A large-area high-position-resolution ultra-pure high-resistance silicon pixel detector and a design method thereof

Abstract

The application discloses a large-area high-position-resolution ultra-pure high-resistance silicon pixel detector and a design method thereof. The silicon pixel detector comprises a right quadrangular prism-shaped silicon base body, a square central anode is etched on the upper bottom surface of the silicon base body, a square central cathode is etched in the middle of the lower bottom surface of the silicon base body, the center of the central anode and the center of the central cathode are overlapped, and a silicon dioxide base body is arranged outside the central cathode. The upper and lower collecting cathode and anode of the silicon pixel detector are similar to parallel-plate capacitors, the calculation method of the parallel-plate capacitors can be used to obtain the optimal wafer thickness, the voltage of the full depletion of the structure is lower, the probability of the chip being biased to be broken down is reduced, the electrode process difficulty is reduced, the chip yield is high, the workload of the external circuit is reduced, the packaging difficulty is reduced, the structure is simple, and the chip process manufacturing and external packaging are easy.

Description

Technical Field

[0001] This invention belongs to the field of medical detection technology and relates to a large-area, high-position-resolution, ultra-pure, high-resistivity silicon pixel detector and its design method. Background Technology

[0002] Currently, detectors are widely used in high-energy physics, astrophysics, aerospace, military, and medical fields. Medical conditions in the medical field place strict requirements on the detectors themselves, namely, high position resolution, and different requirements on the size of the detector array splicing area. Traditional silicon drift detectors have many shortcomings. First, the position resolution of traditional silicon drift detector units cannot meet the accuracy required in the medical field, and the clarity of the detector imaging area is not high. Due to design problems, the splicing area of ​​traditional silicon drift detectors is not large enough, resulting in low position resolution of the entire detector. Therefore, it is urgent to improve the detector structure to meet the needs of medical use.

[0003] Currently, the design of silicon pixel detectors relies on simulation software to calculate the desired dimensions, which is a single source of information. After the detector device is manufactured, its performance, such as charge collection rate, needs to be tested. If the results deviate from the simulation calculations, the simulation parameters need to be readjusted and the detector device needs to be remanufactured. This design process is cumbersome and time-consuming. Furthermore, it is impossible to accurately determine the charge collection rate before the detector is manufactured; only structural factors can indicate that the charge collection rate is low. The design method of this invention can verify simulation calculations, identify problems in advance, reduce repeated manufacturing, and thus simplify the design process. Summary of the Invention

[0004] To achieve the above objectives, the present invention provides a large-area, high-position-resolution, ultra-pure, high-resistivity silicon pixel detector, which solves the problems of low position resolution and insufficient array size of existing detectors.

[0005] This invention also provides a design method for a large-area, high-position-resolution, ultra-pure, high-resistivity silicon pixel detector. This method simplifies the overall design process of the detector, provides data reference for detector testing, and solves the problems of cumbersome and time-consuming silicon pixel detector design process.

[0006] The technical solution adopted in this invention is a large-area, high-position-resolution, ultra-pure, high-resistivity silicon pixel detector, characterized in that it includes a regular square prism-shaped silicon substrate, a square central anode etched on the bottom surface of the silicon substrate, a square central cathode etched in the middle of the bottom surface of the silicon substrate, the center of the central anode and the center of the central cathode overlapping, and a silicon dioxide substrate provided on the outside of the central cathode.

[0007] The upper and lower collecting cathodes and anodes are similar to those of a parallel plate capacitor.

[0008] Furthermore, the central anode has dimensions of 150μm×150μm×20μm, the central cathode has dimensions of 80μm×80μm×20μm, and the vertical distance between the central anode and the central cathode is 260μm.

[0009] Furthermore, the silicon substrate is N-type lightly doped silicon, the central anode is N-type heavily doped silicon, and the central cathode is P-type heavily doped silicon.

[0010] Furthermore, the doping concentration of the silicon substrate is 8 × 10⁻⁶. 11 cm -3 ~1×10 12 cm -3 The doping concentration of both the central anode and the central cathode is 8 × 10⁻⁶. 18 cm -3 ~1×10 19 cm -3 .

