Low-penetration particle low-gain avalanche detector

Abstract

The low-penetration particle, low-gain avalanche detector has a multilayer structure that accepts particles from a radiation source (13). It consists of a thin entry region that accepts particles from the radiation source (13); a low-penetration particle detection region that has a shallow p++ field stop (1) located below the entry region, a p-absorption layer (3) located below the shallow p++ field stop (1), and an n-enhancement layer (4); and a high-penetration particle detection region that is located below the n-enhancement layer and consists of an n-silicon substrate (5). Due to the selected doping polarity, primary electrons (generated by particles from the radiation source (13)) drift away from the entry region. In this way, the signal from the low-penetration particle or radiation undergoes amplification, while the noise is maintained as in a conventional PIN structure, thus increasing the signal-to-noise ratio.

Description

[Technical Field]

[0001] (Object of the invention) It is an object of the present invention to provide a low-penetration particle low-gain avalanche detector with a layered structure, which has the ability to detect low-penetration radiation or particles, good time resolution and low noise, and which can also operate at room temperature. [Background technology]

[0002] (Background of the invention) Silicon radiation detectors based on PIN diodes are well established in high-energy physics experiments. PIN diodes operated in fully depleted conditions with an external reverse bias produce a signal proportional to the energy imparted by the incident radiation.

[0003] Silicon photodetectors are widely used, for example, in synchrotron facilities as X-ray detectors or in large-scale particle physics experiments as particle detectors. Silicon photodetectors are used for their excellent features, including their small size, high quantum efficiency over a wide wavelength range, and high spatial resolution defined by diode segmentation as either pads, strips, or pixels.

[0004] Avalanche photodiodes (APDs) have been developed to detect low-energy particles (e.g., low-energy protons, electrons, and / or neutrons) and low-energy X-rays (light particles, photons). This type of detector exhibits internal signal amplification, with gain proportional to the applied bias voltage and good uniformity over a large detection area. Operation of such devices in the linear region provides a signal output proportional to the absorbed energy, with gain factors between 10 and 100, allowing for the detection of low-intensity signals, even down to a few photons. However, there is noise associated with the amplification process that can distort the signal-to-noise ratio. Furthermore, the leakage current levels associated with APDs are too high to be compatible with modern hybrid pixel readout systems used in particle physics and hybrid pixel detectors.

[0005] Changing the doping level in APDs has allowed the fabrication of devices with lower gains in the 5-10 region that are operated in linear mode for proportional response. Such devices are known as low-gain avalanche detectors (LGADs).

[0006] They operate at low voltages and therefore require high-resistivity silicon substrates (5-10 kΩ cm) to achieve full depletion. The lower gain reduces the noise and detector gain dependence on device temperature and applied bias voltage compared to standard APDs. The diodes can be segmented in the same manner as standard PIN detectors, since the low gain avoids crosstalk. Therefore, hybrid pixel detectors can be fabricated that enable the detection of low-energy X-rays and produce signals below the noise floor of the electronics.

[0007] The LGAD design allows for the detection of subnanosecond signals produced by the smallest ionizing particles, and the ionization is produced uniformly as a function of depth within the detector. For an LGAD with a gain of 10, the same total signal charge is produced from the smallest ionizing particles if the LGAD substrate is thinned by an order of magnitude compared to a standard pin silicon detector. However, the acquisition time is reduced by an order of magnitude.

[0008] Additionally, inverted low-gain avalanche detectors (iLGADs) are known from the state of the art. iLGADs have an APD (Avalanche Photo Diode) structure at the rear contact, while the segmented front surface is created using ohmic contacts. iLGADs are P-on-P position-sensitive detectors with low signal amplification and a uniform electric field over the entire device area, ensuring the same signal amplification no matter where the particle passes through the sensitive bulk.

[0009] This technique is reproducible and well-controlled, but it cannot detect ions, charged particles, or radiation with a low penetration depth, i.e., less than 1 micrometer. Summary of the Invention

[0010] Description of the Invention Low penetration particles are understood to be particles whose penetration depth in silicon is less than 1 micrometer. When trying to detect the signal of low penetration particles, the sensor faces three distinct requirements: first, the inactive layer at the surface must be thin; second, the charge collection efficiency (CCE) at the surface must be high; and third, the readout must have a high signal-to-noise ratio (SNR).

