Abstract:
The present invention relates to a method for processing semiconductor devices with a fine structure, and more particularly, to a processing method suitable for miniaturizing semiconductor devices with a so-called high-k/metal gate structure. In an embodiment of the present invention, a deposited film, which includes an insulating film made of Hf or Zr and a material of Mg, Y or Al existing on, under or in the insulating film, is formed on a Si substrate and is removed by repeating a dry etching process and a wet etching process at least one time. The wet etching process is performed prior to the dry etching process.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese Patent Application JP 2009-253910 filed on Nov. 5, 2009, the content of which is hereby incorporated by reference into this application. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for processing semiconductor devices, and more particularly, to a method for processing transistors with a fine structure or a so-called high-k/metal gate structure with high accuracy. 
     BACKGROUND OF THE INVENTION 
     Japanese Patent Application laid-Open Publication No. 2005-44890 discloses a method for etching high-k dielectric films. The high-k dielectric films are used as gate insulating films of transistors and are expected to decrease current leakage to suppress more power consumption in comparison with conventionally used SiO 2  films. However, processing techniques for fabricating the transistors, including removal of the high-k dielectric films, have not been fully established, and various methods are still in the research-and-development stage. As disclosed in Japanese Patent Application Laid-Open Publication No. 2005-44890, a high-k dielectric film containing Hf is dry-etched with a gas of BCl 3 , HBr, O 2 , fluorocarbon or the like, while a high-k dielectric film including La, Al or the like is wet-etched with a solution containing fluorinated acid and amine. 
     SUMMARY OF THE INVENTION 
     The present invention focuses on a highly-accurate process for semiconductor devices adopting what it called high-k/metal gate technology that has been proposed to enhance the speed of transistors. 
     A known process for such a metal gate structure includes: depositing a metal electrode made of TiN or TaN on a high-k dielectric film, serving as a gate insulating film, made of hafnium oxide (HfO) or zirconium oxide (ZrO); further depositing a conducting material, such as poly Si, W and Mo, to form a deposition structure; and etching the deposition structure with a resist as a mask. 
     The currently required processing size (line width) is 65 nm or less. In addition, in the case of CMOS transistors, the threshold voltages of p-type and n-type transistors need to be equal. The threshold voltages depend on the work function of the material making up a gate interface. 
     In order to control the work function, Mg, Al or Y may be mixed into a high-k dielectric film containing HfO or ZrO, or a film of Mg, Al or Y or a film of an oxide of Mg, Al or Y may be formed on or under the HfO or ZrO film. However, in the related art, there is so far no disclosure about efficient removal methods of the HfO or ZrO high-k dielectric film mixed with the other metal or the multi-layered film of the high-k dielectric film and the other metal film. 
     The present invention provides a method for efficiently removing a film made of a HfO or ZrO high-k dielectric film mixed with Mg, Al or Y, or a multi-layered film of the HfO or ZrO high-k dielectric film and a film of Mg, Al or Y (hereinafter, simply referred to as “deposited film”). 
     One of representative examples of the present invention is shown below. The present invention relates to a method for processing a semiconductor device with a metal gate structure in which a metal electrode is placed on a deposited film formed on a Si substrate, the deposited film including a high-k dielectric film and a material for controlling a work function. The method is characterized in performing at least one wet etching process and at least one dry etching process to remove the deposited film including the high-k dielectric film and the material for controlling the work function. The wet etching process is performed prior to the dry etching process. 
     According to the present invention, the use of the dry etching process subsequent to the wet etching process to remove the high-k dielectric film reduces time required to remove the deposited film, resulting in improvement of throughput. In addition, the number and time of the dry etching can be decreased, thereby reducing wafer damage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart describing a method for processing a semiconductor device according to a first embodiment of the present invention; 
         FIGS. 2A to 2D  are cross-sectional views of a multi-layered film having a high-k/metal gate structure, the views corresponding to the processes of the first embodiment, respectively; 
         FIG. 3  is a vertical cross-sectional view showing an example of a dry etching apparatus used to perform the embodiment of the present invention; 
         FIG. 4  is a vertical cross-sectional view showing an example of a wet etching apparatus used to perform the embodiment of the present invention; 
         FIGS. 5A to 5C  are schematic diagrams showing the high-k dielectric film at an atomic level to explain the action and effect of the embodiment of the present invention; and 
         FIG. 6  is a cross-sectional view of a high-k/metal gate structure according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described in detail referring to the figures. 
     First Embodiment 
     With reference to  FIG. 1  to  FIG. 5C , a method for processing a semiconductor device according to a first embodiment of the present invention will be described. 
