Image device and methods of forming the same

A photodetector is formed in a front surface of a substrate. The substrate is thinned from a back surface of the substrate. A plurality of dopants is introduced into the thinned substrate from the back surface. The plurality of dopants in the thinned substrate is annealed. An anti-reflective layer is deposited over the back surface of the thinned substrate. A micro lens is formed over the anti-reflective layer. At least one ultraviolet (UV) radiation treatment is performed after at least one of the preceding steps.

TECHNICAL FIELD

This disclosure relates to an image sensor device and methods for forming an image sensor device.

BACKGROUND

An image sensor device is one of the building blocks in a digital imaging system such as a digital still or video camera. An image sensor device includes a pixel array (or grid) for detecting light and recording intensity (brightness) of the detected light. The pixel array responds to the light by accumulating a charge—for example, the more light, the higher the charge. The accumulated charge is then used (for example, by other circuitry) to provide a color and brightness signal for use in a suitable application, such as a digital camera. One type of image sensor device is a backside illuminated (BSI) image sensor device. BSI image sensor devices are used for sensing a volume of light projected towards a backside surface of a substrate (which supports the image sensor circuitry of the BSI image sensor device). The pixel grid is located at a front side of the substrate, and the substrate is thin enough so that light projected towards the backside of the substrate can reach the pixel grid. BSI image sensor devices provide a reduced destructive interference, as compared to front-side illuminated (FSI) image sensor devices.

Integrated circuit (IC) technologies are constantly being improved. Such improvements frequently involve scaling down device geometries to achieve lower fabrication costs, higher device integration density, higher speeds, and better performance. Along with the advantages realized from reducing geometry size, improvements are being made directly to the image sensor devices.

Due to device scaling, improvements to image sensor device technology are continually being made to further improve image quality of image sensor devices. Although existing image sensor devices and methods of fabricating image sensor devices have been generally adequate for their intended purposes, as device scaling down continues, they have not been entirely satisfactory in all respects.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components are arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. Moreover, the formation of a first feature over a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiment in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact. Further still, references to relative terms such as “top”, “front”, “bottom”, and “back” are used to provide a relative relationship between elements and are not intended to imply any absolute direction. Various features may be arbitrarily drawn in different scales for simplicity and clarity.

FIG. 1is a top view of an image sensor device100according to various aspects of the present disclosure. In the depicted embodiment, the image sensor device is a backside illuminated (BSI) image sensor device. The image sensor device100includes an array of pixel regions101. Each pixel region101is arranged into a column (for example, C1to Cx) and a row (for example, R1to Ry). The term “pixel region” refers to a unit cell containing features (for example, a photodetector and various circuitry), which may include various semiconductor devices for converting electromagnetic radiation to an electrical signal. Photodetectors in the pixel regions101may include photodiodes, complimentary metal-oxide-semiconductor (CMOS) image sensors, charged coupling device (CCD) sensors, active sensors, passive sensors, and/or other sensors. The pixel regions101may be designed having various sensor types. For example, one group of pixel regions101may be CMOS image sensors and another group of pixel regions101may be passive sensors. In the depicted embodiment, each pixel region101may include a photodetector, such as a photogate-type photodetector, for recording an intensity or brightness of light (radiation). Each pixel region101may also include various semiconductor devices, such as various transistors including a transfer transistor, a reset transistor, a source-follower transistor, a select transistor, some other suitable transistor, or combinations thereof. Additional circuitry, inputs, and/or outputs may be in a periphery region of the image sensor device100. Those circuitry, inputs, and/or outputs in the periphery region are coupled to the pixel regions101to provide an operation environment for the pixel regions101and support external communications with the pixel regions101. For simplicity, an image sensor device including a single pixel region is described in the present disclosure; however, typically an array of such pixel regions may form the image sensor device100illustrated inFIG. 1.

