Abstract:
Systems and methods are provided for a lens or microlens array or non-spherical lens with or without an integrated sensor unit. A dielectric between a substrate and a lens material has curved recesses, which are filled in by the lens material. Light enters the lens material layer and is focused by the curved recess portions.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to microlens arrays and optical lenses, and more particularly to methods for manufacturing microlens arrays or non-spherical lenses.  
         [0003]     2. Related Art  
         [0004]     Microlens arrays provide optical versatility in a miniature package for imaging applications. Traditionally, a microlens is defined as a lens with a diameter less than one millimeter; however, a lens having a diameter as large as five millimeters or more has sometimes also been considered a microlens.  
         [0005]     There are many conventional methods for manufacturing microlens arrays, such as using reflow or diffusion.  FIGS. 1A-1C  show a typical sequence of steps for making a microlens array by depositing material, patterning, and reflowing. In  FIG. 1A , a photosensitive layer  10 , such as a photosensitive resin, is formed on a planarization layer  12  over a silicon substrate (not shown). The material of the photosensitive layer is used to form the microlens array. In  FIG. 1B , photosensitive layer  10  is patterned to form an array of shapes, such as rectangles, stripes, or squares  14 , where the shapes are located where the individual microlenses will be formed. Patterning, for example, can be with a conventional mask and photoresist process, where a photoresist is deposited on photosensitive layer  10 , exposed through a mask having opaque areas, developing (or removing) selected portions of the photoresist, and etching areas of photosensitive layer  10  left exposed by the photoresist. Squares  14  are then heated sufficiently to cause them to reflow, thereby forming an array of semi-spherical microlenses  16 , as shown in  FIG. 1C .  
         [0006]     However, microlens arrays made by thermal reflowing, as described above, have several disadvantages. Typically, photosensitive resins contain components which absorb proportionally more light in the blue region of the visible spectrum. As a result, the color spectrum is distorted, producing an image that is more “yellowish” than it should be. This color distortion increases with time due to oxidation of the resin. Another disadvantage is that the resolution with which the photosensitive resin can be patterned is limited by the thickness of the resin layer. The thicker the resin layer, the farther apart the microlenses in the array, which reduces the light collection efficiency of the array. On the other hand, the resin layer must be thick enough so that, when reflowed, the sag of the resultant microlenses is sufficient to cause the desired focusing effect. Consequently, it is difficult to obtain the highest possible collection efficiency with microlens arrays fabricated in this manner. Yet another disadvantage results from the fact that as the curvature radius of the microlens becomes small, the incident light is focused on a point near the microlens. Thus, the photosensitive layer is patterned to be square or rectangular in shape according to the shape of a cell, using a mask that is simply divided into opaque regions and light-transmissive regions, and is thermally treated to form a microlens. Thus, a curvature radius of the microlens is decreased. Moreover, because a microlens formed in a rectangular shape has a significant difference between its curvature radius in the width and the length directions, it is difficult to focus incident light on the corresponding photodiode without error, and a part of the light is focused on the planarization layer or color filter layer between the photodiode and the microlens, causing loss of light and deterioration of sensitivity and resolution.  
         [0007]     Another conventional method of forming microlens arrays is by diffusion, such as described in “Light Coupling Characteristics of Planar Microlens”, by M. Oikawa et al., Proc. SPIE, 1544, 1991, pp. 226-237, which is incorporated by reference in its entirety.  FIGS. 2A-2G  showsteps for forming a microlens array using two types of diffusion. In  FIG. 2A , a glass substrate  20  is provided. In  FIG. 2B , a metal film  22  is deposited on glass substrate  20 . Metal film  22  is then patterned, such as with conventional processes, to remove portions  24  where individual microlenses are to be formed, as shown in  FIG. 2C .  FIGS. 2D and 2E  show one type of further processing, where the exposed areas  24  are diffused with an appropriate dopant and energy ( FIG. 2D ) and then the remaining metal is removed and the surface is polished, such as with a chemical or machine polish, to form microlenses  26  ( FIG. 2E ).  FIGS. 2F and 2G  show another type of further processing, where ions, protons, or other suitable molecules are used to bombard (e.g., with low energy) ( FIG. 2F ) and diffuse into substrate  20  and the remaining metal portions removed and the irradiated portions “swelled” ( FIG. 2G ), such as with an organic vapor, to form microlenses  28 . The result is a high numeral aperture planar microlens array. One disadvantage to forming microlens arrays using diffusion is that control of the thickness along the optical axis is limited.  