[0011] A design method for a large-area, high-position-resolution, ultra-pure, high-resistivity silicon pixel detector, specifically including the following steps:

[0012] S1, determine the relationship between potential and detector thickness;

[0013] The method for calculating charge collection efficiency is as follows:

[0014] Given the induced current i and the charge drift velocity V dr The calculation formula is as follows:

[0015]

[0016]

[0017] Where q is the drift charge number and μ is a constant. Electric potential is a vector quantity. The specific gravity field is a vector, V dr Let E(x(t)) be the charge drift velocity, E(t) be the drift time of the incident charge carrier, E(x(t)) be the potential at x(t) and E(t) be a scalar, and x(t) be the drift distance of the incident charge carrier after drift time t.

[0018] The central anode (1) and central cathode (3) of the silicon pixel detector are respectively regarded as the two plates of a parallel plate capacitor. The relationship between their potential and the detector thickness is as follows: E(x(t)) is the electric potential at x(t), a scalar quantity; d is the distance between the central anode and the central cathode; and x(t) is the drift distance of the incident carriers after time t. min The minimum potential, i.e., the potential at the central cathode, is a scalar, E. max The maximum potential, i.e., the potential at the central anode, is a scalar quantity;

[0019] S2, determine the wafer thickness;

[0020] The relationship between the collected charge and the electrode spacing is The formula shows that the collected charge is inversely proportional to the electrode spacing. Therefore, the smaller the electrode spacing, the more charge is collected per unit time, i.e., the higher the charge collection rate. Ideally, the electrode spacing can be smaller than but infinitely close to the wafer thickness. Generally, the thinner the silicon wafer, the greater the difficulty of the process. For example, ion implantation may break through the silicon wafer and fail to achieve the implantation effect. In this invention, we selected a wafer with a thickness of 300μm as the substrate for the detector chip.

[0021] Furthermore, S2 specifically includes:

[0022] Substitute E(x(t)) into the induced current i and the charge drift velocity V respectively. dr The calculation formula can be obtained as follows:

[0023]

[0024]

[0025] Let x(t) be the electric potential at point x(t), which is a vector. It is a specific gravity field, and is a vector.

[0026] Formula (2) can be transformed into

[0027] t is a time point, and c is an unknown constant;

[0028] make Y is an intermediate variable, representing the formula that follows;

[0029] but Substituting Y and dx(t) into formula (3), we get:

[0030]

[0031] make D is an intermediate variable, representing the formula that follows;

[0032] From formula (4), we can obtain: Then Y = μ(Dx(t) + E) max (5),

[0033] Formula (5) can be transformed into:

[0034] e is the natural constant e;

[0035] When t is the time t_c of the carrier drifting from the central cathode to the central anode dr Or the time t for the particle to drift from the central anode to the central cathode dr Then, the above formula can be transformed into:

[0036]

[0037] After conversion formula (6), we get Integrating the induced current over time to collect the charge, we can obtain... i(t) is the drift current at time t, and Q represents the current from 0 to t. dr The charge collected at any given time can be derived from the formula to show that the collected charge is inversely proportional to the distance between the electrodes. Therefore, the smaller the distance between the electrodes per unit time, the more charge is collected, that is, the higher the charge collection rate.

[0038] The beneficial effects of this invention are that it proposes a detector structure whose upper and lower collecting cathodes and anodes are similar to those of a parallel-plate capacitor, characterized by easy external packaging and processing. Its low total depletion voltage reduces the probability of chip breakdown due to bias voltage, resulting in high chip yield, reduced external circuitry workload, and lower packaging difficulty. The detector structure of this invention is simple and easy to manufacture and externally package.

[0039] This invention provides a detector size design flow algorithm, which simplifies the design process of silicon pixel detector structure size and shortens the time. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 This is an array diagram of the detectors.

[0042] Figure 2 This is a 3D view of the detection unit.

[0043] Figure 3 This is a front view of the detection unit.

