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Influence of microwave annealing on direct bonded silicon wafers
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Thin Solid Films 516 (2008) 2158 – 2161 www.elsevier.com/locate/tsf
Influence of microwave annealing on direct bonded silicon wafers T.L. Alford ⁎, T. Tang, D.C. Thompson, S. Bhagat, J.W. Mayer School of Materials, Arizona State University, Tempe, Arizona 85287-8706, USA Received 26 April 2006; received in revised form 19 April 2007; accepted 11 June 2007 Available online 20 June 2007
Abstract Strong and nearly void free bonding was achieved using direct bonding followed by microwave annealing. Silicon wafers were cleaned, O2 plasma surface activated, and bonded at room temperature. After microwave annealing at 400 °C, the bond strength of hydrophilic wafers was found to be in the range between 0.2 and 1.6 J/m2. Additional heating of bonded wafers was done at elevated temperatures and for prolonged times using either rapid thermal annealing or microwave annealing. In either case, additional annealing showed no impact on wafer separation area, void, or bond strength. Thus, the initial microwave anneal dictated the ultimate bond strength regardless of subsequent annealing method. The mechanism for wafers bonded in this work involved dipole–dipole bonding and, hydrogen bonding. The initial microwave anneals typically required times less than 60 min. As a result, microwave annealing was shown to be a promising low temperature alternative for wafer bonding when compared to the currently used mechanical furnace anneals. © 2007 Elsevier B.V. All rights reserved. Keywords: Silicon; Direct bonding; Microwave annealing; Rapid thermal annealing; Silicon on insulator
1. Introduction The bonding of silicon wafers enables the fabrication of high-performance silicon-on-insulator (SOI) wafers by subsequent back etching (BESOI) and ion-implant induced cleave of silicon [1]. One of the more appealing approaches is to utilize van der Waals forces to bond wafers (sans adhesives). Among the three types of van der Waals intermolecular attraction forces, the dipole–dipole force between two polar molecules is the strongest. Through exploitation of van der Waals forces, wafer bonding is accomplished bringing mirror polished silicon wafers in direct contact with one another in a clean-room environment [2]. This process is valid for silicon wafer surfaces that are either hydrophobic or hydrophilic [2,3]. Immersing a silicon wafer in a solution of hydrofluoric acid (10% HF) removes the native oxide that terminates the wafer surface and replaces it with a hydrogen terminated surface. Hydrogen terminated surfaces are hydrophobic [2]. By replacing the hydrofluoric dip with an RCA1 clean solution, composed of equal amounts of ammonium hydroxide and hydrogen peroxide (NH4OH:H2O2:H2O = 1:1:5), the wafer ⁎ Corresponding author. Tel.: +1 480 965 7471. E-mail address: [email protected] (T.L. Alford). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.118
surface is terminated by silanol bonds (Si–OH). Silanol bonds leave silicon-wafer surfaces hydrophilic [2]. For applications that require bonded silicon wafers to be covalently bonded, oxide free, electrically and thermally conductive, the wafers are submerged in an HF solution, rinsed in de-ionized water, and immersed in an RCA1 solution. The resulting surface is hydrophilic and contains a small ‘native’ oxide. The wafers are then bonded at room-temperatures by placing them in direct contact, followed by a thermal anneal to enhance the bonding by volatilizing any moisture, hydrogen, or oxide present in the interface between wafers [4]. For the interface between two silicon wafers to result in covalently bonded silicon, anneal temperatures can be as high as 1200 °C for hydrophilic bonding and 600 °C for hydrophobic bonding [5]. Although covalently bonded silicon is possible through wafer bonding and high temperature anneals, high temperatures are not desirable during microelectronics manufacturing [2]. As an example, hydrophilic bonded wafers used in SOI applications can not exceed annealing temperatures of 400 °C [6]. As an alternative to high temperature anneals, low temperature wafer bonding have achieved mechanically stable bonds at temperatures as low as 200 °C. However, the anneal process may take up to several days and results in a wafer bond strength that may not be acceptable for further processing [7].
