WO2014202995A1 - Single-Photon Avalanche Diode and an Array thereof - Google Patents

Single-Photon Avalanche Diode and an Array thereof Download PDF

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Publication number
WO2014202995A1
WO2014202995A1 PCT/GB2014/051895 GB2014051895W WO2014202995A1 WO 2014202995 A1 WO2014202995 A1 WO 2014202995A1 GB 2014051895 W GB2014051895 W GB 2014051895W WO 2014202995 A1 WO2014202995 A1 WO 2014202995A1
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array
photon avalanche
spad
spads
subset
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French (fr)
Inventor
Stuart Mcleod
Pascal Mellot
Lindsay Grant
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STMicroelectronics Grenoble 2 SAS
STMicroelectronics Research and Development Ltd
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STMicroelectronics Grenoble 2 SAS
STMicroelectronics Research and Development Ltd
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Priority to US14/781,333 priority Critical patent/US9960295B2/en
Publication of WO2014202995A1 publication Critical patent/WO2014202995A1/en
Priority to US14/870,108 priority patent/US9728659B2/en
Anticipated expiration legal-status Critical
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    • H10F77/00—Constructional details of devices covered by this subclass
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J1/00—Photometry, e.g. photographic exposure meter
    • G01J1/42—Photometry, e.g. photographic exposure meter using electric radiation detectors
    • G01J1/4228—Photometry, e.g. photographic exposure meter using electric radiation detectors arrangements with two or more detectors, e.g. for sensitivity compensation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J1/00—Photometry, e.g. photographic exposure meter
    • G01J1/42—Photometry, e.g. photographic exposure meter using electric radiation detectors
    • G01J1/44—Electric circuits
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/16—Measuring radiation intensity
    • G01T1/24—Measuring radiation intensity with semiconductor detectors
    • G01T1/248—Silicon photomultipliers [SiPM], e.g. an avalanche photodiode [APD] array on a common Si substrate
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    • H10F30/20—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors
    • H10F30/21—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors the devices being sensitive to infrared, visible or ultraviolet radiation
    • H10F30/22—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors the devices being sensitive to infrared, visible or ultraviolet radiation the devices having only one potential barrier, e.g. photodiodes
    • H10F30/225—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors the devices being sensitive to infrared, visible or ultraviolet radiation the devices having only one potential barrier, e.g. photodiodes the potential barrier working in avalanche mode, e.g. avalanche photodiodes
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    • H10F39/00—Integrated devices, or assemblies of multiple devices, comprising at least one element covered by group H10F30/00, e.g. radiation detectors comprising photodiode arrays
    • H10F39/10—Integrated devices
    • H10F39/103—Integrated devices the at least one element covered by H10F30/00 having potential barriers, e.g. integrated devices comprising photodiodes or phototransistors
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    • H10F39/00—Integrated devices, or assemblies of multiple devices, comprising at least one element covered by group H10F30/00, e.g. radiation detectors comprising photodiode arrays
    • H10F39/10—Integrated devices
    • H10F39/107—Integrated devices having multiple elements covered by H10F30/00 in a repetitive configuration, e.g. radiation detectors comprising photodiode arrays
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    • H10F39/00—Integrated devices, or assemblies of multiple devices, comprising at least one element covered by group H10F30/00, e.g. radiation detectors comprising photodiode arrays
    • H10F39/10—Integrated devices
    • H10F39/12—Image sensors
    • H10F39/18—Complementary metal-oxide-semiconductor [CMOS] image sensors; Photodiode array image sensors
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    • H10F39/00—Integrated devices, or assemblies of multiple devices, comprising at least one element covered by group H10F30/00, e.g. radiation detectors comprising photodiode arrays
    • H10F39/80—Constructional details of image sensors
    • H10F39/802—Geometry or disposition of elements in pixels, e.g. address-lines or gate electrodes
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    • H10F39/00—Integrated devices, or assemblies of multiple devices, comprising at least one element covered by group H10F30/00, e.g. radiation detectors comprising photodiode arrays
    • H10F39/80—Constructional details of image sensors
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    • H10F39/00—Integrated devices, or assemblies of multiple devices, comprising at least one element covered by group H10F30/00, e.g. radiation detectors comprising photodiode arrays