[0044] Figure 4 This is a density field diagram of the detector unit.

[0045] Figure 5 yes Figure 4 Graph showing the change in potential of the cross-section.

[0046] Figure 6 This is a potential distribution diagram of the detector.

[0047] Figure 7 This is the detector's fully depleted voltage diagram.

[0048] In the diagram, 1. Central anode, 2. Silicon substrate, 3. Central cathode, 4. Silicon dioxide substrate. Detailed Implementation

[0049] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0050] A large-area, high-position-resolution, ultra-pure, high-resistivity silicon pixel detector, with the structure as follows: Figure 1 As shown, it is composed of an array of several detection units, and the structure of the detection unit is as follows: Figure 2 , 3 As shown, it includes a regular square prism-shaped silicon substrate 2, a square central anode 1 etched on the bottom surface of the silicon substrate 2, a square central cathode 3 etched in the middle of the bottom surface of the silicon substrate 2, the center of the central anode 1 and the center of the central cathode 3 overlap, and a silicon dioxide substrate 4 is provided on the outside of the central cathode 3.

[0051] This structure, with its upper and lower cathode and anode collection points, is similar to a parallel-plate capacitor. The calculation methods used for parallel-plate capacitors can be applied to determine the optimal wafer thickness. For example... Figure 7 The diagram shows the full depletion voltage of the detector. The lines in the diagram represent voltages of 4V, 6V, 8V, 10V, and 12V from the upper right to the lower left. The full depletion voltage of the detector of this invention is 8.5V. This structure has a lower full depletion voltage, which reduces the probability of the chip being broken down by bias voltage, reduces the difficulty of electrode manufacturing, increases the chip yield, reduces the workload of external circuitry, and reduces packaging difficulty. Its structure is simple and easy to manufacture and package.

[0052] This invention is used in the medical field. It eliminates the need to consider the space charge type transition effect that easily occurs in N-type materials under strong radiation, and uses lightly doped N-type silicon as the silicon substrate 2, with a doping concentration of 8 × 10⁻⁶. 11 cm -3 ~1×10 12 cm -3 Central anode 1 is N-type heavily doped silicon, and central cathode 3 is P-type heavily doped silicon. The doping concentration of both central anode 1 and central cathode 3 is 8 × 10⁻⁶. 18 cm -3 ~1×10 19 cm -3During the detection process, the central anode 1 provides electron carriers, and the central cathode 3 provides hole carriers. The dimensions of the detection unit are 150μm × 150μm × 300μm. In general pixel detectors, the smaller the surface area of ​​the top electrode, the smaller the capacitance. This invention reduces the anode electrode area. The dimensions of the central anode 1 are 150μm × 150μm × 20μm, and the dimensions of the central cathode 3 are 80μm × 80μm × 20μm. The central anode 1 and central cathode 3 serve as carrier collection electrodes. If the dimensions of the central anode 1 and central cathode 3 are too large or too small, it will increase the difficulty of the manufacturing process. If the collection surface is too small, it will affect the carrier collection efficiency, that is, it will be impossible to collect all the excited carriers. The reduced carrier collection efficiency will affect the chip sensitivity. The vertical distance between the central anode 1 and the central cathode 3 is 260μm. If the vertical distance is too high, it will reduce the charge collection rate, thereby reducing the induced current. Figure 6 The diagram shows the potential distribution of the detector unit. As can be seen from the diagram, the detector potential distribution is uniform and there is no obvious dead zone. This means that the detector structure has no obvious defects and is highly feasible to manufacture.

[0053] When using this invention for radiation ion detection, after applying voltage to the central anode 1 and central cathode 3, radiation ions pass through the central anode 1, central cathode 3, and silicon substrate 2. The central anode 1 and silicon substrate 2 provide electron carriers. Electrons drift towards the central anode 1, and holes drift towards the central cathode 3, generating current signals in the central cathode 3 and central anode 1. These signals are displayed through an external circuit. This invention sets the detection unit as a cuboid with square upper and lower surfaces, which makes the surface shape after splicing more regular. Different array shapes can be spliced ​​according to requirements. The area of ​​the array increases after splicing, which can improve the position resolution of the detector.

