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Characterization of pyrolytic boron nitride as a diffusion source for silicon
Mm-I
Solid-Pare Ekctronicr Vol. 21. pp. 987~%8 fJ Pergamon Press Ltd.. 1978. Printed in Great Britain
I01/0701-0987/$02.w/0
NOTES CHARACTERIZATION OF PYROLYTIC BORON NITRIDE AS A DIFFUSION SOURCE FOR SILICON (Received 17 November 1977; in revised form 13 February 1978) There have been several articles[l-4] which have presented the results of the use of boron nitride made by a hot pressed technique as a diffusion source for silicon. This note presents the results of the characterization of a new type of boron nitride, one which is manufactured by a chemical vapor deposition process rather than by a hot pressing technique. The differences between the two techniques are substantial both in terms of the compositional structures involved and in terms of the use, particularly the oxidation procedures used to establish a boric oxide surface layer. The samples used in this work were Union Carbide Boralloy Pyrolytic Boron Nitride (PBN) wafers, two inches in diameter and 0.040 in. thick. For all of the diffusions 2 in. diameter silicon wafers, 3-6 ohm-cm in resistivity, phosphorous doped, having (111) orientation were used. The resistivity values quoted in this note are the average of five measured values on each wafer: center, right, left, top and bottom. The PBN wafers were cleaned before activation. The procedure consisted of a IO min. boil in deionized water followed by a 5 min ultrasonic cleaning in methanol and finally the wafers were dried in nitrogen at about 150°C for one hour. Because there is no boric oxide binder used in PBN there is no problem with using water or methanol for cleaning as there is in the case of some hot pressed materials [2]. There was definite evidence to indicate that this cleaning procedure should be done immediately prior to the activation of the PBN. During some of the work this cleaning procedure had been used but the PBN wafers were allowed to remain for a few days in nitrogen at lSO”C before the activation took place. For those wafers a distinctly poorer activation was noted than for the case when the cleaning took place immediately prior to activation. The activation procedure consisted of placing the cleaned wafers in the diffusion furnace at a temperature of at least 1050°C under an oxygen flow of 500 cm’lmin. for a period of one hour. The PBN wafers did activate satisfactorily under these conditions but temperatures higher than 1050°C lead to a much improved lifetime. The stabilization procedure consisted of leaving the wafers at the activation temperature for about one hour under either a pure nitrogen gas flow or a 24% oxygen flow added to the nitrogen. Some of the tests show that oxygen added during the stabilization period leads to a longer wafer life between reactivations. For example in one test it was observed that at 1050°C when stabilized in pure nitrogen the wafer was in fact barely activated and quickly lost most of its activation. However, when the test was repeated with 4% oxygen added during stabilization it was found that the PBN wafer lasted 9 hr under diffusion conditions. Both test diffusions in this case were done under pure nitrogen. Thus the addition of oxygen seems to play an important role even during stabilization. In order to make an estimate of the time required between activations, several tests were run to determine the useful life as a function of activation temperature and diffusion gas ambient. The lifetime is defined here as the time in hours following the stabilization period until the resistivity of the diffused region rises by about 10% from the nominal value. For these tests three temperatures for activation and three gas ambient conditions for diffusion were chosen. The diffusion conditions were 100% nitrogen, nitrogen plus 2% added oxygen and 4% added oxygen. In all cases the activation time was 60 min and the stabilization time was at least 60min followed by the time necessary for the 987
furnace to change temperature from the activation/stabilization temperature to the diffusion temperature. This was typically about 30 min. The gas ambient during stabilization was the same as that during the subsequent diffusion. The diffusions were all done at a temperature of 1000°C for a period of 20min. The results of the lifetime experiments are plotted in Fii. I. The trend is very clear. The use of higher activation temperatures and oxygen added during the stabilization and diffusion times can substantially increase the lifetime of the PBN between reactivations. The two data points which indicate a range of hours are simply due to the methods by which the data were taken. The wafer was checked at the first time and found to be good and not checked again until the second time when it was found to have deactivated. It was possible to reactivate wafers by leaving them at high temperatures and changing the gas flow to pure oxygen. It appears possible thus to store the wafers in the diffusion tube under a partial oxygen gas flow and to reactivate every day or two for a short period of time without having to re-clean the PBN. One would expect of course, once the PBN had been activated that the diffusions would produce the same resistivity values as
I
x)
I
1050
1100
Temperature. g: Fig. 1. PBN lifetime as a function of activation temperature gas ambient.
and
Notes
988
950
oc
2 12co”c I
IO
20
x)
40 Time,
.
