Patent Publication Number: US-7709386-B2

Title: Atomic layer deposition method and semiconductor device formed by the same

Description:
This application claims the priority of Chinese Patent Application No. 200710042463.4, filed Jun. 22, 2007, the entire disclosure of which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a method for manufacturing a semiconductor, and more particularly, to an atomic layer deposition method and a semiconductor device manufactured by the same. 
   DESCRIPTION OF THE RELATED ART 
   Atomic layer deposition (ALD) is originally referred to as Atomic Layer Epitaxy (ALE), and is also termed Atomic Layer Chemical Vapor Deposition (ALCVD). 
   Atomic layer deposition involves the deposition of successive monolayers over a substrate within a deposition chamber typically maintained at a negative pressure (sub-atmospheric pressure). An exemplary such method is disclosed in the background art of Chinese application No. 03818269 comprising the following steps: feeding a first vaporized precursor to a deposition chamber to form a first monolayer  110  on the semiconductor substrate  100  received in the deposition chamber, as shown in  FIG. 1 ; thereafter, ceasing the flow of the first vaporized precursor, and flowing an inert purge gas through the chamber to remove all remaining first precursor which is not adhering to the semiconductor substrate  100  from the chamber, as shown in  FIG. 2 ; subsequently, flowing a second vaporized precursor which is different from the first precursor to the deposition chamber to form a second monolayer  120  on/with the first monolayer  110 , the second monolayer  120  can react with the first monolayer  110 , as shown in  FIG. 3 ; and ceasing the flow of the second precursor, and flowing an inert purge gas through the chamber to remove all remaining second monolayer  120  which is not adhering to the first layer  110 , as shown in  FIG. 4 . The deposition process for the first and the second monolayer can be repeated until a desired thickness and discrete compound monolayer has been formed on the semiconductor substrate. 
   However, the above atomic layer composed of the first monolayer and the second monolayer and formed on a semiconductor substrate by ALD process is a continuous distribution layer, and is mainly applied in a hole, trench with a high line width and aspect ratio in a conventional process for manufacturing semiconductor. 
   With the development of the processes for manufacturing a semiconductor device, the critical dimension of semiconductor devices becomes smaller and the requirement for the storage density of semiconductor devices becomes higher. In a process for manufacturing a semiconductor device with a three-layer stack structure of medium layer-charge trapping layer-medium layer, the use of a charge trapping layer of discrete atomic islands (nano-dot) instead of a charge trapping layer having a certain thickness in the prior art can decrease the lateral current leakage, reduce the thickness of the charge trapping layer, and improve the memory capacity of the semiconductor device. However, in the prior art, the charge trapping layer of discrete atomic islands (nano-dot) is generally formed by chemical vapor deposition or physical vapor deposition, and the size of the resulting atomic islands is in a range of 10 nm˜100 nm, and the size distribution of the atomic islands is non-uniform. 
   SUMMARY OF THE INVENTION 
   To resolve the above-mentioned problem, the present invention provides an atomic layer deposition method to form a first discrete compound monolayer and a second discrete compound monolayer, and further provides a semiconductor device with a three-layer stack structure of medium layer-charge trapping layer-medium layer, wherein the discrete atomic size islands contained in the charge trapping layer are all at atomic level and have an uniform size distribution. 
   In an aspect according to the present invention, there is provided an atomic layer deposition method comprising the steps of: 
   placing a semiconductor substrate in an atomic layer deposition chamber; 
   flowing a first precursor gas to the semiconductor substrate within the atomic layer deposition chamber to form a first discrete monolayer on the semiconductor substrate; 
   flowing an inert purge gas to the semiconductor substrate within the atomic layer deposition chamber to remove the first precursor gas which does not form the first monolayer on the semiconductor substrate; 
   flowing a second precursor gas to the atomic layer deposition chamber to react with the first precursor gas which forms the first island, thereby forming a first discrete compound monolayer; 
   flowing an inert purge gas to the semiconductor substrate within the atomic layer deposition chamber to remove the second precursor gas that does not react with the first precursor gas and the byproduct of the reaction between the first and the second precursor gases; 
   flowing a third precursor gas to the semiconductor substrate within the atomic layer deposition chamber to form a third monolayer above the semiconductor substrate, there is a discrete distribution between the atoms/molecules of the third precursor gas forming the third monolayer and the atoms/molecules of the first precursor gas forming the first monolayer; 
   flowing an inert purge gas to the atomic layer deposition chamber to remove the third precursor gas that does not form the third monolayer; 
   flowing a forth precursor gas to the atomic layer deposition chamber to react with the third precursor gas that has formed the third monolayer, thereby forming a second discrete compound monolayer; 
   flowing an inert purge gas to the atomic layer deposition chamber to remove the forth precursor gas that does not react with the third monolayer and the byproduct of the reaction between the third and the forth precursor gases. 
   In another aspect according to the present invention, there is provided an atomic layer deposition method comprising the steps of: 
   placing a semiconductor substrate in an atomic layer deposition chamber; 
   flowing a first precursor gas to the semiconductor substrate within the atomic layer deposition chamber to form a first discrete monolayer on the semiconductor substrate; 
   flowing an inert purge gas to the semiconductor substrate within the atomic layer deposition chamber to remove the first precursor gas which does not form the first monolayer on the semiconductor substrate; 
   flowing a second precursor gas to the atomic layer deposition chamber to react with the first precursor gas which forms the first island, thereby forming a first discrete compound monolayer; and 
   flowing an inert purge gas to the semiconductor substrate within the atomic layer deposition chamber to remove the second precursor gas that does not react with the first precursor gas and the byproduct of the reaction between the first and the second precursor gases; 
   flowing a third precursor gas to the semiconductor substrate within the atomic layer deposition chamber to form a third monolayer above the semiconductor substrate, there is a discrete distribution between the atoms/molecules of the third precursor gas forming the third monolayer and the atoms/molecules of the first precursor gas forming the first monolayer; 
   flowing an inert purge gas to the atomic layer deposition chamber to remove the third precursor gas that does not form the third monolayer; 
   flowing a forth precursor gas to the atomic layer deposition chamber to react with the third precursor gas that has formed the third monolayer, thereby forming a second discrete compound monolayer; 
   flowing an inert purge gas to the atomic layer deposition chamber to remove the forth precursor gas that does not react with the third monolayer and the byproduct of the reaction between the third and the forth precursor gases; and 
   forming a dielectric layer to cover the first and second discrete compound monolayers above the semiconductor substrate. 