         [0008]     Microlens arrays are typically used with an underlying array of sensors, such as complementary metal oxide semiconductor (CMOS) or charge couple device (CCD) sensors, to form an imaging device. The microlenses collect and focus light onto corresponding sensors. The microlenses significantly improve the light sensitivity of the imaging device by collecting light from a large light collecting area and focusing it on a small light sensitive area of the sensor (i.e., pixel). One conventional method of generating an image signal is shown in  FIG. 3 . Light rays  30  are collected and focused by a microlens layer  32  comprising an array of microlenses  34  overlying a planarization layer  36 , such as formed by processes described above. After passing through planarization layer  36 , light rays  30  are filtered by color filters  38  in a filter layer  40 , with each color filter allowing only light of a specific color to pass, such as red, green, and blue (RGB). Light through the filters are then passed through a sensor layer  42 , comprising an array of sensors  44 , such as photodiodes or CCD devices. A processor (not shown) combines signals from the sensors to create a color image.  
         [0009]     Such an arrangement of microlenses, filters, and sensors has several disadvantages. Several processing steps are needed to form the separate microlens layer  32 , filter layer  40 , and sensor layer  42 , which increase cost and time. The layers also increase the separation between the microlenses and the sensors, which can increase crosstalk between pixels, due in part to light impinging on adjacent sensors instead of the desired sensor.  
         [0010]     In addition to microlenses, high quality non-spherical lenses are also critical components to many applications in the imaging field. They are widely used in optical systems for controlling critical light propagation and correcting image color quality, such as in professional cameras and video imaging equipment. However, the fabrication of non-spherical lenses is complicated and can only be done through skilled manual operation by highly trained professionals. Unlike spherical lenses which can be manufactured quickly by using conventional machines, non-spherical or specially sized or shaped lenses are typically shaped and polished manually and frequently individually. This can be time consuming and costly.  
         [0011]     Accordingly, there is a need for an improved lens, microlens, or array and method of manufacturing such, including non-spherical lenses, that overcomes the disadvantages of conventional lens arrays or non-spherical lenses and related processes, such as described above. Further, there is a need for an integrated microlens array and sensor array that overcomes the disadvantages as described above with conventional microlens/sensor devices.  
       SUMMARY  
       [0012]     The present invention provides improved microlens arrays or non-spherical lenses and processes of forming microlens arrays or non-spherical lenses. In one aspect, the microlens array is formed on a sensor array, resulting in an integrated microlens/sensor device.  
         [0013]     According to one embodiment, an array of sensors is first fabricated on a substrate. A dielectric layer, such as a spin-on polymer (e.g., polyimide) or an oxide (e.g., SiO 2 ) is deposited over the sensor array. A patterning photosensitive dielectric layer, such as a spin-on photoresist, is next formed over the dielectric layer. Selected portions of the patterning layer are removed to expose areas of the dielectric layer overlying the individual sensors where microlenses are to be formed. The exposed portions are then processed to form curved recesses, such as by using a wet etch, a grey-scale mask, or a shadow mask. The curved recesses may have a controlled shape and range from a shallow recess to a deep spherical recess, depending on the desired characteristics of the microlens. Remaining portions of the patterning layer are then removed. An inorganic lens material having a higher refractive index than the underlying dielectric layer, such as SiO 2 , SiO x N y , Si 3 N 4 , TiO 2 , or a polymer, is deposited over the dielectric layer to form an integrated array of microlenses and sensors. The layer of lens material may be polished, if desired.  
         [0014]     In other embodiments, the dielectric layer can be deposited over any substrate and does not have to be a sensor array. In such embodiments, the process forms and/or can be used to make plastic molding templates to form individual spherical or non-spherical lenses, or an array of spherical and/or non-spherical microlenses of any desired shape or shapes. The process of the present invention allows a lens or microlens array to be formed with different shaped non-spherical and/or spherical lenses. This gives the lens manufacturer more flexibility to fabricate many additional types of lens arrays at discount prices.  