    • H10F39/80—Constructional details of image sensors
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    • H10F39/00—Integrated devices, or assemblies of multiple devices, comprising at least one element covered by group H10F30/00, e.g. radiation detectors comprising photodiode arrays
    • H10F39/80—Constructional details of image sensors
    • H10F39/806—Optical elements or arrangements associated with the image sensors
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    • H10F39/00—Integrated devices, or assemblies of multiple devices, comprising at least one element covered by group H10F30/00, e.g. radiation detectors comprising photodiode arrays
    • H10F39/80—Constructional details of image sensors
    • H10F39/811—Interconnections
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    • H—ELECTRICITY
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    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F77/00—Constructional details of devices covered by this subclass
    • H10F77/30—Coatings
    • H10F77/306—Coatings for devices having potential barriers
    • H10F77/331—Coatings for devices having potential barriers for filtering or shielding light, e.g. multicolour filters for photodetectors
    • H10F77/334—Coatings for devices having potential barriers for filtering or shielding light, e.g. multicolour filters for photodetectors for shielding light, e.g. light blocking layers or cold shields for infrared detectors
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    • H10F77/00—Constructional details of devices covered by this subclass
    • H10F77/95—Circuit arrangements
    • H10F77/953—Circuit arrangements for devices having potential barriers
    • H10F77/959—Circuit arrangements for devices having potential barriers for devices working in avalanche mode
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    • H10F39/00—Integrated devices, or assemblies of multiple devices, comprising at least one element covered by group H10F30/00, e.g. radiation detectors comprising photodiode arrays
    • H10F39/10—Integrated devices
    • H10F39/12—Image sensors
    • H10F39/15—Charge-coupled device [CCD] image sensors
    • H10F39/151—Geometry or disposition of pixel elements, address lines or gate electrodes
    • H10F39/1515—Optical shielding

Definitions

  • This application relates to Single-Photon Avalanche Diodes (SPADs) and in particular to SPADs usable in ambient light sensing and ranging applications where a linear output response to illumination conditions is desirable.
  • SPADs Single-Photon Avalanche Diodes
  • a SPAD is based on a p-n junction device biased beyond its breakdown region.
  • the high reverse bias voltage generates a sufficient magnitude of electric field such that a single charge carrier introduced into the depletion layer of the device can cause a self-sustaining avalanche via impact ionisation.
  • the avalanche is quenched, either actively or passively to allow the device to be "reset” to detect further photons.
  • the initiating charge carrier can be photo-electrically generated by means of a single incident photon striking the high field region. It is this feature which gives rise to the name 'Single Photon Avalanche Diode'. This single photon detection mode of operation is often referred to as 'Geiger Mode'.
  • Single photon counting devices output response to incident light intensity becomes significantly non-linear as intensity increases. This is problematic for many applications such as ambient light sensing and ranging.
  • a Single-Photon Avalanche Diode comprising: an active region for the detection of incident radiation; and a cover partially shielding said active region from said incident radiation.
  • Figure 1 shows an array of SPADs each having a different attenuation profile
  • Figures 2a-2c is a plot of the number of photon arrivals in 50 ms (solid lines) and the number of photon counts recorded in 50 ms (broken lines), against light intensity for unfiltered SPADs, and for SPADs having red, green and blue filters, with no attenuation, 1/1.73x attenuation and 1/1 17x attenuation respectively;
  • Figures 3a-3c is a plot of the non-linearity error against light intensity for unfiltered SPADs, and for SPADs having red, green and blue filters, with no attenuation, 1/1 .73x attenuation and 1/1 17x attenuation respectively;
  • Figure 4 is a plot of count rate (minus dark rate count) against light intensity for SPADs attenuated with varying aperture sizes.
  • the avalanche process in solid-state devices has been known for at least fifty years, as has its application to photo-multiplication.
  • An avalanche is triggered when reverse biasing a PN-junction to around the breakdown voltage. This effect can be used in two modes of operation. Commonly, the avalanche photodiodes are biased just below the breakdown voltage, the photocurrent remaining proportional to the incoming light intensity. Gain values of a few hundreds are obtained in lll-V semiconductors as well as in silicon.
  • Single-Photon Avalanche Diodes are solid-state photo detectors which utilise the fact that p-n diodes can be stable for a finite time above their breakdown voltage.