   Wherein the process for forming the dielectric layer is an atomic layer deposition process, which comprises the following steps: 
   (I) flowing a fifth precursor gas to the atomic layer deposition chamber to form a fifth monolayer above the semiconductor substrate and the first and second discrete compound monolayers, wherein the fifth monolayer fills the spacing in the first discrete compound monolayer, in the second discrete compound monolayer, and between the first and the second discrete compound monolayers; 
   (II) flowing an inert purge gas to the atomic layer deposition chamber to remove the fifth precursor gas that does not form the fifth monolayer; 
   (III) flowing a sixth precursor gas to the atomic layer deposition chamber to react with the fifth precursor gas that has formed the fifth monolayer, thereby forming a dielectric mono-atomic layer; 
   (IV) flowing an inert purge gas to the atomic layer deposition chamber to remove the sixth precursor gas that does not form the dielectric mono-atomic layer and the byproduct of the reaction between the fifth and the sixth precursor gases; and 
   repeating the steps (I) to (IV) until the resulting dielectric layer has a given thickness to cover the first and second discrete compound monolayers. 
   In still another aspect according to the present invention, there is provided a semiconductor device comprising a semiconductor substrate, a three-layer stack structure of medium layer-charge trapping layer-medium layer disposed above the semiconductor substrate, a gate disposed above the three-layer stack structure, and a source and a drain disposed in the semiconductor substrate at either side of the three-layer stack structure. The charge trapping layer is a dielectric layer containing the first and second discrete compound monolayers formed by ALD method, wherein there is a discrete distribution between the molecules in first discrete compound monolayer and in the second discrete compound monolayer. 
   The invention, compared with the prior art, has the following advantages: 
   1. The present invention provides an atomic layer deposition method, by which a first discrete compound monolayer is formed after a first discrete monolayer is formed above a semiconductor substrate, and a third monolayer is formed above the semiconductor substrate, followed by forming a second discrete compound monolayer on the third monolayer, the atoms/molecules in the first discrete compound monolayer, in the second discrete compound monolayer, and between the first and the second discrete compound monolayers show a discrete distribution, thereby achieving a multi-layer discrete distribution of the atomic size islands above the semiconductor substrate. Furthermore, the size of the islands in the first and second discrete compound monolayers is at atomic level and is uniform. 
   Further, in the atomic layer deposition method, a first discrete monolayer is formed on a semiconductor substrate by controlling the process of flowing the first precursor gas to the deposition chamber, for example, by reducing the flow rate and time; and the distribution density of the first monolayer on the semiconductor substrate is adjusted by controlling the flow rate and time for the precursor gas flowing to the deposition chamber. 
   Furthermore, in the atomic layer deposition method, a third discrete monolayer is formed on a semiconductor substrate by controlling the process of flowing the third precursor gas to the deposition chamber, and there is also achieved a discrete distribution between the atoms/molecules of the third precursor gas forming the third monolayer and the atoms/molecules of the first precursor gas forming the first monolayer by decreasing the flow rate and time for the precursor gas flowing to the deposition chamber; the distribution densities of the third monolayer on the first dielectric layer and of the first monolayer and the third monolayer on the semiconductor substrate are controlled by adjusting the flow rate and time for the third precursor gas flowing to the deposition chamber. 
   2. The present invention further provides an atomic layer deposition method, by which a first and second discrete compound monolayers are sealed in a dielectric layer having a thickness which can be controlled in atomic level. The discrete compound monolayer can be used in the manufacture of a semiconductor device, for example, as a charge trapping layer of a semiconductor memory device. 
   3. The present invention further provides a semiconductor device which comprises a three-layer stack structure of medium layer-charge trapping layer-medium layer, the charge trapping layer is a dielectric layer containing the first and second discrete compound monolayers, and there is also a discrete distribution between the atoms/molecules in first discrete compound monolayer and in the second discrete compound monolayer; the size of the islands in the first and second discrete compound monolayers is at atomic level and is controllable. Furthermore, the distribution density of the first and second discrete compound monolayers in the dielectric layer can be adjusted by controlling the flow rate and time for the first and third precursor gases flowing to the deposition chamber during the atomic layer deposition process. 
   The present invention is capable of improving the density of the charge trapping well in the charge trapping layer and the charge trapping capability, even if the semiconductor device has a small line width. And it can further improve the insulating capacity between the discrete islands in the first and second discrete compound monolayers, thereby reducing lateral current leakage of the device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 to 4  are structural diagrams illustrating an atomic layer deposition process according to the prior art; 
       FIGS. 5 to 12  are structural diagrams illustrating an atomic layer deposition process according to a first embodiment of the present invention; 
       FIG. 13  is a flow diagram illustrating the atomic layer deposition process according to the first embodiment of the present invention; 
       FIGS. 14 to 19  are structural diagrams illustrating an atomic layer deposition process according to a second embodiment of the present invention; 
       FIG. 20  is a flow diagram illustrating the atomic layer deposition process according to the second embodiment of the present invention; 
       FIG. 21  is a structural diagram illustrating a semiconductor device according to a third embodiment of the present invention. 
   

   SPECIFIC EMBODIMENTS OF THE INVENTION 
   An object of the present invention is to provide an atomic layer deposition method by which the first and second discrete compound monolayers are formed on a semiconductor substrate, wherein the first and second compound monolayers are composed of discrete atomic size islands (nano-dots), the size and thickness of the islands are at atomic level, and the number of the islands is controllable. 
   Another object of the present invention is to provide a semiconductor device with a three-layer stack structure of medium layer-charge trapping layer-medium layer, wherein the charge trapping layer is a dielectric layer containing the first and second discrete compound monolayers formed by atomic layer deposition method. 
   The above objects, features and advantages of the present invention will become better understood with respect to the following description of the preferred embodiments given in conjunction with the accompanying drawings. 
   First Embodiment 
   An atomic layer deposition method is provided in this embodiment. Referring to the flow diagram shown in  FIG. 13 , the method comprises the following steps: step S 200 , placing a semiconductor substrate in an atomic layer deposition chamber; step S 201 , flowing a first precursor gas to the semiconductor substrate within the atomic layer deposition chamber to form a first discrete monolayer on the semiconductor substrate; step S 202 , flowing an inert purge gas to the semiconductor substrate within the atomic layer deposition chamber to remove the first precursor gas; step S 203 , flowing a second precursor gas to the atomic layer deposition chamber to react with the first precursor gas which has formed the first monolayer to form a first discrete compound monolayer; step S 204 , flowing an inert purge gas to the deposition chamber to remove the second precursor gas that does not react with the first precursor gas and the by product of the reaction between the first and the second precursor gases; step S 205 , flowing a third precursor gas to the atomic layer deposition chamber to form a third monolayer above the semiconductor substrate, there is a discrete distribution between the atoms/molecules of the third precursor gas forming the third monolayer and the atoms/molecules of the first precursor gas forming the first monolayer; step S 206 , flowing an inert purge gas to the atomic layer deposition chamber to remove the third precursor gas that does not form the third monolayer with the first dielectric layer; step S 207 , flowing a forth precursor gas to the atomic layer deposition chamber to react with the third precursor gas that has formed the third monolayer, thereby forming a second discrete compound monolayer; and step S 208 , flowing an inert purge gas to the atomic layer deposition chamber to remove the forth precursor gas that does not react with the third monolayer and the byproduct of the reaction between the third and the forth precursor gases. 
