DIRECT BONDING: FROM AN OPTICAL TECHNOLOGY RESEARCH TOPIC

Transcription

1 Philips J. Res. 49 (1995) 1-10 DIRECT BONDING: FROM AN OPTICAL TECHNOLOGY RESEARCH TOPIC TO A BROAD by U.K.P. BIERMANN, A.A. van GORKUM and J.A. PALS Philips Research Laboratories, Prof Holstlaan 4, 5656 AA Eindhoven, The Netherlands Abstract For more than thirty years Philips Research has been engaged in direct bonding. A survey is given of how what was first the study of an optical technology gradually changed into the development of special applications in semiconductor technology. Cooperation between workshops with mastered dedicated technologies and research groups with incentives is shown to be indispensable for success. The principles and diversity of direct bonding are briefly described, after which an impression is given of its stimulating impact on Philips' research programme. Keywords: direct bonding; optics; gas lasers; workshops; optical technological facility. 1. A brief history of the research at Philips Research Direct bonding, also called 'Van der Waals bonding', 'wringing on' (in English), 'ansprengen' (in German) or 'contact optique' (in French), has been applied at Philips Research since the early 1960s and has had a strong impact on a variety of subjects. In 1962 a short stable gas laser was invented and constructed as part of the 'gas-discharge research project': Javan's one-metre long laser [1] was reduced to a 12cm short, stable one [2, 3]. This flat-mirror laser was stabilized with the aid of a rather thick fused-silica rod and optical flats, bearing the highly reflective mirrors at the centre, directly bonded to the stable fused-silica discharge body to constitute a well-aligned flat-mirror Fabry-Perot interferometer. In that way, all the difficulties involved in the operation of a long flat-mirror gas laser were solved through miniaturization and the first prospects for use were opened up. Pbllips Journal ocresearch Vol. 49 No. 1/

2 U.K.P. Biermann et al. In the second half of the 1960s, researchers at Philips suggested using focused laser light as a sensing probe for recorded signals, and that led to the birth of Video Long Play (the present-day laser disc). The short, stable gas laser of 1962 was the precursor of the later laser-optical closed-circuit system..( ~,"'~,.,~,_; :)J;J! djlbj~'jl if) r: i!i.i,_ After some time, directly bonded gas lasers went out of use in the field of entertainment electronics because of the introduetion of diode lasers. However, Philips Research continued to use the direct-bonding technology as a fine-optical tool, e.g. in interferometric applications. In the mid-1980s silicon-on-insulator (SOl) technology started to be studied worldwide as it was believed that it would become very important. Two technologies were developed for full-wafer-scale SOl applications with some freedom of thickness of both the silicon and the insulator layer: SIMOX, i.e. oxygen implantation and annealing of a silicon wafer, and direct bonding of oxidized silicon wafers followed by thinning and annealing. Since then, direct bonding has evolved into a most advanced and highly valued technique, which is nowadays widely used in the field of SOl applications. At the same time that the above technological advances were being made, the research carried out by M.l. Sparnaay of Philips Research was leading to a better understanding of the theory of Van der Waals forces [4], which are basic forces in the direct bonding technology. As Sparnaay pointed out, Sir Isaac Newton was the first to correctly describe the attraction between two macroscopie bodies. 2. Philips Research's facilities In 1994 Philips Research celebrated its 80th anniversary. During those eighty years, spanning more than three generations of researchers, an even greater number of generations of technologies succeeded one another in theoretical studies, but also in practical implementations in workshops. For the first fifty years, Philips Research's research efforts were supported by 'classical' workshops, which concentrated on mechanical, vacuum, optical and electrical issues. In the 1960s and 1970s small dedicated engineering workshops were formed and in the 1980sthose workshops started to cooperate with scientists on a regular basis. Since then, the workshops have become integral parts of Philips Research, generating technological innovations. They are now indispensable facilities in supporting the development of new technologies; the members of those workshops have evolved from skilled craftsmen into technological specialists. The results of their efforts frequently prove to be of great use to Philips' product divisions. 2 Phllips Journal of Research Vol. 49 No. 1/2 1995

