the oxidation tube wall bulk,

i.e., 10 ppm (~1017 cm-3 ). This consideration clearly

establishes that dry oxidation of silicon made in fused

silica tubes produces oxide films with a significant amount of sodium included; the upper limit is deter- mined by the impurity level in the fused silica bulk. Whether or not this sodium which has chemically

reacted with silica to form sodium silicate in the film (Table VI) influences the electrical and radiation properties of MOS devices is a matter that needs

detailed examination.

It has been reported that structurally included so- dium atoms in silicon dioxide films behave like immobile ionic contamination. Capacitance measure- ments on MOS capacitors with a dielectric of silicon dioxide doped with sodium to density of 1020 cm* 3 showed that most of the included sodium is inactive from the viewpoint of both surface-state density and

ionic migration under high electric field at elevated temperature (15). However, instability of semiconductor devices due to sodium mobility in thermal

silicon dioxide films has been largely recognized as

a central problem in semiconductor technology. So- dium was shown to cause instability under bias-temperature stress in silicon devices made by planar

technology. Thermally stimulated ionic conductivity measurements made on MOS capacitors using alu- minum or gold as the gate metal have shown that

in sodium-doped gate oxide films with surface concentrations in the 10 n -1013 cm -2 range, sodium moves

at temperatures above 200,, C under electric fields as low as 12 / 105 V/cm (20). In practice instabilities due to sodium can be eliminated either by stringent cleanliness during oxide

processing or by the formation of a protective laver such as phosphosilicate glass which can trap sodium

( 20 ). An experimental test was made in a carefully controlled fused silica tube operated at 1000°C and periodically cleaned in situ with chlorine. Silicon dioxide

films 50-100 nm thick were grown on (100) n-type

silicon surfaces using dry electronic grade oxygen

and annealed in situ after oxidation in dry nitrogen

for 30 rr ; n. The metal gate was applied by evaporation

of 1 A*m thick aluminum film in an electron-gun evaporator. MOS capacitors were prepared and capacitance measurements were made at room temperature before and after stressing the capacitors at 300°C for 5 min

with electric field of 106 V/cm (2). The results in- dicate a clear correlation between cleaning of the fused silica oxidation tube and the total mobile

ion surface density in the oxide film. These measure- ments indicate that oxide films grown in recently cleaned tubes show mobile ion densities in the 10 10 cm-2 range. Assuming uniform contamination film in the measurements, these “clean” oxides exhibit an impurity

number density in the 10 15 cm ~ 3 range. This contamination level is caused by impurities accumulated

during the device preparation process. It has been shown that radiation sensitivity of gate oxide films has no correlation with the mobile ion content in the oxide as measured by MOS capacitance measure- ments after bias temperature stressing or high temperature ramping (21) However a correlation of total sodium content with radiation sensitivity in the oxide

of various MOS devices has been reported (22). These

considerations suggest that sodium contamination in- troduced in the oxide film during oxidation and metallization may play a prominent role in determining the device radiation resistance. Phosphorus gettering of the gate region or hydrochloric acid passivation in- creases the radiation sensitivity of MOS devices. Hence

to obtain reliable, radiation-stable MOS devices in the absence of gettering it has been suggested that high standards of cleanliness should be maintained

(21). This is specially applicable to the oxidation

step. Alkali impurity introduced through metallization should also be minimized by careful investigation of the mechanisms through which such contamination

is introduced. Conclusions The results of the present thermodynamic calculations show that sodium included in the bulk of fused

silica oxidation tubes is a significant source of contamination in oxidation atmospheres, even it sodium

surface contamination produced by external agents

in the tube is negligible. The presence of water in the oxidation atmosphere produces a rapid buildup

of the sodium number density from the 107 cm -3 range, corresponding to dry oxidation, of up to two

orders of magnitude greater for water concentrations

of a few parts per million. During the oxidation cycle, sodium is incorporated into the oxide film as sodium

silicate developing a significant contamination level which can be potentially harmful for MOS device

radiation resistance. Water may be incorporated in the oxidation atmosphere through trace water or hy • drogen from hydrocarbon impurities included in oxygen. and by ambient water that diffused into the oxidation chamber through the fused silica tube wall. Preparation of clean oxide films requires the control

of sodium impurity in the oxidation atmosphere This

in turn imposes a strict control of the tube material and appropriate care concerning the environmental

conditions during wafer processing

Acknowledgment

Thanks are due to T C. Rains from the Analytical

Chemistry Division for the results on flame emission

24
Vol. 124, No. 5 SODIUM CONTAMINATION 785

spectrometry. The collaboration of T. F. Leedy and

R. Y. Koyama of the Electronic Technology Division

in preparing MOS capacitors and in measuring their

electrical properties is highly appreciated. This work

was conducted as part of the Semiconductor Technology Program at the National Bureau of Standards and was supported in part by the Defense Nuclear Agency (LACRO 76-816) and the NBS.

