Process for minimizing distortion in multilayer ceramic substrates and the intermediate unsintered green ceramic substrate produced thereby

A method of fabricating a multilayer ceramic substrate with an internal conductive metallurgy circuit network, wherein additional green sheet material is added to the stack of ceramic green sheets during assembly to areas of the substrate outside of the conductive metallurgy to compensate for the volume of conductive metal paste to thereby eliminate or minimize substrate distortion during the sintering operation. An unsintered intermediate green ceramic substrate made up of green ceramic sheets with via holes and conductive metal lines on the surface which collectively form the circuit network where the improvement is additional green ceramic material in the substrate in areas outside of the conductive metallurgy network to compensate for the additional volume of material of the conductive metal which additional material provides a more uniform ceramic material density through the substrate.

FIELD OF INVENTION 
This invention relates to ceramic substrates for integrated circuit 
semiconductor packages, more particularly to methods for preventing 
distortion in laminated ceramic substrates, still more particularly to a 
process for eliminating or minimizing distortion of multilayer ceramic 
substrates used for semiconductor packages. 
BACKGROUND OF INVENTION 
Because of the high package density attainable with multilayer ceramic 
(MLC) substrate circuit structure, it has achieved extensive acceptance in 
the electronics industry for the packaging of integrated circuit 
semiconductor devices and other elements. In general, such conventional 
ceramic structures are formed from ceramic green sheets which are prepared 
from ceramic slurry. The slurry is made by mixing a ceramic particulate, a 
thermoplastic polymer (e.g. polyvinylbutyral) and solvents for the 
polymer. This slurry is then cast or doctor bladed into ceramic sheets 
from which the solvents are subsequently volatilized to provide a coherent 
and self-supporting flexible green sheet, which may be finally fired to 
drive off the binder resin and sinter the ceramic particulates together 
into a densified ceramic unitary substrate. 
In the fabrication of multilevel ceramic structure, the green sheets are 
first punched to form via holes. Subsequently, a pattern of conductive 
material is deposited in the via holes and on the surfaces of the sheets. 
The green sheets are then assembled in the proper order and laminated 
wherein the metallurgy in the via holes and on the green sheets 
collectively form a complex internal metallurgy network. After the 
composite substrates have been pressed to adhere the sheets firmly 
together and indent the conductive metal patterns into the opposed sheets, 
the substrate is fired in an appropriate atmosphere at a temperature to 
first burn off the organic binder, and subsequently to sinter the 
particles of the substrate together to form a unitary ceramic substrate. 
The original green ceramic sheets are formed on a larger scale so that 
upon shrinking the spacing of the various elements conforms to the desired 
standards. The fabrication of multilayer ceramic substrates is described 
in more detail in U.S. Pat. No. 4,245,273. In the manufacture of 
multilayer ceramic substrates for integrated circuit semiconductor 
packages, it is imperative that shrinkage that occurs during the sintering 
operation be predictable and consistent, and also that the shrinkage be 
uniform throughout the multilayer ceramic substrate. Further, the stresses 
and material changes generated by the lamination process operate to cause 
variability during sintering. 
SUMMARY OF THE INVENTION 
An object of this invention is to achieve a shrinkage during sintering of 
an MLC substrate that is uniform throughout the substrate. 
Another object of the invention is to achieve a stable planar sintered 
substrate free of warpage, and where the original geometric shape is 
retained. 
Another object of this invention is to provide a method of producing an MLC 
substrate that is free of warpage, and where the original geometric shape 
is retained. 
Yet another object of this invention is to provide an intermediate product, 
i.e. a green ceramic substrate that will have a uniform shrinkage 
throughout the substrate, be free of warpage, and retain its original 
geometric shape during the sintering operation. 
In accordance with the aforementioned objects of the invention, a method is 
presented for fabricating a multilayer ceramic substrate with an internal 
conductive metallurgy circuit wherein a slurry of particulate ceramic 
material, an organic binder, and a solvent for the binder is formed into a 
green ceramic sheet, holes punched in the sheets, conductive metal 
deposited in the via holes and on the surface of the sheets to form lines, 
a plurality of the green sheets assembled, and the resultant substrate 
sintered, the improvement involving incorporating additional green ceramic 
material in the substrate during assembly in the areas generally outside 
of the areas of the conductive metal lines and filled via holes, 
subjecting the resultant substrate to pressure to laminate the sheets, and 
sintering the resultant substrate. The added green sheet material acting 
to compensate for the additional volume of the conductive metal paste in 
the substrate forming the metallurgy system.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to the Figures of the drawing, in particular FIGS. 1 and 2, 
there is illustrated the general shape, in greatly exaggerated scale, of a 
substrate fabricated by techniques conventional in the art. The centrally 
located green ceramic sheet of a typical MLC substrate has via holes 
punched, filled with conductive paste, and surface lines formed of 
conductive material that joins the via. Also in power planes, an almost 
solid layer of paste may be deposited on the sheet. These vias and surface 
patterns are located generally in the central area of the substrate, in 
general underlying device pads 12 on substrate 10 as shown in FIG. 1. In 
the border area outside of the device pads 12, there are normally no vias 
or conductive patterns. When the green ceramic sheets are assembled and 
laminated, i.e., placed in a press and subjected to pressure, the ceramic 
material in the centrally located volume of the substrate is compacted to 
a greater degree than the volume of the substrate without metallurgy, and 
the density of the ceramic material is greater. While there is some 
plastic flow of the ceramic material to the volume of the substrate 
without metallurgy, the flow is not sufficient to equalize the density of 
the ceramic material. This increased density is not evident in the 
laminated green ceramic substrate since it conforms to the shape of the 
press cavity. However, when the substrate is sintered the geometric shape 
changes to the shape illustrated in FIGS. 1 and 2, the sides of the 
substrate bulge as indicated in exaggerated scale by volumes 14. In 
addition, the edges become tapered, as indicated in Fig. 2, also in 
exaggerated scale. 
