Light block for pixel arrays

Imager devices are formed with light block material between microlenses to enhance the characteristics of image acquisition. The light block material may be deposited over the lenses, and then partially removed to expose central portions of the lenses. The invention is applicable to, among other things, imager devices having pixel arrays formed with the light block material and integrated with one or more processing components in a semiconductor device.

FIELD OF THE INVENTION

The present invention relates to imaging devices and in particular to a light block material and method of forming a light block for use in imaging devices.

BACKGROUND OF THE INVENTION

There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices and hybrid focal plane arrays. CCDs are often employed for image acquisition in small size imaging applications. CCDs are also capable of large formats with small pixel size and they employ low noise charge domain processing techniques. However, CCD imagers have a number of disadvantages. For example, they are susceptible to radiation damage, they exhibit destructive read out over time, they require good light shielding to avoid image smear and they have a high power dissipation for large arrays.

Because of the inherent limitations in CCD technology, there is an interest in complementary metal oxide semiconductor (CMOS) imagers for possible use as low cost imaging devices. A fully compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits would be beneficial to many digital applications such as, for example, in cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detection systems, image stabilization systems and data compression systems for high-definition television.

A typical CMOS imager includes a focal plane array of pixel cells, each one of the cells including either a photodiode, a photogate or a photoconductor overlying a doped region of a substrate for accumulating photo-generated charge in the underlying portion of the substrate.

In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to a floating diffusion region accompanied by charge amplification; (4) resetting the floating diffusion region to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. The charge at the floating diffusion region is typically converted to a pixel output voltage by a source follower output transistor.

There is a need for an effective light blocking structure in imagers which, as discussed below, can reduce the cross-talk and optical noise caused by stray light during image acquisition.

Accordingly, an improved imaging device capable of minimizing if not eliminating cross-talk and optical noise caused by stray light during image acquisition is needed. There is also a need for an improved method for fabricating imaging devices, in which there is reduced cross-talk and optical noise during image acquisition.

SUMMARY OF THE INVENTION

Exemplary embodiments of the invention provide an imaging device, in which a light blocking material is provided between the lenses of a pixel array. The material provided can either absorb or reflect light to block light transmitted between the lenses from reaching the photosensors. In this manner, the material blocks substantially all light transmitted between the lenses, and thus reduces crosstalk and optical noise.

In another exemplary embodiment, a light blocking material is formed surrounding each lens of a pixel array, such that at least an outer portion of each lens remains uncovered by the material. In this manner, the light blocking material reduces cross-talk and optical noise, and the uncovered part of each lens focuses light in an efficient manner.

Also provided are methods of forming an imaging device. In one exemplary method embodiment, an imaging device is produced by forming at least one light blocking material between each lens of a pixel array, in order to reduce cross-talk and optical noise.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following detailed description, reference is made to various specific embodiments in which the invention may be practiced. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be employed, and that structural and logical changes may be made without departing from the spirit or scope of the present invention.

The terms “substrate” and “wafer” can be used interchangeably in the following description and may include any semiconductor-based structure. The structure should be understood to include silicon, silicon-on insulator (SOI), silicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. The semiconductor need not be silicon-based. The semiconductor could be silicon-germanium, germanium, or gallium arsenide. When reference is made to the substrate in the following description, previous process steps may have been utilized to form regions or junctions in or over the base semiconductor or foundation.

The term “pixel” refers to a discrete picture element unit cell containing a photoconductor and transistors for converting electromagnetic radiation to an electrical signal. For purposes of illustration, a representative pixel array is illustrated in the figures and description herein. An array or combination of pixels together comprise a pixel array for use in a CMOS imager device. Typically, fabrication of all pixel arrays in an imager will proceed simultaneously in a similar fashion.

Now referring to the figures, where like reference numbers designate like elements,FIG. 1Adepicts a top view of a section of a pixel array100comprising a plurality of lenses120formed over a base layer, for example a polyimide layer116. At this initial stage of processing, gaps155remain between each lens120. Alternatively, not all lens arrays have gaps between the lenses. The methods of the invention may also be used with arrays having no gaps between the lenses.

FIG. 1Bdepicts a schematic cross-sectional view of theFIG. 1Aarray100taken along line A-A′. The substrate140of imager122may comprise integrated circuits and other semiconductor components that are typically incorporated into an imager device.FIG. 1Bdepicts photosensor112near the surface of the semiconductor substrate140. Photosensors112may be formed beneath the surface of the substrate140. Photosensors112may be fabricated using conventional techniques and are shown to illustrate one environment of many in which the present invention may be employed.

