This relates generally to imaging systems and, more particularly, to positron emission tomography (PET) imaging systems.
Positron emission tomography is a functional imaging technique that uses a radioactive tracer to show how tissues and organs in the human body are functioning. Positron emission tomography has many medical applications. For example, positron emission tomography allows for examination of chemical activity in the body, which may be useful to detect cancer, heart disease, brain disorders, etc.
Positron emission tomography may use an imaging system to sense the position of a radioactive tracer.
It is within this context that the embodiments described herein arise.
Embodiments of the present technology relate to imaging systems that include single-photon avalanche diodes (SPADs).
Some imaging systems include image sensors that sense light by converting impinging photons into electrons or holes that are integrated (collected) in pixel photodiodes within the sensor array. After completion of an integration cycle, collected charge is converted into a voltage, which is supplied to the output terminals of the sensor. In complementary metal-oxide semiconductor (CMOS) image sensors, the charge to voltage conversion is accomplished directly in the pixels themselves, and the analog pixel voltage is transferred to the output terminals through various pixel addressing and scanning schemes. The analog pixel voltage can also be later converted on-chip to a digital equivalent and processed in various ways in the digital domain.
In single-photon avalanche diode (SPAD) devices (such as the ones described in connection with
This concept can be used in two ways. First, the arriving photons may simply be counted (e.g., in low light level applications). Second, the SPAD pixels may be used to measure photon time-of-flight (ToF) from a synchronized light source to a scene object point and back to the sensor, which can be used to obtain a 3-dimensional image of the scene.
Quenching circuitry 206 (sometimes referred to as quenching element 206) may be used to lower the bias voltage of SPAD 204 below the level of the breakdown voltage. Lowering the bias voltage of SPAD 204 below the breakdown voltage stops the avalanche process and corresponding avalanche current. There are numerous ways to form quenching circuitry 206. Quenching circuitry 206 may be passive quenching circuitry or active quenching circuitry. Passive quenching circuitry may, without external control or monitoring, automatically quench the avalanche current once initiated. For example,
This example of passive quenching circuitry is merely illustrative. Active quenching circuitry may also be used in SPAD device 202. Active quenching circuitry may reduce the time it takes for SPAD device 202 to be reset. This may allow SPAD device 202 to detect incident light at a faster rate than when passive quenching circuitry is used, improving the dynamic range of the SPAD device. Active quenching circuitry may modulate the SPAD quench resistance. For example, before a photon is detected, quench resistance is set high and then once a photon is detected and the avalanche is quenched, quench resistance is minimized to reduce recovery time.
SPAD device 202 may also include readout circuitry 212. There are numerous ways to form readout circuitry 212 to obtain information from SPAD device 202. Readout circuitry 212 may include a pulse counting circuit that counts arriving photons. Alternatively or in addition, readout circuitry 212 may include time-of-flight circuitry that is used to measure photon time-of-flight (ToF). The photon time-of-flight information may be used to perform depth sensing. In one example, photons may be counted by an analog counter to form the light intensity signal as a corresponding pixel voltage. The ToF signal may be obtained by also converting the time of photon flight to a voltage. The example of an analog pulse counting circuit being included in readout circuitry 212 is merely illustrative. If desired, readout circuitry 212 may include digital pulse counting circuits. Readout circuitry 212 may also include amplification circuitry if desired.
The example in
Because SPAD devices can detect a single incident photon, the SPAD devices are effective at imaging scenes with low light levels. Each SPAD may detect the number of photons that are received within a given period of time (e.g., using readout circuitry that includes a counting circuit). However, as discussed above, each time a photon is received and an avalanche current initiated, the SPAD device must be quenched and reset before being ready to detect another photon. As incident light levels increase, the reset time becomes limiting to the dynamic range of the SPAD device (e.g., once incident light levels exceed a given level, the SPAD device is triggered immediately upon being reset).
Multiple SPAD devices may be grouped together to help increase dynamic range.
