Abstract:
Disclosed is a method of forming individual thin-film capacitors for embedding inside printed wiring boards or organic semiconductor package substrates, which includes removal of selective portions of the capacitor by sandblasting or other means so that the ceramic dielectric does not come in contact with acid etching solutions.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The technical field is embedded capacitors in printed wiring boards (PWB). More particularly, the technical field includes embedded thin ceramic capacitors in printed wiring boards or semiconductor packages made from formed-on-foil thin dielectrics. 
         [0003]    2. Technical Background of the Invention 
         [0004]    The practice of embedding formed-on-foil (typically, fired-on-foil) ceramic capacitors in printed wiring boards allows for reduced circuit size and improved circuit performance. Ceramic capacitors are typically embedded in inner layer panels that are stacked and connected by interconnection circuitry, the stack of panels forming a multilayer printed wiring board or semiconductor package substrate. Embedded formed-on-foil ceramic capacitors preferably have a high capacitance density. 
         [0005]    The capacitance density of a dielectric is proportional to its dielectric constant, divided by the thickness of the dielectric. A high capacitance density capacitor can therefore be achieved by using a thin, high dielectric constant dielectric in the capacitor. 
         [0006]    Passive circuit components embedded in printed circuit boards formed by formed-on-foil thin ceramic dielectric technology are known. A formed-on-foil thin-film process is disclosed in U.S. Pat. No. 7,029,971 to Borland et al. Referring to  FIG. 1A , formed-on-foil thin ceramic capacitors are formed by first depositing a thin capacitor dielectric material layer  110  onto a metallic foil substrate  100 . The metallic foil substrate  100  may be copper foil and typically may range in thickness between 12 and 36 microns. The thin capacitor dielectric layer may be deposited by screen-printing, slurry casting, chemical solution deposition, sputtering, chemical vapor deposition or any other recognized thin layer deposition method. The thin capacitor dielectric material may be subjected to a firing or annealing process to crystallize the deposited material, and increase the dielectric constant. After firing, the thin dielectric layer will generally be a homogenous ceramic layer and may range in thickness from approximately 12 microns to substantially less than 1 micron. The firing process may be conducted at relatively low temperatures in air or at higher temperatures in reduced oxygen atmospheres. The thin ceramic capacitor dielectric material may comprise a high dielectric constant material and have a high dielectric constant after firing. A metallic electrode material is next deposited over the formed-on-foil thin ceramic capacitor dielectric layer. The metallic electrode generally would cover the entire layer of the ceramic dielectric. The deposition method can be any of a number of deposition methods. Sputtering is generally the preferred choice. The metallic electrode may be plated with additional metal to build the electrode up to a desired thickness, such as that of the metallic foil substrate  100 , and form electrode  120  in  FIG. 1B . The resulting metal/dielectric/metal structure, shown in  FIG. 1B , may be laminated to an underlying organic based dielectric layer such as a glass epoxy or other common organic dielectric used in the printed wiring board industry using a prepreg dielectric layer to form the dielectric layer  130  and create the article shown in  FIG. 1C . 
         [0007]    A series of masking and etching processes is generally undertaken to selectively remove portions of the metallic foil and the thin ceramic capacitor dielectric to form individual capacitors. Two examples of such resulting structures are shown in  FIG. 2A and 2B  wherein the individual capacitors  2000  consist of a first electrode  200 , a thin ceramic capacitor dielectric layer  210  and a second electrode  220 . In the design shown in  FIG. 2A , after incorporation in the printed wiring board, the capacitor electrodes are accessed from top and bottom by use of vias. In the design shown in  FIG. 2B , after incorporation in the printed wiring board, both capacitor electrodes may be accessed from the top using vias. Other designs may be practiced. In order to remove copper, either acid-based or alkaline-based etching chemistries may be used. In general, alkaline etching solutions do not readily remove the ceramic dielectric, but strong acid solutions will. Examples of acids that dissolve ceramic are ferric chloride or cupric chloride in hot 2.4 normal hydrochloric acid, or hydrofluoric acid chemistries. Even with such acids, the rate of removal of the ceramic is slow. However, there is an additional consideration that when using an acid, such as hydrochloric acid, the acid and any products of the dissolution have to be completely removed. It is well known that chloride contamination can cause migration issues under bias and humidity in ceramic dielectrics. Therefore, if the acid etching solution or any products from the ceramic dissolution have not been thoroughly removed from the capacitor, the long-term reliability of the capacitor may be compromised. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a method of making individual formed-on-foil thin ceramic capacitors comprising: providing a structure, said structure comprising a first metallic conductor, a ceramic dielectric, and a second metallic conductor; forming a photo-definable mask over at least one of said metallic conductors, thus forming an article comprising a first photo-definable mask with underlying first metallic conductor, a ceramic dielectric and an opposing second metallic conductor; and removing selective portions of said first photo-definable mask and said underlying first metallic conductor to form a patterned first side of said article comprising a first metallic electrode. 
