Image sensor with a reflective waveguide

An image sensor having shield structures and methods of forming the same are provided. Generally, the image sensor includes: (i) substrate having at least one photosensitive element formed therein; (ii) a dielectric layer overlying the substrate and the photosensitive element; and (iii) an annular reflective waveguide disposed in the dielectric layer above the photosensitive element to reduce cross-talk between adjacent elements of the sensor while increasing sensitivity of the sensor. In certain embodiments, the sensor further includes a photoshield disposed in the dielectric above the photosensitive element and about the waveguide to further reduce the possibility of cross-talk. Other embodiments are also disclosed.

TECHNICAL FIELD

The present invention relates generally to image sensors, and more particularly to image sensors having shield structures to prevent cross-talk between adjacent sensors and to increase photo intensity of light received thereon and methods for forming the same.

BACKGROUND OF THE INVENTION

Solid-state image sensors are widely used in a number of optoelectronic applications including digital cameras, cellular phones, and optical navigation or pointing systems, such as optical computer mice, trackballs, touch screens scroll bars and the like. Conventional solid-state image sensors typically include an array of photosensitive elements, such as charge-coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) photodiodes. Typically, one or more adjacent photosensitive elements are coupled in an elementary unit or picture element commonly known as a pixel.

A conventional image sensor will now be described with reference toFIGS. 1A and 1B. Referring toFIG. 1A, a portion of the image sensor100is shown in cross section. The image sensor100typically includes a semiconductor substrate102in or on which an array of pixels104A,104B has been formed. Each pixel104A,104B shown in these figures can include one or more adjacent photosensitive elements, for example, CCDs or photodiodes (not shown). The pixels104A,104B convert incoming light106from an imaging plane or a light source108into electrical signals via the CCDs or photodiodes. The substrate100is covered by one or more dielectric layers110, which may be substantially transparent to the incoming light106, or may include a number of transparent portions112, as shown, extending therethrough to transmit the incoming light to the pixels102A, and102B.

Incident light106striking the top surface of the sensor100transmitted through the underlying dielectric layer110or the transparent portion112of the dielectric down to the underlying pixel102A or102B. However, it is a common occurrence for the incident light106to strike the top surface of the sensor100surface at a variety of angles. Thus, light106striking the image sensor100at a perpendicular or nearly perpendicular angle to a surface thereof is propagated unimpeded to the underlying pixel102A. However, where the dielectric layer110is substantially transparent to at least some wavelengths of the incoming light106, and part of the light106A strikes the image sensor100at other than a perpendicular angle, a portion of the light can be transmitted to the adjacent pixel102B rather than the pixel102A underlying the strike location. This undesirable effect is commonly called cross-talk and results in reduced accuracy or image resolution of the image sensor100. Moreover, even where this scattering of light in dielectric does not result in striking an adjacent pixel102B causing cross-talk, the loss or non-sensing of the light by the pixel102A underlying the strike location results in reduced photo-efficiency or sensitivity of the image sensor100.

Referring toFIG. 1A, one approach used in conventional image sensors100to minimize if not eliminate cross-talk is the introduction of one or more intermetal dielectric (IMD) layers114A,114B overlying or between the dielectric layers110to reduce or impede the transmission of light therethrough.FIG. 1B, illustrates a top planar view of a portion of the image sensor100ofFIG. 1A, showing a top IMD layer114A and lower IMD layer114B in cut-away. This solution is not wholly satisfactory in that depending on the number, location and size of the IMD layers114A,114B not all of the light106A striking at a highly oblique angle will be stopped, and the IMD layers do not improve the photo-efficiency or sensitivity of the image sensor100. Moreover, the use of multiple metal IMD layers114A,114B requires they be isolated from one another and conducting elements of the image sensor. This requirement in turn causes an increase in the pixel size, or a decrease of the number of pixels within the image sensor.

