NL2011905C2 - Electron-bombarded image sensor device. - Google Patents
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- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
- H01L27/144—Devices controlled by radiation
- H01L27/146—Imager structures
- H01L27/14643—Photodiode arrays; MOS imagers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J31/00—Cathode ray tubes; Electron beam tubes
- H01J31/08—Cathode ray tubes; Electron beam tubes having a screen on or from which an image or pattern is formed, picked up, converted, or stored
- H01J31/26—Image pick-up tubes having an input of visible light and electric output
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Description
Title: Electron-bombarded image sensor device
Field of the invention
The present invention relates to an electron bombarded image sensor device, an image sensor for such a device, a low-light camera comprising such a device, as well as a method of manufacturing such.
Background of the invention
In an image sensor light or incident radiation is converted into an electrical signal. Digital cameras and camera modules in for example mobile phones can comprise such image sensors. Both consumer digital cameras and professional cameras for use in extreme low light situations, mostly comprise either an array of charge-coupled device, CCD, type or complementary metal-oxide-semiconductor, CMOS type pixels, either passive or active by including circuitry arranged for amplification, for example. With respect to the present invention, the term pixel sensor is to be construed as the individual element of the sensor, i.e. a pixel of the sensor. For example an active pixel element of an active pixel sensor. Accordingly, a sensor comprises an array of pixels, i.e. pixel elements or pixel sensors, for sampling the image.
Night vision cameras and other low light applications require high performance image sensors being efficient at low light levels and having high signal-to-noise ratio, for example. CMOS based image sensors are evolving devices that are nowadays arranged to satisfy these requirements.
In particular the use of a backside illuminated type CMOS image sensor adds up to the low light level requirements. In such a backside illuminated, BSI, image sensor the amount of light captured by the pixels is high in comparison with frontside illuminated image sensors due to the fact that circuitry does not take-up a large part of the surface for receiving the incoming light/radiation. In BSI image sensors the circuitry, i.e. wiring, is arranged below the active element because during manufacturing the image sensor is turned over such that the active elements are arranged at the top instead of the bottom side of the wafer. After the wafer is turned over, the backside (now the front side) is thinned in order to remove its substrate. Furthermore, a handle wafer is often used to serve as a new substrate for the turned-over wafer.
To further enhance the sensitivity to low light levels an electron bombarded, EB, configuration can be used. BSI EB CMOS image sensors work on the principle of accelerating (photo)electrons which are generated by a photocathode. These accelerated photoelectrons bombard the pixels of the image sensor and generate therein hole-electron pairs. Each incident photoelectron which is extracted from the photocathode impinges in this way on the sensors surface. The energy of the photoelectron creates hole-electron pairs, i.e. the secondary energy carriers or secondary charge carriers, close to the surface of the sensor. The amount of hole-electron pairs created by the incident photoelectron determines the electron bombardment gain the same way as the quantum efficiency is determined by the amount of hole-electron pairs created by the incident light in a non-EB CMOS image sensor. In the sensor the secondary electrons diffuse towards one or more pixels arranged below for collecting and further conversion to an electrical signal accordingly.
These BSI EB CMOS image sensors are in particular suitable for low light applications and are capable of Single Photon Detection. This means that the signal captured by at least one pixel must exceed the total pixel noise by a factor of 5, although in practise a factor of 3-5 often suffice wherein the total pixel noise is determined by the sum in quadrature of the sensors dark current (shot) noise and readout noise.
As mentioned, BSI EB image sensors are applicable as for example an image sensor device for low light application. Such a device comprises a vacuum envelope having a photocathode for releasing photoelectrons into the vacuum envelope when light impinges the cathode, or more in general, when electromagnetic radiation impinges the cathode. These photoelectrons are released in the vacuum towards an anode which is spaced apart from the photocathode and is in facing relation therewith. At the anode the photoelectrons are received and converted to an electric signal for further processing outside the vacuum envelope. For accelerating the photoelectrons to be received at the anode the device further comprises electric field means for generating an electric field between the anode and cathode such that the photoelectrons are accelerated towards the anode. In this configuration the anode can be a BSI image sensor as described above.
