The embodiments of the present invention relate to a semiconductor device and a manufacturing method thereof.
A three-dimensional stereoscopic memory cell array has been developed to increase the storage capacity of a semiconductor storage device such as a NAND flash memory. Also in the stereoscopic memory cell array, the physical film thickness of a block film between a charge accumulation layer and a control gate needs to be reduced to downscale memory cells.
However, in a case where the film thickness of the block film is small, charges that leak from the control gate to the charge accumulation layer during data erase increase when the number of times of W (Write)/E (Erase) increases. That is, the resistance (endurance property) of the block film is likely to be degraded.
Accordingly, there has been a demand for maintaining or improving the resistance of the block film even when the film thickness of the block film is small.
Embodiments will now be explained with reference to the accompanying drawings. The present invention is not limited to the embodiments. In the embodiments, “an upper direction” or “a lower direction” refers to a relative direction when a direction of a surface of a substrate or a Si column is assumed as “an upper direction”. Therefore, the term “upper direction” or “lower direction” occasionally differs from an upper direction or a lower direction based on a gravitational acceleration direction.
A semiconductor storage device according to an embodiment includes a semiconductor layer. A tunnel dielectric film is formed on the semiconductor layer. A charge accumulation layer is formed on the tunnel dielectric film. A block film is formed on the charge accumulation layer. A control gate is formed on the block film. The block film includes a metal oxide film containing nitrogen in a concentration range equal to or lower than 5×1021 atoms/cm3 and consisting mainly of aluminum.
A NAND flash memory having a three-dimensional structure as an example of a semiconductor storage device according to an embodiment is explained below. However, the present embodiment can be also applied to a NAND flash memory having a two-dimensional structure. The present embodiment is not limited to a NAND flash memory and can be also applied to other semiconductor storage devices.
The memory area 1 has a plurality of blocks BLK and each of the blocks BLK has a plurality of NAND strings NS. The block BLK is a unit of data erase. Each of the NAND strings NS has a plurality of memory cells MC connected in series. Memory cells MC on two ends of each of the NAND strings NS are connected to select gate transistors SGD and SGS, respectively. Memory cells MC on one end of the memory area 1 are connected to bit lines BL via the select gate transistors SGD, respectively, and memory cells MC on the other end of the memory area 1 are connected to a cell source CELSRC via the select gate transistors SGS, respectively.
Each of word lines WL is connected to control gates CG of memory cells MC arrayed in a row direction. Select gate lines SLD and SLS are connected to gates of the select gate transistors SGD and SGS, respectively. The word lines WL and the select gate lines SLS and SLD are driven by a row decoder RD and a word line driver WLD.
Each of the bit lines BL is connected to a plurality of NAND strings NS via the corresponding select gate transistors SGD, respectively. The bit lines BL are connected to a sense amplifier circuit SA. A plurality of memory cells MC connected to one word line WL constitute a page serving as a unit of collective data read and data write.
When the select gate lines SLS and SLD drive the select gate transistors SGS and SGD, each of the NAND strings NS is connected between the corresponding bit line BL and the cell source CELSRC. When the word line driver WLD drives a non-selected word line WL, non-selected memory cells MC are turned on. Accordingly, the sense amplifier circuit SA can apply a voltage to the selected memory cells MC via the corresponding bit lines BL. This enables the sense amplifier circuit SA to detect data in the selected memory cells MC or to write data in the selected memory cells MC. Each of the memory cells MC has a threshold voltage depending on the quantity of charges (the quantity of electrons, for example) accumulated in a charge accumulation layer CA. The sense amplifier circuit SA detects differences in the threshold voltages of the memory cells MC and determines logic of data.
The semiconductor pillar 10 serving as a semiconductor layer extends, for example, in a direction substantially vertical to a surface of a substrate (not shown) and is formed of a conductive material such as doped polysilicon. The semiconductor pillar 10 is formed cylindrically and is provided to pass through the center of the memory cell MC.
