# Patent

## Title
Semiconductor device with amorphous silicon monos memory cell structure and method for manufacturing thereof

## Abstract

A semiconductor device with an amorphous silicon (a-Si) metal-oxide-nitride-oxide-semiconductor (MONOS) memory cell structure. The device includes a substrate, a dielectric layer overlying the substrate, and one or more source or drain regions embedded in the dielectric layer with a co-planar surface of n-type a-Si and the dielectric layer. Additionally, the device includes a p-i-n a-Si diode junction. The device further includes an oxide-nitride-oxide (ONO) charge trapping layer overlying the a-Si p-i-n diode junction and a metal control gate overlying the ONO layer. A method for making the a-Si MONOS memory cell structure is provided and can be repeated to expand the structure three-dimensionally.

## Background
CROSS-REFERENCES TO RELATED APPLICATIONS
This application is a divisional application of U.S. application Ser. No. 11/615,968 filed Dec. 23, 2006, which claims priority to Chinese Patent Application No. 200610147446.2 filed on Dec. 12, 2006, all of which are commonly assigned and hereby incorporated by reference for all purposes.


STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK.
Not Applicable
BACKGROUND OF THE INVENTION
The present invention is directed to integrated circuits and their processing for the manufacture of semiconductor devices. More particularly, the invention provides a semiconductor device having a memory cell and a method for making the device. Merely by way of example, the invention has been applied to a three-dimensional (3D) amorphous silicon (a-Si) metal-oxide-nitride-oxide-semiconductor (MONOS) memory cell structure and a method for making the memory cell structure. But it would be recognized that the invention has a much broader range of applicability. For example, the invention can be applied to a variety of devices such as dynamic random access memory devices, static random access memory devices, Flash memory devices, and others.
Integrated circuits or “ICs” have evolved from a handful of interconnected devices fabricated on a single chip of silicon to millions of devices. Current ICs provide performance and complexity far beyond what was originally imagined. In order to achieve improvements in complexity and circuit density (i.e., the number of devices capable of being packed onto a given chip area), the size of the smallest device feature, also known as the device “geometry”, has become smaller with each generation of ICs. Semiconductor devices are now being fabricated with features less than a quarter of a micron across.
Increasing circuit density has not only improved the complexity and performance of ICs but has also provided lower cost parts to the consumer. An IC fabrication facility can cost hundreds of millions, or even billions, of dollars. Each fabrication facility will have a certain throughput of wafers, and each wafer will have a certain number of ICs on it. Therefore, by making the individual devices of an IC smaller, more devices may be fabricated on each wafer, thus increasing the output of the fabrication facility. Making devices smaller is very challenging, as each process used in IC fabrication has a limit. That is to say, a given process typically only works down to a certain feature size, and then either the process or the device layout needs to be changed.
Over the past, reducing memory devices have been a challenging task. As an example, for non-volatile memory devices the high density memory development is hindered by the inability to scale down the memory cell size without reducing the memory capacitance per unit area. In the past, various conventional techniques have been developed for memory cell structures with reduced dimensions. Unfortunately, these conventional techniques have often been inadequate.
Therefore, it is seen that an improved device design and technique for memory cell structure is desired.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to integrated circuits and their processing for the manufacture of semiconductor devices. More particularly, the invention provides a semiconductor device having a memory cell and a method for making the device. Merely by way of example, the invention has been applied to a three-dimensional (3D) amorphous silicon (a-Si) metal-oxide-nitride-oxide-semiconductor (MONOS) memory cell structure and a method for making the memory cell structure. But it would be recognized that the invention has a much broader range of applicability. For example, the invention can be applied to a variety of devices such as dynamic random access memory devices, static random access memory devices, Flash memory devices, and others.
In a specific embodiment, the invention provides a device with a-Si MONOS memory cell structure. The device includes a substrate, a dielectric layer on the substrate, and one or more source or drain regions being embedded in the dielectric layer. Each of the one or more source or drain regions includes an n-type a-Si layer, a diffusion barrier layer, and a conductive layer. The n-type a-Si layer, which has a co-planar surface with the dielectric layer, is located on the diffusion barrier layer. The diffusion barrier layer overlays the conductive layer. Additionally, the device includes an intrinsic type (i-type) a-Si layer overlying the co-planar surface of the n-type a-Si layer and the dielectric layer. Moreover, the device includes a p-type a-Si layer overlying the i-type a-Si layer. The device further includes an oxide-nitride-oxide (ONO) layer overlying the p-type a-Si layer and at least one control gate overlying the ONO layer.
