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Void-formation in uncured and partially-cured BCB bonding adhesive on patterned surfaces
Microelectronic Engineering 137 (2015) 164–168
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Void-formation in uncured and partially-cured BCB bonding adhesive on patterned surfaces Zhen Song a, Dong Wu a, Huizhong Zhu b, Litian Liu a, Zheyao Wang a,c,⇑ a
Institute of Microelectronics, and Tsinghua National Laboratory for Information Science and Technology, Tsinghua University, Beijing 100084, China Shenzhen Optomechatronics Key Lab., Research Institute of Tsinghua University in Shenzhen, Shenzhen 518057, China c Innovation Center for MicroNanoelectronics and Integrated System, Beijing 1000871, China b
a r t i c l e
i n f o
Article history: Received 22 May 2014 Accepted 11 September 2014 Available online 22 September 2014 Keywords: Wafer bonding Three-dimensional integration Partially-cured Void
a b s t r a c t A void-free bonding interface is critical to yield and reliability for high-quality wafer bonding. Although adhesive bonding using polymers as the bonding interface material is inherently able to restrain voidformation, for the wafers with uneven patterns like metal interconnects and alignment marks, void-free bonding is still challenging. This paper reports the void-formation in uncured and partially-cured benzocyclobutene (BCB) adhesive for bonding wafers with patterns. Experimental results show that uncured and partially-cured BCB has different behaviors in void-formation, and four types of voids, namely center voids, flower voids, micro voids, and floccules voids, are founded and their formation mechanisms are investigated. By optimizing bonding parameters for uncured BCB bonding, the center voids and flower voids are avoided and void-free bonding can be obtained. For partially-cured BCB, the micro voids and floccules voids tend to appear for uneven wafer surfaces. A reflow pretreatment at 140 °C for 2 h before curing process is beneficial to reducing the void areas. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Adhesive wafer bonding, which uses a polymer layer as the bonding interface material (BIM) to join two wafers together, is a intermediate bonding technique that has been widely used in MEMS and three-dimensional (3-D) integration. Compared to other bonding technologies like metal eutectic bonding, metal thermocompression bonding, silicon fusion bonding, and anodic bonding, adhesive bonding has some significant advantages including lower cost, high yield, high bonding strength, wide applicability to various wafer materials, relatively low bonding temperature, without the need of electric voltage or current, and CMOS and MEMS compatibility [1]. More important, the inherently deformable capability of the polymer adhesives enables bonding to be insensitive to particles and topology of the wafers, allowing wide tolerance in process parameters and facilitating high-quality bonding. Various polymers have been used as the intermediate bonding materials for adhesive wafer bonding, such as polyimide [2], photoresist [3], epoxy [4], benzocyclobutene (BCB) [5], etc. Among these polymers, BCB adhesive wafer bonding has been extensively ⇑ Corresponding author at: Institute of Microelectronics, and Tsinghua National Laboratory for Information Science and Technology, Tsinghua University, Beijing 100084, China. Tel.: +86 10 62772748; fax: +86 10 62771130. E-mail address: [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.mee.2014.09.005 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.
used to fabricate 3-D integrated circuits [5–9] and MEMS devices [10–14] due to its excellent chemical resistance, high bond strength, low gas release, and outstanding dielectric properties. BCB is a thermosetting polymer, which flows for a short time when it is heated to undergo cross-linking or curing to form a three dimensional network. After cross-linking is thoroughly completed, it does not melt when being heated again. For high-quality wafer bonding, the voids, which are un-bonded areas in the bonding interface, are strictly prohibited because they may degrade the bonding strength, deteriorate the hermetic capability, and impact the yield and reliability. In some MEMS applications such as wafer transfer, yield may decrease sharply due to the occurrence of voids [15,16]. The voids may also induce localized high stress, leading to possible die cracking [17,18]. Therefore, void-free interface is a critical criterion to evaluate the bonding quality [19,20]. The void-formation mechanisms differ for different bonding technologies. For direct bonding, various reasons may cause voidformation, and so far it has been reported that surface inactivity [21], surface contaminants [22], bonding layer inter-diffusion [19], inappropriate bonding parameters [23], have significant influences on void-formation. To suppress void-formation, special treatments such as O2 plasma activation [21], wet cleaning [22], anti inter-diffusion [19], bonding process optimization [23] are needed. For adhesive wafer bonding, wafer cleaning is also a
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Z. Song et al. / Microelectronic Engineering 137 (2015) 164–168