FIG. 1Balso shows a transfer gate127formed within each pixel. For clarity, other devices are not shown inFIG. 1B. After formation of pixels in array100, the array100is covered with one or more insulating layers, for example layer114. Layer114may comprise, for example, silicon dioxide (SiO2), boro-phospho-silicate glass (BPSG), or other suitable insulating materials. An interlayer dielectric (ILD)115is then formed over layer114. Although only one ILD layer115is shown, more than one ILD layer may be formed. In addition, a passivation layer117is formed comprising, for example, a phospho-silicate-glass (PSG), silicon nitride or oxynitride. Although only one passivation layer117is shown, more than one passivation layer may be formed.

A color filter119is then formed over each pixel. In the embodiment shown inFIG. 1B, each color filter119is formed between a photosensor112and a corresponding lens120. Each color filter119may be formed from a pigmented or dyed material that will only allow a narrow band of light to pass through, for example, red, blue, or green.

Lenses120are formed above the color filters119. As shown inFIG. 1B, each lens120is formed over a corresponding photosensor112(which may be a photodiode, photogate, etc.). Although the lenses120are shown as having a generally rounded or curved outer surface, the lenses120may be formed in any desired shape and/or size suitable for the reception of incoming light.

The process for making exemplary pixel arrays with light block materials according to the invention will now be described with reference toFIGS. 2-4. With reference toFIGS. 2A,2B,3A,3B,4A and4B, like numerals correspond to like numbered parts as described above, with reference toFIGS. 1A and 1B.

Turning toFIG. 2A, after formation and processing of the lenses120, a light block material180is applied to the array100at a thickness sufficient to cover the lenses120. The application of material180may be performed using conventional techniques, including for example surface deposition and micromachining processes.

Different materials can be used to form the light block material180. For example, the light block material180may comprise any material that substantially operates to either absorb or reflect incoming light. For example, the light block material180may comprise at least one layer of black photoresist, or at least one metal layer, such as aluminum. A suitable metal alloy may also be used. The light block material180may also comprise a suitable hard coat material deposited with at least one metal or metal alloy. Light block material180can also be used with any other suitable, non-metallic materials to block stray light. Light block material180may also be formed over any type of lens arrangement including, but not limited to, curved lens as depicted inFIGS. 1-4, or any other shaped lenses of any size and number.

The light block material180may be applied by any suitable technique, including one or more spin-on techniques or any other technique for material deposition. The light block material180may comprise any reflective or high optical density material which can be applied after microlens processing. For example, a high optical density material, such as black photoresist, may be applied by coating over a lens array, then ashed leaving the material over and between each lens, as shown inFIG. 2B. As shown inFIG. 2B, a layer of light block material180is deposited in a substantially uniform manner over and between each lens120to any desired thickness.

Turning toFIGS. 3A and 3B, further processing is performed to remove a portion of the light block material180in a substantially uniform manner to expose the outer surface125of each lens120, while leaving a remaining portion184(FIG. 3B) of the light block material180surrounding and in between each lens120. Dashed lines123represent the outer portion of each lens120that is covered by light block material180. The total area of each lens120that is covered by the light block material180, as shown inFIG. 3A, is represented by the area between dashed lines123and the exposed outer surface125of each lens120. Width “X” represents one portion of this total area for each covered lens120, i.e. the area between dashed lines123and the exposed outer surface125of each lens120. Removal of the portions of the light block material180may be accomplished using any suitable technique, including etching, until the desired thickness of portion184remains between lenses120.

A sufficient amount of light block material180is removed until enough of the outer surface125of each lens120is exposed to efficiently focus incoming light160. The residual light blocking material184is left between the lenses120where light is not to be collected. This substantially prevents unwanted light from entering the pixels, and thus reduces the optical noise of the imager.

As shown inFIG. 3B, incoming light160is transmitted to the outer surface125of each lens120, and then the light160is focused165and propagated by each lens120to the photosensor112. Each lens120formed may receive and propagate light within an interior space of the lens to at least one photosensor112.

AlthoughFIG. 3Bdepicts light block portions184as each having a generally horizontal planar surface, it must be noted that the light block portions184are not limited in shape or dimensions to the depiction in the accompanying figures, but instead can be formed, for example, by any suitable etching technique, to any shape and dimensions desired.

