Method of forming recessed patterns in insulating substrates

A method of forming recessed patterns in insulators is described. One embodiment of the invention is directed to ceramic green sheet fabrication by providing a sculptured plastic tape mold which includes a floor, a plurality of sidewalls adjacent to and extending above the floor and a plurality of protrusions on and extending above the floor, casting a ceramic slurry into the mold such that the slurry contacts the floor, the sidewalls and the protrusions, and drying the slurry so as to produce a ceramic green sheet with a recessed pattern that replicates the shapes of the protrusions. The ceramic green sheet may be removed from the mold and filled with a conductor before firing; alternatively, the ceramic green sheet can be fired before removing the mold to form a rigid ceramic substrate which is then filled with a conductor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the manufacture of electronic components, 
and more particularly to forming recessed patterns such as trenches and 
vias in insulating substrates such as polymers and ceramic green sheets by 
depositing a castable insulator into a mold. 
2. Description of Related Art 
Smaller and faster electronic devices offer obvious advantages. Fast 
electronic devices usually require short connections between the attendant 
integrated circuit chips. This in turn demands that the chips be located 
close together. Close chip spacing also provides for smaller devices. To 
locate chips close together they are commonly mounted on the top layer of 
a flat, insulating substrate and interconnected by conductors routed in 
several underlying layers of the substrate. This arrangement is called a 
multi-chip module ("MCM"). 
In producing a typical MCM, conductors are applied to a single 4 or 5 mm 
thick layer of ceramic substrate and each layer of the substrate-conductor 
combination is then stacked with dozens of other layers. In some methods 
the conductors are routed on the surface of each layer, while in others 
the conductors are embedded or recessed into each layer. Ceramic is well 
suited for substrates because it has low thermal expansion, high thermal 
conductivity, mechanical strength, excellent electrical insulation, and 
reasonable cost. However, it may be difficult to form the recessed 
patterns needed for embedded conductors in ceramic because ceramic is 
dense even before it is fired. After it is fired ceramic is very hard and 
brittle and thus difficult to pattern. 
A ceramic substrate is typically produced by first forming an unpatterned 
mold or "carrier tape" such as by extruding a thermal plastic film. A 
suspension of aluminum particles in a polymer binding is then cast onto 
the carrier tape. This suspension is then cured to a patternless unfired 
sheet ("ceramic green sheet") to be used as one substrate layer in a 
multilayer MCM. After conductors are applied to each layer, the layers are 
stacked together and fired into a rigid unit. 
Several methods are known for forming patterns of conductors in layers of 
ceramic substrates without first forming recesses therein. Typically these 
methods extrude the conductors onto the substrate through a metal mask by 
screen printing. But screen printing cannot produce conductors with 
dimensions that are high relative to width, that is, it cannot widely vary 
the conductor aspect ratio by varying conductor height. Nor can screen 
printing produce conductors of precise dimensions. Furthermore, screen 
printing does not allow the layers of substrate and conductors to be 
tightly stacked before firing because they are not entirely flat. 
Several methods are also known for improving flatness where the conductors 
are formed on the substrate surface. One such method, disclosed in U.S. 
Pat. No. 4,109,377 issued to Blazick et al., places the conductors on the 
surface of each layer of the substrate and presses the layers together, 
thereby somewhat flattening the conductors. Also, the conductors cannot be 
closely spaced because pressing the layers together causes the conductors 
to spread out. This conductor spreading also limits the signal propagation 
speed as described in U.S. Pat. No. 4,581,098 issued to Gregor. Another 
method, disclosed in U.S. Pat. No. 4,825,539 issued to Nagashima et al., 
presses conductors into the top of each layer while cooling the conductors 
and heating the substrate. This does result in a flat surface. It also 
reduces, although does not entirely eliminate, spreading of the 
conductors. Besides not altogether eliminating conductor spreading, a 
further disadvantage is the heating and cooling required. Another method, 
disclosed in U.S. Pat. No. 5,009,744 issued to Mandai et al., reduces the 
thickness of the metal layer on the surface of the substrate by forming it 
first on a "back film" and then transferring it to the substrate. This 
method does not, however, produce an entirely flat surface. 
Another method forms a partial pattern of recesses in the surface of the 
substrates. U.S. Pat. No. 4,715,117 issued to Enomoto describes forming a 
substrate with a regular pattern of through-holes, typically by mechanical 
or laser drilling or by punching; then the unwanted holes are filled; 
finally the selected holes are metal plated at the same time that the 
surface of the substrate is plated. This method does not produce a flat 
surface because it does not provide a complete pattern of recesses. That 
is, it provides through-holes or "vias" for conductors that run between 
the flat surfaces of the substrate layer, but it does not provide 
"trenches" that are needed for the conductors that run parallel to the top 
major surfaces of the substrate. Besides that disadvantage, this method 
does not fill the entire through-hole with conductor material. 
