Hybrid Pixel Detectors for particles detection
The intensive growth of the Hybrid Pixel Detectors (HPDs) was initiated and is still driven by the development for the LHC detectors, where very fast and radiation hardened devices are required .
The fabrication of this type of pixel sensor is very similar to the fabrication of a microstrip sensor. In the pixel case the implants have a higher segmentation. This simple change of the sensor design has many consequences at the system level and offers a variety of applications. The detector part consists essentially of a microstrip detector structure, each strip being subdivided into some number of short pieces, which constitute the pixels. The sensor array and the matching read-out chip are processed independently and are connected together only in the final step. In this way the material and processes can be individually optimized for the actual purpose i.e. detector and electronics. This approach makes it possible to achieve fast enough read-out and radiation hardness compatible with the LHC environment. The detector substrate is high resistivity silicon, although other materials than silicon, e.g. diamond, are also considered.
The read-out electronics is built in an industrial CMOS foundry and it can be similar in architecture to the classical front-end topology for microstrip detectors. The connection of the detector and the read-out electronics is customarily done by means of the flip-chip bonding technique, where small balls of solder, indium or gold, establish the electrical and mechanical connection between each detection element and its read-out circuit.
The two-dimensional high‑density connectivity is the key characteristics of the hybrid pixel detector.
Any electronics chip must have some ancillary logic to extract the signal from the front-end channels, organize the information, and transmit it out. This logic cannot be distributed to all pixel cells, but has to be concentrated and is normally placed close to one edge of the chip.
Since the chip is very close to the sensor, designers must pay special attention to avoid the following:
Large static voltage (i.e. bias voltage) on the front side or on the edge of the sensors that may give rise to destructive sparks. This implies that the guard ring structure which helps to confine the high‑voltage region should be on the backside of the sensor.
Large high-frequency signals on the electronics that may induce detectable signals on the pixel metallization. This implies using low swing logic signals (e.g. LVDS) and minimizing the coupling capacitance between the sensor and the digital busses.
HPDs have the disadvantage of high complexity of millions of interconnections and they introduce extra material in the active area. Moreover, HPDs are characterized by the relatively high power dissipation reaching a few hundred mW/cm2 and relatively large size of a single cell needed to integrate required complex functionality of the read-out circuitry.
Other peculiar characteristics of the pixel detectors are related to the small dimensions of the sensing elements. Each pixel covers, in fact, a very small area (≈ 10^(−4)cm2) over a thin (≈ 300μm) layer of silicon. It therefore exhibits a very low capacitance (≈ 0.2 – 0.4pF), which is dominated by the coupling to the neighboring pixels rather than to the backside plane. The low capacitance is one of the key advantages of pixel detectors since it allows fast signal shaping with very low noise.
It is common to obtain single pixel noise of about 200e for electronics operating at 40MHz and therefore an SNR exceeding 100 for fully depleted 300 μm-thick sensors. This is a very comfortable situation as it allows operation in absence of spurious noise hits. A detection threshold set at, e.g., 10σ noise, gives in fact full efficiency and very low probability that a noise fluctuation exceeds the threshold. This may be looked at as a very idealized situation as other sources of fake hits could be conceived (e.g. electronics pickup, cross talk, low-energy photons), but measurements [1.9] prove that a spurious hit probability of <10−8 per pixel can be reached under experimental conditions. Another way of taking advantage of the excellent SNR is to consider that the detector is robust enough to tolerate even a considerable signal loss.
This extends the application of the hybrid pixel detector in two directions:
To sensors which have a poor charge collection or a limited active thickness (e.g. diamond, GaAs, Cd(Zn)
To crystalline silicon sensors damaged by high irradiation flux.
In the latter case the collected charge is diminished through two effects: the trapping of drifting carriers due to radiation-induced defects in the crystal lattice and the reduction of the depletion depth due to the increase of the space charge .
Finally, smallness of the pixel means smallness of the reverse current flowing through it at depletion (typically 0.1μA/cm2). This reduces the parallel noise and allows operation even after considerable irradiation. After 1015 particles per square centimeter the reverse current density increases to ≈ 30μA/cm2, rendering large sensing elements difficult to operate. In summary, the HPD is the ideal detector to work in the very hostile environment which exists close to the interaction region of a particle accelerator because:
It is radiation hard (i.e. it survives at high fluence of particles);
It provides nonambiguous three-dimensional measurements with good time resolution;
It provides the space resolution which is needed to measure short-lived particles.
HPDs have been shown
to work in particle physics experiments
[3-6]. This success has triggered the design
and the construction of detectors
approaching few square meters of sensitive
area and 100 millions of channels to be
operated in intense particle fluxes.
Freedom in the choice of the sensitive material has also favored the application of HPDs in other fields, like medical diagnostics.
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