Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1999-04-19
2001-09-04
Lee, Eddie C. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S432000, C430S007000
Reexamination Certificate
active
06285065
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to light transmitting filters formed on integrated circuits and in particular to methods of forming color filters on processed wafers and resulting filter structures.
BACKGROUND
Filter material may be placed over photodetector devices formed in an integrated circuit. This filter material may be transparent to selected light frequencies or ranges of frequencies so that only light corresponding to a selected color reaches an underlying device. More than one filter material type may be used so that selected device groups each receive light corresponding to a distinct color. For example, three materials, each transmitting a unique one of red, green, and blue light, may be placed in a pattern over underlying devices.
Light transmitting material is typically formed over photodetector devices using conventional photolithographic methods. The light transmitting material is commonly a thick film of colored resist. Briefly, a material transmitting light for a color to be sensed is spun on to a processed wafer. The material is then exposed, developed, and etched using standard photolithographic methods so that it is patterned as individual islands over corresponding photo detecting devices. As a result, for a large color sensing device array, many individual color filter islands overlie photodetector devices in the integrated circuit. This process may be repeated using different material or materials to form additional individual filters transmitting light of different colors. The molecular structures of the thick films of resist typically produce poor imaging because the resist is usually optimized to transmit the desired color, not for etch resolution. Thick film molecular lengths often exceed the sub-micrometer resolution required in contemporary integrated circuits (ICs) so that precise filter placement is difficult. A more significant problem is that one or more of the resist material islands may loosen or fall off (on the order of a few islands per million).
What is required, therefore, is a method for forming color filters on ICs that allows deep sub-micrometer alignment and etch resolution while allowing the use of materials optimized to transmit colored light. In addition, the structure should offer improved adhesion between the filter and the IC.
SUMMARY OF THE INVENTION
In accordance with the present invention, a contiguous filter structure made of individual filter elements is formed in an integrated circuit passivation layer. Individual filter elements are each aligned over corresponding underlying photodetector devices and each filter element transmits light of a selected color. The filter elements are formed in holes placed in a covering passivation layer by using a via etch.
In one embodiment, three patterns of individual filter elements are formed to make a single filter structure overlying portions of individual chips on a processed wafer. A first pattern of holes, aligned with a first set of underlying photodetector devices, is etched into a covering passivation layer using conventional via etch procedures. The holes do not penetrate the passivation layer. In a three micrometer thick passivation layer, for example, holes are typically one to two micrometers deep. Other hole depths may be used depending on passivation layer thickness and desired filter element thickness. A first light transmitting material, typically a resist that transmits light corresponding to a single color, is spun on to the wafer so that the light transmitting material fills the etched holes. The light transmitting material is baked (or cured) using conventional procedures and is then etched back to expose the passivation layer surface not etched with the first pattern of holes. Thus a layer of the first light transmitting material remains as individual filter elements in each of the first holes.
A second pattern of holes, aligned to a second set of underlying photodetector devices, is then etched into the passivation layer using procedures similar to those used to form the first pattern of holes. A second light transmitting material is spun on, cured, and etched back to expose all remaining passivation layer surface portions not covered by the first or second patterns of holes. Thus a layer of the second material remains as individual filter elements in each of the second holes.
A third pattern of holes is then etched into the passivation layer. This pattern includes holes aligned to a third set of underlying photodetector devices and may include any other portion of the passivation layer not covered by the remaining layers of first or second light transmitting materials. Individual chip areas not containing underlying photodetector devices are masked in some embodiments. In other embodiments, a blanket passivation layer etch is performed. The first and second light transmitting materials in the first and second holes block the third passivation layer etch, so the etch is self-aligning. After etch, a third light transmitting material is spun on to fill the third pattern of etched holes and is cured. A final etch is performed to expose the existing individual filter elements formed in the first and second pattern of holes. In this way a contiguous filter structure is formed of three patterns of individual filter elements. Each unique pattern of filter elements is formed to transmit a unique light color to a corresponding unique set of underlying devices.
Holes are typically square-shaped, although other shapes are possible. In some embodiments, selected holes may be etched deeper than others so that individual filter elements in these deeper holes experience additional adhesion from sidewall contact. In still other embodiments, some holes may be etched to have a bottom wider than a top opening (undercut etch), providing even better filter element adhesion to the passivation layer.
In some embodiments, more or less than three colored light transmitting materials may be used. Portions of the passivation layer may not be etched in some embodiments. Transmitted light is not limited to the visible spectrum; some individual filter elements may be formed to transmit infrared or ultraviolet light, for example. Some embodiments may include a clear material layer over the filter.
This filter has several advantages. Among the advantages, a first is that the contiguous individual filter elements provide lateral support for each other. Junctions between individual filter elements provide greater adhesion and prevent individual elements from becoming dislodged. Second, the light transmitting material requirements are less stringent than those used in current methods because the material has no special photolithographic or etch requirements. Third, this method uses existing photolithographic processes so that sub-micrometer hole dimensions are easily achieved. Finally, the fabrication process may omit a masking step for etching the passivation layer to receive the final colored light transmitting material as the last set of colored filter elements is formed.
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Allenby Christopher B.
Lee Eddie C.
MacPherson Alan H.
Skjerven Morrill & MacPherson LLP
Tower Semiconductor Ltd.
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