US 20070007563 A1
An imager with pixels having a resonant-cavity photodiode. The resonant cavity photodiode increases absorption of light having long wavelengths. A trench is formed for the photodiode and reflective film is grown on the bottom of the trench. The reflective film reflects light that is not initially absorbed back to the active region of the photodiode.
42. A method of forming an image sensor comprising the acts of:
forming a trench within a substrate of the image sensor;
forming a reflective layer on at least one surface of the trench; and
forming a photoconversion device within the trench over the reflective layer.
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68. A method of forming an image sensor comprising:
forming a trench for a photoconversion device within a substrate of the image sensor;
depositing a spacer layer on sidewalls of the trench;
etching the spacer layer such that the bottom surface of the trench is exposed;
growing an oxide layer on the exposed bottom surface of the trench; and
forming a photconversion device within the trench, wherein the oxide layer on the bottom surface of the trench has an index of refraction that reflects at least a portion of incident light.
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The present invention relates to the field of semiconductor devices, particularly to an imager pixel with improved quantum efficiency and reduced cross talk.
Typically, a digital imager array includes a focal plane array of pixel cells, each one of the cells including a photoconversion device such as, e.g., a photogate, photoconductor, or a photodiode. In a complementary metal oxide semiconductor (CMOS) imager a readout circuit is connected to each pixel cell which typically includes a source follower output transistor. The photoconversion device converts photons to electrons which are typically transferred to a floating diffusion region connected to the gate of the source follower output transistor. A charge transfer device (e.g., transistor) can be included for transferring charge from the photoconversion device to the floating diffusion region. In addition, such imager cells typically have a transistor for resetting the floating diffusion region to a predetermined charge level prior to charge transference. The output of the source follower transistor is a voltage output on a column line when a row select transistor for the row containing the pixel is activated.
Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524, and U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc. The disclosures of each of the forgoing patents are herein incorporated by reference in their entirety.
In a typical digital CMOS imager pixel (
Band gap refers to the energy levels separating valence bands and conductive bands. Different materials may have indirect or direct band gap characteristics. For example, silicon has indirect band gap characteristics. Due to the presence of the indirect band gap in silicon, photons have long absorption lengths in silicon compared to direct band gap materials like GaAs or InP. At infrared wavelengths (700 nm), silicon has an absorption coefficient of 3×103 cm−1, which corresponds to an absorption length of slightly more than 3.0 μm. This necessitates a very large photodiode thickness in order to obtain a reasonable response. A large photodiode, however, results in poor bandwidth due to large transit times needed for carrier collection.
In the conventional pixel of
Fabry-Perot resonant cavities have been used in other systems, such as lasers, to build up large field intensities at specified resonant frequencies and to act as spatial and frequency filters. In a resonant cavity, a pair of parallel polished planes act like mirrors to create resonance. What is needed, is an imager that can capture longer wavelengths of light (e.g., 650-750 nm or longer) with improved quantum efficiency and without increased cross talk, using a resonant cavity.
Embodiments of the invention provide an imager pixel comprising a reflective layer formed within a photoconversion device. The photoconversion device is formed within a trench of the pixel's substrate. The reflective layer serves to reflect incident light, not initially absorbed, back up toward the surface of the photoconversion device so that it can be efficiently transferred as charge to a charge collection region. The quantum efficiency of the pixel is thereby improved. Cross talk can be reduced in such a structure due to improved optical and electrical isolation between adjacent pixels.
The above and other features and advantages of the invention will be more readily understood from the following detailed description which is provided in connection with the accompanying drawings, in which:
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof and show by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical, and electrical changes may be made without departing from the spirit and scope of the present invention. The progression of processing steps described is exemplary of embodiments of the invention; however, the sequence of steps is not limited to that set forth herein and may be changed as is known in the art, with the exception of steps necessarily occurring in a certain order.
The terms “wafer” and “substrate,” as used herein, are to be understood as including silicon, silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, and other semiconductor structures. Furthermore, when reference is made to a “wafer” or “substrate” in the following description, previous processing steps may have been utilized to form regions, junctions, or material layers in or over the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but could be based on silicon-germanium, germanium, gallium arsenide or other semiconductors.
