Method of fabricating a binary optics microlens upon a detector array

METHOD OF FABRICATING A BINARY OPTICS MICROLENS

UPON A DETECTOR ARRAY

CROSS-REFERENCE TO A RELATED PATENT APPLICATION;

0004This patent application is related to a commonly assigned patent application S.N. 07/ , filed

0005, entitled "Binary Optic Microlens Array Integral To Detector Array Substrate", by R. Thorn, (Attorney Docket No. PD-S89007) and to a commonly assigned patent appliction S.N. 07/ , filed , entitled "Method and Apparatus for Concentrating Optical Flux in a Focal Plane Array", by P. Norton (Attorney Docket No. PD-S88016) .

FIELD OF THE INVENTION:

0007This invention relates generally to radiation detectors and, in particular, relates to methods of fabricating a binary optics microlens array as an integral part of a backside-illuminated, one or two-dimensional detector array. Each individual microlens is fabricated such that it is disposed relative to an individual detector element of the array for concentrating incident radiation into a relatively small area at the plane of the detectors.

BACKGROUND OF THE INVENTION:

0009Conventional arrays of radiation detectors have been known to include detectors adhesively bonded, for example, to separately fabricated optical elements (or vice-versa) , the optical elements comprising lenticular surfaces or lenses. Typical arrangements have included molded, cast or embossed patterns of lenses. Curved lens surfaces have also been used. Also known are integrating cavities or cones which perform much the same function. "Faceplate" technology employs micrjolenses, the lens patterns being processed on or into a structure separate from the array of detectors. One type -of faceplate technology has been suggested that would provide a binary optics microlens array in proximity to a detector array, but not fabricated in the detector array substrate itself.

0010It can be realized that the fabrication of a binary optic microlens array directly within a radiation receiving surface of an array of detectors would provide a number of advantages over the separately provided lens arrays. For example, the ruggedness of the detector array would be increased while the overall cost related to assembly and fabrication would be reduced. Furthermore, such a lens/detector arrangement would provide for a backside illuminated detector array wherein incident radiation could be focussed to a relatively small spot size at the plane of the detectors, thereby enabling the detector active area to be decreased without sacrificing signal quality. As is known, a reduced area photovoltaic detector is desirable in that it typically exhibits a reduced junction capacitance, an increased operating speed and an improved resistance to ionizing radiation. However, until the integral microlens/detector array fabrication method taught by the invention these benefits were not readily attainable.

SUMMARY OF THE INVENTION

0012The foregoing problems are overcome and other advantages are realized by a binary optics microlens array that is fabricated as an integral part of a backside-illuminated, one or two-dimensional radiation detector array. In accordance with the method of the invention a three dimensional binary optic microlens structure is created within the radiation receiving back surface of the substrate of a radiation detecting array. The microlens has a structure predetermined to achieve a concentration of optical radiation within a desired spot size at the plane of the detectors, thereby facilitating the provision of detectors of reduced active area. The incident radiation may be planar or may be prefocused by externally provided optics. The method of the invention determines a binary optic microlens solution to a Fresnel lens which achieves the desired optical concentration, the method further providing at least one fabrication masking layer upon the back surface of the array and the selective removal of material from or addition of material to the back surface of the array to create the microlens structure.

BRIEF DESCRIPTION OF THE DRAWING

0014The above set forth and other features of the invention will be made more apparent in the ensuing Detailed Description of the Invention when read in conjunction with the attached Drawing, wherein:

0015Fig. la is an elevational view of a back surface of a radiation detector array showing several binary optic microlens structures;

0016Fig. lb is a cross-sectional view, not to scale, taken through one of the binary optic microlens elements of Fig. la;

0017Fig. 2a is an enlarged cross-sectional view of a conventional Fresnel lens showing three Fresnel zone plates;

0018Figs. 2b-2d illustrate a cross-sectional view of a one mask, two mask and four mask binary optic microlens approximation, respectively, of the Fresnel zone plates of Fig. 2a;

0019Fig≤. 3a and 3b schematically illustrate a f/1 embodiment of the binary optic microlens;

0020Figs. 4a and 4b schematically illustrate a f/2 embodiment of the binary optic microlens; and

0021Figs. 5a-5d illustrate steps of a method of the invention of fabricating a binary optic microlens on a back surface of a radiation detector array substrate. DETAILED DESCRIPTION OF THE INVENTION

0022Referring to Figs, la and lb there is shown a backside radiation receiving surface of a radiation detector array 10. The back surface can be seen to be filled with a plurality of Fresnel-type microlens structures which, in accordance with the invention, are comprised of binary optic microlens elements 12. Incident IR radiation (A) is concentrated by the lens elements 12 upon a detector 18 which is disposed on an opposite surface 14 of a transparent substrate 16.

