A MOSAIC LENS ARRAY

FIELD OF THE INVENTION

The invention is directed generally to optical detectors for use in optical detection systems and, more particularly, to a method and apparatus for providing a mosaic lens array adapted for use with a mosaic detector array and an optical system such as an infrared detection system. BACKGROUND OF THE INVENTION

Optical systems and, in particular, infrared detection systems are often used for reconnaissance from aircraft and land vehicles. Such systems typically view a scene of interest and are required to provide data which is processed by electro- optical circuitry. More advanced digital processing systems in this field require increasing amounts of information from such optical systems in order to perform sophisticated tasks. Such tasks may include discriminating targets, differentiating between moving and stationary objects, and providing high resolution display images. As a result, it has become necessary to fabricate detector arrays in densely filled packages in order to detect and transmit signals representing scenes of interest to

the electro-optical processing circuitry without increasing the size and weight of the overall optical system. Known detector arrays typically have an area of about 4 square mils per detector. This area is designed to approximately equal the resolution spot size of the optics. It would be extremely advantageous to reduce the area of each detector in order to (a) reduce the detector area while retaining the same center-to-center spacing on the detector arrays and/or (b) reduce the areas of each detector ^* o increase its radiation properties. In either case, smaller detectors on the same center-to-center space provides more room for lead wires or butting of detector elements. The method and apparatus of the invention accomplishes these advantages by providing a chemically etched mosaic lens array placed at the plane of focus of the optical system which further reduces the size of the optical resolution. The detector array is located at the position of best focus for the lens array.

The use of chemical etchants for fabrication of electronic devices, including semiconductor devices and printed wiring boards has long been known. The usual method of fabricating such electronic devices is to process a wafer of semiconductor material or a

printed wiring board and then at some point delineate a pattern defining an electronic circuit or element by separating individual circuits or elements from one another, either by physical means such as air abrasion or sawing, or by etching. Known chemical etchants typically exhibit a property known as "preferential etching" whereby the etch rates are higher at the edges of the etchant resistant mask than in areas further away from the edges of the mask. This has been considered an undesirable property since uniform etching of semiconductor materials or printed wiring boards, as the case may be, is most preferably done uniformly in order to assure that a uniform amount of material is etched away over the entire surface being etched. In fact, most chemical etchings are buffered in order to reduce preferential etching characteristics.

These known electronics industry etching techniques have also long been used for fabricating multiple detector elements from substrates consisting of optically transmissive substrates. For the first time, the method of the invention takes advantage of the preferential etching property inherent in chemical etchants in order to form an

array of low optical quality, small lenses which are useful for condensing radiation from a scene of interest onto a detector array. The invention provides advantages not previously found in the prior art by providing a mosaic lens array comprised of a plurality of inexpensive, low optical quality lenses which can be installed directly above a mosaic detector array. In an arrangement according to the invention, each detector would receive radiation from a single corresponding lens in the lens array. It is believed that fabrication of such a mosaic lens array assembly of a size useful for an optical detection system was not possible until now.

SUMMARY OF THE INVENTION The invention provides an apparatus and method for providing a mosaic lens array for an optical detection system. In the preferred embodiment, the mosaic lens array is comprised of a two-dimensional array of a plurality of lenses which are etched into an optically transmissive substrate. The optical system has an optical system focus and provides an optical resolution of a predetermined spot size. The lens array is disposed at the plane described by the system focus so as to further reduce the spot size of the optical resolution of the system. A

mosaic detector array is positioned to receive radiation condensed by the lens array so as to have a one-to-one correspondence between each lens in the lens array and each detector in the detector array. The method of the invention includes the steps of depositing an etchant resistant mask on one side of the lens array substrate and then preferentially etching the substrate. The etchant resistant mask is arranged in a grid-like pattern with grid lines having a width of about 1.5 to 2.0 times the thickness of a predetermined etching depth. The substrate is etched into an array of lenses by application of an etchant, typically by emersion, until the etchant resistant mask is removed from the substrate. The lens array substrate may be comprised of CdTe, zinc sulfide (ZnS) , germanium (Ge) or other optically transmissive materials as are known by those skilled in the art.

It is one object of the invention to provide a two-dimensional array of lenses placed at the focus position of an optical system so as to further reduce the size of optical resolution allowing a plurality of detectors of a two-dimensional array positioned to receive radiation focused by the lens array to have detectors of reduced size while still

receiving radiation from the full field of view with no dead space between pixels.

It is yet another object of the invention to provide a mosaic lens array for an optical system fabricated by preferential etching of the lens substrate.

It is yet another object of the invention to prov,ide an optical system having a mosaic lens array placed at the focus position of the optical system. It is a further object of the invention to provide a mosaic lens assembly, allowing a detector array to achieve a nearly 100% fill factor, which concentrates radiation transmitted to a smaller detector mounted below the lens assembly. It is yet another object of the invention to provide an optical detection system having detectors with a reduced detector area, thereby reducing detector noise.

It is yet another object of the invention to provide an optical system having detectors with a reduced detector area but which remain in the same center-to-center spacing to provide room on a detector substrate for lead wires or butting of contacts.

It is yet another object of the invention to provide improved nuclear radiation hardening for an optical system by providing a mosaic lens array which reduces the resolution spot size of the optical system. This allows detectors having a relatively small area to be used thereby having reduced sensitivity to nuclear radiation in the system while maintaining full field coverage by the detectors. Other objects, advantages, and features of the invention will become apparent to those skilled in the art through the Claims, Description of the Preferred Embodiment and Drawings wherein like numerals refer to like elements. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows schematically a mosaic lens array positioned to receive radiation and condense it onto a detector array as provided by the invention. Figures 2A and 2B illustrate a cross-section side view and top view respectively of a mosaic lens array being processed in accordance with the present invention.