Each SPAD device 202 may sometimes be referred to herein as a SPAD pixel 202. Although not shown explicitly in
The example of
While there are a number of possible use cases for SPAD pixels as discussed above, the underlying technology used to detect incident light is the same. All of the aforementioned examples of devices that use SPAD pixels may collectively be referred to as SPAD-based semiconductor devices. A silicon photomultiplier with a plurality of SPAD pixels having a common output may be referred to as a SPAD-based semiconductor device. An array of SPAD pixels with per-pixel readout capabilities may be referred to as a SPAD-based semiconductor device. An array of silicon photomultipliers with per-silicon-photomultiplier readout capabilities may be referred to as a SPAD-based semiconductor device.
It will be appreciated by those skilled in the art that silicon photomultipliers include major bus lines 44 and minor bus lines 45 as illustrated in
An imaging system 10 with a SPAD-based semiconductor device is shown in
PET imaging system 10 may include one or more detector blocks 52. Each detector block 52 may include one or more detector units 54. Each detector unit may include a respective SPAD-based semiconductor device 14 (sometimes referred to as semiconductor device 14, device 14, SPAD-based image sensor 14, image sensor 14, silicon multiplier 14, etc.) and crystal 56 (sometimes referred to as scintillator 56). Crystal 56 may absorb ionizing radiation such as gamma rays (e.g., caused by a radioactive tracer used in the PET imaging system) and emit light in the visible spectrum (e.g., blue light).
Crystal 56 may be formed from lutetium-yttrium oxyorthosilicate (LYSO) or any other desired material.
One or more lenses may optionally cover each semiconductor device 14. During operation, lenses (sometimes referred to as optics) may focus light onto scintillator 46 and/or SPAD-based semiconductor device 14. SPAD-based semiconductor device 14 may include SPAD pixels that convert the light into digital data. The SPAD-based semiconductor device may have any number of SPAD pixels (e.g., hundreds, thousands, millions, or more). In some SPAD-based semiconductor devices, each SPAD pixel may be covered by a respective color filter element and/or microlens.
The imaging system may include circuitry such as control circuitry. Each SPAD-based semiconductor device may optionally include respective control circuitry. Alternatively or in addition, each detector block and/or the imaging system may include control circuitry. The control circuitry for each SPAD-based semiconductor device may be formed either on-chip (e.g., on the same semiconductor substrate as the SPAD devices) or off-chip (e.g., on a different semiconductor substrate as the SPAD devices). The control circuitry may control operation of the SPAD-based semiconductor device. For example, the control circuitry may operate active quenching circuitry within the SPAD-based semiconductor device, may control a bias voltage provided to bias voltage supply terminal 208 of each SPAD, may control/monitor the readout circuitry coupled to the SPAD devices, etc.
Each SPAD-based semiconductor device 14 may optionally include additional circuitry such as logic gates, digital counters, time-to-digital converters, bias circuitry (e.g., source follower load circuits), sample and hold circuitry, correlated double sampling (CDS) circuitry, amplifier circuitry, analog-to-digital (ADC) converter circuitry, data output circuitry, memory (e.g., buffer circuitry), address circuitry, etc. Any of the aforementioned circuits may be considered part of the control circuitry.
Image data from SPAD-based semiconductor device 14 may be provided to image processing circuitry 16. Image processing circuitry 16 may be used to perform image processing functions for PET imaging system 10. In some cases, some or all of the control circuitry within PET imaging system 10 may be formed integrally with image processing circuitry 16.
Imaging system 10 may provide a user with numerous high-level functions. To implement these functions, the imaging system may include input-output devices 22 such as keypads, buttons, input-output ports, joysticks, and displays (e.g., touch-sensitive displays). Additional storage and processing circuitry such as volatile and nonvolatile memory (e.g., random-access memory, flash memory, hard drives, solid state drives, etc.), microprocessors, microcontrollers, digital signal processors, application specific integrated circuits, and/or other processing circuits may also be included in the imaging system.
The subject may be injected with a radiotracer 64. Any desired radiotracer may be used. In general, a radiotracer is formed from a common biological molecule (e.g., glucose, peptides, proteins, etc.) with a radioisotope substituted for one of the components of the molecule. Fluorodeoxyglucose (FDG) is one example of a radiotracer that may be used for PET applications.