  • p-n diodes can be stable for a finite time above their breakdown voltage.
  • SPADs are photodiodes that are biased above the breakdown voltage in the so-called Geiger mode. This mode of operation requires the introduction of a quenching mechanism to stop the avalanche process. Each incoming photon results in a strong current pulse of few nanoseconds duration. The device works as an optical Geiger counter. Quenching is required to stop the avalanche process, which is done by reducing the SPAD's reverse bias below its breakdown voltage.
  • the simplest quenching circuit is commonly referred to as passive quenching. Usually, passive quenching is simply performed by providing a resistance in series to the SPAD.
  • the avalanche current self-quenches simply because it develops a voltage drop across the resistance (a high-value ballast load), reducing the voltage across the SPAD to below its breakdown voltage. After the quenching of the avalanche current, the SPAD's bias slowly recovers to at or above the breakdown voltage and the detector is ready to be triggered again.
  • active quenching An alternative to passive quenching is active quenching.
  • active quenching refers to detection of a breakdown event by some subsequent digital logic connected to the SPAD output, and actively pulling the SPAD moving node to a voltage below breakdown, quenching the avalanche.
  • Active quenching is desirable for several reasons, including a reduction in dead time, and improved photon counting rate at high light levels enabling a dynamic range extension. Active quenching is essential in many applications of SPAD technology. .
  • Dead time is the time interval after detection of a photon, during which it is quenched and returned to its active state. During this time, no photons are detected by the SPAD. While actively quenching the SPAD will bring it to its active state more quickly after a detection event than with passive quenching, under high light levels (with many detection events) the SPAD can still spend a large fraction of its time inactive. This can lead to saturation and reduction in the number of photons detected by the SPAD. The result of this is that the SPAD output does not track the light level linearly which causes problems in some applications.
  • SPADs have no gain control mechanism to address this problem. It is proposed therefore, to attenuate the SPAD to improve output linearity.
  • FIG. 1 shows an overhead view of a SPAD array 100 coupled to logic circuitry 180. Shown are nine SPADs 1 10a-1 10i. Each SPAD comprises a guard ring region 120, a breakdown region 130, an anode 140 and a track area 150 for connection to the anode. Only photons incident on the breakdown region are detected. The breakdown region is shielded by the anode and the track area reducing its active area. Taking one specific, non-limiting, example, the SPAD may have an effective active area of 48.05pm 2 . Attenuation may be achieved by further reducing the SPAD's active area. This may be done by covering more of the breakdown region with a non-transmissive cover which acts to block any photons incident on the cover surface.
  • One way of doing this is to cover part of the breakdown region with a metal layer when forming the metal track and anode.
  • the metal layer may completely cover the active area, except for one or more apertures 160.
  • the only photons detected by the SPAD are those which pass through the aperture and onto the SPAD's active area.
  • one SPAD 1 e is shown uncovered as is conventional. The remainder are shown with their active areas covered, the covers having various aperture 160 configurations, thereby providing varying degrees of attenuation.
  • the cover of SPAD 100a has two apertures 160 providing 1/1 .73 area attenuation.
  • SPADs 100b, 100c, 100d, 100f, 100g all have covers with aperture 160 arrangements providing 1/7.3 attenuation.
  • SPADs 100b, 100c, 100d, 10Of all have a single aperture located in a different quadrant of the active area of the SPAD, while SPAD 100g has four apertures that are each quarter of the size than that of SPADs 100b, 100c, 100d, 10Of.
  • SPAD 100h has four apertures, each of which provides an attenuation of 1/1 17, thereby providing a total attenuation of 1/29.3.
  • SPAD 10Oi has only one aperture which provides an attenuation of 1/1 17. This particular SPAD array arrangement is shown to illustrate the effect of different attenuation levels.
  • FIG. 2a is a plot of the number of photon arrivals in 50 ms (solid lines) and the number of photon counts recorded in 50 ms (broken lines), against light intensity for clear c, red r, green g and blue b SPADs with no attenuation. The counts are given for a 50ms worst case for flicker immunity.
  • the plot shown is for 6500K CCT illumination.
  • the plot shows significant non-linearity with a large number of missed photons even at 500 lux for the clear SPAD.
  • Green, red and blue are all significantly affected above 1000 lux.
