Hermetically sealed micro-device package using cold-gas dynamic spray material deposition

A cover assembly for welding to a package base to form a hermetically sealed micro-device package. The cover assembly includes a sheet of a transparent material having a window portion. A built-up metallic frame adheres to the sheet and circumscribes the window portion, the frame having been deposited as follows: First, powdered metal particles are sprayed onto a prepared area of the sheet using a gas jet at a temperature below the fusing temperature of the particles, the jet having a velocity sufficient to cause the particles to merge with one another upon impact with the sheet and with one another to form an initial continuous metallic coating adhering to the prepared area of the sheet. Next, successive metal particles are applied over the initial coating using the jet to form the frame incorporating the initial continuous metallic coating as its base and having an predetermined overall thickness.

TECHNICAL FIELD OF THE INVENTION

The current invention relates to packages for photonic devices, optical devices, micro-mechanical devices, micro-electromechanical systems (MEMS) devices or micro-optoelectromechanical systems (MOEMS) devices, and more particularly, to methods for manufacturing packages having a hermetically sealed chamber covered by a transparent window using cold-gas dynamic spray material deposition.

BACKGROUND OF THE INVENTION

Photonic, optical and micro-mechanical devices are typically packaged such that the active elements (i.e., the emitters, receivers, micro-mirrors, etc.) are disposed within a sealed chamber to protect them from handling and other environmental hazards. In many cases, it is preferred that the chamber be hermetically sealed to prevent the influx, egress or exchange of gasses between the chamber and the environment. Of course, a window must be provided to allow light or other electromagnetic energy of the desired wavelength to enter and/or leave the package. In some cases, the window will be visibly transparent, e.g. if visible light is involved, but in other cases the window may be visibly opaque while still being “optically” transparent to electromagnetic energy of the desired wavelengths. In many cases, the window is given certain optical properties to enhance the performance of the device. For example, a glass window may be ground and polished to achieve certain flatness specifications in order to avoid distorting the light passing therethrough. In other cases, anti-reflective or anti-refractive coatings may be applied to the window to improve light transmission therethrough.

Hermetically sealed micro-device packages with windows have heretofore been produced using cover assemblies with metal frames and glass window panes. To achieve the required hermetic seal, the glass window pane has heretofore been fused to its metallic frame by heating it in a furnace at a temperature exceeding the glass transition temperature, TG(typically at or above 900° C.). However, because the fusing temperature is above TG, the original surface finish of the glass pane is typically ruined, making it necessary to finish or re-finish (e.g., grinding and polishing) both surfaces of the window pane after fusing in order to obtain the necessary optical characteristics. This polishing of the window panes requires additional process steps during manufacture of the cover assemblies, which steps tend to be relatively time and labor intensive, thus adding significantly to the cost of the cover assembly, and hence to the cost of the overall package. In addition, the need to polish both sides of the glass after fusing requires the glass to project both above and below the attached frame. This restricts the design options for the cover assembly with respect to glass thickness, dimensions, etc., which can also result in increased material costs.

Once a cover assembly with a hermetically sealed window is prepared, it is typically seam welded to the device base (i.e., substrate) in order to produce the finished hermetically sealed package. Seam welding uses a precisely applied AC current to produce localized temperatures of about 1,100° C. at the frame/base junction, thereby welding the metallic cover assembly to the package base and forming a hermetic seal. To prevent distortion of the glass windowpane or package, the metal frame of the cover assembly should be fabricated from Kovar alloy or another alloy having a CTE (i.e., coefficient of thermal expansion) which is similar to that of the transparent window material and to the CTE of the package base.

While the methods described above have heretofore produced useable window assemblies for hermetically sealed micro-device packages, the relatively high cost of these window assemblies is a significant obstacle to their widespread application. A need therefore exists, for package and component designs and assembly methods which reduce the labor costs associated with producing each package.

A need still further exists for package and component designs and assembly methods which will minimize the manufacturing cycle time required to produce a completed package.

A need still further exists for package and component designs and assembly methods which reduce the number of process steps required for the production of each package. It will be appreciated that reducing the number of process steps will reduce the overhead/floor space required in the production facility, the amount of capital equipment necessary for manufacturing, and handling costs associated with transferring the work pieces between various steps in the process. A reduction in the cost of labor may also result. Such reductions would, of course, further reduce the cost of producing these hermetic packages.

A need still further exists for package and component designs and assembly methods which will reduce the overall materials costs associated with each package, either by reducing the initial material cost, by reducing the amount of wastage or loss during production, or both.

SUMMARY OF THE INVENTION

The present invention disclosed and claimed herein comprises, in one aspect thereof, a cover assembly that can be welded to a micro-device package base to form a hermetically sealed micro-device package. The cover assembly includes a sheet of a transparent material having a window portion defined thereupon. A built-up metallic frame adheres to the sheet and circumscribes the window portion, the frame having been deposited onto the sheet as follows: First, a first quantity of powdered metal particles is sprayed onto a prepared frame-attachment area of the sheet using a jet of gas, the gas being at a temperature below the fusing temperature of the metal particles, and the jet of gas having a velocity sufficient to cause the metal particles to merge with one another upon impact with the sheet and with one another so as to form an initial continuous metallic coating adhering to the frame-attachment area of the sheet. Next, successive quantities of powdered metal particles are applied over the initial continuous metallic coating using the jet of gas so as to form the built-up metallic frame incorporating the initial continuous metallic coating as its base and having an overall thickness that is a predetermined thickness.

The present invention disclosed and claimed herein comprises, in another aspect thereof, a micro-device module including a package base, a micro-device supported on the package base, and a cover assembly joined to the package base so as to encapsulate the micro-device in a hermetically sealed cavity formed between the cover assembly and the package base. The cover assembly including a sheet of a transparent material having a window portion defined thereupon and a built-up metallic frame adhering to the sheet and circumscribing the window portion, the frame having been deposited onto the sheet as follows: First, a first quantity of powdered metal particles is sprayed onto the sheet using a jet of gas, the gas being at a temperature below the fusing temperature of the metal particles, and the jet of gas having a velocity sufficient to cause the metal particles to merge with one another upon impact with the sheet and with one another so as to form an initial continuous metallic coating adhering to the sheet. Next, successive quantities of powdered metal particles are applied over the initial continuous metallic coating using the jet of gas so as to form the built-up metallic frame incorporating the initial continuous metallic coating as its base.

DETAILED DESCRIPTION OF THE INVENTION AND BEST MODE

Referring now toFIGS. 1 and 2, there is illustrated a typical hermetically sealed micro-device package for housing photonic devices, optical devices (i.e., including reflective, refractive and diffractive type devices), micro-optoelectromechanical systems (i.e., MOEMS) devices and micro-electromechanical systems (i.e., MEMs) devices. The package102comprises a base or substrate104which is hermetically sealed to a cover assembly106comprising a frame108and a transparent window110. A micro-device112mounted on the base104is encapsulated within a cavity114when the cover assembly106is joined to the base104. One or more electrical leads116may pass through the base104to carry power, ground, and signals to and from the micro-device112inside the package102. It will be appreciated that the electrical leads116must also be hermetically sealed to maintain the integrity of the package102. The window110is formed of an optically or electro-magnetically transparent material. For purposes of this application, the term “transparent” refers to materials which allow the transmission of electromagnetic radiation having predetermined wavelengths, including, but not limited to, visible light, infrared light, ultraviolet light, microwaves, radio waves, or x-rays. The frame108is formed from a material, typically a metal alloy, which preferably has a CTE close to that of both the window110and the package base104.

