Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
1999-06-22
2001-06-05
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S096000
Reexamination Certificate
active
06242278
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing semiconductor elements and particularly for forming a polycrystal silicon film on the capacitor electrode surface.
2. Description of the Related Art
Due to the needs for more highly integrated semiconductor devices, further reduction in the cell size is being sought. Particularly in the field of Dynamic Random Access Memory (DRAM) for which one bit is composed of one transistor and one capacitor, if the cell size is reduced, the electrode area of the capacitor is decreased, hence the capacity value is decreased. As a result, problems such as lowered data hold time and incapability in preventing memory loss caused by an alpha ray will occur.
One method to solve this problem is to use a capacitor with a three-dimensional cylinder structure or a fin structure. However, this method has technical limitations.
As other methods, there is a method to increase the capacity value by using tantal oxide (Ta
2
O
5
) with a high induction rate or barium strontium titanate (Ba
(x)
Sr
(1-x)
TiO
3
) with a strong induction film. However, this method has not been made fit for practical use.
As another notable method, there is a method called the HSG process that increases the capacity value by making the capacitor surface uneven in order to increase the surface area.
FIG. 1
roughly shows how work progresses in the HSG process. As shown in
FIG. 1
(a), the amorphous silicon film (
1
) that is the capacitor understructure electrode is formed on the intercalation layer (
3
) formed on the silicon substrate (
8
). The semiconductor substrate (
8
) and the amorphous silicon film (
1
) are linked by polycrystal silicon (
9
). Also, naturally formed oxide film (
2
) adheres to the amorphous silicon film (
1
). After the naturally formed oxide film (
2
) is washed off during pre-processing, the clean surface of the amorphous silicon film (
1
) is exposed. At this point, hydrogen atoms (
5
) are bonded to the dangling bonds on the surface of the amorphous silicon film (
1
) (
FIG. 1
(b)). This hydrogen (
5
) is desorbed by being heated at a processing temperature of approximately 560° C. and the surface of the amorphous silicon film (
1
) becomes activated (
FIG. 1
(c)). In an atmosphere of monosilane (SiH4) gas, the mixed-phase active layer of amorphous-polycrystal silicon (
6
) is then selectively formed on the activated surface area by surface reaction (
FIG. 1
(d)). At this point, if it is annealed at a temperature of approximately 560° C. for a predetermined time period, the amorphous in the mixed-phase active layer migrate with polycrystal silicon on the surface as a nucleus, crystallizes into polycrystal silicon and the polycrystal silicon grain (
7
) grows. As a result, highly crystalline silicon grains (HSG) (
7
) are formed on the amorphous silicon electrode (
FIG. 1
(e)).
SUMMARY OF THE INVENTION
The present invention has exploited formation of a rough polysilicon film based on selective migration of amorphous silicon out of an amorphous silicon-polysilicon mixed-phase layer. The present inventors have identified problems and resolve the same as follows:
If the HSG method is used to form an amorphous silicon electrode having a rough surface, the following problems occur.
The surface of the amorphous silicon electrode is not completely non-crystalline but partially crystallized in many cases. This problem involves the conditions to form an underlying layer of the intercalation layer, the concentration of dopant phosphorus, the thermal hysteresis prior to HSG formation (e.g., the temperature of the heat-up step), and so forth. Among others, the effect of thermal hysteresis prior to HSG formation is very high.
FIG. 2
is a schematic view showing HSG processes when partial crystallization occurs in the amorphous silicon film. As shown in FIG.
2
(c), an area near the surface of the amorphous silicon where dangling bonds are present is subject to crystallization. As a result, as shown in FIG.
2
(d), ultimately, migration does not occur in the area of the amorphous silicon, i.e., the area remains flat without forming a rough surface. This event is called “bold defect (
21
)”. Because of this bold defect (
21
), the surface of the amorphous silicon electrode is not rough, decreasing the advantageous effects extending the surface area.
In order to prevent such partial crystallization, the phosphorus dopant concentration may be lowered. However, when the phosphorus concentration on the surface decreases and if capacity value is measured by changing voltage, a decrease in the capacitance occurs on the negative voltage side. On the other hand, if the process temperature is decreased to lower thermal hysteresis, reaction speed is decreased, resulting in not only failure to reduce the thermal hysteresis but also decreasing the throughput. Further, employing a device for processing plural pieces may reduce thermal hysteresis. However, reproducibility is not satisfactory due to unstable temperature control under reduced pressure. In addition, during the heating step, hydrogen atoms may be desorbed, and migration is likely to occur where the activated surface is exposed, resulting in partial crystallization.
Consequently, an object of an embodiment of the present invention is to provide with a method to form an amorphous silicon electrode film having a rough surface, without bold defects, made of a polysilicon film (polysilicon grains) by migration of amorphous silicon from an amorphous silicon-polysilicon mixed-phase layer.
Another object of an embodiment of the present invention is to form an amorphous silicon electrode film with a rough surface generated by migration, which can be mass-produced and which excels in stability and reproducibility.
The present invention includes an aspect to provide a method for forming a rough surface made of polysilicon grains on an amorphous silicon film disposed on a semiconductor substrate, comprising the steps of: (i) forming an amorphous silicon-polysilicon mixed-phase layer having a first density on an activated surface of the amorphous silicon film by contacting the surface with a gas containing monosilane at a first flow rate of monosilane and at a first temperature; and (ii) annealing the amorphous silicon-polysilicon mixed-phase layer to form polysilicon grains therefrom, thereby forming a rough surface made of polysilicon grains, wherein the improvement comprises using disilane in place of monosilane at a second flow rate lower than the first flow rate and at a second temperature lower than the first temperature to form an amorphous silicon-polysilicon mixed-phase layer having a second density higher than the first density. According to this aspect, bold defects can be eliminated during formation of the amorphous silicon-polysilicon mixed-phase layer.
In the above, in preferable embodiments, the second flow rate may be 0.01 sccm to 2 sccm, and the second temperature may be approximately 20° C. lower than the first temperature. Further, the improvement can further comprise activating the surface of the amorphous silicon film at a temperature lower than the temperature when using monosilane.
Another aspect of the present invention is to provide a method for forming a rough surface made of a polycrystal silicon film on an amorphous silicon film disposed on a semiconductor substrate, comprising the steps of: (a) activating dangling bonds present on a surface of an amorphous silicon film in a reactor while introducing thereinto hydrogen gas; (b) forming an amorphous silicon-polysilicon mixed-phase layer on the surface of the amorphous silicon film by contacting the dangling bonds with a gas containing silane gas; and (c) annealing the amorphous silicon-polysilicon mixed-phase layer to form a rough surface made of doped polysilicon film. According to this aspect, bold defects can be eliminated during activation of dangling bonds.
In the above, in preferable embodiments, the silane can be monosilane or disilane, the flow rate of hydrogen gas can be s
Fukazawa Atsuki
Shimizu Akira
Takahashi Satoshi
ASM Japan K.K.
Knobbe Marten Olson & Bear LLP
Nelms David
Nhu David
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