Capacitor and method for forming the same

Details Capacitor and method for forming the same Capacitor and method for forming the same

2001-02-16

2002-10-08

Whitehead, Jr., Carl (Department: 2822)

Semiconductor device manufacturing: process

Making passive device

Stacked capacitor

C438S241000, C438S253000

Reexamination Certificate

active

06461930

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to a capacitor for a semiconductor device and a method for forming a capacitor for a semiconductor device. More particularly, the present invention is directed to a capacitor for a semiconductor device having a nodular shaped electrode and a method for forming such an electrode.
BACKGROUND OF THE INVENTION
Advancements in semiconductor manufacture have led to increases in the density and miniaturization of microelectronic circuits. As an example, the manufacture of 1 Gb DRAMs is now possible and 4 Gb prototypes are currently being developed. A key requirement for achieving such high device packing density is the formation of suitable storage capacitors.
With increased packing density of memory cells, however, the area available for storage capacitors (i.e. storage nodes) has decreased. This has necessitated the development of storage capacitors having an increased capacitance. In general, storage capacitors can be formed as stacked structures or as trench structures. The present invention is directed in part to stacked structures and in another part to a combination stacked-trench structure.
Typically, a thin film stacked storage capacitor includes a lower electrode, an upper electrode, and a dielectric layer which is sandwiched between the electrodes. This capacitor structure is stacked on an insulating layer of a substrate. The insulating layer is typically formed from materials such as SiO
2
and Si
3
N
4
that are compatible with a silicon process. The lower electrode of the capacitor is connected to a field effect transistor (FET) formed on the substrate. A polycrystalline silicon layer has been used as the lower electrode of a capacitor. Such a polysilicon layer is sometimes referred to as a polysilicon or silicon electrode.
One way to increase the capacity of this type of capacitor is to use a dielectric layer formed with a high dielectric constant material. These high dielectric constant materials include inorganic non-metallic oxides in the paraelectric or ferro electric phase such as BaSrTiO
3
(BST), BaTiO
3
, SrTiO
3
l PbZrO
3
and others. Such high dielectric constant materials have a dielectric constant greater than 100. This is an order of magnitude larger than traditional dielectric materials, such as SiO
2
and Si
3
N
4
, which have dielectric constants less than 10.
A problem with high capacitance capacitors is that generally high dielectric constant films cannot be formed directly over a polysilicon electrode. This is because an interface layer of silicon dioxide forms between the dielectric film and the polysilicon electrode. Such an interface layer reduces the effective dielectric constant of the dielectric material and defeats its purpose. For this reason, the lower electrode structure is typically formed as a stack comprising a barrier layer formed on the polysilicon electrode and a lower electrode formed on the barrier layer.
The barrier layer is typically formed from a conductive material, such as tantalum (Ta), titanium nitride (TiN), or tungsten nitride (WN). Such a barrier layer, in addition to preventing oxidation of the polysilicon electrode, also functions to prevent silicon diffusion into the lower electrode. Such silicon diffusion increases the resistivity of the lower electrode and could lead to the formation of a thin layer of SiO
2
layer on top of the lower electrode.
Another problem associated with the use of high dielectric constant films is that these films must be deposited at relatively high temperatures (e.g. 600° C. to 700° C.). Because of the high process temperatures that are required, the lower electrode of such a capacitor is typically formed of a high melting point, non-oxidizing metal such as platinum, palladium or rhodium or of a conducting oxide such as ruthenium oxide, iridium oxide, osmium oxide or rhodium oxide. A non-oxidizing material is required for the lower electrode because a traditional electrode material such as aluminum, titanium, nichrome or copper will oxidize at the high temperatures, increasing the resistivity of the electrode.
A prior art stacked capacitor
10
employing a high dielectric constant dielectric film
26
is shown in FIG.
1
. In
FIG. 1
, a semiconductor substrate
12
includes a FET (not shown) formed with a pair of insulated gate electrodes
14
and
16
. An insulating layer
18
is formed over the FET and gate electrodes
14
and
16
. The capacitor
10
is stacked on the insulating layer
18
. A polysilicon plug
20
is formed in a contact hole formed through the insulating layer
18
to the source or drain region
30
of the FET.
The capacitor
10
includes a lower electrode
22
having a rectangular cross section, an upper electrode
24
and a dielectric film
26
formed between the lower electrode
22
and the upper electrode
24
. The capacitor
10
also includes a barrier layer
28
formed between the lower electrode
22
and the polysilicon plug
20
.
Such a capacitor is subject to several limitations. First, the dielectric layer
26
must be formed over the stepped surface contour provided by the stack formed by the lower electrode
22
and the barrier layer
28
. Poor step coverage of the dielectric material
28
over the lower electrode
22
promotes charge leakage at the corners of the dielectric material
26
in the completed capacitor structure. To prevent this leakage, an insulating material such as silicon dioxide is sometimes deposited over the outside corners of the dielectric film
26
.
Second, the sidewalls
34
and
36
of the barrier layer
28
are exposed to oxidation during deposition of the dielectric film
26
. Accordingly, the high temperatures encountered during the dielectric deposition process will cause the sidewalls of the barrier layer
28
to oxidize. Such an oxide increases the contact resistance of the barrier layer
28
. Further, with an oxide formed on the sidewalls
34
and
36
of the barrier layer
28
, the lower electrode
22
will not adhere as well to the barrier layer
28
which results in the lower electrode
22
separating from the barrier layer
28
.
Third, if the barrier layer
28
does not completely overlap the polysilicon plug
20
, then the surface of the polysilicon plug will oxidize during the deposition of the dielectric material
26
. A critical alignment of the barrier layer
28
and the polysilicon plug
20
is required.
A solution to these problems is provided by U.S. Pat. No. 5,335,138 to Sandhu et al. This patent teaches the use of spacers positioned at each end of the lower electrode-barrier layer stack to prevent oxidation of the sidewalls of the barrier layer during deposition of the dielectric film. These spacers also provide a smooth topography for the deposition of the dielectric film. This provides the completed capacitor with a smooth topography which prevents the current leakage which occurs through sharp corners of the dielectric layer. Finally, the spacers provide a larger area for protection of the polysilicon plug from oxidation. However, the process described by Sandhu et al. requires additional processing steps and time to form the spacers.
For these reasons, a need exists in the art for a capacitor which is not subject to the limitations of the prior art. Particularly, a need exists in the art for a capacitor and a method for making a capacitor which has step coverage which eliminates charge leakage. A need also exists for a capacitor and a method for making a capacitor which prevents oxidation of the sidewalls of the lower electrode and the barrier layer. Another need exists for a capacitor and a method for making a capacitor which provide for proper placement of the lower electrode and barrier layer over the polysilicon plug. Finally, a need exists in the art for a method of making such a capacitor which will quickly and efficiently address the current needs in the art.
SUMMARY OF THE INVENTION
The present invention provides a high dielectric constant capacitor and a method for manufacturing such a capacitor. The present invention provides a capacitor with m

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