FIGS. 2A and 2Billustrate another approach used in conventional image sensors200to reduce cross-talk and improve photo-efficiency or sensitivity of the image sensor. In this approach the image sensor200further includes an air-gap202or ring of dielectric material (not shown), in a transparent dielectric layer204surrounding each pixel206A,206B on the substrate208. Referring toFIG. 2A, light210from a light source212striking the image sensor200at a perpendicular or nearly perpendicular angle to a surface of the image sensor200is propagated unimpeded to the underlying pixel206A. Due to differences in refraction between the material of the dielectric layer204and the air-gap202, light210striking at a less than perpendicular angle is reflected from an interface between the dielectric layer204and the air-gap202, and back towards the underlying pixel206A, thereby reducing cross-talk and improving photo-efficiency of the image sensor200. However, where the incoming light210strikes the image sensor200at a highly oblique angle, the difference in refraction is insufficient to reflect all of the light, and a portion of the light210A can be transmitted to the adjacent pixel206B rather than the underlying pixel206A.

Yet another common approach in conventional image sensors to reduce cross-talk and improve photo-efficiency uses a number or an array of micro-lens (not shown) to focus light onto each element or pixel of the image sensor. This approach is also not wholly satisfactory for a number of reasons. First, as with the other approaches outlined above for light striking the micro-lens array at a highly oblique angle, the lens may be insufficient to redirect all of the light toward the underlying pixel, and away from the adjacent pixel. Moreover, the fabrication of the micro-lens array and/or mating the array with the pixels of the image sensor can significantly increase the cost and/or decrease the throughput and yield of working devices.

Accordingly, there is a need for an image sensor and method of fabricating the same that substantially eliminates cross-talk between adjacent pixels while increasing or improving photo-efficiency or sensitivity of the image sensor. It is further desired that the sensor does not include shields that increase pixel size or reduce the number of pixels of the image sensor. It is still further desirable that the sensor does not require micro-lenses, which increase cost of fabricating the image sensor.

The present invention provides a solution to these and other problems, and offers further advantages over conventional image sensor and method of fabricating the same.

DETAILED DESCRIPTION

The present invention is directed to image sensors having an array of multiple photosensitive elements or pixels, and more particularly to image sensors having shield structures to substantially prevent cross-talk between adjacent pixels and to increase photo intensity of light received thereon.

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures, and techniques are not shown in detail or are shown in block diagram form in order to avoid unnecessarily obscuring an understanding of this description.

Reference in the description to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment. The terms “coupling” and “to couple” as used herein may include both to electrically connect directly and indirectly connect through one or more intervening layers or components.

Briefly, an image sensor having an architecture according to an embodiment of the present invention includes: (i) a substrate having a surface with at least one photosensitive element formed therein; (ii) a dielectric layer disposed on the substrate overlying the photosensitive element; and (iii) an annular reflective guard ring or waveguide disposed in the dielectric layer above the photosensitive element. The waveguide is located such that light incident on a top surface of the image sensor at other than a perpendicular angle is reflected from the waveguide toward the underlying photosensitive element, thereby increasing an intensity of light striking the photosensitive element and reducing or eliminating cross-talk between adjacent photosensitive elements.

Optionally, the image sensor further includes a photoshield adapted to substantially prevent light passing therethrough disposed above and about the photosensitive element, to further decrease or reduce the possibility of cross-talk between adjacent photosensitive elements or pixels.

Image sensors having shield structures according to various embodiments of the present invention will now be described in greater detail with reference toFIGS. 3A to 5. For purposes of clarity, many of the details of semiconductor fabrication in general and fabrication of image sensors in particular that are widely known and are not relevant to the present invention have been omitted from the following description.

In one embodiment, illustrated inFIGS. 3A and 3B, the waveguide is formed in a trench in a dielectric overlying and surrounding the sensor or photosensitive element.FIG. 3Adepicts a partial cross-sectional view of an image sensor300having one such a trench surrounding sensor or TSS architecture.FIG. 3Billustrates a top planar view of a portion of the image sensor300ofFIG. 3A. Referring toFIGS. 3A and 3B, the image sensor300generally includes a semiconductor wafer or substrate302having a surface304in or on which an array of one or more photosensitive elements or pixels306has been formed. It will be appreciated that each pixel306can include one or more adjacent photosensitive devices (not shown), such as charge-coupled devices (CCDs), photodiodes (PDs), phototransistors or the like, to convert incoming light from an imaging plane or a light source into electrical signals.

