Method for producing ceramic surfaces with easily removable contact sheets

A method for making multilayer ceramic substrates having substantially reduced planar shrinkage and distortion resulting from the firing or sintering process. Contact sheets are employed in the fabrication process on the surface of the multilayer ceramic substrate to be fired with the contact sheets being prepared from a composition containing a non-sinterable non-metallic inorganic material such as alumina having an average particle size approximately about 1 micron or less and an organic binder and preferably a plasticizer. In a preferred embodiment of the invention, the multilayer structure to be fired containing the contact sheet of the invention is provided with a beveled or chamfered edge at an angle of greater than about 60 degrees. A fabrication process employing only chamfering of the edge or the use of a contact sheet of the invention also provides improved multilayer ceramic substrate products. A further feature is a method to control the surface topography of surface metallization by adjusting the compressibility of the contact sheets during fabrication of the substrate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to compositions and methods for making 
multilayer ceramic substrates from ceramic greensheets by sintering, and, 
in particular, to substantially reducing and controlling planar shrinkage, 
minimizing distortion and controlling surface feature topography of the 
sintered ceramic greensheets caused by the sintering process. 
2. Description of Related Art 
Ceramics have found wide spread use in electronics as a substrate for 
integrated circuit packages. Metallized circuit patterns are applied to 
the ceramic substrate in the form of a greensheet and the ceramic and 
metallization are co-sintered to create a monolith of substrate and 
circuitry. In general, multilayer ceramic packages are constructed by 
combining ceramic particles and organics binders into unfired, or 
greensheet, tapes. Inter-layer conductive paths, known as vias, are then 
inserted (punched) through the greensheet, forming electrical 
interconnection between the circuits on each greensheet layer after they 
are stacked and processed. Metallized circuit patterns are applied to the 
punched greensheets as is well-known in the art and multiple layers of 
punched and metallized tapes are stacked and laminated under pressure. The 
ceramic and metallization laminate is then co-sintered to form a 
monolithic structure with three dimensional circuitry. 
The casting of suspensions of ceramic material to form layers or 
greensheets which are then sintered to produce a ceramic substrate 
material is known in the art. The doctor blade method is one method for 
producing a ceramic greensheet. Typically, ceramic powder is mixed with an 
organic solvent, a plasticizer and a binder forming a slurry, the slurry 
is cast in a regulated thickness on a carrier film with the aid of a 
doctor blade, and the applied layer of the slurry is then dried. The 
ceramic suspension formula is typically alumina and a butyral resin like 
polyvinyl butyral. A cellulose type resin like ethyl cellulose or 
polyvinyl alcohol is also typically used as the binder. 
As part of the multilayer ceramic substrate fabrication process, the tape 
layers are punched and metallized and stacked in registry and pressed 
together at a preselected temperature and pressure to form a monolithic 
structure and then fired at an elevated temperature to drive off the 
organic binder and then finally sintered to densify the multilayer 
substrate. During sintering however, the stacked laminate densifies and 
shrinks and the shrinkage is difficult to control. To control the 
dimensional and electrical circuit integrity of the stacked greensheet 
laminate, pressure sintering or hot pressing the ceramic body during 
sintering with an externally applied load is a well known method for 
controlling the shape (dimensions) of the sintered ceramic part. This 
process however has a tendency for cross-contamination to occur between 
the part and the pressure mold and application of a force during burnout 
of the organic binder may restrict the escape of volatiles causing 
incomplete burn out and/or distortion of the sintered ceramic. 
Other processes include the use of a constraint applied to the outer edges 
(periphery) of the part providing an open escape path for volatiles and an 
entry path for oxygen. In another process a co-extensive force is applied 
to the entire surface of the part to be sintered by either using 
co-extensive porous platens or by application of an air-bearing force. In 
another approach a frictional force is applied to the sintering body by 
use of contact sheets comprised of a porous composition which do not 
sinter or shrink during the heating cycle; thus prevents shrinkage of the 
ceramic in the plane of the contact sheet (XY plane) and allows shrinkage 
in the direction perpendicular to the plane of the contact sheet (Z 
directions). The composition of the contact sheet is selected so that it 
remains porous during firing, does not fuse to the ceramic, is thermally 
stable so that it will essentially not shrink or expand during the 
sintering cycle, and has continuous mechanical integrity/rigidity. The 
contact sheets generally maintain their dimensions during the sintering 
cycle thus restricting or minimizing the ceramic parts from shrinking in 
the plane of the contact sheet (XY direction). 