The term “pixel,” as used herein, refers to a photo-element unit cell containing a photoconversion device and associated transistors for converting photons to an electrical signal. The pixels discussed herein are illustrated and described as inventive modifications to four transistor (4T) pixel circuits for the sake of example only. It should be understood that the invention may be used with other pixel arrangements having fewer (e.g., 3T) or more (e.g., 5T) than four transistors. Although the invention is described herein with reference to the architecture and fabrication of one pixel, it should be understood that this is representative of a plurality of pixels in an array of an imager device. In addition, although the invention is described below with reference to a CMOS imager, the invention has applicability to any solid state imaging device having pixels. The following detailed description is, therefore, not to be taken in a limiting sense.
According to the invention, a resonant cavity is created in a pixel to increase its quantum efficiency, improve absorption of long wavelength photons, improve collection of carriers, minimize carrier generation in neutral regions, remove slow carrier diffusion and reduce cross talk. At 720 nm wavelengths, there is a significant difference between the refractive index of silicon and the index of silicon dioxide films (i.e., nSi=3.8; nSiO2=1.47 at λ=720 nm). Trench depth can be tailored to meet the needs of quantum efficiency and dark currents. According to the invention, an oxide film at the bottom of the trench acts like a mirror to create resonance (described below).
Now referring to the figures, where like reference numbers designate like elements,
The exemplary photodiode 50, as shown in
The remaining structures shown in
The isolation regions 55 are formed to electrically isolate regions of the substrate 60 where pixel cells will later be formed. The isolation regions 55 can be formed by any technique such as thermal oxidation of the underlying silicon in a LOCOS process, or by etching trenches and filling them with oxide in an STI (shallow trench isolation) process. Following the formation of the isolation regions 55, if the p-type well 61 has not yet been formed, it may then be formed by blanket implantation or by masked implantation.
A spacer layer 43 is formed within the trench 45, as illustrated in
As illustrated in
In the illustrated embodiment, the polysilicon layer 40 is a hydrogenated amorphous polysilicon. Leakage levels in amorphous silicon are reduced by increasing the passivation of grain boundaries through hydrogen. As another example, deuteriated amorphous silicon may be used, which also has good passivation characteristics. Yet another exemplary deposition technique employs fluorine incorporated into amorphous silicon after deposition through implantation at a dose of about 1.0×1015/cm2 to about 5×105/cm2 at about 5 keV to about 15 keV energies. As another alternative, the polysilicon layer 40 may be formed by chemical vapor deposition (CVD). The oxide layer 17 has a different refractive index and different band gap through the index of the polysilicon layer 40, which, in the operation of the pixel, allows photons to be reflected back upward into the layer 40.
The pixel sensor cell is essentially complete at this stage, and conventional processing methods may be used to form insulating, shielding, and metallization layers to connect gate lines and other connections to the pixel sensor cells. For example, the entire surface may be covered with a passivation layer 88 (
Another embodiment of the invention is shown in
In another embodiment, shown in
Alternate high and low-refractive index layers offer better optical reflection characteristics for the top mirror. Creating a pattern allows incoming photons to pass through while bottom reflected photons get top reflected by another material that has a different refractive index, which increases quantum efficiency. Creating a pattern using dielectric layers might allow filtering wavelengths of interest. This depends on the pattern's spacing and pitch, which can be defined by lithograpy. This embodiment is useful for special purpose sensors, for example infrared and UV sensors.
System 700, for example a camera system, generally comprises a central processing unit (CPU) 702, such as a microprocessor, that communicates with an input/output (I/O) device 706 over a bus 704. Imaging device 708 also communicates with the CPU 702 over the bus 704. The processor-based system 700 also includes random access memory (RAM) 710, and can include removable memory 715, such as flash memory, which also communicate with the CPU 702 over the bus 704. The imaging device 708 may 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.
The processes and devices described above illustrate preferred methods and typical devices of many that could be used and produced. The above description and drawings illustrate embodiments, which achieve the objects, features, and advantages of the present invention. However, it is not intended that the present invention be strictly limited to the above-described and illustrated embodiments. Any modifications, though presently unforeseeable, of the present invention that come within the spirit and scope of the following claims should be considered part of the present invention.