Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-10-04
2002-10-15
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S787000, C257S666000
Reexamination Certificate
active
06465827
ABSTRACT:
This application is based on application No. H9-9276 filed in Japan, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a resin-encapsulated semiconductor apparatus having a semiconductor device with a ferroelectric film, and a process for its fabrication.
2. Description of the Related Art
In recent years, non-volatile or large-capacity semiconductor memory devices having thin films of ferroelectric substances (dielectric materials having a high dielectric constant, or substances having a perovskite crystalline structure) have been proposed. Ferroelectric films have features such as self polarization and high dielectric constant characteristics. Hence, the ferroelectric films have hysteresis characteristics between polarization and electric fields of ferroelectric substances, and their utilization enables materialization of non-volatile memories. Also, the ferroelectric films have such a larger dielectric constant than silicon oxide films that memory cells can be made to have a smaller area when the ferroelectric films are used as capacitive insulation films, to enable materialization of large-capacitance highly integrated RAMs (random access memories).
The ferroelectric films are comprised of a sintered body of a metal oxide, and contain much oxygen which is rich in reactivity. When capacitors are formed by using such ferroelectric films in the capacitive insulation films, it is indispensable, in the upper and lower electrodes of the capacitive insulation films, to use a substance which is stable to oxidation reaction, as exemplified by an alloy chiefly composed of platinum.
After capacitors, interlayer insulation films and so forth have been formed, passivation films are formed on the outermost surfaces of the devices. Silicon nitride or silicon oxide is used in the interlayer insulation films and passivation films, which are usually formed by CVD (chemical vapor deposition) and hence hydrogen is often incorporated in the films.
When semiconductor apparatuses making use of such ferroelectric films are used in electronic equipment for public use, they are required to be inexpensive resin-encapsulated semiconductor apparatuses having good mass productivity. In particular, ferroelectric non-volatile memories are greatly needed for portable equipment as memories substituting flash memories, because of their properties such as low power, low voltage, and non-volatility making refresh operation unnecessary, and the resin-encapsulated semiconductor apparatuses are also desired in order to provide thin type packages.
At present, however, devices that utilize the ferroelectric films as capacitive insulation films are chiefly held by ceramic-encapsulated products, and almost no resin-encapsulated products are available. Devices with a large capacity are also not yet developed. This is because the polarization characteristics of ferroelectric films deteriorate as a result of heat treatment.
Capacitors having ferroelectric films are known to undergo deterioration of polarization characteristics upon their annealing in an atmosphere of hydrogen (Lecture Collections in '96 Ferroelectric Film Memory Technique Forum, published by K.K. Science Forum, Inc. Page 4-4). This deterioration is presumed to be caused by the platinum of upper and lower electrodes which reacts with hydrogen to act as a reducing catalyst to reduce the ferroelectric film. In particular, in the case of large-capacity highly integrated devices, the ferroelectric films are fine in size, and hence this deterioration of the characteristics of the capacitors is forecasted to greatly affect the characteristics of the overall devices.
In the resin-encapsulating of semiconductor devices by transfer molding, encapsulant resins containing fillers (usually silica) are used. The fillers contained in encapsulant resins, however, have such hard particles that the fillers may damage the device surfaces when encapsulated. Moreover, since ferroelectric materials exhibit piezoelectricity, the characteristics of ferroelectric films ma y change upon application of a pressure to the ferroelectric film inside the devices when encapsulated. In the fabrication of DRAMs (dynamic random access memories), &agr;-rays are emitted from radioactive components contained in the fillers, to cause memory soft errors in some cases. Accordingly, in order to prevent the device surfaces from being damaged by the fillers, to prevent application of pressure to the ferroelectric films and to screen &agr;-rays being emitted from the fillers, protective films comprised of polyimide must be previously formed on the device surfaces. Such surface-protective polyimide films are formed by heat-curing polyimide precursor composition films usually at a temperature of about 350 to 450° C. When such a polyimide precursor is heat-cured, the hydrogen contained in the passivation films or interlayer insulation films may diffuse to cause a deterioration of polarization characteristics of the ferroelectric films. Thus, no resin-encapsulated products of devices in which thermoplastic resins are used as surface-protective films are known at present.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a resin-encapsulated semiconductor apparatus having a ferroelectric film with good polarization characteristics and having a high reliability, and a process for its fabrication.
Studies made on conditions under which ferroelectric films cause the deterioration of polarization characteristics have revealed that the deterioration occurs when heated at above 300° C. The present inventors thought that the surface-protective polyimide films could be heat-cured at below 300° C. However, when conventional polyimide precursors are used, which cure at such a low temperature, the resultant resin-encapsulated semiconductor apparatuses had a problem in their solder reflow resistance.
At present, as methods for packaging resin-encapsulated semiconductor apparatuses on printed-wiring substrates, face-down mounting is prevalent. The face-down mounting employs a solder reflowing method, in which leads of a semiconductor device and wiring of a printed-wiring substrate are provisionally joined with a cream solder followed by heating of the entire semiconductor device and substrate to solder them. As methods sorted according to how heat is applied, infrared reflowing and vapor phase reflowing are known, the former being a method utilizing infrared radiated heat and the latter being a method utilizing condensation heat of fluorinated inert liquid.
As encapsulant resin, epoxy resin is usually used. This epoxy resin always absorbs moisture in an ordinary environment. At the time of solder reflow soldering, resin-encapsulated semiconductor apparatuses are exposed to high temperatures of from 215 to 260° C. Hence, when the resin-encapsulated semiconductor apparatuses are packaged on the substrate by reflow soldering, the abrupt evaporation of water causes cracks in the encapsulant resin to bring about a serious problem in view of the reliability of semiconductor devices. Accordingly, in the past, various improvements have been made from the viewpoint of making the encapsulant resin have a lower moisture absorption and have a high adhesion performance (Thermosetting Resins, Vol. 13, No. 4, published 1992, page 37, right column, lines 8-23).
The present inventors have examined resin cracks produced in conventional resin-encapsulated semiconductor devices, and have found that peeling occurs at the interface between the device surface-protective polyimide film and the encapsulant resin, and that this is the starting point of causing cracks in the encapsulant resin. They have also found that this peeling is influenced by physical properties of surface-protective films, in particular, glass transition temperature and Young's modulus.
Now, as a result of further detailed studies, it has been found that ferroelectric films may cause less deterioration of polarization characteristics when the dev
Isoda Keiko
Ogata Kiyoshi
Tanaka Jun
Clark Sheila V.
Mattingly Stanger & Malur, P.C.
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