FIG. 4Adepicts a different embodiment after further processing of theFIG. 2Adevice. As shown inFIG. 4A, further processing, for example by planarization and etching, may leave a remaining portion194(FIG. 4B) of light block material180surrounding and in between each lens120. For example, a deposited layer of metal may be planarized and then etched, leaving a metal portion194at a level higher than the lens120(depicted as distance “Y” inFIG. 4B) and surrounding and in between each lens120. As shown inFIG. 4B, incoming light160is transmitted to the outer surface125of each lens120, and the light160is focused165and propagated by each lens120to a photosensor112.

AlthoughFIG. 4Bdepicts light block portions194as each having a generally horizontal planar surface, and generally vertical sidewalls, it must be noted that the light block portions194are not limited in shape or dimensions to the depiction in the accompanying figures, but instead can be formed to any shape and dimensions desired. The light block portions194can, for example, each have concave or convex curved upper surfaces and sidewall surfaces.

Any combination or array of pixels can be formed for color processing and imaging by a CMOS imaging device formed in accordance with the present invention.

FIG. 5illustrates a block diagram of a CMOS imager integrated circuit (IC)808having a pixel array800containing a plurality of pixels arranged in rows and columns, including a region802with, for example, two green pixels (G), one blue pixel (B), and one red pixel (R) arranged in a Bayer pattern. The pixels of each row in array800are all turned on at the same time by row select lines811, and the pixels of each column are selectively output by respective column select lines813.

The row lines811are selectively activated by a row driver810in response to row address decoder820. The column select lines813are selectively activated by a column selector860in response to column address decoder870. The pixel array800is operated by the timing and control circuit850, which controls address decoders820,870for selecting the appropriate row and column lines for pixel signal readout.

The pixel column signals, which typically include a pixel reset signal (Vrst) and a pixel image signal (Vsig), are read by a sample and hold circuit861associated with the column selector860. A differential signal (Vrst-Vsig) is produced by differential amplifier862for each pixel that is amplified and digitized by analog-to-digital converter875(ADC). The analog-to-digital converter875supplies the digitized pixel signals to an image processor880.

If desired, the imaging device808described above with respect toFIG. 5may be combined with other components in a single integrated circuit.FIG. 6illustrates an exemplary processor system1100which may include a CMOS imager or other imaging device1108incorporating features illustrated inFIGS. 1-5. Examples of processor systems include, without limitation, computer systems, camera systems, scanners, machine vision systems, vehicle navigation systems, video telephones, surveillance systems, auto focus systems, star tracker systems, motion detection systems, image stabilization systems, and data compression systems for high-definition television, any of which could utilize the invention.

The system1100, for example, a digital camera system, includes an imaging device1108(FIG. 6) comprising a pixel array containing at least one light block material180formed in accordance with any of the various embodiments of the invention. The system1100generally comprises a central processing unit (CPU)1102, such as a microprocessor, that communicates with an input/output (I/O) device1106over a bus1104. Imaging device1108also communicates with the CPU1102over the bus1104. The system1100also includes random access memory (RAM)1110, and can include removable memory1115, such as flash memory, which also communicates with CPU1102over the bus1104. Imaging device1108may be combined with a processor, such as a CPU, digital signal processor, or microprocessor, with or without memory storage on a single integrated circuit or on a different chip than the processor.

While the above-described embodiments of the invention relate to imagers formed with light block materials to enhance the characteristics of image acquisition, one skilled in the art will recognize that the broad scope of the invention includes various other types of imager devices separately or integrated with one or more processing components in a semiconductor device. For example, although the invention is described above for use in a CMOS image sensor, the broad scope of the invention is not limited to such and may be applicable to any suitable image sensor, for example, CCD image sensors. The above-described array embodiments include red, green, and blue pixels, but monochrome or dichrome arrays or other multichrome arrays with these or other wavelength ranges in the visible or invisible EM spectrum could also be implemented with embodiments of the invention.

The above description and drawings illustrate embodiments which achieve the objects of the present invention. Although certain advantages and embodiments have been described above, those skilled in the art will recognize that substitutions, additions, deletions, modifications and/or other changes may be made without departing from the spirit or scope of the invention. Accordingly, the invention is not limited by the foregoing description but is only limited by the scope of the appended claims.