DIRECT BONDING OF ORGANIC POLYMERIC MATERIALS

Philips J. Res. 49 (1995) 139-149 DIRECT BONDING OF ORGANIC POLYMERIC MATERIALS by G.A.C.M. SPIERINGS, J. HAISMA and F.J.H.M. van der KRUIS Philips Research Laboratories, Prof Holstlaan 4, 5656 AA Eindhoven, The Netherlands Abstract Direct bonding of organic polymeric materials can be realized when their surfaces are prepared in such a way that they are clean, smooth and susceptible to direct-bonding. In the surface-preparation process, tribochemical polishing is an essential step. Polymeric materials such as polymethylmethacrylate (PMMA), polyarylate, polyimide and polycarbonate were bonded either to themselves, to another polymer or to an inorganic material such as silicon or fused silica. The surfacial bond energy of the room temperature bond is surprisingly high: 0.1-0.2 J/m 2 Heating strengthens the direct bond; for example, for a bonded PMMA/pMMA wafer pair annealed at the glass-transition temperature of PMMA (105 C), the surfacial bond strength increases to 7.8 J/m 2 This indicates that the bonded surfaces are fused and are interlinked by chemical bonds. When polymers are bonded to low-thermal-expansion materials such as Si and fused silica, during annealing treatments, thermal stresses can induce fracturing of the inorganic part of the bonded wafer pair. By limiting the maximum annealing temperature or the size of the bonded area, fracturing can be avoided. Keywords: direct bonding; organic materials; polymers; surface preparation; bond energy. 1. Introduction Direct bonding is the result of attractive interactions between clean, flat (or enantiomorphic) and smooth surfaces. In nature it is a phenomenon which occurs for many, if not all, classes of materials, including organic materials, as will be shown in this paper. So, if two surfaces are direct-bonded after they have snapped together, they are in contact on a molecular scale. In this case, chemical interactions will occur between molecular units present at the two surfaces. Surfaces of inorganic materials, such as oxides and silicon, are normally covered with OH groups after chemomechanical polishing. Such Philips Journal of Research Vol. 49 No. 1/2 1995 139

G.A.C.M. Spierings et al. OH groups form hydrogen-bond links bridging the two surfaces, which significantly strengthens the bond. Finally, an annealing treatment can be applied to strengthen the direct bond by several orders of magnitude, e.g. in the case of silicon by applying a temperature step above 800 C. For polymeric organic materials the magnitude of Van der Waals forces at room temperature is generally smaller than those which exist between inorganic materials such as metals and oxidic materials. Furthermore, the surfaces of these materials are generally hydrophobic when untreated. For silicon, hydrophobicity has been shown to be a drawback for obtaining tight direct bonding. Nevertheless, in preliminary investigations using PMMA it was observed that direct bonding could be realized, provided the surfaces are properly prepared [1]. This paper considers the direct-bonding characteristics of polymeric materials. Firstly, the surface treatment and the direct-bonding characteristics for bonding two identical polymers, two dissimilar polymers and a polymer to an inorganic material (SijSi0 2 ) are described. The effects of annealing treatments are then discussed. 2. Adhesion between polymers For materials such as rubbers and thermoplasts, e.g. polyethylene, which have a low glass-transition temperature (TrJ, tacky behaviour is a well-known phenomenon [2]. Tackiness can cause spontaneous sticking between surfaces in contact. This behaviour is caused by the relatively high mobility of the macromolecular chains, which leads to enhanced molecular interaction between two surfaces in close contact. Direct bonding can be defined as the formation of a 'perfect' bond over large areas at room temperature, which, after annealing, results in a strong 'chemical' bond. The surfaces must be well prepared, smooth and clean in order to be susceptible to bonding. Nevertheless, tacky behaviour and direct bonding are related as far as the interfacial physical phenomena are concerned. 3. Materials The class of organic polymers comprises materials differing widely in terms of composition and properties. In contrast to low-molecular organic substances, which are in general non-coherent and difficult to polish, polymers are widely used as engineering materials. They can be shaped into products using a wide variety of technologies. For studying the direct bonding of polymers we selected materials which can be prepared in the form of wafer-like 140 Phllips Journal of Research Vol.49 No. 1/2 1995

