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Direct bonding: retrospect and outlook
Philips J. Res. 49 (1995)
171-177
DIRECT BONDING: RETROSPECT
AND OUTLOOK
by JAN HAISMA Philips Research Laboratories, Prof. Holstlaan 4. 5656 AA Eindhoven. The Netherlands
Abstract The characteristic features of direct bonding with respect to its many-sided aspects are briefly enumerated. Nowadays silicon-on-silicon and siliconon-insulator are the trendsetters. The preparative conditions of direct bonding are compatible with silicon technologies. In addition, in the future, direct bonding may find dedicated applications in the field of hybrid material combinations, micromechanics for precision medical tools, sensors and actuators. Keywords:
direct bonding,
retrospective,
state of the art, outlook.
1. Retrospect: a survey
Direct bonding lies somewhere between an art and a technology. It has many characteristic features (see Table I) of a chemical, cryogenic, materialscientific, mechanical, Maxwell-equational, thermal and vacuum nature. It is clear that some of these aspects may have an impact on future unsolved engineering problems. At this point it has to be emphasized that direct bonding is in many cases not a low-cost and simple technology. First of all, the materials engineering problems have to be solved in research, then an industrial route has to be explored and, finally, the implemented technology has to be compatible with existing technologies. For direct bonding, the type of material (metal, semiconductor, carbide, fluoride, oxide, nitride, compound, composite, organic material) and the state of the material (monocrystalline, polycrystalline, amorphous) are generally not significant. However, polycrystalline materials with a second phase are more of a problem. These substances need a special treatment if the hardnesses of the constituent materials are different. Superfinish-polishing of solid-state materials, a necessity for direct bonding,
Philips Journal of Research Vol. 49
No. l/Z
1995
171
J. Haisma
TABLE I A survey of features realizable by direct bonding. Scientific discipline chemical
cryogenic material-scientific
mechanical
Maxwell-equational
thermal vacuum
Direct-bond feature dopant variation buried layers surface-state conservation fusing intentional surface modifications transparency engineering silicon-on-superconductor chemical-composition indifference various compositional states various crystallographic orientations various lattice constants shape-memory materials slitlessness gluelessness interface engineering lossless optical transitions magnetic flux-line conservation electric contactability equal dilatations vacuum tightness
is a skill based on an evaluation of processing routes. Table II gives a strategic map, deploying a large number of successful routes for different goals. This underlines how diverse these itineraries can be. Careful cleaning is also an essential part of the direct-bonding process. The direct-bonded surfaces make contact at atomic distances, so an adsorbed monolayer can have a drastic influence on their susceptibility to direct bonding. Direct bonding as a phenomenon can occur in a manner ranging from spontaneous to very hesitant. The basic causes for these differences are in many cases not clear. One thing, however, is certain: hydrogen bridges play an essential role by adding a chemical bond to the Van der Waals bond as soon as the distance between the bodies is on an atomic scale. Bond-strength enhancement needs a post-treatment, normally annealing.
172
Philips .Journ,,l orResenreh
Vol. 49
No. l/2
1995
Direct bonding: retrospect and outlook
TABLE II General scheme of a polishing strategy. Polishing method
Polishing aim
mechanical polishing dedicated mechanical polishing chemical polishing tribochemical polishing dedicated trib. polishing enhanced trib. polishing organo-liquid-supported trib. pol. oxidation-stimulated trib. pol. recessed-surface pol. zero-removal rate pol.
optical elements refractory metals III-V compounds semiconductors super surface finish hard materials noble metals non-noble metals polycrystalline materials two-sided polishing
Table I gives a survey of the characteristic features we have investigated in the past. A whole gamut of phenomenological and material-scientific possibilities are to be encountered in the context of direct bonding. The list presented here is certainly not exhaustive; unforeseen innovations may stem from it. Property-driven material combinations may emerge; the direct bonding of the metal niobium and sapphire (see Fig. l), which have the same dilatation up to about 1lOO”C, is just one example. 2. State of the art In the fall of 1991 the First International Symposium on Semiconductor Wafer Bonding: Science, Technology and Applications [l] was held in Phoenix, Arizona, USA, and in 1993 the Eighth Biennial Conference on Insulating Films on Semiconductors [2], held in Delft, the Netherlands, dedicated a large part of its programme to direct bonding. From the subject matter of these two conferences, it can easily be seen that direct bonding has become a semiconductor topic, shifting from optics and precision mechanics to modern advanced semiconductor technology. From the papers presented it can be seen that direct bonding is now practised all over the world. From patent literature it can also be learnt that there are three main issues of direct bonding: 0 optical; l silicon-on-insulator (SOI); and
Philips Journal of Research
Vol. 49
No. l/Z
1995
173
J. Haisma
Fig.