Referring now toFIG. 3, there is illustrated an exploded view of a cover assembly manufactured in accordance with one embodiment of the current invention. The cover assembly300includes a frame302and a sheet304of a transparent material. The frame302has a continuous sidewall306which defines a frame aperture308passing therethrough. The frame sidewall306includes a frame seal-ring area310(denoted by crosshatching) circumscribing the frame aperture308. Since the frame302will eventually be welded to the package base104, it is usually formed of a weldable metal or alloy, preferably one having a CTE very close to that of the micro-device package base104. In some embodiments, however, the cover assembly frame304may be formed of a non-metallic material such as ceramic or alumina. Regardless of whether the frame302is formed of a metallic or non-metallic material, the surface of the frame seal-ring area310must be metallic (e.g., metal plated if not solid metal) to facilitate the hermetic sealing of the sheet304to the frame. In a preferred embodiment, the frame is primarily formed of an alloy having a nominal chemical composition of 54% iron (Fe), 29% nickel (Ni) and 17% cobalt (Co). Such alloys are also known by the designation ASTM F-15 alloy and by the trade name Kovar Alloy. As used in this application, the term “Kovar Alloy” will be understood to mean the alloy having the chemical composition just described. In embodiments where a Kovar Alloy frame302is used, it is preferred that the surface of the frame seal-ring area310have a surface layer of gold (Au) overlying a layer of nickel (Ni). The frame302also includes a base seal area320which is adapted for eventual joining, typically by welding, to the package base104. The base seal area320frequently includes a layer of nickel overlaid by a layer of gold to facilitate seam welding to the package base. Although the gold over nickel surface layers are only required along the base seal-ring area320, it will be appreciated that in many cases, for example, where solution bath plating is used to apply the surface materials, the gold over nickel layers may be applied to the entire surface of the frame302. The sheet304can be any type of transparent material, for example, soft glass (e.g., soda-lime glass), hard glass (e.g. borosilicate glass), crystalline materials such as quartz and sapphire, or polymeric materials such as polycarbonate plastic. As previously discussed, it is preferred that the material of the sheet304have a CTE that is similar to that of the frame304and of the package base104to which the cover assembly will eventually be attached. For many semiconductor photonic, MEMS or MOEMS applications, a borosilicate glass is well suited for the material of the sheet304. Examples of suitable glasses include Corning 7052, 7050, 7055, 7056, 7058, 7062, Kimble (Owens Corning) EN-1, and Kimble K650 and K704. Other suitable glasses include Abrisa soda-lime glass, Schott 8245 and Ohara Corporation S-LAM60.

The sheet304has a window portion312defined thereupon, i.e., this is the portion of the sheet302which must remain transparent to allow for the proper functioning of the encapsulated, i.e., packaged, micro-device112. The window portion312of the sheet has top and bottom surfaces314and316, respectively, that are optically finished in the preferred embodiment. The sheet304is preferably obtained with the top and bottom surfaces314and316of the window portion312in ready to use form, however, if necessary the material may be ground and polished or otherwise shaped to the desired surface contour and finish as a preliminary step of the manufacturing process. While in many cases the window portion312will have top and bottom surfaces of314and316that are optically flat and parallel to one another, it will be appreciated that in other embodiments at least one of the finished surfaces of the window portion will be contoured. A sheet seal-ring area318(denoted with cross-hatching) circumscribes the window portion312of the sheet304, and provides a suitable surface for joining to the front seal-ring area310.

Referring now toFIGS. 4aand4b, there are illustrated transparent sheets having contoured sides. InFIG. 4a, transparent sheet304′ has both a curved top side314′ and a curved bottom side316′ producing a window portion312having a curved contour with a constant thickness. InFIG. 4b, sheet304″ has a top side314″ which is curved and a bottom side316″ which is flat, thereby resulting in a window portion312having a plano-convex lens arrangement. It will be appreciated that in similar fashion (not illustrated) the finished surfaces314and316of the window portion312can have the configuration of a refractive lens including a plano-convex lens as previously illustrated, a double convex lens, a plano-concave lens or a double concave lens. Other surface contours may give the finished surfaces of the window portion312the configuration of a Fresnel lens or of a diffraction grating, i.e., “a diffractive lens.”

In many applications, it is desirable that window portion312of the sheet304have enhanced optical or physical properties. To achieve these properties, surface treatments or coatings may be applied to the sheet304prior to or during the assembly process. For example, the sheet304may be treated with siliconoxynitride (SiOn) to provide a harder surface on the window material. Whether or not treated with SiOn, the sheet304may be coated with a scratch resistant/abrasion resistant material such as amorphous diamond-like carbon (DLC) such as that sold by Diamonex, Inc., under the name Diamond Shield®. Other coatings which may be applied in addition to, or instead of, the SiOn or diamond-like carbon include, but are not limited to, optical coatings, anti-reflective coatings, refractive coatings, achromatic coatings, optical filters, electromagnetic interference (EMI) and radio frequency (RF) filters of the type known for use on lenses, windows and other optical elements. It will be appreciated that the optical coatings and/or surface treatments can be applied either on the top surface314or the bottom surface316, or in combination on both surfaces, of the window portion312. It will be further appreciated, that the optical coatings and treatments just described are not illustrated in the figures due to their transparent nature.

In some applications, a visible aperture is formed around the window portion312of the sheet304by first depositing a layer of non-transparent material, e.g., chromium (Cr), over the entire surface of the sheet and then etching the non-transparent material from the desired aperture area. This procedure provides a sharply defined border to the window portion312which is desirable in some applications. This operation may be performed prior to or after the application of other treatments depending on the compatibility and processing economics.

The next step of the process of manufacturing the cover assembly300is to prepare the sheet seal-ring area318for metallization. The sheet seal-ring area318circumscribes the window portion312of the sheet304, and for single aperture covers is typically disposed about the perimeter of the bottom surface316. It will be appreciated, however, that in some embodiments the sheet seal-ring area318can be located in the interior portion of a sheet, for example where the sheet will be diced to form multiple cover assemblies (i.e., as described later herein). The sheet seal-ring area318generally has a configuration which closely matches the configuration of the frame seal-ring area310to which it will eventually be joined. At a minimum, preparing the sheet seal-ring area318involves a thorough cleaning to remove any greases, oils or other contaminants from the surface. More commonly, preparing the sheet seal-ring area318involves roughening the seal-ring area by chemical etching, laser ablating, mechanical grinding or sandblasting this area. This roughening increases the surface area of the sheet seal-ring, thereby providing increased adhesion for the subsequently deposited metallization materials.

Referring now toFIG. 5, there is illustrated a portion of the sheet304which has been placed bottom side up to better illustrate the preparation of the sheet seal-ring area318. In this example the seal-ring area318has been given a roughened surface501to improve adhesion of the metallic layers to be applied. Chemical etching to roughen glass and similar transparent materials is well known. Alternatively, laser ablating, conventional mechanical grinding or sandblasting may be used. A grinding wheel with 325 grit is believed suitable for most glass materials, while a diamond grinding wheel may be used for sapphire and other hardened materials. The depth502to which the roughened surface501of the sheet seal-ring area318penetrates the sheet304is dependent on at least two factors: first, the desired mounting height of the bottom surface316of the window relative to the package bottom and/or the micro-device112mounted inside the package; and second, the required thickness of the frame306including all of the deposited metal layers (described below). It is believed that etching or grinding the sheet seal-ring area318to a depth of502within the range from about 0 inches to about 0.05 inches will provide a satisfactory adhesion for the metallized layers as well as providing an easily detectable “lip” for locating the sheet304in the proper position against the frame306during subsequent joining operations.

It will be appreciated that it may be necessary or desirable to protect the finished surfaces314and/or316in the window portion312of the sheet (e.g., the portions that will be optically active in the finished cover assembly) from damage during the roughening process. If so, the surfaces314and/or316may be covered with semiconductor-grade “tacky tape” or other known masking materials prior to roughening. The mask material must, of course, be removed in areas where the etching/grinding will take place. Sandblasting is probably the most economical method of selectively removing strips of tape or masking material in the regions that will be roughened. If sandblasting is used, it could simultaneously perform the tape removal operation and the roughening of the underlying sheet.