Manuscript submitted Aug. 3, 1976; revised manuscript received Dec. 20, 1976. Any discussion of this paper will appear in a Discussion Section to be published in the December 1977 Journal. All discussions for the December 1977 Discussion Section should be submitted by Aug. 1, 1977.

Publication costs of this article were assisted by the National Bureau of Standards. APPENDIX*

Calculation of equilibrium Reaction Constant from JANAF Data For a reaction xX 4- yY -* wW zZ the thermo- dynamic equilibrium constant may be written as

«w ( W)-a*(Z) _

a

x (X) u>(Y)

where a(X) is the activity of compound X. For cal- culations involved in this work (dealing with very

dilute gaseous species), the activity may be taken as the partial pressure of the vapor phase (measured

in atmospheres). The activity of a solid phase such

as Si02 (c) is taken by convention as unity. For Na2Si 03 solid solution in Si 02 the activity is taken

Table VII. JANAF thermodynamic data used to calculate

equilibrium conditions at 1300°K for reactions listed

in Tables l-VI

— (G°t — H°sse)/T** Compound (kcal/mole) ( cal/ °K mole)

HaO ( g

)

-57.798 51.130 H(g) 52.1 30.879 OH(g) 9.432 48.877 O(g) 59.559 42.044 Oat g) 0 54.283 Ha(g) 0 36.130 Na(g) 25.755 40.201 NaOH(g) -50.4 63.786 NaO(g) 20.0 60.896 Naa(OH)a(g> 154 8 89.141 NaaO SlOa (In SlOa) — 376.8t 50.565 SlOa(c) (quartz) — 217.7 19.918 NaaO ( g

1 1 8.7 72.3 HCl(g) - 22.063 49.624 Cla(g) 0 59.318 Cl(g) 28.922 43.226 NaCllg) - 43.36 61.099 NaCl(l) -92.237 32.199 NaCl(c) -98.26 26.479 NaaCla(g) -135.3 91.387 1(g) 25.537 46.671 Ia(g) 14.924 68.559 HI(g) 6.3 54.425 Nal(g)« -22.7 64.8 Nal(l) -63.695 38.532 Nal(c) - 68.8 32.856

* AH'isss = standard enthalpy of formation.

•• (G't - = Gibbs energy function In the standard

state at temperature 1300°K

t The value of AH'ium for NaaO SlOa (in SlOa) was calculated using the equilibrium constant from reaction [31 Table I [see Ref.

(121). The free energy function was assumed to be that of NaaO SlOa (gls) given by the JANAF tables.

1 The Na.O data were estimated from various lithium, sodium, and potassium oxides data given In the JANAF tables. The Nal

data are likewise estimated using the available alkali halide data.

as the mole fraction (10 5 in the present work) F01 the reaction Naa O • Si02 (gls) t V2 02 (g) - 2NaO(g> 1 Si02 u 1 the equilibrium constant is expressed as K - p

2

( NaO) x lOVp’MOa)

To evaluate K at a particular temperature T (measured in Kelvin) the following relations are used aGt = - RT In K

AGt = AH°29S + T(2(G“t H'"29s ) /T ) where jH‘ 298 is the heat of reaction, calculated from

standard heats of formation as ^HYz98 — 2 AHJ f298 (products) - 2 MTf298 (reactants) [keal/molej and 2 (G“ t — H°298 )/T is calculated from Gibbs en- ergy function in the standard stau <a( tempeiature T

2(G~t — H°298)/T = {2(G', t - tt

0 2U8)/T> prudil.IS — (2iGt — H-m)/T } reactants teal/ K molej

u Thls calculation follows standard thermodynamic formalism

I see Ref (23)) REFERENCES

1. S. Schlegal IEEE Trans Electron Devices ed-14

728 ( 1967) , ibid., ed-15, 951 ( 1968 - 2. K. H Zaininger, RCA Rev., 27, 341 ( 1966).

3. R. J. Kriegler, "12th Annual Proceedings, Reliability Physics 1974," IEEE Catalog No 74CH0839-1

PHY, pp. 250-258 (1974)

4. B. Yurash and B Deal This Journal, 115. 1191 (1968).

5. W. R. Knolle, ibid., 120, 987 (1973)

6. W. R. Knolle and T F. Retajczyk, Jr, ibid. 120, 1106 (1973).

7. K. G Aubuchon, IEEE Trans Nucl Set., ns-18. No. 6. 117 (1971)

8. G Hetherinneton G W. Stephenson, and J A. Winterburn, Electronic Eng., 41, No 495, 52 (1969).

ibid., 41, No. 496, 44 ( 1969)

9. A E. Owen and R. W Douglas. J Soc Glass Technol, 43, 159T (1959).

10. Ii. F. Wolf, “Silicon Semiconductor Data.” pp 526-

527, Pergamon Press Oxford (1969)

11. D. R. Fewer and W. L. Gill, Tech Rep. No. RADCTR-66-345, pp. 33-49 (1966); also available as NTIS Document AD-489-969

12. G. L. Vidale, General Electric Co. Technical In- formation Series, R60SD390 (1960)

13. JANAF Thermochemical Tables D R Stull and

H. Prophet, Editors, 2nd ed. NSRDS-NBS 37

(1971).

