Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-10-03
2004-05-04
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S301000, C257S303000, C257S306000, C257S310000
Reexamination Certificate
active
06730948
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device including a ferroelectric film or a dielectric film with a high dielectric constant (which will be herein referred to as a “high-dielectric-constant film”) and to a method for fabricating the device.
Recently, nonvolatile or large-capacity semiconductor memories have been developed using a ferroelectric material or a dielectric material with a high dielectric constant. Each of these dielectric materials is made by sintering a metal oxide and contains a lot of easily reactive oxygen. Accordingly, when a capacitor, including a capacitive insulating film of such a dielectric material, is formed, the upper and lower electrodes of the capacitor, located over and under the capacitive insulating film, respectively, must be made of some material showing sufficient stability against the oxidation. Examples of the applicable materials include an alloy mainly composed of platinum.
A known semiconductor device includes a passivation film on the uppermost surface thereof. The passivation film is deposited over the structure already including a capacitor and an interlevel dielectric film, and is typically made of silicon nitride or silicon dioxide. Normally, the passivation film is formed by a CVD (chemical vapor deposition) process, and often contains hydrogen or moisture therein.
Also, when a semiconductor device and its associated members are molded together with a resin encapsulant by a transfer molding process, for example, the resin encapsulant used for the process often contains some filler (which is usually silica). However, the particles of the filler have a high hardness, thus possibly doing serious damage on the surface of the device during the resin molding process. In addition, in fabricating a DRAM (dynamic random access memory), an &agr;-ray is emitted from the radioactive components of the filler and sometimes causes soft errors in the memory.
Therefore, to prevent the surface of a semiconductor device from being damaged by the filler particles or to shield the device from the &agr;-rays emitted therefrom, the surface is often covered with a coating of some organic material (e.g., polyimide). Also, the surface of a device is sometimes given double protection. Specifically, a passivation film of an inorganic insulator is deposited over the surface first, and then a surface coating of polyimide is formed on the passivation film. The polyimide surface coating is normally formed by heating and curing a film of a polyimide precursor composition at a temperature of about 350-450° C.
Accordingly, a semiconductor device including a ferroelectric or high-dielectric-constant film also needs to have its surface covered with a polyimide coating because of similar reasons. In the current state of the art, however, where a polyimide coating is formed on the surface of a semiconductor device including a capacitive insulating film made of a ferroelectric material, the polarization properties of the ferroelectric film should degrade while the polyimide is heated to form the coating. Therefore, the polyimide coating is hard to apply to the actual fabrication process of such a device. This is because while the polyimide precursor is being heated and cured, hydrogen or moisture, contained in the passivation or interlevel dielectric film of the device, adversely diffuses into the ferroelectric film due to the heat, thus degrading the polarization properties of the ferroelectric film.
The degradation is believed to occur through one of the following mechanisms. One possibility is that platinum, contained in the upper and lower electrodes, may react with hydrogen and act as a catalyst that reduces the material of the ferroelectric film (i.e., an oxide). Another possibility is that the moisture reacts with the material of metal interconnects made of aluminum, for example, to produce hydrogen and thereby degrade the polarization properties of the capacitor (see The Institute of Electronics, Information and Communication Engineers Transactions, C Vol. J83-G No. 1, pp.53-59).
To solve this problem, a countermeasure process was proposed in Japanese Laid-Open Publication No. 10-270611, for example. In the proposed process, a polyimide film is formed as a surface coating for a semiconductor device including a ferroelectric film by heating and curing a film of a polyimide precursor composition at a temperature of 230-300° C. According to this method, the polarization properties of the ferroelectric film do not degrade so much. It should be noted that the same problem might occur in the high-dielectric-constant film as well as in the ferroelectric film.
Hereinafter, a known semiconductor device and a method for fabricating the device will be described with reference to
FIGS. 6 through 7C
. As an exemplary known semiconductor device,
FIG. 6
schematically illustrates a cross-sectional structure for one of the one-transistor, one-capacitor memory cells of a ferroelectric memory.
The semiconductor device shown in
FIG. 6
includes an MOS transistor
2
and a ferroelectric capacitor
3
that have been formed over a substrate
1
. A surface coating
62
of polyimide has been formed to cover an interconnection layer
5
and a second insulating film
42
that are located over the transistor
2
and capacitor
3
.
The MOS transistor
2
shown in
FIG. 6
is made up of known components including source/drain regions and a polysilicon gate. In the illustrated example, the MOS transistor
2
includes gate electrode
21
of polysilicon, gate oxide film
22
, sidewall
23
, silicon nitride film
24
, source/drain regions (doped regions)
25
and LOCOS
26
.
The ferroelectric capacitor
3
is made up of lower electrode
32
, upper electrode
34
and ferroelectric film
33
interposed between these electrodes
32
and
34
. If necessary, an electrode contact layer
31
is additionally formed under the lower electrode
32
. The ferroelectric film
33
may be made of any arbitrary material such as lead zirconate titanate (Pb(Zr,Ti)O
3
(PZT)) or strontium bismuth tantalate (SrBi
2
Ta
2
O
9
(SBT)).
A first insulating film
41
may be a silicon dioxide film or a silicon nitride film. In the former case, the first insulating film
41
may be a BPSG (borophosphosilicate glass), PSG (phosphosilicate glass) or O
3
-TEOS (tetraethylortho silicate) film, for example. The second insulating film
42
on the first insulating film
41
may be a silicon dioxide film formed by an APCVD (atmospheric-pressure chemical vapor deposition) process, for example. An interconnection layer
5
has been formed on the second insulating layer
42
and electrically connected to the MOS transistor
2
and ferroelectric capacitor
3
.
Hereinafter, a method for fabricating the semiconductor device shown in
FIG. 6
will be described with reference to
FIGS. 7A through 7C
.
FIGS. 7A through 7C
are cross-sectional views illustrating respective process steps for fabricating the known semiconductor device.
First, a semiconductor substrate
1
(which is preferably a wafer in the actual fabrication process) is prepared as shown in FIG.
7
A. Next, MOS transistor
2
, ferroelectric capacitor
3
and so on are formed by a known process on each active region, and then an interconnection layer
5
is formed thereon as shown in FIG.
7
B.
Next, as shown in
FIG. 7C
, a surface coating
62
of polyimide, having a plurality of openings (not shown) over bonding pad regions, is formed to cover the substrate
1
that already includes the transistor
2
, capacitor
3
and interconnection layer
5
thereon.
The surface coating
62
may be formed as follows. First, a photosensitive polyimide material, containing a polyimide precursor composition that will cure when heated to a temperature of 230-300° C., is applied onto the surface of the substrate
1
that already includes the transistor
2
, capacitor
3
and interconnection layer
5
thereon. Next, the film of the polyimide precursor composition is exposed to a radiation while being masked with a predetermined pattern. Subsequentl
Matsunaga Keiichi
Umeda Kazuo
Fenty Jesse A.
Jackson Jerome
Matsushita Electric - Industrial Co., Ltd.
Nixon & Peabody LLP
Studebaker Donald R.
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