The use of contact sheets however, presents other problems in the 
fabrication of the multilayer ceramic substrate. For one, contact sheets 
after sintering must be removed from the sintered substrate by one of 
several abrasive processes (e.g., lapping, polishing, grinding, scrubbing, 
brushing, or media blasting). All of these processes can damage the 
underlying substrate surface metal features and ceramic surfaces. In many 
applications the degree of distortion control and surface finish or 
surface roughness with the conventional contact sheet is adequate; 
however, fine pitch wiring ground rules and thin film metallurgy 
deposition on large ceramic substrates requires improvements in distortion 
control and minimal surface roughness. 
Multilayer ceramic substrates are often produced by combining metallized 
and unfired ceramic layers into a laminate with contact sheets on the 
outer surfaces and then cutting these in the unfired state into smaller 
substrates. These substrates are then heated to consolidate the ceramic 
and metal powders during which time considerable dimensional shrinkage 
occurs. 
These low temperature cofired ceramic substrates built with contact sheets 
from greensheet laminates typically exhibit a concave side profile after 
sintering. This is demonstrated in FIGS. A-1C as described hereinbelow. 
The resulting sharp edges are very fragile and chip very easily. It is 
known to provide a conventional 45.degree. chamfer machined into the edges 
of the substrate prior to sintering so that the edges are less prone to 
chipping. However, a lip or burr often forms on the top and/or bottom 
sides of the substrate after the contact sheet is removed. 
This is demonstrated in FIGS. 3A-3D. This lip or burr can interfere with 
downstream processes requiring a planar surface (e.g., pin or solder ball 
joining). When such a lip or burr occurs the sintered part may need to be 
subjected to an expensive post-sinter machining process, (e.g., lapping, 
polishing, grinding, etc.) to remove the lip or burr and to provide a 
planar surface. The lip or burr will be most severe for thick contact 
sheets. 
Another problem with the use of conventional contact sheets is the 
roughness of the ceramic substrate surface after sintering and removal of 
the contact sheet which may also require a planarizing operation such as 
lapping or grinding to provide the required smooth surface needed for thin 
film wiring integrity, lid sealing or other such electronic component 
fabrication processes. 
A number of patents have issued in this area addressing the problem of 
controlling dimensional integrity of the multilayer ceramic substrate 
during sintering. U.S. Pat. No. 5,277,723 describes a method of 
controlling substrate sintering X-Y orthogonality and edge concavity in 
parts built with non-sintering constraining layers (contact sheets). The 
method essentially controls these parameters by variation of the 
constraining force applied to the substrate during sintering. U.S. Pat. 
Nos. 5,254,191 and 5,387,474 describe the use of a constraining layer to 
control substrate sintering X-Y shrinkage and distortion. The ceramic 
powder used in these patents has an average size between about 1 and 20 
microns with less than 30% being finer than one micron. 
In the Itagaki et al. paper entitled A CoFired Bump Bonding Technique For 
Chip-Scale Package Fabrication Using Zero X-Y Shrinkage Low Temperature 
CoFired Ceramic Substrate published Oct. 12, 1997, a process is described 
to build a ceramic substrate with positive height metal bumps to which 
semiconductor chips can be bonded. The contact sheet used in the process 
is described only as being alumina. 
Still another problem with using non-sintering contact sheets is that often 
their microstructure has very little porosity and when laminated as a 
greensheet onto surface metal features which are located on the outermost 
layers of the multilayer ceramic laminate, the surface features are 
pressed completely into the surface of the ceramic. This may be 
undesirable when electrically conductive metal pads on the surface of the 
substrate which protrude above the ceramic substrate surface are desired 
for solderless electrical connection to Land Grid Array (LGA) conductive 
contacts. 
