LOW-PENETRATING PARTICLES LOW-GAIN AVALANCHE DETECTOR

Abstract

The low-penetrating particles low gain avalanche detector comprises a multi-layered structure and receives particles from a radiation source (13). It consists of a thin entry region that receives the particles from the radiation source (13); a low-penetrating particles detection region, with a p++ shallow field stop (1), positioned beneath the entry region, and a p absorption layer (3), positioned beneath the p++ shallow field stop (1), and an n multiplication layer (4); and a high-penetrating particles detection region positioned beneath the n multiplication layer (4), consisting of a n-- silicon substrate (5). Due to the chosen doping polarities, the primary electrons (created by the particles from the radiation source (13)) drift away from the entry region. That way, signals from low-penetrating particles or radiation experience amplification, while the noise is kept similar to a conventional PIN structure, thus increasing the signal-to-noise ratio.

Claims

1. A low-penetrating particles low gain avalanche detector with a multi-layered structure, configured to receive particles from a radiation source (13), wherein the detector comprises: an entry region, configured to receive the particles from the radiation source (13), and that comprises: a p++ shallow field stop (1), and a low-penetrating particles detection region, which comprises the following layers, in increasing depth: a p absorption layer (3), positioned beneath the p++ shallow field stop (1), an n multiplication layer (4), positioned beneath the p absorption layer (3), a high-penetrating particles detection region, which comprises: an n-- silicon substrate (5), following the n multiplication layer (4), below the low-penetrating particles detection region, a readout region, contacting the high-penetrating particles detection region, and a periphery region, which comprises: a p+ termination doping (2), positioned laterally on both sides of the p++ shallow field stop (1), and one or more metal contacts (7a), joined to the p++ shallow field stop (1).

2. The detector according to claim 1, wherein the entry region additionally comprises a protective layer (6), positioned over the p++ shallow field stop (1).

3. The detector according to claim 2, wherein the protective layer (6) is a conversion layer for incoming neutrons.

4- The detector according to claim 2, wherein the periphery region additionally comprises a silicon oxide layer (8), positioned beneath the protective layer (6).

5. The detector according to claim 1, wherein the p++ shallow field stop (1) is within the range of tens of nanometres of thickness.

6. The detector according to claim 1, wherein the n multiplication layer (4) is within the range of 1 to 5 micrometre thickness.

7. The detector according to claim 1, wherein the n-- silicon substrate (5) is within the range of 10 to 1000 micrometre thickness.

8. The detector according to claim 1, wherein the periphery region additionally comprises n++ ohmic contacts (9), positioned on both sides of the P absorption layer (3).

9. The detector according to claim 1, wherein the readout region additionally comprises segmented electrodes, following the high-penetrating particles detection layer, opposite to the low-penetrating particles detection layer.

10. The detector according to claim 9, wherein the segmented electrodes comprise n++ contacts (10), in contact with the n-- silicon substrate (5), and additional metal contacts (12), connected to the n++ contacts (10).

11. The detector according to claim 9, wherein the readout region additionally comprises p+ stops (11), alternating with the segmented electrodes and in contact with the n-- silicon substrate (5).

Description

DESCRIPTION OF THE DRAWINGS

[0044] To complement the description being made and in order to aid towards a better understanding of the characteristics of the invention, in accordance with a preferred example of practical embodiment thereof, a set of drawings is attached as an integral part of said description wherein, with illustrative and non-limiting character, the following has been represented:

[0045] FIG. 1.- Shows a first embodiment of the low-penetrating particles low gain avalanche detector, in a cross section of one detector half.

[0046] FIG. 2.- Shows a second embodiment of the low-penetrating particles low gain avalanche detector impacted by low-penetrating radiation in a cross section.

[0047] FIG. 3.- Shows a simplified cross section of the device with the electric field profile.

[0048] FIG. 4.- Shows the comparison of the current pulses from a 15 keV proton (top) and a minimum ionizing particle (MIP, bottom), both for normal incidence of the projectile particle.

[0049] FIG. 5.- Shows the current pulse created by a 15 keV proton as a function of time, zoomed into the first peak of FIG. 4.

[0050] FIG. 6.- Shows the undetected protons with different energies at normal incidence, as function of the detection threshold of the data acquisition system in primary electrons.

PREFERRED EMBODIMENT OF THE INVENTION

[0051] With the help of FIGS. 1 to 6, a preferred embodiment of the present invention is described below.