   Firstly, referring to the step  200 , a semiconductor substrate  200  is placed in an atomic layer deposition chamber. The deposition chamber is a deposition chamber of conventional reaction apparatus for atomic layer deposition in accordance with the prior art. In order to distribute the reaction gases on the semiconductor substrate as uniformly as possible during atomic layer deposition process, the reaction gas inlet device of the deposition apparatus is preferably configured to enable gases to flow into the deposition chamber uniformly from various directions, for example, a shower head type inlet device can be employed to make the distribution of discrete islands formed on the semiconductor substrate more uniform. 
   The semiconductor substrate  200  can be made of various semiconductor materials known to those skilled in the field of semiconductor, including silicon or silicon germanium (SiGe) with monocrystal or polycrystal structures, ion-doped Si or SiGe such as N-doped or P-doped Si or SiGe, compound semiconductor such as silicon carbide, indium antimonide, lead telluride, indium arsenide, indium phosphide, gallium arsenide or gallium telluride, alloy semiconductor or combinations thereof, and silicon-on-insulator (SOI). The semiconductor substrate can be a blank semiconductor substrate, or a semiconductor substrate provided with various semiconductor devices and wirings therein. 
   Referring to the step S 201 , a first precursor gas flows to a semiconductor substrate  200  in an atomic layer deposition chamber, as shown in  FIG. 5 , and a first discrete monolayer  210  is formed on the semiconductor substrate  200  due to the physical or chemical adsorption between the first precursor gas and the semiconductor substrate. There is also adsorbed the atoms of the first precursor gas on the first monolayer  210  which is directly contacted with the semiconductor substrate  200  due to the mutual adsorption between the atoms of the first precursor gas. 
   The first precursor gas can be any of reaction gases having nucleation matter and capable of ly forming the first monolayer  210  on the semiconductor substrate  200  via chemical or physical adsorption. It can be selected from one or more of metal materials, semi-conductive materials, or metal coordinated with halogen and organic complex, or semiconductor coordinated with halogen and organic complex or mixtures thereof, for example, the metal materials include Ta, Ti, W, Mo, Nb, Cu, Ni, Pt, Ru, Me, Ni and Al etc., the semi-conductive materials include silicon etc., and the semiconductor coordinated with halogen and organic complex includes SiCl 2 H 2 , Si(OC 2 H 5 ) 4 , SiH 2 [NH(C 4 H 9 )] 2 , and SiH(OC 2 H 5 ) 3  etc. 
   As an illustrative example, the embodiment provides several particular first precursor gases for well understanding and implementing the invention by those skilled in the art. If the final first discrete compound monolayer to be formed is Si 3 N 4  layer, the first precursor gas is a reaction gas having Si atom nucleation matter, such as SiCl 2 H 2 , SiH 4 , Si 2 Cl 6  or SiH 2 [NH(C 4 H 9 )] 2 , etc. 
   If the final first discrete compound monolayer to be formed is SiO 2 , the first precursor gas can be a reaction gas having Si atom nucleation matter, such as Si(OC 2 H 5 )4, SiH 2 [NH(C 4 H 9 )] 2 , SiH(OC 2 H 5 ) 3 , Si 2 Cl 6 , or SiHN[(CH 3 ) 2 ] 3  and the like. 
   If the final first discrete compound monolayer to be formed is HfO layer, the first precursor gas is a reaction gas having Hf atom nucleation matter, such as Hf[N(CH 3 )(C 2 H 5 )] 4 , Hf[N(C 2 H 5 ) 2 ] 4 , Hf[OC(CH 3 ) 3 ] 4 , or HfCl 4 , etc. 
   If the final first discrete compound monolayer to be formed is Al 2 O 3  layer, the first precursor gas is a reaction gas having Al atom nucleation matter, such as Al(CH 3 ) 3 , etc. 
   If the final first discrete compound monolayer to be formed is WN layer, the first precursor gas is a reaction gas having W atom nucleation matter, such as WF 6 , etc. 
   In order to make the first precursor gas form the first discrete monolayer on the semiconductor substrate, the specific process conditions of the first precursor gas flowing to the deposition chamber should be controlled. The conditions contributing to the discrete distribution of the first precursor gas on the semiconductor substrate include the gas flow rate, flowing time, fluid temperature, and pressure etc. of the first precursor gas flowing into the atomic layer deposition chamber. 
   In this embodiment, the flow rate and flowing time of the first precursor gas flowing to the deposition chamber play a key role in achieving a discrete distribution. In order to make the first precursor gas form the first discrete monolayer on the semiconductor substrate, the flow rate and flowing time of the first precursor gas flowing to the deposition chamber should be reduced on the basis of the process of forming a dense first monolayer in prior art, and the flowing of the first precursor gas is ceased before the first precursor gas adsorbed on the semiconductor substrate forms a dense and continuous distribution layer. In a preferred example, the flow rate of the first precursor gas in the deposition chamber is greatly decreased while the flowing time is properly increased, thereby improving the controllability of the process. 
   In this embodiment, the distribution density of the first precursor gas which forms the first monolayer on the semiconductor substrate can be controlled by controlling the flow rate and flowing time of the first precursor gas flowing to the atomic layer deposition chamber. 
   In the prior art, as to different first precursor gases, the flow rate and flowing time required for forming a dense first monolayer on the semiconductor substrate are different; similarly, in the embodiment of present invention, as to different first precursor gases, the flow rate and flowing time required for forming a first discrete monolayer on the semiconductor substrate are different. However, the process of forming a first discrete monolayer according to the present invention is based on the process of forming a dense first monolayer in the prior art and is performed by reducing the flow rate and flowing time of the first precursor gas. In the process of forming a first discrete monolayer according to the present invention, the first monolayer with a discrete distribution can be formed depending on the requirement of process design, and the distribution density of the first precursor gas for forming the first discrete monolayer is controllable. 