prerequisite, but void-formation mechanisms are mostly associated with the polymer properties that depend on the curing temperature profiles [4]. The influences of bonding parameters on void-formation in BCB, PI 2610, and S1818 intermediate layers have been investigated thoroughly, and optimized bonding parameters have been suggested [24]. Trapped air is the most important reason to cause voids in adhesive bonding [15]. Various techniques have been developed to avoid trapped air bubbles. The most straightforward approach is bonding wafers at vacuum condition [15], and auxiliary structures such as ventilation channels also benefit void elimination [25]. Although bonding in vacuum is effective for flat wafers, for uneven wafers voids still exist, and it is reported that long time reflow in 140 °C vacuum is helpful to void suppression for AZ4620 photoresist [3]. For practical applications, bonding uneven wafers with aluminum (Al) interconnects using BCB is a common case. For uneven wafers, the inherent topography makes void-free bonding more challenging. In this paper, void-formation mechanisms are investigated for uneven wafers. As the curing profiles of BCB have significant influences on bonding quality, both uncured BCB and partially-cured BCB are investigated. Upon the different voids in uncured and partially cured BCB, approaches to eliminate voids in BCB adhesive wafer bonding are provided. 2. Experiment methods Fig. 1 shows the process flow for bonding silicon wafers with patterned structures. Al interconnects and alignment marks with thickness of 500 nm are fabricated on 100 mm-diameter silicon wafers using sputtering, photolithography, and reactive ion etching (RIE). The Al patterns are used to simulate the uneven surface topology in practical applications, and the thickness of Al patterns affect the void size but not the void-formation mechanism. Next, a commercial BCB precursor (Cyclotene 3022-46, Dow Chemical) is spin-coated on the wafer surface using 700 rpm for 9 s followed by 4000 rpm for 30 s. Before BCB coating, the wafers are pretreated with AP3000 adhesive promoter. With the increase in the BCB thickness, the protrusion caused by the underneath Al patterns is reduced. Then the wafer with BCB is soft-baked on a hotplate at 110 °C for 2 min to remove solvents and to stabilize the film to avoid BCB flow during subsequent handling. To get uncured and partially-cured BCB, different thermal treatment methods are used. For uncured BCB, no further thermal treatment is needed after soft-baking. As soft-baking does not increase the cross-linking of BCB, the cross-linking extent of BCB is about 35%. For partiallycured BCB, extra thermal treatment at high temperatures is performed. A typical process to obtain partially-cured BCB is to heat the wafer with BCB in a vacuum oven with temperature of 190 °C for 30 min. After the treatment, the cross-linking extent of BCB increases from 35% to 43% [26], and the BCB film is converted from liquid state to gel rubber state [27]. The partially-cured BCB
Al pattern
SiO2/SiN
Si wafer A (a) Al pattern
3. BCB bonding results The uncured BCB and partially-cured BCB have different crosslinking extents, and their properties are different. Uncured BCB has a lower cross-linking extent than partially-cured BCB, so it is softer and stickier and is more deformable and extendable than partially-cured BCB under the same bonding temperature and pressure. This feature makes the void-formation in uncured BCB and partially-cured BCB different. 3.1. Voids in uncured BCB For uncured BCB as the intermediate adhesive layer, two types of voids occur in the experiments, so called center void and flower void. A center void is a large un-bonded area in the center of the wafers with a dimension of several centimeters. Because of the residue stresses, the center void causes crack of thin films after the silicon wafer is thinned to below 50 lm, as shown in Fig. 2(a). The formation of center void is primarily related to the insufficient bonding pressure. As shown in Fig. 2(b), the wafer warpage
Si wafer B
Si wafer A
Wafer
(c) Bonding
BCB
Si wafer A (b) BCB adhesive film
does not reflow distinctly during the following bonding process but it is still adhesive. Then the patterned wafer with BCB is bonded with a Pyrex 7740 glass wafer or a silicon wafer with a 800 nm SiO2/SiN layer. Wafer bonding is implemented with a Suss SB6e wafer bonder. The bonding temperature is 250 °C and the bonding pressures vary. The bonding time with heat and pressure applied is 1 h, which converts the uncured and partially-cured BCB to fully-cured with a crosslinking degree greater than 95%. All the bonding experiments are implemented at a 1 10 4 mbar vacuum condition. For Pyrex 7740 glass wafer, the bonding interface can be inspected directly. For silicon wafer, mechanical grinding was used after wafer bonding to thin the top silicon wafer from the backside to around 50 lm, followed by wet etching to remove the remaining silicon completely. The wet etching was carried out in a 33 wt% KOH solution at 78 °C, and the SiO2/SiN layer is used as the selfstop layer. The etch rate of silicon is about 0.8 lm/min. After the top silicon wafer is removed, the bonding interface can be observed clearly through the transparent SiO2/SiN layer.
BCB
Si wafer A
Wafer
(d)Remove silicon substrate (no this step when glass wafer is used)
Fig. 1. Schematic process flow to investigate the bonding interface.