T.L. Alford et al. / Thin Solid Films 516 (2008) 2158–2161
2159
Because annealing at high temperatures is not conducive to certain microelectronics manufacturing, and low temperature wafer bonding does not always produce the desired bond strengths, a means of thermal annealing at lower temperatures with strong bond strengths is desired. In the following work, a method of direct bonding and microwave annealing is evaluated to determine the effectiveness in low temperature wafer bonding. Experiments are presented which were designed to optimize bond strength at lower processing temperatures. Ultimately this low temperature Si bonding will enable future bonding of Si to polymers and SiO2 coated polymer for flexible electronics. 2. Experimental details In this study, Si–Si bonded wafer pairs were fabricated using (001) p-type, 381±15 μm thick, 1–10 Ω cm, 100 mm silicon wafers. Wafers were cleaned using a solution of sulfuric acid and hydrogen peroxide (H2SO4:H2O2:H20) at 85 °C for 10 min, followed by a 10 min, 10%HF dip. In order to create a clean, uniform hydrophilic surface on each wafer, the wafers were then submerged in RCA1 (NH4OH:H2O2:H2O=1:1:5) and RCA2 (HCl:H2O2:H2O=1:1:5) solutions for 10 min in each. At the end of each cleaning step, the wafers were rinsed with de-ionized water and spin dried. After wafer cleaning and surface preparation, each wafer was placed in oxygen plasma asher for plasma surface activation. Although the underlying mechanism is still debated, previous studies done by Suni et al. indicated that exposing silicon wafers to an oxygen plasma resulted in increased bond strengths [6]. Accordingly for this work wafers were exposed to oxygen plasma with a power of 100 W, pressure of 300 mTorr, oxygen flow rate of 0.8–1.6 ml/s and, RF frequency of 15 MHz for approximately 4 min. By placing the wafers in direct contact, the wafers bonded at room temperature. Without any support weights pressing on the wafers, the bonded wafer set was annealed in a 1000 W microwave annealer. Bonded wafers reached a temperature of 400 °C in approximately 4 s and did not exceed 400 °C regardless of process time. Initially, one of four bonded-wafer sets was microwave annealed for either 15, 30, 45, or 60 min. Additional four sets of bonded wafers were initially microwave annealed for 15 min at 400 °C. However one wafer from this second set also received an additional microwave anneal of either 15, 30, or 45 min at 400 °C in order to determine the effect of piecewise microwave annealing. Finally for comparison, three more sets of samples were cleaned, bonded at room temperature, and microwave annealed for 15 min at 400 °C. In this third case, one bonded-wafer set received an additional rapid thermal anneal (RTA) at either 600, 700, or 900 °C using 60 s increments till a cumulative time of 4 min was obtained. The ‘crack opening’ method is used to examine the bond strength of bonded and annealed wafers [8]. This method is based on the theoretical analysis of crack propagation in an adhered, linearly elastic, double cantilever arrangement. By insertion of a thin blade between the bonded wafers and subsequent measurement of the separation length, a relationship of the surface energy γ, or bond strength, is calculated as shown in [9]: 3Etw3 tb2 g¼ 32L4
ð1Þ
Fig. 1. Infrared image of wafer separation from a pair of bonded wafers using the blade insertion method (or crack-opening method). The darker arc area is induced by the metallic blade wedge between two wafers.
where E is the elastic modulus of silicon, L is the crack length, tw is the wafer thickness, and tb is blade thickness. Measurement of the crack length is done under an infrared (IR) transmission imaging system. Fig. 1 is a photo demonstrating the ‘crack opening’ method used with IR imaging. After insertion of the blade between the bonded wafers, separation progresses in an arc shape manner. The distance between the blade's edge and the separation crack front is designated as the crack length L. Fig. 1 also demonstrates the presence of voids in wafer bonding. Voids are created when contaminants interrupt intimate contact between wafer surfaces during room temperature bonding [2]. The void radius (R) of separation generated by particle contamination has been calculated for (100) silicon as: R ¼ ½0:73Etw3 =g
ð2Þ
where the other variables are the same as in Eq. (1). 3. Results and discussion During direct wafer bonding at room temperature, the hydrophilic surfaces are covered with a layer of native oxide that is approximately 1–2 nm thick. For hydrophilic wafers, the initial bonding begins with hydrogen-bridge bonds between H2O molecules. At room temperature, water can combine with oxygen terminated silicon resulting in surface hydrophilization, or creation of silanol groups on the wafer surfaces [2,7,10]: Si–O–Si þ H2 O→Si–OH þ OH–Si
ð3Þ
When the annealing temperature reaches 100 °C, water molecules at the bonded-wafer interface diffuse through with the native oxide and reacts with Si to form silicon dioxide and hydrogen: Si þ 2H2 O→SiO2 þ 2H2
ð4Þ
For longer anneal times (at lower temperatures) or when using higher temperatures, the hydrogen byproduct out diffuses from the silicon interface completely.