Other methods do provide a complete pattern of recesses. Recesses have 
commonly been formed by pressing a mold into a flat ceramic green sheet 
substrate. However, as the Gregor '098 Patent referred to above points 
out, the green sheet is too hard and dense for this "branding" technique 
to work well. For example, the features of the mold or punch must be 
relatively large in order to be sturdy enough to press a pattern into the 
green sheet. Consequently, other techniques have been developed to 
"thermally machine" the ceramic green sheet, such as by exposing the 
ceramic green sheet to laser or electron beam radiation through a mask. 
These thermal machining methods are effective but require expensive and 
delicate machinery applied under carefully controlled conditions. Another 
method, disclosed in the Gregor '098 Patent, forms a pattern of xeroxed, 
stenciled, or photolithographed lines on the substrate surface and then 
exposes the lined surface to a source that selectively heats the lines 
until the substrate underneath vaporizes. Gregor's method, however, has 
limited reproduceability due to inconsistent thermal decomposition. 
In summary, prior methods of applying conductors to a substrate where the 
resulting substrate-conductor surface is not flat are relatively simple, 
but frequently do not allow stable, compact multilayered structures, 
precise conductor dimensions, or wide variation in conductor aspect 
ratios. The simplest methods to make the layers more stable and compact 
cause conductor spread, which limits the size, spacing, and resistivity of 
the conductors. These spacing and resistivity limitations prevent smaller 
packages and faster devices. By forming recessed patterns in the ceramic 
substrate the layers can be stacked tightly without spreading the 
conductors, but this requires additional steps, and sometimes requires 
expensive, delicate equipment operated under carefully controlled 
conditions. And some simple methods of producing recessed patterns are not 
capable of producing very small features. 
SUMMARY OF THE INVENTION 
A principle object of the present invention is to provide a simple method 
of forming recesses for conductors in a substrate. Another object of the 
invention is to form complex recessed patterns in a substrate while at the 
same time casting the substrate into a planar shape. A further object is 
to embed tightly spaced conductors with large aspect ratios in small 
recessed patterns while maintaining a flat surface on the top of a thin 
substrate so that multiple layers of such substrate-conductor structures 
may be compactly stacked together. 
A feature of the present invention is providing a mold which includes a 
floor, a plurality of sidewalls adjacent to and extending above the floor 
and a plurality of protrusions on and extending above the floor, pouring a 
ceramic slurry into the mold such that the slurry contacts the floor, the 
sidewalls and the protrusions, and drying the slurry so as to produce a 
ceramic green sheet with a recessed pattern that replicates the shapes of 
the protrusions. 
An advantage of the present invention is that few manufacturing steps are 
required to produce a planar substrate with a featured surface. Another 
advantage is that the size of the patterned features can be precise, 
narrow, deep, and closely spaced while the substrate can remain thin. 
Another advantage is that a single metallization step can be used to form 
recessed conductors that are parallel to the planar surface and that 
traverse the surfaces in vias. Also, the conductor aspect ratio can be 
widely varied. 
These and other objects, elements and advantages of the present invention 
will be further described and more readily apparent from a review of the 
detailed description and preferred embodiments which follow.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, FIGS. 1 through 8 illustrate views of 
successive manufacturing stages in accordance with one embodiment of the 
invention directed to forming vias and trenches in a substrate and filling 
the vias and trenches with a conductor. 
Referring now to FIG. 1, a mold with at least one protrusion is provided. 
As may be seen, a sculptured plastic tape mold 10 includes a planar floor 
12, and planar sidewalls 14 adjacent to and extending perpendicularly 
above opposite sides of floor 12. Vertically disposed cylindrical 
protrusions 16 and horizontally disposed rectangular protrusions 18 are 
mounted on and extend above floor 12. Cylindrical protrusions 16 may be 
integral with or separate from rectangular protrusions 18. Rectangular 
protrusions 18 have flat top surfaces 20 positioned in parallel with floor 
12. Top surfaces 22 of cylindrical protrusions 16 and top surfaces 24 of 
side walls 14 are located above surfaces 20. 
With reference now to FIG. 2, a section of mold 10 taken across line 2--2 
of FIG. 1 is illustrated to show floor 12 together with sidewalls 14, 
protrusions 16 and 18, top surfaces 24 of walls 14, top surfaces 20 of 
protrusions 18, and top surfaces 22 of protrusions 16. 
Referring now to FIG. 3, a castable insulator is deposited into the mold 
such as by doctor blading. Suitable insulators include ceramics, slurries 
and polymers generally. Polyimides and epoxies are preferred polymers. 
