Bond interface width

Reduction of distance between supported ends of needles (bond interface width) for two contacted needle-like surfaces in respect to a bonding load was quantitively obtained using a laser triangulation device. For this purpose, a Keyence LC2430 laser triangulation device consisting of a LC-2430 sensor head capable of measuring change of distance in a range of ± 500 µm was used. Four symmetrical points (P1 - P4) on surface of the upper chip holder (which sputtered with 200 nm gold to increase its surface reflectivity) were chosen as reference measurement points (see Figure 41).

After brining the upper chip into an initial contact with the lower chip with no load, the base plate of the bonding apparatus was placed under the laser triangulation device.

The distances between the laser source and reference points were measured while position of sensor head was fixed. After bonding the chips with a bonding load, distances between the laser source and reference points were measured again. The differences between these measurements were then averaged and taken as the distance between two bonded chips as a result of the applied bonding load.

103 Figure 41. Bond interface width measurement setup and position of reference points on the upper chip holder.

Elasticity of adhesive tapes (tesa® handicrafts tapes and Science Service GmbH carbon tapes) used to attach chips to chip holders due to bonding loads were also investigated. The total thickness reduction of four adhesive tapes due to an applied bonding load was obtained using the same approach as measuring the reduction of distance between two bonded chips. For this purpose, a flat metallic conductive disc (with diameter of 10 mm and thickness of 1 mm) was used instead of the bonded chips.

The bond interface width was obtained by subtracting the actual distance between two bonded chips (the measured distance between two bonded chips, minus the measured total thickness reduction of the adhesive tapes) from the distance between two needle-like surfaces at their initial contact when no load applied on them. The distance between two interacting needle-like surfaces at their initial contact was considered as twice the maximum height of their needles (2 × (mean value + standard

deviation value)). This corresponds to 145. 2 µm for the needle-like surface 1,

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52.4 µm for the needle-like surface 2, 54.0 µm for the needle-like surface 3, and 81.4 µm for the needle-like surface 4 (see Table 3). Bond interface width results are shown in Figure 42 and are in a good agreement with the ones measured and extracted optically through SEM images taken from cross-sections of bonded surfaces; they only showed a difference of about 5 - 10 %. A detailed information regarding extraction of bond interface widths and their comparisons with optically measured bond interface widths can be found in Appendix A.3.

Figure 42. The bond interface width of two bonded needle-like surfaces in respect to applied bonding loads for the first bond attempt for needle-like surfaces 1 - 4.

5.2 Summary

Capability of silicon needle-like surfaces generated by anodic etching of lowly doped p-type Si wafers in the transition region for the room temperature Si-Si bonding was experimentally demonstrated. Four different Si needle-like surfaces with different needles properties and densities were generated and used for this purpose. Needle densities and geometrical properties of needles were optically obtained through surface SEM images of generated needle-like surfaces. Surfaces were diced into small chips, and chips from similar surfaces were bonded together at normal clean-room condition by applying different bonding loads through a specially designed bonding apparatus.

105 The bond strength between two bond chips was measured by a special designed force measurement unit and determined by the pull-off force needed to separate the chips. Bonding strengths of 10 - 1032 kPa were obtained from these samples. Two linear relations between bond strength and applied bonding load were observed for all investigated needle-like surfaces. First, bond strength was linearly increased by increasing bonding load and reached to its maximum when bond interface width between two needle-like surfaces reached to average height of needles. After this point, increasing the bonding load was resulted in a linear decrease in the bond interface width and the bond strength.

Interlacing of needles was studied through SEM images taken from cross-sections of bonded surfaces. Interlacing length of needles was increased, and consequently the bond interface width between two needle-like surfaces was decreased upon increasing bonding loads. Maximum bond strengths were obtained at the point, where needles had completely interlaced (maximum interlacing length) for all investigated needle-like surfaces.

Stability and bending behaviors of needles during bonding were studied by comparing their physical conditions at bond interfaces with their physical conditions after separations. A couple hundred needles had broken from their supported ends or experienced some level of shortening during interlacing and detachment processes.

However, most of the needles resisted external forces and did not undergo any physical changes. The number of broken and shortened needles were slightly increased by increasing the bonding load.

Rebondability of surfaces was confirmed by several times reattaching single detached chips using the same bonding load. Up to 10 rebonds between similar chips from needle-like surfaces 1, 2, and 3 and up to 4 rebonds between chips from the needle-like surface 4 were obtained when they bonded with a bonding load larger than 0.33 kg. Bond strengths decreased after each rebond due to breaking and shortening of needles. However, for any applied bonding load larger than 0.3 kg, more than 40 % of bond strengths remained after the 3^rd rebonds for the needle-like surface 1 and 2.

For needle-like surfaces 3 and 4, these values reduced to 10 %.

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6 | Modeling and simulation of the room temperature Si-Si bonding using silicon needle-like surfaces

This chapter presents a bonding mechanism for the room temperature Si-Si direct bonding technique using silicon needle-like surfaces. The proposed bonding mechanism considers both deformation and interaction mechanisms of needles during the bonding. Deformation mechanisms of needles is mainly described by the cantilever beam approach, and interaction mechanisms of needles are considered due to intermolecular forces. Three models based on three different approaches (VdW forces, contact mechanics, and capillary forces) are presented and discussed to describe interaction mechanisms of needles.