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T.L. Alford et al. / Thin Solid Films 516 (2008) 2158–2161
Fig. 2. Infrared images of four sets of bonded wafer. All sets have been microwave annealed at 400 °C for: a) 15, b) 30, c) 45, and d) 60 min, respectively. The darker regions in the wafer are unbonded areas, or voids. Longer initial anneal times reduce the size of the voids.
To examine the quality of the wafer bonded interface, an infrared imaging system is used. Infrared images of wafers bonded at room temperature and annealed in a microwave at 400 °C for increasing times are shown in Fig. 2. As anneal times increase, the amount of silanol and silicon dioxide created also increases. Increased silanol and silicon dioxide molecules production leads to increased van der Waals interactions, which increases the bonding strength γ as prescribed in Eq. (1). This results in a smaller crack length L. Since these wafers were not bonded in a vacuum environment, contaminants in the air would result in void formation during the bonding process [1]. Fig. 3 demonstrates both the magnitude in bond strength and the trend in increasing bond strength as a function of microwave
Fig. 3. Bond strength plotted as a function of microwave annealing time at 400 °C.
anneal time. After using the ‘crack opening’ method and Eq. (1), the bond strength of direct bonded wafers in this work was 0.2 J/m2 and 1.6 J/m2 after 15 and 60 min of microwave annealing at 400 °C, respectively. To characterize the effect of post annealing to the previously microwave annealed (15 min at 400 °C) samples, RTA anneals and microwave anneals were done. Results (not shown) displayed no changes in crack length, void size, or wafer bond strength. Therefore, the wafer-bond strength in these wafer was controlled by the initial microwave annealing process only. Any further attempts of annealing did not enhance the bond strength. This behavior was explained by the concept of hydrogenbridge bonding. Eq. (3) demonstrates that the presence of residual water during room temperature bonding results in the production of silanol bonds upon heating the hydrophilic bonded wafers to temperatures near 100 °C. Previous researchers have asserted that plasma activation enhances the creation of silanol groups, resulting in an ‘activated’ surface [6,7]. In the presence of hydrogen-bridging, these silanol groups serve to increase van der Waals interactions which result in the attraction of each wafer surface to the other. When a particle is present at the interface between bonded wafers, the particle inhibits hydrogen bridge formation near the radius center according to Eq. (1). After initial heating of temperatures above 100 °C, water is no longer present in the void. According to Eq. (4), hydrogen gas is present and serves to terminate dangling silicon bonds on an interior surface of a void and interrupt hydrogen bonding of silanol groups to each other. Hydrogen gas also has a partial pressure which rises with increasing temperature. Until temperatures above 450 °C, hydrogen does not volatize from the Si interface [2,7]. The lack of water, separation distance, and the presence of hydrogen gas results in two internal surfaces with reduced driving force for bonding. As a result the bond strength and void area are unaffected after initial 60 min anneal for the microwave furnace used for this work. This finding does not differ appreciably from other low temperature bonding methods [2,7,8]. Fig. 4 shows the time saturated values of bond energy measured by the ‘crack opening’ method as a function of
Fig. 4. Saturation values of surface or bonding energy measured by the blade insertion (crack opening) method as a function of temperature after heat treatments of up to several days [7,10,11].