Various solutions have been proposed in order to further improve the performance of the EB image sensor for example by increasing the electric field between the anode and cathode or by decreasing the gap between both. However these solutions have several other drawbacks such as the x-ray emission occurring at high voltages and increased field emission probability. Hence, there is a long felt need for a low light level high performance image sensor device wherein at least some of the above described drawbacks are overcome..
Summary of the invention.
Various parameters in the device have influence on the performance of the image sensor therein and various prior art methods are known to increase the performance. As described above, It is for example known to decrease the gap, i.e. the distance between the photocathode and the anode. This has a positive influence on for example the halo size. Wherein halo is considered the effect linked to backscattered electrons that bounce off the sensor surface and under the influence of the electric field enter the sensor surface at a different location. This effect is visible as a ring around high intensity spots.
An object of the present invention is to overcome at least some of the above-mentioned disadvantages of the prior art, and to provide an improved low light level imaging device.
The object is achieved, in a first example, by providing an electron bombarded image sensor device comprising a vacuum envelope, wherein the image sensor device comprises a photocathode for releasing photoelectrons into the vacuum envelope upon electromagnetic radiation impinging the photocathode. And wherein the device further comprises in its vacuum envelope an anode spaced apart from, and in facing relationship with the photocathode for creating multiple hole-electron pairs near the surface of the anode upon receiving the photoelectrons. The anode is an image sensor having an array of pixels for receiving the photoelectrons and generating electrical signals out of the incident photoelectrons. The device further comprises electrical connection means for feeding these electrical signals from the image sensor towards signal processing means arranged outside the vacuum envelope for generating the image, and electric field means for accelerating the photoelectrons from the photocathode towards the anode. The device is characterized in that the image sensor is a hole collecting image sensor.
Prior art EB image sensors collect secondary electrons, i.e. the electrons that are freed in the sensor near the surface thereof by the impinging primary electrons from the photocathode. However, since the impinging primary electrons also create corresponding holes, the inventors found out that in an EB image sensor one could also collect these holes instead of the electrons. The inventors found out that the performance of an electron bombarded image sensor device having an image sensor which is configured to collect the holes results in reduced readout noise, lower dark current and less cross-talk between the pixels.
By collecting holes, amongst others, dark current is reduced, which leads to a lower dark current (shot) noise. Since the dark current noise significantly adds up to the total noise of the sensor the performance of the device as a whole is thereby increased. With lower noise the need for a higher gain in order to achieve higher performance is reduced. This has an effect on the electric field to be applied between the sensor and photocathode. The voltage can be lowered which in its turn reduces the requirements for the power supply powering the electric field means. And accordingly, with a lowered voltage the gap or distance between the photocathode and the image sensor itself can be reduced. This has a positive effect on the halo effect on the image sensor and on image quality, i.e. resolution of the sensor and hence the device.
In another embodiment of the electron bombarded image sensor device the image sensor is arranged on an n-type substrate, and in a further embodiment the image sensor comprises an epitaxial layer.
Prior art EB image sensors are arranged on a p-type substrate. In common practice the minority carriers are collected. Therefore the type of substrate determines the type of charge carrier to be collected and thus in prior art electrons are collected. Holes have a lower mobility than electrons. A lower mobility implies slower diffusion of the charge carrier to the charge collecting element, but also sideways to neighbouring pixels, resulting in less crosstalk. Furthermore, the dark current is reduced. By using an n-type substrate wherein holes are collected, crosstalk and dark current can be reduced compared to prior art.
In even another embodiment of the electron bombarded image sensor device the image sensor comprises a p-type substrate, and in a further embodiment the image sensor comprises n-type wells.
In order for the electron bombarded image sensor device to collect holes instead of electrons it can be comprised of a standard p-type substrate and n-type wells. Deep n-type wells on a p-type substrate enable the image sensor on the one hand to collect holes instead of electrons and on the other hand to implement standard used p-type substrates.
In order to increase the quantum efficiency, or for an electron bombarded image sensor device such as the device of the present invention, electron bombardment gain, the generated holes at the incoming surface need to diffuse towards the pixel(s). Interface states at the incoming surface act as recombination centres for the generated hole-electron pairs. This is considered loss of signal.
