Bond strength and rebondability

Smaller chips (0.25 cm² and 0.36 cm²) were brought into contacts with larger chips (0.64 cm² and 1 cm²) from similar needle-like surfaces and then pressed together at

normal clean-room condition by applying different bonding loads in a range of 0.33 - 12.33 kg using a specially designed bonding apparatus. Figure 27 illustrates 2D

schematic of the designed chip bonding apparatus and its components. The chip bonding apparatus and chip holders are designed in such a way that presses two chips together via a normal force introduced by a bonding load, make chips portable for further investigations (e.g., SEM and bond interface width measurement), and minimize the misalignment between chips during bonding, especially for rebondability tests. The bonding apparatus is consisted of two flat plats, which are called base and roof plates, and a chip bonder unit. The base plate is fixed and holds the chip bonder unit inside. Whereas, the roof plate is movable and can be only moved in z-direction.

87 Bonding loads are applied on the roof plate and press the upper chip towards the lower one. The chip bonder unit (see Figure 28) is consisting of a base plat which fixes and holds the lower chip, two movable plats which move only in x-direction, and a portable chip holder (upper chip holder).

Figure 27. 2D schematic of the designed chip bonding apparatus and its components.

Figure 28. Chip bonder unit and its components and arrangement of chips.

For the bonding experiment, the upper chip (0.25 cm² or 0.36 cm²) was chosen to be 2 - 3 times smaller than the lower chip (0.64 cm² or 1 cm²) in order to have a well-defined active bonding area, and to eliminate misalignment between upper and lower chips for the first bond attempt. However, for the second and further bond attempts, there could be a misalignment of 1 mm in the x-axis and 0.5 mm in the y-axis due to mismatching between alignment pairs on the upper chip holder and the movable

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plates. In this arrangement, chips were attached to base of the chip bonder (lower chip) and the upper chip holder (upper chip) through two 12 mm diameter conductive disks by four double-sided adhesive tapes. 64 mm² square tesa® handicrafts tapes were used for chips from the needle-like surface 1, whereas 63.3 mm² circular carbon tapes from the Science Service GmbH were employed for chips from needle-like surfaces 2 - 4.

The bond strength was measured and extracted through a specially designed force measurement unit consisting of a force sensor (Burster 8523 tension and compression load cell series). A Burster 8523-20 (up to 20 N) force sensor was used to measure low bond strengths (chips from needle-like surfaces 2 - 4), whereas for larger bond strengths (chips from the needle-like surface 1), a Burster 8523-100 (up to 100 N) force sensor was employed. 3D schematic of the designed bond strength measurement unit is shown in Figure 29. The bond strength measurement unit is consisted of an adjusting knob, a force sensor, a chip bonder holder, an applying force bar, and a micro-scale screw. After the bonding process, movable plates of the chip bonder unit are removed, and its base plate which holds bonded chips, is slid and fixed in the chip bonder holder of the bond strength measurement unit. The chip bonder holder can be moved up or down (in z- axis) by the micro-screw to adjust the upper chip holder inside the sensor hook. The sensor hook is then moved in y-axis through the adjusting knob to fine adjusting of the upper chip holder inside the sensor hook. At the end, a pull-off force value is obtained by pushing down the applying force bar and determined from the force needed to detach bonded chips. The actual pull-off force value was extracted from the force sensor data (see Figure 30) by subtracting the offset pull-off force value from the maximum pull-off force value (force at the separation point). The actual pull-off force value was then divided by the actual bonding area (either 0.24 cm² or 0.36 m², depending on the size of the upper chip) and the result was represented as the bond strength in kPa.

89 Figure 29. 3D schematic of the bond strength measurement unit and its components.

Figure 30. Example of a pull-off force as a function of time measured by the Burster 8523 series force sensor.

Temperature and relative humidity (RH) of the room where bonding experiments took place were also monitored prior to bonding for each specific pair of probes.

Temperature was almost stable (21 ± 2°C °C) during all bonding experiments, but relative humidity varied a lot (41 ± 10 %) since bonding experiments had carried out

Force sensor Chip bonder

holder Applying

force bar

Adjusting knob Micro-sacle

screw

Sensor hook

X Z Y

Time (second) Pull-off force (N)

Force due to weight of the upper chip holder and the upper chip Maximum force at the

separation point

Offset value when no force is applied

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at different seasons, dates, and times. Table 4 describes bond conditions for the first bond attempt for every used pair of probes for the needle-like surfaces 1 - 4.