The tunnel dielectric film 20 is provided on a side surface of the semiconductor pillar 10 (around the semiconductor pillar 10). The tunnel dielectric film 20 is formed of an insulating film such as a silicon dioxide film or a silicon nitride film. The tunnel dielectric film 20 can be a multilayered film including insulating films such as a silicon dioxide film and a silicon nitride film (an ONO film (a SiO—SiN—SiO film), for example).
The charge accumulation layer CA is provided on the tunnel dielectric film 20. That is, the charge accumulation layer CA is provided over the side surface of the semiconductor pillar 10 (around the semiconductor pillar 10) with the tunnel dielectric film 20 interposed therebetween. The charge accumulation layer CA is formed of a material such as doped polysilicon or a silicon nitride film.
The charge accumulation layer CA accumulates therein charges (electrons, for example) from the substrate 10 via the tunnel dielectric film 20 or emits charges to the substrate 10 via the tunnel dielectric film 20. Accordingly, the threshold voltage of the memory cell MC changes and thus the memory cell MC can store therein logical data.
The block film 40 is provided on the charge accumulation layer CA. That is, the block film 40 is provided over the side surface of the semiconductor pillar 10 (around the semiconductor pillar 10) with the tunnel dielectric film 20 and the charge accumulation layer CA interposed therebetween. The block film 40 is formed of, for example, a metal oxide film. The block film 40 can be a multilayered film including a silicon dioxide film, a silicon nitride film, or a silicon oxynitride film as well as the metal oxide film. The block film 40 is explained in more detail below with reference to
The control gate CG is provided on the block film 40. That is, the control gate CG is provide over the side surface of the semiconductor pillar 10 with the tunnel dielectric film 20, the charge accumulation layer CA, and the block film 40 interposed therebetween. The control gate CG is formed of a conductive material such as doped polysilicon, tungsten, or a tungsten nitride film.
The tunnel dielectric film 20 is, for example, an ONO film provided on the semiconductor pillar 10. The charge accumulation layer CA is a silicon nitride film provided on the tunnel dielectric film 20. A metal oxide film such as an aluminum oxide film (Al2O3) can be formed on the upper and bottom surfaces (both sides) of the silicon nitride film of the charge accumulation layer CA.
The block film 40 includes a silicon dioxide film 42 provided on the charge accumulation layer CA, a metal oxide film 44 provided on the silicon dioxide film 42, and a silicon nitride film 46 provided on the metal oxide film 44. The block film 40 can include a high-permittivity film having a relative permittivity that is higher than that of the silicon dioxide film (HfO2, for example) instead of or in addition to the silicon dioxide film 42 and the silicon nitride film 44.
In this case, the metal oxide film 44 is an oxide film consisting mainly of aluminum that has a nitrogen concentration equal to or lower than approximately 5×1021 atoms/cm3. More preferably, the metal oxide film 44 is an oxide film consisting mainly of aluminum that has a nitrogen concentration between approximately 4×1021 atoms/cm3 and approximately 5×1021 atoms/cm3. For example, the metal oxide film 44 is an aluminum oxide film (Al2O3) containing nitrogen in the concentration range described above.
Addition of nitrogen to the metal oxide film 44 in the block film 40 enables trapping of charges by the metal oxide film 44. Accordingly, tunneling (back tunneling) of charges from the control gate CG to the charge accumulation layer CA during data erase can be suppressed. Furthermore, the block film 40 can reduce a leak current between the control gate CG and the charge accumulation layer CA.
As shown in
As shown in
As described above, according to the present embodiment, because the metal oxide film 44 has the aluminum oxide film containing nitrogen, the block film 40 can suppress the leak current between the control gate CG and the charge accumulation layer CA and also can increase the quantity of trapped charges. Increase in the quantity of trapped charges leads to suppression in back tunneling (the leak current) during data erase. Furthermore, because the metal oxide film 44 has the aluminum oxide film containing nitrogen, the electric field is relaxed and the back tunneling (the leak current) during data erase is suppressed. That is, according to the present embodiment, the resistance of the block film 40 in a data erase operation and the like can be maintained or improved.
