Abstract:
The present disclosure provides a contiguous microlens array, which consists of a plurality of touching microlenses, wherein the adjacent microlenses are connected to each other to form a contiguous microlens array and curvatures of every angle cross section of each microlens are the same. The shape of the curved surface of a microlens in the microlens array is selectively adjusted according to its position in the array and the incident angle of light incident thereto.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. patent application Ser. No. 11/970,936 filed on Jan. 8, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    This invention relates to an image recording apparatus, and more particularly to a contiguous microlens array, a method of fabricating the same and a photomask for defining the same, which can be applied to any image recording apparatus that requires focusing light on photosensing devices. 
         [0004]    2. Description of Related Art 
         [0005]    In a semiconductor-type image recording apparatus like a charge-coupled device (CCD) or CMOS image recording apparatus, a microlens array is often disposed over the array of photosensing devices to enhance the sensitivity of the same, wherein one microlens focuses light on one photosensing device. 
         [0006]      FIG. 1A  is a local contour plot of a conventional microlens array, and  FIG. 1B  shows the height variations of one microlens in two vertical cross-sectional views at different angles. Because a microlens  110  is formed by reflowing a square photoresist pattern formed on a based layer  10 , the shapes of the lower contours thereof are close to squares, as shown in  FIG. 1A , so that the microlens  110  has different curvatures in vertical cross-sectional views at different angles. For example, the curvature of the 45° cross section (B-B′) is relatively smaller than that of the 0° cross section (A-A′), as shown in  FIG. 1B , so that the microlens  110  is insufficient in the focusing effect. Moreover, because neighboring microlenses  110  are not connected with each other and there are planar sections without a focusing effect between them, the incident light are not fully collected so that the light focusing is not quite effective. 
         [0007]    There is also an issue on the integration of the conventional microlens array with other elements in a image recording apparatus, which is described below with a CMOS image recording apparatus having photodiodes as photosensing devices as an example. Referring to  FIG. 2  schematically showing a part of a CMOS image recording apparatus in the prior art, the microlens array  100  is formed on a transparent base layer  10 , which includes a color filter array  12  and other functional layers on a multi-level interconnect structure  20  including a first-level interconnect layer  22  and a second-level interconnect layer  24  over a photodiode array  30 . The eyepiece  40  of the CMOS image recording apparatus is disposed above the microlens array  100 , apart from it by a certain distance. 
         [0008]    Because the incident angle of the light incident to a microlens  110  in a peripheral portion of the microlens array  100  overly deviates from 90° (the direction of 90° means the normal line direction of the image sensor chip, hereinafter) so that the focus of light is not directly under the microlens  110 , the microlens  110  is laterally shifted relative to the corresponding photodiode  30  to make the light focus on the latter, as shown in  FIG. 2 . However, this makes the exit light  50   a  from the microlens  110  partially blocked by the second-level interconnect layer  24  and thus lowers the recording accuracy of the image. This problem can be solved by laterally shifting portions of the 2 nd -level interconnect layer  24  under the peripheral part of the microlens array  100 , but the interconnect circuit design would become more complicated by doing so. 
       SUMMARY OF THE INVENTION 
       [0009]    In one embodiment, a contiguous microlens array is provided. The contiguous microlens array consists of a plurality of touching microlenses, wherein the adjacent microlenses are connected to each other to form a contiguous microlens array and curvatures of every angle cross section of each microlens are the same. 
         [0010]    In order to make the aforementioned and other objects, features and advantages of the present invention comprehensible, a preferred embodiment accompanied with figures is described in detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1A  is a local contour plot of a conventional microlens array, and  FIG. 1B  shows the height variations of each microlens in two vertical cross-sectional views at different angles. 
           [0012]      FIG. 2  schematically shows a part of a CMOS image recording apparatus in the prior art. 
           [0013]      FIG. 3A  is a local contour plot of a contiguous microlens array of an example in a first embodiment of this invention, and  FIG. 3B  shows the height variations of one microlens in two vertical cross-sectional views at different angles. 
           [0014]      FIGS. 3C and 3D  are local contour plots of the contiguous microlens arrays of two other examples in the first embodiment of this invention. 