[0011] Regarding the first requirement, semiconductor sensors always have a passive layer on their surface to protect them from the environment. These passive layers must be made as thin as possible to allow low-penetration particles to reach the sensor's sensitive layer in the first place. However, even so, a significant portion of the energy of the primary particles is transferred to the passive layer and cannot be measured. Therefore, the thickness and composition of the passive layer determine the lower energy limit of the sensor.

[0012] Regarding the second requirement, the surface of a silicon substrate (below the inactive layer) has a higher defect density than the bulk and is typically heavily doped to form a field stop. Therefore, electron-hole pairs generated from energy applied to the silicon surface region face a higher recombination probability, further reducing the measurable signal. This is evident in the CCE, which is typically significantly lower than 1 near the silicon surface. However, a sensor with high detection efficiency should have a CCE of 1 near the surface.

[0013] Regarding the third requirement, the residual signal charge can easily be an order of magnitude lower than the signal of the minimum ionizing particle (MIP), so a readout with a sufficiently high SNR is of utmost importance.

[0014] In particular, for less invasive particles like low-energy protons, the signal in the sensor is generated close to the sensor's surface, with the majority of the signal generated at a depth of less than 1 micrometer, whereas in conventional silicon sensors, the surface inactive layer is much thicker.

[0015] The proposed low-penetration low-gain avalanche detector (pLGAD) meets these special requirements for low-penetration particles by including a thin entry region, preferably made of aluminum, aluminum oxide, or silicon nitride, on the order of 15 nanometers. The polarity of the signal collection electrode is n-type, and primary electrons drift across the enhancement layer to the readout region. This also implies reversing the polarity of the silicon substrate to keep the p-n junction, and thus the enhancement layer, in the entry region. The p-n junction and enhancement layer are moved away from the entry region into the bulk, and a thin p-type absorption layer is introduced.

[0016] Thus, the low penetration particle low gain avalanche detector (pLGAD) of the present invention enables the detection of ions, charged particles or radiation with a low penetration depth (i.e., less than 1 micrometer). In particular, it includes a multilayer structure that combines an ultra-shallow entry region with a linear enhancement layer and a small capacitance with the possibility of fine segmentation (a few micrometers) of the electrodes in the readout region.

[0017] In pLGAD detectors, only the signal generated near the entry region is amplified. This makes this detector concept less interesting for high-energy particle applications, but perfectly suitable for low-energy physics experiments and for the respective applications, testing, or diagnostics in the fields of materials science and medicine. It should be noted that in this pLGAD detector concept, most of the detector's intrinsic leakage current remains unamplified, since this current is generated within the bulk of the material and only holes cross the enhancement layer. Thus, the pLGAD detector concept cleverly exploits two systematic asymmetries to detect low-penetration particles: on the one hand, the fact that the signal is generated only near the surface (entry region), and on the other hand, the fact that only electrons are enhanced.

[0018] This detector will find applications in areas such as neutron physics, ion physics, medical physics, space research, synchrotron detectors and quantum technologies for the detection of low energy protons, electrons, neutrons and / or X-rays.

[0019] Specifically, the detector comprises a multi-layer structure having five main regions: an entry region, a low invasive particle detection region (p-doped region), a high invasive particle detection region (n-doped region), a readout region, and a peripheral region.

[0020] The entry region faces the radiation source and receives various particles. It can include a protective coating and a shallow p++ field-stop doping, tens of nanometers thick, facing the radiation source or located below the protective coating.

[0021] The low invasive particle detection region is disposed below the entry region and includes a p-absorption layer disposed below a shallow p++ field stop, and an n-enhancement layer disposed below the p-absorption layer and having a thickness of approximately 1 to 5 micrometers.

[0022] The high transmission particle detection region is located directly below the low transmission particle detection region, still near the entry region, and includes an n-- silicon substrate followed by an n-enhancement layer having a thickness of about 10 to 1000 micrometers.

[0023] The device also includes a readout region in contact with the highly invasive particle detection region, which may include a pad, pixel, or strip electrode. The electrode in turn includes an n++ contact in contact with the n-- silicon substrate and a metal contact connected to the n++ contact.

[0024] The readout region may further comprise p+ stops alternating with the strip electrodes and in contact with the n-- silicon substrate.