Referring now toFIG. 6, there is illustrated a view of the seal-ring area318of the sheet304after metallization. The next step of the manufacturing process is to apply one or more metallic layers to the prepared sheet seal-ring area318. The current invention contemplates several options for accomplishing this metallization. A first option is to apply metal layers to the sheet seal-ring area318using conventional chemical vapor deposition (CVD) technology. CVD technology includes atmospheric pressure chemical vapor deposition (APCVD), low pressure chemical vapor deposition (LPCVD), plasma assisted (enhanced) chemical vapor deposition (PACVD, PECVD), photochemical vapor deposition (PCVD), laser chemical vapor deposition (LCVD), metal-organic chemical vapor deposition (MOCVD) and chemical beam epitaxy (CBE). A second option for metallizing the roughened seal-ring area318is using physical vapor deposition (PVD) technology. PVD technology includes sputtering, ion plasma assist, thermal evaporation, vacuum evaporation, and molecular beam epitaxy (MBE). A third option for metallizing the roughened sheet seal-ring area318is using solution bath plating technology (SBP). Solution bath plating includes electroplating, electroless plating and electrolytic plating technology. While solution bath plating cannot be used for depositing the initial metal layer onto a nonmetallic surface such as glass or plastic, it can be used for depositing subsequent layers of metal or metal alloy to the initial layer. Further, it is envisioned that in many cases, solution bath plating will be the most cost effective metal deposition technique. Since the use of chemical vapor deposition, physical vapor deposition and solution bath plating to deposit metals and metal alloys is well known, these techniques will not be further described herein.

A fourth option for metallizing the sheet seal-ring area318of the sheet304is so-called cold-gas dynamic spray technology, also known as “cold-spray”. This technology involves the spraying of powdered metals, alloys, or mixtures of metal and alloys onto an article using a jet of high velocity gas to form continuous metallic coating at temperatures well below the fusing temperatures of the powdered material. Details of the cold-gas dynamic spray deposition technology are disclosed in U.S. Pat. No. 5,302,414 to Alkhimov et al. It has been determined that aluminum provides good results when applied to glass using the cold-gas dynamic spray deposition. The aluminum layer adheres extremely well to the glass and may create a chemical bond in the form of aluminum silicate. However, other materials may also be applied as a first layer using cold-spray, including tin, zinc, silver and gold. Since the cold-gas dynamic spray technology can be used at low temperatures (e.g., near room temperature), it is suitable for metallizing materials having a relatively low melting point, such as polycarbonates or other plastics, as well as for metallizing conventional materials such as glass, alumina, and ceramics.

For the initial metallic layer deposited on the sheet304, it is believed that any of chromium, nickel, aluminum, tin, tin-bismuth alloy, gold, gold-tin alloy can be used, this list being given in order of increasing adhesion to glass. Any of these materials can be applied to the sheet seal-ring area318using any of the CVD or PVD technologies (e.g., sputtering) previously described. After the initial layer602is deposited onto the sheet seal-ring area318of the nonmetallic sheet304, additional metal layers, e.g., second layer604, third layer606and fourth layer608(as applicable) can be added by any of the deposition methods previously described, including solution bath plating. It is believed that the application of the following rules will result in satisfactory thicknesses for the various metal layers. Rule No. 1: the minimum thickness, except for the aluminum or tin-based metals or alloys which will be bonded to the gold-plated Kovar alloy frame: 0.002 microns. Rule 2: the minimum thickness for aluminum or tin-based metals or alloys deposited onto the sheet or as the final layer, which will be bonded to the gold-plated Kovar alloy frame: 0.8 microns. Rule 3: the maximum thickness for aluminum or tin-based metals or alloys deposited onto the sheet or as the final layer, which will be bonded to the gold-plated Kovar alloy frame: 63.5 microns. Rule 4: the maximum thickness for metals, other than chromium, deposited onto the sheet as the first layer and which will have other metals or alloys deposited on top of them: 25 microns. Rule 5: the maximum thickness for metals, other than chromium, deposited onto other metals or alloys as intermediate layers: 6.35 microns. Rule 6: the minimum thickness for metals or alloys deposited onto the sheet or as the final layer, which will act as the solder for attachment to the gold-plated Kovar alloy frame: 7.62 microns. Rule 7: the maximum thickness for metals or alloys deposited onto the sheet or as the final layer, which will act as the solder for attachment to the gold-plated Kovar alloy frame: 101.6 microns. Rule 8: the maximum thickness for chromium: 0.25 microns. Rule 9: the minimum thickness for gold-tin solder, applied via inkjet or supplied as a solder preform: 6 microns. Rule 10: the maximum thickness for gold-tin solder, applied via inkjet or supplied as a solder preform: 101.6 microns. Rule 11: The minimum thickness for immersion zinc; 0.889 microns. Note that the above rules apply to metals deposited using all deposition methods other than cold-gas dynamic spray deposition.

For cold spray applications, the following rules apply: Rule 1: the minimum practical thickness for any metal layer: 2.54 microns. Rule 2: the maximum practical thickness for the first layer, and all additional layers, but not including the final Kovar alloy layer: 127 microns. Rule 3: the maximum practical thickness for the final Kovar alloy layer: 12,700 microns, i.e., 0.5 inches.

By way of example, not to be considered limiting, the following metal combinations are believed suitable for seal-ring area318when bonding the prepared sheet304to a Kovar alloy-nickel-gold frame302(i.e., Kovar alloy core plated first with nickel and then with gold) using thermal compression (TC) bonding, or sonic, ultrasonic or thermosonic bonding.

By way of further example, not to be considered limiting, the following metal combinations and thicknesses are preferred for seal-ring area318when bonding the prepared sheet304to a Kovar alloy-nickel-gold frame302using thermal compression (TC) bonding, or sonic, ultrasonic or thermosonic bounding.

LayersMetalDepositionMin. (microns)Max. (microns)1Sn—BiCVD, PVD150.8
As indicated above, the previous examples are believed suitable for application of, among other processes, thermal compression bonding. TC bonding is a process of diffusion bonding in which two prepared surfaces are brought into intimate contact, and plastic deformation is induced by the combined effect of pressure and temperature, which in turn results in atom movement causing the development of a crystal lattice bridging the gap between facing surfaces and resulting in bonding. TC bonding takes place at significantly lower temperatures than many other forms of bonding such as braze soldering.

Referring now toFIG. 7, there is illustrated a cross-sectional view of the prefabricated frame302suitable for use in this embodiment. The illustrated frame302includes a Kovar alloy core702overlaid with a first metallic layer704of nickel which, in turn, is overlaid by an outer layer706of gold. The use of Kovar alloy for the core702of the frame302is preferred where hard glass, e.g., Corning 7056 or 7058, is used for the sheet304and where Kovar alloy or a similar material is used for the package base104, since these materials have a CTE for the temperature range 30° C. to 300° C. that is within the range from about 5.0·10−6/° K to about 5.6·10−6/° K (e.g. from about 5.0 to about ppm/° K).

Referring still toFIG. 7, another step of the manufacturing process is the preparation of a prefabricated frame302for joining to the sheet304. As previously described, the frame302includes a continuous sidewall306which defines an aperture308therethrough. The sidewall306includes a frame seal-ring area310on its upper surface and a base seal-ring area320on its lower surface. The frame seal-ring area310is generally dimensioned to conform with the sheet seal-ring area318of the transparent sheet304, while the base seal-ring area320is dimensioned to conform against the corresponding seal area on the package base. The frame302may be manufactured using various conventional metal forming technologies, including stamping, casting, die casting, extrusion/parting, and machining. It is contemplated that stamping or die casting will be the most cost effective method for producing the frames302. Depending upon the degree of flatness required for the contemplated bonding procedure and the degree achieved by a particular frame manufacturing method, surface grinding, and possibly even lapping or polishing, maybe required on the frame seal-ring area310or base seal-ring area320, to provide the final flatness necessary for a successful hermetic seal.

In this example, the base seal-ring area320is on the frame face opposite frame seal-ring area310, and utilizes the same layers of nickel704overlaid by gold706to facilitate eventual welding to the package base104.

