Silicon nitride circuit board and semiconductor module

Details Silicon nitride circuit board and semiconductor module Silicon nitride circuit board and semiconductor module

1998-04-17

2001-05-15

Potter, Roy (Department: 2822)

Active solid-state devices (e.g., transistors, solid-state diode

Housing or package

With contact or lead

C257S713000, C257S723000

Reexamination Certificate

active

06232657

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a silicon nitride circuit board and a semiconductor module which is useful as a semiconductor mounted circuit board, semiconductor package and the like.
2. Description of the Related Art
Recently, a ceramic circuit board prepared by bonding a metal plate (metal circuit plate) such as a copper plate to a ceramic plate is used as a substrate for mounting of a semiconductor module such as a power transistor module or semiconductor parts dealing relatively high electric power such as a switching current source module and the like.
As a bonding method of a ceramic substrate with a metal plate cut out in given shape in a production process for the ceramic circuit board as described above, a method using an active metal brazing material (active metal method) prepared by adding 1 to 10 wt. % of active metal such as Ti, Zr, Hf, Nb and the like to an Ag—Cu brazing material and the like, a so-called direct bonding method (DBC method: direct bonding copper method) in which a ceramic substrate is directly bonded to a copper plate using tough pitch electrolytic copper containing 100 to 1000 ppm oxygen or copper of which surface is oxidized in a thickness of 1 to 10 &mgr;m as a metal plate, and the like are known.
For example, in the direct bonding method, a 0.3 to 0.5 mm thick copper circuit plate cut out in given shape is first contacted and placed on a 0.6 to 1.0 mm thick ceramic substrate composed of an aluminum oxide (Al
2
O
3
) sintered material, aluminum nitride (AlN) sintered material and the like, and heated for formation of eutectic crystal liquid phase of Cu—Cu
2
O on bonding interface, the surface of the ceramic substrate is wetted with this liquid phase, then, the liquid phase is cooled to be solidified for direct bonding of the ceramic substrate with the copper circuit plates. The ceramic circuit board obtained by using such a direct bonding method has advantages such as possibility of small size and high-density mounting, and the like, since bonding strength between the ceramic substrate with the copper circuit plates is high and it has simple structure in which a metallized layer or brazing material layer is not required. Further, the production process is intended to be shortened.
However, the above-described ceramic circuit board prepared by bonding a metal plate to a ceramic substrate by the direct bonding method, active metal method and the like has a problem that reliability is poor in heat history, since the metal plate has as large as 0.3 to 0.5 mm thickness for flowing large current. Namely, a ceramic substrate and a metal plate having extremely different heat expansion coefficients are bonded, heat stress derived from the above-described difference in heat expansion coefficient occurs by adding a cooling process and cooling cycle after the bonding. This stress exists in the form of remaining stress distribution of compression and tension on the ceramic substrate side near the bonding portion, and particularly, main stress of the remaining stress acts on ceramic portion adjacent to periphery end of the metal plates. This remaining stress causes cracking on the ceramic substrate, poor withstand voltage, releasing of the metal plate. Further, it exerts a reverse influence that strength of the ceramic substrate is lowered, even if no cracking occurs on the ceramic substrate.
With recent development of high density and highly integrated semiconductor element, miniaturization of a semiconductor module and electronic parts themselves is desired. Under these circumstances, miniaturization of the semiconductor mounted substrate per se is also required.
By the way, an aluminum nitride substrate has a high thermal conductivity and low heat expansion property as compared with other ceramic substrates. However, no aluminum nitride substrate having a sufficient mechanical strength has not been obtained yet. Therefore, when a slight pressure or impact is imparted to the circuit board in mounting process of the circuit board, the circuit board is easily destroyed, and production yield of a semiconductor apparatus is sometimes reduced steeply.
Therefore, in a conventional semiconductor module, it is essential factor to increase thickness of a ceramic substrate and integrate a reinforcing member with a circuit board.
FIG. 4
is a cross-sectional view showing structure of such a conventional semiconductor module
101
. The semiconductor module
101
shown in
FIG. 4
has a large thickness, and is formed by bonding metal circuit plates
103
to a front surface of a ceramic substrate
102
made of an AlN sintered body having a high thermal conductivity, and at the same time by bonding a metal plate
104
as a backing copper plate to the rear surface of the ceramic substrate
102
. Further, a semiconductor element
107
is mounted on a predetermined position of the metal circuit plate
103
by solder bonding, and an electrode portion of the metal circuit plate
103
and the semiconductor element
107
are electrically connected by a bonding wire
108
.
The ceramic circuit board constructed as described above is integrated to the surface of a heat sink plate
105
made of, for example, copper by solder bonding, and a semiconductor module
101
as shown in
FIG. 4
is formed. This semiconductor module
101
is fixed on an apparatus casing
109
, a heat releasing fin or a mounting board by securing with an attaching screw
106
.
However, the above-described conventional semiconductor module
101
or ceramic circuit board has problems that size becomes large when constructed as a semiconductor module and it is difficult to produce a small size module, and production cost for a ceramic substrate increases. In addition, since a heat sink plate which improves heat releasing property and prevents cracking of the ceramic substrate is required, and further, since it is necessary to use a ceramic substrate having a large thickness for enhancing resistance against cracking. Further, since thickness of the substrate increases, there is a problem that heat resistance increases and excellent heat releasing property as expected cannot be obtained even if an AlN substrate having a high thermal conductivity is used.
The present invention has been accomplished to cope with such problems, and a first object thereof is to provide a semiconductor module which does not require both a rear copper plate (rear metal plate) and a heat sink plate, of which structure is simple and can be miniaturized, and of which heat releasing property and heat cycle durability are improved.
On the other hand, with recent development of high density and highly integrated semiconductor element, miniaturization of a semiconductor module and electronic parts themselves is desired, and for high densely mounting, an enlargement of area for mounting part and circuit constituting part is desired.
However, in the conventional ceramic circuit board, since one main surface of a ceramic substrate is only used as a mounting surface, it is necessary to enlarge the ceramic substrate itself for increasing mounting part and circuit constituting part. When the ceramic substrate is enlarged, however, there may be posed problems that the ceramic substrate tends to bend in bonding process for bonding the copper circuit plates and the like. Further, simple enlargement of the ceramic substrate goes against the requirement for miniaturization of a mounting substrate and the electronic parts themselves.
As described above, in the conventional ceramic circuit board, the ceramic substrate should be enlarged for increasing the area of mounting part and the circuit constitutional part, and this invites poor bonding of the copper circuit plates and goes against requirement for miniaturization of the electronic parts. Further, with diversification of the semiconductor element, various properties are required for a semiconductor mounting substrate and semiconductor package, and it is desired to satisfy the various requirements.
On the other hand, high densely integration, hi

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