1. Sapphire
direct-bonded to a sheet of niobium on top of a polish spontaneously. Wafer diameter: 10 cm.
holder;
they bond
0 SO1 wafer manufacturing. These do not necessarily reflect the activities conducted worldwide. The optical avenue, with all its diverse possibilities, has not yet been generally investigated. This is quite understandable if one considers that direct bonding l is in itself not a tight bond; l needs advanced technologies in the context of preparation, bonding and post-treatments; l is (for these reasons) not cost-effective; and l is in most cases easily substitutable by other bonding techniques or technologies. This means that special requirements relating to bonding have to be important in order to make direct bonding a chosen candidate; these requirements may be of a mechanical (precision, thinness), chemical (contaminations), physical (ultrasharp transitions), thermal (equal dilatations) or other origin. In advanced semiconductor technologies practically all these requirements hold: the bond has to be 0 contamination-free; l sharp for SOI; l thin for SO1 in many cases; l homogeneous in thickness, as for SOI; and
174
Philips Journal of Resesrch
Vol. 49
No. 1 I7
19%
Direct bondina: retromect and outlook
in thermal equilibrium of Si to SiOz to Si. All these requirements can be reasonably satisfied by direct bonding, providing a safe basis for today’s status quo, especially in semiconductor technologies. l
3. Outlook 3.1. Direct bonding as a bond technology Direct bonding is unmistakably a high-tech bond technology. The preparation routes are fairly laborious but dedicated engineering may upgrade the bond, not to a simple low-cost one but to an outstanding one. Outstanding means here that it incorporates the features specifically required: precision, accuracy, cleanliness, sharp-cut transition, novelty. These avenues will certainly be the future mainstream for direct bonding of inorganic materials. Many organic materials, on the contrary, have the advantage of a much higher elasticity and, moreover, a reasonable plasticity at relatively low temperatures. The triple combination of temperature, local pressure, lateral or transversal, and direct bonding may constitute a set of interdependent parameters by which organic/inorganic bonds may excel. To mention a case in point: flip-chip technology. Here, direct bonding might be effectively applied as a high-end technology, convergeable to the level of a low-cost bond. 3.2. Disciplinary
fields for direct bonding
Silicon technologies will certainly be trendsetters for direct bonding in the near future although, for SOI, SIMOX is in some areas a formidable competitor. Only where thicker insulating layers (> 0.2 pm) and/or thicker SO1 layers (> 0.5 pm) are required, will direct bonding outperform SIMOX. Direct bonding in SO1 technologies is basically quite feasible. However, cost-effectiveness is governed to a large extent by thinning the SO1 layer. Mass production is feasible and routes have been described [3, 41. A novel method of single-piece fabrication, with a high level of uniformity in thickness of the SO1 layer, is optically and numerically controlled, spatially confined plasma etching [5]. It is, apparently, a promising method. One of the driving forces for SO1 in the future may be energy-effectiveness rather than cost-effectiveness and the increase in circuit density. SO1 integrated circuits can be engineered so as to reduce considerably parasitic capacitance, resulting in reduced ac losses. III-V compounds directly bonded to silicon may surpass largely heteroepitaxial growth in respect of the stress/strain budget. Defect-free (poor)
Philips Journal of Research
Vol. 49
No. l/2
1995
175
J. Haisma
options are feasible and, as soon as integration of optical and IC-actuating components on one chip is required, direct bonding will have a fair chance of success. Full-wafer-scale direct bonding of the two materials concerned, followed by etching open, will prevail over micro-direct-bonding. Micromechanics is the technology of shaping materials in micro dimensions. Until now it has been an area of limited practical value, except for medical applications such as intravenous treatments and measurements. Direct bonding is a very suitable technique for this area. Sensors and actuators are still expected to undergo a major boom in the (near) future. The boom has so far failed to materialize, but this may change. Direct bonding has found applications in pressure sensors [6], and novel devices are emerging. 3.3 Summary Direct bonding has progressed from a technique, via a technology, to a scientific platform, interwoven as it is with many disciplines via diverse routes. Direct bonding is a widely applicable bonding technology, with dedicated applications which have been and are still being evaluated. Direct bonding is a high-tech discipline which is highly compatible with silicon technology. In the future, direct bonding will have an impact primarily on silicon technologies, sensors and actuators, as well as on miniaturized medical applications. Direct bonding is not yet a mainstream technology, more a speciality applicable to particular technological bonding topics.
