Mounting substrate

An LSI mounting substrate having a multilayered thin film wiring portion, with the thin film wiring portion being divided into wiring units each composed of a plurality of wiring layers, with the wirings between the units being electrically connected through connecting pads defined in the same surface as that of a surface conductive layer of the unit.

BACKGROUND OF THE INVENTION 
The present invention relates to a mounting substrate on which a plurality 
of LSIs are mounted in an electronic computer, and more particularly, to a 
mounting substrate having a multi-layered wiring structure formed by a 
thin film process with insulating layers of an organic film composed of 
polyimide or the like. 
In a mounting substrate used in an electronic computer, a problem arises 
when attempting to enhance the mounting density of many LSIs to thereby 
minimize a signal delay in the substrate, and to increase a speed of 
signals transmitted between the LSIs. 
To cope with this problem, attention is paid to a mixed substrate composed 
of a thin film and thick film, which comprises a ceramic substrate on 
which a wiring layer of tungsten or molybdenum formed by a thick film 
process is laminated and sintered, a polyimide interlayer insulating film 
formed thereon, and a conductive layer of copper or aluminium formed 
thereon by a thin film process, and this mixed substrate is in a process 
of development. With this arrangement, a signal can be transmitted at a 
high speed in high density, because a dielectric constant of polyimide in 
a thin film wiring portion is smaller than that of ceramics, copper and 
aluminium of low resistance and a semiconductor process can be used. 
However, the number of laminated layers of a thin film wiring layer must be 
increased to cope with an increase in the number of mounted gates per unit 
area, which is needed by the improvement in performance of computers. 
Several technologies for forming thin film multi-layered wirings have been 
reported. They employ, however, a thin film process as the basic process 
thereof, in which conductive layers, through holes and polyimide layers 
are patterned on a ceramic substrate or silicon substrate by exposing and 
developing photoresist. This thin film process is suitable to miniaturize 
wirings, but has a drawback in that since this process is a so-called 
sequential laminating system by which conductors and through holes are 
individually made a long time is needed to form a thin film wiring 
composed of many laminated layers, and further an entire substrate becomes 
defective if the substrate is determined to be defective in the final 
process and thus yield is lowered and a cost of products is increased. 
Further, a resistance of wirings is suppressed to a low level in the thin 
film wiring, and thus when a width of the wirings is miniaturized, a 
thickness thereof must be increased to provide a given cross sectional 
area. 
As a result, a thickness of a wiring layer is made to be the same or 
greater than a width of wirings, and thus a problem arises in that even if 
fluid polyimide varnish is used, flatness is difficult to obtain, an 
accuracy of a wiring pattern is deteriorated as the number of laminated 
layers is increased, so that the wirings are often cut off or 
short-circuited. Further, a problem also arises in that, since a ceramic 
substrate having I/O terminals and the thin film wiring portion of an 
underlayer are repeatedly heated and immersed in water, chemicals and the 
like, they are deteriorated at the boundary thereof and polluted by the 
ions in impurities, whereby the reliability thereof is lowered. 
To solve these problems, for example, in Japanese Patent Application Kokai 
61-40048, there is disclosed a package comprising an original ceramic 
substrate having I/O terminals and a separate substrate which has a thin 
film wiring formed thereon and connected on the original substrate by 
soldering. This package, which has the thin film wiring portion simply 
formed independently of the ceramic substrate, is fundamentally different 
from that of the present invention, in which a thin film portion is formed 
as a unit. Further, in this prior art, since a tentative substrate used to 
form a thin film serves as an element for forming the package as it is, a 
thin film wiring layer has a thickness which is unnecessarily increased 
and thus this is not preferable in view of the increase of a transmitting 
speed of a signal. 
Further, Japanese Patent Application Kokai No. 63-274199 discloses another 
method by which polyimide films in which wirings are formed are laminated 
in one lot and through hole portions are thermally bonded under pressure. 
This method is effective to reduce a throughput time, but a problem arises 
in that, since a polyimide thin film is treated in a laminating process, 
an aligning accuracy is difficult to improve, the number of contact points 
is greatly increased, and a connecting portion has low reliability. 
SUMMARY OF THE INVENTION 
An object of the present invention is to eliminate the above defects and to 
provide a hybrid substrate arranged so as to enable a thin film wiring 
layer to be formed in a short time with improved yield even if a substrate 
has many thin film wiring layers. 
Another object of the present invention is to provide a method of 
manufacturing the above hybrid substrate. 