T.L. Alford et al. / Thin Solid Films 516 (2008) 2158–2161
temperature [7,10,11]. As can be seen in Fig. 4, bond strength increases exponentially in hydrophobic bonded wafers from 20–700 °C and reaches a maximum of approximately 3.0 J/m2 at 700 °C for mechanical annealing. This annealing behavior makes the mechanical strength of hydrophobic bonded wafers sensitive to process conditions. In a simmer manner, the bond strength in hydrophilic wafers is a constant 1.3 J/m2 between 250 and 800 °C. A bond strength of 1.3 J/m2 has been shown to be strong enough to enable further processing [2]. This constant bond strength behavior in hydrophilic bonded wafers allows for a process window in which hydrophilic bonded wafers can be further processed with a known bond strength and without exceeding the previously noted 400 °C temperature limit for SOI processing. Microwave processing of hydrophilic bonded wafers in this work did not exceed 400 °C, and as Fig. 2 demonstrates, microwave processing does not seem to differ from other methods in the bonding mechanism. Therefore, we assert that microwave processing would be a viable alternative to other more conventional low temperature bonding methods being used today. Fig. 3 demonstrates that the maximum bond strength of direct bonded silicon wafers using microwave annealing is 1.6 J/m2 after 60 min at 400 °C. Inspection of the data compiled from conventional mechanical furnace anneals reveals that the maximum bond strength reaches a plateau only after several hours of annealing. When comparing the microwave anneal behavior with that of the mechanical furnace, it is clearly noted that the microwave anneal takes only fraction of time that is needed in a mechanical furnace to achieve the same bond strength. With an increase in microwave power input, the time needed to achieve desired bond strength can be shortened as well. 4. Conclusion In this work silicon wafers were cleaned with RCA1 and RCA2 solutions and exposed to an O2 plasma and bonded at room temperatures. Silicon direct wafer bonding was determined to be initiated by hydrogen-bridges on plasma activated wafer surfaces. After initial bonding samples were microwave
2161
annealed at 400 °C to determine the benefits of microwave annealing on low temperature wafer bonding. The bond strength of initially microwave annealed hydrophilic wafers was found to be in the range between 0.2 and 1.6 J/m2 depending on the annealing time. Microwave annealing took only 60 min to maximize the bond strength at 400 °C. After which subsequent microwave anneals and RTA processing had no effect on wafer bond strength or void separation area. As a result, the initial 60 min microwave wafer-bonding anneal was crucial to the bond strength in bonded wafer pairs and the 1 hour duration was much less than times needed to obtain similar bond strengths in conventional mechanical furnaces. Acknowledgment This work was partially supported by a grant from the NSF (DMR-0602716, L. Hess). The authors acknowledge critical reviews by S. Chenna. References [1] J. Yu, Y. Wang, J.-Q. Lu, R.J. Gutman, Appl. Phys. Lett. 89 (2006) 092104. [2] Q.-Y. Tong, U. Gösele, Semiconductor Wafer Bonding: Science and Technology, John Wiley and Sons, New York, NY, 1999. [3] Y. Bäcklund, K. Ljungberg, A. Söderbärg, J. Micromechanics Microengineering 2 (1992) 158. [4] Q.-Y. Tong, Q. Gan, G. Hudson, G. Fountain, P. Enquist, R. Scholz, U. Gösele, Appl. Phys. Lett. 83 (2003) 4767. [5] K.D. Hobart, M.E. Twigg, F.J. Kub, C.A. Desmond, Appl. Phys. Lett. 72 (1998) 1095. [6] T. Suni, K. Henttinen, I. Suni, J. Makinen, J. Electrochem. Soc. 149 (2002) G348. [7] U. Gösele, Q.-Y. Tong, Annu. Rev. Mater. Sci. 28 (1998) 215. [8] W.P. Maszara, G. Goetz, A. Caviglia, J.B. McKitterick, J. Appl. Phys. 64 (1988) 4943. [9] U. Gösele, Q.-Y. Tong, A. Schumacher, G. Kräuter, M. Reiche, A. Plößl, P. Kopperschmidt, T.-H. Lee, W.-J. Kim, Sens. Actuators, A 74 (1999) 161. [10] X.X. Zhang, J.P. Raskin, J. Microelectromech. Syst. 14 (2005) 368. [11] Q.-Y. Tong, G. Cha, R. Gafitaenu, U. Gösele, J. Microelectromech. Syst. 3 (1994) 35.