  • clear SPADs have no optical filter, whereas red, green and blue SPADs have optical filters which pass red, green and blue light respectively. Attenuation resultant from the optical filters is discounted when describing a SPAD as having no attenuation.
  • Figure 2b shows a similar graph to that of Figure 2a, but for SPADs having covers providing 1/1.73x attenuation. It shows significantly reduced non-linearity compared to SPADs having no attenuation. The output is essentially linear up to 1000lux for clear SPADs and up to 2500lux for red/green/blue SPADs.
  • Figure 2c shows that same graph for 1/1 17x attenuated SPADs. Non-linearity is greatly reduced for all channels in this case, with the plots for photon arrivals and photon counts largely overlapping, illustrating a linear response.
  • Figures 3a-3c show a plot of the non-linearity error against light intensity for clear c, red r, green g and blue b SPADs, with no attenuation, 1/1 .73x attenuation and 1/1 17x attenuation respectively. It can be seen that 1/1 17x attenuation results in a non-linearity error of less than 1 % (up to 5klux) for red, green and blue SPADs; and a non-linearity error of less than 2% (up to 5klux) for clear SPADs.
  • Figure 4 shows a graph of count rate (corrected for the dark rate count) against light intensity for SPADs attenuated with varying aperture sizes. It can be seen that the area aperture attenuations match actual area reductions.
  • the SPADs disclosed herein will be arranged in arrays.
  • the attenuation levels chosen for particular arrays of SPADs will depend on the specific application, the likely illumination conditions and the acceptable level of non- linearity. For example, if the SPAD array is always likely to be used in situations where illumination levels are high, then an array of SPADs all having high attenuation (such as 1/1 17x attenuation) may be suitable. However, should illumination levels then fall, the detection rate may fall close to zero.
  • an array of SPADs with different attenuation levels may be advantageous. This allows an acceptably linear output to be achievable in a wide range of light conditions.
  • an array may comprise banks of SPADs, one or more having no attenuation, one or more having lesser attenuation (for example 1/1.73x attenuation) and one or more having greater attenuation (for example 1/1 17x attenuation).
  • the unattenuated SPADs will provide a suitably linear response.
  • the output from the 1/1.73x attenuated SPADs should be selected.
  • Suitable logic 180 may be provided to select the appropriate SPAD outputs depending on illumination conditions. There are a number of different ways such logic 180 may be implemented. For example, the outputs of particular banks (having particular attenuation levels) may be selected if their count rate is measured to be over a certain lower threshold level, or between upper and lower threshold levels.
  • SPADs may have, depending on the embodiment, over 40% over 50%, over 75%, over 85%, over 95% or over 99% of the active area covered/shielded.
  • Arrays may comprise any combination of such SPADs.

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Abstract

A sensor includes an array (100) of Single-Photon Avalanche Diodes (SPADs) (110a - HOi). Each SPAD includes an active region (130) for the detection of incident radiation and a cover. The array is divided into two or more subsets of SPADs. The SPADs in the different subsets have covers which shield part of the active region from incident radiation to different degrees.

Description

Single-Photon Avalanche Diode and an Array thereof.
This application relates to Single-Photon Avalanche Diodes (SPADs) and in particular to SPADs usable in ambient light sensing and ranging applications where a linear output response to illumination conditions is desirable.
A SPAD is based on a p-n junction device biased beyond its breakdown region. The high reverse bias voltage generates a sufficient magnitude of electric field such that a single charge carrier introduced into the depletion layer of the device can cause a self-sustaining avalanche via impact ionisation. The avalanche is quenched, either actively or passively to allow the device to be "reset" to detect further photons. The initiating charge carrier can be photo-electrically generated by means of a single incident photon striking the high field region. It is this feature which gives rise to the name 'Single Photon Avalanche Diode'. This single photon detection mode of operation is often referred to as 'Geiger Mode'.
Single photon counting devices output response to incident light intensity becomes significantly non-linear as intensity increases. This is problematic for many applications such as ambient light sensing and ranging.
It would be desirable to provide a SPAD and/or SPAD array with a better linear output response to illumination levels.
In a first aspect of the invention there is provided a Single-Photon Avalanche Diode (SPAD) comprising: an active region for the detection of incident radiation; and a cover partially shielding said active region from said incident radiation.