0023Although the invention is described herein in the context of a photovoltaic Group II-VI radiation detector it should be realized that the teaching of the invention is applicable to detectors comprised of, by example, Group II-VI, Group III-V, silicon, germanium and other materials. In general, the larger the optical index of refraction of the substrate material the higher will be the degree of optical concentration.

0024Each microlens 12 has an associated unit cell width (W) which, in one illustrative embodiment of the invention, is approximately 50 microns. The width (w) of an associated photovoltaic detector 18 is generally reduced in relation to the unit cell width and may be on the order of 12 microns. The thickness of the substrate 16 can vary over a substantially wide range of thicknesses from, for example, 50 microns to 450 microns. In the embodiment of Figs, la and lb the detector 18 is responsive to IR radiation and the substrate 16 comprises material that is substantially transparent to the wavelengths of interest, such as CdTe or CdZnTe, while the surface 14 is a surface of an epitaxial HgCdTe radiation absorbing layer wherein the photovoltaic detector 18 is formed.

0025Referring to Fig. 2a there is shown in cross-section a portion of a conventional Fresnel lens which is comprised of a plurality of Fresnel zone plates 20. Each spherically curved zone plate 20 defines the spatial extent of a region wherein incident radiation makes a (2 pi) phase shift. However, the fabrication of the spherical curvature of the Fresnel zone plates 20 within- the confines of a 50 micron unit cell is extremely difficult if not impossible to accomplish with presently known fabrication techniques. As a result, and in accordance with the invention, a binary optic lens solution is employed to approximate the desired shape of the Fresnel zone plates 20 upon the back surface of an array of radiation detectors. The binary optic lens solution is accomplished by the application of one or more masking layers of predetermined shape in conjunction with the selective removal of material from the back surface of the array. The one or more masking layers define the position, shape and resolution of the resulting binary optic approximation of the Fresnel zone plates.

0026Fig. 2b shows a one-mask embodiment of a binary optic microlens, Fig. 2c shows a two mask embodiment while Fig. 2d shows an enlarged view of one zone plate approximation resulting from a four mask embodiment. As can be seen, as the number of masks employed to create the binary optic microlens increases the overall cross-sectional shape more closely approximates the spherical curvature of the Fresnel zone plate 20. In general, as the number of fabrication mask layers increases the microlens efficiency increases.

0027Example 1

0028Referring to Fig. 3a and 3b there is shown a binary optic microlens 22 which forms a f/1 microlens element. The design considerations of the microlens of Fig. 3a are a substrate comprised of CdTe (n= 2.67), 10 micron incident radiation prefocussed by f/2 optics, a 50 micron unit cell width and a 50 micron substrate thickness. The radiation concentration required is such that approximately 80% of the flux falls within a 6 micron spot at the plane of the detector 24. The overall optical gain of the microlens 22 is approximately 69.

0029A two mask embodiment of microlens 22 achieves an 81% optical efficiency. Table 1 shows the radial dimensions in microns, referenced to the center of the microlens 22, of the first and the second fabrication masks, mask 1 and mask 2. In general, it has been found that the higher order mask, in this case mask 2, is preferably applied and processed first in order that the finer resolution microlens features are applied to a substantially planar surface. Table 1

MASK RADIAL DIMENSIONS (MICRONS) MASK 1 MASK2

00319.8

003213.9 13.9

003317.1

003419.8 19.8

003522.3 24.5 24.5

003626.6

003728.6 28.6

003830.4

003932.2 32.2 ^w 33.9

004035.6 35.6 37.2

0041Example 2

0042Referring to Fig. 4a and 4b there is shown another example of a binary optic microlens 26 which provides an f/2 lens for use with 10 micron wavelength f/2 prefocussed radiation and a 50 micron width unit cell. The CdTe substrate thickness is 100 microns and the microlens 26 achieves an 80% flux concentration within a 10 micron diameter spot at the plane of the detector 28. The optical gain of this two mask configuration is approximately 25 and the optical efficiency is again approximately 81%. Table 2 shows the mask radial dimensions in microns for the first and the second mask. - Table 2

