Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-04-27
2003-11-11
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C257S787000, C257S788000, C257S791000, C257S793000, C257S794000, C523S400000, C523S443000, C523S456000, C523S466000, C525S476000
Reexamination Certificate
active
06645643
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of semiconductor devices, and, more particularly, to a polymeric composition for packaging an electronic semiconductor device. Moreover, the present invention also relates to a plastic packaging material for microelectronic applications, which may be obtained from such polymeric composition, and to a semiconductor device including such packaging material.
BACKGROUND OF THE INVENTION
In the field of microelectronics, it is quite common to encapsulate electronic semiconductor devices, such as power metal oxide semiconductor (MOS) devices, within packages of plastic material. Power MOS electronic devices include a plurality of layers having different chemical structures (e.g., dissipating elements, support frames, die made from semiconductor materials, and plastic packaging materials) whose compatibility determines the performance and reliability of the device.
The reliability of a power device may be measured by a number of tests. In particular, the so-called High Temperature Reverse Bias (HTRB) test allows reliability to be estimated by subjecting the device to high temperatures during reverse biasing. The behavior of the device in this test depends upon the physical-chemical conditions of the circuit die and the interactions with the packaging material.
Usually, plastic packaging material is produced by hardening a polymeric composition including a thermosetting resin and various additives, such as reinforcing fillers based on fused or crystalline silica, for example, and at least one control agent for the rheology of the polymeric composition (generally based on siloxanes). The thermosetting resin usually includes an epoxy resin that is typically obtained from an epoxy pre-cured with phenolic resin or an epoxy pre-cured with an anhydride.
It has been suggested that to have a low ionic content and a high volume resistivity the bulk characteristics of the plastic packaging material of a semiconductor device may require alteration. For example, it has been suggested that the amount of ions, among which Na+ and Br− ions come from raw materials, be reduced to provide a high volume resistivity, preferably higher than 1×10
12
&OHgr;cm. However, the reliability value obtained by subjecting semiconductor devices of the prior art to the HTRB test is inadequate compared to the ever increasing reliability level required from such devices, particularly from power devices.
SUMMARY OF THE INVENTION
An object of the invention is to provide a polymeric composition for packaging a semiconductor electronic device that provides improved reliability of the semiconductor device with respect to prior art devices.
Applicants have determined that to address the above problem it will not suffice simply to alter the bulk characteristics of the die packaging to provide low ionic concentration and a high volume resistivity. Rather, it may be necessary to reduce the polarity of the plastic packaging material layer as much as possible at the interface with the die itself. In fact, applicants have found that the polarity characteristics at the interface between adjoining layers significantly affect the mechanical and electrical performance of the device.
According to the invention, the above technical problem is solved by a polymeric composition including at least one epoxy resin, at least one curing agent in an amount between 30 and 110 parts by weight per 100 parts by weight of epoxy resin, at least one silica-based reinforcing filler in an amount between 300 and 2300 parts by weight per 100 parts by weight of epoxy resin, and at least one control agent for the rheology of the polymeric composition. The at least one control agent may be substantially free from polar groups and may be present in an amount between 0.1 and 50 parts by weight per 100 parts by weight of epoxy resin.
From tests conducted by applicants, the electrical behavior of the interface plastic package material and the semiconductor material die was found to be substantially correlated to the quantity of polar groups existing at the interface. In particular, it was found that a reduction in the quantity of polar groups at the interface, in particular those coming from the control agent for the rheology, provides significant improvement in the reliability of the semiconductor device.
The at least one epoxy resin may be selected from the group including bisphenol A type epoxy resin, phenol-novolac type epoxy resin, creosol-novolac type epoxy resin, glycidyl ester type epoxy resin, biphenyl type epoxy resin, polyfunctional epoxy resin, glycidyl amine type epoxy resin, linear aliphatic epoxy resin, alicyclic epoxy resin, heterocyclic epoxy resin, halogenated epoxy resin, and mixtures thereof. The chemical structures of these resins may be seen in
FIGS. 1A
to
1
L.
Additionally, the epoxy resin may include a concentration of chlorine ions lower than 10 ppm and a concentration of chlorine in hydrolyzed form lower than 0.1% by weight, because such ions may cause electronic failures. Preferred epoxy resins include those of the glycidyl ester type and novolac type epoxy resins, the novolac type including 170 to 300 epoxy equivalents. These provide good workability during molding and also good electronic reliability.
The at least one curing agent may be a phenolic resin selected from the group including phenol-novolac resin, cresol-novolac resin, bisphenol A resin, phenol-aralkyl resin, dicyclopentane phenolic resin, bisphenyl type phenolic resin, polyfunctional phenolic resin, other denatured phenolic resins, and mixtures thereof. The chemical structures of these resins may be seen in
FIGS. 2A
to
2
F.
Advantageously, the epoxy resin and the phenolic resin may be mixed so that the ratio between the number of epoxy equivalents of the epoxy resin and the number of equivalents of the hydroxyl groups of the phenolic resin is between 0.5 and 1.5. It has been determined that if this ratio exceeds the above defined range, the mechanical strength of the cured epoxy resin may be reduced.
The silica-based reinforcing filler may include fused silica powder or crystalline silica. The silica powder may generally assume a spherical, lumpy or fibrous shape. Spherical silica is generally used in combination with other fillers which have non-uniform diameters. The silica-based reinforcing filler is incorporated in the polymeric composition to adequately protect the semiconductor device and to impart improved workability to the polymeric packaging composition during molding. This, in turn, reduces strain in the device itself.
The silica-based reinforcing filler may include spherical silica in an amount between 0.05 and 20% by weight of the total amount of silica-based reinforcing filler. Advantageously, spherical silica provides improved flow of the polymeric composition. In particular, spherical silica reduces the so-called resin-flush phenomenon, i.e., when part of the polymeric packaging composition comes out from the air spaces between the various sections of the mold. The spherical silica preferably has an average particle diameter between 0.3 and 1.5 &mgr;m and a surface area between 3 and 10 m
2
g
−1
.
In particular, it was found that if the surface area value falls below 2 m
2
g
1
, the so-called resin-flush phenomenon cannot be reduced sufficiently. On the other hand, if the surface area value exceeds 10m
2
g
−1
, an undesired moisture absorption by the polymeric composition may take place.
Furthermore, the polymeric composition of the invention may further include at least one silica coupling agent suitable for reacting with the surface hydroxyl groups of silica. Advantageously, the coupling agent performs the function of coupling the silica-based reinforcing filler to the polymeric matrix upon prior reaction with the epoxy polar groups including the same. In this way, as the bond takes place in corresponding polar groups existing in the polymeric matrix, the number of free epoxy groups, and decreases further reduction in the polarity of the polymeric comp
Pignataro Salvatore
Scandurra Antonino
Tenya Yuichi
Yoshizumi Akira
Zafarana Roberto
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Aylward D.
Dawson Robert
Jorgenson Lisa K.
STMicroelectronics S.r.l.
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