In an alternative specific embodiment, the invention provides a method for making an a-Si MONOS memory cell structure. The method includes providing a substrate, forming a first insulation layer on the substrate, and forming one or more source or drain regions on the first insulation layer. Each of the one or more source or drain regions is associated with a first surface and includes an n-type a-Si layer, a barrier layer, and a conductive layer. The n-type a-Si layer is located on the barrier layer. The barrier layer overlays the conductive layer. Additionally, the method includes forming a second insulation layer on the first insulation layer. The second insulation layer is associated with a second surface which is substantially co-planar with the first surface. The method further includes forming an i-type a-Si layer overlying the first surface and the second surface and forming a p-type a-Si layer overlying the i-type a-Si layer. Moreover, the method includes forming an oxide-nitride-oxide (ONO) layer overlying the p-type a-Si layer. The method further includes forming a metal layer overlying the ONO layer and forming at least one control gate by patterning the metal layer.
In yet another specific embodiment, the p-type a-Si overlying the i-type a-Si layer is capable of forming a p-i-n diode junction at the n-type a-Si surface. This p-i-n diode junction can act as an access device for each memory bit with improved signal-to-noise ratio and reduced scaling limitations. In yet another specific embodiment, the combination of n-type a-Si source region, p-type a-Si channel layer, and next n-type a-Si drain region is capable of forming a thin-film-transistor (TFT). The TFT can act as an alternative access device for the memory cell. Additionally, the simplicity of the cell structure design provides an ability to be stacked in 3D. For example, the whole memory cell structure can be implemented in a cross-point memory architecture in which each memory storage element, within the ONO layer, can be sandwiched between an array of orthogonal word lines and bit lines.
Many benefits can be achieved, aside from the ability to be 3D stacked, by way of the present invention over conventional techniques. According to certain embodiments, the present invention combines the advantages of high reliability of ONO charge-trapping memory storage design, small geometric cell size using a p-i-n diode as the access device, and low thermal budget for fabrication and dopant activation within temperature tolerated by the memory cell. In addition, the present invention provides an easy to use process that is compatible with conventional CMOS process technology without substantial modifications to conventional equipment and processes. In some embodiments, the method provides a process to form a p-i-n diode junction by low-temperature deposition of a-Si films that meets the stackability and thermal budget constraint for 3D memory cell structure. Depending upon the embodiment, one or more of these benefits may be achieved. These and other benefits will be described in more throughout the present specification and more particularly below.
Various additional objects, features and advantages of the present invention can be more fully appreciated with reference to the detailed description and accompanying drawings that follow.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified side-view diagram of an a-Si MONOS memory cell according to an embodiment of the present invention;
FIG. 2 is a simplified diagram showing a method of manufacturing a-Si MONOS memory cell structure according to an embodiment of the present invention;
FIG. 3A is a simplified diagram showing a method of forming a first insulation layer on a substrate for making an a-Si MONOS memory cell structure according to an embodiment of the present invention;
FIG. 3B is a simplified diagram showing a method of forming a conductive layer, a diffusion barrier layer, and an n-type a-Si layer sequentially for making an a-Si MONOS memory cell structure according to an embodiment of the present invention;
FIG. 3C is a simplified diagram showing a method of forming a source or drain region for making an a-Si MONOS memory cell structure according to an embodiment of the present invention;
FIG. 3D is a simplified diagram showing a method of forming a second insulation layer around the source or drain region for making an a-Si MONOS memory cell structure according to an embodiment of the present invention;
FIG. 3E is a simplified diagram showing a method of forming a co-planar surface of the source or drain region and the second insulation layer for making an a-Si MONOS memory cell structure according to an embodiment of the present invention;
FIG. 3F is a simplified diagram showing a method of forming a p-i-n diode junction for making an a-Si MONOS memory cell structure according to an embodiment of the present invention;
FIG. 3G is a simplified diagram showing a method of forming an ONO layer for making an a-Si MONOS memory cell structure according to an embodiment of the present invention;
FIG. 3H is a simplified diagram showing a method of forming a metal control gate on the ONO layer for making an a-Si MONOS memory cell structure according to an embodiment of the present invention;