MASK RADIAL DIMENSIONS (MICRONS) MASK 1 MASK2

004413.8

004519.5 19.5 23.9

004627.6 27.6 31.0 ⁰ 34.0 34.0

0047It should be noted that the second example employs three micron rule lithography whereas Example 1 above employs 1.6 micron rule lithography. 5

0048Example 3

0049Although not illustrated Example 3 provides a binary optical microlens for 4.6 micron wavelength f/2 0 prefocussed radiation and a 75 micron width unit cell. The CdTe substrate thickness is 450 microns, thereby providing a f/6 microlens (450/75=6) structure. This microlens concentrates approximately 80% of the incident flux within an approximately 1.9 micron spot ⁵ at the plane of the detector and provides an optical gain of approximately 15.6. Again, an 81% optical efficiency is achieved with two masks.

0050Table 3 illustrates the radial dimensions of the two ° masks. The masks are fabricated with 1.8 micron rule lithography. Table 2

MASK RADIAL DIMENSIONS (MICRONS) MASK 1 MASK2

005213.4

005318.9 18.9

005423.1

005526.7 26.7 29.8

005632.7 32.7

005735.3

005837.7 37.7

005940.0 42.2 42.2

006044.2

006146.2 46.2

006248.1

006349.9 49.9 51.7

0064Referring to Figs 5a-5d there is illustrated, in accordance- with a method of the invention, the fabrication of the binary optic microlens 22 of Example 1 above. It should be noted in the design and fabrication of the microlens 22 that the substrate index of refraction and thickness, the desired detector 24 active area, and the detector center-to-center spacing _j^s known beforehand. Furthermore, the wavelength of the radiation and the degree of prefbcussing, if any, of the incident radiation are also known beforehand. Based on this information, a Fresnel lens that would provide for the required amount of radiation concentration within the detector 24 active area is arrived at by a conventional lens design. After the required Fresnel lens design is obtained the spherical curvature of the zone plates, as in Fig. 2a, is evaluated. From this evaluation a binary optic lens design is determined that most closely approximates the spherical curvature of the Fresnel zone plates within the limitations of the photolithographic and other processing restrictions in effect. After the number of masks and the resolution of each mask is determined the masks are fabricated and the binary optic lens elements are fabricated upon the surface of the substrate.

0065As can be seen in Fig. 5a a substrate 30 has a substantially planar surface 32 to which the highest order mask is applied and processed by known techniques to deposit a masking layer 34. The masking layer 34 may be comprised of photoresist or any suitable type of material. The radial dimensions of the portion of the masking layer 34 that is illustrated in Fig. 5a correspond to that shown in Table 1. After the masking layer 34 is applied the surface is etched with, preferably, a dry etch method such as an ion beam etch to selectively remove the substrate 30 material which is not covered by the masking layer 34. In this regard the etch depth (d) for each mask is determined in accordance with the expression

0066d = (2 pi)/((n-l)*2^N),

0067where n is the index of refraction of the substrate material and N is the mask number or order. P T U 88

006812

0069In Fig. 5b it can be seen that the masking layer 34 has been stripped away and that the surface 32 of the substrate 30 includes a number of depressions 34a. In Fig. 5c a coarser mask, mask 1 of Example 1, is applied and processed to produce the masking layer 36. The surface of the substrate 30 is once more etched. Fig. 5d illustrates the substrate surface after the removal of the masking layer 36. It can be seen that after the second etching step that portions of the substrate 30 have not been etched (32) , portions have been etched once (38) and other portions have been etched twice (34) and, hence, have a greater depth than the once-etched portions.

0070In accordance with the method of the invention a three dimensional binary optic microlens structure is created within the radiation receiving back surface of the substrate of a radiation detecting array. The microlens has a structure predetermined to achieve a concentration of optical radiation within a desired spot size at the plane of the detectors, thereby facilitating the provision and operation of detectors of reduced active area. The incident radiation may be planar or may be prefocused by externally provided optics. The method of the invention determines a binary optic microlens solution to a Fresnel lens which achieves the desired optical concentration, the method further providing at least one fabrication masking layer upon the back surface of the array and the selective removal of material from the back surface of the array to create the microlens structure. While the invention has been particularly shown and described with respect to a preferred embodiment thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention. For example, it is within the scope of the invention to evaporate a material through a reverse image of the mask 34 onto the substrate surface, thereby providing a binary lens structure by an additive as opposed to a subtractive process. The evaporated material is preferably comprised of the same material as the substrate or may be comprised of an optically transparent optical dielectric such as germanium. Furthermore, it can be appreciated that a combination of the etching and additive techniques can be employed.