50
60
min
Fig. 2. Sheet resistance as a function of predeposition time and temperature for PBN wafers using 2% 02 in N2.
mentioned work is considered. Namely it appears that the activation process affects only the surface of the PBN wafer. In hot pressed boron nitride there is a substantially greater volume of activated material. This a conclusion reached from the results of Podobeda et al. [S] which indicates that the rate of oxidation is affected by the amount of boric oxide within the boron nitride wafer. Hence when the gas flow rate is high enough the transfer of B203 from the surface is greater and becomes more nearly rate limited. The rapid depletion of the activated layer then results. Thus this constitutes another piece of evidence indicating the nature of the activation process in PBN wafers. One of the conclusions to be reached is that relatively low gas flow rates are necessary for reasonable wafer life between reactivation periods. For a 70mm tube the flow rate should be less than 500cm3/min with 150-200 cm’lmin being a suggested range. The resistivity uniformity across the diffused wafer is typically in the range of + 2% to 2 3%. Figure 3 shows a plot of average wafer resistivity as a function of position within the boat, with + 3% being easily attainable. The run-to-run resistivity variations were less than 2 4%. Acknowledgements-The
authors express appreciation
to Mr. J.
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I1
IO
I
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I
I
I4
I1
I6
I
I
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position
Fig. 3. Sheet resistance as a function of wafer position in the boat for a lOOO”C/20min diffusion using 150 cm3/min Nz and 8 cm’lmin 02.
those of hot pressed boron nitride. Indeed that is the result which was observed in this work. Shown in Fig. 2 are the curves of sheet resistance as a function of time and temperature. These curves yield the same values as hot pressed boron nitride for the range of temperatures shown. For ordinary boron nitride sources there is a minor effect of the flow rate on the value of resistivity obtained for any particular’diffusion time and temperature[2]. A sequence of tests was performed to determine the effect of flow rate variations on PBN resistivity. In the case of PBN something quite unexpected occurred. Namely, when the gas flow rate exceeded about 0.5 Urnin in a 70 mm tube it simply depleted the surface of the PBN wafer and resulted in very rapid deactivation. This effect was so pronounced that activated PBN wafers which would have normally lasted for 10 or more hours were depleted in a matter of l/2 hour. This effect is not really surprising if some of the previously
M. Dorris and Mr. M. A. Roche of the Union Carbide Corporation for supplying the PBN samples. Solid Stare Eleclronics Laboratory Elecmrical& Computer Engineering Uniuersity of Cincinnati Cincinnati, OH 45221 U.S.A.
JOSEPHH. NEVIN MICHAELT. HELMIG SAUL AGIJIAR,JR.
1. N. Goldsmith, J. Olmstead and J. Scott, Jr., RCA Rev. 28, 344 (1967). 2. D. Rupprecht and J. Stach, 1. Electrochem. Sot. 120, 1266 (1973). 3. J. Stach and A. Turley, 1. Electrochem. Sot. 121,722 (1974). 4. J. Assour, .I. Electrochem. Sot. 119, 1270 (1972). 5. L. Podobeda, A. Tsapuk and A. Buravov, Poroshkouaya, Met. 165, 44 (1976).
Solid-Pare Ekctronicr Vol. 21. pp. 987~%8 fJ Pergamon Press Ltd.. 1978. Printed in Great Britain
I01/0701-0987/$02.w/0
NOTES CHARACTERIZATION OF PYROLYTIC BORON NITRIDE AS A DIFFUSION SOURCE FOR SILICON (Received 17 November 1977; in revised form 13 February 1978) There have been several articles[l-4] which have presented the results of the use of boron nitride made by a hot pressed technique as a diffusion source for silicon. This note presents the results of the characterization of a new type of boron nitride, one which is manufactured by a chemical vapor deposition process rather than by a hot pressing technique. The differences between the two techniques are substantial both in terms of the compositional structures involved and in terms of the use, particularly the oxidation procedures used to establish a boric oxide surface layer. The samples used in this work were Union Carbide Boralloy Pyrolytic Boron Nitride (PBN) wafers, two inches in diameter and 0.040 in. thick. For all of the diffusions 2 in. diameter silicon wafers, 3-6 ohm-cm in resistivity, phosphorous doped, having (111) orientation were used. The resistivity values quoted in this note are the average of five measured values on each wafer: center, right, left, top and bottom. The PBN wafers were cleaned before activation. The procedure consisted of a IO min. boil in deionized water followed by a 5 min ultrasonic cleaning in methanol and finally the wafers were dried in nitrogen at about 150°C for one hour. Because there is no boric oxide binder used in PBN there is no problem with using water or methanol for cleaning as there is in the case of some hot pressed materials [2]. There was definite evidence to indicate that this cleaning procedure should be done immediately prior to the activation of the PBN. During some of the work this cleaning procedure had been used but the PBN wafers were allowed to remain for a few days in nitrogen at lSO”C before the activation took place. For those wafers a distinctly poorer activation was noted than for the case when the cleaning took place immediately prior to activation. The activation procedure consisted of placing the cleaned wafers in the diffusion furnace at a temperature of at least 1050°C under an oxygen flow of 500 cm’lmin. for a period of one hour. The PBN wafers did activate satisfactorily under these conditions but temperatures higher than 1050°C lead to a much improved lifetime. The stabilization procedure consisted of leaving the wafers at the activation temperature for about one hour under either a pure nitrogen gas flow or a 24% oxygen flow added to the nitrogen. Some of the tests show that oxygen added during the stabilization