           [0015]      FIG. 4A  and  FIG. 4B  respectively illustrate, according to the first embodiment of this invention, two examples of a contiguous microlens array wherein neighboring microlenses are entirely connected with each other without a gap between them ( 4 A) or are connected with each other just at their edges ( 4 B). 
           [0016]      FIGS. 5A-1  to  5 A- 3  and  FIGS. 5B-1  to  5 B- 3  respectively illustrate, according to the first embodiment of this invention, two examples of a method of fabricating a contiguous microlens array shown in  FIGS. 4A and 4B , respectively. 
           [0017]    FIG.  5 C/ 5 D illustrates exemplary polygonal photoresist patterns that can be converted to microlenses similar to those shown in FIG.  3 C/ 3 D. 
           [0018]    FIG.  6 A/ 6 B shows exemplary photomask patterns that can define the photoresist patterns of FIG.  5 A- 1 / 5 B- 1  according to the first embodiment of this invention. 
           [0019]    FIG.  6 C/ 6 D shows exemplary photomask patterns that can define the photoresist patterns of FIG.  5 C/ 5 D according to the first embodiment of this invention. 
           [0020]      FIG. 7  schematically illustrates a part of an example of a CMOS image recording apparatus including a contiguous microlens array of a second embodiment of this invention. 
           [0021]      FIG. 8A  shows a local contour plot in a contiguous microlens array according to the second embodiment. 
           [0022]      FIG. 8B  shows a cross-sectional view of some contiguous asymmetric microlenses shown in  FIG. 8A . 
           [0023]      FIG. 9A  shows a top view of some photoresist patterns formed as the precursors of some asymmetric microlenses in a method of fabricating a contiguous microlens array according to the second embodiment of this invention. 
           [0024]      FIG. 9B  shows a cross-sectional view of some photoresist patterns formed as the precursors of some asymmetric microlenses shown in  FIG. 9A . 
           [0025]      FIG. 10  illustrates exemplary photomask patterns that can define the photoresist patterns of  FIG. 9A  according to the second embodiment of this invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       [0026]    In the 1 st  embodiment of the invention, each microlens is substantially symmetric in any vertical cross-sectional view. A microlens in the central part of the microlens array is aligned with the corresponding photosensing device, and a microlens in the at least one peripheral part of the same is laterally shifted relative to the corresponding photosensing device so that light is focused on the latter. Since  FIG. 2  has illustrated such a design, the latter figures relating to the first embodiment do not show again the arrangements of the microlenses in different parts of the microlens array relative to the photosensing devices. 
         [0027]    Moreover, since a microlens in the central part of the array is aligned with the corresponding photosensing device, the photoresist pattern as a precursor of a microlens in the central part and the photomask pattern for defining a microlens in the central part both are aligned with the corresponding photosensing device. Since a microlens in the peripheral part is laterally shifted relative to the corresponding photosensing device, both the photoresist pattern as a precursor of a microlens in the peripheral part and the photomask pattern for defining a microlens in the peripheral part are laterally shifted relative to the corresponding photosensing device. The alignment/shift of a photoresist pattern or a photomask pattern relative to the corresponding photosensing device is not illustrated here because it is easily understandable to one of ordinary skill in the art. 
         [0028]      FIG. 3A  is a local contour plot of a contiguous microlens array in one example of the first embodiment of this invention. The microlens array includes a plurality of contiguous microlenses  310   a  disposed on a base layer  10 . Each microlens  310   a  has substantially circular contours at the heights higher than the connection sections of the same with neighboring microlenses  310   a,  and has substantially partially circular contours at the heights on the connection sections adjacent to the neighboring microlenses  310   a.  As indicated by the contours in  FIG. 3A , each microlens  310   a  is substantially symmetric in any vertical cross-sectional view, and has substantially the same curvature in the vertical cross-sectional views at all angles. As shown in  FIG. 3B , the curvature of the 45° cross section (B-B′) is substantially the same as that of the 0° cross section (A-A′). In this example, neighboring microlenses  310   a  are not entirely connected with teach other, and small gaps are formed between them exposing small portions of the base layer  10 . 
         [0029]      FIGS. 3C and 3D  are local contour plots of two contiguous microlens arrays respectively in two other examples of the first embodiment of this invention. 