[0025] Finally, the device may also include a peripheral region, which may include, in order: one or more metal contacts bonded to the protective coating; one or more metal contacts bonded to the shallow p++ field stop doping, allowing a defined potential to be applied to the shallow p++ field stop doping; a silicon oxide layer disposed below the protective coating; and p+ termination dopings disposed laterally on either side of the shallow p++ field stop.

[0026] The material polarity of the pLGAD causes the primary electrons generated by the incident particle to drift into the readout region, so the enhancement affects the signal generated only in the low invasive particle detection region.

[0027] Due to its design and its application to low invasive particles, pLGAD has many interesting and advantageous properties: Noise behavior and background rejection can be optimized by appropriate selection of detector thickness.

[0028] The SNR of low penetration particles is increased without any drawbacks.

[0029] Current pulse shape allows for fast timing.

[0030] -Can operate at room temperature.

[0031] · Flexibility in choosing the energy range of the incident particle for full enhancement.

[0032] The augmented layer is compatible with many readout methods.

[0033] The following paragraphs explain the listed advantages in detail: The noise behavior and background rejection can be optimized by appropriately selecting the detector thickness. The primary signal is generated near the surface. Therefore, in contrast to high-energy particles, the total charge depends only on the energy of the incident particle and not on the detector thickness. This means that the thickness of the pLGAD can be freely selected within the limits of manufacturing technology, for example, to optimize the noise behavior or the response to background particles.

[0034] The thickness affects the pulse duration and amplitude, as well as the load capacitance seen by the readout electronics. In addition, a thin pLGAD (less than 100 micrometers) allows high-energy particles to pass through and suppresses their signal height, while a thick pLGAD (greater than 1.5 millimeters) enables spectroscopic measurements of deep-penetrating particles by stopping them completely within the sensor.

[0035] The SNR is increased without any drawbacks. With the selection of the polarity of the pLGAD, only the ionizing electrons (primary and secondary electrons) generated by an incoming low-penetration particle are enhanced. Electrons generated in the bulk, such as from a traversing background particle or by thermal excitation, drift away from the enhancement layer. This means that for a traversing particle, the pLGAD behaves like a planar sensor.

[0036] The leakage current and its corresponding noise are not amplified, which means that the SNR for low-penetration particles is increased by the gain without any drawbacks compared to planar sensors.

[0037] The current pulse shape allows for fast timing. The primary electrons traverse the pLGAD thickness on a straight path. In contrast to competing technologies, in pLGAD sensors, the primary electrons traverse the detector thickness on the shortest possible path. Also, only electrons traverse the detector, not holes. This leads to short pulse durations with fast rising edges, which is beneficial for timing.

[0038] They are capable of operating at room temperature. High detection efficiency can be achieved by detectors that accumulate signal charge and repeatedly read it. However, this is a time-consuming process in which leakage currents distort the signal. Therefore, cooling is essential for these sensors. In contrast, pLGADs can operate at room temperature while achieving high detection efficiency.

[0039] The energy range of the penetrating particles for total amplification can be adjusted. The composition of the entry region defines the minimum energy of the low-penetrating particles that can be detected. In contrast, the depth at which amplification occurs defines the upper energy limit that a penetrating particle can have while all of its signal is increased. Ultimately, to achieve full amplification, the incident particle must be completely stopped in the p-absorption layer.

[0040] The enhancement layer is compatible with many readout schemes. Signal generation and readout occur on different sides of the detector. Therefore, pLGADs can be realized with different geometries for the planar readout electrodes, for example, in the form of silicon microstrip sensors or pixel layouts, or in the form of any other readout scheme that can be implemented with n-type electrodes in an n-type substrate. This includes, among others, readout schemes that integrate the first amplification stage into the detector, such as HV-CMOS sensors.