Fig. 2. Appearance and schematic explanation of the center void.
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(a)
Wafer
BCB Residual solvent
Wafer (b) Fig. 3. Flower voids. (a) Appearance, (b) void-formation mechanism.
may exist initially and can be aggravated during bonding due to the mismatch in the coefficients of thermal expansion of different materials on the wafers, such as metal, SiN/SiO2, and BCB. The bonding pressure should be large enough to overcome the initial wafer warpage and thermal induced warpage to achieve complete contact of the two wafers. If the two wafers have opposite curvatures, the perimeters of the wafers contact firstly and then center areas. In case that the bonding pressure is not large enough, a void may occur at wafer center. For the wafers with large initial opposite warpage, even the bonding pressure is large enough to completely bond the wafers, the residual stress arising from wafer deformation could be large enough to delaminate the bonding interface after wafer thinning and cause cracks. Apparently this void-formation is independent of BCB curing extent, and therefore it also occurs in partially-cured BCB. Even though the uncured BCB has more significant deformability, the center void cannot be compensated by the deformation of the uncured BCB if the BCB thickness is much lower than the initial warpage. As the initial warpage varies with wafer to wafer, the minimum bonding pressure changes accordingly. However, for practical applications a much larger bonding pressure is needed to ensure reliable contact and avoid pressure changes frequently. The experiments show that a bonding pressure larger than 1 bar can avoid the center void.
Another type of voids associated with uncured BCB is the flower void, as shown in Fig. 3(a). The flower voids, with a dimension of several millimeters, occur when the soft-bake of BCB is not sufficient. As soft-bake removes solvent in the spin-coated BCB, insufficient soft-bake leaves residual solvent that is not completely removed by soft-bake. The remaining solvent accumulates in the bonding interface under pressure and heating conditions, and the accumulation and the flow of the remaining solvent cause the flower void, as shown in Fig. 3(b). The flower voids can be eliminated by baking the wafers on a hotplate at 110 °C for 2 min before bonding. It should be noted that wafer bonding should be conducted immediately after soft-baking as moisture in atmosphere is adsorbed in the BCB layer. Although the BCB has a rather good moisture resistance, the adsorbed moisture diffuses and accumulates in the bonding interface, and may also lead to flower voids. The protrusion caused by Al patterns is reduced after BCB coating. For wide patterns the protrusion after BCB coating is reduced to 315 nm, and for thin patterns the protrusion after BCB coating is significantly reduced to 30 nm. As the uncured BCB is soft, it exhibits deformable and extendable capability when exerted with high temperature and large pressure, which are capable of compensating the surface topography induced by Al patterns. If the bonding pressure is large enough for wafer bonding, the protrusion does not cause voids. Experiments show that the 500 nm thick Al patterns has no influence on bonding voids. The highest patterns that the uncured BCB can compensate to avoid voids are determined by many factors, among which the BCB thickness and the bonding pressure are dominant. As the conformality and the total deformability of the uncured BCB increases with the BCB thickness, the final protrusion of the patterns decreases with the BCB thickness, and thus thick BCB is beneficial to compensating the pattern protrusions. Besides, as the BCB deformation also increases with the bonding pressure and the temperature, large pressure and high temperature are helpful to avoiding pattern induced voids. After the center voids and the flower voids are eliminated, voidfree wafer bonding using uncured BCB is achieved. Fig. 4 shows a void-free bonding pair after the top silicon wafer is removed to leave only SiO2/SiN layer on the BCB bonding layer. There are neither macroscopic voids nor microscopic voids around the metal structures. The optimized bonding parameters to get a void-free wafer boning using uncured BCB are listed in Table 1. It should be noted that small particles like dusts may exist in the bonding interface, but the particles are pressed into the BCB layer and no void occurs. 3.2. Voids in partially-cured BCB Though void-free wafer bonding can be achieved using uncured BCB with appropriate bonding conditions, bonding with
Fig. 4. Void-free wafer bonding using uncured BCB. (a) Wafer overview. The top silicon wafer was removed. (b) Optical photo around Al patterns. (c) Optical photo around particles.