To reduce this loss, an internal electric field is desirable which accelerates the holes towards the charge collecting element of the pixel, such as a photodiode or photogate. An internal electric field can be produced in several manners, for example by chemisorption, flash gate, UV implementation or delta doping. For producing a positive electric field, i.e. an effective positive charge, at or near the incoming surface. This charge creates an electric field which penetrates into the sensor semiconductor material. Prior art methods of producing an internal electric field for an electron bombarded image sensor comprise a step of either chemisorption or the deposition of a negatively charged oxide.
In an embodiment of the electron bombarded image sensor device according to the present invention chemisorption of a positive ion is presented as an example of providing a positive electric field for a hole collecting electron bombarded image sensor.
In another embodiment of the electron bombarded image sensor device according to the present invention the image sensor comprises a positive charged oxide. In particular a silicon comprised sensor material can be oxidised to form a thin silicon oxide layer. This will create a net positive charge enabling the positive electric field for accelerating the holes towards the charge collecting element of the pixel. The amount of net positive charge is dependent upon the growth temperature and the final silicon oxide thickness.
In another embodiment of the electron bombarded image sensor device according to the present invention the image sensor comprises a deposited flash gate. In prior art, a flash gate comprises a high work function material. The mismatch in work function between the material of the flash gate and the sensor material induces the preferred internal electric field for accelerating the electrons towards the charge collecting element of the pixel. For the electron bombarded image sensor device of the present invention a material with a low work function is preferred for creating a flash gate to drive the holes towards the charge collecting element of the pixel. Several such materials exist and are known by the skilled person.
In a preferred embodiment of the electron bombarded image sensor device according to the present invention the image sensor comprises a thin highly doped region. This can be achieved by delta-doping, but this is expensive and difficult to control. A preferred method is therefore the use of ion implantation followed by annealing.
An electron bombarded image sensor comprising a thin, i.e. shallow, highly doped region provides an electrical field away from the surface, reducing the surface component of the dark current. Hence, in an embodiment the electron bombarded image sensor comprises an n, in particular, an n+, type implanted passivation layer.
In a more specific embodiment of the electron bombarded image sensor device the n-type implanted passivation layer comprises a phosphorus implanted doping profile.
The material to be implanted by doping is depending on the charge carrier to be collected. In case of hole collection, the holes need to be pushed towards the pixel by means of a passivation providing an internal electric field which drives the holes away from the surface. One way of providing this field is by use of an n-type passivation layer. An example of such an n-type dopant which is suitable is phosphorus. The substrate can be an n-type substrate or a p-type substrate comprising n-type wells.
In an alternative embodiment of the electron bombarded image sensor device the n-type passivation layer comprises an arsenic implanted doping profile.
The inventors found out that for EB configured image sensors, for example an EBCMOS, passivation layers arranged by a shallow arsenic doping profile have an improved performance as compared to prior art passivation layers on EB image sensors. Phosphorus and Arsenic doping profiles have a shorter implant range when compared to boron. As indicated, the final junction profile of the passivation layer is dependent upon the implanted species, the original implanted profile and the diffusion constant associated with a specific dopant atom. Dose, implant energy and mass of the dopant determine the doping profile. Since lighter elements travel deeper into the silicon material they result in a deeper profile prior to the step of annealing. For a prior art image sensor having a p-type epimaterial layer the passivation layer is formed by a shallow highly doped p-type layer which is achieved by boron implanting and annealing.
However, boron is a light element and therefore difficult to implant shallowly. With an EB image sensor according to the invention wherein holes are collected and function as secondary carriers, an n-type substrate can be used and hence an n-type passivation layer can be implemented. Since arsenic, as well as phosporus, are heavier elements than boron it has the advantage that they can be implanted very shallow and easier than boron. Furthermore, arsenic has the advantage that it has a smaller diffusion constant than boron. Junctions realised by arsenic implant are more shallow and hence a higher electron bombardment gain is achieved with an image sensor accordingly. Furthermore, the field free region thickness can be kept very thin such that the lateral diffusion in the hole based image sensor is minimal or at least reduced compared to prior art image sensors.