[0054] Currently, charge collection efficiency is obtained by manufacturing physical detectors and testing data. This invention can calculate the charge collection efficiency of silicon pixel detectors in real time during the design process, eliminating the need for simulation software calculations. It can verify the accuracy of software calculations, provide a theoretical basis for physical detector testing, and simplify the design process of silicon pixel detectors by adjusting the structural dimensions according to usage requirements, thus reducing the time required.

[0055] The method for calculating charge collection efficiency is as follows:

[0056] Given the induced current i and the charge drift velocity V dr The calculation formula is as follows:

[0057]

[0058]

[0059] Where q is the drift charge number and μ is a constant. Electric potential is a vector quantity. The specific gravity field is a vector, V dr Let E(x(t)) be the charge drift velocity, E(t) be the drift time of the incident charge carrier, E(x(t)) be the potential at x(t) and E(t) be a scalar, and x(t) be the drift distance of the incident charge carrier after drift time t.

[0060] The central anode 1 and central cathode 3 of the silicon pixel detector are respectively regarded as the two plates of a parallel plate capacitor, and the specific gravity field diagram of the detector unit is as follows. Figure 4 As shown, Figure 4 The specific gravity field diagram of the detector unit was calculated using simulation software under a special potential with an external bias voltage of 1V and a silicon dioxide substrate. Figure 4 The centerline intercept value is obtained Figure 5 Black cut-off line, Figure 5 yes Figure 4 The physical meaning of the potential change diagram of the cross-section is: Figure 4 The numerical variation line shows that the specific gravity field is a special type of electric potential, thus the relationship between potential and thickness is derived as follows: Figure 5 As shown,

[0061] E(x(t)) is the electric potential at x(t), a scalar quantity; d is the distance between the central anode and the central cathode; and x(t) is the drift distance of the incident carriers after time t. min The minimum potential, i.e., the potential at the central cathode, is a scalar, E. max The maximum potential, i.e., the potential at the central anode, is a scalar quantity;

[0062] Substitute E(x(t)) into the induced current i and the charge drift velocity V respectively. dr The calculation formula can be obtained as follows:

[0063]

[0064]

[0065] Let x(t) be the electric potential at point x(t), which is a vector. It is a specific gravity field, and is a vector.

[0066] Formula (2) can be transformed into

[0067] t is a time point, and c is an unknown constant;

[0068] make

[0069] Y is an intermediate variable, representing the formula that follows;

[0070] but Substituting Y and dx(t) into formula (3), we get:

[0071]

[0072] make

[0073] D is an intermediate variable, representing the formula that follows;

[0074] From formula (4), we can obtain: Then Y = μ(Dx(t) + E) max (5),

[0075] Formula (5) can be transformed into:

[0076] e is the natural constant e;

[0077] When t is the time t_c of the carrier drifting from the central cathode to the central anode dr Or the time t for the particle to drift from the central anode to the central cathode dr Then, the above formula can be transformed into:

[0078]

[0079] After conversion formula (6), we get Integrating the induced current over time to collect the charge, we can obtain... i(t) is the drift current at time t, and Q represents the current from 0 to t. dr The charge collected at any given time. Through formula derivation, it can be seen that the collected charge is inversely proportional to the electrode spacing. Therefore, the smaller the electrode spacing per unit time, the more charge is collected, i.e., the higher the charge collection rate.

[0080] A wafer refers to a thin, undoped, oxide-free, high-purity single-crystal silicon wafer of a certain thickness, which is the raw material for detector chips. The wafer thickness determines the maximum value of the electrode spacing. Electrodes are embedded in the wafer; ideally, the electrode spacing can be less than but infinitely close to the wafer thickness. Generally, the thinner the silicon wafer, the greater the manufacturing difficulty. For example, ion implantation may break down the silicon wafer, failing to achieve the desired implantation effect. In this invention, we select a 300μm thick wafer as the detector chip substrate, which allows for higher charge collection efficiency.