Beryllia, mullite, barium titanate, and silicon carbide are suitable 
ceramic substrates where a green sheet is not desired. A suitable ceramic 
slurry for producing green sheets includes, for example, aluminum oxide 
and glass particles suspended in a polymer binding agent together with a 
plasticizer. As an alternative, alumina and aluminum nitride ceramic 
slurries are well suited to the green sheet process. For illustration 
purposes, a castable insulator of ceramic slurry 26 shall be depicted 
herein. Upon being deposited into mold 10, ceramic slurry 26 has a top 
surface 28 and bottom surface 30 on mold floor 12. Slurry 26 conforms to 
the shape of the mold 10. That is, slurry 26 is contained by sidewalls 14 
and covers floor 12 and rectangular protrusions 18 while surrounding and 
partially covering cylindrical protrusions 16. Vibration or some other 
method of compacting may be required in order to assure that the deposited 
slurry forms a uniform, voidless layer in mold 10. As a result, 
cylindrical protrusions 16 and rectangular protrusions 18 displace ceramic 
slurry 26 to respectively form the desired vertically disposed cylindrical 
voids or cavities, referred to herein as vias 32, and the desired 
horizontally disposed rectangular voids or cavities, referred to herein as 
trenches 34. 
It should be noted that the use of sidewalls 14 is not mandatory. That is, 
sidewalls 14 could be eliminated and the surface tension of slurry 26 
could be relied upon to contain the unobstructed edges of the slurry. Such 
unobstructed slurry edges may be formed as fairly straight lines by 
holding the width of the slurry constant during deposition, or by a 
slitting operation after the slurry is deposited. 
With reference now to FIG. 4, the ceramic slurry is solidified into a 
flexible ceramic green sheet 36 by heating, drying and aging to polymerize 
the binding agent and to remove volatile organic compounds. Techniques for 
curing ceramic slurries into ceramic green sheets are well known in the 
art. 
Referring to FIG. 5, ceramic green sheet 36 is removed from mold 10 (not 
shown) and inverted so that surface 30 that was on the floor 12 of mold 10 
is now the exposed top surface. Surface 30 is essentially flat and both 
the top major surface and the patterned surface (with recesses therein) of 
green sheet 36. Likewise, the surface 28 that was the top slurry surface 
in mold 10 is now the bottom surface of the inverted green sheet. This 
inverted position advantageously places vias 32 and trenches 34 in view. 
Vias 32 and trenches 34 are seen to precisely replicate protrusions 16 and 
18, respectively, of mold 10. Thus, the removal of green sheet 36 from 
mold 10 does not disturb the shapes of vias 32 or trenches 34. Clearly, it 
is desirable to have low adhesion between the mold and the substrate, as 
is the case with plastic mold 10 and ceramic green sheet 36. 
With reference to FIG. 6, a section taken across line 6--6 of FIG. 5 is 
illustrated showing the ceramic green sheet 36, major surfaces 28 and 30, 
vias 32 and trenches 34. 
Referring to FIG. 7, ceramic green sheet 36 is inverted, and vias 32 and 
trenches 34 are filled with a metal 40 to produce a planar substrate-metal 
structure 42. Suitable electrically conductive metal pastes for metal 40 
include molybdenum, tungsten, gold and copper. Filling is accomplished 
such as by extruding, rolling, wiping, doctor blading, etc. Liquid or 
powder conductive material could also be used. Alternatively, the trenches 
or vias could be filled with an optical conductor or left empty. 
With reference now to FIG. 8, the planar structure 42 is fired into a 
hardened ceramic 44 containing a hardened metal 46, such as by placing 
structure 42 in a kiln with a reducing atmosphere at a temperature of 
1250.degree. C. to 1500.degree. C. Methods for firing a ceramic green 
sheet into a hardened ceramic structure are well known in the art. Metal 
46 forms a fully inlaid, firmly bonded metallurgical pattern embedded in 
vias 32 and trenches 34 which replicates the shapes of protrusions 16 and 
18. 
FIGS. 9 and 10 illustrate another embodiment for embedding metal conductors 
into a hardened ceramic in accordance with the present invention. In this 
embodiment, the steps of providing mold 10 in FIG. 1 and depositing slurry 
26 into mold 10 in FIG. 3 are repeated as previously described. However, 
instead of curing the slurry into a ceramic green sheet and removing the 
green sheet from the mold before firing the green sheet, the green sheet 
is left in the mold and fired as previously described in FIG. 8 so that a 
hardened ceramic is removed from the mold. Thereafter, a conductor can be 
filled into the vias and trenches in the hardened ceramic. 