In a further embodiment of the electron bombarded image sensor device the image sensor is a complementary MOS, CMOS, image sensor. CMOS image sensors are suitable image sensors that are able to achieve high quantum efficiency, or electron bombardment gain in case of an EB CMOS image sensor such as the image sensor of the present invention. In the EB CMOS image sensor secondary charge is in the photo sensor of each pixel converted by electronic circuitry to an electric signal. This signal can even further be altered by for example a booster stage, amplifier. Implementation of an EB CMOS uses, when compared to CCD, less power and provide a faster readout since a whole row of pixels is readout within a single operation. Further, CMOS production is easier and more commonly used in semiconductor manufacturing when compared to CCD.
In yet a further embodiment of the electron bombarded image sensor device the image sensor is a backside illuminated image sensor.
In an EB configuration an image sensor collects electrons instead of light. Since electrons penetrate less deep, a preferred embodiment of an EB image sensor is in a BackSide Illuminated, BSI, configuration. BSI image sensors are more sensitive to impinging electrons than front illuminated image sensors since the active element, i.e. the photogate or photodiode is not blocked by transistors or other electronic circuitry that block part of the impinging electrons. Part of the impinging electrons are absorbed by the circuitry or transistors and can therefore not be collected by the active elements, thereby not adding up to the signal of the sensor. By use of a BSI image sensor this disadvantage is overcome due to the fact that the image sensor (wafer) in its production process is turned over, thereby bringing the active layer to the top of the wafer and placing the transistors and wiring below. Since the substrate on which the image sensor was created is then still blocking the impinging light or in this invention the impinging electrons this layer is removed as a whole from the wafer bringing the active elements to the surface, with or with the optional use of an additional handling wafer as a new substrate to place the turned over and thinned wafer on.
In yet another embodiment of the electron bombarded image sensor device the image sensor is an active pixel image sensor.
An active pixel image sensor, or Active Pixel Sensor, APS, is called active due to a first amplification stage in the pixel itself. Each pixel is an active element in which the light or other type of radiation is received and converted into a voltage and at least amplified to a first degree at the pixel itself, for further processing by its electronic circuitry outside the pixel. The array as a whole can have a pixel resolution which is determined by the amount of individual pixels in the array, for example an array of 1024 x 1024 pixels results in a pixel resolution of approximately 1 Megapixel, MP. For controlling an active pixel image sensor, the sensor comprises circuitry to select the row and column of the pixels in the array. With the row select circuitry all pixels in a single row can be selected at once and accordingly the pixels in a column with the column select circuitry. As such timing and control of the active pixel sensor is performed by these circuitries respectively. The active pixel image sensor further optionally comprises circuitry for performing correlated double-sampling, CDS, for example for reducing the pixel reset noise..
The active pixel sensor can further comprise a dark signal level arrangement of pixels and circuitry by which the dark current can be determined for comparison with the actual signal of the impinging photoelectrons in the signal pixels in order to subtract both and determine the real photo image signal. The active pixel sensor furthermore comprises amplifier circuitry and analogue to digital converter circuitry.
In yet another embodiment there is provided an electron bombarded image sensor device wherein the pixel sensors comprise a four-transistor pixel sensor layout.
The image sensor comprises an array of pixel sensors. The circuitry of the pixel sensors comprise transistors for processing the signal produced by the photogate or photodiode. In its most basic form the circuitry comprises a so called three-transistor, 3T, layout having a reset node, select node and source follower node. It further comprises a photodiode or other active element. The active element, i.e. the photodiode or the photogate, collects the secondary charge created by the impinging photoelectrons coming from the photocathode. For increased efficiency most of the pixel sensors surface should therefore be reserved for the active element, which is the case in an image sensor having a small amount of transistors, leaving more room for the active element. Each row of pixels is connected to a select transistor for selecting the row to be read out. The reset node is for resetting the photodiode before the charge is collected. The source follower node transfers the charge as a voltage to the column line where it is further processed. A more advanced pixel sensor comprises a four-transistor layout. This is a variant of the three-transistor layout having improved noise figures. In a four-transistor layout the sensor is build-up out of an active element, a transfer node, reset node, select node, and a source follower node. In an example the active element can be arranged as a pinned photodiode with a shallow/thin n-type implant having a low dark current. Intrapixel charge transfer further reduces the noise when making use of correlated double sampling.