         [0009]    According to a next embodiment, a method of making individual thin formed-on-foil ceramic capacitors comprises removal of the ceramic dielectric and metallic conductor (typically copper) by removal means, such as sandblasting as an alternative method to selectively removing ceramic and copper with acid based etchants. The patterning may be achieved by sandblasting through a photo-defined mask, such as a mask made from Riston® photoimageable dry film photoresist or preferably a wholly dry processing resist approach such as the use of RapidMask™ available from IKONICS Company, PhotoBrasive Systems, Duluth, Minn. 
         [0010]    According to a further embodiment, a method of making individual thin formed-on-foil ceramic capacitors comprises patterning a first electrode by use of an alkali etchant and using the patterned first electrode and the photoresist mask (or RapidMask™) as a mask and sandblasting to selectively remove the ceramic between the first electrodes; stripping (defined herein as removal) the first photomask and applying a second photoresist photomask to protect the first electrode and the thin ceramic dielectric and patterning a second electrode by etching. 
         [0011]    According to the above embodiments, the ceramic dielectric of the capacitor does not come into contact with acid etching solutions during fabrication of the individual capacitors. The acid etching solutions may otherwise damage the ceramic dielectric or leave deposits that would compromise the long-term reliability. Capacitor reliability and performance are thereby improved. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The detailed description will refer to the following drawings wherein: 
           [0013]      FIGS. 1A-C  describes formed-on-foil thin ceramic capacitors. 
           [0014]      FIGS. 2A-2B  are individual capacitor structures. 
           [0015]      FIGS. 3A-3F  are a series of views illustrating a method of manufacturing individual formed-on-foil thin ceramic capacitors, according to the first embodiment, wherein selective portions of the copper foil and ceramic dielectric are removed by sandblasting through a photo-defined mask; 
           [0016]      FIGS. 4A-4J  are a series of views illustrating a method of manufacturing individual formed-on-foil thin ceramic capacitors, according to the second embodiment, wherein the first electrode is formed using an alkali etching process. The formed first electrode plus the photo-defined mask protecting it is used to protect the underlying ceramic dielectric when the ceramic dielectric between the first electrode is removed by sandblasting. After sandblasting away the thin ceramic dielectric, the photomask is removed and replaced by another photoresist that is imaged to form another photomask that serves to protect the first electrode and the thin ceramic dielectric while the second electrode is patterned by etching. 
           [0017]      FIGS. 5A-5M  are a series of views illustrating a method of manufacturing individual formed-on-foil thin ceramic capacitors according to a further alternative embodiment, wherein the first electrode is formed on a first side of the formed-on-foil metal/dielectric/metal structure, shown in  FIG. 1B , and said first side is laminated to an additional metal conductor using a prepreg material and then a second electrode is formed on the second side of said formed-on-foil metal/dielectric/metal structure and the exposed thin ceramic dielectric is removed by sandblasting. 
       
    
    
       [0018]    According to common practice, the various features of the drawings are not necessarily drawn to scale. Dimensions of various features may be expanded or reduced to more clearly illustrate the embodiments of the invention. 
       DETAILED DESCRIPTION 
       [0019]    Contiguous sheets of metal are referred to as “metal conductor” and formed capacitor plates as “metal electrodes”. These references are simply done for purposes of clarity and are not intended to be limiting. Additionally, in some embodiments, these metallic conductors and metallic electrodes are copper foil or formed from copper foil. Any reference to copper or copper foil is not intended to be limiting only to copper or copper foil but is to be thought of as an example of a metal conductor. 
         [0020]    The present invention provides a method of making individual formed-on-foil thin ceramic capacitors comprising: providing a structure, said structure comprising a first metallic conductor, a ceramic dielectric, and a second metallic conductor; forming a photo-definable mask over at least one of said metallic conductors, thus forming an article comprising a first photo-definable mask with underlying first metallic conductor, a ceramic dielectric and an opposing second metallic conductor; and removing selective portions of said first photo-definable mask and said underlying first metallic conductor to form a patterned first side of said article comprising a first metallic electrode. A further embodiment of the present invention further comprises the step of essentially completely removing said first photo-definable mask, thus forming an essentially mask-free patterned first side of said article. In another embodiment, the individual formed-on-foil thin ceramic capacitors are laminated to an organic dielectric layer. 