       FIG. 1  is a flow chart describing the method.  FIGS. 2A to 2D  are cross-sectional views showing a multi-layered film having a high-k/metal gate structure fabricated according to the flow chart in  FIG. 1 . In  FIG. 1 , in advance, a HfO film  202  as a high-k dielectric film, a MgO film  203  used to control the work function, a TiN film  204  as a metal gate, a poly Si film  205  as an electrode material and a SiN film  206  as a cap are subsequently deposited on a Si substrate  201 . Then, an antireflection film  207  and a resist film  208  are applied on the SiN film  206  to form a required pattern by lithography, thereby preparing the multi-layered film (step S 1  in  FIG. 1 ). It should be noted that the formation of the multi-layered film is not limited to this example. 
     A cross-sectional view of the multi-layered film having the high-k/metal gate structure formed in step S 1  is shown in  FIG. 2A . As an example, the thickness of each film in the multi-layered film is: 2 nm for the HfO film  202 ; 1 nm for the MgO film  203 ; 10 nm for the TiN film  204 ; 50 nm for the poly Si film  205 ; 50 nm for the SiN film  206 ; 80 nm for the antireflection film  207 ; and 150 nm for the resist film  208 . 
     Next, the multi-layered film is subjected to a dry etching process and a wet etching process to remove undesired parts thereof to complete the high-k/metal gate structure. 
     An exemplary dry etching apparatus and wet etching apparatus required to embody the present invention will be described with  FIGS. 3 and 4 . The dry etching apparatus shown as an example in  FIG. 3  adopts an electron cyclotron resonance (ESR) technique. In the dry etching apparatus, electromagnetic waves are emitted from a plasma power source  301  and discharged through an antenna  302  and a window  303 , which is made of quartz allowing the electromagnetic waves to pass therethrough, into a vacuum chamber  304 . The vacuum chamber  304  is evacuated using a vacuum pump, while a predetermined etching gas is introduced through a gas inlet pipe  309  and gas flow controllers  310  into the chamber whose pressure is maintained constant. A sample stage  305 , which holds a wafer  306 , is connected to a bias power source  307  that accelerates incident ions. With the electromagnetic waves emitted from the antenna  302 , the etching gas is converted into plasma, and reactive ions strike the wafer  306  to etch the wafer  306 . In this apparatus, electromagnetic coils  308  create a magnetic field in the chamber  304 . The magnetic field strength is set so as to match the frequency of the electron cyclotron resonance in plasma and the frequency of the plasma power source  301 , thereby allowing the plasma to efficiently absorb the power and therefore maintaining a high plasma density at a low pressure. The magnetic field strength required to create the ESR can be set by varying the current value to be fed to the electromagnetic coils  308 . 
       FIG. 4  shows an example of a single-wafer wet etching apparatus. In the apparatus, a wafer  403  is placed on a sample stage  402  in a container  401  maintained at atmospheric pressure and an etching solution is supplied through an etchant feeding nozzle  404  onto a surface of the wafer, thereby wet-etching the wafer  403 . 
     Returning to the flow chart in  FIG. 1 , descriptions will be made in detail about the method for processing a semiconductor device according to the first embodiment of the present invention. As described above,  FIG. 2A  shows a multi-layered film to which processes according to the embodiment of the present invention have not been yet applied, in short, a multi-layered film in an initial state. Etching processes are applied to the multi-layered film in such a state. As an example, the antireflection film  207  is etched by Ar/HBr/O 2  gas (S 2 ), and the SiN film  206  is etched by CF 4 /CHF 3 /O 2  gas (S 3 ). Furthermore, the Poly Si film  205  is etched by Ar/CF 4 /CHF 3 /SF 6  gas during a breakthrough etch, by Ar/Cl 2 /HBr/O 2  gas during a main etch and by HBr gas during an over etch (S 4 ). The TiN film is etched by CF 4  gas during a breakthrough etch and by Cl 2  gas during a main etch (S 5 ). 
       FIG. 2B  is a cross-sectional view of the multi-layered film that has been etched to the depth of the TiN film  204  through the dry etching processes. Next step is to remove the deposited film including the high-k dielectric film and MgO film  203  for controlling the work function. In the usual manner, the high-k dielectric film is subjected to dry etching; however, in accordance with this embodiment of the present invention, the MgO film  203  is first wet-etched in HF solution (S 6 ) to bring the multi-layered film into the state shown in  FIG. 2C . The wet etch process is performed with 0.25% HF solution for 120 seconds, for example. Subsequently, the HfO film  202  is dry-etched by BCl 3 /Cl 2  gas (S 7 ). Then, an inspection apparatus (not shown) checks whether there is a residue of the deposited film or the other films on the processed surface of the wafer (semiconductor device) (S 8 ). The presence of a residue is determined by, for example, finding even one residue in a field of view of a scanning electron microscope or substantially detecting a residue on the processed surface. In the case where it is determined there is a residue, the multi-layered film goes back to the wet etching process (S 6 ) to remove the residue with the HF solution and is subjected to dry etching again (S 7 ). Steps S 6  to S 8  are repeatedly performed to the deposited film until the residue disappears, thereby obtaining the semiconductor device in the state shown in  FIG. 2D . 