3 Direct bonding After World War Il, Philips Research paid much attention to precision machining. The study of the direct bonding technology has benefitted greatly from the results of that research. Much ofthe basic knowledge that has been obtained at Philips Research in the many years of research into topics like crystal growth and surface-finishing treatments, and into the many aspects of semiconductor technologies, also proves to be of great value in the further development of the technology of direct bonding. Let us, therefore, take a closer look at the history of the development of this technology. 3. The study of direct bonding at Philips Research Interferometry and gas-discharge technology are the basic elements of gas laser physics. From a geometrical point of view, interferometry is closely related to precision-alignment physical optics, and direct bonding can ensure the precise alignment ofthe optical components concerned. This was the main motive for applying direct bonding in gas-laser physics in the early 1960s.That led to major advances in optical engineering; see Fig. 1 for an example. The aim of the gas laser set-up described was to find out how to control the vacuum tightness of a direct bond and the tuning of a gas laser. A few gas laser experiments showed that controlling the vacuum tightness of a direct bond between sufficiently large areas was no problem, even for long periods of time (10000 hours). In the experiments the discharged gas did not change colour, and the operating parameter remained constant. However, the temperature of the fused-silica body was found to increase slightly during operation, while the laser cavity patterns (modi) of various intensity-distributed forms of the laser cavity spectrum were found to appear and disappear because of the relatively wide spectral range used [5]. In other words, thermal tuning took place. To find out how to control it by electrical cavity tuning, an electrostrictive tuning section was inserted in the cavity, stabilized by an electrical feedback system [6] (Fig. 2). This concept was later used for an external Fabry-Perot interferometer functioning as an optical comb filter having a spectral range of 1 GHz. This device, which included two directly bonded elements, was then used for spectroscopie purposes. See Fig. 3 for an example. In the 1970s, a light-shutter problem in medical x-ray diagnostics had to be solved. X-ray intensities were kept low during visual inspections of patients but high intensities were used in photographing. An optical shutter or a variable optical filter was required to protect the optical image detection system against overexposure. This problem, too, was solved with the aid of Phllips Journal of Research Vol. 49 No. 1/

4 U.K.P. Biermann et al. Fig A helium-neon cross-laser engineered to investigate the intracavity modulation/interaction of two lasing beams. A fine-optical device of high precision manufactured in-house (parallelism of mirrors ~ 2", laser crossing ~ 1'); both cavity lengths 12 cm. Optical flats, bearing dichroic mirrors, are directly bonded to the fused-silica discharge body. a tunable Fabry-Perot interferometer, including directly bonded elements, with the widest spectral range feasible, i.e. with a mirror distance of less than a few microns [7]. See Fig. 4 for a schematic representation of the setup and Fig. 5 for an example of a device used in practice. While carrying out research into the above issue, we encountered the problem of the unbondability of evaporated layers. We found that the only way in which two components bearing dichroic mirrors could be directly bonded was by subjecting the components to a chemomechanical polishing treatment for three minutes, during which only nanometres of material were removed [8]. Figure 6 shows an example of two such directly bonded dichroic mirrors dating from the period of the investigation of intricate - and at that time puzzling - bonding phenomena. The results obtained in this investigation proved to be of essential importance in later research into surface-related effects in direct bonding. They revealed the importance 4 Philips Journal of Research Vol. 49 No. 1/2 1995