Bearing in mind the problems and deficiencies of the prior art, it is 
therefore an object of the present invention to provide a composition and 
method for making multilayer ceramic substrates from ceramic greensheets 
by sintering which multilayer ceramic substrates have improved X-Y 
dimensional integrity and edge and side integrity. 
It is another object of the present invention to provide contact sheets 
which provide enhanced dimensional integrity when used as a top and/or 
bottom layer on a ceramic greensheet stack which is to be sintered in the 
fabrication of multilayer ceramic substrates. 
In a further aspect of the invention, the control of surface metallized 
feature height above the ceramic is made possible by adjusting the 
microstructure of the greensheet to incorporate porosity into the 
greensheet, the thickness of the contact sheet, the compliance of this 
contact sheet and the compliance of the backing material used between the 
contact sheet and rigid plates used during the uniaxial lamination 
process. 
In yet another aspect of the invention, a contact sheet is disclosed which 
is easily removed from the ceramic by ultrasonic cleaning, dry abrasive 
blasting, or wet abrasive blasting leaving the underlying metallurgy free 
from residual contact sheet material. 
Another object of the invention is to provide a method for minimizing the 
sharp edge that results from green sizing parts that are subsequently 
sintered using contact sheets by using an edge chamfer greater than about 
60 degrees. 
Yet another object of the invention is to produce a very fine surface 
finish on the outermost ceramic layers after removal of the contact sheet 
after sintering. 
In a further aspect of the invention, the microstructure of the contact 
sheet is modified to control the topography of metallized features on the 
surface of the substrate produced using contact sheets. 
Another object of the present invention is to provide multilayer ceramic 
substrates made using the contact sheet compositions and/or methods of the 
invention. 
Still other objects and advantages of the invention will in part be obvious 
and will in part be apparent in the specification. 
SUMMARY OF THE INVENTION 
The above and other objects, which will be apparent to those skilled in the 
art, are achieved by the present invention which, in a first aspect, 
relates to a method for making multilayer ceramic substrates having 
enhanced dimensional integrity from ceramic greensheets by sintering 
comprising the steps of: 
stacking a plurality of greensheets having metallurgical circuitry within 
and thereon and which will form a multilayer ceramic substrate when 
sintered; 
forming a contact sheet comprising a non-sinterable, non-metallic inorganic 
material having an average particle size less than about one micron and an 
organic binder; 
positioning the contact sheet on at least one surface of the stack; 
forming a laminate by adhering the contact sheet and greensheets together, 
usually under conditions of elevated temperature and pressure; 
sintering the contact sheet containing laminate to form the multilayer 
ceramic substrate; and 
removing the contact sheet. 
In another aspect, the present invention relates to a method for making 
multilayer ceramic substrates having enhanced dimensional integrity from 
ceramic greensheets by sintering comprising the steps of: 
stacking a plurality of greensheets having metallurgical circuitry within 
and thereon and which will form a multilayer ceramic substrate when 
sintered; 
forming a contact sheet comprising a non-sinterable, non-metallic inorganic 
material and an organic binder; positioning the contact sheet on at least 
one surface of the stack; 
forming a laminate by adhering the contact sheet and greensheets together, 
usually under conditions of elevated temperature and pressure; 
forming a chamfer around preferably all the edges of the laminate 
containing contact sheet, the chamfer angle being greater than about 
60.degree. in relation to the surface of the contact sheet; 
sintering the chamfered contact sheet containing laminate to form a 
multilayer ceramic substrate; and 
removing the contact sheet. 
In another aspect, the present invention relates to a method for making 
multilayer ceramic substrates having enhanced dimensional integrity from 
ceramic greensheets by sintering comprising the steps of: 
stacking a plurality of greensheets having metallurgical circuitry within 
and thereon and which will form a multilayer ceramic substrate when 
sintered; 
forming a contact sheet comprising a non-sinterable, non-metallic inorganic 
material having an average particle size of less than one micron and an 
organic binder; 
positioning the contact sheet on at least one surface of the stack; 
forming a laminate by adhering the contact sheet and greensheets together, 
usually under conditions of elevated temperature and pressure; 
forming a chamfer around preferably all the edges of the laminate 
containing contact sheet, the chamfer angle being greater than about 
60.degree. in relation to the surface of the contact sheet; and 
sintering the chamfered contact sheet containing laminate to form a 
multilayer ceramic substrate; and 
removing the contact sheet. 