         [0015]     The present invention provides numerous advantages over conventional microlens arrays and methods. Since the microlens array is formed directly onto the sensor array with fewer processing steps than conventional methods, microlens/sensor devices of the present invention are easier and less expensive to fabricate than conventional devices. The focal length of the microlenses can be controlled depending on the type of dielectric materials used for the microlenses and/or process control (i.e., curvature of the lens elements.)  
         [0016]     The present invention also provides improved sensor sensitivity due to the ability to make non-spherical lenses using wet etching, grey-scale mask or shadow mask processing. Another advantage is that using non-organic lens materials extends the reliability or useful lifetime of the microlens. The color quality of the image produced by the sensor is also improved because the lens material does not have the adverse characteristics of resin-containing materials, which as discussed above, can absorb proportionally more blue light to make the image yellowier than desired. Yet another advantage the current invention provides is that the resulting microlens/sensor device is thinner and more resistant to environmental effects because the microlens array acts as a protection layer for the sensor elements.  
         [0017]     The resulting microlens array may be used with devices for a variety of application, from a small display screen for a camera, a digital camera sensor, a personal digital assistant, or a laptop to a large display screen for a projection screen, a wall-sized display screen, or a billboard-sized display screen. The processing or fabrication of the array/sensor unit allows high throughput with consistent characteristics between each array/sensor unit.  
         [0018]     The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIGS. 1A-1C  show a typical sequence of steps for making a microlens array by reflowing according to a conventional process;  
         [0020]      FIGS. 2A-2E  show steps for forming a microlens array using one type of diffusion according to a conventional process;  
         [0021]      FIGS. 2A-2C  and  2 F- 2 G show steps for forming a microlens array using another type of conventional process;  
         [0022]      FIG. 3  shows one type of conventional microlens array and sensor array device;  
         [0023]      FIG. 4  is a flow chart showing a process for fabricating a microlens array onto a sensor array according to one embodiment of the present invention;  
         [0024]      FIGS. 5A-5G  show various stages of a process for fabricating a microlens/sensor array according to one embodiment;  
         [0025]      FIGS. 6A and 6B  show a grey scale mask and characteristic of a grey scale mask, respectively, for use in one embodiment of the invention;  
         [0026]      FIGS. 7A-7C  show various stages of a process for forming controlled curvature recesses using a grey scale mask according to one embodiment; and  
         [0027]      FIG. 8  is an angled view of a microlens array according to one embodiment of the present invention. 
     
    
       [0028]     Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.  
       DETAILED DESCRIPTION  
       [0029]      FIG. 4  is a flowchart illustrating a method  400  in accordance with an embodiment of the present invention for fabricating a microlens array or a non-spherical lens. A substrate is first provided in step  402 , where the substrate may include an array of CMOS or CCD sensors. The sensor array may be any suitable size, ranging from small screen applications to large display devices. A dielectric layer is then deposited on the substrate in step  404 . A patterning layer, such as a spin-on photoresist or other photosensitive material, is deposited on the dielectric layer in step  406 . Selected portions of the patterning layer are removed, such as by conventional photolithography processing, in step  408 . The removed portions expose areas of the dielectric layer where the microlenses or non-spherical lenses are to be formed. With embodiments having a sensor array, the exposed areas correspond to locations of individual sensor elements.  
         [0030]     In step  410 , the exposed portions of the dielectric layer are selectively etched, such as with a wet etch, a grey scale mask, or shadow mask, to form controlled curved recesses. The curved recesses deepest in the center and taper up toward the sides or circumference. The etching does not remove all the dielectric material such that the underlying substrate or sensors are exposed. Further, the curved recesses can be any suitable shape, such as semi-spherical or non-spherical, depending on the application. The remaining portions of the patterning layer are removed in step  412 , and the resulting template is ready for further processing steps or can be used for plastic molding of specially designed lenses. When the template is to be continued for further processing, a layer of inorganic lens material is deposited over dielectric layer, in step  414 , to fill in the curved recesses. The lens material, in one embodiment, has a refractive index higher than that of the dielectric layer. Examples of suitable lens materials include, but are not limited to, SiO 2 , SiO x N y , Si 3 N 4 , TiO 2 , a polymer, or plastics in the case of plastic molding. The layer of lens material may then be polished if necessary.  