Figure 3 is a block diagram of an infrared or other electromagnetic energy detection system

employing the mosaic lens array of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figure 1, a mosaic lens array 5 as provided by the method of the invention is shown mounted over a detector array 15 by conventional mounting means (not shown) . The lens array 5 is further comprised of a plurality of lenses 16 having cusps or edges 42 and an apex 44 etched into an optically transmissive substrate 14. The lens array 5 is positioned within an optical system so as to place the two dimensional array of lenses at the focus position of the optical system as designated by dotted line 12. The lenses then receive radiation 10 from the scene of interest through the system optics (shown in Figure 3) . The lenses further reduce the resolution spot size of the optical system onto the detectors 18. T h e substrate 14 may be any optically transmissive substrate such as CdTe, Ge or ZnS. The detector array may be comprised of any optically sensitive detector material such as HgCdTe or other materials well known in the art. It is advantageous to align the lens array substrate directly over the detector array s© as to allow the radiation to be condensed

onto each detector. In the preferred embodiment of the invention, the lens array may be etched into the detector substrate on a first surface opposing a second surface whereon • the detector array is located. In a typical embodiment the lenses may have an area of about 4 square mils while the detectors may have an area in the range of one to 2 square mils. The lenses may be of very low optical quality and still condense nearly all of the radiation onto the detectors if they are located very closely to the detectors. The lens array may be located either on the same substrate as the detector array or mounted over the detector array substrate 21 with a very small gap in between the detectors and the lens array substrate 14. This gap may typically be equivalent to 5 mils in air for the most effective use of the low quality optical lens array. Note that the lens array may be comprised of a plurality of optical lenses which need not have smooth, rounded surfaces. The lenses may be formed from a stepped series of flat surfaces or a combination of round and flat surfaces.

Having described in detail the structure of the invention, the method of the invention for providing

the mosaic lens array as described above will now be explained in detail.

Referring now to Figures 2A and 2B continuously, an etchant resistant mask or pattern 20 is deposited in a well-known manner on the substrate 14. Those skilled in the art will understand that variations of the pattern shown are possible, for example, rectangular patterns may be employed as well as other geometric shapes. In the example shown, the mask is arranged in the form of a square grid. The holes in the mask 40 represent the unmasked substrate areas which are etched by the process of the invention into individual lenses thereby rendering the entire substrate an array of lenses after the etching process is completed. The process of the invention uses the preferential etching properties which are exhibited by all etchant materials generally used in fabricating electronic devices, including optical detectors. Due to the preferential etching properties of typical etchants, the etch rate of the substrate is higher at the edge of the grid line than at the center of the space, therefore, more substrate is removed at the edge of the grid line than at the center. This form a dome, or a lens within the

boundaries of one grid space. The etchant etches in all directions simultaneously and, therefore, undercuts the etchant mask. It is one of the features of the invention to use the undercutting of the grid lines as an indicator of the appropriate etch depth. According to the invention, the gridline width is made to equal twice the desired edge depth. Therefore, when the preferential etching process completely undercuts the grid so as to remove or "float" it from the substrate surface, the lens array is determined to be completely etched and the etching process is stopped. Depending upon the optical qualities desired in the resultant lenses, in accordance with the invention, the grid lines may be adjusted in width to achieve various lens shapes as required by the optical properties desired for the system of application. Optical design of lenses is well known in the art and depends upon considerations such as the lens material, the medium through which the scene of interest is being viewed (i.e. air, water, etc.) and other considerations.

In a typical example of a process using the method of the invention, emersion in an etchant would be on the order of 10 seconds. Grid line

width may be in the range of about .3 mils to about .4 mils. Thus, the depth of the cusps of the lenses which are formed at the edges of the grid lines will be on the order of .15 mils to .2 mils. Those skilled in the art will recognize that other methods of applying an etchant, such as spraying will provide a substantially equivalent result.

Now referring to Figure 3, a block diagram of an infrared or other electromagnetic energy detection system is shown. A typical infrared system normally includes optics 36 for viewing a scene initiating electromagnetic radiation. The optics provide a field of view of the scene. Optics 36 could include an optical scanner; alternatively, the system could employ starring sensors and would not include an optical scanner. The system also includes an array 15 of electro magnetic radiation detectors mounted in the path of the field of view of the,., optics for producing electrical signals representative of the electromagnetic radiation impinging on the detectors. Such a system also includes a shield 17 comprising a cold shield as is well kη*ς>wn in the art. The system also includes a mosaic lens array in accordance with the present invention arranged at the optical system focus in

order to reduce the size of optical size resolution so that the detectors in the detector array 15 may be of reduced size and still view the full field with no dead space between pixels. Those skilled in the art will understand that the mosaic lens array of the invention may be advantageously coated with anti-reflective (A/R) or spectral band pass coatings in order to enhance performance in accordance with well known optical practice. The system also includes cooling means 34 for cooling the array of detectors and the shield. Electro-optics 35 coupled to the electrical output of the array of detectors form a display of the scene viewed by the optics.

This invention has been described herein in considerable detail in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment details and operating procedures, can be accomplished without departing from the scope of the invention itself.-