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Aluminium spearing in silicon integrated circuits
NOTES
S&&St~te
Ekctron~cs,
Aluminium (Receiued
1973, Vol. 16, pp. 1303-1304. Printed in Great Britain
Pergamon
Press.
spearing in silicon integrated circuits
8 January
1973; in revisedform 1973)
26 February
MANY AUTHORShave reported that silicon migrates into the aluminium tracks on integrated circuits. The reverse is also true and aluminium will move into the silicon slice and under certain conditions will form the so called aluminium spears. We have been able to reproduce this effect and this note describes our observations. Procrdure. A 5000 A thick wet oxide layer was grown on 0.4 a-cm n-type polished { 1 11} Si slices. To reproduce the conditions existing in an integrated circuit a 2pm deep p-type diffusion was performed using a transistor test device pattern. Contact windows were etched and a 3000 A thick layer of pure Al evaporated onto the slice from tungsten filaments; interconnection tracks were formed by conventional photolithography. The slices were scribed into 0.5cm-squares which were used in various time-temperature heat cycles. The 0.5cm square chips were placed in a small stainless steel holder, the temperature of which was monitored with a Pt/ 10% Rh-Pt thermocouple. The holder was placed in an open tube furnace with a 12 in. constant temperature zone controllable to -C 1°C. The tube was continuously flushed with dry nitrogen. On removal from the furnace the chips were cooled as quickly as possible. Reslrlts. It was found that spears were produced very rapidly at a temperature of 560°C or greater, while none were produced, even after IOmin at 550°C. The spears were in fact the points of triangles, as shown in Fig. 1, part of the triangle usually being masked by the Al interconnections. The points of the triangles were formed beneath the SiO, and had a silvery appearance when viewed through an optical microscope. The triangles were observed under both the thick oxide covering n-type areas and thinner oxide covering p-type areas of the chip. Figure 2 shows a single triangle as seen with a scanning electron microscope (SEM) after the oxide had been removed with a buffered HF etch (Al tracks remain intact). Removal of the Al, using a phosphoric acid etch, left flat bottomed triangular pits as shown in Fig. 3.
1303
The triangles formed very rapidly at temperature 3 560°C and it was not possible to measure the time precisely with the equipment available. It was possible, however, to remove the slice at a particular temperature as the steel holder was being heated. No triangles were present at 550°C: 54 set later at 557°C alloying had occurred and the alloying fronts could be observed; 20 set later at 560°C the triangles were almost fully formed. After this initial rapid growth, the triangles did not increase greatly in size when subjected to further heat treatment. Repeating the experiment with a proprietory brand of Si doped Al wire containing approximately 12% Si had no apparent effect on the formation of triangles. Pure Al was deposited on polished Si slices without any prior growth of an oxide layer. The Al was deposited in the form of an orthogonal grid containing also small discrete islands approximately 12-pm square. After heating (> 56O”Q triangles appeared along the grid with the largest being formed at the intersections of the grid lines. No triangles were observed to form around the small discrete islands of Al. Discussion. The material removed from the flat bottomed etch pits is obviously rich in Al indicating that Al has moved into the slice. This occurs during the dissolution of Al and Si at a temperature well below the Si : Al eutectic. Neither the conductivity type of the Si nor the presence of an oxide layer appear to have any influence on this reaction. Almost perfect equilateral triangles with sides of approximately 25 pm formed around contact windows 12-pm square. In the case of larger contact windows the alloying fronts could be seen, but they did not generally form complete triangles, possibly because of insufficient Al. The alloying fronts are clearly seen in Fig. 2 and the surface enclosed by these fronts is greatly disturbed compared with the surrounding silicon. This effect is likely to produce scattering of light which enables the triangles to be seen with an optical microscope (they cannot be seen with the SEM if the oxide is left in place). The depression is caused by dissolution of Si and movement of the Si into the Al interconnections as noted by previous authors [ 1,2]. The reaction between Al and Si proceeds rapidly in a direction parallel to the slice surface and much more slowly in the ( 111) direction perpendicular
1304
NOTES
to the slice surface. The reaction planes which move out from the contact area correspond to a rapidly moving { 100) plane and a slowly moving { 1 IO} plane. The { 100) fronts diminish in size. as they are bounded by the { 1 IO} fronts which eventually form the sides of the triangle. The region within these fronts is saturated with Al and can be removed with a conventional Al etch. Conclusions. In { 111) Si triangles are formed at temperatures a 560°C by Si-Al alloying which extends beneath the oxide, the region being bounded by { 110) planes. Complete triangles are only formed around the small square contact windows. The use of %-doped Al reduces the dissolution and movement of Si into the interconnections, but it does not prevent the movement of AI into Si. The most startling observation is the speed with which the triangles are formed, approximately 20 set at 560°C. Consequently if slices in an alloying furnace “see” radiation from the furnace heating coils at a temperature > 560°C for a few seconds,
then spear formation can occur even though the average temperature measured by the furnace thermocouple is below 560°C. This can be prevented by inserting a ceramic linear between the quartz furnace tube and the furnace heating coils and by ensuring that the temperature does not exceed 560°C. Acknowledgement-The
authors wish to thank Dr. McHardy of the MacCaulay Institute for providing the SEM photographs. . -_ T. E. PRICE L. A. BERTHOUD
Robert Gordon’s Institute Aberdeen, AL39 IFR, Scotlund
of Technology,
REFERENCES
‘. G. L. Schnable and R. S. Keen, PVW. Si\-tk AWI.
Reliability Phys. Symp. pp. 170-192, Los Angeles, November (1967). 2, J. 0. McCaldin and H. Sankur. A/>/II. Pltys. fxtts 20 ( 1972).
