Three Dimensional (3D) time-of-flight (TOF) cameras are capable of getting range information for all pixels of the TOF detector in real-time. TOF cameras typically include a light source for illuminating the scene with modulated light, usually light emitting diodes (LEDs) or laser diodes, an optical lens system to form an image of the scene on an image sensor, and the image sensor comprising an array of high-speed demodulation pixels.
Other TOF approaches might use single photon avalanche diodes (SPAD) to detect the time of flight. SPADs measure the travel time of the light from the emitter to the object and back to the camera directly, whereas demodulation pixels are measuring the back-reflected phase information or correlation information representing a mean of the measured distance. Hence, the demodulation pixels measure the distance indirectly.
The light sources of TOF cameras usually operate in the near-infra red light spectra, most commonly between 800 and 900 nanometers (nm), which is outside the visible range. Operating in this range is mainly to avoid bright, visible light bothering people in the surroundings of the camera.
General system setups based on demodulation pixels and full 3D TOF camera implementation has been demonstrated in [LAN01] and [OGG04] as well as a description of several applications in [OGG05]. Further, the SPAD-based system approach has been presented in [NIC03].
Demodulation pixels in image sensors have the function to sample and accumulate the charge carriers generated by the incoming photons. During or after this integration time, the collected charge is then converted into a voltage signal, which is then read out.
Demodulation pixels allow sorting the arriving photons or the respective generated charge according to their arrival time. For that they need at least one fast switching element, and, connected to it, at least one dedicated storage or integration region or integration node, called tap. In these storage nodes, the charge corresponding to the time interval of the open switch is stored and is then read out. Based on these samples, the phase or distance is deduced.
One of the main challenges of 3D TOF pixels is to collect and store quickly as many photo-generated electrons as possible in the storage nodes. A common issue is that near-infra red (NIR) photons penetrate deeper in the substrate compared to photons in the visible spectra. Nevertheless, the electrons that are generated in the substrate by the NIR photons still need to be detected and collected and transferred quickly to corresponding storage nodes.
In recent years, the pixel pitch of standard pixel has been shrinking while the stack of layers on top of the photo-sensitive area have been increasing. This creates the challenge to bring the photons impinging on the pixel through all the stack layers into the photo-sensitive area. The efficiency of converting the photon falling onto a pixel into electrons is drastically lowered by adding more layers on top of the substrate.
A state-of-the-art approach to reduce the problem is the use of micro-lenses to improve the conversion rate from photons impinging on the pixel to electrons. However, the use of micro-lenses has limitations as soon as the layer stack gets too big in relation to the pixel pitch.
Micro-lenses combined with in-pixel lightguides further improve the efficiency. The so-called lightpipes direct all photons through the stack onto the sensitive area [GAM09], [AGA09].
Recent developments propose to backthin the substrate and to illuminate the pixel from the backside [RHO09]. This approach results in most designs having a fill factor of up to 100%, since no electronics is shadowing the photo-sensitive are from the impinging photons. The layer stack on top of the substrate can be increased in that case without losing sensitivity.
As in all solid-state image sensors, the incoming photons generate charge carriers, meaning electron-hole pairs within the substrate. These pairs have to be separated by an electric field, e.g. a p+/n+ doped junction. After the separation one type of charge carriers (normally the electron) is stored in a storage element, e.g. a capacitance, whereas the other type of charge carriers is absorbed by the substrate. In image sensors providing a visual representation of the surrounding, the separation is often slow, as the information is stored just in the amount of the generated charge carriers, giving a gray scale or, with the help of color filters, a color image.
In demodulation pixels, e.g. used for time-of-flight applications, the amount of generated charge carriers is not the main information one is interested in. Instead, the arriving time has to be measured. For this, the separation stage must distinguish the charge carriers depending on their arrival time. Therefore the collection of the charge carriers has to be fast. For this, drift fields are necessary to transfer the carriers to the point through the demodulation region into the corresponding storage node.
The first demodulation pixels, which are mainly dominated by electron diffusion transfer, have been described in [SPI99]. Based on this approach, several prototypes have been built [LAN01], [KAW06], [KAU04]. A similar demodulation principle has been described in [SCH99].
However, in order to improve 3D TOF imaging based on demodulation pixels, a higher electron transfer speed from the sensitive area to the storage nodes is essential if not even required. High speed electron transfer is mainly achieved by applying electric fields across the sensitive area to the storage node. Demodulation pixels based on pure drift transfer have been described in [SEI02], [BUE05B], [NIE05].
All these methods perform demodulation directly in the photo-sensitive area. A new approach separating the sensitive area from the demodulation area but keeping a drift-dominated electron transfer is described in [BUE05A].