In a second aspect of the invention there is provided an array of such Single- Photon Avalanche Diodes. BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which:
Figure 1 shows an array of SPADs each having a different attenuation profile;
Figures 2a-2c is a plot of the number of photon arrivals in 50 ms (solid lines) and the number of photon counts recorded in 50 ms (broken lines), against light intensity for unfiltered SPADs, and for SPADs having red, green and blue filters, with no attenuation, 1/1.73x attenuation and 1/1 17x attenuation respectively;
Figures 3a-3c is a plot of the non-linearity error against light intensity for unfiltered SPADs, and for SPADs having red, green and blue filters, with no attenuation, 1/1 .73x attenuation and 1/1 17x attenuation respectively; and
Figure 4 is a plot of count rate (minus dark rate count) against light intensity for SPADs attenuated with varying aperture sizes.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The avalanche process in solid-state devices has been known for at least fifty years, as has its application to photo-multiplication. An avalanche is triggered when reverse biasing a PN-junction to around the breakdown voltage. This effect can be used in two modes of operation. Commonly, the avalanche photodiodes are biased just below the breakdown voltage, the photocurrent remaining proportional to the incoming light intensity. Gain values of a few hundreds are obtained in lll-V semiconductors as well as in silicon.
Single-Photon Avalanche Diodes (SPADs) are solid-state photo detectors which utilise the fact that p-n diodes can be stable for a finite time above their breakdown voltage. When an incident photon with sufficient energy to liberate an electron arrives, avalanche multiplication of the photo-generated electron occurs due to the high electric field. This produces a measurable current pulse signalling the arrival of the photon which negates the need for amplification due to the internal gain of the device.
Essentially SPADs are photodiodes that are biased above the breakdown voltage in the so-called Geiger mode. This mode of operation requires the introduction of a quenching mechanism to stop the avalanche process. Each incoming photon results in a strong current pulse of few nanoseconds duration. The device works as an optical Geiger counter. Quenching is required to stop the avalanche process, which is done by reducing the SPAD's reverse bias below its breakdown voltage. The simplest quenching circuit is commonly referred to as passive quenching. Usually, passive quenching is simply performed by providing a resistance in series to the SPAD. The avalanche current self-quenches simply because it develops a voltage drop across the resistance (a high-value ballast load), reducing the voltage across the SPAD to below its breakdown voltage. After the quenching of the avalanche current, the SPAD's bias slowly recovers to at or above the breakdown voltage and the detector is ready to be triggered again.
An alternative to passive quenching is active quenching. There are a number of different active quenching arrangements, although in general active quenching refers to detection of a breakdown event by some subsequent digital logic connected to the SPAD output, and actively pulling the SPAD moving node to a voltage below breakdown, quenching the avalanche. Active quenching is desirable for several reasons, including a reduction in dead time, and improved photon counting rate at high light levels enabling a dynamic range extension. Active quenching is essential in many applications of SPAD technology. .
Dead time is the time interval after detection of a photon, during which it is quenched and returned to its active state. During this time, no photons are detected by the SPAD. While actively quenching the SPAD will bring it to its active state more quickly after a detection event than with passive quenching, under high light levels (with many detection events) the SPAD can still spend a large fraction of its time inactive. This can lead to saturation and reduction in the number of photons detected by the SPAD. The result of this is that the SPAD output does not track the light level linearly which causes problems in some applications.
By the nature of their design, SPADs have no gain control mechanism to address this problem. It is proposed therefore, to attenuate the SPAD to improve output linearity.