   In the embodiment, there is provided a particular example for implementing the invention by the skilled in the art. If the final first discrete compound monolayer to be formed is Si 3 N 4 , the first precursor gas SiCl 2 H 2  is flowed into a conventional deposition apparatus, at a flow rate of 0.06˜0.3 slm and for 0˜10 sec, preferably 0˜7 sec. The pressure under which the first precursor gas flows into the deposition chamber is 500-800 Pa, and the temperature in the deposition chamber is in a range of 450-600° C., preferably 550° C. 
   In another particular example, if the final first discrete compound monolayer to be formed is Al 2 O 3 , nitrogen gas carrying the liquid of Al(CH 3 ) 3  is flowed into a conventional atomic layer deposition apparatus as the first precursor gas, wherein the temperature of the nitrogen gas is 25° C., the flow rate of nitrogen gas is 0.03-0.15 slm, and the flowing time is more than 0, but less than 10 sec, preferably more than 0, but less than 7 sec. The pressure under which the first precursor gas flows into the deposition chamber is in a range of 3-5 Pa, and the temperature in the deposition chamber is in a range of 250-450° C., preferably 400° C. 
   In the step S 202 , as shown in  FIG. 6 , an inert purge gas flows to the semiconductor substrate  200  in an atomic layer deposition chamber to remove the first precursor gas that does not form the first monolayer  210  on the semiconductor substrate  200 . 
   In this embodiment, the flowing to the semiconductor substrate in the atomic layer deposition chamber only represents the gas flow direction, and does not means that the gas does always contact the semiconductor substrate directly or react with the same, since the other monolayer or dielectric layer has been formed on the semiconductor substrate after the step  202  of this embodiment. 
   In this step, only the first precursor gas directly contacting the semiconductor substrate  200  is remained to form the first monolayer of the first precursor gas atom/molecule on the semiconductor substrate  200 , and the other first precursor gas which is not adsorbed on the semiconductor substrate  200  directly is removed. The inert gas includes He, Ne, Ar, and the like. 
   After purging by the inert gas, a uniform or non-uniform first monolayer is formed on the semiconductor substrate with a mono-atomic discrete distribution. 
   The purging of the inert gas can be performed according to any of the conventional processes in the prior art. There is provided a particular example for implementing the invention by the skilled in the art, for example, nitrogen (N 2 ) gas flows into and purges the deposition chamber at a flow rate of 5 slm under a pressure of 0.3 Torr. 
   In step S 203 , a second precursor gas flows to the semiconductor substrate in an atomic layer deposition chamber, and reacts with the first precursor gas which forms the first monolayer, thereby forming a discrete compound monolayer. As shown in  FIG. 7 , the second precursor gas flows to the semiconductor substrate in the deposition chamber, and reacts with the first precursor gas to form a discrete compound monolayer  220 . At the same time, due to the adsorption force between atoms, the second precursor gas can be adsorbed on the semiconductor substrate  200 , and the atoms of the second precursor gas can adsorb each other. Due to the chemical reaction between the second precursor gas and the first precursor gas, by product may be generated from the reaction between the second precursor gas and the first precursor gas in the deposition chamber. 
   Depending on the first discrete compound monolayer to be formed and the first precursor gas, the second precursor gas can be any of the conventional substances in the prior art that can react with the first precursor gas to generate the discrete compound monolayer. 
   In a particular example, the second precursor gas is a material containing N or O or metal atoms and is used as a reductant or oxidant, such as NH 3  or O 2 . 
   As an illustrative example only, the embodiment provides several particular second precursor gases for well understanding and implementing the invention by the skilled in the art. If the final first discrete compound monolayer to be formed is Si 3 N 4 , the first precursor gas is reaction gas having Si atom nucleation matter, and the second precursor gas is gas that can react with the nucleation matter of the first monolayer formed by the first precursor gas to form a discrete compound monolayer, such as NH 3 , N 2 O, and N 2  and the like. 
   If the final first discrete compound monolayer to be formed is HfO, the first precursor gas is a reaction gas having Hf atom nucleation matter, and the second precursor gas can be the gas that can react with the nucleation matter of the first monolayer formed by the first precursor gas to form a discrete compound monolayer, such as O 3  and the like. 
   If the final first discrete compound monolayer to be formed is Al 2 O 3 , the first precursor gas is a reaction gas having Al atom nucleation matter, and the second precursor gas can be the gas that can react with the nucleation matter of the first monolayer formed by the first precursor gas to form a discrete compound monolayer, such as H 2 O or O 3  and the like. 
   If the final first discrete compound monolayer to be formed is WN, the first precursor gas is a reaction gas having W atom nucleation matter, and the second precursor gas can be, for example, NH 3  and the like. 
   The process of flowing the second precursor gas into the semiconductor substrate in the deposition chamber can be performed by any conventional techniques known to the skilled in the art. For example, when the first precursor gas is SiCl 2 H 2  and the final first discrete compound monolayer to be formed is Si 3 N 4 , NH 3  is used as the second precursor gas and injected into a conventional atomic layer deposition apparatus in the prior art at a flow rate of 2-5 slm for 0-30 sec, while the pressure in the deposition chamber is 30-50 Pa, and the temperature in the deposition chamber is 450-600° C., preferably 550° C. 
   In the step S 204 , as shown in  FIG. 8 , an inert purge gas flows to the semiconductor substrate in the deposition chamber to remove the second precursor gas that does not react with the first precursor gas and the by product of the reaction between the first precursor gas and the second precursor gas. 
   The inert gas can be He, Ne, Ar and the like. 
   The process conditions for the purging of the inert gas can be achieved through conventional experiments by the skilled in the art, for example, nitrogen can flow into and purge an atomic layer deposition chamber at a flow rate of 5 slm under a pressure of 0.3 Torr. 
   In the step S 205 , a third precursor gas flows to a semiconductor substrate  200  in an atomic layer deposition chamber, as shown in  FIG. 9 , and a third discrete monolayer  230  is formed on the semiconductor substrate  200  due to the physical or chemical adsorption between the third precursor gas and the semiconductor substrate  200 , there is a discrete distribution between the atoms/molecules of the third precursor gas forming the third monolayer and the atoms/molecules of the first precursor gas forming the first monolayer. 
   There is also adsorbed the atoms of the third precursor gas on the third monolayer  260  which is directly contacted with the semiconductor substrate  200  due to the mutual adsorption between the atoms of the first precursor gas. Also, there is adsorbed the atoms of the third precursor gas on the first monolayer of the discrete atomic size islands  220 . 