[0041] If a pLGAD is fabricated with a planar readout electrode, it can easily operate just like any other planar sensor. Or, put another way, an enhancement layer like a pLGAD could, in principle, be added to existing sensor technologies to enhance their detection efficiency for low-penetration particles. [Brief explanation of the drawings]

[0042] DESCRIPTION OF THE DRAWINGS To supplement the description given and to facilitate a better understanding of the nature of the invention, according to a preferred example of its practical embodiment, a set of drawings are attached, with illustrative and non-limiting character, as an integral part of said description, in which: [Figure 1] 1 shows a first embodiment of a low invasive particle low gain avalanche detector in cross section through one detector half. [Figure 2] A second embodiment of a low penetration particle low gain avalanche detector is shown under the influence of low penetration radiation in cross section. [Figure 3] 1 shows a simplified cross section of a device with an electric field profile. [Figure 4] A comparison of current pulses from 15 keV protons (top row) and minimally ionizing particles (MIPs, bottom row) is shown, both for normal incidence of the projected particles. [Figure 5] Zoomed into the first peak in Figure 4, the current pulse produced by 15 keV protons is shown as a function of time. [Figure 6] 1 shows the undetected protons with different energies at normal incidence as a function of the detection threshold of the data collection system for primary electrons. DETAILED DESCRIPTION OF THE INVENTION

[0043] (Preferred embodiment of the present invention) A preferred embodiment of the present invention will be described below with reference to FIGS.

[0044] As shown in Figure 1, the low-intrusion particle low-gain avalanche detector is an entry area, protective layer (6), an entry region comprising a shallow p++ field stop (1) having a thickness of a few tens of nanometers, disposed under a protective layer (6); A low-invasive particle detection area, a p-absorber layer (3) located below a shallow p++ field stop (1); a low-penetration particle detection region comprising an enhancement layer (4) having a thickness of about 1 to 5 micrometers, disposed below the p-absorption layer (3); Highly invasive particle detection area, a highly penetrating particle detection region comprising an n--silicon substrate (5) of about 10 to 1000 micrometers thickness followed by an n-enhancement layer (4); is a readout region, a pixel or strip electrode following the highly invasive particle detection layer; n++ contacts (10) in contact with the n-- silicon substrate (5); a pixel or strip electrode including an additional metal contact (12) connected to the n++ contact (10); a readout region comprising p+ stops (11) alternating with the strip electrodes and in contact with the n-- silicon substrate (5); It is a peripheral area, one or more n++ ohmic contacts (9) disposed adjacent the p absorption layer (3); metal contacts (7a, 7b), where a first metal contact (7a) is bonded to the shallow p++ field stop (1) and one or more second metal contacts (7b) are bonded to the n++ ohmic contacts (9); A silicon oxide layer (8) disposed under the protective layer (6). a peripheral region comprising p+ termination dopings (2) laterally disposed on either side of a shallow p++ field stop (1); The multilayer structure has:

[0045] As shown in Figure 2, low energy protons (or other low invasive particles or radiation) from a radiation source (13) irradiate the detector at the entry region. The protons can pass through a protective layer (6), which may be deposited by atomic layer deposition to obtain Al2O3 or the like, or by low pressure chemical vapor deposition to obtain Si3N4 or the like.

[0046] The protective layer (6) is used to protect the detector surface from moisture or scratches and ensures long-term electrical stability under high bias voltage operation. The protective layer (6) can act as an anti-reflection coating for visible, ultraviolet, and infrared radiation. The protective layer (6) can also be used as a conversion layer for incoming neutrons. The minimum thickness to be deposited is on the order of a few nanometers and can be optimized for different radiation wavelengths.

[0047] The p+ termination doping (2) in the peripheral region must be formed laterally adjacent to a shallow p++ field stop to avoid electric field peaks and ensure stable high voltage operation.

[0048] The incoming protons must also pass through the highly doped region, the shallow p++ field stop (1), which must therefore be kept as thin as possible to avoid recombination of the electron-holes generated by them. The p absorption layer (3) can be formed, for example, by ion implantation and various dopants (B, Ga, Al).

[0049] In particular, because low-energy proton particles penetrate on the order of hundreds of nanometers, the depth of the shallow p++ field stop (1) must be less than 100 nanometers. Therefore, a p-type absorption layer (3), e.g., 1 micrometer thick, is placed underneath to absorb all the energy imparted by the protons (or other low-penetration particles). To obtain an enhanced signal of the collected charge, a low-penetration particle detector and an n-type enhancement layer are placed above the high-penetration particle detector layer, resulting in a final 3x doping profile.