The substrate302may include any known semiconductor material, such as Silicon, Gallium-arsenide, Germanium, Gallium-nitride, Aluminum-phosphide, and mixtures or alloys thereof. Preferably, the substrate302is a doped or undoped silicon-based semiconductor substrate, such as a monocrystalline silicon, silicon-germanium, silicon-on-insulator, or silicon-on-sapphire substrate.

The substrate302is covered by a dielectric layer308, which may be substantially transparent to the incoming light and/or may further include a number of openings310, as shown, extending therethrough to transmit the incoming light to the pixels306. Preferably, the dielectric layer308is a composite dielectric layer made up of one or more layers of dielectric material having desired electrical and optical properties. Suitable dielectric materials can include, for example, silicon dioxide (SiO2), silicon nitride (SiNy), silicon-oxynitride (SiOxNy), or other advanced or low-k dielectric materials such as FSG. More preferably, the finished dielectric layer308has a total thickness from of about 5,000 angstroms ({acute over (Å)}) to about 20,000 {acute over (Å)}, depending on the number of layers in a multi-layer dielectric.

In a preferred embodiment, the annular reflective guard ring or waveguide312can extend from a top surface314of the dielectric layer308to the surface304of the substrate302, as shown inFIG. 3A. In alternative embodiments, not shown, the waveguide312can begin below the top surface314of the dielectric layer308and/or end a predetermined distance from the surface304of the substrate302. Each of the waveguides312form an annular ring about or around each of the pixels306. It will be appreciated that the waveguides312can include a single cylindrical wall surrounding the pixel306or a number of individual, intersecting walls as shown inFIG. 3B.

Generally, in the TSS architecture shown the waveguides312surround the pixel306and may be located from a point immediately above or adjacent to the pixel to a location some distance away, adjacent to the neighboring pixels. The waveguides312can include a square, rectangular or circular cross-section depending on the shape of the pixel306or group of pixels surrounded thereby, and preferably has dimensions selected in relation to the pixel size or pitch, which in turn are generally selected in relation to a wavelength of sensed light.

More preferably, the reflective waveguide312includes a metal. Most preferably, the waveguide312has a composite structure made up of a number of layers of a refractory metal and/or a refractory metal-nitride, such as Titanium (Ti), Titanium-Nitride (TiN), Tungsten (W) and Tungsten-Nitride (WN).

Optionally, as in the embodiment shown inFIGS. 3A and 3B, the image sensor300further includes a photoshield316overlying the dielectric layer308and the waveguide312and disposed about the pixel306. Generally, the photoshield316also includes one or more layers of a reflective material such as a metal and/or a metal nitride to further decrease or reduce cross-talk between adjacent pixels. Preferably, all of the photoshields316are formed from a single patterned layer or number of layers of a refractory metal and/or a refractory metal-nitride, such as Ti, TiN, W and WN. In certain embodiments, described in greater detail below, this patterned refractory metal or refractory metal-nitride layer may also be used as a hardmask layer during fabrication or manufacture of the image sensor300.

It will be appreciated that because the waveguide312and the photoshields316are formed from a substantially reflective and opaque material, and because the waveguide312extends substantially from the top surface316of the dielectric layer308to the surface304of the substrate302, the radial dimensions or thicknesses of both the waveguide and the photoshields can be significantly reduced as compared to the IMD layer114and air-gap202or rings of the prior art. For example, in the TSS architecture shown the waveguide312can have a radial or sidewall thickness of from about 10 {acute over (Å)} to about 100 {acute over (Å)}, while the photoshields316can extend from the opening310or the pixel306to a point substantially adjacent to the neighboring pixels or openings thereto. Thus, avoiding the increase in pixel size or reduction in pixel count required in the prior art while still eliminating cross-talk and increasing the intensity of light received by the pixels306, thereby improving photo-efficiency or sensitivity of the image sensor300.

