Patent Publication Number: US-2022223423-A1

Title: Semiconductor structure manufacturing method and two semiconductor structures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Patent Application No. PCT/CN2021/105239, filed on Jul. 08, 2021, which claims priority to Chinese Patent Application No. 202110049137.6, filed on Jan. 14, 2021. The above-referenced patent applications are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present application relate to the field of semiconductors, and in particular, to a semiconductor structure manufacturing method and two semiconductor structures. 
     BACKGROUND 
     With the continuous development of the scale of integrated circuits, a feature size of a semiconductor structure is decreasing. According to a principle of scale down, a contact area of gate, source and drain structures decreases correspondingly. 
     Contact resistance increases with a decrease in the contact area, which enables series resistance of the semiconductor structure to increase, and leads to the problems such as slowdown of operation and deterioration of amplification characteristics. 
     Therefore, metal silicide is generally used to form ohmic contact, so as to reduce the contact resistance. However, some metal silicide currently still has the problem of a line width effect or high resistance, which may reduce the performance of the semiconductor structure. 
     SUMMARY 
     The embodiments of the present application provide a semiconductor structure manufacturing method and two semiconductor structures, so as to solve the problems of the line width effect and high resistance of the metal silicide, thereby improving the performance of the semiconductor structure. 
     In order to solve the above problems, the embodiments of the present application provide a semiconductor structure manufacturing method, including: providing a substrate and a silicon layer, the substrate exposing a top surface of the silicon layer; performing deposition to form an alloy layer on the silicon layer, the deposition being performed in a nitrogen-containing atmosphere, and a concentration of nitrogen atoms in the nitrogen-containing atmosphere increasing with an increase in deposition time; and annealing the alloy layer and the silicon layer. 
     The embodiments of the present application further provide a semiconductor structure, including: a substrate and a silicon layer, the substrate exposing a top surface of the silicon layer; the silicon layer being provided with an alloy layer, the alloy layer containing nitrogen atoms, and the farther away from the silicon layer, the higher the content of the nitrogen atom. 
     The embodiments of the present application further provide a semiconductor structure, including: a substrate and a silicon layer, the substrate exposing a top surface of the silicon layer; the top surface of the silicon layer being provided with a metal silicide layer, the metal silicide layer containing nitrogen atoms, and the farther away from the silicon layer, the higher the content of the nitrogen atom. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       One or more embodiments are exemplarily described by using figures that are corresponding thereto in the accompanying drawings; the exemplary descriptions do not constitute limitations on the embodiments. Elements with same reference numerals in the accompanying drawings are similar elements. Unless otherwise particularly stated, the figures in the accompanying drawings do not constitute a scale limitation. 
         FIG. 1  to  FIG. 4  are schematic structural diagrams corresponding to steps in a semiconductor structure manufacturing method according to a first embodiment of the present application; 
         FIG. 5  is a schematic diagram of formation of an alloy layer in the semiconductor structure manufacturing method according to the first embodiment of the present application; 
         FIG. 6  is a partial structural diagram corresponding to the formation of the alloy layer in the semiconductor structure manufacturing method according to the first embodiment of the present application; 
         FIG. 7  is a line chart of changes in a deposition rate in the semiconductor structure manufacturing method according to the first embodiment of the present application; 
         FIG. 8  to  FIG. 11  are schematic structural diagrams corresponding to steps in a semiconductor structure manufacturing method according to a second embodiment of the present application; 
         FIG. 12  is a schematic diagram of formation of a first alloy layer and a second alloy layer in the semiconductor structure manufacturing method according to the second embodiment of the present application; and 
         FIG. 13  is a partial structural diagram corresponding to the formation of the first alloy layer and the second alloy layer in the semiconductor structure manufacturing method according to the second embodiment of the present application. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As can be seen from the BACKGROUND, the metal silicide currently still has the problems such as the line width effect and high resistance, and the performance of the semiconductor structure remains to be improved. 
     In order to improve the performance of the semiconductor structure, an alloy layer may react with silicon to form a metal silicide layer including a variety of metal silicide. Different metal silicide can complement one another to optimize the line width effect and reduce the resistance. However, due to a large difference in atomic radii of different metals in the alloy layer and a large difference in degrees of silicification reactions of various metals, a metal silicide layer finally formed is difficult to combine advantages of different metal silicide and weaken disadvantages of different metal silicide. 
     In order to solve the above problems, an embodiment of the present application provides a semiconductor structure manufacturing method, including: forming an alloy layer on a silicon layer, the alloy layer being deposited in a nitrogen-containing atmosphere, and a concentration of nitrogen atoms increasing with an increase in deposition time. The increase in the concentration of nitrogen atoms can control the silicification reaction of the alloy layer, so as to reduce the resistance of the metal silicide layer finally formed and prevent the line width effect, thereby improving the performance of the semiconductor structure. In addition, due to high electronegativity of the nitrogen atom, the nitrogen atom can also attract some metal atoms, so as to reduce the consumption of silicon atoms by the metal atoms to ensure electrical properties of the semiconductor structure. 