FIG. 2is an enlarged top view of a pixel region101in the image sensor device100on a front surface of a substrate (not illustrated inFIG. 2). The pixel region101refers to a unit cell containing at least one photodetector106and various circuitry for converting electromagnetic radiation to an electrical signal. In the depicted embodiment, the photodetector106includes a photodiode for recording an intensity or brightness of light (radiation). The pixel region101may contain various transistors including a transfer transistor110, a reset transistor112, a source-follower transistor114, a select transistor116, or other suitable transistors, or combinations thereof. The pixel region101may also include various doped regions in the substrate, for example doped region113,115and117. The doped regions113,115and117are configured as source/drain regions of previously mentioned transistors. The doped region1117is also referred as a floating diffusion region117. The floating diffusion region117is between the transfer transistor110and the reset transistor112, and is one of the source/drain regions for transfer transistor110and the reset transistor112. A conductive feature132overlaps a portion of a gate stack of the source-follower transistor114and connects to the floating diffusion region117. The image sensor device100also includes various isolation features108formed in the substrate to isolate various regions of the substrate to prevent leakage currents between various regions. In some embodiments, the isolation features includes dielectric isolation features formed by a shallow trench isolation (STI) technique. In certain embodiments, the isolation features may include doped isolation features formed by an implantation technique. In the depicted embodiment, an isolation feature108is formed in the pixel region101to isolate the photodetector106, the transfer transistor110, the reset transistor112, the source-follower transistor114and the select transistor116. The image sensor device100further includes a color filter (not shown) and a lens (not shown) disposed over a back surface of the substrate. The color filter and the lens are aligned with the photodetector106.

In operation of the image sensor device100according to one or more embodiments, the image sensor device100is designed to receive radiation traveling towards the back surface of the substrate. The lens disposed over the back surface of the substrate directs the incident radiation to the corresponding photodetector106in the substrate. The incident radiation generates electron-hole pairs. When exposed to the incident radiation, the photodetector106responds to the incident radiation by accumulating electrons. The holes are trapped by a doped layer over the back surface of the substrate to prevent the re-combination of the electrons and the holes. The electrons are transferred from the photodetector106to the floating diffusion region117when the transfer transistor110is turned on. Through the connection of the conductive feature132, the source-follower transistor114may convert the electrons from the floating diffusion region117to voltage signals. The select transistor116may allow a single row of the pixel array to be read by read-out electronics. The reset transistor112acts as a switch to reset the floating diffusion region117. When the reset transistor112is turned on, the floating diffusion region117is effectively connected to a power supply clearing all accumulated electrons.

FIG. 3is a flowchart of a method300of forming an image sensor device according to one or more embodiments of this disclosure. The flowchart of the method300begins, at operation301, with a substrate that has a front surface and a back surface, wherein a photodetector is formed in the front surface within a pixel region of the substrate. In at least one embodiment, forming the photodetector within the pixel region includes forming a light-sensing region along the front surface of the substrate and forming a doped pinned layer overlapping the light-sensing region at the front surface of the substrate. The forming of the light-sensing region may include introducing a first conductivity type of dopants from the front surface of the substrate. The forming of the doped pinned layer may include introducing a second conductivity type of dopants opposite to the first conductivity type from the front surface into the light-sensing region. Next, the method300continues with operation302in which the substrate is thinned from the back surface of the substrate. In at least one embodiment, a planarization process, such as a chemical mechanical polishing (CMP) process, is applied to the back surface of the substrate to reduce a thickness of the substrate. The method300continues with operation310in which an ultraviolet (UV) radiation treatment is (optionally) performed over the back surface of the thinned substrate. In some embodiments, the UV radiation treatment has a wavelength ranging from about 200 nm to about 410 nm. The UV radiation treatment includes an operation energy ranging from about 3 joule (J) to about 150 J; an operation intensity ranging from about 35 mW/cm2to about 70 mW/cm2; and an operation temperature ranging from about 50° C. to about 120° C.