Table 4. Bond conditions for the first bond attempt for every used pair of probes for the needle-like surfaces 1 - 4.

91 The average and spread of bond strengths for the first bond attempt for the needle-like surfaces 1 - 4 as a function of applied bonding load are shown in Figure 31. The bond strengths were then normalized in respect to the maximum bond strengths and are shown in Figure 32. Two linear relations between bond strength and applied bonding load are observed in all investigated needle-like surfaces: i) a linear increase of bond strength with bonding load, which is the direct result of involvement of more needles in the interaction mechanism between interlacing needles with increasing bonding load, and longer side interaction between the interlaced needles, and ii) a linear decrease of the bond strength, which is the result of compression of the needles, and so shorter side interaction between interlaced needles due to the high bonding loads. For all investigated needle-like surfaces, a maximum bond strength was obtained at a point where the side interaction between interlaced needles (interlacing length) was maximum (corresponding to mean values of height of needles) and most of needles touched the opposite substrate without undergoing a significant compression or shortening. These characteristics can be easily observed through SEM images of cross-sections of the bond interfaces due to applied bonding loads (see Figures 37 and 38).

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Figure 31. First attempt bond strength results as a function of applied bonding load for the needle-like surfaces 1 - 4: a) surface 1, and b) surfaces 2 - 4. Needle-like surfaces 1 and 4 with active bonding area of 0.25 cm² and needle-like surfaces 2 and 3 with active bonding area of 0.36 cm².

0 200 400 600 800 1000

0 2 4 6 8 10 12 14

Bond strength (kPa)

Applied bonding load (kg)

Surface 1

0 20 40 60 80 100 120 140 160

0 2 4 6 8 10 12 14

Bond strength (kPa)

Applied bonding load (kg)

Surface 2 Surface 3 Surface 4

a)

b)

93 Figure 32. Normalized bond strength (first attempt bond strength) as a function of applied bonding load for the needle-like surfaces 1 - 4. Needle-like surfaces 1 and 4 with active bonding area of 0.25 cm² and needle-like surfaces 2 and 3 with active bonding area of 0.36 cm².

Single detached chips were rebonded again several times using the same bonding loads to investigate rebondability of these needle-like surfaces. Figures 33 - 36 show rebondability and corresponding bond strength results obtained at constant applied bonding loads for the needle-like surfaces 1 - 4. Normalized bond strengths were obtained by normalizing bond strengths in respect to the maximum bond strength obtained from each applied 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 greater than 0.33 kg. Bond strengths were decreasing 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 were remained after the 3^rd rebonds for the needle-like surfaces 1 and 2. For needle-like surfaces 3 and 4, these values were reduced to 10 %. However, due to the possible misalignment (0.5 mm²) of placing the smaller chip at the top of the defined area on the larger chip (mismatching between alignment pairs on the upper chip holder and the movable plates), due to the previous bond attempts during rebond process, a slight reduction in a bond strength may need to be considered for every measured rebond bond strength; 1.3 % for needle-like surfaces

0,0

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1 and 4 (0.25 cm² chips) and 2 % for needle-like surfaces 2 and 3 (0.36 cm²).

Rebondability test results indicate that high needle density surfaces result in higher number of rebonds and bond strengths compared to surfaces with lower needle densities.

Figure 33. Rebondability and bond strength results obtained from bonding of chips from the needle-like surface 1 at constant applied bonding loads. Average of 3 bond attempts for every applied bonding load is shown. Inset: normalized bond strength in respect to maximum bond strength.

95 Figure 34. Rebondability and bond strength results obtained from bonding of chips from the needle-like surface 2 at constant applied bonding loads. Average of 2 attempts for every applied bonding load is shown. Inset: normalized bond strength in respect to maximum bond strength.

Figure 35. Rebondability and bond strength results obtained from bonding chips from the needle-like surface 3 at constant applied bonding loads. Average of 2 bond attempts for every applied bonding load is shown. Inset: normalized bond strength in respect to maximum bond strength.

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Figure 36. Rebondability and bond strength results obtained from bonding of chips from the needle-like surface 4 at constant applied bonding loads. Average of 3 bond attempts for every applied bonding load is shown.Inset: normalized bond strength in respect to maximum bond strength.