"Sony had used TSVs in their earlier CMOS image sensors and we had expected the same for the IMX260. But we don’t see them, as they have been replaced by a direct wafer bonding process that we will discuss later.
The Samsung CMOS image sensor has arrays of TSVs along its perimeter and these are used to make the electrical connections to the underlying ASIC.
Simply joining the two wafers together will allow Van der Waal forces (hydrogen bonding) to hold the wafers together, but this bond strength is insufficient for the task. High temperature anneals can convert these bonds to covalent bonding, but high temperature anneals can cause problems with thermal mismatches between the materials making up the two wafers. This is where Ziptronix’s 7,109,092 (‘092) patent comes into play. This patent describes a method for subjecting the wafer’s oxide surface with a fluorinating treatment to promote the covalent bonding of the two wafers in a room temperature process.
This wafer bonding process is likely done prior to the backside thinning of the image sensor wafer that might be about 150 µm thick, as is the control ASIC. The two wafers would brought into alignment using IR microscopes to see through the wafers to their respective alignment marks. This process has yielded a misalignment of the image sensor’s metal 6 pad and the control ASICs metal 8 pad of less than 0.25 µm."
Samsung uses arrays of through silicon vias to connect its S5k2L1SX image sensor to its underlying ASIC. Figure 7 shows a series of Through Silicon Vias (TSVs) arranged on an approximately 5 µm x 8 µm grid pattern. The TSVs are the dark oval shapes seen at the center of each grid point.
The TSVs connecting the image sensor die to the metal 7 traces of the control ASIC was formed after the two dies are affixed to each other, and after the image sensor die had undergone its backside thinning, using a TSV last process. The TSV metallization appears to be tungsten with probably a titanium nitride barrier and possibly titanium adhesion layers.
Using TSVs to electrically connect the image sensor die to the ASIC certainly gets the job done, but at the cost of the added real estate needed for the TSV arrays."
|Sony IMX260 Wafer Bonding Surfaces|
|Top Surface of Samsung S5K2L1SX with TSV Array|
|Samsung S5K2L1SX TSVs|