Achromatic lens structure, method of fabrication, and imaging devices and systems using the same

Lens structures, imaging devices, and methods of making the same that include a lens and a transparent material having different dispersions and used to correct chromatic and spherical aberrations. The transparent material may be a curable polymer used to join the lens to other elements of the lens structure.

FIELD OF THE INVENTION

Embodiments of the invention relate generally to imaging devices, and more particularly, to using a complex lens to correct chromatic and spherical aberration.

BACKGROUND

Solid state imaging devices, e.g., CCD, CMOS, and others, may include a lens or a lens structure to direct incoming light onto a focal plane array of pixels. Each one of the pixels includes a photosensor, for example, a photogate, photoconductor, or photodiode, overlying a substrate for accumulating photo-generated charge in an underlying portion of the substrate. The charge generated by the pixels in the pixel array is then read out and processed to form an image. Often the lens or lens structure is part of a wafer level fabrication and imager module assembly process.

FIG. 1is a diagram of a lens structure100that includes a first convex lens112and a second convex lens114arranged on opposite sides of a substrate110to form a double sided convex lens structure known as “biconvex”. Light rays120passing through the lens structure100are subject to lateral chromatic aberration. Chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light, known as the dispersion of the lens. Since the focal length of a lens is dependent on the refractive index of the lens material, different wavelengths of light will be focused at different positions. Therefore, red120r, green120g, and blue120bcomponents of the light rays120are focused at different distances from the lens structure100, which can result in a blurry image. Chromatic aberration of a lens may manifest as fringes of color around an image, because each color in the optical spectrum cannot be focused at a single common point on the optical axis.

A conventional lens structure100having spherical lenses112,114may also produce spherical aberration. Spherical aberration is an image imperfection that occurs due to the increased refraction of light rays120when the light rays120strike a lens112,114near its edge, in comparison with light rays120that strike nearer the center of the lens112,114. A positive spherical aberration occurs when peripheral rays are bent too much and a negative spherical aberration occurs when peripheral rays are not bent enough.

What is needed is a system and method by which spherical and chromatic aberrations may be corrected in a lens, including one which can be fabricated at a wafer level.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to make and use them, and it is to be understood that structural, logical, or procedural changes may be made to the specific embodiments disclosed herein.

Embodiments of lenses described herein are useful for correcting spherical and chromatic aberrations and include a lens structure, which may be fabricated at a wafer level, and which have a lens coupled to a transparent material having a dispersion different from that of the lens. The transparent material may also be used to bond components of the lens structure, such as substrates and lenses, to each other. In one or more embodiments, the substrates and/or lenses may be bonded together over their entire surface without an air gap. The lens structure thus provides an additional optical active interface, which is used to control chromatic and spherical aberrations.

FIG. 2is a diagram of a lens structure200, which may be fabricated with other like lens structures on a wafer level, having first212, second214, and third216convex lenses. The first convex lens212and third convex lens216are arranged on opposite sides of a first substrate210, and the second convex lens214is arranged on a second substrate218. In various embodiments, the lenses212,214,216may be formed integrally with the substrates210,218. A transparent material230is arranged between the first substrate210and the second substrate218and between the third lens216and the second substrate218. In one embodiment, the transparent material230may be a polymer that bonds or otherwise joins the first substrate210and second substrate218.FIG. 2also illustrates light rays220having red220r, green220g, and blue220bcomponents.

The transparent material230has a dispersion, (inverse to the Abbe number), and a refractive index that is different, i.e., either higher or lower, from that of the third lens216to correct for chromatic aberration. In one embodiment, the transparent material230has a dispersion that is higher than that of the third lens216. As the difference of dispersion (and refractive index) between the third lens216and the transparent material230is made larger, the lens “sag” may be made smaller. As shown inFIG. 2, achromatization of the minimum component220r(red) and maximum component220b(blue) wavelengths is achieved by causing the wavelengths to overlap, while a secondary spectrum is still visible for the middle component220g(green). The secondary spectrum may be corrected as well by using an anomalous dispersion in either the transparent material230or the third lens216.