     In order to make objectives, technical solutions and advantages of the embodiments of the present application clearer, various embodiments of the present application will be described below in details with reference to the drawings. However, those of ordinary skill in the art may understand that, in the embodiments of the present application, numerous technical details are set forth in order to enable a reader to better understand the present application. However, the technical solutions claimed in the present application can be implemented without these technical details and various changes and modifications based on the embodiments below. 
     A first embodiment of the present application provides a semiconductor structure manufacturing method.  FIG. 1  to  FIG. 4  are schematic structural diagrams corresponding to steps in a semiconductor structure manufacturing method according to a first embodiment of the present application;  FIG. 5  is a schematic diagram of formation of an alloy layer in the semiconductor structure manufacturing method according to the first embodiment of the present application;  FIG. 6  is a partial structural diagram corresponding to the formation of the alloy layer in the semiconductor structure manufacturing method according to the first embodiment of the present application; and  FIG. 7  is a line chart of changes in a deposition rate in the semiconductor structure manufacturing method according to the first embodiment of the present application. Specific descriptions are given below with reference to the drawings. 
     Referring to  FIG. 1 , the method includes: providing a substrate  130  and a silicon layer  100 . The substrate  130  exposes a top surface of the silicon layer  100 . 
     To reduce the oxidation of the substrate  130  and the silicon layer  100 , a semiconductor structure may be placed in an environment of inert gas such as nitrogen. 
     The substrate  130  may be made of silicon, germanium, silicon on insulator, sapphire, silicon carbide, gallium arsenide, aluminum nitride, zinc silicon oxide or the like. The substrate  130  may include an isolation structure and an active region. The isolation structure may define a plurality of active regions. The isolation structure may be formed by filling shallow trenches with insulating materials; for example, the isolation structure may be a shallow trench isolation structure. 
     In this embodiment, the substrate  130  also has a gate dielectric layer on a surface, and the silicon layer  100  is located on the gate dielectric layer. The silicon layer  100  and a subsequently-formed alloy layer are configured to form a gate structure. 
     The silicon layer  100  is made of polysilicon, and the gate dielectric layer made of polysilicon and silicon oxide has high affinity and few interface defects. In addition, a threshold voltage and resistivity of the gate structure can be adjusted by doping polysilicon. 
     In other embodiments, the silicon layer may also be a source or drain structure. The silicon layer is located in the substrate and the substrate exposes a surface of the silicon layer. The silicon layer is made of monocrystalline silicon, and the silicon layer includes N-type or P-type doped ions. The subsequently-formed alloy layer is a contact structure of the source or drain structure. 
     In this embodiment, the method further includes the following step: 
     performing surface treatment on the silicon layer  100  based on a plasma of a first gas and a plasma of a second gas. The first gas is configured to increase roughness of the surface of the silicon layer  100 , and the first gas includes an inert gas; the second gas is configured to reduce an oxide on the surface of the silicon layer  100 , and the second gas includes a reducing gas; and a flow rate of the first gas is greater than or equal to that of the second gas. 
     The use of the first gas can increase the roughness of the surface of the silicon layer  100 , so as to increase a contact area between the silicon layer  100  and the subsequently-formed alloy layer, thereby reducing the contact resistance. 
     The oxide has high resistivity, and the use of the second gas to remove the oxide on the surface of the silicon layer  100  can reduce the contact resistance of the silicon layer  100  and the subsequently-formed alloy layer, so as to increase a driving current and reduce a delay effect. 
     The flow rate of the first gas is greater than or equal to that of the second gas based on a main reason below: the second gas bombards the surface of the silicon layer  100  with less strength, while the first gas bombards the surface of the silicon layer  100  with more strength. The flow rate of the first gas being greater than that of the second gas can improve the roughness of the surface of the silicon layer  100  to a large extent. In addition, when the bombardment strength is large, a number of ions bombarded increases accordingly, and a surface area of the silicon layer  100  also increases accordingly, which helps improve a reduction effect of the second gas on the bombarded ions and the surface of the silicon layer  100 . 
     In this embodiment, a ratio of the flow rate of the first gas to the flow rate of the second gas is within a range of 1 to 20. In the above range, the first gas is guaranteed to have large bombardment strength on the surface of the silicon layer  100 , excessive damages to the substrate  130  and the silicon layer  100  can be prevented, and the second gas can also be ensured to reduce the oxide sufficiently, thereby reducing the contact resistance of the silicon layer  100  and the subsequently-formed alloy layer. 
     In this embodiment, the first gas is argon. Argon has large bombardment strength and is cheap. In other embodiments, the first gas may also be helium. 
     In this embodiment, the second gas is hydrogen. Hydrogen is highly reductive and does not produce difficult-to-remove reaction byproducts. In other embodiments, the second gas may be ammonia. 
     A surface treatment temperature of the silicon layer  100  is controlled in a range of 150° C. to 350° C. A higher temperature can improve the activity of the second gas and then increase a removal rate of the oxide. In addition, the above temperature range is higher than the boiling point of water, which is conducive to increasing a removal rate of a volatile impurity such as water on the surface of the silicon layer  100 . 
