Abstract:
According to embodiments of the present invention, an apparatus comprising a beam shaping element (lens) is provided. The apparatus comprises a substrate; a beam shaping element; and an elastic intermediate layer disposed between, and in contact with, the substrate and the beam shaping element, wherein the elastic intermediate layer has a Young&#39;s Modulus in a range of 2-600 MPa and a Poisson&#39;s ratio in a range of 0.2-0.5. Techniques for reducing thermal distortion of lens are described.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to reducing distortion of beam shaping elements. 
       BACKGROUND 
       [0002]    Various types of micro optical beam shaping elements are sometimes incorporated into optical or optoelectronic modules such as cameras and other imagers, LED arrays, proximity sensors and other optical sensors. The beam shaping elements (e.g., lenses) often are formed of a plastic material, which can be significantly less expensive than forming the beam shaping elements of glass. 
         [0003]    Some applications require environmentally stable materials with good optical properties that can be formed on mechanically rigid substrates. Individual lenses or lens arrays, for example, sometimes are formed or bonded directly on a substrate that has a lower coefficient of thermal expansion (CTE) than the plastic lenses. Consequently, the lens is mechanically constrained at the interface with the substrate. At elevated temperatures, this mechanical constraint and the difference in CTE between the two materials can give rise to deleterious shear stresses in the higher-CTE plastic lens. These shear stresses engender significant dimensional distortions in the plastic lens resulting in a loss or severe degradation of optical function. For example, during operation, the temperature of an optoelectronic module (comprising a high-CTE lens bonded directly to a low-CTE substrate) may increase, sometimes significantly. At elevated temperatures the mechanical constraint combined with the different CTE would distort the plastic lens. For example, a dimensional change in the lens in the direction of the optical axis (i.e., in the direction perpendicular to the surface of the substrate to which the lens is attached) can be relatively large and, consequently, the lens focal length can change significantly as a function of temperature. The resulting optical effect or image can become degraded or distorted. 
       SUMMARY 
       [0004]    The present disclosure describes various techniques and arrangements that, in some cases, can help reduce or alleviate the foregoing problems even when the beam shaping elements have a different CTE from the underlying substrate. 
         [0005]    For example, in one aspect, an apparatus includes a substrate, a beam shaping element, and an elastic intermediate layer disposed between, and in contact with, the substrate and the beam shaping element. The elastic intermediate layer has a Young&#39;s Modulus in a range of 2-600 MPa and a Poisson&#39;s ratio in a range of 0.2-0.5, and in some cases a ratio in a range of 0.3-0.5. In some implementations, the elastic intermediate layer has a linear CTE in a range of 30 to 400 E−6/K at room temperature. Further, in some implementations, the elastic intermediate layer has a thickness such that, in combination, the Young&#39;s modulus, the Poisson&#39;s ratio, the linear CTE and the thickness remove the mechanical constraint at the interface minimizing deformation of the lens. Thus, the substrate, beam shaping element and elastic intermediate layer can form a stack having an optical axis, such that the elastic intermediate layer reduces thermally induced distortions of the beam shaping element that would otherwise occur at an elevated temperature in the absence of the elastic intermediate layer. 
         [0006]    According to another aspect, An apparatus includes a substrate and a beam shaping element. The beam shaping element has a base layer composed of the same material as the beam shaping element, and the base layer is supported by the substrate and disposed between the substrate and the beam shaping element. The thickness of the beam shaping element is in a range of 20-200 μm and the thickness of the base layer is in a range of 50-300 μm. In some implementations, the base layer is a pedestal having a shape corresponding to the shape of the beam shaping element. In some instances, the footprint of the base layer is larger than a footprint of the beam shaping element. 
         [0007]    Other aspects, features and advantages will be readily apparent from the following detailed description, the accompanying drawings and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1A  illustrates an example of an assembly that includes a beam shaping element. 
           [0009]      FIG. 1B  illustrates another example of an assembly that includes a beam shaping element. 
           [0010]      FIG. 2  is an example of an optoelectronic module. 
           [0011]      FIG. 3  is another example of an optoelectronic module. 
