A sparse data scan circuit for pixel detector chips

The use of pixel detectors in high rate collider experiments requires the recording and readout of large amounts of data. The architectural choices for managing these large data flows are reviewed and illustrated on specific pixel projects.

The innovation in monolithic and hybrid semiconductor ‘micropattern’ or ‘reactive’ pixel detectors for tracking in particle physics was actually to fit logic and pulse processing electronics with μW power on a pixel area of less than $\text{0.04}\text{mm}{}^2$, retaining the characteristics of a traditional nuclear amplifier chain. The ns timing precision in conjunction with local memory and logic operations allowed event selection at $\text{>10}\text{MHz}$ rates with unambiguous track reconstruction even at particle multiplicities $\text{>10}\text{cm}{}^{\mathrm{-2}}$. The noise in a channel was $\text{∼100}\text{e}{}^{\mathrm{-}}\text{rms}$ and enabled binary operation with random noise ‘hits’ at a level <10⁻⁸. Rectangular pixels from $\text{75}\text{μm}\text{×500}\text{μm}$ down to $\text{34}\text{μm}\text{×125}\text{μm}$ have been used by different teams. In binary mode a tracking precision from 6 to $\text{14}\text{μm}$ was obtained, and using analog interpolation one came close to $\text{1}\text{μm}$. Earlier work, still based on charge integrating imaging circuits, provided a starting point. Two systems each with more than 1 million sensor + readout channels have been built, for WA97-NA57 and for the Delphi very forward tracker. The use of $\text{0.5}\text{μm}$ and $\text{0.25}\text{μm}$ CMOS and enclosed geometry for the transistors in the pixel readout chips resulted in radiation hardness of $\text{∼2}\text{Mrad}$, respectively, $\text{>30}\text{Mrad}$.

The DELPHI Microvertex detector was upgraded in 1996 to reach the higher angular coverage demanded for new particle searches at LEP200. It now consists of an up to 50 cm barrel part and very forward part (VFT), with total angular converage between 11° and 169°. Different parts and layers have particular designs and 11 kinds of silicon sensors are used, including pixels. This paper discusses the layout and performance of the detector.

To improve tracking in the very forward direction for running at LEP200, the angular acceptance of the DELPHI Vertex detector has been extended from 45° to 11° with respect to the beam axis. Pixel detector crowns cover the region between 25° and 13°. Due to very tight space and material thickness constraints it was necessary to develop new techniques (integrated busses in the detector substrate, high density layout on Kapton, etc.). About 1000 cm² of pixels are already installed and working in DELPHI. Techniques, tests and production of these detectors will be described, as well as the main problems encountered during this work.

Modern VLSI readout chips for strip and pixel detectors contain many electronic channels which all should behave exactly alike in order to give the optimal performance. However, good matching is often very difficult to achieve due to many design constraints. In this paper, a simple concept for automatic adjustment of important VLSI circuit properties simultaneously in all readout channels is presented. The tuning is performed by applying a test injection pulse and, if necessary, a reference signal to all cells simultaneously. A simple circuit in each channel adjusts the control voltage on a storage capacitor until matching is optimal. An application of this concept in a self adjusting delay circuit is presented and other possible applications are discussed.

Pixel detectors will be extremely useful in many future experiments. Firstly, due to their very small size, they have a very low intrinsic noise and are thus intrinsically radiation resistant. Secondly, their true 3-D capability improves the tracking efficiency and ghost rejection. They can then be close to the beam axis at the very high planned accelerator luminosities for improved vertexing. Several smart data-driven architectures are being considered for use at LHC and will be tested soon.