   The third precursor gas can be any of reaction gases having nucleation matter and capable of forming the third monolayer on the semiconductor substrate via chemical or physical adsorption. It can be selected from one or more of metal materials, semi-conductive materials, or metal coordinated with halogen and organic complex, or semiconductor coordinated with halogen and organic complex or mixtures thereof, for example, the metal materials include Ta, Ti, W, Mo, Nb, Cu, Ni, Pt, Ru, Me, Ni and Al etc., the semi-conductive materials include silicon etc., and the semiconductor coordinated with halogen and organic complex includes SiCl 2 H 2 , Si(OC 2 H 5 ) 4 , SiH 2 [NH(C 4 H 9 )] 2 , and SiH(OC 2 H 5 ) 3 , etc. 
   As an illustrative example, the embodiment provides several particular third precursor gases for well understanding and implementing the invention by those skilled in the art. If the final second discrete compound monolayer to be formed is Si 3 N 4  layer, the third precursor gas is a reaction gas having Si atom nucleation matter, such as SiCl 2 H 2 , SiH 4 , Si 2 Cl 6  or SiH 2 [NH(C 4 H 9 )] 2 , etc. 
   If the final second discrete compound monolayer to be formed is SiO 2 , the third precursor gas can be a reaction gas having Si atom nucleation matter, such as Si(OC 2 H 5 )4, SiH 2 [NH(C 4 H 9 )] 2 , SiH(OC 2 H 5 ) 3 , Si 2 Cl 6 , or SiHN[(CH 3 ) 2 ] 3  and the like. 
   If the final second discrete compound monolayer to be formed is HfO layer, the third precursor gas is a reaction gas having Hf atom nucleation matter, such as Hf[N(CH 3 )(C 2 H 5 )] 4 , Hf[N(C 2 H 5 ) 2 ] 4 , Hf[OC(CH 3 ) 3 ] 4 , or HfCl 4 , etc. 
   If the final second discrete compound monolayer to be formed is Al 2 O 3  layer, the first precursor gas is a reaction gas having Al atom nucleation matter, such as Al(CH 3 ) 3 , etc. 
   If the final second discrete compound monolayer to be formed is WN layer, the first precursor gas is a reaction gas having W atom nucleation matter, such as WF 6 , etc. 
   In order to make the third precursor gas form the third discrete monolayer on the semiconductor substrate, the specific process conditions of the third precursor gas flowing to the deposition chamber may be controlled. The conditions contributing to the discrete distribution of the third precursor gas on the semiconductor substrate include the gas flow rate, flowing time, fluid temperature, and pressure etc. of the third precursor gas flowing into the atomic layer deposition chamber. 
   In this embodiment, the flow rate and flowing time of the third precursor gas flowing to the deposition chamber play a key role in achieving a discrete distribution. In order to make the third precursor gas form the third discrete monolayer on the semiconductor substrate, the flow rate and flowing time of the third precursor gas flowing to the deposition chamber should be reduced on the basis of the process of forming a dense third monolayer in prior art, and the flowing of the third precursor gas is ceased before the third precursor gas adsorbed on the semiconductor substrate forms a dense and continuous distribution layer. In a preferred example, the flow rate of the third precursor gas in the deposition chamber is greatly decreased while the flowing time is properly increased, thereby improving the controllability of the process. 
   In this embodiment, the distribution density of the third precursor gas which forms the third monolayer on the semiconductor substrate can be controlled by controlling the flow rate and flowing time of the third precursor gas flowing to the atomic layer deposition chamber. 
   In the prior art, as to different third precursor gases, the flow rate and flowing time required for forming a dense third monolayer on the semiconductor substrate are different; similarly, in the embodiment of present invention, as to different third precursor gases, the flow rate and flowing time required for forming a third discrete monolayer on the semiconductor substrate are different. However, the process of forming a third discrete monolayer according to the present invention is based on the process of forming a third dense monolayer in the prior art and is performed by reducing the flow rate and flowing time of the third precursor gas. In the process of forming a third discrete monolayer according to the present invention, the third monolayer with a discrete distribution can be formed depending on the requirement of process design, and the distribution density of the third precursor gas for forming the third discrete monolayer is controllable. 
   In the embodiment, there is provided a particular example for well implementing the invention by the skilled in the art. If the final second discrete compound monolayer to be formed is Si 3 N 4 , the third precursor gas SiCl 2 H 2  is flowed into a conventional deposition apparatus, at a flow rate of 0.06˜0.3 slm and for 0˜10 sec, preferably 0˜7 sec. The pressure under which the third precursor gas flows into the deposition chamber is 500-800 Pa, and the temperature in the deposition chamber is in a range of 450-600° C., preferably 550° C. 
   In another particular example, if the final second discrete compound monolayer to be formed is Al 2 O 3 , nitrogen gas carrying the liquid of Al(CH 3 ) 3  is flowed into a conventional atomic layer deposition apparatus as the third precursor gas, wherein the nitrogen gas is heated through a small heating vessel at a temperature of 25° C., and the heating vessel has a capacity of 300 grams, the flow rate of nitrogen gas is 0.03-0.15 slm, and the flowing time is more than 0, but less than 10 sec, preferably more than 0, but less than 7 sec. The pressure under which the third precursor gas flows into the deposition chamber is in a range of 3-5 Pa, and the temperature in the deposition chamber is in a range of 250-450° C., preferably 400° C. 
   In the step S 206 , as shown in  FIG. 10 , an inert purge gas flows to the semiconductor substrate in an atomic layer deposition chamber to remove the third precursor gas that does not form the third monolayer on the semiconductor substrate. 
   In this step, only the third precursor gas directly contacting the semiconductor substrate  200  is remained, and the other third precursor gas which is not adsorbed on the semiconductor substrate  200  directly is removed. The inert gas includes He, Ne, Ar, and the like. 
   After purging by the inert gas, a uniform or non-uniform third monolayer is formed on the semiconductor substrate  200  with a mono-atomic discrete distribution. Since the first discrete compound monolayer has been formed above the semiconductor substrate, there are formed a first discrete compound monolayer and a first discrete monolayer above the semiconductor substrate, wherein there is a discrete distribution between the atoms/molecules of the first precursor gas forming the first monolayer and the atoms/molecules of the first discrete compound monolayer. 
   The purging of the inert gas can be performed according to any of the conventional processes in the prior art. There is provided a particular example for implementing the invention by the skilled in the art, for example, nitrogen (N 2 ) gas flows into and purges the deposition chamber at a flow rate of 5 slm under a pressure of 0.3 Torr. 
