Pixels for high performance image sensor

Visual and near infrared pixels may have deep photodiodes to ensure sufficient capture of light. The pixels may have a silicon layer that is etched to form a microlens for the pixel. The pixels may include an inversion layer formed over the silicon layer to prevent dark current. Additionally, the pixels may include a conductive layer formed over the inversion layer that further prevents dark current. The conductive layer may be coupled to a bias voltage supply line. The conductive layer may be biased during image acquisition to prevent dark current. During readout, the bias voltage may be pulsed at a lower voltage to ensure all of the collected charge is transferred out of the photodiode during charge transfer.

BACKGROUND

This relates generally to imaging systems and, more particularly, to imaging systems with high performance pixels.

Modern electronic devices such as cellular telephones, cameras, and computers often use digital image sensors. Imager sensors (sometimes referred to as imagers) may be formed from a two-dimensional array of image sensing pixels. Each pixel includes a photosensitive layer that receives incident photons (light) and converts the photons into electrical charge. Image sensors are sometimes designed to provide images to electronic devices using a Joint Photographic Experts Group (JPEG) format.

In certain applications, pixels may be used to sense both visual and near infrared (NIR) light. However, NIR light may travel deep into the photosensitive layer before being converted into electrical charge. Therefore, in these applications it is desirable for photon-generated charge absorption in a pixel to be as deep as possible. Conventional methods of forming photosensitive layers include implanting ions into semiconductor material. However, there is a limit to how deep ions may be implanted. Ion implantation may therefore not produce photosensitive layers optimal for near infrared light sensing.

It would therefore be desirable to provide improved pixels for improved sensing of both visual and near infrared light.

DETAILED DESCRIPTION

Embodiments of the present invention relate to image sensors with pixels. The image sensors may be image sensors with pixels that sense both visual and near infrared light. The pixels may have deep photodiodes to ensure sufficient capture of both visual and near infrared light. The pixels may include an inversion layer to prevent dark current and a conductive layer formed over the inversion layer that further prevents dark current. The conductive layer may also be biased during acquisition to minimize dark current and biased differently during readout to ensure all of the collected charge is transferred out of the photodiode during charge transfer.

An illustrative electronic device that may include these pixels is shown inFIG. 1. Electronic device10may be a digital camera, a computer, a cellular telephone, a medical device, or other electronic device. Camera module12(sometimes referred to as an imaging device) may include image sensor16and one or more lenses14. During operation, lenses14focus light onto image sensor16. Image sensor16includes photosensitive elements (e.g., pixels) that convert the light into digital data. Image sensors may have any number of pixels (e.g., hundreds, thousands, millions, or more). A typical image sensor may, for example, have millions of pixels (e.g., megapixels). As examples, image sensor16may include 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.

Still and video image data from image sensor16may be provided to processing circuitry18. Processing circuitry18may be used to perform image processing functions such as automatic focusing functions, depth sensing, data formatting, adjusting white balance and exposure, implementing video image stabilization, face detection, etc.

Processing circuitry18may also be used to compress raw camera image files if desired (e.g., to Joint Photographic Experts Group or JPEG format). In a typical arrangement, which is sometimes referred to as a system on chip (SOC) arrangement, image sensor16and processing circuitry18are implemented on a common integrated circuit. The use of a single integrated circuit to implement image sensor16and processing circuitry18can help to reduce costs. This is, however, merely illustrative. If desired, image sensor16and processing circuitry18may be implemented using separate integrated circuits. Processing circuitry18may include microprocessors, microcontrollers, digital signal processors, application specific integrated circuits, or other processing circuits.

As shown inFIG. 2, image sensor16may include pixel array20containing image sensor pixels22arranged in rows and columns (sometimes referred to herein as image pixels or pixels) and control and processing circuitry24(which may include, for example, image signal processing circuitry). Array20may contain, for example, hundreds or thousands of rows and columns of image sensor pixels22. Control circuitry24may be coupled to row control circuitry26and image readout circuitry28(sometimes referred to as column control circuitry, readout circuitry, processing circuitry, or column decoder circuitry).