After the radiotracer is injected into the subject, the radiotracer arrives at a target organ and participates in the metabolic process of the subject. The radiotracer decays, causing positrons to be generated. The positrons from the radiotracer collide with electrons of neighboring atoms in an annihilation process. The annihilation generates two gamma rays 66.
The scintillators in detector units 54 of detector blocks 52 may absorb the gamma rays 66 and generate light that is then sensed by the SPAD-based semiconductor devices 14. Data from the SPAD-based semiconductor devices may therefore be used to reconstruct a PET image.
The PET imaging system may include any desired number of detector blocks (e.g., 1, more than 1, more than 5, more than 10, more than 20, more than 30, less than 100, less than 50, between 20 and 50, between 25 and 35, etc.). Each detector block may include any desired number of detector units (e.g., 1, more than 1, more than 5, more than 10, more than 30, more than 100, more than 1,000, less than 5,000, less than 1,000, less than 100, less than 30, etc.).
An incoming gamma ray 66 (caused by the radiotracer as shown in connection with
A transparent layer 74 may be formed over the anti-reflective stack. Transparent layer 74 may cover and protect the underlying SPAD-based sensor 70. Transparent layer 74 may sometimes be referred to as cover layer 74 and may be formed from glass, plastic, or any other desired transparent material. An optical grease layer 76 may be interposed between glass layer 74 and scintillator 56. The optical grease may have an index of refraction that is between the indices of refraction of crystal 56 and transparent layer 74. The optical grease layer 76 may ensure that there is no air gap between crystal 56 and transparent layer 74. Optical grease 76 may be formed from any desired material. As one example, optical grease 76 may be formed from an organic material having an index of refraction between 1.4 and 1.6. The index of refraction of optical grease 76 may be greater than the index of refraction of transparent layer 74 (e.g., between 1.3 and 1.5) and less than the index of refraction of crystal 56 (e.g., between 1.7 and 1.9). These examples are merely illustrative. In general, each component may have any desired index of refraction.
The visible light rays incident upon anti-reflective layer 72 tend to have a high angle of incidence. On-axis light may refer to light that travels parallel to the Z-axis in
The visible light generated by scintillator 56 may predominantly have a high angle of incidence upon anti-reflective layer 72.
The anti-reflective stack 72 may be optimized based on the type of incident light received.
To improve transmittance in imaging system 10, the anti-reflective stack may be optimized for high angle of incidence light. This type of anti-reflective stack (e.g., a high-angle optimized anti-reflective stack) may have a transmittance profile 84. As shown, the transmittance remains higher across a broader range of angles than in profile 82. However, the maximum transmittance (even at the optimal high angles) is less than 90% with this type of arrangement. This may result in the efficiency of imaging system 10 being lower than desired.
A key metric for PET imaging systems is coincidence resolving time (CRT). Improving the transmittance of light from crystal 56 through anti-reflective stack 72 into SPAD-based sensor 70 improves (reduces) the coincidence resolving time for imaging system 10.
Therefore, to improve performance in imaging system 10, the SPAD-based sensor may include a plurality of pyramids etched into its upper surface. This may ensure that light is incident upon the anti-reflective stack at lower angles, improving transmittance through the anti-reflective stack.
As shown in
Semiconductor layer 92 (used to form sensor 70) has an upper surface 94. As shown in
Because of the presence of recesses 270 (which include angled sidewalls), incident light 68 (which approaches upper surface 94 at high angles) is incident upon anti-reflective stack 72 at an angle 102 that is close to 90 degrees (e.g., close to on-axis).
A planarization layer 104 may also be formed in recesses 270. Planarization layer 104 may be formed from silicon dioxide or any other desired material. Planarization layer 104 is interposed between anti-reflective stack 72 and transparent layer 74.
Recesses 270 may have any desired size and shape. Recesses 270 may be pyramid shaped structures (e.g., rectangular-based pyramids such as square-based pyramids, diamond-based pyramids, triangular-based pyramids, etc.). The recesses may be defined by sidewalls 108 that are etched into semiconductor substrate 92. Sidewalls 108 may be at an angle 106 relative to the planar upper surface 94 of semiconductor substrate 92. Angle 106 may be between 30 degrees and 70 degrees, between 40 degrees and 60 degrees, between 45 degrees and 55 degrees, between 50 degrees and 55 degrees, greater than 30 degrees, less than 70 degrees, etc.