In a further aspect of the invention a composition for forming contact 
sheets is provided wherein the composition comprises a non-sinterable, 
non-metallic inorganic material having an average particle of less than 
about one micron and an organic binder. The contact sheet composition is 
used to form the contact sheet by known conventional methods such as the 
doctor blade method used to form ceramic greensheets. 
In a further aspect of the invention, the microstructure of the contact 
sheet is modified to control the topography of metallized features on the 
surface of the substrate produced using contact sheets. 
In a further aspect of the invention multilayer ceramic substrates made 
using the contact sheet composition and methods of the invention are 
provided.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
In describing the preferred embodiment of the present invention, reference 
will be made herein to FIGS. 1A-5C. Features of the invention are not 
necessarily shown to scale in the drawings. 
The ceramic greensheets used to make a multilayer ceramic substrate may be 
made by conventional casting methods using conventional greensheet casting 
compositions. Such compositions and methods are described in U.S. Pat. No. 
2,966,719 to Park, Jr.; U.S. Pat. No. 3,698,923 to Stetson et al.; U.S. 
Pat. No. 4,769,294 to Barringer et al.; and U.S. Pat. No. 5,387,474 to 
Mikesha et al., which patents are hereby incorporated by reference. The 
composition is typically termed a casting slip and may be prepared by 
mixing the ingredients in a ball mill for a number of hours, such as 8 
hours, to ensure that a homogenous mixture is formed and a desired 
viscosity obtained. A low temperature sinterable ceramic composition such 
as a crystallizable cordierite glass ceramic is made and formed into a 
greensheet. The binder may be selected from a large variety of polymers 
such as polyvinyl butyral, polyvinyl alcohol, or an acrylic resin. The 
binder is preferably a thermoplastic which allows joining of greensheets 
under heat and pressure. The solvent may also be selected from a wide 
variety of material such as methyl ethyl ketone (MEK), methyl isobutyl 
ketone (MIBK), methanol, acetone, toluene, etc. and is typically a mixture 
of two solvents which improve the binder solubility. The binder is 
preferably polyvinyl butyral or Butvar from Monsanto Corporation and the 
solvents are a mixture chosen from solvents considered to have good and 
poor solvency for the binder, preferably methanol and methyl isobutyl 
ketone in a ratio of 1 to 3 weight. A plasticizer such as dibutylphthlate 
is also preferably used in the formulation to improve lamination 
properties. 
Typically, the slip composition is prepared in a two-stage milling process. 
In the first stage, the ceramic powder and solvent are mixed. In the 
second stage, the binder and plasticizer are added. The binder serves to 
retain the ceramic-particles in undisrupted position after the organic 
solvent is evaporated from the cast slip. A typical greensheet casting 
composition by weight percent is as follows: 
______________________________________ 
Broad Preferred 
______________________________________ 
Ceramic 40 to 60% 45 to 55% 
Binder 2 to 10% 4 to 8% 
Solvent 20 to 50% 35 to 45% 
Plasticizer 0.5 TO 5% 1 TO 3% 
______________________________________ 
After the ingredients of the casting composition are mixed and homogenized, 
such as in a ball mill, a slip is formed having a viscosity which may vary 
from approximately 400 to approximately 2000 centipoise. The slip can be 
de-aired by means well known in the ceramic art. After de-airing, the slip 
is transferred to a slip reservoir where it is suitably maintained in a 
homogeneous state. From the reservoir, the slip is discharged through a 
small orifice onto a substantially-horizontal flexible tape. The flexible 
supporting tape is typically a tape made of any impervious material, such 
as polyteltrafluoroethylene "Teflon", glycol tetraphthalic acid polyester 
(Mylar) and the like. The casting sheet is pulled across the open bottom 
of the reservoir and under a doctor blade, which is set at a particular 
height to form the desired tape thickness. The casting sheet should be 
supported on a smooth surface and then the solvent vaporized producing a 
leather hard tape. During the casting operation and while evaporating 
solvents from the slurry to form the tape, the temperature and rate of 
removal of the solvents control the consolidation of the powders, binder 
and plasticizer and the development of the tape microstructure. This can 
also be controlled by the slurry composition and viscosity to produce 
microporosity consisting essentially of air bubbles within the tape. This 
can be measured by determining the apparent density of the tape. The tape 
can then be punched before or after removal of the casting sheet to form 
the desired greensheet size. 