Row control circuitry26may receive row addresses from control circuitry24and supply corresponding row control signals such as reset, row-select, charge transfer, dual conversion gain, and readout control signals to pixels22over row control paths30. One or more conductive lines such as column lines32may be coupled to each column of pixels22in array20. Column lines32may be used for reading out image signals from pixels22and for supplying bias signals (e.g., bias currents or bias voltages) to pixels22. If desired, during pixel readout operations, a pixel row in array20may be selected using row control circuitry26and image signals generated by image pixels22in that pixel row can be read out along column lines32.

Image readout circuitry28may receive image signals (e.g., analog pixel values generated by pixels22) over column lines32. Image readout circuitry28may include sample-and-hold circuitry for sampling and temporarily storing image signals read out from array20, amplifier circuitry, analog-to-digital conversion (ADC) circuitry, bias circuitry, column memory, latch circuitry for selectively enabling or disabling the column circuitry, or other circuitry that is coupled to one or more columns of pixels in array20for operating pixels22and for reading out image signals from pixels22. ADC circuitry in readout circuitry28may convert analog pixel values received from array20into corresponding digital pixel values (sometimes referred to as digital image data or digital pixel data). Image readout circuitry28may supply digital pixel data to control and processing circuitry24and/or processor18(FIG. 1) over path25for pixels in one or more pixel columns.

In certain applications, pixels22may be used to sense both visual and near infrared (NIR) light. In these applications, it is desirable for the photodiodes of pixels22to be as deep as possible.FIG. 3is a cross-sectional side view of an illustrative pixel during pixel formation. To form pixels with a deep photodiode, a substrate such as substrate100inFIG. 3may be provided. Substrate100may be a p-type doped silicon substrate. Substrate100may have an n-implanted region102. The n-implanted region may be doped with any desired n-type dopants via ion implantation. Additionally, a layer of epitaxial silicon104may be formed on n-implanted region102. Epitaxial silicon104may be graded epitaxial silicon with a doping profile optimized for pixel performance. In one illustrative example, graded epitaxial silicon104may have a doping concentration at upper surface106different from a doping concentration at lower surface108. The doping concentration may gradually change from the upper surface towards the lower surface.

N-implanted region102may have p-type doped isolation regions110that help isolate adjacent pixels from each other. The p-type doped isolation regions may be formed by implanting p-type dopants into layer102. Pixel22may also include various transistors and signal metal routing lines such as trunk transistors and corresponding metal routing lines112.

FIG. 4is a cross-sectional side view of the pixel ofFIG. 3after etching. As shown, etching may be performed to form trenches114that further isolate adjacent pixels from one another. The etching process may be a wet silicon etch. A wet silicon etching process may minimize silicon crystal damage and reduce dark current in the pixel. Layer104may be etched to form trenches that meet p-type isolation regions110to ensure that each photodiode118is sufficiently isolated from adjacent photodiodes. Reactive ion etching may be used instead of wet silicon etching to form trenches114. However, reactive ion etching may damage the silicon more than the wet silicon etch and result in the pixel having greater dark current.

Etching layer104may also etch n-type epitaxial silicon104into a microlens. In other words, by etching the upper surface of n-type epitaxial silicon104to have curvature, the silicon layer can act as a microlens to focus light116at a focal point. Additionally, trenches114may be filled with a material that has a lower refractive index than silicon (e.g., air or silicon dioxide). Using this low refractive index material will result in a pixel microlens with high optical power.