Acknowledgement Bertus Pals and Bert Spierings read the manuscript critically. Their contributions are gratefully acknowledged.
REFERENCES [1] U. Giisele, T. Abe, J. Haisma and M.A. Schmidt, Semiconductor Wafer Bonding: Science, Technology and Applications, Proc. 1st Int. Symp., Vol. 92-7, publ. The Electrochemical Society, Pennington, 1992. [2] P. Balk and J.J.M. Nijs (eds.), Proc. 8th Bienn. Conf on Insulating Films on Semiconductors, 2-5 June 1993, Delft, The Netherlands; published in Microelectron. Eng., 22, l-420 (1993); in particular: SOI: materials and device technology, pp. 299-410. [3] Eur. Patent application no. 579 298, priority date 15.06.92. [4] Eur. Patent application no. 547 684, priority date 18.12.91.
176
Philips Journal of Research
Vol. 49
No. l/2
1995
Direct bonding: retrospect and outlook
[5] G.J. Gardopee, P.B. Mumola, P.J. Clapis, C.B. Zarowin, L.D. Bollinger and A.M. Ledger, Plasma thinning of silicon-on-insulator wafers, Microelectron. Eng., 22, 347-350 (1993). [6] L. Christel, K. Petersen, P. Barth, F. Pourahmadi, J. Mallon Jr and J. Bryzek, Single-crystal silicon pressure sensors with 500x overpressure protection, Sensors and Actuators, A21-A23, 84-88(1990).
Philips Journal of Research
Vol. 49
No. l/2
1995
177
171-177
DIRECT BONDING: RETROSPECT
AND OUTLOOK
by JAN HAISMA Philips Research Laboratories, Prof. Holstlaan 4. 5656 AA Eindhoven. The Netherlands
Abstract The characteristic features of direct bonding with respect to its many-sided aspects are briefly enumerated. Nowadays silicon-on-silicon and siliconon-insulator are the trendsetters. The preparative conditions of direct bonding are compatible with silicon technologies. In addition, in the future, direct bonding may find dedicated applications in the field of hybrid material combinations, micromechanics for precision medical tools, sensors and actuators. Keywords:
direct bonding,
retrospective,
state of the art, outlook.
1. Retrospect: a survey
Direct bonding lies somewhere between an art and a technology. It has many characteristic features (see Table I) of a chemical, cryogenic, materialscientific, mechanical, Maxwell-equational, thermal and vacuum nature. It is clear that some of these aspects may have an impact on future unsolved engineering problems. At this point it has to be emphasized that direct bonding is in many cases not a low-cost and simple technology. First of all, the materials engineering problems have to be solved in research, then an industrial route has to be explored and, finally, the implemented technology has to be compatible with existing technologies. For direct bonding, the type of material (metal, semiconductor, carbide, fluoride, oxide, nitride, compound, composite, organic material) and the state of the material (monocrystalline, polycrystalline, amorphous) are generally not significant. However, polycrystalline materials with a second phase are more of a problem. These substances need a special treatment if the hardnesses of the constituent materials are different. Superfinish-polishing of solid-state materials, a necessity for direct bonding,
Philips Journal of Research Vol. 49
No. l/Z
1995
171
J. Haisma
TABLE I A survey of features realizable by direct bonding. Scientific discipline chemical
cryogenic material-scientific
mechanical
Maxwell-equational
thermal vacuum
Direct-bond feature dopant variation buried layers surface-state conservation fusing intentional surface modifications transparency engineering silicon-on-superconductor chemical-composition indifference various compositional states various crystallographic orientations various lattice constants shape-memory materials slitlessness gluelessness interface engineering lossless optical transitions magnetic flux-line conservation electric contactability equal dilatations vacuum tightness
is a skill based on an evaluation of processing routes. Table II gives a strategic map, deploying a large number of successful routes for different goals. This underlines how diverse these itineraries can be. Careful cleaning is also an essential part of the direct-bonding process. The direct-bonded surfaces make contact at atomic distances, so an adsorbed monolayer can have a drastic influence on their susceptibility to direct bonding. Direct bonding as a phenomenon can occur in a manner ranging from spontaneous to very hesitant. The basic causes for these differences are in many cases not clear. One thing, however, is certain: hydrogen bridges play an essential role by adding a chemical bond to the Van der Waals bond as soon as the distance between the bodies is on an atomic scale. Bond-strength enhancement needs a post-treatment, normally annealing.