[0052] As shown in FIG. 1, the low-penetrating particles low gain avalanche detector comprises a multi-layered structure with: [0053] an entry region, which comprises: [0054] ◯ a protective layer (6), [0055] ◯ a p++ shallow field stop (1), positioned beneath the protective layer (6) with tens of nanometres of thickness, [0056] a low-penetrating particles detection region, which comprises: [0057] ◯ a p absorption layer (3), positioned beneath the p++ shallow field stop (1), [0058] ◯ a multiplication layer (4), positioned beneath the p absorption layer (3), of approximately 1 to 5 micrometre thickness, [0059] a high-penetrating particles detection region, which comprises: [0060] o a n-- silicon substrate (5), following the n multiplication layer (4), of approximately 10 to 1000 micrometre thickness, [0061] a readout region, which comprises: [0062] ◯ pixel or strip electrodes, following the high-penetrating particles detection layer, which comprise: [0063] .square-solid. n++ contacts (10), in contact with the n-- silicon substrate (5), [0064] .square-solid. additional metal contacts (12), connected to the n++ contacts (10), [0065] ◯ p+ stops (11), alternating with the strip electrodes and in contact with the n-- silicon substrate (5), and [0066] a periphery region, which comprises: [0067] ◯ one or more n++ ohmic contacts (9), positioned next to the p absorption layer (3), [0068] ◯ metal contacts (7a, 7b), wherein a first metal contact (7a) is joined to the p++ shallow field stop (1), and one or more second metal contacts (7b) are joined to the n++ ohmic contacts (9), [0069] ◯ a silicon oxide layer (8), positioned beneath the protective layer (6), and [0070] ◯ a p+ termination doping (2), positioned laterally on both sides of the p++ shallow field stop (1).

[0071] As shown in FIG. 2, low-energy protons (or other low-penetrating particles or radiation) coming from a radiation source (13) illuminate the detector at the entry region. The protons will go through the protective layer (6), which has been deposited for example by atomic layer deposition to obtain for example Al, Al.sub.2O.sub.3, or low-pressure chemical vapour deposition methods to obtain for example Si.sub.3N.sub.4.

[0072] The protective layer (6) is used to protect the surface of the detector from humidity or scratches and insures long-term electrical stability under operation at high bias voltages. The protective layer (6) can act as an antireflective coating for visible, ultraviolet and infrared radiation. The protective layer (6) can also be used as conversion layer for incoming neutrons. The minimum thickness deposited is in the order of a few nanometres and can be optimized for different radiation wavelengths.

[0073] The p+ termination doping (2) of the periphery region must be formed laterally next to the p++ shallow field stop, in order to avoid electric field peaks and ensure stable high-voltage operation.

[0074] Incoming protons will have to go also through the high-doped region, the p++ shallow field stop (1), and therefore it must be kept as thin as possible in order to avoid the recombination of the electron-holes generated by them. The p absorption layer (3) can be created for example by ion implantation and a variety of dopants (B, Ga, Al).

[0075] Particularly, the p++ shallow field stop’s (1) depth must be less than 100 nanometres, since low-energy proton particles will penetrate in the order of several hundred nanometres. Therefore, the p absorption layer (3) of for example 1 micrometre thickness is positioned underneath to absorb all the energy deposited by the proton (or other low-penetrating particles). In order to get a multiplied signal of the collected charge, the low-penetrating particle detection section, and n-type multiplication layer, is positioned above the high-penetrating particle detection layer, leading to a final triple doping profile.

[0076] Thus, a high electric field, i.e. larger than > 3.10.sup.5 V/cm, shown in FIG. 3, is created at the p/n junction, where the primary electrons from the created electron-hole pairs (e.sup.-/h.sup.+) will be multiplied.

[0077] The p absorption layer (3) and the n multiplication layer (4) may be created by ion implantation or by epitaxial growth. The control of the doping profiles and shapes is important for achieving a proper electron gain and avoiding early breakdowns.

[0078] The thickness of the p absorption layer (3) in front of the n multiplication layer (4), which form the p-n junction, can be adapted to match the penetration depth of the incoming particles or radiation.

[0079] The n-- silicon substrate (5) decreases the capacitance and improves the stability with respect to changes in bias voltage, which directly relates to the capacitance. The thickness of the high-penetrating particle detection layer is designed to reduce the input capacitance to the readout electronics and therefore the noise. Possible values of its thickness will range from 10 micrometres to 1000 micrometres (or even above, if the technology permits). The thickness of the high-penetrating particle detection layer defines the minimum operation voltage of the detector, since for getting the minimum capacitance, the full substrate must be depleted from charges.

[0080] The readout region of the detector, which may be on its backside, is segmented with n++ contacts (10) connected to readout electronics via additional metal contacts (12). The n++ contacts (10) can be formed, as shown in FIG. 2, with phosphorus or equivalent dopants by ion implantation and with different geometries such as pads, pixels or strips with different sizes. In principle, any readout scheme compatible with n-in-n contacts can be used, although it depends on the concentration of the n multiplication layer (4) and of the p absorption region (3). That includes, inter alia, monolithic pixels like the ones used in HV-CMOS sensors.