         [0021]    The present invention further provides a method of making individual formed-on-foil thin ceramic capacitors (as noted above) wherein selective portions of at least one of said underlying first metallic conductor and said ceramic dielectric and said opposing metallic conductor are removed by a process selected from the group comprising sandblasting, water impingement, and chemical etching. 
         [0022]    The following descriptions and figures further represent various embodiments of the present invention. 
         [0023]      FIGS. 3A-3F  illustrate a first method of manufacturing individual formed-on-foil thin ceramic capacitors wherein the ceramic dielectric and metallic conductor (typically copper) are removed by sandblasting (or other removal means such as water impingement, water/slurry impingement, water/abrasive impingement, abrasive removal, laser guided water jetting, and chemical etching, etc.) through a mask (photo-definable mask) that is used to protect areas that will form the capacitors. The photo-definable mask as used herein may also be referred to as a mask, photoresist, photoresist mask, etc. Once imaged, the photoresist or photo-definable mask forms the photomask. 
         [0024]    Referring to  FIG. 3A , a thin ceramic capacitor dielectric  310  sandwiched between metallic conductors (typically copper)  300  and  320  and laminated to organic dielectric  330  is provided. Referring to  FIG. 3B , a photoresist material  340  is applied to the copper foil  320 . A dry processing resist, such as RapidMask™ available from PhotoBrasive Systems is preferable but other photoresists that are resistant to impact by sandblasting media (or other media chosen for removal) may be used. As shown in  FIG. 3C , after the photoresist has been exposed with UV radiation, the photoresist becomes brittle in the areas  350  where the photoresist has been exposed. As shown in  FIG. 3D , the abrasive media (typically sand)  360  impinges on the resist surface and breaks up and removes the brittle exposed resist features while the sand bounces off the unexposed, compliant, rubbery resist features without damaging these areas. Once the exposed, brittle resist has been blasted off, the sand removes material in the areas  370  forming individual electrodes  380  and  385  and dielectric  390  as shown in  FIG. 3E . When sandblasting is complete, the non-exposed resist is removed (stripped) from the substrate surface to form the individual capacitors  3000  as shown in  FIG. 3F . One or more individual capacitors may be formed. Those skilled in the art understand that photoresist (and/or metallic electrode and/or ceramic dielectric) removal may be accomplished by means other than sandblasting. For example, removal by water impingement including: 1) water alone; 2) water and slurry or abrasive; 3) laser guided water jetting or by chemical etching. The use of “sandblasting” in the descriptions of the embodiments below is not intended to be limiting. 
         [0025]    The above process may be partially repeated, for example, to form the article shown in  FIG. 2B . In this case, an additional photoresist mask may be applied to all the individual electrodes  380  shown in  FIG. 3F  and a portion of the top electrode  380  and the dielectric  390  removed by sandblasting. The process would be stopped prior to removal of any of the bottom electrode  385  so that the article shown in  FIG. 2B  is formed. 
         [0026]      FIGS. 4A-4J  illustrate a second method of manufacturing individual formed-on-foil thin ceramic capacitors wherein the first electrode is formed using alkali etching processes. The formed first electrode and the photomask protecting it are used to protect the underlying dielectric during sandblasting. The sandblasting removes the dielectric between the formed first electrodes. The photomask is stripped (or removed) and replaced by another photoresist photomask so that the second electrode can be formed using an alkaline etching process. 
         [0027]    Referring to  FIG. 4A , a thin film capacitor dielectric  410  sandwiched between metallic conductors  400  and  420  and laminated to an underlying organic dielectric layer using a prepreg dielectric layer to form organic dielectric  430  is provided. The article is essentially similar to and manufactured in the same manner as the article described in  FIG. 1C . Referring to  FIG. 4B , a photoresist material  440  is applied to the copper foil  420 . The photoresist is imaged and developed to form the photomask  445  as shown in  FIG. 4C . 
         [0028]    The article shown in  FIG. 4C  is now subjected to an alkaline etching bath. The alkali etching process removes the copper in the areas  450  shown in  FIG. 4D  and forms the first electrode  455 . The alkaline etch does not attack or remove any of the dielectric  410 . Referring to  FIG.4E , the article of  FIG. 4D  is subjected to sandblasting. The sandblasting removes the dielectric in the areas  470  shown in  FIG. 4F  and forms the individual dielectric layers  480 . 