Additionally, the silicon surface may be lined with a material or materials that inverts n-type to p-type. For example, inversion layer122may be a conductive or dielectric layer that inverts n-type to p-type. Inversion layer122may be, for example, hafnium oxide (HfO2), a boron oxide, or any other desired material. Inversion layer120may be a conductive or dielectric layer that inverts n-type to p-type. Inversion layer120may be tantalum pentoxide (Ta2O5) or any other desired material. Layers120and122may additionally act as an antireflective coating. Alternatively, an additional antireflective coating may be provided. In other embodiments, only one inversion layer or three or more inversion layers may be provided.

After etching, photodiode118may have a photon collection depth119. Depth119may be more than 1 micron, more than 2 microns, more than 5 microns, more than 8 microns, approximately 8 microns, between 5 and 10 microns, more than 10 microns, less than 10 microns, between 7 and 9 microns, or any other desired depth.

FIG. 5is a cross-sectional side view of an illustrative pixel with an additional conductive layer and a front side reflector. As shown, pixel22may include a thin layer of conductive material formed on top of layer120. Conductive layer124may be formed over photodiode118such that incident light passes through conductive layer124to reach photodiode118. Accordingly, conductive layer124may be substantially transparent to ensure accurate light sensing in photodiode118. Conductive layer124may be formed from polysilicon, aluminum, tungsten, indium tin oxide (ITO), or any other desired material. Conductive layer124may be formed from conductive material meshes (e.g., a thin metal material with a pattern of holes) to ensure the conductive layer is transparent. If desired, different portions of conductive layer124may be formed from different materials. For example, the portion of conductive layer124in trenches114does not need to be transparent since it does not overlay photodiode118. Accordingly, the portions of conductive layer124in trenches114may be formed from a non-transparent material while the portions of conductive layer124that are not formed in trenches114may be formed from a transparent material. In general, any combination of conductive materials may be used to form conductive layer124.

Conductive layer124in trenches114may help isolate adjacent photodiodes. Additionally, conductive layer124may be coupled to contact126which is subject to an electric potential. The potential applied to contact126and conductive layer124may be varied during operation of the pixel to optimize pixel performance. For example, the potential on the contact may be negative during image acquisition to lower dark current. During readout, the potential may be pulsed more negative to push electrons towards transfer gate128for better charge transfer. This may improve the performance of pixel22.

Contact126may be coupled to bias voltage supply line130which supplies a bias voltage (Vbias). During image acquisition, bias voltage supply line130may provide a first bias voltage to contact126and conductive layer124. The first bias voltage may be a negative bias voltage. During readout, bias voltage supply line130may provide a second bias voltage to contact126and conductive layer124. The second bias voltage may be a negative bias voltage that is less than (i.e., more negative than) the first bias voltage. The low bias voltage during readout may funnel electrons towards transfer gate128. Conductive layer124may be held at the second bias voltage throughout readout. Alternatively, the bias voltage supply line may pulse the second bias voltage such that the conductive layer rapidly switches between the second bias voltage and the first bias voltage. Pulsing the bias voltage to a lower value in this manner may increase effectiveness of the conductive layer in funneling electrons to the transfer gate.

To further enhance performance, pixel22may include a front side reflector132. Front side reflector132may be formed from a reflective material (e.g., metal). Reflector132may reflect photons that initially pass through photodiode118without being converted into electrical charge back into photodiode118. Front side reflector132may increase the sensitivity and quantum efficiency of pixel22which has advantageous effects on pixel performance by detecting a larger percent of incident photons.

FIG. 6is a cross-sectional side view of pixel22with trenches surrounding the pixel on all sides. Instead of a trench and a p-type isolation region combining to isolate the pixels (e.g.,FIGS. 4 and 5), layers102and104may both have trenches that isolate the pixels. The trenches in layer102may be formed via etching in the same way as the trenches in layer104. The trenches in layer102may also have inversion layers122and120for inverting n-type to p-type. The trenches in layer102may have conductive layer124similar to the conductive layer in layer104. As shown inFIG. 6, conductive layer124may completely surround the trench in layer102. In certain embodiments, as shown inFIG. 6, trenches114may be filled with an additional conductive material136. Conductive material136may provide additional optical and electrical isolation for photodiodes118. In certain embodiments, contacts126may be coupled to conductive layer124such that contact126directly contacts conductive layer124. In other embodiments, contacts such as contacts134may be coupled to the trench fill material136such that the contacts directly couple conductive material136.