The greensheet may then be punched to form vias and metallized as 
well-known in the art and stacked to form the desired multilayer ceramic 
substrate laminate which is then fired (sintered) to form the final 
multilayer ceramic substrate product. 
Sintering of the multilayer low temperature cofired ceramic laminate is 
conventionally performed using any of a number of heating profile cycles. 
Although not required, the application of pressure on the surface of the 
laminate during sintering to further increase the restraining force for XY 
shrinkage is often used. In general, the sintering of ceramic greensheets, 
and especially, low temperature cofired MLC packages using copper 
metallurgy, is typically performed in three distinct heating phases. The 
first phase pyrolysis breaks down large polymers and volatilizes the 
shorter chain organics. Pyrolysis is usually performed at a temperature 
below 500.degree. C. for about 5 hours. The heating cycle where the binder 
and remaining organics are removed or burned out of the package (termed 
binder burn out "BBO") is usually performed at a temperature above about 
700.degree. C. for at least 2 hours. Sintering is then performed at a 
temperature between about 800.degree. C. and 1000.degree. C. to form the 
final MLC package. The total heating cycle is typically performed in a 
sintering environment which is a reducing atmosphere such as hydrogen or a 
nitrogen/hydrogen mixture. When metals such as gold or silver are used, a 
much simpler sintering process using predominately air atmospheres can be 
used. 
The crux of the subject invention is to fabricate multilayer ceramic 
substrates as described hereinabove by using as the outermost layers of 
the stacked greensheets a contact sheet, the contact sheet comprising a 
non-sinterable, non-metallic inorganic material having an average particle 
size of approximately 1 micron or less in an organic binder. The contact 
sheet is formed preferably and typically using the doctor blade method as 
described hereinabove and then positioned on the stack typically by 
laminating the contact greensheet to the multilayer ceramic stack at the 
same time as all the other greensheet layers are made into a green 
monolithic laminate. 
Preferably, the ceramic material used to form the contact sheet is alumina 
and is dispersed in a matrix of organic binder such as polyvinyl butyral 
and preferably contains a plasticizer such as phthalates or benzoates as 
is well known to those skilled in the art. The slurry formulation more 
particularly is made so that the microstructure of the contact sheet has a 
minimal amount of compressibility. The casting composition after ball 
milling and other dispersing steps is then cast via a conventional doctor 
blade technique to a thickness ranging from about 0.025 to 0.2 mm thick, 
and preferably less than about 0.050 mm thick when a minimal edge lip or 
burr is desired. The contact sheets are then positioned preferably on each 
surface of the greensheet multilayer ceramic substrate stack and laminated 
together under pressure at an appropriate temperature and time resulting 
in a laminate comprising in cross-section a multilayer stack of green 
sheets with preferably each outer surface having thereon a contact sheet. 
This is typically performed in a uniaxial pressing operation. A contact 
sheet or multiple contact sheets may be used on only one surface of the 
multilayer ceramic substrate but it is highly preferred to employ one or 
more contact sheets on each surface, with each surface covered by the same 
number and type of contact sheets. 
Typically, the laminate is then singulated into individual substrates for 
sintering to form the multilayer ceramic product. Referring first to FIGS. 