[0050] In this way, high electric fields, i.e., >3.10, are achieved as shown in Figure 3. 5 A high electric field of more than 1000 V / cm is generated at the p / n junction, and the generated electron-hole pairs (e - / h + ) is increased.

[0051] The p-absorption layer (3) and n-enhancement layer (4) can be formed by ion implantation or epitaxial growth. Control of the doping profile and shape is important to achieve adequate electron gain and avoid premature breakdown.

[0052] The thickness of the p-absorbing layer (3) before the n-enhancing layer (4) that forms the pn junction can be adapted to match the penetration depth of the incoming particles or radiation.

[0053] The n--silicon substrate (5) reduces capacitance and improves stability against changes in bias voltage, which is directly related to capacitance. The thickness of the highly invasive particle detection layer is designed to reduce the input capacitance to the readout electronics and therefore noise. Possible thicknesses range from 10 micrometers to 1000 micrometers (or even higher, if technically feasible). The thickness of the highly invasive particle detection layer determines the minimum operating voltage of the detector, since to obtain a minimum capacitance, the complete substrate must be depleted of charge.

[0054] The detector's readout area may be on its backside and is segmented with n++ contacts (10), which are connected to the readout electronics via additional metal contacts (12). As shown in Figure 2, the n++ contacts (10) can be formed by ion implantation with phosphorus or a comparable dopant 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, depending on the concentrations of the n-enhancement layer (4) and the p-absorber region (3). This includes, among other things, monolithic pixels such as those used in HV-CMOS sensors.

[0055] Preliminary simulation results show that the avalanche process is related to the doping concentrations of the p-absorption region (3) and the n-enhancement region (4) of the proposed structure. In this sense, it is possible to tune its electrical performance by adjusting the maximum amplitude of the electric field, optimizing the gain versus anode voltage in order to increase its linear evolution before the breakdown voltage.

[0056] In a planar sensor, the measurable signal is a current pulse generated by the drift of charge carriers toward the electrodes. We simulate the expected current pulses in our detector by outputting separate current pulses for primary electrons, primary holes, secondary electrons, secondary holes, and the summed current pulse.

[0057] Figure 4 shows an overlay of these current pulses generated by 15 keV protons (top row, low penetration) and MIPs (bottom row, traversing the entire sensor thickness), both at normal incidence. Only the signal close to the entry region is amplified. For high-energy traversing particles, such as MIPs, the pLGAD behaves like a classical planar sensor.

[0058] Figure 5 shows a close-up of the current pulse produced by a 15 keV proton. The most important features of the pulse shape are (sorted chronologically): · Point 1 (1 ps): A small dip (rough dotted line) when the primary holes are collected in the entry region.

[0059] Point 2 (4 ps): The primary electrons are amplified and a sudden increase occurs when secondary electrons are generated (dense dashed line).

[0060] · Point 3 (11 ps): Drop when secondary holes are collected in the entry region (dense dotted line).

[0061] Point 4 (26 ps): The falling shoulder (dense dashed line) when the secondary electrons leave the highly doped enhancement layer. This marks the end of the initial peak.

[0062] Point 5 (30 ps): Plateau (coarse and dense dashed lines) when primary and secondary electrons drift to the readout region in one compact package.

[0063] Point 6 (3 ns, shown in Figure 4): A slight increase occurs when the primary and secondary electrons reach the vicinity of the readout electrode and experience a higher weighted field.

[0064] Point 7 (3.7 ns, shown in Figure 4): A sudden drop when the ionizing electrons (primary and secondary electrons) are collected at the readout electrode.

[0065] The exact shape of the current pulse depends on many things, such as the doping profile, the depth of the pn junction, and the thickness of the enhancement layer. The current pulse shown in Figure 5 is the result of a simplified simulation setup; a realistic doping profile would result in a more washed-out shape. However, the main features of the pulse shape are still present.

[0066] The pLGAD concept targets a specific niche: ultra-low noise silicon sensors, position resolution, and a thin entry area for backside illumination. This niche is also served by two other silicon technologies: silicon drift detectors (SDDs) and depletion-type p-channel field-effect transistors (DEPFETs).

[0067] SDDs are a popular choice in low-energy physics, especially when large areas must be measured and only coarse position resolution is required. A comparison with pLGADs is unfair because SDDs excel due to their large-area sensing elements, on the order of square centimeters. This size is much larger than the target position resolution of pLGAD sensors, so there are no directly comparable noise estimates for SDDs. Therefore, we mention SDDs for completeness but do not provide a detailed comparison of their respective performance.

