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Maxwell-Hiqe

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"Imaging, Stratafied"sm
Recently, image sensors have been provided with additional functionality by adoption of stacked architectures in which two or more wafers are vertically attached.  The processes used are similar to those adopted in the memory business where now in excess of 100 wafers can be stacked to provide high memory capacity in small packages. In these assemblies, all of the bonding is done at the edge of the stack of die cut from the wafers.  These connections are often termed "through-silicon vias" (TSVs).

At the same time, some of the processes long-used in the manufacture of cooled infrared detectors in which a layer of an infrared-sensing material, like HgCdTe, is connected to a silicon readout integrated circuit (ROIC) using bump bonds made of gold or other materials have been extended for use in sensors using a silicon detection layer. 

In 2009, Lumiense built and demonstrated an 8.3 megapixel pixel device with 2.8 µm pixels using a custom bonding process in which an array of photodiodes on a top wafer was each bonded to separate readout circuit on a second wafer below.  This is an image taken by that device to demonstrate the proof of principle:

The image demonstrates some performance shortcomings but it shows only a few missing bonds out of 16.6 million.

In the succeeding years, improvements have been made in both the process and in the qualification of fabrication facilities so that late in 2019, the first commercial device using the Lumiense architecture will begin shipping.  This device will also incorporate additional strata to improve optical performance. Watch for details on the Imagica page in this website.

 

Why "Strata"?

Unlike existing "stacked" image sensors with mostly edge connections, sensors built using the Lumiense architecture have connection for each pixel that runs vertically through the set of bonded wafers.  These vertical connections can extend through five or more wafers to permit color imaging in which the color pixels are vertically aligned so that no reconstruction is needed and to support readout circuitry that is isolated from the incoming light.  This is more than just stacking.

Advantages of strata

Lumiense combines multiple wafer assembly with tailored optical strata to provide:

  • 90%+ fill factor 
  • True antireflectance coatings
  • 120db+ global shuttering
  • Submicron pixel connections
  • High charge well capacity
  • In-pixel CDS and HDR
  • Interwafer optical reflectors