The objects of the present invention will be achieved in such a manner that 
a thin film wiring layer having many laminated layers is first divided 
into unit wiring portions each composed of some layers, the unit wiring 
portions are connected with each other through connecting pads defined to 
through hole portions therebetween, and further a wiring unit is formed on 
tentative substrate. More specifically, after the through hole portions 
have been confronted with each other and the units have been electrically 
connected, a unit substrate is separated from the tentative substrate and 
this separating operation is repeatedly carried out, whereby thin film 
layers can be formed with improved yield in a short time.

DETAILED DESCRIPTION OF THE INVENTION 
A mounting substrate according to the present invention is an LSI mounting 
substrate having a multilayered thin film wiring portion on a ceramic or 
silicon substrate, wherein the thin film wiring portion is divided into 
wiring units each composed of a plurality of wiring layers and wirings 
between the units are electrically connected through connecting pads 
defined in the same surface as that of the surface conductive layer of the 
unit. 
Further, a portion of the wiring unit is composed of a signal layer, a 
power supply layer, and a ground layer, the power supply layer and the 
ground layer are formed on both surfaces, and the respective layers are 
electrically connected via through holes. 
Further, the power supply layer and the ground layer of the wiring unit are 
an overall film and pads electrically insulated from the overall film is 
formed in the overall film. 
The conductive layer of the wiring layer constituting the wiring unit is 
preferably composed of any one metal of Cu, Al, Au, and Ag. 
The insulating layer of the wiring layer constituting the wiring unit is 
preferably composed of polyimide. 
Further, the surfaces of two units disposed in confrontation (e.g., in a 
vertically stacking arrangement) for connection preferably have the same 
conductive pattern configuration. 
Further, the connecting pads between the wiring units preferably have a 
size (e.g., a plan view area) larger than that of through holes for 
connecting the wirings of respective inner layers. 
On the other hand, the unit is preferably formed using a tentative 
substrate comprising a material different from that constituting the thin 
film wiring portion. 
Further, the unit is preferably formed on the tentative substrate by a thin 
film process. 
The reason why the thin film wiring is divided into units each having a 
plurality of wiring layers and the units are connected through connecting 
pads is that each thin film wiring layer can be made as a divided unit, so 
that final yield can be greatly improved by selecting good products and 
bad products before the units are connected. Further, a time needed to 
make the mounting substrate can be greatly reduced by making the 
respective units in parallel. 
The reason why the signal layer is formed as an inner layer and the power 
supply layer and ground layer are formed as an outer layer in the 
arrangement of the wiring unit is that because the power supply layer and 
ground layer are fundamentally an overall film, this arrangement can 
protect signal wiring layers each having a fine wiring width and through 
holes, and connecting pads having a size larger than that of fine through 
holes for connecting wiring layers between the units can be formed in the 
power supply layer and ground layer to thereby increase the reliability of 
the connection between the units. 
Further, the reason why the tentative substrate is used to form the above 
wiring units is that the wiring units can be formed by a sequential 
process, a wiring width and wiring pitch can be easily miniaturized 
because the layers are sequentially laminated, two units can be more 
accurately aligned when they are connected, and the tentative substrate 
can be used as a pressurizing jig when the units are connected. 
Further, the reason why the above tentative substrate is composed of a 
specific material and subjected to a surface treatment whereby the 
substrate can be easily separated from the unit after connecting the units 
with each other is to enable the tentative substrate to be separated from 
the units without injuring the connected portion of the units. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention will be described below with reference to drawings. 
FIG. 1 shows a cross sectional view of a mounting substrate according to 
the present invention. A highly integrated LSI 13 is connected to a thin 
film wiring layer 12, formed on a ceramic substrate 11 by solder balls 14. 
The above thin film layer is divided into four units 121, 122, 123, and 
124. Each of the units except the uppermost unit (124) is composed of two 
signal line layers 28, 29 perpendicular to each other and a power supply 
layer and ground layer 27, 30 disposed on the both sides thereof. The 
uppermost layer unit 124 is composed of pads 15 used for connecting the 
uppermost layer unit to the LSI and enlarged layers 16 for adjusting a 
pitch of the pads and a pitch of through holes in the underlayer unit, but 
sometimes a final resistance layer and power supply layer may be added in 
the uppermost layer. 
The above respective units are connected by confronting connecting pads 31 
formed in the power supply layer and ground layer on the surface of the 
unit. In this case, a size of the connecting pad can be made larger than 
that of a through hole 32, so that the units can be easily connected and 
the reliability of the connected portions is increased. Further, alloy 33 
of a low melting point other than wiring metal is used for the connection, 
and the units are securely connected electrically and mechanically in such 
a manner that the alloy is heated to a temperature higher than the melting 
point thereof under pressure. The respective units may be sequentially 
connected from the ceramic substrate side thereof, or the thin film wiring 
portion may be connected to the ceramic substrate after the respective 
units of the thin film wiring portion have been connected. 