         [0030]    Referring to  FIG. 3C , the structure of the contiguous microlens array of this example is similar to that shown in  FIG. 3A  except that neighboring microlenses  310   c  are entirely connected with each other without a gap exposing the base layer  10  between them. Because neighboring microlenses  310   c  are contiguous in any direction, the incident light can be fully collected to achieve more effective light focusing. 
         [0031]    Referring to  FIG. 3D , the structure of the contiguous microlens array of this example is similar to that shown in  FIG. 3A  except that the connection section of any two neighboring microlenses  310   d  has a thickness of zero, i.e., any two neighboring microlenses  310   d  just contact with each other at their edges. However, because variations are inevitable in a real fabricating process, it is impossible to make any two neighboring microlenses  310   d  just contact with each other at their edges. Accordingly, it is more accurate to state that each connection section has a thickness “close to” zero in consideration of the real fabricating process. The light collection efficiency of such a microlens can be up to about 78%, which is still notably higher than that (−65%) of a conventional microlens with a square-like bottom and a round top as shown in  FIG. 1A . 
         [0032]      FIG. 4A  and  FIG. 4B  respectively illustrate, according to the first embodiment of this invention, two examples of a contiguous microlens array wherein neighboring microlenses are entirely connected with each other without a gap between them ( 4 A) or are connected with each other just at their edges ( 4 B).  FIGS. 5A-1  to  5 A- 3  and  FIGS. 5B-1  to  5 B- 3  respectively illustrate, according to the first embodiment of this invention, two examples of a method of fabricating a contiguous microlens array according to the first embodiment shown in  FIGS. 4A and 4B , respectively. 
         [0033]    Referring to FIGS.  4 A/ 4 B and  5 A- 1 / 5 B- 1 , a plurality of photoresist patterns  302 / 312  are formed, as an array of photoresist patterns, respectively in a plurality of regions  60  predetermined for forming microlenses. Each photoresist pattern  302 / 312  has a substantially circular shape in the top view, and neighboring photoresist patterns as formed are connected with each other ( 302 ) or are close to each other ( 312 ), so that neighboring photoresist patterns are connected with each other after the reflow step. Each photoresist pattern  302 / 312  includes a pillar part  302   a/   312   a  having a substantially circular shape in the top view and a number of annular segments  302   b ( c )/ 312   b  around the pillar part  302   a/   312   a  each having a substantially circular shape in the top view and having a height smaller than that of the pillar part  302   a/   312   a.  The annular segments  302   b ( c )/ 312   b  are different in the height and the height thereof decreases from inner to outer. 
         [0034]    It is particularly noted that the annular segments of a photoresist pattern  302  in  FIG. 4A  include the intact annular segments  302   b  and partial annular segments  302   c  therearound, both of which are generally called “annular segments” in the specification and claims of this invention. The partial annular segments  302   c  of a photoresist patter  302  are connected with those of the neighboring photoresist patterns  302 . For a partial annular segment  302   c,  the center thereof is defined as the center of an imaginary intact annular segment including the partial annular segments  302   c  itself. Moreover, as shown in FIG.  4 A/ 4 B, in each photoresist pattern  302 / 312 , the center of the pillar part  302   a/   312   a  substantially coincides with that of each annular segment  302   b ( c )/ 312   b  and also with that of the corresponding region  60  predetermined for forming the microlens. 
         [0035]    Referring to  FIG. 4A , the outmost annular segments  302   c  of four neighboring photoresist patterns  302  enclose a small gap that exposes a small portion of the base layer  10 . Moreover, the shape of each of the pillar part  302   a  and the annular segments  302   b ( c ) can be changed to a polygonal shape, as indicated by the reference characters  302 ′,  302   a ′,  302   b ′ and  302   c ′ in  FIG. 5 . Since the corners of polygonal photoresist patterns will be rounded in the later reflow step, the microlenses formed therefrom are similar to those formed from circular photoresist patterns. 