In another embodiment, illustrated inFIGS. 4A and 4B, the waveguide is formed in a trench in a dielectric overlying and immediately above the sensor or photosensitive element.FIG. 4Adepicts a partial cross-sectional view of an image sensor400having one such a trench above sensor or TAS architecture.FIG. 4Billustrates a top planar view of a portion of the image sensor400ofFIG. 4A. Referring toFIGS. 4A and 4B, the image sensor400generally includes a semiconductor substrate402having a surface404in or on which an array of one or more photosensitive elements or pixels406has been formed. As in the TSS architecture described above with respect toFIGS. 3A and 3B, each pixel406can include one or more adjacent photosensitive devices (not shown), such as CCDs, PDs, phototransistors or the like.

As also described above, the substrate402may include any known semiconductor material, and preferably is a doped or undoped silicon-based semiconductor substrate.

The substrate402is covered by a dielectric layer408, which may be substantially transparent to the incoming light or may further include a number of openings410, as shown, extending therethrough to transmit the incoming light to the pixels406. As in the TSS architecture described above with respect toFIGS. 3A and 3B, the dielectric layer408is preferably a composite dielectric layer made up of one or more layers of dielectric material including, for example, SiO2, SiNy, SiOxNy, or other advanced dielectric materials such as FSG. More preferably, the dielectric layer408has a total thickness from of about 5,000 {acute over (Å)} to about 20,000 {acute over (Å)}, as in the TSS architecture described above.

Preferably, the annular reflective guard ring or waveguide412includes a number of layers of a refractory metal and/or a refractory metal-nitride, such as Ti, TiN, W and WN. More preferably, the waveguide412extends throughout the dielectric layer408from a top surface414thereof to the surface404of the substrate402, and abuts the opening410to clad or form a sidewall of thereof.

In one embodiment, described in greater detail below, the waveguide412is formed by depositing at least a first glue layer of Ti or TN formed directly on the sidewalls of the opening410in the dielectric layer408, followed by further layers of metal, such as W or Ti, and the subsequent removal of metal on bottom surfaces of the opening to expose the pixel406.

Optionally, the trench or opening410may be filled with a void-less dielectric material substantially transparent to a wavelength of light to which the number of photosensitive elements is sensitive. Suitable materials include, for example, spin-on-glass (SOG), phosphosilicate glass (PSG), or borophosphosilicate glass (BPSG).

As shown inFIG. 4A, the image sensor400can further include a photoshield416overlying the dielectric layer408and disposed about the reflective waveguide412to further decrease or reduce cross-talk between adjacent pixels406. Generally, the photoshield416includes more or more layers of a reflective material, such as a metal and/or a metal-nitride. Preferably, all of the photoshields416in the image sensor400are formed from a single patterned layer or number of layers of a refractory metal and/or a refractory metal-nitride, such as Ti, TiN, W and WN.

In certain embodiments, described in greater detail below, this patterned refractory metal or refractory metal-nitride layer may also be used as a hardmask layer during fabrication or manufacture of the image sensor400to form or etch the openings410in the dielectric layer408.

It will be appreciated that because of the manner in which the waveguide412and the photoshields416are formed the radial dimensions or thicknesses of the waveguide can be significantly less than that of the IMD layer114and air-gap202or rings of the prior art, and even that of the waveguide312used in the TSS architecture. For example, in the TAS architecture shown the waveguide412can have a radial or sidewall thickness of from about 10 {acute over (Å)} to about 100 {acute over (Å)}.

As with the TSS architecture ofFIGS. 3A and 3B, the waveguides412can include a square, rectangular or circular cross-section depending on the shape or of the pixel406or group of pixels enclosed, and can be located from a point immediately above or adjacent to the pixel to a location some distance away, adjacent to the neighboring pixels. More preferably, the waveguides412have cross-sectional dimensions selected in relation to the pixel size or pitch, which in turn are generally selected in relation to a wavelength of sensed light.

