Microelectronic imagers are used in digital cameras, wireless devices with picture-taking capabilities, and many other applications. Mobile phones and personal digital assistants (PDAs), for example, are incorporating microelectronic imagers for capturing and sending pictures. The growth rate of microelectronic imagers has been steadily increasing as they become smaller and produce better images with higher resolution.
Microelectronic imagers include image sensors that typically use charged coupled device (CCD) and complementary metal-oxide semiconductor (CMOS) systems. CCD image sensors have been widely used in digital cameras and other applications. CMOS image sensors are also quickly becoming very popular because they have low production costs, high yields, and small sizes. CMOS image sensors can provide these advantages because they are manufactured using technology and equipment developed for fabricating semiconductor devices.
In practice, imager modules are fabricated in mass rather than individually. Multiple imager dies, each die including a pixel array and associated electronics, are contained on an imager wafer that contains multiple imager dies, and the substrate of the imager wafer is the substrate for the multiple imager dies. During fabrication, backside processes can be conducted substantially concurrently on each imager die on the wafer. Also, lenses may be formed at the wafer level as well, in which a plurality of lens structures, corresponding in location to a plurality of imager dies on an imager wafer, are fabricated on a lens wafer. The lens wafer is attached to the imager wafer with the imager dies and lens structures aligned, allowing plural imager modules to be formed simultaneously. After fabrication, the plurality of imager modules may be separated from the attached wafers into individual imager modules.
Lens systems for microelectronic imagers must be able to correct for various imperfections among imagers, such as chromatic aberration and astigmatism. Chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light, thereby resulting in different wavelengths of light being focused at different positions. One way to correct for chromatic aberration is to increase the focal length of the lens. More commonly, lens systems using two or more lenses having different refractive indices are used to reduce or eliminate chromatic aberration. For example, an achromatic doublet (a lens system having two lenses with different refractive indices) will bring two wavelengths to a common focus, therefore reducing the effects of chromatic aberration, but possibly leaving other wavelengths uncorrected and out of focus.
Known lens systems having two or more lenses that can be used for correcting for chromatic aberration and astigmatism include lens systems that are costly and difficult to produce. Accordingly, there is a need for an optical wafer-level lens that is easily manufactured and corrects for these problems.