The infrared spectrum covers a range of wavelengths longer than the visible wavelengths but shorter than microwave wavelengths. Visible wavelengths are generally regarded as between 0.4 and 0.75 micrometers. The near infrared wavelengths extend from 0.75 micrometers to 10 micrometers. The far infrared wavelengths cover the range from approximately 10 micrometers to 1 millimeter. The function of infrared detectors is to respond to energy of a wavelength within some particular portion of the infrared region.
Heated objects dissipate thermal energy having characteristic wavelengths within the infrared spectrum. Different levels of thermal energy, corresponding to different sources of heat, are characterized by the emission of signals within different portions of the infrared frequency spectrum. No single detector is uniformly efficient over the entire infrared frequency spectrum. Thus, detectors are selected in accordance with their sensitivity in the range of interest to the designer. Similarly, electronic circuitry that receives and processes the signals from the infrared detector must also be selected in view of the intended detection function.
A variety of different types of infrared detectors have been proposed in the art since the first crude infrared detector was constructed in the early 1800's. Virtually all contemporary infrared detectors are solid state devices constructed of materials that respond to infrared frequency energy in one of several ways. Thermal detectors respond to infrared frequency energy by absorbing that energy causing an increase in temperature of the detecting material. The increased temperature in turn causes some other property of the material, such as resistivity, to change. By measuring this change the infrared radiation is measured.
Photo-type detectors (e.g., photoconductive and photovoltaic detectors) absorb the infrared frequency energy directly into the electronic structure of the material, inducing an electronic transition which, in turn, leads to either a change in the electrical conductivity (photoconductors) or to the generation of an output voltage across the terminals of the detector (photovoltaic detectors). The precise change that is effected is a function of various factors including the particular detector material selected, the doping density of that material and the detector area.
By the late 1800's, infrared detectors had been developed that could detect the heat from an animal at one quarter of a mile. The introduction of a focusing lenses constructed of materials transparent to infrared frequency energy, as well as advances in semiconductor materials and highly sensitive electronic circuity have advanced the performance of contemporary infrared detectors close to the ideal photon limit.
Current infrared detection systems incorporate arrays of large numbers of discrete, highly sensitive detector elements the outputs of which are connected to sophisticated processing circuity. By rapidly analyzing the pattern and sequence of detector element excitations, the processing circuitry can identify and monitor sources of infrared radiation. Though the theoretical performance of such systems is satisfactory for many applications, it is difficult to actually construct structures that mate a million or more detector elements and associated circuitry in a reliable and practical manner. Consequently, practical applications for contemporary infrared detection systems have necessitated that further advances be made in areas such as miniaturization of the detector array and accompanying circuitry, minimization of noise intermixed with the electrical signal generated by the detector elements, and improvements in the reliability and economical production of the detector array and accompanying circuitry.
A contemporary subarray of detectors may, for example, contain 256 detectors on a side, or a total off 65,536 detectors, the size of each square detector being approximately 0.0035 inches on a side with 0.0005 inches spacing between detectors. The total width of such a subarray would therefore be 1.024 inches on a side. Thus, interconnection of such a subarray to processing circuitry requires a connective module with sufficient circuitry to connect each of the 65,536 detectors to processing circuitry within a square a little more than one inch on a side. The subarrays may, in turn, be joined to form an array that includes 25 million detectors or more. Considerable difficulties are presented in aligning the detector elements with conductors on the connecting module and in isolating adjacent conductors in such a dense environment.
The outputs of the detectors must undergo a series of processing steps in order to permit derivation of the desired information. The more fundamental processing steps include preamplification, tuned bandpass filtering, clutter and background rejection, multiplexing and fixed noise pattern suppression. By providing a detector connecting module that performs at least a portion of the signal processing functions within the module, i.e., on integrated circuit chips disposed adjacent the detector focal plane, the signal from each detector need be transmitted only a short distance before processing. As a consequence of such on-focal plane or "up front" signal processing, reductions in size, power and cost of the main processor may be achieved. Moreover, up front signal processing helps alleviate performance, reliability and economic problems associated with the construction of millions of closely spaced conductors connecting each detector element to the main signal processing network.
Various constructions have been proposed to support the necessary connectivity and processing functions of the module. Those constructions have heretofore included the formation of a multi-layer passive substrate having metalized patterns formed thereon. Electronic devices such as integrated circuits are mounted on one or more of the substrate layers and connected to the metalized patterns to communicate signals between the electronic devices and the detector elements or external electronics.
The interconnection of conductive conduits formed upon opposite sides of each layer of such a multi-layer substrate is typically accomplished by the use of conductive vias wherein a thin conductive film is sputter-coated into a through-hole interconnecting each side of the layer or substrate. However, the effectiveness and reliability of such conductive via metal interconnects is substantially limited by the aspect ratio (through-hole opening diameter to depth ratio) of the via and is dependent upon the amount of metal deposited within the via. Typically, the sputter-coated metals deposited within a via are substantially thinner than those formed upon the outside surfaces of the substrate, frequently resulting in ineffective and unreliable electrical interconnection.
