This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-081910, filed on Mar. 27, 2007; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a semiconductor device and a method for manufacturing a semiconductor device, and more particularly to a semiconductor device such as a semiconductor memory with its control gate electrode made of polycrystalline silicon and a method for manufacturing the same.
2. Background Art
A nonvolatile semiconductor memory device includes a floating gate electrode between a control gate electrode and a semiconductor substrate, and information is stored in the floating gate electrode by the control gate electrode. The floating gate electrode is opposed to the control gate electrode across an interlayer insulating film, and a silicon thermal oxide film is provided between the floating gate electrode and the semiconductor substrate. The control gate electrode and the floating gate electrode are made of doped polycrystalline silicon. The doped polycrystalline silicon is formed, for example, by forming a nondoped silicon film, which is subjected to doping followed by heat treatment (see, e.g., JP-A 2003-077856 (Kokai)).
With the downsizing of such a semiconductor memory device, the dimensions of the control gate electrode and the floating gate electrode are narrowed, and accordingly the distance between the control gate electrodes and the distance between the floating gate electrodes also decrease.
With such downsizing, the effect of depletion in polycrystalline silicon becomes significant, and electrical interference between adjacent floating gate electrodes increases, causing problems such as changes or variations of threshold voltage or other operating voltage (see, e.g., IEEE ELECTRON DEVICE LETTERS, VOL. 23, no. 5, May 2002, “Effects of Floating-Gate Interference on NAND Flash Memory Cell Operation”).
One of the causes of this depletion is that the decrease of dopant activation ratio (average ratio of the activated dopant concentration to the total dopant concentration) associated with the downsizing results in decrease of carriers in polycrystalline silicon. This number of carriers needs to be increased.
According to an aspect of the invention, there is provided a semiconductor device including: a control gate electrode having a first layer of polycrystalline silicon, the first layer being formed by decreasing a thickness of a first film of doped polycrystalline silicon and retaining a dopant activation ratio of the first film.
According to another aspect of the invention, there is provided a semiconductor device including: a semiconductor substrate; an insulating film provided on the semiconductor substrate; a first layer of polycrystalline silicon provided on the insulating film; a device separation insulating film provided in a device separation trench which penetrates the insulating film and the first layer and reaches the semiconductor substrate; an interlayer insulating film provided on the first layer and the device separation insulating film; and a second layer of polycrystalline silicon provided on the interlayer insulating film, at least one of the first and second layers being formed by decreasing a thickness of a film of doped polycrystalline silicon and retaining a dopant activation ratio of the film.
According to another aspect of the invention, there is provided a method for manufacturing a semiconductor device, including: forming a first film of doped polycrystalline silicon by heat treating an amorphous silicon film provided on an insulating film; and decreasing a thickness of the first film by etching the first film.
An embodiment of the invention will now be described with reference to the drawings. Polycrystalline silicon constituting the control gate electrode and the floating gate electrode of a semiconductor flash memory is illustratively doped with phosphorus (P) and activated by heat treatment to generate carriers.
As shown in
It turns out from
In the embodiment of the invention, a polycrystalline silicon film containing phosphorus (P) or other dopant is formed, and etched back to decrease the film thickness. Thus the crystal grain size and the activation ratio at the time of film formation (before etch back) is retained in the polycrystalline silicon film after etch back.
From
The activation ratio in polycrystalline silicon constituting the control gate electrode and the floating gate electrode of a semiconductor flash memory can be estimated by measuring electrical characteristics such as writing and reading characteristics of the device. For preventing depletion in polycrystalline silicon, the activation ratio is preferably 20% or more.
First, as shown in
Next, a floating gate electrode is formed. As shown in
It is assumed herein that amorphous silicon includes completely amorphous silicon as well as microcrystalline silicon.
Next, on the surface of the amorphous silicon film 40, a cover insulating film, not shown, is formed, followed by heat treatment in a nitrogen atmosphere at 600° C. or more, for example. Then the cover insulating film is entirely peeled off by etching. This heat treatment causes solid-phase diffusion of phosphorus from the phosphorus-containing amorphous silicon film 40 of the second layer into the amorphous silicon film 30 of the first layer, and activates the dopant, phosphorus. Simultaneously, the amorphous silicon is polycrystallized. Thus, as shown in
This polycrystalline silicon film 200 has a thickness of T2, which is larger than the final target thickness T1. Hence, as described above with reference to
Next, as shown in
Next, as shown in
Thus the thickness T1 of the polycrystalline silicon film 100 serving as a floating gate electrode 100a is related to the thickness T2 of the polycrystalline silicon film 200 before etch back (at the time of film formation) as:
T1<T2=T3+T4
Hence, despite its thickness T1 smaller than T2, the floating gate electrode 100a retains the grain size of silicon crystal and the activation ratio of phosphorus in the polycrystalline silicon film 200 with thickness T2. That is, it is possible to obtain a floating gate electrode 100a (polycrystalline silicon film 100) having a larger grain size of silicon crystal and a higher activation ratio of phosphorus than those obtained by formation of the polycrystalline silicon film with thickness T1. Thus, even if the polycrystalline silicon film serving as a floating gate electrode is thinned with the downsizing of the device, depletion in polycrystalline silicon can be prevented.