Although the contacts inFIG. 6are illustrated as all being coupled to bias voltage supply line130, this example is merely illustrative. If desired, each contact may be coupled to an individual bias voltage supply line such that the bias voltage at each contact can be independently controlled.

FIG. 7is a cross-sectional side view of an illustrative image pixel with an additional biasing layer. As shown inFIG. 7, layer138may be provided at the front side of the photodiode. Layer138may be coupled to bias voltage supply line130and may help reduce dark current in the pixel. Layer138may be formed from the same material as conductive layer124or a different material from conductive layer124. In general, layer138may be formed from any desired conductive material (e.g., metal, polysilicon, etc.). It may be formed, for example, from the polysilicon together with trunk transistors112. Layer138may be biased in the same way as contacts126. Alternatively, layer138may be biased independently of the other contacts. For example, layer138may be biased at a higher voltage level than contacts126during pixel readout. Note that inFIGS. 6 and 7, various trunk transistors are not included in the drawings so as to not obfuscate the drawings.

FIGS. 8A, 8B, and 8Cshow top views of an illustrative pixel array with pixels that include biasing conductive layers. As shown inFIG. 8A, each row of pixels (with photodiodes118) may be covered by a corresponding conductive layer. In this embodiment, each row's conductive layer can be independently controlled. As shown, the pixels in the first row are covered by conductive layer124A which is coupled to bias voltage supply line130A. Similarly, the pixels in the second row are covered by conductive layer124B which is coupled to bias voltage supply line130B. During image acquisition, the bias voltage supply lines may default to supplying a first bias voltage. When a row is undergoing readout, that row's conductive layer may be biased to a second bias voltage while the remaining rows continue to supply the first bias voltage. The second bias voltage may be lower than the first bias voltage.

InFIG. 8B, each row has two independently controlled biasing layers. The pixels in the first row have two conductive layers124A and124B which are coupled to bias voltage supply lines130A and130B respectively. The pixels in the second row have two conductive layers124C and124D which are coupled to bias voltage supply lines130C and130D respectively.

InFIG. 8C, a mesh-like configuration of biasing layer per each pixel rows is shown that provides for better light transfer. Conductive mesh material may be thin metal or any other suitable conductive material such as polysilicon or indium tin oxide.

The illustrative examples shown inFIGS. 8A and 8Bare merely a few of the many possible arrangements of conductive biasing layers in a pixel array. A single conductive layer may be used to bias a portion of a single pixel, a single pixel, multiple pixels in the same row, or multiple pixels in multiple rows. Each pixel may be biased by one conductive layer, two conductive layers, three conductive layers, or more than three conductive layers.

The use of conductive layers coupled to bias voltage supply lines has been described herein in the context of visual and near infrared pixels. However, biasing conductive layers of this kind are not limited to these pixels and may be used in any desired type of pixel. For example, the biasing conductive layers may be used in color light pixels. Color light pixels may have color filter elements such as blue color filter elements, red color filter elements, green color filter elements, yellow color filter elements, clear color filter elements, etc. In certain embodiments, the aforementioned biasing conductive layers may be used to adjust spectral response in visible light pixels. For example, a pixel array may be patterned with a Bayer color filter array such that there are red, green, and blue color filters. In this example, it is desirable for the red and green pixels to pass no blue light. However, some blue light may still pass through to the red and green pixels. A bias voltage may be applied to a conductive layer that covers the red and green pixels, which may reduce the amount of blue light collected by those pixels.