FIG. 3I is a simplified diagram showing a method of forming an inter-layer dielectrics for making three-dimensional a-Si MONOS memory cell structures according to an embodiment of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to integrated circuits and their processing for the manufacture of semiconductor devices. More particularly, the invention provides a semiconductor device having a memory cell and a method for making the device. Merely by way of example, the invention has been applied to a three-dimensional (3D) amorphous silicon (a-Si) metal-oxide-nitride-oxide-semiconductor (MONOS) memory cell structure and a method for making the memory cell structure. But it would be recognized that the invention has a much broader range of applicability. For example, the invention can be applied to a variety of devices such as dynamic random access memory devices, static random access memory devices, Flash memory devices, and others.
As discuss above, various conventional techniques have been directed to memory cells with reduced sizes. According to one of the conventional techniques, memory cells are implemented with stacked gate structure. For example, a stack structure is programmed using one or more channel hot-electrons that pass through a source region and a channel region and is then erased by a Fowler-Nordheim tunneling effect.
Unfortunately, stacked gate cell structures, being of two-dimensional array type, usually have a smaller memory capacity per unit area as the cell size scales down. One feasible solution is to stack three-dimensionally several layers of memory arrays on top of a Si substrate containing the CMOS support circuitry. According to various embodiments, the present invention provides three dimensional memory cell structures. For example, certain embodiments of the present invention provides an ability to fabricate stackable access device in the memory cell. This brings up an requirement to improve the design of memory cell structure that can fulfill one or more of the following attributes including, stacking ability, small geometry, low leakage current, bi-directional operable, easy integration into a low temperature backend CMOS flow, cost effectiveness, efficiency, etc. Accordingly, various embodiments of the present invention provide an amorphous silicon metal-oxide-nitride-oxide-semiconductor (a-Si MONOS) memory cell structure. It is to be understood that the term “a-Si MONOS” refers to a class of memory cell structures and is broadly defined. For example, an “a-Si MONOS” may be illustrated according to FIG. 1.
FIG. 1 is a simplified diagram for a semiconductor device 100 with an a-Si MONOS memory cell structure that is capable of being stacked three-dimensionally. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The device 100 includes following components:
1. Substrate 10;
2. Dielectric layer 20;
3. Conductive layer 30;
4. Diffusion barrier layer 35;
5. One or more n-type a-Si source or drain region 40;
6. i-type a-Si middle layer 50;
7. p-type a-Si layer 60;
8. Oxide-nitride-oxide (ONO) layer 70; and
9. Metal gate region 80.
Although the above has been shown using a selected group of components for the device 100, there can be many alternatives, modifications, and variations. For example, some of the components may be expanded and/or combined. Other components may be inserted to those noted above. Depending upon the embodiment, the arrangement of components may be interchanged with others replaced. Further details of these components are found throughout the present specification and more particular below.
In one embodiment, the substrate 10 is made of a semiconductor material. For example, the semiconductor material is silicon. In another example, the substrate 10 includes a plurality of semiconductor devices such as a-Si MONOS memory arrays with dielectric passivation.
The dielectric layer 20 is located on the substrate 10. In one embodiment, at least part of the dielectric layer 20 consists of silicon dioxide formed through thermal oxidation process on a silicon substrate. In another embodiment, the dielectric layer 20 is silicon oxide deposited by high-density-plasma (HDP) assisted chemical vapor deposition or is TEOS deposited silicon oxide.
There are one or more confined regions embedded in the dielectric layer 20. Each of these confined region contains a conductive layer, a diffusion barrier layer, and a semiconductor source or drain region. In an exemplar confined region as shown in FIG. 1, the conductive layer 30 is located at the bottom and the diffusion barrier layer 35 overlays the conductive layer 30, followed by a n-type a-Si layer 40. The n-type a-Si layer 40 has a surface co-planar with the surface of the dielectric layer 20. The n-type a-Si layer 40 is capable of forming a source or drain region for the device 100. The n-type source or drain region 40 is conductively connected to the conductive layer 30 through the diffusion barrier layer 35. The conductive layer 30 is capable of coupling electrically with the memory bitlines (not shown in FIG. 1) to perform programming or erasing function for the memory cell. In one embodiment, the conductive layer 30 is a material containing metal or metal alloy material. For example, the material is titanium silicide. In another embodiment, the diffusion barrier layer 35 is titanium nitride.