[0081] Preliminary simulation results show that the avalanche process relates to the doping concentration of the p absorption region (3) and the n multiplication region (4) of the proposed structure. In this sense, it is possible to tune the amplitude of the electric field maximum in order to adjust its electrical performance, optimizing the gain versus the anode voltage for increasing its linear evolution before the breakdown voltage.

[0082] In a planar sensor, the measurable signal is a current pulse created by the drift of the charge carriers towards the electrodes. We simulate the expected current pulse in the detector of the invention. We separately put out the current pulses for primary electrons, primary holes, secondary electrons, secondary holes and the summed current pulse.

[0083] FIG. 4 shows the overlay of these current pulses created by a 15 keV proton (top, low-penetrating) and a MIP (bottom, traversing the whole sensor thickness), both with normal incidence. Only signals close to the entry region are amplified. For a high-energetic, traversing particle, e.g., a MIP, a pLGAD behaves like a classic planar sensor.

[0084] FIG. 5 shows a zoom into the current pulse created by a 15 keV proton. The most important features of the pulse shape are (sorted chronologically): [0085] Point 1 (1 ps): A small dip, when the primary holes are collected at the entry region (loosely dotted). [0086] Point 2 (4 ps): A sharp increase, when the primary electrons are multiplied and secondary electrons are created (densely dashed). [0087] Point 3 (11 ps): A drop, when the secondary holes are collected at the entry region (densely dotted). [0088] Point 4 (26 ps): A dropping shoulder, when the secondary electrons leave the highly doped multiplication layer (densely dashed). This marks the end of the initial peak. [0089] Point 5 (30 ps): A plateau, when the primary and secondary electrons drift to the readout region in one compact package (loosely dashed and densely dashed). [0090] Point 6 (3 ns, shown in FIG. 4): A slight increase, when the primary and secondary electrons reach the vicinity of the readout electrode and experience a higher weighting field. [0091] Point 7 (3.7 ns, shown in FIG. 4): A sharp drop, when the ionization electrons (primary and secondary electrons) are collected at the readout electrode.

[0092] The exact shape of the current pulse depends on many things, like the doping profiles, the depth of the p-n-junction and the thickness of the multiplication layer. The current pulse shown in FIG. 5 is the result of a simplified simulation setup; realistic doping profiles will lead to a more washed-out shape. However, the main features of the pulse shape will still be present.

[0093] The pLGAD concept targets a specific niche: ultra-low-noise silicon sensor, position-resolving, with thin entry region for back-illumination. This niche is also served by two other silicon technologies: Silicon drift detector (SDD) and DEpleted P-channel Field Effect Transistor (DEPFET).

[0094] SDDs are a common choice in low-energy physics, especially where large areas have to be instrumented, and only a coarse position resolution is required. A comparison with a pLGAD would not be fair, because an SDD shines for large area sensing elements in the order of square centimetres. This size is a lot larger than the target position resolution of pLGAD sensors, so we have no directly comparable noise estimations for the SDD. Consequently, we mention the SDD for completeness, but do not present a detailed comparison of respective performances.

[0095] DEPFET sensors originate from High Energy Particles research and are available with small pixel sizes, down to tens of micrometres. In combination with drift rings (like on an SDD), larger pixel sizes are possible.

[0096] We can directly compare the performance of the pLGAD and the DEPFET in FIG. 6 for a pixel size of 0.4 × 1.6 square millimetres. A DEPFET sensor with appropriate pixel geometry reaches a detection threshold of below 30 primary electrons, if cooled to -50° C. The pLGAD is limited to above 50 primary electrons, but at room temperature.

[0097] Both DEPFET and SDD technologies introduce a number of complications: the sensors are intricate, and therefore expensive, and require at least 7 (5 for SDD) different operation voltages. For a DEPFET sensor, cooling is mandatory to keep the leakage current and its corresponding noise low, and thus increase the detection efficiency (see FIG. 6). In addition, the DEPFET technology involves several different custom readout chips, and has a relatively low frame rate in the order of microseconds.

[0098] In contrast, the proposed pLGAD can be operated with only one supply voltage, works with commercial readout systems that can read planar sensors, can be operated at room temperature, has a high intrinsic time resolution and is a lot cheaper, because of the simple planar production process.

[0099] FIG. 6 shows the percentage of undetected protons as function of the detection threshold of the data acquisition system, for different proton energies at normal incidence, and with a protective layer (6) of 15 nm. For the sake of clarity, FIG. 6 is without error bars, but due to statistical variation of the Monte Carlo method, the represented values can be higher or lower. Protons can go undetected due to backscattering, when they deposit too little energy in the active region of the detector, or when they are completely stopped in the non-active layer.

[0100] For proton energies of 15 keV, the pLGAD outperforms an uncooled DEPFET sensor with the same pixel layout, and achieves almost the same detection efficiency like a cooled one. A proton loss in the order of 0.15 % (or a detection efficiency of 99.85 %) seems possible, as a best-case scenario.