     Referring to  FIG. 2  and  FIG. 5  to  FIG. 7 , deposition is performed to form an alloy layer on the silicon layer  100 , the deposition is performed in a nitrogen-containing atmosphere, and a concentration of nitrogen atoms in the nitrogen-containing atmosphere increases with an increase in deposition time. 
     That is, in the alloy layer  110 , the further away from the silicon layer  100 , the higher the nitrogen content. During subsequent annealing, in the alloy layer  110 , the metal atoms with a high affinity for silicon atoms tend to diffuse towards the silicon layer  100 , while the metal atoms with a low affinity for the silicon atoms tend to diffuse away from the silicon layer  100 . Therefore, the metal atoms with a high affinity for the silicon atoms may cause greater silicon consumption, while the metal atoms with a low affinity for the silicon atoms may not have sufficient silicification reactions. The nitrogen atom is more electronegative, and the further away from the silicon layer  100 , the higher the nitrogen content. As a result, the nitrogen atom becomes increasingly attractive in a direction away from the silicon layer  100 . Therefore, the nitrogen atom can inhibit a diffusion degree of the metal atoms with a higher affinity for the silicon atoms to the silicon layer  100 , thereby reducing silicon consumption. The metal atoms with a high affinity for the silicon atoms occupy fewer diffusion positions, while the metal atoms with a low affinity for the silicon atoms may occupy more diffusion positions towards the silicon layer  100 , thereby improving a degree of the silicification reaction. In this way, the silicification reactions of various metals in the alloy layer  110  are relatively balanced, and the metal silicification layer finally formed may combine advantages of various metal silicides. 
     In this embodiment, the alloy layer  110  is a monolayer structure; that is, the alloy layer  110  has a relatively small thickness, and the alloy layer  110  has a small footprint. Subsequently, a thicker metal layer with low resistance may be formed on the alloy layer  110  to reduce the resistance of the semiconductor structure. 
     The alloy layer  110  is made of one or more combinations of titanium alloys. In this embodiment, the alloy layer  110  is made of a cobalt-titanium alloy, and a titanium atom  112  is reductive and can reduce residual oxides on the surface of the silicon layer  100 . The titanium atom  112  can also improve the contact between a cobalt atom  111  and the surface the silicon layer  100 , which facilitates the silicification reaction in a subsequent annealing process. In other embodiments, the alloy layer may also be made of a tungsten titanium alloy or a tantalum titanium alloy. 
     In the alloy layer  110 , the content of titanium is no more than 10 at %. When the content of titanium is within a range of 1 to 10 at %, a ratio of titanium silicide to cobalt silicide in the metal silicide layer finally formed can be kept within a reasonable range, so that the metal silicide layer has no obvious line width effect and has low resistivity. 
     Since a radius of the cobalt atom  111  is similar to that of a silicon atom  10  land the radius of the titanium atom  112  is quite different from that of the silicon atom  101 , in the subsequent annealing process, more cobalt atoms  111  tend to diffuse towards a direction close to the silicon layer  100 , while more titanium atoms  112  tend to diffuse towards a direction away from the silicon layer  100 . A nitrogen atom  113  is more electronegative than the silicon atom  101 ; when more nitrogen atoms  113  are distributed in a direction away from the silicon layer  100 , the diffusion of the cobalt atoms  111  towards the direction close to the silicon layer  100  can be inhibited to a greater extent, that is, the nitrogen atom  113  can attract some of the cobalt atoms  111  to diffuse towards the direction away from the silicon layer  100 , thereby inhibiting interaction between the cobalt atom  111  and the silicon atom  101 . Since some of the cobalt atoms  111  may diffuse towards the direction away from the silicon layer  100 , positions of some of the titanium atoms  112  may be occupied, so as to enable more titanium atoms  112  to diffuse towards the direction close to the silicon layer  100 , thereby facilitating the formation of the titanium silicide. The titanium silicide has resistivity of 13 to 16 uΩ·cm, low resistance, a simple manufacturing process and good high-temperature stability. The cobalt silicide has no obvious line width effect under a 90 nanometer technology node. As a result, the metal silicide layer has low resistance and no obvious line width effect. 
     In addition, the nitrogen atom  113  has lower conductivity. When more nitrogen atoms  113  are distributed in the direction away from the silicon layer  100 , the nitrogen atom  113  may not have a great influence on the ohmic contact between the silicon layer  100  and the alloy layer  110 , so as to ensure low contact resistance between the silicon layer  100  and the alloy layer  110 . 
     The forming of the alloy layer  110  is specifically described below. 
     The alloy layer  110  is formed by physical vapor deposition. Specifically, the alloy layer  110  is formed by reactive magnetron sputtering in physical vapor deposition. Reactive magnetron sputtering has the advantages of simple equipment, easy control, a large coating area and strong adhesion. Reactive magnetron sputtering takes a cobalt titanium alloy as a target cathode to react with nitrogen to form a nitrogen-containing alloy layer  110  in a film formation process of the silicon layer  100 . 