S&&St~te
Ekctron~cs,
Aluminium (Receiued
1973, Vol. 16, pp. 1303-1304. Printed in Great Britain
Pergamon
Press.
spearing in silicon integrated circuits
8 January
1973; in revisedform 1973)
26 February
MANY AUTHORShave reported that silicon migrates into the aluminium tracks on integrated circuits. The reverse is also true and aluminium will move into the silicon slice and under certain conditions will form the so called aluminium spears. We have been able to reproduce this effect and this note describes our observations. Procrdure. A 5000 A thick wet oxide layer was grown on 0.4 a-cm n-type polished { 1 11} Si slices. To reproduce the conditions existing in an integrated circuit a 2pm deep p-type diffusion was performed using a transistor test device pattern. Contact windows were etched and a 3000 A thick layer of pure Al evaporated onto the slice from tungsten filaments; interconnection tracks were formed by conventional photolithography. The slices were scribed into 0.5cm-squares which were used in various time-temperature heat cycles. The 0.5cm square chips were placed in a small stainless steel holder, the temperature of which was monitored with a Pt/ 10% Rh-Pt thermocouple. The holder was placed in an open tube furnace with a 12 in. constant temperature zone controllable to -C 1°C. The tube was continuously flushed with dry nitrogen. On removal from the furnace the chips were cooled as quickly as possible. Reslrlts. It was found that spears were produced very rapidly at a temperature of 560°C or greater, while none were produced, even after IOmin at 550°C. The spears were in fact the points of triangles, as shown in Fig. 1, part of the triangle usually being masked by the Al interconnections. The points of the triangles were formed beneath the SiO, and had a silvery appearance when viewed through an optical microscope. The triangles were observed under both the thick oxide covering n-type areas and thinner oxide covering p-type areas of the chip. Figure 2 shows a single triangle as seen with a scanning electron microscope (SEM) after the oxide had been removed with a buffered HF etch (Al tracks remain intact). Removal of the Al, using a phosphoric acid etch, left flat bottomed triangular pits as shown in Fig. 3.
1303
The triangles formed very rapidly at temperature 3 560°C and it was not possible to measure the time precisely with the equipment available. It was possible, however, to remove the slice at a particular temperature as the steel holder was being heated. No triangles were present at 550°C: 54 set later at 557°C alloying had occurred and the alloying fronts could be observed; 20 set later at 560°C the triangles were almost fully formed. After this initial rapid growth, the triangles did not increase greatly in size when subjected to further heat treatment. Repeating the experiment with a proprietory brand of Si doped Al wire containing approximately 12% Si had no apparent effect on the formation of triangles. Pure Al was deposited on polished Si slices without any prior growth of an oxide layer. The Al was deposited in the form of an orthogonal grid containing also small discrete islands approximately 12-pm square. After heating (> 56O”Q triangles appeared along the grid with the largest being formed at the intersections of the grid lines. No triangles were observed to form around the small discrete islands of Al. Discussion. The material removed from the flat bottomed etch pits is obviously rich in Al indicating that Al has moved into the slice. This occurs during the dissolution of Al and Si at a temperature well below the Si : Al eutectic. Neither the conductivity type of the Si nor the presence of an oxide layer appear to have any influence on this reaction. Almost perfect equilateral triangles with sides of approximately 25 pm formed around contact windows 12-pm square. In the case of larger contact windows the alloying fronts could be seen, but they did not generally form complete triangles, possibly because of insufficient Al. The alloying fronts are clearly seen in Fig. 2 and the surface enclosed by these fronts is greatly disturbed compared with the surrounding silicon. This effect is likely to produce scattering of light which enables the triangles to be seen with an optical microscope (they cannot be seen with the SEM if the oxide is left in place). The depression is caused by dissolution of Si and movement of the Si into the Al interconnections as noted by previous authors [ 1,2]. The reaction between Al and Si proceeds rapidly in a direction parallel to the slice surface and much more slowly in the ( 111) direction perpendicular
1304
NOTES
to the slice surface. The reaction planes which move out from the contact area correspond to a rapidly moving { 100) plane and a slowly moving { 1 IO} plane. The { 100) fronts diminish in size. as they are bounded by the { 1 IO} fronts which eventually form the sides of the triangle. The region within these fronts is saturated with Al and can be removed with a conventional Al etch. Conclusions. In { 111) Si triangles are formed at temperatures a 560°C by Si-Al alloying which extends beneath the oxide, the region being bounded by { 110) planes. Complete triangles are only formed around the small square contact windows. The use of %-doped Al reduces the dissolution and movement of Si into the interconnections, but it does not prevent the movement of AI into Si. The most startling observation is the speed with which the triangles are formed, approximately 20 set at 560°C. Consequently if slices in an alloying furnace “see” radiation from the furnace heating coils at a temperature > 560°C for a few seconds,
then spear formation can occur even though the average temperature measured by the furnace thermocouple is below 560°C. This can be prevented by inserting a ceramic linear between the quartz furnace tube and the furnace heating coils and by ensuring that the temperature does not exceed 560°C. Acknowledgement-The
authors wish to thank Dr. McHardy of the MacCaulay Institute for providing the SEM photographs. . -_ T. E. PRICE L. A. BERTHOUD
Robert Gordon’s Institute Aberdeen, AL39 IFR, Scotlund
of Technology,
REFERENCES
‘. G. L. Schnable and R. S. Keen, PVW. Si\-tk AWI.
Reliability Phys. Symp. pp. 170-192, Los Angeles, November (1967). 2, J. 0. McCaldin and H. Sankur. A/>/II. Pltys. fxtts 20 ( 1972).