14. E. A. Irene, This Journal, 121, 1613 < 1974)

15. A. G. Revesz and R. J. Evans J Phys Chem

Solids, 30, 551 (1969),

16 S. Mayo, R. A. Keller, J. C. Travis, and R. B Green.

J. Appl. Phys., 47, 4012 (1976).

17. W. Dietze, L. P. Hunt, and D. H Sawyer, This

Journal, 121, 1112 (1974).

18 D H. Sawyer, Private communication.

19. S. Dushman, “Scientific Foundations of Vacuum

Technique," 2nd ed., p. 14, John Wiley & Sons,

Inc., New York (1962).

20. T. W Hickmott, Appl. Phys Lett.. 22 267 (1973); Phys. Rev. Lett., 32, 65 1 1974 ) , J. Appl. Phys.,

46, 2583 (1975)

21. B. L. Gregory, IEEE Trans. Nad Set., ns-22. No. 6. 2295 ( 1975)

22. H. Hughes, R. D Baxter, and B Phillips, ibid., ns- 19, No. 6, 256 (1972).

23. G N. Lewis and M. Randall, • Thermodynamics,'

2nd ed., chap. 15, McGraw Hill Book Co., New

York (1961).

25

106 J. Electrochem. Soc.: SOLID-STATE SCIENCE AND TECHNOLOGY January 1978

Appendix 3

Reprinted from Journal of the Electrochemical Society

Vol. 125, No, 1, January 1978

Printed in the U.S.A.

Thermodynamic Considerations in the Use of Polysilicon

Oxidation Tubes for Clean SiO* Film Preparation

Sontos Mayo*

National Bureau of Standards, Electronic Technology Division,

Institute for Applied Technology, Washington, D C. 20234

and William H. Evans

National Bureau of Standards. Physical Chemistry Division,

Institute for Materials Research, Washington, D.C. 20234

ABSTRACT

The thermodynamic equilibria established in oxidation atmospheres in polycrystalline silicon tubes operated at 1000°C are analyzed Silicon oxidation tubes made by chemical vapor deposition through hydrogen reduction

of pure trichlorosilane have very low sodium content (about 10 ppb or 1000 times less sodium than in transparent fused silica oxidation tubes). Due to the low sodium content in new oxidation tubes, clean (low alkali content) thermal oxide films can be grown on silicon wafers. However, tube contamination developed during semiconductor processing operations imposes the need for appropriate peri. idle tube cleaning to maintain sodium contamination

in the oxidation atmosphere within acceptable levels. Tube cleaning reactions taking place at oxidation temperature are discussed showing that the quality

of thermal oxide films is influenced by tube cleaning efficiency.

Preparation of thermally grown clean oxide films on

-•iiiC'.n wafers has great technological impact on successfc' microelectronic device fabrication. Although

• Elf-cti Khemlcal Society Active Member. Key words: alkali contamination, clean SiO, film, microelectronic device preparation. MOS structures.

the understanding of the role played hy alkali ions

in such films is not complete, especially concerning

the behavior of electrically "a. live” and 'inactive" contamination (1;, the presence of alkali species in thermally grown silicon dioxide films has been ex- 27
Vol. 125, No. 1 POLYSILICON OXIDATION TUBES 107

perimentally demonstrated to cause ionic conductivity and consequently instabilities in metal-oxide semi- conductor (MOS) devices (2-5). The presence of so- dium and other alkali species in thermally grown

oxide films used for semiconductor device preparation has been extensively investigated (6-8). Sources

of alkali contamination have also been investigated in processing materials (9-13), in oxidation furnace refractories (14), and in evaporated gate metal films

(15). Addition of about 5% chlorine or hydrogenchloride to the oxidation atmosphere has been re- ported to enhance the electrical stability of Si-SiOa

structures, both through cleaning and passivating

effects (16-22). HC1 diluted in an inert gas carrier (concentration up to 10%) is used for periods of several hours immediately before oxidations to pro- duce low mobile-ion content thermal oxide films for MOS device fabrication. In a recent paper (23) the development of sodium

contamination in oxidation atmospheres contained in transparent fused silica tubes operated at 1000°C was

analyzed thermodynamically in terms of the impurity

content primarily included in fused silica bulk re- sulting from the tube manufacturing process. Transparent fused silica tubes currently used in most semi- conductor processing furnaces contain about 10 ppm

sodium (24-25). In situ cleaning of fused silica tubes

at 1300° K results in removal of material from the inner tube wall surface creating a sodium-depleted

layer. This in turn reduces the sodium content in the oxidation ambient, allowing the thermal growth of cleaner oxide films on silicon wafers. However, due to

its high diffusivity in Si02 [D = 2 x 10" 6 cm2 /sec at 1000 C (26)] sodium from furnace refractories or from the room atmosphere around the oxidation tube may be incorporated into the fused silica wall contributing to the maintenance of its contamination