1A-1C which show a typical prior art process for making a multilayer 
ceramic substrate product using contact sheets, a multilayer ceramic 
substrate stack is shown generally as 16. The multilayer ceramic substrate 
comprises stacked green sheets shown as 17, 18 and 19 having sides 23a and 
23b and an upper surface 17a and a lower surface 19a. A contact sheet 22 
is placed on each surface 17a and 19a. The top outer surface of the 
contact sheet is shown as 20 and the top outer surface of the substrate 17 
is shown as 21. No edge chamfering is used. When the laminate of FIG. 1A 
is sintered typically under a constraining load, a product as shown in 
FIG. 1B is formed as having concave side walls 23a and 23b. When the 
contact sheets 22 are removed the final multilayer ceramic substrate 
product 16 is shown in FIG. 1C and it can be seen that the multilayer 
ceramic product 16 has concave side walls 23a and 23b formed by extended 
edges 21. In addition, the edges are very sharp and thus subject to 
extensive damage during handling thereafter. The surface smoothness of 
ceramic substrates made with contact sheets comprised of 3 micron alumina 
is typically about 9,000 to 10,000 Angstroms Ra. 
Referring now to FIGS. 3A-3D, to alleviate the sharp edges of ceramic 
substrates, it is typical prior art to chamfer the edges at a 45 degree 
angle. FIG. 3A shows the typical prior art laminate 25 with contact sheets 
15 on the top and bottom of ceramic laminate 25. The contact sheets are 
typically 200 to 400 microns thick. FIG. 3B shows the same laminate after 
an edge chamfer 24 at approximately 45 degrees theta (.theta.) has been 
made in the edges to alleviate the sharp edges anticipated after sintering 
and contact sheet removal. During sintering and while the part is 
constrained under a weight, the glass at the edges of the substrate can 
typically flow during the Z shrinkage of the substrate. Since the contact 
sheet does not sinter and the glass is free to flow around its edges while 
being sintered and pressed under a load, the edge chamfer 24 of the 
substrate after sintering often has a lip or burr 26 as shown in FIG. 3C. 
After the contact sheet is removed as shown in FIG. 3D the burr 26 is very 
pronounced and easily subject to chipping. In addition, the surface is not 
planar which can affect the sealing of the substrate on joining fixtures 
and the integrity of lids attached to the surface. Removal of this lip can 
only be done by expensive abrasive planarizing processes often with 
diamond tooling. The side wall edge 14a and 14b concavity is still very 
noticeable which can affect the edge registration of the substrate in 
sockets as well as create chipping problems in handling. 
Referring now to FIGS. 2A-2C, a preferred fabrication method of the 
invention is shown. A multilayer ceramic product shown generally as 25 
comprises a plurality of stacked green sheets 11, 12 and 13 and a contact 
sheet 15 of the invention on each surface 11a and 13a. Contact sheet 15 is 
less than 3 mils thick and preferably less than 2 mils to minimize any 
edge lip that may occur when the ceramic material flows around the edge of 
the chamfered contact sheet. The substrate 25 is shown having side walls 
14a and 14b. 
In FIG. 2B, each edge of the product 25 is shown having an edge chamfer 24 
at an angle .theta. greater than about 60.degree.. It will be appreciated 
that there are a total of eight edges on the rectangular stack--four on 
top and four on the bottom. At one edge for example the chamfer extends 
from a point 24a on greensheet 11 upward to the surface 11a shown as point 
24b and terminates at point 24c at the upper surface of contact sheet 15. 
Angle .theta. is greater than about 60.degree. and the chamfer may extend 
downward along side walls 14a and 14b of substrate 11 a suitable distance, 
typically about 20% to 40% of the thickness of the laminate 25. Less than 
this amount of chamfer makes the substrate prone to forming concave edges. 
After sintering and removal of the contact sheet 15 the final product 25 is 
shown in FIG. 2C. The Multilayer substrate 25 product is shown having 
substantially straight side walls 14a and 14b. Chamfer 24 is still in the 
product. The surfaces 11a and 13a of the substrate 11 are also smooth due 
to the use of the contact sheet of the invention. 
Comparing the prior art product of FIG. 3D to the product of the invention 
of FIG. 2C, it is clear that the use of a chamfer angle of greater than 
about 60 degrees and a contact sheet of the invention produces a product 
without an edge burr and which has straighter side walls. Thus, the 
product requires less finishing processes to provide the finished product. 