The method300continues with operation303in which dopants are introduced into the thinned substrate from the back surface. In some embodiments, the dopants are implanted from the back surface into the thinned substrate. The dopants have the second conductivity type as the doped pinned layer. In the certain embodiments, the UV radiation treatment (operation310) is (optionally) performed after operation303. The method300continues with operation304in which an anneal process is performed over the thinned substrate. In the certain embodiments, the UV radiation treatment (operation310) is (optionally) performed after operation304. The method300continues with operation305in which an anti-reflective layer is formed over the back surface of the thinned substrate. In the certain embodiments, the UV radiation treatment (operation310) is (optionally) performed after operation305. The method300continues with operation306in which a micro lens is formed over the anti-reflective layer. In the certain embodiments, the UV radiation treatment (operation310) is (optionally) performed after operation306. Further, it is understood that additional steps can be provided before, during, and after the method300. For example, the method300may further include forming a solder bump or copper bump after operation306to complete the forming of an image sensor device.

During the processes in method300, positive charges may accumulate on the back surface of the substrate. The positive charges may penetrate through the substrate near the photodetectors. The positive charges attract the electrons in electron-hole pairs from the incident radiation before the electrons arrive at the photodetectors. A dark current (current that flows in the image sensor device in absence of incident light on the image sensor device) and a white pixel (where an excessive amount of current leakage causes an abnormally high signal from the pixel) may result. By applying the UV radiation treatment (operation310), the positive charges are released by gaining enough energy acquired from the UV radiation to surmount the energy barrier for electron exciting. The UV radiation treatment (operation310) may release the positive charges and generate negative charges over the back surface of the substrate. The negative charges attract the holes in electron-hole pairs from the incident radiation to prevent the re-combination of the electrons and the holes. More electrons are collected by the photodetectors and are converted to voltage signals. Hence, the phenomenon of dark current or white pixel is reduced. The device performance is thus improved.

According to one or more embodiments of this disclosure, the method300includes performing at least one UV radiation treatment (operation310) after at least one of the operations302,303,304,305or306. In certain embodiments, the method300performs one UV radiation treatment (operation310) after introducing a plurality of dopants into the thinned substrate from the back surface (operation303) while performing the UV radiation treatment after the operations302,304to306is optional. In some embodiments, the method300includes more than one UV radiation treatment (operation310). In at least one embodiment, the method300performs UV radiation treatments (operation310) after both operations303and306. Note that, although only a single UV radiation treatment operation310is depicted, which could be performed multiple times, it is also within the contemplated scope of the present disclosure that different radiation treatment operations310(i.e. having differing process parameters such as different wavelengths, different energies, different operation intensities, or operation temperatures, or the like) could be employed at different stages of the method depicted inFIG. 3.

FIGS. 4A to 4Fare cross-sectional views of the pixel region101and a periphery region102in image sensor device100at various stages of manufacture according to various embodiments of the method ofFIG. 3. Various figures have been simplified for a better understanding of the inventive concepts of the preset disclosure

Referring back toFIG. 3, the method300proceeds from operation301in which a photodetector is formed a in front surface a substrate.

FIG. 4Ais a cross-sectional view of the pixel region101along line B-B′ inFIG. 2and the periphery region102in the image sensor device100after performing operation301. The image sensor device100includes a substrate104having a front surface104A and a back surface104B. In the depicted embodiment, the substrate104is a semiconductor substrate including silicon. Alternatively or additionally, the substrate104includes another elementary semiconductor, such as germanium and/or diamond; a compound semiconductor including silicon carbide, gallium arsenide, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. The substrate104may be a semiconductor on insulator (SOI). The substrate104may have various doping configurations depending on design requirements (e.g., p-type substrate or n-type substrate). In some embodiments, the p-type refers to making holes as majority charge carriers in a semiconductor material, and the n-type refers to making electrons as majority charge carriers in a semiconductor material. In the depicted embodiment, the substrate104is a p-type substrate. P-type dopants that the substrate104is doped with include boron, gallium, indium, other suitable p-type dopants, or combinations thereof.