The lens structure200also reduces spherical aberration through use of spherical lenses212,214,216. In another embodiment, one or more of the lenses212,214,216may have aspherical lens profiles to further reduce the spherical aberration and improve the field performance of the lens structure200. In yet another embodiment, several lenses216and transparent material230layers may be joined sequentially together to improve the lens structure200performance over a larger field of view. In another embodiment, several lens structures200may be joined together with air spaces separating the lens structures200.

In one embodiment, the material used to form the first212, second214, and third216lenses may be an ultra-violet curable polymer. One example of such an ultraviolet-curable polymer is Ormocomp® from Micro Resist Technology. The substrates210,218may be made out of materials such as glass or polymer. The transparent material230may be selected based on the material used to form the third lens216so that the transparent material230has a different dispersion from the third lens216. In various embodiments, the transparent material may be TU-7048 or TT8021 from JSR Corporation, Adhesives N61, N63, and N68 from Norland Products, Inc., Vitralit® from Panacol-Elosol GmbH, OPA-20632 from Dymax Corporation, Adhesive 3471-2-136 from DSM Desotech, Inc., Zipcone UA from Gelest, Inc., Adhesive OG134 by Epo-Tek, or Ormocers, e.g., Ormocomp or Ormocore from Micro Resist Technology, GmbH.

FIG. 3is a diagram of a lens structure300having first312, second314, third316, and fourth319convex lenses. The first convex lens312and third convex lens316are arranged on opposite sides of a first substrate310, and the second convex lens314and fourth convex lens319are arranged on opposite sides of a second substrate318. A transparent material330is arranged between the first substrate310and the second substrate318and between the third lens316and the fourth lens319. The transparent material330has a dispersion and refractive index that is different, i.e., either higher or lower, from that of the third lens316and the fourth lens319to correct for chromatic and spherical aberrations.

FIG. 4is a diagram of a lens structure400having first412and second414convex lenses and first416and second419concave lenses. The first convex lens412is arranged on a first substrate410, and the second convex lens414is arranged on a second substrate418. The first concave lens416is arranged on the first substrate410, opposite to the first convex lens412. The second concave lens419is arranged on the second substrate418opposite to the second convex lens414. a transparent material430is arranged between the first416and second419concave lenses. The transparent material430has a dispersion and refractive index that is different, i.e., either higher or lower, from that of the first416and second419concave lenses to correct for chromatic and spherical aberrations. In one embodiment, the transparent material430has a dispersion that is lower than the dispersions of the first416and second419concave lenses.

FIG. 5is a diagram of a lens structure500having first512, second514, and third516convex lenses and a concave lens519. The first convex lens512and third convex lens516are arranged on opposite sides of a first substrate510, and the second convex lens514and concave lens519are arranged on a second substrate518. A transparent material530is arranged between the first substrate510and the second substrate518and between the third lens516and the concave lens519. The transparent material530has a dispersion and refractive index that is different, i.e., either higher or lower, from that of the third lens516and the concave lens519to correct for chromatic and spherical aberrations.

FIG. 6is a diagram of a lens structure600having a first convex lens612arranged on a first substrate610, and a second convex lens614arranged on a second substrate618. A transparent material630is arranged between the first substrate610and the second substrate618. One side618aof the second substrate618is concave and thus forms a concave lens. The second substrate618is formed of a material that has a higher dispersion, and therefore lower Abbe number, than the transparent material630, for example SF6 glass or polycarbonate to correct for chromatic aberration. In another embodiment, the side618aof the second substrate618could be formed into a concave lens.

FIG. 7is a diagram of a lens structure700having a first convex lens712arranged on a first substrate710, and a second convex lens714and a third lens716arranged on a second substrate718. The third lens716is a diffractive or Fresnel lens. A transparent material730is arranged between the first substrate710, the second substrate718, and the third lens716. In the lens structure700of this embodiment, the dispersion of the material used in the third lens716may be the same or different as the dispersion of the transparent material730so long as the index of refraction of the third lens716and the transparent material730is different. In another embodiment, the profile of the third lens716may be formed from the second substrate718itself.

