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 [1].
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
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.
Sensing element
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:
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 [2].
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 10(15)
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:
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 [10-11]
Pixel application
The development of hybrid pixel detectors for particle detection with high spatial resolution in high energy physics experiments has spun off a number of developments with applications in imaging, most notably biomedical imaging, and also imaging in X-ray astronomy.
In the latter, the reconstruction of low-energy X-ray images originating from astronomical point sources with high spatial and high energy resolution at moderate data rates is the challenge. So far this goal has been met best by using fully depleted pn-CCD detectors. For the next generation of X-ray satellites, however, monolithic or semimonolithic pixel devices with excellent spectroscopic performance are in the focus of development.
The largest progress has recently been achieved in the development of pixel detectors for X-ray imaging for applications in radiology and in protein crystallography. Another field in which pixel detector developments have created new possibilities for real-time imaging is that of biomedical autora- diography, where most commonly low-energy β-radiation from radiolabeled tissue must be detected with high spatial resolution and high efficiency. Also γ-emitters are occasionally considered. The use and development of pixel detectors in medical or astrophysical imaging systems like Compton cameras is also addressed by various research groups.
Difference between Particle Tracking and Imaging with Pixel Detectors
While in the case of Particle Tracking individual charged particles, usually triggered by other detectors, have to be identified with high demands on spatial resolution and timing, in imaging applications an image is obtained by the usually untriggered accumulation (integrating or counting) of the quanta of the impinging radiation during some exposure time.
Also, the demands on the detector performance can be quite different. Silicon pixel detectors for high-energy charged particle detection can assume typical signal charges collected at an electrode in the order of 8,000–15,000 electrons even taking into account charge sharing between pixel cells and detector deterioration after irradiation to doses as high as 500 kGy. In X-ray astronomy the amount of charge to be collected with high efficiency can be well below 1, 000 e−.
The spatial resolution is governed by the attainable pixel granularity from a few μm to about 10 μm at best, obtained with pixel dimensions in the order of 50x100 μm2. The requirements from radiology are similar or even more relaxed than those in particle physics, while some applications in autoradiography require submicrometers resolutions, not attainable with present day pixel detectors. However, for applications with lower demands on the spatial resolution (O(10 μm)) but with demands on real-time and time-resolved data acquisition, semiconductor strip or pixel detectors are attractive.
In addition, energy resolution capability may be provided at the same time. While silicon is almost a perfect material for particle physics detectors, allowing the shaping of electric fields by spatially tailored impurity doping, the need of high photon absorption efficiency in radiological applications requires the study and use of semiconductor materials with high atomic charge, such as GaAs, Cd(Zn)Te, or HgI2. For such materials the charge collection properties are much less understood and mechanical issues, in particular those related to hybrid pixels, are abundant. The hybridization of detectors when they are not available in wafer scale sizes can be a real problem. Last but not least, the cost-to-performance ratio is an important factor to consider if an imaging application can be commercially interesting.
References
- P. Garrou, C. Bower and P. Ramm, Handbook of 3D Integration, Wiley-VCH, (2008).
- 1.8 L.H.H.Scharfetter, Active Pixel Detectors for the Large Hadron Collider, PhD thesis, Leopold Franzens Innsbruck University, Austria, (1996).
- E.H.M. Heijne, Semiconductor micropattern pixel detectors: a review of the beginnings, Nucl. Instr. and Meth A 465, (2001) 1–26.
- M. Moll, Radiation damage in silicon particle detectors–microscopic defects and macroscopic properties, PhD Thesis, Universitat Hamburg, Germany (1999). DESY-1999-040.
- E.H.M. Heijne et al, Element Hybrid Silicon Micropattern Pixel Detector Array, Nucl. Instr. and Meth A 349, (1994) 138–155.
- K.H. Becks et al, First operation of a 72 k element hybrid silicon micropattern pixel detector array, Nucl. Instr. and Meth A 418, (1998) 15–21.
- The ATLAS Collaboration, Technical design report of the ATLAS pixel detector, CERN/LHCC/98-13 (1998).
- The CMS Collaboration, CMS tracker technical design report, CERN/LHCC/98-6 (1998).
- The ALICE Collaboration, ALICE technical design report of the inner tracker system, CERN/LHCC/99-12 (1999).
- G. Dipasquale, et al., Characterisation of a single photon counting pixel system for imaging of low-contrast objects, Nucl. Instr. and Meth A 458, (2001) 352–359.
- P. Fischer et al., A counting pixel chip and sensor system for X-ray imaging, IEEE Trans. Nucl. Sci. 46(4), (1999) 1070–1074.
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