         [0029]    Referring to  FIG. 4G , the photomask  445  may now be removed and replaced by another photoresist that is imaged to form the photomask  488  shown in  FIG. 4H . The copper foil  485  may now be removed (i.e., by etching using acid or alkali etching solutions or other methods known to those skilled in the art) in the area  490  to form second electrodes  495  and the article shown in  FIG. 41 . Finally, the photomask  488  can be stripped to form the individual capacitors  4000  as shown in  FIG. 4J . 
         [0030]    The above process may be modified to form alternative designs. For example, the second photomask  488  may be expanded along the foil  485  so that when the foil  485  is etched, the article shown in  FIG. 2B  is formed. 
         [0031]    An alternate technique to form individual capacitors is shown in  FIGS. 5A-5M . The metal/dielectric/metal structure, shown in  FIG. 1B , is reproduced in  FIG. 5A , having metal conductor  500 , dielectric  510 , and metal conductor  520 . 
         [0032]    Referring to  FIG. 5B , a photoresist material  530  is applied to the metal conductor  520 . The photoresist is imaged and developed to form the photomask  535  as shown in  FIG. 5C . 
         [0033]    The article shown in  FIG. 5C  is now subjected to an alkaline etching bath. The alkali etching process removes the copper in the areas  540  shown in  FIG. 5D  and forms the first metal electrodes  525 . The alkaline etch does not attack or remove any of the dielectric  510 . The photomask  535  is subsequently removed by stripping (defined herein as removal), resulting in the structure shown in  FIG. 5E . 
         [0034]    A metal conductor  560  is laminated to the structure shown in  FIG. 5E  using an organic based dielectric layer such as a glass epoxy or other common organic dielectric prepreg  550  used in the printed wiring board industry to form the structure shown in  FIG. 5F . This structure is typically called an innerlayer or subpart having metal conductors on each side and having metal electrodes  525  and thin ceramic dielectric  510  between the metal conductors  500  and  560 . 
         [0035]    In some embodiments of the present invention an innerlayer is formed. This innerlayer comprises individual capacitors formed by an impingement process selected from the group comprising sandblasting, water impingement and chemical etching. 
         [0036]    In further embodiments of the present invention a printed wiring board or organic semiconductor package substrate are formed comprising the inner layer(s) above. 
         [0037]    Referring to  FIG. 5G , a photoresist is applied to metal conductors  500  and  560  and processed to create photomask  570 . Typically, but not always, the metal conductor  560  is removed in the area reserved for the formation of capacitors  5000  shown in  FIG. 5L , thus no photomask  570  is shown on metal conductor  560 . Since a portion of metal conductor  560  may be present on an area not included in the diagrams, it is shown in  FIGS. 5H-5M  by dotted lines  565  for reference. 
         [0038]    The article shown in  FIG. 5G  is now subjected to an alkaline etching bath. The alkali etching process removes portions of the metal conductor  500  in the areas  545  shown in  FIG. 5H  and forms the second metal electrodes  505 . The alkaline etch does not attack or remove any of the dielectric  510 . 
         [0039]    Referring to  FIG. 5I , the photomask  570  may now be removed and replaced by another photoresist that is imaged to form the photomask  580  shown in  FIG. 5J . Portions of the thin ceramic dielectric  510  may now be removed by sandblasting in the areas  548  as shown in  FIG. 5K , leaving thin dielectric  515 . The first metal electrodes  525 , the thin dielectric  515  and second metal electrodes  505  form capacitors  5000  shown in  FIG. 5L . 
         [0040]    Referring to  FIG. 5I , the second metal electrodes  505  may be used as a mask for sandblasting the thin ceramic dielectric  510  to create the structure shown in  FIG. 5M . 
         [0041]    Sandblasting, as used herein, may not necessarily use sand as the blasting media. Sandblasting can use sand, aluminum oxide, glass beads or organic materials that have a hardness (Mohs scale) in the range of 5.5 to 9.0. A high hardness of the blasting media is desirable so that the ceramic dielectric is rapidly removed. Alternative physical means of removing the dielectric and metallic (typically copper) layer may be employed, such as water jet processes that may consist of abrasive slurries or even pure water. 
         [0042]    The invention is capable of use in various other combinations, modifications, and environments, as are within the skill or knowledge of one skilled in the relevant art.