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Z. Song et al. / Microelectronic Engineering 137 (2015) 164–168 Table 1 Optimized bonding parameters for uncured BCB. Step name
Optimized parameter
Spin-coating
The thickness of BCB greater than the height of the patterns 110 °C, 2 min <1 10 4 mbar 250 °C, 1 h >1 bar
Soft-baking Vacuum Heating Pressure
partially-cured BCB is preferred due to its superiority in alignment. The reflow of uncured BCB during bonding, though compensates the pattern induced topography to avoid voids, causes large misalignment in the range of several tens of microns. In comparison, misalignment less than 2 lm can be achieved using partially-cured BCB. Fig. 5 gives a comparison of the bonding results using uncured and partially-cured BCB. Partially-cured BCB has relatively large stiffness and low reflow capability, which benefit to high alignment accuracy but are inferior in avoiding void-formation. Fig. 6 shows the two types of voids occurred in partially-cured BCB, i.e., micro voids and floccules voids. These two voids are microscopic voids around the Al patterns or dust particles. The areas just surrounding the patterns/particles are well-bonded, and the bonded areas duplicate the patterns or particles. Specifically, the bonded areas have similar shapes to the surrounded Al patterns, and those around a particle is a nearly perfect circle. Next to the bonded areas are the micro voids, which look like flames surrounding the inner bonded areas. From the end of the micro voids, floccules voids appear which extend radially. A close observation shows that the in the floccules voids small irregular bonded areas and un-bonded areas interlace one another, resulting in floccular appearance. In some areas, there is no transitional micro voids between the floccules voids and the core bonded areas. The areas far away from the patterns or particles are wellbonded again. Comparison between Figs. 4(c) and 6(a) clearly shows that micro voids and floccules voids appear in partially-cured BCB but not uncured BCB. This implies that the impacts of the curing extent on void-formation is significant, even though the difference between the partially-cured BCB 43% and the uncured BCB 35% is
(a)
Fig. 6. Bonding voids in partially-cured BCB. (a) Voids caused by a particle. (b) Voids caused by Al patterns.
not remarkable. The formation mechanisms for the micro voids and the floccules voids in partially-cured BCB are investigated both qualitatively and numerically. Fig. 7 schematically illustrates the formation of the micro voids and the floccules voids on a protruding pattern. As the partiallycured BCB has very limited deformation and reflow during bonding, the small deflection curvature of the top wafer under the bonding pressure cannot accommodate to the BCB protrusion. Thus the BCB and the top wafer cannot contact in the areas next to the pattern, causing the micro voids. The areas far from the patterns are closely contacted, leading to good bonding. In the transitional areas linking the micro voids and the bonding areas, poor contact appears. In addition, the high bonding temperature induces in situ cross-linking during bonding, which causes shrinkage of BCB and irregular micro bonding areas and finally leads to floccules voids. FEM simulation is performed to verify the void-formation in partially-cured BCB, and the results are shown in Fig. 8. The bonding pressure is uniformly applied to the top wafer, and BCB is treated as elastic materials for simplification. It can be seen that a micro void exists next to the deformed protruding structure.
Micro void Floccules void
Well bonded
Pressure
Structure or Particle Partially-cured BCB
(b) Fig. 5. Comparison of uncured and partially-cured BCB bonding. (a) Void-free wafer bonding using uncured BCB but with a misalignment of 20 lm. (b) Well-aligned wafer bonding using partially-cured BCB but with voids.
Fig. 7. Schematic analysis of the voids in partially-cured BCB bonding.
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315 nm to 285 nm. By decreasing the height of the protrusions, most of the floccules voids vanish and high-quality bonding is achieved, as shown in Fig. 9. As 140 °C thermal treatment does not increase the cross-linking extent, the results indicate that long time reflow at an elevated temperature is helpful to reducing the voids to some extent. 4. Conclusion The features and the mechanisms of four types of voids occurring in uncured and partially-cured BCB wafer bonding have been investigated. To eliminate center voids, the 1 bar bonding pressure should be used to overcome the initial wafer warpage to achieve close contact of the two wafers. A soft-bake with specified temperature and duration can eliminate the flower voids. For partiallycured BCB, the appearances and the mechanisms of micro voids and floccules voids are investigated, and the results show that reflowing the BCB at 140 °C before curing benefits to void control. Acknowledgments
Fig. 8. FEM simulation of wafer deformation and small void.
This work was supported in part by 973 Program under Grant 2011CBA00603, NSFC under Grant 61271130 and Shenzhen Science and Technology Development Fund under Grant CXZZ20130322170740736. Reference
Fig. 9. Bonding of partially-cured BCB. (a) Bonding without thermal treatment. (b) Bonding with 2-h 140 °C treatment.