Direct bonding of organic polymeric materials substrates and which are of importance to the electronics industry. These materials are: polymethylmethacrylate (PMMA) (casocryl moulded acrylate, an Atohaas product); polycarbonate (Apec HT from Bayer, Germany: this is a trade name for an aromatic polyester which contains both polycarbonate and polyarylate units); polyarylate, (Isaryl l5x and Isaryl 25x from Isanova, Wiener Neudorf, Austria: this is an aromatic polyester which shows a high thermal stability) and; polyimide (Sintimid: a sintered polyimide material). The relevant properties (glass-transition temperature, Young's moduli and thermal expansion coefficients) of these materials are given in Table I. 4. Surface preparation The polymer materials were processed into 100 mm-diameter wafers or 40 x 40 mrrr' substrates. For the polyarylate and polyimide substrates the surface was ground using SiC paper prior to polishing. The quality of the PMMA and polycarbonate wafer surfaces were sufficiently homogeneous to be polished directly. PMMA and polycarbonate wafers were polished using TABLEI Young's moduli, glass-transition temperature and thermal expansion coefficient of relevant materials Material Young's modulus (GPa) Thermal expansion coefficient (K.-I x 10-7 ) PMMA Polycarbonate APEC HT (KUl) Polyarylate (Isaryl l5x/25x) Polyimide (Sintimid) Si Fused silica (Si0 2 ) 3.19 2.25 105 184 800 75.0 2.50/2.80 250/325 750 2.60 166 72.5 320 500 30 5 Philips Journal of Research Vol.49 No. 1/2 1995 141

G.A.C.M. Spierings et al. a colloidal silica suspension (25 wt% Si0 2 ) with a ph of 10. A single-sided polishing machine was applied, with a relatively hard polyurethane polishing pad IC40 (Rodel). The polishing pressure was in the 0.05 kgf/cm 2 range. Polymers are known to take up significant quantities of water, and this water take-up will also occur during polishing in aqueous suspensions. During polishing, however, the two surfaces of the substrates are not exposed symmetrically to the water supplied, resulting in an asymmetrical take-up of water, which leads to a geometrical deformation of the substrates. The extent of this problem was minimized by turning the substrate upside down at regular intervals during polishing. After polishing, some scratches were still observed. These are inherent to the relative softness of the polymer, which necessitates extra precautions, such as refreshing the polishing fluid and preventing harder particles from penetrating the polishing area. 4.1. Cleaning Cleaning after polishing is necessary to remove particulate remains present in the polishing liquid. In addition, cleaning is necessary due to organic deposits found after the wafers have been stored for some time in a polymer box. It determines to a large extent the chemical surface state of the wafers. After polishing, the polymer wafers are cleaned by scrubbing on a rotating wet felt-like pad and then carefully rinsed in deionized water. They are dried, for example using the Marangoni method [3].Wafer surfaces prepared in this way are hydrophobic. For studying the behaviour of polymers which are direct bonded to inorganic materials, loomm-diameter n-type silicon and fused silica wafers with a thickness of 525 f.lm are used. They are prepared as described previously [4]. 5. Direct-bonding phenomena The direct-bonding behaviour of two wafers was evaluated by bringing them into contact, as described above, and then investigating the formation of a direct-bonded area over the complete interface of the bonded wafer pair. The direct bonding of different sets of combinations was investigated: wafer pairs formed by two identical polymers, by two different polymers and by a polymer and an inorganic material. All three cases will be described below. The surfacial bond energy of bonded polymer/polymer wafer pairs was determined by the blade-insertion method, as described by Maszara et al. 142 Philip. Journnl of Research Vol.49 No. 1/2 1995

Direct bonding of organic polymeric materials [5].The occurrence of energy-related effects such as viscoelastic relaxation and energy dissipation due to the deformation of macromolecular chains are assumed not to influence this measurement. We used PMMA as a model material for studying the bond energy under different experimental conditions. The adhesive energy can then be increased by several magnitudes up to values corresponding to chemical bonds by a thermal annealing treatment. 5.1. Organic-to-organic: two identical polymers Complete wafer-scale direct bonding was obtained between two PMMA wafers with 100 mm diameter. Although the as-polished surface is hydrophobic, spontaneous direct-bond formation is observed. At room temperature, the bond energy for the PMMA/PMMA wafer pair was 0.16 J/m 2 This is a relatively high bond energy at room temperature for hydrophobic surfaces. It is a value which is comparable to values obtained for bonded hydrophilic silicon wafers (0.1-0.2 J/m 2 ) [6] and which is typical of a direct bond formed primarily by chemical interactions bridging the interface. This interaction causes a surfacial adhesive energy or strength which is higher than that originating from Van der Waals forces (~0.008 J/m 2 ). At the hydrophobic PMMA surface no OH groups are present. The chemical interactions are presumably the result of a mobility of the macromolecular chains of the polymer [7] caused by internal pressure at the bonded interface. This results in increased chemical interactions bridging the interface, as illustrated schematically in Fig. l(a and b). 5.2. Organic-to-organic: two dissimilar polymers It is also possible to bond two dissimilar polymer surfaces, provided directbond-susceptible surfaces (flat, smooth and clean) are brought into contact. Using the methods described above, we realized a direct bond between a PMMA and a polyarylate wafer, both 100 mm in diameter. This is shown in Fig.2. 5.3. Organic-to-inorganic: two different materials PMMA wafers are direct-bonded to a silicon and a fused-silica wafer. The bonded wafer pairs obtained in this way are shown in Fig. 3. For the PMMAI Si wafer pair the bond energy is 0.26 J/m 2, a value even higher than that found for the PMMAfPMMA wafer pair. This indicates that relatively strong Philips Journal of Research Vol. 49 No. 1/2 1995 143