Meanwhile, if the concentration of nitrogen contained in the metal oxide film 44 is too high, the quantity of trapped charges correspondingly increases. If the quantity of trapped charges is too large, horizontal detrapping to an adjacent memory cell MC may occur. That is, charges trapped in the metal oxide film 44 may be dropped out to the metal oxide film 44 or the charge accumulation film CA in an adjacent memory cell MC. Therefore, nitrogen contained in the metal oxide film 44 has an appropriate concentration range as explained with reference to
As shown in
Although not shown in
As described above, according to the present embodiment, addition of nitrogen in the metal oxide film 44 enables the metal oxide film 44 to trap charges. Accordingly, tunneling (back tunneling) of charges from the control gate CG to the charge accumulation layer CA during data erase can be suppressed. Furthermore, the block 40 can reduce the leak current and the quantity of detrapped charges between the control gate CG and the charge accumulation layer CA.
The metal oxide film 44 can be a multilayered film including an aluminum oxide film containing nitrogen and another metal oxide film containing no nitrogen. For example, the metal oxide film 44 can be a multilayered film including AlO (AlO containing no nitrogen) and AlO+N (AlO containing nitrogen) or a multilayered film including AlO, AlO+N (AlO containing nitrogen), and AlO. The embodiment described above can be applied to the memory area 1 in a planar shape in which the memory cells MC are arranged two-dimensionally.
Next, memory holes MH (see
Next, the block film 40, the charge accumulation layer CA, and the tunnel dielectric film 20 are deposited in this order on inner surfaces of the memory holes MH (S30). The semiconductor pillars 10 are then filled in the memory holes MH, respectively (S40). In this way, the structure shown in
A process of forming the metal oxide film 44 in the block film 40 is explained in more detail.
As shown in
A chamber of an ALD film-formation apparatus is purged with inert gas (S32) and then the aluminum film is oxidized using an oxidant (S33). The oxidant is, for example, any of O2, O3, H2O, and an oxygen radical. O3 and the oxygen radical are easily deactivated. Uniform oxidation is difficult to realize with H2O. Therefore, it can be said that it is preferable to use O2 as the oxidant. At Step S33, the aluminum film is oxidized to form an aluminum oxide film at the atomic level.
The chamber of the ALD film-formation apparatus is further purged with an inert gas (S34) and then the processes at Steps S31 to S33 are performed again (NO at S35). By repeatedly performing the processes at Steps S31 to S34, the aluminum oxide film is gradually formed on the inner surfaces of the memory holes MH with respect to each atomic level. At that time, nitrogen contained in the amino group or amidinate remains in the aluminum oxide film so that the metal oxide film 44 becomes an aluminum oxide film containing nitrogen. That is, by forming the aluminum oxide film using the aluminum source containing an amino group or amidinate, nitrogen can be contained in the aluminum oxide film without passing a process of introducing nitrogen purposely. When the aluminum oxide film has reached a desired film thickness (YES at S35), the formation process of the metal oxide film 44 is ended.
The nitrogen concentration of the metal oxide film 44 is controlled to a range from approximately 5×1018 atoms/cm3 to 2×1022 atoms/cm3. The nitrogen concentration can be controlled by adjusting a film formation temperature at Step S31.
The graph in
According to the present embodiment, the aluminum oxide film is formed using an aluminum source containing an amino group or amidinate at the time of formation of the metal oxide film 44 of the block film 40. Accordingly, without performing the process of introducing nitrogen purposely, the aluminum oxide film containing nitrogen can be formed by the ALD method or a CVD (Chemical Vapor Deposition) method. Because the formation process of the metal oxide film 44 according to the present embodiment does not require the process of introducing nitrogen, the process is relatively simple and is completed in a short time.
For example, when the metal oxide film 44 is formed of TMA (trimethylaluminum) (L4 in
On the other hand, when an aluminum source containing an amino group or amidinate is used (L3 in
In the embodiment described above, the metal oxide film 44 is formed by the ALD method. However, the metal oxide film 44 can be formed by the CVD method.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.
This application is based upon and claims the benefit of priority from the prior US Provisional Patent Application No. 61/949,003, filed on Mar. 6, 2014, the entire contents of which are incorporated herein by reference.
Number | Date | Country | |
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61949003 | Mar 2014 | US |