It is important for the frame302to serve as a “heat sink” and “heat spreader” when the cover assembly300is eventually welded to the package base104. It is contemplated that conventional high temperature welding processes (e.g., automatic resistance seam welding or laser welding) will be used for this operation. If the metallized glass sheet304was welded directly to the package base104using these welding processes, the concentrated heat would cause thermal stresses likely to crack the glass sheet or distort its optical properties. However, when a metal frame is attached to the transparent sheet, it acts as both a heat sink, absorbing some of the heat of welding, and as a heat spreader, distributing the heat over a wider area such that the thermal stress on the transparent sheet304is reduced to minimize the likelihood of cracking or optical distortion. Kovar alloy is especially useful in this heat sink and heat spreading role as explained by Kovar alloy's thermal conductivity, 0.0395, which is approximately fourteen times higher than the thermal conductivity of Corning 7052 glass, 0.0028.

Another important aspect of the frame302is that it must be formed from a material having a CTE that is similar to the CTE of the transparent sheet304and the CTE of the package base104. This matching of CTE between the frame302, transparent sheet304and package base104is required to minimize stresses between these components after they are joined to one another so as to ensure the long term reliability of the hermetic seal therebetween under conditions of thermal cycling and/or thermal shock environments.

For window assemblies that will be attached to package bases formed of ceramic, alumina or Kovar alloy, Kovar alloy is preferred for use as the material for the frame304. Although Kovar alloy will be used for the frames in many of the embodiments discussed in detail herein, it will be understood that Kovar alloy is not necessarily suitable for use with all transparent sheet materials. Additionally, other frame materials besides Kovar alloy may be suitable for use with glass. Suitability is determined by the necessity that the material of the transparent sheet304, the material of the frame302and the material of the package base104all have closely matching CTEs to insure maximum long-term reliability of the hermetic seals.

Referring now toFIG. 8, the next step of the manufacturing process is to position the frame302against the sheet304such that at least a portion of the frame seal-ring area310and a least a portion of the sheet seal-ring area318contact one another along a continuous junction region804that circumscribes the window portion312. Actually, in some cases a plasma-cleaning operation is performed on the seal-ring areas and any other sealing surfaces just prior to joining the components to ensure maximum reliability of the joint. InFIG. 8, the sheet304moves from its original position (denoted in broken lines) until it is in contact with the frame302. It is, of course, first necessary to remove any remaining tacky tape or other masking materials left over from operations used to prepare the sheet seal-ring area318if they cannot withstand the elevated temperatures encountered in the joining process without degradation of the mask material and/or its adhesive, if an adhesive is used to attach the mask to the sheet. It will be appreciated that it is not necessary that the sheet seal-ring area318and the frame seal-ring area310have an exact correspondence with regard to their entire areas, rather, it is only necessary that there be some correspondence between the two seal-ring areas forming a continuous junction region804which circumscribes the window portion312. In the embodiment illustrated inFIG. 8, the metallized layers610in the sheet seal-ring area318are much wider than the plated outer layer706of the frame seal-ring area310. Further, the window portion312of the sheet304extends partway through the frame aperture308, providing a means to center the sheet304on the frame302.

The next step of the manufacturing process is to heat the junction region804until a metal-to-metal joint is formed between the frame302and the sheet304all along the junction region, whereby a hermetic seal circumscribing the window portion312is formed. It is necessary that during the step of heating the junction region804, the temperature of the window portion312of the sheet304remain below its glass transition temperature, TGto prevent damage to the finished surfaces314and316. The current invention contemplates several options for accomplishing this heating. A first option is to utilize thermal compression (TC) bonding. As previously described, TC bonding involves the application of high pressures to the materials being joined such that a reduced temperature is required to produce the necessary diffusion bond. Rules for determining the thickness and composition of the metallic layers610on the sheet304were previously provided, for TC bonding to, e.g., a Kovar alloy, nickel or gold frame such as illustrated in FIG.7. The estimated process parameters for the TC bonding of a Kovar alloy/nickel/gold frame302to a metallized sheet304having aluminum as the final layer would be a temperature of approximately 380° C. at an applied pressure of approximately 95,500 psi. Under these conditions, the gold plating706on the Kovar alloy frame302will diffuse into/with the aluminum layer, e.g., layer4in Example 7. Since the 380° C. temperature necessary for TC bonding is below the approximately 500° C. to 900° C. TGfor hard glasses such as Corning 7056, the TC bonding process could be performed in a single or batch mode by fixturing the cover assembly components302,304together in compression and placing the compressed assemblies into a furnace (or oven, etc.) at approximately 380° C. The hermetic bond would be obtained without risking the finished surfaces314and316of the window portion312.

Alternatively, employing resistance welding at the junction area804to add additional heat in addition to the TC bonding could allow preheating the window assemblies to less than 380° C. and possibly reduce the overall bonding process time. In another method, the TC bonding could be accomplished by fixturing the cover assembly components302and304using heated tooling that would heat the junction area304by conduction. In yet another alternative method, resistance welding can be used to supply 100% of the heat required to achieve the necessary TC bonding temperature, thereby eliminating the need for furnaces, ovens, etc. or specialized thermally conductive tooling.

After completion of TC bonding or other welding processes, the window assembly300is ready for final processing, for example, chamfering the edges of the cover assembly to smooth them and prevent chipping, scratching, marking, etc., during post-assembly, cleaning, marking or other operations.

Referring now toFIG. 9, there is illustrated a block diagram of the manufacturing process just described in accordance with one embodiment of the current invention. Block902represents the step of obtaining a sheet of transparent material, e.g., glass or other material, having finished top and bottom surfaces as previously described. The process then proceeds to block904as indicated by the arrow.

Block904represents the step of applying surface treatments to the sheet, e.g., scratch-resistant or anti-reflective coatings, as previously described. In addition to these permanent surface treatments, block904also represents the sub-steps of applying tape or other temporary masks to the surfaces of the sheet to protect them during the subsequent steps of the process. It will be appreciated that the steps represented by block904are optional and that one or more of these steps may not be present in every embodiment of the invention. The process then proceeds to block906as indicated by the arrow.

Block906represents the step of preparing the seal-ring area on the sheet to provide better adhesion for the required metallic layers. This step usually involves roughening the seal-ring area using chemical etching, mechanical grinding, laser ablating or sandblasting as previously described. To the extent necessary, block906also represents the sub-steps of removing any masking material from the seal-ring area. It will be appreciated that the steps represented by block906are optional and that some or all of these steps may not be present in every embodiment of the invention. The process then proceeds to block908as indicated by the arrow.

Block908represents the step of metallizing the seal-ring areas of the sheet. The step represented by block908is mandatory since at least one metallic layer must be applied to the seal-ring area of the sheet. In most embodiments, block908will represent numerous sub-steps for applying successive metallic layers to the sheet, where the layers of each sub-step may be applied by processes including CVD, PVD, cold-spray or solution bath plating as previously described. Following the steps represented by block908, the sheet is ready for joining to the frame. However, before the process can proceed to this joining step (i.e., block916), a suitable frame must first be prepared.

Block910represents the step of obtaining a pre-fabricated frame having a CTE that closely matches the CTE of the transparent sheet from block902and the CTE of the package base. In most cases where the base is alumina or Kovar alloy, a frame formed of Kovar alloy will be suitable. As previously described, the frame may be formed using, e.g., stamping, die-casting or other known metal-forming processes. The process then proceeds to block912as indicated by the arrow.

Block912represents the step of grinding, polishing and/or otherwise flattening the seal-ring areas of the frame as necessary to increase its flatness so that it will fit closely against the seal-ring areas of the transparent sheet. It will be appreciated that the steps represented by block912are optional and may not be necessary or present in every embodiment of the invention. The process then proceeds to block914as indicated by the arrow.

Block914represents the step of applying additional metallic layers to the seal-ring areas of the frame. These metallic layers are frequently necessary to achieve compatible chemistry for bonding with the metallized seal-ring areas of the transparent sheet. In most embodiments, block914will represent numerous sub-steps for applying successive metallic layers to the frame. Once the steps represented by block914are completed, the frame is ready for joining to the transparent sheet. Thus, the results of process block908and block914both proceed to block916as indicated by the arrows.

