Sony’s World-first two-layer image sensor: TechInsights preliminary analysis and results

Image Sensors World        Go to the original article...

By TechInsights Image Sensor Experts: Eric Rutulis, John Scott-Thomas, PhD

We first heard about it at IEDM 2021, and Sony provided more details at the 2022 IEEE Symposium on VLSI Technology and Circuits conference. Now it’s on the market and TechInsights has had our first look at the “world’s first” two-layer image sensor and we present our preliminary results here. The device was found in a Sony Xperia 1V smartphone main camera having a 48 MP, 1.12 µm pixel pitch and we can confirm it has dual photodiodes (a Left and Right photodiode in each pixel for full array PDAF). The die size measures 11.37 x 7.69 mm edge-to-edge.

In fact, the sensor actually has three layers of active silicon, with an Image Signal Processor (ISP) stacked in a conventional arrangement using a Direct Bond Interface (DBI) to the “second layer” (we will use Sony’s nomenclature when possible) of the CMOS Image Sensor (CIS). Figure 1 shows a SEM cross-section through the array. Light enters from the bottom of the image, through the microlenses and color filters. Each pixel is separated by an aperture grid (with compound layers) to increase the quantum efficiency. Front Deep Trench Isolation is used between each photodiode and it appears that Sony is using silicon dioxide in the deep trench to improve Full Well Capacity and Quantum Efficiency (this will be confirmed with further analysis).This layer also has the planar Transfer Gate used to transfer photocharge from the diode to the floating diffusion. Above the first layer is the “second layer” of silicon that contains three transistors for each pixel; the Reset, Amp (Source-Follower) and Select transistors. These transistors sit above the second layer silicon and connection to the first layer is achieved using “Deep contacts” which pass through the second layer essentially forming Through Silicon Vias (TSVs). Finally, the ISP sits above the metallization of the second layer, connected using Hybrid (Direct) Bonding. The copper of the ISP used for connection to the CIS DBI Cu is not visible in this image.

Figure 1: SEM Cross-section through the sensor indicating the three active silicon layers.

Key to this structure is a process flow that can withstand the thermal cycling needed to create the thermal oxide and activate the implants on the second layer. Sony has described the process flow in some detail (IEDM 2021, “3D Sequential Process Integration for CMOS Image Sensor”).

Figure 2 is an image from this paper showing the process flow. The first layer photodiodes and Transfer Gate are formed, and the second layer is wafer bonded and thinned. Only then are the second layer gate oxides formed and the implants are activated. Finally, the deep contacts are formed, etching through the second layer, and contacting the first layer devices.

Figure 2: Process flow for two-layer CIS described in “3D Sequential Process Integration for CMOS Image Sensor”, IEDM 2021.

The interface between the first and second layer is shown in more detail in Figure 3. The Transfer Gate (TG in the image) is connected to the first metal layer of the second layer. Slightly longer deep contacts lie below the sample surface and are partially visible in the image. These connect the floating diffusion node between the first and second layer. A sublocal connection (below the sample surface) is used to interconnect four photodiodes just above the first layer to the source of the Reset FET and gate of the AMP (Source-Follower) FET.

                    Figure 3: SEM cross-section detail of the first and second layer interface.

The sublocal connection is explored more in Figure 4. This is a planar SEM image of the first layer at the substrate level. Yellow boxes outline the pixel, with PDL and PDR indicating the left and right photodiodes. One microlense covers each pixel. Sublocal connections are indicated and are used to interconnect the Floating Diffusion for two pixels and ground for four pixels. The sublocal connection appears to be polysilicon; this is currently being confirmed with further analysis.

Figure 4: SEM planar view of the pixel first layer at the substrate level.

The motivation for the two-layer structure is multiple. The photodiode full well capacity can be maintained even with the reduced pixel pitch. The use of sublocal contacts reduces the capacitance of the floating diffusion, increasing the conversion gain of the pixels. The increased area available on the second layer allows the AMP (Source-Follower) transistor area to be increased, reducing noise (flicker and telegraph) create in the channel of this device.

It's worth taking a moment to appreciate Sony’s achievement here. The new process flow and deep contact technology allow two layers of active devices to be interconnected with an impressive 0.46 µm (center-to-center) spacing of the deep contacts (or Through Silicon Vias). Even the hybrid bonding to the ISP is just 1.12 µm; the smallest pitch TechInsights has seen to date. At the recent International Image Sensors Workshop, Sony described an upcoming generation that will use “buried” sublocal connections embedded in the first layer and pixel FinFets in the second layer (to be published). Perhaps we are seeing the first stages of truly three-dimensional circuitry, with active devices on multiple layers of silicon, all interconnected. Congratulations, Sony!

TechInsights' first Device Essentials analysis on this device will be published shortly with more analyses underway.

Access the TechInsights Platform for more content and reports on image sensors.

Go to the original article...

Leave a Reply