Even more configurations exist and can be arranged in the pixel sensor, such as five-transistor and six-transistor configuration in which additional transistors are added in order to configure the sensor as a global shutter sensor instead of a rolling shutter in which rows are readout subsequently.
In a further embodiment of the electron bombarded image sensor device the pixel sensors comprise a photogate or a pinned photodiode. The active element of the sensor can comprise either a photogate or a photodiode, more particular a pinned photodiode having a low dark current.
In a second example there is provided an electron bombarded image sensor for use in an electron bombarded image sensor device according to any of the previous descriptions wherein the image sensor is a hole collecting image sensor.
In a third example there is provided a low-light level camera comprising an electron bombarded image sensor according to any of the descriptions above.
In a fourth example there is provided a method of manufacturing an image sensor for use in an electron bombarded image sensor device according to any of the descriptions above and comprising the steps of: providing an n-type substrate, or a p-type substrate for comprising deep n-type wells; forming a detection layer comprising an array of pixel sensors; forming an electric circuitry layer comprising at least three transistors per pixel sensor as well as read out electronics; removing the backside of the image sensor by removing the substrate; forming an n-type passivation layer on the detection layer; characterized in that the image sensor is a hole collecting image sensor.
As mentioned above in the case of an EB BSI (CMOS) image sensor an optional handle wafer could be introduced to which the substrate is bonded, for increasing the mechanical strength before the backside is removed.
The invention will now be described in more detail using a drawing, which drawing depict in:
Brief description of the drawings
Figure 1, a schematic view of an electron bombarded image sensor device according to an example of the invention;
Figure 2, a detailed view of the pixel sensors of the electron bombarded image sensor of figure 1;
Figure 3, a detailed view of the layers and elements of a pixel sensor of the electron bombarded image sensor of figure 1 with a n-type substrate;
Figure 4, a detailed view of the layers and elements of a pixel sensor of the electron bombarded image sensor of figure 1 with a p-type substrate with deep n-wells.
Detailed description
For an enhanced clarification of the invention in the following description the same parts will be indicated and referred to with the same referral numerals.
Figure 1 shows a schematic view of a simplified electron bombarded image sensor such as an electron bombarded backside illuminated complementary metal oxide semiconductor image sensor, EB BSI CMOS image sensor. It comprises a support layer 11, i.e. a support substrate that forms the vacuum chamber or vacuum envelope 14 by a support and wall structure. The vacuum envelope 14 is on the other, topside formed and closed by the entrance window 12. The entrance window 12 acts as a photocathode entrance window because it comprises on its inside a photocathode layer 13. The photocathode 13 is in this embodiment shown as a different feature with respect to the entrance window 12. However, in an alternative embodiment the entrance window 12 and the photocathode 15 can also be integrated.
Opposite to the photocathode, at a certain distance thereof and in facing relationship thereof, an image sensor 15 is shown. The image sensor is arranged on the support and can be either a CMOS or CMOS like image sensor or even a charge-coupled device, CCD or the like. The image sensor functions as an anode for receiving the incident photoelectrons from the photocathode 13 and comprises a 2D array of pixel sensors, or pixels for short.
The image sensor device 10 is in particular arranged for low light level situations such as for use in a night vision device. Light falls on the entrance window 12 of the device 10 and photoelectrons are generated in the photosensitive layer 13 of the window 12. During operation the photocathode is negatively charged in respect to the anode, i.e. the image sensor 15. The charge is provided by electric field means not shown in the figure. These electric field means apply a voltage potential between the anode and cathode at which single photon detection is achieved, in practice in most prior art devices this can be as high as 2kV or more In the present invention a voltage potential is used which also enables single photon detection, however, this could be achieved at lower voltages, even below 1.5kV. In order to achieve single photon detection the voltage cannot be increased unlimitedly since there is the risk of a voltage breakdown when using high voltage, in which breakdown current starts to flow between the anode and cathode. Decreasing the gap 14 between the cathode and anode also adds to the risk of voltage breakdown..
By applying the voltage potential between the anode and the cathode the photoelectrons are accelerated towards the anode. The anode, constructed as an image sensor receives the impinging photoelectrons and they penetrate into the surface of the sensor It is intended that the penetration is to be of sufficient depth in the surface in order to reduce the risk of recombination at the surface of the secondary charges generated in the semiconductor layer as this is reducing the gain of the device.