[0081] The present invention, through the above method, can determine the relationship between charge collection efficiency and detector thickness during the silicon pixel detector design process, thereby enabling the selection of wafer thickness.

[0082] The various embodiments in this specification are described in a related manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions of the method embodiments.

[0083] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of protection of the present invention.

Claims

1. A large area high position resolution ultra-pure high resistivity silicon pixel detector, characterized in that, The silicon substrate (2) includes a right quadrangular prism, a square central anode (1) is etched on the bottom surface of the silicon substrate (2), a square central cathode (3) is etched in the middle of the lower bottom surface of the silicon substrate (2), the centers of the central anode (1) and the central cathode (3) are overlapped, and a silicon dioxide substrate (4) is arranged outside the central cathode (3); The length, width and height of the central anode (1) are 150µm×150µm×20µm, the length, width and height of the central cathode (3) are 80µm×80µm×20µm, and the vertical distance between the central anode (1) and the central cathode (3) is 260µm; The silicon substrate (2) is N-type lightly doped silicon, the central anode (1) is N-type heavily doped silicon, and the central cathode (3) is P-type heavily doped silicon; The doping concentration of the silicon substrate (2) is 8 × 10⁻⁶. 11 cm -3 ~1×10 12 cm -3 The doping concentration of both the central anode (1) and the central cathode (3) is 8 × 10⁻⁶. 18 cm -3 ~1×10 19 cm -3 .

2. The method of designing a large area high position resolution ultra-pure high resistivity silicon pixel detector according to claim 1, wherein, Specifically, the method comprises the following steps: S1, determining the relationship between the potential and the thickness of the detector; Known induced current And the charge drift velocity The calculation formula is as follows: ； ； wherein is the number of drift charges, is a constant, is the electric potential, is a vector, is the specific gravity field, is a vector, is the drift velocity of the electric charge, drift time of the charge carrier, is is the electric potential at the point, is a scalar, is the drift distance of the incident charge carrier after the time instant, is the drift distance of the incident charge carrier after the time instant, The central anode (1) and central cathode (3) of the silicon pixel detector are respectively regarded as the two plates of a parallel plate capacitor, and their potential is related to the detector thickness as follows: , for The electric potential at a point is a scalar quantity. The distance between the central anode and the central cathode. For incident carrier drift Drift distance after the time point, The minimum potential, i.e., the potential at the central cathode, is a scalar. The maximum potential, i.e., the potential at the central anode, is a scalar quantity; S2, determining the thickness of the wafer; The relationship between the collected charge and the electrode spacing is , for Drift current at any moment Indicates from 0 to The charge collected at any time can be derived from the formula to show that the collected charge is inversely proportional to the electrode spacing. Therefore, the smaller the electrode spacing per unit time, the more charge is collected, that is, the higher the charge collection rate. Ideally, the electrode spacing can be smaller than but infinitely close to the wafer thickness. Therefore, a wafer with a thickness of 300µm is selected as the substrate of the detector chip.

3. The design method of a large-area high-position-resolution ultra-pure high- resistance silicon pixel detector according to claim 2, characterized in that, The S2 specifically comprises: Substituting the calculated values of the induced current and the charge drift velocity into the formula gives: （1）； （2）； For The potential at a point is a vector, The specific gravity field is a vector; Equation (2) can be transformed into (3); t is a time node, and c is an unknown constant; Let Y is an intermediate variable, representing the formula below; Then Substituting Y and into equation (3) gives: （4）； Let , D is an intermediate variable, representing the formula below; From equation (4) we have: , then (5); Equation (5) can be converted to: ; e is a natural constant e; When the time for a carrier to drift from the central cathode to the central anode or from the central anode to the central cathode then the above equation can be transformed into: （6）； The converted formula (6) is obtained The time integral of the collected charge of the induced current is obtained , is the drift current at time t, represents the collected charge from 0 to time t, through formula derivation, it can be obtained that the collected charge is inversely proportional to the electrode spacing, so the smaller the electrode spacing, the more the collected charge per unit time, that is, the higher the charge collection rate.

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