1.  Field of the Invention
The present invention relates to a circuit substrate and a method for fabricating the circuit substrate, more specifically to a circuit substrate and a method for fabricating the circuit substrate which are adaptable to higher density and higher speed. 
2.  Description of the Related Art
Recently, semiconductor parts used in computers, etc.  are rapidly made increasingly adaptable to higher density and higher speed.  Circuit substrates as well are accordingly required to be adaptable to the higher density and the higher speed. 
One example of the conventional circuit substrates will be explained with reference to FIGS.  7A and 7B. FIG.  7A is a sectional view of the mounted conventional circuit substrate, which shows the structure of the circuit substrate.  FIG.  7B is a perspective view of the circuit substrate, which shows the structure of the circuit substrate. 
As shown in FIG.  7A, the circuit substrate 100 is mounted on a packaging substrate 106.  The circuit substrate 100 and the packaging substrate 106 are electrically connected to each other via solder balls 104a, etc.  A decoupling capacitor 108 is formed on the upper surface of the circuit substrate 100.  An LSI substrate 110 is mounted on the circuit substrate 100 mounted on the packaging substrate 106.  The circuit substrate 100 and the LSI substrate 110 are electrically connected to each other via solder balls 104b, etc. 
As shown in FIG.  7B, through-holes 114 are formed in the circuit substrate 100 at a certain pitch.  Vias 116 of a metal are buried in the through-holes 114.  Usually, passive elements, active elements, etc.,  such as decoupling capacitors, etc.,  are formed on the circuit substrate 100 mounted on a packaging substrate 106.  In FIG.  7B, these elements, etc.  are omitted. 
Prescribed interconnections of the LSI substrate 110 and prescribed interconnections of the packaging substrate 106 are electrically connected via the vias 116, electrode pads 102a, 102b and solder balls 104a, 104b.  
The above-described circuit substrate generally comprises a resinous substrate, ceramic substrate, as of alumina ceramics, glass ceramics, or other substrates. 
The resinous substrate is formed in the following way.  First, copper foil internal plates which function as the interconnections, and sheets of carbon fibers impregnated with a thermosetting resin, which is called a prepreg are laid alternately the former on the latter.  Then, the laid cooper foil internal layers plates and the prepregs are pressed and sintered to form the resinous substrate.  Then the through-holes are formed by mechanical processing using a drill.  Next, the surface is plated with copper. 
The ceramic substrate is fabricated in the following way.  First, a ceramic sheet before sintered, which is called a green sheet is punched to form openings.  Then, the surface is plated with copper.  Next, a plurality of the green sheets are laid one on another, and pressed and sintered. 
Metal is buried by plating in the through-holes formed in the respective substrates to electrically connect the surface of the substrate and the backside of the substrate. 
For higher density packaging, the through-holes formed in the substrate are required to have a smaller diameter and a smaller pitch.  However, the circuit substrate formed of the resinous substrate or the sintered ceramic substrate has a limit to a smaller diameter of the through-holes and a pitch of the through-holes. 
For the ceramic substrate, the through-holes are formed by mechanical processing using a punch, which makes it difficult to form the through-holes at a pitch smaller than a feed pitch of the punch. 
For the resinous substrate, the through-holes are formed by mechanical processing using a drill, which makes it difficult to form the through-holes at a pitch smaller than a feed pitch of the drill.  In a case where a thin drill is used to make the through-holes micronizsed, there is a risk that the drill may be broken when the through-holes are formed or the substrate itself may be broken.  The risk that the substrate itself may be broken is higher when the through-holes are formed of a high aspect ratio are formed at a small pitch. 
To bury a metal in the through-holes by plating, the growing velocity of the metal film is low.  Accordingly, a long plating time is required.  For example, when a metal is buried in the through-holes of a 50 μm-diameter and a 300 μm-depth, about 3 day of the plating are required. 
In the case where the through-holes have a high aspect ratio, burying metal by the plating has the following disadvantage.  That is, as an aspect ratio of the through-holes is higher, it is more difficult for a plating liquid to intrude into the through-holes, and part of the inside walls of the through-holes are not plated.  Resultantly, the electric conductivity is less reliable. 
For the resinous substrate, thermal processing of high temperatures generates contaminative gases from the substrate, and melts the substrate itself, which makes it difficult to form passive elements, such as capacitors, etc.,  on the resinous substrate.  The ceramic substrate is formed by sintering, which causes disadvantages of inferior dimensional stability due to variable shrinkages, etc.,  voids formed in the meal interconnections, and other disadvantages.  Such disadvantages make it difficult to apply the ceramic substrate to the micronized devices. 
Here, it is considered to use a silicon substrate or a glass substrate in place of the above-described resinous substrate and ceramic substrate.  For the silicon substrates, etc.,  micronized processing by photolithography can be used.  Accordingly, more micronized through-holes can be formed in the silicon substrate, etc.  than in the resinous substrate, etc.  Furthermore, the silicon substrate and the glass substrate are the same material as the LSI chips, or the coefficients of linear expansion of the former are approximate to a linear expansion coefficient of the LSI chips, whereby the generation of stresses due to temperature changes are depressed.  Advantageously, the reliability can be improved. 
However, to bury metal by plating into the micronized through-holes formed in the silicon substrate, etc.  it takes a long time, as does it in the case of the resinous substrates.  There is a risk that simply forming the micronized through-holes at a small pitch may break the substrates themselves.  Burying metal in the through-holes formed simply at a small pitch will damage the substrates due to stresses applied to the substrates due to the difference of thermal expansion coefficients.