5 Direct bonding optical flat + - discharge tube + - tuning element a, b, c: direct bonds Fig Schematic representation of the set-up of an electrostrictive1y tunable gas laser. of surface treatments for optimizing the effects of direct bonding, and also demonstrated the wide potentialof the technique itself. 4. Direct-bonding technologies In the 1980s Silicon-On-Insulator (SOl) applications became important. A wide range of technologies were evaluated to determine their suitability for these SOl applications [9]. Two full-wafer-scale technologies were found to Fig A scanning Fabry-Perot interferometer, engineered in-house for spectroscopie purposes. An optical comb filter. Cavity length 15 cm. Optical flats, bearing the dichroic mirrors, are directly bonded to the cavity body. Phiüps Joumet of Research Vol. 49 No. 1/

6 u.k.p. Biermann et al. electrostrictive element mirror distance f/ f/ direct bond of dichroic mirrors f/ Fig Schematic representation of the set-up of the tunable Fabry-Perot interferometer with the wide-spectral-range interferometer of Fig. 5. be suitable: SIMOX, i.e. Separation by IMplanted OXygen, and Direct Bonding (DB). The results of Philips' research efforts in the direct bonding of silicon and a series of other materials are the subject of this special issue. Let us take a brief look at the history of this research at Philips Research. As mentioned above, research into direct bonding was started for the purpose of finding solutions to problems encountered in the field of optics. In 1980, however, different aspects of the technology of direct bonding itself started to be investigated. Figure 7 shows a GaAs substrate directly bonded Fig Tunable Fabry-Perot interterorneter with a wide spectral range; engineered in-house, applicable in x-ray displays. 6 Philip. Journalof Research Vol.49 No. 1/2 1995

7 Direct bonding Fig Two directly bonded optical flats bearing deposited (optical) layers. It was found that as-deposited layers were not directly bondable; nor were as-deposited and scrubbed layers, but asdeposited briefly polished layers were. to a silicon fiat body, GaAs directly bonded to fused silica and silicon directly bonded to fused silica. The experiments showed that the principles of direct bonding could be used in other fields, besides that of optics. In 1984/1985 the first efforts were made to realize full-wafer scale SOl applications through the direct bonding of two oxidized silicon wafers, followed by annealing and appropriate chemical thinning, using an etchstop. Figure 8 shows our first 'primitive' SOl sample prepared in this way. Between 1984 and 1989 several technologies of direct bonding for SOl purposes were investigated; they are described in detail in ref. [10]. Figure 9 shows an SOl sample that was prepared in 1988 by polishing only: only a few years' research had led to the development of a technology with which single SOl samples of a quality comparable with that of bulk monocrystalline silicon could be fabricated [11]. So far, research efforts have concentrated on silicon and oxidized silicon. Direct bonding is a complex process. The silicon must be absolutely free of Fig Ga As substrate directly bonded to a silicon flat (left); GaAs directly bonded to a fused-silica flat (middle); silicon directly bonded to a fused-silica flat (right). Philips Journalof Research Vol. 49 No. 1/

8 U.K.P. Biermann et al. Fig First example of a pair of directly bonded oxidized 3" silicon wafers, one ofwhich has been thinned chemically - the first step towards directly bonded SOL contaminations, which means that pretreatment methods like grinding, polishing and cleaning for obtaining hydrophobic or hydrophilic surface states, and aftertreatment methods like annealing, are of great importance; accurate control of these methods is a prerequisite. The geometrical, mechanical, chemical, physical and subsurface states ofthe wafer surface, which are so important in direct bonding, will be discussed in detail elsewhere in this special issue. The numerous applications of silicon directly bonded to a second compound include silicon-on-silicon, silicon-on-insulator, silicon-on-metal, silicon-on-interdiffusing-solids, silicon-on-superconductor, silicon-on-diamond, silicon-on-ferroelectric and silicon-on-polymer. The feasibility of all these examples has been proven; some will be described in some detail below, others Fig Directly bonded, annealed and thinned oxidized silicon wafer pair: SOl, insulator thickness 2 ia», silicon layer thickness 5.0 ± 0.2 i-'m. 8 Philip" Jouroal of Research Vol. 49 No. 1/2 1995