The pixel region101includes at least one photodetector106formed adjacent the front surface104A of the substrate. The at least one photodetector106, such as a photodiode, includes a light-sensing region106A and a doped pinned layer106B. The forming of the light-sensing region106A may include introducing a first conductivity type of dopants from the front surface104A of the substrate104. In the depicted embodiment, the light-sensing region106A is an n-type doped region. An implantation process may be performed on the light-sensing region106A with n-type dopants into the substrate104. The n-type dopants may include phosphorus, arsenic, other suitable n-type dopants or combinations thereof. The doped pinned layer106B overlaps the light-sensing region106A at the front surface104A of the substrate104. The forming of the doped pinned layer106B may include introducing a second conductivity type of dopants opposite to the first conductivity type of the light-sensing region106A from the front surface104A into the light-sensing region106A. In the depicted embodiment, the doped pinned layer106B is a p-type implanted layer. An implantation process may be performed on doped pinned layer106B with p-type dopants into the substrate104. The p-type dopants may include boron, gallium, indium, other suitable p-type dopants, or combinations thereof.

The pixel region101further includes various transistors, such as the transfer transistor110(shown inFIG. 2), the reset transistor112(shown inFIG. 2), the source-follower transistor114and the select transistor116(shown inFIG. 2). Each transistor has a corresponding gate stack disposed over the front surface104A of the substrate104. The gate stack of each transistor includes a gate dielectric layer and a gate electrode layer. The gate stacks are formed by suitable processes, including deposition, lithography patterning and etching processes. The gate dielectric layer includes a dielectric material, such as silicon oxide, a high-k dielectric material, other dielectric material, or combinations thereof. Examples of high-k dielectric material include HfO2, HfSiO, HfSiON, HfTaO, HfTiO, HfZrO, zirconium oxide, aluminum oxide, hafnium dioxide-alumina (HfO2—Al2O3) alloy or combinations thereof. The gate electrode layer includes polysilicon and/or a metal including Al, Cu, Ti, Ta, W, Mo, TaN, NiSi, CoSi, TiN, WN, TiAl, TiAlN, TaCN, TaC, TaSiN or combinations thereof.

In some embodiments, the pixel region101includes an isolation well109underlying the gate stack of each transistor. A top surface of the isolation well109is under the front surface104A with a distance W1. The distance W1is in a range from about 1000 Å to about 3000 Å. The isolation well109has the second conductivity type opposite to the first conductivity type of the light-sensing region106A. In the depicted embodiment, the isolation well109is a p-type doped region formed by lithography patterning and implantation process. The isolation well109surrounds the light-sensing region106A of photodetector106.

The periphery region102may include readout circuitry and/or control circuitry coupled to the pixel region101to provide an operation environment for the pixel region101. In the depicted embodiment, a PMOS transistor122and a NMOS transistor124are shown. The PMOS transistor122includes a gate stack122A and source/drain regions122B having p-type conductivity formed in a n-type well122C. The NMOS transistor124includes a gate stack124A and source/drain regions124B having n-type conductivity formed in a p-type well124C.

The image sensor device100further includes a plurality of isolation features108formed in substrate104of the pixel region101and a plurality of dielectric isolation features126formed in substrate104of the periphery region102. The isolation features108and the dielectric isolation features126isolate various regions of the substrate104to prevent leakage currents between various regions. In the depicted embodiment, the isolation features108and the dielectric isolation features126isolate the PMOS transistor122and the NMOS transistor124, the photodetector106, the transfer transistor110(shown inFIG. 2), the reset transistor112(shown inFIG. 2), the source-follower transistor114and the select transistor116(shown inFIG. 2).

The dielectric isolation features126in the periphery region102include silicon oxide, silicon nitride, silicon oxynitride, other insulating material, or combination thereof. Each of the dielectric isolation features126extends from the front surface104A into the substrate104. The formation of dielectric isolation features126may include a photolithography process, an etching process to etch a trench from the front surface104A into the substrate104and a deposition process to fill the trench (for example, by using a chemical vapor deposition process) with dielectric material.

In some embodiments, the isolation features108in the pixel region101include dielectric isolation features108formed by a shallow trench isolation (STI) technique similar to the technique for forming the dielectric isolation features126. In certain embodiments, the isolation features108in the pixel region101may include doped isolation features108formed by an implantation technique. Each of the isolation features108extends from the front surface104A into the substrate104. The doped isolation feature108has the second conductivity type, as does the isolation well109. The doped isolation features108and the isolation well109surround the light-sensing region106A of the photodetector106to prevent horizontal leakage paths between the photodetector106and other regions. In the depicted embodiment, doped isolation feature108is a p-type doped region. P-type dopants of the doped isolation feature108include boron (B), BF2, gallium, indium, other suitable p-type dopants or combination thereof.