     The feature of the first embodiment is that the wet etching process is always performed prior to the dry etching process used to remove the deposited film including the high-k dielectric film and MgO film for controlling the work function from the multi-layered film. Since the deposited film shown in  FIGS. 2A to 2D  includes the MgO film  203  placed on the HfO film  202 , the procedure in which the dry etch of the HrO film  202  is performed after the wet etch of the MgO film  203  is the natural course. However, even though the deposited film contains HfO and Mg mixed thereto or includes a HfO film and a MgO film placed under the HfO film, the wet etching process with the HF solution is performed in advance in the first embodiment. 
     This procedure reduces wafer damage caused by the dry etching process as well as removes the high-k dielectric film by dry etching with a small amount of residue left on the wafer. 
     The action and effect produced by the method according to the first embodiment of the present invention will be described below with reference to  FIGS. 5A to 5C . The method was found experimentally and there is explicit evidence. Since some points need to be clarified; however, the description accompanied by  FIGS. 5A to 5C  will be about a mechanism assumed by the inventors.  FIGS. 5A to 5C  are diagrams of the interface between the HfO film and MgO film, which are magnified to an atomic level. This deposited film is made by depositing a MgO film  501  and a HfO film  502 , in this order, on a Si substrate  201 . Unlike the exemplary multi-layered film shown in  FIGS. 2A to 2D , the MgO film  501  is deposited under the HfO film  502 . 
     Immediately after formation of the deposited film, Mg atoms and Hf atoms are separated from each other, like they are in different layers, as shown in  FIG. 5A . In general, the high-k dielectric film is subjected to heat treatment after the HfO film and MgO film are deposited in step S 1  in  FIG. 1  for the purpose of stabilizing the interface, and the TiN film is deposited thereon. As is apparent from  FIG. 5B  showing the state of the heat-treated deposited film, the Mg atoms  503  and Hf atoms are mixed to a degree due to the interdiffusion. It is conceivable that the deposited film in this state, i.e., the film of the HfO atoms mixed with a certain amount of the Mg atoms  503  may suffer delays in the progress of the etching process using BCl 3 /Cl 2  gas. On the other hand, in a wet etching process using the HF solution, F ions penetrate into the deposited film while cleaving the bonds of atoms whose charges are greatly unbalanced, in short, MgO bonds, and eliminate the Mg atoms as shown in  FIG. 5C . The atom density of the remaining HfO film  504  becomes low because of elimination of the Mg atoms, thereby realizing easy removal of the HfO film  504  by the dry etching process using the BCl 3 /Cl 2  gas. 
     According to the experiments by the inventors and others, in order to remove a deposited film including a HfO film of 2 nm, but not MgO film, through a dry etching process, the deposited film needs to be dry-etched with a mixed gas of BCl 3  of 80 ml/min and Cl 2  of 20 ml/min, in a plasma at a pressure of 0.2 Pa, for 20 seconds with the wafer applied with a bias power of 10 W and subsequently for 60 seconds at a bias power of 0 W. In the case where a deposited film including a HfO film of 2 nm and a MgO film is first wet-etched in HF solution to remove Mg atoms based on the method of the first embodiment of the present invention, the HfO film can be removed by dry etching it for 60 seconds at a bias power of 0 W. It is apparent that removal of the Mg atoms enhances the dry etching of the HfO film. 
     It is also possible to remove the high-k dielectric film, as in the case of conventional technique, by adding a dry etching process using BCl 3 /Cl 2  gas before performing the HF wet etching process to the deposited film and then performing a HF wet etching process and dry etching process with the BCl 3 /Cl 2  gas again; however, this extends process time as well as decreases throughput. As a result, the substrate may suffer more damage due to an increase in the number of the dry etching process by one time. 
     Although the etching method of the first embodiment is used for a deposited film having the MgO film under the HfO film, quite the same method can be used for a deposited film having the MgO film on the HfO film or a deposited film having the HfO film mixed with the Mg atoms in advance. 
     In other words, the method according to the first embodiment in which a dry etching process with the BCl 3 /Cl 2  gas is performed after a wet etching process with the HF solution produces the above-described effect for any deposited films including the HfO film and MgO film, i.e., a deposited film in the state shown in  FIG. 2B , regardless of vertical arrangement of the HfO film and MgO film and mixed conditions of HfO and MgO. 