level. When fused silica oxidation tubes are replaced

by silicon tubes, the conditions established in the oxi- dation ambient are expected to be more favorable

for growing clean thermal oxide films on silicon wafers. This is caused by the low sodium content in sili- con tubes fabricated by chemical vapor deposition

through hydrogen reduction of trichlorosilane. This

process produces high purity polysilicon with only 10 ppb sodium contamination, 1000 times less sodium

content than in fused silica tubes i28). However, due

to external contamination sources (i.e., sodium from

refractories and room ambient), periodic in situ cleaning of the tube is necessary to maintain appropriate

conditions in the oxidation atmosphere. The cleaning

interval for both fused silica or silicon tubes is deter- mined by the boundary conditions at the tube wall outer surface and the diffusion of sodium in the tube

wall. The diffusion coefficient of sodium in polycrystalline silicon is higher than in single crystal silicon due to grain boundary effects. At 1000‘C the latter is D ^ 3 x 10' s cm2/sec (26). Similar effects were re- ported on the diffusion of boron in silicon at 1050°C

(27). The work presented here is an extension of the previous analysis on the behavior of fused silica oxidation

tubes at 1000 C (23). 1 Silicon oxidation tube behavior resembles the fused silica tube behavior except for the initial low sodium content when the tube is new. The effects of HC1 cleaning on these tubes are also analyzed. Oxidation of Silicon Wafers in Silicon Oxidation Tubes The kinetics of silicon oxidation have been extensively studied by different authors for a number of temperatures in the range of 800°-1200°C and for several oxidation atmosphere compositions contained

in fused silica tubes [most early work on this subject

is listed in bibliographic compilations on MOS tech- nology (9,30)]. It was shown that the addition of trace amounts of water to the oxidation ambient

causes a significant increase of silicon oxidation rate

for all silicon orientations (31). It has also been shown

that the equilibrium sodium density in oxidation at- mospheres at 1300 K significantly increases when a few ppm water are added to the oxidation atmosphere (23). From these results it can be stated that the preparation of clean, low mobile ion content oxide

films on silicon wafers is controlled primarily by

four parameters: the oxidation temperature, the amount of sodium (in general, alkali) contamination

in the tube wall bulk, the amount of water content in the oxidation atmosphere, and the furnace environment. The use of strictly controlled hydrocarbon-free

dry oxygen in addition to alkali-free oxidation tubes operated at high temperatures (above 1000‘C) may

result in clean thermal oxide preparations if appropi iate care is taken concerning clean room environ- ments around oxidation facilities. For high temperature (close to 1200 V C) processes where devitrification

of fused silica may produce serious problems, silicon tubes are preferred due to their better mechanical be- havior and low sodium contamination level. In the present work, the equilibrium atmosphere

established at 1300‘K in a silicon oxidation tube was

studied theoretically for a number of cases of practical interest. 2 Table I shows the equilibrium atmo1 For these thermodynamic calculations data at 1300*K are used The scheme for these calculations is outlined In the appendix

of Kef. 23 because of errors and approximations involved In ther- modynamic data used to evaluate the equilibrium results, an ab- solute error of ±50% could be expected In these calculations. The

relative error is much smaller.

Table I. Equilibrium atmosphere in silicon wet oxidation tube at I300‘K Sodium content in silicon bulk: 10 ppb, oxygen at 1 atm, water at 10* atm

Sl(c) 4 Oi(g) 4 HaO(g) —* StOa(c) 4- ‘/sHa(g) 4 OH(g)

2Ha( g ) 4 Oat g) - 2HX)(g)

Site) 4 Oa(gt -» SlOa(C) SIO(g| 4 Via Os(g) -• StOa(c) NaaO SICMci -> SlOa(c) 4 NaaOigl

StOa(C) 4 HsO-SlOaic) 4 2Na(g) 4 2 0Hlgl

SiOaic) 4 HaO - SiOlOH) Igl 4 NaaOig) 4 Oll(g)

SIOa(C) 4 HtO -• Sl(OHlltg) 4 la O/l g I 4 NaaOigl

SlOa(C) 4 H,0 -• SlOlOH)a(g) T NaaOigl _ SlOa(C) 4 2HaO -4 SI(OH).lg) 4 NaaOigl 2Na.O SlOaici 3HaO SlsOi OHm g| 4 2NaaOig) Silo 4 2HaO(gj -4 SlOa(c) 4 2Ha(gl

Stic i a H.O(g) -4 SiH(g) 4 OHigi

Slid 4 2HsO(g) -4 SiH.lg) 4 Oaig)

Sl(c) 4 3/2 Oa(g) 4 2Na(g) -• NaaO SiOa(c)

Site) 4 5/2 0aigi 4 2Na(g) -• Na.O 25103.0 NaaO SIO.(C) 4 SiO.'i c ) - Na,0 2SiOa(C) NaaO(g) 4 Va Oa(gi -4 2NaO(g/

NaO(g) 4 HaOig) -4 NaOH(g) 4 OHig)