Referring to FIG. 1, the n-type a-Si source or drain region 40 contains hydrogen-free amorphous silicon which is located at the top part within the confined region embedded in dielectric region 20. In one embodiment, the amorphous silicon is doped with n-type dopants (e.g., As, P, etc) to be conductive with electrons as its majority carrier. As seen in FIG. 1, in another embodiment, the n-type a-Si source or drain region 40 has a co-planar surface with the dielectric layer 20.
Referring again to FIG. 1, the i-type a-Si layer 50 is located on the co-planar surface of the n-type a-Si source or drain region 40 and the dielectric layer 20. The i-type a-Si layer 50 is an intrinsic semiconductor without impurity doping. In one embodiment, the i-type a-Si layer 50 is at least partially in direct contact with the n-type a-Si source or drain region 40. Additionally, the p-type a-Si layer 60 is located on the i-type a-Si layer 50. In one example, the p-type a-Si layer is doped with p-type dopants (e.g., B, Ga, etc) with holes as its majority carrier. Referring to FIG. 1, in another embodiment, p-type a-Si layer 60 and i-type a-Si layer 50 overlying the confined n-type a-Si source or drain region 40 form an a-Si p-i-n diode junction in the device 100. In yet another embodiment, the p-type a-Si layer 60 located over two neighboring n-type a-Si source or drain regions 40 is capable of forming a p-channel thin-film transistor (TFT). Either the a-Si p-i-n diode junction or p-channel TFT can be utilized as a memory access device for the device 100.
Referring to FIG. 1, an ONO layer 70 is located on the p-type a-Si layer 60. The ONO layer 70 includes a bottom oxide layer 72 overlying the p-type a-Si layer 60, followed by a nitride layer 75 overlying the bottom oxide layer 72, and an upper oxide layer 78 overlying the nitride layer 75. For example, the bottom oxide layer 72 and the upper oxide layer 78 are made of atomic-layer deposited silicon dioxide. The nitride layer 75 is made of atomic-layer deposited silicon nitride. In one embodiment, the lower oxide layer 72 is a tunneling oxide of the device 100 for hot carriers to be injected into the charge trapping nitride layer 75 for memory storage. In another embodiment, the upper oxide layer 78 is a blocking dielectric or gate insulation layer for blocking the charge leakage out of the charge trapping nitride layer 75. The ONO layer 70 can be modified to optimize certain device design requirements. For example, the composition in silicon nitride layer and the thickness of the tunnel oxide and blocking dielectric can be adjusted to provide enhanced performance for the memory device. In another example, the charge amount trapped in the silicon nitride layer may be fine tuned and controlled to store 4 or more bits per cell.
Referring back to FIG. 1, the device 100 further includes at least one metal gate region 80 as a control gate. The metal gate region 80 is formed by patterning a metal layer which is formed overlying the ONO layer 70. The patterned metal gate region 80 is located at least over one confined n-type a-Si source or drain region 40 where a p-i-n a-Si diode junction is formed. The pattern geometry of the metal gate region 80 is not specifically shown in FIG. 1, which is merely an example and one of ordinary skill in the art would recognize many variations, alternatives, and modifications of the metal gate regions 80 as well as their interconnections. For example, the metal gate region 80 can be electrically coupled to the memory array wordlines (not shown) which may be orthogonal to the memory bitlines coupled to the conductive layer 30. In one embodiment, the metal gate region 80 as a control gate of the memory cell can be made of a material with high work function to suppress the parasitic gate erase current. In one example, the metal gate region 80 includes titanium material. In another example, the metal gate region 80 includes an aluminum material.
According to an embodiment of the present invention, the device 100 with an a-Si MONOS memory cell structure can be repeated laterally to form a memory array. The memory array further can be passivated with an inter-layer dielectrics with a plurality of metal interconnects and/or contacts to the gate, source or drain regions. In another embodiment, the passivation layer can be further planarized to form a substrate for stacking a plurality of the devices 100 again. In yet another embodiment, the present invention provides an a-Si MONOS memory cell structure that can be stacked in multiple layers to form a three-dimensional memory array.
FIG. 2 is a simplified diagram showing a method 2000 for manufacturing an a-Si MONOS memory cell structure according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. The method includes the following processes:
1. Process 2100 for forming a first insulation layer on a substrate;
2. Process 2200 for forming n-type a-Si source or drain region;
3. Process 2300 for forming p-type a-Si layer overlying an i-type a-Si layer;
4. Process 2400 for forming ONO layer;
5. Process 2500 for forming metal control gate; and
6. Process 2600 for forming inter-layer dielectrics.
The above sequence of processes provides a method according to an embodiment of the present invention. Other alternatives can also be provided where processes are added, one or more processes are removed, or one or more processes are provided in a different sequence without departing from the scope of the claims herein. For example, the semiconductor device with an a-Si MONOS memory cell structure made by the method 2000 is the device 100 in FIG. 1. Further details of the present invention can be found throughout the present specification and more particularly below.