Upon this void-formation, increasing the bonding pressure may not be helpful to reducing the micro-voids and floccules voids due to the limited deformation and poor reflow feature of partiallycured BCB. Experiments show that a pressure as large as 7 bar has no apparent improvement on the bonding interface. It seems that the effective way to reduce the micro voids and the floccules voids is to improve the surface topology. For this purpose, chemical–mechanical polishing (CMP) of the BCB film before bonding is the most effective approach. The problem associated with CMP is that the polishing abrasives will be embedded in the BCB layer under the pressure in CMP. The residual abrasives may cause extra voids. Besides CMP, experiments show that 2 h thermal treatment at 140 °C before curing at 190 °C decreases a protrusion from
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Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Void-formation in uncured and partially-cured BCB bonding adhesive on patterned surfaces Zhen Song a, Dong Wu a, Huizhong Zhu b, Litian Liu a, Zheyao Wang a,c,⇑ a
Institute of Microelectronics, and Tsinghua National Laboratory for Information Science and Technology, Tsinghua University, Beijing 100084, China Shenzhen Optomechatronics Key Lab., Research Institute of Tsinghua University in Shenzhen, Shenzhen 518057, China c Innovation Center for MicroNanoelectronics and Integrated System, Beijing 1000871, China b
a r t i c l e
i n f o
Article history: Received 22 May 2014 Accepted 11 September 2014 Available online 22 September 2014 Keywords: Wafer bonding Three-dimensional integration Partially-cured Void
a b s t r a c t A void-free bonding interface is critical to yield and reliability for high-quality wafer bonding. Although adhesive bonding using polymers as the bonding interface material is inherently able to restrain voidformation, for the wafers with uneven patterns like metal interconnects and alignment marks, void-free bonding is still challenging. This paper reports the void-formation in uncured and partially-cured benzocyclobutene (BCB) adhesive for bonding wafers with patterns. Experimental results show that uncured and partially-cured BCB has different behaviors in void-formation, and four types of voids, namely center voids, flower voids, micro voids, and floccules voids, are founded and their formation mechanisms are investigated. By optimizing bonding parameters for uncured BCB bonding, the center voids and flower voids are avoided and void-free bonding can be obtained. For partially-cured BCB, the micro voids and floccules voids tend to appear for uneven wafer surfaces. A reflow pretreatment at 140 °C for 2 h before curing process is beneficial to reducing the void areas. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Adhesive wafer bonding, which uses a polymer layer as the bonding interface material (BIM) to join two wafers together, is a intermediate bonding technique that has been widely used in MEMS and three-dimensional (3-D) integration. Compared to other bonding technologies like metal eutectic bonding, metal thermocompression bonding, silicon fusion bonding, and anodic bonding, adhesive bonding has some significant advantages including lower cost, high yield, high bonding strength, wide applicability to various wafer materials, relatively low bonding temperature, without the need of electric voltage or current, and CMOS and MEMS compatibility [1]. More important, the inherently deformable capability of the polymer adhesives enables bonding to be insensitive to particles and topology of the wafers, allowing wide tolerance in process parameters and facilitating high-quality bonding. Various polymers have been used as the intermediate bonding materials for adhesive wafer bonding, such as polyimide [2], photoresist [3], epoxy [4], benzocyclobutene (BCB) [5], etc. Among these polymers, BCB adhesive wafer bonding has been extensively ⇑ Corresponding author at: Institute of Microelectronics, and Tsinghua National Laboratory for Information Science and Technology, Tsinghua University, Beijing 100084, China. Tel.: +86 10 62772748; fax: +86 10 62771130. E-mail address: [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.mee.2014.09.005 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.
used to fabricate 3-D integrated circuits [5–9] and MEMS devices [10–14] due to its excellent chemical resistance, high bond strength, low gas release, and outstanding dielectric properties. BCB is a thermosetting polymer, which flows for a short time when it is heated to undergo cross-linking or curing to form a three dimensional network. After cross-linking is thoroughly completed, it does not melt when being heated again. For high-quality wafer bonding, the voids, which are un-bonded areas in the bonding interface, are strictly prohibited because they may degrade the bonding strength, deteriorate the hermetic capability, and impact the yield and reliability. In some MEMS applications such as wafer transfer, yield may decrease sharply due to the occurrence of voids [15,16]. The voids may also induce localized high stress, leading to possible die cracking [17,18]. Therefore, void-free interface is a critical criterion to evaluate the bonding quality [19,20]. The void-formation mechanisms differ for different bonding technologies. For direct bonding, various reasons may cause voidformation, and so far it has been reported that surface inactivity [21], surface contaminants [22], bonding layer inter-diffusion [19], inappropriate bonding parameters [23], have significant influences on void-formation. To suppress void-formation, special treatments such as O2 plasma activation [21], wet cleaning [22], anti inter-diffusion [19], bonding process optimization [23] are needed. For adhesive wafer bonding, wafer cleaning is also a