The image sensor device100further includes a multilayer interconnect (MLI)128disposed over the front surface104A of the substrate104, including over the photodetector106. The MLI128is coupled to various components of the image sensor device100, for example the photodetector106, such that the various components of the image sensor device100are operable to properly respond to illuminated light (imaging radiation). The MLI128includes various conductive features130and132, which may be vertical interconnects130, such as contacts and/or vias130, and horizontal interconnects132, such as lines132. The various conductive features130and132include conductive materials, such as aluminum, aluminum/silicon/copper alloy, titanium, titanium nitride, tungsten, polysilicon, metal silicide, or combinations thereof.

The image sensor device100may further include a carrier wafer136disposed over the front surface104A of the substrate104. In the depicted embodiment, the carrier wafer136is bonded to the MLI128. The carrier wafer136includes silicon or glass. The carrier wafer136can provide protection for the various features (such as the photodetector106) formed on the front surface104A of the substrate104, and can also provide mechanical strength and support for processing the back surface104B of the substrate104.

Referring back toFIG. 3, the method300continues with operation302. The substrate is thinned from the back surface of the substrate. In at least one embodiment, a planarization process, such as a chemical mechanical polishing (CMP) process, is applied to the back surface of the substrate to reduce a thickness of the substrate.

FIG. 4Bis a cross-sectional view of the image sensor device100after performing operation302. In some embodiments, the substrate104is thinned from the back surface104A to a thickness in a range from about 1 μm to about 4 μm. The substrate104may be thinned by grinding, polishing, and/or chemical etching. The thinned substrate104allows more photons to reach the light-sensing region106A from a back surface104C of the thinned substrate104.

Referring back toFIG. 3, the method300continues with operation310in which an ultraviolet (UV) radiation treatment is optionally performed after operation302.

FIG. 4Bis also the cross-sectional view of the image sensor device100while performing operation310. An ultraviolet (UV) radiation treatment150is optionally performed over the back surface104C of the thinned substrate104. By applying the UV radiation treatment150, negative charges may be generated on the back surface104C of the thinned substrate104. The negative charges attract the holes in electron-hole pairs from the incident radiation to prevent the re-combination of the electrons and the holes. Hence, the phenomenon of dark current or white pixel is reduced. The device performance is improved.

In some embodiments, the UV radiation treatment150has a wavelength ranging from about 200 nm to about 410 nm. With wavelengths out of this rage, the positive charges may not gain enough energy acquired from the UV radiation to surmount the energy barrier for electron exciting. In at least one embodiment, the UV radiation treatment150has a wavelength at about 254 nm. The UV radiation treatment150includes an operation energy ranging from about 3 joule (J) to about 150 J; an operation intensity ranging from about 35 mW/cm2to about 70 mW/cm2; and an operation temperature ranging from about 50° C. to about 120° C.

Referring back toFIG. 3, the method300continues with operation303. A plurality of dopants is introduced into the thinned substrate from the back surface.

FIG. 4Cis a cross-sectional view of the image sensor device100while performing operation303. In some embodiments, a plurality of dopants160having the second conductivity type as the doped pinned layer106B is implanted into the thinned substrate104from the back surface104C to form a doped layer138. In the depicted embodiment, the doped layer138is a p-type layer including one p-type dopant, such as boron, gallium, indium, other suitable p-type dopants, or combinations thereof. In the operation of the image sensor device100, the light radiation toward the back surface104C generates electron-hole pairs. The electrons travel toward the light-sensing region106A and are converted to a signal. The holes are trapped by the doped layer138to prevent the re-combination of the electron and the hole. Hence, the quantum efficiency of the image sensor device100is increased.

In the certain embodiments, an UV radiation treatment150is optionally performed after operation303of the method300as shown inFIG. 4D.