2NaOH(g) * NaaiOH)a(g)

NaaO

NaaO

NaaO

NaaO

NaaO

K Product p ( atm;

2 54 x 10»« 0. 1.0 1.35 x 10'* Haig) 8 61 x 10 " 18x110*4 OH 4 82 x 10 7 5 59 x 10"» HaO 10-

7.8J x 10 A. NaaO 7.9 a 10 =» 5.85 x 10 " Na 5 02 A 10 •* 1.19 x 10-" SiOiOH) 3 13 x 10 “

1 68 » 10" Si 1 OH 1 a 2 13 x 10 =» 272 -. 10* SIOlOHi, 3 44 x 10 "

1.57 x 10-^ SKOH ). 2.00 x 10 ->•

1 38 * 10 SlaOiOHl. 2.21 X 10 38

1 33 x 10"

5.18 x io->* S1H 1.08 X 10-*» 8 00 x 10-' SIH. 8.00 X 10 38 7.14 x lu*

1.21 x 10*

0 944

0 42 NaO 1 82 x 10 “

4.69 NaOH 1.77 X 10 lJ 4 78 N«m OH 1.50 » io-*1

Naicm-*)

* Rapid formation of a silica him on the silicon tube. 28
108 J. Electrochem. Soc.: SOLID-STATE SCIENCE AND TECHNOLOGY January 1978

sphere resulting in a typical silicon oxidation tube operated at 1300°K using pure oxygen at 1 atm plus

100 ppm water. Sodium in the tube wall might be as- sumed to be sodium metasilicate [Na20 • Si 02 (c)] and sodium silicate [Na20 • 2Si02(c) ]. Measurements of sodium vaporization from silica glasses (32) indicate

that sodium metasilicate might be dominant; deviations of the value ^H°f298 = —37 6.8 kcal/mole calculated for Na20 • Si02(c) from the JANAF value (33)

is ascribed to solution effects of Na20 xSiO-2 species. Silicon and water react to form SiC>2 (c) (equilibrium

reaction constant K = 1.33 x 10 13 ). Sodium metasilicate is incorporated in the oxide film developed on

the tube wall and evaporates to build up in the oxidation chamber an atmosphere containing about 108 so- dium atoms/cm3

, including both atomic and molecular

species; NaOH is the most abundant compound. If no water is present in the oxidation ambient, the sodium

content is negligible (P(Na20) = 7.9 x 10~ 28 atm]. This condition prevails as long as the sodium level in

silicon bulk is only 10 ppb.

Oxidation Tube Cleaning

Periodic in situ cleaning at oxidation temperature is required to keep silicon oxidation tubes clean under

operative conditions. Dilute hydrogen chloride is generally used for rather long periods in the oxidation chamber to assure proper cleaning of the wall. As- suming 100 ppm water (10~ 4 atm) in addition to 10% HC1 in a silicon tube operated at 1300°K, the relevant

reactions occurring are shown in Table II. Sodium

chloride is formed from the interaction with sodium

glasses contained in the silica film formed on the sili- con tube wall. At equilibrium, the sodium number

density in the atmosphere is 3.66 x 10 15 atoms/cm3. This is 107 times larger than the equilibrium sodium

density normally present in the oxidation ambient. It should be emphasized that these results refer to equi- librium conditions. Although most reactions proceed

rapidly at the inner tube wall surface, diffusion effects in the bulk may limit to some extent the applicability

of this analysis. However, the enhanced sodium diffusion in polysilicon will reduce this limitation. The

density of the resulting species at the surface could be

lower, thus reducing the amount of sodium (or other

species) actually removed. No attempts are made

here to explore the kinetics of these reactions al- though it can be seen that hydrogen chloride cleaning

13 predicted to be very effective in agreement with

current empirical observation. The use of dry chlorine as a cleaning agent of sili- con tubes should be avoided because of the risk in- volved due to its high reactivity with silicon. Table

III shows that the tube could be severely damaged

by chlorine if the protective silica film deposited on

the inner tube wall has pinholes through which chlorine may come in contact with silicon to produce

quantities of SiCMg), SiCLfg), and SiCl2 ( g ) . However, if water is present in the system, hydrogen chloride is generated, moderating the aggressive behavior

of dry chlorine.

Oxidation Tube Conditioning

The silica film formed on the tube wall can be re- moved by hydrogen at oxidation temperature. The

formation of water generated through the hydrogen

silica interaction contributes to the leaching of so- dium from the tube bulk. Table rV shows the relevant

reactions taking place in the tube when 100 ppm water

(10 " 3 atm) is present in the hydrogen atmosphere. So- dium compounds such as NaO, Na20, NaOH, and Na2 <OH) 2 are formed in addition to atomic sodium. The total sodium equilibrium pressure in the tube atmosphere is 1.82 x 10 -8 atm resulting in a number

density of 1.03 x 10“ atoms/cm3 where the atomic

species is the most abundant one due to the reducing

action of hydrogen. Although the use of hydrogen

around an oxidation facility involves risks and diffi- culties which may make it impractical, it is of value to observe that the use of diluted (less than 4%) hydrogen in an inert gas carrier could make this procedure

feasible by eliminating the risk of forming an explosive air-hydrogen mixture. If such an atmosphere

is maintained in silicon oxidation tubes when they are

Table II. Equilibrium atmosphere in silicon oxidation tube cleaned with hydrogen chloride at I300°K Sodium content in silicon bulk: 10 ppb, hydrogen chloride at 0.1 atm, water at 10-* atm