The contact sheets of the current invention can also be used to control the 
topography of surface metallization. This is desirable when the metal 
features on the surface of the substrate are required to protrude or 
desired to be level with the surface of the substrate for interconnection 
to, for instance, a Land Grid Array connector. 
At the time the contact sheets are made during the casting operation the 
microstructure of the contact sheets is controlled by the slurry 
composition and casting conditions. A very slow casting rate at low 
temperature with slow evaporating solvents can be used to produce a very 
dense contact sheet. A rapid casting rate at elevated temperatures can be 
used to produce a porous contact sheet. During lamination a contact sheet 
with a porous microstructure compresses slightly. The amount the contact 
sheet compresses after lamination compared to its initial thickness is 
expressed as the compressibility, typically in percent. Sheets that have 
little porosity have very little compressibility and conversely sheets 
with high porosity have higher compressibility. 
During lamination surface metal features are embedded into the contact 
sheet. This is shown in FIGS. 4A-4C. Referring to FIG. 4A a typical 
lamination process includes placing contact sheets 76 on each side of a 
stack 70 of metallized ceramic layers. The top and bottom layers 71 and 73 
respectively have surface metallization features 74 and 75 such as IO pads 
or solderable features for attaching components. The entire stack is then 
pressed in a uniaxial press under elevated temperature and pressure for a 
fixed time to thermoplastically bond the layers together. If contact 
sheets with high compressibility are used the surface metal features 74 
and 75 are embedded into the contact sheets. A compressibility greater 
than 6% is considered high. This is shown in FIG. 4B. After sintering and 
removal of the contact sheets the surface metal features are found to be 
raised above the ceramic surface as shown in FIG. 4C. Now referring to 
FIG. 5A, contact sheets 76 are shown having low porosity and 
compressibility, typically less than 6%. Again these are used on the top 
and bottom of a stack of metallized ceramic layers 71, 72 and 73. After 
lamination the surface metal features are embedded into the outermost 
ceramic layers as shown in FIG. 5B. After sintering and contact sheet 
removal the metal surface features are found to be essentially level with 
the surface of the ceramic. This is shown in FIG. 5C. 
When planar ceramic and metal surfaces are desired, thin contact sheets 
with low compressibility are used. Additionally, a hard surface is used to 
contact the contact sheet during the lamination process. This is typically 
a metal plate with a release coating or Mylar layer between it and the 
contact sheet to prevent sticking. 
Referring to FIG. 2C the surfaces 11a and 13a of the multilayer ceramic 
substrate products made using the method of the invention are smoother 
than the comparable prior art multilayer ceramic substrate product shown 
in FIGS. 1C and 3D. This smoothness is the result of using the contact 
sheets of the invention which are made using a fine particle size ceramic 
which has been found to enhance the smoothness of the products of the 
invention. The smoothness is typically 3 to 10 times smoother than 
obtained with contact sheets made using conventional particle size 
alumina. The submicron alumina contact sheet also provides at least about 
a 50% reduction in sintering distortion referred to as the difference 
between the desired location of a surface feature and the actual location 
of that feature as compared to typical contact sheets formulated with 
typical "coarse" particle size alumina as shown in FIGS. 1C and 3D. This 
difference can be measured by an optical measurement system. 
An additional feature of the invention is that contact sheets formulated 
with the sub micron size alumina powder are much more completely removed 
after sintering by a non-contact method such as an aqueous ultrasonic bath 
than for conventionally made contact sheets. Further, if any of the 
contact sheet particles are not removed from metal features, subsequent 
metallurgical processes, particularly nickel and gold plating operations 
are not significantly adversely affected due to the small particle size of 
the ceramic contact sheet residue. It has been found that typical coarse 
particle size alumina contact sheets can be removed with ultrasonics 
however, a 3-4 time factor reduction has been experienced when using the 
contact sheets of the invention. 
While the present invention has been particularly described, in conjunction 
with a specific preferred embodiment, it is evident that many 
alternatives, modifications and variations will be apparent to those 
skilled in the art in light of the foregoing description. It is therefore 
contemplated that the appended claims will embrace any such alternatives, 
modifications and variations as falling within the true scope and spirit 
of the present invention.