period leads to a longer wafer life between reactivations. For example in one test it was observed that at 1050°C when stabilized in pure nitrogen the wafer was in fact barely activated and quickly lost most of its activation. However, when the test was repeated with 4% oxygen added during stabilization it was found that the PBN wafer lasted 9 hr under diffusion conditions. Both test diffusions in this case were done under pure nitrogen. Thus the addition of oxygen seems to play an important role even during stabilization. In order to make an estimate of the time required between activations, several tests were run to determine the useful life as a function of activation temperature and diffusion gas ambient. The lifetime is defined here as the time in hours following the stabilization period until the resistivity of the diffused region rises by about 10% from the nominal value. For these tests three temperatures for activation and three gas ambient conditions for diffusion were chosen. The diffusion conditions were 100% nitrogen, nitrogen plus 2% added oxygen and 4% added oxygen. In all cases the activation time was 60 min and the stabilization time was at least 60min followed by the time necessary for the 987
furnace to change temperature from the activation/stabilization temperature to the diffusion temperature. This was typically about 30 min. The gas ambient during stabilization was the same as that during the subsequent diffusion. The diffusions were all done at a temperature of 1000°C for a period of 20min. The results of the lifetime experiments are plotted in Fii. I. The trend is very clear. The use of higher activation temperatures and oxygen added during the stabilization and diffusion times can substantially increase the lifetime of the PBN between reactivations. The two data points which indicate a range of hours are simply due to the methods by which the data were taken. The wafer was checked at the first time and found to be good and not checked again until the second time when it was found to have deactivated. It was possible to reactivate wafers by leaving them at high temperatures and changing the gas flow to pure oxygen. It appears possible thus to store the wafers in the diffusion tube under a partial oxygen gas flow and to reactivate every day or two for a short period of time without having to re-clean the PBN. One would expect of course, once the PBN had been activated that the diffusions would produce the same resistivity values as
I
x)
I
1050
1100
Temperature. g: Fig. 1. PBN lifetime as a function of activation temperature gas ambient.
and
Notes
988
950
oc
2 12co”c I
IO
20
x)
40 Time,
.
50
60
min
Fig. 2. Sheet resistance as a function of predeposition time and temperature for PBN wafers using 2% 02 in N2.
mentioned work is considered. Namely it appears that the activation process affects only the surface of the PBN wafer. In hot pressed boron nitride there is a substantially greater volume of activated material. This a conclusion reached from the results of Podobeda et al. [S] which indicates that the rate of oxidation is affected by the amount of boric oxide within the boron nitride wafer. Hence when the gas flow rate is high enough the transfer of B203 from the surface is greater and becomes more nearly rate limited. The rapid depletion of the activated layer then results. Thus this constitutes another piece of evidence indicating the nature of the activation process in PBN wafers. One of the conclusions to be reached is that relatively low gas flow rates are necessary for reasonable wafer life between reactivation periods. For a 70mm tube the flow rate should be less than 500cm3/min with 150-200 cm’lmin being a suggested range. The resistivity uniformity across the diffused wafer is typically in the range of + 2% to 2 3%. Figure 3 shows a plot of average wafer resistivity as a function of position within the boat, with + 3% being easily attainable. The run-to-run resistivity variations were less than 2 4%. Acknowledgements-The
authors express appreciation
to Mr. J.
35 t
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2
t
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1
6
I
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6
Wafer
I1
IO
I
I
12
I
I
I4
I1
I6
I
I
18
position
Fig. 3. Sheet resistance as a function of wafer position in the boat for a lOOO”C/20min diffusion using 150 cm3/min Nz and 8 cm’lmin 02.
those of hot pressed boron nitride. Indeed that is the result which was observed in this work. Shown in Fig. 2 are the curves of sheet resistance as a function of time and temperature. These curves yield the same values as hot pressed boron nitride for the range of temperatures shown. For ordinary boron nitride sources there is a minor effect of the flow rate on the value of resistivity obtained for any particular’diffusion time and temperature[2]. A sequence of tests was performed to determine the effect of flow rate variations on PBN resistivity. In the case of PBN something quite unexpected occurred. Namely, when the gas flow rate exceeded about 0.5 Urnin in a 70 mm tube it simply depleted the surface of the PBN wafer and resulted in very rapid deactivation. This effect was so pronounced that activated PBN wafers which would have normally lasted for 10 or more hours were depleted in a matter of l/2 hour. This effect is not really surprising if some of the previously
M. Dorris and Mr. M. A. Roche of the Union Carbide Corporation for supplying the PBN samples. Solid Stare Eleclronics Laboratory Elecmrical& Computer Engineering Uniuersity of Cincinnati Cincinnati, OH 45221 U.S.A.
JOSEPHH. NEVIN MICHAELT. HELMIG SAUL AGIJIAR,JR.
1. N. Goldsmith, J. Olmstead and J. Scott, Jr., RCA Rev. 28, 344 (1967). 2. D. Rupprecht and J. Stach, 1. Electrochem. Sot. 120, 1266 (1973). 3. J. Stach and A. Turley, 1. Electrochem. Sot. 121,722 (1974). 4. J. Assour, .I. Electrochem. Sot. 119, 1270 (1972). 5. L. Podobeda, A. Tsapuk and A. Buravov, Poroshkouaya, Met. 165, 44 (1976).