Next, as shown in
Next, as shown in
The interlayer insulating film 60 is illustratively made of a laminated film of silicon oxide film/silicon nitride film/silicon oxide film.
Next, a control gate electrode is formed. As shown in
The nondoped amorphous silicon film 70 is formed so that its thickness T7 is half or more the distance between adjacent floating gate electrodes, t1. Nondoped amorphous silicon has better step coverage than doped amorphous silicon and serves to prevent voids from occurring at the step between the floating gate electrode 100a and the device separation insulating film 50. Presumably, with the downsizing of the device, the ratio of the step dimension t2 to the distance between adjacent floating gate electrodes, t1, further increases. Hence occurrence of voids can be effectively prevented by forming a nondoped amorphous silicon film in the underlying layer.
Next, on the surface of the amorphous silicon film 80, a cover insulating film, not shown, is formed, followed by heat treatment in a nitrogen atmosphere at 600° C. or more, for example. Then the cover insulating film is entirely peeled off by etching. This heat treatment causes solid-phase diffusion of phosphorus from the phosphorus-containing amorphous silicon film 80 of the second layer into the amorphous silicon film 70 of the first layer, and activates the dopant, phosphorus. Simultaneously, the amorphous silicon is polycrystallized. Thus, as shown in
This polycrystalline silicon film 600 has a thickness of T6, which is larger than the final target thickness T5. Hence, as described above with reference to
Next, as shown in
Then the polycrystalline silicon film 500 is patterned by lithography and etching processes. Thus a plurality of control gate electrodes 500a made of the polycrystalline silicon film 500 are formed. Here, the control gate electrode 500a may be configured as a polycide structure by forming a silicide film on the polycrystalline silicon film 500 and patterning this silicide film and the polycrystalline silicon film 500.
Thus the thickness T5 of the polycrystalline silicon film 500 serving as a control gate electrode 500a is related to the thickness T6 of the polycrystalline silicon film 600 before etch back (at the time of film formation) as:
T5<T6=T7+T8
Hence, despite its thickness T5 smaller than T6, the control gate electrode 500a retains the grain size of silicon crystal and the activation ratio of phosphorus in the polycrystalline silicon film 600 with thickness T6. That is, it is possible to obtain a control gate electrode 500a (polycrystalline silicon film 500) having a larger grain size of silicon crystal and a higher activation ratio of phosphorus than those obtained by formation of the polycrystalline silicon film with thickness T5. Thus, even if the polycrystalline silicon film serving as a control gate electrode is thinned with the downsizing of the device, depletion in polycrystalline silicon can be prevented.
The control gate electrode 500a is formed on the recess (step) produced by the floating gate electrode 100a and the device separation insulating film 50. Depletion of polycrystalline silicon prominently occurs in this recess. Hence the effect of preventing depletion according to this embodiment is manifested more prominently in the floating gate electrode formed on the step than in the control gate electrode formed on the flat portion.
As described above, according to the embodiment of the invention, it is possible to increase the grain size of silicon crystal and the dopant activation ratio even if the polycrystalline silicon film constituting the floating gate electrode and the control gate electrode is thinned. Hence depletion of polycrystalline silicon can be prevented. Thus, despite the downsizing of the device, electrical interference between adjacent floating gate electrodes can be reduced, and changes or variations of threshold voltage or other operating voltage can be restrained.
In the above embodiment of the invention, a doped amorphous silicon film is formed on a nondoped amorphous silicon film, followed by heat treatment to cause solid-phase diffusion of dopant from the overlying layer into the underlying layer for activating the dopant, along with polycrystallizing the amorphous silicon. Thus a doped polycrystalline silicon film is formed. However, the following method can be also used as the process for forming such a doped polycrystalline silicon film.
A nondoped amorphous silicon film is formed by chemical vapor deposition, followed by heat treatment in a dopant-containing gas to cause gas-phase diffusion of dopant for activating the dopant, along with polycrystallizing the amorphous silicon. Thus a doped polycrystalline silicon film can be formed. In this case, dopant may be attached from gas phase to the surface of the nondoped amorphous silicon film, followed by the above heat treatment.
Alternatively, a doped amorphous silicon film is formed by chemical vapor deposition, followed by heat treatment to cause gas-phase diffusion of dopant for activating the dopant, along with crystal growth of amorphous silicon. Thus a doped polycrystalline silicon film can be formed.
The above embodiment of the invention is illustrated with reference to a semiconductor flash memory. However, the examples of the invention can be suitably modified without departing from the spirit of the invention. The invention is applicable to semiconductor memory devices with control gate electrodes of polycrystalline silicon. Besides semiconductor memory devices, the invention is also applicable to semiconductor logic circuit devices and semiconductor arithmetic circuit devices. Likewise, the method for manufacturing a semiconductor memory device according to the invention is applicable to methods for manufacturing a semiconductor memory device, semiconductor logic circuit device, or semiconductor arithmetic circuit device by forming a polycrystalline silicon film.
Number | Date | Country | Kind |
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2007-081910 | Mar 2007 | JP | national |