Referring now to FIG. 9, mold 10 is provided, ceramic slurry 26 is 
deposited into mold 10, and ceramic green sheet 36 is formed as previously 
described. In this embodiment, however, ceramic green sheet 36 is not 
removed from mold 10 prior to the firing operation. Instead, ceramic green 
sheet 36 is left in mold 10 and then fired. Thereafter, as is shown, a 
hardened ceramic 44 is produced and removed from the mold. In addition, if 
desired, the mold may be removed from hardened ceramic 44 as part of the 
firing operation by using a mold with a low melting or vaporization 
temperature. For example, the ceramic green sheet 36 and the plastic mold 
10 as shown in FIG. 4 may be placed into a kiln at a high temperature 
(1250.degree. C. to 1500.degree. C.) so that the mold is incinerated while 
the ceramic is hardened. 
With reference now to FIG. 10, the hardened ceramic 44 is inverted so that 
the surface 30 that was on the floor 12 of mold 10 (not shown) is now the 
exposed top surface. Likewise, the surface 28 that was the top surface 
prior to inverting the ceramic is now the bottom surface. Vias 32 and 
trenches 34 in hardened ceramic 44 are filled with conductor 47. In this 
embodiment the conductor does not have to undergo firing with the ceramic 
so it may be different than the metal paste illustrated in FIG. 7. 
Electroless deposition, for example, could be used to plate metal into the 
vias and trenches. Furthermore, vias 32 and trenches 34 can be filled with 
an optical conductor or left empty. 
Referring now to FIG. 11, a section across line 11--11 of FIG. 10 is 
illustrated showing the hardened ceramic 44, the major surfaces 28 and 30, 
and conductor 47 as a fully inlaid, firmly bonded metallurgical pattern 
embedded in vias 32 and trenches 34 which replicates the protrusions of 
mold 10. 
Referring now to FIG. 12, the mold 10 of FIG. 1 is shown wherein a first 
rectangular protrusion 18a has a width 48 no larger than 4 mils, a height 
50 of at least 2 mils and, therefore, an aspect ratio of at least 0.5. 
First rectangular protrusion 18a also has a separation 52 of no more than 
4 mils from a second rectangular protrusion 18b. In addition, cylindrical 
protrusion 16 has a diameter 54 of no more than 4 mils. 
With reference now to FIG. 13, ceramic slurry 26 is filled or leveled to a 
specified depth 56 in the mold 10. Depth 56 may be, for instance, less 
than 7 mils. 
Referring now to FIG. 14, the ceramic slurry of FIG. 13 is cured to a 
ceramic green sheet 36 and is removed from the mold 10 (not shown). The 
ceramic green sheet 36 is shown inverted so that essentially flat surface 
30 that was on the floor 12 of the mold 10 is now the exposed top surface. 
Likewise, the surface 28 that was the top surface is now the bottom 
surface. Vias 32 and trenches 34 are filled with a metal paste 40 to 
produce a planar substrate-metal structure 42. The green sheet 36 is a 
specified thickness 58 corresponding to the fill depth 56 in FIG. 13. That 
is, green sheet thickness 58 is less than 7 mils. Likewise, the metal 
paste 40 is a specified width 60 and height 62 corresponding to the width 
48 and height 50 of protrusion 18a in FIG. 12. The ratio of the metal 
height 62 to metal width 60 defines the aspect ratio for the metal in 
trench 18a. 
Referring now to FIG. 15, before firing, a compact multilayer ceramic 
structure 64 can be composed by stacking ceramic layers 66, 68 and 70, 
each of which may be formed in accordance with the embodiments previously 
described herein. Of course, more than three layers can be used (as shown 
by the broken lines), such as 12 layers stacked into a structure no more 
than 60 mils thick before firing. Thus, the present invention provides via 
and/or trench formation as an integral part of casting ceramic green 
sheets in the fabrication of multilayer ceramic packaging, particularly 
packages which require vertical conductors and/or vias such as substrates, 
multi-chip modules and pin grid arrays. 
Of course, numerous changes and variations in the aforementioned 
embodiments will be apparent to those skilled in the art. For instance, 
the patterned mold protrusions can take whatever shapes, heights etc. that 
can be sculptered into the mold. Also, the use of extruded or injected 
plastic molds allows for a wide variety of protrusion patterns as well as 
low adherence (easy separation) with the flexible ceramic green sheets 
and/or the fired, hardened ceramics. 
The present invention, therefore, is well adapted to carry out the objects 
and attain the ends and advantages mentioned, as well as others inherent 
therein. While presently preferred embodiments of the present invention 
have been described for the purpose of disclosure, numerous other changes 
in the details of construction, arrangement of parts, compositions and 
materials selection, processing steps can be carried out without departing 
from the spirit of the present invention which is intended to be limited 
only by the scope of the appended claims.