At process 2100, an insulation layer is formed on a substrate. FIG. 3A shows a simplified method for forming an insulation layer for manufacturing a semiconductor device with a-Si MONOS memory cell structures according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.
As shown in FIG. 3A, a substrate 110 is provided. For example, substrate 110 includes silicon. In another example, substrate 110 includes a plurality of semiconductor devices including a plurality of CMOS memory devices embedded in inter-layer dielectrics with passivation. On substrate 110, a first insulation layer 120 is formed. In one embodiment, the first insulation layer 120 includes silicon dioxide. For example, the silicon dioxide is formed by thermal oxidation process. In another example, the silicon dioxide layer is deposited by using high-density plasma chemical vapor deposition (HDP-CVD) technique.
Referring to FIG. 2, at process 2200, one or more n-type a-Si source or drain regions are formed. FIGS. 3B, 3C, 3D, and 3E show a simplified method for forming one n-type a-Si source or drain region for manufacturing a semiconductor device with a-Si MONOS memory cell structures according to an embodiment of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, process 2200 can be implemented to make device 100 in FIG. 1.
As shown in FIG. 3B, on first insulation layer 120 a conductive layer 130, a diffusion barrier layer 135, and an n-type a-Si layer 140 are formed sequentially. In one embodiment, conductive layer 130 is made of a metal silicide material. The metal silicide material is capable of forming a contact pad for electric coupling. For example, the conductive layer 130 can be electrically coupled with memory array bitlines (not shown) embedded in the first insulation layer 120 along a certain direction. In another example, the conductive layer 130 is titanium silicide (TiSi2). In another embodiment, the TiSi2 layer can be formed by various deposition techniques including evaporation, sputtering or CVD. For example, TiSi2 layer is formed through thermal CVD using gas mixture of SiH4 and TiCl4 etc followed by a thermal annealing at 600-800° C.
In yet another embodiment, to reduce the metal inter-diffusion issue, a diffusion barrier layer 135 is deposited overlying conductive layer 130 prior the formation of a semiconductor layer. For example, diffusion barrier layer 135 is made of titanium nitride (TiN) material. In another example, the TiN material is deposited using low-pressure chemical vapor deposition (LPCVD) or physical vapor deposition (PVD). Referring again to FIG. 3B, an n-type a-Si layer 140 is formed on the diffusion barrier layer 135. In still another embodiment, n-type a-Si layer 140 is deposited through a low temperature (<400° C.) plasma-enhanced CVD with flowing of SiH4/PH3/H2 gas mixture, wherein phosphorus is the n-type dopant impurity. Other alternative methods such as LPCVD and atomic layer deposition (ALD) techniques can be used to form the n-type a-Si layer. Obviously, one of ordinary skill in the art would recognize many other alternatives of the n-type a-Si formation including its dopant.
Still at process 2200, referring to FIG. 3C, according to an embodiment of the present invention, patterning and etching are performed with the sequential layers of 130, 135, and 140. In one embodiment, the patterning is performed with an application of a photoresist layer followed by an UV light exposure under a patterned photo mask. Developing the photoresist layer and strip-cleaning the exposed resist material lead to a surface consisting of partially revealed n-type a-Si layer 140 and one or more confined regions that are still covered by the resist layer. Furthermore, plasma etching is performed to remove the layers of 130, 135, and 140 that are unmasked until the first insulation layer 120 is revealed. The etching process is anisotropic so that the regions covered by the resist layer pattern remain. After the resist layer is removed, one or more confined regions 150 are formed as shown in FIG. 3C. In one embodiment, each of the one or more confined regions 150 includes confined portions of n-type a-Si layer 140a, a diffusion barrier layer 135a, and a conductive layer 130a. The confined n-type a-Si layer 140a is located on the confined diffusion barrier layer 135a which overlays the confined conductive layer 130a. 
Additionally at process 2200, a second insulation layer 160 is deposited to completely cover the formed one or more confined regions 150 and the exposed area of the first insulation layer 120, as shown in FIG. 3D. In one embodiment, second insulation layer 160 includes silicon oxide. For example, the silicon oxide is deposited using a high-density-plasma (HDP) Chemical Vapor Deposition process. In another example, the silicon oxide is a tetraethyl orthosilicate (commonly known as TEOS) deposited silicon oxide.