Block916represents the step of clamping the prepared frame together with the prepared transparent sheet so that their respective metallized seal-ring areas are in contact with one another under conditions producing a predetermined contact pressure at the junction region circumscribing the window portion. This predetermined contact pressure between the seal-ring surfaces allows thermal compression (TC) bonding of the metallized surfaces to occur at a lower temperature than would be required for conventional welding (including most soldering and brazing processes). The process then proceeds to block918as indicated by the arrow.

Block918represents the step of applying heat to the junction between the frame and the transparent sheet while maintaining the predetermined contact pressure until the temperature is sufficient to cause thermal compression bonding to occur. In some embodiments, block918will represent a single heating step, e.g., heating the fixtured assembly in a furnace. In other embodiments, block918will represent several sub-steps for applying heat to the junction area, for example, first preheating the fixtured assembly (e.g., in a furnace) to an intermediate temperature, and then using resistance welding techniques along the junction to raise the temperature of the localized area of the metallic layers the rest of the way to the temperature where thermal compression bonding will occur. The thermal compression bonding creates a hermetic seal between the transparent sheet material and the frame. The process then proceeds to block920as indicated by the arrow.

Block920represents the step of completing the window assembly. Block920may represent merely cooling the window assembly after thermal compression bonding, or it may represent additional finishing processes including chamfering the edges of the assembly to prevent chipping, cracking, etc., marking the assembly, or other post-assembly procedures. The process of this embodiment has thus been described.

It will be appreciated that in alternative embodiments of the invention, conventional welding techniques (including soldering and/or brazing) may be used instead of thermal compression bonding to join the frame to the transparent sheet. In such alternative embodiments, the steps represented by blocks916and918ofFIG. 9would be replaced by the steps of fixturing the frame and transparent sheet together so that the metallized seal-ring areas are in contact with one another (but not necessarily producing a predetermined contact pressure along the junction) and then applying heat to the junction area using conventional means until the temperature is sufficient to cause the melting and diffusing of the metallic layers necessary to achieve the welded bond.

In an alternative embodiment, braze-soldering is used to join the frame302to the metallized sheet304. In this embodiment, a solder metal or solder alloy is utilized as the final layer of the metallic layers610on the metallized sheet304, and clamping the sheet304to the frame302at a high predetermined contact pressure is not required. Light to moderate clamping pressure can be used: 1) to insure alignment during the solder's molten phase; and 2) to promote even distribution of the molten solder all along the junction region between the respective seal-ring areas; thereby helping to insure a hermetic seal, however, this clamping pressure does not contribute to the bonding process itself as in TC bonding. In most other respects, however, this embodiment is substantially similar to that previously described.

The following examples, not to be considered limiting, are provided to illustrate the details of the metallic layers610in the sheet seal-ring area318that are suitable for braze-soldering to a Kovar alloy/nickel/gold frame302such as that illustrated in FIG.7.

By way of further examples, not to be considered limiting, the following combinations are preferred for the metallic layers610in the sheet seal-ring area318for braze-soldering to a Kovar alloy/nickel/gold frame302such as that illustrated in FIG.7.

Referring now toFIG. 10, there is illustrated yet another embodiment of the current invention. Note that in this embodiment, the cover assembly300is circular in configuration rather than rectangular. It will be appreciated that this is simply another possible configuration for a cover assembly manufactured in accordance with this invention, and that this embodiment is not limited to configurations of any particular shape. As in the embodiment previously described, this embodiment also uses braze-soldering to hermetically join the transparent sheet304to the frame302. However, in this embodiment, the solder is provided in the form of a separate solder preform1000having the shape of the sheet seal-ring area318or the frame seal-ring area310.

In this embodiment, instead of positioning the frame and the sheet directly against one another, the frame302and the sheet304are instead positioned against opposite sides of the solder preform1000such that the solder preform is interposed between the frame seal-ring area310and the sheet seal-ring are318along a continuous junction region that circumscribes the window portion312. After the frame302and sheet304are positioned against the solder preform1000, the junction region is heated until the solder preform fuses forming a solder joint between the frame and sheet all along the junction region. The heating of the junction region may be performed by any of the procedures previously described, including heating or preheating in a furnace, oven, etc., either alone or in combination with other heating methods including resistance welding. It is required that during the step of heating the junction region, the temperature of the window portion312of the sheet304remain below the glass transition temperature TGsuch that the finished surfaces314and316on the sheet are not adversely affected.

The current embodiment using a solder preform1000can be used for joining a metallized sheet304to a Kovar alloy/nickel/gold frame such as that illustrated in FIG.7. In accordance with a preferred embodiment, the solder preform1000is formed of a gold-tin (Au—Sn) alloy, and in a more preferred embodiment, the gold-tin alloy is the eutectic composition. The thickness of the gold-tin preform1000will be within the range from about 6 microns to about 101.2 microns.

The following examples, not to be considered limiting, are provided to illustrate the details of the metallic layers610and the sheet seal-ring area318that are suitable for braze-soldering to a Kovar alloy/nickel/gold frame in combination with a gold-tin solder preform.

By way of further examples, not to be considered limiting, the following combinations are preferred for the metallic layers610and the sheet seal-ring area318for braze-soldering to a Kovar alloy/nickel/gold frame in combination with a gold-tin soldered preform.

Referring now toFIG. 11, there is illustrated yet another embodiment of the current invention. This embodiment also uses soldering, however, in this embodiment the solder is applied via inkjet technology to either the metallized area610in the sheet seal-ring area318or the sheet seal-ring310of the frame assembly.FIG. 11shows a portion of the Kovar alloy/nickel/gold frame302and an inkjet dispensing head1102which is dispensing overlapping drops of solder1104onto the frame seal-ring area310as the dispensing head moves around the frame aperture308as indicated by arrow1106. Preferably, the inkjet dispensed solder is a gold-tin (Au—Sn) alloy, and more preferably it is the eutectic composition. The thickness of the gold-tin solder applied by dispensing head1102in this embodiment is within the range from about 6 microns to about 101.2 microns. It will be appreciated that while the example illustrated inFIG. 11shows the dispensing head1102depositing the solder droplets1104onto the frame302, in other embodiments the inkjet deposited solder may be applied to the sheet seal-ring area318, either alone or in combination with applications on the frame seal-ring area310. In still other embodiments, the inkjet deposited solder may be used to create a discrete solder preform that would be employed as described in the previous examples herein. Details of the metallic layers610in the sheet seal-ring area318that are suitable for a soldering to a Kovar alloy/nickel/gold frame302such as that illustrated inFIG. 7using inkjet supplied solder are substantially identical to those layers illustrated in previous Examples 21 through 32.

Referring now toFIGS. 12athrough12candFIGS. 13athrough13c, there is illustrated yet another alternative method for manufacturing cover assemblies constituting another embodiment of the current invention. Whereas, in the previous embodiments a separate prefabricated metal frame was joined to the transparent sheet to act as a heat spreader/heat sink needed for subsequent welding, in this embodiment a cold gas dynamic spray deposition process is used to fabricate a metallic frame/heat spreader directly on the transparent sheet material. In other words, in this embodiment the frame is fabricated directly on the transparent sheet as an integral part, no subsequent joining operation is required. In addition, since cold gas dynamic spray deposition can be accomplished at near room temperature, this method is especially useful where the transparent sheet material and/or surface treatments thereto have a relatively low TG, melting temperature, or other heat tolerance parameter.

Referring specifically toFIG. 12a, there is illustrated a sheet of transparent material304having a window portion312defined thereupon. The window portion312has finished top and bottom surfaces314and316(note that the304sheet appears bottom side up inFIGS. 12athrough12c). A frame attachment area1200is defined on the sheet304, the frame attachment area circumscribing the window portion312. It will be appreciated in the embodiment illustrated inFIG. 12that the frame attachment area1200need not follow the specific boundaries of the window area312(i.e., which in this case are circular) as long as the frame attachment area1200completely circumscribes the window portion.

It will be appreciated that, unless specifically noted otherwise, the initial steps of obtaining a transparent sheet having a window portion with finished top and bottom surfaces, preparing the seal-ring area of the sheet and metallizing the seal-ring area of the sheet are substantially identical to those described for the previous embodiments and will not be described in detail again.