     Distribution positions and nitrogen content of the nitrogen atoms  113  in the alloy layer  110  can be controlled by controlling the type, time and flow rate of an introduced nitrogen-containing gas. 
     In this embodiment, the nitrogen-containing gas is nitrogen which may not introduce other impurities and can prevent the impact on the performance of the semiconductor structure. In other embodiments, the nitrogen-containing gas may also be ammonia. 
     In this embodiment, during the formation of the alloy layer  110 , the concentration of the nitrogen atom  113  in the nitrogen-containing atmosphere increases step by step. The step-by-step increase means that the concentration of the nitrogen atom  113  in a same time period is the same, but the concentration of the nitrogen atom  113  in different time periods is different and the concentration of the nitrogen atom  113  in a later time period is greater than that in a previous time period. In this way, in the formed alloy layer  110 , the nitrogen content also increases step by step in a direction further away from the silicon layer  100 . The step-by-step increase may reduce the setting of control parameters, thereby simplifying a manufacturing process. 
     In other embodiments, the concentration of nitrogen atoms in the nitrogen-containing atmosphere increases linearly. The linear increase means that the concentration of nitrogen atoms is different in a same time period, and the concentration of nitrogen atoms also increases with an increase in time. In this way, in the formed alloy layer, the nitrogen content also increases linearly in the direction further away from the silicon layer. 
     In addition, during the reactive magnetron sputtering, the content of cobalt and titanium atoms can be controlled by controlling the content of cobalt and titanium in the cobalt titanium alloy and magnitude of sputtering power. 
     It may be understood that an increase in the sputtering power can increase a sputtering rate and enable sputtered ions to have higher energy, thereby improving adhesion and density of a film. However, if the sputtering power is too large, the kinetic energy of the sputtered ions may be greatly increased, thereby causing damages to the film. In this embodiment, the sputtering power ranges from 3000 W to 4000 W. The sputtering power within the above range can ensure that the formed alloy layer  110  has greater adhesion and density, and can also prevent greater damages to the silicon layer  100 . 
     In this embodiment, the deposition has a variable deposition rate, and the deposition rate decreases with an increase in the deposition time. The deposition rate refers to a film thickness of a material sputtered from a target material deposited on the silicon layer  100  in unit time, that is, a film thickness formed by deposition of the cobalt atom  111  and the titanium atom  112  on the silicon layer  100  in unit time. 
     The deposition rate is positively correlated with the sputtering rate which is affected by a surface state of the target material. The sputtering rate may be reduced if poisoning occurs in the target material. The poisoning of the target material means that a reaction gas reacts with a surface of the target material to form an impurity film. The impurity film is easy to produce cold field arc discharge, that is, an arcing phenomenon, so that cathode sputtering cannot proceed, thereby affecting a sputtering effect. 
     In this embodiment, the deposition rate decreases with an increase in the deposition time based on a main reason below: with an increase in the nitrogen concentration, the nitrogen atom may react with the surface of the target material; therefore, in order to keep the deposition rate constant, the sputtering rate should be increased to some extent; however, a large sputtering rate may cause damages to the target material, and the damages to the target material damage may deepen the poisoning of the target material. On the contrary, if the deposition rate is controlled to decrease with an increase in the deposition time, the sputtering rate can be kept in a low range, so as to reduce the damages to the target material to alleviate the poisoning of the target material, thereby preventing the influence on the sputtering effect. 
     Further, referring to  FIG. 7 , in this embodiment, the deposition rate has a three-stage change interval, and the deposition rates of the change intervals show a linear decreasing function relationship. Specifically, in this embodiment, the deposition rate has a first change interval a, a second change interval b and a third change interval c. The first change interval a corresponds to a metal mode, and the nitrogen content in the interval is minimum. The second change interval b corresponds to a transition mode, and the nitrogen content in this interval is greater than that in the previous interval. The third change interval c corresponds to a compound pattern (Co—Ti—N compound), and the nitrogen content in this interval is greater than that in the first two intervals. The deposition rates of the three change intervals show a linear decreasing function relationship, so the three change intervals enable the sputtering rate to be kept in a low range, so as to reduce the damages to the target material to alleviate the poisoning of the target material, thereby making the sputtering effect better in the three change intervals. 
     Linear decreasing functions of the deposition rates of the change intervals have different slopes. Specifically, a slope of the second change interval b is greater than that of the first change interval a, and the slope of the first change interval a is greater than that of the third change interval c. It may be understood that the smaller slope of the first change interval a can ensure that a target thickness of the film can be reached more quickly and can also prevent the poisoning of the target material to some extent; the maximum slope of the second change interval b can ensure that the sputtering rate can be rapidly reduced after the target thickness is reached, thereby greatly alleviating the poisoning of the target material under the high nitrogen concentration; the minimum slope of the third change interval c can ensure that a particular amount of sputtered ions bombard the surface of the film, thereby achieving a uniform nitrogen doping effect and also protecting the target material at a low sputtering rate. 