         [0031]      FIGS. 5A-5G  show various stages of fabricating a microlens array according to one embodiment of the invention.  FIG. 5A  shows a top view of a substrate  500  onto which the microlens array will be formed. In one embodiment, substrate  500  is a glass or silicon substrate, in which the resulting device is a microlens array. In another embodiment, substrate  500  is a sensor array formed on top of a supporting substrate, such as glass or silicon, in which the resulting device is an integrated sensor/microlens array. The sensor array can be an array of CMOS or CCD sensors, such as photodiodes or other sensor elements. Fabrication of the sensor array is with conventional methods.  FIG. 5A  shows the embodiment where a sensor array  502  with individual sensor elements  504  is formed on a supporting substrate  506 . A dielectric layer  508 , such as an oxide (e.g., SiO 2 , TiO 2 ), nitride (e.g., SiO x N y ), spin-on polymer, is deposited on sensor array  502 , as shown in  FIG. 5B . The thickness of the dielectric layer  508  depends on specific application requirements. In one embodiment for integrated sensor/microlens array, dielectric layer  508  is between 1 μm and several millimeters thick. In another embodiment for individual non-spherical lens, dielectric layer  508  can be up to one centimeter or thicker.  
         [0032]     Next, in  FIG. 5C , a patterning layer  510  is deposited over dielectric layer  508 , where patterning layer  510  will be used to expose portions of the dielectric layer where microlenses or non-spherical lenses will be formed. Patterning layer  510  is a photosensitive dielectric material and is selected based on the type of patterning process used. For example, for a photolithography process, patterning layer  510  can be a spin-on photoresist or other photosensitive material. The desired pattern can then formed on patterning layer  510  by exposure through a photomask. The photomask, if the photoresist is positive, may have an array of circular openings, where the circular openings correspond to locations of the microlenses to be formed. If the microlenses are to have different shapes and/or sizes, the individual openings of the photomask can be adjusted accordingly. Exposed portions of patterning layer  510  are then removed to expose portions  512  of dielectric layer  508  where microlenses or non-spherical lenses are to be formed, as shown in  FIG. 5D . With an underlying sensor array, portions  512  correspond to individual sensor elements  504 .  
         [0033]     In  FIG. 5E , exposed portions  512  of dielectric layer  508  are then etched to form curved recesses  514  overlying sensor elements  504 . Curved recesses  514  can be semi-spherical, as shown in  FIG. 5F , which is a top view of  FIG. 5E . As noted above, the shape of individual curved recesses  514  can be varied according to the microlens application. Further, curved recesses  514  are formed, in one embodiment, by controlled etches, such as a wet etch or etching after patterning using a grey scale mask or shadow mask. Other etching processes for tapered etching may also be suitable with the present invention. The depth and taper of the etch also determines the optical characteristics, such as focal length, of the microlens or lens. Thus, by controlling the etch of the dielectric layer, different types of microlens arrays can be easily fabricated.  
         [0034]      FIGS. 6A and 6B  and  7 A- 7 C show a method of forming controlled curved recesses using a grey scale mask process according to one embodiment.  FIG. 6A  shows an example of one opening  600  of a grey scale mask, where a typical grey scale mask will have many such openings  600  separated by opaque sections in between. A grey scale mask lets different amounts of light through different radius locations of the opening, such as shown in  FIG. 6B . The degree of grey at different radii of the opening  600  on the grey scale mask determines the degree of light exposure at corresponding locations of the underlying photosensitive dielectric such as photoresist. As shown, less light passes through radially outward from the center of the opening, from a maximum of approximately 100% at the center to approximately 0% at the edge or outer circumference. The light transmission curve “a” can be any suitable shape for forming the desired microlens or lens.  
         [0035]      FIGS. 7A-7C  show a sequence of steps using a grey scale mask to form the controlled curved recesses. In  FIG. 7A , a small portion of patterning layer  510  (such as a positive photoresist) is exposed through one opening  600  of a grey scale mask. Note that the portions between openings of the grey scale mask in the x-direction are opaque. Patterning layer  510  is developed and a dry etch is performed to transfer the exposed pattern to underlying dielectric layer  508 , as shown in  FIGS. 7B and 7C , to form curved recesses  514 . Thus, by controlling the scale of the grey on the grey scale mask and dry etch, both spherical and non-spherical microlenses and lenses of different designs can be formed quickly and inexpensively.  