The structure of the thin film wiring portion composed of several divided 
units connected to each other, which is arranged as described above, 
enables the respective units to be individually made and inspected at the 
same time, whereby the yield thereof as a mounting substrate is improved 
and the mounting substrate can be made in a short time. 
Next, a method of manufacturing the above wiring unit will be described. 
Each wiring unit is made using a thin film process. Since the thin film 
process can miniaturize a wiring more easily than a thick film process, it 
can miniaturize a minimum width of the wiring to about 10 to 20 
micrometers. Since a wiring resistance is increased as a width of the 
wiring is reduced, however, a thickness thereof must be increased to 
prevent an increase in the resistance. For example, when the wiring has a 
width of 20 micrometers, a thickness thereof is preferably 20 micrometers 
or more, and thus an aspect ratio of a cross section of the wiring is 1 or 
more. Therefore, when a unit substrate is to be made from this thick film, 
a plating method is used to form a conductor and a method of coating 
polyimide varnish to a thick film and thermosetting the same is used to 
form an insulating film. 
FIGS. 2(a) to 2(e) show the various processing steps of a method of 
manufacturing a wiring unit composed of four layers using copper and 
polyimide. First, a tentative substrate 21 having sufficient rigidity and 
flatness not to be deformed by the formation of a polyimide film and a 
surface 22 the bonding force of which to a wiring unit is adjusted to a 
suitable range is prepared, and a copper film 23 serving as a power supply 
layer or ground layer is formed thereon by a plating method. Next, after a 
pattern has been formed using photoresist 24, the copper film 23 is 
etched. Further, after polyimide varnish has been coated and thermally 
set, a portion of a polyimide layer 25 is opened by being etched to form a 
through hole 26. Further, the through hole is filled with copper by a 
plating method to complete the formation of the wiring of a first layer 
27. The conductive wirings of a second layer 28, third layer 29, and 
fourth layer 30 are formed by repeating the same process. Finally, an 
alloy film 32 of Au-Sn, Au-Si, Pb-Sn or the like formed on the surface 31 
of connecting pads provided in the conductive layer of the fourth layer. 
The reason why the substrate needs the sufficient flatness and rigidity in 
this process is to uniformly arrange a height of the connecting pads 
formed in the power supply layer of the fourth layer and to securely 
connect the units thereafter. Further, the when using the tentative 
substrate in connection with the formation of the wiring units, one 
bonding force of which to the under surface of the units i.e., to the 
copper film of the first layer 27 is adjusted to a suitable range so as to 
prevent the tentative substrate from being exfoliated from the unit during 
the formation of the wiring units and to enable the tentative substrate to 
be separated from the units without damaging the connected portions after 
the respective units have been connected. For this purpose, a material of 
the tentative substrate and surface roughness thereof must be selected. In 
the case of a copper film, stainless is most suitable and a bonding force 
can be controlled by changing the surface roughness. The bonding force is 
such that it does not cause exfoliation and separation in the process in 
which the above wiring units are made and must be adjusted to a range 
smaller than a bonding force between the units. 
FIG. 3(a) and 3(b) show a method of connecting the units. The tentative 
substrate 21 used for making the units is also used as a carrier and 
connecting jig as it is to connect the units. More specifically, the units 
122 are connected in such a manner that the units formed on the tentative 
substrate are arranged so that they are confronted to each other, pressure 
is applied thereto after the connecting pads have been aligned, and then 
solder 33 on the pad surfaces is heated to the melting temperature 
thereof. Thereafter, only the tentative substrate is separated from the 
wiring unit to complete the connection of a single unit. All the units can 
be connected by repeating this process. In this case, if there is a fear 
that the connecting pads may be broken when the tentative substrate is 
separated, a bonding force between the connected units may be reinforced 
by filling other portions therebetween with solder, thermosetting resin, 
or the like. 
Further, when the units must be aligned with pin-point accuracy, the 
pattern position on the tentative substrate is read by an image sensor 
before the units are confronted to each other, and then the units are 
subject to an automatic alignment based on the data obtained by reading 
the pattern. 
As described above, according to the present invention, there are 
advantages in that a thin film multi-layered wiring having many laminated 
layers can be made as units divided with respect to each other and each 
unit can be independently inspected. As a result, therefore yield as a 
whole is greatly improved and throughput can be reduced. 
Further, there is also an advantage in that a wiring unit formed by using a 
tentative substrate enables a signal layer having a fine wiring width to 
be easily formed, and an accuracy, with which the respective units to be 
connected to each other are aligned, is improved. Therefore, computers 
having high reliability can be produced by use of the mounting substrate 
of the present invention.