Referring back toFIG. 3, the method300continues with operation304. An anneal process is performed on the thinned substrate. In some embodiments, a laser annealing process is performed to repair crystal defects caused by the ion implantation of operation303and to activate the dopants in operation303. In the certain embodiments, an UV radiation treatment (operation310) is optionally performed after operation304of the method300.

Referring back toFIG. 3, the method300continues with operation305in which an anti-reflective layer is formed over the back surface of the thinned substrate.

FIG. 4Eis a cross-sectional view of the image sensor device100after performing operation305. An anti-reflective layer140is deposited over the back surface104C of the thinned substrate104. The anti-reflective layer140allows light to pass through to the photodiode106while minimizing reflection that would decrease the efficiency of the image sensor device100. The anti-reflective layer140may include a dielectric material, such as silicon nitride or silicon oxy-nitride.

In the certain embodiments, an UV radiation treatment150is optionally performed after operation305of the method300as shown inFIG. 4E.

Referring back toFIG. 3, the method300continues with operation306in which a micro lens is formed over the anti-reflective layer.

FIG. 4Fis a cross-sectional view of the image sensor device100after performing operation306. A color filter142and a lens144are formed over the back surface104C of the substrate104. The color filter142is formed over the antireflective layer140, and is substantially aligned with the light-sensing region106A of the photodetector106. The color filter142is configured to pass through light of a predetermined wavelength. For example, the color filter142may pass through visible light of a red wavelength, a green wavelength, or a blue wavelength to the photodetector106. In an example, the color filter142includes a dye-based (or pigment-based) polymer for filtering out a specific frequency band (for example, a desired wavelength of light).

The lens144is formed over the color filter142and is also substantially aligned with the light-sensing region106A of the photodetector106. The lens144may be in various positional arrangements with the photodetector106and color filter142, such that the lens144focuses incident radiation146on the light-sensing region106A of the photodetector106. Alternatively, the position of the color filter layer142and the lens144may be reversed, such that the lens144is disposed between the antireflective layer140and color filter142.

In the certain embodiments, an UV radiation treatment150is optionally performed after the formation of the lens144as shown inFIG. 4F.

In the above depicted embodiments, image sensor device100includes a p-type doped substrate104. Various doping configurations for various features, such as the light-sensing regions106A, the doped isolation feature108, the isolation well region109and the doped layer138, described above should be read consistent with the formation of the image sensor device100with a p-type doped substrate. Alternatively, image sensor device100may include a n-type doped substrate104or a n-type material in the substrate104. Various doping configurations for various features described above should be read consistent with the formation of the image sensor device100with a n-type doped substrate.

One aspect of the disclosure describes a method of forming an image sensor device. At step a, a photodetector is formed in a front surface of a substrate. At step b, the substrate is thinned from a back surface of the substrate. At step c, a plurality of dopants is introduced into the thinned substrate from the back surface. At step d, the plurality of dopants in the thinned substrate is annealed. At step e, an anti-reflective layer is deposited over the back surface of the thinned substrate. At step f, a micro lens is formed over the anti-reflective layer. At step g, at least one ultraviolet (UV) radiation treatment is performed after the steps of b, c, d, e or f.

A further aspect of the disclosure describes a method of forming an image sensor device. A photodetector is formed in a front surface of a substrate. The substrate is thinned from a back surface of the substrate. A plurality of dopants is introduced into the substrate from the back surface. An ultraviolet (UV) radiation treatment is performed after introducing the plurality of dopants. The substrate is annealed. An anti-reflective layer is deposited over the back surface of the substrate. A micro lens is formed over the anti-reflective layer.

Another aspect of the disclosure describes a method of forming an image sensor device. At step a, a photodetector is formed in a front surface of a substrate. At step b, the substrate is polished from a back surface of the substrate. At step c, p-type dopants are introduced into the polished substrate from the back surface. At step d, the p-type dopants in the polished substrate are laser annealed. At step e, an anti-reflective layer is deposited over the polished substrate after the laser annealing. At step f, a color filter is formed over the anti-reflective layer. The color filter substantially aligns with the photodetector. At step g, a micro lens is formed over the anti-reflective layer. At step h, at least one ultraviolet (UV) radiation treatment is performed after the steps of b, c, d, e, f or g.