         [0036]     Depending on the type of patterning and etch, curved recesses  514  may need to be treated to smooth out irregularities on the surface of the curved recesses. The “roughness” of the curved recesses should be small compared to the wavelength of the visible light. In one embodiment, the roughness should be approximately 1/10 the wavelength of the visible light. “Roughness” as defined herein refers to the distance or variation between peaks and troughs on the surface of the curved recesses. For example, when using dry etch to form curved recesses  514 , a quick wet etch or wash may be added to smooth out any roughness of the surface of curved recesses  514 . An alternative to the quick wet etch is to coat the surface of curved recesses  514  with a thin dielectric material of the same refractive index as underlying dielectric layer  508 . Other suitable methods to smooth out the surface areas of the recesses  514  include those such as properly designed chemical mechanical polishing (CMP) and the like.  
         [0037]     After forming curved recesses  514  of dielectric layer  508  (and polished if necessary), the structure can be used as a template for making plastic lenses through plastic molding, or to continue further processing for microlens/sensor integration. For plastic molding of lenses, multiple templates of the same pattern design and curved shapes or different design and shapes may be used depending on specific applications. When used for microlens/sensor integration, referring back to  FIG. 5G , after curved recesses  514  of dielectric layer  508  are formed (and polished if necessary), a layer of transparent lens material  516  is deposited, as shown in  FIG. 5G , to form the microlens array. In one embodiment, the lens material is inorganic and has a higher index of refraction than that of underlying dielectric layer  508 . Some suitable materials for lens material  516  include dielectrics, such as SiO 2 , SiO x N y , Si 3 N 4 , TiO 2 , a polymer, plastics or a combination of them. Thus, depending on the microlens requirements, dielectric layer  508  and lens material  516  are selected accordingly. In one embodiment, the deposited thickness of lens material  516  is approximately the same as the depth at the center of the curved recesses or thicker depending on the application requirement. Use of inorganic lens materials, as opposed to resin-based reflow processes, produces lenses that create a truer color image. That is, there is no extra absorption in the blue spectrum, which produces yellowier images. Further, forming the microlens by deposition instead of diffusion provides better control of the lens shape and the thickness along the optical axis. After deposition of lens material  516 , the upper surface can be polished to produce a flat smooth surface if necessary.  
         [0038]      FIG. 8  is an angled view of a microlens array  800  having integrated sensors/microlenses. Transparent lens material  516  can act as a protection layer for the underlying microlenses  802  and sensor array  502 . Each microlens  802  corresponds to an underlying sensor element  808 , which are supported by substrate  506 . Light entering microlens array  800  is directed toward individual sensors in the sensor array by corresponding microlenses  802 . The process of making the microlens array allows more light to be received by the sensors, thereby improving image sensitivity and color quality. However, as noted above, microlens array  800  or an individual non-spherical lens does not require an underlying array of sensors.  
         [0039]     The present invention allows a microlens array or individual lens having non-spherical or different sized/shaped microlenses/lens to be manufactured easily. In conventional processes for making non-spherical or specially sized or shaped lenses, the lenses are typically shaped and polished manually and sometimes individually. This can be costly in terms of time and effort. On the other hand, spherical lens arrays can be manufactured quickly by using conventional machines. However, the machines do not allow non-spherical lenses to be formed nor do they allow lenses of different shapes or sizes to be formed on the same array. Advantageously, the present invention allows microlens arrays or lenses having non-spherical microlenses or lenses of different shapes or sizes to be made quickly and inexpensively.  
         [0040]     Embodiments described above illustrate but do not limit the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. For example, the above embodiments describe the use of a patterning layer over a dielectric layer. However, the dielectric layer can be excluded if the patterning photosensitive dielectric layer can be directly used to form usable curved recesses or to form the curved recesses using other means such as suitable chemical processes or ion beam sputtering and the like. Accordingly, the scope of the invention is defined only by the following claims.