A method of making the lens structure200in the embodiment shown inFIG. 2at a wafer level is now described. As shown inFIG. 8A, a first lens stamp832created from an original master may be used to form the first convex lenses212on the first substrate210using a UV (ultra-violet) replication method. Although only three first lenses212are shown inFIG. 8A, it should be understood that tens, hundreds, or thousands of lens structures may be formed at the same time by this method.

As shown inFIG. 8B, second convex lenses216may be formed on the opposite side of the first substrate210using a second lens stamp834via a UV replication method. As shown inFIG. 8C, third convex lenses214are formed on the second substrate218using a third lens stamp836via a UV replication method. In various embodiments, the first832, second834, and third836lens stamps may be the same or different stamps depending on whether the lens profiles of the first212, second216, and third214lenses have the same or different profiles. The lenses212,214,216may be optionally bonded to their respective substrates210,218by an adhesive agent, such as Hexamethyldisilazane (HMDS).

As shown inFIG. 8D, the transparent material230is applied to the second substrate218on the side opposite the third lenses214. In another embodiment, the transparent material could be applied to the first substrate210and second lenses216. As shown inFIG. 8E, the first substrate210and the second substrate218are aligned and joined together via transparent material230and then the transparent material230is cured to bond the substrates210,218and lenses216together.

As shown inFIG. 8F, the substrates210,218are aligned and joined to a plurality of spacers804on a wafer806containing a plurality of pixel arrays946. As shown inFIG. 8G, the completed stack may be diced along the dashed lines860by any method to separate and complete the imaging device820. Alternatively, only the stack of lenses and spacer wafers are separated and individually placed on image sensors. A similar method can be used to form imaging devices that include lens structures300,400,500,600, and700.

FIG. 9shows a block diagram of an imaging device900, e.g. a CMOS imager, that may be used in conjunction with one of the lens structures200,300,400,500,600,700according to embodiments described herein. A timing and control circuit932provides timing and control signals for enabling the reading out of signals from pixels of the pixel array946in a manner commonly known to those skilled in the art. The pixel array946has dimensions of M rows by N columns of pixels, with the size of the pixel array946depending on a particular application.

Signals from the imaging device900are typically read out a row at a time using a column parallel readout architecture. The timing and control circuit932selects a particular row of pixels in the pixel array946by controlling the operation of a row addressing circuit934and row drivers940. Signals stored in the selected row of pixels are provided to a readout circuit942. The signals are read from each of the columns of the array sequentially or in parallel using a column addressing circuit944. The pixel signals, which include a pixel reset signal Vrst and image pixel signal Vsig, are provided as outputs of the readout circuit942, and are typically subtracted in a differential amplifier960and the result digitized by an analog to digital converter964to provide a digital pixel signal. The digital pixel signals represent an image captured by pixel array946and are processed in an image processing circuit968to provide an output image.

FIG. 10shows a system1000that includes an imaging device900and one of lens structure200,300,400,500,600,700constructed and operated in accordance with the various embodiments described above. The system1000is a system having digital circuits that include imaging device900. Without being limiting, such a system could include a computer system, camera system, e.g., a camera system incorporated into an electronic device, such as a cell phone, scanner, machine vision, vehicle navigation, video phone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system, or other image acquisition system.

System1000, e.g., a digital still or video camera system, generally comprises a central processing unit (CPU)1002, such as a control circuit or microprocessor for conducting camera functions, that communicates with one or more input/output (I/O) devices1006over a bus1004. Imaging device900also communicates with the CPU1002over the bus1004. The processor system1000also includes random access memory (RAM)1010, and can include removable memory1015, such as flash memory, which also communicates with the CPU1002over the bus1004. The imaging device900may be combined with the CPU processor with or without memory storage on a single integrated circuit or on a different chip than the CPU processor. In a camera system, a lens structure200,300,400,500,600,700according to various embodiments described herein may be used to focus image light onto the pixel array946of the imaging device900and an image is captured when a shutter release button1022is pressed.

While embodiments have been described in detail in connection with the embodiments known at the time, it should be readily understood that the claimed invention is not limited to the disclosed embodiments. Rather, the embodiments can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described. For example, while some embodiments are described in connection with a CMOS pixel imaging device, they can be practiced with any other type of imaging device (e.g., CCD, etc.) employing a pixel array.