As discussed above, the average angle 110 of incoming light 68 relative to on-axis light (e.g., 0 degree light) may be 50 degrees, between 40 degrees and 60 degrees, between 45 degrees and 55 degrees, between 30 degrees and 70 degrees, etc.
The difference between angle 106 of the recess-defining sidewall and the average angle 110 of incident light 68 may be less than 20 degrees, less than 10 degrees, less than 5 degrees, less than 3 degrees, less than 1 degree, etc. Minimizing this difference results in incident light 68 reaching the sidewalls 108 at angle 102 that may be between 80 degrees and 100 degrees, between 70 degrees and 110 degrees, etc. In other words, the average angle of incidence for light 68 upon sidewalls 108 is within 20 degrees, within 10 degrees, within 5 degrees, within 3 degrees, within 1 degree, etc. of the surface normal for sidewalls 108 (e.g., on-axis for sidewalls 108).
Because pyramidal recesses 270 cause incident light to be incident at on-axis angles relative to the anti-reflective stack (instead of off-axis angles), the on-axis optimized anti-reflective stack 72 (e.g., having transmittance profile 82 in
Recesses 270 may be formed using trenches (e.g., etched trenches) or using any other desired structures/techniques. The trenches may extend from surface 94 (e.g., an upper surface) towards the opposing surface (e.g., a lower surface) of semiconductor layer 92. The recesses each have a height 272 (sometimes referred to as depth) and a width 274. The recesses also have a pitch 276 (e.g., the center-to-center separation between each recess). In general, each recess may have a height 272 of less than 5 micron, less than 3 micron, less than 2 micron, less than 1 micron, less than 0.5 micron, less than 0.2 micron, less than 0.1 micron, greater than 0.01 micron, greater than 0.5 micron, greater than 1 micron, between 1 and 2 micron, between 0.5 and 3 micron, between 0.3 micron and 10 micron, between 0.01 micron and 0.2 micron, etc. Each recess may have a width 274 of less than 5 micron, less than 3 micron, less than 2 micron, less than 1 micron, less than 0.5 micron, less than 0.2 micron, less than 0.1 micron, greater than 0.01 micron, greater than 0.5 micron, greater than 1 micron, between 1 and 2 micron, between 0.5 and 3 micron, between 0.3 micron and 10 micron, between 0.01 micron and 0.2 micron, etc. The pitch 276 may be less than 5 micron, less than 3 micron, less than 2 micron, less than 1 micron, less than 0.5 micron, less than 0.3 micron, less than 0.1 micron, greater than 0.01 micron, great than 0.1 micron, greater than 0.5 micron, greater than 1 micron, between 1 and 2 micron, between 0.5 and 3 micron, between 0.3 micron and 10 micron, between 0.01 and 0.3 micron, between 0.01 micron and 1 micron, etc.
The ratio of the width 274 to the pitch 276 may be referred to as the duty cycle or the etch percentage for the substrate. The duty cycle (etch percentage) indicates how much unetched substrate is present between each pair of recesses and how much of the upper surface of the substrate is etched to form the recesses. The ratio may be 100% (e.g., each recess is immediately adjacent to surrounding recesses), lower than 100%, lower than 90%, lower than 70%, lower than 60%, greater than 50%, greater than 70%, between (and including) 50% and 100%, etc. The semiconductor substrate 92 may have a thickness of greater than 4 micron, greater than 6 micron, greater than 8 micron, greater than 10 micron, greater than 12 micron, less than 12 micron, between 4 and 10 micron, between 5 and 20 micron, less than 10 micron, less than 6 micron, less than 4 micron, less than 2 micron, greater than 1 micron, etc.
Each SPAD may be covered by any desired number of recesses 270 (e.g., 1 recess, more than 1 recess, more than 4 recesses, more than 9 recesses, more than 25 recesses, more than 50 recesses, more than 100 recesses, more than 300 recesses, more than 1,000 recesses, less than 1,000 recesses, less than 300 recesses, less than 100 recesses, less than 50 recesses, less than 25 recesses, etc.
The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art. The foregoing embodiments may be implemented individually or in any combination.