Multi-layer Z-modules, as disclosed in U.S. Pat. No. 4,703,170 issued to SCHMITZ on Oct. 27, 1987 and entitled INFRARED FOCAL PLANE MODULE and disclosed in U.S. Pat. No. 5,093,708 issued to SOLOMON on Mar. 3, 1992 and entitled MULTI-LAYER INTEGRATED CIRCUIT MODULE, the contents of both of which are hereby incorporated by reference, utilize ceramic substrates having two-sided metalization wherein interconnection of the conductive conduits formed upon opposing sides of the substrate is effected with gold thin-film metalized through-holes drilled by a small laser beam with inner diameters of less than 50 microns. Sputter-coating is typically applied from both sides of the substrate so as to provide more complete coverage of the metalization layer within the via through-hole.
Since the via through-hole depth, i.e., the thickness of the substrate, is several times greater than the diameter of the via openings, the amount of metal deposited within the via through-hole is typically substantially less than that formed upon the exterior surfaces of the substrate. For example, it has been found that in a 100 micron thick substrate with 40 micron diameter via holes, that a 1 micron thick deposit on the exterior surface typically results in an equivalent thickness of less than 0.1 micron inside the via through-hole.
Subsequent processing of the substrate frequently results in damage to the thin-film metalization layer formed within the via. Such damage occurs due to physical, chemical, and thermal stresses inherent to such subsequent processing. Physical damage may result from foreign materials being undesirably introduced into the via during such processing. Chemical damage may result from the effects of corrosive agents and/or solvents being deposited within the via through-hole during such subsequent processing. Thermal stress is typically inherent to subsequent processing of the substrate wherein various materials are deposited upon the substrate, typically via thermal processes, and wherein various components may be attached to the substrate via thermal bonding. As such, failure of the via interconnection may occur, typically where the metalization layer is thinnest, i.e., proximate the mid-point of the via through-hole.
Multi-layered micro circuit boards such as ceramic layers utilized in the fabrication of infrared detectors z-modules, metalized through-hole interconnects are commonly used to provide electrical interconnection for circuitry formed upon the two sides of the substrate. Laser-drilled vias with sizes between 20-50 microns in diameter are typically formed upon ceramic substrates of approximately 80-120 microns in thickness to facilitate this process.
Sputter-coated thin-film metalization has been used as one method of metalizing such via through-holes. However, such sputtering is typically effective only when the aspect ratio (the ratio of the depth of the via through-hole to the diameter thereof) of the vias is small, typically less than 2, and the processing temperature is low, typically less than 300.degree. C. For via interconnects to be processed at higher temperatures, thick-film via-fill materials must be utilized.
Even when thin-film via interconnect successfully pass electrical acceptance testing subsequent to such processing, it is possible for the via interconnect to degrade due to the effects of aging and thermal stress, thereby causing latent failures.
One way to overcome the problems inherent in the use of such thin-films deposited within via through-holes is to fill the via through-holes so as to create impermeable plugs which provide high conductivity.
However, previous attempts to fill through-holes so as to create impermeable plugs while providing high conductivity have, to date, been ineffective in providing a satisfactory product.
When such plugs or thick-film gold conductive materials are used, the via-fill materials usually shrink considerably upon sintering. To fully plug and fill up the vias, repetitive fill and sintering steps are typically required. Alternatively, an excess of the via-fill material may be left on the surface of the substrate. After sintering, the excess surface material must be removed by an appropriate method such as grinding and/or lapping. This approach, however, often results in smearing of the via-fill material upon the substrate surface. Even when the top layer of the substrate is lapped off, the remaining surface may not be sufficiently cleaned to meet the requirements for subsequent thin-filled sputter coating. Furthermore, such lapping of the surface itself creates additional problems. For example, the substrate with which the process is commenced must be substantially thicker than otherwise required, so as to compensate for the thickness of the substrate which is removed during the lapping process. As those skilled in the art will appreciate, the lapping process further creates additional yield loss due to breakage and thickness variations.
According to contemporary methodology, laser drilling is applied to either bare substrates or substrates precoated with a thin metallic coating. When laser energy is applied to a substrate directly, the localized intensive heat cause thereby often result in recasting of the material surrounding the perimeter of the laser beam. For brittle materials, such as ceramics, the sudden thermal shock may result in shattering of the surrounding materials.
Application of a thin metalized coating helps to better dissipate the thermal shock and thus reduce the amount of shattering but slag and cracking still persist. However, in any case, via through-holes having irregularly shaped, shattered, or recast edges resulted from the laser drilling process can cause imperfect metalization patterns. Thus, reduced reliability occurs in the contact between the surface metalization and the via-fill material.
As such, it is beneficial to provide a method for forming conductive via interconnects wherein a sacrificial coating is applied to the substrate so as to prevent recasting and shattering of the substrate during laser drilling and also so as to facilitate accurate screen printing of conductive materials within the via through-hole.
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