172
Philips .Journ,,l orResenreh
Vol. 49
No. l/2
1995
Direct bonding: retrospect and outlook
TABLE II General scheme of a polishing strategy. Polishing method
Polishing aim
mechanical polishing dedicated mechanical polishing chemical polishing tribochemical polishing dedicated trib. polishing enhanced trib. polishing organo-liquid-supported trib. pol. oxidation-stimulated trib. pol. recessed-surface pol. zero-removal rate pol.
optical elements refractory metals III-V compounds semiconductors super surface finish hard materials noble metals non-noble metals polycrystalline materials two-sided polishing
Table I gives a survey of the characteristic features we have investigated in the past. A whole gamut of phenomenological and material-scientific possibilities are to be encountered in the context of direct bonding. The list presented here is certainly not exhaustive; unforeseen innovations may stem from it. Property-driven material combinations may emerge; the direct bonding of the metal niobium and sapphire (see Fig. l), which have the same dilatation up to about 1lOO”C, is just one example. 2. State of the art In the fall of 1991 the First International Symposium on Semiconductor Wafer Bonding: Science, Technology and Applications [l] was held in Phoenix, Arizona, USA, and in 1993 the Eighth Biennial Conference on Insulating Films on Semiconductors [2], held in Delft, the Netherlands, dedicated a large part of its programme to direct bonding. From the subject matter of these two conferences, it can easily be seen that direct bonding has become a semiconductor topic, shifting from optics and precision mechanics to modern advanced semiconductor technology. From the papers presented it can be seen that direct bonding is now practised all over the world. From patent literature it can also be learnt that there are three main issues of direct bonding: 0 optical; l silicon-on-insulator (SOI); and
Philips Journal of Research
Vol. 49
No. l/Z
1995
173
J. Haisma
Fig.
1. Sapphire
direct-bonded to a sheet of niobium on top of a polish spontaneously. Wafer diameter: 10 cm.
holder;
they bond
0 SO1 wafer manufacturing. These do not necessarily reflect the activities conducted worldwide. The optical avenue, with all its diverse possibilities, has not yet been generally investigated. This is quite understandable if one considers that direct bonding l is in itself not a tight bond; l needs advanced technologies in the context of preparation, bonding and post-treatments; l is (for these reasons) not cost-effective; and l is in most cases easily substitutable by other bonding techniques or technologies. This means that special requirements relating to bonding have to be important in order to make direct bonding a chosen candidate; these requirements may be of a mechanical (precision, thinness), chemical (contaminations), physical (ultrasharp transitions), thermal (equal dilatations) or other origin. In advanced semiconductor technologies practically all these requirements hold: the bond has to be 0 contamination-free; l sharp for SOI; l thin for SO1 in many cases; l homogeneous in thickness, as for SOI; and
174
Philips Journal of Resesrch
Vol. 49
No. 1 I7
19%
Direct bondina: retromect and outlook
in thermal equilibrium of Si to SiOz to Si. All these requirements can be reasonably satisfied by direct bonding, providing a safe basis for today’s status quo, especially in semiconductor technologies. l
3. Outlook 3.1. Direct bonding as a bond technology Direct bonding is unmistakably a high-tech bond technology. The preparation routes are fairly laborious but dedicated engineering may upgrade the bond, not to a simple low-cost one but to an outstanding one. Outstanding means here that it incorporates the features specifically required: precision, accuracy, cleanliness, sharp-cut transition, novelty. These avenues will certainly be the future mainstream for direct bonding of inorganic materials. Many organic materials, on the contrary, have the advantage of a much higher elasticity and, moreover, a reasonable plasticity at relatively low temperatures. The triple combination of temperature, local pressure, lateral or transversal, and direct bonding may constitute a set of interdependent parameters by which organic/inorganic bonds may excel. To mention a case in point: flip-chip technology. Here, direct bonding might be effectively applied as a high-end technology, convergeable to the level of a low-cost bond. 3.2. Disciplinary