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prerequisite, but void-formation mechanisms are mostly associated with the polymer properties that depend on the curing temperature profiles [4]. The influences of bonding parameters on void-formation in BCB, PI 2610, and S1818 intermediate layers have been investigated thoroughly, and optimized bonding parameters have been suggested [24]. Trapped air is the most important reason to cause voids in adhesive bonding [15]. Various techniques have been developed to avoid trapped air bubbles. The most straightforward approach is bonding wafers at vacuum condition [15], and auxiliary structures such as ventilation channels also benefit void elimination [25]. Although bonding in vacuum is effective for flat wafers, for uneven wafers voids still exist, and it is reported that long time reflow in 140 °C vacuum is helpful to void suppression for AZ4620 photoresist [3]. For practical applications, bonding uneven wafers with aluminum (Al) interconnects using BCB is a common case. For uneven wafers, the inherent topography makes void-free bonding more challenging. In this paper, void-formation mechanisms are investigated for uneven wafers. As the curing profiles of BCB have significant influences on bonding quality, both uncured BCB and partially-cured BCB are investigated. Upon the different voids in uncured and partially cured BCB, approaches to eliminate voids in BCB adhesive wafer bonding are provided. 2. Experiment methods Fig. 1 shows the process flow for bonding silicon wafers with patterned structures. Al interconnects and alignment marks with thickness of 500 nm are fabricated on 100 mm-diameter silicon wafers using sputtering, photolithography, and reactive ion etching (RIE). The Al patterns are used to simulate the uneven surface topology in practical applications, and the thickness of Al patterns affect the void size but not the void-formation mechanism. Next, a commercial BCB precursor (Cyclotene 3022-46, Dow Chemical) is spin-coated on the wafer surface using 700 rpm for 9 s followed by 4000 rpm for 30 s. Before BCB coating, the wafers are pretreated with AP3000 adhesive promoter. With the increase in the BCB thickness, the protrusion caused by the underneath Al patterns is reduced. Then the wafer with BCB is soft-baked on a hotplate at 110 °C for 2 min to remove solvents and to stabilize the film to avoid BCB flow during subsequent handling. To get uncured and partially-cured BCB, different thermal treatment methods are used. For uncured BCB, no further thermal treatment is needed after soft-baking. As soft-baking does not increase the cross-linking of BCB, the cross-linking extent of BCB is about 35%. For partiallycured BCB, extra thermal treatment at high temperatures is performed. A typical process to obtain partially-cured BCB is to heat the wafer with BCB in a vacuum oven with temperature of 190 °C for 30 min. After the treatment, the cross-linking extent of BCB increases from 35% to 43% [26], and the BCB film is converted from liquid state to gel rubber state [27]. The partially-cured BCB
Al pattern
SiO2/SiN
Si wafer A (a) Al pattern
3. BCB bonding results The uncured BCB and partially-cured BCB have different crosslinking extents, and their properties are different. Uncured BCB has a lower cross-linking extent than partially-cured BCB, so it is softer and stickier and is more deformable and extendable than partially-cured BCB under the same bonding temperature and pressure. This feature makes the void-formation in uncured BCB and partially-cured BCB different. 3.1. Voids in uncured BCB For uncured BCB as the intermediate adhesive layer, two types of voids occur in the experiments, so called center void and flower void. A center void is a large un-bonded area in the center of the wafers with a dimension of several centimeters. Because of the residue stresses, the center void causes crack of thin films after the silicon wafer is thinned to below 50 lm, as shown in Fig. 2(a). The formation of center void is primarily related to the insufficient bonding pressure. As shown in Fig. 2(b), the wafer warpage
Si wafer B
Si wafer A
Wafer
(c) Bonding
BCB
Si wafer A (b) BCB adhesive film
does not reflow distinctly during the following bonding process but it is still adhesive. Then the patterned wafer with BCB is bonded with a Pyrex 7740 glass wafer or a silicon wafer with a 800 nm SiO2/SiN layer. Wafer bonding is implemented with a Suss SB6e wafer bonder. The bonding temperature is 250 °C and the bonding pressures vary. The bonding time with heat and pressure applied is 1 h, which converts the uncured and partially-cured BCB to fully-cured with a crosslinking degree greater than 95%. All the bonding experiments are implemented at a 1 10 4 mbar vacuum condition. For Pyrex 7740 glass wafer, the bonding interface can be inspected directly. For silicon wafer, mechanical grinding was used after wafer bonding to thin the top silicon wafer from the backside to around 50 lm, followed by wet etching to remove the remaining silicon completely. The wet etching was carried out in a 33 wt% KOH solution at 78 °C, and the SiO2/SiN layer is used as the selfstop layer. The etch rate of silicon is about 0.8 lm/min. After the top silicon wafer is removed, the bonding interface can be observed clearly through the transparent SiO2/SiN layer.
BCB
Si wafer A
Wafer
(d)Remove silicon substrate (no this step when glass wafer is used)
Fig. 1. Schematic process flow to investigate the bonding interface.
Fig. 2. Appearance and schematic explanation of the center void.