Referring now also toFIG. 13a, there is illustrated a partial cross-sectional view to the edge of the sheet304. In this example, the step of preparing a frame attachment area1200on the sheet304comprises roughening the frame attachment area by roughening and/or grinding the surface from its original level (shown in broken line) to produce a recessed area1302. After the frame attachment area1200has been prepared, metal layers are deposited into the frame attachment area of the sheet using cold gas dynamic spray deposition. InFIG. 12b, an initial metal layer1202has been applied into the frame attachment area1200using cold gas dynamic spray deposition.

Referring now also toFIG. 13b, the cold gas dynamic spray nozzle1304is shown depositing a stream of metal particles1306onto the frame attachment area1200. The initial layer1202has now been overlaid with a secondary layer1204and the spray nozzle1304is shown as it begins to deposit the final Kovar alloy layer1206.

Referring now toFIG. 12c, the completed cover assembly1210is illustrated including the integral frame/heat spreader1212, which has been built up from layer1206to a predetermined height, denoted by reference numeral1308, above the finished surface of the sheet. In a preferred embodiment, the predetermined height1308of the built-up metal frame above the frame attachment area1200is within the range from about 5% to about 100% of the thickness denoted by reference numeral1310of the sheet304beneath the frame attachment area. In the embodiment shown, the step of depositing metal using cold gas dynamic spray included depositing a layer of Kovar alloy onto the sheet to fabricate the built-up frame/heat spreader1212. The use of cold gas dynamic spray deposition allows a tremendous range of thickness for this Kovar alloy layer, which thickness may be within the range from about 2.54 microns to about 12,700 microns. It will, of course, be appreciated that the frame/heat spreader1212may be fabricated through the deposition of materials other than Kovar alloy, depending upon the characteristics of the transparent sheet304and of the package base104, especially their respective CTEs.

The following examples, not to be considered limiting, are provided to illustrate the details of the metallic layers, denoted collectively by reference numeral1207for forming a frame/heat spreader compatible with hard glass transparent sheets and Kovar alloy or ceramic package bases.

By way of further examples, not to be considered limiting, the following combinations are preferred for the metallic layers1207for forming a frame/heat spreader compatible with hard glass transparent sheets and Kovar alloy or ceramic package bases.

After the deposition of the metal layers using the cold gas dynamic spray deposition, it may be necessary to grind or shape the top surface of the built-up frame1212to a predetermined flatness before performing additional steps to ensure that a good contact will be made in later bonding. Another process which may be used, either alone or in combination with shaping the top surface of the built-up frame, is the depositing of additional metal layers onto the built-up frame/heat spreader1212using solution bath plating. The most common reason for such plated layers is to promote a good bonding when the frame/heat spreader is adjoined to the package base104. In a preferred embodiment, the additional metallic layers applied to the built-up frame1212include a layer of nickel directly over the cold gas dynamic spray deposited metal having a thickness within the range of about 0.002 microns to about 25 microns and then solution bath plating a layer of gold over the nickel layer until the gold layer has a thickness within the range from about 0.0508 microns to about 0.508 microns.

Referring now toFIG. 14, there is illustrated a block diagram of the alternative embodiment utilizing cold gas dynamic spray deposition. It will be appreciated that, unless specifically noted otherwise, the initial steps of obtaining a transparent sheet having finished surfaces, applying surface treatments to the sheet, cleaning, roughening or otherwise preparing the frame attachment area of the sheet are substantially identical to those described for the previous embodiments and will not be described in detail again. For example, block1402ofFIG. 14represents the step of obtaining a sheet of transparent material having finished surfaces and corresponds directly with block902, and with the description of suitable transparent materials. Similarly, except as noted, blocks1404,1406and1408ofFIG. 14correspond directly with blocks904,906and908, respectively, of FIG.9and with the previous descriptions of the steps and sub-steps provided herein. Thus, it will be understood that all of the options described for performing the various steps and sub-steps represented by the blocks902-908in the previous (i.e., prefabricated frame) embodiments are applicable to the blocks1402-1408in the current (i.e., cold spray) embodiment.

The next step of the process is to use cold gas dynamic spray deposition to deposit frame/heat spreader metal onto any previously deposited metal layers in the frame attachment area1200. This step is represented by block1410. As previously described in connection withFIGS. 13band13c, the high velocity particles1306from the gas nozzle1304form a new layer on the previous metallic layers, and by directing the cold spray jet across the frame attachment area1200repeatedly, the new material can become a continuous metallic layer around the entire periphery of the frame attachment area, i.e., it will circumscribe the window portion312of the transparent sheet304. Where the material of the package base104(to which the cover assembly1210will eventually be joined) is Kovar alloy or appropriately metallized alumina, Kovar alloy is preferred for the material1206to be cold sprayed to form the integral frame. In other cases, a heat spreader material must be selected which has a CTE that is closely matched to the CTE of the package base104. Of course, that material must also be compatible with the cold gas dynamic spray process.

The cold spraying of the powdered heat spreader material is continued until the new layer1206reaches the thickness required to serve as a heat spreader/integral frame. This would represent the end of the process represented by block1410. For some applications, the built-up heat spreader/frame1212is now complete and ready for use. For other applications, however, performing further finishing operations on the heat spreader/frame1212may be desirable.

For example, it is known that significant residual stresses may be encountered in metal structures deposited using cold-gas dynamic spray technology as a result of the mechanics of the spray process. These stresses may make the resulting structure prone to dimensional changes, cracking or other stress-related problems during later use. Annealing by controlled heating and cooling is known to reduce or eliminate residual stresses. Thus, in some applications, the integral heat spreader/frame1212is annealed following its deposition on the sheet304. This optional step is represented by block1411in FIG.14. In some embodiments, the annealing step1411may include the annealing of the totality of the sprayed-on metals and alloys constituting the heat spreader/frame1212. In other embodiments, however, the annealing step1411includes annealing only the outermost portions of the integral built-up heat spreader/frame1212, while the inner layers are left unannealed.

It will be appreciated that there are flatness requirements for the sealing surface at the “top” of the heat spreader (which is actually projecting from the bottom surface316of the sheet). If these flatness requirements are not met via the application of the heat spreader material by the cold spray process, it will be necessary to flatten the sealing surface at the next step of the process. This step is represented by block1412in FIG.14. There are a number of options for achieving the required surface flatness. First, it is possible to remove surface material from the heat spreader to achieve the required flatness. This may be accomplished by conventional surface grinding, by other traditional mechanical means, or it may be accomplished by the laser removal of high spots. Where material removal is used, care must be taken to avoid damaging the finished window surfaces314and316during the material removal operations. Special fixturing and/or masking of the window portion312may be required. Alternatively, if the cold spray deposited heat spreader1212is ductile enough, the surface may be flattened using a press operation, i.e., pressing the frame against a flat pattern. This would reduce the handling precautions as compared to using a surface grinder or laser operations.

Finally, as previously described, in some embodiments additional metal layers are plated onto the integral frame/heat spreader1212. These optional plating operations, such as solution bath plating layers of nickel and gold onto a Kovar alloy frame, are represented by block1414in FIG.14. In the embodiment shown inFIG. 14, the optional plating operation1414is performed after the optional flattening operation1412, which in turn is performed after the optional annealing operation1411. While such order is preferred, it will be appreciated that in other embodiments the order of the optional finishing steps1411,1412and1414may be rearranged. The primary considerations for the ordering of these finishing steps is whether later steps will damage the results of earlier steps. For example, it would be impractical to perform plating step1414before the flattening step1412if the flattening was to be carried out by grinding, while it might be acceptable if the flattening was to be carried out by pressing.