     Second Embodiment 
     With reference to  FIG. 6 , a method for processing a semiconductor device according to a second embodiment of the present invention will be described.  FIG. 6  shows a multi-layered film, which has a deposited film including a Y 2 O 3  film  601  under a HfO film  202 , and is etched to the depth of a TiN film  204 . The HfO film  202  has a thickness of 2 nm, while the Y 2 O 3  film  601  has a thickness of 1 nm. In order to remove the HfO film  202  and Y 2 O 3  film  601 , the Y 2 O 3  film  601  is first wet-etched in 1.4% HNO 3  solution for 120 seconds. Then, a dry etching process is performed mainly for the HfO film  202  with a mixed gas of BCl 3  of 80 ml/min and Cl 2  of 20 ml/min, in plasma at a pressure of 0.2 Pa, for 20 seconds with the wafer applied with a bias power of 10 W and subsequently for 60 seconds at a bias power of 0 W. Then, the Y 2 O 3  film  601  is wet-etched in the 1.4% HNO 3  solution for 120 seconds. Furthermore, a dry etching process is performed mainly for the HfO film  202  with a mixed gas of BCl 3  of 80 ml/min and Cl 2  of 20 ml/min, in plasma at a pressure of 0.2 Pa, for 60 seconds at a bias power of 0 W. Through the above-described processes, the HfO film  202  and Y 2 O 3  film  601  are removable. The reason why the removal of the Y 2 O 3  film requires the wet and dry etching processes one more time than the removal of the MgO film in the first embodiment does is that the Y 2 O 3  film has a high resistance to wet etching. 
     Removal of a deposited film that is thicker than that of the above example is possible by increasing the number of cycles of wet etching and dry etching (Steps S 6  to S 8  in  FIG. 1 ). It is deemed that the mechanism of removing the films through the method of the second embodiment is the same as the mechanism of removing the MgO film in the first embodiment. 
     Third Embodiment 
     Next, a method for processing a semiconductor device according to a third embodiment of the present invention will be described. In the third embodiment, a deposited film having an Al 2 O 3  film under a HfO film  202  is removed. In the case of the materials, the Al 2 O 3  film is first wet-etched in HF solution (concentration: 0.25%, time: 120 seconds). Then, the HfO film  202  is dry-etched by a mixed gas of BCl 3  of 80 ml/min and Cl 2  of 20 ml/min, in plasma at a pressure of 0.2 Pa, for 20 seconds with the wafer applied with a bias power of 10 W and subsequently for 60 seconds at a bias power of 0 W. The deposited film can be removed through the processes. It is known that completely crystallized Al 2 O 3  films are insoluble in acid; however, if the Al 2 O 3  film is a thin film formed by CVD (chemical vapor deposition) as an insulating film or the like used in a semiconductor device, such a thin Al 2 O 3  film can be removed by the HF solution. It is deemed that the mechanism of removing the films through the method of the third embodiment is the same as the mechanism of removing the MgO film in the first embodiment. 
     As described above, removal of a deposited film having a HfO film mixed with or placed on or under Mg, Y or Al is possible by alternately repeating the wet etching process and dry etching process. In addition, starting the removal procedure of the high-k dielectric film with the wet etching process can reduce the process time and damage in comparison to a procedure starting with dry etching. Even if a ZrO film is used instead of the HfO film, the deposited film including the ZrO film can be dry-etched by BCl 3 /Cl 2  gas. 
     In the above-described embodiments, the conditions and the number of the wet etching processes and dry etching processes vary according to the thickness of the deposited film and therefore need to be optimized appropriately. 
     Fourth Embodiment 
     A method for processing a semiconductor device according to a fourth embodiment of the present invention will be described. This description explains the relationship of metal gate material, etching gas associated with the material and a high-k dielectric film in terms of removal. Although the metal gate is made of a TiN film in the above-described embodiments, the metal gate can be made of TaN, TaSiN, MoN, MoSiN and other various kinds of metal or nitride thereof. Gas used to etch the metal gate varies according to the material of the metal gate. For smooth removal of the high-k dielectric film, it is preferable to choose hard-to-deposit etching gases. Specifically, the appropriate gas to etch TiN, for the purpose of demonstrating the effect available by the method according to the embodiments of the present invention, is CF 4  gas for a breakthrough etching and Cl 2  gas or Cl 2  gas added with noble gas for the subsequent etching. The Cl 2  gas is also desirable to etch TaN. For materials, such as TaSiN, which contains a large amount of Si, the appropriate gas is CF 4 , SF 6  or NF 3 . In the case of MoN, it is preferable to use Cl 2  gas mixed with a miniscule amount of oxygen. If gases with a high deposition property, such as HBr and CHF 3  are added, a reaction product is deposited on a surface of the etched high-k dielectric film and may hinder the high-k dielectric film from being removed thereafter. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.