[0068] DEPFET sensors originate from high-energy particle research and are available with pixel sizes as small as tens of micrometers. In combination with drift rings (as on SDDs), larger pixel sizes are possible.

[0069] For a pixel size of 0.4 × 1.6 square millimeters, we can directly compare the performance of the pLGAD and DEPFET in Figure 6. A DEPFET sensor with the appropriate pixel geometry reaches a detection threshold of less than 30 primary electrons when cooled to -50 °C. The pLGAD is limited to more than 50 primary electrons, but at room temperature.

[0070] Both DEPFET and SDD technologies introduce several complexities: the sensors are complex and therefore expensive, and require at least seven (five for SDD) different operating voltages. Cooling is mandatory for DEPFET sensors to keep leakage currents and corresponding noise low and to increase detection efficiency (see Figure 6). Furthermore, DEPFET technology involves several different custom readout chips and has a relatively low frame rate, on the order of microseconds.

[0071] In contrast, the proposed pLGAD can operate with only one supply voltage, can work with commercially available readout systems that can read planar sensors, can operate at room temperature, has high intrinsic time resolution, and is very inexpensive due to a simple planar fabrication process.

[0072] Figure 6 shows the percentage of undetected protons as a function of the detection threshold of the data collection system for different proton energies at normal incidence with a 15 nm protective layer (6). For clarity, Figure 6 is shown without error bars, but due to statistical fluctuations in the Monte Carlo method, the values ​​represented may be higher or lower. Protons may not be detected due to backscattering when they impart too little energy to the active region of the detector or when they stop completely within the inactive layer.

[0073] For proton energies of 15 keV, the pLGAD outperforms uncooled DEPFET sensors with the same pixel layout, achieving nearly the same detection efficiency as their cooled counterparts. In a best-case scenario, proton losses on the order of 0.15% (or a detection efficiency of 99.85%) appear possible.

Claims

1. A low penetration particle low gain avalanche detector having a multi-layer structure configured to receive particles from a radiation source (13), comprising: an entry area configured to receive the particles from the radiation source (13), an entry region comprising a shallow p++ field stop (1); The low invasive particle detection area is as follows: a p-absorber layer (3) located below the shallow p++ field stop (1); a low-penetration particle detection region comprising an n-enhancement layer (4) located below the p-absorption layer (3); A highly invasive particle detection area, a highly invasive particle detection region below the low invasive particle detection region, comprising an n-- silicon substrate (5) following the n-enhancement layer (4); a readout region in contact with the highly invasive particle detection region; A peripheral area, p+ termination dopings (2) laterally disposed on either side of said shallow p++ field stop (1); a peripheral region comprising one or more metal contacts (7a) bonded to said shallow p++ field stop (1); A detector comprising:

2. 2. The detector of claim 1, wherein the entry region further comprises a protective layer (6) disposed above the shallow p++ field stop (1).

3. Detector according to claim 2, characterized in that the protective layer (6) is a conversion layer for incoming neutrons.

4. 3. The detector of claim 2, wherein the peripheral region further comprises a silicon oxide layer (8) disposed below the protective layer (6).

5. Detector according to claim 1, wherein the shallow p++ field stop (1) is less than 100 nanometers thick.

6. Detector according to claim 1, wherein the n-enhancing layer (4) is in the range of 1 to 5 micrometers thick.

7. Detector according to claim 1, wherein said n-- silicon substrate (5) is in the range of thickness from 10 to 1000 μm.

8. Detector according to claim 1, wherein the peripheral region further comprises n++ ohmic contacts (9) located on either side of the p-absorbing layer (3).

9. The detector of claim 1 , wherein the readout region further comprises a segment electrode following the high invasive particle detection region and opposite the low invasive particle detection region.

10. 10. The detector of claim 9, wherein the segment electrode comprises an n++ contact (10) in contact with the n-- silicon substrate (5) and an additional metal contact (12) connected to the n++ contact (10).

11. 10. The detector of claim 9, wherein the readout regions further comprise p+ stops (11) alternating with the segment electrodes and in contact with the n-- silicon substrate (5).

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