Reaction K Product P (atm) Na (cm-4)

NaaO SiOa(c) + 2HCl(g) -* SIO><c) + 2NaCl(g) + HaO(g) 0.42 NaCl 6 48 x 10-‘ 3.66 x 10 15 NaaO SiOatc) + 2HCl(g) SMOH)a(g) + 2NaCl(g) Va Oa(g) 9.03 x 10"2* Sil OH )a 1 66 x 10-1S NaaO SiOa(c) + 2HCl(g) - SIO(OHHg) + 2NaCi(g) + OHig) 6.39 x 10-1S SiO(OH) 8.45 X 10-“ NaaO SiOa(C) + 2HCl(g) + HaO(g) -* SUOHi.tgi + 2NaCl(g> 8 45 x 10® Si(OH), 2.01 x 10-‘= 2NaaO SiOa(C) + 4HCl(g) + lHX)(g) SIX) • (OH)o(g) + 4NaCl(g) 3.98 x 10-1 * SlaO(OH)» 2.26 x 10-“ NaaO SiOa(c) + 3HCllg) -> SiClHj(g) + 3/2 0a(g) + 2NaCl(g) 4 98 x 10-^ SiCIHj 5 50 x 10-“ NaaO SlOaic) + 4HCl(g) -* SiCbHa(g) + lOalg) + 2NaCl(g) + HaOigi 7.38 x 10---® SlCl.Hs 1.05 x 10-^ NaaO SlOa(c) + 5HCl(g) -» SiCbH(g) + VaOalg) + 2NaCl(gl + 2HaO(g) 3.53 x 10-*“ SICbH 6 51 x 10-® NaaO SiOa(c) + 5HCl(g> -* SlChlg) + OH(g) 2NaCI ( g 1 t 2 HaO(g) 4.14 x 10- :o SiCb 5.48 x 10-* NaaO SiOa(c) + 4HCl(g) -» SlCblg) + laOalgl + 2NaC!(g) + 2HXXg) 5.38 x 10-21 SiCb 9.91 x 10-w

NaaO SiOa(c) + 6HCl(g) -* SlCblg) + 2NaCl(g) t 3H.O ( g

)

3.80 x 10-“ SiCl, 9.04 x 10-7 2NaCl(g) -* NaaCh(g) 27.1 Na-CL 1.14 x 10-* 6.43 x 10“

St(C) + HClig) SICl.lg) + 2H*( g

)

1.20 x 103 Si (OH) a 1.67 x 10-1 * SiOa(C) + HaO(g) - SilOHla + Va Oafg) 2.15 x 10* Oa 1.67 x 10-“ HaO(g) + Cb(g) - 2HC1 + Va Oa 1.29 x 10-~ HCI 0.1 HCl(g) -> 'aHa(g) + ViCb(g) 7.5 x 10* CL 9.19 x 10-“ HClig) -* H ( g ) + Cllg) 1.0 X 10-“ Cl 1.17 x lO-o Ha( g ) 2H(g) 1.2 X 10-“ H 8.57 x 10-° HaO(g) -» Hlg) + OH(g) 1 54 x 10-1 * H“ 6 12 x 10-3 2HaOig) H’l g ) + 2 OHlg) 2.0 X 10-“ OH 1.80 x 10-“ HaO(g) + '/a Oa(g) -» 2 OH(g) 2.5 x 10-“ HaO 10-‘

Table III. Reaction constants resulting in silicon oxidation tubes cleaned with chlorine at 1300‘K Sodium content In silicon bulk: 10 ppb, chlorine at 0.1 atm

Reaction K Product P (atm)

Site) + 2Cb(g) - SiCh(g)

2Si(ci + 3Ch(g) - 2S;Cb(g>

Sicc) + Clalg) SiCMg)

SiOa(c) + 2Cb(g) - SlCl.(g) + Oa(g)

2SiOa(C) + 3Cb(g) -• 2SiCh(g) + 2 Oa(g)

SiOa(c) + Chi g ) -> SiCb + Oalgi NeuO - SiOa(c) + Cb(gJ -» SiOa(C) + 2NaCl(g) + la 02 (g)

3.77 x 1(M SiCl, 3.77 x 10“*

4.74 x 10* SiCb 6.91 x 10“'

3.50 x 109 SiCb 3.50 x 10’ 2.11 x 10-9 1.46 x 10-*

1.96 x 10-‘“ 6.43

* Rapid formation of these compounds. 29
Vol. 125, No 1 POLYSILICON OXIDATION TUBES 1 09