Referring now toFIGS. 15aand15b, there is illustrated a method for manufacturing multiple cover assemblies simultaneously in accordance with another embodiment of the current invention. Shown inFIG. 15ais an exploded view of a multi-unit assembly which can be subdivided after fabrication to produce individual cover assemblies. The multi-unit assembly1500includes a frame1502and a sheet1504of a transparent material. The frame1502has sidewalls1506defining a plurality of frame apertures1508therethrough. Each frame aperture1508is circumscribed by a continuous sidewall section having a frame seal-ring area1510(denoted by cross-hatching). Each frame seal-ring area1510has a metallic surface, which may result from the inherent material of the frame1502or it may result from metal layers which have been applied to the surface of the frame. In some embodiments, the frame1502includes reduced cross-sectional thickness areas1509formed on the frame sidewalls1506between adjacent frame apertures1508.FIG. 15bshows the bottom side of the frame1502, to better illustrate the reduced cross-sectional thickness areas1509formed between each aperture1508. Also illustrated is the base seal-ring area1520(denoted by cross-hatching) which surrounds each aperture1508to allow joining to the package bases104.

Except for the details just described, the multiple-aperture frame1502of this embodiment shares material, fabrication and design details with the single aperture frame302previously described. In this regard, a preferred embodiment of the frame1502is primarily formed of Kovar alloy or similar materials and more preferably, will have a Kovar alloy core with a surface layer of gold overlaying an intermediate layer of nickel as previously described.

The transparent sheet1504for the multi-unit assembly can be formed from any type of transparent material as previously discussed for sheet304. In this embodiment, however, the sheet1504has a plurality of window portions1512defined thereupon, with each window portion having finished top and bottom surface1514and1516, respectively. A plurality of sheet seal-ring areas1518are denoted by cross-hatching surrounding each window portion inFIG. 15a. With respect to the material of the sheet1504, with respect to the finished configuration of the top and bottom surfaces1514and1516, respectively, of each window portion1512, with respect to surface treatments, and/or coatings, the sheet1504is substantially identical to the single window portion sheet304previously discussed.

The next step of the process of manufacturing the multi-unit assembly1500is to prepare the sheet seal-ring areas1518for metallization. As noted earlier, each sheet seal-ring area1518circumscribes a window portion of the sheet1504. The sheet seal-ring areas1508typically have a configuration which closely matches the configuration of the frame seal-ring areas1510to which they will eventually be joined. It will be appreciated, however, that in some cases other considerations will affect the configuration of the frame grid, e.g., when electrical resistance heating is used to produce bonding, then the seal-ring areas1518must be connected to form the appropriate circuits. The steps of preparing the sheet seal-ring areas1518for metallization is substantially identical to the steps and options presented during discussion of preparing the frame seal-ring area310on the single aperture frame302. Thus, at a minimum, preparing the sheet seal-ring area1518involves a thorough (e.g., plasma) cleaning to remove any contaminants from the surfaces and typically also involves roughening the seal-ring area by chemical etching, laser ablating, mechanical grinding or sandblasting this area.

The step of metallizing the prepared sheet seal-ring areas1510of the sheet1502are substantially identical to the steps described for metallizing the frame seal-ring area310on the single aperture frame302. For example, the metal layers shown in Examples 1 through 8 can be used in connection with thermal compression bonding, the metal layers of Examples 9 through 20 can be used for soldering where the solder material is plated onto the sheet as a final metallic layer, the metal layer configurations of Examples 21 through 32 can be used in connection with soldering in combination with a separate gold-tin of solder preform and also for soldering in connection with solders deposited or formed using inkjet technology.

The next step of the process is to position the frame1502against the sheet1504(it being understood that solder preforms or solder layers would be interposed between the frame and the sheet) such that each of the window portions1512overlays one of the frame apertures1508, and that for each such window portion/frame aperture combination, at least a portion of the associated frame seal-ring area1510and at least a portion of the associated sheet seal-ring area1518contact one another along a continuous junction region that circumscribes the associated window portion. This operation is generally analogous to the steps of positioning the frame against the sheet in the single aperture embodiment previously described.

Referring now toFIG. 16a, there is illustrated the positioning of a multi-window sheet1504(in this case having window portions1512with contoured surfaces) against a multi-aperture frame1502using compliant tooling in accordance with another embodiment. The compliant tooling includes a compliant element1650and upper and lower support plates1652,1654, respectively. The support plates1652and1654receive compressive force, denoted by arrows1656, at discrete locations from tooling fixtures (not shown). The compliant member1650is positioned between one of the support plates and the cover assembly pre-fab (i.e., frame1502and sheet1504). The compliant member1650yields elastically when a force is applied, and therefore can conform to irregular surfaces (such as the sheet1504) while at the same time applying a distributed force against the irregular surface to insure that the required contact pressure is achieved all along the frame/sheet junction. Such compliant tooling can also be used to press a sheet or frame against the other member when the two members are not completely flat, taking advantage of the inherent flexibility (even if small) present in all materials. In the illustrated example, the compliant member1650is formed from a solid block of an elastomer material, e.g., rubber, however in other embodiments the compliant member may also be fabricated from discrete elements, e.g., springs.

The next step of the process is heating all of the junction regions until a metal-to-metal joint is formed between the frame1502and the sheet1504all along each junction region, thus creating the multi-unit assembly1500having a hermetic frame/sheet seal circumscribing each window portion1512. It will be appreciated that any of the heating technologies previously described for joining the single aperture frame302to the single sheet304are applicable to joining the multi-aperture frame1502to the corresponding multi-window sheet1504.

Referring now toFIG. 16b, the final step of the current process is to divide the multi-unit assembly1500along each junction region that is common between two window portions1512taking care to preserve and maintain the hermetic seal circumscribing each window portion. A plurality of individual cover assemblies are thereby produced.FIG. 16b, illustrates a side view of a multi-unit assembly1500following the hermetic bonding of the sheet1504to the frame1502. Where the frame1502includes reduced cross-sectional thickness areas1509, the step of dividing the multi-unit assembly may include scoring the frame along the back side of the reduced cross-sectional thickness area at the position indicated by arrow1602, preferably breaking through or substantially weakening the remaining frame material below area1509, and also simultaneously scoring the sheet1504along a line vertically adjacent to area1509, i.e., at the point indicated by arrow1604, followed by flexing the assembly1500, e.g., in the direction indicated by arrows1606such that a fracture will propagate away from the score along line1608, thereby separating the assembly into two pieces. This procedure can be repeated along each area of reduced cross-sectional thickness1509until the multi-unit assembly1500has been completely subdivided into single aperture cover assemblies that are substantially identical to those produced by the earlier method described herein. In other embodiments, instead of using the score-and-break method, the cover assemblies may be cut apart, preferably from the frame side along the path indicated by arrow1602(i.e., between the window portions1512), using mechanical cutting, laser, water jet or other parting technology.

Referring now toFIGS. 17aand17b, there is illustrated yet another method for simultaneously manufacturing multiple cover assemblies. This method expands upon the cold gas dynamic spray technique used to build an integral frame/heat spreader directly upon the transparent sheet material as previously illustrated in connection withFIGS. 12athrough12candFIGS. 13athrough13c. As shown inFIG. 17a, the process starts with a sheet of nonmetallic transparent material1704having a plurality of window portions1712defined thereupon, each window portion having finished top and bottom surfaces1714and1716, respectively. The properties and characteristics of the transparent sheet1704are substantially identical to those in the embodiments previously discussed. The next step of the process involves preparing a plurality of frame attachment areas1720(denoted by the path of the broken line surrounding each window portion1712), each frame attachment area1720circumscribing one of the window portions1712. As in previous embodiments, the step of preparing the frame attachment areas may comprise cleaning, roughening, grinding or otherwise modifying the frame attachment areas in preparation for metallization.

The next step in this process is metallizing the prepared frame attachment areas on the sheet, i.e., this metallization may be performed using a cold gas dynamic spray technology or where the layers are relatively thin, using a CVD, physical vapor deposition or other conventional metal deposition techniques. It will be appreciated that the primary purpose of this step is to apply metal layers necessary to obtain good adhesion to the transparent sheet1704and/or to meet the metallurgical requirements for corrosion prevention, etc.