G.A.C.M. Spierings et al. a. b. c. Fig. 1. A schematic drawing of the molecular-chain configuration at the polymer-polymer interface: a) prior to direct bonding; b) after direct bonding, showing some molecular rearrangements at the bonded interface; c) after an annealing step at a temperature higher than Tg, showing intertwining of the macromolecular chains, resulting in an inseparable interface. chemical interactions [8] are taking place at the PMMA-Si(02) interface. The Si is covered with a thin (0.5-1 run) native oxide layer. Direct bonding between polycarbonate, polyarylate and polyimide substrates and Si, as well as Ta, was also realized. In all these cases a spontaneous direct-bond behaviour was observed. 144 PhlUpsJournnl of Research Vol.49 No. 1/2 1995

Direct bonding of organic polymeric materials Fig. 2. Polyarylate direct-bonded to PMMA. 6. Annealing treatments 6.1. PMMA-to-PMMA wafer pair The surfacial bond energy can be increased by a thermal treatment. Figure 4 shows the surfacial bond energy at room temperature as a function of annealing temperature after annealing the PMMA/PMMA wafer pair for 2 h in air. A gradual increase in bond energy can be observed. After 2 h at 105 C (= Tg of PMMA) the surfacial bond strength had increased to 7.8 J/m 2 and after 2 h at 120 C an uncleavable bond (:;:'40 J/m 2 ) was obtained. A bond energy of 7.8 J/m 2 is substantially larger than typical bond energies found for hydrogen bond formation (0.1-0.2 J 1m 2 ), but also substantially smaller than the fracture energy of PMMA (200-400 J 1m2). It indicates that at T:;:' Tg relatively strong chemical bonds form over the original PMMAI PMMA interface. The increased mobility at higher temperatures results in increasing interdiffusion of macromolecular chains over the bonded interface. At Tg the number of bridging chains is small, resulting in a bond energy which is significantly smaller than the fracture energy of PMMA. At 120 C the concentration of bridging chains has increased to such a level that an inseparable bond is obtained by complete intertwining of the macromolecular chains. This situation is shown schematically in Fig. le. These processes occurring at such a polymer-polymer interface have been reviewed recently by Wool et al. [7]. Philips Journal of Research Vol.49 No. 1/2 1995 145

G.A.C.M. Spierings et al. Fig. 3. PMMA wafers 100 mm in diameter bonded to a) PMMA, b) Si and c) Si0 2. 146 Philip, Journalof Research Vol. 49 No. 1/2 1995

Direct bonding of organic polymeric materials I 10 N 5..._ E.., >- 0') 2... Q) c Q) "0 C 0 al 0.5 0.2 PMMA/PMMA 0.1 o PMMA/Si 20 40 60 80 100 Annealing temperature (OC).. Fig. 4. The bond energy as a function of annealing temperature for PMMA/pMMA and PMMA/ Si wafer pairs. In the latter case, fracturing of the Si wafer occurs above 60 C. Annealing time is 2 h in air. 6.2. PMMA-to-Si wafer pair When two material bodies with different dilatations are bonded, heating results in a build-up of stresses. Depending on the degree of mismatch between the thermal dilatations, the bonding forces and the elastic coefficients of the two materials, this can result in a fracture of one or both wafers of the bonded pair. These effects were studied in more detail for wafer-pair combinations of polymers bonded to Si. The dilatations of polymers and Si differ by more than a factor of 10 (see Table I). Fortunately, the effect of the high dilatation of polymers is compensated by their low elastic coefficients and viscoelastic stress relaxation. Therefore, a bonded PMMAjSi wafer can still be heated for 2 h at 60 C without fracturing. Heating to higher temperatures and cooling to room temperature resulted in fracturing ofthe Si wafer. Fracturing occurs particularly at the edge of the wafer along the (110) crystal orientation. Annealing polyarylate, polycarbonate and polyimide bonded to 100 mmdiameter silicon wafers already resulted in fracturing of the silicon at low temperatures. Using smaller silicon samples, e.g. 5 x 5mrrr', the maximum Philip. Journal of Research Vol. 49 No. 1/2 1995 147