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(a)
Wafer
BCB Residual solvent
Wafer (b) Fig. 3. Flower voids. (a) Appearance, (b) void-formation mechanism.
may exist initially and can be aggravated during bonding due to the mismatch in the coefficients of thermal expansion of different materials on the wafers, such as metal, SiN/SiO2, and BCB. The bonding pressure should be large enough to overcome the initial wafer warpage and thermal induced warpage to achieve complete contact of the two wafers. If the two wafers have opposite curvatures, the perimeters of the wafers contact firstly and then center areas. In case that the bonding pressure is not large enough, a void may occur at wafer center. For the wafers with large initial opposite warpage, even the bonding pressure is large enough to completely bond the wafers, the residual stress arising from wafer deformation could be large enough to delaminate the bonding interface after wafer thinning and cause cracks. Apparently this void-formation is independent of BCB curing extent, and therefore it also occurs in partially-cured BCB. Even though the uncured BCB has more significant deformability, the center void cannot be compensated by the deformation of the uncured BCB if the BCB thickness is much lower than the initial warpage. As the initial warpage varies with wafer to wafer, the minimum bonding pressure changes accordingly. However, for practical applications a much larger bonding pressure is needed to ensure reliable contact and avoid pressure changes frequently. The experiments show that a bonding pressure larger than 1 bar can avoid the center void.
Another type of voids associated with uncured BCB is the flower void, as shown in Fig. 3(a). The flower voids, with a dimension of several millimeters, occur when the soft-bake of BCB is not sufficient. As soft-bake removes solvent in the spin-coated BCB, insufficient soft-bake leaves residual solvent that is not completely removed by soft-bake. The remaining solvent accumulates in the bonding interface under pressure and heating conditions, and the accumulation and the flow of the remaining solvent cause the flower void, as shown in Fig. 3(b). The flower voids can be eliminated by baking the wafers on a hotplate at 110 °C for 2 min before bonding. It should be noted that wafer bonding should be conducted immediately after soft-baking as moisture in atmosphere is adsorbed in the BCB layer. Although the BCB has a rather good moisture resistance, the adsorbed moisture diffuses and accumulates in the bonding interface, and may also lead to flower voids. The protrusion caused by Al patterns is reduced after BCB coating. For wide patterns the protrusion after BCB coating is reduced to 315 nm, and for thin patterns the protrusion after BCB coating is significantly reduced to 30 nm. As the uncured BCB is soft, it exhibits deformable and extendable capability when exerted with high temperature and large pressure, which are capable of compensating the surface topography induced by Al patterns. If the bonding pressure is large enough for wafer bonding, the protrusion does not cause voids. Experiments show that the 500 nm thick Al patterns has no influence on bonding voids. The highest patterns that the uncured BCB can compensate to avoid voids are determined by many factors, among which the BCB thickness and the bonding pressure are dominant. As the conformality and the total deformability of the uncured BCB increases with the BCB thickness, the final protrusion of the patterns decreases with the BCB thickness, and thus thick BCB is beneficial to compensating the pattern protrusions. Besides, as the BCB deformation also increases with the bonding pressure and the temperature, large pressure and high temperature are helpful to avoiding pattern induced voids. After the center voids and the flower voids are eliminated, voidfree wafer bonding using uncured BCB is achieved. Fig. 4 shows a void-free bonding pair after the top silicon wafer is removed to leave only SiO2/SiN layer on the BCB bonding layer. There are neither macroscopic voids nor microscopic voids around the metal structures. The optimized bonding parameters to get a void-free wafer boning using uncured BCB are listed in Table 1. It should be noted that small particles like dusts may exist in the bonding interface, but the particles are pressed into the BCB layer and no void occurs. 3.2. Voids in partially-cured BCB Though void-free wafer bonding can be achieved using uncured BCB with appropriate bonding conditions, bonding with
Fig. 4. Void-free wafer bonding using uncured BCB. (a) Wafer overview. The top silicon wafer was removed. (b) Optical photo around Al patterns. (c) Optical photo around particles.