   In step S 207 , as shown in  FIG. 11 , a fourth precursor gas flows to the semiconductor substrate  200  in an atomic layer deposition chamber, and reacts with the third precursor gas which forms the third monolayer  230 , thereby forming a second discrete compound monolayer  250 . At the same time, due to the adsorption force between atoms, the fourth precursor gas can also be adsorbed on the semiconductor substrate  200 , and there is also adsorption between atoms of the fourth precursor gas. Due to the chemical reaction between the fourth precursor gas and the third precursor gas, by products may be generated from the reaction between the fourth precursor gas and the third precursor gas in the deposition chamber. 
   Depending on the second discrete compound monolayer to be formed and the third precursor gas, the fourth precursor gas can be any of the conventional substances in the prior art that can react with the third precursor gas to generate the second discrete compound monolayer. 
   In a particular example, the fourth precursor gas may be a material containing N or O or metal atoms and is used as a reductant or oxidant, such as NH 3  or O 2 . 
   As an illustrative example only, the embodiment provides several particular fourth precursor gases for well understanding and implementing the invention by the skilled in the art. If the final second discrete compound monolayer to be formed is Si 3 N 4 , the third precursor gas is a reaction gas having Si atom nucleation matter, and the fourth precursor gas is a gas that can react with the nucleation matter of the third monolayer formed by the third precursor gas to form a second discrete compound monolayer, such as NH 3 , N 2 O, and N 2  and the like. 
   If the final second discrete compound monolayer to be formed is SiO 2 , the third precursor gas is a reaction gas having Si atom nucleation matter, and the fourth precursor gas is a gas that can react with the nucleation matter of the third monolayer formed by the third precursor gas to form a second discrete compound monolayer, such as NH 3 , N 2 O, and O 2  and the like. 
   If the final second discrete compound monolayer to be formed is HfO, the third precursor gas is a reaction gas having Hf atom nucleation matter, and the fourth precursor gas may be the gas that can react with the nucleation matter of the third monolayer formed by the third precursor gas to form a second discrete compound monolayer, such as O 3  and the like. 
   If the final second discrete compound monolayer to be formed is Al 2 O 3 , the third precursor gas is a reaction gas having Al atom nucleation matter, and the fourth precursor gas may be the gas that can react with the nucleation matter of the third monolayer formed by the third precursor gas to form a second discrete compound monolayer, such as H 2 O or O 3  and the like. 
   If the final second discrete compound monolayer to be formed is WN, the third precursor gas is a reaction gas having W atom nucleation matter, and the fourth precursor gas can be, for example, NH 3  and the like. 
   The process of flowing the fourth precursor gas into the semiconductor substrate in the deposition chamber can be performed by any conventional techniques known to the skilled in the art. For example, when the third precursor gas is SiCl 2 H 2  and the final discrete compound monolayer is Si 3 N 4 , NH 3  is used as the fourth precursor gas and injected into a conventional atomic layer deposition apparatus in the prior art at a flow rate of 2-5 slm for 0-30 sec, while the pressure in the deposition chamber is 30-50 Pa, and the temperature in the deposition chamber is 450-600° C., preferably 550° C. 
   In this embodiment, there are formed more discrete compound monolayers (nano-dot) above the semiconductor substrate. The first precursor gas and the third precursor gas all can be selected from the various conventional gases available in the atomic layer deposition process and known to those skilled in the art. In the specific examples provided in this embodiment, the first precursor gas is different from the third precursor gas. In this embodiment, if the first discrete compound monolayer is made of silicon nitride, the second discrete compound monolayer can be made of materials other than silicon nitride, such as silicon oxide, aluminum oxide. 
   In the step S 208 , as shown in  FIG. 12 , an inert purge gas flows to the semiconductor substrate in the deposition chamber to remove the fourth precursor gas that does not react with the third precursor gas and the by product of the reaction between the third precursor gas and the fourth precursor gas. After purging by the inert gas, a first and second discrete compound monolayers are formed with a mono-atomic discrete distribution on the semiconductor substrate, that is, the first discrete compound monolayer has a discrete distribution, the second discrete compound monolayer has a discrete distribution, and there is a discrete distribution between the first and the second discrete compound monolayers. 
   Two discrete compound monolayers can be formed above the semiconductor substrate by the atomic deposition process described in the embodiment. In the light of this embodiment and principle, more than two kinds of discrete compound monolayer can be formed continuously above the semiconductor substrate by referring the process of forming the first or the second discrete compound monolayer, and the more than two kinds of discrete compound monolayer can be made of any materials formed by the conventional atomic layer deposition process in the prior art. 
   In this embodiment, more than one kind of discrete compound monolayer is formed above the semiconductor substrate, such as the first and second discrete compound monolayers, and the first and the second discrete compound monolayers are in a discrete distribution on the semiconductor substrate. The distributions of the first and the second discrete compound monolayers on the semiconductor substrate may be controlled by controlling the flow rates and time of the first precursor gas for forming the first discrete compound monolayer and of the third precursor gas for forming the second discrete compound monolayer. When used in a semiconductor device, the more than one kind of discrete compound monolayer can improve the performance of the semiconductor device. For example, when the first and the second discrete compound monolayers are made of aluminum oxide and hafnium oxide respectively, they can be used as dielectric layers of a capacitor in DRAM, wherein if the content of the aluminum oxide is higher, the breakdown of the capacitor can be improved, and if the content of the hafnium oxide is higher, the capacitance of the capacitor can be improved. 
   Second Embodiment 
   Referring the flow diagram in  FIG. 20 , there is provided an atomic layer deposition process in this embodiment, comprising the following steps: 
   Step S 300 , placing a semiconductor substrate in an atomic layer deposition chamber; 
   Step S 301 , flowing a first precursor gas to the semiconductor substrate within the atomic layer deposition chamber to form a first discrete monolayer on the semiconductor substrate; 
   Step S 302 , flowing an inert purge gas to the semiconductor substrate within the atomic layer deposition chamber to remove the first precursor gas which does not form the first monolayer on the semiconductor substrate; 
   Step S 303 , flowing a second precursor gas to the atomic layer deposition chamber to react with the first precursor gas which forms the first island, thereby forming a first discrete compound monolayer; 
   Step S 304 , flowing an inert purge gas to the semiconductor substrate within the atomic layer deposition chamber to remove the second precursor gas that does not react with the first precursor gas and the byproduct of the reaction between the first and the second precursor gases; 
   Step S 305 , flowing a third precursor gas to the semiconductor substrate within the atomic layer deposition chamber to form a third monolayer above the semiconductor substrate, there is a discrete distribution between the atoms/molecules of the third precursor gas forming the third monolayer and the atoms/molecules of the first precursor gas forming the first monolayer; 
   Step S 306 , flowing an inert purge gas to the atomic layer deposition chamber to remove the third precursor gas that does not form the third monolayer; 
   Step S 307 , flowing a forth precursor gas to the atomic layer deposition chamber to react with the third precursor gas that has formed the third monolayer, thereby forming a second discrete compound monolayer; 
   Step S 308 , flowing an inert purge gas to the atomic layer deposition chamber to remove the forth precursor gas that does not react with the third monolayer and the byproduct of the reaction between the third and the forth precursor gases; and 
   Step S 309 , forming a dielectric layer to cover the first and second discrete compound monolayers above the semiconductor substrate. 
