Integrated optics are critically important for the fast growing opto-electronics field. Solid state diode lasers often require beam shaping optics, and solid state photodetectors commonly utilize collection optics. These and other minaturized opto-electronic devices have created a demand for small, short focal length lenses (referred to herein as "microlenses"), so substantial effort and expense have been devoted to the development of microlenses and microlens arrays. Unfortunately, however, the process technology for producing such lenses has lagged behind the development of the LSI (large scale integrated) and the VLSI (very large scale integrated) semiconductor fabrication technology which is now being used to manufacture opto-electronic devices.
As a general rule, microlenses and microlens arrays are molded parts produced by replicating the contours of precisely machined dies. See, for example, I. N. Ozerov, et al. "Shaping the Contours of Dies for Manufacturing Lens Arrays Having Spherical Elements," Soviet Journal of Optical Technology (U.S.A.), Vol. 48, No. 1, January 1981, pp. 49-50. It is noted, however, that semiconductor fabrication technology has been proposed for the production of molds for replica-type production of stepped index zone plate lenses. See, L. d'Auria, et al., "Photolithographic Fabrication of Thin Film Lenses," Optics Communications, Vol. 5, No. 4, July 1972, pp. 232-5.
Replication is a suitable process for economically producing high quality, relatively uniform microlenses and microlens arrays, but the lenses still have to be mounted and optically aligned after being removed from the mold, so extreme care must be taken to ensure that they are properly aligned and firmly bonded or otherwise held in place when they are used for opto-electronics applications. Even then, the necessary optical alignment may be difficult to maintain over extended periods of time due to environmental factors, such a large temperature swings, which may cause differential thermal expansion of the lenses and the opto-electronic devices upon which they are mounted.
It also has been reported that planar arrays of distributed index microlenses have been produced. See, Oikawa, M. et al, "Array of Distributed-Index Planar Micro-Lenses Prepared from Ion Exchange Technique," Japanese Journal of Applied Physics, Vol. 20, No. 4, April 1981, pp. L296-8. Furthermore, criteria and procedures for evaluating microlenses and microlens arrays have been proposed. See, G. P. Shadurskii et al., "The Effect of Errors in the Fabrication of Lens Array Elements on the Effective Depth of Field," Soviet Journal of Optical Technology, Vol. 41, No. 11, November 1974, pp. 507-9, and G. S. Glukhovskiy, et al., "Evaluation of the Integrated Quality of Fine-Structure Lenslet Arrays," Soviet Journal of Optical Technology, Vol. 40, No. 7, July 1973, pp. 413-5. However, the foregoing references generally relate to the independent (i.e., non-monolithic) production and testing of microlens and microlens array technology, so they do not purport to materially alleviate the bonding and alignment problems that are encountered when conventional microlenses are employed.
An integrated microlens structure and fabrication process was described by Y. Ishihara et al., "A High Photosensitivity IL-CCD Image Sensor with Monolithic Resin Lens Array," International Electron Devices Meeting, 1983, pp. 497-500. They reported on developing a process for producing strip lenses for areal CCD (charge coupled device) image sensor arrays through the use of more or less standard semiconductor fabrication technology. Briefly, according to their description of the process, they (1) deposited a second layer of resin on an annealed, thermally hardened, smooth base resin layer, (2) photolithographically patterned the second resin layer to form a stripe-like pattern, and (3) then heated the second layer resin sufficiently to cause it to flow, thereby transforming its stripe-like pattern into a series of semicylinderical rolls or convex lenses. The strip lenses are, therefore, integrated with the CCD arrays, thereby avoiding the bonding and alignment problems of discrete microlenses.
There is, however, an entirely separate issue that must be addressed with respect to controlling such an integrated lens fabrication process sufficiently to obtain repeatable results for the production of microlenses and microlens arrays to reasonably exacting optical specifications. In particular, one of the principal shortcomings of the abovedescribed process is that the flow of the second layer, lens forming resin is inadequately controlled to form microlenses having well defined geometries or to form geometrically uniform, high density arrays of spatially distributed, individually addressable microlenses. Instead, it appears from the description of the process that the thermal flow of the lens forming resin is virtually unbounded by anything except the thermal characteristics of the system, so the lenses tend to flow into the merge with one another, thereby making it extremely difficult, if not impossible, to predict their optical boundary characteristics with any substantial precision. While precise process control may not be required for the manufacture of some microlenses, the optical specifications for microlenses and microlens arrays usually are sufficiently exacting to require adequate process controls for ensuring that the optical characteristics of each microlens are independently determined.