Since a particular application of the present image sensor device is for usage in a night vision device, it is of upmost importance to have a sufficient gain to enable single photon detection. Since altering the electric field means for applying a higher voltage potential between the anode and cathode has abovementioned disadvantage and risks, it is an object of image sensor devices to increase the gain as much as possible. A known method for increasing the gain of an image sensor is by passivation of the semiconductor. An example of a passivation layer on an image sensor semiconductor is an assembly of an electrically conductive layer and an insulating layer which layers are provided on the surface area of the image sensor facing toward the vacuum envelope and the photocathode. An example of such state of the art passivation assemblies is an insulating layer of silicon oxide (Si02) and an metal comprising electrically conductive layer. The electrically conductive layer can in an example also be connected with the secondary electrical field means in order to provide a voltage potential thereupon. More particularly the electrically conductive layer of the passivation assembly of the image sensor is provided with a negative voltage potential in order to reduce carrier recombination, thereby significantly increasing its gain. The person skilled in the art will understand from the description above that various other types of passivation assemblies, or the like, exist and the assembly described here is merely an example thereof.
Figure 2 shows a close-up of the image sensor 20, more specific, a part of an image sensor 20 according to the image sensor shown in Figure 1. In the image sensor 20 at least two layers can be recognised. The first, top layer 21 is the layer in which the incident photons, or in this invention the photoelectrons, impinge and penetrate the surface. The interrupted line 23 depicts an exemplary photoelectron. This single photoelectron 23 is extracted from the photocathode shown in Figure 1 with reference number 13 and impinges the silicon surface. The kinetic energy of the photoelectron (determined by the acceleration voltage) creates electron hole-pairs in the silicon surface of the image sensor. The electrons and corresponding holes are the secondary carriers (secondary charge carriers) and the amount of secondary carriers, hence the amount of electrons and holes created by a single incident photoelectrons is stated as G, the electron bombardment gain.
Not only the energy of the photoelectron but also the quality of the passivation layer of the image sensor affects the electron bombardment gain. A more effective passivation layer adds up to the performance of the device by increasing the gain since less secondary carriers recombine at the surface.
As stated, each incident photoelectron creates multiple electron hole-pairs. The holes of the hole collector diffuse laterally prior to the capture by the image sensors pixels 22a, 22b, 22c below the surface. Hence, an incident electron impinging at for example the corresponding pixel 22b does also create holes that are captured by the adjacent pixels 22a, 22c. This effect is known as the crosstalk effect. Cross talk depends upon many factors like carrier mobility and internal electric field due to a potential graded doping profile. In case of using deep n-wells, the pixel collection areas (the deep n-wells) are separated by small p-type barriers (the non-transferred part of the original p-type substrate) providing isolation between pixels.
According to an aspect of the present invention the passivation layer of the image sensor 20 can be arranged by a technique of backside doping in which dopants are implanted into the semiconductors surface and subsequently annealed. Other types of passivation processes also exist and are considered known by the person skilled in the art. For the passivation layer to be efficient, the doping profile has to be shallow. To achieve such a shallow doping profile a process known as delta doping can be performed. Delta doping can for example be achieved by molecular beam epitaxy. This will create shallow and very thin doping profiles; however, it is very costly and hence less complex processes such as standard doping implantation and subsequent annealing can also be used.
The type of dopant however depends on the original material in which the dopant is to be implanted. Amongst others the type of doping atom influences the formation of the passivation layer and thus the final doping profile depth and hence the performance of the device, i.e. its gain. Standard prior art electron bombarded image sensors comprise a p-type material layer and hence the most common dopant is boron. Since boron is a lightweight element it is difficult to implant shallowly.
By making use of an n-type substrate or deep n-wells in a p-substrate, the electron bombarded image sensor according to the invention can be provided with a n-type passivation layer, for example doped with arsenic or phosphorus atoms. Since these elements are heavier than boron it is easier to implant them more shallowly. Arsenic furthermore also has a smaller diffusion constant than boron making it highly suitable for formation of the passivation layer since it creates more shallow junctions, hence increasing the electron bombardment gain. Accordingly a low-light level imaging device comprising an electron bombarded image sensor can be provided with an increased gain. If however the gain of a prior art electron bombarded image sensor is, for certain applications, already sufficient, the increased gain of the n-type passivation layer allows the use of a lower applied voltage and hence decreases stress on the power supply, i.e. the voltage supply means. Furthermore, due to the decreased voltage potential the gap between the photocathode and the anode, i.e. the image sensor, can also be decreased without the risk of voltage breakdown between both. Due to the decreased gap the halo effect described before is also decreased.
