Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
2000-08-07
2004-11-23
Kang, Donghee (Department: 2811)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S060000, C438S070000, C438S071000, C438S072000, C438S075000, C257S432000, C257S435000, C257S436000
Reexamination Certificate
active
06821810
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to the optical design and microelectronic fabrication of high transmittance overcoat material(s) to increase focal length and optimize performance of long focal length microlens arrays in semiconductor color imagers.
(2) Description of Prior Art
Semiconductor array color image sensors for video cameras are conventionally comprised of complementary metal-oxide semiconductor (CMOS), charge-coupled devices (CCD), or, charge-injection devices (CID) integrated with optical structures consisting of planar arrays of microlenses, spacers, and primary color filters mutually aligned to an area array of photodiodes patterned onto a semiconductor substrate. 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. The microlens on top of each pixel focuses light rays onto the photosensitive one of the pixel. The optical performance of semiconductor imaging arrays depends on pixel size and the geometrical optical design of the camera lens, microlenses, color filter combinations, spacers, and photodiode active area size and shape. The function of the microlens is to efficiently collect incident light falling within the acceptance cone and refract this light in an image formation process onto a focal plane at a depth defined by the planar array of photodiode elements. Significant depth of focus may be required to achieve high resolution images and superior spectral signal contrast since the typical configuration positions the microlens array at the top light collecting surface and the photosensors at the semiconductor substrate surface.
When a microlens element forms an image of an object passed by a video camera lens, the amount of radiant energy (light) collected is directly proportional to the area of the clear aperture, or entrance pupil, of the microlens. At the image falling on the photodiode active area, the illumination (energy per unit area) is inversely proportional to the image area over which the object light is spread. The aperture area is proportional to the square of the pupil diameter and the image area is proportional to the square of the image distance, or focal length. The ratio of the focal length to the clear aperture of the microlens is known in Optics as the relative aperture or f-number. The illumination in the image arriving at the plane of the photodetectors is inversely proportional to the square of the ratio of the focal length to clear aperture. An alternative description uses the definition that the numerical aperture (NA) of the lens is the reciprocal of twice the f-number. The concept of depth of focus is that there exists an acceptable range of blur (due to defocussing) that will not adversely affect the performance of the optical system.
The depth of focus is dependent on the wavelength of light, and, falls off inversely with the square of the numerical aperture. Truncation of illuminance patterns falling outside the microlens aperture results in diffractive spreading and clipping or vignetting, producing undesirable nonuniformities and a dark ring around the image.
The limiting numerical aperture or f-stop of the imaging camera's optical system is determined by the smallest aperture element in the convolution train. Typically, the microlens will be the limiting aperture in video camera systems. Prior Art is characterized by methods and structures to maximize the microlens aperture by increasing the radius of curvature, employing lens materials with increased refractive index, or, using compound lens arrangements to extend the focal plane deeper to match the multilayer span required to image light onto the buried photodiodes at the surface of the semiconductor substrate. Light falling between photodiode elements or on insensitive outer zones of the photodiodes, known as dead zones, may cause image smear or noise. With industry trends to increased miniaturization, smaller photodiodes are associated with decreasing manufacturing cost, and, similarly, mitigate against the extra steps of forming layers for Prior Art compound lens arrangements to gain increased focal length imaging. Since the microlens is aligned and matched in physical size to shrinking pixel sizes, larger microlens sizes are not a practical direction. Higher refractive index materials for the microlens would increase the reflection-loss at the air-microlens interface and result in decreased light collection efficiency and reduced spectral signal contrast or reduced signal-to-noise ratio. Limits to the numerical aperture value of the microlens are imposed by the inverse relationship of the depth of focus decreasing as the square of the numerical aperture, a strong quadratic sensitivity on the numerical aperture. For these physical reasons, microlens optical design properties need to be kept within practical value-windows to achieve engineering design objectives for spectral resolution and signal-to-noise.
The design challenge for creating superior solid-state color imagers is, therefore, to optimize spectral collection efficiency by a single microlens to maximize the fill-factor of the photosensor array elements with the minimum number of microelectronic fabrication process steps. The present invention is clearly distinguished from Prior Art by introducing a high transmittance overcoat to optimize long focal length single layer microlens performance without significant optoelectronic design changes.
FIG. 1
exhibits the conventional Prior Art vertical semiconductor cross-sectional profile and optical configuration for color image formation. Microlens
1
residing on a planarization layer which serves as a spacer
2
collects a bundle of light rays from the image presented to the video camera and converges the light into focal cone
3
onto photodiode
8
after passing through color filter
4
residing on planarization layer
5
, passivation layer
6
, and metallization layer
7
. The purpose of the microlens' application in CCD and CMOS imaging devices is to increase imager sensing efficiency.
FIG. 2
illustrates the geometrical optics for incident image light
9
converged by microlens element
10
, color filter
11
, into focal cone
12
, to the focal area
13
within a photoactive area
14
surrounded by a dead or non-photosensitive area
15
, wherein the sum of the areas of
14
and
15
comprise the region of the pixel.
Huang et al in U.S. Pat. No. 6,001,540 teaches an optical imaging array device with two principal process embodiments to form a biconvex microlens or a converted plano-convex microlens version. In the primary embodiment, a layer of silicon dioxide is deposited onto a substrate, followed by a deposition of polysilicon and a layer of silicon nitride. Patterning and etching the silicon nitride, a circular opening is formed and the exposed polysilicon is oxidized to form a lenticular body of silicon oxide. The surrounding silicon nitride is removed by etching to form a biconvex microlens. In the second method, a sequence of steps is employed wherein spin on glass is deposited to a thickness equal to half said lenticular body's thickness; a process is described and claimed for manufacturing a plano-convex microlens. In either embodiment, the top surface of the microlens is at an air interface with unit refractive index , n=1.0. Control of focal length is done by adjusting the thickness of a spacer layer on which the microlens is formed.
An alternative Prior Art approach to microlens formation for solid-state image sensors is provided by Song et al in U.S. Pat. No. 5,672,519. Song et al. teach an image sensor with a compound regular-shape microlens which extends conventional prior art from square to rectangular illuminance areas to account for CCD structures where the dimensions of a pixel or photodiode are different in the vertical and horizontal directions of the semiconductor. Song e
Chang Bii-Juno
Hsiao Yu-Kung
Lu Kuo-Liang
Pan Sheng-Liang
Kang Donghee
Taiwan Semiconductor Manufacturing Company
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