3.1. Shear Fracture Strength
Figure 3 shows a bonded substrate pair, which has the Al bonding layer of Al with thin Sn layer insertion (
Figure 1c). The bonding was performed at 360 °C under 60.1 MPa pressure for 2 h. As found in
Figure 3a, 8 out of 16 dies were hermetically sealed, since the concave shape of the diaphragm shown in
Figure 3b is the result of hermetic sealing [
9]. Yield of hermetic seal depends on bonding temperature, pressure, and thickness of bonding metal layer, and in addition, depending on layer structure [
6,
7,
8]. The distribution of non-hermetic seal die in the 2 × 2 cm
2 Si substrate might be caused by particle contaminations or ununiform bonding pressure.
Figure 3c shows the die after saw dicing. The dies with and without the diaphragm were subjected to shear fracture strength measurement.
Figure 4 shows the shear fracture strength distribution of the dies with the bonding layer in
Figure 1c (Al/Sn/Al/Sn/Al//Al/Sn/Al/Sn/Al). The substrates were bonded at 360 °C or 380 °C, and the hermeticity of the cavity was checked by the diaphragm deformation. The vertical axis represents the number of dies, and the horizontal axis represents measured shear fracture strength. One group of the dies were not hermetically sealed (
Figure 4a), and the other group were hermetically sealed (
Figure 4b). Both groups show a similar distribution. The average and standard deviation are 51.4 MPa and 36.2 MPa for the non-hermetic dies, and 52.2 MPa and 37.8 MPa for the hermetic dies, respectively. The minimum shear fracture strength is 19.3 MPa (10.5 N in shear force), which was measured for a non-hermetic die, but that of the hermetic dies is 25.2 MPa.
Figure 5a,b show the distributions of shear fracture strength of the dies with the bonding layer in
Figure 1d (Al/Sn//Sn/Al) bonded at 370 to 390 °C. There was no diaphragm on the dies, and thus, the hermeticity of the cavity did not be confirmed. The average of all dies is 72.0 MPa, and the standard deviation is 31.3 MPa. The average for each bonding temperature is shown in
Figure 5b. No clear bonding temperature dependency of shear fracture strength is observed. The minimum shear fracture strength is 32.8 MPa, which was observed for the die bonded at 370 °C.
Figure 5c,d show the distributions of shear fracture strength of the die with the bonding layer in
Figure 1c (Al/Sn/Al/Sn/Al//Al/Sn/Al/Sn/Al) bonded at 360 to 390 °C. The average of all dies is 49.6 MPa, and the standard deviation is 32.5 MPa. The minimum shear fracture strength is 19.3 MPa. No clear bonding temperature dependency of shear fracture strength is observed also in
Figure 5d. If two bonding layer structures are compared, the Al/Sn//Sn/Al structure exhibits higher shear fracture strength in both average and minimum.
The reason why structure of “Al/Sn//Sn/Al” structure is stronger than that of “Al/Sn/Al/Sn/Al//Al/Sn/Al/Sn/Al” could be due to the difference of the amount of direct contact area of Al between both substrates. The former has wider non-oxidized Al surface in pre-compression. Whereas, the latter is oxidized in pre-compression, and then oxide is partly broken by compression, as observed by TEM (transmission electron microscope; JEOL Ltd; Akishima, Japan) [
7]. Therefore, Al direct contact area of bonding interfaces of the former could be wider than that of the latter, and then Al diffusion between bonding interfaces of the former could be more than that of the latter.
3.2. Observation Results of Shear-Fractured Surface
To clarify the relationship between die shear fracture strength and fracture mechanism, we observed the surface of shear-fractured dies by an optical microscope, SEM, EDX, and a surface profiler.
Figure 6 shows the observation results of the bonded ridge frame substrate and the opposed flat substrate with the Sn-capped Al bonding layer. The bonding was carried out at 390 °C under 65.3 MPa pressure. The measured die shear fracture strength is 57.1 MPa, which corresponds to a shear fracture force of 20.0 N. Yellow lines in
Figure 6a,b represent the observation areas by SEM, EDX, and the surface profiler. In the optical micrographs, the ridge frame and the bonding trace show white color. The color and the EDX analysis suggest that the fractured surfaces mainly consist of silicon oxide. We have already reported the results of SEM and EDX observation of bonding interface cross section after slightly separated by tension stress for bonded die with Al bonding layer capped with thin Sn at 390°C under 77.7 MPa pressure for 2h in
Figure 6 of reference [
6]. It was observed that the Al bonding interlayer became as thin as a thickness of 0.5 to 0.6 μm and its thin interlayer consisted with Al. According to these results, Al with Sn is supposed to exist on the surface as shown in
Figure 6f, although EDX signal from Al was not clearly observed.
