Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-10-12
2002-12-17
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S216000
Reexamination Certificate
active
06495813
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to the optical design and microelectronic fabrication of microlens arrays for the optimization of spectral collection efficiency and related photodetector signal contrast in the color filter process of semiconductor imaging devices.
(2) Description of Prior Art
Image sensors for color digital still and analog or digital video cameras are typically charge-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) photodiode array structures which comprise a spectrally photosensitive layer below one or more layers comprised of an array of color filters and a plane-array of microlens elements. The elementary unit of the imager is defined as a pixel, characterized as an addressable area element with intensity and chroma attributes related to the spectral signal contrast derived from the photon collection efficiency of the microlens array, spectral transmission or reflection properties of the color filter array materials, and spectral response and electrical noise characteristics of the photodetectors and signal processing train.
The ability of a sensor to capture images in low-level irradiance conditions is critical in applications. The primary attributes of the sensor that determine its ability to capture low-level image light are the geometrical optics of the lens arrangement, fill-factors of lenses and photodiodes, and, the photoelectron quantum efficiency of the semiconductor in which the photodiodes are fabricated. The quantum efficiency is a measure of the photon-to-electron conversion ratio, and, for most CCD's these quantum efficiencies are similar. But, the physical size of the photosensitive area, coupled with the geometry of the lenses for collecting light and imaging this light onto the useful photosensitive area, create superior or inferior solid-state imagers. Responsivity is a measure of the signal that each pixel can produce and is directly proportional to pixel area. Another benefit of increased responsivity is that less illumination is needed to achieve a desired signal- to-noise contrast. With low-level illumination, the image will appear less grainy, and, the imager's frame rate can also be increased, providing increased video rates.
The optical performance of a solid-state imager is seen to depend on pixel size. Pixel size also affects the interaction of the camera lens with the microlens array. The microlens on top of each pixel focuses light rays onto the photosensitive zone of the pixel. The microlens significantly increases responsivity but it also limits the angular range of good responsivity. Typically, a 5-micron pixel has a severe drop in responsivity at 5 degrees from normal incidence. This leads to an optical effect whereby pixels near the edge of the camera lens field of view collect light less efficiently than at the center. This effect is reduced with larger 9-micron pixels, which have high responsivity at angles out to 15 degrees or more. Truncation of illuminance patterns falling outside the microlens aperture results in diffractive spreading and clipping or vignetting, producing nonuniformities and a dark ring around the image. Larger pixels use more silicon area which drives up the solid-state imager device manufacturing cost. The size of the active area can be set to optimize three factors: low light sensitivity, overall sensor size, and, the size of the optics necessary to project the desired image over the entire array. Instead of increasing the active area, some sensor manufacturers add extra steps to the manufacturing process to apply a microlens over each pixel. A microlens captures most of the incident light and focuses it onto the active area, which increases the effective fill ratio. The trade-off is, therefore, between the added cost of the microlens processing steps and the cost of the larger active areas. Typically, a pixel with a microlens requires a narrower incident light angle than a pixel that does not use a microlens, imposing additional optical design implications for the lens of the camera.
Associated with the microelectronic fabrication processes and materials used in forming solid-state array imagers, there often results gradients of spectral sensitivity and/or responsivity across the individual sensor elements comprising the imaging matrix, as well as related variations from sensor to sensor. The intrasensor variation and the intersensor variation are convolved with the imaging optics of the camera and the microlens arrays overlaying the photodiode arrays, such that the resultant contrast or modulation transfer function which manifests itself as an optoelectronic signal distribution mapping the illuminance distribution of the image formed across the imager-array surface, is an electronic signal contrast function which is fed into an electronic signal processing train which again further convolves noise and other contributions into the output of the imager. In particular, the regions within and between pixels which suffer significant variations ranging from peak sensitivity and responsivity to “dead zones” of practically no photosensitivity, or, vignetted light from the optics which truncated and diffracted the original image light-intensity distribution function, direct the attention of solid-state imager designers to improve the geometrical optical configurations to optimize light collection to focus images onto active areas. More particularly, in cases where sensing areas are formed into non-regular or L-type geometries, conventional microlens designs typically yield relatively low light collection efficiencies and result in degraded optoelectronic signal contrast. Images formed may suffer from known types of classical lens aberrations, produce motional smearing, pixel gaps, or, have other significant undesirable effects on image quality. Practical, manufacturable imaging arrays must, therefore, be designed to increase flexibility in the layout of the imaging device. Central to this goal is the optimization of the design of the multi-microlens array configuration geometry and the formation process steps for the microelectronic fabrication of these optics overlaying the photodiode elements in the matrix comprising the solid-state imager. Variations in lens curvature, index of refraction, and, light-scattering centers in the materials which are caused to flow to form the microlens arrays will combine to determine the resultant image quality and signal strength. Unless the microlenses are ideally diffraction- limited, imager defects will always be introduced in practical situations.
The design and fabrication of the multi-microlens arrays over the microelectronic photosensors of the solid-state imager, with enhanced optical collection efficiency for non-regular and L-shaped sensing areas, is thus a goal to which the present invention is directed. In conventional configurations for the color-filter process, only one microlens covers one pixel, and, the center of the microlens is aligned to the center of the pixel. It is, therefore, another goal of the present invention to disclose multi-microlens structures and configurations suitable for L-shaped and non-regular sensing areas applicable to manufacturing CMOS and CCD image devices. Park et al in U.S. Pat. No. 5,877,040 shows a CCD with a convex microlens formed integrally on the planarization layer above a photodiode element of a CCD array such that the focal-distance of the lens may be positioned by adjustment of the microelectronic fabrication process of intervening film-layer thicknesses. By fixing the irradiance pattern comprising the image on only the photodiode, image smear caused by the photoelectric effect when diffracted peripheral light is incident on the interpixel regions is said to be minimized. The microlens formation process described is derived from the flow of a microlens material which flows thermally at 100 degrees to 200 degrees Centigrade. Following a dry-etch step of the substrate, the set of convex microlenses are formed in a second planarizati
Chang Bii-Cheng
Fan Yang-Tung
Lu Kuo-Liang
Pan Sheng-Liang
Ackerman Stephen B.
Le Que T.
Saile George O.
Taiwan Semiconductor Manufacturing Company
LandOfFree
Multi-microlens design for semiconductor imaging devices to... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Multi-microlens design for semiconductor imaging devices to..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Multi-microlens design for semiconductor imaging devices to... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2995977