Substrate 10 is thus distorted in the X and Y directions, which complicates 
placement of devices on pads 12 on the surface pattern. It makes testing 
the substrate for electrical internal defects difficult since the probes 
must be placed over and contact the pad pattern. When the top surface 
metallurgy pattern is distorted, fewer contact points can be made at a 
time. If the pattern is undistorted, theoretically all the pads could be 
contacted at once. The greater the distortion the fewer electrical 
contacts can be made. 
As indicated in FIG. 2, the substrate is also distorted in the Z direction. 
This distortion also complicates testing the substrate for electrical 
defects. The pads along the edge are lower than the inner pads. The 
contact probe may fail to establish electrical contact with the outer pad 
since the probe will contact the inner pad first and prevent the probe 
from descending to the level of the outer pads. The seal that ultimately 
will be established between the substrate and cap may also be impacted 
because of this surface irregularity. Still further, the top surfaces of 
the outermost devices bonded to the substrate will not be in the same 
plane as the inner devices. When the conduction pistons as disclosed in 
U.S. Pat. No. 3,993,123 are used to cool the devices, the cooling 
efficiency is impacted because the piston is tilted relative to the 
surface, thus limiting the area contact. 
When the stack of green ceramic sheets is laminated in a press, the flow of 
ceramic material caused by the application of pressure moves the center 
layers outwardly. This essentially bows the vertical lines of the internal 
metallurgy outwardly where the bow is greatest along the outer peripheral 
areas. the 
The aforementioned problems become more serious as the number of sheets in 
the substrate increase since the complexity of the substrate is 
increasing. Due to increased microminiaturization of the device geometry, 
the number of sheets in a substrate will increase which will also increase 
the problems associated therewith. 
FIG. 3 illustrates the desired profile of a sintered multilayer ceramic 
substrate 20, which can be achieved by this invention. The substrate 
claimed is substantially free of distortion in the X, Y and Z directions. 
Referring now to FIG. 4 there is illustrated a first preferred embodiment 
of the method of the invention. FIG. 4 shows an unsintered multilayer 
ceramic substrate 30 formed of green ceramic sheets 32, formed in 
accordance with known methods. The top sheet has a suitable surface 
metallurgy pattern 34 (shown schematically) adapted to provide electrical 
connections to a plurality of semiconductor devices. In order to 
compensate for the volume of conductive metal paste in the central area of 
sheets 32, a sheet 36 is disposed in the stack of sheets. Sheet 36 is 
preferably formed of the same ceramic material as sheet 32, but has a 
large central opening 38. Generally, the opening 38 corresponds to the 
area provided on sheets 32 for vias and conduction metallurgy patterns. 
Thus, the use of sheet 36 does not require vias or conductive patterns to 
be formed thereon. Any suitable number of sheets 36 can be placed 
internally or externally in the stack of sheets 30 in order to minimize or 
eliminate density variations in the laminated substrate. The sheet 36 can 
have the same thickness as sheets 32 or it can be thicker or thinner 
depending on the need. The number of sheets 36 in the substrate can be 
determined by trial and error, and is dependent on the nature of the 
conductive metal paste, the paste thickness and the total number of sheets 
in the substrate being fabricated. 
Referring now to FIG. 5, there is illustrated another embodiment of the 
invention. FIG. 5 shows a stack 40 of green ceramic sheets 32 formed in 
accordance with known methods. Sheets 32 have via holes (not shown) filled 
with conductive metal paste, and metallurgy patterns (not shown) which 
collectively form an internal metallurgy pattern for an MLC substrate. 
Inserted in the stack 40 are two sheets 42 with a thinned central portion 
44. The sheets 44 are preferably positioned with the direction of casting 
at right angles to each other. Sheets 42 can be conveniently formed by 
doctor blading, where the doctor blade is shaped to the compliment of the 
desired sheet profile. As in the previous method embodiment, the number of 
sheets inserted in the substrate can vary to meet the individual substrate 
requirements. Sheets 42 must have via holes punched and filled with 
conductive metal paste to join the metallurgy patterns on the associated 
green sheets 32. 
Referring now to FIG. 6 of the drawing, there is illustrated yet another 
embodiment of the invention. FIG. 6 shows a stack 50 of green ceramic 
sheets 52. Sheets 52 each have a profile designed to compensate for 
conductive metal paste on,the sheets and in the via holes. Here the 
profiled sheets 52 have via holes (not shown) with conductive metal paste, 
and conductive lines (not shown) that collectively form an internal 
metallurgy system. Since the profile variation of sheets 52 vary only 
slightly from the flat sheets 32 in FIGS. 4 and 5, the conventional 
punching and screening present no significant problem. The stack of green 
sheets 50 shown in FIG. 6 thus are comprised of uniformly shaped green 
sheets with a thinned central profile. As indicated, the direction of 
casting which corresponds to the profile is varied with each sheet where 
the direction is advanced 90.degree. relative to the preceding sheet. 
In the practice of the invention, any suitable ceramic material combined 
with an appropriate organic binder and solvent for the binder can be used 
including alumina ceramic, mullite, glass ceramic, etc. After the stacks 
of green ceramic sheets have been assembled, they are laminated i.e. 
pressed between two platens, preferably with a confining edge. 
While the invention has been illustrated and described with reference to 
preferred embodiments thereof, it is to be understood that the invention 
is not limited to the precise construction herein disclosed and the right 
is reserved to all changes and modifications coming within the scope of 
the invention as defined in the appended claims.