         [0036]    Differently, the annular segments of a photoresist pattern  312  in  FIG. 5B-1  include intact annular segments  312   b  only, wherein the outmost intact annular segments  312   b  of neighboring photoresist patterns  312  are sufficiently close to each other so that the neighboring photoresist patterns  312  are connected with each other after the reflow step. The shape of each of the pillar part  312   a  and the annular segments  312   b  can also be changed to a polygonal shape in the example, as indicated by the reference characters  312 ′,  312   a ′ and  312   b ′ in  FIG. 5D . Since the corners of the polygonal photoresist patterns will be rounded in the later reflow step, the microlenses formed therefrom are similar to those formed from circular photoresist patterns. 
         [0037]    Referring to FIG.  5 A- 2 / 5 B- 2 , a reflow step  304  is performed, including heating the above photoresist patterns  302 / 312  to round their surfaces and thereby form surface-rounded photoresist patterns  306 / 316 . The reflow step  304  may be conducted at a temperature of about 120-140° C. for about 10-15 minutes. When the surface of each photoresist pattern  302 / 312  has a proper height distribution, the surface of each surface-rounded photoresist pattern  306 / 316  can be close to a part of a spherical surface  307 / 317  that is namely a partial spherical surface. 
         [0038]    Moreover, for the photoresist pattern array in  FIG. 4A  and  FIG. 5A-1 , the photoresist material of the outmost annular segments  302   c  of four neighboring photoresist patters  302  flows into the small gap between them in the reflow step so that each microlens has a curved surface covering all the corresponding region  60  predetermined for its formation. For the photoresist pattern array in  FIG. 4B  and  FIG. 5B-1 , the photoresist material of the outmost annular segment  312   b  of each photoresist pattern  312  flows outward in the reflow step so that neighboring photoresist patterns  312  which are not connected with each other as formed are connected with each other. 
         [0039]    Referring to FIG.  5 A- 3 / 5 B- 3 , a fixing step  308  is then performed to remove the residual solvent in each surface-rounded photoresist patterns  306 / 316  and thereby fix the shape of the same to form a microlens  310   c/   310   d.  In an embodiment, the fixing step  308  uses UV-light to irradiate the photoresist patterns  306 / 316 , wherein the wavelength of the UV-light used may be about  365  angstroms, the intensity of the UV-light may be about 300 mJ/cm 2  and the processing time may be about 10-15 minutes. In another embodiment, the fixing step  308  includes further heating the surface-rounded photoresist patterns  306 / 316  at a temperature higher than that set in the reflow step  304 , such as a temperature within the range of 180-200° C. The processing time may be about 10-15 minutes. 
         [0040]    Exemplary photomask patterns that can define the photoresist patterns in FIG.  4 A/ 4 B are illustrated in FIG.  6 A/ 6 B. The photomask includes a transparent substrate  500 / 530 , and a plurality of photomask patterns  510 / 540  thereon that are disposed in the regions  502 / 532  corresponding to the regions  60  predetermined for the microlenses and constitute a photomask pattern array corresponding to the microlens array to be defined. 
         [0041]    In the example of  FIG. 6A , each photomask pattern  510  is typically a square unit pattern apart from the neighboring photomask patterns  510  and has therein a number of annular partition lines  520  that expose portions of the transparent substrate  500  and are for defining the annular segments of the photoresist pattern. In a photomask pattern  510 , one of any two neighboring annular partition lines  520  is surrounded by the other of the two neighboring annular partition lines  520 . The annular partition lines  520  include intact annular partition lines  520   a  and partial annular partition lines  520   b  therearound, both of which are generally called “annular partition lines” in the specification and claims of this invention. 
         [0042]    Moreover, the distance between neighboring photomask patterns  510  is small enough so that the neighboring photoresist patterns defined thereby are not disconnected. Each annular partition line  520  is sufficiently narrow such that no annular trench pattern is formed in the photoresist layer but the irradiation on the region around the portion of the photoresist corresponding to the partition line  520  is raised, so that the photoresist layer in the region is partially removed to form an annular segment of a photoresist pattern. Accordingly, when there are two or more annular partition lines  520 , the photoresist pattern defined by the photomask pattern  510  has a number of annular segments that descend stepwise in the height from inner to outer, as shown in  FIG. 5A-1 . Moreover, because there are two straight partition lines crossing in the region between four neighboring photomask patterns  510 , the photoresist material in the region is entirely removed to form the small gap between the corresponding four neighboring photoresist patterns, as shown in  FIG. 4A . 