Referring to FIG. 3E, still at process 2200, a chemical-mechanical planarization (CMP) process is performed to remove a portion of second insulation layer 160 until n-type a-Si layer 140a in confined regions 150 is revealed and a co-planarized surface is formed. The CMP planarized surface includes at least partially a first surface 141 of the n-type a-Si layer in the regions 140a and partially a second surface 161 of second insulation layer 160. In another embodiment, combination of a CMP process and a dry etching process or a dry etching process alone can be used to remove the portion of the second insulation layer. In yet another embodiment of the present invention, second insulation layer 160 is deposited around confined regions 150 (as shown in FIG. 3C) until second surface 161 of second insulation layer 160 is substantially co-planar with first surface 141 of n-type a-Si in regions 140a. N-type a-Si layer 140a within each of the one or more confined regions 150 is embedded in second insulation layer 150 with a co-planar surface and capable of forming a source or drain region of a memory device. For example, the memory device is device 100 in FIG. 1.
Referring back to FIG. 2 at process 2300, a sequential layer of i-type a-Si and p-type a-Si is formed. FIG. 3F shows a simplified method for forming a p-type a-Si layer overlying an i-type a-Si layer for manufacturing a semiconductor device with a-Si MONOS memory cell structures according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.
As shown in FIG. 3F, an intrinsic or undoped (i.e., the i-type) a-Si layer 170 is formed overlying surface 141 of n-type a-Si layer 140a in confined region 150 and surface 161 of the second insulation layer 160. In one embodiment, i-type a-Si layer 170 is at least partially in contact with surface 141 of the n-type a-Si layer 140a in the confined regions 150. In another embodiment, i-type a-Si layer 170 is deposited using a low temperature (<400° C.) plasma enhanced CVD using a SiH4/H2 gas mixture. In another example, i-type a-Si layer 170 is formed by depositing amorphous silicon using Si2H6 using a LPCVD process under 450° C. Other alternative method can be used, such as an ALD method using SiH4 or Si2H6.
Additionally at process 2300, a p-type a-Si layer 180 is deposited overlying the i-type a-Si layer. For example, p-type a-Si layer 180 is deposited using a low temperature (<400° C.) plasma-enhanced CVD process using SiH4/BF3/H2 gas mixture. Other alternative methods such as LPCVD and ALD techniques can be used to form the p-type a-Si layer. In one specific embodiment, overlying the confined n-type a-Si layer 140a there exists a sequential layer of i-type a-Si and p-type a-Si, which is capable of forming an a-Si p-i-n diode junction. The a-Si p-i-n diode junction can be used as an access device for a memory cell. In another specific embodiment, confined n-type a-Si layer 140a in one of confined region 150 forms a source region and confined n-type a-Si layer 140a in a neighboring region 150 forms a drain region. Over both of the regions 150 the p-type a-Si layer 180 on the i-type a-Si layer 170 forms a thin-film-transistor channel region connecting the n-type source region to the n-type drain region. Such an architecture including an a-Si p-channel thin-film-transistor, according to an embodiment of the invention, can also functions as an access device for a memory cell. Of course, one skilled in the art would recognize other variations, modifications, and alternatives.
Referring again to FIG. 2 at process 2400, an ONO layer is formed. FIG. 3G shows a simplified method for forming an ONO layer for manufacturing a semiconductor device with a-Si MONOS memory cell structures according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.
Referring to FIG. 3G, an ONO layer 190 is formed overlying the p-type a-Si layer 180. The ONO layer is a nitride layer 195 sandwiched by a bottom oxide layer 192 and an upper oxide layer 198. Nitride layer 195 can be used as a charge trapping insulation layer for the memory storage. For example, nitride layer 195 is silicon nitride deposited by ALD technique. In another example, the silicon nitride can be silicon-rich nitride. The bottom oxide layer 192 is a tunnel oxide separating nitride layer 195 from p-type a-Si layer 180 where hot electrons from n-type a-Si layer drain region 140a are to be injected into nitride layer 195 by programming of an electric field. Upper oxide layer 198 is a blocking oxide or gate insulation layer, setting a high barrier for the retention of charges in nitride layer 195. In one example, both the bottom oxide layer 192 and upper oxide layer 198 are made of silicon dioxide. In another example, the silicon dioxide is grown by ALD technique. In one embodiment, upper oxide layer 198 is thicker than bottom oxide layer 192. Of course there can be other variations, modifications, and alternatives.
At process 2500, a metal control gate is formed. FIG. 3H shows a simplified method for forming a metal control gate for manufacturing a semiconductor device having a-Si MONOS memory cell structures according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the process 2500 of the invented method is implemented to make the metal control gate for device 100 in FIG. 1.
As shown in FIG. 3H, a metal layer 200 is deposited overlying ONO layer 190. In one embodiment, metal layer 200 overlies upper oxide layer 198. Metal layer 200 includes a material with a high work function. In one example, metal layer 200 is made of aluminum material. In another example, metal layer 200 includes titanium material. The metal deposition can be performed by various techniques including evaporation, sputtering, and electrochemical deposition (ECD), among others.