Referring now toFIG. 17b, the next step of the process is depositing metal onto the prepared/metallized frame attachment areas of the sheet1704using cold gas dynamic spray deposition techniques until a built-up metal frame1722is formed upon the sheet having a seal-ring area1726that is a predetermined vertical thickness above the frame attachment areas, thus creating a multi-unit assembly having an inherent hermetic seal between the frame1722and the sheet1704circumscribing each window portion1712. In some embodiments, reduced cross-sectional thickness areas1724are formed by selectively depositing the metal during the cold spray deposition. In other embodiments, the reduced cross-sectional area sections1724may be formed following deposition of the frame/heat spreader1722through the use of grinding, cutting or other mechanical techniques such as laser ablation and water jet.

The next step of the process which, while not required is strongly preferred, is to flatten, if necessary, the seal-ring area1726of the sprayed-on frame1722to meet the flatness requirements for joining it to the package base104. This flattening can be accomplished by mechanical means, e.g., grinding, lapping, polishing, etc., or by other techniques such as laser ablation.

The next step of the process, which, while not required, is strongly preferred, is to add additional metallic layers, e.g., a nickel layer and preferably also a gold layer, to the seal-ring area1726of the sprayed-on frame1722to facilitate welding the cover assembly to the package base104. These metallic layers are preferably added using a solution bath plating process, e.g., solution bath plating, although other techniques may be used.

The next step of the process is dividing the multi-unit assembly1700along each frame wall section common between two window portions1712while, at the same time, preserving and maintaining the hermetic seal circumscribing each window portion. After dividing the multi-unit1700, a plurality of single aperture cover assemblies1728(shown in broken line) will be produced, each one being substantially identical to the single aperture cover assemblies produced using the method described inFIGS. 12athrough12candFIGS. 13athrough13c. All of the options, characteristics and techniques described for use in the single unit cover assembly produced using cold gas dynamic spray technology are applicable to this embodiment. It will be appreciated that certain operations for example, the flattening of the frame and the plating of the frame with additional metallic layers, maybe performed on the multi-unit assembly1700, prior to separation of the individual units, or on the individual units after separation.

As previously described, heating the junction region between the metallized seal-ring area of the transparent sheet and the seal-ring area of the frame is required for forming the hermetic seal therebetween. Also as previously described, this heating may be accomplished using a furnace, oven, or various electrical heating techniques, including electrical resistance heating (ERH). Referring now toFIGS. 18a-18c, there is illustrated methods of utilizing electric resistance heating to manufacture multiple cover assemblies simultaneously.

Referring first toFIG. 18a, there is illustrated a transparent sheet1804having a plurality of seal-ring areas1818laid out in a rectangular arrangement around a plurality of window portions1812. These seal-ring areas1818have been first prepared, and then metallized with one or more metal or metal alloy layers, as previously described herein. The transparent sheet1804further includes an electrode portion1830which has been metallized, but does not circumscribe any window portions1812. This electrode portion is electrically connected to the metallized seal-ring areas1818of the sheet. One or more electrode pads1832may be provided on the electrode portion1830to receive electrical energy from electrodes during the subsequent ERH procedure.

Referring now toFIG. 18b, there is illustrated a frame1802having a plurality of sidewalls1806laid out in a rectangular arrangement around a plurality of frame apertures1808. The apertures1808are disposed so as to correspond with the positions of the window portions1812of the sheet1804, and the sidewalls1806are disposed so that frame seal-ring areas1810(located thereupon) correspond with the positions of the sheet seal-ring areas1818of the sheet. The frame is metallic or metallized in order to facilitate joining as previously described herein. The frame1802further includes an electrode portion1834that does not circumscribe any frame apertures1808. This frame electrode portion1834is positioned so as not to correspond to the position of the sheet electrode portion1830, and preferably is disposed on an opposing side of the sheet-window/frame-grid assembly (i.e., when the sheet is assembled against the frame). The frame electrode portion1834is electrically connected to the metallized frame seal-ring areas1810. One or more electrode pads1836may be provided on the electrode portion1834to receive electrical energy from electrodes during the subsequent ERH procedure.

Referring now toFIG. 18c, the sheet1804is shown positioned against the frame1802in preparation for heating to produce the hermetic seal therebetween. If applicable, solder or a solder preform has been positioned therebetween as previously described. It will be appreciated that when the transparent sheet1804is brought against the frame1802, the metallized seal-ring areas1818on the lower surface of the sheet will be in electrical contact with the metallized seal-ring areas1810on the upper surface of the frame. However, the sheet electrode portion1830and the frame electrode portion1834will not be in direct contact with one another, but instead will be electrically connected only through the metallized seal-ring areas1818and1810to which they are, respectively, electrically connected. When an electrical potential is applied from electrode pads1832to electrode pads1836(denoted by the “+” and “−” symbols adjacent to the electrodes), electrical current flows through the junction region of the entire sheet-window/frame-grid assembly. This current flow produces electrical resistance heating (ERH) due to the resistance inherent in the metallic layers. In some embodiments, this electrical resistance heating may be sufficient, in and of itself, to result in TC bonding, soldering, or other hermetic seal formation between the sheet1804and the frame1802in order to form a multi-unit assembly. In other embodiments, however, electrical resistance heating may be combined with other heating forms such as furnace or oven pre-heating in order to supply the necessary heat required for bonding to form the multi-unit assembly.

After bonding the sheet1804to the frame1802to form the multi-unit assembly, the sheet electrode portion1830and the frame electrode portion1834can be cut away and discarded, having served their function of providing electrical access for external electrodes (or other electrical supply members) to the metallized seal-ring areas of the sheet and frame, respectively. The removal of these “sacrificial” electrode portions1830and1834may occur before or during the “dicing” process step, i.e., the separating of the multi-unit assembly into individual cover assemblies. It will be appreciated that any of the technologies previously described herein for separating a multi-unit assembly into individual cover assemblies can be used for the dicing step of separating a multi-unit assembly fabricated using ERH heating.

Where ERH is to be used for manufacturing multiple cover assemblies simultaneously, the configuration of the sheet-window/frame-grid array and/or the placement of the electrodes portions within the sheet-window/frame-grid array may be selected to modify the flow of current through the junction region during heating. The primary type of modification is to even the flow of current through the various portions of the sheet-window/frame-grid during heating to produce more even temperatures, i.e., to avoid “hot spots” or “cold spots.”

Referring now toFIGS. 19a-19f, there are illustrated various sheet-window/frame-grid configurations adapted for producing more even temperatures during ERH. In each ofFIGS. 19a-19f, there is shown a sheet-window/frame-grid array1900comprising a prepared, metallized transparent sheet1904overlying a prepared, metallic/metallized frame1902. The window portions of the sheet1904directly overlie the frame apertures of the frame1902, and the metallized seal-ring areas of the sheet directly overlie the seal-ring areas of the frame (it will be appreciated that metallized portions of the sheet1904and the frame1902appear coincident in these figures). Metallized electrode portions formed on the transparent sheet1904are denoted by reference letters A, B, C and D. These electrode portions A, B, C and D are electrically connected to the adjoining sheet seal-ring areas of the sheet, but are electrically insulated from one another by non-metallized areas1906of the sheet. An external electrode is applied to the top of the metallic/metallized frame (on the side opposite from the sheet) across the area denoted by reference letter E. For bonding or soldering, electrical power is applied at the electrodes, e.g., one line to electrodes A, B, C and D simultaneously, and the other line to electrode E, or alternatively, one line in sequence to each of electrode A, B, C and D, and the other line to electrode E. It will be appreciated that many other combinations of electrode powering are within the scope of the invention.

Referring toFIG. 19f, this embodiment illustrates a sheet-window/frame-grid1900having a “shingle” configuration, i.e., where the seal-ring areas between the window portions/frame apertures do not form continuous straight lines across the assembly array. Shingle-arrangement frame assemblies are more labor-intensive to separate using scribe-and-break or cutting procedures. Separating such assemblies requires that each row first be separated from the overall grid, and then that individual cover assemblies be separated from the row by separate scribe-and-break or cutting operations. Nevertheless, use of shingle-arrangement assemblies may have benefits relating to heating using ERH techniques.

While the invention has been shown or described in a variety of its forms, it should be apparent to those skilled in the art that it is not limited to these embodiments, but is susceptible to various changes without departing from the scope of the invention.