fields for direct bonding
Silicon technologies will certainly be trendsetters for direct bonding in the near future although, for SOI, SIMOX is in some areas a formidable competitor. Only where thicker insulating layers (> 0.2 pm) and/or thicker SO1 layers (> 0.5 pm) are required, will direct bonding outperform SIMOX. Direct bonding in SO1 technologies is basically quite feasible. However, cost-effectiveness is governed to a large extent by thinning the SO1 layer. Mass production is feasible and routes have been described [3, 41. A novel method of single-piece fabrication, with a high level of uniformity in thickness of the SO1 layer, is optically and numerically controlled, spatially confined plasma etching [5]. It is, apparently, a promising method. One of the driving forces for SO1 in the future may be energy-effectiveness rather than cost-effectiveness and the increase in circuit density. SO1 integrated circuits can be engineered so as to reduce considerably parasitic capacitance, resulting in reduced ac losses. III-V compounds directly bonded to silicon may surpass largely heteroepitaxial growth in respect of the stress/strain budget. Defect-free (poor)
Philips Journal of Research
Vol. 49
No. l/2
1995
175
J. Haisma
options are feasible and, as soon as integration of optical and IC-actuating components on one chip is required, direct bonding will have a fair chance of success. Full-wafer-scale direct bonding of the two materials concerned, followed by etching open, will prevail over micro-direct-bonding. Micromechanics is the technology of shaping materials in micro dimensions. Until now it has been an area of limited practical value, except for medical applications such as intravenous treatments and measurements. Direct bonding is a very suitable technique for this area. Sensors and actuators are still expected to undergo a major boom in the (near) future. The boom has so far failed to materialize, but this may change. Direct bonding has found applications in pressure sensors [6], and novel devices are emerging. 3.3 Summary Direct bonding has progressed from a technique, via a technology, to a scientific platform, interwoven as it is with many disciplines via diverse routes. Direct bonding is a widely applicable bonding technology, with dedicated applications which have been and are still being evaluated. Direct bonding is a high-tech discipline which is highly compatible with silicon technology. In the future, direct bonding will have an impact primarily on silicon technologies, sensors and actuators, as well as on miniaturized medical applications. Direct bonding is not yet a mainstream technology, more a speciality applicable to particular technological bonding topics.
Acknowledgement Bertus Pals and Bert Spierings read the manuscript critically. Their contributions are gratefully acknowledged.
REFERENCES [1] U. Giisele, T. Abe, J. Haisma and M.A. Schmidt, Semiconductor Wafer Bonding: Science, Technology and Applications, Proc. 1st Int. Symp., Vol. 92-7, publ. The Electrochemical Society, Pennington, 1992. [2] P. Balk and J.J.M. Nijs (eds.), Proc. 8th Bienn. Conf on Insulating Films on Semiconductors, 2-5 June 1993, Delft, The Netherlands; published in Microelectron. Eng., 22, l-420 (1993); in particular: SOI: materials and device technology, pp. 299-410. [3] Eur. Patent application no. 579 298, priority date 15.06.92. [4] Eur. Patent application no. 547 684, priority date 18.12.91.
176
Philips Journal of Research
Vol. 49
No. l/2
1995
Direct bonding: retrospect and outlook
[5] G.J. Gardopee, P.B. Mumola, P.J. Clapis, C.B. Zarowin, L.D. Bollinger and A.M. Ledger, Plasma thinning of silicon-on-insulator wafers, Microelectron. Eng., 22, 347-350 (1993). [6] L. Christel, K. Petersen, P. Barth, F. Pourahmadi, J. Mallon Jr and J. Bryzek, Single-crystal silicon pressure sensors with 500x overpressure protection, Sensors and Actuators, A21-A23, 84-88(1990).
Philips Journal of Research
Vol. 49
No. l/2
1995
177