Figure 1 shows an overhead view of a SPAD array 100 coupled to logic circuitry 180. Shown are nine SPADs 1 10a-1 10i. Each SPAD comprises a guard ring region 120, a breakdown region 130, an anode 140 and a track area 150 for connection to the anode. Only photons incident on the breakdown region are detected. The breakdown region is shielded by the anode and the track area reducing its active area. Taking one specific, non-limiting, example, the SPAD may have an effective active area of 48.05pm2. Attenuation may be achieved by further reducing the SPAD's active area. This may be done by covering more of the breakdown region with a non-transmissive cover which acts to block any photons incident on the cover surface. One way of doing this is to cover part of the breakdown region with a metal layer when forming the metal track and anode. The metal layer may completely cover the active area, except for one or more apertures 160. In this way, the only photons detected by the SPAD are those which pass through the aperture and onto the SPAD's active area. In Figure 1 , one SPAD 1 e is shown uncovered as is conventional. The remainder are shown with their active areas covered, the covers having various aperture 160 configurations, thereby providing varying degrees of attenuation. The cover of SPAD 100a has two apertures 160 providing 1/1 .73 area attenuation. SPADs 100b, 100c, 100d, 100f, 100g all have covers with aperture 160 arrangements providing 1/7.3 attenuation. SPADs 100b, 100c, 100d, 10Of all have a single aperture located in a different quadrant of the active area of the SPAD, while SPAD 100g has four apertures that are each quarter of the size than that of SPADs 100b, 100c, 100d, 10Of. SPAD 100h has four apertures, each of which provides an attenuation of 1/1 17, thereby providing a total attenuation of 1/29.3. SPAD 10Oi has only one aperture which provides an attenuation of 1/1 17. This particular SPAD array arrangement is shown to illustrate the effect of different attenuation levels.
The effect of the attenuation is to reduce the number of photons detected by the SPAD. In doing this, the SPAD does not need to be reset so often, and therefore the likelihood is reduced, of a photon arriving during a SPAD reset following a previous detection event. Consequently, fewer photons are missed by the SPAD, and linearity is maintained at higher incident light levels. This results in a more linear output response to light intensity. Figure 2a is a plot of the number of photon arrivals in 50 ms (solid lines) and the number of photon counts recorded in 50 ms (broken lines), against light intensity for clear c, red r, green g and blue b SPADs with no attenuation. The counts are given for a 50ms worst case for flicker immunity. The plot shown is for 6500K CCT illumination. The plot shows significant non-linearity with a large number of missed photons even at 500 lux for the clear SPAD. Green, red and blue are all significantly affected above 1000 lux. In this example clear SPADs have no optical filter, whereas red, green and blue SPADs have optical filters which pass red, green and blue light respectively. Attenuation resultant from the optical filters is discounted when describing a SPAD as having no attenuation.
Figure 2b shows a similar graph to that of Figure 2a, but for SPADs having covers providing 1/1.73x attenuation. It shows significantly reduced non-linearity compared to SPADs having no attenuation. The output is essentially linear up to 1000lux for clear SPADs and up to 2500lux for red/green/blue SPADs. Figure 2c shows that same graph for 1/1 17x attenuated SPADs. Non-linearity is greatly reduced for all channels in this case, with the plots for photon arrivals and photon counts largely overlapping, illustrating a linear response.
Figures 3a-3c show a plot of the non-linearity error against light intensity for clear c, red r, green g and blue b SPADs, with no attenuation, 1/1 .73x attenuation and 1/1 17x attenuation respectively. It can be seen that 1/1 17x attenuation results in a non-linearity error of less than 1 % (up to 5klux) for red, green and blue SPADs; and a non-linearity error of less than 2% (up to 5klux) for clear SPADs.
Figure 4 shows a graph of count rate (corrected for the dark rate count) against light intensity for SPADs attenuated with varying aperture sizes. It can be seen that the area aperture attenuations match actual area reductions.
In practical applications, the SPADs disclosed herein will be arranged in arrays. The attenuation levels chosen for particular arrays of SPADs will depend on the specific application, the likely illumination conditions and the acceptable level of non- linearity. For example, if the SPAD array is always likely to be used in situations where illumination levels are high, then an array of SPADs all having high attenuation (such as 1/1 17x attenuation) may be suitable. However, should illumination levels then fall, the detection rate may fall close to zero.
Consequently, for many practical applications, an array of SPADs with different attenuation levels may be advantageous. This allows an acceptably linear output to be achievable in a wide range of light conditions. In one embodiment, an array may comprise banks of SPADs, one or more having no attenuation, one or more having lesser attenuation (for example 1/1.73x attenuation) and one or more having greater attenuation (for example 1/1 17x attenuation). In very low light levels (less than 500 lux as illustrated in Figure 2a), the unattenuated SPADs will provide a suitably linear response. At medium light levels the output from the 1/1.73x attenuated SPADs should be selected. At the highest light levels, the outputs of the 1/1 17x attenuated SPADs should be selected. Suitable logic 180 may be provided to select the appropriate SPAD outputs depending on illumination conditions. There are a number of different ways such logic 180 may be implemented. For example, the outputs of particular banks (having particular attenuation levels) may be selected if their count rate is measured to be over a certain lower threshold level, or between upper and lower threshold levels.