   Wherein the steps S 300  to  308  can be performed by referring to the corresponding processes in the first embodiment, a structure comprising a first discrete compound monolayer and a second discrete compound monolayer is formed above the semiconductor substrate after the step S 308 , as shown in  FIG. 14 . 
   In the step S 309 , the process for forming the dielectric layer on the semiconductor substrate to cover the first and second discrete compound monolayers can be any techniques well known to those skilled in the art, such as physical vapor deposition process, chemical vapor deposition process. In this embodiment, there is provided an atomic layer deposition process to form the first dielectric layer, which comprises the following steps: 
   (I) flowing a fifth precursor gas to the atomic layer deposition chamber to form a fifth monolayer above the semiconductor substrate and the first and second discrete compound monolayers, wherein the fifth monolayer fills the spacing in the first and second discrete compound monolayers, and between the first discrete compound monolayer and the second discrete compound monolayer; 
   (II) flowing an inert purge gas to the atomic layer deposition chamber to remove the fifth precursor gas that does not form the fifth monolayer; 
   (III) flowing a sixth precursor gas to the atomic layer deposition chamber to react with the fifth precursor gas that has formed the fifth monolayer, thereby forming a dielectric mono-atomic layer; 
   (IV) flowing an inert purge gas to the atomic layer deposition chamber to remove the sixth precursor gas that does not form the dielectric mono-atomic layer and the byproduct of the reaction between the fifth and the sixth precursor gases; and repeating the steps (I) to (IV) until the resulting dielectric layer has a given thickness to cover the first and second discrete compound monolayers. 
   Referring to  FIG. 15 , the fifth precursor gas flows to the semiconductor substrate  300  in the atomic layer deposition chamber. Since the first discrete compound monolayer  320  and the second discrete compound monolayer  350  have been formed on the semiconductor substrate  300 , the fifth precursor gas forms a fifth monolayer  330  above the semiconductor substrate  300  and the first discrete compound monolayer  320  and the second discrete compound monolayer  350 ; furthermore, due to the discrete distribution of the islands in the first discrete compound monolayer  320  and the second discrete compound monolayer  350  on the semiconductor substrate  300 , the third monolayer  330  on the semiconductor substrate  300  fills the space in the first discrete compound monolayer  320 , the second discrete compound monolayer  350 , and between the first discrete compound monolayer  320  and the second discrete compound monolayer  350 . The fifth precursor gas positioned on the first discrete compound monolayer  320  and the second discrete compound monolayer  350  combines with the first discrete compound monolayer  320  and the second discrete compound monolayer  350  via interatomic force or chemical bonds. 
   The fifth precursor gas can be any of the reaction gases that has nucleation matter in the prior art and can form the fifth monolayer on the semiconductor substrate, the first discrete compound monolayer  320  and the second discrete compound monolayer  350  through chemical or physical adsorption. Furthermore, the fifth precursor gas can react with a sixth precursor gas to form insulation materials, such as silicon oxide, silicon nitride, and silicon oxynitride, etc. 
   For better understanding and implementing the invention by the skilled in the art, the embodiment provides several particular examples. If the final dielectric layer to be formed is Si 3 N 4  layer, the fifth precursor gas is a reaction gas having Si atom nucleation matter, such as SiCl 2 H 2 , SiH 4 , Si 2 Cl 6  or SiH 2 [NH(C 4 H 9 )] 2  etc. 
   If the final dielectric layer to be formed is SiO 2 , the fifth precursor gas can be Si(OC 2 H 5 ) 4 , SiH 2 [NH(C 4 H 9 )] 2 , SiH(OC 2 H 5 ) 3 , Si 2 Cl 6 , or SiHN[(CH 3 ) 2 ] 3  and the like. 
   The processes of the third precursor gas flowing to the semiconductor substrate in the atomic layer deposition chamber can be performed by any techniques in the prior art known to those skilled in the art. 
   Referring to the  FIG. 16 , the inert purge gas flows to the semiconductor substrate  300  in the atomic layer deposition chamber to remove the fifth precursor gas which does not form the fifth monolayer on the semiconductor substrate  200 , the first discrete compound monolayer  320  and the second discrete compound monolayer  350 . The inert purge gas can be, foe example, He, Ne, Ar, and the like, and the process conditions of purging can be determined by those skilled in the art. 
   Referring to the  FIG. 17 , the sixth precursor gas flows to the semiconductor substrate in the atomic layer deposition chamber and reacts with the fifth precursor gas that has formed the fifth monolayer  330  to form a dielectric mono-atomic layer  340 . 
   The fifth precursor gas forming the third monolayer  330  includes two portions, the first portion is that fills the spacings in the first discrete compound monolayer  320 , the second discrete compound monolayer  350 , and between the first discrete compound monolayer  320  and the second discrete compound monolayer  350 , and the second portion is that lies on the first discrete compound monolayer  320 , the second discrete compound monolayer  350  and combines with the first portion via adsorption. 
   The sixth precursor gas reacts with the fifth precursor gas to form a dielectric mono-atomic layer  340 , which can be a mono-atomic layer of insulation materials such as silicon oxide, silicon nitride, and silicon oxynitride, etc. 
   As an illustrative example, if the final dielectric layer to be formed is Si 3 N 4  layer, the sixth precursor gas can be NH 3 , N 2 O, N 2  and the like. 
   If the final dielectric layer to be formed is SiO 2 , the forth precursor gas can be NH 3 , N 2 O, O 2  and the like. 
   The processes of the sixth precursor gas flowing to the semiconductor substrate in the atomic layer deposition chamber can be performed by any techniques in the prior art known to those skilled in the art. 
   Referring to the  FIG. 18 , the inert purge gas flows to the semiconductor substrate in the atomic layer deposition chamber to remove the sixth precursor gas that does not form the dielectric mono-atomic layer and the by product of the reaction between the fifth and the sixth precursor gases. 
   The inert purge gas is He, Ne, Ar and the like, and the process conditions of purging can be determined by those skilled in the art. 