     During the reactive magnetron sputtering, argon is also introduced, and a flow rate of the argon ranges from 10 sccm to 80 sccm, which may be, for example, 20 sccm, 40 sccm or 70 sccm. Argon can be used as a discharge carrier and can also ensure that the semiconductor structure is in an inert gas atmosphere, thereby preventing the oxidation of the semiconductor structure. 
     A chamber pressure is controlled in a range of 0.0015 Torr to 0.0090 Torr, which may be, for example, 0.0030 Torr, 0.0050 Torr or 0.0080 Torr. The deposition rate and uniformity of the alloy layer  110  are correlated with the chamber pressure. When the chamber pressure is higher, the deposition rate is higher. However, the uniformity of the alloy layer  110  is greater when the chamber pressure is lower. When the chamber pressure is in the above range, the deposition rate can be accelerated to some extent and the uniformity of the alloy layer  110  can be guaranteed. 
     In other embodiments, the alloy layer may also be formed by physical vapor deposition such as vacuum evaporation or ion deposition. 
     Referring to  FIG. 3 , the deposition further includes forming a metal layer  120  on the alloy layer  110 . 
     Resistivity of the metal layer  120  is lower than that of the alloy layer  110 , which can further reduce the resistance of the semiconductor structure and increase an operating speed of the semiconductor structure. For example, the metal layer  120  may be made of a material including a low-resistance metal such as tungsten, titanium, gold or silver. 
     In this embodiment, the metal layer  120  is formed by physical vapor deposition. In other embodiments, the metal layer may also be formed by chemical vapor deposition. 
     Referring to  FIG. 4 , the alloy layer  110  (refer to  FIG. 3 ) and the silicon layer  100  are annealed to form a metal silicide layer  110   a.  In this embodiment, the substrate  130  and the metal layer  120  are also annealed. 
     The metal silicide layer  110   a  includes cobalt silicide, titanium silicide, titanium cobalt nitride, titanium silicon nitride and other substances. The metal silicide layer  110   a  may further include a cobalt titanium alloy not completely reacting. The titanium silicide has lower resistance, while the cobalt silicide can reduce the line width effect. 
     The metal silicide layer  110   a  finally formed contains nitrogen atoms, and in the metal silicide layer  110   a,  the farther away from the silicon layer  100 , the higher the content of the nitrogen atom. 
     In other embodiments, the metal silicide layer may also include tantalum silicide, tungsten silicide, titanium tungsten nitride, titanium tantalum nitride and other substances. The metal silicide layer may further include a tantalum titanium alloy or a tungsten titanium alloy not completely reacting. 
     In a solid phase reaction of cobalt silicon, with the constant increase in the temperature, a formation phase sequence of cobalt silicide is as follows: dicobalt silicide (Co 2 Si)→cobalt silicate (CoSi)→cobalt disilicide (CoSi 2 ). During the formation of dicobalt silicide (Co 2 Si) and cobalt disilicide (CoSi 2 ), cobalt atoms are motion atoms; while during the formation of cobalt silicate (CoSi), silicon atoms are motion atoms. The increase in the concentration of the nitrogen atom can slow down the interdiffusion and interaction of cobalt and silicon, which is conducive to ordered growth of crystal lattice and can also reduce ohmic contact resistance and silicon consumption. In addition, the increase in the concentration of the nitrogen atom can also facilitate the formation of titanium silicide and further reduce the ohmic contact resistance. 
     An annealing temperature ranges from 500° C. to 900° C., which may be, for example, 600° C., 700° C. or 800° C. A higher annealing temperature can improve a migration rate of atoms. The annealing temperature in the above range can ensure the silicification reaction is more thorough and can prevent the problem of current leakage or short circuit caused by excessive silicon diffusion. 
     The time is controlled in a range of 10 s to 100 s, which may be, for example, 20 s, 30 s or 80 s. The annealing time in the above range can ensure the silicification reaction is more thorough and can prevent the problem of current leakage or short circuit caused by excessive silicon diffusion. 
     The annealing is performed in an argon atmosphere to prevent the oxidation of the semiconductor structure. 
     Based on the above, in the first embodiment of the present application, the alloy layer  110  is formed on a surface of the silicon layer, the alloy layer  110  is deposited in a nitrogen-containing atmosphere, and the concentration of nitrogen atoms in the nitrogen-containing atmosphere increases with an increase in deposition time. The increase in the concentration of the nitrogen atom  113  can control the silicification reaction process, so as to reduce the contact resistance and the line width effect of the metal silicification layer  110   a  and then improve the electrical properties and operating speed of the semiconductor structure. 
     A second embodiment of the present application provides a semiconductor structure manufacturing method.  FIG. 8  to  FIG. 11  are schematic structural diagrams corresponding to steps in a semiconductor structure manufacturing method according to a second embodiment of the present application;  FIG. 12  is a schematic diagram of formation of a first alloy layer and a second alloy layer in the semiconductor structure manufacturing method according to the second embodiment of the present application; and  FIG. 13  is a partial structural diagram corresponding to the formation of the first alloy layer and the second alloy layer in the semiconductor structure manufacturing method according to the second embodiment of the present application. Referring to  FIG. 8  to  FIG. 13 , the manufacturing method according to the second embodiment is substantially the same as the method according to the first embodiment, and main differences are as follows: the deposition according to the second embodiment, as the two-stage deposition, includes first deposition and second deposition, the first deposition has first initial nitrogen atom concentration, the second deposition has a second initial nitrogen atom concentration, and the second initial nitrogen atom concentration is greater than the first initial nitrogen atom concentration. 