It should be appreciated that the above description is for illustration only and other embodiments and variations may be envisaged without departing from the spirit and scope of the invention. In particular the specific levels of attenuation may be varied from those illustrated. SPADs according to the concepts described herein may have, depending on the embodiment, over 40% over 50%, over 75%, over 85%, over 95% or over 99% of the active area covered/shielded. Arrays may comprise any combination of such SPADs.

Claims

1 . A Single-Photon Avalanche Diode (SPAD) comprising:
an active region for the detection of incident radiation; and
a cover shielding part of said active region from said incident radiation.
2. The Single-Photon Avalanche Diode as claimed in claim 1 wherein said cover comprises a material which is non-transmissive to incident photons.
3. A Single-Photon Avalanche Diode as claimed in claim 1 or 2 wherein over 40% of the active area is shielded by said cover.
4. A Single-Photon Avalanche Diode as claimed in claim 1 or 2 wherein over 75% of the active area is shielded by said cover.
5. A Single-Photon Avalanche Diode as claimed in claim 1 or 2 wherein over 98% of the active area is shielded by said cover.
6. A Single-Photon Avalanche Diode as claimed in any preceding claim wherein said cover comprises one or more apertures exposing the portion of the active area not shielded by said cover.
7. A Single-Photon Avalanche Diode as claimed in any preceding claim wherein said cover is comprised of the same material as the SPAD's anode and/or electrical connections.
8. A Single-Photon Avalanche Diode as claimed in claim 7 wherein said cover has been formed during the same process as the formation of the SPAD's anode and electrical connection.
9. An array of Single-Photon Avalanche Diodes (SPADs), comprising any of the SPADs of any of claims 1 to 8.
10. An array of Single-Photon Avalanche Diodes as claimed in claim 9 wherein the SPADs comprised in said array all have the same proportion of their active areas shielded.
1 1 . An array of Single-Photon Avalanche Diodes as claimed in claim 9 wherein different subsets of said SPADs comprised in said array have different proportions of their active areas shielded.
12. An array of Single-Photon Avalanche Diodes as claimed in claim 1 1 comprising a low attenuation subset of SPADs, each SPAD of said low attenuation subset comprising no cover, or having less that 25% of its active area shielded.
13. An array of Single-Photon Avalanche Diodes as claimed in claim 1 1 or 12 comprising a medium attenuation subset of SPADs, each SPAD of said medium attenuation subset having between 40% and 95% of its active area shielded.
14. An array of Single-Photon Avalanche Diodes as claimed in claim 13 wherein each SPAD of said medium attenuation subset has between 75% and 95% of its active area shielded.
15. An array of Single-Photon Avalanche Diodes as claimed in any of claims 1 1 to 14 comprising a high attenuation subset of SPADs, each SPAD of said high attenuation subset having over 95% of its active area shielded.
16. An array of Single-Photon Avalanche Diodes as claimed in claim 15 wherein each SPAD of said high attenuation subset has over 98% of its active area shielded.
17. An array of Single-Photon Avalanche Diodes as claimed in any of claims 1 1 to 16 further comprising logic circuitry operable to select output from the appropriate subset depending on the incident radiation intensity.
18. A sensor comprising an array of Single-Photon Avalanche Diodes as claimed in any of claims 1 1 to 17.
19. An apparatus, comprising:
an array of Single-Photon Avalanche Diodes(SPADs);
wherein said array is divided into at least a first subset of SPADs and a second subset of SPADs;
wherein each SPAD of said array comprises an active region for the detection of incident radiation; and
wherein each SPAD of said first subset includes a cover shielding part of said active region from said incident radiation to a first degree; and
wherein each SPAD of said second subset includes a cover shielding part of said active region from said incident radiation to a second degree different from said first degree.
PCT/GB2014/051895 2013-06-21 2014-06-20 Single-Photon Avalanche Diode and an Array thereof Ceased WO2014202995A1 (en)

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