Thin Solid Films 516 (2008) 2158 – 2161 www.elsevier.com/locate/tsf
Influence of microwave annealing on direct bonded silicon wafers T.L. Alford ⁎, T. Tang, D.C. Thompson, S. Bhagat, J.W. Mayer School of Materials, Arizona State University, Tempe, Arizona 85287-8706, USA Received 26 April 2006; received in revised form 19 April 2007; accepted 11 June 2007 Available online 20 June 2007
Abstract Strong and nearly void free bonding was achieved using direct bonding followed by microwave annealing. Silicon wafers were cleaned, O2 plasma surface activated, and bonded at room temperature. After microwave annealing at 400 °C, the bond strength of hydrophilic wafers was found to be in the range between 0.2 and 1.6 J/m2. Additional heating of bonded wafers was done at elevated temperatures and for prolonged times using either rapid thermal annealing or microwave annealing. In either case, additional annealing showed no impact on wafer separation area, void, or bond strength. Thus, the initial microwave anneal dictated the ultimate bond strength regardless of subsequent annealing method. The mechanism for wafers bonded in this work involved dipole–dipole bonding and, hydrogen bonding. The initial microwave anneals typically required times less than 60 min. As a result, microwave annealing was shown to be a promising low temperature alternative for wafer bonding when compared to the currently used mechanical furnace anneals. © 2007 Elsevier B.V. All rights reserved. Keywords: Silicon; Direct bonding; Microwave annealing; Rapid thermal annealing; Silicon on insulator
1. Introduction The bonding of silicon wafers enables the fabrication of high-performance silicon-on-insulator (SOI) wafers by subsequent back etching (BESOI) and ion-implant induced cleave of silicon [1]. One of the more appealing approaches is to utilize van der Waals forces to bond wafers (sans adhesives). Among the three types of van der Waals intermolecular attraction forces, the dipole–dipole force between two polar molecules is the strongest. Through exploitation of van der Waals forces, wafer bonding is accomplished bringing mirror polished silicon wafers in direct contact with one another in a clean-room environment [2]. This process is valid for silicon wafer surfaces that are either hydrophobic or hydrophilic [2,3]. Immersing a silicon wafer in a solution of hydrofluoric acid (10% HF) removes the native oxide that terminates the wafer surface and replaces it with a hydrogen terminated surface. Hydrogen terminated surfaces are hydrophobic [2]. By replacing the hydrofluoric dip with an RCA1 clean solution, composed of equal amounts of ammonium hydroxide and hydrogen peroxide (NH4OH:H2O2:H2O = 1:1:5), the wafer ⁎ Corresponding author. Tel.: +1 480 965 7471. E-mail address: [email protected] (T.L. Alford). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.118
surface is terminated by silanol bonds (Si–OH). Silanol bonds leave silicon-wafer surfaces hydrophilic [2]. For applications that require bonded silicon wafers to be covalently bonded, oxide free, electrically and thermally conductive, the wafers are submerged in an HF solution, rinsed in de-ionized water, and immersed in an RCA1 solution. The resulting surface is hydrophilic and contains a small ‘native’ oxide. The wafers are then bonded at room-temperatures by placing them in direct contact, followed by a thermal anneal to enhance the bonding by volatilizing any moisture, hydrogen, or oxide present in the interface between wafers [4]. For the interface between two silicon wafers to result in covalently bonded silicon, anneal temperatures can be as high as 1200 °C for hydrophilic bonding and 600 °C for hydrophobic bonding [5]. Although covalently bonded silicon is possible through wafer bonding and high temperature anneals, high temperatures are not desirable during microelectronics manufacturing [2]. As an example, hydrophilic bonded wafers used in SOI applications can not exceed annealing temperatures of 400 °C [6]. As an alternative to high temperature anneals, low temperature wafer bonding have achieved mechanically stable bonds at temperatures as low as 200 °C. However, the anneal process may take up to several days and results in a wafer bond strength that may not be acceptable for further processing [7].