     The parts of the second embodiment the same as or similar to those of the first embodiment can be obtained with reference to the first embodiment and are not described in detail herein. 
     Specific description is given below with reference to the drawings. 
     Referring to  FIG. 8 , a substrate  230  and a silicon layer  200  are provided. The substrate  230  exposes a top surface of the silicon layer  200 . 
     In this embodiment, the silicon layer  200  and subsequently formed first and second alloy layers are configured to form a gate structure. The silicon layer  200  contains doped atoms such as boron and phosphorus atoms. The boron and phosphorus atoms can increase the conductivity of the gate structure and regulate a threshold voltage of the gate structure. 
     In other embodiments, the silicon layer is configured to form a source structure or a drain structure. The boron or phosphorus atoms can provide holes or electrons for the source structure and the drain structure. When the gate structure is open, the source structure and the drain structure are switched on. The subsequently formed first and second alloy layers are contact structures of the source or drain structure. 
     Surface treatment is performed on the silicon layer  200  based on a plasma of a first gas and a plasma of a second gas, the first gas includes an inert gas, the second gas includes a reducing gas, and a flow rate of the first gas is greater than or equal to that of the second gas. 
     A specific description of the above steps can be obtained with reference to the first embodiment and is not described in detail here. 
     Referring to  FIG. 9 ,  FIG. 12  and  FIG. 13 , in this embodiment, the alloy layer is a double-layer structure, including a first alloy layer  210  and a second alloy layer  220 . First deposition is performed on the first alloy layer  210  located on the silicon layer  200 , and second deposition is performed on the second alloy layer  220  located on the first alloy layer  210 ; and the second initial nitrogen atom concentration is greater than the first initial nitrogen atom concentration. 
     The second initial nitrogen atom concentration is greater than the first initial nitrogen atom concentration based on a reason below: during the annealing, the doped atoms, such as a boron atom  202  and a phosphorus atom  203 , tend to diffuse towards the first alloy layer  210  and the second alloy layer  220 ; the nitrogen atom  213  is chemically inert with the doped atoms such as the boron atom  202  and the phosphorus atom  203 ; therefore, a nitrogen concentration gradient of the second alloy layer  220  and the first alloy layer  210  can inhibit the diffusion of the boron atom  202  and the phosphorus atom  203  to a large extent, thereby reducing a variation degree of the threshold voltage of the gate structure. 
     In addition, the nitrogen concentration gradient of the second alloy layer  220  and the first alloy layer  210  can further reduce the silicon consumption to ensure the excellent electrical properties of the gate structure. 
     Besides, the nitrogen concentration gradient of the second alloy layer  220  and the first alloy layer  210  can also inhibit the diffusion of oxides on a surface of the silicon layer  200 , so as to further reduce the resistance of a subsequently formed metal silicide layer. 
     In other embodiments, when the silicon layer is configured to form the source or drain structure, the nitrogen concentration gradient of the second metal layer and the first alloy layer can greatly inhibit the diffusion of the boron and phosphorus atoms, so that when the gate structure is open, conduction between the source and drain structures can be achieved; that is, enough electrons or holes pass between the source and drain structures. 
     In this embodiment, the first alloy layer  210  and the second alloy layer  220  are made of a same material, so as to reduce interface defects between them. The first alloy layer  210  and the second alloy layer  220  are both made of a cobalt titanium alloy. 
     In other embodiments, the first alloy layer and the second alloy layer may be made of different materials, which may be, for example, combinations of a cobalt titanium alloy, a tungsten titanium alloy and tantalum titanium alloy. 
     In this embodiment, in the first alloy layer  210 , the further from the silicon layer  200 , the higher the nitrogen content; and in the second alloy layer  220 , the further from the silicon layer  200 , the higher the nitrogen content. In this embodiment, the nitrogen content increases step by step. In other embodiments, the nitrogen content may also increase linearly. 
     The further from the silicon layer  200 , the higher the nitrogen content, so that the diffusion of a cobalt atom  211  towards the direction of the silicon layer  200  can be inhibited to a greater extent, thereby reducing a reaction degree of cobalt silicon and reducing a degree of consumption of a silicon atom  201  by the cobalt atom  211 . The decrease in the cobalt atom  211  diffusing towards the silicon layer  200  can provide more diffusion positions for a titanium atom  212 , thereby facilitating the diffusion of the titanium atom  212  towards the silicon layer  200  to form more titanium silicon alloys. 
     In addition, the closer to the silicon layer  200 , the lower the nitrogen content, so that the first alloy layer  210  and the second alloy layer  220  can form good ohmic contact with the silicon layer  200 . 