According to an aspect of the present invention the electron bombarded image sensor based on the collection of holes instead of conventional electron collection as secondary carriers has the further advantage of reduced lateral carrier spreading. This reduces the (electrical) cross-talk effect increasing the quality of the picture generated by the device. Furthermore, reduced lateral spreading means the package of generated secondary charges spreads among less pixels. The middle pixel receives more signal of the package and thus single photon detection is easier to achieve, that is, lower gain is needed. This could be a further way to reduce the electron acceleration voltage. In a further embodiment the image sensor can also be provided with a vertical gradient doping profile from the surface of the image sensor towards the active elements in the pixels, i.e. the photogate or photodiode.
In figure 3 and 4 cross sections of sensor pixels 30, 40 of an electron bombarded image sensor device according to two examples of the present invention are shown. The pixel sensors are a pixel sensor of which multiple exist in the form of an array in the electron bombarded image sensor according to the invention and as shown in Figure 1. The cross sections disclose the multiple (semiconductor) layers of which the pixels are build-up.
In figure 3 a first example of a sensor pixel 30 of an image sensor according to the invention is shown which is build-up of an n-type substrate. In figure 4 a second example of a sensor pixel 40 of an image sensor according to the invention is shown which is build-up of p-type substrate with a deep n-type well.
The sensor pixel 30 shown in figure 3 is formed on a substrate 31 which according to an example of the invention is comprised of n-type material. In the case of the use of an epi-layer, the substrate 31 is the remaining epi-layer of the wafer. In an example this could also be the silicon remaining after BOX removal, if the image sensor is fabricated on a silicon-on-insulator (SOI) wafer. This technique is suitable for backside illuminated image sensors since the wafer thinning can be controlled accurately due to the buried oxide layer of the SOI providing an effective stop for the etching of the silicon.
The sensor pixel 30 of figure 3 further comprises a p-well 33 which forms the photodiode. The photodiode can be provided with a pinning layer 34 which forms the pinned photodiode. The pinning layer electrically separates the silicon surface from the photodiode, thus reducing the surface component of the dark current.
The sensor pixel 30 of figure 3 is further comprised of a transfer gate TX 35 and floating diffusion node FD 36. The integration of the charge carriers of the array takes place in the pinned photodiodes, and the charge carriers are moved towards the floating diffusion node FD 36 by activating the transfer gate TX 35. Sensor pixels according to the invention could comprise further elements, e.g. transistors/nodes not shown in the figures. These could for example be a reset transistor for resetting the storage node, and a source follower to convert the charge to a voltage on the column bus.
The single pixel sensor, as part of the image sensor shown in figure 3 is further comprised of a passivation layer 32.
In figure 4 a second example of a sensor pixel 40 of an image sensor according to the invention is shown which is build-up of an p-type substrate. The p-type substrate comprises a deep n-type well 38 which is enclosed by the remaining parts of the p-type substrate. Again, in accordance with the sensor pixel shown in figure 3, an epi-layer could be used, and in that case, the remaining substrate 37 is an epi-layer. Further, also SOI wafer could be used, wherein the remaining substrate 37 in that case is the silicon remaining after the BOX removal.
Figure 4 further discloses a deep n-well 38, a p-well 33 which forms the photodiode and a pinning layer 34 configuring the photodiode as a pinned photodiode. Further the sensor pixel 40 also comprises a transfer gate, TX, 35 and a floating diffusion node, FD, 36 according to the sensor pixel 30 shown in figure 3. Hence, the sensor pixel 40 of figure 4 could also be comprised of additional transistors / nodes such as a reset transistor and source follower not shown in figure 4.
Based on the above description, a person skilled in the art may provide modifications and additions to the device disclosed, which modifications and additions are all comprised by the scope of the appended claims.
Claims (18)
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