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Z. Song et al. / Microelectronic Engineering 137 (2015) 164–168 Table 1 Optimized bonding parameters for uncured BCB. Step name
Optimized parameter
Spin-coating
The thickness of BCB greater than the height of the patterns 110 °C, 2 min <1 10 4 mbar 250 °C, 1 h >1 bar
Soft-baking Vacuum Heating Pressure
partially-cured BCB is preferred due to its superiority in alignment. The reflow of uncured BCB during bonding, though compensates the pattern induced topography to avoid voids, causes large misalignment in the range of several tens of microns. In comparison, misalignment less than 2 lm can be achieved using partially-cured BCB. Fig. 5 gives a comparison of the bonding results using uncured and partially-cured BCB. Partially-cured BCB has relatively large stiffness and low reflow capability, which benefit to high alignment accuracy but are inferior in avoiding void-formation. Fig. 6 shows the two types of voids occurred in partially-cured BCB, i.e., micro voids and floccules voids. These two voids are microscopic voids around the Al patterns or dust particles. The areas just surrounding the patterns/particles are well-bonded, and the bonded areas duplicate the patterns or particles. Specifically, the bonded areas have similar shapes to the surrounded Al patterns, and those around a particle is a nearly perfect circle. Next to the bonded areas are the micro voids, which look like flames surrounding the inner bonded areas. From the end of the micro voids, floccules voids appear which extend radially. A close observation shows that the in the floccules voids small irregular bonded areas and un-bonded areas interlace one another, resulting in floccular appearance. In some areas, there is no transitional micro voids between the floccules voids and the core bonded areas. The areas far away from the patterns or particles are wellbonded again. Comparison between Figs. 4(c) and 6(a) clearly shows that micro voids and floccules voids appear in partially-cured BCB but not uncured BCB. This implies that the impacts of the curing extent on void-formation is significant, even though the difference between the partially-cured BCB 43% and the uncured BCB 35% is
(a)
Fig. 6. Bonding voids in partially-cured BCB. (a) Voids caused by a particle. (b) Voids caused by Al patterns.
not remarkable. The formation mechanisms for the micro voids and the floccules voids in partially-cured BCB are investigated both qualitatively and numerically. Fig. 7 schematically illustrates the formation of the micro voids and the floccules voids on a protruding pattern. As the partiallycured BCB has very limited deformation and reflow during bonding, the small deflection curvature of the top wafer under the bonding pressure cannot accommodate to the BCB protrusion. Thus the BCB and the top wafer cannot contact in the areas next to the pattern, causing the micro voids. The areas far from the patterns are closely contacted, leading to good bonding. In the transitional areas linking the micro voids and the bonding areas, poor contact appears. In addition, the high bonding temperature induces in situ cross-linking during bonding, which causes shrinkage of BCB and irregular micro bonding areas and finally leads to floccules voids. FEM simulation is performed to verify the void-formation in partially-cured BCB, and the results are shown in Fig. 8. The bonding pressure is uniformly applied to the top wafer, and BCB is treated as elastic materials for simplification. It can be seen that a micro void exists next to the deformed protruding structure.
Micro void Floccules void
Well bonded
Pressure
Structure or Particle Partially-cured BCB
(b) Fig. 5. Comparison of uncured and partially-cured BCB bonding. (a) Void-free wafer bonding using uncured BCB but with a misalignment of 20 lm. (b) Well-aligned wafer bonding using partially-cured BCB but with voids.
Fig. 7. Schematic analysis of the voids in partially-cured BCB bonding.
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315 nm to 285 nm. By decreasing the height of the protrusions, most of the floccules voids vanish and high-quality bonding is achieved, as shown in Fig. 9. As 140 °C thermal treatment does not increase the cross-linking extent, the results indicate that long time reflow at an elevated temperature is helpful to reducing the voids to some extent. 4. Conclusion The features and the mechanisms of four types of voids occurring in uncured and partially-cured BCB wafer bonding have been investigated. To eliminate center voids, the 1 bar bonding pressure should be used to overcome the initial wafer warpage to achieve close contact of the two wafers. A soft-bake with specified temperature and duration can eliminate the flower voids. For partiallycured BCB, the appearances and the mechanisms of micro voids and floccules voids are investigated, and the results show that reflowing the BCB at 140 °C before curing benefits to void control. Acknowledgments
Fig. 8. FEM simulation of wafer deformation and small void.
This work was supported in part by 973 Program under Grant 2011CBA00603, NSFC under Grant 61271130 and Shenzhen Science and Technology Development Fund under Grant CXZZ20130322170740736. Reference
Fig. 9. Bonding of partially-cured BCB. (a) Bonding without thermal treatment. (b) Bonding with 2-h 140 °C treatment.
Upon this void-formation, increasing the bonding pressure may not be helpful to reducing the micro-voids and floccules voids due to the limited deformation and poor reflow feature of partiallycured BCB. Experiments show that a pressure as large as 7 bar has no apparent improvement on the bonding interface. It seems that the effective way to reduce the micro voids and the floccules voids is to improve the surface topology. For this purpose, chemical–mechanical polishing (CMP) of the BCB film before bonding is the most effective approach. The problem associated with CMP is that the polishing abrasives will be embedded in the BCB layer under the pressure in CMP. The residual abrasives may cause extra voids. Besides CMP, experiments show that 2 h thermal treatment at 140 °C before curing at 190 °C decreases a protrusion from
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