T.L. Alford et al. / Thin Solid Films 516 (2008) 2158–2161
2159
Because annealing at high temperatures is not conducive to certain microelectronics manufacturing, and low temperature wafer bonding does not always produce the desired bond strengths, a means of thermal annealing at lower temperatures with strong bond strengths is desired. In the following work, a method of direct bonding and microwave annealing is evaluated to determine the effectiveness in low temperature wafer bonding. Experiments are presented which were designed to optimize bond strength at lower processing temperatures. Ultimately this low temperature Si bonding will enable future bonding of Si to polymers and SiO2 coated polymer for flexible electronics. 2. Experimental details In this study, Si–Si bonded wafer pairs were fabricated using (001) p-type, 381±15 μm thick, 1–10 Ω cm, 100 mm silicon wafers. Wafers were cleaned using a solution of sulfuric acid and hydrogen peroxide (H2SO4:H2O2:H20) at 85 °C for 10 min, followed by a 10 min, 10%HF dip. In order to create a clean, uniform hydrophilic surface on each wafer, the wafers were then submerged in RCA1 (NH4OH:H2O2:H2O=1:1:5) and RCA2 (HCl:H2O2:H2O=1:1:5) solutions for 10 min in each. At the end of each cleaning step, the wafers were rinsed with de-ionized water and spin dried. After wafer cleaning and surface preparation, each wafer was placed in oxygen plasma asher for plasma surface activation. Although the underlying mechanism is still debated, previous studies done by Suni et al. indicated that exposing silicon wafers to an oxygen plasma resulted in increased bond strengths [6]. Accordingly for this work wafers were exposed to oxygen plasma with a power of 100 W, pressure of 300 mTorr, oxygen flow rate of 0.8–1.6 ml/s and, RF frequency of 15 MHz for approximately 4 min. By placing the wafers in direct contact, the wafers bonded at room temperature. Without any support weights pressing on the wafers, the bonded wafer set was annealed in a 1000 W microwave annealer. Bonded wafers reached a temperature of 400 °C in approximately 4 s and did not exceed 400 °C regardless of process time. Initially, one of four bonded-wafer sets was microwave annealed for either 15, 30, 45, or 60 min. Additional four sets of bonded wafers were initially microwave annealed for 15 min at 400 °C. However one wafer from this second set also received an additional microwave anneal of either 15, 30, or 45 min at 400 °C in order to determine the effect of piecewise microwave annealing. Finally for comparison, three more sets of samples were cleaned, bonded at room temperature, and microwave annealed for 15 min at 400 °C. In this third case, one bonded-wafer set received an additional rapid thermal anneal (RTA) at either 600, 700, or 900 °C using 60 s increments till a cumulative time of 4 min was obtained. The ‘crack opening’ method is used to examine the bond strength of bonded and annealed wafers [8]. This method is based on the theoretical analysis of crack propagation in an adhered, linearly elastic, double cantilever arrangement. By insertion of a thin blade between the bonded wafers and subsequent measurement of the separation length, a relationship of the surface energy γ, or bond strength, is calculated as shown in [9]: 3Etw3 tb2 g¼ 32L4
ð1Þ
Fig. 1. Infrared image of wafer separation from a pair of bonded wafers using the blade insertion method (or crack-opening method). The darker arc area is induced by the metallic blade wedge between two wafers.
where E is the elastic modulus of silicon, L is the crack length, tw is the wafer thickness, and tb is blade thickness. Measurement of the crack length is done under an infrared (IR) transmission imaging system. Fig. 1 is a photo demonstrating the ‘crack opening’ method used with IR imaging. After insertion of the blade between the bonded wafers, separation progresses in an arc shape manner. The distance between the blade's edge and the separation crack front is designated as the crack length L. Fig. 1 also demonstrates the presence of voids in wafer bonding. Voids are created when contaminants interrupt intimate contact between wafer surfaces during room temperature bonding [2]. The void radius (R) of separation generated by particle contamination has been calculated for (100) silicon as: R ¼ ½0:73Etw3 =g
ð2Þ
where the other variables are the same as in Eq. (1). 3. Results and discussion During direct wafer bonding at room temperature, the hydrophilic surfaces are covered with a layer of native oxide that is approximately 1–2 nm thick. For hydrophilic wafers, the initial bonding begins with hydrogen-bridge bonds between H2O molecules. At room temperature, water can combine with oxygen terminated silicon resulting in surface hydrophilization, or creation of silanol groups on the wafer surfaces [2,7,10]: Si–O–Si þ H2 O→Si–OH þ OH–Si
ð3Þ
When the annealing temperature reaches 100 °C, water molecules at the bonded-wafer interface diffuse through with the native oxide and reacts with Si to form silicon dioxide and hydrogen: Si þ 2H2 O→SiO2 þ 2H2
ð4Þ
For longer anneal times (at lower temperatures) or when using higher temperatures, the hydrogen byproduct out diffuses from the silicon interface completely.
