Saturday, December 7, 2013

IFTLE 172 Sony TSV Stacked CMOS Image Sensors Finally Arrive in 2013

Many of us can recall 2008 when Toshiba commercialized the first CMOS image sensor with TSV last /backside. We called these the first 3DIC products, when in reality they were only 1 layer devices.  More recently we have discussed Sony’s plans to release CMOS image sensors where the circuitry and sensors are fabricated on separate wafers and joined by TSV – i.e. true 3DIC structures. [ see IFTLE 112,”TSMC Staffing up for 2.5/3D Expansion ; Semi 3D Standards;Sony shows off 3D stacked Image Sensors” and IFTLE  137, “

Somehow IFTLE missed detailing the actual product technology release of the Sony ISX014 (link)  at the ISSCC earlier in the year. This is important enough in terms of 3DIC product introductions that we will cover it now.

The Sony ISX014 8MP sensor features 1.12um pixels and integrated high speed ISP.The pixel layer and logic layer part are manufactured as separate chips and stacked by using TSVs. Previously the pixel and logic circuit of Sony's back side  illuminated (BSI) CMOS image sensor were formed during the same fabrication process. This is compared below.

Actual layers are shown below:

Using separate pixel and logic layers allows the use an optimal process technology for each separate layer. Sony fabricated the pixel chip and logic chip using 90nm and 65nm process technologies, respectively. Stacking the chips, reduced chip area by 30%, compared with the previous image sensor made using 90nm process technology.
TSVs are used connect the row drivers on the pixel chip with the row decoders on the logic chip and connect the comparators on the pixel chip and the counters on the logic chip. TSVs are formed in areas to reduce the influence of noise.  For example, comparators are arranged on the pixel chip, which can be manufactured by using Sony's matured process technology, rather than on the logic chip.
The stacked vs conventional technologies are compared below:
Since the logic chip can be manufactured at Si foundries, Sony does not have to invest in advanced logic process technologies.
Sony is in volume production of the new CMOS image sensor for its smartphone, other companies' tablet computers, etc. The size, pixel count and pixel pitch of the sensor are 1/4 inch, 8.08 million and 1.12μm, respectively. Characteristics of the CMOS device are shown below:

Sony has not disclosed details on TSV processing. The total number of TSVs is a few thousand. The following figure shows the stacked chips cross-section.The insulators of the upper and lower chips are attached together. It seems that TSVs are formed later to connect the circuit layers of the chips. IFTLE assumes they are using the Ziptronics oxide bonding technology that they licensed previously [ see IFTLE 65, “Samsung’s 32GB RDIMM DDR3, GLOBALFOUNDRIES Packaging Alliance, ZiptronixLicensing News”.

A recent cross section produced by chipworks helps us understand the interlayer connections done with 6um pitch TSV [link].
IFTLE would expect other CIS manufacturers to move in this direction shortly.
For all the latest in 3DIC and advanced packaging stay linked to IFTLE……………………………….
Coming soon:
- An end of year update from Lester the Lightbulb         - more on IMAPS 2013
- more on the GIT Interposer Workshop                           - coverage of IWLPC                       
- more on IEEE 3DIC                                                              - coverage of RTI’s 3D ASIP                            
                               - Christmas coverage of Hannah and Madeline