Toble IV. Equilibrium atmosphere in silicon oxidorion tube cleaned with wet hydrogen at 1300 K Sodium content in silicon bulk. 10 ppb. hydrogen at 1 atm. water at 10 * atm

Reaction

SICMci . H=igi 1 H.Olgl -.SiHlg) x 3 OHtgi

SiO;it. . 2H Igi -.SiHngi Ouig, NasO SiOsici H;( g * -* SiH i g i -t OH(g) i inigi i 2Naigi NasO SiO.ici . H.lg) SiHigl + OH(g) * 2NaO(gi

N'ai-0 SIO.icl . 5'2H=(gi - SIH(g) + HjOigi i 2NaOH(gi

Si(c> + HXngi SlO:IC) r 2H.-(g) 2NaOH(gi -> Na.iOHlatgi

NasOig) Na(l(gi t Naigi

2HsO Haig i + 2 OH(g)

HiO(g) + Vi Oil g j -* 2 OH(g)

Table V. JANAF thermodynamic data used to calculate

equilibrium conditions at 1300 K lor reactions listed

in Tables I -IV

Compound AH 'nm>

Ikcal mole) (cal/ 'K mole;

HsO(g) — 57.758 51.136 Ulgl 52 i 30 879 OHl g) e 432 48.877 O(g) 59 559 42.044 Oiigi 0.1) 54.283 Haig) 0.0 36.130 Nat gj NaOH(g)

25.755

50 4 40.201 6J.786 NaO(g)

Naai OH)»t

20.0 60.896

154 8 89.141 NaaO SiOviglsi 1 376 8 50.565 NaX) 2SIO;lc) - 590.36 73 554 SiOaicj (quart/' -217 7 19.918 NajOigi* 8 7 72 30 HCl (

g

) 22.063 49.624 Cblg) 0 0 59.318 eng) 29 922 43.226 NaCllg) 43 36 61.099 NaCKUqi 92.237 32 199 NaCKci -98 26 26.479 NaX'iatgi -135.3 91 387 Sngi 107.7 43.715 Sid) 1 i .585 15.188 Stic) 0.0 8.393 SlH(g) 90.0 52.515 SiH, 7.3 58 969 SiOig) -24.0 55 994

SiOa< g; 73.0 63.266 SlOalti 215 74 21.134 SlCHg ' 45.7 62 985 SiClllMg 48.0 71 412 SiCl.lgi 157 1 95.763 SlCl.H-lgi -75.0 81.596

SlClali' gl 119.6 89 <22 SlCl.lgj 1 39.3 76.581 SiCb'gi- 960 89.134 SK» (OH. 1* 118.0 73 63

SI iOHi-A 101.0 74.42 sio ioH'.e 222 0 82.65 Si OH). 322.3 95.43

SiOtr . OH » + 812.1 132.83

- standard enthalpy ot formation.

* l C i *1 *... T

v.t 1300* K

- fj'hbc* energy function In the standard sla

the v.ili.o of •.»/ . for Na.O SiO; 'in Siu.i was calculated

us ofc t tie equilibrium i instant from reaction Nx-O SiO* tin SiO1

-2Naiei * 'S Oilg SIOiICI, K 25 x 10 see Ref <32il The tree energy function was assumed to be that of NaX) SICK

iglsi erw hv the JANAF tables.

1 the Na.O data were estimated from various lithium, sodium, and potassium oxides data given in the JANAF tables -The heals of formation were calculated from bond energies as outlined In Kef i36i. The thermodynamic functions were ob- tained bv interpolation of the values given there.

not bvi no used for semiconductor processing, the sodium impurity from the tube wall hulk and the

silica Him on the tube are constantly removed from

toe hoi zone The enhanced sodium diffusion in polys'licon increases the sodium migiation into the silica Him A inch is being removed by the hydrogen reducing

action. Tins tuoe conditioning would reduce the need

for periodic HCl cleaning and consequently minimize me incidence of other species (such as chlorine) in the oxidation chamber. Experimental evidence shows

I •!* ' h'.vrine is incorporated into oxide films thermally

/r.iv ,i on silicon wafers processed in oxidation tubes oieMiunly cleaned with HCl (34). This has proved to oe detrimental for radiation-hard devices resulting in electrical instabilities (35).

K Pioduct P (atmi

7 86 \ 10 *• SiH 2.80 * 10 '“ 6.00 « 10 •” SIH* 9.38 - 10->* 8.43 x 10 Na 1 82 * 10 ‘

1 10 10 NaO 5.28 • 10 »

8.49 < 10 <“ NaOH » 74 v 10 "

1.33 x 10 IS 4.78 NaMOHjj 1.45 a 10-** 1.16 NasO 8.28 x 10 24 Hj 1.0 HjO 10 -* 2.0 x 10-n OH 1.41 x 10 12 2.5 x 10-* o> 6.40 a 10-»