     In other embodiments, the nitrogen content in the first alloy layer may be constant, and the nitrogen content in the second alloy layer may also be constant. 
     The formation of the first alloy layer  210  and the second alloy layer  220  is described in detail below. 
     The first alloy layer  210  and the second alloy layer  220  are formed by physical vapor deposition. Specifically, the first alloy layer  210  and the second alloy layer  220  are formed by reactive magnetron sputtering in physical vapor deposition. 
     Nitrogen is introduced during the formation of the first alloy layer  210 , and nitrogen has a first flow rate, so that the first deposition has a first initial nitrogen atom concentration. Nitrogen is introduced during the formation of the second alloy layer  220 , and nitrogen has a second flow rate, so that the second deposition has a second initial nitrogen atom concentration. The second flow rate is greater than the first flow rate, so that the second initial nitrogen atom concentration is greater than the first initial nitrogen atom concentration. 
     With the prolongation of the process time, the first flow rate increases and the second flow rate increases. In this embodiment, the first flow rate increases step by step, and the second flow rate increases step by step. In other embodiments, the first flow rate may increase linearly, and the second flow rate may increase linearly. 
     In other embodiments, the first flow rate and the second flow rate may also change with time, provided that the second flow rate is greater than the first flow rate. 
     In this embodiment, the first flow rate ranges from 1 sccm to 5 sccm, which may be, for example, 2 sccm, 3 sccm or 4 sccm; the second flow rate ranges from 3 sccm to 15 sccm, which may be, for example, 5 sccm, 7 sccm or 10 sccm. When the first flow rate and the second flow rate are in the above ranges, a reasonable nitrogen concentration gradient can be ensured between the first alloy layer  210  and the second alloy layer  220 . 
     In this embodiment, the deposition has a variable deposition rate, and the deposition rate decreases with an increase in the deposition time. The deposition rate refers to a film thickness formed by deposition of the cobalt atom  211  and the titanium atom  212  on the silicon layer  200  in unit time. 
     The deposition rate is positively correlated with the sputtering rate which is affected by a surface state of the target material. The sputtering rate may be reduced if poisoning occurs in the target material. 
     In this embodiment, the deposition rate decreases with an increase in the deposition time based on a main reason below: if the deposition rate is controlled to decrease with an increase in the deposition time, the sputtering rate can be kept in a low range, so as to reduce the damages to the target material to alleviate the poisoning of the target material, thereby preventing the influence on the sputtering effect. 
     Further, in this embodiment, the deposition rate has a three-stage change interval, and the deposition rates of the change intervals show a linear decreasing function relationship. The three change intervals enable the sputtering rate to be kept in a low range, so as to reduce the damages to the target material to alleviate the poisoning of the target material, thereby making the sputtering effect better in the three change intervals. 
     Linear decreasing functions of the deposition rates of the change intervals have different slopes. Specifically, the small slope of the first change interval a can ensure that a target thickness of the film can be reached relatively quickly and can also prevent the poisoning of the target material to some extent; the maximum slope of the second change interval b can ensure that the sputtering rate can be rapidly reduced after the target thickness is reached, thereby greatly alleviating the poisoning of the target material under the high nitrogen concentration; the minimum slope of the third change interval c can ensure that a particular amount of sputtered ions bombard the surface of the film, thereby achieving a uniform nitrogen doping effect and also protecting the target material at a low sputtering rate. 
     Refer to the first embodiment for a detailed description of the change process of the deposition rate. In addition, it is to be noted that, in this embodiment, the first deposition and the second deposition have the above three change intervals. 
     During the formation of the first alloy layer  210 , the sputtering power ranges from 3000 W to 4000 W, a flow rate of argon ranges from 10 sccm to 80 sccm, and a chamber pressure is controlled in a range of 0.0015 Torr to 0.0090 Torr. 
     During the formation of the second alloy layer  220 , the sputtering power ranges from 3000 W to 4000 W, the flow rate of argon ranges from 10 sccm to 80 sccm, and the chamber pressure is controlled in the range of 0.0015 Torr to 0.0090 Torr. 
     Referring to  FIG. 10 , the deposition further includes forming a metal layer  240  on the alloy layer. In this embodiment, the metal layer  240  is located on the second alloy layer  220 . Refer to the first embodiment for a detailed description of the metal layer  240 . 
     Referring to  FIG. 11 , the first alloy layer  210  (refer to  FIG. 10 ), the second alloy layer  220  (refer to  FIG. 10 ) and the silicon layer  200  are annealed to form a metal silicide layer  250 . The metal silicide layer  250  can greatly reduce the contact resistance and then increase the operating speed of the semiconductor structure. 
     The metal silicide layer  250  includes a first region  210   a  and a second region  220   a.  Since the first region  210   a  is closer to the silicon layer  200 , the content of the metal silicide in the first region  210   a  is higher than that in the second region  220   a.    
     The metal silicide layer  250  mainly includes titanium silicide, cobalt silicide, titanium cobalt nitride, titanium nitride, a cobalt titanium alloy and other substances. 