As described in the previous example, biasing conductive layers may be formed in any desired pattern over a pixel array. For example, a biasing conductive layer may only cover pixels of a certain color or colors. In general, any number of biasing conductive layers may be formed in any combination, pattern, or shape, depending on the specific design needs of the image sensor.

In various embodiments of the invention, an imaging pixel may include a substrate, a photodiode formed on the substrate, a conductive layer that covers the imaging pixel such that incident light passes through the conductive layer to reach the photodiode, and a bias voltage supply line that is coupled to the conductive layer and provides a bias voltage to the conductive layer. The conductive layer may include polysilicon, aluminum, tungsten, indium tin oxide, or a metal mesh. The imaging pixel may include doped silicon, and the doped silicon may be curved to form a microlens for the imaging pixel. The imaging pixel may also include at least one inversion layer formed on the doped silicon that is interposed between the doped silicon and the conductive layer. The at least one inversion layer may include a layer of hafnium oxide and/or a layer of tantalum pentoxide.

The imaging pixel may be a back side illuminated imaging pixel. The imaging pixel may also include a reflector formed on a front side of the imaging pixel. The conductive layer may be formed on a back side of the imaging pixel. The imaging pixel may also include an additional conductive layer formed on a front side of the imaging pixel. The additional conductive layer may be coupled to the bias voltage supply line. The conductive layer may be formed on a back side of the imaging pixel. The imaging pixel may also include an additional conductive layer formed on a front side of the imaging pixel. The additional conductive layer may be coupled to an additional bias voltage supply line that provides a different bias voltage than the bias voltage supply line. The bias voltage supply line may be configured to provide the bias voltage to the conductive layer during an image acquisition period. The bias voltage supply line may be configured to provide an additional bias voltage to the conductive layer during a readout period. The additional bias voltage may be lower than the bias voltage.

In various embodiments, an image sensor with an array of imaging pixels may include a doped silicon substrate, an implanted region formed on the doped silicon substrate, epitaxial silicon formed on the implanted region, a conductive layer formed over the epitaxial silicon such that incident light passes through the conductive layer to reach the epitaxial silicon, and a bias voltage supply line that is coupled to the conductive layer and provides a bias voltage to the conductive layer. The epitaxial silicon may have etched portions that form trenches between each imaging pixel in the array of imaging pixels. The image sensor may also include doped isolation regions formed in the implanted region. The doped isolation regions may be formed between each imaging pixel in the array of imaging pixels such that the trenches in the epitaxial silicon overlap the doped isolation regions. The implanted region may have etched portions that form additional trenches between each imaging pixel in the array of imaging pixels such that the trenches in the epitaxial silicon overlap the additional trenches.

The image sensor may also include an inversion layer interposed between the conductive layer and the epitaxial silicon. The array of imaging pixels may include a first row of imaging pixels, and the conductive layer may only cover the first row of imaging pixels. The array of imaging pixels may include a first row of imaging pixels and an additional plurality of rows of imaging pixels. The conductive layer may cover the first row of imaging pixels, and the image sensor may also include a plurality of additional conductive layers and a plurality of additional bias voltage supply lines. Each conductive layer of the plurality of additional conductive layers may cover a respective row of imaging pixels of the additional plurality of rows of imaging pixels, and each bias voltage supply line of the plurality of additional bias voltage supply lines may be coupled to a respective conductive layer of the plurality of additional conductive layers.

In various embodiments, a method of operating an imaging pixel with a photodiode, a conductive layer that covers the imaging pixel such that incident light passes through the conductive layer to reach the photodiode, and a bias voltage supply line may include using the bias voltage supply line to supply a first bias voltage to the conductive layer during a first time period, and using the bias voltage supply line to supply a second bias voltage to the conductive layer during a second time period. The second bias voltage may be different than the first bias voltage. The second bias voltage may be lower than the first bias voltage. The first time period may include an image acquisition period during which the photodiode collects charge, and the second time period may include a readout period during which the charge is transferred out of the photodiode using a transfer gate.