Semiconductor device

Details Semiconductor device Semiconductor device

2003-01-24

2004-05-25

Nelms, David (Department: 2818)

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

Field effect device

Having insulated electrode

C257S017000, C257S024000, C257S261000, C257S321000, C365S185300, C365S185330

Reexamination Certificate

active

06740928

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device including microparticles and the like for holding charge to permit use of the semiconductor device as a memory.
Existing ultralarge-scale integrated circuits (ULSI) have a memory section where a number of memory devices each composed of a MOS transistor are integrated. In recent years, demands have increased on the memory devices for higher operation speed, reduced power consumption, and prolonged recording retention. To respond to these demands, development of MOS transistors satisfying these demands is in progress.
Memory devices proposed and trial-produced so far employ a technique that quite a small amount of charge is injected into a microparticle of a semiconductor or the like during write or erase operation of the memory and the charge is held therein. An example of this conventional technique is a study by S. Tiwari et al. on a memory using a plurality of silicon microparticles (dots) (Appl. Phys. Lett. 68 (1996) 1377).
FIG. 57
is a cross-sectional view of the conventional semiconductor memory device functioning as a memory using a plurality of silicon microparticles. The semiconductor memory device includes a tunnel oxide film
6202
made of SiO
2
and a SiO
2
film
6204
deposited in this order on a p-type silicon substrate
6201
. An n-type polysilicon electrode
6205
is formed on the SiO
2
film
6204
. Silicon microparticles
6203
are buried between the tunnel oxide film
6202
and the SiO
2
film
6204
. Source/drain regions
6206
are formed in portions of the p-type silicon substrate
6201
located below both sides of the n-type polysilicon electrode
6205
.
In the above conventional semiconductor memory device, by applying a positive voltage to the n-type polysilicon electrode
6205
, electrons can be injected into the silicon microparticles
6203
via the tunnel oxide film
6202
. By applying a negative voltage to the n-type polysilicon electrode
6205
, electrons in the silicon microparticles
6203
can be released. This enables the threshold voltage of the device to change depending on whether or not electrons exist in the silicon microparticles
6203
. The levels (high or low) of the threshold voltage are made associated with information H (high) and information L (low), respectively. In this way, information writing/reading is realized.
The thickness of the tunnel oxide film
6202
is extremely small (about 1.5 nm to 4 nm). Therefore, the above electron injection process is by direct tunneling, not Fowler-Nordheim (FN) tunneling.
According to a study by the inventors of the present invention, a highly sophisticated and fine fabrication technique is required to attempt to implement the above proposed semiconductor device with practical performance.
For example, if the tunnel oxide film
6202
is too thick, charge injection by tunneling is difficult, resulting in difficulty in obtaining low-voltage operation and high-speed operation. On the contrary, if the tunnel oxide film
6202
is too thin, charge confinement is insufficient during charge holding, resulting in difficulty in long-term charge holding, that is, long-term information recording.
In addition, in order to provide practical characteristics for the conventional semiconductor device, required is a fabrication technique allowing for high-level control of the diameter of the silicon microparticles
6203
as well as the dispersion thereof. If the diameter of the silicon microparticles
6203
is too small or large to provide a sufficient surface density of the silicon microparticles
6203
, the charge holding duration is too short or the allowable amount of charge held is too small, resulting in reduction of the reliability of the semiconductor device.
Moreover, in the case of an increase of thermal energy due to temperature rise, for example, charge accumulated in the silicon microparticles
6203
is released spontaneously due to tunneling from the silicon microparticles
6203
to the p-type silicon substrate
6201
.
In view of the above, in order to provide practical device characteristics for the above conventional semiconductor device, it is necessary to control the quality and thickness of the tunnel oxide film
6202
uniformly with markedly high precision. It is also necessary to provide the silicon microparticles
6203
at a high surface density in a uniform dispersion state while ensuring a uniform diameter of the silicon microparticles
6203
. A highly sophisticated fabrication technique is required to realize the above control over the entire surface of the p-type silicon substrate. In consideration of the above, if the conventional semiconductor device is actually fabricated in the above fabrication process, the possibility that the resultant semiconductor device has practical characteristics is small. Further, the fabricated conventional semiconductor device will be poor in reliability. In short, according to the study by the inventors of the present invention, it is difficult for the conventional semiconductor device to realize high-speed charge injection and release and long-term charge holding.
SUMMARY OF THE INVENTION
An object of the present invention is providing a semiconductor device with high reliability that can be easily fabricated, and a method for fabricating such a semiconductor device.
The first semiconductor device according to the present invention include: a substrate having a conductive layer; and a charge holding region including a barrier layer formed on the conductive layer for functioning as a barrier against charge transfer and a plurality of particles dispersed in the barrier layer so that the particles have different distances from the conductive layer from each other. The capacitance of the particles is larger as the distance from the conductive layer is smaller.
As the capacitance is larger, potential rise during charge holding is smaller and thus charge transfer is easier. Therefore, it becomes easy for the particles located farther from the conductive layer to hold charge, or to release charge upon application of a voltage. This charge holding state can be used as information.
In the first semiconductor device, the capacitance is larger and charge transfer is easier as the particles are dispersed at a higher dispersion density or as the mean diameter of the particles is larger. Therefore, by varying these factors depending on the distance from the conductive layer, spontaneous release of accumulated charge can be effectively suppressed, resulting in prolonged charge holding in the charge holding region. This increases the reliability.
In the first semiconductor device, the particles are quantized. This facilitates control of charge injection and release with a voltage.
Preferably, the plurality of particles are divided into a plurality of particle groups each composed of a plurality of particles common in the distance from the conductive layer.
The semiconductor device further includes: an insulating layer formed on the barrier layer; a gate electrode formed on the insulating layer; and source/drain regions formed by introducing an impurity into regions of the substrate located below both sides of the gate electrode. The resultant semiconductor device functions as a MIS transistor.
The second semiconductor device according to the present invention includes: a substrate having a conductive layer; and a charge holding region including a barrier layer formed on the conductive layer for functioning as a barrier against charge transfer and a plurality of particles dispersed in the barrier layer so that the particles have different distances from the conductive layer from each other. The barrier height of inter-particle portions of the charge holding region is smaller as the distance from the conductive layer is smaller.
As the barrier height is smaller, charge transfer is easier. Therefore, it becomes easy for the particles located farther from the conductive layer to hold charge, or to release charge upon application of a voltage. This charge holding state can be used as info

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