Capacitively coupled pixel detectors
Hybrid pixel detectors
Complex in-pixel signal processing is of great importance for many applications. In the case of particle tracking (high-energy physics), it may be necessary to measure the particle-hit time in every pixel. In the case of the medical- and the synchrotron-imaging, it may be useful to count the detected photons. When the pixel readout electronics exceeds a certain level of complexity it may be advantageous to implement the readout-part of the pixel on a separate chip. In such a device, every sensor pixel has its own readout cell on the readout chip. The RO cells are arranged as a matrix and, in most cases, have the same geometry as the sensor pixels. The contacts between the electrodes of the pixel- and the readout cells are traditionally established by the small spherical metal contacts - the bump bonds. Such a detector type is called the hybrid pixel detector.
The main advantage of the hybrid pixel detectors is the possibility to implement very complex in-pixel data processing. The readout chip can be designed in a modern deep-submicron technology with very small device size. Generally speaking, since the senor- and the readout chips are produced separately, we can choose the optimal technology for both parts.
The drawback of the hybrid technology is the need for a complicated chip-to-chip connection technology. The bump-bonds are difficult to produce when a small pitch i.e. bump to bump distance are required.
Capacitively coupled pixel detector
We have proposed an alternative hybrid detector concept that does not require any complicate chip-to-chip connection technology and allows for the construction of low-cost and large-area hybrid pixel detectors. The signal transmission from the sensor to the readout chip is obtained by capacitive coupling. The capacitive chip-to-chip connection can be established without any complicated bonding technology - the readout chip is simply glued onto the sensor. The coupling occurs between the electrodes realized in the last metal layers of both chips. The signals are transmitted through the air gap between chips. We refer to the new detector type as capacitively coupled pixel detector - CCPD.
Since there is no need for bump-bonding, a CCPD is cheaper and easier to produce than a standard bump-bonded hybrid detectors. CCPDs are thus a large-volume, low-cost and a low-mass alternative to the standard detectors. Another advantage would be the possibility to implement pixels of smaller size, since the electrode for capacitive signal transmission usually occupies a smaller area than a typical bump-bond. Pixel sizes down to 20 micrometers can be easily achieved.
Implementation
The sensor for a CCPD can be designed in any active pixel technology. A sensor technology based on a high resistive semiconductor can be used as well, provided the implementation of a bias-circuit on the sensor is possible.
We have designed almost all sensors for CCPDs as the SDA structures. The pixel electronics contains a charge sensitive amplifier and, optionally, a discriminator. The voltage output of the amplifier is connected to an electrode implemented in the last metal layer. A readout chip is flipped and glued onto the sensor so that the sensor electrodes and the input pads of the readout channels form capacitors. The voltage signals are transmitted from the sensor- to the readout chip by means of AC coupling.