Figure 6e is the surface topologies of a separated pair of the bonded substrate. The bumps at both sides of the bonded and fractured region is the metal which was squeezed out from the bonding layer [
6,
7].
Figure 7 shows the surface observation results of the shear-fractured dies, which showed a large fracture strength of 130 MPa (45.6 N in shear fracture force), more than twice larger than that of the above sample. The bonding layer is made of Al capped with thin Sn, and the bonding was carried out at 390 °C under 65.3 MPa pressure for 2 h. Yellow lines and red lines in
Figure 7a–c represent the observation areas by SEM and EDX, and the measured position by the surface profiler, respectively. In these figures, there are green arrows and “Fractured Si area”, which will be mentioned later.
Figure 7c is the magnified photographs at the center right area of
Figure 7a, and the center left area of
Figure 7b. The black color areas of the bonding region have concave profile, and mainly consist of Si and a small amount of Al and Sn according to EDX observation (
Figure 7d,e). We have already reported that the black color area of a bonding region after forcefully separated by tension stress consisted of fractured Si and that Si of the ridge was tore off to the opposed substrate [
6]. On the other hand, the cross-section profiles in
Figure 7f show that the bonding regions of both substrates have concave profiles. This is probably because Si around the bonding region of the flat substrate is simultaneously torn off, as schematically depicted in
Figure 7g.
Figure 8 shows the results of EDX, SEM, and surface profiles of the lower right part of
Figure 7a and the lower left part of
Figure 7b. The fractured region of the ridge frame substrate at R5 consists of Si, showing black color and concave profile. The EDX signals from O and Al are very small. On the other hand, the fractured region of the flat substrate at F6 shows light blue color and flat profile, accompanying with small bumps on both sides. Furthermore, its surface consists of Si and O judging from EDX analysis, which reveals that the fractured surface is SiO
2. Since the color of deposited SiO
2 of 100 nm thickness on the Si substrates was dark blue, light blue color can be explained as the color of SiO
2 covered with a thin layer like Al, which does not conflict with the previously reported results [
6].
Figure 8e shows a predicted fracture mode. Shear fracture which happened in the Al bonding metal and the fracture of the root of the ridge also happened due to the strong impact of shear force.
Figure 8f shows the surface profiles at R7 and F8. The color of both fractured regions is black, and the surface profile of the ridge frame substrate is concave, and that of the flat substrate is largely protruded. These results suggest that shear fracture in Si appeared at the root of ridge, and the fractured Si around the ridge moved to the flat substrate, which agrees with the previously reported results [
6,
7].
According to the results above mentioned, when the colors of the bonding area of the ridge frame substrate and the opposed flat substrate are black, shear fractures happened in both Si substrates. This mode of fracture is observed when the shear fracture strength is larger than about 42 MPa. In
Figure 7a,b, this mode is observed in the regions where green arrows indicate.
As mentioned above, the Si bulk fracture mode appears when the shear fracture strength is large. Here, the Si fracture length is defined as the length where this fracture mode is observed, i.e., the length of the green arrows in
Figure 7a or
Figure 7b.
Figure 9 shows the die shear fracture strength dependence of the ratio of the Si fracture length to the total length of the ridge frame for the die with bonding metal Al capped by thin Sn layer, which is called fractured Si frame ratio. The obtained results are categorized in three regions. In region (A), the fractured Si frame ratio has a positive linear correlation with the shear fracture strength with an
x-axis segment of 17.5 MPa. The shear fracture strength ranges from 41.6 to 209 MPa. In regions (B) and (C), the fractured Si frame ratio has no dependence on die shear fracture strength. All the dies in region (B) have white and/or light blue color frames, i.e., frames fractured in the bonding metal. The obtained shear fracture strength ranges from 32.8 to 97.4 MPa. In region (C), the dies have a few % of the fractured Si frame ratio, and the obtained shear strength ranges from 62.6 to 93.8 MPa.