Conclusions The above results show that the dominant factor in the preparation of clean thermal oxides is the purity

of the oxidation tube wall if either process steps are kept under strict control. Such processes involve the wafer cleaning and handling prior to oxidation and

metallization immediately after oxidation. When re- cently installed, new silicon tubes have a low sodium

content and thus constitute an excellent material to enclose the oxidation atmosphere. This is due to the tube fabrication process (chemical vapor deposition)

employing highly purified (low alkali) materials (28) Preparation of clean low moDile inn content thermal

oxide films by wet oxidation of silicon wafers is in principle possible if silicon oxidation tubes are used, provided that the tube environment is clean enough to avoid inclusion of ambient contamination in the tube However a silicon oxidation tube will tend lo de- velop alkali contamination during normal operation

unless special care is taken to isolate it from external contamination. These conditions are hard to maintain even in clean production-oriented facilities so periodic

in situ lube cleaning is required. At oxidation temperatures sodium deposited on the exteiior of the tube diffuses through the wall into the oxidation at- mosphere contaminating the thermal oxide films grown on silicon wafers The contamination process may be originated by the fuinace refractories impurity, the room ambient, and other factors external

lo the tube The main advantage of employing silicon oxidation lubes lies in the possibility of using temperatures higher than tolerated by fused silica tubes.

Acknowledgment

The authors aie very much indebted to Drs. W

Murray Bulbs and Kenneth F Galloway of the Electronic Technology Division tor constant stimulus during this work and nitical renew of the manuscript

Tins work was conducted as part of the Semicon

ductor Technology Program at the National Bureau

of Standaids and was supported by the Defense Nuclear Agency.

Manuscript submitted June 17 1977 lev.sco manuscript received Sept 8, 1977. Any discussion of this paper wnl appeal m a Discussion Section to be published in the December 1978 Journal. All discussions for the Decembei '978 Discussion Section should be submitted by Aug I. 1978. Publication costs of this article were assisted by the National Bureau of Standards. REFERENCES

1. B E. Deal, This Journal 121 198C • 1974)

2. T W Hickmott, J Appl Phys . 46 2583 . !‘>75i

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5 0 1 Derbenwick, J. Appl Phys , 48, 1127 (1977;

6. K G. Aubuchon, IEEE Trans. Nucl Sci., NS-18, No

6. 117 (1971). 30
110 J. Electrochem. Soc.: SOLID-STATE SCIENCE AND TECHNOLOGY January 1978

7. E. H. Snow, A. S. Grove, B. E. Deal, and C. T. Sah,

J. Appl. Phys., 36, 1664 (1965).

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ibid., 41, No. 496, 44 (1969).

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137, 526-527 Pergamon Press, Oxford (1969).

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30. A. H. Agajanian, IBM Tech. Rep., 22, 1559 (1972).

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(1976).

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from Sodium Silicate Glasses,” Generali Electric

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31

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NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. 1 . PUBLICATION OR REPORT NO. 2. Gov’t Accession BIBLIOGRAPHIC DATA llr,„ Tr, No. sheet NBSIR 77-1404

3. Recipient's Accession No.

4. TITLE AND SUBTITLE Control of Mobile-Ion Contamination in Oxidation Ambients

for MOS Device Processing

5. Publication Date January 1978

6. Performing Organization Code

7. AUTHOR(S)

Santos Mayo, Richard Y. Ko.yama, and Thomas F. Leedy

8. Performing Organ. Report No. NBSIR -77-1404

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. Z99QAXTD072 W.U. 66

11. Contract/Grant No. 9NA IACR0 77-809

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Defense Nuclear Agency

Washington, D. C. 20305

13. Type of Report & Period Coveted Final

Oct. 1 976-Oct. 1977

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) An alternative method for controlling the mobile-ion contamination in the oxidation

ambients for MOS device processing is explored. Mobile-ion contamination in silicon

dioxide films thermally grown in dry oxygen at 1000°C on silicon substrates has been

studied by use of a double-wall fused-silica oxidation tube. The space between the

tubes were alternatively filled with chlorine, room air, or sodium hydroxide gas

to determine if a correlation exists between the presence of these substances in the

jacket and the mobile-ion density in the oxide films. MOS capacitors were prepared

on these films and mobile-ion densities were measured using conventional C-V techniques. The ion densities ranged from 10 13 to 10 i0 cm

-2

as a function of the jacket

atmosphere. These preliminary results suggest that there is a correlation between

the presence of cleaning or contaminating agents in the jacket and the mobile-ion

density in the oxide films. Both cleaning and contaminating actions occur through

the tube wal 1

.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Double-wall oxidation tube; dry oxidation; mobile-ion contamination; MOS device pro- cessingj MOS devices; oxidation ambientcontrol ; oxide growth; semiconductor device

process ing_; gr silicon dioxide; thermal. si 1 icon dioxide filn

18. AVAILABILITY [XI Uni imited

I | For Official Distribution. Do Not Release to NTIS

j) Order From Sup. of Doc., U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. Cl 3

IX ! Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

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(THIS REPORT)

UNCI. ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

43

22. Price

$4.50

USCOMM-DC 29042-P74