The ability of an image sensor having shield structures according to the present invention to prevent cross-talk between adjacent photosensitive elements and to increase photo intensity of light received thereon will now be illustrated with reference toFIG. 5.FIG. 5is a block diagram of a portion of an image sensor500having a trench surrounding a sensor (TSS) architecture similar to that described in detail above with reference toFIGS. 3A and 3B. To review, the image sensor500generally includes a substrate502having a surface504in which an array of pixels506A and506B has been formed. The substrate502is covered by a dielectric layer508, which includes a number of openings510through which incoming light is transmitted to the pixels506A and506B. The shield structures shown inFIG. 5include an annular reflective guard ring or waveguide512extending from a top surface514of the dielectric layer508to the surface504of the substrate502, and a photoshield516overlying the dielectric layer and the waveguide and disposed about the pixels506A and506B.

Referring toFIG. 5, light518A from a light source520striking the image sensor500at a perpendicular or nearly perpendicular angle to a surface of the image sensor200is propagated unimpeded to the underlying pixel506A. Light518B striking at a less than perpendicular angle is reflected from the waveguide512back towards the underlying pixel506A. Because substantially the entire light incident on the reflective waveguide512is reflected, unlike with the air-gap202of the prior art, the photo-efficiency of the image sensor500is improved. Moreover, even where the incoming light518C strikes the image sensor500at a highly oblique angle, photoshield516blocks propagation of the light to an adjacent pixel506B, thereby substantially reducing if not eliminating cross-talk.

Methods for fabricating image sensors having shield structures to substantially prevent cross-talk between adjacent pixels and to increase photo intensity of light received thereon according to various embodiments of the present invention will now be described in greater detail with reference toFIGS. 6 and 7, whereFIG. 6is a flowchart of a method for forming an image sensor having a TSS architecture, andFIG. 7is a flowchart of a method for forming an image sensor having a TAS architecture.

Referring toFIG. 6, the method begins with forming a number of photosensitive elements or pixels on a surface of a substrate (step602). Generally, the photosensitive elements are fabricated using standard semiconductor fabrication processes, and can each include a plurality of photosensitive devices, such as photo-diodes or CCDs, as described above. Next, a dielectric layer is formed on the substrate overlying the photosensitive elements (step604). The dielectric layer may be a composite of one or more layers of dielectric material, such as SiO2, SiN or SiON, and is generally formed any suitable standard processing means including, for example, being thermally grown, or deposited using Chemical Vapor Deposition (CVD).

A number of trenches are formed in the dielectric layer, each of the trenches disposed about one of the photosensitive elements (step606). The trenches may be formed or etched in the dielectric layer using any suitable, highly anisotropic dielectric etch process including, for example, it may be etched using a low pressure radiofrequency (RF) or microwave (MW) plasma containing fluorinated hydrocarbon and/or fluorinated carbon compounds.

Next, the trenches are filed with a metal comprising reflective material to form annular reflective guard rings or waveguides in the dielectric layer disposed about each of the number of photosensitive elements (step608). Preferably, as noted above, the trenches are filled with one or more layers of Ti, TiN or W, using any suitable standard processing means including, for example, CVD, Metal Organic CVD (MOCVD), or atomic layer deposition (ALD).

Preferably, the method further includes the step of forming a number of photoshields overlying the top surface of the dielectric layer, the photoshields including a number of openings extending therethrough and disposed about the number of photosensitive elements (step610). Generally, the photoshields are formed from a patterned layer of a refractory metal and/or a refractory metal nitride, such as Ti, TiN or W, which is deposited using any suitable standard processing means including, for example, CVD, MOCVD, and ALD. The refractory metal/refractory metal nitride layer is then patterned using standard photolithography techniques and a suitable wet or dry etch process that is selective to the underlying dielectric and semiconducting materials.