2160
T.L. Alford et al. / Thin Solid Films 516 (2008) 2158–2161
Fig. 2. Infrared images of four sets of bonded wafer. All sets have been microwave annealed at 400 °C for: a) 15, b) 30, c) 45, and d) 60 min, respectively. The darker regions in the wafer are unbonded areas, or voids. Longer initial anneal times reduce the size of the voids.
To examine the quality of the wafer bonded interface, an infrared imaging system is used. Infrared images of wafers bonded at room temperature and annealed in a microwave at 400 °C for increasing times are shown in Fig. 2. As anneal times increase, the amount of silanol and silicon dioxide created also increases. Increased silanol and silicon dioxide molecules production leads to increased van der Waals interactions, which increases the bonding strength γ as prescribed in Eq. (1). This results in a smaller crack length L. Since these wafers were not bonded in a vacuum environment, contaminants in the air would result in void formation during the bonding process [1]. Fig. 3 demonstrates both the magnitude in bond strength and the trend in increasing bond strength as a function of microwave
Fig. 3. Bond strength plotted as a function of microwave annealing time at 400 °C.
anneal time. After using the ‘crack opening’ method and Eq. (1), the bond strength of direct bonded wafers in this work was 0.2 J/m2 and 1.6 J/m2 after 15 and 60 min of microwave annealing at 400 °C, respectively. To characterize the effect of post annealing to the previously microwave annealed (15 min at 400 °C) samples, RTA anneals and microwave anneals were done. Results (not shown) displayed no changes in crack length, void size, or wafer bond strength. Therefore, the wafer-bond strength in these wafer was controlled by the initial microwave annealing process only. Any further attempts of annealing did not enhance the bond strength. This behavior was explained by the concept of hydrogenbridge bonding. Eq. (3) demonstrates that the presence of residual water during room temperature bonding results in the production of silanol bonds upon heating the hydrophilic bonded wafers to temperatures near 100 °C. Previous researchers have asserted that plasma activation enhances the creation of silanol groups, resulting in an ‘activated’ surface [6,7]. In the presence of hydrogen-bridging, these silanol groups serve to increase van der Waals interactions which result in the attraction of each wafer surface to the other. When a particle is present at the interface between bonded wafers, the particle inhibits hydrogen bridge formation near the radius center according to Eq. (1). After initial heating of temperatures above 100 °C, water is no longer present in the void. According to Eq. (4), hydrogen gas is present and serves to terminate dangling silicon bonds on an interior surface of a void and interrupt hydrogen bonding of silanol groups to each other. Hydrogen gas also has a partial pressure which rises with increasing temperature. Until temperatures above 450 °C, hydrogen does not volatize from the Si interface [2,7]. The lack of water, separation distance, and the presence of hydrogen gas results in two internal surfaces with reduced driving force for bonding. As a result the bond strength and void area are unaffected after initial 60 min anneal for the microwave furnace used for this work. This finding does not differ appreciably from other low temperature bonding methods [2,7,8]. Fig. 4 shows the time saturated values of bond energy measured by the ‘crack opening’ method as a function of
Fig. 4. Saturation values of surface or bonding energy measured by the blade insertion (crack opening) method as a function of temperature after heat treatments of up to several days [7,10,11].