         [0043]    In the example of  FIG. 6B , the photomask patterns  540  are circular patterns, each of which includes a number of annular partition lines  550  exposing portions of the transparent substrate  530  that are all intact annular partition lines, wherein any two neighboring annular partition lines  550  are in the relationship of inner and outer rings. Neighboring photomask patterns  540  are properly spaced from each other such that the neighboring photoresist patterns defined thereby are not connected with each other until the reflow step is conducted. The effect of the annular partition lines  550  is the same as that of the annular partition lines  520  in  FIG. 6A , so that the photoresist pattern defined by such a photomask pattern  540  also has a number of annular segments that descend stepwise in the height from inner to outer, as shown in  FIG. 5B-1 . 
         [0044]    Exemplary photomask patterns that can define the photoresist patterns in FIG.  5 C/ 5 D are illustrated in FIG.  6 A/ 6 D. The photomask patterns can be derived from FIG.  6 A/ 6 B by changing the above circular annular partition lines  520 ( a/b ), circular photomask patterns  540  and circular annular partition lines  550  to polygonal annular partition lines  520 ′ (including  520   a ′ and  520   b ′), polygonal photomask patterns  540 ′ and polygonal annular partition lines  550 ′. In addition, the transparent substrate is labeled with  500 ′/ 530 ′, the region corresponding to a region  60  predetermined for forming a microlens is labeled with  502 ′/ 532 ′, and the square unit pattern corresponding to a microlens to be defined is labeled with  510 ′. 
         [0045]    Moreover, by properly adjusting at least one of the thickness and the absorption coefficient of the photoresist layer as well as the number and the width of the partition line(s), the envelop of the disk-like portions of a photoresist pattern can be close to a partial spherical surface with a required curvature so that the microlens formed from the photoresist pattern through the reflow step has a surface close to the partial spherical surface with the required curvature. 
         [0046]    In this embodiment, since each microlens has substantially circular or regular-polygonal contours at the heights higher than the connection sections of the microlens with neighboring microlenses, has substantially partially circular contours at heights on the connection sections adjacent to the neighboring microlenses and is substantially symmetric in any vertical cross-sectional view, the curvature variation over the cross sections of all angles in the microlens is smaller than that in a conventional microlens with a squire-like bottom and a circular top so that the microlens provides better focusing than the conventional one. Moreover, in a case where no gap is present between neighboring microlenses, the incident light can be fully collected to increase the light collection efficiency because there is no planar section in the contiguous microlens array. 
       Second Embodiment 
       [0047]    In the second embodiment of this invention, each microlens is aligned with the corresponding photosensing device, a microlens in a central part of the microlens array is substantially symmetric in any vertical cross-sectional view, and a microlens in the peripheral part of the microlens array has an asymmetric vertical cross section. 
         [0048]    A CMOS image recording apparatus including photodiodes as the photosensing devices is taken as an example again in the second embodiment.  FIG. 7  schematically shows a part of an example of such a CMOS image recording apparatus. The structure of the CMOS image recording apparatus is similar to that shown in  FIG. 2  except the shapes and positions of microlenses  610   a/b/c  in the microlens array  600 . Specifically, the microlens array  600  is formed on a transparent base layer  10 , which includes a color filter array  12  and other functional layers and is disposed on a multi-level interconnect structure  20  including a first-level interconnect layer  22  and a second-level interconnect layer  24  over the array of photodiodes  30 . The eyepiece  40  of the image recording apparatus is disposed above the microlens array  600 , apart from it by a certain distance. 
         [0049]    A microlens  610   a  in the central part of the microlens array  600  is substantially symmetric in any vertical cross-sectional view, and a microlens  610   b/c  in the peripheral part of the array  600  has an asymmetric vertical cross section. It is noted that the center-shift direction of an asymmetric microlens  610   b/c  is set according to the incident angle of light, such that the incident light  50  overly deviating from 90° is converted to exit light  51  having an average exit angle close to 90° that focuses on the photodiode  30  directly under the microlens  610   b/c.  For example, the center of the left asymmetric microlens  610   b  is shifted left as being subjected to incident light inclining toward the right side, while the center of the right asymmetric microlens  610   c  is shifted right as being subjected to incident light inclining toward the left side. 