Optionally, the method may further include the step of forming a number of openings extending through dielectric layer from the top surface thereof to the surface of the substrate to expose the number of photosensitive elements (step612). The openings may be formed or etched in the dielectric layer using any suitable, highly anisotropic dielectric etch process such as that described above for forming the trenches around the photosensitive elements. That is the openings may be etched using a low pressure, RF or MW coupled plasma containing fluorinated hydrocarbon and/or fluorinated carbon compounds. In certain embodiments, where the step of forming a number of photoshields, step610, precedes the step of forming a number of openings extending through the dielectric layer, step612, and the photoshields may be advantageously used to serve as a hardmask for etching the openings through the dielectric layer.

Methods for fabricating image sensors having shield structures with a TAS or trench above sensor architecture will now be described with reference toFIG. 7.

Referring toFIG. 7, the method begins with forming a number of photosensitive elements or pixels on a surface of a substrate (step702), followed by forming a dielectric layer on the substrate overlying the photosensitive elements (step704). The photosensitive elements and dielectric layer may be formed using any suitable processing method as described above in connection with the TSS architecture.

Next, a patterned hardmask layer is formed overlying the dielectric layer (step706). In one preferred embodiment, the patterned hardmask layer is formed a refractory metal and/or a refractory metal nitride, such as Ti, TiN or W, which also serves photoshields in completed image sensor, further reducing cross-talk between adjacent photosensitive elements or pixels. The patterned hardmask layer may be formed using any suitable standard deposition and patterning techniques as described above in connection with the TSS architecture.

A number of openings or trenches are formed in the dielectric layer extending from a top surface thereof to the surface of the substrate using the hardmask layer, each of the number of trenches disposed above one of the number of photosensitive elements (step708). The trenches may be formed or etched using any suitable, highly anisotropic dielectric etch process and selective to the material of the hardmask, such as a low pressure, RF or MW coupled plasma containing fluorinated hydrocarbon and/or fluorinated carbon compounds as described above.

Finally, annular reflective guard rings or waveguides are formed in the dielectric layer above each of the photosensitive elements by forming or depositing a metal comprising layer on surfaces of the number of trenches (step710), followed by removing the metal layer on bottom surfaces of the trenches to expose the photosensitive elements (step712). Preferably, the waveguides are formed by depositing one or more layers of a refractory metal and/or a refractory metal nitride, such as Ti, TiN or W, using any suitable processing method including, for example, CVD, MOCVD, and ALD. More preferably, the metal layer on bottom surfaces of the trenches is removed to form the waveguides using a highly directional etch process, such as Reactive Ion Etching (RIE) or sputtering.

Optionally, the method may further include the step of filling the trenches inside of the annular reflective waveguides with a void-less dielectric material substantially transparent to at least one wavelength of light to which the number of photosensitive elements are sensitive (step714). Suitable dielectric material can include, for example, a flowable oxide, such as SOG, PSG or BPSG, and can be formed by any suitable, processing technique, such as a spin-on-coating, or CVD.

The advantages of the image sensor and methods of forming the same of the present invention over previous or conventional approaches include: (i) substantially eliminating cross-talk between adjacent pixels, thereby improving the accuracy or image resolution of the image sensor; (ii) increasing the intensity of light received by the pixels, thereby improving photo-efficiency or sensitivity of the image sensor; while (iii) avoiding the increase in pixel size or reduction in pixel count required with image sensors having metal shielding used heretofore; and (iv) avoiding the need for a plurality of micro-lens or a micro-lens array to individually focus light on each pixel, thereby reducing fabrication costs and increasing throughput and/or yield of working sensors.

The foregoing description of specific embodiments and examples of the invention have been presented for the purpose of illustration and description, and although the invention has been described and illustrated by certain of the preceding examples, it is not to be construed as being limited thereby. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications, improvements and variations within the scope of the invention are possible in light of the above teaching. For example, it will be appreciated by those skilled in the art that an image sensor according to the present invention can further include or be combined with a single lens or a small array of lens, the purpose of which is to focus light on the array of one or more photosensitive elements or pixels, rather than on an individual pixel, thereby further increasing the photo-efficiency of the image sensor. It is intended that the scope of the invention encompass the generic area as herein disclosed, and by the claims appended hereto and their equivalents. The scope of the present invention is defined by the claims, which includes known equivalents and unforeseeable equivalents at the time of filing of this application.