Image sensor having large micro-lenses at the peripheral...

Details Image sensor having large micro-lenses at the peripheral... Image sensor having large micro-lenses at the peripheral...

2002-10-25

2003-10-28

Mulpuri, Savitri (Department: 2812)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Responsive to electromagnetic radiation

C438S065000, C438S066000, C438S073000

Reexamination Certificate

active

06638786

ABSTRACT:

TECHNICAL FIELD
The present invention relates to image sensors, and more particularly, towards an image sensor that has taller micro-lenses in the outer regions of the pixel array.
BACKGROUND
Image sensors are electronic integrated circuits that can be used to produce still or video images. Solid state image sensors can be either of the charge coupled device (CCD) type or the complimentary metal oxide semiconductor (CMOS) type. In either type of image sensor, a light gathering pixel is formed in a substrate and arranged in a two-dimensional array. Modern image sensors typically contain millions of pixels to provide a high resolution image. Important parts of the image sensor are the color filters and micro-lens structures formed atop of the pixels. The color filters, as the name implies, are operative, in conjunction with signal processing, to provide a color image. The micro-lenses serve to focus the incident light onto the pixels, and thus to improve the fill factor of each pixel.
Conventionally, micro-lenses are formed by spin coating a layer of micro-lens material onto a planarized layer. The micro-lens material is then developed to form cylindrical or other shaped regions that are centered above each pixel. Then, the micro-lens material is heated and reflowed to form a hemispherical micro-lens.
FIG. 1
shows a prior art cross-sectional simplified diagram of an image sensor
101
having micro-lenses formed thereon. As seen in
FIG. 1
, the image sensor includes a plurality of pixels that have light detecting elements
103
formed in the substrate. The light detecting elements
103
may be one of several types, such as a photodiode, a photogate, or other solid state light sensitive element. Formed atop of each pixel is a micro-lens
105
. The micro-lens
105
focuses incident light onto the light detecting elements
103
. Moreover, in the region between the light detecting elements
103
and the micro-lens
105
, denoted by reference numeral
107
, there are various intervening layers that would typically include the color filter layers and various metal conducting lines. These components are excluded from the diagram in order to simplify the explanation herein and not to obscure the invention.
In the prior art, the formation of the micro-lenses is controlled such that the shape of the micro-lenses exhibits uniformity throughout all of the pixels of the image sensor. However, the applicant has observed a variation in the amount of light captured by the pixels, with the pixels near the center of the image sensor collecting more light than the pixels in the periphery of the image sensor.
FIG. 3
illustrates one reason why this occurs. In
FIG. 3
, an image sensor
101
typically works in conjunction with an imaging lens
204
to capture an image. The imaging lens
204
takes incident light and transmits it to the image sensor
101
as shown in FIG.
2
. As seen, for those pixels that are located in the center of the image sensor
101
, the incident light from the imaging lens
204
is focused correctly to the light detecting elements. However, for those pixels that are located at the outer regions of the image sensor
101
, the incident light from the imaging lens
204
is not perpendicular to the micro-lens, thereby causing the focused incident light to be misaligned to the light detecting element. This in turn results in relatively less light being captured by the light detecting element. This is referred to herein as the “dark corner” phenomena.


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