According to an embodiment of the present invention, a control gate can be formed by patterning metal layer 200 located on ONO layer 190 over the a-Si p-i-n diode junction formed at process 2300. In another embodiment, the patterned control gate is positioned over a p-channel TFT formed alternatively at process 2300. In still another embodiment, dual metal gates can be formed within each memory cell. The patterning and etching processes for forming each of the control gate include applying photoresist layer, masking, exposing, developing, striping exposed resist residue, metal etching and resist layer removing, and others.
In another embodiment, each of the patterned control gate can be electrically coupled with memory array wordlines. The memory array wordlines may be configured to be orthogonal to the directions of bitlines of the memory array thereof. Although the detail pattern geometry of the control gate is not explicitly illustrated in FIG. 3H, one of ordinary skilled in the art would recognize many variations, alternatives, and modifications of the gate architectures which should not unduly limit the scope of the claims. At the end of the process 2500, the formation of the control gate completes the formation of the a-Si MONOS memory cell structure. For example, arrays of devices 100 are made by method 2000 including sequential processes from the process 2100 to the process 2500.
Referring back to FIG. 2 at process 2600, an inter-layer dielectrics is formed. FIG. 3I shows a simplified method of forming an inter-layer dielectrics 210 covering a device with a-Si MONOS memory cell structures formed at the end of the process 2500. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, a plurality of devices with a-Si MONOS memory cell structures can be formed at the end of the process 2500 prior to the formation of the inter-layer dielectrics 210. Additionally, metal interconnects (not shown) can be embedded in inter-layer dielectrics 210 for either bitlines or wordlines of the memory arrays. In another specific embodiment, the a-Si MONOS memory cell structure formed though processes 2100 to 2600 is three-dimensionally stackable. For example, the inter-layer dielectrics may be planarized by a CMP or an etch back process, or a combination. The planarized surface of the dielectrics can be used as a substrate in a specific embodiment. The sequence of processes (2100 through 2500) of method 2000 can be repeated to form another layer of memory cell arrays.
As shown in FIG. 3I, in a specific embodiment, the present invention provides a device with a-Si MONOS memory cell structures. The device includes a substrate, a dielectric layer on the substrate, and one or more source or drain regions embedded in the dielectric layer. Each of the one or more source or drain regions includes an n-type a-Si layer, a diffusion barrier layer, and a conductive layer. The n-type a-Si layer, which has a co-planar surface with the dielectric layer, is located on the diffusion barrier layer. The diffusion barrier layer overlies the conductive layer. Additionally, the device includes an intrinsic type (i-type) a-Si layer overlying the co-planar surface of the n-type a-Si layer and the dielectric layer. Moreover, the device includes a p-type a-Si layer overlying the i-type a-Si layer. The device further includes an oxide-nitride-oxide (ONO) layer overlying the p-type a-Si layer and at least one control gate overlying the ONO layer.
The processes described above for manufacturing a semiconductor device with a-Si MONOS memory cell structures are merely examples which should not unduly limit the scope of the claims herein. There can be many alternatives, modifications, and variations for an ordinary skill in the art. For example, some of the processes may be expanded and/or combined. Other processes may be inserted to those mentioned above. According to a specific embodiment, method 2000 provides a two-dimensional array of memory cells having the same structure of device 100. According to another specific embodiment, the method 2000 can be repeated to stack the memory cell structure in multi-layers, so that a three-dimensional array of memory cell structures can be manufactured. The simplicity of the formation of n-type a-Si source or drain regions, the a-Si p-i-n diode junction or p-channel TFT, and the ONO charge trapping layer followed by a metal control gate provides easy 3D stackability of memory cells. For example, the device 100 having a-Si MONOS memory cell structure can be embedded in larger chips three-dimensionally with increased memory density per unit area in a vertical direction.
The present invention has various advantages. Some embodiments of the present invention provide an a-Si MONOS memory cell that is capable of 3D stacking. Certain embodiments of the present invention provide an a-Si p-i-n diode junction in the memory cell as the memory access device. For example, the a-Si p-i-n diode junction is formed using low temperature (<450° C.) CVD processes which meets the stackability and thermal budget constraints for the 3D memory cells. Some embodiments of the present invention can reduce transistor leakage current and improve a charge retention time for the memory cell. Certain embodiments of the present invention provide a simple method for making 3D memory cells using processes that are fully compatible with established CMOS technology.
It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.