     Based on the above, the gradient nitrogen content of the second alloy layer  220  and the first alloy layer  210  can reduce the diffusion degree of the doped ions such as boron and phosphorus atoms towards the first alloy layer  210  and the second alloy layer  220 , so as to ensure good electrical properties of the semiconductor structure. 
     A third embodiment of the present application provides a semiconductor structure.  FIG. 10  is a schematic diagram of the semiconductor structure. Referring to  FIG. 10 , the semiconductor structure includes: a substrate  230  and a silicon layer  200 , the substrate  230  exposing a top surface of the silicon layer  200 ; the silicon layer  200  being provided with an alloy layer. 
     The silicon layer  200  and the alloy layer are configured to form a gate structure. In other embodiments, the silicon layer may be configured to form a source or drain structure, and the alloy layer is configured to form a contact structure of the source or drain structure. 
     The alloy layer contains nitrogen atoms in the alloy layer, and the further away from the silicon layer  200 , the higher the content of nitrogen atoms. In this embodiment, the alloy layer is a double-layer structure, including a first alloy layer  210  and a second alloy layer  220  sequentially stacked, and nitrogen content of the second alloy layer  220  is greater than that of the first alloy layer  210 . 
     The nitrogen concentration gradient of the first alloy layer  210  and the second alloy layer  220  can inhibit the diffusion of doped ions such as boron and phosphorus atoms in the silicon layer  200  towards the first alloy layer  210  and the second alloy layer  220 , thereby ensuring good electrical properties of the semiconductor structure. 
     In the first alloy layer  210  and the second alloy layer  220 , the further away from the silicon layer  200 , the higher the content of nitrogen atoms. The increase in the nitrogen content can control the silicification reaction of the first alloy layer  210  and the second alloy layer  220 , so as to reduce the contact resistance and prevent the line width effect of the metal silicide layer finally formed, thereby improving the performance of the semiconductor structure. 
     In other embodiments, the nitrogen atoms in the first alloy layer and the second alloy layer may also be evenly distributed, provided that the nitrogen content in the second alloy layer is greater than that in the first alloy layer. 
     The nitrogen content in the first alloy layer  210  ranges from 1 at % to 5 at %, and the nitrogen content in the second alloy layer  220  is no more than 7 at %. That is, a difference between the nitrogen content of the first alloy layer  210  and the nitrogen content of the second alloy layer  220  ranges from 2 at % to 6 at %, which may be, for example, 3 at %, 4 at % or 5 at %. The difference between the nitrogen content in the above range can inhibit the diffusion of the doped atoms to a greater extent, and can also reduce the influence of the nitrogen atoms on the resistance of the first alloy layer  210  and the second alloy layer  220 . 
     In other embodiments, the alloy layer may also be a monolayer structure; 
     that is, the first alloy layer may not be provided with the second alloy layer. 
     Based on the above, the nitrogen concentration gradient of the first alloy layer  210  and the second alloy layer  220  can reduce the diffusion of the doped ions in the silicon layer  200 ; and the increase in the nitrogen content in the first alloy layer  210  and the second alloy layer  220  can control the silicification reaction of the first alloy layer  210  and the second alloy layer  220 , thereby reducing the contact resistance and preventing the line width effect. 
     A fourth embodiment of the present application provides a semiconductor structure. The semiconductor structure according to this embodiment is the semiconductor structure according to the third embodiment after annealing.  FIG. 11  is a schematic diagram of the semiconductor structure according to this embodiment. Referring to  FIG. 11 , the semiconductor structure includes: a substrate  230  and a silicon layer  200 , the substrate  230  exposing a top surface of the silicon layer  200 ; the top surface of the silicon layer  200  being provided with a metal silicide layer  250 , the metal silicide layer  250  containing nitrogen atoms, and the farther away from the silicon layer  200 , the higher the content of a nitrogen atom. 
     Specific description is given below with reference to the drawings. 
     In this embodiment, the metal silicide layer  250  includes cobalt silicide, titanium silicide, titanium cobalt nitride, and titanium silicon nitride. The metal silicide layer  250  may further include a cobalt titanium alloy not completely reacting. 
     The cobalt silicide has no significant line width effect, titanium silicide has low resistance, and metal silicide layer  250  can combine the advantages of both. 
     In other embodiments, the metal silicide layer may also include tantalum silicide, tungsten silicide, titanium tungsten nitride, and titanium tantalum nitride. The metal silicide layer may further include a tantalum titanium alloy or a tungsten titanium alloy not completely reacting. 
     Based on the above, in this embodiment, the metal silicide layer  250  formed includes a variety of metal silicide, which can prevent the high resistance or line width effect of single-metal silicide, thereby improving the performance of the semiconductor structure. 
     Those of ordinary skill in the art may understand that the above implementations are specific embodiments for implementing the present application. However, in practical applications, various changes in forms and details may be made thereto without departing from the spirit and scope of the present application. Any person skilled in the art can make respective changes and modifications without departing from the spirit and scope of the present application. Therefore, the protection scope of the present application should be subject to the scope defined by the claims.