T.L. Alford et al. / Thin Solid Films 516 (2008) 2158–2161
temperature [7,10,11]. As can be seen in Fig. 4, bond strength increases exponentially in hydrophobic bonded wafers from 20–700 °C and reaches a maximum of approximately 3.0 J/m2 at 700 °C for mechanical annealing. This annealing behavior makes the mechanical strength of hydrophobic bonded wafers sensitive to process conditions. In a simmer manner, the bond strength in hydrophilic wafers is a constant 1.3 J/m2 between 250 and 800 °C. A bond strength of 1.3 J/m2 has been shown to be strong enough to enable further processing [2]. This constant bond strength behavior in hydrophilic bonded wafers allows for a process window in which hydrophilic bonded wafers can be further processed with a known bond strength and without exceeding the previously noted 400 °C temperature limit for SOI processing. Microwave processing of hydrophilic bonded wafers in this work did not exceed 400 °C, and as Fig. 2 demonstrates, microwave processing does not seem to differ from other methods in the bonding mechanism. Therefore, we assert that microwave processing would be a viable alternative to other more conventional low temperature bonding methods being used today. Fig. 3 demonstrates that the maximum bond strength of direct bonded silicon wafers using microwave annealing is 1.6 J/m2 after 60 min at 400 °C. Inspection of the data compiled from conventional mechanical furnace anneals reveals that the maximum bond strength reaches a plateau only after several hours of annealing. When comparing the microwave anneal behavior with that of the mechanical furnace, it is clearly noted that the microwave anneal takes only fraction of time that is needed in a mechanical furnace to achieve the same bond strength. With an increase in microwave power input, the time needed to achieve desired bond strength can be shortened as well. 4. Conclusion In this work silicon wafers were cleaned with RCA1 and RCA2 solutions and exposed to an O2 plasma and bonded at room temperatures. Silicon direct wafer bonding was determined to be initiated by hydrogen-bridges on plasma activated wafer surfaces. After initial bonding samples were microwave
2161
annealed at 400 °C to determine the benefits of microwave annealing on low temperature wafer bonding. The bond strength of initially microwave annealed hydrophilic wafers was found to be in the range between 0.2 and 1.6 J/m2 depending on the annealing time. Microwave annealing took only 60 min to maximize the bond strength at 400 °C. After which subsequent microwave anneals and RTA processing had no effect on wafer bond strength or void separation area. As a result, the initial 60 min microwave wafer-bonding anneal was crucial to the bond strength in bonded wafer pairs and the 1 hour duration was much less than times needed to obtain similar bond strengths in conventional mechanical furnaces. Acknowledgment This work was partially supported by a grant from the NSF (DMR-0602716, L. Hess). The authors acknowledge critical reviews by S. Chenna. References [1] J. Yu, Y. Wang, J.-Q. Lu, R.J. Gutman, Appl. Phys. Lett. 89 (2006) 092104. [2] Q.-Y. Tong, U. Gösele, Semiconductor Wafer Bonding: Science and Technology, John Wiley and Sons, New York, NY, 1999. [3] Y. Bäcklund, K. Ljungberg, A. Söderbärg, J. Micromechanics Microengineering 2 (1992) 158. [4] Q.-Y. Tong, Q. Gan, G. Hudson, G. Fountain, P. Enquist, R. Scholz, U. Gösele, Appl. Phys. Lett. 83 (2003) 4767. [5] K.D. Hobart, M.E. Twigg, F.J. Kub, C.A. Desmond, Appl. Phys. Lett. 72 (1998) 1095. [6] T. Suni, K. Henttinen, I. Suni, J. Makinen, J. Electrochem. Soc. 149 (2002) G348. [7] U. Gösele, Q.-Y. Tong, Annu. Rev. Mater. Sci. 28 (1998) 215. [8] W.P. Maszara, G. Goetz, A. Caviglia, J.B. McKitterick, J. Appl. Phys. 64 (1988) 4943. [9] U. Gösele, Q.-Y. Tong, A. Schumacher, G. Kräuter, M. Reiche, A. Plößl, P. Kopperschmidt, T.-H. Lee, W.-J. Kim, Sens. Actuators, A 74 (1999) 161. [10] X.X. Zhang, J.P. Raskin, J. Microelectromech. Syst. 14 (2005) 368. [11] Q.-Y. Tong, G. Cha, R. Gafitaenu, U. Gösele, J. Microelectromech. Syst. 3 (1994) 35.