   Referring to the  FIG. 19 , the deposition process of the dielectric mono-atomic layer is performed on the dielectric mono-atomic layer one or more times again to form a dielectric layer having a given thickness to cover the first and second discrete compound monolayers. 
   The process steps for continuously forming dielectric layers on the dielectric mono-atomic layer are the same as the process steps described in the  FIGS. 15-19 . The embodiment provides a process for forming another dielectric mono-atomic layer on the dielectric mono-atomic layer, comprising the steps of: 
   1) flowing a seventh precursor gas to the semiconductor substrate in the atomic layer deposition chamber to form a seventh monolayer on the dielectric mono-atomic layer and the discrete compound monolayer, and the seventh monolayer covers the dielectric mono-atomic layer and the discrete compound monolayer; 
   wherein, the seventh precursor gas is the same as the fifth precursor gas for forming the fifth monolayer, which comprises any of the reaction gases in the prior art that have nucleation matter and can form the seventh monolayer on the dielectric mono-atomic layer and the discrete compound monolayer through chemical or physical adsorption. Furthermore, the seventh precursor gas can react with a eighth precursor gas to form insulation materials, such as silicon oxide, silicon nitride, or silicon oxynitride, etc. 
   2) flowing an inert purge gas to the semiconductor substrate in the atomic layer deposition chamber to remove the seventh precursor gas that does not form the seventh monolayer on the dielectric mono-atomic layer and the discrete compound monolayer. 
   3) flowing a eighth precursor gas to the semiconductor substrate in the atomic layer deposition chamber to react with the seventh precursor gas which has formed the seventh monolayer, thereby forming another dielectric mono-atomic layer  360  above the dielectric mono-atomic layer  340 , 
   wherein the eighth precursor gas is the same as the seventh precursor gas, and reacts with the seventh precursor gas to form the dielectric mono-atomic layer  360 , which is a mono-atomic layer of insulation materials such as silicon oxide and the like; and 
   4) flowing an inert purge gas to the semiconductor substrate in the deposition atomic layer chamber to remove the eighth precursor gas that does not form the dielectric mono-atomic layer and the by product of the reaction between the seventh and the eighth precursor gases. 
   Another dielectric mono-atomic layer  360  is formed to cover the dielectric mono-atomic layer  340  and the first discrete compound monolayer  320 , the second discrete compound monolayer  350  by the processes from step 1) to step 4). 
   In this embodiment, the flowing to the semiconductor substrate in the atomic layer deposition chamber only means that the gas flows to the position where the semiconductor substrate is placed. 
   If necessary, the processes of forming the dielectric mono-atomic layer  340  or the dielectric mono-atomic layer  360  can be repeatedly performed many times to obtain dielectric layers having desired thickness. 
   In the process described in the embodiment, after forming a first discrete compound monolayer and a second discrete compound monolayer, a dielectric layer is formed on the first discrete compound monolayer and the second discrete compound monolayer to cover and seal the first discrete compound monolayer and the second discrete compound monolayer. The discrete compound monolayers can be used in the manufacture of a semiconductor device, for example, as a charge trapping layer of a semiconductor memory device. 
   Third Embodiment 
   As shown in  FIG. 21 , there is provided a semiconductor device in this embodiment, comprising a semiconductor substrate  400 , a three layer stack structure of medium layer  430 -charge trapping layer  440 -medium layer  450  arranged on the semiconductor substrate  400 , and a gate  460  arranged on the three layer structure, and a source  410  and a drain  420  arranged in the semiconductor substrate at either side of the three layer structure, wherein the charge trapping layer  440  is a dielectric layer containing the first discrete compound monolayer and the second discrete compound monolayer formed by atomic layer deposition. Here the word “containing” means that the first discrete compound monolayer and the second discrete compound monolayer are embedded in dielectric layers and covered by the same. 
   The semiconductor substrate  400  can include silicon(Si) or silicon germanium (SiGe) with monocrystal or polycrystal structure, ion-doped Si or SiGe such as N-doped or P-doped Si or SiGe, compound semiconductor such as silicon carbide, indium antimonide, lead telluride, indium arsenide, indium phosphide, gallium arsenide or gallium telluride, alloy semiconductor or combinations thereof, and silicon-on-insulator (SOI). 
   In the three layer stack structure, the medium layer  430  or  450  can be insulation material, such as SiO 2 ; the charge trapping layer  440  can be a dielectric layer containing a first discrete compound monolayer and a second discrete compound monolayer, and the dielectric layer is made of insulation materials such as silicon oxide, silicon nitride, and silicon oxynitride; the discrete compound monolayer is sealed in dielectric layer, and the islands constituting the first discrete compound monolayer and the second discrete compound monolayer are in uniform or nonuniform atom distribution and form discrete nano-dots. 
   The first and the second compound monolayers can be any of the substance used for trapping charges in a charge trapping layer of a semiconductor device in the prior art, such as SiO 2 , Si 3 N 4 , Al 2 O 3 , HfO, or WN. 
   In this embodiment, the material of the first discrete compound monolayer is different from that of the second discrete compound monolayer, for example, if the first discrete compound monolayer is made of Si 3 N 4 , the second discrete compound monolayer can be made of Al 2 O 3 , HfO, or WN etc. 
   The first discrete compound monolayer and the second discrete compound monolayer are formed by an atomic layer deposition process, referring to the description in the first embodiment. 
   The gate  460  can be a multilayer structure containing semiconductor materials, such as Si, Ge, metal or their combinations. 
   The source  410  and the drain  420  are located in the semiconductor substrate  400  at either side of the three layer stack structure. In the  FIG. 17 , the position of the source  410  and the drain  420  can be exchanged with each other, and the doping ions can be one or more of phosphorus ion, arsenic ion, boron ion, and indium ion. 
   In the semiconductor device provided by the embodiment, the charge trapping layer is a dielectric layer containing a first discrete compound monolayer and a second discrete compound monolayer, the size of the islands in the first discrete compound monolayer and the second discrete compound monolayer are at atomic level and are controllable; furthermore, the sizes of each first discrete compound monolayer or each second discrete compound monolayer are the same. 
   The distribution densities of the first discrete compound monolayer and the second discrete compound monolayer in the dielectric layer can be controlled through controlling the atomic layer deposition processes for forming the first discrete compound monolayer and the second discrete compound monolayer, for example controlling the flow rates and time of the first precursor gas for forming the first discrete compound monolayer and of the third precursor gas for forming the second discrete compound monolayer. 
   The present invention is capable of improving the density of the charge trapping well in the charge trapping layer and the charge trapping capability, even if the semiconductor device has a small line width. 
   While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.