G.A.C.M. Spierings et al. t Fig. 5. Detail of 5 x 5 mm? silicon chip bonded to a polycarbonate disk annealed for 2 h in air at 175 C. A shrinkage mark is present but the contact is over the complete interface. This effect is caused by a large difference in thermal-expansion coefficients and strong bonding forces. ;;~///,) Polycarbonate T = 175 0 T = 25 0 Shrinkage ring / Fig. 6. Thermal effects observed after annealing a 5 x 5 mrrr' silicon chip bonded to a polycarbonate disk and annealed for 2 h in air at 175 C.Situation after: a) direct bonding at room temperature; b) heating to 175 C (at this temperature the bond is strengthened); c) cooling to room temperature. Due to the large thermal-expansion-coefficient difference the bonded interface at the polymer side has to shrink. A plastic deformation process occurs, causing the loop shown in Fig. 4. In addition, a reflow process takes place around the edge of the chip in order to accommodate stresses. 148 Philip. Journalof Research Vol. 49 No. 1/2 1995

Direct bonding of organic polymeric materials annealing temperature was increased to several hundred degrees. It was thus found to be possible to anneal at 250 C 100 mm diameter polyimide or polyarylate wafers to which 5 x 5 mm 2 silicon chips were bonded, and cool them to room temperature without any fracturing problems. Nevertheless, it was observed that in the polymer wafer surrounding the bonded silicon chip, plastic flow had occurred in order to accommodate the stress from the difference in dilatation. Figure 5 shows a silicon chip bonded to a polycarbonate wafer and annealed at 175 C. The silicon chip is bonded over its complete surface area. A faint loop was observed in this figure. It is most probably caused by a shrinkage/rebonding effect originating from the large difference in thermal-expansion coefficients. In addition, plastic deformation has taken place in the polycarbonate around the bonded silicon. These effects are illustrated schematically in Fig. 6. 7. Conclusions This paper describes an exploratory investigation of the direct bonding of polymers to themselves and to inorganic materials such as silicon and fused silica. The effect of an annealing treatment on the quality and strength of the bond was investigated in some detail. The possibility of direct-bonding polymer materials opens up new routes for processing these important materials in today's optoelectronics and electronics industries. Acknowledgment critically. Their contri- Dick Broer and Ans Seppen read the manuscript bution is gratefully acknowledged. REFERENCES [1] G.A.C.M. Spierings and J. Haisma, Direct bonding of organic materials, Appl. Phys. Lett., 64,3246-3248 (1993). [2] R.P. Wool, Welding, tackand green strength of polymers, in L.H. Lee (ed.) Fundamentals of Adhesion, Plenum Press, New York, pp. 207-248, 1991. [3] A.F.M. Leenaars, J.A.M. Huethorst and J.J. van Oekel, Marangoni drying: a new extremely clean drying process, Langmuir, 6, 1701 (1990). [4] J. Haisma, G.A.C.M. Spierings, U.K.P. Biermann and J.A. Pals, Silicon-on-insulator, wafer bonding-wafer thinning, technological evaluations, Jpn. J. Appl. Phys., 28,1426-1443 (1989). [5] W.P. Maszara, G. Goetz, A. Caviglia and J.B. McKitterick, Bonding of silicon wafers for silicon-on-insulator, J. Appl. Phys., 64, 4943-4950 (1988). [6] R. StengI, T. Tan and U. Gösele, A model for the silicon wafer bondingprocess, Jpn. J. Appl. Phys.,28, 1735-1741 (1989). [7] R.P. Wool, B.L. Yuan and O.J. McGarel, Welding of polymer interfaces, Polymer Eng. Se., 29, 1340 (1989). [8] J.F. Watts, M.M. Chehimi and E.M. Gibson, Acid-base interactions in adhesion: the characterization of surfaces and interfaces by XPS, J. Adhesion, 39, 145-156 (1992). Philips Journal of Research Vol. 49 No. 1/2 1995 149