           [0012]      FIG. 4  is a flow chart showing an example of providing a PDMS layer. 
           [0013]      FIG. 5  is a flow chart showing another example of providing a PDMS layer. 
           [0014]      FIG. 6  illustrates another example of an assembly that includes a beam shaping element. 
           [0015]      FIG. 7  illustrates example of a further assembly that includes a beam shaping element 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    As shown in  FIG. 1A , in accordance with a first implementation, an assembly  20  includes a beam shaping element (e.g., a lens)  10  that is decoupled mechanically from an underlying substrate  12  by an intermediate layer  14  composed, for example, of polydimethlysiloxane (PDMS) or a flexible epoxy or acrylate. The assembly thus forms a stack including the beam shaping element  10 , the intermediate layer  14  and the substrate  12 , where the intermediate layer  14  is disposed between the substrate  12  and the beam shaping element  10 . The intermediate layer  14  should be highly elastic (e.g., Young&#39;s Modulus=2-600 MPa), should have a Poisson&#39;s ratio in the range of 0.2-0.5 (and preferably in the range of 0.3-0.5), should possess a linear CTE of 30 to 400E−6/K at room temperature, and should have a thickness such that the combined features effectively eliminate or reduce the mechanical constraint at the lens-intermediate layer interface—diminishing shear stress at the lens-intermediate layer interface—with reduced or eliminated distortion. Preferably, the intermediate layer  14  is present below the entirety of the beam shaping element  10  so that it supports, and is in contact with, the entire lower surface of the beam shaping element  10  that faces the substrate  12 . 
         [0017]    A light beam that passes through the beam shaping element  10  may be shaped in a predefined manner For example, in some implementations, the beam shaping element  10  may focus the light beam toward a light sensitive element  22  in an optoelectronic module  24  (see  FIG. 2 ) or may direct light emitted by a light emitting element  26  out of an optoelectronic module  28  (see  FIG. 3 ). Examples of beam shaping elements that can be used include optical lenses, optical prisms and optical diffraction gratings. 
         [0018]    To allow the light that passes through the beam shaping element  10  to pass through the substrate  12  and the intermediate layer  14  as well, the substrate  12  should be composed of a material that is substantially transparent to light at the particular wavelength(s) of interest (e.g., infra-red (IR) or visible). In some implementations, the substrate  12  is composed of glass or other transparent inorganic materials, or of an organic plastic material. 
         [0019]    By providing the intermediate layer  14  between the beam shaping element  10  and the substrate  12 , even if the assembly  20  is placed in an environment that is at an elevated temperature, any resulting thermally induced distortion (e.g., expansion in the direction perpendicular to the surface of the substrate  12 ) can be significantly reduced, as the base of the optical element  10  is less rigidly constrained, as shown in  FIG. 1B . In particular, the elastic material diminishes the lateral restraint on expansion at the intermediate layer-lens interface. 
         [0020]    Various techniques can be used to provide the intermediate layer. Some implementations use a wafer-level process in which a glass or other wafer is coated with an intermediate (PDMS-type) material ( FIG. 4 , block  102 ), which then is cured (block  104 ). In this context, a wafer refers generally to a substantially disk- or plate-like shaped item, its extension in one direction (z-direction or vertical direction) is small with respect to its extension in the other two directions (x- and y-, or lateral directions). In some implementations, the diameter of the wafer is between 5 cm and 40 cm, and can be, for example, between 10 cm and 31 cm. The wafer may be cylindrical with a diameter, for example, of 2, 4, 6, 8, or 12 inches, one inch being about 2.54 cm. In some implementations of a wafer level process, there can be provisions for at least ten modules in each lateral direction, and in some cases at least thirty or even fifty or more modules in each lateral direction. 
         [0021]    After depositing the intermediate layer on the wafer, multiple beam optical shaping elements (e.g., lenses) can be formed, for example, by replication, on the intermediate layer (block  106 ). Replication generally refers to techniques by means of which a given structure or a negative thereof is reproduced (e.g., etching, embossing, or molding). 