         [0050]      FIG. 8A  shows a local contour plot and a cross-sectional view of some contiguous asymmetric microlenses  610   b  and  FIG. 8B  shows a cross-sectional view of  FIG. 8A , while the structures of the asymmetric microlenses  610   c  and those in other portions of the peripheral part can be known based on  FIGS. 8A and 8B . As shown in  FIGS. 8A and 8B , each microlens  610   b  also has substantially circular contours at heights above the connection sections of the same with neighboring microlenses  610   b.  As in the cases of  FIG. 3D , it is also possible that the thickness of each connection section is alternatively close to zero. 
         [0051]      FIG. 9A  depicts a top view and  FIG. 9B  depicts a cross-sectional view of exemplary photoresist patterns that can serve as the precursors of the asymmetric microlenses  610   b,  wherein each photoresist pattern  602  includes a pillar part  602   a  having a substantially circular shape in the top view and a number of annular segments  602   b/c  therearound that are lower than the pillar part  602   a  and different in the heights, wherein the heights thereof decreases from inner to outer. It is particularly noted that the intact annular segments  602   b  and the partial annular segments  602   c  therearound both are generally called “annular segments” in the specification and claims of this invention. Meanwhile, the center of a partial annular segment  602   c  is defined as the center of an imaginary intact annular segment that includes the partial annular segment  602   c  itself. 
         [0052]    In the top view of a photoresist patter  602 , the center of the pillar part  602   a  and that of the annular partition line  602   b/c  both are shifted relative to the center of the region  60  in which the photoresist patter  602  is located. Moreover, it is also possible to form a photoresist pattern including a polygonal pillar part and at least one polygonal annular segment therearound, which can be easily understood based on the above mentioned and are not illustrated in the drawings. 
         [0053]      FIG. 10  illustrates exemplary photomask patterns that can define the photoresist patterns in  FIG. 8  according to the second embodiment. The photomask patterns  910  are formed on a transparent substrate  900  in the areas  902  corresponding to the regions  60  predetermined for forming the microlenses  610   b,  along with the photomask patterns for defining the microlenses  610   a,    610   c  and so forth. 
         [0054]    Each photomask pattern  910  is substantially a square unit pattern and has therein annular partition lines  920  that expose portions of the transparent substrate  900  and are for defining the annular segments of a photoresist pattern. The annular partition lines  920  include intact annular partition lines  920   a  and partial annular partition lines  920   b  therearound. Moreover, the distance between neighboring photomask patterns  910  is sufficiently small so that the neighboring photoresist patterns defined thereby are not disconnected from each other. It is particularly noted that the intact annular partition lines  920   a  and the partial annular partition lines  920   b  both are generally called “annular partition lines” in the specification and claims of this invention, while the center of a partial annular partition line  920   b  is defined as the center of an imaginary intact annular partition line that includes the partial annular partition line  920   b  itself. 
         [0055]    Moreover, in a photomask pattern  910 , the center of each annular partition line  920   a/b  is laterally shifted relative to that of the region  902  in which the photomask pattern  910  is located. In addition, each annular partition line  920   a/b  is sufficiently narrow so that the photoresist pattern defined by a photomask pattern  910  has a number of annular surfaces that descend stepwise in the height from inner to outer, as mentioned in the descriptions of  FIG. 6A-6D . Moreover, if a photoresist pattern including a polygonal pillar part and at least one polygonal annular segment therearound is to be formed, the shape of each partition line  920   a/b  must be made polygonal. This is easily understood based on the above mentioned and is therefore not illustrated in the drawings. 
         [0056]    As mentioned above, in the second embodiment, a microlens in the peripheral portion with incident angles of light overly deviating from 90° has an asymmetric vertical cross section to make the exit angle of light from the microlens close to 90°. Hence, each microlens in the central part and the peripheral part are allowed to align with the corresponding photodiode without a lateral shift relative thereto. Thereby, the interconnect structures under the peripheral part of the microlens array are not necessary to shift, so that no modification is required for the interconnect circuit design. 
         [0057]    This invention has been disclosed above in the preferred embodiments, but is not limited to those. It is known to persons skilled in the art that some modifications and innovations may be made without departing from the spirit and scope of this invention. Hence, the scope of this invention should be defined by the following claims.