9 Direct bonding will be dealt with only briefly. These examples give a good impression of the great potentialof silicon direct bonding. Silicon is not the only material that is suitable for direct bonding; a whole range of combinations of the same or different materials have been investigated and discussed in the literature [12]. The physiognomic conditions (i.e. the geometrical, mechanical, physical, chemical and thermal conditions) of the materials to be directly bonded have to be compatible. Monocrystalline, polycrystalline, amorphous, composite, organic and inorganic materials can all be made suitable for direct bonding via different preparation, cleaning, direct-bonding and optionally annealing routes, depending on the material concerned. A good deal of research has been done into polishing strategies, in particular because the finish of a surface that is to be bonded directly to another surface is a good criterion for the definition of a superpolished state. That eventually led to an optimum polishing strategy, which proved to be applicable on a much wider scale than originally foreseen. The research into direct bonding has led to many spin-offs! Direct bonding can be used to enhance the materials involved with special intrinsic properties, for example special magnetic (conservation of flux lines), electric (combinations of superconductors and semiconductors) and electromagnetic properties (a lossless transition from the core glass of a fibre plate to a bulk glass). A separate paper will focus on some non-silicon applications enhanced with some of the above intrinsic properties, i.e. a chemical, a magnetic and two electromagnetic applications. Direct bonding has been a major research topic at Philips Research for more than three decades now. In the last decade in particular, the subject has received continuous attention. The outcomes of the efforts of craftsmen and scientists have been combined and optimized, the practical results of the former and the incentives of the latter both being indispensable for the success of research into this challenging topic. We are very pleased that we have found specialists of both groups willing to participate in the composition of this special issue, which aims to demonstrate the diversity and fundamental importance of direct bonding. The skills of a wide variety of technologists and scientists have contributed to the mastering of this technology and have revealed its wide potential. REFERENCES [1] A. Javan, W.R. Bennett Jr. and D.R. Herriott, Population inversion and continuous optical PbIIIps JoumaI of Research Vol. 49 No. 1/

10 U.K.P. Biermann et al. maser oscillation in a gas discharge containing a He-Ne mixture, Phys. Rev. Lett., 6, (1961). [2] J. Haisma, Construction and properties of short stable gas lasers, Philips Research Reports number 1,1967, pp [3] H.G. van Bueren, J. Haisma and H. de Lang, A small and stable continuous gas laser, Phys. Lett., 2, (1962). [4] M.J. Sparnaay, Four notes.on -Van.der Waals forces; induction effect, non-additivity, attraction between a cone 'anda flat plate (asperities), history, J. of Colloid and Interface Sci., 91, (1983). [5] J. Haisma and G. Bouwhuis, Mode competition and combination tones in a gas laser, Phys. Rev. Lett. 12, (1964). [6] US patent no 3,477,036 dated Nov.4, 1969; no 3,501,713 dated March 17,1970. [7] US patent no 4,810,318 dated March 7, [8] US patent no 4,547,801 dated Oct.15, [9] J. Haisma, SOl technologies: their past, present and future, J. de Physique, Colloque C4, supplément au no.9, 49, C4-3 to C4-12 (1988). [10] J. Haisma, G.A.C.M. Spierings, U.K.P. Biermann and J.A. Pals, Silicon-on-insulator wafer bonding - wafer thinning; technological evaluations, Jap. J. Appl. Phys., 28, (1989). [11] J. Haisma, T.M. Miehielsen and G.A.C.M. Spierings, High-quality SOl by bonding of standard Si wafers and thinning by polishing techniques only, Jap. J. Appl. Phys., 28, L725-L726 (1989). [12] J. Haisma, G.A.C.M. Spierings, U.K.P. Biermann and A.A. van Gorkum, Diversity and feasibility of direct bonding: a survey of a dedicated optical technology, Appl. Opt., 33, (1994). 10 Pbllips Journal of Research Vol. 49 No. 1/2 1995