         [0022]    In other implementations, the elastic intermediate layer can be provided by forming individual pedestals and then placing them (e.g., using pick-and-place equipment) onto a glass or other wafer ( FIG. 5 , block  120 ). An optical beam shaping element subsequently can be formed on each of the pedestals, for example, by a replication process (block  122 ). 
         [0023]    The results of the processes of  FIG. 4  and  FIG. 5  are optics wafers that include multiple beam shaping elements, such as lenses, formed on individual pedestals each of which separates one of the lenses from the underlying wafer. An example is illustrated in  FIG. 6 , which shows a beam shaping element (e.g., a lens)  150  on a pedestal  154 , which in turn is on a transparent substrate  152 . In this example, the beam shaping element  150  and pedestal  154  are composed of different materials. The beam shaping elements may be arranged, for example, as an array. The optics wafer then can be used, for example, in a wafer-level process for fabricating multiple optoelectronic modules, such as those illustrated in  FIGS. 2 and 3 . 
         [0024]      FIG. 7  illustrates another implementation of an assembly  200  that includes a beam shaping element (e.g., a lens)  210  formed on a substrate  212 . In this example, the beam shaping element  210  includes a relatively thick base layer  214  that can be composed of the same material (e.g., a particular plastic material) as the beam shaping element itself. The base layer  214 , in some cases, is a cylindrical or other shaped pedestal. Thus, the base layer  214  can be a pedestal having a shape corresponding generally to the shape of the beam shaping element  210 . At elevated temperatures, the thicker base layer  214  may be subject to shear stress at the interface with the lower-CTE substrate; however, the resulting deformations decrease/dissipate with the height of the base layer such that the shape of  210  is not significantly deformed. The thickness of the base layer  214  can depend on various factors, including, for example, the CTE. Poisson&#39;s ratio, and modulus of elasticity of the material of the beam shaping element  210  and the base layer  214 . In an illustrative example, the lens  210  has a diameter (d) in the range of 750-1000 μm and a thickness (t) of about 150 μm. The thickness (h) of the base layer  214  in the illustrated example is on the order of about 200 μm, and the height (H) of the substrate  212  is in the range of 300-500 μm. In some implementations, the thickness (t) of the lens  210  is in the range of 20-200 μm, and the thickness (h) of the base layer  214  is in the range of 50-300 μm. The footprint of the base layer  214  preferably is somewhat larger than the footprint of the lens  210 . Different dimensions may be appropriate for other implementations. 
         [0025]    In some implementations, the base layer  214  can be made as part of the same processing step(s) as the beam shaping element  210  itself, whereas in other implementations, the base layer  214  may be formed in separate step(s). 
         [0026]    Various techniques can be used to provide the base layer  214  and beam shaping element  210  of  FIG. 7 . In a first technique, both the base  214  and the beam shaping element  210  are formed as part of the same replication process. For example, the pedestal shape of the base  214  can be incorporated into a single point diamond turn (SDPT) master tool used for wafer-level replication of the combined base  214  and beam shaping element  210 . In accordance with a second technique, the beam shaping element and pedestal-shaped base pairs are formed by vacuum injection molding in a wafer-level process. The wafer-level injection molding tool can include channels for receiving injectable material that forms the beam shaping elements and corresponding pedestal-shaped bases. Alternatively, in accordance with a third technique, the pedestal-shaped bases  214  alone can be made by vacuum-injection, and the beam shaping elements (manufactured by the same or other process) can be added in a subsequent step, again as part of a wafer-level process. In some implementations, instead of a wafer-level process, single or multiple beam shaping elements  210  with corresponding pedestal-shaped bases  214  are made by injection molding. In yet other implementations, pedestal-shaped bases  214  are made by a photolithographic process, and then combined with the beam shaping elements  210 . 
         [0027]    The beam shaping elements in each of the foregoing implementations can be, for example, diffractive, refractive or reflective lenses. They can have concave, convex, or other shapes depending on the desired beam shaping. The beam shaping elements can be composed, for example, of a plastic or composite material. 
         [0028]    Various modifications can be made within the spirit of the disclosure. Accordingly, other implementations are within the scope of the claims.