## Claims

1. A device with amorphous silicon (a-Si) metal-oxide-nitride-oxide-semiconductor (MONOS) memory cell structure, the device comprising:
a substrate;
a dielectric layer on the substrate, the dielectric layer being associated with a first surface;
one or more source or drain regions being embedded in the dielectric layer, each of the one or more source or drain regions being associated with a second surface and including an n-type a-Si layer, a diffusion barrier layer, and a conductive layer, the n-type a-Si layer being located on the diffusion barrier layer, the diffusion barrier layer overlying the conductive layer, the second surface consisting of n-type a-Si and being substantially co-planar with the first surface;
an intrinsic (i-type) a-Si layer overlying the first surface and the second surface;
a p-type a-Si layer overlying the i-type a-Si layer;
an oxide-nitride-oxide (ONO) layer overlying the p-type a-Si layer; and
at least one control gate overlying the ONO layer.

2. The device of claim 1 wherein the dielectric layer comprises silicon oxide.

3. The device of claim 1 wherein the conductive layer is metal silicide comprising TiSi2.

4. The device of claim 3 wherein the metal silicide layer is capable of coupling with memory array bitlines electrically.

5. The device of claim 1 wherein the diffusion barrier layer is metal-nitride comprising TiN.

6. The device of claim 1 wherein the p-type a-Si layer overlying the i-type a-Si layer is capable of forming an amorphous silicon p-i-n diode junction at the second surface.

7. The device of claim 1 wherein the ONO layer comprises a silicon nitride layer for charge trapping, sandwiched by an upper silicon oxide block layer and a bottom silicon oxide tunnel layer.

8. The device of claim 1 wherein the control gate is characterized by a metal layer pattern overlying the ONO layer.

9. The device of claim 8 wherein the metal layer comprises titanium.

10. The device of claim 8 wherein the metal layer comprises aluminum.

11. The device of claim 8 wherein the metal layer couples with memory array wordlines electrically.

12. The device of claim 8 wherein the control gate is positioned over at least one source and one drain region.